A MAGNETIC RIBBON MODEL FOR STAR-FORMING FILAMENTS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Auddy, Sayantan; Basu, Shantanu; Kudoh, Takahiro, E-mail: sauddy3@uwo.ca, E-mail: basu@uwo.ca, E-mail: kudoh@nagasaki-u.ac.jp

2016-11-01

We develop a magnetic ribbon model for molecular cloud filaments. These result from turbulent compression in a molecular cloud in which the background magnetic field sets a preferred direction. We argue that this is a natural model for filaments and is based on the interplay between turbulence, strong magnetic fields, and gravitationally driven ambipolar diffusion, rather than pure gravity and thermal pressure. An analytic model for the formation of magnetic ribbons that is based on numerical simulations is used to derive a lateral width of a magnetic ribbon. This differs from the thickness along the magnetic field direction, which ismore » essentially the Jeans scale. We use our model to calculate a synthetic observed relation between apparent width in projection versus observed column density. The relationship is relatively flat, similar to observations, and unlike the simple expectation based on a Jeans length argument.« less

GMC COLLISIONS AS TRIGGERS OF STAR FORMATION. I. PARAMETER SPACE EXPLORATION WITH 2D SIMULATIONS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wu, Benjamin; Loo, Sven Van; Tan, Jonathan C.

We utilize magnetohydrodynamic (MHD) simulations to develop a numerical model for giant molecular cloud (GMC)–GMC collisions between nearly magnetically critical clouds. The goal is to determine if, and under what circumstances, cloud collisions can cause pre-existing magnetically subcritical clumps to become supercritical and undergo gravitational collapse. We first develop and implement new photodissociation region based heating and cooling functions that span the atomic to molecular transition, creating a multiphase ISM and allowing modeling of non-equilibrium temperature structures. Then in 2D and with ideal MHD, we explore a wide parameter space of magnetic field strength, magnetic field geometry, collision velocity, andmore » impact parameter and compare isolated versus colliding clouds. We find factors of ∼2–3 increase in mean clump density from typical collisions, with strong dependence on collision velocity and magnetic field strength, but ultimately limited by flux-freezing in 2D geometries. For geometries enabling flow along magnetic field lines, greater degrees of collapse are seen. We discuss observational diagnostics of cloud collisions, focussing on {sup 13}CO(J = 2–1), {sup 13}CO(J = 3–2), and {sup 12}CO(J = 8–7) integrated intensity maps and spectra, which we synthesize from our simulation outputs. We find that the ratio of J = 8–7 to lower-J emission is a powerful diagnostic probe of GMC collisions.« less

Polarization models of filamentary molecular clouds.

NASA Astrophysics Data System (ADS)

Carlqvist, P.; Kristen, H.

1997-08-01

We study numerically the linear polarization and extinction of light from background stars in three types of models of elongated molecular clouds by following the development of the Stokes parameters. The clouds are assumed to be of cylindrical shape and penetrated by a helical magnetic field {vec}(B). In the first two models we study only the relative magnitude of the polarization assuming that the polarization is proportional to Bmu^, where primarily μ=2. Provided there is no background/foreground polarization present we find from the cylindrically symmetric Model I that the angle of polarization has a bimodal character with the polarization being either parallel with or perpendicular to the axis of the filament. For some magnetic-field geometries both angles may exist in one and the same filament. It is concluded that it is not a straightforward task to find the magnetic-field-line pattern from the polarization pattern. If a background/foreground polarization exists or, as in Model II, the filament is not cylindrically symmetric, the bimodal character of the angle of polarization is lost. By means of Model III we have, using semi-empirical methods based on the Davis-Greenstein mechanism, estimated the absolute degree of polarization in the filamentary molecular cloud L204. It is found that the polarization produced by the model is much less than the polarization observed. We therefore conclude that most of the polarization measured in the L204 cloud is not produced in the cloud itself but is constituted by a large-scale background/foreground polarization.

The physics of interstellar shock waves

NASA Technical Reports Server (NTRS)

Shull, J. Michael; Draine, Bruce T.

1987-01-01

This review discusses the observations and theoretical models of interstellar shock waves, in both diffuse cloud and molecular cloud environments. It summarizes the relevant gas dynamics, atomic, molecular and grain processes, radiative transfer, and physics of radiative and magnetic precursors in shock models. It then describes the importance of shocks for observations, diagnostics, and global interstellar dynamics. It concludes with current research problems and data needs for atomic, molecular and grain physics.

Very Large Array H I Zeeman Observations of the Cygnus X Region: DR 22 and ON 2

NASA Astrophysics Data System (ADS)

Mayo, E. A.; Troland, T. H.

2012-02-01

We have used the Very Large Array to study the Zeeman effect in 21 cm H I absorption lines from two star-forming regions in the Cygnus X complex, DR 22 and ON 2. We measure the line-of-sight magnetic field toward these regions, finding B los = -84 ± 11 μG toward the DR 22 H II region and B los < 50 μG toward each of the two H II regions in ON 2. We interpret these results in terms of two different models. In one model, we assume that the H I Zeeman effect is a measure of magnetic fields in the associated molecular clouds. If so, then the DR 22 molecular cloud is magnetically subcritical, that is, magnetically dominated. The ON 2 molecular clouds are magnetically supercritical. In a second model, we assume that the H I Zeeman effect is a measure of magnetic fields in photon-dominated regions where the gas has been compressed (and the field amplified) by absorption of stellar radiation. We find that this second model, where the measured field strength has been affected by star formation, accounts well for the DR 22 H I Zeeman effect. This same model, however, overpredicts the magnetic field in ON 2. ON 2 may be a region where the magnetic field is energetically insignificant or where the field happens to lie nearly in the plane of the sky.

Fragmentation of interstellar clouds and star formation

NASA Technical Reports Server (NTRS)

Silk, J.

1982-01-01

The principal issues are addressed: the fragmentation of molecular clouds into units of stellar mass and the impact of star formation on molecular clouds. The observational evidence for fragmentation is summarized, and the gravitational instability described of a uniform spherical cloud collapsing from rest. The implications are considered of a finite pressure for the minimum fragment mass that is attainable in opacity-limited fragmentation. The role of magnetic fields is discussed in resolving the angular momentum problem and in making the collapse anisotropic, with notable consequences for fragmentation theory. Interactions between fragments are described, with emphasis on the effect of protostellar winds on the ambient cloud matter and on inhibiting further star formation. Such interactions are likely to have profound consequences for regulating the rate of star formation and on the energetics and dynamics of molecular clouds.

Collapse and Fragmentation of Molecular Cloud Cores. VII. Magnetic Fields and Multiple Protostar Formation

NASA Astrophysics Data System (ADS)

Boss, Alan P.

2002-04-01

Recent observations of star-forming regions suggest that binary and multiple young stars are the rule rather than the exception and implicate fragmentation as the likely mechanism for their formation. Most numerical hydrodynamic calculations of fragmentation have neglected the possibly deleterious effects of magnetic fields, despite ample evidence for the importance of magnetic support of precollapse clouds. We present here the first numerical hydrodynamic survey of the collapse and fragmentation of initially magnetically supported clouds that takes into account several magnetic field effects in an approximate manner. The models are calculated with a three-dimensional, finite differences code that solves the equations of hydrodynamics, gravitation, and radiative transfer in the Eddington and diffusion approximations. Magnetic field effects are included through two simple approximations: magnetic pressure is added to the gas pressure, and magnetic tension is approximated by gravity dilution once collapse is well underway. Ambipolar diffusion of the magnetic field leading to cloud collapse is treated approximately as well. Models are calculated for a variety of initial cloud density profiles, shapes, and rotation rates. We find that in spite of the inclusion of magnetic field effects, dense cloud cores are capable of fragmenting into binary and multiple protostar systems. Initially prolate clouds tend to fragment into binary protostars, while initially oblate clouds tend to fragment into multiple protostar systems containing a small number (of the order of 4) of fragments. The latter are likely to be subject to rapid orbital evolution, with close encounters possibly leading to the ejection of fragments. Contrary to expectation, magnetic tension effects appear to enhance fragmentation, allowing lower mass fragments to form than would otherwise be possible, because magnetic tension helps to prevent a central density singularity from forming and producing a dominant single object. Magnetically supported dense cloud cores thus seem to be capable of collapsing and fragmenting into sufficient numbers of binary and multiple protostar systems to be compatible with observations of the relative rarity of single protostars.

Magnetization of Cloud Cores and Envelopes and Other Observational Consequences of Reconnection Diffusion

NASA Astrophysics Data System (ADS)

Lazarian, A.; Esquivel, A.; Crutcher, R.

2012-10-01

Recent observational results for magnetic fields in molecular clouds reviewed by Crutcher seem to be inconsistent with the predictions of the ambipolar diffusion theory of star formation. These include the measured decrease in mass to flux ratio between envelopes and cores, the failure to detect any self-gravitating magnetically subcritical clouds, the determination of the flat probability distribution function (PDF) of the total magnetic field strengths implying that there are many clouds with very weak magnetic fields, and the observed scaling Bvpropρ2/3 that implies gravitational contraction with weak magnetic fields. We consider the problem of magnetic field evolution in turbulent molecular clouds and discuss the process of magnetic field diffusion mediated by magnetic reconnection. For this process that we termed "reconnection diffusion," we provide a simple physical model and explain that this process is inevitable in view of the present-day understanding of MHD turbulence. We address the issue of the expected magnetization of cores and envelopes in the process of star formation and show that reconnection diffusion provides an efficient removal of magnetic flux that depends only on the properties of MHD turbulence in the core and the envelope. We show that as the amplitude of turbulence as well as the scale of turbulent motions decrease from the envelope to the core of the cloud, the diffusion of the magnetic field is faster in the envelope. As a result, the magnetic flux trapped during the collapse in the envelope is being released faster than the flux trapped in the core, resulting in much weaker fields in envelopes than in cores, as observed. We provide simple semi-analytical model calculations which support this conclusion and qualitatively agree with the observational results. Magnetic reconnection is also consistent with the lack of subcritical self-gravitating clouds, with the observed flat PDF of field strengths, and with the scaling of field strength with density. In addition, we demonstrate that the reconnection diffusion process can account for the empirical Larson relations and list a few other implications of the reconnection diffusion concept. We argue that magnetic reconnection provides a solution to the magnetic flux problem of star formation that agrees better with observations than the long-standing ambipolar diffusion paradigm. Due to the illustrative nature of our simplified model we do not seek quantitative agreement, but discuss the complementary nature of our approach to the three-dimensional MHD numerical simulations.

CHANGE OF MAGNETIC FIELD-GAS ALIGNMENT AT THE GRAVITY-DRIVEN ALFVÉNIC TRANSITION IN MOLECULAR CLOUDS: IMPLICATIONS FOR DUST POLARIZATION OBSERVATIONS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chen, Che-Yu; King, Patrick K.; Li, Zhi-Yun

Diffuse striations in molecular clouds are preferentially aligned with local magnetic fields, whereas dense filaments tend to be perpendicular to them. When and why this transition occurs remain uncertain. To explore the physics behind this transition, we compute the histogram of relative orientation (HRO) between the density gradient and the magnetic field in three-dimensional magnetohydrodynamic (MHD) simulations of prestellar core formation in shock-compressed regions within giant molecular clouds. We find that, in the magnetically dominated (sub-Alfvénic) post-shock region, the gas structure is preferentially aligned with the local magnetic field. For overdense sub-regions with super-Alfvénic gas, their elongation becomes preferentially perpendicularmore » to the local magnetic field. The transition occurs when self-gravitating gas gains enough kinetic energy from the gravitational acceleration to overcome the magnetic support against the cross-field contraction, which results in a power-law increase of the field strength with density. Similar results can be drawn from HROs in projected two-dimensional maps with integrated column densities and synthetic polarized dust emission. We quantitatively analyze our simulated polarization properties, and interpret the reduced polarization fraction at high column densities as the result of increased distortion of magnetic field directions in trans- or super-Alfvénic gas. Furthermore, we introduce measures of the inclination and tangledness of the magnetic field along the line of sight as the controlling factors of the polarization fraction. Observations of the polarization fraction and angle dispersion can therefore be utilized in studying local magnetic field morphology in star-forming regions.« less

Circumstellar Disks and Outflows in Turbulent Molecular Cloud Cores: Possible Formation Mechanism for Misaligned Systems

DOE Office of Scientific and Technical Information (OSTI.GOV)

Matsumoto, Tomoaki; Machida, Masahiro N.; Inutsuka, Shu-ichiro, E-mail: matsu@hosei.ac.jp

2017-04-10

We investigate the formation of circumstellar disks and outflows subsequent to the collapse of molecular cloud cores with the magnetic field and turbulence. Numerical simulations are performed by using an adaptive mesh refinement to follow the evolution up to ∼1000 years after the formation of a protostar. In the simulations, circumstellar disks are formed around the protostars; those in magnetized models are considerably smaller than those in nonmagnetized models, but their size increases with time. The models with stronger magnetic fields tend to produce smaller disks. During evolution in the magnetized models, the mass ratios of a disk to amore » protostar is approximately constant at ∼1%–10%. The circumstellar disks are aligned according to their angular momentum, and the outflows accelerate along the magnetic field on the 10–100 au scale; this produces a disk that is misaligned with the outflow. The outflows are classified into two types: a magnetocentrifugal wind and a spiral flow. In the latter, because of the geometry, the axis of rotation is misaligned with the magnetic field. The magnetic field has an internal structure in the cloud cores, which also causes misalignment between the outflows and the magnetic field on the scale of the cloud core. The distribution of the angular momentum vectors in a core also has a non-monotonic internal structure. This should create a time-dependent accretion of angular momenta onto the circumstellar disk. Therefore, the circumstellar disks are expected to change their orientation as well as their sizes in the long-term evolutions.« less

Gravitational instability of filamentary molecular clouds, including ambipolar diffusion; non-isothermal filament

NASA Astrophysics Data System (ADS)

Hosseinirad, Mohammad; Abbassi, Shahram; Roshan, Mahmood; Naficy, Kazem

2018-04-01

Recent observations of the filamentary molecular clouds show that their properties deviate from the isothermal equation of state. Theoretical investigations proposed that the logatropic and the polytropic equations of state with negative indexes can provide a better description for these filamentary structures. Here, we aim to compare the effects of these softer non-isothermal equations of state with their isothermal counterpart on the global gravitational instability of a filamentary molecular cloud. By incorporating the ambipolar diffusion, we use the non-ideal magnetohydrodynamics framework for a filament that is threaded by a uniform axial magnetic field. We perturb the fluid and obtain the dispersion relation both for the logatropic and polytropic equations of state by taking the effects of magnetic field and ambipolar diffusion into account. Our results suggest that, in absence of the magnetic field, a softer equation of state makes the system more prone to gravitational instability. We also observed that a moderate magnetic field is able to enhance the stability of the filament in a way that is sensitive to the equation of state in general. However, when the magnetic field is strong, this effect is suppressed and all the equations of state have almost the same stability properties. Moreover, we find that for all the considered equations of state, the ambipolar diffusion has destabilizing effects on the filament.

Interplay of dust alignment, grain growth, and magnetic fields in polarization: lessons from the emission-to-extinction ratio

NASA Astrophysics Data System (ADS)

Fanciullo, L.; Guillet, V.; Boulanger, F.; Jones, A. P.

2017-06-01

Context. Polarized extinction and emission from dust in the interstellar medium (ISM) are hard to interpret, as their dependence on dust optical properties, grain alignment, and magnetic field orientation is complex. This is particularly true in molecular clouds. The aforementioned phenomena are usually considered independently in polarization studies, while it is likely that they all contribute and their effects have yet to be disentangled. Aims: The data available today are not yet used to their full potential. The combination of emission and extinction, in particular, provides information not available from either of them alone. We combine data from the scientific literature on polarized dust extinction with Planck data on polarized emission, and we use them to constrain the possible variations in dust and environmental conditions inside molecular clouds, and especially translucent lines of sight, taking the magnetic field orientation into account. Methods: We focused on the dependence between λmax (the wavelength of maximum polarization in extinction) and other observables such as the extinction polarization, the emission polarization, and the ratio between the two. We set out to reproduce these correlations using Monte Carlo simulations in which we varied the relevant quantities in a dust model, which are grain alignment, size distribution, and magnetic field orientation, to mimic the diverse conditions that are expected inside molecular clouds. Results: None of the quantities we chose can explain the observational data on their own: the best results are obtained when all quantities vary significantly across and within clouds. However, some of the data, most notably the stars with a low ratio of polarization in emission to polarization in extinction, are not reproduced by our simulation. Conclusions: Our results suggest not only that dust evolution is necessary to explain polarization in molecular clouds, but that a simple change in size distribution is not sufficient to explain the data. Our results also point the way for future and more sophisticated models.

Relationship between turbulence energy and density variance in the solar neighbourhood molecular clouds

NASA Astrophysics Data System (ADS)

Kainulainen, J.; Federrath, C.

2017-11-01

The relationship between turbulence energy and gas density variance is a fundamental prediction for turbulence-dominated media and is commonly used in analytic models of star formation. We determine this relationship for 15 molecular clouds in the solar neighbourhood. We use the line widths of the CO molecule as the probe of the turbulence energy (sonic Mach number, ℳs) and three-dimensional models to reconstruct the density probability distribution function (ρ-PDF) of the clouds, derived using near-infrared extinction and Herschel dust emission data, as the probe of the density variance (σs). We find no significant correlation between ℳs and σs among the studied clouds, but we cannot rule out a weak correlation either. In the context of turbulence-dominated gas, the range of the ℳs and σs values corresponds to the model predictions. The data cannot constrain whether the turbulence-driving parameter, b, and/or thermal-to-magnetic pressure ratio, β, vary among the sample clouds. Most clouds are not in agreement with field strengths stronger than given by β ≲ 0.05. A model with b2β/ (β + 1) = 0.30 ± 0.06 provides an adequate fit to the cloud sample as a whole. Based on the average behaviour of the sample, we can rule out three regimes: (i) strong compression combined with a weak magnetic field (b ≳ 0.7 and β ≳ 3); (ii) weak compression (b ≲ 0.35); and (iii) a strong magnetic field (β ≲ 0.1). When we include independent magnetic field strength estimates in the analysis, the data rule out solenoidal driving (b < 0.4) for the majority of the solar neighbourhood clouds. However, most clouds have b parameters larger than unity, which indicates a discrepancy with the turbulence-dominated picture; we discuss the possible reasons for this.

The magnetic field of molecular clouds

NASA Astrophysics Data System (ADS)

Padoan, P.

2018-01-01

The magnetic field of molecular clouds (MCs) plays an important role in the process of star formation: it determines the statistical properties of supersonic turbulence that controls the fragmentation of MCs, controls the angular momentum transport during the protostellar collapse, and affects the stability of circumstellar disks. In this work, we focus on the problem of the determination of the magnetic field strength. We review the idea that the MC turbulence is super-Alfvénic, and we argue that MCs are bound to be born super-Alfvénic. We show that this scenario is supported by results from a recent simulation of supernova-driven turbulence on a scale of 250 pc, where the turbulent cascade is resolved on a wide range of scales, including the interior of MCs.

Discovery of Molecular and Atomic Clouds Associated with the Magellanic Superbubble 30 Doradus C

NASA Astrophysics Data System (ADS)

Sano, H.; Yamane, Y.; Voisin, F.; Fujii, K.; Yoshiike, S.; Inaba, T.; Tsuge, K.; Babazaki, Y.; Mitsuishi, I.; Yang, R.; Aharonian, F.; Rowell, G.; Filipović, M. D.; Mizuno, N.; Tachihara, K.; Kawamura, A.; Onishi, T.; Fukui, Y.

2017-07-01

We analyzed the 2.6 mm CO and 21 cm H I lines toward the Magellanic superbubble 30 Doradus C, in order to reveal the associated molecular and atomic gas. We uncovered five molecular clouds in a velocity range from 251 to 276 km s-1 toward the western shell. The non-thermal X-rays are clearly enhanced around the molecular clouds on a parsec scale, suggesting possible evidence for magnetic field amplification via shock-cloud interaction. The thermal X-rays are brighter in the eastern shell, where there are no dense molecular or atomic clouds, opposite to the western shell. The TeV γ-ray distribution may spatially match the total interstellar proton column density as well as the non-thermal X-rays. If the hadronic γ-ray is dominant, the total energy of the cosmic-ray protons is at least ˜ 1.2× {10}50 erg with the estimated mean interstellar proton density ˜60 cm-3. In addition, the γ-ray flux associated with the molecular cloud (e.g., MC3) could be detected and resolved by the Cherenkov Telescope Array (CTA). This should permit CTA to probe the diffusion of cosmic-rays into the associated dense ISM.

Magnetic Fields Versus Gravity

NASA Astrophysics Data System (ADS)

Hensley, Kerry

2018-04-01

Deep within giant molecular clouds, hidden by dense gas and dust, stars form. Unprecedented data from the Atacama Large Millimeter/submillimeter Array (ALMA) reveal the intricate magnetic structureswoven throughout one of the most massive star-forming regions in the Milky Way.How Stars Are BornThe Horsehead Nebulasdense column of gas and dust is opaque to visible light, but this infrared image reveals the young stars hidden in the dust. [NASA/ESA/Hubble Heritage Team]Simple theory dictates that when a dense clump of molecular gas becomes massive enough that its self-gravity overwhelms the thermal pressure of the cloud, the gas collapses and forms a star. In reality, however, star formation is more complicated than a simple give and take between gravity and pressure. Thedusty molecular gas in stellar nurseries is permeated with magnetic fields, which are thought to impede the inward pull of gravity and slow the rate of star formation.How can we learn about the magnetic fields of distant objects? One way is by measuring dust polarization. An elongated dust grain will tend to align itself with its short axis parallel to the direction of the magnetic field. This systematic alignment of the dust grains along the magnetic field lines polarizes the dust grains emission perpendicular to the local magnetic field. This allows us to infer the direction of the magnetic field from the direction of polarization.Magnetic field orientations for protostars e2 and e8 derived from Submillimeter Array observations (panels a through c) and ALMA observations (panels d and e). Click to enlarge. [Adapted from Koch et al. 2018]Tracing Magnetic FieldsPatrick Koch (Academia Sinica, Taiwan) and collaborators used high-sensitivity ALMA observations of dust polarization to learn more about the magnetic field morphology of Milky Way star-forming region W51. W51 is one of the largest star-forming regions in our galaxy, home to high-mass protostars e2, e8, and North.The ALMA observations reveal polarized emission toward all three sources. By extracting the magnetic field orientations from the polarization vectors, Koch and collaborators found that the molecular cloud contains an ordered magnetic field with never-before-seen structures. Several small clumps on the perimeter of the massive star-forming cores exhibit comet-shaped magnetic field structures, which could indicate that these smaller cores are being pulled toward the more massive cores.These findings hint that the magnetic field structure can tell us about the flow of material within star-forming regions key to understanding the nature of star formation itself.Maps of sin for two of the protostars (e2 and e8) and their surroundings. [Adapted from Koch et al. 2018]Guiding Star FormationDo the magnetic fields in W51 help or hinder star formation? To explore this question,Koch and collaborators introduced the quantity sin , where is the angle between the local gravity and the local magnetic field.When the angle between gravity and the magnetic field is small (sin 0), the magnetic field has little effect on the collapse of the cloud. If gravity and the magnetic field are perpendicular (sin 1), the magnetic field can slow the infall of gas and inhibit star formation.Based on this parameter, Koch and collaborators identified narrow channels where gravity acts unimpeded by the magnetic field. These magnetic channels may funnel gas toward the dense cores and aid the star-formation process.The authors observations demonstrate just one example of the broad realm ALMAs polarimetry capabilities have opened to discovery. These and future observations of dust polarization will continue to reveal more about the delicate magnetic structure within molecular clouds, furtherilluminating the role that magnetic fields play in star formation.CitationPatrick M. Koch et al 2018 ApJ 855 39. doi:10.3847/1538-4357/aaa4c1

MAGNETIC FIELD OF THE VELA C MOLECULAR CLOUD

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kusune, Takayoshi; Sugitani, Koji; Nakamura, Fumitaka

We have performed extensive near-infrared ( JHK {sub s}) imaging polarimetry toward the Vela C molecular cloud, which covers the five high-density sub-regions (North, Centre-Ridge, Centre-Nest, South-Ridge, and South-Nest) with distinct morphological characteristics. The obtained polarization vector map shows that three of these sub-regions have distinct plane-of-the-sky (POS) magnetic-field characteristics according to the morphological characteristics. (1) In the Centre-Ridge sub-region, a dominating ridge, the POS magnetic field is mostly perpendicular to the ridge. (2) In the Centre-Nest sub-region, a structure having a slightly extended nest of filaments, the POS magnetic field is nearly parallel to its global elongation. (3) Inmore » the South-Nest sub-region, which has a network of small filaments, the POS magnetic field appears to be chaotic. By applying the Chandrasekhar–Fermi method, we derived the POS magnetic field strength as ∼70–310 μ G in the Centre-Ridge, Centre-Nest, and South-Ridge sub-regions. In the South-Nest sub-region, the dispersion of polarization angles is too large to apply the C-F method. Because the velocity dispersion in this sub-region is not greater than those in the other sub-regions, we suggest that the magnetic field in this sub-region is weaker than those in other sub-regions. We also discuss the relationship between the POS magnetic field (configuration and strength) and the cloud structure of each sub-region.« less

Collapse and Fragmentation Models of Tidally Interacting Molecular Cloud Cores. IV. Initial Slow Rotation and Magnetic Field Support

NASA Astrophysics Data System (ADS)

Sigalotti, Leonardo Di G.; Klapp, Jaime

2000-03-01

Fragmentation has long been advocated as the primary mechanism for explaining the observed binary frequency among pre-main-sequence stars and, more recently, for explaining the emerging evidence for binary and multiple protostellar systems. The role of magnetic fields and ambipolar diffusion is essential to understand how dense cloud cores begin dynamic collapse and eventually fragment into protostars. Here we consider new numerical models of the gravitational collapse and fragmentation of slowly rotating molecular cloud cores, including the effects of magnetic support and ambipolar diffusion. The starting point of the evolution is provided by a magnetically stable (subcritical) condensation that results from adding a magnetic field pressure, B2/8π [with the field strength given by the scaling relation B=B0(ρ/ρ0)1/2], to a reference state consisting of a thermally supercritical (α~0.36), slowly rotating (β~0.037), Gaussian cloud core of prolate shape and central density ρ0. The effects of ambipolar diffusion are approximated by allowing the reference field strength B0 to gradually decrease over a timescale of 10 free-fall times. The models also include the effects of tidal interaction due to a gravitational encounter with another protostar, and so they may apply to low-mass star formation within a cluster-forming environment. The results indicate that the magnetic forces delay the onset of dynamic collapse, and hence of fragmentation, by an amount of time that depends on the initial central mass-to-flux ratio. Compared with previous magnetic collapse calculations of rapidly rotating (β=0.12) clouds, lower initial rotation (β~0.037) is seen to result in much shorter delay periods, thus anticipating binary fragmentation. In general, the results show that the models are still susceptible to fragment into binary systems. Intermediate magnetic support (η~0.285) and low tidal forces (τ<~0.201) may lead to final triple or quadruple protostellar systems, while increasing the size of η and τ always results in final binary protostellar cores. The formed binary systems have separations of ~200-350 AU, suggesting that the recently observed peaks around ~90 AU and 215 AU for T Tauri stars may be explained by the collapse and fragmentation of initially slowly rotating magnetic cloud cores with β<~0.04.

Giant molecular cloud collisions as triggers of star formation. VI. Collision-induced turbulence

NASA Astrophysics Data System (ADS)

Wu, Benjamin; Tan, Jonathan C.; Nakamura, Fumitaka; Christie, Duncan; Li, Qi

2018-05-01

We investigate collisions between giant molecular clouds (GMCs) as potential generators of their internal turbulence. Using magnetohydrodynamic (MHD) simulations of self-gravitating, magnetized, turbulent GMCs, we compare kinematic and dynamic properties of dense gas structures formed when such clouds collide compared to those that form in non-colliding clouds as self-gravity overwhelms decaying turbulence. We explore the nature of turbulence in these structures via distribution functions of density, velocity dispersions, virial parameters, and momentum injection. We find that the dense clumps formed from GMC collisions have higher effective Mach number, greater overall velocity dispersions, sustain near-virial equilibrium states for longer times, and are the conduit for the injection of turbulent momentum into high density gas at high rates.

Giant molecular cloud collisions as triggers of star formation. VI. Collision-induced turbulence

NASA Astrophysics Data System (ADS)

Wu, Benjamin; Tan, Jonathan C.; Nakamura, Fumitaka; Christie, Duncan; Li, Qi

2018-01-01

We investigate collisions between giant molecular clouds (GMCs) as potential generators of their internal turbulence. Using magnetohydrodynamic (MHD) simulations of self-gravitating, magnetized, turbulent GMCs, we compare kinematic and dynamic properties of dense gas structures formed when such clouds collide compared to those that form in non-colliding clouds as self-gravity overwhelms decaying turbulence. We explore the nature of turbulence in these structures via distribution functions of density, velocity dispersions, virial parameters, and momentum injection. We find that the dense clumps formed from GMC collisions have higher effective Mach number, greater overall velocity dispersions, sustain near-virial equilibrium states for longer times, and are the conduit for the injection of turbulent momentum into high density gas at high rates.

TWO-STAGE FRAGMENTATION FOR CLUSTER FORMATION: ANALYTICAL MODEL AND OBSERVATIONAL CONSIDERATIONS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bailey, Nicole D.; Basu, Shantanu, E-mail: nwityk@uwo.ca, E-mail: basu@uwo.ca

2012-12-10

Linear analysis of the formation of protostellar cores in planar magnetic interstellar clouds shows that molecular clouds exhibit a preferred length scale for collapse that depends on the mass-to-flux ratio and neutral-ion collision time within the cloud. We extend this linear analysis to the context of clustered star formation. By combining the results of the linear analysis with a realistic ionization profile for the cloud, we find that a molecular cloud may evolve through two fragmentation events in the evolution toward the formation of stars. Our model suggests that the initial fragmentation into clumps occurs for a transcritical cloud onmore » parsec scales while the second fragmentation can occur for transcritical and supercritical cores on subparsec scales. Comparison of our results with several star-forming regions (Perseus, Taurus, Pipe Nebula) shows support for a two-stage fragmentation model.« less

Equiparatition of energy for turbulent astrophysical fluids: Accounting for the unseen energy in molecular clouds

NASA Technical Reports Server (NTRS)

Zweibel, Ellen G.; Mckee, Christopher F.

1995-01-01

Molecular clouds are observed to be partially supported by turbulent pressure. The kinetic energy of the turbulence is directly measurable, but the potential energy, which consists of magnetic, thermal, and gravitational potential energy, is largly unseen. We have extended previous results on equipartition between kinetic and potential energy to show that it is likely to be a very good approximation in molecular clouds. We have used two separate approaches to demonstrate this result: For small-amplitude perturbations of a static equilibrium, we have used the energy principle analysis of Bernstein et al. (1958); this derivation applies to perturbations of arbitary wavelength. To treat perturbations of a nonstatic equilibrium, we have used the Lagrangian analysis of Dewar (1970); this analysis applies only to short-wavelength perturbations. Both analysis assume conservation of energy. Wave damping has only a small effect on equipartition if the wave frequency is small compared to the neutral-ion collision frequency; for the particular case we considered, radiative losses have no effect on equipartition. These results are then incorporated in a simple way into analyses of cloud equilibrium and global stability. We discuss the effect of Alfvenic turbulence on the Jeans mass and show that it has little effect on the magnetic critical mass.

A Deep X-ray Survey of Low-Mass PMS Stars in NGC 2264

NASA Technical Reports Server (NTRS)

Simon, Theodore

2005-01-01

Two X-ray images were obtained with the XMM-Newton spacecraft of more than 300 members of the NGC 2264 Open Cluster and its associated molecular cloud in order to investigate their magnetic activity. The X-ray fluxes extracted from those observations were used to study the dependence of stellar dynamo activity upon age and rotation for the optically revealed T Tauri stars and to place empirical constraints on theoretical models of angular momentum/dynamo evolution. The observations were also used to study the role of magnetic fields in the formation of low mass stars through the observation of very young protostars that are deeply embedded in the molecular cloud located behind the visible open cluster.

The structure and statistics of interstellar turbulence

NASA Astrophysics Data System (ADS)

Kritsuk, A. G.; Ustyugov, S. D.; Norman, M. L.

2017-06-01

We explore the structure and statistics of multiphase, magnetized ISM turbulence in the local Milky Way by means of driven periodic box numerical MHD simulations. Using the higher order-accurate piecewise-parabolic method on a local stencil (PPML), we carry out a small parameter survey varying the mean magnetic field strength and density while fixing the rms velocity to observed values. We quantify numerous characteristics of the transient and steady-state turbulence, including its thermodynamics and phase structure, kinetic and magnetic energy power spectra, structure functions, and distribution functions of density, column density, pressure, and magnetic field strength. The simulations reproduce many observables of the local ISM, including molecular clouds, such as the ratio of turbulent to mean magnetic field at 100 pc scale, the mass and volume fractions of thermally stable Hi, the lognormal distribution of column densities, the mass-weighted distribution of thermal pressure, and the linewidth-size relationship for molecular clouds. Our models predict the shape of magnetic field probability density functions (PDFs), which are strongly non-Gaussian, and the relative alignment of magnetic field and density structures. Finally, our models show how the observed low rates of star formation per free-fall time are controlled by the multiphase thermodynamics and large-scale turbulence.

Probes of turbulent driving mechanisms in molecular clouds from fluctuations in synchrotron intensity

NASA Astrophysics Data System (ADS)

Herron, C. A.; Federrath, C.; Gaensler, B. M.; Lewis, G. F.; McClure-Griffiths, N. M.; Burkhart, Blakesley

2017-04-01

Previous studies have shown that star formation depends on the driving of molecular cloud turbulence, and differences in the driving can produce an order of magnitude difference in the star formation rate. The turbulent driving is characterized by the parameter ζ, with ζ = 0 for compressive, curl-free driving (e.g. accretion or supernova explosions), and ζ = 1 for solenoidal, divergence-free driving (e.g. Galactic shear). Here we develop a new method to measure ζ from observations of synchrotron emission from molecular clouds. We calculate statistics of mock synchrotron intensity images produced from magnetohydrodynamic simulations of molecular clouds, in which the driving was controlled to produce different values of ζ. We find that the mean and standard deviation of the log-normalized synchrotron intensity are sensitive to ζ, for values of ζ between 0 (curl-free driving) and 0.5 (naturally mixed driving). We quantify the dependence of zeta on the direction of the magnetic field relative to the line of sight. We provide best-fitting formulae for ζ in terms of the log-normalized mean and standard deviation of synchrotron intensity, with which ζ can be determined for molecular clouds that have similar Alfvénic Mach number to our simulations. These formulae are independent of the sonic Mach number. Signal-to-noise ratios larger than 5, and angular resolutions smaller than 5 per cent of the cloud diameter, are required to apply these formulae. Although there are no firm detections of synchrotron emission from molecular clouds, by combining Green Bank Telescope and Very Large Array observations it should be possible to detect synchrotron emission from molecular clouds, thereby constraining the value of ζ.

Star-Forming Clouds Feed, Churn, and Fall

NASA Astrophysics Data System (ADS)

Kohler, Susanna

2017-12-01

Molecular clouds, the birthplaces of stars in galaxies throughout the universe, are complicated and dynamic environments. A new series of simulations has explored how these clouds form, grow, and collapse over their lifetimes.This composite image shows part of the Taurus Molecular Cloud. [ESO/APEX (MPIfR/ESO/OSO)/A. Hacar et al./Digitized Sky Survey]Stellar BirthplacesMolecular clouds form out of the matter in between stars, evolving through constant interactions with their turbulent environments. These interactions taking the form of accretion flows and surface forces, while gravity, turbulence, and magnetic fields interplay are thought to drive the properties and evolution of the clouds.Our understanding of the details of this process, however, remains fuzzy. How does mass accretion affect these clouds as they evolve? What happens when nearby supernova explosions blast the outsides of the clouds? What makes the clouds churn, producing the motion within them that prevents them from collapsing? The answers to these questions can tellus about the gas distributed throughout galaxies, revealing information about the environments in which stars form.A still from the simulation results showing the broader population of molecular clouds that formed in the authors simulations, as well as zoom-in panels of three low-mass clouds tracked in high resolution. [Ibez-Meja et al. 2017]Models of TurbulenceIn a new study led by Juan Ibez-Meja (MPI Garching and Universities of Heidelberg and Cologne in Germany, and American Museum of Natural History), scientists have now explored these questions using a series of three-dimensional simulations of a population of molecular clouds forming and evolving in the turbulent interstellar medium.The simulations take into account a whole host of physics, including the effects of nearby supernova explosions, self-gravitation, magnetic fields, diffuse heating, and radiative cooling. After looking at the behavior of the broader population of clouds, the authors zoom in and explore three clouds in high-resolution to learn more about the details.Watching Clouds EvolveIbez-Meja and collaborators find that mass accretion occurring after the molecular clouds form plays an important role in the clouds evolution, increasing the mass available to form stars and carrying kinetic energy into the cloud. The accretion process is driven both by background turbulent flows and gravitational attraction as the cloud draws in the gas in its nearby environment.Plots of the cloud mass and radius (top) and mass accretion rate (bottom) for one of the three zoomed-in clouds, shown as a function of time over the 10-Myr simulation. [Adapted from Ibez-Meja et al. 2017]The simulations show that nearby supernovae have two opposing effects on a cloud. On one hand, the blast waves from supernovae compress the envelope of the cloud, increasing the instantaneous rate of accretion. On the other hand, the blast waves disrupt parts of the envelope and erode mass from the clouds surface, decreasing accretion overall. These events ensure that the mass accretion rate of molecular clouds is non-uniform, regularly punctuated by sporadic increases and decreases as the clouds are battered by nearby explosions.Lastly, Ibez-Meja and collaborators show that mass accretion alone isnt enough to power the turbulent internal motions we observe inside molecular clouds. Instead, they conclude, the cloud motions must be primarily powered by gravitational potential energy being converted into kinetic energy as the cloud contracts.The authors simulations therefore show that molecular clouds exist in a state of precarious balance, prevented from collapsing by internal turbulence driven by interactions with their environment and by their own contraction. These results give us an intriguing glimpse into the complex environments in which stars are born.BonusCheck out the animated figure below, which displays how the clouds in the authors simulations evolve over the span of 10 million years.http://cdn.iopscience.com/images/0004-637X/850/1/62/Full/apjaa93fef1_video.mp4CitationJuan C. Ibez-Meja et al 2017 ApJ 850 62. doi:10.3847/1538-4357/aa93fe

Polarization of seven MBM clouds at high Galactic latitude

NASA Astrophysics Data System (ADS)

Neha, S.; Maheswar, G.; Soam, A.; Lee, C. W.

2018-06-01

We made R-band polarization measurements of 234 stars towards the direction of the MBM 33-39 cloud complex. The distance of the MBM 33-39 complex was determined as 120 ± 10 pc using polarization results and near-infrared photometry from the 2MASS survey. The magnetic field geometry of the individual clouds inferred from our polarimetric results reveals that the field lines are in general consistent with the global magnetic field geometry of the region obtained from previous studies. This implies that the clouds in the complex are permeated by the interstellar magnetic field. Multi-wavelength polarization measurements of a few stars projected on to the complex suggest that the size of the dust grains in these clouds is similar to those found in the normal interstellar medium of the Milky Way. We studied a possible formation scenario of the MBM 33-39 complex by combining the polarization results from our study with those from the literature and by identifying the distribution of ionized, atomic and molecular (dust) components of material in the region.

GRAVITATIONAL CONTRACTION VERSUS SUPERNOVA DRIVING AND THE ORIGIN OF THE VELOCITY DISPERSION–SIZE RELATION IN MOLECULAR CLOUDS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ibáñez-Mejía, Juan C.; Mac Low, Mordecai-Mark; Klessen, Ralf S.

Molecular cloud (MC) observations show that clouds have non-thermal velocity dispersions that scale with the cloud size as σ ∝ R {sup 1/2} at a constant surface density, and for varying surface density scale with both the cloud’s size and surface density, σ {sup 2} ∝ R Σ. The energy source driving these chaotic motions remains poorly understood. We describe the velocity dispersions observed in a cloud population formed in a numerical simulation of a magnetized, stratified, supernova (SN)-driven, interstellar medium, including diffuse heating and radiative cooling, before and after we include the effects of the self-gravity of the gas.more » We compare the relationships between velocity dispersion, size, and surface density measured in the simulated cloud population to those found in observations of Galactic MCs. Our simulations prior to the onset of self-gravity suggest that external SN explosions alone do not drive turbulent motions of the observed magnitudes within dense clouds. On the other hand, self-gravity induces non-thermal motions as gravitationally bound clouds begin to collapse in our model, approaching the observed relations between velocity dispersion, size, and surface density. Energy conservation suggests that the observed behavior is consistent with the kinetic energy being proportional to the gravitational energy. However, the clouds in our model show no sign of reaching a stable equilibrium state at any time, even for strongly magnetized clouds. We conclude that gravitationally bound MCs are always in a state of gravitational contraction and their properties are a natural result of this chaotic collapse. In order to agree with observed star formation efficiencies, this process must be terminated by the early destruction of the clouds, presumably from internal stellar feedback.« less

Quantifying the interplay between gravity and magnetic field in molecular clouds - a possible multiscale energy equipartition in NGC 6334

NASA Astrophysics Data System (ADS)

Li, Guang-Xing; Burkert, Andreas

2018-02-01

The interplay between gravity, turbulence and the magnetic field determines the evolution of the molecular interstellar medium (ISM) and the formation of the stars. In spite of growing interests, there remains a lack of understanding of the importance of magnetic field over multiple scales. We derive the magnetic energy spectrum - a measure that constraints the multiscale distribution of the magnetic energy, and compare it with the gravitational energy spectrum derived in Li & Burkert. In our formalism, the gravitational energy spectrum is purely determined by the surface density probability density distribution (PDF), and the magnetic energy spectrum is determined by both the surface density PDF and the magnetic field-density relation. If regions have density PDFs close to P(Σ) ˜ Σ-2 and a universal magnetic field-density relation B ˜ ρ1/2, we expect a multiscale near equipartition between gravity and the magnetic fields. This equipartition is found to be true in NGC 6334, where estimates of magnetic fields over multiple scales (from 0.1 pc to a few parsec) are available. However, the current observations are still limited in sample size. In the future, it is necessary to obtain multiscale measurements of magnetic fields from different clouds with different surface density PDFs and apply our formalism to further study the gravity-magnetic field interplay.

Evidence for Decay of Turbulence by MHD Shocks in the ISM via CO Emission

NASA Astrophysics Data System (ADS)

Larson, Rebecca L.; Evans, Neal J., II; Green, Joel D.; Yang, Yao-Lun

2015-06-01

We utilize observations of sub-millimeter rotational transitions of CO from a Herschel Cycle 2 open time program (“COPS”, PI: J. Green) to identify previously predicted turbulent dissipation by magnetohydrodynamic (MHD) shocks in molecular clouds. We find evidence of the shocks expected for dissipation of MHD turbulence in material not associated with any protostar. Two models fit about equally well: model 1 has a density of 103 cm-3, a shock velocity of 3 km s-1, and a magnetic field strength of 4 μG model 2 has a density of 103.5 cm-3, a shock velocity of 2 km s-1, and a magnetic field strength of 8 μG. Timescales for decay of turbulence in this region are comparable to crossing times. Transitions of CO up to J of 8, observed close to active sites of star formation, but not within outflows, can trace turbulent dissipation of shocks stirred by formation processes. Although the transitions are difficult to detect at individual positions, our Herschel-SPIRE survey of protostars provides a grid of spatially distributed spectra within molecular clouds. We averaged all spatial positions away from known outflows near seven protostars. We find significant agreement with predictions of models of turbulent dissipation in slightly denser (103.5 cm-3) material with a stronger magnetic field (24 μG) than in the general molecular cloud.

The Hall effect in star formation

NASA Astrophysics Data System (ADS)

Braiding, C. R.; Wardle, M.

2012-05-01

Magnetic fields play an important role in star formation by regulating the removal of angular momentum from collapsing molecular cloud cores. Hall diffusion is known to be important to the magnetic field behaviour at many of the intermediate densities and field strengths encountered during the gravitational collapse of molecular cloud cores into protostars, and yet its role in the star formation process is not well studied. We present a semianalytic self-similar model of the collapse of rotating isothermal molecular cloud cores with both Hall and ambipolar diffusion, and similarity solutions that demonstrate the profound influence of the Hall effect on the dynamics of collapse. The solutions show that the size and sign of the Hall parameter can change the size of the protostellar disc by up to an order of magnitude and the protostellar accretion rate by 50 per cent when the ratio of the Hall to ambipolar diffusivities is varied between -0.5 ≤ηH/ηA≤ 0.2. These changes depend upon the orientation of the magnetic field with respect to the axis of rotation and create a preferred handedness to the solutions that could be observed in protostellar cores using next-generation instruments such as ALMA. Hall diffusion also determines the strength and position of the shocks that bound the pseudo and rotationally supported discs, and can introduce subshocks that further slow accretion on to the protostar. In cores that are not initially rotating (not examined here), Hall diffusion can even induce rotation, which could give rise to disc formation and resolve the magnetic braking catastrophe. The Hall effect clearly influences the dynamics of gravitational collapse and its role in controlling the magnetic braking and radial diffusion of the field merits further exploration in numerical simulations of star formation.

VLA OH Zeeman Observations of the NGC 6334 Complex Source A

NASA Astrophysics Data System (ADS)

Mayo, E. A.; Sarma, A. P.; Troland, T. H.; Abel, N. P.

2004-12-01

We present a detailed analysis of the NGC 6334 complex source A, a compact continuum source in the SW region of the complex. Our intent is to determine the significance of the magnetic field in the support of the surrounding molecular cloud against gravitational collapse. We have performed OH 1665 and 1667 MHz observations taken with the Very Large Array in the BnA configuration and combined these data with the lower resolution CnB data of Sarma et al. (2000). These observations reveal magnetic fields with values of the order of 350 μ G toward source A, with maximum fields reaching 500 μ G. We have also theoretically modeled the molecular cloud surrounding source A using Cloudy, with the constraints to the model based on observation. This model provides significant information on the density of H2 through the cloud and also the relative density of H2 to OH which is important to our analysis of the region. We will combine the knowledge gained through the Cloudy modeling with Virial estimates to determine the significance of the magnetic field to the dynamics and evolution of source A.

The formation of massive molecular filaments and massive stars triggered by a magnetohydrodynamic shock wave

NASA Astrophysics Data System (ADS)

Inoue, Tsuyoshi; Hennebelle, Patrick; Fukui, Yasuo; Matsumoto, Tomoaki; Iwasaki, Kazunari; Inutsuka, Shu-ichiro

2018-05-01

Recent observations suggest an that intensive molecular cloud collision can trigger massive star/cluster formation. The most important physical process caused by the collision is a shock compression. In this paper, the influence of a shock wave on the evolution of a molecular cloud is studied numerically by using isothermal magnetohydrodynamics simulations with the effect of self-gravity. Adaptive mesh refinement and sink particle techniques are used to follow the long-time evolution of the shocked cloud. We find that the shock compression of a turbulent inhomogeneous molecular cloud creates massive filaments, which lie perpendicularly to the background magnetic field, as we have pointed out in a previous paper. The massive filament shows global collapse along the filament, which feeds a sink particle located at the collapse center. We observe a high accretion rate \\dot{M}_acc> 10^{-4} M_{⊙}yr-1 that is high enough to allow the formation of even O-type stars. The most massive sink particle achieves M > 50 M_{⊙} in a few times 105 yr after the onset of the filament collapse.

Prospects for Studying Interstellar Magnetic Fields with a Far-Infrared Polarimeter for SAFIR

NASA Technical Reports Server (NTRS)

Dowell, C. Darren; Chuss, D. T.; Dotson, J. L.

2008-01-01

Polarimetry at mid-infrared through millimeter wavelengths using airborne and ground-based telescopes has revealed magnetic structures in dense molecular clouds in the interstellar medium, primarily in regions of star formation. Furthermore, spectropolarimetry has offered clues about the composition of the dust grains and the mechanism by which they are aligned with respect to the local magnetic field. The sensitivity of the observations to date has been limited by the emission from the atmosphere and warm telescopes. A factor of 1000 in sensitivity can be gained by using instead a cold space telescope. With 5 arcminute resolution, Planck will make the first submillimeter polarization survey of the full Galaxy early in the next decade. We discuss the science case for and basic design of a far-infrared polarimeter on the SAFIR space telescope, which offers resolution in the few arcsecond range and wavelength selection of cold and warm dust components. Key science themes include the formation and evolution of molecular clouds in nearby spiral galaxies, the magnetic structure of the Galactic center, and interstellar turbulence.

Magnetic seismology of interstellar gas clouds: Unveiling a hidden dimension

NASA Astrophysics Data System (ADS)

Tritsis, Aris; Tassis, Konstantinos

2018-05-01

Stars and planets are formed inside dense interstellar molecular clouds by processes imprinted on the three-dimensional (3D) morphology of the clouds. Determining the 3D structure of interstellar clouds remains challenging because of projection effects and difficulties measuring the extent of the clouds along the line of sight. We report the detection of normal vibrational modes in the isolated interstellar cloud Musca, allowing determination of the 3D physical dimensions of the cloud. We found that Musca is vibrating globally, with the characteristic modes of a sheet viewed edge on, not the characteristics of a filament as previously supposed. We reconstructed the physical properties of Musca through 3D magnetohydrodynamic simulations, reproducing the observed normal modes and confirming a sheetlike morphology.

The violent interstellar medium in Milky-Way like disk galaxies

NASA Astrophysics Data System (ADS)

Karoline Walch, Stefanie

2015-08-01

Molecular clouds are cold, dense, and turbulent filamentary structures that condense out of the multi-phase interstellar medium. They are also the sites of star formation. The minority of new-born stars is massive, but these stars are particularly important for the fate of their parental molecular clouds as their feedback drives turbulence and regulates star formation.I will present results from the SILCC project (SImulating the Life Cycle of molecular Clouds), in which we study the formation and dispersal of molecular clouds within the multi-phase ISM using high-performance, three-dimensional simulations of representative pieces of disk galaxies. Apart from stellar feedback, self-gravity, an external stellar potential, and magnetic fields, we employ an accurate description of gas heating and cooling as well as a small chemical network including molecule formation and (self-)shielding from the interstellar radiation field. We study the impact of the supernova rate and the positioning of the supernova explosions with respect to the molecular gas in a well defined set of simulations. This allows us to draw conclusions on structure of the multi-phase ISM, the amount of molecular gas formed, and the onset of galactic outflows. Furthermore, we show how important stellar wind feedback is for regulating star formation in these disks.

FIRST OPTICAL AND NEAR-INFRARED POLARIMETRY OF A MOLECULAR CLOUD FORMING A PROTO-BROWN DWARF CANDIDATE

DOE Office of Scientific and Technical Information (OSTI.GOV)

Soam, A.; Maheswar, G.; Kwon, Jugmi

2015-04-20

LDN 328 is cited as an example of a fairly isolated clump contracting to form multiple sub-cores, possibly through gravitational fragmentation. In one of these sub-cores, a proto-brown dwarf (L328-IRS) candidate is in the process of formation through the self-gravitating contraction, similar to the formation scenario of a low-mass star. We present results of our optical and near-infrared polarization observations of regions toward LDN 328. This is the first observational attempt to map the magnetic field geometry of a cloud harboring a proto-brown dwarf candidate associated with a sub-parsec-scale molecular outflow. On a parsec scale, the magnetic field is foundmore » to follow the curved structure of the cloud showing a head–tail morphology. The magnetic field is found to be well ordered over a 0.02–0.2 pc scale around L328-IRS. Taking into account the uncertainties in the determination of position angles, the projected angular offset between the magnetic field direction and the outflow axis is found to be in the range of 0°–70°. Considering outflow to be the proxy for the rotation axis, the result obtained in this study implies that the rotation axis in L328 is preferably parallel to the local magnetic field. The magnetic field strength estimated in the close vicinity of L328-IRS is ∼20 μG. Results from the present study suggest that the magnetic field may be playing a vital role even in the cores that are forming sub-stellar sources.« less

High Resolution IRS Mapping of the Star-Forming Region NGC 6334 A

NASA Astrophysics Data System (ADS)

Sarma, Anuj; Abel, Nicholas; Ferland, Gary; Mayo, Elizabeth; Troland, Thomas

2005-06-01

Star formation involves the interplay of thermal, gravitational and magnetic forces. These processes lead to a dynamically evolving region in which O stars ionize the surrounding medium, and the ionized gas expands into the molecular cloud. Of these forces, magnetic effects are the least understood. A detailed analysis of the conditions in star-forming environments requires that one combine magnetic field observations with observations of the ionized, atomic, and molecular gas along with dust. We propose to carry out high-resolution IRS spectroscopy between 9.9-37.2 microns of the nearby (1.7 kpc) star-forming region NGC 6334 A. Maps of the magnetic field strength in the molecular gas exist for NGC 6334 A, yet the conditions in the H II region, the surrounding photodissociated region (PDR), and the dynamical interaction between the two regions are poorly understood. In the H II region, our proposed observation will allow us to use well-known infrared diagnostic ratios to determine the electron density, temperature, and the hardness of the continuum source. Spitzer observations of rotational transitions of molecular hydrogen and PAH emission, combined with previous observations, will allow us to determine the hydrogen density, UV radiation flux, and temperature in the PDR. We will combine our observations with theoretical calculations, using the spectral synthesis code Cloudy. Recent improvements to Cloudy include a ~1000 reaction molecular network, the ability to treat the dynamical flow of ionized gas into a molecular cloud, and the effects of magnetic pressure. Matching the observed spectra with theoretical calculations will tell us the physical conditions in the H II region and PDR, the role of magnetic fields in NGC 6334 A, and the importance of dynamics in the region. Overall, IRS observations of NGC 6334 A offers a unique opportunity to study, at high spatial resolution, many of the physical processes in star-forming regions.

Spatial and Temporal Energy Characterization of Precipitating Electrons for the January 10th, 1997 Magnetic Cloud Event

NASA Technical Reports Server (NTRS)

Spann, J. F., Jr.; Germany, G. A.; Brittnacher, M. J.; Parks, G. K.; Elsen, R.

1997-01-01

The January 10-11, 1997 magnetic cloud event provided a rare opportunity to study auroral energy deposition under varying but intense IMF conditions. The Wind spacecraft located about 100 RE upstream monitored the IMF and plasma parameters during the passing of the cloud. The Polar Ultraviolet Imager (UVI) observed the aurora[ precipitation during the first encounter of the cloud with Earth's magnetosphere and during several subsequent substorm events. The UVI has the unique capability of measuring the energy flux and characteristic energy of the precipitating electrons through the use of narrow band filters that distinguish short and long wavelength molecular nitrogen emissions. The spatial and temporal characteristics of the precipitating electron energy will be discussed beginning with the inception of the event at the Earth early January 1 Oth and continuing through the subsidence of auroral activity on January 11th.

The effect of extreme ionization rates during the initial collapse of a molecular cloud core

NASA Astrophysics Data System (ADS)

Wurster, James; Bate, Matthew R.; Price, Daniel J.

2018-05-01

What cosmic ray ionization rate is required such that a non-ideal magnetohydrodynamics (MHD) simulation of a collapsing molecular cloud will follow the same evolutionary path as an ideal MHD simulation or as a purely hydrodynamics simulation? To investigate this question, we perform three-dimensional smoothed particle non-ideal MHD simulations of the gravitational collapse of rotating, one solar mass, magnetized molecular cloud cores, which include Ohmic resistivity, ambipolar diffusion, and the Hall effect. We assume a uniform grain size of ag = 0.1 μm, and our free parameter is the cosmic ray ionization rate, ζcr. We evolve our models, where possible, until they have produced a first hydrostatic core. Models with ζcr ≳ 10-13 s-1 are indistinguishable from ideal MHD models, and the evolution of the model with ζcr = 10-14 s-1 matches the evolution of the ideal MHD model within 1 per cent when considering maximum density, magnetic energy, and maximum magnetic field strength as a function of time; these results are independent of ag. Models with very low ionization rates (ζcr ≲ 10-24 s-1) are required to approach hydrodynamical collapse, and even lower ionization rates may be required for larger ag. Thus, it is possible to reproduce ideal MHD and purely hydrodynamical collapses using non-ideal MHD given an appropriate cosmic ray ionization rate. However, realistic cosmic ray ionization rates approach neither limit; thus, non-ideal MHD cannot be neglected in star formation simulations.

Dynamics of the interstellar matter in galaxies : isolated barred spiral galaxies : cloud formation processes

NASA Astrophysics Data System (ADS)

Helmuth, Kristen

1998-12-01

The dynamical components of six isolated barred spiral (SB) galaxies are investigated. No evidence is found supporting the hypothesis of a low amount of dark matter being characteristic of SB galaxies. The presence of companion galaxies is found to correlate with an increased statistical spread in the neutral hydrogen (HI) extent. It is concluded that the selection of galaxies with large HI extent may introduce a bias towards tidally interacting systems. The circumnuclear region of the SB galaxy NGC 1365 is studied with the Hubble Space Telescope (HST). Numerous bright "super star clusters" (SSCs) are detected, surrounding the active nucleus. The bright compact radio source NGC 1365:A is found to coincide spatially with one of the SSCs. We conclude that the source is a "radio supernova". In the [OIII] l 5007 line, the HST resolves individual clouds within the conical outflow from the nucleus, some of which gather in larger agglomerations. An in-depth study of the dynamics of the SB galaxy NGC 1300 is presented. Multi-wavelength data yield an estimate of the velocity field and gravitational potential. Subsequent hydrodynamical simulations are able to reproduce the morphology and kinematics in the bar region using a pure bar perturbing potential. To reproduce the spiral structure a weak spiral component has to be added, indicative of stellar spiral response to the bar and/or self-gravitating gas in the arms. Two separate models, differing mainly with respect to pattern speed and associated resonance structure, are found to reproduce the observations. We study numerically the linear polarization and extinction of light from background stars passing through molecular clouds, illuminating the intricacies of the derivation of the magnetic-field-line pattern in a cloud from the observed polarization pattern: Due to a higher gas-grain collision frequency within the cloud, the polarization caused by the cloud may well be dominated by background/foreground polarization. Furthermore, variations in field-orientation along the line-of-sight may cause notable differences between the observed polarization vectors and the true magnetic-field-line pattern. Small-scale, helical, interstellar filaments are discussed on the basis of optical observations of an "elephant trunk" structure in the Rosette nebula. The observed sinusoidal filaments are suggested to be helices lined up by magnetic fields. We propose that the Rosette elephant trunks form an interconnected system of rope-like structures which are relics from filamentary skeletons of magnetic fields in the primordial cloud. Stochastic mass fractionation of a molecular cloud is simulated numerically. It is found that geometry alone may constrain the resulting mass spectrum of molecular cloud clumps. We demonstrate that further fragmentation of the cloud clumps, under the assumption of a lower limit of the self-similar regime, produces a mass spectrum that has qualitative and quantitative similarities with the empirically determined stellar initial mass function.

Planck intermediate results: XXXIV. The magnetic field structure in the Rosette Nebula

DOE PAGES

Aghanim, N.; Alves, M. I. R.; Arnaud, M.; ...

2016-02-09

Planck has mapped the polarized dust emission over the whole sky, making it possible to trace the Galactic magnetic field structure that pervades the interstellar medium (ISM). In this paper, we combine polarization data from Planck with rotation measure (RM) observations towards a massive star-forming region, the Rosette Nebula in the Monoceros molecular cloud, to study its magnetic field structure and the impact of an expanding H ii region on the morphology of the field. We derive an analytical solution for the magnetic field, assumed to evolve from an initially uniform configuration following the expansion of ionized gas and themore » formation of a shell of swept-up ISM. From the RM data we estimate a mean value of the line-of-sight component of the magnetic field of about 3 μG (towards the observer) in the Rosette Nebula, for a uniform electron density of about 12 cm -3. The dust shell that surrounds the Rosette H ii region is clearly observed in the Planck intensity map at 353 GHz, with a polarization signal significantly different from that of the local background when considered asa whole. The Planck observations constrain the plane-of-the-sky orientation of the magnetic field in the Rosette’s parent molecular cloud to be mostly aligned with the large-scale field along the Galactic plane. The Planck data are compared with the analytical model, which predicts the mean polarization properties of a spherical and uniform dust shell for a given orientation of the field. This comparison leads to an upper limit of about 45° on the angle between the line of sight and the magnetic field in the Rosette complex, for an assumed intrinsic dust polarization fraction of 4%. This field direction can reproduce the RM values detected in the ionized region if the magnetic field strength in the Monoceros molecular cloud is in the range 6.5–9 μG. Finally, the present analytical model is able to reproduce the RM distribution across the ionized nebula, as well as the mean dust polarization properties of the swept-up shell, and can be directly applied to other similar objects.« less

Planck intermediate results: XXXIV. The magnetic field structure in the Rosette Nebula

DOE Office of Scientific and Technical Information (OSTI.GOV)

Aghanim, N.; Alves, M. I. R.; Arnaud, M.

Planck has mapped the polarized dust emission over the whole sky, making it possible to trace the Galactic magnetic field structure that pervades the interstellar medium (ISM). In this paper, we combine polarization data from Planck with rotation measure (RM) observations towards a massive star-forming region, the Rosette Nebula in the Monoceros molecular cloud, to study its magnetic field structure and the impact of an expanding H ii region on the morphology of the field. We derive an analytical solution for the magnetic field, assumed to evolve from an initially uniform configuration following the expansion of ionized gas and themore » formation of a shell of swept-up ISM. From the RM data we estimate a mean value of the line-of-sight component of the magnetic field of about 3 μG (towards the observer) in the Rosette Nebula, for a uniform electron density of about 12 cm -3. The dust shell that surrounds the Rosette H ii region is clearly observed in the Planck intensity map at 353 GHz, with a polarization signal significantly different from that of the local background when considered asa whole. The Planck observations constrain the plane-of-the-sky orientation of the magnetic field in the Rosette’s parent molecular cloud to be mostly aligned with the large-scale field along the Galactic plane. The Planck data are compared with the analytical model, which predicts the mean polarization properties of a spherical and uniform dust shell for a given orientation of the field. This comparison leads to an upper limit of about 45° on the angle between the line of sight and the magnetic field in the Rosette complex, for an assumed intrinsic dust polarization fraction of 4%. This field direction can reproduce the RM values detected in the ionized region if the magnetic field strength in the Monoceros molecular cloud is in the range 6.5–9 μG. Finally, the present analytical model is able to reproduce the RM distribution across the ionized nebula, as well as the mean dust polarization properties of the swept-up shell, and can be directly applied to other similar objects.« less

Supernova Remnant Kes 17: An Efficient Cosmic Ray Accelerator inside a Molecular Cloud

NASA Astrophysics Data System (ADS)

Gelfand, Joseph D.; Castro, Daniel; Slane, Patrick O.; Temim, Tea; Hughes, John P.; Rakowski, Cara

2013-11-01

The supernova remnant Kes 17 (SNR G304.6+0.1) is one of a few but growing number of remnants detected across the electromagnetic spectrum. In this paper, we analyze recent radio, X-ray, and γ-ray observations of this object, determining that efficient cosmic ray acceleration is required to explain its broadband non-thermal spectrum. These observations also suggest that Kes 17 is expanding inside a molecular cloud, though our determination of its age depends on whether thermal conduction or clump evaporation is primarily responsible for its center-filled thermal X-ray morphology. Evidence for efficient cosmic ray acceleration in Kes 17 supports recent theoretical work concluding that the strong magnetic field, turbulence, and clumpy nature of molecular clouds enhance cosmic ray production in supernova remnants. While additional observations are needed to confirm this interpretation, further study of Kes 17 is important for understanding how cosmic rays are accelerated in supernova remnants.

Bondi-Hoyle accretion in an isothermal magnetized plasma

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lee, Aaron T.; McKee, Christopher F.; Klein, Richard I.

2014-03-01

In regions of star formation, protostars and newborn stars will accrete mass from their natal clouds. These clouds are threaded by magnetic fields with a strength characterized by the plasma β—the ratio of thermal and magnetic pressures. Observations show that molecular clouds have β ≲ 1, so magnetic fields have the potential to play a significant role in the accretion process. We have carried out a numerical study of the effect of large-scale magnetic fields on the rate of accretion onto a uniformly moving point particle from a uniform, non-self-gravitating, isothermal gas. We consider gas moving with sonic Mach numbersmore » of up to M≈45; magnetic fields that are either parallel, perpendicular, or oriented 45° to the flow; and β as low as 0.01. Our simulations utilize adaptive mesh refinement in order to obtain high spatial resolution where it is needed; this also allows the boundaries to be far from the accreting object to avoid unphysical effects arising from boundary conditions. Additionally, we show that our results are independent of our exact prescription for accreting mass in the sink particle. We give simple expressions for the steady-state accretion rate as a function of β and M for the parallel and perpendicular orientations. Using typical molecular cloud values of M∼5 and β ∼ 0.04 from the literature, our fits suggest that a 0.4 M {sub ☉} star accretes ∼4 × 10{sup –9} M {sub ☉} yr{sup –1}, almost a factor of two less than accretion rates predicted by hydrodynamic models. This disparity can grow to orders of magnitude for stronger fields and lower Mach numbers. We also discuss the applicability of these accretion rates versus accretion rates expected from gravitational collapse, and under what conditions a steady state is possible. The reduction in the accretion rate in a magnetized medium leads to an increase in the time required to form stars in competitive accretion models, making such models less efficient than predicted by Bondi-Hoyle rates. Our results should find application in numerical codes, enabling accurate sub-grid models of sink particles accreting from magnetized media.« less

The formation of high-mass binary star systems

NASA Astrophysics Data System (ADS)

Lund, Kristin; Bonnell, Ian A.

2018-06-01

We develop a semi-analytic model to investigate how accretion onto wide low-mass binary stars can result in a close high-mass binary system. The key ingredient is to allow mass accretion while limiting the gain in angular momentum. We envision this process as being regulated by an external magnetic field during infall. Molecular clouds are made to collapse spherically with material either accreting onto the stars or settling in a disk. Our aim is to determine what initial conditions are needed for the resulting binary to be both massive and close. Whether material accretes, and what happens to the binary separation as a result, depends on the relative size of its specific angular momentum, compared to the specific angular momentum of the binary. When we add a magnetic field we are introducing a torque to the system which is capable of stripping the molecular cloud of some of its angular momentum, and consequently easing the formation of high-mass binaries. Our results suggest that clouds in excess of 1000 M⊙ and radii of 0.5 pc or larger, can easily form binary systems with masses in excess of 25 M⊙ and separations of order 10 R⊙ with magnetic fields of order 100 μG (mass-to-flux ratios of order 5).

The next generation balloon-borne large aperture submillimeter telescope (BLAST-TNG)

NASA Astrophysics Data System (ADS)

Dober, Bradley Jerald

Large areas of astrophysics, such as precision cosmology, have benefited greatly from large maps and datasets, yielded by telescopes of ever-increasing number and ability. However, due to the unique challenges posed by submillimeter polarimetry, the study of molecular cloud dynamics and star formation remain stunted. Previously, polarimetry data was limited to a few vectors on only the brightest areas of molecular clouds. This made drawing statistically-driven conclusions a daunting task. However, the successful flight of the Balloon-born Large Aperture Submillimeter Telescope for Polarimetry (BLASTPol) generated maps with thousands of independent polarization measurements of molecular clouds, and ushered in a new era of empirical modeling of molecular cloud dynamics. Now that the potential benefits from large-scale maps of magnetic fields in molecular clouds had been identified, a successor that would truly unlock the secrets must be born. The Next Generation Balloon-borne Large Aperture Submillimeter Telescope (BLAST-TNG), the successor to BLASTPol, has the ability to make larger and more detailed maps of magnetic fields in molecular clouds. It will push the field of star formation into a statistics-driven, empirical realm. With these large, detailed datasets, astronomers will be able to find new relationships between the dust dynamics and the magnetic fields. The field will surge to a new level of understanding. One of the key enabling technologies of BLAST-TNG is its three arrays of polarization-sensitive Microwave Kinetic Inductance Detectors (MKIDs). MKIDs are superconducting RLC circuits with a resonant frequency that shifts proportionally to the amount of incident radiation. The key feature of MKIDs is that thousands of detectors, each with their own unique resonant frequency, can be coupled to the same readout line. This technology will be able to drive the production of large-scale monolithic arrays, containing tens or hundreds of thousands of detectors, resulting in an ever-increasing rate of scientific progress. The current limiting factor that determines how many MKIDs can be placed on the same readout line is the bandwidth and processing limitations of the readout hardware. BLAST-TNG has pushed this technology forward by implementing the first Reconfigurable Open-Architecture Computing Hardware (ROACH2) based readout system. This has significantly raised the processing abilities of the MKID readout electronics, enabling over 1000 MKIDs to be read out on a single line. It is also the first ever ROACH (1 or 2) based system to ever be flown on a long duration balloon (LDB) payload. This thesis documents the first-ever deployment of MKIDs on a balloon payload. This is a significant technological step towards an MKID-based satellite payload. This thesis overviews the balloon payload, details the underlying detector physics, catalogs the detector and full-scale array development, and ends with the room-temperature readout electronics.

The Magnetic Field Structure of W3(OH)

NASA Astrophysics Data System (ADS)

El-Batal, Adham M.; Clemens, Dan P.; Montgomery, Jordan

2018-06-01

Situated in the Perseus arm of the Galaxy, the W3 molecular cloud is a high-mass star-forming region with low foreground optical extinction. Near-infrared H- and K-band polarimetric observations of a 10' × 10' field of view of W3 were obtained using the Mimir instrument on the 1.8 m Perkins Telescope. This field of view encompasses W3(OH), a region of OH and H2O masers as well as an HII region. The H-band data were used in conjunction with Spitzer M-band data to map extinction via H-M color excess. In total, 2654 stellar objects were found in the Mimir field of view, of which 1013 had H and M magnitudes with low errors. Using the extinction map and the individual stellar H-M color excess values, 429 stars with polarized signals were found to be background to the molecular cloud. These were useful for mapping the magnetic field structure and estimating the magnetic field strength of the cloud. Mid- to far-infrared (70 - 870 μm) archival Herschel and Planck data were used to map dust extinction at 850 µm and create an H2 column density map. Combined, maps of magnetic field strength and hydrogen column density can be used to infer the ratio of gravitational potential to magnetic potential ( M/Φ ). Findings are discussed in the context of M/Φ ratio in models and the stability of high-mass star-forming regions.This work has been supported by NSF AST14-12269 and NASA NNX15AE51G grants.

Magnetic field structure in Monoceros R2

NASA Technical Reports Server (NTRS)

Jarrett, T. H.; Novak, G.; Xie, T.; Goldsmith, P. F.

1994-01-01

We have carried out polarimetric observations to investigate the geometry of the magnetic field in the giant molecular cloud Monoceros R2. This study is based upon deep R-band charge coupled device (CCD) polarimetry, covering a total area of 0.5 deg(exp 2) of the giant molecular cloud. The data were calibrated using a new technique that relies on obtaining broad-band photometry of stars simultaneously with polarimetric photometry of the Mon R2 fields, thus providing an accurate means of measuring the electric vectors of starlight which is polarized by the fore-ground dust grains aligned by the magnetic field in the Mon R2 GMC. In this work, (1) we were able to continuously trace magnetic field lines from the largest scales in Mon R2 to the detailed structure of the field in the dense core, as determined from infrared polarimetry; and (2) we have found that the ambient field is apparently modified by a large-scale structure in the Mon R2 cloud. The mean angle of polarization for the complete sample we measured is 158 deg, which is roughly coincident with the local Galactic magnetic field (155 deg). The dispersion in the angle of polarization is 33 deg, similar to that found in the Orion GMC. The dispersion in angle of polarization for stars located along the western side of the three CCD fields is 22 deg. The CCD fields are bisected by a dense ridge of gas defining the boundary of an expanding gas shell that recent observational results at millimeter wavelengths now reveal dominates the Mon R2 GMC. Our results suggest th at the expanding shell has distorted the magnetic field lines extending from the core to the northern gas structure comprising Mon R2.

Comparison of prestellar core elongations and large-scale molecular cloud structures in the Lupus I region

DOE Office of Scientific and Technical Information (OSTI.GOV)

Poidevin, Frédérick; Ade, Peter A. R.; Hargrave, Peter C.

2014-08-10

Turbulence and magnetic fields are expected to be important for regulating molecular cloud formation and evolution. However, their effects on sub-parsec to 100 parsec scales, leading to the formation of starless cores, are not well understood. We investigate the prestellar core structure morphologies obtained from analysis of the Herschel-SPIRE 350 μm maps of the Lupus I cloud. This distribution is first compared on a statistical basis to the large-scale shape of the main filament. We find the distribution of the elongation position angle of the cores to be consistent with a random distribution, which means no specific orientation of themore » morphology of the cores is observed with respect to the mean orientation of the large-scale filament in Lupus I, nor relative to a large-scale bent filament model. This distribution is also compared to the mean orientation of the large-scale magnetic fields probed at 350 μm with the Balloon-borne Large Aperture Telescope for Polarimetry during its 2010 campaign. Here again we do not find any correlation between the core morphology distribution and the average orientation of the magnetic fields on parsec scales. Our main conclusion is that the local filament dynamics—including secondary filaments that often run orthogonally to the primary filament—and possibly small-scale variations in the local magnetic field direction, could be the dominant factors for explaining the final orientation of each core.« less

Dynamics of the Interstellar Matter in Galaxies

NASA Astrophysics Data System (ADS)

Kristen, H.

The dynamical components of six isolated barred spiral (SB) galaxies are investigated. No evidence is found supporting the hypothesis of a low amount of dark matter being characteristic of SB galaxies. The presence of companion galaxies is found to correlate with an increased statistical spread in the neutral hydrogen (HI) extent. It is concluded that the selection of galaxies with large HI~extent may introduce a bias towards tidally interacting systems. The circumnuclear region of the SB galaxy NGC 1365 is studied with the Hubble Space Telescope (HST). Numerous bright ``super star clusters'' (SSCs) are detected, surrounding the active nucleus. The bright compact radio source NGC 1365:A is found to coincide spatially with one of the SSCs. We conclude that the source is a ``radio supernova''. In the [OIII] 5007 line, the HST resolves individual clouds within the conical outflow from the nucleus, some of which gather in larger agglomerations. An in-depth study of the dynamics of the SB galaxy NGC 1300 is presented. Multi-wavelength data yield an estimate of the velocity field and gravitational potential. Subsequent hydrodynamical simulations are able to reproduce the morphology and kinematics in the bar region using a pure bar perturbing potential. To reproduce the spiral structure a weak spiral component has to be added, indicative of stellar spiral response to the bar and/or self-gravitating gas in the arms. Two separate models, differing mainly with respect to pattern speed and associated resonance structure, are found to reproduce the observations. We study numerically the linear polarization and extinction of light from background stars passing through molecular clouds, illuminating the intricacies of the derivation of the magnetic-field-line pattern in a cloud from the observed polarization pattern: Due to a higher gas-grain collision frequency within the cloud, the polarization caused by the cloud may well be dominated by background/foreground polarization. Furthermore, variations in field-orientation along the line-of-sight may cause notable differences between the observed polarization vectors and the true magnetic-field-line pattern. Small-scale, helical, interstellar filaments are discussed on the basis of optical observations of an ``elephant trunk'' structure in the Rosette nebula. The observed sinusoidal filaments are suggested to be helices lined up by magnetic fields. We propose that the Rosette elephant trunks form an interconnected system of rope-like structures which are relics from filamentary skeletons of magnetic fields in the primordial cloud. Stochastic mass fractionation of a molecular cloud is simulated numerically. It is found that geometry alone may constrain the resulting mass spectrum of molecular cloud clumps. We demonstrate that further fragmentation of the cloud clumps, under the assumption of a lower limit of the self-similar regime, produces a mass spectrum that has qualitative and quantitative similarities with the empirically determined stellar initial mass function.

EVIDENCE FOR DECAY OF TURBULENCE BY MHD SHOCKS IN THE ISM VIA CO EMISSION

DOE Office of Scientific and Technical Information (OSTI.GOV)

Larson, Rebecca L.; Evans II, Neal J.; Green, Joel D.

2015-06-10

We utilize observations of sub-millimeter rotational transitions of CO from a Herschel Cycle 2 open time program (“COPS”, PI: J. Green) to identify previously predicted turbulent dissipation by magnetohydrodynamic (MHD) shocks in molecular clouds. We find evidence of the shocks expected for dissipation of MHD turbulence in material not associated with any protostar. Two models fit about equally well: model 1 has a density of 10{sup 3} cm{sup −3}, a shock velocity of 3 km s{sup −1}, and a magnetic field strength of 4 μG; model 2 has a density of 10{sup 3.5} cm{sup −3}, a shock velocity of 2more » km s{sup −1}, and a magnetic field strength of 8 μG. Timescales for decay of turbulence in this region are comparable to crossing times. Transitions of CO up to J of 8, observed close to active sites of star formation, but not within outflows, can trace turbulent dissipation of shocks stirred by formation processes. Although the transitions are difficult to detect at individual positions, our Herschel-SPIRE survey of protostars provides a grid of spatially distributed spectra within molecular clouds. We averaged all spatial positions away from known outflows near seven protostars. We find significant agreement with predictions of models of turbulent dissipation in slightly denser (10{sup 3.5} cm{sup −3}) material with a stronger magnetic field (24 μG) than in the general molecular cloud.« less

Star formation induced by cloud-cloud collisions and galactic giant molecular cloud evolution

NASA Astrophysics Data System (ADS)

Kobayashi, Masato I. N.; Kobayashi, Hiroshi; Inutsuka, Shu-ichiro; Fukui, Yasuo

2018-05-01

Recent millimeter/submillimeter observations towards nearby galaxies have started to map the whole disk and to identify giant molecular clouds (GMCs) even in the regions between galactic spiral structures. Observed variations of GMC mass functions in different galactic environments indicates that massive GMCs preferentially reside along galactic spiral structures whereas inter-arm regions have many small GMCs. Based on the phase transition dynamics from magnetized warm neutral medium to molecular clouds, Kobayashi et al. (2017, ApJ, 836, 175) proposes a semi-analytical evolutionary description for GMC mass functions including a cloud-cloud collision (CCC) process. Their results show that CCC is less dominant in shaping the mass function of GMCs than the accretion of dense H I gas driven by the propagation of supersonic shock waves. However, their formulation does not take into account the possible enhancement of star formation by CCC. Millimeter/submillimeter observations within the Milky Way indicate the importance of CCC in the formation of star clusters and massive stars. In this article, we reformulate the time-evolution equation largely modified from Kobayashi et al. (2017, ApJ, 836, 175) so that we additionally compute star formation subsequently taking place in CCC clouds. Our results suggest that, although CCC events between smaller clouds are more frequent than the ones between massive GMCs, CCC-driven star formation is mostly driven by massive GMCs ≳ 10^{5.5} M_{⊙} (where M⊙ is the solar mass). The resultant cumulative CCC-driven star formation may amount to a few 10 percent of the total star formation in the Milky Way and nearby galaxies.

The Central Molecular Zone of the Milky Way: Lessons about Star Formation from an extreme Environment

NASA Astrophysics Data System (ADS)

Kauffmann, Jens; Thushara Pillai, G. S.; Zhang, Qizhou; Lu, Xing; Immer, Katharina

2015-08-01

The Central Molecular Zone of the Milky Way (CMZ; innermost ~100pc) hosts a number of remarkably dense and massive clouds. These are subject to extreme environmental conditions, including very high cosmic ray fluxes and strong magnetic fields. Exploring star formation under such exceptional circumstances is essential for several of reasons. First, the CMZ permits to probe an extreme point in the star formation parameter space, which helps to test theoretical models. Second, CMZ clouds might help to understand the star formation under extreme conditions in more distant environments, such as in starbursts and the early universe.One particularly striking aspect is that — compared to the solar neighborhood — CMZ star formation in dense gas is suppressed by more than an order of magnitude (Longmore et al. 2012, Kauffmann et al. 2013). This questions current explanations for relations between the dense gas and the star formation rate (e.g., Gao & Solomon 2004, Lada et al. 2012). In other words, the unusually dense and massive CMZ molecular clouds form only very few stars, if any at all. Why is this so?Based on data from ALMA, CARMA, and SMA interferometers, we present results from the Galactic Center Molecular Cloud Survey (GCMS), the first study of a comprehensive sample of molecular clouds in the CMZ. This research yields a curious result: most of the major CMZ clouds are essentially devoid of significant substructure of the sort usually found in regions of high-mass star formation (Kauffmann et al. 2013). Preliminary analysis indicates that some clouds rather resemble homogeneous balls of gas. This suggests a highly dynamic picture of cloud evolution in the CMZ where clouds form, disperse, and re-assemble constantly. This concept is benchmarked against a new ALMA survey and first results from a legacy survey on the SMA.It is plausible that dense clouds in other galaxies have a similar internal structure. Instruments like ALMA and the JWST will soon permit to resolve such regions in nearby galaxies.

Discovery of massive star formation quenching by non-thermal effects in the centre of NGC 1097

NASA Astrophysics Data System (ADS)

Tabatabaei, F. S.; Minguez, P.; Prieto, M. A.; Fernández-Ontiveros, J. A.

2018-01-01

Observations show that massive star formation quenches first at the centres of galaxies. To understand quenching mechanisms, we investigate the thermal and non-thermal energy balance in the central kpc of NGC 1097—a prototypical galaxy undergoing quenching—and present a systematic study of the nuclear star formation efficiency and its dependencies. This region is dominated by the non-thermal pressure from the magnetic field, cosmic rays and turbulence. A comparison of the mass-to-magnetic flux ratio of the molecular clouds shows that most of them are magnetically critical or supported against the gravitational collapse needed to form the cores of massive stars. Moreover, the star formation efficiency of the clouds drops with the magnetic field strength. Such an anti-correlation holds with neither the turbulent nor the thermal pressure. Hence, a progressive build up of the magnetic field results in high-mass stars forming inefficiently, and this may be the cause of the low-mass stellar population in the bulges of galaxies.

Jeans instability of rotating magnetized dusty plasma

NASA Astrophysics Data System (ADS)

Sharma, S.; Sutar, D. L.; Kumar, V.; Pensia, R. K.

