Opening the CHOCBOX: clumpy stellar winds in Cyg X-1

NASA Astrophysics Data System (ADS)

Grinberg, V.; Uttley, P.; Wilms, J.; Miller-Jones, J.; Pottschmidt, K.; Niu, S.; Hirsch, M.; Chocbox Collaboration

2017-10-01

Winds of O/B-stars are key drivers of enrichment and star formation and evolution. Yet, our understanding of their clumpy structure is limited. Luckily, high mass X-ray binaries, where the compact object accretes from the stellar wind of the companion, are perfect laboratories to study such winds: the X-ray radiation from the vicinity of the compact object is quasi-pointlike and effectively X-rays the clumps crossing the line of sight. We observed the high mass X-ray binary Cyg X-1 with XMM for 7 consecutive days with simultaneous coverage with NuSTAR, INTEGRAL and VLBA. One of our main aims was to probe the wind of the O-type companion in an unprecedented uninterrupted campaign, spanning more than an orbital period and including two superior conjunctions where we expect the densest wind. Here, we present first results from the CHOCBOX (Cyg X-1 Hard state Observations of a Complete Binary Orbit in X-rays) campaign and compare them to previous work, in particular multi-year studies of absorption variability and high resolution snapshots with Chandra-HETG. We argue that the clumps have a complex structure with hotter outer and colder inner layers and are not symmetrical.

STScI-PRC98-38 GREAT BALLS OF FIRE! HUBBLE SEES BRIGHT KNOTS EJECTED FROM BRILLIANT STAR

NASA Technical Reports Server (NTRS)

2002-01-01

Resembling an aerial fireworks explosion, this dramatic NASA Hubble Space Telescope picture of the energetic star WR124 reveals it is surrounded by hot clumps of gas being ejected into space at speeds of over 100,000 miles per hour. Also remarkable are vast arcs of glowing gas around the star, which are resolved into filamentary, chaotic substructures, yet with no overall global shell structure. Though the existence of clumps in the winds of hot stars has been deduced through spectroscopic observations of their inner winds, Hubble resolves them directly in the nebula M1-67 around WR124 as 100 billion-mile wide glowing gas blobs. Each blob is about 30 times the mass of the Earth. The massive, hot central star is known as a Wolf-Rayet star. This extremely rare and short-lived class of super-hot star (in this case 50,000 degrees Kelvin) is going through a violent, transitional phase characterized by the fierce ejection of mass. The blobs may result from the furious stellar wind that does not flow smoothly into space but has instabilities which make it clumpy. The surrounding nebula is estimated to be no older than 10,000 years, which means that it is so young it has not yet slammed into the gasses comprising the surrounding interstellar medium. As the blobs cool they will eventually dissipate into space and so don't pose any threat to neighboring stars. The star is 15,000 light-years away, located in the constellation Sagittarius. The picture was taken with Hubble's Wide Field Planetary Camera 2 in March 1997. The image is false-colored to reveal details in the nebula's structure. Credit: Yves Grosdidier (University of Montreal and Observatoire de Strasbourg), Anthony Moffat (Universitie de Montreal), Gilles Joncas (Universite Laval), Agnes Acker (Observatoire de Strasbourg), and NASA

Star Formation at z = 2.481 in the Lensed Galaxy SDSS J1110+6459. II. What is Missed at the Normal Resolution of the Hubble Space Telescope?

NASA Astrophysics Data System (ADS)

Rigby, J. R.; Johnson, T. L.; Sharon, K.; Whitaker, K.; Gladders, M. D.; Florian, M.; Lotz, J.; Bayliss, M.; Wuyts, E.

2017-07-01

For lensed galaxy SGAS J111020.0+645950.8 at redshift z = 2.481, which is magnified by a factor of 28 ± 8, we analyze the morphology of star formation, as traced by rest-frame ultraviolet emission, in both the highly magnified source plane and simulations of how this galaxy would appear without lensing magnification. Were this galaxy not lensed, but rather drawn from a Hubble Space Telescope deep field, we would conclude that almost all its star formation arises from an exponential disk (Sérsic index of 1.0 ± 0.4) with an effective radius of {r}e=2.7+/- 0.3 {kpc} measured from two-dimensional fitting to F606W using Galfit, and {r}e=1.9+/- 0.1 {kpc} measured by fitting a radial profile to F606W elliptical isophotes. At the normal spatial resolution of the deep fields, there is no sign of clumpy star formation within SGAS J111020.0+645950.8. However, the enhanced spatial resolution enabled by gravitational lensing tells a very different story; much of the star formation arises in two dozen clumps with sizes of r = 30-50 pc spread across the 7 kpc length of the galaxy. The color and spatial distribution of the diffuse component suggests that still-smaller clumps are unresolved. Despite this clumpy, messy morphology, the radial profile is still well-characterized by an exponential profile. In this lensed galaxy, stars are forming in complexes with sizes well below 100 pc such sizes are wholly unexplored by surveys of galaxy evolution at 1< z< 3.

Cold, clumpy accretion onto an active supermassive black hole

NASA Astrophysics Data System (ADS)

Tremblay, Grant R.; Oonk, J. B. Raymond; Combes, Françoise; Salomé, Philippe; O'Dea, Christopher P.; Baum, Stefi A.; Voit, G. Mark; Donahue, Megan; McNamara, Brian R.; Davis, Timothy A.; McDonald, Michael A.; Edge, Alastair C.; Clarke, Tracy E.; Galván-Madrid, Roberto; Bremer, Malcolm N.; Edwards, Louise O. V.; Fabian, Andrew C.; Hamer, Stephen; Li, Yuan; Maury, Anaëlle; Russell, Helen R.; Quillen, Alice C.; Urry, C. Megan; Sanders, Jeremy S.; Wise, Michael W.

2016-06-01

Supermassive black holes in galaxy centres can grow by the accretion of gas, liberating energy that might regulate star formation on galaxy-wide scales. The nature of the gaseous fuel reservoirs that power black hole growth is nevertheless largely unconstrained by observations, and is instead routinely simplified as a smooth, spherical inflow of very hot gas. Recent theory and simulations instead predict that accretion can be dominated by a stochastic, clumpy distribution of very cold molecular clouds—a departure from the ‘hot mode’ accretion model—although unambiguous observational support for this prediction remains elusive. Here we report observations that reveal a cold, clumpy accretion flow towards a supermassive black hole fuel reservoir in the nucleus of the Abell 2597 Brightest Cluster Galaxy (BCG), a nearby (redshift z = 0.0821) giant elliptical galaxy surrounded by a dense halo of hot plasma. Under the right conditions, thermal instabilities produce a rain of cold clouds that fall towards the galaxy’s centre, sustaining star formation amid a kiloparsec-scale molecular nebula that is found at its core. The observations show that these cold clouds also fuel black hole accretion, revealing ‘shadows’ cast by the molecular clouds as they move inward at about 300 kilometres per second towards the active supermassive black hole, which serves as a bright backlight. Corroborating evidence from prior observations of warmer atomic gas at extremely high spatial resolution, along with simple arguments based on geometry and probability, indicate that these clouds are within the innermost hundred parsecs of the black hole, and falling closer towards it.

Cold, clumpy accretion onto an active supermassive black hole.

PubMed

Tremblay, Grant R; Oonk, J B Raymond; Combes, Françoise; Salomé, Philippe; O'Dea, Christopher P; Baum, Stefi A; Voit, G Mark; Donahue, Megan; McNamara, Brian R; Davis, Timothy A; McDonald, Michael A; Edge, Alastair C; Clarke, Tracy E; Galván-Madrid, Roberto; Bremer, Malcolm N; Edwards, Louise O V; Fabian, Andrew C; Hamer, Stephen; Li, Yuan; Maury, Anaëlle; Russell, Helen R; Quillen, Alice C; Urry, C Megan; Sanders, Jeremy S; Wise, Michael W

2016-06-09

Supermassive black holes in galaxy centres can grow by the accretion of gas, liberating energy that might regulate star formation on galaxy-wide scales. The nature of the gaseous fuel reservoirs that power black hole growth is nevertheless largely unconstrained by observations, and is instead routinely simplified as a smooth, spherical inflow of very hot gas. Recent theory and simulations instead predict that accretion can be dominated by a stochastic, clumpy distribution of very cold molecular clouds--a departure from the 'hot mode' accretion model--although unambiguous observational support for this prediction remains elusive. Here we report observations that reveal a cold, clumpy accretion flow towards a supermassive black hole fuel reservoir in the nucleus of the Abell 2597 Brightest Cluster Galaxy (BCG), a nearby (redshift z = 0.0821) giant elliptical galaxy surrounded by a dense halo of hot plasma. Under the right conditions, thermal instabilities produce a rain of cold clouds that fall towards the galaxy's centre, sustaining star formation amid a kiloparsec-scale molecular nebula that is found at its core. The observations show that these cold clouds also fuel black hole accretion, revealing 'shadows' cast by the molecular clouds as they move inward at about 300 kilometres per second towards the active supermassive black hole, which serves as a bright backlight. Corroborating evidence from prior observations of warmer atomic gas at extremely high spatial resolution, along with simple arguments based on geometry and probability, indicate that these clouds are within the innermost hundred parsecs of the black hole, and falling closer towards it.

MAPPING THE CLUMPY STRUCTURES WITHIN SUBMILLIMETER GALAXIES USING LASER-GUIDE STAR ADAPTIVE OPTICS SPECTROSCOPY

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

Menendez-Delmestre, Karin; Goncalves, Thiago S.; Blain, Andrew W.

2013-04-20

We present the first integral-field spectroscopic observations of high-redshift submillimeter-selected galaxies (SMGs) using Laser-Guide Star Adaptive Optics. We target H{alpha} emission of three SMGs at redshifts z {approx} 1.4-2.4 with the OH-Suppressing Infrared Imaging Spectrograph on Keck. The spatially resolved spectroscopy of these galaxies reveals unresolved broad-H{alpha} line regions (FWHM >1000 km s{sup -1}) likely associated with an active galactic nucleus (AGN) and regions of diffuse star formation traced by narrow-line H{alpha} emission (FWHM {approx}< 500 km s{sup -1}) dominated by multiple H{alpha}-bright stellar clumps, each contributing 1%-30% of the total clump-integrated H{alpha} emission. We find that these SMGs hostmore » high star formation rate surface densities, similar to local extreme sources, such as circumnuclear starbursts and luminous infrared galaxies. However, in contrast to these local environments, SMGs appear to be undergoing such intense activity on significantly larger spatial scales as revealed by extended H{alpha} emission over 4-16 kpc. H{alpha} kinematics show no evidence of ordered global motion as would be found in a disk, but rather large velocity offsets ({approx}few Multiplication-Sign 100 km s{sup -1}) between the distinct stellar clumps. Together with the asymmetric distribution of the stellar clumps around the AGN in these objects, it is unlikely that we are unveiling a clumpy disk structure as has been suggested in other high-redshift populations of star-forming galaxies. The SMG clumps in this sample may correspond to remnants of originally independent gas-rich systems that are in the process of merging, hence triggering the ultraluminous SMG phase.« less

The evidence for clumpy accretion in the Herbig Ae star HR 5999

NASA Technical Reports Server (NTRS)

Perez, M. R.; Grady, C. A.; The, P. S.

1993-01-01

Analysis of IUE high- and low-dispersion spectra of the young Herbig Ae star HR 5999 (HD 144668) covering 1978-1992 revealed dramatic changes in the Mg II h and k (2795.5, 2802.7 A) emission profiles, changes in the column density and distribution in radial velocity of accreting gas, and flux in the Ly(alpha), O I, and C IV emission lines, which are correlated with the UV excess luminosity. Variability in the spectral type inferred from the UV spectral energy distribution, ranging from A5 IV-III in high state to A7 III in the low state, was also observed. The trend of earlier inferred spectral type with decreasing wavelength and with increasing UV continuum flux has previously been noted as a signature of accretion disks in lower mass pre-main sequence stars (PMS) and in systems undergoing FU Orionis-type outbursts. Our data represent the first detection of similar phenomena in an intermediate mass (M greater than or equal to 2 solar mass) PMS star. Recent IUE spectra show gas accreting toward the star with velocities as high as plus 300 km/s, much as is seen toward beta Pic, and suggest that we also view this system through the debris disk. The absence of UV lines with the rotational broadening expected given the optical data (A7 IV, V sini=180 plus or minus 20 km/s for this system) also suggests that most of the UV light originates in the disk, even in the low continuum state. The dramatic variability in the column density of accreting gas, is consistent with clumpy accretion, such as has been observed toward beta Pic, is a hallmark of accretion onto young stars, and is not restricted to the clearing phase, since detectable amounts of accretion are present for stars with 0.5 Myr less than t(sub age) less than 2.8 Myr. The implications for models of beta Pic and similar systems are briefly discussed.