2018-05-01

It has been shown that rotation has to play a predominant important role in the formation of many astrophysical objects and the stability of molecular clouds. In this paper the theoretical investigation of the presence of rotation in the magnetized dusty plasma. The general dispersion relation is obtained normal mode analysis technique, and we found the Alfven mode is modified due to the presence of rotation and magnetic field. The graphical presentation shows that rotation and Alfven wave velocity have a stabilizing in the system.

Magnetic seismology of interstellar gas clouds: Unveiling a hidden dimension.

PubMed

Tritsis, Aris; Tassis, Konstantinos

2018-05-11

Stars and planets are formed inside dense interstellar molecular clouds by processes imprinted on the three-dimensional (3D) morphology of the clouds. Determining the 3D structure of interstellar clouds remains challenging because of projection effects and difficulties measuring the extent of the clouds along the line of sight. We report the detection of normal vibrational modes in the isolated interstellar cloud Musca, allowing determination of the 3D physical dimensions of the cloud. We found that Musca is vibrating globally, with the characteristic modes of a sheet viewed edge on, not the characteristics of a filament as previously supposed. We reconstructed the physical properties of Musca through 3D magnetohydrodynamic simulations, reproducing the observed normal modes and confirming a sheetlike morphology. Copyright © 2018 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.

Shocked molecular gas and the origin of cosmic rays

NASA Astrophysics Data System (ADS)

Reach, William; Gusdorf, Antoine; Richter, Matthew

2018-06-01

When massive stars reach the end of their ability to remain stable with core nuclear fusion, they explode in supernovae that drive powerful shocks into their surroundings. Because massive stars form in and remain close to molecular clouds they often drive shocks into dense gas, which is now believed to be the origin of a significant fraction of galactic cosmic rays. The nature of the supernova-molecular cloud interaction is not well understood, though observations are gradually elucidating their nature. The range of interstellar densities, and the inclusion of circumstellar matter from the late-phase mass-loss of the stars before their explosions, leads to a wide range of possible appearances and outcomes. In particular, it is not even clear what speed or physical type of shocks are present: are they dense, magnetically-mediated shocks where H2 is not dissociated, or are they faster shocks that dissociate molecules and destroy some of the grains? SOFIA is observing some of the most significant (in terms of cosmic ray production potential and infrared energy output) supernova-molecular cloud interactions for measurement of the line widths of key molecular shocks tracers: H2, [OI], and CO. The presence of gas at speeds 100 km/s or greater would indicate dissociative shocks, while speeds 30 km/s and slower retain most molecules. The shock velocity is a key ingredient in modeling the interaction between supernovae and molecular clouds including the potential for formation of cosmic rays.

The Tai Chi in Star Formation

NASA Astrophysics Data System (ADS)

Li, Hua-bai

2017-10-01

Tai Chi, a Chinese martial art developed based on the laws of nature, emphasises how 'to conquer the unyielding with the yielding'. The recent observation of star formation shows that stars result from the interaction between gravity, turbulence and magnetic fields. This interaction again follows the nature rules that inspired Tai Chi. For example, if self-gravity is the force that dominates, the molecular cloud will collapse isotropically, which compresses magnetic field lines. The density of the yielding field lines increases until magnetic pressure reaches the critical value to support the cloud against the gravitational force in directions perpendicular to the field lines (Lorentz force). Then gravity gives way to Lorentz force, accumulating gas only along the field lines till the gas density achieves the critical value to again compress the field lines. The Tai Chi goes on in a self-similar way.

Penetration of Cosmic Rays into Dense Molecular Clouds: Role of Diffuse Envelopes

NASA Astrophysics Data System (ADS)

Ivlev, A. V.; Dogiel, V. A.; Chernyshov, D. O.; Caselli, P.; Ko, C.-M.; Cheng, K. S.

2018-03-01

A flux of cosmic rays (CRs) propagating through a diffuse ionized gas can excite MHD waves, thus generating magnetic disturbances. We propose a generic model of CR penetration into molecular clouds through their diffuse envelopes, and identify the leading physical processes controlling their transport on the way from a highly ionized interstellar medium to the dense interior of the cloud. The model allows us to describe a transition between a free streaming of CRs and their diffusive propagation, determined by the scattering on the self-generated disturbances. A self-consistent set of equations, governing the diffusive transport regime in an envelope and the MHD turbulence generated by the modulated CR flux, is characterized by two dimensionless numbers. We demonstrate a remarkable mutual complementarity of different mechanisms leading to the onset of the diffusive regime, which results in a universal energy spectrum of the modulated CRs. In conclusion, we briefly discuss implications of our results for several fundamental astrophysical problems, such as the spatial distribution of CRs in the Galaxy as well as the ionization, heating, and chemistry in dense molecular clouds. This paper is dedicated to the memory of Prof. Vadim Tsytovich.

Observing Interstellar and Intergalactic Magnetic Fields

NASA Astrophysics Data System (ADS)

Han, J. L.

2017-08-01

Observational results of interstellar and intergalactic magnetic fields are reviewed, including the fields in supernova remnants and loops, interstellar filaments and clouds, Hii regions and bubbles, the Milky Way and nearby galaxies, galaxy clusters, and the cosmic web. A variety of approaches are used to investigate these fields. The orientations of magnetic fields in interstellar filaments and molecular clouds are traced by polarized thermal dust emission and starlight polarization. The field strengths and directions along the line of sight in dense clouds and cores are measured by Zeeman splitting of emission or absorption lines. The large-scale magnetic fields in the Milky Way have been best probed by Faraday rotation measures of a large number of pulsars and extragalactic radio sources. The coherent Galactic magnetic fields are found to follow the spiral arms and have their direction reversals in arms and interarm regions in the disk. The azimuthal fields in the halo reverse their directions below and above the Galactic plane. The orientations of organized magnetic fields in nearby galaxies have been observed through polarized synchrotron emission. Magnetic fields in the intracluster medium have been indicated by diffuse radio halos, polarized radio relics, and Faraday rotations of embedded radio galaxies and background sources. Sparse evidence for very weak magnetic fields in the cosmic web is the detection of the faint radio bridge between the Coma cluster and A1367. Future observations should aim at the 3D tomography of the large-scale coherent magnetic fields in our Galaxy and nearby galaxies, a better description of intracluster field properties, and firm detections of intergalactic magnetic fields in the cosmic web.

Fantastic Striations and Where to Find Them: The Origin of Magnetically Aligned Striations in Interstellar Clouds

NASA Astrophysics Data System (ADS)

Chen, Che-Yu; Li, Zhi-Yun; King, Patrick K.; Fissel, Laura M.

2017-10-01

Thin, magnetically aligned striations of relatively moderate contrast with the background are commonly observed in both atomic and molecular clouds. They are also prominent in MHD simulations with turbulent converging shocks. The simulated striations develop within a dense, stagnated sheet in the midplane of the post-shock region where magnetically induced converging flows collide. We show analytically that the secondary flows are an inevitable consequence of the jump conditions of oblique MHD shocks. They produce the stagnated, sheet-like sub-layer through a secondary shock when, roughly speaking, the Alfvénic speed in the primary converging flows is supersonic, a condition that is relatively easy to satisfy in interstellar clouds. The dense sub-layer is naturally threaded by a strong magnetic field that lies close to the plane of the sub-layer. The substantial magnetic field makes the sheet highly anisotropic, which is the key to the striation formation. Specifically, perturbations of the primary inflow that vary spatially perpendicular to the magnetic field can easily roll up the sheet around the field lines without bending them, creating corrugations that appear as magnetically aligned striations in column density maps. On the other hand, perturbations that vary spatially along the field lines curve the sub-layer and alter its orientation relative to the magnetic field locally, seeding special locations that become slanted overdense filaments and prestellar cores through enhanced mass accumulation along field lines. In our scenario, the dense sub-layer, which is unique to magnetized oblique shocks, is the birthplace for both magnetically aligned diffuse striations and massive star-forming structures.

Studies for the Europagenic Plasma Source in Jupiter's Inner Magnetosphere during the Galileo Europa Mission

NASA Technical Reports Server (NTRS)

Smyth, William H.

2004-01-01

Progress in research to understand the three-dimensional nature of the Europagenic plasma torus is summarized. Efforts to improve the plasma torus description near Europa's orbit have included a better understanding of Europa's orbit and an improved description of the planetary magnetic field. New plasma torus chemistry for molecular and atomic species has been introduced and implemented in Europa neutral cloud models. Preliminary three-dimensional model calculations for Europa's neutral clouds and their plasma sources are presented.

Interferometric Mapping of Perseus Outflows with MASSES

NASA Astrophysics Data System (ADS)

Stephens, Ian; Dunham, Michael; Myers, Philip C.; MASSES Team

2017-01-01

The MASSES (Mass Assembly of Stellar Systems and their Evolution with the SMA) survey, a Submillimeter Array (SMA) large-scale program, is mapping molecular lines and continuum emission about the 75 known Class 0/I sources in the Perseus Molecular Cloud. In this talk, I present some of the key results of this project, with a focus on the CO(2-1) maps of the molecular outflows. In particular, I investigate how protostars inherit their rotation axes from large-scale magnetic fields and filamentary structure.

Magnetic flux in modeled magnetic clouds at 1 AU and some specific comparisons to associated photospheric flux

NASA Technical Reports Server (NTRS)

Lepping, R. P.; Szabo, A.; DeForest, C. E.; Thompson, B. J.

1997-01-01

In order to better understand the solar origins of magnetic clouds, statistical distributions of the estimated axial magnetic flux of 30 magnetic clouds at 1 AU, separated according to their occurrence during the solar cycle, were obtained and a comparison was made of the magnetic flux of a magnetic cloud to the aggregate flux of apparently associated photospheric magnetic flux tubes, for some specific cases. The 30 magnetic clouds comprise 12 cases from WIND, and the remainder from IMP-8, earlier IMPs, the International Sun-Earth Explorer (ISEE) 3 and HELIOS. The total magnetic flux along the cloud axis was estimated using a constant alpha, cylindrical, force-free flux rope model to determine cloud diameter and axial magentic field strength. The distribution of magentic fluxes for the 30 clouds is shown to be in the form of a skewed Gaussian.

Simultaneous observations of solar MeV particles in a magnetic cloud and in the earth's northern tail lobe - Implications for the global field line topology of magnetic clouds and for the entry of solar particles into the magnetosphere during cloud passage

NASA Technical Reports Server (NTRS)

Farrugia, C. J.; Richardson, I. G.; Burlaga, L. F.; Lepping, R. P.; Osherovich, V. A.

1993-01-01

Simultaneous ISEE 3 and IMP 8 spacecraft observations of magnetic fields and flow anisotropies of solar energetic protons and electrons during the passage of an interplanetary magnetic cloud show various particle signature differences at the two spacecraft. These differences are interpretable in terms of the magnetic line topology of the cloud, the connectivity of the cloud field lines to the solar surface, and the interconnection between the magnetic fields of the magnetic clouds and of the earth. These observations are consistent with a magnetic cloud model in which these mesoscale configurations are curved magnetic flux ropes attached at both ends to the sun's surface, extending out to 1 AU.

Evidence for Decay of Turbulence by MHD Shocks in the ISM via CO Emission

NASA Astrophysics Data System (ADS)

Larson, Rebecca; Evans, Neal J.; Green, Joel; Yang, Yao-Lun

2015-08-01

Star formation rates in molecular clouds are about 100 times slower than simple estimates based on Jeans mass and free-fall time arguments. A leading candidate to explain the slowness of star formation is MHD turbulence. Such turbulence should decay via low-velocity shocks. Until recently, these shocks have resisted detection because of confusion with emission excited by PDRs. We present evidence for shocks at levels predicted from simulations (Pon et al. 2012), and distinguished from PDR emission by the pattern of emission in rotational levels of CO up to J = 8. The data come from observations of sub-millimeter rotational transitions of CO in molecular clouds. We find evidence of the shocks expected for dissipation of MHD turbulence in material not associated with any protostar, at a density of about 103 cm-3 to 103.5 cm-3, a shock velocity of 2 to 3 km s-1, and a magnetic field strength of 4 to 8 μG. We calculate the dissipation timescale to be around 1.5 million years which is about 3 times less than the flow crossing timescale and agrees with predictions by Pon et al. 2012. Transitions of CO observed close to active sites of star formation, but not within outflows, can trace turbulent dissipation of shocks stirred by formation processes. Although the transitions are difficult to detect at individual positions, our Herschel-SPIRE survey of protostars provides a grid of spatially-distributed spectra within molecular clouds. We averaged all spatial positions away from known outflows near seven protostars from a Herschel Cycle 2 open time program ("COPS'', PI: J. Green). We find significant agreement with predictions of models of turbulent dissipation in slightly denser (103.5 cm-3) material and stronger magnetic field (24 μG) than in the general molecular cloud.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Frau, P.; Girart, J. M.; Padovani, M.

The Pipe nebula is a massive, nearby, filamentary dark molecular cloud with a low star formation efficiency threaded by a uniform magnetic field perpendicular to its main axis. It harbors more than a hundred, mostly quiescent, very chemically young starless cores. The cloud is therefore a good laboratory to study the earliest stages of the star formation process. We aim to investigate the primordial conditions and the relation among physical, chemical, and magnetic properties in the evolution of low-mass starless cores. We used the IRAM 30 m telescope to map the 1.2 mm dust continuum emission of five new starlessmore » cores, which are in good agreement with previous visual extinction maps. For the sample of nine cores, which includes the four cores studied in a previous work, we derived an A {sub V} to N{sub H{sub 2}} factor of (1.27 {+-} 0.12) Multiplication-Sign 10{sup -21} mag cm{sup 2} and a background visual extinction of {approx}6.7 mag possibly arising from the cloud material. We derived an average core diameter of {approx}0.08 pc, density of {approx}10{sup 5} cm{sup -3}, and mass of {approx}1.7 M {sub Sun }. Several trends seem to exist related to increasing core density: (1) the diameter seems to shrink, (2) the mass seems to increase, and (3) the chemistry tends to be richer. No correlation is found between the direction of the surrounding diffuse medium magnetic field and the projected orientation of the cores, suggesting that large-scale magnetic fields seem to play a secondary role in shaping the cores. We also used the IRAM 30 m telescope to extend the previous molecular survey at 1 and 3 mm of early- and late-time molecules toward the same five new Pipe nebula starless cores, and analyzed the normalized intensities of the detected molecular transitions. We confirmed the chemical differentiation toward the sample and increased the number of molecular transitions of the 'diffuse' (e.g., the 'ubiquitous' CO, C{sub 2}H, and CS), 'oxo-sulfurated' (e.g., SO and CH{sub 3}OH), and 'deuterated' (e.g., N{sub 2}H{sup +}, CN, and HCN) starless core groups. The chemically defined core groups seem to be related to different evolutionary stages: 'diffuse' cores present the cloud chemistry and are less dense, while 'deuterated' cores are the densest and present a chemistry typical of evolved dense cores. 'Oxo-sulfurated' cores might be in a transitional stage exhibiting intermediate properties and a very characteristic chemistry.« less

Magnetic clouds, helicity conservation, and intrinsic scale flux ropes

NASA Technical Reports Server (NTRS)

Kumar, A.; Rust, D. M.

1995-01-01

An intrinsic-scale flux-rope model for interplanetary magnetic clouds, incorporating conservation of magnetic helicity, flux and mass is found to adequately explain clouds' average thermodynamic and magnetic properties. In spite their continuous expansion as they balloon into interplanetary space, magnetic clouds maintain high temperatures. This is shown to be due to magnetic energy dissipation. The temperature of an expanding cloud is shown to pass through a maximum above its starting temperature if the initial plasma beta in the cloud is less than 2/3. Excess magnetic pressure inside the cloud is not an important driver of the expansion as it is almost balanced by the tension in the helical field lines. It is conservation of magnetic helicity and flux that requires that clouds expand radially as they move away from the Sun. Comparison with published data shows good agreement between measured cloud properties and theory. Parameters determined from theoretical fits to the data, when extended back to the Sun, are consistent with the origin of interplanetary magnetic clouds in solar filament eruptions. A possible extension of the heating mechanism discussed here to heating of the solar corona is discussed.

Structures Of Magnetically-Supported Filaments And Their Appearance In The Linear Polarization

NASA Astrophysics Data System (ADS)

Tomisaka, Kohji

2017-10-01

Dust thermal emissions observed with Herschel have revealed that interstellar molecular clouds consist of many filaments. Polarization observation of interstellar extinctions in the optical and near IR wavelengths shows that the dense filaments are extending perpendicular to the interstellar magnetic field. Magnetohydrostatic structures of such filaments are studied. It is well known that a hydrostatic filament without magnetic field has a maximum line mass of ¥lambda_max=2c_s^2/G (c_s:the isothermal sound speed and G: the gravitational constant). On the other hand, the magnetically-supported maximum line mass increases in proportion to the magnetic flux per unit length threading the filament (¥phi), as ¥lambda_max 2c_s^2/G + ¥phi/(2¥pi G^1/2). Comparison is made with 3D clouds. Stability of these magnetized filaments is studied using time-dependent 3D MHD simulations to discuss star formation in the filaments. Polarization pattern expected for the magnetically subcritical filaments is calculated. The distribution function of the angle between B-field and the axis of the filament, which is obtained with Planck Satellite, is compared with this mock observation.

General analytic results for nonlinear waves and solitons in molecular clouds

NASA Technical Reports Server (NTRS)

Adams, Fred C.; Fatuzzo, Marco; Watkins, Richard

1994-01-01

We study nonlinear wave phenomena in self-gravitating fluid systems, with a particular emphasis on applications to molecular clouds. This paper presents analytical results for one spatial dimension. We show that a large class of physical systems can be described by theories with a 'charge density' q(rho); this quantity replaces the density on the right-hand side of the Poisson equation for the gravitational potential. We use this formulation to prove general results about nonlinear wave motions in self-gravitating systems. We show that in order for stationary waves to exist, the total charge (the integral of the charge density over the wave profile) must vanish. This 'no-charge' property for solitary waves is related to the capability of a system to be stable to gravitational perturbations for arbitrarily long wavelengths. We find necessary and sufficient conditions on the charge density for the existence of solitary waves and stationary waves. We study nonlinear wave motions for Jeans-type theories (where q(rho) = rho-rho(sub 0)) and find that nonlinear waves of large amplitude are confined to a rather narrow range of wavelengths. We also study wave motions for molecular clouds threaded by magnetic fields and show how the allowed range of wavelengths is affected by the field strength. Since the gravitational force in one spatial dimension does not fall off with distance, we consider two classes of models with more realistic gravity: Yukawa potentials and a pseudo two-dimensional treatment. We study the allowed types of wave behavior for these models. Finally, we discuss the implications of this work for molecular cloud structure. We argue that molecular clouds can support a wide variety of wave motions and suggest that stationary waves (such as those considered in this paper) may have already been observed.

A Holistic Perspective on the Dynamics of G035.39-00.33: The Interplay between Gas and Magnetic Fields

NASA Astrophysics Data System (ADS)

Liu, Tie; Li, Pak Shing; Juvela, Mika; Kim, Kee-Tae; Evans, Neal J., II; Di Francesco, James; Liu, Sheng-Yuan; Yuan, Jinghua; Tatematsu, Ken’ichi; Zhang, Qizhou; Ward-Thompson, Derek; Fuller, Gary; Goldsmith, Paul F.; Koch, P. M.; Sanhueza, Patricio; Ristorcelli, I.; Kang, Sung-ju; Chen, Huei-Ru; Hirano, N.; Wu, Yuefang; Sokolov, Vlas; Lee, Chang Won; White, Glenn J.; Wang, Ke; Eden, David; Li, Di; Thompson, Mark; Pattle, Kate M.; Soam, Archana; Nasedkin, Evert; Kim, Jongsoo; Kim, Gwanjeong; Lai, Shih-Ping; Park, Geumsook; Qiu, Keping; Zhang, Chuan-Peng; Alina, Dana; Eswaraiah, Chakali; Falgarone, Edith; Fich, Michel; Greaves, Jane; Gu, Q.-L.; Kwon, Woojin; Li, Hua-bai; Malinen, Johanna; Montier, Ludovic; Parsons, Harriet; Qin, Sheng-Li; Rawlings, Mark G.; Ren, Zhi-Yuan; Tang, Mengyao; Tang, Y.-W.; Toth, L. V.; Wang, Jiawei; Wouterloot, Jan; Yi, H.-W.; Zhang, H.-W.

2018-06-01

Magnetic field plays a crucial role in shaping molecular clouds and regulating star formation, yet the complete information on the magnetic field is not well constrained owing to the limitations in observations. We study the magnetic field in the massive infrared dark cloud G035.39-00.33 from dust continuum polarization observations at 850 μm with SCUBA-2/POL-2 at JCMT for the first time. The magnetic field tends to be perpendicular to the densest part of the main filament (F M), whereas it has a less defined relative orientation in the rest of the structure, where it tends to be parallel to some diffuse regions. A mean plane-of-the-sky magnetic field strength of ∼50 μG for F M is obtained using the Davis–Chandrasekhar–Fermi method. Based on 13CO (1–0) line observations, we suggest a formation scenario of F M due to large-scale (∼10 pc) cloud–cloud collision. Using additional NH3 line data, we estimate that F M will be gravitationally unstable if it is only supported by thermal pressure and turbulence. The northern part of F M, however, can be stabilized by a modest additional support from the local magnetic field. The middle and southern parts of F M are likely unstable even if the magnetic field support is taken into account. We claim that the clumps in F M may be supported by turbulence and magnetic fields against gravitational collapse. Finally, we identified for the first time a massive (∼200 M ⊙), collapsing starless clump candidate, “c8,” in G035.39-00.33. The magnetic field surrounding “c8” is likely pinched, hinting at an accretion flow along the filament.

Fantastic Striations and Where to Find Them: The Origin of Magnetically Aligned Striations in Interstellar Clouds

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chen, Che-Yu; Li, Zhi-Yun; King, Patrick K.

2017-10-01

Thin, magnetically aligned striations of relatively moderate contrast with the background are commonly observed in both atomic and molecular clouds. They are also prominent in MHD simulations with turbulent converging shocks. The simulated striations develop within a dense, stagnated sheet in the midplane of the post-shock region where magnetically induced converging flows collide. We show analytically that the secondary flows are an inevitable consequence of the jump conditions of oblique MHD shocks. They produce the stagnated, sheet-like sub-layer through a secondary shock when, roughly speaking, the Alfvénic speed in the primary converging flows is supersonic, a condition that is relativelymore » easy to satisfy in interstellar clouds. The dense sub-layer is naturally threaded by a strong magnetic field that lies close to the plane of the sub-layer. The substantial magnetic field makes the sheet highly anisotropic, which is the key to the striation formation. Specifically, perturbations of the primary inflow that vary spatially perpendicular to the magnetic field can easily roll up the sheet around the field lines without bending them, creating corrugations that appear as magnetically aligned striations in column density maps. On the other hand, perturbations that vary spatially along the field lines curve the sub-layer and alter its orientation relative to the magnetic field locally, seeding special locations that become slanted overdense filaments and prestellar cores through enhanced mass accumulation along field lines. In our scenario, the dense sub-layer, which is unique to magnetized oblique shocks, is the birthplace for both magnetically aligned diffuse striations and massive star-forming structures.« less

Probing the magnetic topologies of magnetic clouds by means of solar energetic particles

NASA Technical Reports Server (NTRS)

Kahler, S. W.; Reames, D. V.

1991-01-01

Solar energetic particles (SEPs) have been used as probes of magnetic cloud topologies. The rapid access of SEPs to the interiors of many clouds indicates that the cloud field lines extend back to the sun and hence are not plasmoids. The small modulation of galactic cosmic rays associated with clouds also suggests that the magnetic fields of clouds are not closed.

Fragmentation of a Filamentary Cloud Permeated by a Perpendicular Magnetic Field

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hanawa, Tomoyuki; Kudoh, Takahiro; Tomisaka, Kohji

We examine the linear stability of an isothermal filamentary cloud permeated by a perpendicular magnetic field. Our model cloud is assumed to be supported by gas pressure against self-gravity in the unperturbed state. For simplicity, the density distribution is assumed to be symmetric around the axis. Also for simplicity, the initial magnetic field is assumed to be uniform, and turbulence is not taken into account. The perturbation equation is formulated to be an eigenvalue problem. The growth rate is obtained as a function of the wavenumber for fragmentation along the axis and the magnetic field strength. The growth rate dependsmore » critically on the outer boundary. If the displacement vanishes in regions very far from the cloud axis (fixed boundary), cloud fragmentation is suppressed by a moderate magnetic field, which means the plasma beta is below 1.67 on the cloud axis. If the displacement is constant along the magnetic field in regions very far from the cloud, the cloud is unstable even when the magnetic field is infinitely strong. The cloud is deformed by circulation in the plane perpendicular to the magnetic field. The unstable mode is not likely to induce dynamical collapse, since it is excited even when the whole cloud is magnetically subcritical. For both boundary conditions, the magnetic field increases the wavelength of the most unstable mode. We find that the magnetic force suppresses compression perpendicular to the magnetic field especially in regions of low density.« less

A new mechanical stellar wind feedback model for the Rosette Nebula

NASA Astrophysics Data System (ADS)

Wareing, C. J.; Pittard, J. M.; Wright, N. J.; Falle, S. A. E. G.

2018-04-01

The famous Rosette Nebula has an evacuated central cavity formed from the stellar winds ejected from the 2-6 Myr old codistant and comoving central star cluster NGC 2244. However, with upper age estimates of less than 110 000 yr, the central cavity is too young compared to NGC 2244 and existing models do not reproduce its properties. A new proper motion study herein using Gaia data reveals the ejection of the most massive star in the Rosette, HD 46223, from NGC 2244 occurred 1.73 (+0.34, -0.25) Myr (1σ uncertainty) in the past. Assuming this ejection was at the birth of the most massive stars in NGC 2244, including the dominant centrally positioned HD 46150, the age is set for the famous ionized region at more than 10 times that derived for the cavity. Here, we are able to reproduce the structure of the Rosette Nebula, through simulation of mechanical stellar feedback from a 40 M⊙ star in a thin sheet-like molecular cloud. We form the 135 000 M⊙ cloud from thermally unstable diffuse interstellar medium (ISM) under the influence of a realistic background magnetic field with thermal/magnetic pressure equilibrium. Properties derived from a snapshot of the simulation at 1.5 Myr, including cavity size, stellar age, magnetic field, and resulting inclination to the line of sight, match those derived from observations. An elegant explanation is thus provided for the stark contrast in age estimates based on realistic diffuse ISM properties, molecular cloud formation and stellar wind feedback.

Generation of zonal magnetic fields by low-frequency dispersive electromagnetic waves in a nonuniform dusty magnetoplasma.

PubMed

Shukla, P K

2004-04-01

It is shown that zonal magnetic fields can be parametrically excited by low-frequency dispersive driftlike compressional electromagnetic (DDCEM) modes in a nonuniform dusty magnetoplasma. For this purpose, we derive a pair of coupled equations which exhibits the nonlinear coupling between DDCEM modes and zonal magnetic fields. The coupled mode equations are Fourier analyzed to derive a nonlinear dispersion relation. The latter depicts that zonal magnetic fields are nonlinearly generated at the expense of the low-frequency DDCEM wave energy. The relevance of our investigation to the transfer of energy from short scale DDCEM waves to long scale zonal magnetic field structures in dark molecular clouds is discussed.

Filament formation in wind-cloud interactions- II. Clouds with turbulent density, velocity, and magnetic fields

NASA Astrophysics Data System (ADS)

Banda-Barragán, W. E.; Federrath, C.; Crocker, R. M.; Bicknell, G. V.

2018-01-01

We present a set of numerical experiments designed to systematically investigate how turbulence and magnetic fields influence the morphology, energetics, and dynamics of filaments produced in wind-cloud interactions. We cover 3D, magnetohydrodynamic systems of supersonic winds impacting clouds with turbulent density, velocity, and magnetic fields. We find that lognormal density distributions aid shock propagation through clouds, increasing their velocity dispersion and producing filaments with expanded cross-sections and highly magnetized knots and subfilaments. In self-consistently turbulent scenarios, the ratio of filament to initial cloud magnetic energy densities is ∼1. The effect of Gaussian velocity fields is bound to the turbulence Mach number: Supersonic velocities trigger a rapid cloud expansion; subsonic velocities only have a minor impact. The role of turbulent magnetic fields depends on their tension and is similar to the effect of radiative losses: the stronger the magnetic field or the softer the gas equation of state, the greater the magnetic shielding at wind-filament interfaces and the suppression of Kelvin-Helmholtz instabilities. Overall, we show that including turbulence and magnetic fields is crucial to understanding cold gas entrainment in multiphase winds. While cloud porosity and supersonic turbulence enhance the acceleration of clouds, magnetic shielding protects them from ablation and causes Rayleigh-Taylor-driven subfilamentation. Wind-swept clouds in turbulent models reach distances ∼15-20 times their core radius and acquire bulk speeds ∼0.3-0.4 of the wind speed in one cloud-crushing time, which are three times larger than in non-turbulent models. In all simulations, the ratio of turbulent magnetic to kinetic energy densities asymptotes at ∼0.1-0.4, and convergence of all relevant dynamical properties requires at least 64 cells per cloud radius.

Magnetic flux rope versus the spheromak as models for interplanetary magnetic clouds

NASA Technical Reports Server (NTRS)

Farrugia, C. J.; Osherovich, V. A.; Burlaga, L. F.

1995-01-01

Magnetic clouds form a subset of interplanetary ejecta with well-defined magnetic and thermodynamic properties. Observationally, it is well established that magnetic clouds expand as they propagate antisunward. The aim of this paper is to compare and contrast two models which have been proposed for the global magnetic field line topology of magnetic clouds: a magnetic flux tube geometry, on the one hand, and a spheromak geometry (including possible higher multiples), on the other. Traditionally, the magnetic structure of magnetic clouds has been modeled by force-free configurations. In a first step, we therefore analyze the ability of static force-free models to account for the asymmetries observed in the magnetic field profiles of magnetic clouds. For a cylindrical flux tube the magnetic field remains symmetric about closest approach to the magnetic axis on all spacecraft orbits intersecting it, whereas in a spheromak geometry one can have asymmetries in the magnetic field signatures along some spacecraft trajectories. The duration of typical magnetic cloud encounters at 1 AU (1 to 2 days) is comparable to their travel time from the Sun to 1 AU and thus magnetic clouds should be treated as strongly nonstationary objects. In a second step, therefore, we abandon the static approach and model magnetic clouds as self-similarly evolving MHD configurations. In our theory, the interaction of the expanding magnetic cloud with the ambient plasma is taken into account by a drag force proportional to the density and the velocity of expansion. Solving rigorously the full set of MHD equations, we demonstrate that the asymmetry in the magnetic signature may arise solely as a result of expansion. Using asymptotic solutions of the MHD equations, we least squares fit both theoretical models to interplanetary data. We find that while the central part of the magnetic cloud is adequately described by both models, the 'edges' of the cloud data are modeled better by the magnetic flux tube. Further comparisons of the two models necessarily involve thermodynamic properties, since real magnetic configurations are never exactly force-free and gas pressure plays an essential role. We consider a polytropic gas. Our theoretical analysis shows that the self-similar expansion of a magnetic flux tube requires the polytropic index gamma to be less than unity. For the spheromak, however, self-similar, radially expanding solutions are known only for gamma equal to 4/3. This difference, therefore, yields a good way of distinguishing between the two geometries. It has been shown recently that the polytropic relationship is applicable to magnetic clouds and that the corresponding polytropic index is approximately 0.5. This observational result is consistent with the self-similar model of the magnetic flux rope but is in conflict with the self-similar spheromak model.

THE FORMATION OF FILAMENTARY BUNDLES IN TURBULENT MOLECULAR CLOUDS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Moeckel, Nickolas; Burkert, Andreas, E-mail: nickolas1@gmail.com, E-mail: burkert@usm.uni-muenchen.de

2015-07-01

The classical picture of a star-forming filament is a near-equilibrium structure with its collapse dependent on its gravitational criticality. Recent observations have complicated this picture, revealing filaments to be a mess of apparently interacting subfilaments with transsonic internal velocity dispersions and mildly supersonic intra-subfilament dispersions. How structures like this form is unresolved. Here, we study the velocity structure of filamentary regions in a simulation of a turbulent molecular cloud. We present two main findings. First, the observed complex velocity features in filaments arise naturally in self-gravitating hydrodynamic simulations of turbulent clouds without the need for magnetic or other effects. Second,more » a region that is filamentary only in projection and is in fact made of spatially distinct features can display these same velocity characteristics. The fact that these disjoint structures can masquerade as coherent filaments in both projection and velocity diagnostics highlights the need to continue developing sophisticated filamentary analysis techniques for star formation observations.« less

Study of an expanding magnetic cloud

NASA Astrophysics Data System (ADS)

Nakwacki, M. S.; Dasso, S.; Mandrini, C. H.; Démoulin, P.

Magnetic Clouds (MCs) transport into the interplanetary medium the magnetic flux and helicity released in coronal mass ejections by the Sun. At 1 AU from the Sun, MCs are generally modelled as static flux ropes. However, the velocity profile of some MCs presents signatures of expansion. We analise here the magnetic structure of an expanding magnetic cloud observed by Wind spacecraft. We consider a dynamical model, based on a self-similar behaviour for the cloud radial velocity. We assume a free expansion for the cloud, and a cylindrical linear force free field (i.e., the Lundquist's field) as the initial condition for its magnetic configuration. We derive theoretical expressions for the magnetic flux across a surface perpendicular to the cloud axis, for the magnetic helicity and magnetic energy per unit length along the tube using the self-similar model. Finally, we compute these magntitudes with the fitted parameters. FULL TEXT IN SPANISH

Behavior of nanoparticle clouds around a magnetized microsphere under magnetic and flow fields.

PubMed

Magnet, C; Kuzhir, P; Bossis, G; Meunier, A; Nave, S; Zubarev, A; Lomenech, C; Bashtovoi, V

2014-03-01

When a micron-sized magnetizable particle is introduced into a suspension of nanosized magnetic particles, the nanoparticles accumulate around the microparticle and form thick anisotropic clouds extended in the direction of the applied magnetic field. This phenomenon promotes colloidal stabilization of bimodal magnetic suspensions and allows efficient magnetic separation of nanoparticles used in bioanalysis and water purification. In the present work, the size and shape of nanoparticle clouds under the simultaneous action of an external uniform magnetic field and the flow have been studied in detail. In experiments, a dilute suspension of iron oxide nanoclusters (of a mean diameter of 60 nm) was pushed through a thin slit channel with the nickel microspheres (of a mean diameter of 50 μm) attached to the channel wall. The behavior of nanocluster clouds was observed in the steady state using an optical microscope. In the presence of strong enough flow, the size of the clouds monotonically decreases with increasing flow speed in both longitudinal and transverse magnetic fields. This is qualitatively explained by enhancement of hydrodynamic forces washing the nanoclusters away from the clouds. In the longitudinal field, the flow induces asymmetry of the front and the back clouds. To explain the flow and the field effects on the clouds, we have developed a simple model based on the balance of the stresses and particle fluxes on the cloud surface. This model, applied to the case of the magnetic field parallel to the flow, captures reasonably well the flow effect on the size and shape of the cloud and reveals that the only dimensionless parameter governing the cloud size is the ratio of hydrodynamic-to-magnetic forces-the Mason number. At strong magnetic interactions considered in the present work (dipolar coupling parameter α≥2), the Brownian motion seems not to affect the cloud behavior.

Behavior of nanoparticle clouds around a magnetized microsphere under magnetic and flow fields

NASA Astrophysics Data System (ADS)

Magnet, C.; Kuzhir, P.; Bossis, G.; Meunier, A.; Nave, S.; Zubarev, A.; Lomenech, C.; Bashtovoi, V.

2014-03-01

When a micron-sized magnetizable particle is introduced into a suspension of nanosized magnetic particles, the nanoparticles accumulate around the microparticle and form thick anisotropic clouds extended in the direction of the applied magnetic field. This phenomenon promotes colloidal stabilization of bimodal magnetic suspensions and allows efficient magnetic separation of nanoparticles used in bioanalysis and water purification. In the present work, the size and shape of nanoparticle clouds under the simultaneous action of an external uniform magnetic field and the flow have been studied in detail. In experiments, a dilute suspension of iron oxide nanoclusters (of a mean diameter of 60 nm) was pushed through a thin slit channel with the nickel microspheres (of a mean diameter of 50 μm) attached to the channel wall. The behavior of nanocluster clouds was observed in the steady state using an optical microscope. In the presence of strong enough flow, the size of the clouds monotonically decreases with increasing flow speed in both longitudinal and transverse magnetic fields. This is qualitatively explained by enhancement of hydrodynamic forces washing the nanoclusters away from the clouds. In the longitudinal field, the flow induces asymmetry of the front and the back clouds. To explain the flow and the field effects on the clouds, we have developed a simple model based on the balance of the stresses and particle fluxes on the cloud surface. This model, applied to the case of the magnetic field parallel to the flow, captures reasonably well the flow effect on the size and shape of the cloud and reveals that the only dimensionless parameter governing the cloud size is the ratio of hydrodynamic-to-magnetic forces—the Mason number. At strong magnetic interactions considered in the present work (dipolar coupling parameter α ≥2), the Brownian motion seems not to affect the cloud behavior.

Magnetic Cloud Field Intensities and Solar Wind Velocities

NASA Technical Reports Server (NTRS)

Gonzalez, Walter D.; Clau de Gonzalez, Alicia D.; Tsurutani, Bruce T.; Arballo, John K.

1997-01-01

For the sets of magnetic clouds studied in this work we have shown that there is a general relationship between their magnetic fields strength and velocities. With a clear tendency that the faster the speed of the cloud the higher the magnetic field.

NEAR-INFRARED POLARIZATION SOURCE CATALOG OF THE NORTHEASTERN REGIONS OF THE LARGE MAGELLANIC CLOUD

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kim, Jaeyeong; Pak, Soojong; Jeong, Woong-Seob

2016-01-15

We present a near-infrared band-merged photometric and polarimetric catalog for the 39′ × 69′ fields in the northeastern part of the Large Magellanic Cloud (LMC), which were observed using SIRPOL, an imaging polarimeter of the InfraRed Survey Facility. This catalog lists 1858 sources brighter than 14 mag in the H band with a polarization signal-to-noise ratio greater than three in the J, H, or K{sub s} bands. Based on the relationship between the extinction and the polarization degree, we argue that the polarization mostly arises from dichroic extinctions caused by local interstellar dust in the LMC. This catalog allows usmore » to map polarization structures to examine the global geometry of the local magnetic field, and to show a statistical analysis of the polarization of each field to understand its polarization properties. In the selected fields with coherent polarization position angles, we estimate magnetic field strengths in the range of 3−25 μG using the Chandrasekhar–Fermi method. This implies the presence of large-scale magnetic fields on a scale of around 100 parsecs. When comparing mid- and far-infrared dust emission maps, we confirmed that the polarization patterns are well aligned with molecular clouds around the star-forming regions.« less

Evolutionary Description of Giant Molecular Cloud Mass Functions on Galactic Disks

NASA Astrophysics Data System (ADS)

Kobayashi, Masato I. N.; Inutsuka, Shu-ichiro; Kobayashi, Hiroshi; Hasegawa, Kenji

2017-02-01

Recent radio observations show that giant molecular cloud (GMC) mass functions noticeably vary across galactic disks. High-resolution magnetohydrodynamics simulations show that multiple episodes of compression are required for creating a molecular cloud in the magnetized interstellar medium. In this article, we formulate the evolution equation for the GMC mass function to reproduce the observed profiles, for which multiple compressions are driven by a network of expanding shells due to H II regions and supernova remnants. We introduce the cloud-cloud collision (CCC) terms in the evolution equation in contrast to previous work (Inutsuka et al.). The computed time evolution suggests that the GMC mass function slope is governed by the ratio of GMC formation timescale to its dispersal timescale, and that the CCC effect is limited only in the massive end of the mass function. In addition, we identify a gas resurrection channel that allows the gas dispersed by massive stars to regenerate GMC populations or to accrete onto pre-existing GMCs. Our results show that almost all of the dispersed gas contributes to the mass growth of pre-existing GMCs in arm regions whereas less than 60% contributes in inter-arm regions. Our results also predict that GMC mass functions have a single power-law exponent in the mass range <105.5 {M}⊙ (where {M}⊙ represents the solar mass), which is well characterized by GMC self-growth and dispersal timescales. Measurement of the GMC mass function slope provides a powerful method to constrain those GMC timescales and the gas resurrecting factor in various environments across galactic disks.

Transparency of a magnetic cloud boundary for cosmic rays

NASA Astrophysics Data System (ADS)

Petukhov, I. S.; Petukhov, S. I.

2013-02-01

We have suggested a model of magnetic cloud presented as a torus with magnetic flux rope structure situated inside the interplanetary corona mass ejecta expanding radially away from the Sun through the interplanetary medium. The magnetic field of the torus changing during its propagation has been obtained. The magnetic cloud — solar wind boundary transparency for cosmic rays with different energies depending on the cloud orientation and properties of the torus magnetic field has been determined by means of calculation of the particle trajectories at the boundary.

Magnetohydrodynamic Models of Molecular Tornadoes

NASA Astrophysics Data System (ADS)

Au, Kelvin; Fiege, Jason D.

2017-07-01

Recent observations near the Galactic Center (GC) have found several molecular filaments displaying striking helically wound morphology that are collectively known as molecular tornadoes. We investigate the equilibrium structure of these molecular tornadoes by formulating a magnetohydrodynamic model of a rotating, helically magnetized filament. A special analytical solution is derived where centrifugal forces balance exactly with toroidal magnetic stress. From the physics of torsional Alfvén waves we derive a constraint that links the toroidal flux-to-mass ratio and the pitch angle of the helical field to the rotation laws, which we find to be an important component in describing the molecular tornado structure. The models are compared to the Ostriker solution for isothermal, nonmagnetic, nonrotating filaments. We find that neither the analytic model nor the Alfvén wave model suffer from the unphysical density inversions noted by other authors. A Monte Carlo exploration of our parameter space is constrained by observational measurements of the Pigtail Molecular Cloud, the Double Helix Nebula, and the GC Molecular Tornado. Observable properties such as the velocity dispersion, filament radius, linear mass, and surface pressure can be used to derive three dimensionless constraints for our dimensionless models of these three objects. A virial analysis of these constrained models is studied for these three molecular tornadoes. We find that self-gravity is relatively unimportant, whereas magnetic fields and external pressure play a dominant role in the confinement and equilibrium radial structure of these objects.

Supernova Remnant Kes 17: An Efficient Cosmic Ray Accelerator inside a Molecular Cloud

NASA Astrophysics Data System (ADS)

Gelfand, Joseph; Slane, Patrick; Hughes, John; Temim, Tea; Castro, Daniel; Rakowski, Cara

Supernova remnant are believed to be the dominant source of cosmic rays protons below the "knee" in the energy spectrum. However, relatively few supernova remnants have been identified as efficient producers of cosmic ray protons. In this talk, I will present evidence that the production of cosmic ray protons is required to explain the broadband non-thermal spectrum of supernova remnant Kes 17 (SNR G304.6+0.1). Evidence for efficient cosmic ray acceleration in Kes 17 supports recent theoretical work concluding that the strong magnetic field, turbulence, and clumpy nature of molecular clouds enhance cosmic ray production in supernova remnants. While additional observations are needed to confirm this interpretation, further study of Kes 17 and similar sources are important for understanding how cosmic rays are accelerated in supernova remnants.

A study of an expanding interplanatary magnetic cloud and its interaction with the earth's magnetosphere - The interplanetary aspect

NASA Technical Reports Server (NTRS)

Farrugia, C. J.; Burlaga, L. F.; Osherovich, V. A.; Richardson, I. G.; Freeman, M. P.; Lepping, R. P.; Lazarus, A. J.

1993-01-01

High time resolution interplanetary magnetic field and plasma measurements of an interplanetary magnetic cloud and its interaction with the earth's magnetosphere on January 14/15, 1988 are interpreted and discussed. It is argued that the data are consistent with the theoretical model of magnetic clouds as flux ropes of local straight cylindrical geometry. The data also suggest that this cloud is aligned with its axis in the ecliptic plane and pointing in the east-west direction. Evidence consisting of the intensity and directional distribution of energetic particle in the magnetic cloud argues in favor of the connectedness of the magnetic field lines to the sun's surface. The intensities of about 0.5 MeV ions is rapidly enhanced and the particles stream in a collimated beam along the magnetic field preferentially from the west of the sun. The particles travel form a flare site along the cloud magnetic field lines, which are thus presumably still attached to the sun.

Shaken and stirred: the effects of turbulence and rotation on disc and outflow formation during the collapse of magnetized molecular cloud cores

NASA Astrophysics Data System (ADS)

Lewis, Benjamin T.; Bate, Matthew R.

2018-07-01

We present the results of 18 magnetohydrodynamical calculations of the collapse of a molecular cloud core to form a protostar. Some calculations include radiative transfer in the flux-limited diffusion approximation, while others employ a barotropic equation of state. We cover a wide parameter space, with mass-to-flux ratios ranging from μ = 5 to 20; initial turbulent amplitudes ranging from a laminar calculation (i.e. where the Mach number, M = 0) to transonic M = 1; and initial rotation rates from βrot = 0.005 to 0.02. We first show that using a radiative transfer scheme produces warmer pseudo-discs than the barotropic equation of state, making them more stable. We then `shake' the core by increasing the initial turbulent velocity field, and find that at all three mass-to-flux ratios transonic cores are weakly bound and do not produce pseudo-discs; M = 0.3 cores produce very disrupted discs; and M = 0.1 cores produce discs broadly comparable to a laminar core. In our previous paper, we showed that a pseudo-disc coupled with sufficient magnetic field is necessary to form a bipolar outflow. Here, we show that only weakly turbulent cores exhibit collimated jets. We finally take the M = 1.0, μ = 5 core and `stir' it by increasing the initial angular momentum, finding that once the degree of rotational energy exceeds the turbulent energy in the core the disc returns, with a corresponding (though slower), outflow. These conclusions place constraints on the initial mixtures of rotation and turbulence in molecular cloud cores which are conducive to the formation of bipolar outflows early in the star formation process.

Energetic ion and cosmic ray characteristics of a magnetic cloud

NASA Astrophysics Data System (ADS)

Sanderson, T. R.; Beeck, J.; Marsden, R. G.; Tranquille, C.; Wenzel, K.-P.; McKibben, R. B.; Smith, E. J.

The large interplanetary shock event of February 11, 1982, has yielded ISEE-3 energetic ion and magnetic field data as well as ground-based neutron-monitor cosmic-ray data. The timing and the onset of the Forbush decrease associated with this shock event coincide with the arrival at the earth of its magnetic cloud component; the duration of the decrease, similarly, corresponds to that of the cloud's passage past the earth. The large scattering mean free path readings suggest that while magnetic cloud ions can easily travel along magnetic field lines, they cannot travel across them, so that they cannot escape the cloud after entering it. Similarly, the cloud field lines prevented cosmic ray entrance, and could have prevented their reaching the earth. The cloud is therefore a major basis for the Forbush decrease.

Morphological diagnostics of star formation in molecular clouds

NASA Astrophysics Data System (ADS)

Beaumont, Christopher Norris

Molecular clouds are the birth sites of all star formation in the present-day universe. They represent the initial conditions of star formation, and are the primary medium by which stars transfer energy and momentum back to parsec scales. Yet, the physical evolution of molecular clouds remains poorly understood. This is not due to a lack of observational data, nor is it due to an inability to simulate the conditions inside molecular clouds. Instead, the physics and structure of the interstellar medium are sufficiently complex that interpreting molecular cloud data is very difficult. This dissertation mitigates this problem, by developing more sophisticated ways to interpret morphological information in molecular cloud observations and simulations. In particular, I have focused on leveraging machine learning techniques to identify physically meaningful substructures in the interstellar medium, as well as techniques to inter-compare molecular cloud simulations to observations. These contributions make it easier to understand the interplay between molecular clouds and star formation. Specific contributions include: new insight about the sheet-like geometry of molecular clouds based on observations of stellar bubbles; a new algorithm to disambiguate overlapping yet morphologically distinct cloud structures; a new perspective on the relationship between molecular cloud column density distributions and the sizes of cloud substructures; a quantitative analysis of how projection effects affect measurements of cloud properties; and an automatically generated, statistically-calibrated catalog of bubbles identified from their infrared morphologies.

Supernova Driving. IV. The Star-formation Rate of Molecular Clouds

NASA Astrophysics Data System (ADS)

Padoan, Paolo; Haugbølle, Troels; Nordlund, Åke; Frimann, Søren

2017-05-01

We compute the star-formation rate (SFR) in molecular clouds (MCs) that originate ab initio in a new, higher-resolution simulation of supernova-driven turbulence. Because of the large number of well-resolved clouds with self-consistent boundary and initial conditions, we obtain a large range of cloud physical parameters with realistic statistical distributions, which is an unprecedented sample of star-forming regions to test SFR models and to interpret observational surveys. We confirm the dependence of the SFR per free-fall time, SFRff, on the virial parameter, α vir, found in previous simulations, and compare a revised version of our turbulent fragmentation model with the numerical results. The dependences on Mach number, { M }, gas to magnetic pressure ratio, β, and compressive to solenoidal power ratio, χ at fixed α vir are not well constrained, because of random scatter due to time and cloud-to-cloud variations in SFRff. We find that SFRff in MCs can take any value in the range of 0 ≤ SFRff ≲ 0.2, and its probability distribution peaks at a value of SFRff ≈ 0.025, consistent with observations. The values of SFRff and the scatter in the SFRff-α vir relation are consistent with recent measurements in nearby MCs and in clouds near the Galactic center. Although not explicitly modeled by the theory, the scatter is consistent with the physical assumptions of our revised model and may also result in part from a lack of statistical equilibrium of the turbulence, due to the transient nature of MCs.

The Mass Surface Density Distribution of a High-Mass Protocluster forming from an IRDC and GMC

NASA Astrophysics Data System (ADS)

Lim, Wanggi; Tan, Jonathan C.; Kainulainen, Jouni; Ma, Bo; Butler, Michael

2016-01-01

We study the probability distribution function (PDF) of mass surface densities of infrared dark cloud (IRDC) G028.36+00.07 and its surrounding giant molecular cloud (GMC). Such PDF analysis has the potential to probe the physical processes that are controlling cloud structure and star formation activity. The chosen IRDC is of particular interest since it has almost 100,000 solar masses within a radius of 8 parsecs, making it one of the most massive, dense molecular structures known and is thus a potential site for the formation of a high-mass, "super star cluster". We study mass surface densities in two ways. First, we use a combination of NIR, MIR and FIR extinction maps that are able to probe the bulk of the cloud structure that is not yet forming stars. This analysis also shows evidence for flattening of the IR extinction law as mass surface density increases, consistent with increasing grain size and/or growth of ice mantles. Second, we study the FIR and sub-mm dust continuum emission from the cloud, especially utlizing Herschel PACS and SPIRE images. We first subtract off the contribution of the foreground diffuse emission that contaminates these images. Next we examine the effects of background subtraction and choice of dust opacities on the derived mass surface density PDF. The final derived PDFs from both methods are compared, including also with other published studies of this cloud. The implications for theoretical models and simulations of cloud structure, including the role of turbulence and magnetic fields, are discussed.

HAWCPol: a first-generation far-infrared polarimeter for SOFIA

NASA Astrophysics Data System (ADS)

Dowell, C. Darren; Cook, Brant T.; Harper, D. Al; Lin, Lung-Sheng; Looney, Leslie W.; Novak, Giles; Stephens, Ian; Berthoud, Marc; Chuss, David T.; Crutcher, Richard M.; Dotson, Jessie L.; Hildebrand, Roger H.; Houde, Martin; Jones, Terry J.; Krejny, Megan; Lazarian, Alexandre; Moseley, S. Harvey; Tassis, Kostas; Vaillancourt, John E.; Werner, Michael W.

2010-07-01

We describe our ongoing project to build a far-infrared polarimeter for the HAWC instrument on SOFIA. Far-IR polarimetry reveals unique information about magnetic fields in dusty molecular clouds and is an important tool for understanding star formation and cloud evolution. SOFIA provides flexible access to the infrared as well as good sensitivity to and angular resolution of continuum emission from molecular clouds. We are making progress toward outfitting HAWC, a first-generation SOFIA camera, with a four-band polarimeter covering 50 to 220 microns wavelength. We have chosen a conservative design which uses quartz half-wave plates continuously rotating at ~0.5 Hz, ball bearing suspensions, fixed wire-grid polarizers, and cryogenic motors. Design challenges are to fit the polarimeter into a volume that did not originally envision one, to minimize the heating of the cryogenic optics, and to produce negligible interference in the detector system. Here we describe the performance of the polarimeter measured at cryogenic temperature as well as the basic method we intend for data analysis. We are on track for delivering this instrument early in the operating lifetime of SOFIA.

Dynamics of aging magnetic clouds. [interacted with solar wind

NASA Technical Reports Server (NTRS)

Osherovich, V. A.; Farrugia, C. J.; Burlaga, L. F.

1993-01-01

The dynamics of radially expanding magnetic clouds is rigorously analyzed within the framework of ideal MHD. The cloud is modelled as a cylindrically symmetric magnetic flux rope. In the force balance we include the gas pressure gradient and the Lorentz force. Interaction with the ambient solar wind due to expansion of the magnetic cloud is represented by a drag force proportional to the bulk velocity. We consider the self-similar expansion of a polytrope, and reduce the problem to an ordinary nonlinear differential equation for the evolution function. Analyzing the asymptotic behavior of the evolution function, we formulate theoretical expectations for the long-term behavior of cloud parameters. We focus on the temporal evolution of (1) the magnetic field strength; (2) the twist of the field lines; (3) the asymmetry of the total field profile; and (4) the bulk flow speed. We present data from two magnetic clouds observed at 1 AU and 2 AU, respectively, and find good agreement with theoretical expectations. For a peak magnetic field strength at 1 AU of 25 nT and a polytropic index of 0.5, we find that a magnetic cloud can be distinguished from the background interplanetary field up to a distance of about 5 AU. Taking larger magnetic fields and bigger polytropic indices this distance can double.

The first in situ observation of torsional Alfvén waves during the interaction of large-scale magnetic clouds

NASA Astrophysics Data System (ADS)

Raghav, Anil N.; Kule, Ankita

2018-05-01

The large-scale magnetic cloud such as coronal mass ejections (CMEs) is the fundamental driver of the space weather. The interaction of the multiple-CMEs in interplanetary space affects their dynamic evolution and geo-effectiveness. The complex and merged multiple magnetic clouds appear as the in situ signature of the interacting CMEs. The Alfvén waves are speculated to be one of the major possible energy exchange/dissipation mechanism during the interaction. However, no such observational evidence has been found in the literature. The case studies of CME-CME collision events suggest that the magnetic and thermal energy of the CME is converted into the kinetic energy. Moreover, magnetic reconnection process is justified to be responsible for merging of multiple magnetic clouds. Here, we present unambiguous evidence of sunward torsional Alfvén waves in the interacting region after the super-elastic collision of multiple CMEs. The Walén relation is used to confirm the presence of Alfvén waves in the interacting region of multiple CMEs/magnetic clouds. We conclude that Alfvén waves and magnetic reconnection are the possible energy exchange/dissipation mechanisms during large-scale magnetic clouds collisions. This study has significant implications not only in CME-magnetosphere interactions but also in the interstellar medium where interactions of large-scale magnetic clouds are possible.

Propagation of a Toroidal Magnetic Cloud through the Inner Heliosphere

NASA Astrophysics Data System (ADS)

Romashets, Eugene; Vandas, Marek

2003-09-01

An analytical solution for a potential magnetic field with arbitrary intensity around a toroidal magnetic cloud has been found. The background external field may have a gradient. The solution is used for calculation of magnetic cloud propagation. Obtained velocity profiles show a good agreement with in situ observations near the Earth's orbit.

Energetic ion observations in the magnetic cloud of 14-15 January 1988 and their implications for the magnetic field topology

NASA Technical Reports Server (NTRS)

Richardson, I. G.; Farrugia, C. J.; Burlaga, L. F.

1991-01-01

On 14-15 January 1988, a magnetic cloud with a local field topology consistent with an east-west aligned cylindrical flux-rope and which formed the driver of an interplanetary shock passed the earth. Using 0.5-4 MeV/n ion data from the instrument on IMP 8, the paper addresses the question of whether or not magnetic field lines within the magnetic cloud were connected to the sun. An impulsive solar particle event was detected inside the magnetic cloud strongly suggesting that the field lines were rooted at the sun.

The Structure and Evolution of Self-Gravitating Molecular Clouds

NASA Astrophysics Data System (ADS)

Holliman, John Herbert, II

1995-01-01

We present a theoretical formalism to evaluate the structure of molecular clouds and to determine precollapse conditions in star-forming regions. Models consist of pressure-bounded, self-gravitating spheres of a single -fluid ideal gas. We treat the case without rotation. The analysis is generalized to consider states in hydrostatic equilibrium maintained by multiple pressure components. Individual pressures vary with density as P_i(r) ~ rho^{gamma {rm p},i}(r), where gamma_{rm p},i is the polytropic index. Evolution depends additionally on whether conduction occurs on a dynamical time scale and on the adiabatic index gammai of each component, which is modified to account for the effects of any thermal coupling to the environment of the cloud. Special attention is given to properly representing the major contributors to dynamical support in molecular clouds: the pressures due to static magnetic fields, Alfven waves, and thermal motions. Straightforward adjustments to the model allow us to treat the intrinsically anisotropic support provided by the static fields. We derive structure equations, as well as perturbation equations for performing a linear stability analysis. The analysis provides insight on the nature of dynamical motions due to collapse from an equilibrium state and estimates the mass of condensed objects that form in such a process. After presenting a set of general results, we describe models of star-forming regions that include the major pressure components. We parameterize the extent of ambipolar diffusion. The analysis contributes to the physical understanding of several key results from observations of these regions. Commonly observed quantities are explicitly cross-referenced with model results. We theoretically determine density and linewidth profiles on scales ranging from that of molecular cloud cores to that of giant molecular clouds (GMCs). The model offers an explanation of the mean pressures in GMCs, which are observed to be high relative to that in the intercloud medium. We estimate what fraction of a cloud on the verge of gravitational collapse will ultimately form a condensed object, and we predict the qualitative appearance of the collapse. Finally, we simulate fragmentation--a key step in the star-forming process whereby molecular clouds or clumps within more massive clouds break up into substantially less massive cores that can in turn condense into stars. Fragmentation occurs in the context of dynamical collapse--a highly nonlinear process--so it has been difficult to reach a consensus on its specific appearance or on the influence of initial conditions. Increases in density by several orders of magnitude and the unknown, time-dependent positions of the rapidly evolving fragments present difficulties for the simulation of fragmentation. In order to increase the efficiency and effective resolution with which we can model this process, we have assembled can adaptive mesh refinement (AMR) hydrodynamics algorithm and an adaptive elliptical solver for self-gravity. The code is adaptive in the sense that it can dynamically and automatically alter the configuration of a recursively finer mesh in the computational domain. A test suite helps confirm the proper operation of the algorithm. Using initial conditions adopted in previous fragmentation studies, we simulate the collapse of a molecular cloud core. (Abstract shortened by UMI.).

Masses, luminosities and dynamics of galactic molecular clouds

NASA Technical Reports Server (NTRS)

Solomon, P. M.; Rivolo, A. R.; Mooney, T. J.; Barrett, J. W.; Sage, L. J.

1987-01-01

Star formation in galaxies takes place in molecular clouds and the Milky Way is the only galaxy in which it is possible to resolve and study the physical properties and star formation activity of individual clouds. The masses, luminosities, dynamics, and distribution of molecular clouds, primarily giant molecular clouds in the Milky Way are described and analyzed. The observational data sets are the Massachusetts-Stony Brook CO Galactic Plane Survey and the IRAS far IR images. The molecular mass and infrared luminosities of glactic clouds are then compared with the molecular mass and infrared luminosities of external galaxies.

The numerical modelling of MHD astrophysical flows with chemistry

NASA Astrophysics Data System (ADS)

Kulikov, I.; Chernykh, I.; Protasov, V.

2017-10-01

The new code for numerical simulation of magnetic hydrodynamical astrophysical flows with consideration of chemical reactions is given in the paper. At the heart of the code - the new original low-dissipation numerical method based on a combination of operator splitting approach and piecewise-parabolic method on the local stencil. The chemodynamics of the hydrogen while the turbulent formation of molecular clouds is modeled.

Electron Cloud Trapping in Recycler Combined Function Dipole Magnets

DOE Office of Scientific and Technical Information (OSTI.GOV)

Antipov, Sergey A.; Nagaitsev, S.

2016-10-04

Electron cloud can lead to a fast instability in intense proton and positron beams in circular accelerators. In the Fermilab Recycler the electron cloud is confined within its combined function magnets. We show that the field of combined function magnets traps the electron cloud, present the results of analytical estimates of trapping, and compare them to numerical simulations of electron cloud formation. The electron cloud is located at the beam center and up to 1% of the particles can be trapped by the magnetic field. Since the process of electron cloud build-up is exponential, once trapped this amount of electronsmore » significantly increases the density of the cloud on the next revolution. In a Recycler combined function dipole this multiturn accumulation allows the electron cloud reaching final intensities orders of magnitude greater than in a pure dipole. The multi-turn build-up can be stopped by injection of a clearing bunch of 1010 p at any position in the ring.« less

A comparative study of dynamically expanding force-free, constant-alpha magnetic configurations with applications to magnetic clouds

NASA Technical Reports Server (NTRS)

Farrugia, C. J.; Burlaga, L. F.; Osherovich, V. A.; Lepping, R. P.

1992-01-01

We contrast two different solutions of the constant alpha, force-free MHD equation, both of which have been suggested as models for magnetic clouds: a solution in cylindrical coordinates and one in spherical coordinates. In line with the observation that magnetic clouds expand, we generalize these static models and construct their expanding counterparts. We find that expansion introduces in both cases a large asymmetry in the field strength signature which is in the same sense as that seen the the data, i.e. towards the leading edge of the cloud. We then do a least squares fit of the respective models to one-spacecraft data on a magnetic cloud. We find that the fitting routine converges in both cases. However, while purely formally we cannot distinguish between the two models using data from one spacecraft, the field components in the 'spherical' model have features not compatible with data on magnetic clouds.

Enhancement of Feedback Efficiency by Active Galactic Nucleus Outflows via the Magnetic Tension Force in the Inhomogeneous Interstellar Medium

DOE Office of Scientific and Technical Information (OSTI.GOV)

Asahina, Yuta; Ohsuga, Ken; Nomura, Mariko, E-mail: asahina@cfca.jp

By performing three-dimensional magnetohydrodynamics simulations of subrelativistic jets and disk winds propagating into the magnetized inhomogeneous interstellar medium (ISM), we investigate the magnetic effects on the active galactic nucleus feedback. Our simulations reveal that the magnetic tension force promotes the acceleration of the dense gas clouds, since the magnetic field lines, which are initially straight, bend around the gas clouds. In the jet models, the velocity dispersion of the clouds increases with an increase in the initial magnetic fields. The increment of the kinetic energy of the clouds is proportional to the initial magnetic fields, implying that the magnetic tensionmore » force increases the energy conversion efficiency from the jet to the gas clouds. Through simulations of the mildly collimated disk wind and the funnel-shaped disk wind, we confirm that such an enhancement of the energy conversion efficiency via the magnetic fields appears even if the energy is injected via the disk winds. The enhancement of the acceleration of the dense part of the magnetized ISM via the magnetic tension force will occur wherever the magnetized inhomogeneous matter is blown away.« less

Numerical Simulation of Interacting Magnetic Flux Ropes

NASA Astrophysics Data System (ADS)

Odstrcil, Dusan; Vandas, Marek; Pizzo, Victor J.; MacNeice, Peter

2003-09-01

A 212-D MHD numerical model is used to investigate the dynamic interaction between two flux ropes (clouds) in a homogeneous magnetized plasma. One cloud is set into motion while the other is initially at rest. The moving cloud generates a shock which interacts with the second cloud. Two cases with different characteristic speeds within the second cloud are presented. The shock front is significantly distorted when it propagates faster (slower) in the cloud with larger (smaller) characteristic speed. Correspondingly, the density behind the shock front becomes smaller (larger). Later, the clouds approach each other and by a momentum exchange they come to a common speed. The oppositely directed magnetic fields are pushed together, a driven magnetic reconnection takes a place, and the two flux ropes gradually coalescence into a single flux rope.

Does the Alfvén wave wreck the large-scale magnetic cloud structure?

NASA Astrophysics Data System (ADS)

Raghav, Anil N.; Kule, Ankita

2018-06-01

Alfvén waves are primal and pervasive in space plasmas and significantly contributes to microscale fluctuations in the solar wind and some heliospheric processes. Here, we demonstrate the first observable distinct feature of Alfvén wave while propagating from magnetic cloud to trailing solar wind. The Walén test is used to confirm their presence in selected regions. The amplitude ratio of inward to outward Alfvén waves is employed to establish their flow direction. The dominant inward flow is observed in magnetic cloud whereas trailing solar wind shows the dominant outward flow of Alfvén waves. The observed reduction in Walén slope and correlation coefficient within magnetic cloud suggest (i) the simultaneous presence of an inward & outward Alfvén waves and/or (ii) a possibility of magnetic reconnection and/or (iii) development of thermal anisotropy and/or (iv) dissipation of Alfvénic fluctuations. The study implies that either the Alfvén waves dissipate in the magnetic cloud or its presence can lead to disruption of the magnetic cloud structure.

Reconnection Remnants in the Magnetic Cloud of October 18-19, 1995: A Shock, Monochromatic Wave, Heat Flux Dropout and Energetic Ion Beam

NASA Technical Reports Server (NTRS)

Collier, Michael R.; Szabo, A.; Farrell, W.; Slavin, J. A.; Lepping, R. P.; Fitzenreiter, R.; Thompson, B.; Hamilton, D. C.; Gloeckler, G.; Ho, G. C.

2000-01-01

Evidence is presented that the WIND spacecraft observed particle and field signatures on October 18-19, 1995 due to reconnection near the footpoints of a magnetic cloud (i.e., between 1 and 5 solar radii). These signatures include: (1) an internal shock traveling approximately along the axis of the magnetic cloud, (2) a simple compression of the magnetic field consistent with the footpoint magnetic fields being thrust outwards at speeds much greater than the solar wind speed, (3) an electron heat flux dropout occurring within minutes of the shock indicating a topological change resulting from disconnection from the solar surface, (4) a very cold 5 keV proton beam and (5) an associated monochromatic wave. We expect that, given observations of enough magnetic clouds, Wind and other spacecraft will see signatures similar to the ones reported here indicating reconnection. However, these observations require the spacecraft to be fortuitously positioned to observe the passing shock and other signatures and will therefore be associated with only a small fraction of magnetic clouds. Consistent with this, a few magnetic clouds observed by Wind have been found to possess internal shock waves.

The Distribution of Mass Surface Densities in a High-mass Protocluster

NASA Astrophysics Data System (ADS)

Lim, Wanggi; Tan, Jonathan C.; Kainulainen, Jouni; Ma, Bo; Butler, Michael J.

2016-09-01

We study the probability distribution function (PDF) of mass surface densities, Σ, of infrared dark cloud (IRDC) G028.37+00.07 and its surrounding giant molecular cloud. This PDF constrains the physical processes, such as turbulence, magnetic fields, and self-gravity, that are expected to be controlling cloud structure and star formation activity. The chosen IRDC is of particular interest since it has almost 100,000 solar masses within a radius of 8 pc, making it one of the most massive, dense molecular structures known and is thus a potential site for the formation of a “super star cluster.” We study Σ in two ways. First, we use a combination of NIR and MIR extinction maps that are able to probe the bulk of the cloud structure up to Σ ˜ 1 g cm-2(A V ≃ 200 mag). Second, we study the FIR and submillimeter dust continuum emission from the cloud utilizing Herschel-PACS and SPIRE images and paying careful attention to the effects of foreground and background contamination. We find that the PDFs from both methods, applied over a ˜20‧(30 pc)-sized region that contains ≃1.5 × 105 M ⊙ and enclosing a minimum closed contour with Σ ≃ 0.013 g cm-2 (A V ≃ 3 mag), shows a lognormal shape with the peak measured at Σ ≃ 0.021 g cm-2 (A V ≃ 4.7 mag). There is tentative evidence for the presence of a high-Σ power-law tail that contains from ˜3% to 8% of the mass of the cloud material. We discuss the implications of these results for the physical processes occurring in this cloud.

VLA Zeeman Observations of the NGC 6334 Complex

NASA Astrophysics Data System (ADS)

Mayo, E. A.; Sarma, A. P.; Troland, T. H.

2004-05-01

We present OH 1665 and 1667 MHz observations of the NGC 6334 complex taken with the Very Large Array in the BnA configuration. We have combined our data with the lower resolution CnB data of Sarma et al (1999), in order to perform a detailed study of Source A, a compact continuum source in the SW region of the complex. Our observations reveal magnetic fields with peak values of the order of 700μ G toward Source A. Virial estimates presented indicate the significance of the magnetic field in the support of the molecular cloud against gravitational collapse.

Chemistry in Magnetohydrodynamic Shock Waves in Diffuse Molecular Clouds

NASA Astrophysics Data System (ADS)

Peimbert, Antonio

1998-09-01

Absorption observations of the CH+ molecule with column densities of up to 1014 cm-2 in diffuse molecular clouds in many lines of sight are reviewed, and compared to the reddening and to abundances and velocity shifts of molecules like CH. Special attention is placed on the observations of the line of sight towards ς Ophiuchi where high quality observations of many chemical species are available. The problem of the required CH+ is described, and many formation mechanisms from the literature are reviewed, finding that none of them is particularly apt at describing the observations towards ς-Oph. Two fluid J-type shock models are studied as an alternative. The necessary conditions for their formation are discussed, and it is shown how they are expected to be present widely in the interstellar medium. Plane parallel numerical integrations, for the particular case in which the magnetic field is perpendicular to the shock velocity, are employed to study the region of phase-space of initial conditions that will produce 2 fluid shocks. A chemical network is developed and formation of key molecules like CH+, CH and OH, along with the excited roto-vibrational levels of H2, are studied under the shock dynamics. These models are then compared to the observations of the different lines of sight, showing they are capable of reproducing the features of the observations towards most of those clouds. An attempt to model the line of sight towards ς-Oph is done, finding that a shock with a shock speed vs = 9.0km/s going through a cloud with a density of nH = 14cm-3 with a magnetic field of B = 4.7μG does a reasonable job at satisfying most of the observations with the exception of the highest rotational excited states of molecular hydrogen for which observations are available. There is a small family of solutions capable of explaining the observed results which make specific predictions for the velocity profiles of the H2 lines of various excited levels. New observations with the Interstellar Medium Absorption Profile Spectrograph (IMAPS) camera would be useful in confirming or rejecting these models.

On the stability of self-gravitating magnetized dusty plasmas

NASA Astrophysics Data System (ADS)

Salimullah, M.; Shukla, P. K.

1999-03-01

The effects of a homogeneous magnetic field and the plasma nonuniformity on the dispersion relations of various electrostatic waves in self-gravitating magnetized dusty plasmas have been investigated. For this purpose, the kinetic dielectric response functions for the electrons and ions distributions have been used and the dielectric response function for the magnetized dust grains has been derived from the hydrodynamic equations that include the self-gravitational potential. Thus, extremely massive charged dust grains are subjected to both the electromagnetic and gravitational forces. Analytical studies of the dispersion relations in various frequency and wave number regimes reveal that both the magnetic fields and plasma inhomogeneities contribute to the stability of a self-gravitating dusty plasma system. The results of this investigation should be useful in understanding the stability of dusty proto-stars and dusty dark molecular clouds, which are held in strong magnetic fields and equilibrium density gradients.

The giant molecular cloud Monoceros R2. 1: Shell structure

NASA Technical Reports Server (NTRS)

Xie, Taoling; Goldsmith, Paul F.

1994-01-01

We have obtained a 45 sec resolution, Nyquist-sampled map in CO J = 1-0 covering approximately a 3 deg x 3 deg region of the giant molecular cloud Monoceros R2. The map consists of 167,000 spectra observed with the 15 element focal-plane array system on the FCRAO 14 m telescope. The data reveal that the large-scale structure of Mon R2 is dominated by a is approximately 30 pc diameter largely hemispherical shell containing approximately 4 x 10(exp 4) solar mass of molecular material and expanding at approximately 3-4 km s(exp -1) with symmetric axis roughly along the line of sight. The dynamical timescale of the shell is estimated to be approximately 4 x 10(exp 6) yr, which is consistent with the age of main-sequence stars powering the clusters of reflection nebulea in this region. There is no evidence for a redshifted shell on the far side of the interior 'bubble,' which is largely devoid of molecular material. Distortions of the shell are obvious, suggesting inhomogeneity of the cloud and possible presence of a magnetic field prior to its formation. Dense clumps in Mon R2, including the main core and the GGD 12-15 core, appear to be condensations located on the large shell. The reflection nebulea with their illuminating stars as well as embedded IRAS sources suggest that triggered star formation has taken place over a large part of the Mon R2 shell.

He2+ molecular ion and the He- atomic ion in strong magnetic fields

NASA Astrophysics Data System (ADS)

Vieyra, J. C. Lopez; Turbiner, A. V.

2017-08-01

We study the question of existence, i.e., stability with respect to dissociation of the spin-quartet permutation- and reflection-symmetric 4(-3) +g (Sz=-3 /2 ,M =-3 ) state of the (α α e e e ) Coulomb system: the He2 + molecular ion, placed in a magnetic field 0 ≤B ≤10 000 a.u. We assume that the α particles are infinitely massive (Born-Oppenheimer approximation of zero order) and adopt the parallel configuration, when the molecular axis and the magnetic field direction coincide, as the optimal configuration. The study of the stability is performed variationally with a physically adequate trial function. To achieve this goal, we explore several helium-containing compounds in strong magnetic fields, in particular; we study the spin-quartet ground state of the He- ion and the ground (spin-triplet) state of the helium atom, both for a magnetic field in 100 ≤B ≤10 000 a.u. The main result is that the He2 + molecular ion in the state 4(-3) +g is stable towards all possible decay modes for magnetic fields B ≳120 a .u . and with the magnetic field increase the ion becomes more tightly bound and compact with a cigar-type form of electronic cloud. At B =1000 a .u . , the dissociation energy of He2 + into He-+α is ˜702 eV and the dissociation energy for the decay channel to He +α +e is ˜729 eV , and both energies are in the energy window for one of the observed absorption features of the isolated neutron star 1E1207.4-5209.

Magnetic Fields and Multiple Protostar Formation

NASA Astrophysics Data System (ADS)

Boss, A. P.

2001-12-01

Recent observations of star-forming regions suggest that binary and multiple young stars are the rule rather than the exception, and implicate fragmentation as the likely mechanism for their formation. Most numerical hydrodynamical calculations of fragmentation have neglected the possibly deleterious effects of magnetic fields, in spite of ample evidence for the importance of magnetic support of pre-collapse clouds. We present here the first numerical hydrodynamical survey of the full effects of magnetic fields on the collapse and fragmentation of dense cloud cores. The models are calculated with a three dimensional, finite differences code which solves the equations of hydrodynamics, gravitation, and radiative transfer in the Eddington and diffusion approximations. Magnetic field effects are included through two simple approximations: magnetic pressure is added to the gas pressure, and magnetic tension is approximated by gravity dilution once collapse is well underway. Ambipolar diffusion of the magnetic field leading to cloud collapse is treated approximately as well. Models are calculated for a variety of initial cloud density profiles, shapes, and rotation rates. We find that in spite of the inclusion of magnetic field effects, dense cloud cores are capable of fragmenting into binary and multiple protostar systems. Initially prolate clouds tend to fragment into binary protostars, while initially oblate clouds tend to fragment into multiple protostar systems containing a small number (of order four) of fragments. The latter are likely to be subject to rapid orbital evolution, with close encounters possibly leading to the ejection of fragments. Contrary to expectation, magnetic tension effects appear to enhance fragmentation, allowing lower mass fragments to form than would otherwise be possible, because magnetic tension helps to prevent a central density singularity from forming and producing a dominant single object. Magnetically-supported dense cloud cores thus seem to be capable of collapsing and fragmenting into sufficient numbers of binary and multiple protostar systems to be compatible with observations of the relative rarity of single protostars. This work was partially supported by NSF grants AST-9983530 and MRI-9976645.

Galactic and Intergalactic Magnetic Fields

NASA Astrophysics Data System (ADS)

Klein, U.; Fletcher, A.

This course-tested textbook conveys the fundamentals of magnetic fields and relativistic plasma in diffuse cosmic media, with a primary focus on phenomena that have been observed at different wavelengths. Theoretical concepts are addressed wherever necessary, with derivations presented in sufficient detail to be generally accessible. In the first few chapters the authors present an introduction to various astrophysical phenomena related to cosmic magnetism, with scales ranging from molecular clouds in star-forming regions and supernova remnants in the Milky Way, to clusters of galaxies. Later chapters address the role of magnetic fields in the evolution of the interstellar medium, galaxies and galaxy clusters. The book is intended for advanced undergraduate and postgraduate students in astronomy and physics and will serve as an entry point for those starting their first research projects in the field.

Magnetohydrodynamic Models of Molecular Tornadoes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Au, Kelvin; Fiege, Jason D., E-mail: fiege@physics.umanitoba.ca

Recent observations near the Galactic Center (GC) have found several molecular filaments displaying striking helically wound morphology that are collectively known as molecular tornadoes. We investigate the equilibrium structure of these molecular tornadoes by formulating a magnetohydrodynamic model of a rotating, helically magnetized filament. A special analytical solution is derived where centrifugal forces balance exactly with toroidal magnetic stress. From the physics of torsional Alfvén waves we derive a constraint that links the toroidal flux-to-mass ratio and the pitch angle of the helical field to the rotation laws, which we find to be an important component in describing the molecularmore » tornado structure. The models are compared to the Ostriker solution for isothermal, nonmagnetic, nonrotating filaments. We find that neither the analytic model nor the Alfvén wave model suffer from the unphysical density inversions noted by other authors. A Monte Carlo exploration of our parameter space is constrained by observational measurements of the Pigtail Molecular Cloud, the Double Helix Nebula, and the GC Molecular Tornado. Observable properties such as the velocity dispersion, filament radius, linear mass, and surface pressure can be used to derive three dimensionless constraints for our dimensionless models of these three objects. A virial analysis of these constrained models is studied for these three molecular tornadoes. We find that self-gravity is relatively unimportant, whereas magnetic fields and external pressure play a dominant role in the confinement and equilibrium radial structure of these objects.« less

First Near-infrared Imaging Polarimetry of Young Stellar Objects in the Circinus Molecular Cloud

NASA Astrophysics Data System (ADS)

Kwon, Jungmi; Nakagawa, Takao; Tamura, Motohide; Hough, James H.; Choi, Minho; Kandori, Ryo; Nagata, Tetsuya; Kang, Miju

2018-02-01

We present the results of near-infrared (NIR) linear imaging polarimetry in the J, H, and K s bands of the low-mass star cluster-forming region in the Circinus Molecular Cloud Complex. Using aperture polarimetry of point-like sources, positive detection of 314, 421, and 164 sources in the J, H, and K s bands, respectively, was determined from among 749 sources whose photometric magnitudes were measured. For the source classification of the 133 point-like sources whose polarization could be measured in all 3 bands, a color–color diagram was used. While most of the NIR polarizations of point-like sources are well-aligned and can be explained by dichroic polarization produced by aligned interstellar dust grains in the cloud, 123 highly polarized sources have also been identified with some criteria. The projected direction on the sky of the magnetic field in the Cir-MMS region is indicated by the mean polarization position angles (70°) of the point-like sources in the observed region, corresponding to approximately 1.6× 1.6 pc2. In addition, the magnetic field direction is compared with the outflow orientations associated with Infrared Astronomy Satellite sources, in which two sources were found to be aligned with each other and one source was not. We also show prominent polarization nebulosities over the Cir-MMS region for the first time. Our polarization data have revealed one clear infrared reflection nebula (IRN) and several candidate IRNe in the Cir-MMS field. In addition, the illuminating sources of the IRNe are identified with near- and mid-infrared sources.

Characteristics of magnetised plasma flow around stationary and expanding magnetic clouds

NASA Astrophysics Data System (ADS)

Dalakishvili, Giorgi

Studies of interplanetary magnetic clouds have shown that the characteristics of the region ahead of these objects, which are moving away from the Sun in the solar wind, play a role in determining their geo-efficiency, i.e. the kind and the degree of their effects on the Earth environment. Therefore, our main goal is to model and study the plasma parameters in the vicinity of interplanetary magnetic clouds. To this end we present a model in which the magnetic clouds are immersed in a magnetised plasma flow with a homogeneous magnetic field. We first calculate the resulting distortion of the external magnetic field and then determine the plasma velocity by employing the frozen-in condition. Subsequently, the plasma density and pressure are expressed as functions of the magnetic field and the velocity field. The plasma flow parameters are determined by solving the time-independent ideal MHD equations for both the stationary regime and for the case of an expand-ing cylindrical magnetic cloud, thus extending previous results that appeared in the literature.

Electron temperatures within magnetic clouds between 2 and 4 AU: Voyager 2 observations

NASA Astrophysics Data System (ADS)

Sittler, E. C.; Burlaga, L. F.

1998-08-01

We have performed an analysis of Voyager 2 plasma electron observations within magnetic clouds between 2 and 4 AU identified by Burlaga and Behannon [1982]. The analysis has been confined to three of the magnetic clouds identified by Burlaga and Behannon that had high-quality data. The general properties of the plasma electrons within a magnetic cloud are that (1) the moment electron temperature anticorrelates with the electron density within the cloud, (2) the ratio Te/Tp tends to be >1, and (3) on average, Te/Tp~7.0. All three results are consistent with previous electron observations within magnetic clouds. Detailed analyses of the core and halo populations within the magnetic clouds show no evidence of either an anticorrelation between the core temperature TC and the electron density Ne or an anticorrelation between the halo temperature TH and the electron density. Within the magnetic clouds the halo component can contribute more than 50% of the electron pressure. The anticorrelation of Te relative to Ne can be traced to the density of the halo component relative to the density of the core component. The core electrons dominate the electron density. When the density goes up, the halo electrons contribute less to the electron pressure, so we get a lower Te. When the electron density goes down, the halo electrons contribute more to the electron pressure, and Te goes up. We find a relation between the electron pressure and density of the form Pe=αNeγ with γ~0.5.