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

Iono, Daisuke; Hatsukade, Bunyo; Kawabe, Ryohei

The central structure in three of the brightest unlensed z = 3–4 submillimeter galaxies is investigated through 0.″015–0.″05 (120–360 pc) 860 μ m continuum images obtained using the Atacama Large Millimeter/submillimeter Array (ALMA). The distribution in the central kiloparsec in AzTEC1 and AzTEC8 is extremely complex, and they are composed of multiple ∼200 pc clumps. AzTEC4 consists of two sources that are separated by ∼1.5 kpc, indicating a mid-stage merger. The peak star formation rate densities in the central clumps are ∼300–3000 M {sub ⊙} yr{sup −1} kpc{sup −2}, suggesting regions with extreme star formation near the Eddington limit. Bymore » comparing the flux obtained by ALMA and Submillimeter Array, we find that 68%–90% of the emission is extended (≳1 kpc) in AzTEC4 and 8. For AzTEC1, we identify at least 11 additional compact (∼200 pc) clumps in the extended 3–4 kpc region. Overall, the data presented here suggest that the luminosity surface densities observed at ≲150 pc scales are roughly similar to that observed in local ULIRGs, as in the eastern nucleus of Arp 220. Between 10% and 30% of the 860 μ m continuum is concentrated in clumpy structures in the central kiloparsec, while the remaining flux is distributed over ≳1 kpc regions, some of which could also be clumpy. These sources can be explained by a rapid inflow of gas such as a merger of gas-rich galaxies, surrounded by extended and clumpy starbursts. However, the cold mode accretion model is not ruled out.« less

Ultrafaint Dwarf Galaxies—the Lowest-mass Relics from Before Reionization

NASA Astrophysics Data System (ADS)

Bland-Hawthorn, Joss; Sutherland, Ralph; Webster, David

2015-07-01

New observations suggest that ultrafaint dwarf galaxies (UFDs)—the least luminous systems bound by dark matter halos (≲ {10}5 {L}⊙ )—may have formed before reionization. The extrapolated virial masses today are uncertain, with estimates ranging from 108 {M}⊙ to as high as 109 {M}⊙ depending on the assumed form of the underlying potential. We show that the progenitor halo masses of UFDs can, in principle, be as low as {M}{vir} ≈ 107 {M}⊙ . Under the right conditions, such a halo can survive the energy input of a supernova (SN) and its radiative progenitor. A clumpy (fractal) medium is much less susceptible to both internal and external injections of energy. It is less prone to SN sweeping (particularly if it is off-centered) because the coupling efficiency of the explosive energy is much lower than for a diffuse interstellar medium. With the aid of the 3D hydro/ionization code Fyris, we show that sufficient baryons are retained to form stars following a single SN event in dark matter halos down to {M}{vir} ≈ 107 {M}⊙ in the presence of radiative cooling. In these models, the gas survives the SN explosion, is enriched with the specific abundance yields of the discrete events, and reaches surface densities where low-mass stars can form. Our highest-resolution simulations reveal why cooling is so effective in retaining gas compared to any other factor. In the early stages, the super-hot metal-enriched SN ejecta exhibit strong cooling, leading to much of the explosive energy being lost. Consistent with earlier work, the baryons do not survive in smooth or adiabatic models in the event of an SN. The smallest galaxies may not contribute a large fraction of matter to the formation of galaxies, but they carry signatures of the earliest epochs of star formation, as we show. These signatures may allow us to distinguish a small primordial galaxy from one that was stripped down to its present size through tidal interaction. We discuss these results in the context of local UFDs and damped Lyα systems (z ˜ 2) at very low metallicity ([Fe/H] ˜ -3). We show that both classes of objects are consistent with primordial low-mass systems that have experienced only a few enrichment events.

Massive star clusters in a z=1 star-forming galaxy seen at a 100 pc scale thanks to strong gravitational lensing

NASA Astrophysics Data System (ADS)

Dessauges-Zavadsky, Miroslava; Cava, Antonio; Richard, Johan; Schaerer, Daniel; Egami, Eiichi

2015-08-01

Deep and high-resolution imaging has revealed clumpy, rest-frame UV morphologies among z=1-3 galaxies. The majority of these galaxies has been shown to be dominated by ordered disk rotation, which led to the conclusion that the observed giant clumps, resolved on kpc-scales, are generated from disk fragmentation due to gravitational instability. State-of-the-art numerical simulations show that they may occupy a relevant role in galaxy evolution, contributing to the galactic bulge formation. Despite the high resolution attained by the most advanced ground- and space-based facilities, as well as in numerical simulations, the intrinsic typical masses and scale sizes of these star-forming clumps remain unconstrained, since they are barely resolved at z=1-3.Thanks to the amplification and stretching power provided by strong gravitational lensing, we are likely to reach the spatial resolving power for unveiling the physics of these star-forming regions. We report on the study of clumpy star formation observed in the Cosmic Snake, a strongly lensed galaxy at z=1, representative of the typical star-forming population close to the peak of Universe activity. About 20 clumps are identified in the HST images. Benefiting from extreme amplification factors up to 100, they are resolved down to an intrinsic scale of 100 pc, never reached before at z=1.The HST multi-wavelength analysis of these individual star clusters allows us to determine their intrinsic physical properties, showing stellar masses (Ms) from 106 to 108.3 Msun, sizes from 100 to 400 pc, and ages from 106 to 108.5 yr. The masses we find are in line with the new, very high resolution numerical simulations, which also suggest that the massive giant clumps previously observed at high redshift with Ms as high as 109-10 Msun may suffer from low resolution effects, being unresolved conglomerates of less massive star clusters. We also compare our results with those of massive young clusters in nearby galaxies. Our approved ALMA observations will reach the same 100 pc scale, which is essential for the study of associated giant molecular clouds in this galaxy.

Ionization processes in a local analogue of distant clumpy galaxies: VLT MUSE IFU spectroscopy and FORS deep images of the TDG NGC 5291N

NASA Astrophysics Data System (ADS)

Fensch, J.; Duc, P.-A.; Weilbacher, P. M.; Boquien, M.; Zackrisson, E.

2016-01-01

Context. We present Integral Field Unit (IFU) observations with MUSE and deep imaging with FORS of a dwarf galaxy recently formed within the giant collisional HI ring surrounding NGC 5291. This Tidal Dwarf Galaxy (TDG) -like object has the characteristics of typical z = 1-2 gas-rich spiral galaxies: a high gas fraction, a rather turbulent clumpy interstellar medium, the absence of an old stellar population, and a moderate metallicity and star formation efficiency. Aims: The MUSE spectra allow us to determine the physical conditions within the various complex substructures revealed by the deep optical images and to scrutinize the ionization processes at play in this specific medium at unprecedented spatial resolution. Methods: Starburst age, extinction, and metallicity maps of the TDG and the surrounding regions were determined using the strong emission lines Hβ, [OIII], [OI], [NII], Hα, and [SII] combined with empirical diagnostics. Different ionization mechanisms were distinguished using BPT-like diagrams and shock plus photoionization models. Results: In general, the physical conditions within the star-forming regions are homogeneous, in particular with a uniform half-solar oxygen abundance. On small scales, the derived extinction map shows narrow dust lanes. Regions with atypically strong [OI] emission line immediately surround the TDG. The [OI]/ Hα ratio cannot be easily accounted for by the photoionization by young stars or shock models. At greater distances from the main star-foming clumps, a faint diffuse blue continuum emission is observed, both with the deep FORS images and the MUSE data. It does not have a clear counterpart in the UV regime probed by GALEX. A stacked spectrum towards this region does not exhibit any emission line, excluding faint levels of star formation, or stellar absorption lines that might have revealed the presence of old stars. Several hypotheses are discussed for the origin of these intriguing features. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile: ESO MUSE programme 60.A-9320(A) and FORS programme 382.B-0213(A).

AN OBSERVED LINK BETWEEN ACTIVE GALACTIC NUCLEI AND VIOLENT DISK INSTABILITIES IN HIGH-REDSHIFT GALAXIES

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

Bournaud, Frederic; Juneau, Stephanie; Le Floc'h, Emeric

2012-09-20

We provide evidence for a correlation between the presence of giant clumps and the occurrence of active galactic nuclei (AGNs) in disk galaxies. Giant clumps of 10{sup 8}-10{sup 9} M{sub Sun} arise from violent gravitational instability in gas-rich galaxies, and it has been proposed that this instability could feed supermassive black holes (BHs). We use emission line diagnostics to compare a sample of 14 clumpy (unstable) disks and a sample of 13 smoother (stable) disks at redshift z {approx} 0.7. The majority of clumpy disks in our sample have a high probability of containing AGNs. Their [O III] {lambda}5007 emissionmore » line is strongly excited, inconsistent with low-metallicity star formation (SF) alone. [Ne III] {lambda}3869 excitation is also higher. Stable disks rarely have such properties. Stacking ultra sensitive Chandra observations (4 Ms) reveals an X-ray excess in clumpy galaxies, which confirms the presence of AGNs. The clumpy galaxies in our intermediate-redshift sample have properties typical of gas-rich disk galaxies rather than mergers, being in particular on the main sequence of SF. This suggests that our findings apply to the physically similar and numerous gas-rich unstable disks at z > 1. Using the observed [O III] and X-ray luminosities, we conservatively estimate that AGNs hosted by clumpy disks have typical bolometric luminosities of the order of a few 10{sup 43} erg s{sup -1}, BH growth rates m-dot{sub BH}{approx}10{sup -2} M{sub Sun} yr{sup -1}, and that these AGNs are substantially obscured in X-rays. This moderate-luminosity mode could provide a large fraction of today's BH mass with a high duty cycle (>10%), accretion bursts with higher luminosities being possible over shorter phases. Violent instabilities at high redshift (giant clumps) are a much more efficient driver of BH growth than the weak instabilities in nearby spirals (bars), and the evolution of disk instabilities with mass and redshift could explain the simultaneous downsizing of SF and of BH growth.« less

NGC 2548: clumpy spatial and kinematic structure in an intermediate-age Galactic cluster

NASA Astrophysics Data System (ADS)

Vicente, Belén; Sánchez, Néstor; Alfaro, Emilio J.

2016-09-01

NGC 2548 is a ˜400-500 Myr old open cluster with evidence of spatial substructures likely caused by its interaction with the Galactic disc. In this work we use precise astrometric data from the Carte du Ciel - San Fernando (CdC-SF) catalogue to study the clumpy structure in this cluster. We confirm the fragmented structure of NGC 2548 but, additionally, the relatively high precision of our kinematic data lead us to the first detection of substructures in the proper motion space of a stellar cluster. There are three spatially separated cores each of which has its own counterpart in the proper motion distribution. The two main cores lie nearly parallel to the Galactic plane whereas the third one is significantly fainter than the others and it moves towards the Galactic plane separating from the rest of the cluster. We derive core positions and proper motions, as well as the stars belonging to each core.

Resolving molecular gas to ~500 pc in a unique star forming disk galaxy at z~2

NASA Astrophysics Data System (ADS)

Brisbin, Drew; Aravena, Manuel; Hodge, Jacqueline; Carilli, Chris Luke; Daddi, Emanuele; Dannerbauer, Helmut; Riechers, Dominik; Wagg, Jeff

2018-06-01

We have resolved molecular gas in a 'typical' star forming disk galaxy at z>2 down to the scale of ~500 pc. Previous observations of CO and [CI] lines on larger spatial scales have revealed bulk molecular and atomic gas properties indicating that the target is a massive disk galaxy with large gas reserves. Unlike many galaxies studied at high redshift, it is undergoing modest quiescent star formation rather than bursty centrally concentrated star formation. Therefore this galaxy represents an under-studied, but cosmologically important population in the early universe. Our new observations of CO (4-3) highlight the clumpy molecular gas fuelling star formation throughout the disk. Underlying continuum from cold dust provides a key constraint on star formation rate surface densities, allowing us to examine the star formation rate surface density scaling law in a never-before-tested regime of early universe galaxies.These observations enable an unprecedented view of the obscured star formation that is hidden to optical/UV imaging and trace molecular gas on a fine enough scale to resolve morphological traits and provide a view akin to single dish surveys in the local universe.