Effects of interplanetary magnetic clouds, interaction regions, and high-speed streams on the transient modulation of galactic cosmic rays

NASA Astrophysics Data System (ADS)

Singh, Y. P.; Badruddin

2007-02-01

Interplanetary manifestations of coronal mass ejections (CMEs) with specific plasma and field properties, called ``interplanetary magnetic clouds,'' have been observed in the heliosphere since the mid-1960s. Depending on their associated features, a set of observed magnetic clouds identified at 1 AU were grouped in four different classes using data over 4 decades: (1) interplanetary magnetic clouds moving with the ambient solar wind (MC structure), (2) magnetic clouds moving faster than the ambient solar wind and forming a shock/sheath structure of compressed plasma and field ahead of it (SMC structure), (3) magnetic clouds ``pushed'' by the high-speed streams from behind, forming an interaction region between the two (MIH structure), and (4) shock-associated magnetic clouds followed by high-speed streams (SMH structure). This classification into different groups led us to study the role, effect, and the relative importance of (1) closed field magnetic cloud structure with low field variance, (2) interplanetary shock and magnetically turbulent sheath region, (3) interaction region with large field variance, and (4) the high-speed solar wind stream coming from the open field regions, in modulating the galactic cosmic rays (GCRs). MC structures are responsible for transient decrease with fast recovery. SMC structures are responsible for fast decrease and slow recovery, MIH structures produce depression with slow decrease and slow recovery, and SMH structures are responsible for fast decrease with very slow recovery. Simultaneous variations of GCR intensity, solar plasma velocity, interplanetary magnetic field strength, and its variance led us to study the relative effectiveness of different structures as well as interplanetary plasma/field parameters. Possible role of the magnetic field, its topology, field turbulence, and the high-speed streams in influencing the amplitude and time profile of resulting decreases in GCR intensity have also been discussed.

Vector velocity profiles of the solar wind within expanding magnetic clouds at 1 AU: Some surprises

NASA Astrophysics Data System (ADS)

Wu, C.; Lepping, R. P.; Berdichevsky, D.; Ferguson, T.; Lazarus, A. J.

2002-12-01

We investigated the average vector velocity profile of 36 carefully chosen WIND interplanetary magnetic clouds occurring over about a 7 year period since spacecraft launch, to see if a differential pattern of solar wind flow exists. Particular cases were chosen of clouds whose axes were generally within 45 degrees of the ecliptic plane and of relatively well determined characteristics obtained from cloud-parameter (cylindrically symmetric force free) fitting. This study was motivated by the desire to understand the manner in which magnetic clouds expand, a well know phenomenon revealed by most cloud speed-profiles at 1 AU. One unexpected and major result was that, even though cloud expansion was confirmed, it was primarily along the Xgse axis; i.e., neither the Ygse or Zgse velocity components reveal any noteworthy pattern. After splitting the full set of clouds into a north-passing set (spacecraft passing above the cloud, where Nn = 21) and south-passing set (Ns = 15), to study the plasma expansion of the clouds with respect to the position of the observer, it was seen that the Xgse component of velocity differs for these two sets in a rather uniform and measurable way for most of the average cloud's extent. This does not appear to be the case for the Ygse or Zgse velocity components where little measurable differences exists, and clearly no pattern, across the average cloud between the north and south positions. It is not clear why such a remarkably non-axisymmetric plasma flow pattern within the "average magnetic cloud" at 1 AU should exist. The study continues from the perspective of magnetic cloud coordinate representation. ~ ~ ~

Planck intermediate results: XXXII. The relative orientation between the magnetic field and structures traced by interstellar dust

DOE PAGES

Adam, R.; Ade, P. A. R.; Aghanim, N.; ...

2016-02-09

The role of the magnetic field in the formation of the filamentary structures observed in the interstellar medium (ISM) is a debated topic owing to the paucity of relevant observations needed to test existing models. The Planck all-sky maps of linearly polarized emission from dust at 353 GHz provide the required combination of imaging and statistics to study the correlation between the structures of the Galactic magnetic field and of interstellar matter over the whole sky, both in the diffuse ISM and in molecular clouds. The data reveal that structures, or ridges, in the intensity map have counterparts in themore » Stokes Q and/or U maps. In this paper, we focus our study on structures at intermediate and high Galactic latitudes, which cover two orders of magnitude in column density, from 10 20 to 10 22 cm -2. We measure the magnetic field orientation on the plane ofthe sky from the polarization data, and present an algorithm to estimate the orientation of the ridges from the dust intensity map. We use analytical models to account for projection effects. Comparing polarization angles on and off the structures, we estimate the mean ratio between the strengths of the turbulent and mean components of the magnetic field to be between 0.6 and 1.0, with a preferred value of 0.8. We find that the ridges are usually aligned with the magnetic field measured on the structures. This statistical trend becomes more striking for increasing polarization fraction and decreasing column density. There is no alignment for the highest column density ridges. We interpret the increase in alignment with polarization fraction as a consequence of projection effects. We present maps to show that the decrease in alignment for high column density is not due to a loss of correlation between the distribution of matter and the geometry of the magnetic field. In molecular complexes, we also observe structures perpendicular to the magnetic field, which, statistically, cannot be accounted for by projection effects. This first statistical study of the relative orientation between the matter structures and the magnetic field in the ISM points out that, at the angular scales probed by Planck, the field geometry projected on the plane of the sky is correlated with the distribution of matter. In the diffuse ISM, the structures of matter are usually aligned with the magnetic field, while perpendicular structures appear in molecular clouds. Finally, we discuss our results in the context of models and MHD simulations, which attempt to describe the respective roles of turbulence, magnetic field, and self-gravity in the formation of structures in the magnetized ISM.« less

Planck intermediate results. XXXII. The relative orientation between the magnetic field and structures traced by interstellar dust

NASA Astrophysics Data System (ADS)

Planck Collaboration; Adam, R.; Ade, P. A. R.; Aghanim, N.; Alves, M. I. R.; Arnaud, M.; Arzoumanian, D.; Ashdown, M.; Aumont, J.; Baccigalupi, C.; Banday, A. J.; Barreiro, R. B.; Bartolo, N.; Battaner, E.; Benabed, K.; Benoit-Lévy, A.; Bernard, J.-P.; Bersanelli, M.; Bielewicz, P.; Bonaldi, A.; Bonavera, L.; Bond, J. R.; Borrill, J.; Bouchet, F. R.; Boulanger, F.; Bracco, A.; Burigana, C.; Butler, R. C.; Calabrese, E.; Cardoso, J.-F.; Catalano, A.; Chamballu, A.; Chiang, H. C.; Christensen, P. R.; Colombi, S.; Colombo, L. P. L.; Combet, C.; Couchot, F.; Crill, B. P.; Curto, A.; Cuttaia, F.; Danese, L.; Davies, R. D.; Davis, R. J.; de Bernardis, P.; de Rosa, A.; de Zotti, G.; Delabrouille, J.; Dickinson, C.; Diego, J. M.; Dole, H.; Donzelli, S.; Doré, O.; Douspis, M.; Ducout, A.; Dupac, X.; Efstathiou, G.; Elsner, F.; Enßlin, T. A.; Eriksen, H. K.; Falgarone, E.; Ferrière, K.; Finelli, F.; Forni, O.; Frailis, M.; Fraisse, A. A.; Franceschi, E.; Frejsel, A.; Galeotta, S.; Galli, S.; Ganga, K.; Ghosh, T.; Giard, M.; Gjerløw, E.; González-Nuevo, J.; Górski, K. M.; Gregorio, A.; Gruppuso, A.; Guillet, V.; Hansen, F. K.; Hanson, D.; Harrison, D. L.; Henrot-Versillé, S.; Hernández-Monteagudo, C.; Herranz, D.; Hildebrandt, S. R.; Hivon, E.; Hobson, M.; Holmes, W. A.; Hovest, W.; Huffenberger, K. M.; Hurier, G.; Jaffe, A. H.; Jaffe, T. R.; Jones, W. C.; Juvela, M.; Keihänen, E.; Keskitalo, R.; Kisner, T. S.; Kneissl, R.; Knoche, J.; Kunz, M.; Kurki-Suonio, H.; Lagache, G.; Lamarre, J.-M.; Lasenby, A.; Lattanzi, M.; Lawrence, C. R.; Leonardi, R.; Levrier, F.; Liguori, M.; Lilje, P. B.; Linden-Vørnle, M.; López-Caniego, M.; Lubin, P. M.; Macías-Pérez, J. F.; Maffei, B.; Maino, D.; Mandolesi, N.; Maris, M.; Marshall, D. J.; Martin, P. G.; Martínez-González, E.; Masi, S.; Matarrese, S.; Mazzotta, P.; Melchiorri, A.; Mendes, L.; Mennella, A.; Migliaccio, M.; Miville-Deschênes, M.-A.; Moneti, A.; Montier, L.; Morgante, G.; Mortlock, D.; Munshi, D.; Murphy, J. A.; Naselsky, P.; Natoli, P.; Nørgaard-Nielsen, H. U.; Noviello, F.; Novikov, D.; Novikov, I.; Oppermann, N.; Oxborrow, C. A.; Pagano, L.; Pajot, F.; Paoletti, D.; Pasian, F.; Perdereau, O.; Perotto, L.; Perrotta, F.; Pettorino, V.; Piacentini, F.; Piat, M.; Plaszczynski, S.; Pointecouteau, E.; Polenta, G.; Ponthieu, N.; Popa, L.; Pratt, G. W.; Prunet, S.; Puget, J.-L.; Rachen, J. P.; Reach, W. T.; Reinecke, M.; Remazeilles, M.; Renault, C.; Ristorcelli, I.; Rocha, G.; Roudier, G.; Rubiño-Martín, J. A.; Rusholme, B.; Sandri, M.; Santos, D.; Savini, G.; Scott, D.; Soler, J. D.; Spencer, L. D.; Stolyarov, V.; Sudiwala, R.; Sunyaev, R.; Sutton, D.; Suur-Uski, A.-S.; Sygnet, J.-F.; Tauber, J. A.; Terenzi, L.; Toffolatti, L.; Tomasi, M.; Tristram, M.; Tucci, M.; Umana, G.; Valenziano, L.; Valiviita, J.; Van Tent, B.; Vielva, P.; Villa, F.; Wade, L. A.; Wandelt, B. D.; Wehus, I. K.; Wiesemeyer, H.; Yvon, D.; Zacchei, A.; Zonca, A.

2016-02-01

The role of the magnetic field in the formation of the filamentary structures observed in the interstellar medium (ISM) is a debated topic owing to the paucity of relevant observations needed to test existing models. The Planck all-sky maps of linearly polarized emission from dust at 353 GHz provide the required combination of imaging and statistics to study the correlation between the structures of the Galactic magnetic field and of interstellar matter over the whole sky, both in the diffuse ISM and in molecular clouds. The data reveal that structures, or ridges, in the intensity map have counterparts in the Stokes Q and/or U maps. We focus our study on structures at intermediate and high Galactic latitudes, which cover two orders of magnitude in column density, from 1020 to 1022 cm-2. We measure the magnetic field orientation on the plane ofthe sky from the polarization data, and present an algorithm to estimate the orientation of the ridges from the dust intensity map. We use analytical models to account for projection effects. Comparing polarization angles on and off the structures, we estimate the mean ratio between the strengths of the turbulent and mean components of the magnetic field to be between 0.6 and 1.0, with a preferred value of 0.8. We find that the ridges are usually aligned with the magnetic field measured on the structures. This statistical trend becomes more striking for increasing polarization fraction and decreasing column density. There is no alignment for the highest column density ridges. We interpret the increase in alignment with polarization fraction as a consequence of projection effects. We present maps to show that the decrease in alignment for high column density is not due to a loss of correlation between the distribution of matter and the geometry of the magnetic field. In molecular complexes, we also observe structures perpendicular to the magnetic field, which, statistically, cannot be accounted for by projection effects. This first statistical study of the relative orientation between the matter structures and the magnetic field in the ISM points out that, at the angular scales probed by Planck, the field geometry projected on the plane of the sky is correlated with the distribution of matter. In the diffuse ISM, the structures of matter are usually aligned with the magnetic field, while perpendicular structures appear in molecular clouds. We discuss our results in the context of models and MHD simulations, which attempt to describe the respective roles of turbulence, magnetic field, and self-gravity in the formation of structures in the magnetized ISM.

Planck intermediate results: XXXII. The relative orientation between the magnetic field and structures traced by interstellar dust

DOE Office of Scientific and Technical Information (OSTI.GOV)

Adam, R.; Ade, P. A. R.; Aghanim, N.

The role of the magnetic field in the formation of the filamentary structures observed in the interstellar medium (ISM) is a debated topic owing to the paucity of relevant observations needed to test existing models. The Planck all-sky maps of linearly polarized emission from dust at 353 GHz provide the required combination of imaging and statistics to study the correlation between the structures of the Galactic magnetic field and of interstellar matter over the whole sky, both in the diffuse ISM and in molecular clouds. The data reveal that structures, or ridges, in the intensity map have counterparts in themore » Stokes Q and/or U maps. In this paper, we focus our study on structures at intermediate and high Galactic latitudes, which cover two orders of magnitude in column density, from 10 20 to 10 22 cm -2. We measure the magnetic field orientation on the plane ofthe sky from the polarization data, and present an algorithm to estimate the orientation of the ridges from the dust intensity map. We use analytical models to account for projection effects. Comparing polarization angles on and off the structures, we estimate the mean ratio between the strengths of the turbulent and mean components of the magnetic field to be between 0.6 and 1.0, with a preferred value of 0.8. We find that the ridges are usually aligned with the magnetic field measured on the structures. This statistical trend becomes more striking for increasing polarization fraction and decreasing column density. There is no alignment for the highest column density ridges. We interpret the increase in alignment with polarization fraction as a consequence of projection effects. We present maps to show that the decrease in alignment for high column density is not due to a loss of correlation between the distribution of matter and the geometry of the magnetic field. In molecular complexes, we also observe structures perpendicular to the magnetic field, which, statistically, cannot be accounted for by projection effects. This first statistical study of the relative orientation between the matter structures and the magnetic field in the ISM points out that, at the angular scales probed by Planck, the field geometry projected on the plane of the sky is correlated with the distribution of matter. In the diffuse ISM, the structures of matter are usually aligned with the magnetic field, while perpendicular structures appear in molecular clouds. Finally, we discuss our results in the context of models and MHD simulations, which attempt to describe the respective roles of turbulence, magnetic field, and self-gravity in the formation of structures in the magnetized ISM.« less

Sub-structure formation in starless cores

NASA Astrophysics Data System (ADS)

Toci, C.; Galli, D.; Verdini, A.; Del Zanna, L.; Landi, S.

2018-02-01

Motivated by recent observational searches of sub-structure in starless molecular cloud cores, we investigate the evolution of density perturbations on scales smaller than the Jeans length embedded in contracting isothermal clouds, adopting the same formalism developed for the expanding Universe and the solar wind. We find that initially small amplitude, Jeans-stable perturbations (propagating as sound waves in the absence of a magnetic field) are amplified adiabatically during the contraction, approximately conserving the wave action density, until they either become non-linear and steepen into shocks at a time tnl, or become gravitationally unstable when the Jeans length decreases below the scale of the perturbations at a time tgr. We evaluate analytically the time tnl at which the perturbations enter the non-linear stage using a Burgers' equation approach, and we verify numerically that this time marks the beginning of the phase of rapid dissipation of the kinetic energy of the perturbations. We then show that for typical values of the rms Mach number in molecular cloud cores, tnl is smaller than tgr, and therefore density perturbations likely dissipate before becoming gravitational unstable. Solenoidal modes grow at a faster rate than compressible modes, and may eventually promote fragmentation through the formation of vortical structures.

Origin of coronal mass ejection and magnetic cloud: Thermal or magnetic driven?

NASA Technical Reports Server (NTRS)

Zhang, Gong-Liang; Wang, Chi; He, Shuang-Hua

1995-01-01

A fundamental problem in Solar-Terrestrial Physics is the origin of the solar transient plasma output, which includes the coronal mass ejection and its interplanetary manifestation, e.g. the magnetic cloud. The traditional blast wave model resulted from solar thermal pressure impulse has faced with challenge during recent years. In the MHD numerical simulation study of CME, the authors find that the basic feature of the asymmetrical event on 18 August 1980 can be reproduced neither by a thermal pressure nor by a speed increment. Also, the thermal pressure model fails in simulating the interplanetary structure with low thermal pressure and strong magnetic field strength, representative of a typical magnetic cloud. Instead, the numerical simulation results are in favor of the magnetic field expansion as the likely mechanism for both the asymmetrical CME event and magnetic cloud.

First Results on Interstellar Magnetic Fields from the HAWC+ Instrument for SOFIA

NASA Astrophysics Data System (ADS)

Dowell, C. Darren; HAWC+ Instrument Team; HAWC+ Science Team

2018-06-01

HAWC+, a second-generation SOFIA instrument designed to map far-infrared intensity and polarization, was commissioned in late 2016 and made first science observations in SOFIA Cycles 4 and 5. We describe basic characteristics of the instrument, report on the commissioning flights and data analysis pipeline, and show some example science products resulting from Guaranteed-Time Observations (GTO). HAWC+ and SOFIA provide unique access to the far-infrared (50 - 250 micron) spectral range for polarimetry. Far-IR polarization arises from dust grains aligned with respect to the magnetic field, as well as synchrotron radiation, and the GTO program focuses primarily on the magnetic field structure of nearby molecular clouds and the Galactic center, and the physical characteristics of dust.

What are we learning from the relative orientation between density structures and the magnetic field in molecular clouds?

NASA Astrophysics Data System (ADS)

Soler, J. D.; Hennebelle, P.

2017-10-01

>⟨Bˆ⊥⟩-> being mostly parallel at low NH to mostly perpendicular at the highest NH, is related to the magnetic field strength and constitutes a crucial piece of information for determining the role of the magnetic field in the dynamics of MCs.

Magnetic fields, stellar feedback, and the geometry of H II regions

NASA Astrophysics Data System (ADS)

Ferland, Gary J.

2009-04-01

Magnetic pressure has long been known to dominate over gas pressure in atomic and molecular regions of the interstellar medium. Here I review several recent observational studies of the relationships between the H+, H0 and H2 regions in M42 (the Orion complex) and M17. A simple picture results. When stars form they push back surrounding material, mainly through the outward momentum of starlight acting on grains, and field lines are dragged with the gas due to flux freezing. The magnetic field is compressed and the magnetic pressure increases until it is able to resist further expansion and the system comes into approximate magnetostatic equilibrium. Magnetic field lines can be preferentially aligned perpendicular to the long axis of quiescent cloud before stars form. After star formation and pushback occurs ionized gas will be constrained to flow along field lines and escape from the system along directions perpendicular to the long axis. The magnetic field may play other roles in the physics of the H II region and associated PDR. Cosmic rays may be enhanced along with the field and provide additional heating of atomic and molecular material. Wave motions may be associated with the field and contribute a component of turbulence to observed line profiles.

Photoionization-regulated star formation and the structure of molecular clouds

NASA Technical Reports Server (NTRS)

Mckee, Christopher F.

1989-01-01

A model for the rate of low-mass star formation in Galactic molecular clouds and for the influence of this star formation on the structure and evolution of the clouds is presented. The rate of energy injection by newly formed stars is estimated, and the effect of this energy injection on the size of the cloud is determined. It is shown that the observed rate of star formation appears adequate to support the observed clouds against gravitational collapse. The rate of photoionization-regulated star formation is estimated and it is shown to be in agreement with estimates of the observed rate of star formation if the observed molecular cloud parameters are used. The mean cloud extinction and the Galactic star formation rate per unit mass of molecular gas are predicted theoretically from the condition that photionization-regulated star formation be in equilibrium. A simple model for the evolution of isolated molecular clouds is developed.

THE STAR FORMATION RATE OF TURBULENT MAGNETIZED CLOUDS: COMPARING THEORY, SIMULATIONS, AND OBSERVATIONS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Federrath, Christoph; Klessen, Ralf S., E-mail: christoph.federrath@monash.edu

2012-12-20

The role of turbulence and magnetic fields is studied for star formation in molecular clouds. We derive and compare six theoretical models for the star formation rate (SFR)-the Krumholz and McKee (KM), Padoan and Nordlund (PN), and Hennebelle and Chabrier (HC) models, and three multi-freefall versions of these, suggested by HC-all based on integrals over the log-normal distribution of turbulent gas. We extend all theories to include magnetic fields and show that the SFR depends on four basic parameters: (1) virial parameter {alpha}{sub vir}; (2) sonic Mach number M; (3) turbulent forcing parameter b, which is a measure for themore » fraction of energy driven in compressive modes; and (4) plasma {beta}=2M{sub A}{sup 2}/M{sup 2} with the Alfven Mach number M{sub A}. We compare all six theories with MHD simulations, covering cloud masses of 300 to 4 Multiplication-Sign 10{sup 6} M{sub Sun} and Mach numbers M=3-50 and M{sub A}=1-{infinity}, with solenoidal (b = 1/3), mixed (b = 0.4), and compressive turbulent (b = 1) forcings. We find that the SFR increases by a factor of four between M=5 and 50 for compressive turbulent forcing and {alpha}{sub vir} {approx} 1. Comparing forcing parameters, we see that the SFR is more than 10 times higher with compressive than solenoidal forcing for M=10 simulations. The SFR and fragmentation are both reduced by a factor of two in strongly magnetized, trans-Alfvenic turbulence compared to hydrodynamic turbulence. All simulations are fit simultaneously by the multi-freefall KM and multi-freefall PN theories within a factor of two over two orders of magnitude in SFR. The simulated SFRs cover the range and correlation of SFR column density with gas column density observed in Galactic clouds, and agree well for star formation efficiencies SFE = 1%-10% and local efficiencies {epsilon} = 0.3-0.7 due to feedback. We conclude that the SFR is primarily controlled by interstellar turbulence, with a secondary effect coming from magnetic fields.« less

Role of the magnetosheath in the interaction of magnetic clouds with the Earth's magnetosphere

NASA Astrophysics Data System (ADS)

Fontaine, Dominique; Turc, Lucile; Savoini, Philippe; Modolo, Ronan

2016-04-01

Magnetic clouds are among the most geoeffective solar events capable to trigger strong magnetic storms in the terrestrial magnetosphere. However, their characteristics and those of the surrounding media are not always capable to explain their high level of geoeffectivity. From observations and simulations, we investigate here the role of the bow shock and of the magnetosheath. Conjugated observations upstream (ACE) and downstream (CLUSTER) of the bow shock show that the magnetic clouds' magnetic structure in the magnetosheath can strongly depart from their pristine structure upstream of the bow shock. This modification depends on the shock configuration (quasi-perpendicular, quasi-parallel). We also discuss this question from hybrid simulations of the interaction of magnetic clouds with the bow shock. We show that this interaction may produce unexpected characteristics in the magnetosheath, such as asymmetric distributions of magnetic field, density, temperature, velocity. They thus lead to interactions with the magnetosphere which were not expected from the pristine characteristics of the magnetic clouds in the solar wind upstream of bow shock. We here discuss the effects of such an asymmetric magnetosheath on key parameters for the interaction with the magnetopause (reconnection, instabilities), responsible in turn for the development of geomagnetic activity inside the magnetosphere.

The role of magnetic fields in the collapse of protostellar gas clouds

NASA Technical Reports Server (NTRS)

Scott, E. H.; Black, D. C.

1980-01-01

The paper presents the results of a numerical calculation of the collapse of an idealized protostellar gas cloud including the effects of a 'frozen-in' magnetic field. The 'traditional' picture of magnetic effects on gas clouds and recent observational and theoretical work on the subject are summarized. Attention is given to the method of calculation and the results are interpreted. It is found that the central magnetic field in the collapsing cloud model follows a rho to the 1/2 power relation, and the discussion implies that this is a general result which should hold true for some range of initial conditions around those chosen. In addition, it is found that the outer envelope of the cloud will be held up by tension in the field lines.

AceCloud: Molecular Dynamics Simulations in the Cloud.

PubMed

Harvey, M J; De Fabritiis, G

2015-05-26

We present AceCloud, an on-demand service for molecular dynamics simulations. AceCloud is designed to facilitate the secure execution of large ensembles of simulations on an external cloud computing service (currently Amazon Web Services). The AceCloud client, integrated into the ACEMD molecular dynamics package, provides an easy-to-use interface that abstracts all aspects of interaction with the cloud services. This gives the user the experience that all simulations are running on their local machine, minimizing the learning curve typically associated with the transition to using high performance computing services.

Understanding the Twist Distribution Inside Magnetic Flux Ropes by Anatomizing an Interplanetary Magnetic Cloud

NASA Astrophysics Data System (ADS)

Wang, Yuming; Shen, Chenglong; Liu, Rui; Liu, Jiajia; Guo, Jingnan; Li, Xiaolei; Xu, Mengjiao; Hu, Qiang; Zhang, Tielong

2018-05-01

Magnetic flux rope (MFR) is the core structure of the greatest eruptions, that is, the coronal mass ejections (CMEs), on the Sun, and magnetic clouds are posteruption MFRs in interplanetary space. There is a strong debate about whether or not a MFR exists prior to a CME and how the MFR forms/grows through magnetic reconnection during the eruption. Here we report a rare event, in which a magnetic cloud was observed sequentially by four spacecraft near Mercury, Venus, Earth, and Mars, respectively. With the aids of a uniform-twist flux rope model and a newly developed method that can recover a shock-compressed structure, we find that the axial magnetic flux and helicity of the magnetic cloud decreased when it propagated outward but the twist increased. Our analysis suggests that the "pancaking" effect and "erosion" effect may jointly cause such variations. The significance of the pancaking effect is difficult to be estimated, but the signature of the erosion can be found as the imbalance of the azimuthal flux of the cloud. The latter implies that the magnetic cloud was eroded significantly leaving its inner core exposed to the solar wind at far distance. The increase of the twist together with the presence of the erosion effect suggests that the posteruption MFR may have a high-twist core enveloped by a less-twisted outer shell. These results pose a great challenge to the current understanding on the solar eruptions as well as the formation and instability of MFRs.

Molecular Diagnostics of the Internal Motions of Massive Cores

NASA Astrophysics Data System (ADS)

Pineda, Jorge; Velusamy, T.; Goldsmith, P.; Li, D.; Peng, R.; Langer, W.

2009-12-01

We present models of the internal kinematics of massive cores in the Orion molecular cloud. We use a sample of cores studied by Velusamy et al. (2008) that show red, blue, and no asymmetry in their HCO+ line profiles in equal proportion, and which therefore may represent a sample of cores in different kinematic states. We use the radiative transfer code RATRAN (Hogerheijde & van der Tak 2000) to model several transitions of HCO+ and H13CO+ as well as the dust continuum emission, of a spherical model cloud with radial density, temperature, and velocity gradients. We find that an excitation and velocity gradients are prerequisites to reproduce the observed line profiles. We use the dust continuum emission to constrain the density and temperature gradients. This allows us to narrow down the functional forms of the velocity gradient giving us the opportunity to test several theoretical predictions of velocity gradients produced by the effect of magnetic fields (e.g. Tassis et. al. 2007) and turbulence (e.g. Vasquez-Semanedi et al 2007).

First Observation of the Submillimeter Polarization Spectrum in a Translucent Molecular Cloud

NASA Astrophysics Data System (ADS)

Ashton, Peter C.; Ade, Peter A. R.; Angilè, Francesco E.; Benton, Steven J.; Devlin, Mark J.; Dober, Bradley; Fissel, Laura M.; Fukui, Yasuo; Galitzki, Nicholas; Gandilo, Natalie N.; Klein, Jeffrey; Korotkov, Andrei L.; Li, Zhi-Yun; Martin, Peter G.; Matthews, Tristan G.; Moncelsi, Lorenzo; Nakamura, Fumitaka; Netterfield, Calvin B.; Novak, Giles; Pascale, Enzo; Poidevin, Frédérick; Santos, Fabio P.; Savini, Giorgio; Scott, Douglas; Shariff, Jamil A.; Soler, Juan D.; Thomas, Nicholas E.; Tucker, Carole E.; Tucker, Gregory S.; Ward-Thompson, Derek

2018-04-01

Polarized emission from aligned dust is a crucial tool for studies of magnetism in the ISM, but a troublesome contaminant for studies of cosmic microwave background polarization. In each case, an understanding of the significance of the polarization signal requires well-calibrated physical models of dust grains. Despite decades of progress in theory and observation, polarized dust models remain largely underconstrained. During its 2012 flight, the balloon-borne telescope BLASTPol obtained simultaneous broadband polarimetric maps of a translucent molecular cloud at 250, 350, and 500 μm. Combining these data with polarimetry from the Planck 850 μm band, we have produced a submillimeter polarization spectrum, the first for a cloud of this type. We find the polarization degree to be largely constant across the four bands. This result introduces a new observable with the potential to place strong empirical constraints on ISM dust polarization models in a previously inaccessible density regime. Compared to models by Draine & Fraisse, our result disfavors two of their models for which all polarization arises due only to aligned silicate grains. By creating simple models for polarized emission in a translucent cloud, we verify that extinction within the cloud should have only a small effect on the polarization spectrum shape, compared to the diffuse ISM. Thus, we expect the measured polarization spectrum to be a valid check on diffuse ISM dust models. The general flatness of the observed polarization spectrum suggests a challenge to models where temperature and alignment degree are strongly correlated across major dust components.

THE INFLUENCE OF NONUNIFORM CLOUD COVER ON TRANSIT TRANSMISSION SPECTRA

DOE Office of Scientific and Technical Information (OSTI.GOV)

Line, Michael R.; Parmentier, Vivien, E-mail: mrline@ucsc.edu

2016-03-20

We model the impact of nonuniform cloud cover on transit transmission spectra. Patchy clouds exist in nearly every solar system atmosphere, brown dwarfs, and transiting exoplanets. Our major findings suggest that fractional cloud coverage can exactly mimic high mean molecular weight atmospheres and vice versa over certain wavelength regions, in particular, over the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) bandpass (1.1–1.7 μm). We also find that patchy cloud coverage exhibits a signature that is different from uniform global clouds. Furthermore, we explain analytically why the “patchy cloud-high mean molecular weight” degeneracy exists. We also explore the degeneracy ofmore » nonuniform cloud coverage in atmospheric retrievals on both synthetic and real planets. We find from retrievals on a synthetic solar composition hot Jupiter with patchy clouds and a cloud-free high mean molecular weight warm Neptune that both cloud-free high mean molecular weight atmospheres and partially cloudy atmospheres can explain the data equally well. Another key finding is that the HST WFC3 transit transmission spectra of two well-observed objects, the hot Jupiter HD 189733b and the warm Neptune HAT-P-11b, can be explained well by solar composition atmospheres with patchy clouds without the need to invoke high mean molecular weight or global clouds. The degeneracy between high molecular weight and solar composition partially cloudy atmospheres can be broken by observing the molecular Rayleigh scattering differences between the two. Furthermore, the signature of partially cloudy limbs also appears as a ∼100 ppm residual in the ingress and egress of the transit light curves, provided that the transit timing is known to seconds.« less

Some Peculiar Properties of Magnetic Clouds as Observed by the WIND Spacecraft

NASA Technical Reports Server (NTRS)

Berdichevsky, D.; Lepping, R. P.; Szabo, A.; Burlaga, L. F.; Thompson, B. J.; Lazarus, A. J.; Steinburg, J. T.; Mariani, F.

1999-01-01

We aimed at understanding the common characteristics of magnetic clouds, relevant to solar-interplanetary connections, but exceptional ones were noted and are stressed here through a short compendium. The study is based on analyses of 28 good or better events (Out of 33 candidates) as identified in WIND magnetic field and plasma data. These cloud intervals are provided by WIND-MFI's Website under the URL (http://lepmfi.gsfc.nasa.gov/mfi/mag_cloud_publ.html#table). The period covered is from early 1995 to November 1998. A force free, cylindrically symmetric, magnetic field model has been applied to the field data in usually 1-hour averaged form for the cloud analyses. Some of the findings are: (1) one small duration event turned out to have an approximately normal size which was due to a distant almost "skimming" passage by the spacecraft; (2) One truly small event was observed, where 10 min averages had to be used in the model fitting; it had an excellent model fit and the usual properties of a magnetic cloud, except it possessed a small axial magnetic flux; (3) One cloud ha a dual axial-field-polarity, in the sense that the "core" had one polarity and the annular region around it had an opposite polarity. This event also satisfied the model and with a ve3ry good chi-squared value. Some others show a hint of this dual polarity; (4) The temporal distribution of occurrence clouds over the 4 years show a dip in 1996; (5) About 50 % of the clouds had upstream shocks; any others had upstream pressure pulses; (6) The overall average speed (390 km/s) of the best 28 events is less than the normally quoted for the average solar wind speed (420 km/s) The average of central cloud speed to the upstream solar wind speed was not much greater than one (1.08), even though many of these clouds were drivers of interplanetary shocks. Cloud expansion is partly the reason for the existence of upstream shocks; (7) The cloud axes often (about 50 % of the time) revealed reasonable attitudes with respect to the interpreted solar source, from simple geometry, but many bore no relationship, suggesting that their observations at 1 AU were probably those of the legs of the global cloud often having near-radial axes; (8) many clouds appear to have magnetic holes at or their their boundaries.

Molecular clouds without detectable CO

NASA Technical Reports Server (NTRS)

Blitz, Leo; Bazell, David; Desert, F. Xavier

1990-01-01

The clouds identified by Desert, Bazell, and Boulanger (DBB clouds) in their search for high-latitude molecular clouds were observed in the CO (J = 1-0) line, but only 13 percent of the sample was detected. The remaining 87 percent are diffuse molecular clouds with CO abundances of about 10 to the -6th, a typical value for diffuse clouds. This hypothesis is shown to be consistent with Copernicus data. The DBB clouds are shown to ben an essentially complete catalog of diffuse molecular clouds in the solar vicinity. The total molecular surface density in the vicinity of the sun is then only about 20 percent greater than the 1.3 solar masses/sq pc determined by Dame et al. (1987). Analysis of the CO detections indicates that there is a sharp threshold in extinction of 0.25 mag before CO is detectable and is derived from the IRAS I(100) micron threshold of 4 MJy/sr. This threshold is presumably where the CO abundance exhibits a sharp increase

Molecular clouds without detectable CO

DOE Office of Scientific and Technical Information (OSTI.GOV)

Blitz, L.; Bazell, D.; Desert, F.X.

1990-03-01

The clouds identified by Desert, Bazell, and Boulanger (DBB clouds) in their search for high-latitude molecular clouds were observed in the CO (J = 1-0) line, but only 13 percent of the sample was detected. The remaining 87 percent are diffuse molecular clouds with CO abundances of about 10 to the -6th, a typical value for diffuse clouds. This hypothesis is shown to be consistent with Copernicus data. The DBB clouds are shown to be an essentially complete catalog of diffuse molecular clouds in the solar vicinity. The total molecular surface density in the vicinity of the sun is thenmore » only about 20 percent greater than the 1.3 solar masses/sq pc determined by Dame et al. (1987). Analysis of the CO detections indicates that there is a sharp threshold in extinction of 0.25 mag before CO is detectable and is derived from the IRAS I(100) micron threshold of 4 MJy/sr. This threshold is presumably where the CO abundance exhibits a sharp increase 18 refs.« less

Time Evolution of the Giant Molecular Cloud Mass Functions across Galactic Disks

NASA Astrophysics Data System (ADS)

Kobayashi, Masato I. N.; Inutsuka, Shu-Ichiro; Kobayashi, Hiroshi; Hasegawa, Kenji

2017-01-01

We formulate and conduct the time-integration of time evolution equation for the giant molecular cloud mass function (GMCMF) including the cloud-cloud collision (CCC) effect. Our results show that the CCC effect is only limited in the massive-end of the GMCMF and indicate that future high resolution and sensitivity radio observations may constrain giant molecular cloud (GMC) timescales by observing the GMCMF slope in the lower mass regime.

The Galactic Distribution of OB Associations in Molecular Clouds

NASA Astrophysics Data System (ADS)

Williams, Jonathan P.; McKee, Christopher F.

1997-02-01

Molecular clouds account for half of the mass of the interstellar medium interior to the solar circle and for all current star formation. Using cloud catalogs of two CO surveys of the first quadrant, we have fitted the mass distribution of molecular clouds to a truncated power law in a similar manner as the luminosity function of OB associations in the companion paper to this work. After extrapolating from the first quadrant to the entire inner Galaxy, we find that the mass of cataloged clouds amounts to only 40% of current estimates of the total Galactic molecular mass. Following Solomon & Rivolo, we have assumed that the remaining molecular gas is in cold clouds, and we normalize the distribution accordingly. The predicted total number of clouds is then shown to be consistent with that observed in the solar neighborhood where cloud catalogs should be more complete. Within the solar circle, the cumulative form of the distribution is \\Nscrc(>M)=105[(Mu/M)0.6-1], where \\Nscrc is the number of clouds, and Mu = 6 × 106 M⊙ is the upper mass limit. The large number of clouds near the upper cutoff to the distribution indicates an underlying physical limit to cloud formation or destruction processes. The slope of the distribution corresponds to d\\Nscrc/dM~M-1.6, implying that although numerically most clouds are of low mass, most of the molecular gas is contained within the most massive clouds. The distribution of cloud masses is then compared to the Galactic distribution of OB association luminosities to obtain statistical estimates of the number of massive stars expected in any given cloud. The likelihood of massive star formation in a cloud is determined, and it is found that the median cloud mass that contains at least one O star is ~105 M⊙. The average star formation efficiency over the lifetime of an association is about 5% but varies by more than 2 orders of magnitude from cloud to cloud and is predicted to increase with cloud mass. O stars photoevaporate their surrounding molecular gas, and even with low rates of formation, they are the principal agents of cloud destruction. Using an improved estimate of the timescale for photoevaporation and our statistics on the expected numbers of stars per cloud, we find that 106 M⊙ giant molecular clouds (GMCs) are expected to survive for about 3 × 107 yr. Smaller clouds are disrupted, rather than photoionized, by photoevaporation. The porosity of H II regions in large GMCs is shown to be of order unity, which is consistent with self-regulation of massive star formation in GMCs. On average, 10% of the mass of a GMC is converted to stars by the time it is destroyed by photoevaporation.

Evidence in Magnetic Clouds for Systematic Open Flux Transport on the Sun

NASA Technical Reports Server (NTRS)

Crooker, N. U.; Kahler, S. W.; Gosling, J. T.; Lepping, R. P.

2008-01-01

Most magnetic clouds encountered by spacecraft at 1 AU display a mix of unidirectional suprathermal electrons signaling open field lines and counterstreaming electrons signaling loops connected to the Sun at both ends. Assuming the open fields were originally loops that underwent interchange reconnection with open fields at the Sun, we determine the sense of connectedness of the open fields found in 72 of 97 magnetic clouds identified by the Wind spacecraft in order to obtain information on the location and sense of the reconnection and resulting flux transport at the Sun. The true polarity of the open fields in each magnetic cloud was determined from the direction of the suprathermal electron flow relative to the magnetic field direction. Results indicate that the polarity of all open fields within a given magnetic cloud is the same 89% of the time, implying that interchange reconnection at the Sun most often occurs in only one leg of a flux rope loop, thus transporting open flux in a single direction, from a coronal hole near that leg to the foot point of the opposite leg. This pattern is consistent with the view that interchange reconnection in coronal mass ejections systematically transports an amount of open flux sufficient to reverse the polarity of the heliospheric field through the course of the solar cycle. Using the same electron data, we also find that the fields encountered in magnetic clouds are only a third as likely to be locally inverted as not. While one might expect inversions to be equally as common as not in flux rope coils, consideration of the geometry of spacecraft trajectories relative to the modeled magnetic cloud axes leads us to conclude that the result is reasonable.

Radio Observations of Sgr B2

NASA Astrophysics Data System (ADS)

Mao, Xin-Jie; Su, Jiang-Tao

2001-10-01

The 13CO (J=1-0) map of the molecular cloud Sgr B2 reveals that the mass center of the molecular cloud nucleus does not coincide with that of compact HII regions which are likely to be the outcome of a shock on the cloud. We find evidence of cloud contraction probably resulting from cloud-cloud collision at subsonic speed.

The collapse of a molecular cloud core to stellar densities using radiation non-ideal magnetohydrodynamics

NASA Astrophysics Data System (ADS)

Wurster, James; Bate, Matthew R.; Price, Daniel J.

2018-04-01

We present results from radiation non-ideal magnetohydrodynamics (MHD) calculations that follow the collapse of rotating, magnetized, molecular cloud cores to stellar densities. These are the first such calculations to include all three non-ideal effects: ambipolar diffusion, Ohmic resistivity, and the Hall effect. We employ an ionization model in which cosmic ray ionization dominates at low temperatures and thermal ionization takes over at high temperatures. We explore the effects of varying the cosmic ray ionization rate from ζcr = 10-10 to 10-16 s-1. Models with ionization rates ≳10-12 s-1 produce results that are indistinguishable from ideal MHD. Decreasing the cosmic ray ionization rate extends the lifetime of the first hydrostatic core up to a factor of 2, but the lifetimes are still substantially shorter than those obtained without magnetic fields. Outflows from the first hydrostatic core phase are launched in all models, but the outflows become broader and slower as the ionization rate is reduced. The outflow morphology following stellar core formation is complex and strongly dependent on the cosmic ray ionization rate. Calculations with high ionization rates quickly produce a fast (≈14 km s-1) bipolar outflow that is distinct from the first core outflow, but with the lowest ionization rate, a slower (≈3-4 km s-1) conical outflow develops gradually and seamlessly merges into the first core outflow.

THE DISTRIBUTION OF MASS SURFACE DENSITIES IN A HIGH-MASS PROTOCLUSTER

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lim, Wanggi; Tan, Jonathan C.; Ma, Bo

2016-09-20

We study the probability distribution function (PDF) of mass surface densities, Σ, of infrared dark cloud (IRDC) G028.37+00.07 and its surrounding giant molecular cloud. This PDF constrains the physical processes, such as turbulence, magnetic fields, and self-gravity, that are expected to be controlling cloud structure and star formation activity. The chosen IRDC is of particular interest since it has almost 100,000 solar masses within a radius of 8 pc, making it one of the most massive, dense molecular structures known and is thus a potential site for the formation of a “super star cluster.” We study Σ in two ways.more » First, we use a combination of NIR and MIR extinction maps that are able to probe the bulk of the cloud structure up to Σ ∼ 1 g cm{sup −2}( A {sub V}≃ 200 mag). Second, we study the FIR and submillimeter dust continuum emission from the cloud utilizing Herschel -PACS and SPIRE images and paying careful attention to the effects of foreground and background contamination. We find that the PDFs from both methods, applied over a ∼20′(30 pc)-sized region that contains ≃1.5 × 10{sup 5} M {sub ⊙} and enclosing a minimum closed contour with Σ ≃ 0.013 g cm{sup −2} ( A {sub V} ≃ 3 mag), shows a lognormal shape with the peak measured at Σ ≃ 0.021 g cm{sup −2} ( A {sub V}≃ 4.7 mag). There is tentative evidence for the presence of a high-Σ power-law tail that contains from ∼3% to 8% of the mass of the cloud material. We discuss the implications of these results for the physical processes occurring in this cloud.« less

Studying the Relationship between High-Latitude Geomagnetic Activity and Parameters of Interplanetary Magnetic Clouds with the Use of Artificial Neural Networks

NASA Astrophysics Data System (ADS)

Barkhatov, N. A.; Revunov, S. E.; Vorobjev, V. G.; Yagodkina, O. I.

2018-03-01

The cause-and-effect relations of the dynamics of high-latitude geomagnetic activity (in terms of the AL index) and the type of the magnetic cloud of the solar wind are studied with the use of artificial neural networks. A recurrent neural network model has been created based on the search for the optimal physically coupled input and output parameters characterizing the action of a plasma flux belonging to a certain magnetic cloud type on the magnetosphere. It has been shown that, with IMF components as input parameters of neural networks with allowance for a 90-min prehistory, it is possible to retrieve the AL sequence with an accuracy to 80%. The successful retrieval of the AL dynamics by the used data indicates the presence of a close nonlinear connection of the AL index with cloud parameters. The created neural network models can be applied with high efficiency to retrieve the AL index, both in periods of isolated magnetospheric substorms and in periods of the interaction between the Earth's magnetosphere and magnetic clouds of different types. The developed model of AL index retrieval can be used to detect magnetic clouds.

The deformation of flux tubes in the solar wind with applications to the structure of magnetic clouds and CMEs

NASA Technical Reports Server (NTRS)

Cargill, Peter J.; Chen, James; Spicer, D. S.; Zalesak, S. T.

1994-01-01

Two dimensional magnetohydrodynamic simulations of the distortion of a magnetic flux tube, accelerated through ambient solar wind plasma, are presented. Vortices form on the trailing edge of the flux tube, and couple strongly to its interior. If the flux tube azimuthal field is weak, it deforms into an elongated banana-like shape after a few Alfven transit times. A significant azimuthal field component inhibits this distortion. In the case of magnetic clouds in the solar wind, it is suggested that the shape observed at 1 AU was determined by distortion of the cloud in the inner heliosphere. Distortion of the cloud beyond 1 AU takes many days. It is estimated that effective drag coefficients slightly greater than unity are appropriate for modeling flux tube propagation. Synthetic magnetic field profiles as would be seen by a spacecraft traversing the cloud are presented.

Radio Emission from a Young Supernova Remnant Interacting with an Interstellar Cloud: Magnetohydrodynamic Simulation with Relativistic Electrons

NASA Astrophysics Data System (ADS)

Jun, Byung-Il; Jones, T. W.

1999-02-01

We present two-dimensional MHD simulations of the evolution of a young Type Ia supernova remnant (SNR) during its interaction with an interstellar cloud of comparable size at impact. We include for the first time in such simulations explicit relativistic electron transport. This was done using a simplified treatment of the diffusion-advection equation, thus allowing us to model injection and acceleration of cosmic-ray electrons at shocks and their subsequent transport. From this information we also model radio synchrotron emission, including spectral information. The simulations were carried out in spherical coordinates with azimuthal symmetry and compare three different situations, each incorporating an initially uniform interstellar magnetic field oriented in the polar direction on the grid. In particular, we modeled the SNR-cloud interactions for a spherical cloud on the polar axis, a toroidal cloud whose axis is aligned with the polar axis, and, for comparison, a uniform medium with no cloud. We find that the evolution of the overrun cloud qualitatively resembles that seen in simulations of simpler but analogous situations: that is, the cloud is crushed and begins to be disrupted by Rayleigh-Taylor and Kelvin-Helmholtz instabilities. However, we demonstrate here that, in addition, the internal structure of the SNR is severely distorted as such clouds are engulfed. This has important dynamical and observational implications. The principal new conclusions we draw from these experiments are the following. (1) Independent of the cloud interaction, the SNR reverse shock can be an efficient site for particle acceleration in a young SNR. (2) The internal flows of the SNR become highly turbulent once it encounters a large cloud. (3) An initially uniform magnetic field is preferentially amplified along the magnetic equator of the SNR, primarily because of biased amplification in that region by Rayleigh-Taylor instabilities. A similar bias produces much greater enhancement to the magnetic energy in the SNR during an encounter with a cloud when the interstellar magnetic field is partially transverse to the expansion of the SNR. The enhanced magnetic fields have a significant radial component, independent of the field orientation external to the SNR. This leads to a strong equatorial bias in synchrotron brightness that could easily mask any enhancements to electron-acceleration efficiency near the magnetic equator of the SNR. Thus, to establish the latter effect, it will be essential to establish that the magnetic field in the brightest regions are actually tangential to the blast wave. (4) The filamentary radio structures correlate well with ``turbulence-enhanced'' magnetic structures, while the diffuse radio emission more closely follows the gas-density distribution within the SNR. (5) At these early times, the synchrotron spectral index due to electrons accelerated at the primary shocks should be close to 0.5 unless those shocks are modified by cosmic-ray proton pressures. While that result is predictable, we find that this simple result can be significantly complicated in practice by SNR interactions with clouds. Those events can produce regions with significantly steeper spectra. Especially if there are multiple cloud encounters, this interaction can lead to nonuniform spatial spectral distributions or, through turbulent mixing, produce a spectrum that is difficult to relate to the actual strength of the blast wave. (6) Interaction with the cloud enhances the nonthermal electron population in the SNR in our simulations because of additional electron injection taking place in the shocks associated with the cloud. Together with point 3, this means that SNR-cloud encounters can significantly increase the radio emission from the SNR.

Formation history of open clusters constrained by detailed asteroseismology of red giant stars observed by Kepler

NASA Astrophysics Data System (ADS)

Corsaro, Enrico; Lee, Yueh-Ning; García, Rafael A.; Hennebelle, Patrick; Mathur, Savita; Beck, Paul G.; Mathis, Stephane; Stello, Dennis; Bouvier, Jérôme

2017-10-01

Stars originate by the gravitational collapse of a turbulent molecular cloud of a diffuse medium, and are often observed to form clusters. Stellar clusters therefore play an important role in our understanding of star formation and of the dynamical processes at play. However, investigating the cluster formation is diffcult because the density of the molecular cloud undergoes a change of many orders of magnitude. Hierarchical-step approaches to decompose the problem into different stages are therefore required, as well as reliable assumptions on the initial conditions in the clouds. We report for the first time the use of the full potential of NASA Kepler asteroseismic observations coupled with 3D numerical simulations, to put strong constraints on the early formation stages of open clusters. Thanks to a Bayesian peak bagging analysis of about 50 red giant members of NGC 6791 and NGC 6819, the two most populated open clusters observed in the nominal Kepler mission, we derive a complete set of detailed oscillation mode properties for each star, with thousands of oscillation modes characterized. We therefore show how these asteroseismic properties lead us to a discovery about the rotation history of stellar clusters. Finally, our observational findings will be compared with hydrodynamical simulations for stellar cluster formation to constrain the physical processes of turbulence, rotation, and magnetic fields that are in action during the collapse of the progenitor cloud into a proto-cluster.

Molecular Gas toward the Gemini OB1 Molecular Cloud Complex. II. CO Outflow Candidates with Possible WISE Associations

NASA Astrophysics Data System (ADS)

Li, Yingjie; Li, Fa-Cheng; Xu, Ye; Wang, Chen; Du, Xin-Yu; Yang, Wenjin; Yang, Ji

2018-03-01

We present a large-scale survey of CO outflows in the Gem OB1 molecular cloud complex and its surroundings, using the Purple Mountain Observatory Delingha 13.7 m telescope. A total of 198 outflow candidates were identified over a large area (∼58.5 square degrees), of which 193 are newly detected. Approximately 68% (134/198) are associated with the Gem OB1 molecular cloud complex, including clouds GGMC 1, GGMC 2, BFS 52, GGMC 3, and GGMC 4. Other regions studied are: the Local arm (Local Lynds, West Front), Swallow, Horn, and Remote cloud. Outflow candidates in GGMC 1, BFS 52, and Swallow are mainly located at ring-like or filamentary structures. To avoid excessive uncertainty in distant regions (≳3.8 kpc), we only estimated the physical parameters for clouds in the Gem OB1 molecular cloud complex and in the Local arm. In those clouds, the total kinetic energy and the energy injection rate of the identified outflow candidates are ≲1% and ≲3% of the turbulent energy and the turbulent dissipation rate of each cloud, indicating that the identified outflow candidates cannot provide enough energy to balance turbulence of their host cloud at the scale of the entire cloud (several to dozens of parsecs). The gravitational binding energy of each cloud is ≳135 times the total kinetic energy of the identified outflow candidates within the corresponding cloud, indicating that the identified outflow candidates cannot cause major disruptions to the integrity of their host cloud at the scale of the entire cloud.

X-ray and IR Surveys of the Orion Molecular Clouds and the Cepheus OB3b Cluster

NASA Astrophysics Data System (ADS)

Megeath, S. Thomas; Wolk, Scott J.; Pillitteri, Ignazio; Allen, Tom

2014-08-01

X-ray and IR surveys of molecular clouds between 400 and 700 pc provide complementary means to map the spatial distribution of young low mass stars associated with the clouds. We overview an XMM survey of the Orion Molecular Clouds, at a distance of 400 pc. By using the fraction of X-ray sources with disks as a proxy for age, this survey has revealed three older clusters rich in diskless X-ray sources. Two are smaller clusters found at the northern and southern edges of the Orion A molecular cloud. The third cluster surrounds the O-star Iota Ori (the point of Orion's sword) and is in the foreground to the Orion molecular cloud. In addition, we present a Chandra and Spitzer survey of the Cep OB3b cluster at 700 pc. These data show a spatially variable disk fraction indicative of age variations within the cluster. We discuss the implication of these results for understanding the spread of ages in young clusters and the star formation histories of molecular clouds.

Biogenic, urban, and wildfire influences on the molecular composition of dissolved organic compounds in cloud water

NASA Astrophysics Data System (ADS)

Cook, Ryan D.; Lin, Ying-Hsuan; Peng, Zhuoyu; Boone, Eric; Chu, Rosalie K.; Dukett, James E.; Gunsch, Matthew J.; Zhang, Wuliang; Tolic, Nikola; Laskin, Alexander; Pratt, Kerri A.

2017-12-01

Organic aerosol formation and transformation occurs within aqueous aerosol and cloud droplets, yet little is known about the composition of high molecular weight organic compounds in cloud water. Cloud water samples collected at Whiteface Mountain, New York, during August-September 2014 were analyzed by ultra-high-resolution mass spectrometry to investigate the molecular composition of dissolved organic carbon, with a focus on sulfur- and nitrogen-containing compounds. Organic molecular composition was evaluated in the context of cloud water inorganic ion concentrations, pH, and total organic carbon concentrations to gain insights into the sources and aqueous-phase processes of the observed high molecular weight organic compounds. Cloud water acidity was positively correlated with the average oxygen : carbon ratio of the organic constituents, suggesting the possibility for aqueous acid-catalyzed (prior to cloud droplet activation or during/after cloud droplet evaporation) and/or radical (within cloud droplets) oxidation processes. Many tracer compounds recently identified in laboratory studies of bulk aqueous-phase reactions were identified in the cloud water. Organosulfate compounds, with both biogenic and anthropogenic volatile organic compound precursors, were detected for cloud water samples influenced by air masses that had traveled over forested and populated areas. Oxidation products of long-chain (C10-12) alkane precursors were detected during urban influence. Influence of Canadian wildfires resulted in increased numbers of identified sulfur-containing compounds and oligomeric species, including those formed through aqueous-phase reactions involving methylglyoxal. Light-absorbing aqueous-phase products of syringol and guaiacol oxidation were observed in the wildfire-influenced samples, and dinitroaromatic compounds were observed in all cloud water samples (wildfire, biogenic, and urban-influenced). Overall, the cloud water molecular composition depended on air mass source influence and reflected aqueous-phase reactions involving biogenic, urban, and biomass burning precursors.

The Bastille Day Magnetic Clouds and Upstream Shocks: Near Earth Interplanetary Observations

NASA Technical Reports Server (NTRS)

Lepping, R. P.; Berdichevsky, D. B.; Burlaga, L. F.; Lazarus, A. J.; Kasper, J.; Desch, M. D.; Wu, C.-C.; Reames, D. V.; Singer, H. J.; Singer, H. J.;

Excitation and Disruption of a Giant Molecular Cloud by the Sepurnova Remnant 3C 391

NASA Technical Reports Server (NTRS)

Reach, W. T.; Rho, J.

1998-01-01

The ambient molecular gas at the distance of the remnant comprises a giant molecular cloud whose edge is closely parallel to a ridge of bright non-thermal radio continuum, which evidently delineates the blast-wave into the cloud.

Evolution of magnetic fields in collapsing star-forming clouds under different environments

NASA Astrophysics Data System (ADS)

Higuchi, Koki; Machida, Masahiro N.; Susa, Hajime

2018-04-01

In nearby star-forming clouds, amplification and dissipation of the magnetic field are known to play crucial roles in the star-formation process. The star-forming environment varies from place to place and era to era in galaxies. In this study, amplification and dissipation of magnetic fields in star-forming clouds are investigated under different environments using magnetohydrodynamics (MHD) simulations. We consider various star-forming environments in combination with the metallicity and the ionization strength, and prepare prestellar clouds having two different mass-to-flux ratios. We calculate the cloud collapse until protostar formation using ideal and non-ideal (inclusion and exclusion of ohmic dissipation and ambipolar diffusion) MHD calculations to investigate the evolution of the magnetic field. We perform 288 runs in total and show the diversity of the density range within which the magnetic field effectively dissipates, depending on the environment. In addition, the dominant dissipation process (Ohmic dissipation or ambipolar diffusion) is shown to strongly depend on the star-forming environment. Especially, for the primordial case, magnetic field rarely dissipates without ionization source, while it efficiently dissipates when very weak ionization sources exist in the surrounding environment. The results of this study help to clarify star formation in various environments.

Influence of Heliospheric Current Sheet presence on geomagnetic storm originated by Magnetic Clouds.

NASA Astrophysics Data System (ADS)

Hidalgo, M. A.; Blanco, J. J.

2006-12-01

It is well known the importance of Magnetic Cloud (MC) on the Magnetosphere and its influence as cause of strong geomagnetic activity, especially fast magnetic cloud. Sometimes magnetic cloud travels in solar wind close to the Heliospheric Current Sheet (HCS). We wonder if the HCS presence plays some role on the geomagnetic storm development. In this work we will try to respond to this question comparing the effect on the Magnetosphere of MC+HCS and MC without HCS, detected by WIND instruments. This work has been supported by the Spanish Comisión Internacional de Ciencia y Tecnología (CICYT), grant ESP2005-07290-C02-01 and ESP2006-08459 and Madrid Autonomous Community / University of Alcala grant CAM-UAH 2005/007.

Physical conditions in molecular clouds

NASA Technical Reports Server (NTRS)

Evans, Neal J., II

1989-01-01

Recent developments have complicated the picture of the physical conditions in molecular clouds. The discoveries of widespread emission from high-J lines of CD and 12-micron IRAS emission have revealed the presence of considerably hotter gas and dust near the surfaces of molecular clouds. These components can complicate interpretation of the bulk of the cloud gas. Commonly assumed relations between column density or mean density and cloud size are called into question by conflicting results and by consideration of selection effects. Analysis of density and density structure through molecular excitation has shown that very high densities exist in star formation regions, but unresolved structure and possible chemical effects complicate the interpretation. High resolution far-IR and submillimeter observations offer a complementary approach and are beginning to test theoretical predictions of density gradients in clouds.

RE-EXAMINING LARSON'S SCALING RELATIONSHIPS IN GALACTIC MOLECULAR CLOUDS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Heyer, Mark; Krawczyk, Coleman; Duval, Julia

The properties of Galactic molecular clouds tabulated by Solomon et al. (SRBY) are re-examined using the Boston University-FCRAO Galactic Ring Survey of {sup 13}CO J = 1-0 emission. These new data provide a lower opacity tracer of molecular clouds and improved angular and spectral resolution compared with previous surveys of molecular line emission along the Galactic Plane. We calculate giant molecular cloud (GMC) masses within the SRBY cloud boundaries assuming local thermodynamic equilibrium (LTE) conditions throughout the cloud and a constant H{sub 2} to {sup 13}CO abundance, while accounting for the variation of the {sup 12}C/{sup 13}C with galactocentric radius.more » The LTE-derived masses are typically five times smaller than the SRBY virial masses. The corresponding median mass surface density of molecular hydrogen for this sample is 42 M{sub sun} pc{sup -2}, which is significantly lower than the value derived by SRBY (median 206 M{sub sun} pc{sup -2}) that has been widely adopted by most models of cloud evolution and star formation. This discrepancy arises from both the extrapolation by SRBY of velocity dispersion, size, and CO luminosity to the 1 K antenna temperature isophote that likely overestimates the GMC masses and our assumption of constant {sup 13}CO abundance over the projected area of each cloud. Owing to the uncertainty of molecular abundances in the envelopes of clouds, the mass surface density of GMCs could be larger than the values derived from our {sup 13}CO measurements. From velocity dispersions derived from the {sup 13}CO data, we find that the coefficient of the cloud structure functions, v{sup 0} = {sigma}{sub v}/R {sup 1/2}, is not constant, as required to satisfy Larson's scaling relationships, but rather systematically varies with the surface density of the cloud as {approx}{sigma}{sup 0.5} as expected for clouds in self-gravitational equilibrium.« less

Star formation in evolving molecular clouds

NASA Astrophysics Data System (ADS)

Völschow, M.; Banerjee, R.; Körtgen, B.

2017-09-01

Molecular clouds are the principle stellar nurseries of our universe; they thus remain a focus of both observational and theoretical studies. From observations, some of the key properties of molecular clouds are well known but many questions regarding their evolution and star formation activity remain open. While numerical simulations feature a large number and complexity of involved physical processes, this plethora of effects may hide the fundamentals that determine the evolution of molecular clouds and enable the formation of stars. Purely analytical models, on the other hand, tend to suffer from rough approximations or a lack of completeness, limiting their predictive power. In this paper, we present a model that incorporates central concepts of astrophysics as well as reliable results from recent simulations of molecular clouds and their evolutionary paths. Based on that, we construct a self-consistent semi-analytical framework that describes the formation, evolution, and star formation activity of molecular clouds, including a number of feedback effects to account for the complex processes inside those objects. The final equation system is solved numerically but at much lower computational expense than, for example, hydrodynamical descriptions of comparable systems. The model presented in this paper agrees well with a broad range of observational results, showing that molecular cloud evolution can be understood as an interplay between accretion, global collapse, star formation, and stellar feedback.

The Embedded Ring-like Feature and Star Formation Activities in G35.673-00.847

NASA Astrophysics Data System (ADS)

Dewangan, L. K.; Devaraj, R.; Ojha, D. K.

2018-02-01

We present a multiwavelength study to probe the star formation (SF) process in the molecular cloud linked with the G35.673-00.847 site (hereafter MCG35.6), which is traced in a velocity range of 53–62 km s‑1. Multiwavelength images reveal a semi-ring-like feature (associated with ionized gas emission) and an embedded face-on ring-like feature (without the NVSS 1.4 GHz radio emission, where 1σ ∼ 0.45 mJy beam‑1) in MCG35.6. The semi-ring-like feature is originated by the ionizing feedback from a star with spectral type B0.5V–B0V. The central region of the ring-like feature does not contain detectable ionized gas emission, indicating that the ring-like feature is unlikely to be produced by the ionizing feedback from a massive star. Several embedded Herschel clumps and young stellar objects (YSOs) are identified in MCG35.6, tracing the ongoing SF activities within the cloud. The polarization information from the Planck and GPIPS data trace the plane-of-sky magnetic field, which is oriented parallel to the major axis of the ring-like feature. At least five clumps (having M clump ∼ 740–1420 M ⊙) seem to be distributed in an almost regularly spaced manner along the ring-like feature and contain noticeable YSOs. Based on the analysis of the polarization and molecular line data, three subregions containing the clumps are found to be magnetically supercritical in the ring-like feature. Altogether, the existence of the ring-like feature and the SF activities on its edges can be explained by the magnetic field mediated process as simulated by Li & Nakamura.

A Herschel [C ii] Galactic plane survey. II. CO-dark H2 in clouds

NASA Astrophysics Data System (ADS)

Langer, W. D.; Velusamy, T.; Pineda, J. L.; Willacy, K.; Goldsmith, P. F.

2014-01-01

Context. H i and CO large scale surveys of the Milky Way trace the diffuse atomic clouds and the dense shielded regions of molecular hydrogen clouds, respectively. However, until recently, we have not had spectrally resolved C+ surveys in sufficient lines of sight to characterize the ionized and photon dominated components of the interstellar medium, in particular, the H2 gas without CO, referred to as CO-dark H2, in a large sample of interstellar clouds. Aims: We use a sparse Galactic plane survey of the 1.9 THz (158 μm) [C ii] spectral line from the Herschel open time key programme, Galactic Observations of Terahertz C+ (GOT C+), to characterize the H2 gas without CO in a statistically significant sample of interstellar clouds. Methods: We identify individual clouds in the inner Galaxy by fitting the [C ii] and CO isotopologue spectra along each line of sight. We then combine these spectra with those of H i and use them along with excitation models and cloud models of C+ to determine the column densities and fractional mass of CO-dark H2 clouds. Results: We identify1804 narrow velocity [C ii] components corresponding to interstellar clouds in different categories and evolutionary states. About 840 are diffuse molecular clouds with no CO, ~510 are transition clouds containing [C ii] and 12CO, but no 13CO, and the remainder are dense molecular clouds containing 13CO emission. The CO-dark H2 clouds are concentrated between Galactic radii of ~3.5 to 7.5 kpc and the column density of the CO-dark H2 layer varies significantly from cloud to cloud with a global average of 9 × 1020 cm-2. These clouds contain a significant fraction by mass of CO-dark H2, that varies from ~75% for diffuse molecular clouds to ~20% for dense molecular clouds. Conclusions: We find a significant fraction of the warm molecular ISM gas is invisible in H i and CO, but is detected in [C ii]. The fraction of CO-dark H2 is greatest in the diffuse clouds and decreases with increasing total column density, and is lowest in the massive clouds. The column densities and mass fraction of CO-dark H2 are less than predicted by models of diffuse molecular clouds using solar metallicity, which is not surprising as most of our detections are in Galactic regions where the metallicity is larger and shielding more effective. There is an overall trend towards a higher fraction of CO-dark H2 in clouds with increasing Galactic radius, consistent with lower metallicity there. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

Featured Image: A Molecular Cloud Outside Our Galaxy

NASA Astrophysics Data System (ADS)

Kohler, Susanna

2018-06-01

What do molecular clouds look like outside of our own galaxy? See for yourself in the images above and below of N55, a molecular cloud located in the Large Magellanic Cloud (LMC). In a recent study led by Naslim Neelamkodan (Academia Sinica Institute of Astronomy and Astrophysics, Taiwan), a team of scientists explore N55 to determine how its cloud properties differ from clouds within the Milky Way. The image above reveals the distribution of infrared-emitting gas and dust observed in three bands by the Spitzer Space Telescope. Overplotted in cyan are observations from the Atacama Submillimeter Telescope Experiment tracing the clumpy, warm molecular gas. Below, new observations from the Atacama Large Millimeter/submillimeter Array (ALMA) reveal the sub-parsec-scale molecular clumps in greater detail, showing the correlation of massive clumps with Spitzer-identified young stellar objects (crosses). The study presented here indicates that this cloud in the LMC is the site of massive star formation, with properties similar to equivalent clouds in the Milky Way. To learn more about the authors findings, check out the article linked below.CitationNaslim N. et al 2018 ApJ 853 175. doi:10.3847/1538-4357/aaa5b0

A search for pre-main sequence stars in the high-latitude molecular clouds. II - A survey of the Einstein database

NASA Technical Reports Server (NTRS)

Caillault, Jean-Pierre; Magnani, Loris

1990-01-01

The preliminary results are reported of a survey of every EINSTEIN image which overlaps any high-latitude molecular cloud in a search for X-ray emitting pre-main sequence stars. This survey, together with complementary KPNO and IRAS data, will allow the determination of how prevalent low mass star formation is in these clouds in general and, particularly, in the translucent molecular clouds.

The Destructive Birth of Massive Stars and Massive Star Clusters

NASA Astrophysics Data System (ADS)

Rosen, Anna; Krumholz, Mark; McKee, Christopher F.; Klein, Richard I.; Ramirez-Ruiz, Enrico

2017-01-01

Massive stars play an essential role in the Universe. They are rare, yet the energy and momentum they inject into the interstellar medium with their intense radiation fields dwarfs the contribution by their vastly more numerous low-mass cousins. Previous theoretical and observational studies have concluded that the feedback associated with massive stars' radiation fields is the dominant mechanism regulating massive star and massive star cluster (MSC) formation. Therefore detailed simulation of the formation of massive stars and MSCs, which host hundreds to thousands of massive stars, requires an accurate treatment of radiation. For this purpose, we have developed a new, highly accurate hybrid radiation algorithm that properly treats the absorption of the direct radiation field from stars and the re-emission and processing by interstellar dust. We use our new tool to perform a suite of three-dimensional radiation-hydrodynamic simulations of the formation of massive stars and MSCs. For individual massive stellar systems, we simulate the collapse of massive pre-stellar cores with laminar and turbulent initial conditions and properly resolve regions where we expect instabilities to grow. We find that mass is channeled to the massive stellar system via gravitational and Rayleigh-Taylor (RT) instabilities. For laminar initial conditions, proper treatment of the direct radiation field produces later onset of RT instability, but does not suppress it entirely provided the edges of the radiation-dominated bubbles are adequately resolved. RT instabilities arise immediately for turbulent pre-stellar cores because the initial turbulence seeds the instabilities. To model MSC formation, we simulate the collapse of a dense, turbulent, magnetized Mcl = 106 M⊙ molecular cloud. We find that the influence of the magnetic pressure and radiative feedback slows down star formation. Furthermore, we find that star formation is suppressed along dense filaments where the magnetic field is amplified. Our results suggest that the combined effect of turbulence, magnetic pressure, and radiative feedback from massive stars is responsible for the low star formation efficiencies observed in molecular clouds.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Asahina, Yuta; Kawashima, Tomohisa; Furukawa, Naoko

The formation mechanism of CO clouds observed with the NANTEN2 and Mopra telescopes toward the stellar cluster Westerlund 2 is studied by 3D magnetohydrodynamic simulations, taking into account the interstellar cooling. These molecular clouds show a peculiar shape composed of an arc-shaped cloud on one side of the TeV γ -ray source HESS J1023-575 and a linear distribution of clouds (jet clouds) on the other side. We propose that these clouds are formed by the interaction of a jet with clumps of interstellar neutral hydrogen (H i). By studying the dependence of the shape of dense cold clouds formed bymore » shock compression and cooling on the filling factor of H i clumps, we found that the density distribution of H i clumps determines the shape of molecular clouds formed by the jet–cloud interaction: arc clouds are formed when the filling factor is large. On the other hand, when the filling factor is small, molecular clouds align with the jet. The jet propagates faster in models with small filling factors.« less

Cool Star Beginnings: YSOs in the Perseus Molecular Cloud

NASA Astrophysics Data System (ADS)

Young, Kaisa E.; Young, Chadwick H.

2015-01-01

Nearby molecular clouds, where there is considerable evidence of ongoing star formation, provide the best opportunity to observe stars in the earliest stages of their formation. The Perseus molecular cloud contains two young clusters, IC 348 and NGC 1333 and several small dense cores of the type that produce only a few stars. Perseus is often cited as an intermediate case between quiescent low-mass and turbulent high-mass clouds, making it perhaps an ideal environment for studying ``typical low-mass star formation. We present an infrared study of the Perseus molecular cloud with data from the Spitzer Space Telescope as part of the ``From Molecular Cores to Planet Forming Disks (c2d) Legacy project tep{eva03}. By comparing Spitzer's near- and mid-infrared maps, we identify and classify the young stellar objects (YSOs) in the cloud using updated extinction corrected photometry. Virtually all of the YSOs in Perseus are forming in the clusters and other smaller associations at the east and west ends of the cloud with very little evidence of star formation in the midsection even in areas of high extinction.

The Physics of Molecular Shocks in Star-Forming Regions

NASA Technical Reports Server (NTRS)

Hollenbach, David; Cuzzi, Jeffrey (Technical Monitor)

1996-01-01

Molecular shocks are produced by the impact of the supersonic infall of gas and dust onto protostars and by the interaction of the supersonic outflow from the protostar with the circumstellar material. Infalling gas creates an accretion shock around the circumstellar disk which emits a unique infrared spectrum and which processes the interstellar dust as it enters the disk. The winds and jets from protostars also impact the disk, the infalling material, and the ambient molecular cloud core creating shocks whose spectrum and morphology diagnose the mass loss processes of the protostar and the orientation and structure of the star forming system. We discuss the physics of these shocks, the model spectra derived from theoretical models, and comparisons with observations of H2O masers, H2 emission, as well as other shocks tracers. We show the strong effect of magnetic fields on molecular shock structure, and elucidate the chemical changes induced by the shock heating and compression.

Making and Breaking Clouds

NASA Astrophysics Data System (ADS)

Kohler, Susanna

2017-10-01

Molecular clouds which youre likely familiar with from stunning popular astronomy imagery lead complicated, tumultuous lives. A recent study has now found that these features must be rapidly built and destroyed.Star-Forming CollapseA Hubble view of a molecular cloud, roughly two light-years long, that has broken off of the Carina Nebula. [NASA/ESA, N. Smith (University of California, Berkeley)/The Hubble Heritage Team (STScI/AURA)]Molecular gas can be found throughout our galaxy in the form of eminently photogenic clouds (as featured throughout this post). Dense, cold molecular gas makes up more than 20% of the Milky Ways total gas mass, and gravitational instabilities within these clouds lead them to collapse under their own weight, resulting in the formation of our galaxys stars.How does this collapse occur? The simplest explanation is that the clouds simply collapse in free fall, with no source of support to counter their contraction. But if all the molecular gas we observe collapsed on free-fall timescales, star formation in our galaxy would churn a rate thats at least an order of magnitude higher than the observed 12 solar masses per year in the Milky Way.Destruction by FeedbackAstronomers have theorized that there may be some mechanism that supports these clouds against gravity, slowing their collapse. But both theoretical studies and observations of the clouds have ruled out most of these potential mechanisms, and mounting evidence supports the original interpretation that molecular clouds are simply gravitationally collapsing.A sub-mm image from ESOs APEX telescope of part of the Taurus molecular cloud, roughly ten light-years long, superimposed on a visible-light image of the region. [ESO/APEX (MPIfR/ESO/OSO)/A. Hacar et al./Digitized Sky Survey 2. Acknowledgment: Davide De Martin]If this is indeed the case, then one explanation for our low observed star formation rate could be that molecular clouds are rapidly destroyed by feedback from the very stars they create. But to match with observations, this wouldsuggest that molecular clouds are short-lived objects that are built (and therefore replenished) just as quickly as they are destroyed. Is this possible?Speedy Building?In a recent study, a team of scientists led by Mordecai-Mark Mac Low (American Museum of Natural History and Heidelberg University, Germany) explore whether there is a way to create molecular clouds rapidly enough to match the necessary rate of destruction.Mac Low and collaborators find that some common mechanisms used to explain the formation of molecular clouds like gas being swept up by supernovae cant quite operate quickly enough to combat the rate of cloud destruction. On the other hand, the Toomre gravitational instability,which is a large-scale gravitational instability that occurs in gas disks,can very rapidly assemble gas into clumps dense enough to form molecules.A composite of visible and near-infrared images from the VLT ANTU telescope of the Barnard 68 molecular cloud, roughly half a light-year in diameter. [ESO]A Rapid CycleBased on their findings, the authors argue that dense, star-forming molecular clouds persist only for a short time before collapsing into stars and then being blown apart by stellar feedback but these very clouds are built equally quickly via gravitational instabilities.Conveniently, this model has a very testable prediction: the Toomre instability is expected to become even stronger at higher redshift, which suggests that the fraction of gas in the form of molecules should increase at high redshifts. This appears to agree with observations, supporting the authors picture of a rapid cycle of cloud assembly and destruction.CitationMordecai-Mark Mac Low et al 2017 ApJL 847 L10. doi:10.3847/2041-8213/aa8a61

Chemical evolution of molecular clouds

NASA Technical Reports Server (NTRS)

Prasad, Sheo S.; Tarafdar, Sankar P.; Villere, Karen R.; Huntress, Wesley T., Jr.

1987-01-01

The principles behind the coupled chemical-dynamical evolution of molecular clouds are described. Particular attention is given to current problems involving the simplest species (i.e., C. CO, O2, and H2) in quiescent clouds. The results of a comparison made between the molecular abundances in the Orion ridge and the hot core (Blake, 1986) are presented.

The XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST)

NASA Technical Reports Server (NTRS)

Guedel, M.; Briggs, K. R.; Arzner, K.; Audard, M.; Bouvier, J.; Feigelson, E. D.; Franciosini, E.; Glauser, A.; Grosso, N.; Micela, G.;

Biogenic, urban, and wildfire influences on the molecular composition of dissolved organic compounds in cloud water

DOE PAGES

Cook, Ryan D.; Lin, Ying-Hsuan; Peng, Zhuoyu; ...

2017-12-21

Organic aerosol formation and transformation occurs within aqueous aerosol and cloud droplets, yet little is known about the composition of high molecular weight organic compounds in cloud water. Cloud water samples collected at Whiteface Mountain, New York, during August-September 2014 were analyzed by ultra-high-resolution mass spectrometry to investigate the molecular composition of dissolved organic carbon, with a focus on sulfur- and nitrogen-containing compounds. Organic molecular composition was evaluated in the context of cloud water inorganic ion concentrations, pH, and total organic carbon concentrations to gain insights into the sources and aqueous-phase processes of the observed high molecular weight organic compounds.more » Cloud water acidity was positively correlated with the average oxygen : carbon ratio of the organic constituents, suggesting the possibility for aqueous acid-catalyzed (prior to cloud droplet activation or during/after cloud droplet evaporation) and/or radical (within cloud droplets) oxidation processes. Many tracer compounds recently identified in laboratory studies of bulk aqueous-phase reactions were identified in the cloud water. Organosulfate compounds, with both biogenic and anthropogenic volatile organic compound precursors, were detected for cloud water samples influenced by air masses that had traveled over forested and populated areas. Oxidation products of long-chain (C 10-12) alkane precursors were detected during urban influence. Influence of Canadian wildfires resulted in increased numbers of identified sulfur-containing compounds and oligomeric species, including those formed through aqueous-phase reactions involving methylglyoxal. Light-absorbing aqueous-phase products of syringol and guaiacol oxidation were observed in the wildfire-influenced samples, and dinitroaromatic compounds were observed in all cloud water samples (wildfire, biogenic, and urban-influenced). Overall, the cloud water molecular composition depended on air mass source influence and reflected aqueous-phase reactions involving biogenic, urban, and biomass burning precursors.« less

Biogenic, urban, and wildfire influences on the molecular composition of dissolved organic compounds in cloud water

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cook, Ryan D.; Lin, Ying-Hsuan; Peng, Zhuoyu

Organic aerosol formation and transformation occurs within aqueous aerosol and cloud droplets, yet little is known about the composition of high molecular weight organic compounds in cloud water. Cloud water samples collected at Whiteface Mountain, New York, during August-September 2014 were analyzed by ultra-high-resolution mass spectrometry to investigate the molecular composition of dissolved organic carbon, with a focus on sulfur- and nitrogen-containing compounds. Organic molecular composition was evaluated in the context of cloud water inorganic ion concentrations, pH, and total organic carbon concentrations to gain insights into the sources and aqueous-phase processes of the observed high molecular weight organic compounds.more » Cloud water acidity was positively correlated with the average oxygen : carbon ratio of the organic constituents, suggesting the possibility for aqueous acid-catalyzed (prior to cloud droplet activation or during/after cloud droplet evaporation) and/or radical (within cloud droplets) oxidation processes. Many tracer compounds recently identified in laboratory studies of bulk aqueous-phase reactions were identified in the cloud water. Organosulfate compounds, with both biogenic and anthropogenic volatile organic compound precursors, were detected for cloud water samples influenced by air masses that had traveled over forested and populated areas. Oxidation products of long-chain (C 10-12) alkane precursors were detected during urban influence. Influence of Canadian wildfires resulted in increased numbers of identified sulfur-containing compounds and oligomeric species, including those formed through aqueous-phase reactions involving methylglyoxal. Light-absorbing aqueous-phase products of syringol and guaiacol oxidation were observed in the wildfire-influenced samples, and dinitroaromatic compounds were observed in all cloud water samples (wildfire, biogenic, and urban-influenced). Overall, the cloud water molecular composition depended on air mass source influence and reflected aqueous-phase reactions involving biogenic, urban, and biomass burning precursors.« less

NARROW Na AND K ABSORPTION LINES TOWARD T TAURI STARS: TRACING THE ATOMIC ENVELOPE OF MOLECULAR CLOUDS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pascucci, I.; Simon, M. N.; Edwards, S.

2015-11-20

We present a detailed analysis of narrow Na i and K i absorption resonance lines toward nearly 40 T Tauri stars in Taurus with the goal of clarifying their origin. The Na i λ5889.95 line is detected toward all but one source, while the weaker K i λ7698.96 line is detected in about two-thirds of the sample. The similarity in their peak centroids and the significant positive correlation between their equivalent widths demonstrate that these transitions trace the same atomic gas. The absorption lines are present toward both disk and diskless young stellar objects, which excludes cold gas within themore » circumstellar disk as the absorbing material. A comparison of Na i and CO detections and peak centroids demonstrates that the atomic gas and molecular gas are not co-located, the atomic gas being more extended than the molecular gas. The width of the atomic lines corroborates this finding and points to atomic gas about an order of magnitude warmer than the molecular gas. The distribution of Na i radial velocities shows a clear spatial gradient along the length of the Taurus molecular cloud filaments. This suggests that absorption is associated with the Taurus molecular cloud. Assuming that the gradient is due to cloud rotation, the rotation of the atomic gas is consistent with differential galactic rotation, whereas the rotation of the molecular gas, although with the same rotation axis, is retrograde. Our analysis shows that narrow Na i and K i absorption resonance lines are useful tracers of the atomic envelope of molecular clouds. In line with recent findings from giant molecular clouds, our results demonstrate that the velocity fields of the atomic and molecular gas are misaligned. The angular momentum of a molecular cloud is not simply inherited from the rotating Galactic disk from which it formed but may be redistributed by cloud–cloud interactions.« less

Cloud-cloud collision in the Galactic center 50 km s-1 molecular cloud

NASA Astrophysics Data System (ADS)

Tsuboi, Masato; Miyazaki, Atsushi; Uehara, Kenta

2015-12-01

We performed a search of star-forming sites influenced by external factors, such as SNRs, H II regions, and cloud-cloud collisions (CCCs), to understand the star-forming activity in the Galactic center region using the NRO Galactic Center Survey in SiO v = 0, J = 2-1, H13CO+J = 1-0, and CS J = 1-0 emission lines obtained with the Nobeyama 45 m telescope. We found a half-shell-like feature (HSF) with a high integrated line intensity ratio of ∫TB(SiO v = 0, J = 2-1)dv/∫TB(H13CO+J = 1-0)dv ˜ 6-8 in the 50 km s-1 molecular cloud; the HSF is a most conspicuous molecular cloud in the region and harbors an active star-forming site where several compact H II regions can be seen. The high ratio in the HSF indicates that the cloud contains huge shocked molecular gas. The HSF can be also seen as a half-shell feature in the position-velocity diagram. A hypothesis explaining the chemical and kinetic properties of the HSF is that the feature originates from a CCC. We analyzed the CS J = 1-0 emission line data obtained with the Nobeyama Millimeter Array to reveal the relation between the HSF and the molecular cloud cores in the cloud. We made a cumulative core mass function (CMF) of the molecular cloud cores within the HSF. The CMF in the CCC region is not truncated at least up to ˜2500 M⊙, although the CMF of the non-CCC region reaches the upper limit of ˜1500 M⊙. Most massive molecular cores with Mgas > 750 M⊙ are located only around the ridge of the HSF and adjoin the compact H II region. These may be a sign of massive star formation induced by CCCs in the Galactic center region.