A Widespread, Clumpy Starburst in the Isolated Ongoing Dwarf Galaxy Merger dm1647+21

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

Privon, G. C.; Stierwalt, S.; Johnson, K. E.

Interactions between pairs of isolated dwarf galaxies provide a critical window into low-mass hierarchical, gas-dominated galaxy assembly and the build-up of stellar mass in low-metallicity systems. We present the first Very Large Telescope/Multi Unit Spectroscopic Explorer (VLT/MUSE) optical integral field unit (IFU) observations of the interacting dwarf pair dm1647+21 selected from the TiNy Titans survey. The H α emission is widespread and corresponds to a total unobscured star formation rate (SFR) of 0.44 M {sub ⊙} yr{sup −1}, which is 2.7 times higher than the SFR inferred from Sloan Digital Sky Survey (SDSS) data. The implied specific SFR (sSFR) formore » the system is elevated by more than an order of magnitude above non-interacting dwarfs in the same mass range. This increase is dominated by the lower-mass galaxy, which has a sSFR enhancement of >50. Examining the spatially resolved maps of classic optical line diagnostics, we find that the interstellar medium (ISM) excitation can be fully explained by star formation. The velocity field of the ionized gas is not consistent with simple rotation. Dynamical simulations indicate that the irregular velocity field and the stellar structure is consistent with the identification of this system as an ongoing interaction between two dwarf galaxies. The widespread, clumpy enhancements in the star formation in this system point to important differences in the effect of mergers on dwarf galaxies, compared to massive galaxies; rather than the funneling of gas to the nucleus and giving rise to a nuclear starburst, starbursts in low-mass galaxy mergers may be triggered by large-scale ISM compression, and thus may be more distributed.« less

Evidence for hot clumpy accretion flow in the transitional millisecond pulsar PSR J1023+0038

NASA Astrophysics Data System (ADS)

Shahbaz, T.; Dallilar, Y.; Garner, A.; Eikenberry, S.; Veledina, A.; Gandhi, P.

2018-06-01

We present simultaneous optical and near-infrared (IR) photometry of the millisecond pulsar PSR J1023+0038 during its low-mass X-ray binary phase. The r΄- and Ks-band light curves show rectangular, flat-bottomed dips, similar to the X-ray mode-switching (active-passive state transitions) behaviour observed previously. The cross-correlation function (CCF) of the optical and near-IR data reveals a strong, broad negative anticorrelation at negative lags, a broad positive correlation at positive lags, with a strong, positive narrow correlation superimposed. The shape of the CCF resembles the CCF of black hole X-ray binaries but the time-scales are different. The features can be explained by reprocessing and a hot accretion flow close to the neutron star's magnetospheric radius. The optical emission is dominated by the reprocessed component, whereas the near-IR emission contains the emission from plasmoids in the hot accretion flow and a reprocessed component. The rapid active-passive state transition occurs when the hot accretion flow material is channelled on to the neutron star and is expelled from its magnetosphere. During the transition the optical reprocessing component decreases resulting in the removal of a blue spectral component. The accretion of clumpy material through the magnetic barrier of the neutron star produces the observed near-IR/optical CCF and variability. The dip at negative lags corresponds to the suppression of the near-IR synchrotron component in the hot flow, whereas the broad positive correlation at positive lags is driven by the increased synchrotron emission of the outflowing plasmoids. The narrow peak in the CCF is due to the delayed reprocessed component, enhanced by the increased X-ray emission.

Early dynamical evolution of substructured stellar clusters

NASA Astrophysics Data System (ADS)

Dorval, Julien; Boily, Christian

2015-08-01

It is now widely accepted that stellar clusters form with a high level of substructure (Kuhn et al. 2014, Bate 2009), inherited from the molecular cloud and the star formation process. Evidence from observations and simulations also indicate the stars in such young clusters form a subvirial system (Kirk et al. 2007, Maschberger et al. 2010). The subsequent dynamical evolution can cause important mass loss, ejecting a large part of the birth population in the field. It can also imprint the stellar population and still be inferred from observations of evolved clusters. Nbody simulations allow a better understanding of these early twists and turns, given realistic initial conditions. Nowadays, substructured, clumpy young clusters are usually obtained through pseudo-fractal growth (Goodwin et al. 2004) and velocity inheritance. Such models are visually realistics and are very useful, they are however somewhat artificial in their velocity distribution. I introduce a new way to create clumpy initial conditions through a "Hubble expansion" which naturally produces self consistent clumps, velocity-wise. A velocity distribution analysis shows the new method produces realistic models, consistent with the dynamical state of the newly created cores in hydrodynamic simulation of cluster formation (Klessen & Burkert 2000). I use these initial conditions to investigate the dynamical evolution of young subvirial clusters, up to 80000 stars. I find an overall soft evolution, with hierarchical merging leading to a high level of mass segregation. I investigate the influence of the mass function on the fate of the cluster, specifically on the amount of mass loss induced by the early violent relaxation. Using a new binary detection algorithm, I also find a strong processing of the native binary population.

Fluid dynamics of stellar jets in real time: Third Epoch Hubble Space Telescope images of HH 1, HH 34, AND HH 47

DOE PAGES

Hartigan, P.; Frank, A.; Foster, J. M.; ...

2011-07-01

We present new, third-epoch Hubble Space Telescope Hα and [S II] images of three Herbig-Haro (HH) jets (HH 1&2, HH 34, and HH 47) and compare the new images with those from previous epochs. The high spatial resolution, coupled with a time series whose cadence is of order both the hydrodynamic and radiative cooling timescales of the flow, allows us to follow the hydrodynamic/magnetohydrodynamic evolution of an astrophysical plasma system in which ionization and radiative cooling play significant roles. Cooling zones behind the shocks are resolved, so it is possible to identify which way material flows through a given shockmore » wave. The images show that heterogeneity is paramount in these jets, with clumps dominating the morphologies of both bow shocks and their Mach disks. This clumpiness exists on scales smaller than the jet widths and determines the behavior of many of the features in the jets. Evidence also exists for considerable shear as jets interact with their surrounding molecular clouds, and in several cases we observe shock waves as they form and fade where material emerges from the source and as it proceeds along the beam of the jet. Fine structure within two extended bow shocks may result from Mach stems that form at the intersection points of oblique shocks within these clumpy objects. Taken altogether, these observations represent the most significant foray thus far into the time domain for stellar jets, and comprise one of the richest data sets in existence for comparing the behavior of a complex astrophysical plasma flow with numerical simulations and laboratory experiments.« less

The Structure of the Young Star Cluster NGC 6231. II. Structure, Formation, and Fate

NASA Astrophysics Data System (ADS)

Kuhn, Michael A.; Getman, Konstantin V.; Feigelson, Eric D.; Sills, Alison; Gromadzki, Mariusz; Medina, Nicolás; Borissova, Jordanka; Kurtev, Radostin

2017-12-01

The young cluster NGC 6231 (stellar ages ˜2-7 Myr) is observed shortly after star formation activity has ceased. Using the catalog of 2148 probable cluster members obtained from Chandra, VVV, and optical surveys (Paper I), we examine the cluster’s spatial structure and dynamical state. The spatial distribution of stars is remarkably well fit by an isothermal sphere with moderate elongation, while other commonly used models like Plummer spheres, multivariate normal distributions, or power-law models are poor fits. The cluster has a core radius of 1.2 ± 0.1 pc and a central density of ˜200 stars pc-3. The distribution of stars is mildly mass segregated. However, there is no radial stratification of the stars by age. Although most of the stars belong to a single cluster, a small subcluster of stars is found superimposed on the main cluster, and there are clumpy non-isotropic distributions of stars outside ˜4 core radii. When the size, mass, and age of NGC 6231 are compared to other young star clusters and subclusters in nearby active star-forming regions, it lies at the high-mass end of the distribution but along the same trend line. This could result from similar formation processes, possibly hierarchical cluster assembly. We argue that NGC 6231 has expanded from its initial size but that it remains gravitationally bound.

Warm water vapour in the sooty outflow from a luminous carbon star.

PubMed

Decin, L; Agúndez, M; Barlow, M J; Daniel, F; Cernicharo, J; Lombaert, R; De Beck, E; Royer, P; Vandenbussche, B; Wesson, R; Polehampton, E T; Blommaert, J A D L; De Meester, W; Exter, K; Feuchtgruber, H; Gear, W K; Gomez, H L; Groenewegen, M A T; Guélin, M; Hargrave, P C; Huygen, R; Imhof, P; Ivison, R J; Jean, C; Kahane, C; Kerschbaum, F; Leeks, S J; Lim, T; Matsuura, M; Olofsson, G; Posch, T; Regibo, S; Savini, G; Sibthorpe, B; Swinyard, B M; Yates, J A; Waelkens, C

2010-09-02

The detection of circumstellar water vapour around the ageing carbon star IRC +10216 challenged the current understanding of chemistry in old stars, because water was predicted to be almost absent in carbon-rich stars. Several explanations for the water were postulated, including the vaporization of icy bodies (comets or dwarf planets) in orbit around the star, grain surface reactions, and photochemistry in the outer circumstellar envelope. With a single water line detected so far from this one carbon-rich evolved star, it is difficult to discriminate between the different mechanisms proposed. Here we report the detection of dozens of water vapour lines in the far-infrared and sub-millimetre spectrum of IRC +10216 using the Herschel satellite. This includes some high-excitation lines with energies corresponding to approximately 1,000 K, which can be explained only if water is present in the warm inner sooty region of the envelope. A plausible explanation for the warm water appears to be the penetration of ultraviolet photons deep into a clumpy circumstellar envelope. This mechanism also triggers the formation of other molecules, such as ammonia, whose observed abundances are much higher than hitherto predicted.

Cooling Timescale of Dust Tori in Dying Active Galactic Nuclei

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

Ichikawa, Kohei; Tazaki, Ryo, E-mail: k.ichikawa@astro.columbia.edu

We estimate the dust torus cooling timescale once the active galactic nucleus (AGN) is quenched. In a clumpy torus system, once the incoming photons are suppressed, the cooling timescale of one clump from T {sub dust} = 1000 K to several 10 K is less than 10 years, indicating that the dust torus cooling time is mainly governed by the light crossing time of the torus from the central engine. After considering the light crossing time of the torus, the AGN torus emission at 12 μ m becomes over two orders of magnitude fainter within 100 years after the quenching.more » We also propose that those “dying” AGNs could be found using the AGN indicators with a different physical scale R such as 12 μ m band luminosity tracing AGN torus ( R ∼ 10 pc) and the optical [O iii] λ 5007 emission line tracing narrow line regions ( R = 10{sup 2–4} pc).« less

Resolving the Circumgalactic Medium in the NEPHTHYS Simulations

NASA Astrophysics Data System (ADS)

Richardson, Mark Lawrence Albert; Devriendt, Julien; Slyz, Adrianne; Rosdahl, Karl Joakim; Kimm, Taysun

2018-01-01

NEPHTHYS is a RAMSES Cosmological-zoom galaxy simulation suite investigating the impact of stellar feedback (winds, radiation, and type Ia and II SNe) on z > 1 ~L* galaxies and their environments. NEPHTHYS has ~10 pc resolution in the galaxy, where the scales driving star formation and the interaction of stellar feedback with the ISM can begin to be resolved. As outflows, winds, and radiation permeate through the circumgalactic medium (CGM) they can heat or cool gas, and deposit metals throughout the CGM. Such material in the CGM is seen by spectroscopic studies of distant quasars, where CGM gas of foreground galaxies is observed in absorption. It is still unclear what the origin and evolution of this gas is. To help answer this, NEPHTHYS includes additional refinement in the CGM, refining it to an unrivaled 80 pc resolution. I will discuss how this extra resolution is crucial for resolving the complex structure of outflows and accretion in the CGM. Specifically, the metal mass and covering fraction of metals and high energy ions is increased, while the better resolved outflows leads to a decrease in the overall baryon content of galaxy halos, and individual outflow events can have larger velocities. Our results suggest that absorption observations of CGM are tracing a clumpy column of gas with multiple kinematic components.

A spin-down clock for cool stars from observations of a 2.5-billion-year-old cluster.