Analytic solutions for single and multiple cylinders of gravitating polytropes in magnetostatic equilibrium

NASA Technical Reports Server (NTRS)

Lerche, I.; Low, B. C.

1980-01-01

Exact analytic solutions for the static equilibrium of a gravitating plasma polytrope in the presence of magnetic fields are presented. The means of generating various equilibrium configurations to illustrate directly the complex physical relationships between pressure, magnetic fields, and gravity in self-gravitating systems is demonstrated. One of the solutions is used to model interstellar clouds suspended by magnetic fields against the galactic gravity such as may be formed by the Parker (1966) instability. It is concluded that the pinching effect of closed loops of magnetic fields in the clouds may be a dominant agent in further collapsing the clouds following their formation.

Ionisation in turbulent magnetic molecular clouds. I. Effect on density and mass-to-flux ratio structures

NASA Astrophysics Data System (ADS)

Bailey, Nicole D.; Basu, Shantanu; Caselli, Paola

2017-05-01

Context. Previous studies show that the physical structures and kinematics of a region depend significantly on the ionisation fraction. These studies have only considered these effects in non-ideal magnetohydrodynamic simulations with microturbulence. The next logical step is to explore the effects of turbulence on ionised magnetic molecular clouds and then compare model predictions with observations to assess the importance of turbulence in the dynamical evolution of molecular clouds. Aims: In this paper, we extend our previous studies of the effect of ionisation fractions on star formation to clouds that include both non-ideal magnetohydrodynamics and turbulence. We aim to quantify the importance of a treatment of the ionisation fraction in turbulent magnetised media and investigate the effect of the turbulence on shaping the clouds and filaments before star formation sets in. In particular, here we investigate how the structure, mass and width of filamentary structures depend on the amount of turbulence in ionised media and the initial mass-to-flux ratio. Methods: To determine the effects of turbulence and mass-to-flux ratio on the evolution of non-ideal magnetised clouds with varying ionisation profiles, we have run two sets of simulations. The first set assumes different initial turbulent Mach values for a fixed initial mass-to-flux ratio. The second set assumes different initial mass-to-flux ratio values for a fixed initial turbulent Mach number. Both sets explore the effect of using one of two ionisation profiles: step-like (SL) or cosmic ray only (CR-only). We compare the resulting density and mass-to-flux ratio structures both qualitatively and quantitatively via filament and core masses and filament fitting techniques (Gaussian and Plummer profiles). Results: We find that even with almost no turbulence, filamentary structure still exists although at lower density contours. Comparison of simulations shows that for turbulent Mach numbers above 2, there is little structural difference between the SL and CR-only models, while below this threshold the ionisation structure significantly affects the formation of filaments. This holds true for both sets of models. Analysis of the mass within cores and filaments shows that the mass decreases as the degree of turbulence increases. Finally, observed filaments within the Taurus L1495/B213 complex are best reproduced by models with supercritical mass-to-flux ratios and/or at least mildly supersonic turbulence, however, our models show that the sterile fibres observed within Taurus may occur in highly ionised, subcritical environments. Conclusions: From the analysis of the simulations, we conclude that in the presence of low turbulent velocities, the ionisation structure of the medium still plays a role in shaping the structure of the cloud, however, above Mach 2, the differences between the two profiles become indistinguishable. However, differences may be present in the underlying velocity structure. Kinematics studies will be the focus of the next paper in this series. Regions with fertile fibres likely indicate a trans- or supercritical mass-to-flux ratio within the region while sterile fibres are likely subcritical and transient.

Is Molecular Cloud Turbulence Driven by External Supernova Explosions?

NASA Astrophysics Data System (ADS)

Seifried, Daniel; Walch, Stefanie; Haid, Sebastian; Girichidis, Philipp; Naab, Thorsten

2018-03-01

We present high-resolution (∼0.1 pc), hydrodynamical and magnetohydrodynamical simulations to investigate whether the observed level of molecular cloud (MC) turbulence can be generated and maintained by external supernova (SN) explosions. The MCs are formed self-consistently within their large-scale galactic environment following the non-equilibrium formation of H2 and CO, including (self-) shielding and important heating and cooling processes. The MCs inherit their initial level of turbulence from the diffuse ISM, where turbulence is injected by SN explosions. However, by systematically exploring the effect of individual SNe going off outside the clouds, we show that at later stages the importance of SN-driven turbulence is decreased significantly. This holds for different MC masses as well as for MCs with and without magnetic fields. The SN impact also decreases rapidly with larger distances. Nearby SNe (d ∼ 25 pc) boost the turbulent velocity dispersions of the MC by up to 70% (up to a few km s‑1). For d > 50 pc, however, their impact decreases fast with increasing d and is almost negligible. For all probed distances the gain in velocity dispersion decays rapidly within a few 100 kyr. This is significantly shorter than the average timescale for an MC to be hit by a nearby SN under solar neighborhood conditions (∼2 Myr). Hence, at these conditions SNe are not able to sustain the observed level of MC turbulence. However, in environments with high gas surface densities and SN rates, like the Central Molecular Zone, observed elevated MC dispersions could be triggered by external SNe.

Molecular clouds and galactic spiral structure

NASA Technical Reports Server (NTRS)

Dame, T. M.

1984-01-01

Galactic CO line emission at 115 GHz was surveyed in order to study the distribution of molecular clouds in the inner galaxy. Comparison of this survey with similar H1 data reveals a detailed correlation with the most intense 21 cm features. To each of the classical 21 cm H1 spiral arms of the inner galaxy there corresponds a CO molecular arm which is generally more clearly defined and of higher contrast. A simple model is devised for the galactic distribution of molecular clouds. The modeling results suggest that molecular clouds are essentially transient objects, existing for 15 to 40 million years after their formation in a spiral arm, and are largely confined to spiral features about 300 pc wide.

The molecular universe: from astronomy to laboratory astrophysics and back

NASA Astrophysics Data System (ADS)

van Dishoeck, Ewine

2015-08-01

Molecules are found in a wide range of astronomical environments, fromour Solar System to distant starburst galaxies at the highest redshifts. Thanks to the opening up of the infrared and (sub)millimeter wavelength regime, culminating with Herschel and ALMA, more than 180 different species have now been found throughout the various stages of stellar birth and death: diffuse and dense interstellar clouds, protostars and disks, the envelopes of evolved stars and planetary nebulae, and exo-planetary atmospheres. Molecules and solid-state features are now also routinely detected in the interstellar medium of external galaxies, near and far.There are many motivations for studying this molecular universe. From the chemical perspective, interstellar space provides a unique laboratory to study basic molecular processes under very different conditions from those normally found in a laboratory on Earth. For astronomers, molecules are unique probes of the many environments where they are found, providing information on density, temperature, dynamics, ionization fractions and magnetic fields. Molecules also play an important role in the cooling of clouds allowing them to collapse, including the formation of the very first stars and galaxies. Finally, the molecular composition is sensitive to the history of the material, and ultimately provides critical information on our origins.This talk will summarize a number of recent observational highlights and provide examples of cases where the availability of new laboratory data proved crucial in the analysis. This includes basic data such as spectroscopy and collisional rate coefficients, but also an improved understanding of photoprocesses in the gaseous and solid state. Much of the chemistry in star- and planet-forming regions is now thought to be driven by gas-grain chemistry rather than pure gas-phase chemistry, and a few examples of the close link between models and laboratory experiments will be given. In spite of lingering uncertainties, the future of molecular astrophysics is bright and will allow increased understanding of the journey of gas and solids from clouds to comets and planets.

ROSAT X-ray sources embedded in the rho Ophiuchi cloud core

NASA Astrophysics Data System (ADS)

Casanova, Sophie; Montmerle, Thierry; Feigelson, Eric D.; Andre, Philippe

1995-02-01

We present a deep ROSAT Position Sensitive Proportional Counter (PSPC) image of the central region of the rho Oph star-forming region. The selected area, about 35 x 35 arcmins in size, is rich with dense molecular cores and young stellar objects (YSOs). Fifty-five reliable X-ray sources are detected (and up to 50 more candidates may be present) above approximately 1 keV,, doubling the number of Einstein sources in this area. These sources are cross-identified with an updated list of 88 YSOs associated with the rho Oph cloud core. A third of the reliable X-ray sources do not have optical counterparts on photographic plates. Most can be cross-identified wth Class II and Class III infrared (IR) sources, which are embedded T Tauri stars, but three reliable X-ray sources and up to seven candidate sources are tentatively identified with Class I protostars. Eighteen reliable, and up to 20 candidate, X-ray sources are probably new cloud members. The overall detection rate of the bona fide cloud population is very high (73% for the Class II and Class III objects). The spatial distribution of the X-ray sources closely follows that of the moleclar gas. The visual extinctions Av estimated from near-IR data) of the ROSAT sources can be as high as 50 or more, confirming that most are embedded in the cloud core and are presumably very young. Using bolometric luminosities Lbol estimated from J-magnitudes a tight correlation between Lx and Lbol is found, similar to that seen for older T Tauri stars in the Cha I cloud: Lx approximately 10-4 Lbol. A general relation Lxproportional to LbolLj seems to apply to all T Tauri-like YSOs. The near equality of the extintion in the IR J band and in the keV X-ray rage implies that this relation is valid for the detected fluxes as well as for the dereddened fluxes. The X-ray luminosity function of the embedded sourced in rho Oph spans a range of Lx approximately 1028.5 to approximately equal to or greater than 1031.5 ergs/s and is statistically indistinguishable from that of X-ray-detected visile T Tauri stars. We estimate a total X-ray luminosity Lx, Oph approximately equal to or greater than 6 x 10 32 ergs/s from approximately equal to 200 X-ray sources in the cloud core, down to Lbol approximately 0.1 solar luminosity or Mstar approximately 0.3 solar mass. We discuss several consequences of in situ irradiation of molecular clouds by X-rays from embedded YSOs. These X-rays must partially ionize the inner regions of circumstellar disk coronae, possibly playing an important role in coupling magnetic ionize the fields and wind or bipolar outflows. Photon-stimulated deportion of large molecules by YSO X-rays may be partly responsible for the bright 12 micrometer halos seen in some molecular clouds.

Model experiment of magnetic field amplification in laser-produced plasmas via the Richtmyer-Meshkov instability

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kuramitsu, Y., E-mail: yasu@ncu.edu.tw; Moritaka, T.; Ohnishi, N.

2016-03-15

A model experiment of magnetic field amplification (MFA) via the Richtmyer-Meshkov instability (RMI) in supernova remnants (SNRs) was performed using a high-power laser. In order to account for very-fast acceleration of cosmic rays observed in SNRs, it is considered that the magnetic field has to be amplified by orders of magnitude from its background level. A possible mechanism for the MFA in SNRs is stretching and mixing of the magnetic field via the RMI when shock waves pass through dense molecular clouds in interstellar media. In order to model the astrophysical phenomenon in laboratories, there are three necessary factors formore » the RMI to be operative: a shock wave, an external magnetic field, and density inhomogeneity. By irradiating a double-foil target with several laser beams with focal spot displacement under influence of an external magnetic field, shock waves were excited and passed through the density inhomogeneity. Radiative hydrodynamic simulations show that the RMI evolves as the density inhomogeneity is shocked, resulting in higher MFA.« less

Formation of young massive clusters from turbulent molecular clouds

NASA Astrophysics Data System (ADS)

Fujii, Michiko; Portegies Zwart, Simon

2015-08-01

We simulate the formation and evolution of young star clusters using smoothed-particle hydrodynamics (SPH) and direct N-body methods. We start by performing SPH simulations of the giant molecular cloud with a turbulent velocity field, a mass of 10^4 to 10^6 M_sun, and a density between 17 and 1700 cm^-3. We continue the SPH simulations for a free-fall time scale, and analyze the resulting structure of the collapsed cloud. We subsequently replace a density-selected subset of SPH particles with stars. As a consequence, the local star formation efficiency exceeds 30 per cent, whereas globally only a few per cent of the gas is converted to stars. The stellar distribution is very clumpy with typically a dozen bound conglomerates that consist of 100 to 10000 stars. We continue to evolve the stars dynamically using the collisional N-body method, which accurately treats all pairwise interactions, stellar collisions and stellar evolution. We analyze the results of the N-body simulations at 2 Myr and 10 Myr. From dense massive molecular clouds, massive clusters grow via hierarchical merging of smaller clusters. The shape of the cluster mass function that originates from an individual molecular cloud is consistent with a Schechter function with a power-law slope of beta = -1.73 at 2 Myr and beta = -1.67 at 10 Myr, which fits to observed cluster mass function of the Carina region. The superposition of mass functions have a power-law slope of < -2, which fits the observed mass function of star clusters in the Milky Way, M31 and M83. We further find that the mass of the most massive cluster formed in a single molecular cloud with a mass of M_g scales with 6.1 M_g^0.51 which also agrees with recent observation in M51. The molecular clouds which can form massive clusters are much denser than those typical in the Milky Way. The velocity dispersion of such molecular clouds reaches 20 km/s and it is consistent with the relative velocity of the molecular clouds observed near NGC 3603 and Westerlund 2, for which a triggered star formation by cloud-cloud collisions is suggested.

Experimental Investigation of Electron Cloud Containment in a Nonuniform Magnetic Field

NASA Technical Reports Server (NTRS)

Eninger, J. E.

1974-01-01

Dense clouds of electrons were generated and studied in an axisymmetric, nonuniform magnetic field created by a short solenoid. The operation of the experiment was similar to that of a low-pressure (approximately 0.000001 Torr) magnetron discharge. Discharge current characteristics are presented as a function of pressure, magnetic field strength, voltage, and cathode end-plate location. The rotation of the electron cloud is determined from the frequency of diocotron waves. In the space charge saturated regime of operation, the cloud is found to rotate as a solid body with frequency close to V sub a/phi sub a where V sub a is the anode voltage and phi suba is the total magnetic flux. This result indicates that, in regions where electrons are present, the magnetic field lines are electrostatic equipotentials (E bar, B bar = 0). Equilibrium electron density distributions suggested by this conditions are integrated with respect to total ionizing power and are found consistent with measured discharge currents.

The size-line width relation and the mass of molecular hydrogen

DOE Office of Scientific and Technical Information (OSTI.GOV)

Issa, M.; Maclaren, I.; Wolfendale, A. W.

Some difficulties associated with the problem of cloud definition are considered, with particular regard to the crowded distribution of clouds and the difficulty of choosing an appropriate boundary in such circumstances. A number of tests carried out on the original data suggest that the delta(v) - S relation found by Solomon et al. (1987) is not a genuine reflection of the dynamical state of Giant Molecular Clouds. The Solomon et al. parameters, are insensitive to the actual cloud properties and are unable to distinguish true clouds from the consequences of sampling any crowded region of emission down to a lowmore » threshold temperature. The overall effect of such problems is to overestimate both the masses of Giant Molecular Clouds and the number of very large clouds. 24 refs.« less

Cosmic-ray ionisation of dense molecular clouds

NASA Astrophysics Data System (ADS)

Vaupre, Solenn

2015-07-01

Cosmic rays (CR) are of tremendous importance in the dynamical and chemical evolution of interstellar molecular clouds, where stars and planets form. CRs are likely accelerated in the shells of supernova remnants (SNR), thus molecular clouds nearby can be irradiated by intense fluxes of CRs. CR protons have two major effects on dense molecular clouds: 1) when they encounter the dense medium, high-energy protons (>280 MeV) create pions that decay into gamma-rays. This process makes SNR-molecular cloud associations intense GeV and/or TeV sources whose spectra mimic the CR spectrum. 2) at lower energies, CRs penetrate the cloud and ionise the gas, leading to the formation of molecular species characteristic of the presence of CRs, called tracers of the ionisation. Studying these tracers gives information on low-energy CRs that are unaccessible to any other observations. I studied the CR ionisation of molecular clouds next to three SNRs: W28, W51C and W44. These SNRs are known to be interacting with the nearby clouds, from the presence of shocked gas, OH masers and pion-decay induced gamma-ray emission. My work includes millimeter observations and chemical modeling of tracers of the ionisation in these dense molecular clouds. In these three regions, we determined an enhanced CR ionisation rate, supporting the hypothesis of an origin of the CRs in the SNR nearby. The evolution of the CR ionisation rate with the distance to the SNR brings valuable constraints on the propagation properties of low-energy CRs. The method used relies on observations of the molecular ions HCO+ and DCO+, which shows crucial limitations at high ionisation. Therefore, I investigated, both through modeling and observations, the chemical abundances of several other species to try and identity alternative tracers of the ionisation. In particular, in the W44 region, observations of N2H+ bring additional constraints on the physical conditions, volatile abundances in the cloud, and the ionisation state. This research brought valuable insight in to the CR induced chemistry in the interstellar medium. It also brought new perspectives of interdisciplinary research towards the understanding of CRs, from millimeter to gamma-ray observations.

The heliospheric sector boundary as a distented magnetic cloud

NASA Technical Reports Server (NTRS)

Crooker, N. U.; Intriligator, D. S.

1995-01-01

A magnetic cloud was detected both near Earth and by Pioneer 11 located 43 deg east of Earth at 4.8 AU. The magnetic field within the cloud rotated smoothly from toward to away polarity, marking sector boundary passage. Interpreted as a flux rope, the cloud had a vertical axis, implying that its cylindrical cross-section in the ecliptic plane was distended along the sector boundary by at least 43, forming an extensive occlusion in the heliospheric current sheet. At 1 AU the cloud had plasma signatures typical of a fast coronal mass ejection with low temperature and a leading shock. In contrast, at 4.8 AU, only the cloud signature remained. Its radial dimension was the same at both locations, consistent with little expansion beyond 1 AU. Energetic particle data at 4.8 AU show high fluxes preceding the cloud but not extending forward to the corotating shock that marked entry into the interaction region containing the cloud. The streaming direction was antisunward, consistent with possible acceleration in a low-beta region of field line draping around the cloud's western (upstream) end. The fluxes dropped upon entry into the cloud and became essentially isotropic one third of the way through it. On the basis of sector boundary characteristics published in the past, we suggest that distended clouds may be common heliospheric current sheet occlusions.

A FAR-INFRARED OBSERVATIONAL TEST OF THE DIRECTIONAL DEPENDENCE IN RADIATIVE GRAIN ALIGNMENT

DOE Office of Scientific and Technical Information (OSTI.GOV)

Vaillancourt, John E.; Andersson, B.-G., E-mail: jvaillancourt@sofia.usra.edu, E-mail: bg@sofia.usra.edu

The alignment of interstellar dust grains with magnetic fields provides a key method for measuring the strength and morphology of the fields. In turn, this provides a means to study the role of magnetic fields from diffuse gas to dense star-forming regions. The physical mechanism for aligning the grains has been a long-term subject of study and debate. The theory of radiative torques, in which an anisotropic radiation field imparts sufficient torques to align the grains while simultaneously spinning them to high rotational velocities, has passed a number of observational tests. Here we use archival polarization data in dense regionsmore » of the Orion molecular cloud (OMC-1) at 100, 350, and 850 μm to test the prediction that the alignment efficiency is dependent upon the relative orientations of the magnetic field and radiation anisotropy. We find that the expected polarization signal, with a 180-degree period, exists at all wavelengths out to radii of 1.5 arcmin centered on the Becklin–Neugebauer Kleinmann-Low (BNKL) object in OMC-1. The probabilities that these signals would occur due to random noise are low (≲1%), and are lowest toward BNKL compared to the rest of the cloud. Additionally, the relative magnetic field to radiation anisotropy directions accord with theoretical predictions in that they agree to better than 15° at 100 μm and 4° at 350 μm.« less

The interaction of a magnetic cloud with the Earth - Ionospheric convection in the Northern and Southern Hemispheres for a wide range of quasi-steady interplanetary magnetic field conditions

NASA Technical Reports Server (NTRS)

Freeman, M. P.; Farrugia, C. J.; Burlaga, L. F.; Hairston, M. R.; Greenspan, M. E.; Ruohoniemi, J. M.; Lepping, R. P.

1993-01-01

Observations are presented of the ionospheric convection in cross sections of the polar cap and auroral zone as part of the study of the interaction of the Earth's magnetosphere with the magnetic cloud of January 13-15, 1988. For strongly northward IMF, the convection in the Southern Hemisphere is characterized by a two-cell convection pattern comfined to high latitudes with sunward flow over the pole. The strength of the flows is comparable to that later seen under southward IMF. Superimposed on this convection pattern there are clear dawn-dusk asymmetries associated with a one-cell convection component whose sense depends on the polarity of the magnetic cloud's large east-west magnetic field component. When the cloud's magnetic field turns southward, the convection is characterized by a two-cell pattern extending to lower latitude with antisunward flow over the pole. There is no evident interhemispheric difference in the structure and strength of the convection. Superimposed dawn-dusk asymmetries in the flow patterns are observed which are only in part attributable to the east-west component of the magnetic field.

Erratum to "Solar Sources and Geospace Consequences of Interplanetary Magnetic Clouds Observed During Solar Cycle 23-Paper 1" [J. Atmos. Sol.-Terr. Phys. 70(2-4) (2008) 245-253

NASA Technical Reports Server (NTRS)

Gopalswamy, N.; Akiyama, S.; Yashiro, S.; Michalek, G.; Lepping, R. P.

2009-01-01

One of the figures (Fig. 4) in "Solar sources and geospace consequences of interplanetary magnetic Clouds observed during solar cycle 23 -- Paper 1" by Gopalswamy et al. (2008, JASTP, Vol. 70, Issues 2-4, February 2008, pp. 245-253) is incorrect because of a software error in t he routine that was used to make the plot. The source positions of various magnetic cloud (MC) types are therefore not plotted correctly.

Dust scattering from the Taurus Molecular Cloud

NASA Astrophysics Data System (ADS)

Narayan, Sathya; Murthy, Jayant; Karuppath, Narayanankutty

2017-04-01

We present an analysis of the diffuse ultraviolet emission near the Taurus Molecular Cloud based on observations made by the Galaxy Evolution Explorer. We used a Monte Carlo dust scattering model to show that about half of the scattered flux originates in the molecular cloud with 25 per cent arising in the foreground and 25 per cent behind the cloud. The best-fitting albedo of the dust grains is 0.3, but the geometry is such that we could not constrain the phase function asymmetry factor (g).

The emerging role of cloud computing in molecular modelling.

PubMed

Ebejer, Jean-Paul; Fulle, Simone; Morris, Garrett M; Finn, Paul W

2013-07-01

There is a growing recognition of the importance of cloud computing for large-scale and data-intensive applications. The distinguishing features of cloud computing and their relationship to other distributed computing paradigms are described, as are the strengths and weaknesses of the approach. We review the use made to date of cloud computing for molecular modelling projects and the availability of front ends for molecular modelling applications. Although the use of cloud computing technologies for molecular modelling is still in its infancy, we demonstrate its potential by presenting several case studies. Rapid growth can be expected as more applications become available and costs continue to fall; cloud computing can make a major contribution not just in terms of the availability of on-demand computing power, but could also spur innovation in the development of novel approaches that utilize that capacity in more effective ways. Copyright © 2013 Elsevier Inc. All rights reserved.

Isotopic evidence for primordial molecular cloud material in metal-rich carbonaceous chondrites.

PubMed

Van Kooten, Elishevah M M E; Wielandt, Daniel; Schiller, Martin; Nagashima, Kazuhide; Thomen, Aurélien; Larsen, Kirsten K; Olsen, Mia B; Nordlund, Åke; Krot, Alexander N; Bizzarro, Martin

2016-02-23

The short-lived (26)Al radionuclide is thought to have been admixed into the initially (26)Al-poor protosolar molecular cloud before or contemporaneously with its collapse. Bulk inner Solar System reservoirs record positively correlated variability in mass-independent (54)Cr and (26)Mg*, the decay product of (26)Al. This correlation is interpreted as reflecting progressive thermal processing of in-falling (26)Al-rich molecular cloud material in the inner Solar System. The thermally unprocessed molecular cloud matter reflecting the nucleosynthetic makeup of the molecular cloud before the last addition of stellar-derived (26)Al has not been identified yet but may be preserved in planetesimals that accreted in the outer Solar System. We show that metal-rich carbonaceous chondrites and their components have a unique isotopic signature extending from an inner Solar System composition toward a (26)Mg*-depleted and (54)Cr-enriched component. This composition is consistent with that expected for thermally unprocessed primordial molecular cloud material before its pollution by stellar-derived (26)Al. The (26)Mg* and (54)Cr compositions of bulk metal-rich chondrites require significant amounts (25-50%) of primordial molecular cloud matter in their precursor material. Given that such high fractions of primordial molecular cloud material are expected to survive only in the outer Solar System, we infer that, similarly to cometary bodies, metal-rich carbonaceous chondrites are samples of planetesimals that accreted beyond the orbits of the gas giants. The lack of evidence for this material in other chondrite groups requires isolation from the outer Solar System, possibly by the opening of disk gaps from the early formation of gas giants.

Isotopic evidence for primordial molecular cloud material in metal-rich carbonaceous chondrites

PubMed Central

Van Kooten, Elishevah M. M. E.; Wielandt, Daniel; Schiller, Martin; Nagashima, Kazuhide; Thomen, Aurélien; Olsen, Mia B.; Nordlund, Åke; Krot, Alexander N.; Bizzarro, Martin

2016-01-01

The short-lived 26Al radionuclide is thought to have been admixed into the initially 26Al-poor protosolar molecular cloud before or contemporaneously with its collapse. Bulk inner Solar System reservoirs record positively correlated variability in mass-independent 54Cr and 26Mg*, the decay product of 26Al. This correlation is interpreted as reflecting progressive thermal processing of in-falling 26Al-rich molecular cloud material in the inner Solar System. The thermally unprocessed molecular cloud matter reflecting the nucleosynthetic makeup of the molecular cloud before the last addition of stellar-derived 26Al has not been identified yet but may be preserved in planetesimals that accreted in the outer Solar System. We show that metal-rich carbonaceous chondrites and their components have a unique isotopic signature extending from an inner Solar System composition toward a 26Mg*-depleted and 54Cr-enriched component. This composition is consistent with that expected for thermally unprocessed primordial molecular cloud material before its pollution by stellar-derived 26Al. The 26Mg* and 54Cr compositions of bulk metal-rich chondrites require significant amounts (25–50%) of primordial molecular cloud matter in their precursor material. Given that such high fractions of primordial molecular cloud material are expected to survive only in the outer Solar System, we infer that, similarly to cometary bodies, metal-rich carbonaceous chondrites are samples of planetesimals that accreted beyond the orbits of the gas giants. The lack of evidence for this material in other chondrite groups requires isolation from the outer Solar System, possibly by the opening of disk gaps from the early formation of gas giants. PMID:26858438

AMR Studies of Star Formation: Simulations and Simulated Observations

NASA Astrophysics Data System (ADS)

Offner, Stella; McKee, C. F.; Klein, R. I.

2009-01-01

Molecular clouds are typically observed to be approximately virialized with gravitational and turbulent energy in balance, yielding a star formation rate of a few percent. The origin and characteristics of the observed supersonic turbulence are poorly understood, and without continued energy injection the turbulence is predicted to decay within a cloud dynamical time. Recent observations and analytic work have suggested a strong connection between the initial stellar mass function, the core mass function, and turbulence characteristics. The role of magnetic fields in determining core lifetimes, shapes, and kinematic properties remains hotly debated. Simulations are a formidable tool for studying the complex process of star formation and addressing these puzzles. I present my results modeling low-mass star formation using the ORION adaptive mesh refinement (AMR) code. I investigate the properties of forming cores and protostars in simulations in which the turbulence is driven to maintain virial balance and where it is allowed to decay. I will discuss simulated observations of cores in dust emission and in molecular tracers and compare to observations of local star-forming clouds. I will also present results from ORION cluster simulations including flux-limited diffusion radiative transfer and show that radiative feedback, even from low-mass stars, has a significant effect on core fragmentation, disk properties, and the IMF. Finally, I will discuss the new simulation frontier of AMR multigroup radiative transfer.

On the physical mechanisms governing the cloud lifecycle in the Central Molecular Zone of the Milky Way

NASA Astrophysics Data System (ADS)

Jeffreson, S. M. R.; Kruijssen, J. M. D.; Krumholz, M. R.; Longmore, S. N.

2018-05-01

We apply an analytic theory for environmentally-dependent molecular cloud lifetimes to the Central Molecular Zone of the Milky Way. Within this theory, the cloud lifetime in the Galactic centre is obtained by combining the time-scales for gravitational instability, galactic shear, epicyclic perturbations and cloud-cloud collisions. We find that at galactocentric radii ˜45-120 pc, corresponding to the location of the `100-pc stream', cloud evolution is primarily dominated by gravitational collapse, with median cloud lifetimes between 1.4 and 3.9 Myr. At all other galactocentric radii, galactic shear dominates the cloud lifecycle, and we predict that molecular clouds are dispersed on time-scales between 3 and 9 Myr, without a significant degree of star formation. Along the outer edge of the 100-pc stream, between radii of 100 and 120 pc, the time-scales for epicyclic perturbations and gravitational free-fall are similar. This similarity of time-scales lends support to the hypothesis that, depending on the orbital geometry and timing of the orbital phase, cloud collapse and star formation in the 100-pc stream may be triggered by a tidal compression at pericentre. Based on the derived time-scales, this should happen in approximately 20 per cent of all accretion events onto the 100-pc stream.

Electron-impact excitation and recombination of molecular cations in edge fusion plasma: application to H2+and BeD+

NASA Astrophysics Data System (ADS)

Pop, Nicolina; Iacob, Felix; Mezei, Zsolt; Motapon, Ousmanou; Niyonzima, Sebastien; Schneider, Ioan

2017-10-01

Dissociative recombination, ro-vibrational excitation and dissociative excitation of molecular cations with electrons are major elementary process in the kinetics and in the energy balance of astrophysically-relevant ionized media (supernovae, interstellar molecular clouds, planetary ionospheres, early Universe), in edge fusion and in many other cold media of technological interest. For the fusion plasma edge, extensive cross sections and rate coefficients have been produced for reactions induced on HD+, H2+ and BeD+ using the Multichannel Quantum Defect Theory (MQDT). Our calculations resulted in good agreement with the CRYRING (Stockholm) and TSR (Heidelberg) magnetic storage ring results, and our approach is permanently improved in order to face the new generation of electrostatic storage rings, as CSR (Heidelberg) and DESIREE (Stockholm). Member of APS Reciprocal Society: European Physics Society.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cook, Ryan D.; Lin, Ying-Hsuan; Peng, Zhuoyu

Organic aerosol formation and transformation occurs within aqueous aerosol and cloud droplets, yet little is known about the composition of high molecular weight organic compounds in cloud water. Cloud water samples collected at Whiteface Mountain, New York, during August-September 2014 were analyzed by ultra-high-resolution mass spectrometry to investigate the molecular composition of dissolved organic carbon, with a focus on sulfur- and nitrogen-containing compounds. Organic molecular composition was evaluated in the context of cloud water inorganic ion concentrations, pH, and total organic carbon concentrations to gain insights into the sources and aqueous-phase processes of the observed high molecular weight organic compounds.more » Cloud water acidity was positively correlated with the average oxygen : carbon ratio of the organic constituents, suggesting the possibility for aqueous acid-catalyzed (prior to cloud droplet activation or during/after cloud droplet evaporation) and/or radical (within cloud droplets) oxidation processes. Many tracer compounds recently identified in laboratory studies of bulk aqueous-phase reactions were identified in the cloud water. Organosulfate compounds, with both biogenic and anthropogenic volatile organic compound precursors, were detected for cloud water samples influenced by air masses that had traveled over forested and populated areas. Oxidation products of long-chain (C 10-12) alkane precursors were detected during urban influence. Influence of Canadian wildfires resulted in increased numbers of identified sulfur-containing compounds and oligomeric species, including those formed through aqueous-phase reactions involving methylglyoxal. Light-absorbing aqueous-phase products of syringol and guaiacol oxidation were observed in the wildfire-influenced samples, and dinitroaromatic compounds were observed in all cloud water samples (wildfire, biogenic, and urban-influenced). Overall, the cloud water molecular composition depended on air mass source influence and reflected aqueous-phase reactions involving biogenic, urban, and biomass burning precursors.« less

Molecular clouds in galaxies with different Z - Fragmentation of diffuse clouds driven by opacity

NASA Technical Reports Server (NTRS)

Franco, Jose; Cox, Donald P.

1986-01-01

Molecular clouds are formed from diffuse interstellar clouds when the external ultraviolet radiation field is prevented from penetrating into the cloud. The opacity is provided mainly by dust grains and the required column density to the cloud center is larger than about 5 x 10 to the 20th (solar Z/Z)/sq cm. This high-opacity criterion could have a significant impact on the radial trends observed in spiral galaxies, and on the distinctions between spiral and dwarf irregular galaxies.

OH 18 cm TRANSITION AS A THERMOMETER FOR MOLECULAR CLOUDS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ebisawa, Yuji; Inokuma, Hiroshi; Yamamoto, Satoshi

We have observed the four hyperfine components of the 18 cm OH transition toward the translucent cloud eastward of Heiles Cloud 2 (HCL2E), the cold dark cloud L134N, and the photodissociation region of the ρ-Ophiuchi molecular cloud with the Effelsberg 100 m telescope. We have found intensity anomalies among the hyperfine components in all three regions. In particular, an absorption feature of the 1612 MHz satellite line against the cosmic microwave background has been detected toward HCL2E and two positions of the ρ-Ophiuchi molecular cloud. On the basis of statistical equilibrium calculations, we find that the hyperfine anomalies originate frommore » the non-LTE population of the hyperfine levels, and can be used to determine the kinetic temperature of the gas over a wide range of H{sub 2} densities (10{sup 2}–10{sup 7} cm{sup −3}). Toward the center of HCL2E, the gas kinetic temperature is determined to be 53 ± 1 K, and it increases toward the cloud peripheries (∼60 K). The ortho-to-para ratio of H{sub 2} is determined to be 3.5 ± 0.9 from the averaged spectrum for the eight positions. In L134N, a similar increase of the temperature is also seen toward the periphery. In the ρ-Ophiuchi molecular cloud, the gas kinetic temperature decreases as a function of the distance from the exciting star HD 147889. These results demonstrate a new aspect of the OH 18 cm line that can be used as a good thermometer of molecular cloud envelopes. The OH 18 cm line can be used to trace a new class of warm molecular gas surrounding a molecular cloud, which is not well traced by the emission of CO and its isotopologues.« less

Study of Molecular Clouds, Variable Stars and Related Topics at NUU and UBAI

NASA Astrophysics Data System (ADS)

Hojaev, A. S.

2017-07-01

The search of young PMS stars made by our team at Maidanak, Lulin and Beijing observatories, especially in NGC 6820/23 area, as well as monitoring of a sample of open clusters will be described and results will be presented. We consider physical conditions in different star forming regions, particularly in TDC and around Vul OB1, estimate SFE and SFR, energy balance and instability processes in these regions. We also reviewed all data on molecular clouds in the Galaxy and in other galaxies where the clouds were observed to prepare general catalog of molecular clouds, to study physical conditions, unsteadiness and possible star formation in them, the formation and evolution of molecular cloud systems, to analyze their role in formation of different types of galaxies and structural features therein.

Characterization of molecular structural changes in pectin during juice cloud destabilization in frozen concentrated orange juice

USDA-ARS?s Scientific Manuscript database

Pectin comprises one of the major components of cloud material in citrus juices. Juice cloud is a complex mixture of polysaccharides, proteins and lower molecular weight compounds that are responsible for the turbid appearance of citrus juices. The stability of juice cloud depends on a number of fac...

The temperature of large dust grains in molecular clouds

NASA Technical Reports Server (NTRS)

Clark, F. O.; Laureijs, R. J.; Prusti, T.

1991-01-01

The temperature of the large dust grains is calculated from three molecular clouds ranging in visual extinction from 2.5 to 8 mag, by comparing maps of either extinction derived from star counts or gas column density derived from molecular observations to I(100). Both techniques show the dust temperature declining into clouds. The two techniques do not agree in absolute scale.

The simulation of molecular clouds formation in the Milky Way

NASA Astrophysics Data System (ADS)

Khoperskov, S. A.; Vasiliev, E. O.; Sobolev, A. M.; Khoperskov, A. V.

2013-01-01

Using 3D hydrodynamic calculations we simulate formation of molecular clouds in the Galaxy. The simulations take into account molecular hydrogen chemical kinetics, cooling and heating processes. Comprehensive gravitational potential accounts for contributions from the stellar bulge, two- and four-armed spiral structure, stellar disc, dark halo and takes into account self-gravitation of the gaseous component. Gas clouds in our model form in the spiral arms due to shear and wiggle instabilities and turn into molecular clouds after t ≳ 100 Myr. At the times t ˜ 100-300 Myr the clouds form hierarchical structures and agglomerations with the sizes of 100 pc and greater. We analyse physical properties of the simulated clouds and find that synthetic statistical distributions like mass spectrum, `mass-size' relation and velocity dispersion are close to those observed in the Galaxy. The synthetic l-v (galactic longitude-radial velocity) diagram of the simulated molecular gas distribution resembles observed one and displays a structure with appearance similar to molecular ring of the Galaxy. Existence of this structure in our modelling can be explained by superposition of emission from the galactic bar and the spiral arms at ˜3-4 kpc.

Analysis of 20 magnetic clouds at 1 AU during a solar minimum

NASA Astrophysics Data System (ADS)

Gulisano, A. M.; Dasso, S.; Mandrini, C. H.; Démoulin, P.

We study 20 magnetic clouds, observed in situ by the spacecraft Wind, at the Lagrangian point L1, from 22 August, 1995, to 7 November, 1997. In previous works, assuming a cylindrical symmetry for the local magnetic configuration and a satellite trajectory crossing the axis of the cloud, we obtained their orientations using a minimum variance analysis. In this work we compute the orientations and magnetic configurations using a non-linear simultaneous fit of the geometric and physical parameters for a linear force-free model, including the possibility of a not null impact parameter. We quantify global magnitudes such as the relative magnetic helicity per unit length and compare the values found with both methods (minimum variance and the simultaneous fit). FULL TEXT IN SPANISH

Typical and Unusual Properties of Magnetic Clouds during the WIND Era

NASA Technical Reports Server (NTRS)

Lepping, R. P.; Berdichevsky, D.; Szabo, A.; Burlaga, L. F.; Thompson, B. J.; Mariani, F.; Lazarus, A. J.; Steinberg, J. T.

1999-01-01

A list of 33 magnetic clouds as identified in WIND magnetic field and plasma data has been compiled. The intervals for these events are provided as part of NASA/GSFC, WIND-MFI's Website under the URL http://lepmfi.qsfc.nasa.gov/mfi/mag_cloud publ.html#table The period covered in this study is from early 1995 to November 1998 which primarily occurs in the quiet part of the solar cycle. A force free, cylindrically symmetric, magnetic field model has been applied to the field data in 1-hour averaged form for all of these events (except one small event where 10 min avg's were used) and the resulting fit-parameters examined. Each event was provided a semi-quantitatively determined quality factor (excellent, good or poor). A set of 28 good or better cases, spanning a surprisingly large range of values for its various properties, was used for further analysis. These properties are, for example, durations, attitudes, sizes, asymmetries, axial field strengths, speeds, and relative impact parameters. They will be displayed and analyzed, along with some related derived quantities, with emphasis on typical vs unusual properties and on the magnetic fields magnetic clouds' relationships to the Sun and to upstream interplanetary shocks, where possible. For example, it is remarkable how narrowly distributed the speeds of these clouds are, and the overall average speed (390 techniques km/s) is less than that normally quoted for the average solar wind speed (420 km/s) despite the fact that many of these clouds are d"drivers" of interplanetary shocks. On average, a cloud appears to be a little less symmetric when the spacecraft is able to pass close to the cloud's axis as compared to a farther out passage. The average longitude and latitude (in GSE) of the axes of the clouds are 85 degrees and 8 degrees, respectively, with standard deviations near 40 degrees. Also, the half=yearly averaged axial magnetic flux has approximately tripled. almost monotonically, from about 6 to 17 X 10(exp 29) Mx over the first 3.5 years of consideration, but with a large uncertainty on each of the half-year estimates, because of small sampling. If true,this finding implies an approximate tripling of the events' solar fluxes over this period as it goes into solar maximum.

The small-scale turbulent dynamo in smoothed particle magnetohydrodynamics

NASA Astrophysics Data System (ADS)

Tricco, T. S.; Price, D. J.; Federrath, C.

2016-05-01

Supersonic turbulence is believed to be at the heart of star formation. We have performed smoothed particle magnetohydrodynamics (SPMHD) simulations of the small- scale dynamo amplification of magnetic fields in supersonic turbulence. The calculations use isothermal gas driven at rms velocity of Mach 10 so that conditions are representative of starforming molecular clouds in the Milky Way. The growth of magnetic energy is followed for 10 orders in magnitude until it reaches saturation, a few percent of the kinetic energy. The results of our dynamo calculations are compared with results from grid-based methods, finding excellent agreement on their statistics and their qualitative behaviour. The simulations utilise the latest algorithmic developments we have developed, in particular, a new divergence cleaning approach to maintain the solenoidal constraint on the magnetic field and a method to reduce the numerical dissipation of the magnetic shock capturing scheme. We demonstrate that our divergence cleaning method may be used to achieve ∇ • B = 0 to machine precision, albeit at significant computational expense.

Variation of z-height of the molecular clouds on the Galactic Plane

NASA Astrophysics Data System (ADS)

Lee, Y.; Stark, A. A.

2002-12-01

Using the Bell Laboratories Galactic plane in the J=1-0 transition of 13CO, (l, b) = (-5o to 117o, -1o to +1o), and cloud identification code, 13CO clouds have been identified and cataloged as a function of threshold temperature. Distance estimates to the identified clouds have been made with several criteria. Minimum and maximum distances to each identified cloud are determined from a set of all the possible distances of a cloud. Several physical parameters can be determined with distances, such as z-height [D sin (b)], CO luminosity, virial mass and so forth. We select the clouds with a ratio of maximum and minimum of CO luminosities less than 3. The number of selected clouds is 281 out of 1400 identified clouds with 1 K threshold temperature. These clouds are mostly located on the tangential positions in the inner Galaxy, and some are in the Outer Galaxy. It is found that the z-height of lower luminosity clouds (less massive clouds) is systimatically larger than that of high-luminosity clouds (more massive clouds). We claim that this is the first observational evidence of the z-height variation depending on the luminosities (or masses) of molecular clouds on the Galactic plane. Our results could be a basis explaining the formation mechanism of massive clouds, such as giant molecular clouds.

Turbulence and star formation in molecular clouds

NASA Astrophysics Data System (ADS)

Larson, R. B.

1981-03-01

Consideration is given to the turbulence properties of molecular clouds and their implications for star formation. Data for 54 molecular clouds and condensations is presented which reveals cloud velocity dispersion and region size to follow a power-law relation, similar to the Kolmogoroff law for subsonic turbulence. Examination of the dynamics of the molecular clouds for which mass determinations are available reveals essentially all of them to be gravitationally bound, and to approximately satisfy the virial theorem. The observation of moderate scatter in the dispersion-size relation is noted to imply that most regions have not collapsed much since formation, suggesting that processes of turbulent hydrodynamics have played an important role in producing the observed substructures. A lower limit to the size of subcondensations at which their internal motions are no longer supersonic is shown to predict a minimum protostellar mass on the order of a few tenths of a solar mass, while massive protostellar clumps are found to develop complex internal structures, probably leading to the formation of prestellar condensation nuclei. The observed turbulence of molecular clouds is noted to imply lifetimes of less than 10 million years.

Diffusion of Magnetic Field and Removal of Magnetic Flux from Clouds Via Turbulent Reconnection

NASA Astrophysics Data System (ADS)

Santos-Lima, R.; Lazarian, A.; de Gouveia Dal Pino, E. M.; Cho, J.

2010-05-01

The diffusion of astrophysical magnetic fields in conducting fluids in the presence of turbulence depends on whether magnetic fields can change their topology via reconnection in highly conducting media. Recent progress in understanding fast magnetic reconnection in the presence of turbulence reassures that the magnetic field behavior in computer simulations and turbulent astrophysical environments is similar, as far as magnetic reconnection is concerned. This makes it meaningful to perform MHD simulations of turbulent flows in order to understand the diffusion of magnetic field in astrophysical environments. Our studies of magnetic field diffusion in turbulent medium reveal interesting new phenomena. First of all, our three-dimensional MHD simulations initiated with anti-correlating magnetic field and gaseous density exhibit at later times a de-correlation of the magnetic field and density, which corresponds well to the observations of the interstellar media. While earlier studies stressed the role of either ambipolar diffusion or time-dependent turbulent fluctuations for de-correlating magnetic field and density, we get the effect of permanent de-correlation with one fluid code, i.e., without invoking ambipolar diffusion. In addition, in the presence of gravity and turbulence, our three-dimensional simulations show the decrease of the magnetic flux-to-mass ratio as the gaseous density at the center of the gravitational potential increases. We observe this effect both in the situations when we start with equilibrium distributions of gas and magnetic field and when we follow the evolution of collapsing dynamically unstable configurations. Thus, the process of turbulent magnetic field removal should be applicable both to quasi-static subcritical molecular clouds and cores and violently collapsing supercritical entities. The increase of the gravitational potential as well as the magnetization of the gas increases the segregation of the mass and magnetic flux in the saturated final state of the simulations, supporting the notion that the reconnection-enabled diffusivity relaxes the magnetic field + gas system in the gravitational field to its minimal energy state. This effect is expected to play an important role in star formation, from its initial stages of concentrating interstellar gas to the final stages of the accretion to the forming protostar. In addition, we benchmark our codes by studying the heat transfer in magnetized compressible fluids and confirm the high rates of turbulent advection of heat obtained in an earlier study.

Science Questions for the Post-SIRTF and Herschel Era

NASA Technical Reports Server (NTRS)

Werner, Michael

2004-01-01

The contents include the following: 1. SIRTF. Long wavelength surveys planned for SIRTF. Galaxy Discovery Rates for Future Missions. Impact of SIRTF s Improved Resolution at 160um: Resolving the Background. 2. Polarimetry. Submillimeter Polarimetry - The State of Play. Magnetic Vectors Across the Orion Molecular Cloud Core. Neutral and Ionized Molecular Spectral Lines. Variation of Polarization With Wavelength. The Polarization Spectrum. Submillimeter Polarimetry - Looking Ahead. 3.Confusion. Confusion at 500, 600 micron. 4. Extragalactic Science. Do Massive Black Holes and Galaxy Bulges form Together? 5. Galactic Science. Can We See the First Generations of Stars and Metal Formation? The Birth of Planets and the Origins of Life. Spatial Resolution at 100 microns. Far-ir/Sub-mm Transitions of Linear Carbon Clusters. Predicted Spectra of Glycine.

Impact-generated magnetic fields on the Moon : a magnetohydrodynamic numerical investigation

NASA Astrophysics Data System (ADS)

Oran, Rona; Shprits, Yuri; Weiss, Benjamin; Gombosi, Tamas

2015-04-01

Natural remanent magnetization has been identified in lunar rocks, the lunar crust, and a diversity of meteorites. Much of this magnetization is thought to have been produced by cooling a core dynamo mag-netic field. However, the identification of lunar crustal magnetic anomalies at the antipodes of four of the five youngest large (>600 km diameter) impact basins has motivated the alternative hypothesis that the lunar crust could have been magnetized by the impacts. In particular, it has been proposed that highly conducting ionized vapor produced by a basin-forming impact interacts with the ambient solar wind plasma surrounding the Moon to amplify the ambient solar wind magnetic field or any core dynamo field. In this picture, as the ionized vapor cloud expands around the Moon, it pushes and compresses the solar wind plasma into a small region at the antipodal point. The conservation of magnetic flux then leads to an enhanced magnetic field in the compressed plasma. This field can then be recorded as shock remanent magnetization by crustal materials at the antipodal point following the impact of converging basin ejecta. A key requirement for the impact-generated fields hypothesis is that the compressed field be suffi-ciently strong to explain the lunar paleointensities (at least tens of μT) and maintained at the antipodal point for a sufficiently long time (several hours) for the ejecta to arrive and impact the surface. Previous simulations of the expansion of the vapor cloud found that the enhanced field will be strong enough (per-haps reaching hundreds of μT) and will remain at the antipodal site for a sufficiently long time (>1 day) for the arrival of incoming ejecta. However, these studies did not include an explicit calculation of the interaction of the magnetized solar wind plasma with the vapor cloud. Rather, the cloud evolution under the lunar gravity was simulated in the purely hydrodynamic regime. The vapor cloud structure at certain times was used to derive a simplified picture of what the effects would be on an ambient magnetized plasma using general magnetohydrodynamic (MHD) arguments. The solar wind drag acting on the cloud, as well as MHD effects such as field lines stretching and magnetic reconnection were not taken into ac-count. With the advances made in computational MHD models in recent years, we can now revisit these ear-lier important models. Our goal is to perform the first MHD simulations of an impact-generated vapor cloud expanding in the solar wind around the Moon, using BATSRUS, a 3D highly-parallelized versatile MHD code developed at the University of Michigan, in order to self-consistently test the previous estima-tions of the strength and duration of the magnetic field enhancement at the antipodal points. We will con-sider different MHD processes, such as: 1) the finite resistivity of the lunar mantle 2) magnetic diffusion between the solar wind and the initially non-magnetized cloud, 3) magnetic reconnection at the antipode, and 4) viscous drag and the transport of magnetic flux due to solar wind motion, and 4) MHD instabili-ties. This will allow us to systematically examine whether impact-generated fields can indeed be respon-sible for the formation of crustal field enhancements on the Moon.

A quantitative analysis of IRAS maps of molecular clouds

NASA Technical Reports Server (NTRS)

Wiseman, Jennifer J.; Adams, Fred C.

1994-01-01

We present an analysis of IRAS maps of five molecular clouds: Orion, Ophiuchus, Perseus, Taurus, and Lupus. For the classification and description of these astrophysical maps, we use a newly developed technique which considers all maps of a given type to be elements of a pseudometric space. For each physical characteristic of interest, this formal system assigns a distance function (a pseudometric) to the space of all maps: this procedure allows us to measure quantitatively the difference between any two maps and to order the space of all maps. We thus obtain a quantitative classification scheme for molecular clouds. In this present study we use the IRAS continuum maps at 100 and 60 micrometer(s) to produce column density (or optical depth) maps for the five molecular cloud regions given above. For this sample of clouds, we compute the 'output' functions which measure the distribution of density, the distribution of topological components, the self-gravity, and the filamentary nature of the clouds. The results of this work provide a quantitative description of the structure in these molecular cloud regions. We then order the clouds according to the overall environmental 'complexity' of these star-forming regions. Finally, we compare our results with the observed populations of young stellar objects in these clouds and discuss the possible environmental effects on the star-formation process. Our results are consistent with the recently stated conjecture that more massive stars tend to form in more 'complex' environments.

VizieR Online Data Catalog: M33 molecular clouds and young stellar clusters (Corbelli+, 2017)

NASA Astrophysics Data System (ADS)

Corbelli, E.; Braine, J.; Bandiera, R.; Brouillet, N.; Combes, F.; Druard, C.; Gratier, P.; Mata, J.; Schuster, K.; Xilouris, M.; Palla, F.

2017-04-01

Table 5 : Physical parameters for the 566 molecular clouds identified through the IRAM 30m CO J=2-1 survey of the star forming disk of M33. For each cloud the cloud type and the following properties are listed: celestial coordinates, galactocentric radius, cloud deconvolved effective radius and its uncertainty, CO(2-1) line velocity dispersion from CPROPS and its uncertainty, line velocity dispersion from a Gaussian fit, CO luminous mass and its uncertainty, and virial mass from a Gaussian fit. In the last column the identification number of the young stellar cluster candidates associated with the molecular cloud are listed. Notes: We identify up to four young stellar cluster candidates (YSCCs) associated with each molecular cloud and we list them according to the identification number of Sharma et al. (2011, Cat. J/A+A/545/A96) given also in Table 6. Table 6 : Physical parameters for the 630 young stellar cluster candidates identified via their mid-infrared emission in the star forming disk of M33. For each YSCC we list the type of source, the identified number of the molecular clouds associated with it (if any) and the corresponding cloud classes. In addition, for each YSCC we give the celestial coordinates, the bolometric, total infrared, FUV and Halpha luminosities, the estimated mass and age, the visual extinction, the galactocentric radius, the source size, and its flux at 24μm. (2 data files).

1.0 Mm Maps and Radial Density Distributions of Southern Hii/molecular Cloud Complexes

NASA Technical Reports Server (NTRS)

Cheung, L. H.; Frogel, J. A.; Gezar, D. Y.; Hauser, M. G.

1980-01-01

Several 1.0 continuum mapping observations were made of seven southern hemisphere h12/molecular cloud complexes with 65 arcsec resolution. The radial density distribution of the clouds with central luminosity sources was determined observationally. Strong similarities in morphology and general physical conditions were found to exist among all of the southern clouds in the sample.

Turbulence in molecular clouds - A new diagnostic tool to probe their origin

NASA Technical Reports Server (NTRS)

Canuto, V. M.; Battaglia, A.

1985-01-01

A method is presented to uncover the instability responsible for the type of turbulence observed in molecular clouds and the value of the physical parameters of the 'placental medium' from which turbulence originated. The method utilizes the observational relation between velocities and sizes of molecular clouds, together with a recent model for large-scale turbulence (constructed by Canuto and Goldman, 1985).

Carbon Isotope Chemistry in Molecular Clouds

NASA Technical Reports Server (NTRS)

Robertson, Amy N.; Willacy, Karen

2012-01-01

Few details of carbon isotope chemistry are known, especially the chemical processes that occur in astronomical environments like molecular clouds. Observational evidence shows that the C-12/C-13 abundance ratios vary due to the location of the C-13 atom within the molecular structure. The different abundances are a result of the diverse formation pathways that can occur. Modeling can be used to explore the production pathways of carbon molecules in an effort to understand and explain the chemical evolution of molecular clouds.

STAR FORMATION LAWS: THE EFFECTS OF GAS CLOUD SAMPLING

DOE Office of Scientific and Technical Information (OSTI.GOV)

Calzetti, D.; Liu, G.; Koda, J., E-mail: calzetti@astro.umass.edu

Recent observational results indicate that the functional shape of the spatially resolved star formation-molecular gas density relation depends on the spatial scale considered. These results may indicate a fundamental role of sampling effects on scales that are typically only a few times larger than those of the largest molecular clouds. To investigate the impact of this effect, we construct simple models for the distribution of molecular clouds in a typical star-forming spiral galaxy and, assuming a power-law relation between star formation rate (SFR) and cloud mass, explore a range of input parameters. We confirm that the slope and the scattermore » of the simulated SFR-molecular gas surface density relation depend on the size of the sub-galactic region considered, due to stochastic sampling of the molecular cloud mass function, and the effect is larger for steeper relations between SFR and molecular gas. There is a general trend for all slope values to tend to {approx}unity for region sizes larger than 1-2 kpc, irrespective of the input SFR-cloud relation. The region size of 1-2 kpc corresponds to the area where the cloud mass function becomes fully sampled. We quantify the effects of selection biases in data tracing the SFR, either as thresholds (i.e., clouds smaller than a given mass value do not form stars) or as backgrounds (e.g., diffuse emission unrelated to current star formation is counted toward the SFR). Apparently discordant observational results are brought into agreement via this simple model, and the comparison of our simulations with data for a few galaxies supports a steep (>1) power-law index between SFR and molecular gas.« less

Magnetic field structure around cores with very low luminosity objects

NASA Astrophysics Data System (ADS)

Soam, A.; Maheswar, G.; Lee, Chang Won; Dib, Sami; Bhatt, H. C.; Tamura, Motohide; Kim, Gwanjeong

2015-01-01

Aims: We carried out optical polarimetry of five dense cores, (IRAM 04191, L1521F, L328, L673-7, and L1014) which are found to harbour very low luminosity objects (VeLLOs; Lint ≲ 0.1 L⊙). This study was conducted mainly to understand the role played by the magnetic field in the formation of very low and substellar mass range objects. Methods: Light from the stars, while passing through the dust grains that are aligned with their short axis parallel to an external magnetic field, becomes linearly polarised. The polarisation position angles measured for the stars can provide the plane-of-the sky magnetic field orientation. Because the light in the optical wavelength range is most efficiently polarised by the dust grains typically found at the outer layers of the molecular clouds, optical polarimetry mostly traces the magnetic field orientation of the core envelope. Results: The polarisation observations of stars projected on IRAM 04191, L328, L673-7, and L1014 were obtained in the R-band and those of L1521F were obtained in the V-band. The angular offsets between the envelope magnetic field direction (inferred from optical polarisation measurements) and the outflow position angles from the VeLLOs in IRAM 04191, L1521F, L328, L673-7, and L1014 are found to be 84°, 53°, 24°, 08°, and 15°, respectively. The mean value of the offsets for all the five clouds is ~ 37°. If we exclude IRAM 04191, the mean value reduces to become ~ 25°. In IRAM 04191, the offset between the projected envelope and the inner magnetic field (inferred from the submillimetre data obtained using SCUBA-POL) is found to be ~ 68°. The inner magnetic field, however, is found to be nearly aligned with the projected position angles of the minor axis, the rotation axis of the cloud, and the outflow from the IRAM 04191-IRS. We discuss a possible explanation for the nearly perpendicular orientation between the envelope and core scale magnetic fields in IRAM 04191. The angular offset between the envelope magnetic field direction and the minor axis of IRAM 04191, L1521F, L673-7, and L1014 are 82°, 60°, 47°, and 55°, respectively. The mean value of the offsets between the envelope magnetic field and the minor axis position angles for the four cores is found to be ~ 60°. Conclusions: The results obtained from our study on the limited sample of five cores with VeLLOs show that the outflows in three of them tend to nearly align with the envelope magnetic field. Table 4 is available in electronic form at http://www.aanda.org

Cloud Structure of Galactic OB Cluster-forming Regions from Combining Ground- and Space-based Bolometric Observations

NASA Astrophysics Data System (ADS)

Lin, Yuxin; Liu, Hauyu Baobab; Li, Di; Zhang, Zhi-Yu; Ginsburg, Adam; Pineda, Jaime E.; Qian, Lei; Galván-Madrid, Roberto; McLeod, Anna Faye; Rosolowsky, Erik; Dale, James E.; Immer, Katharina; Koch, Eric; Longmore, Steve; Walker, Daniel; Testi, Leonardo

2016-09-01

We have developed an iterative procedure to systematically combine the millimeter and submillimeter images of OB cluster-forming molecular clouds, which were taken by ground-based (CSO, JCMT, APEX, and IRAM-30 m) and space telescopes (Herschel and Planck). For the seven luminous (L\\gt {10}6 L ⊙) Galactic OB cluster-forming molecular clouds selected for our analyses, namely W49A, W43-Main, W43-South, W33, G10.6-0.4, G10.2-0.3, and G10.3-0.1, we have performed single-component, modified blackbody fits to each pixel of the combined (sub)millimeter images, and the Herschel PACS and SPIRE images at shorter wavelengths. The ˜10″ resolution dust column density and temperature maps of these sources revealed dramatically different morphologies, indicating very different modes of OB cluster-formation, or parent molecular cloud structures in different evolutionary stages. The molecular clouds W49A, W33, and G10.6-0.4 show centrally concentrated massive molecular clumps that are connected with approximately radially orientated molecular gas filaments. The W43-Main and W43-South molecular cloud complexes, which are located at the intersection of the Galactic near 3 kpc (or Scutum) arm and the Galactic bar, show a widely scattered distribution of dense molecular clumps/cores over the observed ˜10 pc spatial scale. The relatively evolved sources G10.2-0.3 and G10.3-0.1 appear to be affected by stellar feedback, and show a complicated cloud morphology embedded with abundant dense molecular clumps/cores. We find that with the high angular resolution we achieved, our visual classification of cloud morphology can be linked to the systematically derived statistical quantities (I.e., the enclosed mass profile, the column density probability distribution function (N-PDF), the two-point correlation function of column density, and the probability distribution function of clump/core separations). In particular, the massive molecular gas clumps located at the center of G10.6-0.4 and W49A, which contribute to a considerable fraction of their overall cloud masses, may be special OB cluster-forming environments as a direct consequence of global cloud collapse. These centralized massive molecular gas clumps also uniquely occupy much higher column densities than what is determined by the overall fit of power-law N-PDF. We have made efforts to archive the derived statistical quantities of individual target sources, to permit comparisons with theoretical frameworks, numerical simulations, and other observations in the future.

Speeding Clouds May Reveal Invisible Black Holes

NASA Astrophysics Data System (ADS)

Kohler, Susanna

2017-07-01

Several small, speeding clouds have been discovered at the center of our galaxy. A new study suggests that these unusual objects may reveal the lurking presence of inactive black holes.Peculiar Cloudsa) Velocity-integrated intensity map showing the location of the two high-velocity compact clouds, HCN0.0090.044 and HCN0.0850.094, in the context of larger molecular clouds. b) and c) Latitude-velocity and longitude-velocity maps for HCN0.0090.044 and HCN0.0850.094, respectively. d) and e) spectra for the two compacts clouds, respectively. Click for a closer look. [Takekawa et al. 2017]Sgr A*, the supermassive black hole marking the center of our galaxy, is surrounded by a region roughly 650 light-years across known as the Central Molecular Zone. This area at the heart of our galaxy is filled with large amounts of warm, dense molecular gas that has a complex distribution and turbulent kinematics.Several peculiar gas clouds have been discovered within the Central Molecular Zone within the past two decades. These clouds, dubbed high-velocity compact clouds, are characterized by their compact sizes and extremely broad velocity widths.What created this mysterious population of energetic clouds? The recent discovery of two new high-velocity compact clouds, reported on in a paper led by Shunya Takekawa (Keio University, Japan), may help us to answer this question.Two More to the CountUsing the James Clerk Maxwell Telescope in Hawaii, Takekawa and collaborators detected the small clouds near the circumnuclear disk at the centermost part of our galaxy. These two clouds have velocity spreads of -80 to -20 km/s and -80 to 0 km/s and compact sizes of just over 1 light-year. The clouds similar appearances and physical properties suggest that they may both have been formed by the same process.Takekawa and collaborators explore and discard several possible origins for these clouds, such as outflows from massive protostars (no massive, luminous stars have been detected affiliated with these clouds), interaction with supernova remnants (no supernova remnants have been detected toward the clouds), and cloudcloud collisions (such collisions leave other signs, like cavities in the parent cloud, which are not detected here).Masses and velocities of black holes that could create the two high-velocity compact clouds fall above the red and blue lines here. [Takekawa et al. 2017]Revealed on the PlungeAs an alternative explanation, Takekawa and collaborators propose that these two small,speeding cloudswere each created when a massive compact object plunged into a nearby molecular cloud. Since we dont seeany luminous stellar counterparts to the high-velocity compact clouds, this suggests that the responsibleobjects were invisible black holes. As each black hole tore through a molecular cloud, it dragged some of the clouds gas along behind it to form the high-velocity compact cloud.Does this explanation make sense statistically? The authors point out that the number of black holes predicted to silently lurk in the central 30 light-years of the Milky Way is around 10,000. This makes it entirely plausible that we could have caught sight of two of them as they revealed their presence while plunging through molecular clouds.If the authors interpretation is correct, then high-velocity compact clouds provide an excellent opportunity: we can search for these speeding bodiesto potentially discover inactive black holes that would otherwise go undetected.CitationShunya Takekawa et al 2017 ApJL 843 L11. doi:10.3847/2041-8213/aa79ee

Compression and ablation of the photo-irradiated molecular cloud the Orion Bar.

PubMed

Goicoechea, Javier R; Pety, Jérôme; Cuadrado, Sara; Cernicharo, José; Chapillon, Edwige; Fuente, Asunción; Gerin, Maryvonne; Joblin, Christine; Marcelino, Nuria; Pilleri, Paolo

2016-09-08

The Orion Bar is the archetypal edge-on molecular cloud surface illuminated by strong ultraviolet radiation from nearby massive stars. Our relative closeness to the Orion nebula (about 1,350 light years away from Earth) means that we can study the effects of stellar feedback on the parental cloud in detail. Visible-light observations of the Orion Bar show that the transition between the hot ionized gas and the warm neutral atomic gas (the ionization front) is spatially well separated from the transition between atomic and molecular gas (the dissociation front), by about 15 arcseconds or 6,200 astronomical units (one astronomical unit is the Earth-Sun distance). Static equilibrium models used to interpret previous far-infrared and radio observations of the neutral gas in the Orion Bar (typically at 10-20 arcsecond resolution) predict an inhomogeneous cloud structure comprised of dense clumps embedded in a lower-density extended gas component. Here we report one-arcsecond-resolution millimetre-wave images that allow us to resolve the molecular cloud surface. In contrast to stationary model predictions, there is no appreciable offset between the peak of the H 2 vibrational emission (delineating the H/H 2 transition) and the edge of the observed CO and HCO + emission. This implies that the H/H 2 and C + /C/CO transition zones are very close. We find a fragmented ridge of high-density substructures, photoablative gas flows and instabilities at the molecular cloud surface. The results suggest that the cloud edge has been compressed by a high-pressure wave that is moving into the molecular cloud, demonstrating that dynamical and non-equilibrium effects are important for the cloud evolution.

Compression and ablation of the photo-irradiated molecular cloud the Orion Bar

NASA Astrophysics Data System (ADS)

Goicoechea, Javier R.; Pety, Jérôme; Cuadrado, Sara; Cernicharo, José; Chapillon, Edwige; Fuente, Asunción; Gerin, Maryvonne; Joblin, Christine; Marcelino, Nuria; Pilleri, Paolo

2016-09-01

The Orion Bar is the archetypal edge-on molecular cloud surface illuminated by strong ultraviolet radiation from nearby massive stars. Our relative closeness to the Orion nebula (about 1,350 light years away from Earth) means that we can study the effects of stellar feedback on the parental cloud in detail. Visible-light observations of the Orion Bar show that the transition between the hot ionized gas and the warm neutral atomic gas (the ionization front) is spatially well separated from the transition between atomic and molecular gas (the dissociation front), by about 15 arcseconds or 6,200 astronomical units (one astronomical unit is the Earth-Sun distance). Static equilibrium models used to interpret previous far-infrared and radio observations of the neutral gas in the Orion Bar (typically at 10-20 arcsecond resolution) predict an inhomogeneous cloud structure comprised of dense clumps embedded in a lower-density extended gas component. Here we report one-arcsecond-resolution millimetre-wave images that allow us to resolve the molecular cloud surface. In contrast to stationary model predictions, there is no appreciable offset between the peak of the H2 vibrational emission (delineating the H/H2 transition) and the edge of the observed CO and HCO+ emission. This implies that the H/H2 and C+/C/CO transition zones are very close. We find a fragmented ridge of high-density substructures, photoablative gas flows and instabilities at the molecular cloud surface. The results suggest that the cloud edge has been compressed by a high-pressure wave that is moving into the molecular cloud, demonstrating that dynamical and non-equilibrium effects are important for the cloud evolution.

COLLISIONS BETWEEN DARK MATTER CONFINED HIGH VELOCITY CLOUDS AND MAGNETIZED GALACTIC DISKS: THE SMITH CLOUD

DOE Office of Scientific and Technical Information (OSTI.GOV)

Galyardt, Jason; Shelton, Robin L., E-mail: jeg@uga.edu, E-mail: rls@physast.uga.edu

2016-01-01

The Galaxy’s population of High Velocity Clouds (HVCs) may include a subpopulation that is confined by dark matter minihalos and falling toward the Galactic disk. We present the first magnetohydrodynamic simulational study of dark-matter-dominated HVCs colliding with a weakly magnetized galactic disk. Our HVCs have baryonic masses of 5 × 10{sup 6}M{sub ⊙} and dark matter minihalo masses of 0, 3 × 10{sup 8}, or 1 × 10{sup 9} M{sub ⊙}. They are modeled on the Smith Cloud, which is said to have collided with the disk 70 Myr ago. We find that, in all cases, the cloud’s collision with the galactic disk creates a hole inmore » the disk, completely disperses the cloud, and forms a bubble-shaped structure on the far side of the disk. In contrast, when present, the dark matter minihalo continues unimpeded along its trajectory. Later, as the minihalo passes through the bubble structure and galactic halo, it accretes up to 6.0 × 10{sup 5} M{sub ⊙} in baryonic material, depending on the strengths of the magnetic field and minihalo gravity. These simulations suggest that if the Smith Cloud is associated with a dark matter minihalo and collided with the Galactic disk, the minihalo has accreted the observed gas. However, if the Smith Cloud is dark-matter-free, it is on its first approach toward the disk. These simulations also suggest that the dark matter is most concentrated either at the head of the cloud or near the cloud, depending upon the strength of the magnetic field, a point that could inform indirect dark matter searches.« less

Resolving the substructure of molecular clouds in the LMC

NASA Astrophysics Data System (ADS)

Wong, Tony; Hughes, Annie; Tokuda, Kazuki; Indebetouw, Remy; Wojciechowski, Evan; Bandurski, Jeffrey; MC3 Collaboration

2018-01-01

We present recent wide-field CO and 13CO mapping of giant molecular clouds in the Large Magellanic Cloud with ALMA. Our sample exhibits diverse star-formation properties, and reveals comparably diverse molecular cloud properties including surface density and velocity dispersion at a given scale. We first present the results of a recent study comparing two GMCs at the extreme ends of the star formation activity spectrum. Our quiescent cloud exhibits 10 times lower surface density and 5 times lower velocity dispersion than the active 30 Doradus cloud, yet in both clouds we find a wide range of line widths at the smallest resolved scales, spanning nearly the full range of line widths seen at all scales. This suggests an important role for feedback on sub-parsec scales, while the energetics on larger scales are dominated by clump-to-clump relative velocities. We then extend our analysis to four additional clouds that exhibit intermediate levels of star formation activity.

The wavelet transform as an analysis tool for structure identification in molecular clouds

NASA Astrophysics Data System (ADS)

Gill, Arnold Gerald

1993-01-01

Of the many methods used to attempt to understand the complex structure of giant molecular clouds, perhaps the most commonly used are the autocorrelation functions (ACF), the structure function, and the power spectrum. However, these do not give unique interpretations of structure, as is shown by explicit examples compared to the Taurus Molecular Complex. Thus, another, independent method of analysis is indicated. Here, the wavelet transform is presented, a relatively new technique less than 10 years old. It can be thought of as a band-pass filter that identifies structures of specific sizes. In addition, its mathematical properties allow it to be used to identify fractal structures and accurately identify the scaling exponent. This is shown by the wavelet transform identifying the fractal dimension of a hierarchical rain cloud model first proposed by Frisch et al. (1978). A wavelet analysis is then carried out for a range of astronomical CO data, including the clouds Orion A and B and NGC 7538 (in (12)CO) and Orion A and B, Mon R2, and L1551 (in (13)CO). The data analyzed consists of the velocities of the fitted Gaussians to the individual spectra, the halfwidths and amplitude of these Gaussians, and the total area of the spectral line. For most of the clouds investigated, each of these data types showed a very high degree of scaling coherence over a wide range of scales, from down at the beam spacing up to the full size of the cloud. The analysis carried out uses both the scaling and structure identification strengths of the wavelet transform The fragmentation parameters used by Scalo (1985) and the parameters of the geometric molecular cloud description introduced by Henriksen (1986) are calculated for each cloud. These results are all consistent with previous observations of these and other molecular clouds, though they are obtained individually for each cloud investigated. It is found that the uncertainties are of a magnitude that the differentiation of various theories of molecular cloud structure is not possible. It is noted that the effects of projection and superposition strongly affect the values of some of these parameters, thus hampering a thorough understanding of the underlying physics. The strengths and weaknesses of the wavelet transform in the analysis of molecular cloud data are presented, as well as directions for future work.

The XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST)

NASA Astrophysics Data System (ADS)

Feigelson, Eric; Guedel, M.

2007-12-01

The XMM-Newton Extended Survey of the Taurus Molecular Cloud is an exceptionally large and growing X-ray survey of the Taurus Molecular Cloud (TMC). Now comprising 31 1/2-degree diameter fields, observed with the three XMM-Newton EPIC cameras. High-resolution spectroscopy has been obtained for about ten T Tauri stars (TTS) with the RGS instruments, and the Optical Monitor secured an optical/UV survey. XEST detects essentially the entire surveyed TTS population of the TMC in X-rays including about half of the observed (8/16) brown dwarfs and Class I protostars (8/20). Several new candidate members are identified. The X-ray luminosity (LX) of TTS shows related correlations with both stellar bolometric luminosity and mass. Classical TTS show suppressed X-ray output in the CCD band by a factor of about 2. These statistical results confirm results from other star formation regions. Different from previous reports on TMC, XEST identifies no activity-rotation relation. Brown dwarfs are found to follow trends set by TTS, both for accreting and non-accreting objects. But a decrease of the fractional luminosity, LX/Lbol, is seen with decreasing mass indicating weakened heating efficiency in the substellar domain. XEST reports five members of the class of "Two-Absorber X-Ray" (TAX) sources which reveal a double-peaked spectrum originating from two unrelated sources with different absorption column densities. The softer emission is thought to be related to jets, as explicitly seen in DG Tau. RGS spectroscopy shows a systematic "X-ray soft excess" in classical TTS, suggesting excessive cool (1-2 MK) plasma due to accretion, although the excess seems to correlate with magnetic activity as well. XEST has been supported by the Space Science Institute (Bern/Switz.).

Boundary conditions for the paleoenvironment: Chemical and physical processes in the pre-solar nebula. [molecular clouds, interstellar matter, and abundance

NASA Technical Reports Server (NTRS)

Irvine, W. M.; Schloerb, F. P.

1985-01-01

Two additional hyperfine components of the interstellar radical C3H were detected. In addition, methanol was discovered in interstellar clouds. The abundance of HCCN and various chemical isomers in molecular clouds was investigated.

Formation of compact HII regions possibly triggered by cloud-cloud collision

NASA Astrophysics Data System (ADS)

Ohama, Akio; Torii, Kazufumi; Hasegawa, Keisuke; Fukui, Yasuo

2015-08-01

Compact HII regions are ionized by young high-mass star(s) and ~1000 compact HII regions are cataloged in the Galaxy (Urquhart et al. MNRAS 443, 1555-1586 (2014)). Compact HII regions are one of the major populations of Galactic HII regions. The molecular environments around compact HII regions are however not well understood due to lack of extensive molecular surveys. In order to better understand formation of exciting stars and compact HII regions, we have carried out a systematic study of molecular clouds toward compact HII regions by using the 12CO datasets obtained with the JCMT and NANTEN2 telescopes for l = 10 - 56, and present here the first results.In one of the present samples, RCW166, we have discovered that the HII region is associated with two molecular clouds whose velocity separation is ~10 km s-1 the two clouds show complimentary spatial distributions, where one of the clouds have a cavity-like distribution apparently embracing the other. We present an interpretation that the two clouds collided with each other and the cavity-like distribution represents a hole created by the collision in the larger cloud as modeled by Habe and Ohta (1992). Similar molecular distributions are often found in the other compact HII regions in the present study.A recent study by Torii et al. (2015, arXiv:1503.00070) indicates that the Spitzer bubble RCW120 was formed by cloud-cloud collision where the inside of the cavity is fully ionized by the exiting stars. RCW166, on the other hand, shows that only a small part of the cavity, the compact HII region, is ionized. We thus suggest that RCW166 represents an evolutionary stage corresponding to an earlier phase of RCW120 in the collision scenario.

MAGNETIZED GAS IN THE SMITH HIGH VELOCITY CLOUD

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hill, Alex S.; McClure-Griffiths, Naomi M.; Mao, S. A.

2013-11-01

We report the first detection of magnetic fields associated with the Smith High Velocity Cloud. We use a catalog of Faraday rotation measures toward extragalactic radio sources behind the Smith Cloud, new H I observations from the Robert C. Byrd Green Bank Telescope, and a spectroscopic map of Hα from the Wisconsin H-Alpha Mapper Northern Sky Survey. There are enhancements in rotation measure (RM) of ≈100 rad m{sup –2} which are generally well correlated with decelerated Hα emission. We estimate a lower limit on the line-of-sight component of the field of ≈8 μG along a decelerated filament; this is amore » lower limit due to our assumptions about the geometry. No RM excess is evident in sightlines dominated by H I or Hα at the velocity of the Smith Cloud. The smooth Hα morphology of the emission at the Smith Cloud velocity suggests photoionization by the Galactic ionizing radiation field as the dominant ionization mechanism, while the filamentary morphology and high (≈1 Rayleigh) Hα intensity of the lower-velocity magnetized ionized gas suggests an ionization process associated with shocks due to interaction with the Galactic interstellar medium. The presence of the magnetic field may contribute to the survival of high velocity clouds like the Smith Cloud as they move from the Galactic halo to the disk. We expect these data to provide a test for magnetohydrodynamic simulations of infalling gas.« less

Effect of magnetic field inhomogeneity on ion cyclotron motion coherence at high magnetic field.

PubMed

Vladimirov, Gleb; Kostyukevich, Yury; Hendrickson, Christopher L; Blakney, Greg T; Nikolaev, Eugene

2015-01-01

A three-dimensional code based on the particle-in-cell algorithm modified to account for the inhomogeneity of the magnetic field was applied to determine the effect of Z(1), Z(2), Z(3), Z(4), X, Y, ZX, ZY, XZ(2) YZ(2), XY and X(2)-Y(2) components of an orthogonal magnetic field expansion on ion motion during detection in an FT-ICR cell. Simulations were performed for magnetic field strengths of 4.7, 7, 14.5 and 21 Tesla, including experimentally determined magnetic field spatial distributions for existing 4.7 T and 14.5 T magnets. The effect of magnetic field inhomogeneity on ion cloud stabilization ("ion condensation") at high numbers of ions was investigated by direct simulations of individual ion trajectories. Z(1), Z(2), Z(3) and Z(4) components have the largest effect (especially Z(1)) on ion cloud stability. Higher magnetic field strength and lower m/z demand higher relative magnetic field homogeneity to maintain cloud coherence for a fixed time period. The dependence of mass resolving power upper limit on Z(1) inhomogeneity is evaluated for different magnetic fields and m/z. The results serve to set the homogeneity requirements for various orthogonal magnetic field components (shims) for future FT-ICR magnet design.

High Energy Particle Acceleration and Turbulent Magnetic Field Amplification in Shell Type Supernova Remnants. Degree awarded by Minnesota Univ.

NASA Technical Reports Server (NTRS)

Keohane, Jonathan Wilmore

1998-01-01

Thesis submitted to the faculty of the Graduate School of the University of Minnesota in partial fulfillment of the requirements for the degree of Doctor of Philosophy. Part I discusses the spatial correlation between the x-ray and radio morphologies of Cas A, and in the process address: the effect of inhomogeneous absorption on the apparent x-ray morphology, the interaction between the SNR and a molecular cloud, and the rapid move toward equipartition between the magnetic and gas energy densities. Discussions of the x-ray./radio correlation continues in Chapter 5, where we present a new, deep, ROSAT HRI image of Cas A. Chapter 7 presents ASCA spectra, with non-thermal spectral fits for 13 of the youngest SNRs in the Galaxy.

High Energy Particle Acceleration and Turbulent Magnetic Field Amplification in Shell Type Supernova Remnants

NASA Astrophysics Data System (ADS)

Keohane, Jonathan Wilmore

1998-07-01

Thesis submitted to the faculty of the Graduate School of the University of Minnesota in partial fulfillment of the requirements for the degree of Doctor of Philosophy. Part I discusses the spatial correlation between the x-ray and radio morphologies of Cas A, and in the process address: the effect of inhomogeneous absorption on the apparent x-ray morphology, the interaction between the SNR and a molecular cloud, and the rapid move toward equipartition between the magnetic and gas energy densities. Discussions of the x-ray./radio correlation continues in Chapter 5, where we present a new, deep, ROSAT HRI image of Cas A. Chapter 7 presents ASCA spectra, with non-thermal spectral fits for 13 of the youngest SNRs in the Galaxy.

Water in dense molecular clouds

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wannier, P.G.; Kuiper, T.B.H.; Frerking, M.A.

1991-08-01

The G.P. Kuiper Airborne Observatory (KAO) was used to make initial observations of the half-millimeter ground-state transition of water in seven giant molecular clouds and in two late-type stars. No significant detections were made, and the resulting upper limits are significantly below those expected from other, indirect observations and from several theoretical models. The implied interstellar H2O/CO abundance is less than 0.003 in the cores of three giant molecular clouds. This value is less than expected from cloud chemistry models and also than estimates based on HDO and H3O(+) observations. 78 refs.