PubMed

Meibom, Søren; Barnes, Sydney A; Platais, Imants; Gilliland, Ronald L; Latham, David W; Mathieu, Robert D

2015-01-29

The ages of the most common stars--low-mass (cool) stars like the Sun, and smaller--are difficult to derive because traditional dating methods use stellar properties that either change little as the stars age or are hard to measure. The rotation rates of all cool stars decrease substantially with time as the stars steadily lose their angular momenta. If properly calibrated, rotation therefore can act as a reliable determinant of their ages based on the method of gyrochronology. To calibrate gyrochronology, the relationship between rotation period and age must be determined for cool stars of different masses, which is best accomplished with rotation period measurements for stars in clusters with well-known ages. Hitherto, such measurements have been possible only in clusters with ages of less than about one billion years, and gyrochronology ages for older stars have been inferred from model predictions. Here we report rotation period measurements for 30 cool stars in the 2.5-billion-year-old cluster NGC 6819. The periods reveal a well-defined relationship between rotation period and stellar mass at the cluster age, suggesting that ages with a precision of order 10 per cent can be derived for large numbers of cool Galactic field stars.

Investigating the 3D Structure of the Winds of Hot Supergiants

NASA Astrophysics Data System (ADS)

Klement, Robert

2018-04-01

An observational effort targeting supergiant stars of spectral classes B and A has been started using the VEGA high spectral resolution visible beam combiner at the CHARA array. The H-alpha emission from the structured stellar winds was resolved with respect to the surrounding continuum, showing signs of inhomogenities in the circumstellar environments as well as temporal variability on different time scales. We have begun a radiative transfer modelling effort to investigate the clumpy structure of the stellar winds and the origin of the inhomogenities, probably linked to the stellar photosphere features.

Stacked Star Formation Rate Profiles of Bursty Galaxies Exhibit “Coherent” Star Formation

NASA Astrophysics Data System (ADS)

Orr, Matthew E.; Hayward, Christopher C.; Nelson, Erica J.; Hopkins, Philip F.; Faucher-Giguère, Claude-André; Kereš, Dušan; Chan, T. K.; Schmitz, Denise M.; Miller, Tim B.

2017-11-01

In a recent work based on 3200 stacked Hα maps of galaxies at z˜ 1, Nelson et al. find evidence for “coherent star formation”: the stacked star formation rate (SFR) profiles of galaxies above (below) the “star formation main sequence” (MS) are above (below) that of galaxies on the MS at all radii. One might interpret this result as inconsistent with highly bursty star formation and evidence that galaxies evolve smoothly along the MS rather than crossing it many times. We analyze six simulated galaxies at z˜ 1 from the Feedback in Realistic Environments (FIRE) project in a manner analogous to the observations to test whether the above interpretations are correct. The trends in stacked SFR profiles are qualitatively consistent with those observed. However, SFR profiles of individual galaxies are much more complex than the stacked profiles: the former can be flat or even peak at large radii because of the highly clustered nature of star formation in the simulations. Moreover, the SFR profiles of individual galaxies above (below) the MS are not systematically above (below) those of MS galaxies at all radii. We conclude that the time-averaged coherent star formation evident stacks of observed galaxies is consistent with highly bursty, clumpy star formation of individual galaxies and is not evidence that galaxies evolve smoothly along the MS.

Orbital Circularization of Hot and Cool Kepler Eclipsing Binaries

NASA Astrophysics Data System (ADS)

Van Eylen, Vincent; Winn, Joshua N.; Albrecht, Simon

2016-06-01

The rate of tidal circularization is predicted to be faster for relatively cool stars with convective outer layers, compared to hotter stars with radiative outer layers. Observing this effect is challenging because it requires large and well-characterized samples that include both hot and cool stars. Here we seek evidence of the predicted dependence of circularization upon stellar type, using a sample of 945 eclipsing binaries observed by Kepler. This sample complements earlier studies of this effect, which employed smaller samples of better-characterized stars. For each Kepler binary we measure e cos ω based on the relative timing of the primary and secondary eclipses. We examine the distribution of e cos ω as a function of period for binaries composed of hot stars, cool stars, and mixtures of the two types. At the shortest periods, hot-hot binaries are most likely to be eccentric; for periods shorter than four days, significant eccentricities occur frequently for hot-hot binaries, but not for hot-cool or cool-cool binaries. This is in qualitative agreement with theoretical expectations based on the slower dissipation rates of hot stars. However, the interpretation of our results is complicated by the largely unknown ages and evolutionary states of the stars in our sample.

Star formation across cosmic time and its influence on galactic dynamics

NASA Astrophysics Data System (ADS)

Freundlich, Jonathan

2015-12-01

Observations show that ten billion years ago, galaxies formed their stars at rates up to twenty times higher than now. As stars are formed from cold molecular gas, a high star formation rate means a significant gas supply, and galaxies near the peak epoch of star formation are indeed much more gas-rich than nearby galaxies. Is the decline of the star formation rate mostly driven by the diminishing cold gas reservoir, or are the star formation processes also qualitatively different earlier in the history of the Universe? Ten billion years ago, young galaxies were clumpy and prone to violent gravitational instabilities, which may have contributed to their high star formation rate. Stars indeed form within giant, gravitationally-bound molecular clouds. But the earliest phases of star formation are still poorly understood. Some scenarii suggest the importance of interstellar filamentary structures as a first step towards core and star formation. How would their filamentary geometry affect pre-stellar cores? Feedback mechanisms related to stellar evolution also play an important role in regulating star formation, for example through powerful stellar winds and supernovae explosions which expel some of the gas and can even disturb the dark matter distribution in which each galaxy is assumed to be embedded. This PhD work focuses on three perspectives: (i) star formation near the peak epoch of star formation as seen from observations at sub-galactic scales; (ii) the formation of pre-stellar cores within the filamentary structures of the interstellar medium; and (iii) the effect of feedback processes resulting from star formation and evolution on the dark matter distribution.

Hubble Space Telescope imaging of Eta Carinae

NASA Technical Reports Server (NTRS)

Hester, J. J.; Westphal, James A.; Light, Robert M.; Currie, Douglas G.; Groth, Edward J.