Peering Inside the Pillars of Creation

NASA Astrophysics Data System (ADS)

Kohler, Susanna

2018-06-01

On 1 April 1995, Hubble captured one of its most well-known images: a stunning photo of towering features known as the Pillars of Creation, located in the Eagle Nebula just 7,000 light-years away. A new study explores how these iconic columns are influenced by the magnetic fields within them.Pillars from ShocksAn illustrative figure of the BISTRO magnetic-field vectors observed in the Pillars of Creation, overlaid on a Hubble composite of the pillars. [Pattle et al. 2018]In the Hubble image, we see the result of young, hot stars that have driven a photoionization shock into the cloud around them, forming complex structures in the dense gas at the shock interfaces. These structures in this case, dense columns of neutral gas and dust are then bombarded with hot radiation from the young stars, giving the structures a misty, ethereal look as they photoevaporate.Though we have a rough picture, the specifics of how the Pillars of Creation were formed and how they evolve in this harsh radiation environment arent yet fully understood. In particular, the role of magnetic fields in shaping and sustaining these pillars is poorly constrained, both observationally and theoretically.To address this problem, a team of scientists led by Kate Pattle (University of Central Lancashire, UK and National Tsing Hua University, Taiwan), has now made the first direct observations of the magnetic-field morphology within the Pillars of Creation.The authors proposed formation scenario: a) an ionization front approaches an overdensity in the molecular gas, b) the front is slowed at the overdensity, causing the magnetic field lines to bend, c) the compressed magnetic field supports the pillar against radial collapse, but cant support against longitudinal erosion. [Adapted from Pattle et al. 2018]Observing FieldsPattle and collaborators imaged the pillars as a part of the B-Fields in Star-Forming Region Observations (BISTRO) project, which uses a camera and polarimeter mounted on the James Clerk Maxwell Telescope in Hawaii. The high-resolution, submillimeter-wavelength polarimetric observations allowed the team to measure the orientations of the magnetic fields within the pillars.Pattle and collaborators found that the magnetic fields inside the Pillars of Creation are actually quite organized: they generally run along the length of the pillars, perpendicular to and decoupled from the field in the surrounding cloud. The authors use their observations to estimate the strength of the fields: roughly 170320 G in the pillars.Magnetic SupportWhat do these results tell us? First, the strength of the fields is consistent with a formation scenario in which very weakly magnetized gas was compressed to form columns. The authors propose that the Pillars of Creation were formed when an ionization front driven by radiation from nearby young, hot stars encountered a dense clump as it moved through the cloud of molecular gas. The overdensity slowed the front, causing the magnetic field to bend as the surrounding gas moved. The compressed magnetic field then supported the resulting column from collapse.Pattle and collaborators argue that the magnetic fields in the Pillars of Creation are supporting the pillars radially against collapse even now. They may also be preventing the pillar ends from breaking off into disconnected clumps known as cometary globules, a process that could eventually disintegrate the pillars.So whats BISTRO up to now? The project is continuing to survey magnetic fields in the dense gas of other nearby high-mass star-forming regions. This may help confirm the results found for the Pillars of Creation, bringing us another step closer to understanding how magnetic fields influence the some of the striking features that Hubble and other telescopes have revealed in our astronomical backyard.CitationKate Pattle et al 2018 ApJL 860 L6. doi:10.3847/2041-8213/aac771

The Abundance of Molecular Hydrogen and Its Correlation with Midplane Pressure in Galaxies: Non-equilibrium, Turbulent, Chemical Models

NASA Astrophysics Data System (ADS)

Mac Low, Mordecai-Mark; Glover, Simon C. O.

2012-02-01

Observations of spiral galaxies show a strong linear correlation between the ratio of molecular to atomic hydrogen surface density R mol and midplane pressure. To explain this, we simulate three-dimensional, magnetized turbulence, including simplified treatments of non-equilibrium chemistry and the propagation of dissociating radiation, to follow the formation of H2 from cold atomic gas. The formation timescale for H2 is sufficiently long that equilibrium is not reached within the 20-30 Myr lifetimes of molecular clouds. The equilibrium balance between radiative dissociation and H2 formation on dust grains fails to predict the time-dependent molecular fractions we find. A simple, time-dependent model of H2 formation can reproduce the gross behavior, although turbulent density perturbations increase molecular fractions by a factor of few above it. In contradiction to equilibrium models, radiative dissociation of molecules plays little role in our model for diffuse radiation fields with strengths less than 10 times that of the solar neighborhood, because of the effective self-shielding of H2. The observed correlation of R mol with pressure corresponds to a correlation with local gas density if the effective temperature in the cold neutral medium of galactic disks is roughly constant. We indeed find such a correlation of R mol with density. If we examine the value of R mol in our local models after a free-fall time at their average density, as expected for models of molecular cloud formation by large-scale gravitational instability, our models reproduce the observed correlation over more than an order-of-magnitude range in density.

The Nonisothermal Stage of Magnetic Star Formation. I. Formulation of the Problem and Method of Solution

NASA Astrophysics Data System (ADS)

Kunz, Matthew W.; Mouschovias, Telemachos Ch.

2009-03-01

We formulate the problem of the formation and subsequent evolution of fragments (or cores) in magnetically supported, self-gravitating molecular clouds in two spatial dimensions. The six-fluid (neutrals, electrons, molecular and atomic ions, positively charged, negatively charged, and neutral grains) physical system is governed by the radiation, nonideal magnetohydrodynamic equations. The magnetic flux is not assumed to be frozen in any of the charged species. Its evolution is determined by a newly derived generalized Ohm's law, which accounts for the contributions of both elastic and inelastic collisions to ambipolar diffusion and Ohmic dissipation. The species abundances are calculated using an extensive chemical-equilibrium network. Both MRN and uniform grain size distributions are considered. The thermal evolution of the protostellar core and its effect on the dynamics are followed by employing the gray flux-limited diffusion approximation. Realistic temperature-dependent grain opacities are used that account for a variety of grain compositions. We have augmented the publicly available Zeus-MP code to take into consideration all these effects and have modified several of its algorithms to improve convergence, accuracy, and efficiency. Results of magnetic star formation simulations that accurately track the evolution of a protostellar fragment from a density sime103 cm-3 to a density sime1015 cm-3, while rigorously accounting for both nonideal MHD processes and radiative transfer, are presented in a separate paper.

Laboratory Investigations of the Physical and Optical Properties of the Analogs of Individual Cosmic Dust Grains

NASA Technical Reports Server (NTRS)

Abbas, M. M.; Tankosic, D.; Craven, P. D.; Spann, J. F.; LeClair, A.; West, E. A.

2005-01-01

Microdsub-micron size cosmic dust grains play an important role in the physical and dynamical process in the galaxy, the interstellar medium, and the interplanetary and planetary environments. The dust grains in various astrophysical environments are generally charged by a variety of mechanisms that include collisional process with electrons and ions, and photoelectric emissions with UV radiation. The photoelectric emission process is believed to be the dominant process in many astrophysical environments with nearby UV sources, such as the interstellar medium, diffuse clouds, the outer regions of the dense molecular clouds, interplanetary medium, dust in planetary environments and rings, cometary tails, etc. Also, the processes and mechanisms involved in the rotation and alignment of interstellar dust grains are of great interest in view of the polarization of observed starlight as a probe for evaluation of the galactic magnetic field.

Modeling Jet and Outflow Feedback during Star Cluster Formation

NASA Astrophysics Data System (ADS)

Federrath, Christoph; Schrön, Martin; Banerjee, Robi; Klessen, Ralf S.

2014-08-01

Powerful jets and outflows are launched from the protostellar disks around newborn stars. These outflows carry enough mass and momentum to transform the structure of their parent molecular cloud and to potentially control star formation itself. Despite their importance, we have not been able to fully quantify the impact of jets and outflows during the formation of a star cluster. The main problem lies in limited computing power. We would have to resolve the magnetic jet-launching mechanism close to the protostar and at the same time follow the evolution of a parsec-size cloud for a million years. Current computer power and codes fall orders of magnitude short of achieving this. In order to overcome this problem, we implement a subgrid-scale (SGS) model for launching jets and outflows, which demonstrably converges and reproduces the mass, linear and angular momentum transfer, and the speed of real jets, with ~1000 times lower resolution than would be required without the SGS model. We apply the new SGS model to turbulent, magnetized star cluster formation and show that jets and outflows (1) eject about one-fourth of their parent molecular clump in high-speed jets, quickly reaching distances of more than a parsec, (2) reduce the star formation rate by about a factor of two, and (3) lead to the formation of ~1.5 times as many stars compared to the no-outflow case. Most importantly, we find that jets and outflows reduce the average star mass by a factor of ~ three and may thus be essential for understanding the characteristic mass of the stellar initial mass function.

EMBEDDED CLUSTERS IN THE LARGE MAGELLANIC CLOUD USING THE VISTA MAGELLANIC CLOUDS SURVEY

DOE Office of Scientific and Technical Information (OSTI.GOV)

Romita, Krista; Lada, Elizabeth; Cioni, Maria-Rosa, E-mail: k.a.romita@ufl.edu, E-mail: elada@ufl.edu, E-mail: mcioni@aip.de

We present initial results of the first large-scale survey of embedded star clusters in molecular clouds in the Large Magellanic Cloud (LMC) using near-infrared imaging from the Visible and Infrared Survey Telescope for Astronomy Magellanic Clouds Survey. We explored a ∼1.65 deg{sup 2} area of the LMC, which contains the well-known star-forming region 30 Doradus as well as ∼14% of the galaxy’s CO clouds, and identified 67 embedded cluster candidates, 45 of which are newly discovered as clusters. We have determined the sizes, luminosities, and masses for these embedded clusters, examined the star formation rates (SFRs) of their corresponding molecularmore » clouds, and made a comparison between the LMC and the Milky Way. Our preliminary results indicate that embedded clusters in the LMC are generally larger, more luminous, and more massive than those in the local Milky Way. We also find that the surface densities of both embedded clusters and molecular clouds is ∼3 times higher than in our local environment, the embedded cluster mass surface density is ∼40 times higher, the SFR is ∼20 times higher, and the star formation efficiency is ∼10 times higher. Despite these differences, the SFRs of the LMC molecular clouds are consistent with the SFR scaling law presented in Lada et al. This consistency indicates that while the conditions of embedded cluster formation may vary between environments, the overall process within molecular clouds may be universal.« less

Probing a dusty magnetized plasma with self-excited dust-density waves

NASA Astrophysics Data System (ADS)

Tadsen, Benjamin; Greiner, Franko; Piel, Alexander

2018-03-01

A cloud of nanodust particles is created in a reactive argon-acetylene plasma. It is then transformed into a dusty magnetized argon plasma. Plasma parameters are obtained with the dust-density wave diagnostic introduced by Tadsen et al. [Phys. Plasmas 22, 113701 (2015), 10.1063/1.4934927]. A change from an open to a cylindrically enclosed nanodust cloud, which was observed earlier, can now be explained by a stronger electric confinement if a vertical magnetic field is present. Using two-dimensional extinction measurements and the inverse Abel transform to determine the dust density, a redistribution of the dust with increasing magnetic induction is found. The dust-density profile changes from being peaked around the central void to being peaked at an outer torus ring resulting in a hollow profile. As the plasma parameters cannot explain this behavior, we propose a rotation of the nanodust cloud in the magnetized plasma as the origin of the modified profile.

Angular momentum of the N2H+ cores in the Orion A cloud

NASA Astrophysics Data System (ADS)

Tatematsu, Ken'ichi; Ohashi, Satoshi; Sanhueza, Patricio; Nguyen Luong, Quang; Umemoto, Tomofumi; Mizuno, Norikazu

2016-04-01

We have analyzed the angular momentum of the molecular cloud cores in the Orion A giant molecular cloud observed in the N2H+ J = 1-0 line with the Nobeyama 45 m radio telescope. We have measured the velocity gradient using position-velocity diagrams passing through core centers, and made sinusoidal fits against the position angle. Twenty-seven out of 34 N2H+ cores allowed us to measure the velocity gradient without serious confusion. The derived velocity gradient ranges from 0.5 to 7.8 km s-1 pc-1. We marginally found that the specific angular momentum J/M (against the core radius R) of the Orion N2H+ cores tends to be systematically larger than that of molecular cloud cores in cold dark clouds obtained by Goodman et al., in the J/M-R relation. The ratio β of rotational to gravitational energy is derived to be β = 10-2.3±0.7, and is similar to that obtained for cold dark cloud cores in a consistent definition. The large-scale rotation of the ∫-shaped filament of the Orion A giant molecular cloud does not likely govern the core rotation at smaller scales.

VizieR Online Data Catalog: Very Low-Luminosity Objects (VeLLOs) from 1.25-850um (Kim+, 2016)

NASA Astrophysics Data System (ADS)

Kim, M.-R.; Lee, C. W.; Dunham, M. M.; Evans, N. J., II; Kim, G.; Allen, L. E.

2016-10-01

The Spitzer Gould Belt Survey (GBS) is a project to survey about 21 square degrees of 11 nearby molecular clouds at 3.6-160um to provide a census of star formation in nearby large clouds (P.I. L. Allen). Spitzer has mapped a total of 11 molecular clouds, CMC, Chamaeleon I, Chamaeleon III, Musca, Lupus V, Lupus VI, Ophiuchus North, Aquila, CrA, Cepheus, and IC 5146 with the IRAC and MIPS between 2004 March and 2008 October. We utilized the data provided by the c2d/GBS projects (Evans et al. 2009, J/ApJS/181/321; Dunham et al. 2015, J/ApJS/220/11). There are two cloud complexes which were not listed in the c2d/GBS projects, but observed by other projects, the Taurus molecular clouds and the Orion molecular clouds. The Taurus molecular clouds have been observed over an area of ~44 square degrees by one of the GTO programs (P.I. D. Padgett) with the IRAC and the MIPS instruments. The Orion molecular clouds have been surveyed in ~9°2 area by Spitzer (P.I. T. Megeath). See section 2.1 for further details. Complementary archive infrared data were retrieved from 2MASS and Herschel PACS and SPIRE and JCMT SCUBA-2; see section 2.2. We observed our sources with the N2H+(1-0) line with the Korean Very Long Baseline Interferometry Network (KVN) 21m radio telescopes from 2011 October to 2016 May for the northern hemisphere sources, and the Mopra 22m telescope in 2012 April for the southern hemisphere sources. See section 2.3 for further explanations. (8 data files).

Evolution of magnetic flux ropes associated with flux transfer events and interplanetary magnetic clouds

NASA Technical Reports Server (NTRS)

Wei, C. Q.; Lee, L. C.; Wang, S.; Akasofu, S.-I.

1991-01-01

Spacecraft observations suggest that flux transfer events and interplanetary magnetic clouds may be associated with magnetic flux ropes which are magnetic flux tubes containing helical magnetic field lines. In the magnetic flux ropes, the azimuthal magnetic field is superposed on the axial field. The time evolution of a localized magnetic flux rope is studied. A two-dimensional compressible MHD simulation code with a cylindrical symmetry is developed to study the wave modes associated with the evolution of flux ropes. It is found that in the initial phase both the fast magnetosonic wave and the Alfven wave are developed in the flux rope. After this initial phase, the Alfven wave becomes the dominant wave mode for the evolution of the magnetic flux rope and the radial expansion velocity of the flux rope is found to be negligible. Numerical results further show that even for a large initial azimuthal component of the magnetic field, the propagation velocity along the axial direction of the flux rope remains the Alfven velocity. It is also found that the localized magnetic flux rope tends to evolve into two separate magnetic ropes propagating in opposite directions. The simulation results are used to study the evolution of magnetic flux ropes associated with flux transfer events observed at the earth's dayside magnetopause and magnetic clouds in the interplanetary space.

[Cii] emission from L1630 in the Orion B molecular cloud.

PubMed

Pabst, C H M; Goicoechea, J R; Teyssier, D; Berné, O; Ochsendorf, B B; Wolfire, M G; Higgins, R D; Riquelme, D; Risacher, C; Pety, J; Le Petit, F; Roueff, E; Bron, E; Tielens, A G G M

2017-10-01

L1630 in the Orion B molecular cloud, which includes the iconic Horsehead Nebula, illuminated by the star system σ Ori, is an example of a photodissociation region (PDR). In PDRs, stellar radiation impinges on the surface of dense material, often a molecular cloud, thereby inducing a complex network of chemical reactions and physical processes. Observations toward L1630 allow us to study the interplay between stellar radiation and a molecular cloud under relatively benign conditions, that is, intermediate densities and an intermediate UV radiation field. Contrary to the well-studied Orion Molecular Cloud 1 (OMC1), which hosts much harsher conditions, L1630 has little star formation. Our goal is to relate the [Cii] fine-structure line emission to the physical conditions predominant in L1630 and compare it to studies of OMC1. The [Cii] 158 μ m line emission of L1630 around the Horsehead Nebula, an area of 12' × 17', was observed using the upgraded German Receiver for Astronomy at Terahertz Frequencies (upGREAT) onboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). Of the [Cii] emission from the mapped area 95%, 13 L ⊙ , originates from the molecular cloud; the adjacent Hii region contributes only 5%, that is, 1 L ⊙ . From comparison with other data (CO(1-0)-line emission, far-infrared (FIR) continuum studies, emission from polycyclic aromatic hydrocarbons (PAHs)), we infer a gas density of the molecular cloud of n H ∼ 3 · 10 3 cm -3 , with surface layers, including the Horsehead Nebula, having a density of up to n H ∼ 4 · 10 4 cm -3 . The temperature of the surface gas is T ∼ 100 K. The average [Cii] cooling efficiency within the molecular cloud is 1.3 · 10 -2 . The fraction of the mass of the molecular cloud within the studied area that is traced by [Cii] is only 8%. Our PDR models are able to reproduce the FIR-[Cii] correlations and also the CO(1-0)-[Cii] correlations. Finally, we compare our results on the heating efficiency of the gas with theoretical studies of photoelectric heating by PAHs, clusters of PAHs, and very small grains, and find the heating efficiency to be lower than theoretically predicted, a continuation of the trend set by other observations. In L1630 only a small fraction of the gas mass is traced by [Cii]. Most of the [Cii] emission in the mapped area stems from PDR surfaces. The layered edge-on structure of the molecular cloud and limitations in spatial resolution put constraints on our ability to relate different tracers to each other and to the physical conditions. From our study, we conclude that the relation between [Cii] emission and physical conditions is likely to be more complicated than often assumed. The theoretical heating efficiency is higher than the one we calculate from the observed [Cii] emission in the L1630 molecular cloud.

[Cii] emission from L1630 in the Orion B molecular cloud

PubMed Central

Pabst, C. H. M.; Goicoechea, J. R.; Teyssier, D.; Berné, O.; Ochsendorf, B. B.; Wolfire, M. G.; Higgins, R. D.; Riquelme, D.; Risacher, C.; Pety, J.; Le Petit, F.; Roueff, E.; Bron, E.; Tielens, A. G. G. M.

2017-01-01

Context L1630 in the Orion B molecular cloud, which includes the iconic Horsehead Nebula, illuminated by the star system σ Ori, is an example of a photodissociation region (PDR). In PDRs, stellar radiation impinges on the surface of dense material, often a molecular cloud, thereby inducing a complex network of chemical reactions and physical processes. Aims Observations toward L1630 allow us to study the interplay between stellar radiation and a molecular cloud under relatively benign conditions, that is, intermediate densities and an intermediate UV radiation field. Contrary to the well-studied Orion Molecular Cloud 1 (OMC1), which hosts much harsher conditions, L1630 has little star formation. Our goal is to relate the [Cii] fine-structure line emission to the physical conditions predominant in L1630 and compare it to studies of OMC1. Methods The [Cii] 158 μm line emission of L1630 around the Horsehead Nebula, an area of 12′ × 17′, was observed using the upgraded German Receiver for Astronomy at Terahertz Frequencies (upGREAT) onboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). Results Of the [Cii] emission from the mapped area 95%, 13 L⊙, originates from the molecular cloud; the adjacent Hii region contributes only 5%, that is, 1 L⊙. From comparison with other data (CO(1-0)-line emission, far-infrared (FIR) continuum studies, emission from polycyclic aromatic hydrocarbons (PAHs)), we infer a gas density of the molecular cloud of nH ∼ 3 · 103 cm−3, with surface layers, including the Horsehead Nebula, having a density of up to nH ∼ 4 · 104 cm−3. The temperature of the surface gas is T ∼ 100 K. The average [Cii] cooling efficiency within the molecular cloud is 1.3 · 10−2. The fraction of the mass of the molecular cloud within the studied area that is traced by [Cii] is only 8%. Our PDR models are able to reproduce the FIR-[Cii] correlations and also the CO(1-0)-[Cii] correlations. Finally, we compare our results on the heating efficiency of the gas with theoretical studies of photoelectric heating by PAHs, clusters of PAHs, and very small grains, and find the heating efficiency to be lower than theoretically predicted, a continuation of the trend set by other observations. Conclusions In L1630 only a small fraction of the gas mass is traced by [Cii]. Most of the [Cii] emission in the mapped area stems from PDR surfaces. The layered edge-on structure of the molecular cloud and limitations in spatial resolution put constraints on our ability to relate different tracers to each other and to the physical conditions. From our study, we conclude that the relation between [Cii] emission and physical conditions is likely to be more complicated than often assumed. The theoretical heating efficiency is higher than the one we calculate from the observed [Cii] emission in the L1630 molecular cloud. PMID:28989177

A composite large-scale CO survey at high galactic latitudes in the second quadrant

NASA Technical Reports Server (NTRS)

Heithausen, A.; Stacy, J. G.; De Vries, H. W.; Mebold, U.; Thaddeus, P.

1993-01-01

Surveys undertaken in the 2nd quadrant of the Galaxy with the CfA 1.2 m telescope have been combined to produce a map covering about 620 sq deg in the 2.6 mm CO(J = 1 - 0) line at high galactic latitudes. There is CO emission from molecular 'cirrus' clouds in about 13 percent of the region surveyed. The CO clouds are grouped together into three major cloud complexes with 29 individual members. All clouds are associated with infrared emission at 100 micron, although there is no one-to-one correlation between the corresponding intensities. CO emission is detected in all bright and dark Lynds' nebulae cataloged in that region; however not all CO clouds are visible on optical photographs as reflection or absorption features. The clouds are probably local. At an adopted distance of 240 pc cloud sizes range from O.1 to 30 pc and cloud masses from 1 to 1600 solar masses. The molecular cirrus clouds contribute between 0.4 and 0.8 M solar mass/sq pc to the surface density of molecular gas in the galactic plane. Only 26 percent of the 'infrared-excess clouds' in the area surveyed actually show CO and about 2/3 of the clouds detected in CO do not show an infrared excess.

NOEMA Observations of a Molecular Cloud in the Low-metallicity Galaxy Kiso 5639

NASA Astrophysics Data System (ADS)

Elmegreen, Bruce G.; Herrera, Cinthya; Rubio, Monica; Elmegreen, Debra Meloy; Sánchez Almeida, Jorge; Muñoz-Tuñón, Casiana; Olmo-García, Amanda

2018-06-01

A giant star-forming region in a metal-poor dwarf galaxy has been observed in optical lines with the 10 m Gran Telescopio Canarias (GTC) and in the emission line of CO(1–0) with the Northern Extended Millimeter Array (NOEMA) mm-wave interferometer. The metallicity was determined to be 12+{log}({{O}}/{{H}})=7.83+/- 0.09, from which we estimate a conversion factor of α CO ∼ 100 M ⊙ pc‑2(K km s‑1)‑1 and a molecular cloud mass of ∼2.9 × 107 M ⊙. This is an enormous concentration of molecular mass at one end of a small galaxy, suggesting a recent accretion. The molecular cloud properties seem normal: the surface density, 120 M ⊙ pc‑2, is comparable to that of a standard giant molecular cloud; the cloud’s virial ratio of ∼1.8 is in the star formation range; and the gas consumption time, 0.5 Gyr, at the present star formation rate is typical for molecular regions. The low metallicity implies that the cloud has an average visual extinction of only 0.8 mag, which is close to the threshold for molecule formation. With such an extinction threshold, molecular clouds in metal-poor regions should have high surface densities and high internal pressures. If high pressure is associated with the formation of massive clusters, then metal-poor galaxies such as dwarfs in the early universe could have been the hosts of metal-poor globular clusters.

The dependence of stellar age distributions on giant molecular cloud environment

NASA Astrophysics Data System (ADS)

Dobbs, C. L.; Pringle, J. E.; Naylor, T.

2014-01-01

In this Letter, we analyse the distributions of stellar ages in giant molecular clouds (GMCs) in spiral arms, interarm spurs and at large galactic radii, where the spiral arms are relatively weak. We use the results of numerical simulations of galaxies, which follow the evolution of GMCs and include star particles where star formation events occur. We find that GMCs in spiral arms tend to have predominantly young (<10 Myr) stars. By contrast, clouds which are the remainders of spiral arm giant molecular asssociations that have been sheared into interarm GMCs contain fewer young (<10 Myr) stars and more ˜20 Myr stars. We also show that clouds which form in the absence of spiral arms, due to local gravitational and thermal instabilities, contain preferentially young stars. We propose that the age distributions of stars in GMCs will be a useful diagnostic to test different cloud evolution scenarios, the origin of spiral arms and the success of numerical models of galactic star formation. We discuss the implications of our results in the context of Galactic and extragalactic molecular clouds.

Chemical abundances in cold, dark interstellar clouds

NASA Technical Reports Server (NTRS)

Irvine, William M.; Kaifu, Norio; Ohishi, Masatoshi

1991-01-01

Current tabulations are presented of the entire range of known interstellar molecules, giving attention to that subset which has been identified in the cold, dark interstellar clouds out of which the sun has been suggested to have formed. The molecular abundances of two such clouds, Taurus Molecular Cloud 1 and Lynd's 134N, exhibit prepossessing chemical differences despite considerable physical similarities. This discrepancy may be accounted for by the two clouds' differing evolutionary stages. Two novel classes of interstellar molecules are noted: sulfur-terminated carbon chains and silicon-terminated ones.

Haloing in bimodal magnetic colloids: The role of field-induced phase separation

NASA Astrophysics Data System (ADS)

Magnet, C.; Kuzhir, P.; Bossis, G.; Meunier, A.; Suloeva, L.; Zubarev, A.

2012-07-01

If a suspension of magnetic micrometer-sized and nanosized particles is subjected to a homogeneous magnetic field, the nanoparticles are attracted to the microparticles and form thick anisotropic halos (clouds) around them. Such clouds can hinder the approach of microparticles and result in effective repulsion between them [M. T. López-López, A. Yu. Zubarev, and G. Bossis, Soft Matter10.1039/c0sm00261e 6, 4346 (2010)]. In this paper, we present detailed experimental and theoretical studies of nanoparticle concentration profiles and of the equilibrium shapes of nanoparticle clouds around a single magnetized microsphere, taking into account interactions between nanoparticles. We show that at a strong enough magnetic field, the ensemble of nanoparticles experiences a gas-liquid phase transition such that a dense liquid phase is condensed around the magnetic poles of a microsphere while a dilute gas phase occupies the rest of the suspension volume. Nanoparticle accumulation around a microsphere is governed by two dimensionless parameters—the initial nanoparticle concentration (φ0) and the magnetic-to-thermal energy ratio (α)—and the three accumulation regimes are mapped onto a α-φ0 phase diagram. Our local thermodynamic equilibrium approach gives a semiquantitative agreement with the experiments on the equilibrium shapes of nanoparticle clouds. The results of this work could be useful for the development of the bimodal magnetorheological fluids and of the magnetic separation technologies used in bioanalysis and water purification systems.

STAR FORMATION IN THE MOLECULAR CLOUD ASSOCIATED WITH THE MONKEY HEAD NEBULA: SEQUENTIAL OR SPONTANEOUS?

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chibueze, James O.; Imura, Kenji; Omodaka, Toshihiro

2013-01-01

We mapped the (1,1), (2,2), and (3,3) lines of NH{sub 3} toward the molecular cloud associated with the Monkey Head Nebula (MHN) with a 1.'6 angular resolution using a Kashima 34 m telescope operated by the National Institute of Information and Communications Technology (NICT). The kinetic temperature of the molecular gas is 15-30 K in the eastern part and 30-50 K in the western part. The warmer gas is confined to a small region close to the compact H II region S252A. The cooler gas is extended over the cloud even near the extended H II region, the MHN. Wemore » made radio continuum observations at 8.4 GHz using the Yamaguchi 32 m radio telescope. The resultant map shows no significant extension from the H{alpha} image. This means that the molecular cloud is less affected by the MHN, suggesting that the molecular cloud did not form by the expanding shock of the MHN. Although the spatial distribution of the Wide-field Infrared Survey Explorer and Two Micron All Sky Survey point sources suggests that triggered low- and intermediate-mass star formation took place locally around S252A, but the exciting star associated with it should be formed spontaneously in the molecular cloud.« less

Fast instability caused by electron cloud in combined function magnets

DOE PAGES

Antipov, S. A.; Adamson, P.; Burov, A.; ...

2017-04-10

One of the factors which may limit the intensity in the Fermilab Recycler is a fast transverse instability. It develops within a hundred turns and, in certain conditions, may lead to a beam loss. The high rate of the instability suggest that its cause is electron cloud. Here, we studied the phenomena by observing the dynamics of stable and unstable beam, simulating numerically the build-up of the electron cloud, and developed an analytical model of an electron cloud driven instability with the electrons trapped in combined function di-poles. We also found that beam motion can be stabilized by a clearingmore » bunch, which confirms the electron cloud nature of the instability. The clearing suggest electron cloud trapping in Recycler combined function mag-nets. Numerical simulations show that up to 1% of the particles can be trapped by the magnetic field. Since the process of electron cloud build-up is exponential, once trapped this amount of electrons significantly increases the density of the cloud on the next revolution. Furthermore, in a Recycler combined function dipole this multi-turn accumulation allows the electron cloud reaching final intensities orders of magnitude greater than in a pure dipole. The estimated resulting instability growth rate of about 30 revolutions and the mode fre-quency of 0.4 MHz are consistent with experimental observations and agree with the simulation in the PEI code. The created instability model allows investigating the beam stability for the future intensity upgrades.« less

Fast instability caused by electron cloud in combined function magnets

DOE Office of Scientific and Technical Information (OSTI.GOV)

Antipov, S. A.; Adamson, P.; Burov, A.

One of the factors which may limit the intensity in the Fermilab Recycler is a fast transverse instability. It develops within a hundred turns and, in certain conditions, may lead to a beam loss. The high rate of the instability suggest that its cause is electron cloud. Here, we studied the phenomena by observing the dynamics of stable and unstable beam, simulating numerically the build-up of the electron cloud, and developed an analytical model of an electron cloud driven instability with the electrons trapped in combined function di-poles. We also found that beam motion can be stabilized by a clearingmore » bunch, which confirms the electron cloud nature of the instability. The clearing suggest electron cloud trapping in Recycler combined function mag-nets. Numerical simulations show that up to 1% of the particles can be trapped by the magnetic field. Since the process of electron cloud build-up is exponential, once trapped this amount of electrons significantly increases the density of the cloud on the next revolution. Furthermore, in a Recycler combined function dipole this multi-turn accumulation allows the electron cloud reaching final intensities orders of magnitude greater than in a pure dipole. The estimated resulting instability growth rate of about 30 revolutions and the mode fre-quency of 0.4 MHz are consistent with experimental observations and agree with the simulation in the PEI code. The created instability model allows investigating the beam stability for the future intensity upgrades.« less

A Catalog of Distances to Molecular Clouds from Pan-STARRS1

NASA Astrophysics Data System (ADS)

Schlafly, Eddie; Green, G.; Finkbeiner, D. P.; Rix, H.

2014-01-01

We present a catalog of distances to molecular clouds, derived from PanSTARRS-1 photometry. We simultaneously infer the full probability distribution function of reddening and distance of the stars towards these clouds using the technique of Green et al. (2013) (see neighboring poster). We fit the resulting measurements using a simple dust screen model to infer the distance to each cloud. The result is a large, homogeneous catalog of distances to molecular clouds. For clouds with heliocentric distances greater than about 200 pc, typical statistical uncertainties in the distances are 5%, with systematic uncertainty stemming from the quality of our stellar models of about 10%. We have applied this analysis to many of the most well-studied clouds in the δ > -30° sky, including Orion, California, Taurus, Perseus, and Cepheus. We have also studied the entire catalog of Magnani, Blitz, and Mundy (1985; MBM), though for about half of those clouds we can provide only upper limits on the distances. We compare our distances with distances from the literature, when available, and find good agreement.

H2, CO, and dust absorption through cold molecular clouds

NASA Astrophysics Data System (ADS)

Lacy, John H.; Sneden, Chris; Kim, Hwihyun; Jaffe, Daniel Thomas

2017-06-01

We have made observations with IGRINS on the Harlan J. Smith telescope at McDonald Observatory of near-infrared absorption by H2, CO, and dust toward stars behind molecular clouds, primarily the TMC. Prior to these observations, the abundance of H2 in molecular clouds, relative to the commonly used tracer CO, had only been measured toward a few embedded stars, which may be surrounded by atypical gas. The new observations provide a representative sample of these molecules in cold molecular gas. We find N(H2)/Av ~ 0.9e+21, N(CO)/Av ~ 1.6e+17, and H2/CO ~ 6000. The measured H2/CO ratio is consistent with that measured toward embedded stars in various molecular clouds, but half that derived from mm-wave observations of CO emission and star counts or other determinations of Av.

Galactic Cosmic Ray Intensity Response to Interplanetary Coronal Mass Ejections/Magnetic Clouds in 1995-2009

NASA Technical Reports Server (NTRS)

Richardson, I. G.; Cane, H. V.

2011-01-01

We summarize the response of the galactic cosmic ray (CGR) intensity to the passage of the more than 300 interplanetary coronal mass ejections (ICMEs) and their associated shocks that passed the Earth during 1995-2009, a period that encompasses the whole of Solar Cycle 23. In approx.80% of cases, the GCR intensity decreased during the passage of these structures, i.e., a "Forbush decrease" occurred, while in approx.10% there was no significant change. In the remaining cases, the GCR intensity increased. Where there was an intensity decrease, minimum intensity was observed inside the ICME in approx.90% of these events. The observations confirm the role of both post-shock regions and ICMEs in the generation of these decreases, consistent with many previous studies, but contrary to the conclusion of Reames, Kahler, and Tylka (Astrophys. 1. Lett. 700, L199, 2009) who, from examining a subset of ICMEs with flux-rope-like magnetic fields (magnetic clouds) argued that these are "open structures" that allow free access of particles including GCRs to their interior. In fact, we find that magnetic clouds are more likely to participate in the deepest GCR decreases than ICMEs that are not magnetic clouds.

The standard model and some new directions. [for scientific theory of Active Galactic Nuclei

NASA Technical Reports Server (NTRS)

Blandford, R. D.; Rees, M. J.

1992-01-01

A 'standard' model of Active Galactic Nuclei (AGN), based upon a massive black hole surrounded by a thin accretion disk, is defined. It is argued that, although there is good evidence for the presence of black holes and orbiting gas, most of the details of this model are either inadequate or controversial. Magnetic field may be responsible for the confinement of continuum and line-emitting gas, for the dynamical evolution of accretion disks and for the formation of jets. It is further argued that gaseous fuel is supplied in molecular form and that this is responsible for thermal re-radiation, equatorial obscuration and, perhaps, the broad line gas clouds. Stars may also supply gas close to the black hole, especially in low power AGN and they may be observable in discrete orbits as probes of the gravitational field. Recent observations suggest that magnetic field, stars, dusty molecular gas and orientation effects must be essential components of a complete description of AGN. The discovery of quasars with redshifts approaching 5 is an important clue to the mechanism of galaxy formation.

A Herschel-SPIRE Survey of the MonR2 Giant Molecular Cloud

NASA Astrophysics Data System (ADS)

Pokhrel, Riwaj; Gutermuth, Robert A.; Ali, Babar; Megeath, S. Thomas; Pipher, Judith; Myers, Philip C.; Fischer, William J.; Henning, Thomas; Wolk, Scott J.; Allen, Lori; Tobin, John J.

2014-06-01

We present a new survey of the MonR2 giant molecular cloud with SPIRE on the Herschel Space Observatory. We cross-calibrated SPIRE data with Planck-HFI and accounted for its absolute offset and zero point correction. We fixed emissivity with the help of flux-error and flux ratio plots. As the best representation of cold dusty molecular clouds, we did greybody fits of the SEDs. We studied the nature of distribution of column densities above and below certain critical limit, followed by the mass and temperature distributions for different regions. We isolated the filaments and studied radial column density profile in this cloud.

ALMA Observations of a Quiescent Molecular Cloud in the Large Magellanic Cloud

NASA Astrophysics Data System (ADS)

Wong, Tony; Hughes, Annie; Tokuda, Kazuki; Indebetouw, Rémy; Bernard, Jean-Philippe; Onishi, Toshikazu; Wojciechowski, Evan; Bandurski, Jeffrey B.; Kawamura, Akiko; Roman-Duval, Julia; Cao, Yixian; Chen, C.-H. Rosie; Chu, You-hua; Cui, Chaoyue; Fukui, Yasuo; Montier, Ludovic; Muller, Erik; Ott, Juergen; Paradis, Deborah; Pineda, Jorge L.; Rosolowsky, Erik; Sewiło, Marta

2017-12-01

We present high-resolution (subparsec) observations of a giant molecular cloud in the nearest star-forming galaxy, the Large Magellanic Cloud. ALMA Band 6 observations trace the bulk of the molecular gas in 12CO(2-1) and the high column density regions in 13CO(2-1). Our target is a quiescent cloud (PGCC G282.98-32.40, which we refer to as the “Planck cold cloud” or PCC) in the southern outskirts of the galaxy where star formation activity is very low and largely confined to one location. We decompose the cloud into structures using a dendrogram and apply an identical analysis to matched-resolution cubes of the 30 Doradus molecular cloud (located near intense star formation) for comparison. Structures in the PCC exhibit roughly 10 times lower surface density and five times lower velocity dispersion than comparably sized structures in 30 Dor, underscoring the non-universality of molecular cloud properties. In both clouds, structures with relatively higher surface density lie closer to simple virial equilibrium, whereas lower surface-density structures tend to exhibit supervirial line widths. In the PCC, relatively high line widths are found in the vicinity of an infrared source whose properties are consistent with a luminous young stellar object. More generally, we find that the smallest resolved structures (“leaves”) of the dendrogram span close to the full range of line widths observed across all scales. As a result, while the bulk of the kinetic energy is found on the largest scales, the small-scale energetics tend to be dominated by only a few structures, leading to substantial scatter in observed size-line-width relationships.

The Global Evolution of Giant Molecular Clouds. I. Model Formulation and Quasi-Equilibrium Behavior

NASA Astrophysics Data System (ADS)

Krumholz, Mark R.; Matzner, Christopher D.; McKee, Christopher F.

2006-12-01

We present semianalytic dynamical models for giant molecular clouds evolving under the influence of H II regions launched by newborn star clusters. In contrast to previous work, we neither assume that clouds are in virial or energetic equilibrium, nor do we ignore the effects of star formation feedback. The clouds, which we treat as spherical, can expand and contract homologously. Photoionization drives mass ejection; the recoil of cloud material both stirs turbulent motions and leads to an effective confining pressure. The balance between these effects and the decay of turbulent motions through isothermal shocks determines clouds' dynamical and energetic evolution. We find that for realistic values of the rates of turbulent dissipation, photoevaporation, and energy injection by H II regions, the massive clouds where most molecular gas in the Galaxy resides live for a few crossing times, in good agreement with recent observational estimates that large clouds in Local Group galaxies survive roughly 20-30 Myr. During this time clouds remain close to equilibrium, with virial parameters of 1-3 and column densities near 1022 H atoms cm-2, also in agreement with observed cloud properties. Over their lives they convert 5%-10% of their mass into stars, after which point most clouds are destroyed when a large H II region unbinds them. In contrast, small clouds like those found in the solar neighborhood only survive ~1 crossing time before being destroyed.

Waves on the surface of the Orion molecular cloud.

PubMed

Berné, Olivier; Marcelino, Núria; Cernicharo, José

2010-08-19

Massive stars influence their parental molecular cloud, and it has long been suspected that the development of hydrodynamical instabilities can compress or fragment the cloud. Identifying such instabilities has proved difficult. It has been suggested that elongated structures (such as the 'pillars of creation') and other shapes arise because of instabilities, but alternative explanations are available. One key signature of an instability is a wave-like structure in the gas, which has hitherto not been seen. Here we report the presence of 'waves' at the surface of the Orion molecular cloud near where massive stars are forming. The waves seem to be a Kelvin-Helmholtz instability that arises during the expansion of the nebula as gas heated and ionized by massive stars is blown over pre-existing molecular gas.

Molecular clouds toward three Spitzer bubbles S116, S117, and S118: Evidence for a cloud-cloud collision which formed the three H II regions and a 10 pc scale molecular cavity

NASA Astrophysics Data System (ADS)

Fukui, Yasuo; Ohama, Akio; Kohno, Mikito; Torii, Kazufumi; Fujita, Shinji; Hattori, Yusuke; Nishimura, Atsushi; Yamamoto, Hiroaki; Tachihara, Kengo

2018-05-01

We carried out a molecular-line study toward the three Spitzer bubbles S116, S117, and S118, which show active formation of high-mass stars. We found molecular gas consisting of two components with a velocity difference of ˜5 km s-1. One of them, the small cloud, has a typical velocity of -63 km s-1 and the other, the large cloud, has one of -58 km s-1. The large cloud has a nearly circular intensity depression, the size of which is similar to that of the small cloud. We present an interpretation that its cavity was created by a collision between the two clouds and that this collision compressed the gas into a dense layer elongating along the western rim of the small cloud. In this scenario, the O stars including those in the three Spitzer bubbles were formed in the interface layer compressed by the collision. Assuming that the relative motion of the clouds has a tilt of 45° to the line of sight, we estimate that the collision continued for the last 1 Myr at a relative velocity of ˜10 km s-1. In the S116-S117-S118 system the H II regions are located outside of the cavity. This morphology is ascribed to the density-bound distribution of the large cloud which caused the H II regions to expand more easily toward the outer part of the large cloud than towards the inside of the cavity. The present case proves that a cloud-cloud collision creates a cavity without the action of O-star feedback, and suggests that the collision-compressed layer is highly filamentary.

74 MHz nonthermal emission from molecular clouds: evidence for a cosmic ray dominated region at the galactic center.

PubMed

Yusef-Zadeh, F; Wardle, M; Lis, D; Viti, S; Brogan, C; Chambers, E; Pound, M; Rickert, M

2013-10-03

We present 74 MHz radio continuum observations of the Galactic center region. These measurements show nonthermal radio emission arising from molecular clouds that is unaffected by free–free absorption along the line of sight. We focus on one cloud, G0.13-0.13, representative of the population of molecular clouds that are spatially correlated with steep spectrum (α(327MHz)(74MHz) = 1.3 ± 0.3) nonthermal emission from the Galactic center region. This cloud lies adjacent to the nonthermal radio filaments of the Arc near l 0.2° and is a strong source of 74 MHz continuum, SiO (2-1), and Fe I Kα 6.4 keV line emission. This three-way correlation provides the most compelling evidence yet that relativistic electrons, here traced by 74 MHz emission, are physically associated with the G0.13-0.13 molecular cloud and that low-energy cosmic ray electrons are responsible for the Fe I Kα line emission. The high cosmic ray ionization rate 10(–1)3 s(–1) H(–1) is responsible for heating the molecular gas to high temperatures and allows the disturbed gas to maintain a high-velocity dispersion. Large velocity gradient (LVG) modeling of multitransition SiO observations of this cloud implies H2 densities 10(4–5) cm(–3) and high temperatures. The lower limit to the temperature of G0.13-0.13 is 100 K, whereas the upper limit is as high as 1000 K. Lastly, we used a time-dependent chemical model in which cosmic rays drive the chemistry of the gas to investigate for molecular line diagnostics of cosmic ray heating. When the cloud reaches chemical equilibrium, the abundance ratios of HCN/HNC and N2H+/HCO+ are consistent with measured values. In addition, significant abundance of SiO is predicted in the cosmic ray dominated region of the Galactic center. We discuss different possibilities to account for the origin of widespread SiO emission detected from Galactic center molecular clouds.

Magnetic Field Generation During the Collision of Narrow Plasma Clouds

NASA Astrophysics Data System (ADS)

Sakai, Jun-ichi; Kazimura, Yoshihiro; Haruki, Takayuki

1999-06-01

We investigate the dynamics of the collision of narrow plasma clouds,whose transverse dimension is on the order of the electron skin depth.A 2D3V (two dimensions in space and three dimensions in velocity space)particle-in-cell (PIC) collisionless relativistic code is used toshow the generation of a quasi-staticmagnetic field during the collision of narrow plasma clouds both inelectron-ion and electron-positron (pair) plasmas. The localizedstrong magnetic fluxes result in the generation of the charge separationwith complicated structures, which may be sources of electromagneticas well as Langmuir waves. We also present one applicationof this process, which occurs during coalescence of magnetic islandsin a current sheet of pair plasmas.

The Monoceros R2 Molecular Cloud

NASA Astrophysics Data System (ADS)

Carpenter, J. M.; Hodapp, K. W.

2008-12-01

The Monoceros R2 region was first recognized as a chain of reflection nebulae illuminated by A- and B-type stars. These nebulae are associated with a giant molecular cloud that is one of the closest massive star forming regions to the Sun. This chapter reviews the properties of the Mon R2 region, including the namesake reflection nebulae, the large scale molecula= r cloud, global star formation activity, and properties of prominent star forming regions in the cloud.

Supernovae-generated high-velocity compact clouds

NASA Astrophysics Data System (ADS)

Yalinewich, A.; Beniamini, P.

2018-05-01

Context. A previous study claimed the discovery of an intermediate-mass black hole (IMBH). This hypothetical black hole was invoked in order to explain the high-velocity dispersion in one of several dense molecular clouds near the Galactic center. The same study considered the possibility that this cloud was due to a supernova explosion, but disqualified this scenario because no X-rays were detected. Aims: We here check whether a supernova explosion could have produced that cloud, and whether this explanation is more likely than an IMBH. More specifically, we wish to determine whether a supernova inside a dense molecular cloud would emit in the X-rays. Methods: We have approached this problem from two different directions. First, we performed an analytic calculation to determine the cooling rate by thermal bremsstrahlung and compared this time to the lifetime of the cloud. Second, we estimated the creation rate of these dense clouds in the central molecular zone (CMZ) region near the Galactic center, where they were observed. Based on this rate, we can place lower bounds on the total mass of IMBHs and clouds and compare this to the masses of the components of the CMZ. Results: We find that the cooling time of the supernova remnant inside a molecular cloud is shorter than its dynamical time. This means that the temperature in such a remnant would be much lower than that of a typical supernova remnant. At such a low temperature, the remnant is not expected to emit in the X-rays. We also find that to explain the rate at which such dense clouds are created requires fine-tuning the number of IMBHs. Conclusions: We find the supernova model to be a more likely explanation for the formation of high-velocity compact clouds than an IMBH.

The 27-28 October 1986 FIRE IFO Cirrus Case Study: Cloud Optical Properties Determined by High Spectral Resolution Lidar

NASA Technical Reports Server (NTRS)

Grund, C. J.; Eloranta, E. W.

1996-01-01

During the First ISCCP Region Experiment (FIRE) cirrus intensive field observation (IFO) the High Spectral Resolution Lidar was operated from a roof top site on the University of Wisconsin-Madison campus. Because the HSRL technique separately measures the molecular and cloud particle backscatter components of the lidar return, the optical thickness is determined independent of particle backscatter. This is accomplished by comparing the known molecular density distribution to the observed decrease in molecular backscatter signal with altitude. The particle to molecular backscatter ratio yields calibrated measurements of backscatter cross sections that can be plotted ro reveal cloud morphology without distortion due to attenuation. Changes in cloud particle size, shape, and phase affect the backscatter to extinction ratio (backscatter-phase function). The HSRL independently measures cloud particle backscatter phase function. This paper presents a quantitative analysis of the HSRL cirrus cloud data acquired over an approximate 33 hour period of continuous near zenith observations. Correlations between small scale wind structure and cirrus cloud morphology have been observed. These correlations can bias the range averaging inherent in wind profiling lidars of modest vertical resolution, leading to increased measurement errors at cirrus altitudes. Extended periods of low intensity backscatter were noted between more strongly organized cirrus cloud activity. Optical thicknesses ranging from 0.01-1.4, backscatter phase functions between 0.02-0.065 sr (exp -1) and backscatter cross sections spanning 4 orders of magnitude were observed. the altitude relationship between cloud top and bottom boundaries and the cloud optical center altitude was dependent on the type of formation observed Cirrus features were observed with characteristic wind drift estimated horizontal sizes of 5-400 km. The clouds frequently exhibited cellular structure with vertical to horizontal dimension ratios of 1:5-1:1.

High spectral resolution observations of fluorescent molecular hydrogen in molecular clouds

NASA Technical Reports Server (NTRS)

Burton, Michael G.; Geballe, T. R.; Brand, P. W. J. L.; Moorhouse, A.

1990-01-01

The 1-0 S(1) line of molecular hydrogen has been observed at high spectral resolution in several sources where the emission was suspected of being fluorescent. In NGC 2023, the Orion Bar, and Parsamyan 18, the S(1) line is unresolved, and the line center close to the rest velocity of the ambient molecular cloud. Such behavior is expected for UV-excited line emission. The H2 line widths in molecular clouds thus can serve as diagnostic for shocked and UV-excitation mechanisms. If the lines are broader than several km/s or velocity shifts are observed across a source it is likely that shocks are responsible for the excitation of the gas.

Triggered O Star Formation in M20 via Cloud-Cloud Collision: Comparisons between High-resolution CO Observations and Simulations

NASA Astrophysics Data System (ADS)

Torii, K.; Hattori, Y.; Hasegawa, K.; Ohama, A.; Haworth, T. J.; Shima, K.; Habe, A.; Tachihara, K.; Mizuno, N.; Onishi, T.; Mizuno, A.; Fukui, Y.

2017-02-01

Understanding high-mass star formation is one of the top-priority issues in astrophysics. Recent observational studies have revealed that cloud-cloud collisions may play a role in high-mass star formation in several places in the Milky Way and the Large Magellanic Cloud. The Trifid Nebula M20 is a well-known Galactic H II region ionized by a single O7.5 star. In 2011, based on the CO observations with NANTEN2, we reported that the O star was formed by the collision between two molecular clouds ˜0.3 Myr ago. Those observations identified two molecular clouds toward M20, traveling at a relative velocity of 7.5 {km} {{{s}}}-1. This velocity separation implies that the clouds cannot be gravitationally bound to M20, but since the clouds show signs of heating by the stars there they must be spatially coincident with it. A collision is therefore highly possible. In this paper we present the new CO J = 1-0 and J = 3-2 observations of the colliding clouds in M20 performed with the Mopra and ASTE telescopes. The high-resolution observations revealed that the two molecular clouds have peculiar spatial and velocity structures, I.e., a spatially complementary distribution between the two clouds and a bridge feature that connects the two clouds in velocity space. Based on a new comparison with numerical models, we find that this complementary distribution is an expected outcome of cloud-cloud collisions, and that the bridge feature can be interpreted as the turbulent gas excited at the interface of the collision. Our results reinforce the cloud-cloud collision scenario in M20.

Theory of grain alignment in molecular clouds

NASA Technical Reports Server (NTRS)

Roberge, Wayne G.

1993-01-01

Research accomplishments are presented and include the following: (1) mathematical theory of grain alignment; (2) super-paramagnetic alignment of molecular cloud grains; and (3) theory of grain alignment by ambipolar diffusion.

Characterization of organic matter in cloud waters sampled at the puy de Dôme mountain using FT-ICR-MS

NASA Astrophysics Data System (ADS)

Bianco, A.; Chaumerliac, N.; Vaitilingom, M.; Deguillaume, L.; Bridoux, M. C.

2017-12-01

The chemical composition of organic matter in cloud water is highly complex. The organic species result from their dissolution from the gas phase or from the soluble fraction of the particle phase. They are also produced by aqueous phase reactivity. Several low molecular weight organic species have been quantified such as aldehydes and carboxylic acids. Recently, amino acids were also detected in cloud water and their presence is related to the presence of microorganisms. Compounds presenting similarities with high molecular weight organic substances or HULIS found in aerosols were also observed in clouds. Overall, these studies mainly focused on individual compounds or functional groups rather than the complex mixture at the molecular level. This study presents a non-targeted approach to characterize the organic matter in clouds. Samples were collected at the puy de Dôme Mountain (France). Two cloud water samples (June & July 2016) were analyzed using high resolution mass spectrometry (ESI-FT-ICR-MS 9.4T). A reversed solid phase extraction (SPE) procedure was performed to concentrate dissolved organic matter components. Composer (v.1.5.3) software was used to filter the mass spectral data, recalibrate externally the dataset and calculate all possible formulas for detected anions. The first cloud sample (June) resulted from air mass coming from the North (North Sea) while the second one (July) resulted from air mass coming from the West (Atlantic Ocean). Thus, both cloud events derived from marine air masses but were characterized by different hydrogen peroxide concentration and dissolved organic carbon content and were sampled at different periods during the day. Elemental compositions of 6487 and 3284 unique molecular species were identified in each sample. Nitrogen-containing compounds (CHNO compounds), sulfur-containing compounds (CHOS & CHNOS compounds) and other oxygen-containing compounds (CHO compounds) with molecular weights up to 800 Da were detected. The main class is CHNO (53% for both samples) while sulfur-containing compounds represent for the two samples respectively 21 & 14% of the total assigned molecular formulas. CHO compounds molecular formulas are respectively 25 & 32%. Among the two samples, only 2490 molecular formulas were found common to the two samples.

Fast Molecular Cloud Destruction Requires Fast Cloud Formation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mac Low, Mordecai-Mark; Burkert, Andreas; Ibáñez-Mejía, Juan C., E-mail: mordecai@amnh.org, E-mail: burkert@usm.lmu.de, E-mail: ibanez@ph1.uni-koeln.de

A large fraction of the gas in the Galaxy is cold, dense, and molecular. If all this gas collapsed under the influence of gravity and formed stars in a local free-fall time, the star formation rate in the Galaxy would exceed that observed by more than an order of magnitude. Other star-forming galaxies behave similarly. Yet, observations and simulations both suggest that the molecular gas is indeed gravitationally collapsing, albeit hierarchically. Prompt stellar feedback offers a potential solution to the low observed star formation rate if it quickly disrupts star-forming clouds during gravitational collapse. However, this requires that molecular cloudsmore » must be short-lived objects, raising the question of how so much gas can be observed in the molecular phase. This can occur only if molecular clouds form as quickly as they are destroyed, maintaining a global equilibrium fraction of dense gas. We therefore examine cloud formation timescales. We first demonstrate that supernova and superbubble sweeping cannot produce dense gas at the rate required to match the cloud destruction rate. On the other hand, Toomre gravitational instability can reach the required production rate. We thus argue that, although dense, star-forming gas may last only around a single global free-fall time; the dense gas in star-forming galaxies can globally exist in a state of dynamic equilibrium between formation by gravitational instability and disruption by stellar feedback. At redshift z ≳ 2, the Toomre instability timescale decreases, resulting in a prediction of higher molecular gas fractions at early times, in agreement with the observations.« less

[C II] emission from L1630 in the Orion B molecular cloud

NASA Astrophysics Data System (ADS)

Pabst, C. H. M.; Goicoechea, J. R.; Teyssier, D.; Berné, O.; Ochsendorf, B. B.; Wolfire, M. G.; Higgins, R. D.; Riquelme, D.; Risacher, C.; Pety, J.; Le Petit, F.; Roueff, E.; Bron, E.; Tielens, A. G. G. M.

2017-10-01

Context. L1630 in the Orion B molecular cloud, which includes the iconic Horsehead Nebula, illuminated by the star system σ Ori, is an example of a photodissociation region (PDR). In PDRs, stellar radiation impinges on the surface of dense material, often a molecular cloud, thereby inducing a complex network of chemical reactions and physical processes. Aims: Observations toward L1630 allow us to study the interplay between stellar radiation and a molecular cloud under relatively benign conditions, that is, intermediate densities and an intermediate UV radiation field. Contrary to the well-studied Orion Molecular Cloud 1 (OMC1), which hosts much harsher conditions, L1630 has little star formation. Our goal is to relate the [C II] fine-structure line emission to the physical conditions predominant in L1630 and compare it to studies of OMC1. Methods: The [C II] 158 μm line emission of L1630 around the Horsehead Nebula, an area of 12' × 17', was observed using the upgraded German Receiver for Astronomy at Terahertz Frequencies (upGREAT) onboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). Results: Of the [C II] emission from the mapped area 95%, 13 L⊙, originates from the molecular cloud; the adjacent H II region contributes only 5%, that is, 1 L⊙. From comparison with other data (CO (1 - 0)-line emission, far-infrared (FIR) continuum studies, emission from polycyclic aromatic hydrocarbons (PAHs)), we infer a gas density of the molecular cloud of nH 3 × 103 cm-3, with surface layers, including the Horsehead Nebula, having a density of up to nH 4 × 104 cm-3. The temperature of the surface gas is T 100 K. The average [C II] cooling efficiency within the molecular cloud is 1.3 × 10-2. The fraction of the mass of the molecular cloud within the studied area that is traced by [C II] is only 8%. Our PDR models are able to reproduce the FIR-[C II] correlations and also the CO (1 - 0)-[C II] correlations. Finally, we compare our results on the heating efficiency of the gas with theoretical studies of photoelectric heating by PAHs, clusters of PAHs, and very small grains, and find the heating efficiency to be lower than theoretically predicted, a continuation of the trend set by other observations. Conclusions: In L1630 only a small fraction of the gas mass is traced by [C II]. Most of the [C II] emission in the mapped area stems from PDR surfaces. The layered edge-on structure of the molecular cloud and limitations in spatial resolution put constraints on our ability to relate different tracers to each other and to the physical conditions. From our study, we conclude that the relation between [C II] emission and physical conditions is likely to be more complicated than often assumed. The theoretical heating efficiency is higher than the one we calculate from the observed [C II] emission in the L1630 molecular cloud.

Measuring Filament Orientation: A New Quantitative, Local Approach

NASA Astrophysics Data System (ADS)

Green, C.-E.; Dawson, J. R.; Cunningham, M. R.; Jones, P. A.; Novak, G.; Fissel, L. M.

2017-09-01

The relative orientation between filamentary structures in molecular clouds and the ambient magnetic field provides insight into filament formation and stability. To calculate the relative orientation, a measurement of filament orientation is first required. We propose a new method to calculate the orientation of the one-pixel-wide filament skeleton that is output by filament identification algorithms such as filfinder. We derive the local filament orientation from the direction of the intensity gradient in the skeleton image using the Sobel filter and a few simple post-processing steps. We call this the “Sobel-gradient method.” The resulting filament orientation map can be compared quantitatively on a local scale with the magnetic field orientation map to then find the relative orientation of the filament with respect to the magnetic field at each point along the filament. It can also be used for constructing radial profiles for filament width fitting. The proposed method facilitates automation in analyses of filament skeletons, which is imperative in this era of “big data.”

The Properties and Fate of the Galactic Center G2 Cloud

NASA Astrophysics Data System (ADS)

Shcherbakov, Roman V.

2014-03-01

The object G2 was recently discovered descending into the gravitational potential of the supermassive black hole (BH) Sgr A*. We test the photoionized cloud scenario, determine the cloud properties, and estimate the emission during the pericenter passage. The incident radiation is computed starting from the individual stars at the locations of G2. The radiative transfer calculations are conducted with CLOUDY code and 2011 broadband and line luminosities are fitted. The spherically symmetric, tidally distorted, and magnetically arrested cloud shapes are tested with both the interstellar medium dust and 10 nm graphite dust. The best-fitting magnetically arrested model has the initial density n init = 1.8 × 105 cm-3, initial radius R init = 2.2 × 1015 cm = 17 mas, mass m cloud = 4 M Earth, and dust relative abundance A = 0.072. It provides a good fit to 2011 data, is consistent with the luminosities in 2004 and 2008, and reaches an agreement with the observed size. We revise down the predicted radio and X-ray bow shock luminosities to be below the quiescent level of Sgr A*, which readily leads to non-detection in agreement to observations. The magnetic energy dissipation in the cloud at the pericenter coupled with more powerful irradiation may lead to an infrared source with an apparent magnitude m_{L^{\\prime }}\\approx 13.0. No shock into the cloud and no X-rays are expected from cloud squeezing by the ambient gas pressure. Larger than previously estimated cloud mass m cloud = (4-20) M Earth may produce a higher accretion rate and a brighter state of Sgr A* as the debris descend onto the BH.

Molecules in interstellar clouds. [physical and chemical conditions of star formation and biological evolution

NASA Technical Reports Server (NTRS)

Irvine, W. M.; Hjalmarson, A.; Rydbeck, O. E. H.

1981-01-01

The physical conditions and chemical compositions of the gas in interstellar clouds are reviewed in light of the importance of interstellar clouds for star formation and the origin of life. The Orion A region is discussed as an example of a giant molecular cloud where massive stars are being formed, and it is pointed out that conditions in the core of the cloud, with a kinetic temperature of about 75 K and a density of 100,000-1,000,000 molecules/cu cm, may support gas phase ion-molecule chemistry. The Taurus Molecular Clouds are then considered as examples of cold, dark, relatively dense interstellar clouds which may be the birthplaces of solar-type stars and which have been found to contain the heaviest interstellar molecules yet discovered. The molecular species identified in each of these regions are tabulated, including such building blocks of biological monomers as H2O, NH3, H2CO, CO, H2S, CH3CN and H2, and more complex species such as HCOOCH3 and CH3CH2CN.

Dispersal of Giant Molecular Clouds by Photoionization and Radiation Pressure

NASA Astrophysics Data System (ADS)

Kim, Jeong-Gyu; Kim, Woong-Tae; Ostriker, Eve C.

2018-01-01

UV radiation feedback from young massive stars plays a key role in the evolution of giant molecular clouds (GMCs) by forming HII regions and driving their expansion. We present the results of radiation hydrodynamic simulations of star cluster formation in turbulent GMCs, focusing on the effects of photoionization and radiation pressure on regulating the net star formation efficiency (SFE) and lifetime of clouds. We find that the net SFE depends primarily on the initial gas surface density, $\\Sigma_0$, such that the net SFE increases from 4% to 50% as $\\Sigma_0$ increases from $20\\,M_{\\odot}\\,{\\rm pc}^{-2}$ to $1300\\,M_{\\odot}\\,{\\rm pc}^{-2}$. Cloud dispersal occurs within $10\\,{\\rm Myr}$ after the onset of radiation feedback, or within 0.7--4.0 free-fall times that increases with $\\Sigma_0$. Photoionization plays a dominant role in destroying molecular clouds typical of the Milky Way, while radiation pressure takes over in massive, dense clouds. Based on the analysis of mass loss processes by photoevaporation or momentum injection, we develop a semi-analytic model for cloud dispersal and compare it with the numerical results.

Red Fluorescent Line Emission from Hydrogen Molecules in Diffuse Molecular Clouds

NASA Technical Reports Server (NTRS)

Neufeld, David A.; Spaans, Marco

1996-01-01

We have modeled the fluorescent pumping of electronic and vibrational emissions of molecular hydrogen (H2) within diffuse molecular clouds that are illuminated by ultraviolet continuum radiation. Fluorescent line intensities are predicted for transitions at ultraviolet, infrared, and red visible wavelengths as functions of the gas density, the visual extinction through the cloud, and the intensity of the incident UV continuum radiation. The observed intensity in each fluorescent transition is roughly proportional to the integrated rate of H2 photodissociation along the line of sight. Although the most luminous fluorescent emissions detectable from ground-based observatories lie at near-infrared wavelengths, we argue that the lower sky brightness at visible wavelengths makes the red fluorescent transitions a particularly sensitive probe. Fabry-Perot spectrographs of the type that have been designed to observe very faint diffuse Ha emissions are soon expected to yield sensitivities that will be adequate to detect H2 vibrational emissions from molecular clouds that are exposed to ultraviolet radiation no stronger than the mean radiation field within the Galaxy. Observations of red H2 fluorescent emission together with cospatial 21 cm H I observations could serve as a valuable probe of the gas density in diffuse molecular clouds.

Giant molecular cloud scaling relations: the role of the cloud definition

NASA Astrophysics Data System (ADS)

Khoperskov, S. A.; Vasiliev, E. O.; Ladeyschikov, D. A.; Sobolev, A. M.; Khoperskov, A. V.

2016-01-01

We investigate the physical properties of molecular clouds in disc galaxies with different morphologies: a galaxy without prominent structure, a spiral barred galaxy and a galaxy with flocculent structure. Our N-body/hydrodynamical simulations take into account non-equilibrium H2 and CO chemical kinetics, self-gravity, star formation and feedback processes. For the simulated galaxies, the scaling relations of giant molecular clouds, or so-called Larson's relations, are studied for two types of cloud definition (or extraction method): the first is based on total column density position-position (PP) data sets and the second is indicated by the CO (1-0) line emission used in position-position-velocity (PPV) data. We find that the cloud populations obtained using both cloud extraction methods generally have similar physical parameters, except that for the CO data the mass spectrum of clouds has a tail with low-mass objects M ˜ 103-104 M⊙. Owing toa varying column density threshold, the power-law indices in the scaling relations are significantly changed. In contrast, the relations are invariant to the CO brightness temperature threshold. Finally, we find that the mass spectra of clouds for PPV data are almost insensitive to the galactic morphology, whereas the spectra for PP data demonstrate significant variation.

Magnetic field in the NGC7023 photodissociation region

NASA Astrophysics Data System (ADS)

Alves, Marta

2015-10-01

The far-UV radiation of massive stars illuminates molecular clouds creating photodissociation regions (PDRs), the transition layers between atomic and molecular media. Recent results based on Herschel observations reveal the presence of small regions at high gas pressure in the PDRs, whose origin is still not well understood, while polarization measurements towards a few PDRs indicate that magnetic fields can play a significant role in their structure. The limited number of existing polarization observations suggest that, when subject to a high gas and radiation pressure from the stars, the magnetic field tends to align and to be compressed in the PDR. As a consequence, bright PDRs should be magnetically dominated. However, this possibility has been the subject of very few studies due to the sparsity of relevant data. We propose to map the magnetic field in a nearby bright PDR, NGC 7023, using the unique capabilities of HAWC+ onboard SOFIA. For one, we wish to test the hypothesis that the magnetic field should be parallel to this PDR, which is illuminated by a radiation field of 2600 (in Habing units). Secondly, since NGC 7023 is a well studied region, its physical conditions (density, temperature) are known and can thus be related to the magnetic field across the PDR. We can investigate the relation between the field structure and the geometry of the PDR, and aided by Herschel observations we can also explore a possible connection between the magnetic field and the existence of high density regions in the PDR. SOFIA HAWC+ is the only instrument capable of imaging the polarized emission of extended objects, with structure at arcsecond scales. Moreover, it allows us trace the magnetic field within the PDR, owing to its 63micron band that traces the warm (40K) dust present at the illuminated surface. Our observations will be complementary to those led by the instrument team, who will observe NGC 7023 using the three highest wavelength filters.

A Catalog of Molecular Clouds in the Milky Way Galaxy

NASA Astrophysics Data System (ADS)

Wahl, Matthew; Koda, J.

2010-01-01

We have created a complete catalog of molecular clouds in the Milky Way Galaxy. This is an extension of our previous study (Koda et al. 2006) which used a preliminary data set from The Boston University Five College Radio Astronomy Observatory Galactic Ring Survey (BUFCRAO GRS). This work is of the complete data set from this GRS. The data covers the inner part of the northern Galactic disk between galactic longitudes 15 to 56 degrees, galactic latitudes -1.1 to 1.1 degrees, and the entire Galactic velocities. We used the standard cloud identification method. This method searches the data cube for a peak in temperature above a specified value, and then searches around that peak in all directions until the extents of the cloud are found. This method is iterated until all clouds are found. We prefer this method over other methods, because of its simplicity. The properties of our molecular clouds are very similar to those based on a more evolved method (Rathborne et al. 2009).

Molecular Cloud Evolution VI. Measuring cloud ages

NASA Astrophysics Data System (ADS)

Vázquez-Semadeni, Enrique; Zamora-Avilés, Manuel; Galván-Madrid, Roberto; Forbrich, Jan

2018-06-01

In previous contributions, we have presented an analytical model describing the evolution of molecular clouds (MCs) undergoing hierarchical gravitational contraction. The cloud's evolution is characterized by an initial increase in its mass, density, and star formation rate (SFR) and efficiency (SFE) as it contracts, followed by a decrease of these quantities as newly formed massive stars begin to disrupt the cloud. The main parameter of the model is the maximum mass reached by the cloud during its evolution. Thus, specifying the instantaneous mass and some other variable completely determines the cloud's evolutionary stage. We apply the model to interpret the observed scatter in SFEs of the cloud sample compiled by Lada et al. as an evolutionary effect so that, although clouds such as California and Orion A have similar masses, they are in very different evolutionary stages, causing their very different observed SFRs and SFEs. The model predicts that the California cloud will eventually reach a significantly larger total mass than the Orion A cloud. Next, we apply the model to derive estimated ages of the clouds since the time when approximately 25% of their mass had become molecular. We find ages from ˜1.5 to 27 Myr, with the most inactive clouds being the youngest. Further predictions of the model are that clouds with very low SFEs should have massive atomic envelopes constituting the majority of their gravitational mass, and that low-mass clouds (M ˜ 103-104M⊙) end their lives with a mini-burst of star formation, reaching SFRs ˜300-500 M⊙ Myr-1. By this time, they have contracted to become compact (˜1 pc) massive star-forming clumps, in general embedded within larger GMCs.

Beam induced electron cloud resonances in dipole magnetic fields

DOE PAGES

Calvey, J. R.; Hartung, W.; Makita, J.; ...

2016-07-01

The buildup of low energy electrons in an accelerator, known as electron cloud, can be severely detrimental to machine performance. Under certain beam conditions, the beam can become resonant with the cloud dynamics, accelerating the buildup of electrons. This paper will examine two such effects: multipacting resonances, in which the cloud development time is resonant with the bunch spacing, and cyclotron resonances, in which the cyclotron period of electrons in a magnetic field is a multiple of bunch spacing. Both resonances have been studied directly in dipole fields using retarding field analyzers installed in the Cornell Electron Storage Ring. Thesemore » measurements are supported by both analytical models and computer simulations.« less

KINEMATIC STRUCTURE OF MOLECULAR GAS AROUND HIGH-MASS YSO, PAPILLON NEBULA, IN N159 EAST IN THE LARGE MAGELLANIC CLOUD: A NEW PERSPECTIVE WITH ALMA

DOE Office of Scientific and Technical Information (OSTI.GOV)

Saigo, Kazuya; Harada, Ryohei; Kawamura, Akiko

We present the ALMA Band 3 and Band 6 results of {sup 12}CO(2-1), {sup 13}CO(2-1), H30 α recombination line, free–free emission around 98 GHz, and the dust thermal emission around 230 GHz toward the N159 East Giant Molecular Cloud (N159E) in the Large Magellanic Cloud (LMC). LMC is the nearest active high-mass star-forming face-on galaxy at a distance of 50 kpc and is the best target for studing high-mass star formation. ALMA observations show that N159E is the complex of filamentary clouds with the width and length of ∼1 pc and several parsecs. The total molecular mass is 0.92 ×more » 10{sup 5} M {sub ⊙} from the {sup 13}CO(2-1) intensity. N159E harbors the well-known Papillon Nebula, a compact high-excitation H ii region. We found that a YSO associated with the Papillon Nebula has the mass of 35 M {sub ⊙} and is located at the intersection of three filamentary clouds. It indicates that the formation of the high-mass YSO was induced by the collision of filamentary clouds. Fukui et al. reported a similar kinematic structure toward two YSOs in the N159 West region, which are the other YSOs that have the mass of ≳35 M {sub ⊙}. This suggests that the collision of filamentary clouds is a primary mechanism of high-mass star formation. We found a small molecular hole around the YSO in Papillon Nebula with a sub-parsec scale. It is filled by free–free and H30 α emission. The temperature of the molecular gas around the hole reaches ∼80 K. It indicates that this YSO has just started the distruction of parental molecular cloud.« less

Kinematic Structure of Molecular Gas around High-mass YSO, Papillon Nebula, in N159 East in the Large Magellanic Cloud: A New Perspective with ALMA

NASA Astrophysics Data System (ADS)

Saigo, Kazuya; Onishi, Toshikazu; Nayak, Omnarayani; Meixner, Margaret; Tokuda, Kazuki; Harada, Ryohei; Morioka, Yuuki; Sewiło, Marta; Indebetouw, Remy; Torii, Kazufumi; Kawamura, Akiko; Ohama, Akio; Hattori, Yusuke; Yamamoto, Hiroaki; Tachihara, Kengo; Minamidani, Tetsuhiro; Inoue, Tsuyoshi; Madden, Suzanne; Galametz, Maud; Lebouteiller, Vianney; Chen, C.-H. Rosie; Mizuno, Norikazu; Fukui, Yasuo

2017-01-01

We present the ALMA Band 3 and Band 6 results of 12CO(2-1), 13CO(2-1), H30α recombination line, free-free emission around 98 GHz, and the dust thermal emission around 230 GHz toward the N159 East Giant Molecular Cloud (N159E) in the Large Magellanic Cloud (LMC). LMC is the nearest active high-mass star-forming face-on galaxy at a distance of 50 kpc and is the best target for studing high-mass star formation. ALMA observations show that N159E is the complex of filamentary clouds with the width and length of ˜1 pc and several parsecs. The total molecular mass is 0.92 × 105 M⊙ from the 13CO(2-1) intensity. N159E harbors the well-known Papillon Nebula, a compact high-excitation H II region. We found that a YSO associated with the Papillon Nebula has the mass of 35 M⊙ and is located at the intersection of three filamentary clouds. It indicates that the formation of the high-mass YSO was induced by the collision of filamentary clouds. Fukui et al. reported a similar kinematic structure toward two YSOs in the N159 West region, which are the other YSOs that have the mass of ≳35 M⊙. This suggests that the collision of filamentary clouds is a primary mechanism of high-mass star formation. We found a small molecular hole around the YSO in Papillon Nebula with a sub-parsec scale. It is filled by free-free and H30α emission. The temperature of the molecular gas around the hole reaches ˜80 K. It indicates that this YSO has just started the distruction of parental molecular cloud.

Carbon chemistry in dense molecular clouds: Theory and observational constraints

NASA Technical Reports Server (NTRS)

Blake, Geoffrey A.

1990-01-01

For the most part, gas phase models of the chemistry of dense molecular clouds predict the abundances of simple species rather well. However, for larger molecules and even for small systems rich in carbon these models often fail spectacularly. Researchers present a brief review of the basic assumptions and results of large scale modeling of the carbon chemistry in dense molecular clouds. Particular attention is to the influence of the gas phase C/O ratio in molecular clouds, and the likely role grains play in maintaining this ratio as clouds evolve from initially diffuse objects to denser cores with associated stellar and planetary formation. Recent spectral line surveys at centimeter and millimeter wavelengths along with selected observations in the submillimeter have now produced an accurate inventory of the gas phase carbon budget in several different types of molecular clouds, though gaps in our knowledge clearly remain. The constraints these observations place on theoretical models of interstellar chemistry can be used to gain insights into why the models fail, and show also which neglected processes must be included in more complete analyses. Looking toward the future, larger molecules are especially difficult to study both experimentally and theoretically in such dense, cold regions, and some new methods are therefore outlined which may ultimately push the detectability of small carbon chains and rings to much heavier species.

Molecular gas in the H II-region complex RCW 166: Possible evidence for an early phase of cloud-cloud collision prior to the bubble formation

NASA Astrophysics Data System (ADS)

Ohama, Akio; Kohno, Mikito; Fujita, Shinji; Tsutsumi, Daichi; Hattori, Yusuke; Torii, Kazufumi; Nishimura, Atsushi; Sano, Hidetoshi; Yamamoto, Hiroaki; Tachihara, Kengo; Fukui, Yasuo

2018-05-01

Young H II regions are an important site for the study of O star formation based on distributions of ionized and molecular gas. We reveal that two molecular clouds at ˜48 km s-1 and ˜53 km s-1 are associated with the H II regions G018.149-00.283 in RCW 166 by using the JCMT CO High-Resolution Survey (COHRS) of the 12CO(J = 3-2) emission. G018.149-00.283 comprises a bright ring at 8 μm and an extended H II region inside the ring. The ˜48 km s-1 cloud delineates the ring, and the ˜53 km s-1 cloud is located within the ring, indicating a complementary distribution between the two molecular components. We propose a hypothesis that high-mass stars within G018.149-00.283 were formed by triggering during cloud-cloud collision at a projected velocity separation of ˜5 km s-1. We argue that G018.149-00.283 is in an early evolutionary stage, ˜0.1 Myr after the collision according to the scheme detailed by Habe and Ohta (1992, PASJ, 44, 203), which will be followed by a bubble formation stage like RCW 120. We also suggest that nearby H II regions N21 and N22 are candidates for bubbles possibly formed by cloud-cloud collision. Inoue and Fukui (2013, ApJ, 774, L31) showed that the interface gas becomes highly turbulent and realizes a high-mass accretion rate of 10-3-10-4 M⊙ yr-1 by magnetohydrodynamical numerical simulations, which offers an explanation of the O-star formation. The fairly high frequency of cloud-cloud collision in RCW 166 is probably due to the high cloud density in this part of the Scutum arm.

Using baryon octet magnetic moments and masses to fix the pion cloud contribution

DOE PAGES

Franz L. Gross; Ramalho, Gilberto T. F.; Tsushima, Kazuo

2010-05-12

In this study, using SU(3) symmetry to constrain themore » $$\\pi BB'$$ couplings, assuming SU(3) breaking comes only from one-loop pion cloud contributions, and using the the covariant spectator theory to describe the photon coupling to the quark core, we show how the experimental masses and magnetic moments of the baryon octet can be used to set a model independent constraint on the strength of the pion cloud contributions to the octet, and hence the nucleon, form factors at $Q^2=0$.« less

Properties and rotation of molecular clouds in M 33

NASA Astrophysics Data System (ADS)

Braine, J.; Rosolowsky, E.; Gratier, P.; Corbelli, E.; Schuster, K.-F.

2018-04-01

The sample of 566 molecular clouds identified in the CO(2-1) IRAM survey covering the disk of M 33 is explored in detail. The clouds were found using CPROPS and were subsequently catalogued in terms of their star-forming properties as non-star-forming (A), with embedded star formation (B), or with exposed star formation (C, e.g., presence of Hα emission). We find that the size-linewidth relation among the M 33 clouds is quite weak but, when comparing with clouds in other nearby galaxies, the linewidth scales with average metallicity. The linewidth and particularly the line brightness decrease with galactocentric distance. The large number of clouds makes it possible to calculate well-sampled cloud mass spectra and mass spectra of subsamples. As noted earlier, but considerably better defined here, the mass spectrum steepens (i.e., higher fraction of small clouds) with galactocentric distance. A new finding is that the mass spectrum of A clouds is much steeper than that of the star-forming clouds. Further dividing the sample, this difference is strong at both large and small galactocentric distances and the A vs. C difference is a stronger effect than the inner vs. outer disk difference in mass spectra. Velocity gradients are identified in the clouds using standard techniques. The gradients are weak and are dominated by prograde rotation; the effect is stronger for the high signal-to-noise clouds. A discussion of the uncertainties is presented. The angular momenta are low but compatible with at least some simulations. Finally, the cloud velocity gradients are compared with the gradient of disk rotation. The cloud and galactic gradients are similar; the cloud rotation periods are much longer than cloud lifetimes and comparable to the galactic rotation period. The rotational kinetic energy is 1-2% of the gravitational potential energy and the cloud edge velocity is well below the escape velocity, such that cloud-scale rotation probably has little influence on the evolution of molecular clouds.

MOLECULAR CLOUDS AND CLUMPS IN THE BOSTON UNIVERSITY-FIVE COLLEGE RADIO ASTRONOMY OBSERVATORY GALACTIC RING SURVEY

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rathborne, J. M.; Johnson, A. M.; Jackson, J. M.

2009-05-15

The Boston University-Five College Radio Astronomy Observatory (BU-FCRAO) Galactic Ring Survey (GRS) of {sup 13}CO J = 1 {yields} 0 emission covers Galactic longitudes 18{sup 0} < l < 55.{sup 0}7 and Galactic latitudes |b| {<=} 1{sup 0}. Using the SEQUOIA array on the FCRAO 14 m telescope, the GRS fully sampled the {sup 13}CO Galactic emission (46'' angular resolution on a 22'' grid) and achieved a spectral resolution of 0.21 km s{sup -1}. Because the GRS uses {sup 13}CO, an optically thin tracer, rather than {sup 12}CO, an optically thick tracer, the GRS allows a much better determination ofmore » column density and also a cleaner separation of velocity components along a line of sight. With this homogeneous, fully sampled survey of {sup 13}CO emission, we have identified 829 molecular clouds and 6124 clumps throughout the inner Galaxy using the CLUMPFIND algorithm. Here we present details of the catalog and a preliminary analysis of the properties of the molecular clouds and their clumps. Moreover, we compare clouds inside and outside of the 5 kpc ring and find that clouds within the ring typically have warmer temperatures, higher column densities, larger areas, and more clumps compared with clouds located outside the ring. This is expected if these clouds are actively forming stars. This catalog provides a useful tool for the study of molecular clouds and their embedded young stellar objects.« less

Numerical Simulation on a Possible Formation Mechanism of Interplanetary Magnetic Cloud Boundaries

NASA Astrophysics Data System (ADS)

Fan, Quan-Lin; Wei, Feng-Si; Feng, Xue-Shang

2003-08-01

The formation mechanism of the interplanetary magnetic cloud (MC) boundaries is numerically investigated by simulating the interactions between an MC of some initial momentum and a local interplanetary current sheet. The compressible 2.5D MHD equations are solved. Results show that the magnetic reconnection process is a possible formation mechanism when an MC interacts with a surrounding current sheet. A number of interesting features are found. For instance, the front boundary of the MCs is a magnetic reconnection boundary that could be caused by a driven reconnection ahead of the cloud, and the tail boundary might be caused by the driving of the entrained flow as a result of the Bernoulli principle. Analysis of the magnetic field and plasma data demonstrates that at these two boundaries appear large value of the plasma parameter β, clear increase of plasma temperature and density, distinct decrease of magnetic magnitude, and a transition of magnetic field direction of about 180 degrees. The outcome of the present simulation agrees qualitatively with the observational results on MC boundary inferred from IMP-8, etc. The project supported by National Natural Science Foundation of China under Grant Nos. 40104006, 49925412, and 49990450

RCW 36 in the Vela Molecular Ridge: Evidence for high-mass star-cluster formation triggered by cloud-cloud collision

NASA Astrophysics Data System (ADS)

Sano, Hidetoshi; Enokiya, Rei; Hayashi, Katsuhiro; Yamagishi, Mitsuyoshi; Saeki, Shun; Okawa, Kazuki; Tsuge, Kisetsu; Tsutsumi, Daichi; Kohno, Mikito; Hattori, Yusuke; Yoshiike, Satoshi; Fujita, Shinji; Nishimura, Atsushi; Ohama, Akio; Tachihara, Kengo; Torii, Kazufumi; Hasegawa, Yutaka; Kimura, Kimihiro; Ogawa, Hideo; Wong, Graeme F.; Braiding, Catherine; Rowell, Gavin; Burton, Michael G.; Fukui, Yasuo

2018-02-01

A collision between two molecular clouds is one possible candidate for high-mass star formation. The H II region RCW 36, located in the Vela molecular ridge, contains a young star cluster (˜ 1 Myr old) and two O-type stars. We present new CO observations of RCW 36 made with NANTEN2, Mopra, and ASTE using 12CO(J = 1-0, 2-1, 3-2) and 13CO(J = 2-1) emission lines. We have discovered two molecular clouds lying at the velocities VLSR ˜ 5.5 and 9 km s-1. Both clouds are likely to be physically associated with the star cluster, as verified by the good spatial correspondence among the two clouds, infrared filaments, and the star cluster. We also found a high intensity ratio of ˜ 0.6-1.2 for CO J = 3-2/1-0 toward both clouds, indicating that the gas temperature has been increased due to heating by the O-type stars. We propose that the O-type stars in RCW 36 were formed by a collision between the two clouds, with a relative velocity separation of 5 km s-1. The complementary spatial distributions and the velocity separation of the two clouds are in good agreement with observational signatures expected for O-type star formation triggered by a cloud-cloud collision. We also found a displacement between the complementary spatial distributions of the two clouds, which we estimate to be 0.3 pc assuming the collision angle to be 45° relative to the line-of-sight. We estimate the collision timescale to be ˜ 105 yr. It is probable that the cluster age found by Ellerbroek et al. (2013b, A&A, 558, A102) is dominated by the low-mass members which were not formed under the triggering by cloud-cloud collision, and that the O-type stars in the center of the cluster are explained by the collisional triggering independently from the low-mass star formation.

RCW 36 in the Vela Molecular Ridge: Evidence for high-mass star-cluster formation triggered by cloud-cloud collision

NASA Astrophysics Data System (ADS)

Sano, Hidetoshi; Enokiya, Rei; Hayashi, Katsuhiro; Yamagishi, Mitsuyoshi; Saeki, Shun; Okawa, Kazuki; Tsuge, Kisetsu; Tsutsumi, Daichi; Kohno, Mikito; Hattori, Yusuke; Yoshiike, Satoshi; Fujita, Shinji; Nishimura, Atsushi; Ohama, Akio; Tachihara, Kengo; Torii, Kazufumi; Hasegawa, Yutaka; Kimura, Kimihiro; Ogawa, Hideo; Wong, Graeme F.; Braiding, Catherine; Rowell, Gavin; Burton, Michael G.; Fukui, Yasuo

2018-05-01

A collision between two molecular clouds is one possible candidate for high-mass star formation. The H II region RCW 36, located in the Vela molecular ridge, contains a young star cluster (˜ 1 Myr old) and two O-type stars. We present new CO observations of RCW 36 made with NANTEN2, Mopra, and ASTE using 12CO(J = 1-0, 2-1, 3-2) and 13CO(J = 2-1) emission lines. We have discovered two molecular clouds lying at the velocities VLSR ˜ 5.5 and 9 km s-1. Both clouds are likely to be physically associated with the star cluster, as verified by the good spatial correspondence among the two clouds, infrared filaments, and the star cluster. We also found a high intensity ratio of ˜ 0.6-1.2 for CO J = 3-2/1-0 toward both clouds, indicating that the gas temperature has been increased due to heating by the O-type stars. We propose that the O-type stars in RCW 36 were formed by a collision between the two clouds, with a relative velocity separation of 5 km s-1. The complementary spatial distributions and the velocity separation of the two clouds are in good agreement with observational signatures expected for O-type star formation triggered by a cloud-cloud collision. We also found a displacement between the complementary spatial distributions of the two clouds, which we estimate to be 0.3 pc assuming the collision angle to be 45° relative to the line-of-sight. We estimate the collision timescale to be ˜ 105 yr. It is probable that the cluster age found by Ellerbroek et al. (2013b, A&A, 558, A102) is dominated by the low-mass members which were not formed under the triggering by cloud-cloud collision, and that the O-type stars in the center of the cluster are explained by the collisional triggering independently from the low-mass star formation.

Boundary conditions for the paleoenvironment: Chemical and Physical Processes in dense interstellar clouds

NASA Technical Reports Server (NTRS)

Irvine, W. M.; Schloerb, F. P.; Ziurys, L. M.

1986-01-01

The present research includes searches for important new interstellar constituents; observations relevant to differentiating between different models for the chemical processes that are important in the interstellar environment; and coordinated studies of the chemistry, physics, and dynamics of molecular clouds which are the sites or possible future sites of star formation. Recent research has included the detection and study of four new interstellar molecules; searches which have placed upper limits on the abundance of several other potential constituents of interstellar clouds; quantitative studies of comparative molecular abundances in different types of interstellar clouds; investigation of reaction pathways for astrochemistry from a comparison of theory and the observed abundance of related species such as isomers and isotopic variants; studies of possible tracers of energenic events related to star formation, including silicon and sulfur containing molecules; and mapping of physical, chemical, and dynamical properties over extended regions of nearby cold molecular clouds.

On the origin of the Orion and Monoceros molecular cloud complexes

NASA Technical Reports Server (NTRS)

Franco, J.; Tenorio-Tagle, G.; Bodenheimer, P.; Rozyczka, M.; Mirabel, I. F.

1988-01-01

A detailed model for the origin of the Orion and Monoceros cloud complexes is presented, showing that a single high-velocity H I cloud-galaxy collision can explain their main observed features. The collision generates massive shocked layers, and self-gravity can then provide the conditions for the transformation of these layers into molecular clouds. The clouds formed by the collision maintain the motion of their parental shocked gas and reach positions located far away from the plane. According to this model, both the Orion and Monoceros complexes were formed some 60 million yr ago, when the original shocked layer was fragmented by Galactic tidal forces.

Measuring the Internal Structure and Physical Conditions in Star and Planet Forming Clouds Core: Toward a Quantitative Description of Cloud Evolution

NASA Technical Reports Server (NTRS)

Lada, Charles J.

2005-01-01

This grant funds a research program to use infrared extinction measurements to probe the detailed structure of dark molecular cloud cores and investigate the physical conditions which give rise to star and planet formation. The goals of this program are to acquire, reduce and analyze deep infrared and molecular-line observations of a carefully selected sample of nearby dark clouds in order to internal structure of starless cloud cores and to quantitatively investigate the evolution of such structure through the star and planet formation process. During the second year of this grant, progress toward these goals is discussed.

Giant molecular clouds as regions of particle acceleration

NASA Technical Reports Server (NTRS)

Dogiel, V. A.; Gurevich, A. V.; Istomin, Y. N.; Zybin, K. A.

1985-01-01

One of the most interesting results of investigations carried out on the satellites SAS-II and COS-B is the discovery of unidentified discrete gamma sources. Possibly a considerable part of them may well be giant molecular clouds. Gamma emission from clouds is caused by the processes with participation of cosmic rays. The estimation of the cosmic ray density in clouds has shown that for the energy E approx. = I GeV their density can 10 to 1000 times exceed the one in intercloud space. We have made an attempt to determine the mechanism which could lead to the increase in the cosmic ray density in clouds.

Measuring the Internal Structure and Physical Conditions in Star and Planet Forming Clouds Cores: Towards a Quantitative Description of Cloud Evolution

NASA Technical Reports Server (NTRS)

Lada, Charles J.

2004-01-01

This grant funds a research program to use infrared extinction measurements to probe the detailed structure of dark molecular cloud cores and investigate the physical conditions which give rise to star and planet formation. The goals of this program are to acquire, reduce and analyze deep infrared and molecular-line observations of a carefully selected sample of nearby dark clouds in order to determine the detailed initial conditions for star formation from quantitative measurements of the internal structure of starless cloud cores and to quantitatively investigate the evolution of such structure through the star and planet formation process.

The inception of star cluster formation revealed by [C II] emission around an Infrared Dark Cloud

NASA Astrophysics Data System (ADS)

Bisbas, Thomas G.; Tan, Jonathan C.; Csengeri, Timea; Wu, Benjamin; Lim, Wanggi; Caselli, Paola; Güsten, Rolf; Ricken, Oliver; Riquelme, Denise

2018-07-01

We present SOFIA-upGREAT observations of [C II] emission of Infrared Dark Cloud (IRDC) G035.39-00.33, designed to trace its atomic gas envelope and thus test models of the origins of such clouds. Several velocity components of [C II] emission are detected, tracing structures that are at a wide range of distances in the Galactic plane. We find a main component that is likely associated with the IRDC and its immediate surroundings. This strongest emission component has a velocity similar to that of the 13CO(2-1) emission of the IRDC, but offset by ˜3 km s-1 and with a larger velocity width of ˜9 km s-1. The spatial distribution of the [C II] emission of this component is also offset predominantly to one side of the dense filamentary structure of the IRDC. The C II column density is estimated to be of the order of ˜1017-1018 cm-2. We compare these results to the [C II] emission from numerical simulations of magnetized, dense gas filaments formed from giant molecular cloud (GMC) collisions, finding similar spatial and kinematic offsets. These observations and modellingof [C II] add further to the evidence that IRDC G035.39-00.33 has been formed by a process of GMC-GMC collision, which may thus be an important mechanism for initiating star cluster formation.

Testing the Two-Layer Model for Correcting Clear Sky Reflectance near Clouds

NASA Technical Reports Server (NTRS)

Wen, Guoyong; Marshak, Alexander; Evans, Frank; Varnai, Tamas; Levy, Rob

2015-01-01

A two-layer model (2LM) was developed in our earlier studies to estimate the clear sky reflectance enhancement due to cloud-molecular radiative interaction at MODIS at 0.47 micrometers. Recently, we extended the model to include cloud-surface and cloud-aerosol radiative interactions. We use the LES/SHDOM simulated 3D true radiation fields to test the 2LM for reflectance enhancement at 0.47 micrometers. We find: The simple model captures the viewing angle dependence of the reflectance enhancement near cloud, suggesting the physics of this model is correct; the cloud-molecular interaction alone accounts for 70 percent of the enhancement; the cloud-surface interaction accounts for 16 percent of the enhancement; the cloud-aerosol interaction accounts for an additional 13 percent of the enhancement. We conclude that the 2LM is simple to apply and unbiased.

Optical observations related to the molecular chemistry in diffuse interstellar clouds

NASA Technical Reports Server (NTRS)

Federman, S. R.

1987-01-01

Observations, which have been published since 1979, of molecular species in diffuse clouds are discussed. Particular attention is given to the ultraviolet measurements of CO with the Copernicus and IUE satellites and to ground-based optical measurements of CH, CH(+), CN, and 02. These data encompass large enough samples to test the chemical schemes expected to occur in diffuse clouds. Upper limits for other species (e.g., H2O, H2O(+), and C3) place restrictions on the pathways for molecular production. Moreover, analysis of the rotational distribution of the C2 molecule results in the determination of the physical conditions of the cloud. These parameters, including density, temperature, and the intensity of the radiation field, are necessary for modeling the chemistry.

Pillars of Creation among Destruction: Star Formation in Molecular Clouds near R136 in 30 Doradus

NASA Astrophysics Data System (ADS)

Kalari, Venu M.; Rubio, Mónica; Elmegreen, Bruce G.; Guzmán, Viviana V.; Zinnecker, Hans; Herrera, Cinthya N.

2018-01-01

We present new sensitive CO(2–1) observations of the 30 Doradus region in the Large Magellanic Cloud. We identify a chain of three newly discovered molecular clouds that we name KN1, KN2, and KN3 lying within 2–14 pc in projection from the young massive cluster R136 in 30 Doradus. Excited H2 2.12 μm emission is spatially coincident with the molecular clouds, but ionized Brγ emission is not. We interpret these observations as the tails of pillar-like structures whose ionized heads are pointing toward R136. Based on infrared photometry, we identify a new generation of stars forming within this structure.

Structure and extent of the giant molecular cloud near M17

DOE Office of Scientific and Technical Information (OSTI.GOV)

Elmegreen, B.G.; Lada, C.J.; Dickinson, D.F.

1979-06-01

Carbon monoxide emission at ..nu../sub LSR/ = 20 +- 2 km s/sup -1/ is found to extend 4/sup 0/ (approx.170 pc) southwest of M17, and is studied in an attempt to understand the internal structure and dynamics of a giant molecular cloud complex. The region contains two primary clouds. The first has at least 2 x 10/sup 5/ M/sub sun/ of molecular gas and extends for 1./sup 0/8 (72 pc) parallel to, but below the galactic plane southwest of M17. The second, located above the plane approximately 2./sup 0/5 southwest of M17, is about 1./sup 0/7 in extent, but containsmore » considerably less molecular mass (> or approx. =3 x 10/sup 4/ M/sub sun/). Between these two clouds is a 1/sup 0/ long region of relatively low intensity, clumpy CO emission which appears to bridge the two main clouds. The molecular mass within this bridge is estimated to be 2 x 10/sup 4/ M/sub sun/. The cloud associated with M17 is itself divided into four discrete fragments of approximately equal mass (4 x 10/sup 4/ M/sub sun/). The /sup 12/CO and /sup 13/CO line widths are higher in these four fragments than they are between the fragments. OB star formation is active only in the northeastern two of these fragments. The /sup 13/CO line widths in the discrete fragments satisfy the virial theorem for the derived masses. (b) The /sup 13/CO velocity structure in the large complex containing M17 shows a gradual change from regularity in the northeast to irregularity and occasionally multipeaked profiles in the southwest. This change corresponds to a gradient in the degree of compactness and intensity of star formation in the four fragments. A massive (10/sup 5/ M/sub sun/) molecular cloud complex associated with M16, 2/sup 0/ north of M17, and the two clouds southwest of M17, form a pattern of equally spaced star-forming clouds whose positions alternate above and below the galactic plane. Patchy CO emission is found between these three objects. The entire region of molecular emission is approx.250 pc long.« less

Fast Transverse Beam Instability Caused by Electron Cloud Trapped in Combined Function Magnets

DOE Office of Scientific and Technical Information (OSTI.GOV)

Antipov, Sergey

Electron cloud instabilities affect the performance of many circular high-intensity particle accelerators. They usually have a fast growth rate and might lead to an increase of the transverse emittance and beam loss. A peculiar example of such an instability is observed in the Fermilab Recycler proton storage ring. Although this instability might pose a challenge for future intensity upgrades, its nature had not been completely understood. The phenomena has been studied experimentally by comparing the dynamics of stable and unstable beam, numerically by simulating the build-up of the electron cloud and its interaction with the beam, and analytically by constructing a model of an electron cloud driven instability with the electrons trapped in combined function dipoles. Stabilization of the beam by a clearing bunch reveals that the instability is caused by the electron cloud, trapped in beam optics magnets. Measurements of microwave propagation confirm the presence of the cloud in the combined function dipoles. Numerical simulations show that up to 10more » $$^{-2}$$ of the particles can be trapped by their magnetic field. Since the process of electron cloud build-up is exponential, once trapped this amount of electrons significantly increases the density of the cloud on the next revolution. In a combined function dipole this multi-turn accumulation allows the electron cloud reaching final intensities orders of magnitude greater than in a pure dipole. The estimated fast instability growth rate of about 30 revolutions and low mode frequency of 0.4 MHz are consistent with experimental observations and agree with the simulations. The created instability model allows investigating the beam stability for the future intensity upgrades.« less

Cloud-based calculators for fast and reliable access to NOAA's geomagnetic field models

NASA Astrophysics Data System (ADS)

Woods, A.; Nair, M. C.; Boneh, N.; Chulliat, A.

2017-12-01

While the Global Positioning System (GPS) provides accurate point locations, it does not provide pointing directions. Therefore, the absolute directional information provided by the Earth's magnetic field is of primary importance for navigation and for the pointing of technical devices such as aircrafts, satellites and lately, mobile phones. The major magnetic sources that affect compass-based navigation are the Earth's core, its magnetized crust and the electric currents in the ionosphere and magnetosphere. NOAA/CIRES Geomagnetism (ngdc.noaa.gov/geomag/) group develops and distributes models that describe all these important sources to aid navigation. Our geomagnetic models are used in variety of platforms including airplanes, ships, submarines and smartphones. While the magnetic field from Earth's core can be described in relatively fewer parameters and is suitable for offline computation, the magnetic sources from Earth's crust, ionosphere and magnetosphere require either significant computational resources or real-time capabilities and are not suitable for offline calculation. This is especially important for small navigational devices or embedded systems, where computational resources are limited. Recognizing the need for a fast and reliable access to our geomagnetic field models, we developed cloud-based application program interfaces (APIs) for NOAA's ionospheric and magnetospheric magnetic field models. In this paper we will describe the need for reliable magnetic calculators, the challenges faced in running geomagnetic field models in the cloud in real-time and the feedback from our user community. We discuss lessons learned harvesting and validating the data which powers our cloud services, as well as our strategies for maintaining near real-time service, including load-balancing, real-time monitoring, and instance cloning. We will also briefly talk about the progress we achieved on NOAA's Big Earth Data Initiative (BEDI) funded project to develop API interface to our Enhanced Magnetic Model (EMM).

Light-element nucleosynthesis in a molecular cloud interacting with a supernova remnant and the origin of beryllium-10 in the protosolar nebula

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tatischeff, Vincent; Duprat, Jean; De Séréville, Nicolas, E-mail: Vincent.Tatischeff@csnsm.in2p3.fr

The presence of short-lived radionuclides (t {sub 1/2} < 10 Myr) in the early solar system provides important information about the astrophysical environment in which the solar system formed. The discovery of now extinct {sup 10}Be (t {sub 1/2} = 1.4 Myr) in calcium-aluminum-rich inclusions (CAIs) with Fractionation and Unidentified Nuclear isotope anomalies (FUN-CAIs) suggests that a baseline concentration of {sup 10}Be in the early solar system was inherited from the protosolar molecular cloud. In this paper, we investigate various astrophysical contexts for the nonthermal nucleosynthesis of {sup 10}Be by cosmic-ray-induced reactions. We first show that the {sup 10}Be recordedmore » in FUN-CAIs cannot have been produced in situ by irradiation of the FUN-CAIs themselves. We then show that trapping of Galactic cosmic rays (GCRs) in the collapsing presolar cloud core induced a negligible {sup 10}Be contamination of the protosolar nebula, the inferred {sup 10}Be/{sup 9}Be ratio being at least 40 times lower than that recorded in FUN-CAIs ({sup 10}Be/{sup 9}Be ∼ 3 × 10{sup –4}). Irradiation of the presolar molecular cloud by background GCRs produced a steady-state {sup 10}Be/{sup 9}Be ratio ≲ 1.3 × 10{sup –4} at the time of the solar system formation, which suggests that the presolar cloud was irradiated by an additional source of CRs. Considering a detailed model for CR acceleration in a supernova remnant (SNR), we find that the {sup 10}Be abundance recorded in FUN-CAIs can be explained within two alternative scenarios: (1) the irradiation of a giant molecular cloud by CRs produced by ≳ 50 supernovae exploding in a superbubble of hot gas generated by a large star cluster of at least 20,000 members, and (2) the irradiation of the presolar molecular cloud by freshly accelerated CRs escaped from an isolated SNR at the end of the Sedov-Taylor phase. In the second picture, the SNR resulted from the explosion of a massive star that ran away from its parent OB association, expanded during most of its adiabatic phase in an intercloud medium of density of about 1 H-atom cm{sup –3}, and eventually interacted with the presolar molecular cloud only during the radiative stage. This model naturally provides an explanation for the injection of other short-lived radionuclides of stellar origin into the cold presolar molecular cloud ({sup 26}Al, {sup 41}Ca, and {sup 36}Cl) and is in agreement with the solar system originating from the collapse of a molecular cloud shocked by a supernova blast wave.« less

Very Large Array OH Zeeman Observations of the Star-forming Region S88B

NASA Astrophysics Data System (ADS)

Sarma, A. P.; Brogan, C. L.; Bourke, T. L.; Eftimova, M.; Troland, T. H.

2013-04-01

We present observations of the Zeeman effect in OH thermal absorption main lines at 1665 and 1667 MHz taken with the Very Large Array toward the star-forming region S88B. The OH absorption profiles toward this source are complicated, and contain several blended components toward a number of positions. Almost all of the OH absorbing gas is located in the eastern parts of S88B, toward the compact continuum source S88B-2 and the eastern parts of the extended continuum source S88B-1. The ratio of 1665/1667 MHz OH line intensities indicates the gas is likely highly clumped, in agreement with other molecular emission line observations in the literature. S88-B appears to present a similar geometry to the well-known star-forming region M17, in that there is an edge-on eastward progression from ionized to molecular gas. The detected magnetic fields appear to mirror this eastward transition; we detected line-of-sight magnetic fields ranging from 90 to 400 μG, with the lowest values of the field to the southwest of the S88B-1 continuum peak, and the highest values to its northeast. We used the detected fields to assess the importance of the magnetic field in S88B by a number of methods; we calculated the ratio of thermal to magnetic pressures, we calculated the critical field necessary to completely support the cloud against self-gravity and compared it to the observed field, and we calculated the ratio of mass to magnetic flux in terms of the critical value of this parameter. All these methods indicated that the magnetic field in S88B is dynamically significant, and should provide an important source of support against gravity. Moreover, the magnetic energy density is in approximate equipartition with the turbulent energy density, again pointing to the importance of the magnetic field in this region.

The Spitzer Survey of Interstellar Clouds in the Gould Belt. VI. The Auriga-California Molecular Cloud Observed with IRAC and MIPS

NASA Technical Reports Server (NTRS)

Broekhoven-Fiene, Hannah; Matthews, Brenda C.; Harvey, Paul M.; Gutermuth, Robert A.; Huard, Tracy L.; Tothill, Nicholas F. H.; Nutter, David; Bourke, Tyler L.; DiFrancesco, James; Jorgensen, Jes K.;

ALMA Observations of Molecular Clouds in Three Group-centered Elliptical Galaxies: NGC 5846, NGC 4636, and NGC 5044

NASA Astrophysics Data System (ADS)

Temi, Pasquale; Amblard, Alexandre; Gitti, Myriam; Brighenti, Fabrizio; Gaspari, Massimo; Mathews, William G.; David, Laurence

2018-05-01

We present new ALMA CO(2–1) observations of two well-studied group-centered elliptical galaxies: NGC 4636 and NGC 5846. In addition, we include a revised analysis of Cycle 0 ALMA observations of the central galaxy in the NGC 5044 group. We find evidence that molecular gas is a common presence in bright group-centered galaxies (BGG). CO line widths are broader than Galactic molecular clouds, and using the reference Milky Way X CO, the total molecular mass ranges from 2.6 × 105 M ⊙ in NGC 4636 to 6.1 × 107 M ⊙ in NGC 5044. Complementary observations using the ALMA Compact Array do not exhibit any detection of a CO diffuse component at the sensitivity level achieved by current exposures. The origin of the detected molecular features is still uncertain, but these ALMA observations suggest that they are the end product of the hot gas cooling process and not the result of merger events. Some of the molecular clouds are associated with dust features as revealed by HST dust extinction maps, suggesting that these clouds formed from dust-enhanced cooling. The global nonlinear condensation may be triggered via the chaotic turbulent field or buoyant uplift. The large virial parameter of the molecular structures and correlation with the warm ({10}3{--}{10}5 {{K}})/hot (≥106) phase velocity dispersion provide evidence that they are unbound giant molecular associations drifting in the turbulent field, consistent with numerical predictions of the chaotic cold accretion process. Alternatively, the observed large CO line widths may be generated by molecular gas flowing out from cloud surfaces due to heating by the local hot gas atmosphere.

Millimeter-wave Molecular Line Observations of the Tornado Nebula

NASA Astrophysics Data System (ADS)

Sakai, D.; Oka, T.; Tanaka, K.; Matsumura, S.; Miura, K.; Takekawa, S.

2014-08-01

We report the results of millimeter-wave molecular line observations of the Tornado Nebula (G357.7-0.1), which is a bright radio source behind the Galactic center region. A 15' × 15' area was mapped in the J = 1-0 lines of CO, 13CO, and HCO+ with the Nobeyama Radio Observatory 45 m telescope. The Very Large Array archival data of OH at 1720 MHz were also reanalyzed. We found two molecular clouds with separate velocities, V LSR = -14 km s-1 and +5 km s-1. These clouds show rough spatial anti-correlation. Both clouds are associated with OH 1720 MHz emissions in the area overlapping with the Tornado Nebula. The spatial and velocity coincidence indicates violent interaction between the clouds and the Tornado Nebula. Modestly excited gas prefers the position of the Tornado "head" in the -14 km s-1 cloud, also suggesting the interaction. Virial analysis shows that the +5 km s-1 cloud is more tightly bound by self-gravity than the -14 km s-1 cloud. We propose a formation scenario for the Tornado Nebula; the +5 km s-1 cloud collided into the -14 km s-1 cloud, generating a high-density layer behind the shock front, which activates a putative compact object by Bondi-Hoyle-Lyttleton accretion to eject a pair of bipolar jets.

ALMA Detects CO(3-2) within a Super Star Cluster in NGC 5253

NASA Astrophysics Data System (ADS)

Turner, Jean L.; Consiglio, S. Michelle; Beck, Sara C.; Goss, W. M.; Ho, Paul. T. P.; Meier, David S.; Silich, Sergiy; Zhao, Jun-Hui

2017-09-01

We present observations of CO(3-2) and 13CO(3-2) emission near the supernebula in the dwarf galaxy NGC 5253, which contains one of the best examples of a potential globular cluster in formation. The 0.″3 resolution images reveal an unusual molecular cloud, “Cloud D1,” that is coincident with the radio-infrared supernebula. The ˜6 pc diameter cloud has a linewidth, Δ v = 21.7 {km} {{{s}}}-1, that reflects only the gravitational potential of the star cluster residing within it. The corresponding virial mass is 2.5 × 105 {M}⊙ . The cluster appears to have a top-heavy initial mass function, with M * ≳ 1-2 {M}⊙ . Cloud D1 is optically thin in CO(3-2), probably because the gas is hot. Molecular gas mass is very uncertain but constitutes <35% of the dynamical mass within the cloud boundaries. In spite of the presence of an estimated ˜1500-2000 O stars within the small cloud, the CO appears relatively undisturbed. We propose that Cloud D1 consists of molecular clumps or cores, possibly star-forming, orbiting with more evolved stars in the core of the giant cluster.

Ambiguities in the identification of giant molecular cloud complexes from longitude-velocity diagrams

NASA Technical Reports Server (NTRS)

Adler, David S.; Roberts, William W., Jr.

1992-01-01

Techniques which use longitude-velocity diagrams to identify molecular cloud complexes in the disk of the Galaxy are investigated by means of model Galactic disks generated from N-body cloud-particle simulations. A procedure similar to the method used to reduce the low-level emission in Galactic l-v diagrams is employed to isolate complexes of emission in the model l-v diagram (LVCs) from the 'background'clouds. The LVCs produced in this manner yield a size-line-width relationship with a slope of 0.58 and a mass spectrum with a slope of 1.55, consistent with Galactic observations. It is demonstrated that associations identified as LVCs are often chance superpositions of clouds spread out along the line of sight in the disk of the model system. This indicates that the l-v diagram cannot be used to unambiguously determine the location of molecular cloud complexes in the model Galactic disk. The modeling results also indicate that the existence of a size-line-width relationship is not a reliable indicator of the physical nature of cloud complexes, in particular, whether the complexes are gravitationally bound objects.

Dust clouds and plasmoids in Saturn's magnetosphere as seen with four Cassini instruments

NASA Astrophysics Data System (ADS)

Khalisi, Emil

2017-03-01

We revisit the evidence for a ;dust cloud; observed by the Cassini spacecraft at Saturn in 2006. The data of four instruments are simultaneously compared to interpret the signatures of a coherent swarm of dust that would have remained near the equatorial plane for as long as six weeks. The conspicuous pattern, as seen in the dust counters of the Cosmic Dust Analyser (CDA), clearly repeats on three consecutive revolutions of the spacecraft. That particular cloud is estimated to about 1.36 Saturnian radii in size, and probably broadening. We also present a reconnection event from the magnetic field data (MAG) that leave behind several plasmoids like those reported from the Voyager flybys in the early 1980s. That magnetic bubbles happened at the dawn side of Saturn's magnetosphere. At their nascency, the magnetic field showed a switchover of its alignment, disruption of flux tubes and a recovery on a time scale of about 30 days. However, we cannot rule out that different events might have taken place. Empirical evidence is shown at another occasion when a plasmoid was carrying a cloud of tiny dust particles such that a connection between plasmoids and coherent dust clouds is probable.

The impact of different interstellar medium structures on the dynamical evolution of supernova remnants

NASA Astrophysics Data System (ADS)

Wang, Yueyang; Bao, Biwen; Yang, Chuyuan; Zhang, Li

2018-05-01

The dynamical properties of supernova remnants (SNRs) evolving with different interstellar medium structures are investigated through performing extensive two-dimensional magnetohydrodynamic (MHD) simulations in the cylindrical symmetry. Three cases of different interstellar medium structures are considered: the uniform medium, the turbulent medium and the cloudy medium. Large-scale density and magnetic fluctuations are calculated and mapped into the computational domain before simulations. The clouds are set by random distribution in advance. The above configuration allows us to study the time-dependent dynamical properties and morphological evolution of the SNR evolving with different ambient structures, along with the development of the instabilities at the contact discontinuity. Our simulation results indicate that remnant morphology deviates from symmetry if the interstellar medium contains clouds or turbulent density fluctuations. In the cloudy medium case, interactions between the shock wave and clouds lead to clouds' fragmentation. The magnetic field can be greatly enhanced by stretching field lines with a combination of instabilities while the width of amplification region is quite different among the three cases. Moreover, both the width of amplification region and the maximum magnetic-field strength are closely related to the clouds' density.

The SOLA Team: A Star Formation Project To Study the Soul of Lupus with ALMA

NASA Astrophysics Data System (ADS)

De Gregorio-Monsalvo, Itziar; Saito, M.; Rodon, J.; Takahashi, S.

2017-06-01

The SOLA team is a multi-national and multi-wavelength collaboration composed by scientists with technical expertise in ALMA and in infrared and optical techniques. The aim of the team is to establish a low-mass star formation scenario based on the Lupus molecular clouds. In this talk I will present our unique catalog of pre-stellar and proto-stellar cores toward Lupus molecular clouds, the results on our latest studies in protoplanetary disks, as well as our ALMA Cycle 3 data aiming at testing the formation mechanism of sub-stellar objects in Lupus molecular clouds.

Low-Frequency Carbon Recombination Lines in the Orion Molecular Cloud Complex

NASA Astrophysics Data System (ADS)

Tremblay, Chenoa D.; Jordan, Christopher H.; Cunningham, Maria; Jones, Paul A.; Hurley-Walker, Natasha

2018-05-01

We detail tentative detections of low-frequency carbon radio recombination lines from within the Orion molecular cloud complex observed at 99-129 MHz. These tentative detections include one alpha transition and one beta transition over three locations and are located within the diffuse regions of dust observed in the infrared at 100 μm, the Hα emission detected in the optical, and the synchrotron radiation observed in the radio. With these observations, we are able to study the radiation mechanism transition from collisionally pumped to radiatively pumped within the H ii regions within the Orion molecular cloud complex.

Embedded Clusters

NASA Astrophysics Data System (ADS)

Ascenso, Joana

The past decade has seen an increase of star formation studies made at the molecular cloud scale, motivated mostly by the deployment of a wealth of sensitive infrared telescopes and instruments. Embedded clusters, long recognised as the basic units of coherent star formation in molecular clouds, are now seen to inhabit preferentially cluster complexes tens of parsecs across. This chapter gives an overview of some important properties of the embedded clusters in these complexes and of the complexes themselves, along with the implications of viewing star formation as a molecular-cloud scale process rather than an isolated process at the scale of clusters.

Wide-field 12CO (J=2-1) and 13CO (J=2-1) Observations toward the Aquila Rift and Serpens Molecular Cloud Complexes. I. Molecular Clouds and Their Physical Properties

NASA Astrophysics Data System (ADS)

Nakamura, Fumitaka; Dobashi, Kazuhito; Shimoikura, Tomomi; Tanaka, Tomohiro; Onishi, Toshikazu

2017-03-01

We present the results of wide-field 12CO (J=2{--}1) and 13CO (J=2{--}1) observations toward the Aquila Rift and Serpens molecular cloud complexes (25^\\circ < l< 33^\\circ and 1^\\circ < b< 6^\\circ ) at an angular resolution of 3.‧4 (≈ 0.25 pc) and at a velocity resolution of 0.079 km s-1 with velocity coverage of -5 {km} {{{s}}}-1< {V}{LSR}< 35 {km} {{{s}}}-1. We found that the 13CO emission better traces the structures seen in the extinction map, and derived the {X}{13{CO}}-factor of this region. Applying SCIMES to the 13CO data cube, we identified 61 clouds and derived their mass, radii, and line widths. The line width-radius relation of the identified clouds basically follows those of nearby molecular clouds. The majority of the identified clouds are close to virial equilibrium, although the dispersion is large. By inspecting the 12CO channel maps by eye, we found several arcs that are spatially extended to 0.°2-3° in length. In the longitude-velocity diagrams of 12CO, we also found two spatially extended components that appear to converge toward Serpens South and the W40 region. The existence of two components with different velocities and arcs suggests that large-scale expanding bubbles and/or flows play a role in the formation and evolution of the Serpens South and W40 cloud.

Fourier transform ion cyclotron resonance mass resolution and dynamic range limits calculated by computer modeling of ion cloud motion.

PubMed

Vladimirov, Gleb; Hendrickson, Christopher L; Blakney, Greg T; Marshall, Alan G; Heeren, Ron M A; Nikolaev, Eugene N

2012-02-01

Particle-in-Cell (PIC) ion trajectory calculations provide the most realistic simulation of Fourier transform ion cyclotron resonance (FT-ICR) experiments by efficient and accurate calculation of the forces acting on each ion in an ensemble (cloud), including Coulomb interactions (space charge), the electric field of the ICR trap electrodes, image charges on the trap electrodes, the magnetic field, and collisions with neutral gas molecules. It has been shown recently that ion cloud collective behavior is required to generate an FT-ICR signal and that two main phenomena influence mass resolution and dynamic range. The first is formation of an ellipsoidal ion cloud (termed "condensation") at a critical ion number (density), which facilitates signal generation in an FT-ICR cell of arbitrary geometry because the condensed cloud behaves as a quasi-ion. The second phenomenon is peak coalescence. Ion resonances that are closely spaced in m/z coalesce into one resonance if the ion number (density) exceeds a threshold that depends on magnetic field strength, ion cyclotron radius, ion masses and mass difference, and ion initial spatial distribution. These two phenomena decrease dynamic range by rapid cloud dephasing at small ion density and by cloud coalescence at high ion density. Here, we use PIC simulations to quantitate the dependence of coalescence on each critical parameter. Transitions between independent and coalesced motion were observed in a series of the experiments that systematically varied ion number, magnetic field strength, ion radius, ion m/z, ion m/z difference, and ion initial spatial distribution (the present simulations begin from elliptically-shaped ion clouds with constant ion density distribution). Our simulations show that mass resolution is constant at a given magnetic field strength with increasing ion number until a critical value (N) is reached. N dependence on magnetic field strength, cyclotron radius, ion mass, and difference between ion masses was determined for two ion ensembles of different m/z, equal abundance, and equal cyclotron radius. We find that N and dynamic range depend quadratically on magnetic field strength in the range 1-21 Tesla. Dependences on cyclotron radius and Δm/z are linear. N depends on m/z as (m/z)(-2). Empirical expressions for mass resolution as a function of each of the experimental parameters are presented. Here, we provide the first exposition of the origin and extent of trade-off between FT-ICR MS dynamic range and mass resolution (defined not as line width, but as the separation between the most closely resolved masses). © American Society for Mass Spectrometry, 2011

Tidal disruption of open clusters in their parent molecular clouds

NASA Technical Reports Server (NTRS)

Long, Kevin

1989-01-01

A simple model of tidal encounters has been applied to the problem of an open cluster in a clumpy molecular cloud. The parameters of the clumps are taken from the Blitz, Stark, and Long (1988) catalog of clumps in the Rosette molecular cloud. Encounters are modeled as impulsive, rectilinear collisions between Plummer spheres, but the tidal approximation is not invoked. Mass and binding energy changes during an encounter are computed by considering the velocity impulses given to individual stars in a random realization of a Plummer sphere. Mean rates of mass and binding energy loss are then computed by integrating over many encounters. Self-similar evolutionary calculations using these rates indicate that the disruption process is most sensitive to the cluster radius and relatively insensitive to cluster mass. The calculations indicate that clusters which are born in a cloud similar to the Rosette with a cluster radius greater than about 2.5 pc will not survive long enough to leave the cloud. The majority of clusters, however, have smaller radii and will survive the passage through their parent cloud.

Cosmic-rays, gas, and dust in nearby anticentre clouds. II. Interstellar phase transitions and the dark neutral medium

NASA Astrophysics Data System (ADS)

Remy, Q.; Grenier, I. A.; Marshall, D. J.; Casandjian, J. M.

2018-03-01

Aim. H I 21-cm and 12CO 2.6-mm line emissions trace the atomic and molecular gas phases, respectively, but they miss most of the opaque H I and diffuse H2 present in the dark neutral medium (DNM) at the transition between the H I-bright and CO-bright regions. Jointly probing H I, CO, and DNM gas, we aim to constrain the threshold of the H I-H2 transition in visual extinction, AV, and in total hydrogen column densities, NHtot. We also aim to measure gas mass fractions in the different phases and to test their relation to cloud properties. Methods: We have used dust optical depth measurements at 353 GHz, γ-ray maps at GeV energies, and H I and CO line data to trace the gas column densities and map the DNM in nearby clouds toward the Galactic anticentre and Chamaeleon regions. We have selected a subset of 15 individual clouds, from diffuse to star-forming structures, in order to study the different phases across each cloud and to probe changes from cloud to cloud. Results: The atomic fraction of the total hydrogen column density is observed to decrease in the (0.6-1) × 1021 cm-2 range in NHtot (AV ≈ 0.4 mag) because of the formation of H2 molecules. The onset of detectable CO intensities varies by only a factor of 4 from cloud to cloud, between 0.6 × 1021 cm-2 and 2.5 × 1021 cm-2 in total gas column density. We observe larger H2 column densities than linearly inferred from the CO intensities at AV > 3 mag because of the large CO optical thickness; the additional H2 mass in this regime represents on average 20% of the CO-inferred molecular mass. In the DNM envelopes, we find that the fraction of diffuse CO-dark H2 in the molecular column densities decreases with increasing AV in a cloud. For a half molecular DNM, the fraction decreases from more than 80% at 0.4 mag to less than 20% beyond 2 mag. In mass, the DNM fraction varies with the cloud properties. Clouds with low peak CO intensities exhibit large CO-dark H2 fractions in molecular mass, in particular the diffuse clouds lying at high altitude above the Galactic plane. The mass present in the DNM envelopes appears to scale with the molecular mass seen in CO as MHDNM = 62 ± 7 MH2CO0.51 ± 0.02 across two decades in mass. Conclusions: The phase transitions in these clouds show both common trends and environmental differences. These findings will help support the theoretical modelling of H2 formation and the precise tracing of H2 in the interstellar medium.

The Chemistry and Excitation of Water in Molecular Clouds

NASA Technical Reports Server (NTRS)

Hollenbach, David

2003-01-01

We model the chemistry and thermal balance of opaque molecular clouds exposed to an external flux of ultraviolet photons. We include the processes of gas phase and grain surface chemical reactions; in particular we examine closely the freezing of atoms and molecules onto grain surfaces and the desorption of molecules from grain surfaces as a function of depth into a molecular cloud. We find that on the surface of a molecular cloud the gas phase water abundances are low because of photodissociation, and the grain phase water (ice) abundance is low because of photodesorption of water from the grain surfaces. Deeper into the cloud, at A(sub v) less than or approximately 2-8 depending on the strength of the external ultraviolet flux, the gas phase water abundance increases with depth as the photodissociation rates decline due to dust attenuation of the ultraviolet field. However, beyond A(sub v) less than or approximately 2-8 the gas phase water abundance declines because the water freezes as water ice on the grains, and photodesorption is no longer effective in clearing the ice. A peak water abundance of about 10(exp -6) to 10(exp -7) occurs at about A(sub v) approximately 2-8, relatively independent of the gas density and the ultraviolet field. We show that such a model matches very closely the observations of the Submillimeter Wave Astronomical Satellite (SWAS), a NASA Small Explorer Mission. The model elucidates several mechanisms that have been recently invoked to understand gas phase chemistry in clouds, including-the freeze-out of molecules onto grain surface, the desorption of these molecules from the surfaces, and the abundance gradients of molecules as functions of depth into molecular clouds.

The bird: A pressure-confined explosion in the interstellar medium

NASA Technical Reports Server (NTRS)

Lane, A. P.; Stark, A. A.; Helfand, D. J.

1986-01-01

The non-thermal radio continuum source G5.3-1.0, mapped at 20 cm with the Very Large Array (VLA) by Becker and Helfand, has an unusual bird-like shape. In order to determine possible interaction of this source with adjacent cold gas, we have mapped this region in the J=1-0 line of CO using the AT and T Bell Laboratories 7m antenna and the FCRAO 14m antenna. The map shown contains 1859 spectra sampled on a 1.5 arcminute grid; each spectrum has an rms noise of 0.2 K in 1 MHz channels. There are several molecular clouds at different velocities along the line of sight. The outer regions of a previously unknown Giant Molecular Cloud (GMC) at l=4.7 deg., b=-0.85 deg., v=200 km s(-1) appears to be interacting with G5.3-10: the molecular cloud has a bird-shaped hole at the position of the continuum source, except that the brightest continuum point (the bird's head) appears to be embedded in the cloud. The velocity of this GMC indicates it is within 2 kpc of the galactic center. The morphology suggests that a supernova or other explosive event occurred near the outside of the GMC, in a region where (n) is approximately 300 cm(-3), and expanded into a region of lower density and pressure. The pressures, densities, and velocity gradients of molecular clouds near the galactic center are on average higher than those of clouds near the Sun. We therefore expect that Type II supernovae near the galactic center would be distorted by their interactions with their parent molecular clouds.

Infrared Polarization of the Molecular Cloud Associated to IRAS 18236-1205

NASA Astrophysics Data System (ADS)

Luna, A.; Retes, R.; Devaraj, R.; Maya, Y. D.; Carrasco, L.

2017-07-01

We present the near-infrared polarization observations towards the star forming molecular cloud associated with the IRAS source 18236-1205, obtained with the near-infrared (NIR) imaging polarimeter POLICAN at the Guillermo Haro Astrophysical Observatory in Cananea, Sonora, México.

Disruption of Giant Molecular Clouds by Massive Star Clusters

NASA Astrophysics Data System (ADS)

Harper-Clark, Elizabeth

The lifetime of a Giant Molecular Cloud (GMC) and the total mass of stars that form within it are crucial to the understanding of star formation rates across a whole galaxy. In particular, the stars within a GMC may dictate its disruption and the quenching of further star formation. Indeed, observations show that the Milky Way contains GMCs with extensive expanding bubbles while the most massive stars are still alive. Simulating entire GMCs is challenging, due to the large variety of physics that needs to be included, and the computational power required to accurately simulate a GMC over tens of millions of years. Using the radiative-magneto-hydrodynamic code Enzo, I have run many simulations of GMCs. I obtain robust results for the fraction of gas converted into stars and the lifetimes of the GMCs: (A) In simulations with no stellar outputs (or "feedback''), clusters form at a rate of 30% of GMC mass per free fall time; the GMCs were not disrupted but contained forming stars. (B) Including ionization gas pressure or radiation pressure into the simulations, both separately and together, the star formation was quenched at between 5% and 21% of the original GMC mass. The clouds were fully disrupted within two dynamical times after the first cluster formed. The radiation pressure contributed the most to the disruption of the GMC and fully quenched star formation even without ionization. (C) Simulations that included supernovae showed that they are not dynamically important to GMC disruption and have only minor effects on subsequent star formation. (D) The inclusion of a few micro Gauss magnetic field across the cloud slightly reduced the star formation rate but accelerated GMC disruption by reducing bubble shell disruption and leaking. These simulations show that new born stars quench further star formation and completely disrupt the parent GMC. The low star formation rate and the short lifetimes of GMCs shown here can explain the low star formation rate across the whole galaxy.

Influence of galactic arm scale dynamics on the molecular composition of the cold and dense ISM. I. Observed abundance gradients in dense clouds

NASA Astrophysics Data System (ADS)

Ruaud, M.; Wakelam, V.; Gratier, P.; Bonnell, I. A.

2018-04-01

Aim. We study the effect of large scale dynamics on the molecular composition of the dense interstellar medium during the transition between diffuse to dense clouds. Methods: We followed the formation of dense clouds (on sub-parsec scales) through the dynamics of the interstellar medium at galactic scales. We used results from smoothed particle hydrodynamics (SPH) simulations from which we extracted physical parameters that are used as inputs for our full gas-grain chemical model. In these simulations, the evolution of the interstellar matter is followed for 50 Myr. The warm low-density interstellar medium gas flows into spiral arms where orbit crowding produces the shock formation of dense clouds, which are held together temporarily by the external pressure. Results: We show that depending on the physical history of each SPH particle, the molecular composition of the modeled dense clouds presents a high dispersion in the computed abundances even if the local physical properties are similar. We find that carbon chains are the most affected species and show that these differences are directly connected to differences in (1) the electronic fraction, (2) the C/O ratio, and (3) the local physical conditions. We argue that differences in the dynamical evolution of the gas that formed dense clouds could account for the molecular diversity observed between and within these clouds. Conclusions: This study shows the importance of past physical conditions in establishing the chemical composition of the dense medium.

Gas-grain chemical models of star-forming molecular clouds as constrained by ISO and SWAS observations

NASA Astrophysics Data System (ADS)

Charnley, S. B.; Rodgers, S. D.; Ehrenfreund, P.

2001-11-01

We have investigated the gaseous and solid state molecular composition of dense interstellar material that periodically experiences processing in the shock waves associated with ongoing star formation. Our motivation is to confront these models with the stringent abundance constraints on CO2, H2O and O2, in both gas and solid phases, that have been set by ISO and SWAS. We also compare our results with the chemical composition of dark molecular clouds as determined by ground-based telescopes. Beginning with the simplest possible model needed to study molecular cloud gas-grain chemistry, we only include additional processes where they are clearly required to satisfy one or more of the ISO-SWAS constraints. When CO, N2 and atoms of N, C and S are efficiently desorbed from grains, a chemical quasi-steady-state develops after about one million years. We find that accretion of CO2 and H2O cannot explain the [CO2/H2O]ice ISO observations; as with previous models, accretion and reaction of oxygen atoms are necessary although a high O atom abundance can still be derived from the CO that remains in the gas. The observational constraints on solid and gaseous molecular oxygen are both met in this model. However, we find that we cannot explain the lowest H2O abundances seen by SWAS or the highest atomic carbon abundances found in molecular clouds; additional chemical processes are required and possible candidates are given. One prediction of models of this type is that there should be some regions of molecular clouds which contain high gas phase abundances of H2O, O2 and NO. A further consequence, we find, is that interstellar grain mantles could be rich in NH2OH and NO2. The search for these regions, as well as NH2OH and NO2 in ices and in hot cores, is an important further test of this scenario. The model can give good agreement with observations of simple molecules in dark molecular clouds such as TMC-1 and L134N. Despite the fact that S atoms are assumed to be continously desorbed from grain surfaces, we find that the sulphur chemistry independently experiences an ``accretion catastrophe''. The S-bearing molecular abundances cease to lie within the observed range after about 3 x 106 years and this indicates that there may be at least two efficient surface desorption mechanisms operating in dark clouds - one quasi-continous and the other operating more sporadically on this time-scale. We suggest that mantle removal on short time-scales is mediated by clump dynamics, and by the effects of star formation on longer time-scales. The applicability of this type of dynamical-chemical model for molecular cloud evolution is discussed and comparison is made with other models of dark cloud chemistry.

NEAR-IR IMAGING POLARIMETRY TOWARD A BRIGHT-RIMMED CLOUD: MAGNETIC FIELD IN SFO 74

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kusune, Takayoshi; Sugitani, Koji; Miao, Jingqi

2015-01-01

We have made near-infrared (JHK {sub s}) imaging polarimetry of a bright-rimmed cloud (SFO 74). The polarization vector maps clearly show that the magnetic field in the layer just behind the bright rim is running along the rim, quite different from its ambient magnetic field. The direction of the magnetic field just behind the tip rim is almost perpendicular to that of the incident UV radiation, and the magnetic field configuration appears to be symmetric as a whole with respect to the cloud symmetry axis. We estimated the column and number densities in the two regions (just inside and farmore » inside the tip rim) and then derived the magnetic field strength, applying the Chandrasekhar-Fermi method. The estimated magnetic field strength just inside the tip rim, ∼90 μG, is stronger than that far inside, ∼30 μG. This suggests that the magnetic field strength just inside the tip rim is enhanced by the UV-radiation-induced shock. The shock increases the density within the top layer around the tip and thus increases the strength of the magnetic field. The magnetic pressure seems to be comparable to the turbulent one just inside the tip rim, implying a significant contribution of the magnetic field to the total internal pressure. The mass-to-flux ratio was estimated to be close to the critical value just inside the tip rim. We speculate that the flat-topped bright rim of SFO 74 could be formed by the magnetic field effect.« less

The relation between the column density structures and the magnetic field orientation in the Vela C molecular complex

NASA Astrophysics Data System (ADS)

Soler, J. D.; Ade, P. A. R.; Angilè, F. E.; Ashton, P.; Benton, S. J.; Devlin, M. J.; Dober, B.; Fissel, L. M.; Fukui, Y.; Galitzki, N.; Gandilo, N. N.; Hennebelle, P.; Klein, J.; Li, Z.-Y.; Korotkov, A. L.; Martin, P. G.; Matthews, T. G.; Moncelsi, L.; Netterfield, C. B.; Novak, G.; Pascale, E.; Poidevin, F.; Santos, F. P.; Savini, G.; Scott, D.; Shariff, J. A.; Thomas, N. E.; Tucker, C. E.; Tucker, G. S.; Ward-Thompson, D.

2017-07-01

We statistically evaluated the relative orientation between gas column density structures, inferred from Herschel submillimetre observations, and the magnetic field projected on the plane of sky, inferred from polarized thermal emission of Galactic dust observed by the Balloon-borne Large-Aperture Submillimetre Telescope for Polarimetry (BLASTPol) at 250, 350, and 500 μm, towards the Vela C molecular complex. First, we find very good agreement between the polarization orientations in the three wavelength-bands, suggesting that, at the considered common angular resolution of 3.´0 that corresponds to a physical scale of approximately 0.61 pc, the inferred magnetic field orientation is not significantly affected by temperature or dust grain alignment effects. Second, we find that the relative orientation between gas column density structures and the magnetic field changes progressively with increasing gas column density, from mostly parallel or having no preferred orientation at low column densities to mostly perpendicular at the highest column densities. This observation is in agreement with previous studies by the Planck collaboration towards more nearby molecular clouds. Finally, we find a correspondencebetween (a) the trends in relative orientation between the column density structures and the projected magnetic field; and (b) the shape of the column density probability distribution functions (PDFs). In the sub-regions of Vela C dominated by one clear filamentary structure, or "ridges", where the high-column density tails of the PDFs are flatter, we find a sharp transition from preferentially parallel or having no preferred relative orientation at low column densities to preferentially perpendicular at highest column densities. In the sub-regions of Vela C dominated by several filamentary structures with multiple orientations, or "nests", where the maximum values of the column density are smaller than in the ridge-like sub-regions and the high-column density tails of the PDFs are steeper, such a transition is also present, but it is clearly less sharp than in the ridge-like sub-regions. Both of these results suggest that the magnetic field is dynamically important for the formation of density structures in this region.

A Low-metallicity Molecular Cloud in the Lower Galactic Halo

NASA Astrophysics Data System (ADS)

Hernandez, Audra K.; Wakker, Bart P.; Benjamin, Robert A.; French, David; Kerp, Juergen; Lockman, Felix J.; O'Toole, Simon; Winkel, Benjamin

2013-11-01

We find evidence for the impact of infalling, low-metallicity gas on the Galactic disk. This is based on FUV absorption line spectra, 21 cm emission line spectra, and far-infrared (FIR) mapping to estimate the abundance and physical properties of IV21 (IVC135+54-45), a galactic intermediate-velocity molecular cloud that lies ~300 pc above the disk. The metallicity of IV21 was estimated using observations toward the subdwarf B star PG1144+615, located at a projected distance of 16 pc from the cloud's densest core, by measuring ion and H I column densities for comparison with known solar abundances. Despite the cloud's bright FIR emission and large column densities of molecular gas as traced by CO, we find that it has a sub-solar metallicity of log (Z/Z ⊙) = -0.43 ± 0.12 dex. IV21 is thus the first known sub-solar metallicity cloud in the solar neighborhood. In contrast, most intermediate-velocity clouds (IVC) have near-solar metallicities and are believed to originate in the Galactic Fountain. The cloud's low metallicity is also atypical for Galactic molecular clouds, especially in light of the bright FIR emission which suggest a substantial dust content. The measured I 100 μm/N(H I) ratio is a factor of three below the average found in high latitude H I clouds within the solar neighborhood. We argue that IV21 represents the impact of an infalling, low-metallicity high-velocity cloud that is mixing with disk gas in the lower Galactic halo. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from MAST at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program No. 12275. The Green Bank Telescope is part of the National Radio Astronomy Observatory which is a Facility of the National Science Foundation, operated by Associated Universities, Inc.

Ionisation and discharge in cloud-forming atmospheres of brown dwarfs and extrasolar planets

NASA Astrophysics Data System (ADS)

Helling, Ch; Rimmer, P. B.; Rodriguez-Barrera, I. M.; Wood, Kenneth; Robertson, G. B.; Stark, C. R.

2016-07-01

Brown dwarfs and giant gas extrasolar planets have cold atmospheres with rich chemical compositions from which mineral cloud particles form. Their properties, like particle sizes and material composition, vary with height, and the mineral cloud particles are charged due to triboelectric processes in such dynamic atmospheres. The dynamics of the atmospheric gas is driven by the irradiating host star and/or by the rotation of the objects that changes during its lifetime. Thermal gas ionisation in these ultra-cool but dense atmospheres allows electrostatic interactions and magnetic coupling of a substantial atmosphere volume. Combined with a strong magnetic field \\gg {{B}\\text{Earth}} , a chromosphere and aurorae might form as suggested by radio and x-ray observations of brown dwarfs. Non-equilibrium processes like cosmic ray ionisation and discharge processes in clouds will increase the local pool of free electrons in the gas. Cosmic rays and lighting discharges also alter the composition of the local atmospheric gas such that tracer molecules might be identified. Cosmic rays affect the atmosphere through air showers in a certain volume which was modelled with a 3D Monte Carlo radiative transfer code to be able to visualise their spacial extent. Given a certain degree of thermal ionisation of the atmospheric gas, we suggest that electron attachment to charge mineral cloud particles is too inefficient to cause an electrostatic disruption of the cloud particles. Cloud particles will therefore not be destroyed by Coulomb explosion for the local temperature in the collisional dominated brown dwarf and giant gas planet atmospheres. However, the cloud particles are destroyed electrostatically in regions with strong gas ionisation. The potential size of such cloud holes would, however, be too small and might occur too far inside the cloud to mimic the effect of, e.g. magnetic field induced star spots.

Molecular Cloud Structures and Massive Star Formation in N159

NASA Astrophysics Data System (ADS)

Nayak, O.; Meixner, M.; Fukui, Y.; Tachihara, K.; Onishi, T.; Saigo, K.; Tokuda, K.; Harada, R.

2018-02-01

The N159 star-forming region is one of the most massive giant molecular clouds (GMCs) in the Large Magellanic Cloud (LMC). We show the 12CO, 13CO, CS molecular gas lines observed with ALMA in N159 west (N159W) and N159 east (N159E). We relate the structure of the gas clumps to the properties of 24 massive young stellar objects (YSOs) that include 10 newly identified YSOs based on our search. We use dendrogram analysis to identify properties of the molecular clumps, such as flux, mass, linewidth, size, and virial parameter. We relate the YSO properties to the molecular gas properties. We find that the CS gas clumps have a steeper size–linewidth relation than the 12CO or 13CO gas clumps. This larger slope could potentially occur if the CS gas is tracing shocks. The virial parameters of the 13CO gas clumps in N159W and N159E are low (<1). The threshold for massive star formation in N159W is 501 M ⊙ pc‑2, and the threshold for massive star formation in N159E is 794 M ⊙ pc‑2. We find that 13CO is more photodissociated in N159E than N159W. The most massive YSO in N159E has cleared out a molecular gas hole in its vicinity. All the massive YSO candidates in N159E have a more evolved spectral energy distribution type in comparison to the YSO candidates in N159W. These differences lead us to conclude that the giant molecular cloud complex in N159E is more evolved than the giant molecular cloud complex in N159W.

CRRES: The combined release and radiation effects satellite program directory

NASA Technical Reports Server (NTRS)

Layman, Laura D.; Miller, George P.

1992-01-01

As a result of natural processes, plasma clouds are often injected into the magnetosphere. These chemical releases can be used to study many aspects of such injections. When a dense plasma is injected into the inner magnetosphere, it is expected to take up the motion of the ambient plasma. However, it has been observed in previous releases at moderate altitudes that the cloud preserved its momentum for some time following the release and that parts of the cloud peeled off from the main cloud presumable due to the action of an instability. As one moves outward into the magnetosphere, the mirror force becomes less dominant and the initial conditions following a release are dominated by the formation of a diamagnetic cavity since the initial plasma pressure from the injected Ba ions is greater than the magnetic field energy density. A previous high-altitude release (31,300 km) showed this to be the case initially, but at later times there was evidence for acceleration of the Ba plasma to velocities corresponding to 60,000 K. This effect is not explained. This series of experiments is therefore designed to inject plasma clouds into the magnetosphere under widely varying conditions of magnetic field strength and ambient plasma density. In this way the coupling of injected clouds to the ambient plasma and magnetic field, the formation of striations due to instabilities, and possible heating and acceleration of the injected Ba plasma can be studied over a wide range of magnetosphere parameters. Adding to the scientific yield will be the availability of measurements for the DOD/SPACERAD instruments which can monitor plasma parameters, electric and magnetic fields, and waves before, during and after the releases.

Probing Massive Star Cluster Formation with ALMA

NASA Astrophysics Data System (ADS)

Johnson, Kelsey

2015-08-01

Observationally constraining the physical conditions that give rise to massive star clusters has been a long-standing challenge. Now with the ALMA Observatory coming on-line, we can finally begin to probe the birth environments of massive clusters in a variety of galaxies with sufficient angular resolution. In this talk I will give an overview of ALMA observations of galaxies in which candidate proto-super star cluster molecular clouds have been identified. These new data probe the physical conditions that give rise to super star clusters, providing information on their densities, pressures, and temperatures. In particular, the observations indicate that these clouds may be subject to external pressures of P/k > 108 K cm-3, which is consistent with the prevalence of optically observed adolescent super star clusters in interacting galaxy systems and other high pressure environments. ALMA observations also enable an assessement of the molecular cloud chemical abundances in the regions surrounding super star clusters. Molecular clouds associated with existing super star clusters are strongly correlated with HCO+ emission, but appear to have relatively low ratio of CO/HCO+ emission compared to other clouds, indicating that the super star clusters are impacting the molecular abundances in their vicinity.

Physical properties and scaling relations of molecular clouds: the effect of stellar feedback

NASA Astrophysics Data System (ADS)

Grisdale, Kearn; Agertz, Oscar; Renaud, Florent; Romeo, Alessandro B.

2018-06-01

Using hydrodynamical simulations of entire galactic discs similar to the Milky Way, reaching 4.6{ pc} resolution, we study the origins of observed physical properties of giant molecular clouds (GMCs). We find that efficient stellar feedback is a necessary ingredient in order to develop a realistic interstellar medium (ISM), leading to molecular cloud masses, sizes, velocity dispersions and virial parameters in excellent agreement with Milky Way observations. GMC scaling relations observed in the Milky Way, such as the mass-size (M-R), velocity dispersion-size (σ-R), and the σ-RΣ relations, are reproduced in a feedback driven ISM when observed in projection, with M∝R2.3 and σ∝R0.56. When analysed in 3D, GMC scaling relations steepen significantly, indicating potential limitations of our understanding of molecular cloud 3D structure from observations. Furthermore, we demonstrate how a GMC population's underlying distribution of virial parameters can strongly influence the scatter in derived scaling relations. Finally, we show that GMCs with nearly identical global properties exist in different evolutionary stages, where a majority of clouds being either gravitationally bound or expanding, but with a significant fraction being compressed by external ISM pressure, at all times.

Cold Water Vapor in the Barnard 5 Molecular Cloud

NASA Technical Reports Server (NTRS)

Wirstrom, E. S.; Charnley, S. B.; Persson, C. M.; Buckle, J. V.; Cordiner, M. A.; Takakuwa, S.

2014-01-01

After more than 30 yr of investigations, the nature of gas-grain interactions at low temperatures remains an unresolved issue in astrochemistry. Water ice is the dominant ice found in cold molecular clouds; however, there is only one region where cold ((is) approximately 10 K) water vapor has been detected-L1544. This study aims to shed light on ice desorption mechanisms under cold cloud conditions by expanding the sample. The clumpy distribution of methanol in dark clouds testifies to transient desorption processes at work-likely to also disrupt water ice mantles. Therefore, the Herschel HIFI instrument was used to search for cold water in a small sample of prominent methanol emission peaks. We report detections of the ground-state transition of o-H2O (J = 110-101) at 556.9360 GHz toward two positions in the cold molecular cloud, Barnard 5. The relative abundances of methanol and water gas support a desorption mechanism which disrupts the outer ice mantle layers, rather than causing complete mantle removal.

Pillars and globules at the edges of H ii regions. Confronting Herschel observations and numerical simulations

NASA Astrophysics Data System (ADS)

Tremblin, P.; Minier, V.; Schneider, N.; Audit, E.; Hill, T.; Didelon, P.; Peretto, N.; Arzoumanian, D.; Motte, F.; Zavagno, A.; Bontemps, S.; Anderson, L. D.; André, Ph.; Bernard, J. P.; Csengeri, T.; Di Francesco, J.; Elia, D.; Hennemann, M.; Könyves, V.; Marston, A. P.; Nguyen Luong, Q.; Rivera-Ingraham, A.; Roussel, H.; Sousbie, T.; Spinoglio, L.; White, G. J.; Williams, J.

2013-12-01

Context. Herschel far-infrared imaging observations have revealed the density structure of the interface between H ii regions and molecular clouds in great detail. In particular, pillars and globules are present in many high-mass star-forming regions, such as the Eagle nebula (M 16) and the Rosette molecular cloud, and understanding their origin will help characterize triggered star formation. Aims: The formation mechanisms of these structures are still being debated. The initial morphology of the molecular cloud and its turbulent state are key parameters since they generate deformations and curvatures of the shell during the expansion of the H ii region. Recent numerical simulations have shown how pillars can arise from the collapse of the shell in on itself and how globules can be formed from the interplay of the turbulent molecular cloud and the ionization from massive stars. The goal here is to test this scenario through recent observations of two massive star-forming regions, M 16 and the Rosette molecular cloud. Methods: First, the column density structure of the interface between molecular clouds and associated H ii regions was characterized using column density maps obtained from far-infrared imaging of the Herschel HOBYS key programme. Then, the DisPerSe algorithm was used on these maps to detect the compressed layers around the ionized gas and pillars in different evolutionary states. Column density profiles were constructed. Finally, their velocity structure was investigated using CO data, and all observational signatures were tested against some distinct diagnostics established from simulations. Results: The column density profiles have revealed the importance of compression at the edge of the ionized gas. The velocity properties of the structures, i.e. pillars and globules, are very close to what we predict from the numerical simulations. We have identified a good candidate of a nascent pillar in the Rosette molecular cloud that presents the velocity pattern of the shell collapsing on itself, induced by a high local curvature. Globules have a bulk velocity dispersion that indicates the importance of the initial turbulence in their formation, as proposed from numerical simulations. Altogether, this study re-enforces the picture of pillar formation by shell collapse and globule formation by the ionization of highly turbulent clouds. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

A magnetic torsional wave near the Galactic Centre traced by a 'double helix' nebula.

PubMed

Morris, Mark; Uchida, Keven; Do, Tuan

2006-03-16

The magnetic field in the central few hundred parsecs of the Milky Way has a dipolar geometry and is substantially stronger than elsewhere in the Galaxy, with estimates ranging up to a milligauss (refs 1-6). Characterization of the magnetic field at the Galactic Centre is important because it can affect the orbits of molecular clouds by exerting a drag on them, inhibit star formation, and could guide a wind of hot gas or cosmic rays away from the central region. Here we report observations of an infrared nebula having the morphology of an intertwined double helix about 100 parsecs from the Galaxy's dynamical centre, with its axis oriented perpendicular to the Galactic plane. The observed segment is about 25 parsecs in length, and contains about 1.25 full turns of each of the two continuous, helically wound strands. We interpret this feature as a torsional Alfvén wave propagating vertically away from the Galactic disk, driven by rotation of the magnetized circumnuclear gas disk. The direct connection between the circumnuclear disk and the double helix is ambiguous, but the images show a possible meandering channel that warrants further investigation.

Giant Molecular Clouds with High Abundance of Atomic Carbon and Cyano Radical in the Milky Way's Central Molecular Zone

NASA Astrophysics Data System (ADS)

Tanaka, Kunihiko; Oka, Tomoharu; Nagai, Makoto; Kamegai, Kazuhisa

2015-08-01

The central 400 pc region of the Milky Way Galaxy is the closest galactic central region to us, providing a unique opportunity to detailedly investigate gas dynamics, star formation activity, and chemistry under the extreme environment of galactic centers, where the presence of bar, intense UV/cosmic-ray fluxes, high degree of turbulence may significantly affect those processes. We report the results of molecular line surveys toward the Milky Way's central molecular zone (CMZ) performed with the ASTE 10m telescope, the Mopra 22m telescope, and the Nobeyama 45 m telescope. With the observations of the 500 GHz [CI] fine structure line of atomic carbon (C0), we have found a molecular cloud structure with remarkably bright [CI] emission in the Sgr A comlex in the innermost 20 pc region. The [CI] cloud is more extended than the GMCs in the region, and appears to connect the northern part of the 50 kms-1 (M-0.02-0.07) and the circumnuclear disk (CND), though no corresponding structures are visible in other molecular lines. The [C0]/[CO] abundance ratio is measured to be 0.5-2, which is 2-10 times those measured to the clouds at larger Galactic radii. This high ratio is close to the values measured toward centers of galaxies with starburst and AGN, suggesting that the chemical state of the cloud is similar to that in those active galaxies. We have also found a large scale gradient of the cyano radical (CN) abundance toward the Galactic center in the innermost 100 pc radius, showing near the Sgr A complex. We suggest that the cloud with high C0 and CN abundance is a feature formed as a result of inward transfer of diffuse molecular gas by the bar potential in the inner Galaxy, in which PDR-like chemical composition remains preserved, and that thus the [CI] cloud could be deeply related to formation of the GMCs and star formation in the CMZ. We also discuss other possible mechanisms to enhance C0 and CN abundances, including the enhanced cosmic-ray dissociation ratio.

Time evolution of giant molecular cloud mass functions with cloud-cloud collisions and gas resurrection in various environments

NASA Astrophysics Data System (ADS)

Kobayashi, M. I. N.; Inutsuka, S.; Kobayashi, H.; Hasegawa, K.

We formulate the evolution equation for the giant molecular cloud (GMC) mass functions including self-growth of GMCs through the thermal instability, self-dispersal due to massive stars born in GMCs, cloud-cloud collisions (CCCs), and gas resurrection that replenishes the minimum-mass GMC population. The computed time evolutions obtained from this formulation suggest that the slope of GMC mass function in the mass range <105.5 Mȯ is governed by the ratio of GMC formation timescale to its dispersal timescale, and that the CCC process modifies only the massive end of the mass function. Our results also suggest that most of the dispersed gas contributes to the mass growth of pre-existing GMCs in arm regions whereas less than 60 per cent contributes in inter-arm regions.

Millimeter-wave molecular line observations of the Tornado nebula

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sakai, D.; Oka, T.; Tanaka, K.

We report the results of millimeter-wave molecular line observations of the Tornado Nebula (G357.7-0.1), which is a bright radio source behind the Galactic center region. A 15' × 15' area was mapped in the J = 1-0 lines of CO, {sup 13}CO, and HCO{sup +} with the Nobeyama Radio Observatory 45 m telescope. The Very Large Array archival data of OH at 1720 MHz were also reanalyzed. We found two molecular clouds with separate velocities, V{sub LSR} = –14 km s{sup –1} and +5 km s{sup –1}. These clouds show rough spatial anti-correlation. Both clouds are associated with OH 1720more » MHz emissions in the area overlapping with the Tornado Nebula. The spatial and velocity coincidence indicates violent interaction between the clouds and the Tornado Nebula. Modestly excited gas prefers the position of the Tornado 'head' in the –14 km s{sup –1} cloud, also suggesting the interaction. Virial analysis shows that the +5 km s{sup –1} cloud is more tightly bound by self-gravity than the –14 km s{sup –1} cloud. We propose a formation scenario for the Tornado Nebula; the +5 km s{sup –1} cloud collided into the –14 km s{sup –1} cloud, generating a high-density layer behind the shock front, which activates a putative compact object by Bondi-Hoyle-Lyttleton accretion to eject a pair of bipolar jets.« less

Collisional excitation of molecules in dense interstellar clouds

NASA Technical Reports Server (NTRS)

Green, S.

1985-01-01

State transitions which permit the identification of the molecular species in dense interstellar clouds are reviewed, along with the techniques used to calculate the transition energies, the database on known molecular transitions and the accuracy of the values. The transition energies cannot be measured directly and therefore must be modeled analytically. Scattering theory is used to determine the intermolecular forces on the basis of quantum mechanics. The nuclear motions can also be modeled with classical mechanics. Sample rate constants are provided for molecular systems known to inhabit dense interstellar clouds. The values serve as a database for interpreting microwave and RF astrophysical data on the transitions undergone by interstellar molecules.

Global Studies of Molecular Clouds in the Galaxy, the Magellanic Cloud and M31

NASA Technical Reports Server (NTRS)

Thaddeus, Patrick

1998-01-01

Over the past five years we have used our extensive CO surveys of the Galaxy and M31 in conjunction with spacecraft observations to address central problems in galactic structure and the astrophysics of molecular clouds. These problems included the nature of the molecular ring and its relation to the spiral arms and central bar, the cosmic ray distribution, the origin of the diffuse X-ray background, the distribution and properties of x-ray sources and supernova remnants, and the Galactic stellar mass distribution. For many of these problems, the nearby spiral M31 provided an important complementary perspective.

A Herschel-SPIRE Survey of the MonR2 Giant Molecular Cloud

NASA Astrophysics Data System (ADS)

Pokhrel, Riwaj; Gutermuth, Robert; Ali, Babar; Megeath, Thomas; Pipher, Judith; Myers, Philip; Fischer, William; Henning, Thomas; Wolk, Scott; Allen, Lori; Tobin, John

2015-08-01

We present a new survey of the MonR2 giant molecular cloud with SPIRE on the Herschel Space Observatory. We cross-calibrated SPIRE data with Planck-HFI and accounted for its absolute offset and zero point correction. We fixed emissivity with the help of flux-error and flux ratio plots. As the best representation of cold dusty molecular clouds, we did greybody fits of the SEDs. We studied the nature of distribution of column densities above and below certain critical limit, followed by the mass and temperature distributions for different regions. We used dendrograms as a technique to study the hierarchical structures in the GMC.

FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN): Molecular clouds toward W ; possible evidence for a cloud-cloud collision triggering O star formation

NASA Astrophysics Data System (ADS)

Kohno, Mikito; Torii, Kazufumi; Tachihara, Kengo; Umemoto, Tomofumi; Minamidani, Tetsuhiro; Nishimura, Atsushi; Fujita, Shinji; Matsuo, Mitsuhiro; Yamagishi, Mitsuyoshi; Tsuda, Yuya; Kuriki, Mika; Kuno, Nario; Ohama, Akio; Hattori, Yusuke; Sano, Hidetoshi; Yamamoto, Hiroaki; Fukui, Yasuo

2018-05-01

We observed molecular clouds in the W 33 high-mass star-forming region associated with compact and extended H II regions using the NANTEN2 telescope as well as the Nobeyama 45 m telescope in the J = 1-0 transitions of 12CO, 13CO, and C18O as part of the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN) legacy survey. We detected three velocity components at 35 km s-1, 45 km s-1, and 58 km s-1. The 35 km s-1 and 58 km s-1 clouds are likely to be physically associated with W 33 because of the enhanced 12CO J = 3-2 to J = 1-0 intensity ratio as R_3-2/1-0 > 1.0 due to the ultraviolet irradiation by OB stars, and morphological correspondence between the distributions of molecular gas and the infrared and radio continuum emissions excited by high-mass stars. The two clouds show complementary distributions around W 33. The velocity separation is too large to be gravitationally bound, and yet not explained by expanding motion by stellar feedback. Therefore, we discuss whether a cloud-cloud collision scenario likely explains the high-mass star formation in W 33.

FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN): Molecular clouds toward W 33; possible evidence for a cloud-cloud collision triggering O star formation

NASA Astrophysics Data System (ADS)

Kohno, Mikito; Torii, Kazufumi; Tachihara, Kengo; Umemoto, Tomofumi; Minamidani, Tetsuhiro; Nishimura, Atsushi; Fujita, Shinji; Matsuo, Mitsuhiro; Yamagishi, Mitsuyoshi; Tsuda, Yuya; Kuriki, Mika; Kuno, Nario; Ohama, Akio; Hattori, Yusuke; Sano, Hidetoshi; Yamamoto, Hiroaki; Fukui, Yasuo

2018-01-01

We observed molecular clouds in the W 33 high-mass star-forming region associated with compact and extended H II regions using the NANTEN2 telescope as well as the Nobeyama 45 m telescope in the J = 1-0 transitions of 12CO, 13CO, and C18O as part of the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN) legacy survey. We detected three velocity components at 35 km s-1, 45 km s-1, and 58 km s-1. The 35 km s-1 and 58 km s-1 clouds are likely to be physically associated with W 33 because of the enhanced 12CO J = 3-2 to J = 1-0 intensity ratio as R3-2/1-0 > 1.0 due to the ultraviolet irradiation by OB stars, and morphological correspondence between the distributions of molecular gas and the infrared and radio continuum emissions excited by high-mass stars. The two clouds show complementary distributions around W 33. The velocity separation is too large to be gravitationally bound, and yet not explained by expanding motion by stellar feedback. Therefore, we discuss whether a cloud-cloud collision scenario likely explains the high-mass star formation in W 33.

FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN): Molecular clouds toward W 33; possible evidence for a cloud-cloud collision triggering O star formation

NASA Astrophysics Data System (ADS)

Kohno, Mikito; Torii, Kazufumi; Tachihara, Kengo; Umemoto, Tomofumi; Minamidani, Tetsuhiro; Nishimura, Atsushi; Fujita, Shinji; Matsuo, Mitsuhiro; Yamagishi, Mitsuyoshi; Tsuda, Yuya; Kuriki, Mika; Kuno, Nario; Ohama, Akio; Hattori, Yusuke; Sano, Hidetoshi; Yamamoto, Hiroaki; Fukui, Yasuo

2018-05-01

We observed molecular clouds in the W 33 high-mass star-forming region associated with compact and extended H II regions using the NANTEN2 telescope as well as the Nobeyama 45 m telescope in the J = 1-0 transitions of 12CO, 13CO, and C18O as part of the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN) legacy survey. We detected three velocity components at 35 km s-1, 45 km s-1, and 58 km s-1. The 35 km s-1 and 58 km s-1 clouds are likely to be physically associated with W 33 because of the enhanced 12CO J = 3-2 to J = 1-0 intensity ratio as R_3-2/1-0} > 1.0 due to the ultraviolet irradiation by OB stars, and morphological correspondence between the distributions of molecular gas and the infrared and radio continuum emissions excited by high-mass stars. The two clouds show complementary distributions around W 33. The velocity separation is too large to be gravitationally bound, and yet not explained by expanding motion by stellar feedback. Therefore, we discuss whether a cloud-cloud collision scenario likely explains the high-mass star formation in W 33.

Cold Atomic Hydrogen, Narrow Self-Absorption, and the Age of Molecular Clouds

NASA Technical Reports Server (NTRS)

Goldsmith, Paul F.

2006-01-01

This viewgraph presentation reviews the history, and current work on HI and its importance in star formation. Through many observations of HI Narrow Self Absorption (HINSA) the conclusions are drawn and presented. Local molecular clouds have HI well-mixed with molecular constituents This HI is cold, quiescent, and must be well-shielded from the UV radiation field The density and fractional abundance (wrt H2) of the cold HI are close to steady state values The time required to convert these starless clouds from purely HI initial state to observed present composition is a few to ten million years This timescale is a lower limit - if dense clouds being swept up from lower density regions by shocks, the time to accumulate material to get A(sub v) is approximately 1 and provide required shielding may be comparable or longer

Molecular clouds in the NGC 6334 and NGC 6357 region: Evidence for a 100 pc-scale cloud-cloud collision triggering the Galactic mini-starbursts

NASA Astrophysics Data System (ADS)

Fukui, Yasuo; Kohno, Mikito; Yokoyama, Keiko; Torii, Kazufumi; Hattori, Yusuke; Sano, Hidetoshi; Nishimura, Atsushi; Ohama, Akio; Yamamoto, Hiroaki; Tachihara, Kengo

2018-03-01

We carried out new CO (J = 1-0, 2-1, and 3-2) observations with NANTEN2 and ASTE in the region of the twin Galactic mini-starbursts NGC 6334 and NGC 6357. We detected two velocity molecular components of 12 km s-1 velocity separation, which is continuous over 3° along the plane. In NGC 6334 the two components show similar two-peaked intensity distributions toward the young H II regions and are linked by a bridge feature. In NGC 6357 we found spatially complementary distribution between the two velocity components as well as a bridge feature in velocity. Based on these results we hypothesize that the two clouds in the two regions collided with each other in the past few Myr and triggered the formation of the starbursts over ˜ 100 pc. We suggest that the formation of the starbursts happened toward the collisional region of extent ˜ 10 pc with initial high molecular column densities. For NGC 6334 we present a scenario which includes spatial variation of the colliding epoch due to non-uniform cloud separation. The scenario possibly explains the apparent age differences among the young O stars in NGC 6334, which range from 104 yr to 106 yr; the latest collision happened within 105 yr toward the youngest stars in NGC 6334 I(N) and I which exhibit molecular outflows without H II regions. For NGC 6357 the O stars were formed a few Myr ago, and the cloud dispersal by the O stars is significant. We conclude that cloud-cloud collision offers a possible explanation of the mini-starburst over a 100 pc scale.

Molecular clouds in the NGC 6334 and NGC 6357 region; Evidence for a 100-pc-scale cloud-cloud collision triggering the Galactic mini-starbursts

NASA Astrophysics Data System (ADS)

Fukui, Yasuo; Kohno, Mikito; Yokoyama, Keiko; Torii, Kazufumi; Hattori, Yusuke; Sano, Hidetoshi; Nishimura, Atsushi; Ohama, Akio; Yamamoto, Hiroaki; Tachihara, Kengo

2018-03-01

We carried out new CO (J = 1-0, 2-1, and 3-2) observations with NANTEN2 and ASTE in the region of the twin Galactic mini-starbursts NGC 6334 and NGC 6357. We detected two velocity molecular components of 12 km s-1 velocity separation, which is continuous over 3° along the plane. In NGC 6334 the two components show similar two-peaked intensity distributions toward the young H II regions and are linked by a bridge feature. In NGC 6357 we found spatially complementary distribution between the two velocity components as well as a bridge feature in velocity. Based on these results we hypothesize that the two clouds in the two regions collided with each other in the past few Myr and triggered the formation of the starbursts over ˜ 100 pc. We suggest that the formation of the starbursts happened toward the collisional region of extent ˜ 10 pc with initial high molecular column densities. For NGC 6334 we present a scenario which includes spatial variation of the colliding epoch due to non-uniform cloud separation. The scenario possibly explains the apparent age differences among the young O stars in NGC 6334, which range from 104 yr to 106 yr; the latest collision happened within 105 yr toward the youngest stars in NGC 6334 I(N) and I which exhibit molecular outflows without H II regions. For NGC 6357 the O stars were formed a few Myr ago, and the cloud dispersal by the O stars is significant. We conclude that cloud-cloud collision offers a possible explanation of the mini-starburst over a 100-pc scale.

Molecular clouds in the NGC 6334 and NGC 6357 region: Evidence for a 100 pc-scale cloud-cloud collision triggering the Galactic mini-starbursts

NASA Astrophysics Data System (ADS)

Fukui, Yasuo; Kohno, Mikito; Yokoyama, Keiko; Torii, Kazufumi; Hattori, Yusuke; Sano, Hidetoshi; Nishimura, Atsushi; Ohama, Akio; Yamamoto, Hiroaki; Tachihara, Kengo

2018-05-01

We carried out new CO (J = 1-0, 2-1, and 3-2) observations with NANTEN2 and ASTE in the region of the twin Galactic mini-starbursts NGC 6334 and NGC 6357. We detected two velocity molecular components of 12 km s-1 velocity separation, which is continuous over 3° along the plane. In NGC 6334 the two components show similar two-peaked intensity distributions toward the young H II regions and are linked by a bridge feature. In NGC 6357 we found spatially complementary distribution between the two velocity components as well as a bridge feature in velocity. Based on these results we hypothesize that the two clouds in the two regions collided with each other in the past few Myr and triggered the formation of the starbursts over ˜ 100 pc. We suggest that the formation of the starbursts happened toward the collisional region of extent ˜ 10 pc with initial high molecular column densities. For NGC 6334 we present a scenario which includes spatial variation of the colliding epoch due to non-uniform cloud separation. The scenario possibly explains the apparent age differences among the young O stars in NGC 6334, which range from 104 yr to 106 yr; the latest collision happened within 105 yr toward the youngest stars in NGC 6334 I(N) and I which exhibit molecular outflows without H II regions. For NGC 6357 the O stars were formed a few Myr ago, and the cloud dispersal by the O stars is significant. We conclude that cloud-cloud collision offers a possible explanation of the mini-starburst over a 100 pc scale.

Hydrodynamic simulations of mechanical stellar feedback in a molecular cloud formed by thermal instability

NASA Astrophysics Data System (ADS)

Wareing, C. J.; Pittard, J. M.; Falle, S. A. E. G.

2017-09-01

We have used the AMR hydrodynamic code, mg, to perform 3D hydrodynamic simulations with self-gravity of stellar feedback in a spherical clumpy molecular cloud formed through the action of thermal instability. We simulate the interaction of the mechanical energy input from 15, 40, 60 and 120 M⊙ stars into a 100 pc diameter 16 500 M⊙ cloud with a roughly spherical morphology with randomly distributed high-density condensations. The stellar winds are introduced using appropriate non-rotating Geneva stellar evolution models. In the 15 M⊙ star case, the wind has very little effect, spreading around a few neighbouring clumps before becoming overwhelmed by the cloud collapse. In contrast, in the 40, 60 and 120 M⊙ star cases, the more powerful stellar winds create large cavities and carve channels through the cloud, breaking out into the surrounding tenuous medium during the wind phase and considerably altering the cloud structure. After 4.97, 3.97 and 3.01 Myr, respectively, the massive stars explode as supernovae (SNe). The wind-sculpted surroundings considerably affect the evolution of these SN events as they both escape the cloud along wind-carved channels and sweep up remaining clumps of cloud/wind material. The 'cloud' as a coherent structure does not survive the SN from any of these stars, but only in the 120 M⊙ case is the cold molecular material completely destabilized and returned to the unstable thermal phase. In the 40 and 60 M⊙ cases, coherent clumps of cold material are ejected from the cloud by the SN, potentially capable of further star formation.

Role of Magnetic Diffusion Induced by Turbulent Magnetic Reconnection for Star Formation

NASA Astrophysics Data System (ADS)

Lazarian, Alex; Santos de Lima, R.; de Gouveia Dal Pino, E.

2010-01-01

The diffusion of astrophysical magnetic fields in conducting fluids in the presence of turbulence depends on whether magnetic fields can change their topology or reconnect in highly conducting media. Recent progress in understanding fast magnetic reconnection in the presence of turbulence is reassuring that the magnetic field behavior in the computer simulations and turbulent astrophysical environments is similar, as far as the magnetic reconnection is concerned. This makes it meaningful to perform MHD simulations of turbulent flows in order to understand the diffusion of magnetic field in astrophysical environments. Our study of magnetic field diffusion reveals important propertie s of the process. First of all, our 3D MHD simulations initiated with anti-correlating magnetic field and gaseous density exhibit at later times a decorrelation of the magnetic field and density, which corresponds well to the observations of the interstellar media. In the presence of gravity, our 3D simulations show the decrease of the flux to mass ratio with density concentration when turbulence is present. We observe this effect both in the situations when we start with the equilibrium distributions of gas and magnetic field and when we start with collapsing dynamically unstable configurations. Thus the process of turbulent magnetic field removal should be applicable both to quasistatic subcritical molecular clouds and cores and violently collapsing supercritical entities. The increase of the gravitational potential as well as the magnetization of the gas increases the segregation of the mass and flux in the saturated final state of simulations, supporting the notion that turbulent diffusivity relaxes the magnetic field + gas system in the gravitational field to its minimal energy state. At the same time, turbulence of high level may get the system unbound making the flux to mass ratio more uniform through the simulation box.

Distortion of Magnetic Fields in a Starless Core: Near-infrared Polarimetry of FeSt 1–457

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kandori, Ryo; Tamura, Motohide; Kusakabe, Nobuhiko

Magnetic fields are believed to play an important role in controlling the stability and contraction of the dense condensations of gas and dust that lead to the formation of stars and planetary systems. In the present study, the magnetic field of FeSt 1–457, a cold starless molecular cloud core, was mapped on the basis of the polarized near-infrared light from 185 background stars after being dichroically absorbed by dust aligned with the magnetic field in the core. A distinct “hourglass-shaped” magnetic field was identified in the region of the core, and was interpreted as the first evidence of a magneticmore » field structure distorted by mass condensation in a starless core. The steep curvature of the magnetic field lines obtained in the present study indicates that the distortion was mainly created during the formation phase of the dense core. The derived mass-to-magnetic flux ratio indicates that the core is in a magnetically supercritical state. However, the stability of the core can be considered to be in a nearly critical state if the additional contributions from the thermal and turbulent support are included. Further diffusion of the magnetic field and/or turbulent dissipation would cause the onset of the dynamical collapse of the core. The geometrical relationship between the direction of the magnetic field lines and the elongation of the core was found to be in good agreement with theoretical predictions for the formation of Sun-like stars under the influence of a magnetic field.« less

Mapping of the extinction in giant molecular clouds using optical star counts

NASA Astrophysics Data System (ADS)

Cambrésy, L.

1999-05-01

This paper presents large scale extinction maps of most nearby Giant Molecular Clouds of the Galaxy (Lupus, rho Ophiuchus, Scorpius, Coalsack, Taurus, Chamaeleon, Musca, Corona Australis, Serpens, IC 5146, Vela, Orion, Monoceros R1 and R2, Rosette, Carina) derived from a star count method using an adaptive grid and a wavelet decomposition applied to the optical data provided by the USNO-Precision Measuring Machine. The distribution of the extinction in the clouds leads to estimate their total individual masses M and their maximum of extinction. I show that the relation between the mass contained within an iso-extinction contour and the extinction is similar from cloud to cloud and allows the extrapolation of the maximum of extinction in the range 5.7 to 25.5 magnitudes. I found that about half of the mass is contained in regions where the visual extinction is smaller than 1 magnitude. The star count method used on large scale ( ~ 250 square degrees) is a powerful and relatively straightforward method to estimate the mass of molecular complexes. A systematic study of the all sky would lead to discover new clouds as I did in the Lupus complex for which I found a sixth cloud of about 10(4) M_⊙.

Polaron-like vortices, dissociation transition, and self-induced pinning in magnetic superconductors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bulaevskii, L. N., E-mail: lnb@lanl.gov; Lin, S.-Z.

2013-09-15

Vortices in magnetic superconductors polarize spins nonuniformly and repolarize them when moving. At a low spin relaxation rate and at low bias currents, vortices carrying magnetic polarization clouds become polaron-like and their velocities are determined by the effective drag coefficient that is significantly bigger than the Bardeen-Stephen (BS) one. As the current increases, vortices release polarization clouds and the velocity as well as the voltage in the I-V characteristics jump to values corresponding to the BS drag coefficient at a critical current J{sub c}. The nonuniform components of the magnetic field and magnetization drop as the velocity increases, resulting inmore » weaker polarization and a discontinuous dynamic dissociation depinning transition. Experimentally, the jump shows up as a depinning transition and the corresponding current at the jump is the depinning current. As the current decreases, on the way back, vortices are retrapped by polarization clouds at the current J{sub r} < J{sub c}. As a result, the polaronic effect suppresses dissipation and enhances the critical current. Borocarbides (RE)Ni{sub 2}B{sub 2}C with a short penetration length and highly polarizable rare earth spins seem to be optimal systems for a detailed study of vortex polaron formation by measuring I-V characteristics. We also propose to use a superconductor-magnet multilayer structure to study polaronic mechanism of pinning with the goal to achieve high critical currents. The magnetic layers should have large magnetic susceptibility to enhance the coupling between vortices and magnetization in magnetic layers while the relaxation of the magnetization should be slow. For Nb and a proper magnet multilayer structure, we estimate the critical current density J{sub c} {approx} 10{sup 9} A/m{sup 2} at the magnetic field B Almost-Equal-To 1 T.« less

H2 emission as a tracer of molecular hydrogen: Large-scale observations of Orion

NASA Technical Reports Server (NTRS)

Luhman, M. L.; Jaffe, D. T.; Keller, L. D.; Pak, Soojong

1994-01-01

We have detected extremely extended (greater than 1.5 deg, or 12 pc) near-infrared H2 line emission from the Orion A molecular cloud. We have mapped emission in the 1.601 micrometer(s) upsilon = 6 - 4 Q(1) and 2.121 micrometer(s) upsilon = 1 - 0 S(1) lines of H2 along a approx. 2 deg R.A. cut and from a 6' x 6' region near theta(sup 1) Ori C. The surface brightness of the extended H2 line emission is 10(exp -6) to 10(exp -5) ergs/s/sq. cm/sr. Based on the distribution and relative strengths of the H2 lines, we conclude that UV fluorescene is most likely the dominant H2 emission mechanism in the outer parts of the Orion cloud. Shock-heated gas does not make a major contribution to the H2 emission in this region. The fluorescent component of the total H2 upsilon = 1 - 0 S(1) luminosity from Orion is 30-40 solar luminosity. Molecular hydrogen excited by UV radiation from nearby OB stars contributes 98%-99% of the global H2 line emission from the Orion molecular cloud, even though this cloud has a powerful shock-excited H2 source in its core. The ability to detect large-scale H2 directly opens up new possibilities for the study of molecular clouds.

Measuring Filament Orientation: A New Quantitative, Local Approach

DOE Office of Scientific and Technical Information (OSTI.GOV)

Green, C.-E.; Cunningham, M. R.; Jones, P. A.

The relative orientation between filamentary structures in molecular clouds and the ambient magnetic field provides insight into filament formation and stability. To calculate the relative orientation, a measurement of filament orientation is first required. We propose a new method to calculate the orientation of the one-pixel-wide filament skeleton that is output by filament identification algorithms such as filfinder. We derive the local filament orientation from the direction of the intensity gradient in the skeleton image using the Sobel filter and a few simple post-processing steps. We call this the “Sobel-gradient method.” The resulting filament orientation map can be compared quantitativelymore » on a local scale with the magnetic field orientation map to then find the relative orientation of the filament with respect to the magnetic field at each point along the filament. It can also be used for constructing radial profiles for filament width fitting. The proposed method facilitates automation in analyses of filament skeletons, which is imperative in this era of “big data.”.« less

Kinematics of the Ultra-High-Velocity Gas in the Expanding Molecular Shell Adjacent to the W44 Supernova Remnant

NASA Astrophysics Data System (ADS)

Yamada, Masaya; Oka, Tomoharu; Tanaka, Kunihiko; Nomura, Mariko; Takekawa, Shunya; Iwata, Yuhei; Tokuyama, Sekito; Tanabe, Keisuke; Tsujimoto, Shiho; Furusawa, Maiko

2017-01-01

High-velocity compact cloud (HVCC) is a peculiar category of molecular clouds detected in the central molecular zone of our Galaxy (Oka et al. 1998, 2007, and 2012). They are characterized by compact appearances (d < 5 pc) and very large velocity widths (Δ V > 50 km s-1). Some of them show high CO J=3-2/J=1-0 intensity ratios (>= 1.5), indicating that they consist of dense and warm molecular gas. Dispite a number of efforts, we have not reached a comprehensive interpretation of HVCCs. Recently, we detected an extraordinaly broad velocity width feature, the `Bullet', in the molecular cloud interacting with the W44 supernova remnant. The Bullet shares essential properties with HVCCs. Because of its proximity, a close inspection of the Bullet must contribute to the understanding of HVCCs.

Effect of Ambipolar Diffusion on Ion Abundances in Contracting Protostellar Cores

NASA Astrophysics Data System (ADS)

Ciolek, Glenn E.; Mouschovias, Telemachos Ch.

1998-09-01

Numerical simulations and analytical solutions have established that ambipolar diffusion can reduce the dust-to-gas ratio in magnetically and thermally supercritical cores during the epoch of core formation. We study the effect that this has on the ion chemistry in contracting protostellar cores and present a simplified analytical method that allows one to calculate the ion power-law exponent k (≡d ln ni/d ln nn, where ni and nn are the ion and neutral densities, respectively) as a function of core density. We find that, as in earlier numerical simulations, no single value of k can adequately describe the ion abundance for nn <~ 109 cm-3, a result that is contrary to the ``canonical'' value of k = 1/2 found in previous static equilibrium chemistry calculations and often used to study the effect of ambipolar diffusion in interstellar clouds. For typical cloud and grain parameters, reduction of the abundance of grains results in k > 1/2 during the core formation epoch (densities <~105 cm-3). As a consequence, observations of the degree of ionization in cores could be used, in principle, to determine whether ambipolar diffusion is responsible for core formation in interstellar molecular clouds. For densities >>105 cm-3, k is generally <<1/2.

DOES MAGNETIC-FIELD-ROTATION MISALIGNMENT SOLVE THE MAGNETIC BRAKING CATASTROPHE IN PROTOSTELLAR DISK FORMATION?

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li Zhiyun; Krasnopolsky, Ruben; Shang, Hsien

2013-09-01

Stars form in dense cores of molecular clouds that are observed to be significantly magnetized. In the simplest case of a laminar (non-turbulent) core with the magnetic field aligned with the rotation axis, both analytic considerations and numerical simulations have shown that the formation of a large, 10{sup 2} AU scale, rotationally supported protostellar disk is suppressed by magnetic braking in the ideal MHD limit for a realistic level of core magnetization. This theoretical difficulty in forming protostellar disks is termed the ''magnetic braking catastrophe''. A possible resolution to this problem, proposed by Hennebelle and Ciardi and Joos et al.,more » is that misalignment between the magnetic field and rotation axis may weaken the magnetic braking enough to enable disk formation. We evaluate this possibility quantitatively through numerical simulations. We confirm the basic result of Joos et al. that the misalignment is indeed conducive to disk formation. In relatively weakly magnetized cores with dimensionless mass-to-flux ratio {approx}> 4, it enabled the formation of rotationally supported disks that would otherwise be suppressed if the magnetic field and rotation axis are aligned. For more strongly magnetized cores, disk formation remains suppressed, however, even for the maximum tilt angle of 90 Degree-Sign . If dense cores are as strongly magnetized as indicated by OH Zeeman observations (with a mean dimensionless mass-to-flux ratio {approx}2), it would be difficult for the misalignment alone to enable disk formation in the majority of them. We conclude that, while beneficial to disk formation, especially for the relatively weak field case, misalignment does not completely solve the problem of catastrophic magnetic braking in general.« less

Cool neutral hydrogen in the direction of an anonymous OB association

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bania, T.M.

1983-08-01

H I self-absorption is seen in the direction l = 55./sup 0/6 probably physically associated with an anonymous OB association which has the Cepheid GY Sagittae as a member. The cool H I is in two clouds at least 15 pc in diameter located 3.25 kpc from the Sun. If their temperature is approx. =50 K, the cloud masses are approx. =10/sup 3/ M/sub sun/. The neutral atomic hydrogen clouds are probably warm envelopes surrounding cold molecular cloud cores because CO observations in this region show two molecular clouds nearly coincident with the absorbing H i gas. Since the OBmore » association is only approx. =10/sup 7/ years old, these clouds are likely to be part of the original cloud complex from which the stellar cluster formed. The H i clouds are part of the larger Arecibo survey of self-absorption which suggests that many of the Arecibo clouds are associated with heretofore unidentified star clusters. Even if this is generally not the case, the Arecibo objects have accurate kinematic distances and thus provide a new sample of cool H I clouds whose thermodynamic properties can be studied.« less

The Of?p stars of the Magellanic Clouds: Are they strongly magnetic?

NASA Astrophysics Data System (ADS)

Munoz, M.; Wade, G. A.; Nazé, Y.; Bagnulo, S.; Puls, J.

2018-01-01

All known Galactic Of?p stars have been shown to host strong, organized, magnetic fields. Recently, five Of?p stars have been discovered in the Magellanic Clouds. They posses photometric (Nazé et al., 2015) and spectroscopic (Walborn et al., 2015) variability compatible with the Oblique Rotator Model (ORM). However, their magnetic fields have yet to be directly detected. We have developed an algorithm allowing for the synthesis of photometric observables based on the Analytic Dynamical Magnetosphere (ADM) model by Owocki et al. (2016). We apply our model to OGLE photometry in order to constrain their magnetic geometries and surface dipole strengths. We predict that the field strengths for some of theses candidate extra-Galactic magnetic stars may be within the detection limits of the FORS2 instrument

The Photoevaporation of a Neutral Structure by an EUV+FUV Radiation Field

NASA Astrophysics Data System (ADS)

Lora, Veronica; Vasconcelos, M. J.; Raga, A. C.; Cerqueira, A. H.; Esquivel, A.

The expansion of an HII region into a surrounding inhomogeneous molecular cloud, leads to the formation of elongated "elephant trunk" structures. The EUV photo-ionising radiation and FUV dissociating radiation from newly born stars photo-evaporate their parental neutral cloud, leading to the formation of dense clumps in the tips of elephant trunks, that could in principle eventually form stars. We study th effects of including a photo-dissociating FUV flux in models of fragmentation of a photo-evaporating, self-gravitating molecular cloud.

Infrared Extinction and the Initial Conditions For Star and Planet Formation

NASA Technical Reports Server (NTRS)

Lada, Charles J.

2003-01-01

This grant funds a research program to use infrared extinction measurements to probe the detailed structure of dark molecular clouds and investigate the physical conditions which give rise to star and planet formation. The goals of the this program are to: 1) acquire deep infrared and molecular-line observations of a carefully selected sample of nearby dark clouds, 2) reduce and analyze the data obtained in order to produce detailed extinction maps of the clouds, 3) prepare results, where appropriate, for publication.

Infrared Extinction and the Initial Conditions for Star and Planet Formation

NASA Technical Reports Server (NTRS)

Lada, Charles J.

2002-01-01

This grant funds a research program to use infrared extinction measurements to probe the detailed structure of dark molecular clouds and investigate the physical conditions which give rise to star and planet formation. The goals of the this program are to: (1) acquire deep infrared and molecular-line observations of a carefully selected sample of nearby dark clouds; (2) reduce and analyze the data obtained in order to produce detailed extinction maps of the clouds; and (3) prepare results, where appropriate, for publication.

Labeled drawing of Jupiter showing its core and composition

NASA Technical Reports Server (NTRS)

1989-01-01

Labeled drawing of Jupiter identifies fluid molecular hydrogen, transition zone, fluid metallic hydrogen, and possible core and the composition of its atmosphere - cloud tops - aerosols, ammonia crystals, ammonium hydrosulfide clouds, ice crystal clouds, and water droplets.

Clustering the Orion B giant molecular cloud based on its molecular emission.

PubMed

Bron, Emeric; Daudon, Chloé; Pety, Jérôme; Levrier, François; Gerin, Maryvonne; Gratier, Pierre; Orkisz, Jan H; Guzman, Viviana; Bardeau, Sébastien; Goicoechea, Javier R; Liszt, Harvey; Öberg, Karin; Peretto, Nicolas; Sievers, Albrecht; Tremblin, Pascal

2018-02-01

Previous attempts at segmenting molecular line maps of molecular clouds have focused on using position-position-velocity data cubes of a single molecular line to separate the spatial components of the cloud. In contrast, wide field spectral imaging over a large spectral bandwidth in the (sub)mm domain now allows one to combine multiple molecular tracers to understand the different physical and chemical phases that constitute giant molecular clouds (GMCs). We aim at using multiple tracers (sensitive to different physical processes and conditions) to segment a molecular cloud into physically/chemically similar regions (rather than spatially connected components), thus disentangling the different physical/chemical phases present in the cloud. We use a machine learning clustering method, namely the Meanshift algorithm, to cluster pixels with similar molecular emission, ignoring spatial information. Clusters are defined around each maximum of the multidimensional Probability Density Function (PDF) of the line integrated intensities. Simple radiative transfer models were used to interpret the astrophysical information uncovered by the clustering analysis. A clustering analysis based only on the J = 1 - 0 lines of three isotopologues of CO proves suffcient to reveal distinct density/column density regimes ( n H ~ 100 cm -3 , ~ 500 cm -3 , and > 1000 cm -3 ), closely related to the usual definitions of diffuse, translucent and high-column-density regions. Adding two UV-sensitive tracers, the J = 1 - 0 line of HCO + and the N = 1 - 0 line of CN, allows us to distinguish two clearly distinct chemical regimes, characteristic of UV-illuminated and UV-shielded gas. The UV-illuminated regime shows overbright HCO + and CN emission, which we relate to a photochemical enrichment effect. We also find a tail of high CN/HCO + intensity ratio in UV-illuminated regions. Finer distinctions in density classes ( n H ~ 7 × 10 3 cm -3 ~ 4 × 10 4 cm -3 ) for the densest regions are also identified, likely related to the higher critical density of the CN and HCO + (1 - 0) lines. These distinctions are only possible because the high-density regions are spatially resolved. Molecules are versatile tracers of GMCs because their line intensities bear the signature of the physics and chemistry at play in the gas. The association of simultaneous multi-line, wide-field mapping and powerful machine learning methods such as the Meanshift clustering algorithm reveals how to decode the complex information available in these molecular tracers.

Theory of peak coalescence in Fourier transform ion cyclotron resonance mass spectrometry.

PubMed

Boldin, Ivan A; Nikolaev, Eugene N

2009-10-01

Peak coalescence, i.e. the merging of two close peaks in a Fourier transform ion cyclotron resonance (FTICR) mass spectrum at a high number of ions, plays an important role in various FTICR experiments. In order to describe the coalescence phenomenon we would like to propose a new theory of motion for ion clouds with close mass-to-charge ratios, driven by a uniform magnetic field and Coulomb interactions between the clouds. We describe the motion of the ion clouds in terms of their averaged drift motion in crossed magnetic and electric fields. The ion clouds are considered to be of constant size and their motion is studied in two dimensions. The theory deals with the first-order approximation of the equations of motion in relation to dm/m, where dm is the mass difference and m is the mass of a single ion. The analysis was done for an arbitrary inter-cloud interaction potential, which makes it possible to analyze finite-size ion clouds of any shape. The final analytical expression for the condition of the onset of coalescence is found for the case of uniformly charged spheres. An algorithm for finding this condition for an arbitrary interaction potential is proposed. The critical number of ions for the peak coalescence to take place is shown to depend quadratically on the magnetic field strength and to be proportional to the cyclotron radius and inversely proportional to the ion masses. Copyright (c) 2009 John Wiley & Sons, Ltd.

Multipoint Data Analysis of the Poloidal Motion inside a Magnetic Cloud

NASA Astrophysics Data System (ADS)

Zhao, A. K.

2015-12-01

The dynamical evolution of the magnetic clouds (MCs) has been the focus of the research since the MCs were identified several decades ago. Recently, some phenomena indicate the existence of the poloidal motion of plasma in MCs. In order to study the possible cause of this motion, we use in-situ data from STEREO-A, STEREO-B and Wind spacecraft to analyze the magnetic cloud on November 19 to 20, 2007. A velocity-modified cylindrical force-free flux rope model is used to fit the MC at three spacecraft and the relation between the value of fitting velocity and observed velocity in the x'-y' plane in MC frame is obtained. Through the analysis, we find the difference of the speed of the ambient solar wind before the leading edge and after the trailing edge of the MC is the major cause of the poloidal motion of plasma in MC.

Experimental Demonstration of a Synthetic Lorentz Force by Using Radiation Pressure.

PubMed

Šantić, N; Dubček, T; Aumiler, D; Buljan, H; Ban, T

2015-09-02

Synthetic magnetism in cold atomic gases opened the doors to many exciting novel physical systems and phenomena. Ubiquitous are the methods used for the creation of synthetic magnetic fields. They include rapidly rotating Bose-Einstein condensates employing the analogy between the Coriolis and the Lorentz force, and laser-atom interactions employing the analogy between the Berry phase and the Aharonov-Bohm phase. Interestingly, radiation pressure - being one of the most common forces induced by light - has not yet been used for synthetic magnetism. We experimentally demonstrate a synthetic Lorentz force, based on the radiation pressure and the Doppler effect, by observing the centre-of-mass motion of a cold atomic cloud. The force is perpendicular to the velocity of the cold atomic cloud, and zero for the cloud at rest. Our novel concept is straightforward to implement in a large volume, for a broad range of velocities, and can be extended to different geometries.

Calculation of gyrosynchrotron radiation brightness temperature for outer bright loop of ICME

NASA Astrophysics Data System (ADS)

Sun, Weiying; Wu, Ji; Wang, C. B.; Wang, S.

:Solar polar orbit radio telescope (SPORT) is proposed to detect the high density plasma clouds of outer bright loop of ICMEs from solar orbit with large inclination. Of particular interest is following the propagation of the plasma clouds with remote sensor in radio wavelength band. Gyrosynchrotron emission is a main radio radiation mechanism of the plasma clouds and can provide information of interplanetary magnetic field. In this paper, we statistically analyze the electron density, electron temperature and magnetic field of background solar wind in time of quiet sun and ICMEs propagation. We also estimate the fluctuation range of the electron density, electron temperature and magnetic field of outer bright loop of ICMEs. Moreover, we calculate and analyze the emission brightness temperature and degree of polarization on the basis of the study of gyrosynchrotron emission, absorption and polarization characteristics as the optical depth is less than or equal to 1.

Elves and associated electron density changes due to cloud-to-ground and in-cloud lightning discharges

NASA Astrophysics Data System (ADS)

Marshall, R. A.; Inan, U. S.; Glukhov, V. S.

2010-04-01

A 3-D finite difference time domain model is used to simulate the lightning electromagnetic pulse (EMP) and its interaction with the lower ionosphere. Results agree with the frequently observed, doughnut-shaped optical signature of elves but show that the structure exhibits asymmetry due to the presence of Earth's ambient magnetic field. Furthermore, in-cloud (horizontal) lightning channels produce observable optical emissions without the doughnut shape and, in fact, produce a much stronger optical output for the same channel current. Electron density perturbations associated with elves are also calculated, with contributions from attachment and ionization. Results presented as a function of parameters such as magnetic field direction, dipole current orientation, altitude and amplitude, and ambient ionospheric density profile demonstrate the highly nonlinear nature of the EMP-ionosphere interaction. Ionospheric effects of a sequence of in-cloud discharges are calculated, simulating a burst of in-cloud lightning activity and resulting in large density changes in the overlying ionosphere.

Cyanide and isocyanide abundances in the cold, dark cloud TMC-1

NASA Technical Reports Server (NTRS)

Irvine, W. M.; Schloerb, F. P.

1984-01-01

Cold, dark molecular clouds are particularly useful for the study of interstellar chemistry because their physical parameters are better understood than those of heterogeneous, complex giant molecular clouds. Another advantage is their relatively small distance from the solar system. The present investigaation has the objective to provide accurate abundance ratios for several cyanides and isocyanides in order to constrain models of dark cloud chemistry. The relative abundances of such related species can be particularly useful for the study of chemical processes. The cloud TMC-1 considered in the current study has a remarkably high abundance of acetylene and polyacetylene derivatives. Data at 3 mm, corresponding to the J = 1 to 0 transitions of HCN, H(C-13)N, HN(C-13), HC(N-15), and H(N-15)C were obtained.

Studying the Formation and Development of Molecular Clouds: With the CCAT Heterodyne Array Instrument (CHAI)

NASA Technical Reports Server (NTRS)

Goldsmith, Paul F.

2012-01-01

Surveys of all different types provide basic data using different tracers. Molecular clouds have structure over a very wide range of scales. Thus, "high resolution" surveys and studies of selected nearby clouds add critical information. The combination of large-area and high resolution allows Increased spatial dynamic range, which in turn enables detection of new and perhaps critical morphology (e.g. filaments). Theoretical modeling has made major progress, and suggests that multiple forces are at work. Galactic-scale modeling also progressing - indicates that stellar feedback is required. Models must strive to reproduce observed cloud structure at all scales. Astrochemical observations are not unrelated to questions of cloud evolution and star formation but we are still learning how to use this capability.

Interstellar molecules and dense clouds.

NASA Technical Reports Server (NTRS)

Rank, D. M.; Townes, C. H.; Welch, W. J.

1971-01-01

Current knowledge of the interstellar medium is discussed on the basis of recent published studies. The subjects considered include optical identification of interstellar molecules, radio molecular lines, interstellar clouds, isotopic abundances, formation and disappearance of interstellar molecules, and interstellar probing techniques. Diagrams are plotted for the distribution of galactic sources exhibiting molecular lines, for hydrogen molecule, hydrogen atom and electron abundances due to ionization, for the densities, velocities and temperature of NH3 in the direction of Sagitarius B2, for the lower rotational energy levels of H2CO, and for temporal spectral variations in masing H2O clouds of the radio source W49. Future applications of the maser and of molecular microscopy in this field are visualized.

A general theory for the lifetimes of giant molecular clouds under the influence of galactic dynamics

NASA Astrophysics Data System (ADS)

Jeffreson, Sarah M. R.; Kruijssen, J. M. Diederik

2018-05-01

We propose a simple analytic theory for environmentally dependent molecular cloud lifetimes, based on the large-scale (galactic) dynamics of the interstellar medium. Within this theory, the cloud lifetime is set by the time-scales for gravitational collapse, galactic shear, spiral arm interactions, epicyclic perturbations, and cloud-cloud collisions. It is dependent on five observable quantities, accessible through measurements of the galactic rotation curve, the gas and stellar surface densities, and the gas and stellar velocity dispersions of the host galaxy. We determine how the relative importance of each dynamical mechanism varies throughout the space of observable galactic properties, and conclude that gravitational collapse and galactic shear play the greatest role in setting the cloud lifetime for the considered range of galaxy properties, while cloud-cloud collisions exert a much lesser influence. All five environmental mechanisms are nevertheless required to obtain a complete picture of cloud evolution. We apply our theory to the galaxies M31, M51, M83, and the Milky Way, and find a strong dependence of the cloud lifetime upon galactocentric radius in each case, with a typical cloud lifetime between 10 and 50 Myr. Our theory is ideally suited for systematic observational tests with the Atacama Large Millimetre/submillimetre array.

[CII] observations of H2 molecular layers in transition clouds

NASA Astrophysics Data System (ADS)

Velusamy, T.; Langer, W. D.; Pineda, J. L.; Goldsmith, P. F.; Li, D.; Yorke, H. W.

2010-10-01

We present the first results on the diffuse transition clouds observed in [CII] line emission at 158 μm (1.9 THz) towards Galactic longitudes near 340° (5 LOSs) & 20° (11 LOSs) as part of the HIFI tests and GOT C+ survey. Out of the total 146 [CII] velocity components detected by profile fitting we identify 53 as diffuse molecular clouds with associated 12CO emission but without 13CO emission and characterized by AV < 5 mag. We estimate the fraction of the [CII] emission in the diffuse HI layer in each cloud and then determine the [CII] emitted from the molecular layers in the cloud. We show that the excess [CII] intensities detected in a few clouds is indicative of a thick H2 layer around the CO core. The wide range of clouds in our sample with thin to thick H2 layers suggests that these are at various evolutionary states characterized by the formation of H2 and CO layers from HI and C+, respectively. In about 30% of the clouds the H2 column densities (“dark gas”) traced by the [CII] is 50% or more than that traced by 12CO emission. On the average ~25% of the total H2 in these clouds is in an H2 layer which is not traced by CO. We use the HI, [CII], and 12CO intensities in each cloud along with simple chemical models to obtain constraints on the FUV fields and cosmic ray ionization rates. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

Distant Tail Behavior During High Speed Solar Wind Streams and Magnetic Storms

NASA Technical Reports Server (NTRS)

Ho, C. M.; Tsurutani, B. T.

1997-01-01

We have examined the ISEE 3 distant tail data during three intense magnetic storms and have identified the tail response to high-speed solar wind streams, interplanetary magnetic clouds, and near-Earth storms.

HOW GALACTIC ENVIRONMENT REGULATES STAR FORMATION

DOE Office of Scientific and Technical Information (OSTI.GOV)

Meidt, Sharon E.

2016-02-10

In a new simple model I reconcile two contradictory views on the factors that determine the rate at which molecular clouds form stars—internal structure versus external, environmental influences—providing a unified picture for the regulation of star formation in galaxies. In the presence of external pressure, the pressure gradient set up within a self-gravitating turbulent (isothermal) cloud leads to a non-uniform density distribution. Thus the local environment of a cloud influences its internal structure. In the simple equilibrium model, the fraction of gas at high density in the cloud interior is determined simply by the cloud surface density, which is itselfmore » inherited from the pressure in the immediate surroundings. This idea is tested using measurements of the properties of local clouds, which are found to show remarkable agreement with the simple equilibrium model. The model also naturally predicts the star formation relation observed on cloud scales and at the same time provides a mapping between this relation and the closer-to-linear molecular star formation relation measured on larger scales in galaxies. The key is that pressure regulates not only the molecular content of the ISM but also the cloud surface density. I provide a straightforward prescription for the pressure regulation of star formation that can be directly implemented in numerical models. Predictions for the dense gas fraction and star formation efficiency measured on large-scales within galaxies are also presented, establishing the basis for a new picture of star formation regulated by galactic environment.« less

G30.79 FIR 10: a gravitationally bound infalling high-mass star-forming clump

NASA Astrophysics Data System (ADS)

Cortes, P. C.; Parra, R.; Cortes, J. R.; Hardy, E.

2010-09-01

Context. The process of high-mass star formation is still shrouded in controversy. Models are still tentative and current observations are just beginning to probe the densest inner regions of giant molecular clouds. Aims: The study of high-mass star formation requires the observation and analysis of high-density gas. This can be achieved by the detection of emission from higher rotational transitions of molecules in the sub-millimeter. Here, we studied the high-mass clump G30.79 FIR 10 by observing molecular emission in the 345 GHz band. The goal is to understand the gravitational state of this clump, considering turbulence and magnetic fields, and to study the kinematics of dense gas. Methods: We approached this region by mapping the spatial distribution of HCO+(J = 4 →) 3, H13CO+ (J = 4 → 3), CS(J = 7 → 6), 12CO(J = 3 → 2), and 13CO(J = 3 → 2) molecular emission by using the ASTE telescope and by observing the 12C18O(J = 3 → 2), HCN(J = 4 → 3), and H13CN(J = 4 → 3) molecular transitions with the APEX telescope. Results: Infalling motions were detected and modeled toward this source. A mean infall velocity of 0.5 km s-1 with an infall mass rate of 5 × 10-3 M⊙ yr-1 was obtained. Also, a previously estimated value for the magnetic field strength in the plane of the sky was refined to be 855 μG which we used to calculate a mass-to-magnetic flux ratio, λ = 1.9, or super-critical. The virial mass from turbulent motions was also calculated finding Mvir = 563 M⊙, which gives a ratio of Msubmm/Mvir = 5.9. Both values strongly suggest that this clump must be in a state of gravitational collapse. Additionally, we estimated the HCO+ abundance, obtaining X(HCO+) = 2.4 × 10-10.

C3H2 observations as a diagnostic probe for molecular clouds

NASA Technical Reports Server (NTRS)

Avery, L. W.

1986-01-01

Recently the three-membered ring molecule, cyclopropenylidene, C3H2, has been identified in the laboratory and detected in molecular clouds by Thaddeus, Vrtilek and Gottlieb (1985). This molecule is wide-spread throughout the Galaxy and has been detected in 25 separate sources including cold dust clouds, circumstellar envelopes, HII regions, and the spiral arms observed against the Cas supernova remnant. In order to evaluate the potential of C3H2 as a diagnostic probe for molecular clouds, and to attempt to identify the most useful transitions, statistical equilibrium calculations were carried out for the lowest 24 levels of the ortho species and the lowest 10 levels of the para species. Many of the sources observed by Matthews and Irvine (1985) show evidence of being optically thick in the 1(10)-1(01) line. Consequently, the effects of radiative trapping should be incorporated into the equilibrium calculations. This was done using the Large Velocity Gradient approximation for a spherical cloud of uniform density. Some results of the calculations for T(K)=10K are given. Figures are presented which show contours of the logarithm of the ratio of peak line brightness temperatures for ortho-para pairs of lines at similar frequencies. It appears that the widespread nature of C3H2, the relatively large strength of its spectral lines, and their sensitivity to density and molecular abundance combine to make this a useful molecule for probing physical conditions in molecular clouds. The 1(10)-1(01) and 2(20)-2(11) K-band lines may be especially useful in this regard because of the ease with which they are observed and their unusual density-dependent emission/absorption properties.

Observational Approach to Molecular Cloud Evolutation with the Submillimeter-Wave CI Lines

NASA Astrophysics Data System (ADS)

Oka, T.; Yamamoto, S.

Neutral carbon atoms (CI) play important roles both in chemistry and cooling processes of interstellar molecular clouds. It is thus crucial to explore its large area distribution to obtain information on formation processes and thermal balance of molecular clouds. However, observations of the submillimeter-wave CI lines have been limited to small areas around some representative objects. We have constructed a 1.2 m submillimeter-wave telescope at the summit of Mt.Fuji. The telescope was designed for the exclusive use of surveying molecular clouds in two submillimeter-wave CI lines, 3 P1 -3 P0 (492GHz) and 3 P2 -3 P1 (809 GHz), of atomic carbon. A superconductor-insulator-superconductor (SIS) mixer receiver was equipped on the Nasmyth focus of the telescope. The receiver noise temperatures [Trx(DSB)] are 300 K and 1000 K for the 492 GHz and the 809 GHz mixers, respectively. The intermediate frequency is centered at 2 GHz, having a 700 MHz bandwidth. An acousto-optical spectrometer (AOS) with 1024 channel outputs is used as a receiver backend. The telescope was installed at Nishi-yasugawara (alt. 3725 m), which is 200 m north of the highest peak, Kengamine (3776 m), in July 1998. It has b en operatede successfully during 4 observing seasons in a remote way from the Hongo campus of the University of Tokyo. We have already observed more than 40 square degrees of the sky with the CI 492 GHz line. The distribution of CI emission is found to be different from those of the 13 CO or C1 8 O emission in some clouds. These differences are discussed in relation to formation processes of molecular clouds.

GEOMETRY-INDEPENDENT DETERMINATION OF RADIAL DENSITY DISTRIBUTIONS IN MOLECULAR CLOUD CORES AND OTHER ASTRONOMICAL OBJECTS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Krčo, Marko; Goldsmith, Paul F., E-mail: marko@astro.cornell.edu

2016-05-01

We present a geometry-independent method for determining the shapes of radial volume density profiles of astronomical objects whose geometries are unknown, based on a single column density map. Such profiles are often critical to understand the physics and chemistry of molecular cloud cores, in which star formation takes place. The method presented here does not assume any geometry for the object being studied, thus removing a significant source of bias. Instead, it exploits contour self-similarity in column density maps, which appears to be common in data for astronomical objects. Our method may be applied to many types of astronomical objectsmore » and observable quantities so long as they satisfy a limited set of conditions, which we describe in detail. We derive the method analytically, test it numerically, and illustrate its utility using 2MASS-derived dust extinction in molecular cloud cores. While not having made an extensive comparison of different density profiles, we find that the overall radial density distribution within molecular cloud cores is adequately described by an attenuated power law.« less

Interstellar C2, CH, and CN in translucent molecular clouds

NASA Technical Reports Server (NTRS)

Black, John H.; Van Dishoeck, Ewine F.

1989-01-01

Optical absorption-line techniques have been applied to the study of a number of translucent molecular clouds in which the total column densities are large enough that substantial molecular abundances can be maintained. Results are presented for a survey of absorption lines of interstellar C2, CH, and CN. Detections of CN through the A 2Pi-X 2Sigma(+) (1,O) and (2,O) bands of the red system are reported and compared with observations of the violet system for one line of sight. The population distributions in C2 provide diagnostic information on temperature and density. The measured column densities of the three species can be used to test details of the theory of molecule formation in clouds where photoprocesses still play a significant role. The C2 and CH column densities are strongly correlated with each other and probably also with the H2 column density. In contrast, the CN column densities are found to vary greatly from cloud to cloud. The observations are discussed with reference to detailed theoretical models.

Chemistry and Evolution of Interstellar Clouds

NASA Technical Reports Server (NTRS)

Wooden, D. H.; Charnley, S. B.; Ehrenfreund, P.

2003-01-01

In this chapter we describe how elements have been and are still being formed in the galaxy and how they are transformed into the reservoir of materials present at the time of formation of our protosolar nebula. We discuss the global cycle of matter, beginning at its formation site in stars, where it is ejected through winds and explosions into the diffuse interstellar medium. In the next stage of the global cycle occurs in cold, dense molecular clouds, where the complexity of molecules and ices increases relative to the diffuse ISM.. When a protostar forms in a dense core within a molecular cloud, it heats the surrounding infalling matter warms and releases molecules from the solid phase into the gas phase in a warm, dense core, sponsoring a rich gas-phase chemistry. Some material from the cold and warm regions within molecular clouds probably survives as interstellar matter in the protostellar disk. For the diffuse ISM, for cold, dense clouds, and for dense-warm cores, the physio-chemical processes that occur within the gas and solid phases are discussed in detail.

Synthetic observations of molecular clouds in a galactic centre environment - I. Studying maps of column density and integrated intensity

NASA Astrophysics Data System (ADS)

Bertram, Erik; Glover, Simon C. O.; Clark, Paul C.; Ragan, Sarah E.; Klessen, Ralf S.

2016-02-01

We run numerical simulations of molecular clouds, adopting properties similar to those found in the central molecular zone (CMZ) of the Milky Way. For this, we employ the moving mesh code AREPO and perform simulations which account for a simplified treatment of time-dependent chemistry and the non-isothermal nature of gas and dust. We perform simulations using an initial density of n0 = 103 cm-3 and a mass of 1.3 × 105 M⊙. Furthermore, we vary the virial parameter, defined as the ratio of kinetic and potential energy, α = Ekin/|Epot|, by adjusting the velocity dispersion. We set it to α = 0.5, 2.0 and 8.0, in order to analyse the impact of the kinetic energy on our results. We account for the extreme conditions in the CMZ and increase both the interstellar radiation field (ISRF) and the cosmic ray flux (CRF) by a factor of 1000 compared to the values found in the solar neighbourhood. We use the radiative transfer code RADMC-3D to compute synthetic images in various diagnostic lines. These are [C II] at 158 μm, [O I] (145 μm), [O I] (63 μm), 12CO (J = 1 → 0) and 13CO (J = 1 → 0) at 2600 and 2720 μm, respectively. When α is large, the turbulence disperses much of the gas in the cloud, reducing its mean density and allowing the ISRF to penetrate more deeply into the cloud's interior. This significantly alters the chemical composition of the cloud, leading to the dissociation of a significant amount of the molecular gas. On the other hand, when α is small, the cloud remains compact, allowing more of the molecular gas to survive. We show that in each case the atomic tracers accurately reflect most of the physical properties of both the H2 and the total gas of the cloud and that they provide a useful alternative to molecular lines when studying the interstellar medium in the CMZ.

Evolution of fractality in space plasmas of interest to geomagnetic activity

NASA Astrophysics Data System (ADS)

Muñoz, Víctor; Domínguez, Macarena; Alejandro Valdivia, Juan; Good, Simon; Nigro, Giuseppina; Carbone, Vincenzo

2018-03-01

We studied the temporal evolution of fractality for geomagnetic activity, by calculating fractal dimensions from the Dst data and from a magnetohydrodynamic shell model for turbulent magnetized plasma, which may be a useful model to study geomagnetic activity under solar wind forcing. We show that the shell model is able to reproduce the relationship between the fractal dimension and the occurrence of dissipative events, but only in a certain region of viscosity and resistivity values. We also present preliminary results of the application of these ideas to the study of the magnetic field time series in the solar wind during magnetic clouds, which suggest that it is possible, by means of the fractal dimension, to characterize the complexity of the magnetic cloud structure.

Sequential Star Formation in RCW 34: A Spectroscopic Census of the Stellar Content of High-Mass Star-Forming Regions

NASA Astrophysics Data System (ADS)

Bik, A.; Puga, E.; Waters, L. B. F. M.; Horrobin, M.; Henning, Th.; Vasyunina, T.; Beuther, H.; Linz, H.; Kaper, L.; van den Ancker, M.; Lenorzer, A.; Churchwell, E.; Kurtz, S.; Kouwenhoven, M. B. N.; Stolte, A.; de Koter, A.; Thi, W. F.; Comerón, F.; Waelkens, Ch.

2010-04-01

In this paper, we present VLT/SINFONI integral field spectroscopy of RCW 34 along with Spitzer/IRAC photometry of the surroundings. RCW 34 consists of three different regions. A large bubble has been detected in the IRAC images in which a cluster of intermediate- and low-mass class II objects is found. At the northern edge of this bubble, an H II region is located, ionized by 3 OB stars, of which the most massive star has spectral type O8.5V. Intermediate-mass stars (2-3 M sun) are detected of G- and K-spectral type. These stars are still in the pre-main-sequence (PMS) phase. North of the H II region, a photon-dominated region is present, marking the edge of a dense molecular cloud traced by H2 emission. Several class 0/I objects are associated with this cloud, indicating that star formation is still taking place. The distance to RCW 34 is revised to 2.5 ± 0.2 kpc and an age estimate of 2 ± 1 Myr is derived from the properties of the PMS stars inside the H II region. Between the class II sources in the bubble and the PMS stars in the H II region, no age difference could be detected with the present data. The presence of the class 0/I sources in the molecular cloud, however, suggests that the objects inside the molecular cloud are significantly younger. The most likely scenario for the formation of the three regions is that star formation propagated from south to north. First the bubble is formed, produced by intermediate- and low-mass stars only, after that, the H II region is formed from a dense core at the edge of the molecular cloud, resulting in the expansion similar to a champagne flow. More recently, star formation occurred in the rest of the molecular cloud. Two different formation scenarios are possible. (1) The bubble with the cluster of low- and intermediate-mass stars triggered the formation of the O star at the edge of the molecular cloud, which in its turn induces the current star formation in the molecular cloud. (2) An external triggering is responsible for the star formation propagating from south to north. Based on observations collected at the European Southern Observatory at Paranal, Chile (ESO program 078.C-0780).

Relationship between the column density distribution and evolutionary class of molecular clouds as viewed by ATLASGAL

NASA Astrophysics Data System (ADS)

Abreu-Vicente, J.; Kainulainen, J.; Stutz, A.; Henning, Th.; Beuther, H.

2015-09-01

We present the first study of the relationship between the column density distribution of molecular clouds within nearby Galactic spiral arms and their evolutionary status as measured from their stellar content. We analyze a sample of 195 molecular clouds located at distances below 5.5 kpc, identified from the ATLASGAL 870 μm data. We define three evolutionary classes within this sample: starless clumps, star-forming clouds with associated young stellar objects, and clouds associated with H ii regions. We find that the N(H2) probability density functions (N-PDFs) of these three classes of objects are clearly different: the N-PDFs of starless clumps are narrowest and close to log-normal in shape, while star-forming clouds and H ii regions exhibit a power-law shape over a wide range of column densities and log-normal-like components only at low column densities. We use the N-PDFs to estimate the evolutionary time-scales of the three classes of objects based on a simple analytic model from literature. Finally, we show that the integral of the N-PDFs, the dense gas mass fraction, depends on the total mass of the regions as measured by ATLASGAL: more massive clouds contain greater relative amounts of dense gas across all evolutionary classes. Appendices are available in electronic form at http://www.aanda.org

The Origin of the X-Ray Emission from the High-velocity Cloud MS30.7-81.4-118

NASA Astrophysics Data System (ADS)

Henley, David B.; Shelton, Robin L.; Kwak, Kyujin

2014-08-01

A soft X-ray enhancement has recently been reported toward the high-velocity cloud MS30.7-81.4-118 (MS30.7), a constituent of the Magellanic Stream. In order to investigate the origin of this enhancement, we have analyzed two overlapping XMM-Newton observations of this cloud. We find that the X-ray enhancement is ~6' or ~100 pc across, and is concentrated to the north and west of the densest part of the cloud. We modeled the X-ray enhancement with a variety of spectral models. A single-temperature equilibrium plasma model yields a temperature of (3.69^{+0.47}_{-0.44}) \\times 10^6 \\,K and a 0.4-2.0 keV luminosity of 7.9 × 1033 erg s-1. However, this model underpredicts the on-enhancement emission around 1 keV, which may indicate the additional presence of hotter plasma (T >~ 107 K), or that recombination emission is important. We examined several different physical models for the origin of the X-ray enhancement. We find that turbulent mixing of cold cloud material with hot ambient material, compression or shock heating of a hot ambient medium, and charge exchange reactions between cloud atoms and ions in a hot ambient medium all lead to emission that is too faint. In addition, shock heating in a cool or warm medium leads to emission that is too soft (for reasonable cloud speeds). We find that magnetic reconnection could plausibly power the observed X-ray emission, but resistive magnetohydrodynamical simulations are needed to test this hypothesis. If magnetic reconnection is responsible for the X-ray enhancement, the observed spectral properties could potentially constrain the magnetic field in the vicinity of the Magellanic Stream.

Statistical properties of the polarized emission of Planck Galactic cold clumps

NASA Astrophysics Data System (ADS)

Ristorcelli, Isabelle; Planck Collaboration

2015-08-01

The Galactic magnetic fields are considered as one of the key components regulating star formation, but their actual role on the dense cores formation and evolution remains today an open question.Dust polarized continuum emission is particularly well suited to probe the dense and cold medium and study the magnetic field structure. Such observations also provide tight constraints to better understand the efficiency of the dust alignment along the magnetic field lines, which in turn relate on our grasp to properly interpret the B-field properties.With the Planck all-sky survey of dust submillimeter emission in intensity and polarization, we can investigate the intermediate scales, between that of molecular cloud and of prestellar cores, and perform a statistical analysis on the polarization properties of cold clumps.Combined with the IRAS map at 100microns, the Planck survey has allowed to build the first all-sky catalogue of Galactic Cold Clumps (PGCC, Planck 2015 results XXVIII 2015). The corresponding 13188 sources cover a broad range in physical properties, and correspond to different evolutionary stages, from cold and starless clumps, nearby cores, to young protostellar objects still embedded in their cold surrounding cloud.I will present the main results of our polarization analysis obtained on different samples of sources from the PGCC catalogue, based on the 353GHz polarized emission measured with Planck. The statistical properties are derived from a stacking method, using optimized estimators for the polarization fraction and angle parameters. These properties are determined and compared according to the nature of the sources (starless or YSOs), their size or density range. Finally, I will present a comparison of our results with predictions from MHD simulations of clumps including radiative transfer and the dust radiative torque alignment mechanism.

Distortion of Magnetic Fields in a Starless Core II: 3D Magnetic Field Structure of FeSt 1-457

NASA Astrophysics Data System (ADS)

Kandori, Ryo; Tamura, Motohide; Tomisaka, Kohji; Nakajima, Yasushi; Kusakabe, Nobuhiko; Kwon, Jungmi; Nagayama, Takahiro; Nagata, Tetsuya; Tatematsu, Ken'ichi

2017-10-01

Three-dimensional (3D) magnetic field information on molecular clouds and cores is important for revealing their kinematical stability (magnetic support) against gravity, which is fundamental for studying the initial conditions of star formation. In the present study, the 3D magnetic field structure of the dense starless core FeSt 1-457 is determined based on the near-infrared polarimetric observations of the dichroic polarization of background stars and simple 3D modeling. With an obtained angle of line-of-sight magnetic inclination axis {θ }{inc} of 45^\\circ +/- 10^\\circ and previously determined plane-of-sky magnetic field strength {B}{pol} of 23.8 ± 12.1 μ {{G}}, the total magnetic field strength for FeSt 1-457 is derived to be 33.7 ± 18.0 μ {{G}}. The critical mass of FeSt 1-457, evaluated using both magnetic and thermal/turbulent support is {M}{cr}=3.70+/- 0.92 {M}⊙ , which is identical to the observed core mass, {M}{core}=3.55+/- 0.75 {M}⊙ . We thus conclude that the stability of FeSt 1-457 is in a condition close to the critical state. Without infalling gas motion and no associated young stars, the core is regarded to be in the earliest stage of star formation, I.e., the stage just before the onset of dynamical collapse following the attainment of a supercritical condition. These properties could make FeSt 1-457 one of the best starless cores for future studies of the initial conditions of star formation.

(abstract) The Distant Tail Behavior During High Speed Solar Wind Streams and Magnetic Storms

NASA Technical Reports Server (NTRS)

Ho, C. M.; Tsurutani, B. T.

1996-01-01

We have examined the ISEE-3 distant tail data during three intense magnetic storms and have identified the tail response to high speed solar wind streams, interplanetary magnetic clouds, and near-Earth storms.

STAR FORMATION IN TURBULENT MOLECULAR CLOUDS WITH COLLIDING FLOW

DOE Office of Scientific and Technical Information (OSTI.GOV)

Matsumoto, Tomoaki; Dobashi, Kazuhito; Shimoikura, Tomomi, E-mail: matsu@hosei.ac.jp

2015-03-10

Using self-gravitational hydrodynamical numerical simulations, we investigated the evolution of high-density turbulent molecular clouds swept by a colliding flow. The interaction of shock waves due to turbulence produces networks of thin filamentary clouds with a sub-parsec width. The colliding flow accumulates the filamentary clouds into a sheet cloud and promotes active star formation for initially high-density clouds. Clouds with a colliding flow exhibit a finer filamentary network than clouds without a colliding flow. The probability distribution functions (PDFs) for the density and column density can be fitted by lognormal functions for clouds without colliding flow. When the initial turbulence ismore » weak, the column density PDF has a power-law wing at high column densities. The colliding flow considerably deforms the PDF, such that the PDF exhibits a double peak. The stellar mass distributions reproduced here are consistent with the classical initial mass function with a power-law index of –1.35 when the initial clouds have a high density. The distribution of stellar velocities agrees with the gas velocity distribution, which can be fitted by Gaussian functions for clouds without colliding flow. For clouds with colliding flow, the velocity dispersion of gas tends to be larger than the stellar velocity dispersion. The signatures of colliding flows and turbulence appear in channel maps reconstructed from the simulation data. Clouds without colliding flow exhibit a cloud-scale velocity shear due to the turbulence. In contrast, clouds with colliding flow show a prominent anti-correlated distribution of thin filaments between the different velocity channels, suggesting collisions between the filamentary clouds.« less

Large scale IRAM 30 m CO-observations in the giant molecular cloud complex W43

NASA Astrophysics Data System (ADS)

Carlhoff, P.; Nguyen Luong, Q.; Schilke, P.; Motte, F.; Schneider, N.; Beuther, H.; Bontemps, S.; Heitsch, F.; Hill, T.; Kramer, C.; Ossenkopf, V.; Schuller, F.; Simon, R.; Wyrowski, F.

2013-12-01

We aim to fully describe the distribution and location of dense molecular clouds in the giant molecular cloud complex W43. It was previously identified as one of the most massive star-forming regions in our Galaxy. To trace the moderately dense molecular clouds in the W43 region, we initiated W43-HERO, a large program using the IRAM 30 m telescope, which covers a wide dynamic range of scales from 0.3 to 140 pc. We obtained on-the-fly-maps in 13CO (2-1) and C18O (2-1) with a high spectral resolution of 0.1 km s-1 and a spatial resolution of 12''. These maps cover an area of ~1.5 square degrees and include the two main clouds of W43 and the lower density gas surrounding them. A comparison to Galactic models and previous distance calculations confirms the location of W43 near the tangential point of the Scutum arm at approximately 6 kpc from the Sun. The resulting intensity cubes of the observed region are separated into subcubes, which are centered on single clouds and then analyzed in detail. The optical depth, excitation temperature, and H2 column density maps are derived out of the 13CO and C18O data. These results are then compared to those derived from Herschel dust maps. The mass of a typical cloud is several 104 M⊙ while the total mass in the dense molecular gas (>102 cm-3) in W43 is found to be ~1.9 × 106 M⊙. Probability distribution functions obtained from column density maps derived from molecular line data and Herschel imaging show a log-normal distribution for low column densities and a power-law tail for high densities. A flatter slope for the molecular line data probability distribution function may imply that those selectively show the gravitationally collapsing gas. Appendices are available in electronic form at http://www.aanda.orgThe final datacubes (13CO and C18O) for the entire survey are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/560/A24

Discovery of a Molecular Collision Front in Interacting Galaxies NGC 4567/4568 with ALMA

NASA Astrophysics Data System (ADS)

Kaneko, Hiroyuki; Kuno, Nario; Saitoh, Takayuki R.

2018-06-01

We present results of 12CO(J = 1–0) imaging observations of NGC 4567/4568, a galaxy pair in a close encounter, with the Atacama Large Millimeter/Submillimeter Array (ALMA). For the first time, we find clear evidence of a molecular collision front with a velocity dispersion that is 16.8 ± 1.4 km s‑1 at the overlapping region, owing to high spatial and velocity resolution. By integrating over the velocity width that corresponds to the molecular collision front, we find a long filamentary structure with a size of 1800 pc × 350 pc at the collision front. This filamentary molecular structure spatially coincides with a dark lane seen in the R-band image. We find four molecular clouds in the filament, each with a radius of 30 pc and mass of 106 M ⊙ the radii matching a typical value for giant molecular clouds (GMCs) and the masses corresponding to those between GMCs and giant molecular associations (GMAs). All four clouds are gravitationally bound. The molecular filamentary structure and its physical conditions are similar to the structure expected via numerical simulation. The filament could be a progenitor of super star clusters.

Understanding star formation in molecular clouds. I. Effects of line-of-sight contamination on the column density structure

NASA Astrophysics Data System (ADS)

Schneider, N.; Ossenkopf, V.; Csengeri, T.; Klessen, R. S.; Federrath, C.; Tremblin, P.; Girichidis, P.; Bontemps, S.; André, Ph.

2015-03-01

Column-density maps of molecular clouds are one of the most important observables in the context of molecular cloud- and star-formation (SF) studies. With the Herschel satellite it is now possible to precisely determine the column density from dust emission, which is the best tracer of the bulk of material in molecular clouds. However, line-of-sight (LOS) contamination from fore- or background clouds can lead to overestimating the dust emission of molecular clouds, in particular for distant clouds. This implies values that are too high for column density and mass, which can potentially lead to an incorrect physical interpretation of the column density probability distribution function (PDF). In this paper, we use observations and simulations to demonstrate how LOS contamination affects the PDF. We apply a first-order approximation (removing a constant level) to the molecular clouds of Auriga and Maddalena (low-mass star-forming), and Carina and NGC 3603 (both high-mass SF regions). In perfect agreement with the simulations, we find that the PDFs become broader, the peak shifts to lower column densities, and the power-law tail of the PDF for higher column densities flattens after correction. All corrected PDFs have a lognormal part for low column densities with a peak at Av ~ 2 mag, a deviation point (DP) from the lognormal at Av(DP) ~ 4-5 mag, and a power-law tail for higher column densities. Assuming an equivalent spherical density distribution ρ ∝ r- α, the slopes of the power-law tails correspond to αPDF = 1.8, 1.75, and 2.5 for Auriga, Carina, and NGC 3603. These numbers agree within the uncertainties with the values of α ≈ 1.5,1.8, and 2.5 determined from the slope γ (with α = 1-γ) obtained from the radial column density profiles (N ∝ rγ). While α ~ 1.5-2 is consistent with a structure dominated by collapse (local free-fall collapse of individual cores and clumps and global collapse), the higher value of α > 2 for NGC 3603 requires a physical process that leads to additional compression (e.g., expanding ionization fronts). From the small sample of our study, we find that clouds forming only low-mass stars and those also forming high-mass stars have slightly different values for their average column density (1.8 × 1021 cm-2 vs. 3.0 × 1021 cm-2), and they display differences in the overall column density structure. Massive clouds assemble more gas in smaller cloud volumes than low-mass SF ones. However, for both cloud types, the transition of the PDF from lognormal shape into power-law tail is found at the same column density (at Av ~ 4-5 mag). Low-mass and high-mass SF clouds then have the same low column density distribution, most likely dominated by supersonic turbulence. At higher column densities, collapse and external pressure can form the power-law tail. The relative importance of the twoprocesses can vary between clouds and thus lead to the observed differences in PDF and column density structure. Appendices are available in electronic form at http://www.aanda.orgHerschel maps as FITS files are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/575/A79

Physical properties of CO-dark molecular gas traced by C+

NASA Astrophysics Data System (ADS)

Tang, Ningyu; Li, Di; Heiles, Carl; Wang, Shen; Pan, Zhichen; Wang, Jun-Jie

2016-09-01

Context. Neither Hi nor CO emission can reveal a significant quantity of so-called dark gas in the interstellar medium (ISM). It is considered that CO-dark molecular gas (DMG), the molecular gas with no or weak CO emission, dominates dark gas. Determination of physical properties of DMG is critical for understanding ISM evolution. Previous studies of DMG in the Galactic plane are based on assumptions of excitation temperature and volume density. Independent measurements of temperature and volume density are necessary. Aims: We intend to characterize physical properties of DMG in the Galactic plane based on C+ data from the Herschel open time key program, namely Galactic Observations of Terahertz C+ (GOT C+) and Hi narrow self-absorption (HINSA) data from international Hi 21 cm Galactic plane surveys. Methods: We identified DMG clouds with HINSA features by comparing Hi, C+, and CO spectra. We derived the Hi excitation temperature and Hi column density through spectral analysis of HINSA features. The Hi volume density was determined by utilizing the on-the-sky dimension of the cold foreground Hi cloud under the assumption of axial symmetry. The column and volume density of H2 were derived through excitation analysis of C+ emission. The derived parameters were then compared with a chemical evolutionary model. Results: We identified 36 DMG clouds with HINSA features. Based on uncertainty analysis, optical depth of HiτHi of 1 is a reasonable value for most clouds. With the assumption of τHi = 1, these clouds were characterized by excitation temperatures in a range of 20 K to 92 K with a median value of 55 K and volume densities in the range of 6.2 × 101 cm-3 to 1.2 × 103 cm-3 with a median value of 2.3 × 102 cm-3. The fraction of DMG column density in the cloud (fDMG) decreases with increasing excitation temperature following an empirical relation fDMG =-2.1 × 10-3Tex,(τHi = 1) + 1.0. The relation between fDMG and total hydrogen column density NH is given by fDMG = 1.0-3.7 × 1020/NH. We divided the clouds into a high extinction group and low extinction group with the dividing threshold being total hydrogen column density NH of 5.0 × 1021 cm-2 (AV = 2.7 mag). The values of fDMG in the low extinction group (AV ≤ 2.7 mag) are consistent with the results of the time-dependent, chemical evolutionary model at the age of ~10 Myr. Our empirical relation cannot be explained by the chemical evolutionary model for clouds in the high extinction group (AV > 2.7 mag). Compared to clouds in the low extinction group (AV ≤ 2.7 mag), clouds in the high extinction group (AV > 2.7 mag) have comparable volume densities but excitation temperatures that are 1.5 times lower. Moreover, CO abundances in clouds of the high extinction group (AV > 2.7 mag) are 6.6 × 102 times smaller than the canonical value in the Milky Way. Conclusions: The molecular gas seems to be the dominate component in these clouds. The high percentage of DMG in clouds of the high extinction group (AV > 2.7 mag) may support the idea that molecular clouds are forming from pre-existing molecular gas, I.e., a cold gas with a high H2 content but that contains a little or no CO content.

Are We Observing Coronal Mass Ejections in OH/IR AGB Stars?

NASA Astrophysics Data System (ADS)

Heiles, Carl

2017-05-01

Solar Coronal Mass Ejections (CMEs) are magnetic electron clouds that are violently ejected by the same magnetic reconnection events that produce Solar flares. CMEs are the major driving source of the hazardous space weather environments near the Earth. In exoplanet systems, the equivalent of Solar wind and CMEs can affect a planet's atmosphere, and in extreme cases can erode it, as probably happened with Mars, or disrupt the cosmic-ray shielding aspect of the planet's magnetic field.We (Jensen et al. 2013SoPh..285...83J, 2016SoPh..291..465J) have developed a new way to observe the electron column density and magnetic field of CMEs, namely to measure the frequency change and Faraday rotation of a spacecraft downlink carrier produced by propagation effects in the plasma. Surprisingly, this can work on other stars if they have the equivalent of the spacecraft carrier, as do OH/IR stars.OH/IR stars are Asymptotic Giant Branch (AGB) stars, which are red giant stars burning He in their final stages of stellar evolution. They have highly convective surfaces and large mass-ejection rates in the form of expanding dense shells of molecular gas and obscuring dust, which were ejected from the star by chaotic turbulent motions and then accelerated by radiation pressure. OH masers reside in these shells, pumped by the IR emission from the dust. The OH masers on the far side of the star (i.e., the positive-velocity masers) are the surrogate for the Solar-case spacecraft signal.The big question: Can we see CMEs in OH/IR stars? We have observed six OH/IR stars with the Arecibo Observatory for a total of about 150 hours over the past 1.5 years. We see changes in OH maser frequency and in the position angle of linear polarization. Both can be produced by electron clouds moving across the line of sight. We will present statistical summaries of the variability and interpret them in terms of CME models.

Infrared Extinction and the Initial Conditions for Star and Planet Formation

NASA Technical Reports Server (NTRS)

Lada, Charles J.

2004-01-01

This grant funds a research program to use infrared extinction measurements to probe the detailed structure of dark molecular clouds and investigate the physical conditions which give rise to star and planet formation. The goals of the this program are to: 1) acquire deep infrared and molecular-line observations of a carefully selected sample of nearby dark clouds, 2) reduce and analyze the data obtained in order to produce detailed extinction maps of the clouds, 3) prepare results, where appropriate, for publication. A description of how these goals were met are included.

Magnetic conveyor belt for transporting and merging trapped atom clouds.

PubMed

Hänsel, W; Reichel, J; Hommelhoff, P; Hänsch, T W

2001-01-22

We demonstrate an integrated magnetic device which transports cold atoms near a surface with very high positioning accuracy. Time-dependent currents in a lithographic conductor pattern create a moving chain of potential wells; atoms are transported in these wells while remaining confined in all three dimensions. We achieve mean fluxes up to 10(6) s(-1) with a negligible heating rate. An extension of this device allows merging of atom clouds by unification of two Ioffe-Pritchard potentials. The unification, which we demonstrate experimentally, can be performed without loss of phase space density. This novel, all-magnetic atom manipulation offers exciting perspectives, such as trapped-atom interferometry.

MR-based detection of individual histotripsy bubble clouds formed in tissues and phantoms.

PubMed

Allen, Steven P; Hernandez-Garcia, Luis; Cain, Charles A; Hall, Timothy L

2016-11-01

To demonstrate that MR sequences can detect individual histotripsy bubble clouds formed inside intact tissues. A line-scan and an EPI sequence were sensitized to histotripsy by inserting a bipolar gradient whose lobes bracketed the lifespan of a histotripsy bubble cloud. Using a 7 Tesla, small-bore scanner, these sequences monitored histotripsy clouds formed in an agar phantom and in vitro porcine liver and brain. The bipolar gradients were adjusted to apply phase with k-space frequencies of 10, 300 or 400 cm -1 . Acoustic pressure amplitude was also varied. Cavitation was simultaneously monitored using a passive cavitation detection system. Each image captured local signal loss specific to an individual bubble cloud. In the agar phantom, this signal loss appeared only when the transducer output exceeded the cavitation threshold pressure. In tissues, bubble clouds were immediately detected when the gradients created phase with k-space frequencies of 300 and 400 cm -1 . When the gradients created phase with a k-space frequency of 10 cm -1 , individual bubble clouds were not detectable until many acoustic pulses had been applied to the tissue. Cavitation-sensitive MR-sequences can detect single histotripsy bubble clouds formed in biologic tissue. Detection is influenced by the sensitizing gradients and treatment history. Magn Reson Med 76:1486-1493, 2016. © 2015 International Society for Magnetic Resonance in Medicine. © 2015 International Society for Magnetic Resonance in Medicine.

A two-dimensional theory of plasma contactor clouds used in the ionosphere with an electrodynamic tether

NASA Technical Reports Server (NTRS)

Hastings, D. E.; Gatsonis, N. A.; Rivas, D. A.

1988-01-01

Plasma contactors have been proposed as a means of making good electrical contact between biased surfaces such as found at the ends of an electrodynamic tether and the space environment. A plasma contactor is a plasma source which emits a plasma cloud which facilitates the electrical connection. The physics of this plasma cloud is investigated for contactors used as electron collectors and it is shown that contactor clouds in space will consist of a spherical core possibly containing a shock wave. Outside of the core the cloud will expand anisotropically across the magnetic field leading to a turbulent cigar shape structure along the field. This outer region is itself divided into two regions by the ion response to the electric field. A two-dimensional theory of the motion of the cloud across the magnetic field is developed. The current voltage characteristic of an Argon plasma contactor cloud is estimated for several ion currents in the range of 1-100 Amperes. It is shown that small ion current contactors are more efficient than large ion current contactors. This suggests that if a plasma contactor is used on an electrodynamic tether then a miltiple tether array will be more efficient than a single tether.

A search for pre-main-sequence stars in high-latitude molecular clouds. 3: A survey of the Einstein database

NASA Technical Reports Server (NTRS)

Caillault, Jean-Pierre; Magnani, Loris; Fryer, Chris

1995-01-01

In order to discern whether the high-latitude molecular clouds are regions of ongoing star formation, we have used X-ray emission as a tracer of youthful stars. The entire Einstein database yields 18 images which overlap 10 of the clouds mapped partially or completely in the CO (1-0) transition, providing a total of approximately 6 deg squared of overlap. Five previously unidentified X-ray sources were detected: one has an optical counterpart which is a pre-main-sequence (PMS) star, and two have normal main-sequence stellar counterparts, while the other two are probably extragalactic sources. The PMS star is located in a high Galactic latitude Lynds dark cloud, so this result is not too suprising. The translucent clouds, though, have yet to reveal any evidence of star formation.

Factor analysis as a tool for spectral line component separation 21cm emission in the direction of L1780

NASA Technical Reports Server (NTRS)

Toth, L. V.; Mattila, K.; Haikala, L.; Balazs, L. G.

1992-01-01

The spectra of the 21cm HI radiation from the direction of L1780, a small high-galactic latitude dark/molecular cloud, were analyzed by multivariate methods. Factor analysis was performed on HI (21cm) spectra in order to separate the different components responsible for the spectral features. The rotated, orthogonal factors explain the spectra as a sum of radiation from the background (an extended HI emission layer), and from the L1780 dark cloud. The coefficients of the cloud-indicator factors were used to locate the HI 'halo' of the molecular cloud. Our statistically derived 'background' and 'cloud' spectral profiles, as well as the spatial distribution of the HI halo emission distribution were compared to the results of a previous study which used conventional methods analyzing nearly the same data set.

Detailed CO(J = 1-0, 2-1, and 3-2) observations toward an H II region RCW 32 in the Vela Molecular Ridge

NASA Astrophysics Data System (ADS)

Enokiya, Rei; Sano, Hidetoshi; Hayashi, Katsuhiro; Tachihara, Kengo; Torii, Kazufumi; Yamamoto, Hiroaki; Hattori, Yusuke; Hasegawa, Yutaka; Ohama, Akio; Kimura, Kimihiro; Ogawa, Hideo; Fukui, Yasuo

2018-05-01

We performed CO(J = 1-0, 2-1, and 3-2) observations toward an H II region RCW 32 in the Vela Molecular Ridge. The CO gas distribution associated with the H II region was revealed for the first time at a high resolution of 22″. The results revealed three distinct velocity components which show correspondence with the optical dark lanes and/or Hα distribution. Two of the components show complementary spatial distribution which suggests collisional interaction between them at a relative velocity of ˜ 4 km s-1. Based on these results, we present a hypothesis that a cloud-cloud collision determined the cloud distribution and triggered formation of the exciting star ionizing RCW 32. The collision time scale is estimated from the cloud size and the velocity separation to be ˜2 Myr and the collision terminated ˜1 Myr ago, which is consistent with the age of the exciting star and the associated cluster. By combing the previous works on the H II regions in the Vela Molecular Ridge, we argue that the majority (at least four) of the H II regions in the Ridge were formed by triggering of cloud-cloud collision.

CO line ratios in molecular clouds: the impact of environment

NASA Astrophysics Data System (ADS)

Peñaloza, Camilo H.; Clark, Paul C.; Glover, Simon C. O.; Klessen, Ralf S.

2018-04-01

Line emission is strongly dependent on the local environmental conditions in which the emitting tracers reside. In this work, we focus on modelling the CO emission from simulated giant molecular clouds (GMCs), and study the variations in the resulting line ratios arising from the emission from the J = 1-0, J = 2-1, and J = 3-2 transitions. We perform a set of smoothed particle hydrodynamics simulations with time-dependent chemistry, in which environmental conditions - including total cloud mass, density, size, velocity dispersion, metallicity, interstellar radiation field (ISRF), and the cosmic ray ionization rate (CRIR) - were systematically varied. The simulations were then post-processed using radiative transfer to produce synthetic emission maps in the three transitions quoted above. We find that the cloud-averaged values of the line ratios can vary by up to ±0.3 dex, triggered by changes in the environmental conditions. Changes in the ISRF and/or in the CRIR have the largest impact on line ratios since they directly affect the abundance, temperature, and distribution of CO-rich gas within the clouds. We show that the standard methods used to convert CO emission to H2 column density can underestimate the total H2 molecular gas in GMCs by factors of 2 or 3, depending on the environmental conditions in the clouds.

Formaldehyde in the Diffuse Interstellar Cloud MBM40

NASA Astrophysics Data System (ADS)

Joy, Mackenzie; Magnani, Loris A.

2018-06-01

MBM40, a high-latitude molecular cloud, has been extensively studied using different molecular tracers. It appears that MBM40 is composed of a relatively dense, helical filament embedded in a more diffuse substrate of low density molecular gas. In order to study the transition between the two regimes, this project presents the first high-resolution mapping of MBM40 using the 110-111 hyperfine transition of formaldehyde (H2CO) at 4.83 GHz. We used H2CO spectra obtained with the Arecibo telescope more than a decade ago to construct this map. The results can be compared to previous maps made from the CO(1-0) transition to gain further understanding of the structure of the cloud. The intensity of the H2CO emission was compared to the CO emission. Although a correlation exists between the H2CO and CO emissivity, there seems to be a saturation of H2CO line strength for stronger CO emissivity. This is probably a radiative transfer effect of the CO emission. We have also found that the velocity dispersion of H2CO in the lower ridge of the cloud is significantly lower than in the rest of the cloud. This may indicate that this portion of the cloud is a coherent structure (analogous to an eddy) in a turbulent flow.

Morphology and ionization of the interstellar cloud surrounding the solar system.

PubMed

Frisch, P C

1994-09-02

The first encounter between the sun and the surrounding interstellar cloud appears to have occurred 2000 to 8000 years ago. The sun and cloud space motions are nearly perpendicular, an indication that the sun is skimming the cloud surface. The electron density derived for the surrounding cloud from the carbon component of the anomalous cosmic ray population in the solar system and from the interstellar ratio of Mg(+) to Mg degrees toward Sirius support an equilibrium model for cloud ionization (an electron density of 0.22 to 0.44 per cubic centimeter). The upwind magnetic field direction is nearly parallel to the cloud surface. The relative sun-cloud motion indicates that the solar system has a bow shock.

Identification of prominence ejecta by the proton distribution function and magnetic fine structure in interplanetary coronal mass ejections in the inner heliosphere

NASA Astrophysics Data System (ADS)

Yao, Shuo; Marsch, Eckart; Tu, Chuan-Yi; Schwenn, Rainer

2010-05-01

This work presents in situ solar wind observations of three magnetic clouds (MCs) that contain cold high-density material when Helios 2 was located at 0.3 AU on 9 May 1979, 0.5 AU on 30 March 1976, and 0.7 AU on 24 December 1978. In the cold high-density regions embedded in the interplanetary coronal mass ejections we find (1) that the number density of protons is higher than in other regions inside the magnetic cloud, (2) the possible existence of He+, (3) that the thermal velocity distribution functions are more isotropic and appear to be colder than in the other regions of the MC, and the proton temperature is lower than that of the ambient plasma, and (4) that the associated magnetic field configuration can for all three MC events be identified as a flux rope. This cold high-density region is located at the polarity inversion line in the center of the bipolar structure of the MC magnetic field (consistent with previous solar observation work that found that a prominence lies over the neutral line of the related bipolar solar magnetic field). Specifically, for the first magnetic cloud event on 8 May 1979, a coronal mass ejection (CME) was related to an eruptive prominence previously reported as a result of the observation of Solwind (P78-1). Therefore, we identify the cold and dense region in the MC as the prominence material. It is the first time that prominence ejecta were identified by both the plasma and magnetic field features inside 1 AU, and it is also the first time that the thermal ion velocity distribution functions were used to investigate the microstate of the prominence material. Moreover, from our three cases, we also found that this material tended to fall behind the magnetic cloud and become smaller as it propagated farther away from the Sun, which confirms speculations in previous work. Overall, our in situ observations are consistent with three-part CME models.

Formation of iron metal and grain coagulation in the solar nebula

NASA Technical Reports Server (NTRS)

Nuth, Joseph A., III; Berg, Otto

1994-01-01

The interstellar grain population in the giant molecular cloud from which the sun formed contained little or no iron metal. However, thermal processing of individual interstellar silicates in the solar nebula is likely to result in the formation of a population of very small iron metal grains. If such grains are exposed to even transient magnetic fields, each will become a tiny dipole magnet capable of interacting with other such dipoles over spatial scale orders of magnitude larger than the radii of individual grains. Such interactions will greatly increase the coagulation cross-section for this grain population. Furthermore, the magnetic attraction between two iron dipoles will significantly increase both the collisional sticking coefficient and the strength of the interparticle binding energy for iron aggregates. Formation of iron metal may therefore be a key step in the aggregation of planetesimals in a protoplanetary nebula. Such aggregates may have already been observed in protoplanetary systems. The enhancement in the effective interaction distance between two magnetic dipoles is directly proportional to the strength of the magnetic dipoles and inversely proportional to the relative velocity. It is less sensitive to the reduced mass of the interacting particles (alpha M(exp -1/2)) and almost insensitive to the initial number density of magnetic dipoles (alpha n(sub o)(exp 1/6)). We are in the process of measuring the degree of coagulation in our condensation flow apparatus as a function of applied magnetic field and correlating these results by means of magnetic remanance acquisition measurements on our iron grains with the strength of the magnetic field to which the grains are exposed. Results of our magnetic remanance acquisition measurements and the magnetic-induced coagulation study will be presented as well as an estimate of the importance of such processes near the nebular midplane.

Toward Measuring Galactic Dense Molecular Gas Properties and 3D Distribution with Hi-GAL

NASA Astrophysics Data System (ADS)

Zetterlund, Erika; Glenn, Jason; Maloney, Phil

2016-01-01

The Herschel Space Observatory's submillimeter dust continuum survey Hi-GAL provides a powerful new dataset for characterizing the structure of the dense interstellar medium of the Milky Way. Hi-GAL observed a 2° wide strip covering the entire 360° of the Galactic plane in broad bands centered at 70, 160, 250, 350, and 500 μm, with angular resolution ranging from 10 to 40 arcseconds. We are adapting a molecular cloud clump-finding algorithm and a distance probability density function distance-determination method developed for the Bolocam Galactic Plane Survey (BGPS) to the Hi-GAL data. Using these methods we expect to generate a database of 105 cloud clumps, derive distance information for roughly half the clumps, and derive precise distances for approximately 20% of them. With five-color photometry and distances, we will measure the cloud clump properties, such as luminosities, physical sizes, and masses, and construct a three-dimensional map of the Milky Way's dense molecular gas distribution.The cloud clump properties and the dense gas distribution will provide critical ground truths for comparison to theoretical models of molecular cloud structure formation and galaxy evolution models that seek to emulate spiral galaxies. For example, such models cannot resolve star formation and use prescriptive recipes, such as converting a fixed fraction of interstellar gas to stars at a specified interstellar medium density threshold. The models should be compared to observed dense molecular gas properties and galactic distributions.As a pilot survey to refine the clump-finding and distance measurement algorithms developed for BGPS, we have identified molecular cloud clumps in six 2° × 2° patches of the Galactic plane, including one in the inner Galaxy along the line of sight through the Molecular Ring and the termination of the Galactic bar and one toward the outer Galaxy. Distances have been derived for the inner Galaxy clumps and compared to Bolocam Galactic Plane Survey results. We present the pilot survey clump catalog, distances, clump properties, and a comparison to BGPS.

Diffuse cloud chemistry. [in interstellar matter

NASA Technical Reports Server (NTRS)

Van Dishoeck, Ewine F.; Black, John H.

1988-01-01

The current status of models of diffuse interstellar clouds is reviewed. A detailed comparison of recent gas-phase steady-state models shows that both the physical conditions and the molecular abundances in diffuse clouds are still not fully understood. Alternative mechanisms are discussed and observational tests which may discriminate between the various models are suggested. Recent developments regarding the velocity structure of diffuse clouds are mentioned. Similarities and differences between the chemistries in diffuse clouds and those in translucent and high latitude clouds are pointed out.

ALMA Reveals Molecular Cloud N55 in the Large Magellanic Cloud as a Site of Massive Star Formation

NASA Astrophysics Data System (ADS)

Naslim, N.; Tokuda, K.; Onishi, T.; Kemper, F.; Wong, T.; Morata, O.; Takada, S.; Harada, R.; Kawamura, A.; Saigo, K.; Indebetouw, R.; Madden, S. C.; Hony, S.; Meixner, M.

2018-02-01

We present the molecular cloud properties of N55 in the Large Magellanic Cloud using 12CO(1–0) and 13CO(1–0) observations obtained with Atacama Large Millimeter Array. We have done a detailed study of molecular gas properties, to understand how the cloud properties of N55 differ from Galactic clouds. Most CO emission appears clumpy in N55, and molecular cores that have young stellar objects (YSOs) show larger linewidths and masses. The massive clumps are associated with high and intermediate mass YSOs. The clump masses are determined by local thermodynamic equilibrium and virial analysis of the 12CO and 13CO emissions. These mass estimates lead to the conclusion that (a) the clumps are in self-gravitational virial equilibrium, and (b) the 12CO(1–0)-to-H2 conversion factor, {X}{CO}, is 6.5 × 1020 cm‑2 (K km s‑1)‑1. This CO-to-H2 conversion factor for N55 clumps is measured at a spatial scale of ∼0.67 pc, which is about two times higher than the {X}{CO} value of the Orion cloud at a similar spatial scale. The core mass function of N55 clearly show a turnover below 200 {M}ȯ , separating the low-mass end from the high-mass end. The low-mass end of the 12CO mass spectrum is fitted with a power law of index 0.5 ± 0.1, while for 13CO it is fitted with a power law index 0.6 ± 0.2. In the high-mass end, the core mass spectrum is fitted with a power index of 2.0 ± 0.3 for 12CO, and with 2.5 ± 0.4 for 13CO. This power law behavior of the core mass function in N55 is consistent with many Galactic clouds.

Characteristics of magnetic clouds/magnetic-cloud-like structures during the years 1995-2003

NASA Astrophysics Data System (ADS)

Wu, C. C.; Lepping, R.

Using nine years of solar wind plasma and magnetic field data we investigated the characteristics of both magnetic clouds MCs and magnetic cloud like structures MCLs during 1995-2003 A MCL structure is an event which was identified by an automatic scheme Lepping et al 2005 with the same criteria as for a MC but is not identifiable as a flux rope by using the MC Burlaga 1981 fitting model developed by Lepping et al 1990 The average occurrence rate is sim 9 5 for MCs and sim 13 6 for MCLs per year for the overall period of interest and there were 82 MCs and 122 MCLs identified during this period The characteristics of MCs and MCL structures are as follows 1 The averaged duration Dt of MCs is 21 1 hour which is 40 longer than MCLs Dt 15 hrs 2 The averaged Bz min minimum Bz found in MC MCL is -10 2 nT for MCs and -6 nT for MCLs 3 The averaged Dst min minimum Dst caused by MC MCL is -82 nT for MCs and -37 nT for MCLs 4 The average of the solar wind velocity is 453 km s for MCs and 413 km s for MCLs 5 The average of the thermal speed is 24 6 km s for MCs and 27 7 km s for MCLs 6 The average of the magnetic field intensity is 12 7 nT for MCs and 9 8 nT for MCLs and 7 The average of the solar wind density is 9 4 cm -3 for MCs and 6 3 cm -3 for MCLs The longer duration more intense magnetic field and higher solar wind speed and denisty of MCs compared to those of the MCLs might be the major reason for generally causing geomagnetic storms with higher

Three-dimensional particle-in-cell simulations of a plasma jet/cloud streaming across a transverse magnetic field

NASA Astrophysics Data System (ADS)

Voitcu, Gabriel; Echim, Marius

2014-05-01

The dynamics of collisionless plasma jets/clouds in magnetic field configurations typical for the terrestrial magnetotail and frontside magnetosheath is a topic of interest for understanding the physics of the magnetosphere and its interaction with the solar wind. The presence of high-speed jets in the frontside magnetosheath has been recently proved experimentally by Cluster and THEMIS spacecrafts. There is increasing evidence that the bursty bulk flows in the magnetotail have jet-like features. In the present paper we use fully electromagnetic 3D explicit particle-in-cell (PIC) simulations to investigate the interaction of a localized three-dimensional plasma element/jet/cloud with a transverse magnetic field. We consider a plasma jet/cloud that moves in vacuum and perpendicular to an ambient magnetic field. Ampère and Faraday's laws are used to compute the self-consistent electric and magnetic fields on a three-dimensional spatial grid having a step-size of the order of the Debye length and using a time-step that resolves the plasma frequency. The initial magnetic field inside the simulation domain is uniform and the plasma bulk velocity at the beginning of the simulation is normal to the magnetic field direction. The total time scale of the simulation is of the order of few ion Larmor periods. Space and time variations of the plasma parameters and of the electromagnetic field are analyzed and discussed. We emphasize non-MHD effects like the energy-dispersion signatures at the edges of the plasma element, similar to results previously reported by Voitcu and Echim (2012) using test-kinetic simulations. Acknowledgments: Research supported by the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 313038/STORM, and a grant of the Romanian Ministry of National Education, CNCS - UEFISCDI, project number PN-II-ID-PCE-2012-4-0418.

Infrared Extinction and the Initial Conditions for Star and Planet Formation

NASA Technical Reports Server (NTRS)

Lada, Charles J.

2005-01-01

This grant funded a research program to use infrared extinction measurements to probe the detailed structure of dark molecular clouds and investigate the physical conditions which give rise to star and planet formation. The goals of the this program were to: 1) acquire deep infrared and molecular-line observations of a carefully selected sample of nearby dark clouds, 2) reduce and analyze the data obtained in order to produce detailed extinction maps of the clouds, 3) use the results to measure and quantitatively describe the physical conditions of the dense gas and dust that produce stars and their accompanying planetary systems in molecular clouds. The goals of this project were met and exceeded as described below. 1) The infrared data for the project were obtained in a number of observing runs using the 3.5-meter NTT and 8-meter VLT telescopes of the European Southern Observatory in Chile and the 1.2-meter telescope of the Smithsonian Astrophysical Observatory in Arizona, the 1 0-meter Keck telescope in Hawaii, the 6.5-meter MMT of the Smithsonian Astrophysical Observatory in Arizona, and the NASA Hubble Space Telescope. The molecular-line data was obtained in three runs using the IRAM 30-meter telescope in Spain and one run with the ESO-15 meter millimeter-wave telescope in Chile. Millimeter-wave continuum measurements were obtained with the 15-meter JCMT in Hawaii. 2) Considerable effort was expended to reduce the infrared imaging observations including the development of custom software to produce high quality photometry and source astrometry. All the millimeter-line data was reduced using standard reduction routines. The highlights of the infrared analysis were the production of detailed extinction maps and the construction of profiles of the density structure of the B68, Coalsack, B335 and Lupus clouds. 3) The principal scientific accomplishments of this research program include the following: We were able to use our infrared observations to determine the density structure of the B68 cloud to an unprecedented level of precision. This lead to a major breakthrough in the study of molecular cloud structure. For the first time we have been able to characterize the structure of a dark cloud in a detail only exceeded by that known for a star. We determined that the cloud's structure is exquisitely well described by the equations of a Bonner-Ebert sphere (a pressure confined isothermal sphere). We were able to show that the cloud is very nearly in equilibrium with the internal thermal pressure of the cloud balancing gravity and the external pressure of the surrounding interstellar medium. We were able to determine for the first time the gas-to-dust ratio in a dense cloud core. We also demonstrated a new method to determine extremely precise distances to such clouds by combining knowledge of the properties of Bonner-Ebert Spheres with our infrared and millimeter-wave observations.

Galactic kinematics of molecuar clouds

DOE Office of Scientific and Technical Information (OSTI.GOV)

Stark, A.A.

1979-01-01

The kinematics of molecular clouds in the galactic disk are studied to determine the magnitude of cloud-to-coud velocity dispersions and systematic streaming motions. Three observational programs are reported: (i) a strip map of 1 = 180/sup 0/ from b = -9/sup 0/ to +8/sup 0/ observed in CO J = 1 greater than or equal to 0 to an rms noise level of 0.1 K in 250 kHz filters; (ii) a 20-point map of the minor axis of M31, observed in CO J = 1 greater than or equal to 0 to an rms noise level of 20 mK inmore » 1 MHz filters; (iii) three maps in the molecular ring, centered at 1 = 34/sup 0/, 1 = 36/sup 0/ and 1 = 51/sup 0/, each about one degree square, sampled every 0.05/sup 0/ in /sup 13/CO J = 1 greater than or equal to 0 to an rms noise level of 0.1 K in 250 kHz filters. Molecular clouds typically have one dimensional cloud-to-cloud velocity dispersions of about 8 km s/sup -1/. This dispersion is independent of cloud mass over a range of 10/sup 2/M /sub solar/ < M/sub C < 10/sup 5/ /sup 5/M /sub solar/. Clouds more massive than about 10 /sup 5/ /sup 5/M /sub solar/ have a markedly smaller dispersion. These most massive clouds occur preferentially in spiral arms, and result in spiral arm CO emissivities several times that of interarm regions. Also associated with spiral arms are large-scale streaming motions, which amount to 100 km s/sup -1/ in one arm in M31.« less

How Does Dense Molecular Gas Contribute to Star Formation in the Starburst Galaxy NGC 2146?

NASA Astrophysics Data System (ADS)

Wofford, Alia

2017-01-01

The starburst galaxy NGC 2146 is believed to have been formed approximately 800 Myr ago, when two galaxies collided with each other possibly leading to a burst of star formation. NGC 2146 is known as a starburst galaxy for the high frequency of star formation going on in its molecular clouds. These clouds serve as nurseries for star formation to occur. Hydrogen Cyanide (HCN) and Carbon monoxide (CO) are molecules found in molecular gas clouds. HCN molecules are tracers for high density star forming gas. Whereas, CO molecules are tracers for low density star forming gas. In this project, we are observing these two molecules and their proximity to where the stars are forming in the galaxy to determine if the star formation is occurring in the same area as the high and low density molecular gas areas in starburst galaxy NGC 2146.

Cosmic rays, gas and dust in nearby anticentre clouds. I. CO-to-H2 conversion factors and dust opacities

NASA Astrophysics Data System (ADS)

Remy, Q.; Grenier, I. A.; Marshall, D. J.; Casandjian, J. M.

2017-05-01

Aims: We aim to explore the capabilities of dust emission and γ rays for probing the properties of the interstellar medium in the nearby anti-centre region, using γ-ray observations with the Fermi Large Area Telescope (LAT), and the thermal dust optical depth inferred from Planck and IRAS observations. We also aim to study massive star-forming clouds including the well known Taurus, Auriga, Perseus, and California molecular clouds, as well as a more diffuse structure which we refer to as Cetus. In particular, we aim at quantifying potential variations in cosmic-ray density and dust properties per gas nucleon across the different gas phases and different clouds, and at measuring the CO-to-H2 conversion factor, XCO, in different environments. Methods: We have separated six nearby anti-centre clouds that are coherent in velocities and distances, from the Galactic-disc background in H I 21-cm and 12CO 2.6-mm line emission. We have jointly modelled the γ-ray intensity recorded between 0.4 and 100 GeV, and the dust optical depth τ353 at 353 GHz as a combination of H I-bright, CO-bright, and ionised gas components. The complementary information from dust emission and γ rays was used to reveal the gas not seen, or poorly traced, by H I, free-free, and 12CO emissions, namely (I) the opaque H iand diffuse H2 present in the Dark Neutral Medium at the atomic-molecular transition, and (II) the dense H2 to be added where 12CO lines saturate. Results: The measured interstellar γ-ray spectra support a uniform penetration of the cosmic rays with energies above a few GeV through the clouds, from the atomic envelopes to the 12CO-bright cores, and with a small ± 9% cloud-to-cloud dispersion in particle flux. We detect the ionised gas from the H iiregion NGC 1499 in the dust and γ-ray emissions and measure its mean electron density and temperature. We find a gradual increase in grain opacity as the gas (atomic or molecular) becomes more dense. The increase reaches a factor of four to six in the cold molecular regions that are well shielded from stellar radiation. Consequently, the XCO factor derived from dust is systematically larger by 30% to 130% than the γ-ray estimate. We also evaluate the average γ-ray XCO factor for each cloud, and find that XCO tends to decrease from diffuse to more compact molecular clouds, as expected from theory. We find XCO factors in the anti-centre clouds close to or below 1020 cm-2 K-1 km-1 s, in agreement with other estimates in the solar neighbourhood. Together, they confirm the long-standing unexplained discrepancy, by a factor of two, between the mean XCO values measured at parsec scales in nearby clouds and those obtained at kiloparsec scale in the Galaxy. Our results also highlight large quantitative discrepancies in 12CO intensities between simulations and observations at low molecular gas densities.

Chemical evolution of giant molecular clouds in simulations of galaxies

NASA Astrophysics Data System (ADS)

Richings, Alexander J.; Schaye, Joop

2016-08-01

We present an analysis of giant molecular clouds (GMCs) within hydrodynamic simulations of isolated, low-mass (M* ˜ 109 M⊙) disc galaxies. We study the evolution of molecular abundances and the implications for CO emission and the XCO conversion factor in individual clouds. We define clouds either as regions above a density threshold n_{H, min} = 10 {cm}^{-3}, or using an observationally motivated CO intensity threshold of 0.25 {K} {km} {s}^{-1}. Our simulations include a non-equilibrium chemical model with 157 species, including 20 molecules. We also investigate the effects of resolution and pressure floors (I.e. Jeans limiters). We find cloud lifetimes up to ≈ 40 Myr, with a median of 13 Myr, in agreement with observations. At one-tenth solar metallicity, young clouds ( ≲ 10-15 Myr) are underabundant in H2 and CO compared to chemical equilibrium, by factors of ≈3 and one to two orders of magnitude, respectively. At solar metallicity, GMCs reach chemical equilibrium faster (within ≈ 1 Myr). We also compute CO emission from individual clouds. The mean CO intensity, ICO, is strongly suppressed at low dust extinction, Av, and possibly saturates towards high Av, in agreement with observations. The ICO-Av relation shifts towards higher Av for higher metallicities and, to a lesser extent, for stronger UV radiation. At one-tenth solar metallicity, CO emission is weaker in young clouds ( ≲ 10-15 Myr), consistent with the underabundance of CO. Consequently, XCO decreases by an order of magnitude from 0 to 15 Myr, albeit with a large scatter.

IRAS and the Boston University Arecibo Galactic H I Survey: A catalog of cloud properties

NASA Technical Reports Server (NTRS)

Bania, Thomas M.

1992-01-01

The Infrared Astronomy Satellite (IRAS) Galactic Plane Surface Brightness Images were used to identify infrared emission associated with cool, diffuse H I clouds detected by the Boston University-Arecibo Galactic H I Survey. These clouds are associated with galactic star clusters, H II regions, and molecular clouds. Using emission-absorption experiments toward galactic H II regions, we determined the H I properties of cool H I clouds seen in absorption against the thermal continuum, including their kinematic distances. Correlations were then made between IRAS sources and these H II regions, thus some of the spatial confusion associated with the IRAS fields near the galactic plane was resolved since the distances to these sources was known. Because we can also correlate the BU-Arecibo clouds with existing CO surveys, these results will allow us to determine the intrinsic properties of the gas (neutral and ionized atomic as well as molecular) and dust for interstellar clouds in the inner galaxy. For the IRAS-identified H II region sample, we have established the far infrared (FIR) luminosities and galactic distribution of these sources.

Effect of surface conductivity on the peak magnetic field radiated by first return strokes in cloud-to-ground lightning

NASA Technical Reports Server (NTRS)

Tyahla, Lori J.; Lopez, Raul E.

1994-01-01

The effect of surface conductivity on the peak magnetic field radiated by the first return stroke in cloud-to-ground lightning was investigated by comparing the peak magnetic fields from return strokes that struck water with those that struck land. The data were obtained from a network of three gated, wideband magnetic direction finders (DFs) at the NASA Kennedy Space Center during the summer of 1985. Two geographical areas that were equidistant from two of the direction finders were compared where the flash distances ranged from approximately 40 to 60 km. An unbiased data set was obtained by correcting site errors, equalizing differences in sensor gain, eliminating directional biases in DF triggering, and keeping differences in signal attenuation over the two surfaces to a minimum. When a statistical analysis was performed on the frequency distributions of the signal amplitudes, there was no statistically significant difference in the peak amplitudes of first return strokes over land (lambda = 8.2 x 10(exp -3) mho/m) and over water (lambda = 4 mho/m). Therefore we infer that the conductivity of the underlying surface does not significantly affect the magnitude of the peak magnetic field, and hence the peak current, in the first return stroke of a cloud-to-ground lightning flash.

Torsional Alfvén Wave Embedded ICME Magnetic Cloud and Corresponding Geomagnetic Storm

NASA Astrophysics Data System (ADS)

Raghav, Anil N.; Kule, Ankita; Bhaskar, Ankush; Mishra, Wageesh; Vichare, Geeta; Surve, Shobha

2018-06-01

Energy transfer during the interaction of large-scale solar wind structure and the Earth’s magnetosphere is a chronic issue in space-weather studies. To understand this, researchers widely studied the geomagnetic storm and substorm phenomena. The present understanding suggests that the long duration of the southward interplanetary magnetic field component is the most important parameter for the geomagnetic storm. Such a long duration strong southward magnetic field is often associated with ICMEs, torsional Alfvén fluctuations superposed corotating interacting regions (CIRs), and fast solar wind streams. Torsional Alfvén fluctuations embedded CIRs have been known of for a long time; however, magnetic clouds embedded with such fluctuations are rarely observed. The presence of Alfvén waves in the ICME/MC and the influence of these waves on the storm evolution remains an interesting topic of study. The present work confirms the torsional Alfvén waves in a magnetic cloud associated with a CME launched on 2011 February 15, which impacted the Earth’s magnetosphere on 2011 February 18. Furthermore, observations indicate that these waves inject energy into the magnetosphere during the storm and contribute to the long recovery time of geomagnetic storms. Our study suggests that the presence of torsional Alfvén waves significantly controls the storm dynamics.

Distortion of Magnetic Fields in a Starless Core. III. Polarization–Extinction Relationship in FeSt 1-457

NASA Astrophysics Data System (ADS)

Kandori, Ryo; Tamura, Motohide; Nagata, Tetsuya; Tomisaka, Kohji; Kusakabe, Nobuhiko; Nakajima, Yasushi; Kwon, Jungmi; Nagayama, Takahiro; Tatematsu, Ken’ichi

2018-04-01

The relationship between dust polarization and extinction was determined for the cold dense starless molecular cloud core FeSt 1-457 based on the background star polarimetry of dichroic extinction at near-infrared wavelengths. Owing to the known (three-dimensional) magnetic field structure, the observed polarizations from the core were corrected by considering (a) the subtraction of the ambient polarization component, (b) the depolarization effect of inclined distorted magnetic fields, and (c) the magnetic inclination angle of the core. After these corrections, a linear relationship between polarization and extinction was obtained for the core in the range up to A V ≈ 20 mag. The initial polarization versus extinction diagram changed dramatically after the corrections of (a) to (c), with the correlation coefficient being refined from 0.71 to 0.79. These corrections should affect the theoretical interpretation of the observational data. The slope of the finally obtained polarization–extinction relationship is {P}H/{E}H-{Ks}=11.00+/- 0.72 % {mag}}-1, which is close to the statistically estimated upper limit of the interstellar polarization efficiency. This consistency suggests that the upper limit of interstellar polarization efficiency might be determined by the observational viewing angle toward polarized astronomical objects.

Multi-wavelength study of NGC 281 A

NASA Technical Reports Server (NTRS)

Henning, TH.; Martin, K.; Reimann, H.-G.; Launhardt, R.; Leisawitz, D.; Zinnecker, H.

1994-01-01

We present a study of the molecular cloud NGC 281 A and the associated compact and young star cluster NGC 281 (AS 179). Optical photometry leads to a new distance of 3500 pc for the star cluster which is in good agreement with the kinematical distance of the adjacent molecular cloud NGC 281 A. The exciting star HD 5005 of the optical nebulosity is a Trapezium system with O6 III as photometric spectral type for the component HD 5005 AB. For the age of the star cluster we estimated a value of about 3 x 10(exp 6) yr. The (12)CO (2 to 1), (13)CO (2 to 1), and (12)CO (3 to 2) emission shows that the molecular cloud NGC 281 A consists of two cloud fragments. The western fragment is more compact and massive than the eastern fragment and contains an NH3 core. This core is associated with the IRAS source 00494+5617, an H2O maser, and 1.3 millimeter dust continuum radiation. Both cloud fragments contain altogether 22 IRAS point sources which mostly share the properties of young stellar objects. They have luminosities between 150 and 8800 solar luminosity. The maxima of the 60 and 100 micrometers HIRES maps correspond to the maxima of the (12)CO (3 to 2) emission. The NGC 281 A region shares many properties with the Orion Trapezium-BN/KL region the main differences being a larger separation between the cluster centroid and the new site of star formation as well as a lower mass and luminosity of the molecular cloud and the infrared cluster.

Clustering the Orion B giant molecular cloud based on its molecular emission

PubMed Central

Bron, Emeric; Daudon, Chloé; Pety, Jérôme; Levrier, François; Gerin, Maryvonne; Gratier, Pierre; Orkisz, Jan H.; Guzman, Viviana; Bardeau, Sébastien; Goicoechea, Javier R.; Liszt, Harvey; Öberg, Karin; Peretto, Nicolas; Sievers, Albrecht; Tremblin, Pascal

2017-01-01

Context Previous attempts at segmenting molecular line maps of molecular clouds have focused on using position-position-velocity data cubes of a single molecular line to separate the spatial components of the cloud. In contrast, wide field spectral imaging over a large spectral bandwidth in the (sub)mm domain now allows one to combine multiple molecular tracers to understand the different physical and chemical phases that constitute giant molecular clouds (GMCs). Aims We aim at using multiple tracers (sensitive to different physical processes and conditions) to segment a molecular cloud into physically/chemically similar regions (rather than spatially connected components), thus disentangling the different physical/chemical phases present in the cloud. Methods We use a machine learning clustering method, namely the Meanshift algorithm, to cluster pixels with similar molecular emission, ignoring spatial information. Clusters are defined around each maximum of the multidimensional Probability Density Function (PDF) of the line integrated intensities. Simple radiative transfer models were used to interpret the astrophysical information uncovered by the clustering analysis. Results A clustering analysis based only on the J = 1 – 0 lines of three isotopologues of CO proves suffcient to reveal distinct density/column density regimes (nH ~ 100 cm−3, ~ 500 cm−3, and > 1000 cm−3), closely related to the usual definitions of diffuse, translucent and high-column-density regions. Adding two UV-sensitive tracers, the J = 1 − 0 line of HCO+ and the N = 1 − 0 line of CN, allows us to distinguish two clearly distinct chemical regimes, characteristic of UV-illuminated and UV-shielded gas. The UV-illuminated regime shows overbright HCO+ and CN emission, which we relate to a photochemical enrichment effect. We also find a tail of high CN/HCO+ intensity ratio in UV-illuminated regions. Finer distinctions in density classes (nH ~ 7 × 103 cm−3 ~ 4 × 104 cm−3) for the densest regions are also identified, likely related to the higher critical density of the CN and HCO+ (1 – 0) lines. These distinctions are only possible because the high-density regions are spatially resolved. Conclusions Molecules are versatile tracers of GMCs because their line intensities bear the signature of the physics and chemistry at play in the gas. The association of simultaneous multi-line, wide-field mapping and powerful machine learning methods such as the Meanshift clustering algorithm reveals how to decode the complex information available in these molecular tracers. PMID:29456256

The shapes of column density PDFs. The importance of the last closed contour

NASA Astrophysics Data System (ADS)

Alves, João; Lombardi, Marco; Lada, Charles J.

2017-10-01

The probability distribution function of column density (PDF) has become the tool of choice for cloud structure analysis and star formation studies. Its simplicity is attractive, and the PDF could offer access to cloud physical parameters otherwise difficult to measure, but there has been some confusion in the literature on the definition of its completeness limit and shape at the low column density end. In this letter we use the natural definition of the completeness limit of a column density PDF, the last closed column density contour inside a surveyed region, and apply it to a set of large-scale maps of nearby molecular clouds. We conclude that there is no observational evidence for log-normal PDFs in these objects. We find that all studied molecular clouds have PDFs well described by power laws, including the diffuse cloud Polaris. Our results call for a new physical interpretation of the shape of the column density PDFs. We find that the slope of a cloud PDF is invariant to distance but not to the spatial arrangement of cloud material, and as such it is still a useful tool for investigating cloud structure.

Rigid-body rotation of an electron cloud in divergent magnetic fields

DOE PAGES

Fruchtman, A.; Gueroult, R.; Fisch, N. J.

2013-07-10

For a given voltage across a divergent poloidal magnetic field, two electric potential distributions, each supported by a rigid-rotor electron cloud rotating with a different frequency, are found analytically. The two rotation frequencies correspond to the slow and fast rotation frequencies known in uniform plasma. Due to the centrifugal force, the equipotential surfaces, that correspond to the two electric potential distributions, diverge more than the magnetic surfaces do, the equipotential surfaces in the fast mode diverge largely in particular. The departure of the equipotential surfaces from the magnetic field surfaces may have a significant focusing effect on the ions acceleratedmore » by the electric field. Furthermore, the focusing effect could be important for laboratory plasma accelerators as well as for collimation of astrophysical jets.« less

Rigid-body rotation of an electron cloud in divergent magnetic fields

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fruchtman, A.; Gueroult, R.; Fisch, N. J.

2013-07-15

For a given voltage across a divergent poloidal magnetic field, two electric potential distributions, each supported by a rigid-rotor electron cloud rotating with a different frequency, are found analytically. The two rotation frequencies correspond to the slow and fast rotation frequencies known in uniform plasma. Due to the centrifugal force, the equipotential surfaces, that correspond to the two electric potential distributions, diverge more than the magnetic surfaces do, the equipotential surfaces in the fast mode diverge largely in particular. The departure of the equipotential surfaces from the magnetic field surfaces may have a significant focusing effect on the ions acceleratedmore » by the electric field. The focusing effect could be important for laboratory plasma accelerators as well as for collimation of astrophysical jets.« less

GIANT MOLECULAR CLOUD FORMATION IN DISK GALAXIES: CHARACTERIZING SIMULATED VERSUS OBSERVED CLOUD CATALOGS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Benincasa, Samantha M.; Pudritz, Ralph E.; Wadsley, James

We present the results of a study of simulated giant molecular clouds (GMCs) formed in a Milky Way-type galactic disk with a flat rotation curve. This simulation, which does not include star formation or feedback, produces clouds with masses ranging between 10{sup 4} M{sub ☉} and 10{sup 7} M{sub ☉}. We compare our simulated cloud population to two observational surveys: the Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey and the BIMA All-Disk Survey of M33. An analysis of the global cloud properties as well as a comparison of Larson's scaling relations is carried out. We find that simulatedmore » cloud properties agree well with the observed cloud properties, with the closest agreement occurring between the clouds at comparable resolution in M33. Our clouds are highly filamentary—a property that derives both from their formation due to gravitational instability in the sheared galactic environment, as well as to cloud-cloud gravitational encounters. We also find that the rate at which potentially star-forming gas accumulates within dense regions—wherein n{sub thresh} ≥ 10{sup 4} cm{sup –3}—is 3% per 10 Myr, in clouds of roughly 10{sup 6} M{sub ☉}. This suggests that star formation rates in observed clouds are related to the rates at which gas can be accumulated into dense subregions within GMCs via filamentary flows. The most internally well-resolved clouds are chosen for listing in a catalog of simulated GMCs—the first of its kind. The cataloged clouds are available as an extracted data set from the global simulation.« less

An evaluation of atmospheric corrections to advanced very high resolution radiometer data

USGS Publications Warehouse

Meyer, David; Hood, Joy J.

1993-01-01

A data set compiled to analyze vegetation indices is used to evaluate the effect of atmospheric correction to AVHRR measurement in the solar spectrum. Such corrections include cloud screening and "clear sky" corrections. We used the "clouds from AVHRR" (CLAVR) method for cloud detection and evaluated its performance over vegetated targets. Clear sky corrections, designed to reduce the effects of molecular scattering and absorption due to ozone, water vapor, carbon dioxide, and molecular oxygen, were applied to data values determine to be cloud free. Generally, it was found that the screening and correction of the AVHRR data did not affect the maximum NDVI compositing process adversely, while at the same time improving estimates of the land-surface radiances over a compositing period.

VizieR Online Data Catalog: Molecular clouds with GLIMPSE/MIPSGAL data (Retes-Romero+, 2017)

NASA Astrophysics Data System (ADS)

Retes-Romero, R.; Mayya, Y. D.; Luna, A.; Carrasco, L.

2017-11-01

All of the 12 selected molecular clouds have GLIMPSE and MIPSGAL public data available. Typical Spitzer RGB images (3.6um, 8.0um, and 24um) of the resulting sample of clouds are shown in Figure 1, where the position of the IRAS source is identified. In order to define the parent molecular cloud that harbors the high-mass star-forming regions, we used 13CO(J=1-0) emission data from the Galactic Ring Survey (GRS) database (Jackson+ 2006ApJS..163..145J). The survey data have a velocity resolution of 0.21km/s, a typical (1σ) rms sensitivity of ~0.13K, a main beam efficiency of {eta}mb=0.48, and a beam of 46". The 13CO emission spectra for the line of sight (LOS) to the selected IRAS sources are shown in Figure 2, where the observed velocity of the CS(J=2-1) emission line (Bronfman+ 1996, J/A+AS/115/81) is also marked. (3 data files).

On the distribution of sodium in the vicinity of Io

NASA Technical Reports Server (NTRS)

Trafton, L.; Macy, W., Jr.

1978-01-01

The contribution of scattering in a telescope to measurements of the size of Io's sodium cloud and to the distribution of emission intensity in the cloud is investigated. The brightest regions, within 30 arcsec of Io near opposition and along the equatorial plane, are relatively undistorted, but regions further than 45 arcsec away and not close to the equatorial plane are very likely to consist mainly of scattered light. Portions of the cloud in the vicinity of the magnetic equator are also mostly scattered light when Io is near extreme magnetic latitude. The equatorial torus, however, extends up to 20 arcmin from Jupiter. The large size of the cloud is thus confirmed. High-resolution line profile shapes indicate that sodium streams from Io preferentially in the forward direction with velocities distributed up to 18 km/sec. The observed wavelength shifts of the peak intensities from Io's rest frame are compatible with a cloud streaming through a bound atmospheric component, but they could also be caused by a velocity distribution peaked at very low velocities.

Chemical Impact of Solar Energetic Particle Event From The Young Sun: Implications for the Origin of Prebiotic Chemistry and the Fain Young Sun Paradox

NASA Astrophysics Data System (ADS)

Airapetian, V.; Gronoff, G.; Hébrard, E.; Danchi, W.

2015-12-01

Understanding how the simple molecules present on the early Earth and possibly Mars may have set a path for complex biological molecules, the building blocks of life, represents one of greatest unsolved questions. Here we present a new model of the rise of the abiotic nitrogen fixation and associated pre-biotic chemistry in the early Earth and Mars atmosphere mediated by solar eruptive events. Our physical models of interaction of magnetic clouds ejected from the young Sun with magnetospheres of the early Earth show significant perturbations of geomagnetic fields that produce extended polar caps. These polar caps provide pathways for energetic particles associated with magnetic clouds to penetrate into the nitrogen-rich weakly reducing atmosphere and initiate the reactive chemistry by breaking molecular nitrogen, carbon dioxide, methane and producing hydrogen cyanide, the essential compound for life. The model also shows that contrary to the current models of warming of early Earth and Mars, major atmospheric constituents, CO2 and CH4 will be destroyed due to collisional dissociation with energetic particles. Instead, efficient formation of the potent greenhouse gas, nitrous oxide, as a by-product of these processes is expected. This mechanism can consistently explain the Faint Young Sun's paradox for the early atmospheres of Earth and Mars. Our new model provides insight into how life may have initiated on Earth and Mars and how to search for the spectral signatures on planets "pregnant" with the potential for life.

Boundary Conditions for the Paleoenvironment: Chemical and Physical Processes in the Pre-Solar Nebula

NASA Technical Reports Server (NTRS)

Irvine, William M.; Schloerb, F. Peter

1997-01-01

The basic theme of this program is the study of molecular complexity and evolution in interstellar clouds and in primitive solar system objects. Research has included the detection and study of a number of new interstellar molecules and investigation of reaction pathways for astrochemistry from a comparison of theory and observed molecular abundances. The latter includes studies of cold, dark clouds in which ion-molecule chemistry should predominate, searches for the effects of interchange of material between the gas and solid phases in interstellar clouds, unbiased spectral surveys of particular sources, and systematic investigation of the interlinked chemistry and physics of dense interstellar clouds. In addition, the study of comets has allowed a comparison between the chemistry of such minimally thermally processed objects and that of interstellar clouds, shedding light on the evolution of the biogenic elements during the process of solar system formation.

TYCHO SN 1572: A NAKED Ia SUPERNOVA REMNANT WITHOUT AN ASSOCIATED AMBIENT MOLECULAR CLOUD

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tian, W. W.; Leahy, D. A., E-mail: tww@bao.ac.cn

The historical supernova remnant (SNR) Tycho SN 1572 originates from the explosion of a normal Type Ia supernova that is believed to have originated from a carbon-oxygen white dwarf in a binary system. We analyze the 21 cm continuum, H I, and {sup 12}CO-line data from the Canadian Galactic Plane Survey in the direction of SN 1572 and the surrounding region. We construct H I absorption spectra to SN 1572 and three nearby compact sources. We conclude that SN 1572 has no molecular cloud interaction, which argues against previous claims that a molecular cloud is interacting with the SNR. Thismore » new result does not support a recent claim that dust, newly detected by AKARI, originates from such an SNR-cloud interaction. We suggest that the SNR has a kinematic distance of 2.5-3.0 kpc based on a nonlinear rotational curve model. Very high energy {gamma}-ray emission from the remnant has been detected by the VERITAS telescope, so our result shows that its origin should not be an SNR-cloud interaction. Both radio and X-ray observations support that SN 1572 is an isolated Type Ia SNR.« less

Plasma Clouds and Snowplows: Bulk Plasma Escape from Mars Observed by MAVEN

NASA Technical Reports Server (NTRS)

Halekas, J. S.; Brain, D. A.; Ruhunusiri, S.; McFadden, J. P.; Mitchell, D. L.; Mazelle, C.; Connerney, J. E. P.; Harada, Y.; Hara, T.; Espley, J. R.;

High resolution far-infrared observations of the evolved H II region M16

DOE Office of Scientific and Technical Information (OSTI.GOV)

McBreen, B.; Fazio, G.G.; Jaffe, D.T.

1982-03-01

M16 is an evolved, extremely density bounded H II region, which now consists only of a series of ionization fronts at molecular cloud boundaries. The source of ionization is the OB star cluster (NGC 6611) which is about 5 x 10/sup 6/ years old. We used the CFA/UA 102 cm balloon-borne telescope to map this region and detected three far-infrared (far-IR) sources embedded in an extended ridge of emission. Source I is an unresolved far-IR source embedded in a molecular cloud near a sharp ionization front. An H/sub 2/O maser is associated with this source, but no radio continuum emissionmore » has been observed. The other two far-IR sources (II and III) are associated with ionized gas-molecular cloud interfaces, with the far-IR radiation arising from dust at the boundary heated by the OB cluster. Source II is located at the southern prominent neutral intrusion with its associated bright rims and dark ''elephant trunk'' globules that delineate the current progress of the ionization front into the neutral material, and Source III arises at the interface of the northern molecular cloud fragment.« less

Bi-directional streaming of halo electrons in interplanetary plasma clouds observed between 0.3 and 1 AU

NASA Technical Reports Server (NTRS)

Ivory, K.; Schwenn, R.

1995-01-01

The solar wind data obtained from the two Helios solar probes in the years 1974 to 1986 were systematically searched for the occurrence of bi-directional electron events. Most often these events are found in conjunction with shock associated magnetic clouds. The implications of these observations for the topology of interplanetary plasma clouds are discussed.

The molecular gas content of the Pipe Nebula. I. Direct evidence of outflow-generated turbulence in B59?

NASA Astrophysics Data System (ADS)

Duarte-Cabral, A.; Chrysostomou, A.; Peretto, N.; Fuller, G. A.; Matthews, B.; Schieven, G.; Davis, G. R.

2012-07-01

Context. Star forming regions may share many characteristics, but the specific interplay between gravity, magnetic fields, large-scale dynamics, and protostellar feedback will have an impact on the star formation history of each region. The importance of feedback from outflows is a particular subject to debate, as we are yet to understand the details of their impact on clouds and star formation. Aims: The Pipe Nebula is a nearby molecular cloud hosting the B59 region as its only active star-forming clump. This paper focuses on the global dynamics of B59, its temperature structure, and its outflowing gas, with the goal of revealing the local and global impact of the protostellar outflows. Methods: Using HARP at the James Clerk Maxwell Telescope, we have mapped the B59 region in the J = 3 → 2 transition of 12CO to study the kinematics and energetics of the outflows, and the same transitions of 13CO and C18O to study the overall dynamics of the ambient cloud, the physical properties of the gas, and the hierarchical structure of the region. Results: The B59 region has a total of ~30 M⊙ of cold and quiescent material, mostly gravitationally bound, with narrow line widths throughout. Such low levels of turbulence in the non-star-forming regions within B59 are indicative of the intrinsic initial conditions of the cloud. On the other hand, close to the protostars the impact of the outflows is observed as a localised increase of both C18O line widths from ~0.3 km s-1 to ~1 km s-1, and 13CO excitation temperatures by ~2-3 K. The impact of the outflows is also evident in the low column density material which shows signs of being shaped by the outflow bow shocks as they pierce their way out of the cloud. Much of this structure is readily apparent in a dendrogram analysis of the cloud and demonstrates that when decomposing clouds using such techniques a careful interpretation of the results is needed. Conclusions: The low mass of B59 together with its intrinsically quiescent gas and small number of protostars, allows the identification of specific regions where the outflows from the embedded sources interact the dense gas. Our study suggests that outflows are an important mechanism for injecting and sustaining supersonic turbulence at sub-parsec size scales. We find that less than half of the outflow energy is deposited as turbulent energy of the gas, however this turbulent energy is sufficient to slow down the collapse of the region.

Antipodal Magnetic Anomalies on the Moon, Contributions from Impact Induced Currents Due to Positive Holes and Flexoelectric Phenomina and Dynamo

NASA Technical Reports Server (NTRS)

Kletetschka, G.; Freund, F.; Wasilewski, P. J.; Mikula, V.; Kohout, Tomas

2005-01-01

Large impacts on the Moon generate large pressure pulses that penetrate the whole body. Several of these large impacts may have generated antipodal structure with anomalous magnetic intensity.These regions can be more than a thousand km across, with fields of the order of tens to hundreds of nT. This is the case of Orientale, Imbrium, Serenitatis, Crisium, and Nectaris impact basins. The production of large-scale magnetic fields and associated crustal magnetization due to lunar basin-forming impacts was hypothesized to have an origin in fields external to the impact plasma cloud that are produced by the magnetohydrodynamic interaction of the cloud with ambient magnetic fields and plasmas. During the period of compressed antipodal field amplification, seismic compressional waves from the impact converge at the antipode resulting in transient shock pressures that reach 2 GPa (20 kbar). This can produce conditions for shock magnetic acquisition of the crust antipodal to impact basins.

Shocking Changes to Molecular Clouds

NASA Astrophysics Data System (ADS)

Melnick, Gary J.

1998-05-01

Supersonic motions are commonly observed in molecular clouds as evidenced by larger-than-thermal line widths measured in most species. The shocks that ensue can profoundly effect these clouds, not only dynamically, but chemically. Because shocks compress and heat the gas, chemical reactions that are extremely slow at typical molecular cloud temperatures (T ~ 10-30 K) can proceed rapidly in the wake of a shock. In many cases, compositional changes brought on by a passing shock can endure long after the gas has cooled and returned to its pre-shock state. We have used a coupled time-dependent chemical and dynamical model to investigate the lifetime of such chemical relics in the wake of non-dissociative shocks. Using a Monte Carlo cloud simulation, we explore the effects of stochastic shock activity on molecular gas over a cloud lifetime. Particular attention is paid to the chemistry of H_2O and O_2, two molecules which are predicted to have abundances that are significantly affected by shock-heated gas. Both pure gas-phase and gas-grain chemistry are considered. In agreement with previous studies, we find that shocks with velocities in excess of 10 km s(-1) can chemically process all oxygen not locked in CO into H_2O on timescales of a shock passage time ( ~ \\:few hundred years). For pure gas-phase models, the high water abundance lingers for ~ (4-7) x 10(5) yr, independent of the gas density. A density dependence for the lifetime of H_2O is found in gas-grain models as the water molecules deplete onto grains at the depletion timescale. We demonstrate that the time-averaged abundance of H_2O and O_2 (as well as other tracers, such as SiO and CH_3OH) is a sensitive function of the frequency of shocks. As such, the abundance of H_2O, and to a lesser extent O_2, can be used to trace the shock history in molecular clouds. Equally important, we find that depletion of shock-produced water onto grains can be quite large and is comparable to that observed in molecular clouds. This offers an alternative method to create water-ice mantles without resorting to grain surface chemistry. Observationally, a combination of space-based (for H_2O and O_2) and ground-based (for SiO, CH_3OH, and others) telescopes will be needed to investigate these predictions.

HAWC+/SOFIA observations of Rho Oph A: far-infrared polarization spectrum

NASA Astrophysics Data System (ADS)

Santos, Fabio; Dowell, Charles D.; Houde, Martin; Looney, Leslie; Lopez-Rodriguez, Enrique; Novak, Giles; Ward-Thompson, Derek; HAWC+ Science Team

2018-01-01

In this work, we present preliminary results from the HAWC+ far-infrared polarimeter that operates on the SOFIA airborne observatory. The densest portions of the Rho Ophiuchi molecular complex, known as Rho Oph A, have been mapped using HAWC+ bands C (89 microns) and D (155 microns). Rho Oph A is a well known nearby star forming region. At the target's distance of approximately 130 pc, our observations provide excellent spatial resolution (~5 mpc in band C).The magnetic field map suggests a compressed and distorted field morphology around Oph S1, a massive B3 star that is the main heat source of Rho Oph A. We compute the ratio p(D)/p(C), where p(C) and p(D) are the polarization degree maps at bands C and D, respectively. This ratio estimates the slope of the polarization spectrum in the far-infrared. Although the slope is predicted to be positive by dust grain models, previous observations of other molecular clouds have revealed that negative slopes are common. In Rho Oph A, we find that there is a smooth gradient of p(D)/p(C) across the mapped field. The change in p(D)/p(C) is well correlated with the integrated NH3 (1,1) emission. A positive slope dominates the lower density and well illuminated portions of the cloud, whereas a transition to a negative slope is observed at the denser and less evenly illuminated cloud core.We interpret the positive to negative slope transition as being consistent with the radiative torques (RATs) grain alignment theory. For the sight lines of higher column density, polarized emission from the warmer outer cloud layers is added to emission from the colder inner well-shielded layers lying along the same line-of-sight. Given that the outer layers receive more radiation from Oph S1, their grain alignment efficiency is expected to be higher according to RATs. The combination of warmer, well aligned grains with cooler, poorly aligned grains is what causes the negative slope. This effect is not present in the sight lines of lower column density, due to the much lower extinction.

The Infrared Reflection Nebula Around the Protostellar System in S140

NASA Technical Reports Server (NTRS)

Harker, D.; Bregman, J.; Tielens, A. G. G. M.; Temi, P.; Rank, D.; Morrison, David (Technical Monitor)

1994-01-01

We have studied the protostellar system in S140 at 2.2, 3.1 and 3.45 microns using a 128x128 InSb array at the Lick Observatory 3m telescope. Besides the protostellar sources, the data reveal a bright infrared reflection nebula. We have developed a simple model of this region and derived the physical conditions. IRSI is surrounded by a dense dusty disk viewed almost edge-on. Photons leaking out through the poles illuminate almost directly north and south the inner edge of a surrounding shell of molecular gas, Analysis of the observed colors and intensities of the NIR light, using Mie scattering theory, reveal that the dust grains in the molecular cloud are somewhat larger than in the general diffuse interstellar medium. Moreover, the incident light has a "cool" color temperature, approximately equals 800K, and likely originates from a dust photosphere close to the protostar. Finally, we find little H2O ice associated with the dusty disk around IRSI. Most of the 3.1 micron ice extinction arises instead from cool intervening molecular cloud material. We have compared our infrared dust observations with millimeter and radio observations of molecular gas associated with this region. The large scale structure observable in the molecular gas is indicative of the interaction between the protostellar wind and the surrounding molecular cloud rather than the geometry of the protostellar disk. We conclude that S140 is a young blister formed by this outflow on the side of a molecular cloud and viewed edge-on.

Wide-field Infrared Polarimetry of the ρ Ophiuchi Cloud Core

NASA Astrophysics Data System (ADS)

Kwon, Jungmi; Tamura, Motohide; Hough, James H.; Nakajima, Yasushi; Nishiyama, Shogo; Kusakabe, Nobuhiko; Nagata, Tetsuya; Kandori, Ryo

2015-09-01

We conducted wide and deep simultaneous JHKs-band imaging polarimetry of the ρ Ophiuchi cloud complex. Aperture polarimetry in the JHKs band was conducted for 2136 sources in all three bands, of which 322 sources have significant polarizations in all the JHKs bands and have been used for a discussion of the core magnetic fields. There is a positive correlation between degrees of polarization and H - Ks color up to H - Ks ≈ 3.5. The magnetic field structures in the core region are revealed up to at least AV ≈ 47 mag and are unambiguously defined in each sub-region (core) of Oph-A, Oph-B, Oph-C, Oph-E, Oph-F, and Oph-AC. Their directions, degrees of polarization, and polarization efficiencies differ but their changes are gradual; thus, the magnetic fields appear to be connected from core to core, rather than as a simple overlap of the different cloud core components. Comparing our results with the large-scale field structures obtained from previous optical polarimetric studies, we suggest that the magnetic field structures in the core were distorted by the cluster formation in this region, which may have been induced by shock compression due to wind/radiation from the Scorpius-Centaurus association.

A quantitative assessment of the role of the post-shock turbulent region in the formation of Forbush decreases

NASA Technical Reports Server (NTRS)

Sanderson, T. R.; Heras, A. M.; Marsden, R. G.; Winterhalter, D.

1992-01-01

We present results of a survey of the relation between Forbush decreases, magnetic clouds, and interplanetary shocks during the period August 1978 to November 1982. We have used data from the ISEE-3 study of bidirectional ions associated with magnetic structures or clouds of Marsden et al. (1987), and ground-based observations of Forbush decreases from several neutron monitors. We use the two-step model of a Forbush decrease. We assume that the first step is due to the passage of the postshock turbulent region, and that the second is due to the passage of the magnetic cloud or structure which usually follows the postshock turbulent region. To determine the effectiveness of the postshock turbulent region in causing a Forbush decrease, we have evaluated the radial diffusion coefficient of the postshock turbulent region for the eight largest events during the above period using observations of the magnetic field. We have made a quantitative assessment of the relative importance of the postshock turbulent region in the formation of the Forbush decrease, concluding that the postshock turbulent region alone is not sufficient to cause a Forbush decrease.

CO excitation in four IR luminous galaxies

NASA Technical Reports Server (NTRS)

Radford, Simon J. E.; Solomon, P. M.; Downes, Dennis

1990-01-01

The correlation between the CO and far infrared luminosities of spiral galaxies is well established. The luminosity ration, L sub FIR/L sub CO in IR luminous active galaxies is, however, systematically five to ten times higher than in ordinary spirals and molecular clouds in our Galaxy. Furthermore, the masses of molecular hydrogen in luminous galaxies are large, M (H2) approx. equals 10(exp 10) solar magnitude, which indicates the observed luminosity ratios are due to an excess of infrared output, rather than a deficiency of molecular gas. These large amounts of molecular gas may fuel luminous galaxies through either star formation or nuclear activity. This interpretation rests on applying the M (H2)/L sub CO ratio calibrated in our Galaxy to galaxies with strikingly different luminosity ratios. But are the physical conditions of the molecular gas different in galaxies with different luminosity ratios. And, if so, does the proportionality between CO and H2 also vary among galaxies. To investigate these questions researchers observed CO (2 to 1) and (1 to 0) emission from four luminous galaxies with the Institute for Radio Astronomy in the Millimeter range (IRAM) 30 m telescope. Researchers conclude that most of the CO emission from these Arp 193, Arp 220, and Mrk 231 arises in regions with moderate ambient densities similar to the clouds in the Milky Way molecular ring. The emission is neither from dense hot cloud cores nor from the cold low density gas characteristic of the envelopes of dark clouds.

Neutral hydrogen self-absorption in the Milky Way Galaxy

NASA Astrophysics Data System (ADS)

Kavars, Dain William

2006-06-01

To develop a better understanding of the cold neutral medium phase of the interstellar medium, we present a detailed analysis of neutral hydrogen self- absorption (HISA) clouds in the Milky Way Galaxy. These HISA clouds are in the Southern Galactic Plane Survey (SGPS), spanning the region l = 253°--358° and | b | <= 1.3°, and in the VLA Galactic Plane Survey (VGPS), spanning the region l = 18°--67° and | b | <= 1.3°--2.3°. The SGPS and VGPS have an angular resolution of ~1 arcminute and a velocity channel spacing of 0.82 km s -1 . With the recent completion of these surveys, we can study HISA features across the Galaxy at a much better resolution and sensitivity than any previous work. To analyze HISA in detail, catalogs of clouds of all sizes, including those undetectable by eye alone, are required. We present an automated search routine to detect all HISA clouds in the SGPS. We compare HISA to CO data and find some HISA clouds associated with CO, but others have no associated CO. This suggests that HISA clouds are in a transition between molecular and atomic gas, bridging the gap between dense molecular clouds and warmer, diffuse atomic clouds. HISA thus plays an important role in the overall evolution of the Galaxy. To study this transition further, we present observations of the OH molecule toward a select sample of HISA clouds in the VGPS, using the Green Bank Telescope (GBT). We present an analysis of the molecular properties of this sample, including a derivation of an OH to H 2 conversion factor and H 2 to H I abundance ratios. We discuss the complex relationship between H I, OH, 12 CO, and 13 CO emission. Finally we present a statistical analysis comparing HISA with infrared data from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) project. The GLIMPSE data reveal a large number of compact, dark infrared clouds believed to be in the early stages of star formation. If GLIMPSE clouds are associated with HISA, they provide valuable information on the evolution of HISA clouds.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xu, Siyao; Lazarian, A.; Yan, Huirong, E-mail: hryan@pku.edu.cn

We address the problem of the different line widths of coexistent neutrals and ions observed in molecular clouds and explore whether this difference can arise from the effects of magnetohydrodynamic (MHD) turbulence acting on partially ionized gas. Among the three fundamental modes of MHD turbulence, we find that fast and slow modes do not contribute to line width differences. We focus on the Alfvénic component, and consider the damping of Alfvén modes by taking into account both neutral-ion collisions and neutral viscosity. We confirm that the line width difference can be explained by the differential damping of the Alfvénic turbulencemore » in ions and the hydrodynamic turbulence in neutrals, and find it strongly depends on the properties of MHD turbulence. We consider various regimes of turbulence corresponding to different media magnetizations and turbulent drivings. In the case of super-Alfvénic turbulence, when the damping scale of Alfvénic turbulence is below the Alfvénic scale l{sub A}, the line width difference does not depend on magnetic field strength. In other turbulence regimes, however, the dependence is present and evaluation of magnetic field from the observed line width difference is possible.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Osiry, H.; Cano, A.; Lemus-Santana, A.A.

This contribution discusses the intercalation of imidazole and its 2-ethyl derivative, and pyridine in 2D copper nitroprusside. In the interlayer region, neighboring molecules remain interacting throu gh their dipole and quadrupole moments, which supports the solid 3D crystal structure. The crystal structure of this series of intercalation compounds was solved and refined from powder X-ray diffraction patterns complemented with spectroscopic information. The intermolecular interactions were studied from the refined crystal structures and low temperature magnetic measurements. Due to strong attractive forces between neighboring molecules, the resulting π–π cloud overlapping enables the ferromagnetic coupling between metal centers on neighboring layers, which wasmore » actually observed for the solids containing imidazole and pyridine as intercalated molecules. For these two solids, the magnetic data were properly described with a model of six neighbors. For the solid containing 2-ethylimidazole and for 2D copper nitroprusside, a model of four neighbors in a plane is sufficient to obtain a reliable data fitting. - Highlights: • Intercalation of organic molecules in 2D copper (II) nitroprusside. • Molecular properties of intercalation compounds of 2D copper (II) nitroprusside. • Magnetic properties of hybrid inorganic–organic solids. • Hybrid inorganic–organic 3D framework.« less

HIGH-RESOLUTION 8 mm AND 1 cm POLARIZATION OF IRAS 4A FROM THE VLA NASCENT DISK AND MULTIPLICITY (VANDAM) SURVEY

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cox, Erin G.; Harris, Robert J.; Looney, Leslie W.

2015-12-01

Magnetic fields can regulate disk formation, accretion, and jet launching. Until recently, it has been difficult to obtain high-resolution observations of the magnetic fields of the youngest protostars in the critical region near the protostar. The VANDAM survey is observing all known protostars in the Perseus Molecular Cloud. Here we present the polarization data of IRAS 4A. We find that with ∼0.″2 (50 AU) resolution at λ = 8.1 and 10.3 mm, the inferred magnetic field is consistent with a circular morphology, in marked contrast with the hourglass morphology seen on larger scales. This morphology is consistent with frozen-in fieldmore » lines that were dragged in by rotating material entering the infall region. The field morphology is reminiscent of rotating circumstellar material near the protostar. This is the first polarization detection of a protostar at these wavelengths. We conclude from our observations that the dust emission is optically thin with β ∼ 1.3, suggesting that millimeter-/centimeter-sized grains have grown and survived in the short lifetime of the protostar.« less

The formation of relativistic plasma structures and their potential role in the generation of cosmic ray electrons

NASA Astrophysics Data System (ADS)

Dieckmann, M. E.

2008-11-01

Recent particle-in-cell (PIC) simulation studies have addressed particle acceleration and magnetic field generation in relativistic astrophysical flows by plasma phase space structures. We discuss the astrophysical environments such as the jets of compact objects, and we give an overview of the global PIC simulations of shocks. These reveal several types of phase space structures, which are relevant for the energy dissipation. These structures are typically coupled in shocks, but we choose to consider them here in an isolated form. Three structures are reviewed. (1) Simulations of interpenetrating or colliding plasma clouds can trigger filamentation instabilities, while simulations of thermally anisotropic plasmas observe the Weibel instability. Both transform a spatially uniform plasma into current filaments. These filament structures cause the growth of the magnetic fields. (2) The development of a modified two-stream instability is discussed. It saturates first by the formation of electron phase space holes. The relativistic electron clouds modulate the ion beam and a secondary, spatially localized electrostatic instability grows, which saturates by forming a relativistic ion phase space hole. It accelerates electrons to ultra-relativistic speeds. (3) A simulation is also revised, in which two clouds of an electron-ion plasma collide at the speed 0.9c. The inequal densities of both clouds and a magnetic field that is oblique to the collision velocity vector result in waves with a mixed electrostatic and electromagnetic polarity. The waves give rise to growing corkscrew distributions in the electrons and ions that establish an equipartition between the electron, the ion and the magnetic energy. The filament-, phase space hole- and corkscrew structures are discussed with respect to electron acceleration and magnetic field generation.

IRAM 30 m large scale survey of {sup 12}CO(2-1) and {sup 13}CO(2-1) emission in the Orion molecular cloud

DOE Office of Scientific and Technical Information (OSTI.GOV)

Berné, O.; Cernicharo, J.; Marcelino, N., E-mail: olivier.berne@irap.omp.eu

2014-11-01

Using the IRAM 30 m telescope, we have surveyed a 1 × 0.°8 part of the Orion molecular cloud in the {sup 12}CO and {sup 13}CO (2-1) lines with a maximal spatial resolution of ∼11'' and spectral resolution of ∼0.4 km s{sup –1}. The cloud appears filamentary, clumpy, and with a complex kinematical structure. We derive an estimated mass of the cloud of 7700 M {sub ☉} (half of which is found in regions with visual extinctions A{sub V} below ∼10) and a dynamical age for the nebula of the order of 0.2 Myr. The energy balance suggests that magneticmore » fields play an important role in supporting the cloud, at large and small scales. According to our analysis, the turbulent kinetic energy in the molecular gas due to outflows is comparable to turbulent kinetic energy resulting from the interaction of the cloud with the H II region. This latter feedback appears negative, i.e., the triggering of star formation by the H II region is inefficient in Orion. The reduced data as well as additional products such as the column density map are made available online (http://userpages.irap.omp.eu/∼oberne/Olivier{sub B}erne/Data).« less

Carbon monoxide in clouds at low metallicity in the dwarf irregular galaxy WLM.

PubMed

Elmegreen, Bruce G; Rubio, Monica; Hunter, Deidre A; Verdugo, Celia; Brinks, Elias; Schruba, Andreas

2013-03-28

Carbon monoxide (CO) is the primary tracer for interstellar clouds where stars form, but it has never been detected in galaxies in which the oxygen abundance relative to hydrogen is less than 20 per cent of that of the Sun, even though such 'low-metallicity' galaxies often form stars. This raises the question of whether stars can form in dense gas without molecules, cooling to the required near-zero temperatures by atomic transitions and dust radiation rather than by molecular line emission; and it highlights uncertainties about star formation in the early Universe, when the metallicity was generally low. Here we report the detection of CO in two regions of a local dwarf irregular galaxy, WLM, where the metallicity is 13 per cent of the solar value. We use new submillimetre observations and archival far-infrared observations to estimate the cloud masses, which are both slightly greater than 100,000 solar masses. The clouds have produced stars at a rate per molecule equal to 10 per cent of that in the local Orion nebula cloud. The CO fraction of the molecular gas is also low, about 3 per cent of the Milky Way value. These results suggest that in small galaxies both star-forming cores and CO molecules become increasingly rare in molecular hydrogen clouds as the metallicity decreases.

On the star-forming ability of Molecular Clouds

NASA Astrophysics Data System (ADS)

Anathpindika, S.; Burkert, A.; Kuiper, R.

2018-02-01

The star-forming ability of a molecular cloud depends on the fraction of gas it can cycle into the dense-phase. Consequently, one of the crucial questions in reconciling star formation in clouds is to understand the factors that control this process. While it is widely accepted that the variation in ambient conditions can alter significantly the ability of a cloud to spawn stars, the observed variation in the star-formation rate in nearby clouds that experience similar ambient conditions, presents an interesting question. In this work, we attempted to reconcile this variation within the paradigm of colliding flows. To this end we develop self-gravitating, hydrodynamic realizations of identical flows, but allowed to collide off-centre. Typical observational diagnostics such as the gas-velocity dispersion, the fraction of dense-gas, the column density distribution (N-PDF), the distribution of gas mass as a function of K-band extinction and the strength of compressional/solenoidal modes in the post-collision cloud were deduced for different choices of the impact parameter of collision. We find that a strongly sheared cloud is terribly inefficient in cycling gas into the dense phase and that such a cloud can possibly reconcile the sluggish nature of star formation reported for some clouds. Within the paradigm of cloud formation via colliding flows this is possible in case of flows colliding with a relatively large impact parameter. We conclude that compressional modes - though probably essential - are insufficient to ensure a relatively higher star-formation efficiency in a cloud.

Molecular gas excitation in Kleinmann-Low nebula

NASA Astrophysics Data System (ADS)

Callejo, Gonzague; Lemaire, Jean-Louis; Le Petit, F.; Pineau Des Forets, Guillaume; Field, David

2004-12-01

The Orion molecular cloud OMC-1 is the perfect object for the study of the dense interstellar medium accounting for active star formation; the intrinsic complexity of this region serves as a stallion, both for observing techniques and for interstellar medium modelling. A detailed dynamic and spectroscopic study has been performed using VLT and CFHT observations of the infrared Kleinmann-Low nebula; yielding a complete small-scale structure in velocity and most importantly powerful diagnosis tools in order to put a new light into the gas behaviour. These results allow to build up a consistent model of the gas excitation, and a clear dynamical view of the region. The crucial action of the shock waves is confirmed, and the discrepancies between the observations and the standard models are discussed. The consequences of this modelling will be discussed in terms of extinction, magnetic field, and other quantities related to induced star formation. The goodness of the models used for the interpretations will be also discussed and some future directions of investigation enhanced.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li Zhiyun; Krasnopolsky, Ruben; Shang Hsien

Dense, star-forming cores of molecular clouds are observed to be significantly magnetized. A realistic magnetic field of moderate strength has been shown to suppress, through catastrophic magnetic braking, the formation of a rotationally supported disk (RSD) during the protostellar accretion phase of low-mass star formation in the ideal MHD limit. We address, through two-dimensional (axisymmetric) simulations, the question of whether realistic levels of non-ideal effects, computed with a simplified chemical network including dust grains, can weaken the magnetic braking enough to enable an RSD to form. We find that ambipolar diffusion (AD), the dominant non-ideal MHD effect over most ofmore » the density range relevant to disk formation, does not enable disk formation, at least in two dimensions. The reason is that AD allows the magnetic flux that would be dragged into the central stellar object in the ideal MHD limit to pile up instead in a small circumstellar region, where the magnetic field strength (and thus the braking efficiency) is greatly enhanced. We also find that, on the scale of tens of AU or more, a realistic level of Ohmic dissipation does not weaken the magnetic braking enough for an RSD to form, either by itself or in combination with AD. The Hall effect, the least explored of these three non-ideal MHD effects, can spin up the material close to the central object to a significant, supersonic rotation speed, even when the core is initially non-rotating, although the spun-up material remains too sub-Keplerian to form an RSD. The problem of catastrophic magnetic braking that prevents disk formation in dense cores magnetized to realistic levels remains unresolved. Possible resolutions of this problem are discussed.« less

Widespread SiO and CH3OH emission in filamentary infrared dark clouds

NASA Astrophysics Data System (ADS)

Cosentino, G.; Jiménez-Serra, I.; Henshaw, J. D.; Caselli, P.; Viti, S.; Barnes, A. T.; Fontani, F.; Tan, J. C.; Pon, A.

2018-03-01

Infrared dark clouds (IRDCs) are cold, dense regions of high (optical and infrared) extinction, believed to be the birthplace of high-mass stars and stellar clusters. The physical mechanisms leading to the formation of these IRDCs are not completely understood and it is thus important to study their molecular gas kinematics and chemical content to search for any signature of the IRDCs formation process. Using the 30-m-diameter antenna at the Instituto de Radioastronomía Milimétrica (IRAM), we have obtained emission maps of dense gas tracers (H13CO+ and HN13C) and typical shock tracers (SiO and CH3OH) towards three IRDCs, G028.37+00.07, G034.43+00.24, and G034.77-00.55 (clouds C, F, and G, respectively). We have studied the molecular gas kinematics in these clouds and, consistent with previous works towards other IRDCs, the clouds show complex gas kinematics with several velocity-coherent substructures separated in velocity space by a few km s-1. Correlated with these complex kinematic structures, widespread (parsec-scale) emission of SiO and CH3OH is present in all the three clouds. For clouds C and F, known to be actively forming stars, widespread SiO and CH3OH is likely associated with on-going star formation activity. However, for cloud G, which lacks either 8 or 24 μm sources and 4.5 μm H2 shock-excited emission, the detected widespread SiO and CH3OH emission may have originated in a large-scale shock interaction, although a scenario involving a population of low-mass stars driving molecular outflows cannot be fully ruled out.

CAWSES November 7-8, 2004, Superstorm: Complex Solar and Interplanetary Features in the Post-Solar Maximum Phase

NASA Technical Reports Server (NTRS)

Tsurutani, Bruce T.; Echer, Ezequiel; Guarnieri, Fernando L.; Kozyra, J. U.

2008-01-01

The complex interplanetary structures during 7 to 8 Nov 2004 are analyzed to identify their properties as well as resultant geomagnetic effects and the solar origins. Three fast forward shocks, three directional discontinuities and two reverse waves were detected and analyzed in detail. The three fast forward shocks 'pump' up the interplanetary magnetic field from a value of approx.4 nT to 44 nT. However, the fields after the shocks were northward, and magnetic storms did not result. The three ram pressure increases were associated with major sudden impulses (SI + s) at Earth. A magnetic cloud followed the third forward shock and the southward Bz associated with the latter was responsible for the superstorm. Two reverse waves were detected, one at the edge and one near the center of the magnetic cloud (MC). It is suspected that these 'waves' were once reverse shocks which were becoming evanescent when they propagated into the low plasma beta MC. The second reverse wave caused a decrease in the southward component of the IMF and initiated the storm recovery phase. It is determined that flares located at large longitudinal distances from the subsolar point were the most likely causes of the first two shocks without associated magnetic clouds. It is thus unlikely that the shocks were 'blast waves' or that magnetic reconnection eroded away the two associated MCs. This interplanetary/solar event is an example of the extremely complex magnetic storms which can occur in the post-solar maximum phase.

Protostars and Disks

NASA Technical Reports Server (NTRS)

Ho, Paul

1997-01-01

The research concentrated on high angular resolution (arc-second scale) studies of molecular cloud cores associated with very young star formation. New ways to study disks and protoplanetary systems were explored. Findings from the areas studied are briefly summarized: (1) molecular clouds; (2) gravitational contraction; (3) jets, winds, and outflows; (4) Circumstellar Disks (5) Extrasolar Planetary Systems. A bibliography of publications and submitted papers produced during the grant period is included.

Investigation of plasma contactors for use with orbiting wires

NASA Technical Reports Server (NTRS)

Estes, Robert D.; Grossi, Mario D.; Hohlfeld, Robert

1987-01-01

The proposed Shuttle-based short tether experiments with hollow cathodes have the potential for providing important data that will not be obtained in long tether experiments. A critical property for hollow cathode effectiveness as a plasma contactor is the cross magnetic field conductivity of the emitted plasma. The different effects of hollow cathode cloud overlap in the cases of motion-driven and battery-driven operation are emphasized. The calculations presented on the size and shape of the hollow cathode cloud improve the qualitative picture of hollow cathodes in low Earth orbit and provide estimates of time constants for establishing the fully-expanded cloud. The magnetic boundary value problem calculations indicate the way in which the magnetic field will effect the shape of the cloud by resisting expansion in the direction perpendicular to the field. The large-scale interactions of the system were also considered. It was concluded that recent plasma chamber experiments by Stenzel and Urrutia do not model an electrodynamic tether well enough to apply the results to tethered system behavior. Orbiting short tether experiments on hollow cathodes will provide critical information on hollow cathode performance and the underlying physics that cannot be obtained any other way. Experiments should be conducted as soon as funding and a suitable space vehicle are available.

Recent observations of organic molecules in nearby cold, dark interstellar clouds

NASA Technical Reports Server (NTRS)

Suzuki, H.; Ohishi, M.; Morimoto, M.; Kaifu, N.; Friberg, P.

1985-01-01

Recent investigations of the organic chemistry of relatively nearby cold, dark interstellar clouds are reported. Specifically, the presence of interstellar tricarbon monoxide (C3O) in Taurus Molecular Cloud 1 (TMC-1) is confirmed. The first detection in such regions of acetaldehyde (CH3CHO), the most complex oxygen-containing organic molecule yet found in dark clouds is reported, as well as the first astronomical detection of several molecular rotational transitions, including the J = 18-17 and 14-13 transitions of cyanodiacetylene (HC5N), the 1(01)-0(00) transition of acetaldehyde, and the J = 5-4 transition of C3O. A significant upper limit is set on the abundance of cyanocarbene (HCCN) as a result of the first reported interstellar search for this molecule.

Radiation Pressure-Driven Magnetic Disk Winds in Broad Absorption Line Quasi-Stellar Objects

NASA Technical Reports Server (NTRS)

DeKool, Martin; Begelman, Mitchell C.

1995-01-01

We explore a model in which QSO broad absorption lines (BALS) are formed in a radiation pressure-driven wind emerging from a magnetized accretion disk. The magnetic field threading the disk material is dragged by the flow and is compressed by the radiation pressure until it is dynamically important and strong enough to contribute to the confinement of the BAL clouds. We construct a simple self-similar model for such radiatively driven magnetized disk winds, in order to explore their properties. It is found that solutions exist for which the entire magnetized flow is confined to a thin wedge over the surface of the disk. For reasonable values of the mass-loss rate, a typical magnetic field strength such that the magnetic pressure is comparable to the inferred gas pressure in BAL clouds, and a moderate amount of internal soft X-ray absorption, we find that the opening angle of the flow is approximately 0.1 rad, in good agreement with the observed covering factor of the broad absorption line region.

New experimental measurements of electron clouds in ion beams with large tune depression

DOE Office of Scientific and Technical Information (OSTI.GOV)

Molvik, A W; Covo, M K; Cohen, R H

We study electron clouds in high perveance beams (K = 8E-4) with a large tune depression of 0.2 (defined as the ratio of a single particle oscillation response to the applied focusing fields, with and without space charge). These 1 MeV, 180 mA, K+ beams have a beam potential of +2 kV when electron clouds are minimized. Simulation results are discussed in a companion paper [J-L. Vay, this Conference]. We have developed the first diagnostics that quantitatively measure the accumulation of electrons in a beam [1]. This, together with measurements of electron sources, will enable the electron particle balance tomore » be measured, and electron-trapping efficiencies determined. We, along with colleagues from GSI and CERN, have also measured the scaling of gas desorption with beam energy and dE/dx [2]. Experiments where the heavy-ion beam is transported with solenoid magnetic fields, rather than with quadrupole magnetic or electrostatic fields, are being initiated. We will discuss initial results from experiments using electrode sets (in the middle and at the ends of magnets) to either expel or to trap electrons within the magnets. We observe electron oscillations in the last quadrupole magnet when we flood the beam with electrons from an end wall. These oscillations, of order 10 MHz, are observed to grow from the center of the magnet while drifting upstream against the beam, in good agreement with simulations.« less

ULTRAVIOLET ESCAPE FRACTIONS FROM GIANT MOLECULAR CLOUDS DURING EARLY CLUSTER FORMATION

DOE Office of Scientific and Technical Information (OSTI.GOV)

Howard, Corey; Pudritz, Ralph; Klessen, Ralf

2017-01-01

The UV photon escape fraction from molecular clouds is a key parameter for understanding the ionization of the interstellar medium and extragalactic processes such as cosmic reionization. We present the ionizing photon flux and the corresponding photon escape fraction ( f {sub esc}) arising as a consequence of star cluster formation in a turbulent, 10{sup 6} M {sub ⊙} giant molecular cloud, simulated using the code FLASH. We make use of sink particles to represent young, star-forming clusters coupled with a radiative transfer scheme to calculate the emergent UV flux. We find that the ionizing photon flux across the cloudmore » boundary is highly variable in time and space due to the turbulent nature of the intervening gas. The escaping photon fraction remains at ∼5% for the first 2.5 Myr, followed by two pronounced peaks at 3.25 and 3.8 Myr with a maximum f {sub esc} of 30% and 37%, respectively. These peaks are due to the formation of large H ii regions that expand into regions of lower density, some of which reaching the cloud surface. However, these phases are short-lived, and f {sub esc} drops sharply as the H ii regions are quenched by the central cluster passing through high-density material due to the turbulent nature of the cloud. We find an average f {sub esc} of 15% with factor of two variations over 1 Myr timescales. Our results suggest that assuming a single value for f {sub esc} from a molecular cloud is in general a poor approximation, and that the dynamical evolution of the system leads to large temporal variation.« less

Extinction and Star Formation Study in Molecular Clouds with DENIS infrared data and USNO optical data

NASA Astrophysics Data System (ADS)

Cambrésy, Laurent

1999-11-01

This thesis consists in a study of molecular clouds, essentially of the point of view of the interstellar environment, but also of the one of the star formation. The original method to estimate extinction presented here is based on adaptive star counts as well as on a wavelet decomposition. For the first time, an extinction map of the whole sky is proposed (USNO-PMM optical data). Access to very large field maps offers the opportunity to analyze the interstellar matter distribution in various environments. A first result is that the contained mass in regions for which AV > 1 would not exceed half of the total cloud mass. Using DENIS data, it becomes possible to probe dense regions of clouds. For instance, star counts in the Chamaeleon complex show cores which were not resolved before. Moreover, the selection of stars with a strong infrared excess yields about fifty T Tauri candidates. From their luminosity function, I derived the average lifetime of circumstellar disc of low--mass stars: ~4cdot 106 years. It is difficult to understand the relation between extinction and molecular emission, but it appears clearly that molecular emission is a bad estimator of the column density for low extinction area. Actually, thresholds exist in the CO detection and I conclude that photodissociation, density and cloud geometry have important consequences on the CO emission when AV < 2. Investigation of the relation between extinction and far--infrared emission in Polaris leads to a four times larger emissivity in cold areas than in hot areas. This result explains the low temperatures in this cloud and implies severe restrictions concerning the use of far--infrared fluxes as an extinction estimator.

Discovery of recombining plasma from the faintest GeV supernova remnant HB 21 and a possible scenario for cosmic rays escaping from supernova remnant shocks

NASA Astrophysics Data System (ADS)

Suzuki, Hiromasa; Bamba, Aya; Nakazawa, Kazuhiro; Furuta, Yoshihiro; Sawada, Makoto; Yamazaki, Ryo; Koyama, Katsuji

2018-06-01

We present an X-ray study of the GeV gamma-ray supernova remnant (SNR) HB 21 with Suzaku. HB 21 is interacting with molecular clouds, and is the faintest in the GeV band among known GeV SNRs. We discovered strong radiative recombination continua of Si and S from the center of the remnant, which provide direct evidence of a recombining plasma (RP). The total emission can be explained with the RP and ionizing plasma components. The electron temperature and recombination timescale of the RP component were estimated as 0.17 (0.15-0.18) keV and 3.2 (2.0-4.8) × 1011 s cm-3, respectively. The estimated age of the RP (˜170 kyr) is the longest among known recombining GeV SNRs, because of a very low density of electrons (˜0.05 cm-3). We have examined the dependencies of GeV spectral indices on each of RP ages and SNR diameters for nine recombining GeV SNRs. Both showed possible positive correlations, indicating that both the parameters can be good indicators of properties of accelerated protons, for instance the degree of escape from SNR shocks. A possible scenario for a process of proton escape is introduced: interaction with molecular clouds makes weaker magnetic turbulence and cosmic-ray protons escape, simultaneously cooling down the thermal electrons and generating an RP.

X-ray spectroscopy of the mixed morphology supernova remnant W 28 with XMM-Newton

NASA Astrophysics Data System (ADS)

Nakamura, Ryoko; Bamba, Aya; Ishida, Manabu; Yamazaki, Ryo; Tatematsu, Ken'ichi; Kohri, Kazunori; Pühlhofer, Gerd; Wagner, Stefan J.; Sawada, Makoto

2014-06-01

We report on spatially resolved X-ray spectroscopy of the north-eastern part of the mixed morphology supernova remnant (SNR) W 28 with XMM-Newton. The observed field of view includes a prominent and twisted shell emission forming the edge of this SNR as well as part of the center-filled X-ray emission brightening toward the south-west edge of the field of view. The shell region spectra are in general represented by an optically thin thermal plasma emission in collisional ionization equilibrium with a temperature of ˜ 0.3 keV and a density of ˜ 10 cm-3, which is much higher than the density obtained for inner parts. In contrast, we detected no significant X-ray flux from one of the TeV γ-ray peaks with an upper-limit flux of 2.1 × 10-14 erg cm-2 s-1 in the 2-10 keV band. The large flux ratio of TeV to X-ray, larger than 16, and the spatial coincidence of the molecular cloud and the TeV γ-ray emission site indicate that the TeV γ-ray of W 28 is π0-decay emission originating from collisions between accelerated protons and molecular cloud protons. Comparing the spectrum in the TeV band and the X-ray upper limit, we obtained a weak upper limit on the magnetic field strength B ≲ 1500 μG.

SUPERNOVA DRIVING. I. THE ORIGIN OF MOLECULAR CLOUD TURBULENCE

DOE Office of Scientific and Technical Information (OSTI.GOV)

Padoan, Paolo; Pan, Liubin; Haugbølle, Troels

2016-05-01

Turbulence is ubiquitous in molecular clouds (MCs), but its origin is still unclear because MCs are usually assumed to live longer than the turbulence dissipation time. Interstellar medium (ISM) turbulence is likely driven by supernova (SN) explosions, but it has never been demonstrated that SN explosions can establish and maintain a turbulent cascade inside MCs consistent with the observations. In this work, we carry out a simulation of SN-driven turbulence in a volume of (250 pc){sup 3}, specifically designed to test if SN driving alone can be responsible for the observed turbulence inside MCs. We find that SN driving establishesmore » a velocity scaling consistent with the usual scaling laws of supersonic turbulence, suggesting that previous idealized simulations of MC turbulence, driven with a random, large-scale volume force, were correctly adopted as appropriate models for MC turbulence, despite the artificial driving. We also find that the same scaling laws extend to the interiors of MCs, and that the velocity–size relation of the MCs selected from our simulation is consistent with that of MCs from the Outer-Galaxy Survey, the largest MC sample available. The mass–size relation and the mass and size probability distributions also compare successfully with those of the Outer Galaxy Survey. Finally, we show that MC turbulence is super-Alfvénic with respect to both the mean and rms magnetic-field strength. We conclude that MC structure and dynamics are the natural result of SN-driven turbulence.« less

ALMA’s Polarized View of 10 Protostars in the Perseus Molecular Cloud

NASA Astrophysics Data System (ADS)

Cox, Erin G.; Harris, Robert J.; Looney, Leslie W.; Li, Zhi-Yun; Yang, Haifeng; Tobin, John J.; Stephens, Ian

2018-03-01

We present 870 μm ALMA dust polarization observations of 10 young Class 0/I protostars in the Perseus Molecular Cloud. At ∼0.″35 (80 au) resolution, all of our sources show some degree of polarization, with most (9/10) showing significantly extended emission in the polarized continuum. Each source has incredibly intricate polarization signatures. In particular, all three disk-candidates have polarization vectors roughly along the minor axis, which is indicative of polarization produced by dust scattering. On ∼100 au scales, the polarization is at a relatively low level (≲1%) and is quite ordered. In sources with significant envelope emission, the envelope is typically polarized at a much higher (≳5%) level and has a far more disordered morphology. We compute the cumulative probability distributions for both the small (disk-scale) and large (envelope-scale) polarization percentage. We find that the two are intrinsically different, even after accounting for the different detection thresholds in the high/low surface brightness regions. We perform Kolmogorov–Smirnov and Anderson–Darling tests on the distributions of angle offsets of the polarization from the outflow axis. We find disk-candidate sources are different from the non-disk-candidate sources. We conclude that the polarization on the 100 au scale is consistent with the signature of dust scattering for disk-candidates and that the polarization on the envelope-scale in all sources may come from another mechanism, most likely magnetically aligned grains.

Comparing the Atmospheric Losses at Io and Europa

NASA Astrophysics Data System (ADS)

Dols, V. J.; Bagenal, F.; Crary, F. J.; Cassidy, T.

2017-12-01

At Io and Europa, the interaction of the Jovian plasma with the moon atmosphere leads to a significant loss of atomic/molecular neutrals and ions to space. The processes that lead to atmospheric escape are diverse: atmospheric sputtering, molecular dissociation, molecular ion recombination, Jeans escape etc. Each process leads to neutrals escaping at different velocities (i.e. electron impact dissociation leads to very slow atomic neutrals, sputtering might eject faster molecular neutrals). Some neutrals will be ejected out of the Jovian system; others will form extended neutral clouds along the orbit of the moons. These atomic/molecular extended neutral clouds are probably the main source of plasma for the Jovian magnetosphere. They are difficult to observe directly thus their composition and density are still poorly constrained. A future modeling of the formation of these extended clouds requires an estimate of their atmospheric sources. We estimate the atmospheric losses at Io and Europa for each loss process with a multi-species chemistry model, using a prescribed atmospheric distribution consistent with the observations. We compare the neutral losses at Io and Europa.

High Spectral Resolution Lidar Measurements of Multiple Scattering

NASA Technical Reports Server (NTRS)

Eloranta, E. W.; Piironen, P.

1996-01-01

The University of Wisconsin High Spectral Resolution Lidar (HSRL) provides unambiguous measurements of backscatter cross section, backscatter phase function, depolarization, and optical depth. This is accomplished by dividing the lidar return into separate particulate and molecular contributions. The molecular return is then used as a calibration target. We have modified the HSRL to use an I2 molecular absorption filter to separate aerosol and molecular signals. This allows measurement in dense clouds. Useful profiles extend above the cloud base until the two way optical depth reaches values between 5 and 6; beyond this, photon counting errors become large. In order to observe multiple scattering, the HSRL includes a channel which records the combined aerosol and molecular lidar return simultaneously with the spectrometer channel measurements of optical properties. This paper describes HSRL multiple scattering measurements from both water and ice clouds. These include signal strengths and depolarizations as a function of receiver field of view. All observations include profiles of extinction and backscatter cross sections. Measurements are also compared to predictions of a multiple scattering model based on small angle approximations.

New concepts for molecular magnets

NASA Astrophysics Data System (ADS)

Pilawa, Bernd

1999-03-01

Miller and Epstein (1994) define molecular magnets as magnetic materials which are prepared by the low-temperature methods of the preparative chemistry. This definition includes molecular crystals of neutral radicals, radical salts and charge transfer complexes as well as metal complexes and polymers with unpaired spins (Dormann 1995). The challenge of molecular magnets consists in tailoring magnetic properties by specific modifications of the molecular units. The combination of magnetism with mechanical or electrical properties of molecular compounds promise materials of high technical interest (Gatteschi 1994a and 1994b, Möhwald 1996) and both the chemical synthesis of new molecular materials with magnetic properties as well as the physical investigation and explanation of these properties is important, in order to achieve any progress. This work deals with the physical characterization of the magnetic properties of molecular materials. It is organized as follows. In the first part molecular crystals of neutral radicals are studied. After briefly discussing the general magnetic properties of these materials and after an overview over the physical principles of exchange interaction between organic radicals I focus on the interplay between the crystallographic structure and the magnetic properties of various derivatives of the verdazyl and nitronyl nitroxide radicals. The magnetic properties of metal complexes are the subject of the second part. After an overview over the experimental and theoretical tools which are used for the investigation of the magnetic properties I shortly discuss the exchange coupling of transition metal ions and the magnetic properties of complexes of two and three metal ions. Special emphasis is given to spin cluster compounds. Spin cluster denote complexes of many magnetic ions. They are attractive as building blocks of molecular magnets as well as magnetic model compounds for the study of spin frustration, molecular super-paramagnetism and quasi one-dimensional magnets.

Composition, structure and chemistry of interstellar dust

NASA Technical Reports Server (NTRS)

Tielens, Alexander G. G. M.; Allamandola, Louis J.

1986-01-01

The observational constraints on the composition of the interstellar dust are analyzed. The dust in the diffuse interstellar medium consists of a mixture of stardust (amorphous silicates, amorphous carbon, polycyclic aromatic hydrocarbons, and graphite) and interstellar medium dust (organic refractory material). Stardust seems to dominate in the local diffuse interstellar medium. Inside molecular clouds, however, icy grain mantles are also important. The structural differences between crystalline and amorphous materials, which lead to differences in the optical properties, are discussed. The astrophysical consequences are briefly examined. The physical principles of grain surface chemistry are discussed and applied to the formation of molecular hydrogen and icy grain mantles inside dense molecular clouds. Transformation of these icy grain mantles into the organic refractory dust component observed in the diffuse interstellar medium requires ultraviolet sources inside molecular clouds as well as radical diffusion promoted by transient heating of the mantle. The latter process also returns a considerable fraction of the molecules in the grain mantle to the gas phase.