1991-01-01

New high spatial resolution observations of the material around Eta Carinae, obtained with the Hubble Space Telescope Wide Field/Planetary Camera, are presented. The star Eta Carinae is one of the most massive and luminous stars in the Galaxy, and has been episodically expelling significant quantities of gas over the last few centuries. The morphology of the bright central nebulosity (the homunculus) indicates that it is a thin shell with very well defined edges, and is clumpy on 0.2 arcsec (about 10 to the 16th cm) scales. An extension to the northeast of the star (NN/NS using Walborn's 1976 nomenclature) appears to be a stellar jet and its associated bow shock. The bow shock is notable for an intriguing series of parallel linear features across its face. The S ridge and the W arc appear to be part of a 'cap' of emission located to the SW and behind the star. Together, the NE jet and the SW cap suggest that the symmetry axis for the system runs NE-SW rather than SE-NW, as previously supposed. Overall, the data indicate that the material around the star may represent an oblate shell with polar blowouts, rather than a bipolar flow.

A Circumstellar Disk around HD 169142 in the Mid-Infrared (N-Band)

NASA Astrophysics Data System (ADS)

Okamoto, Yoshiko Kataza; Kataza, Hirokazu; Honda, M.; Yamashita, T.; Fujiyoshi, T.; Miyata, T.; Sako, S.; Fujiwara, H.; Sakon, I.; Fukagawa, M.; Momose, M.; Onaka, T.

2017-07-01

The Herbig Ae star HD 169142 is one of the objects that show complex structure, such as multiple (innermost, middle, and outer) disks, gaps, and unresolved sources. We made N-band (8-13 μm) observations of HD 169142 with the Cooled Mid-Infrared Camera and Spectrometer on the 8.2 m Subaru Telescope. The images are spatially resolved out to an ˜1″ radius in all the observed bands. We made a simple disk model composed of an unresolved central source (representing the innermost disk/halo) and the ring at a radius r ˜ 25 au (corresponding to the inner wall or edge of a middle disk at ˜25-40 au). The radial intensity profile within the central region (≲0.″3 or ≲ 40 au) is well reproduced by the model. Furthermore, we subtracted the model image from the observed one to search for additional structures. In the model-subtracted images, we found an unresolved west source separated by 17.0 ± 2.9 au in the direction of position angle 260° ± 5° from the original emission peak, which is supposed to correspond to the position of the central star, and a bright east arc located at r ˜ 60 au. The west source is different from the L‧-band unresolved source recently found in coronagraphic observations. It could be a structure related to planet formation in the disk, such as a circumplanetary disk or clumpy disk structure. The east arc corresponds to the inner wall or edge of the outer disk. Based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.

Chromospheric activity of cool giant stars

NASA Technical Reports Server (NTRS)

Steiman-Cameron, T. Y.

1986-01-01

During the seventh year of IUE twenty-six spectra of seventeen cool giant stars ranging in spectral type from K3 thru M6 were obtained. Together with spectra of fifteen stars observed during the sixth year of IUE, these low-resolution spectra have been used to: (1) examine chromospheric activity in the program stars and late type giants in general, and (2) evaluate the extent to which nonradiative heating affects the upper levels of cool giant photospheres. The stars observed in this study all have well determined TiO band strengths, angular diameters (determined from lunar occulations), bolometric fluxes, and effective temperatures. Chromospheric activity can therefore be related to effective temperatures providing a clearer picture of activity among cool giant stars than previously available. The stars observed are listed.

Hidden Lair at the Heart of Galaxy NGC 1068

NASA Image and Video Library

2015-12-17

Galaxy NGC 1068 can be seen in close-up in this view from NASA's Hubble Space Telescope. NuSTAR's high-energy X-rays eyes were able to obtain the best view yet into the hidden lair of the galaxy's central, supermassive black hole. This active black hole -- shown as an illustration in the zoomed-in inset -- is one of the most obscured known, meaning that it is surrounded by extremely thick clouds of gas and dust. The NuSTAR data revealed that the torus of gas and dust surrounding the black hole, also referred to as a doughnut, is more clumpy than previously thought. doughnuts around active, supermassive black holes were originally proposed in the mid-1980s to be smooth entities. More recently, researchers have been finding that doughnuts are not so smooth but have lumps. NuSTAR's latest finding shows that this is true for even the thickest of doughnuts. http://photojournal.jpl.nasa.gov/catalog/PIA20058

A Local Laboratory for Studying Positive Feedback from Supermassive Black Holes

NASA Astrophysics Data System (ADS)

Croft, Steve

2016-10-01

AGN feedback is a critical regulator of galaxy growth. As well as curtailing star formation in diffuse, hot gas, it is increasingly understood to sometimes enhance star formation in the clumpy ISM through shock-induced collapse of clouds. Simulations have shown that such positive feedback may play a significant role in determining the stellar populations of galaxies. Minkowsi's Object (MO) provides an excellent local laboratory to probe this poorly-studied process in detail. The detection of a Type II supernova in MO (unexpected given the low mass of MO) suggests that jet-induced star formation may overproduce massive stars, and that models of the initial mass function in such systems may need to be revised. Recent results also suggest that star formation efficiency is enhanced in MO. Using WFC3, we will obtain morphologies, SEDs, H-a luminosities, equivalent widths, sizes, and population synthesis models of star forming regions across MO in order to address these questions, critical for understanding not just this single object, but the general process: 1. Does jet induced star formation change the luminosities and initial mass functions of star clusters? 2. What do the age gradients of the star clusters tell us about the process of conversion of gas (HI, CO) into stars as the radio jet progressed through the parent cloud? Does this match numerical simulations? 3. By using observations to refine simulations, what can we learn about intrinsic properties of these kinds of radio jets, such as propagation speed, age, pressure and jet energy flux?

Analysing neutron star in HESS J1731-347 from thermal emission and cooling theory

NASA Astrophysics Data System (ADS)

Ofengeim, D. D.; Kaminker, A. D.; Klochkov, D.; Suleimanov, V.; Yakovlev, D. G.

2015-12-01

The central compact object in the supernova remnant HESS J1731-347 appears to be the hottest observed isolated cooling neutron star. The cooling theory of neutron stars enables one to explain observations of this star by assuming the presence of strong proton superfluidity in the stellar core and the existence of the surface heat blanketing envelope which almost fully consists of carbon. The cooling model of this star is elaborated to take proper account of the neutrino emission due to neutron-neutron collisions which is not suppressed by proton superfluidity. Using the results of spectral fits of observed thermal spectra for the distance of 3.2 kpc and the cooling theory for the neutron star of age 27 kyr, new constraints on the stellar mass and radius are obtained which are more stringent than those derived from the spectral fits alone.

Self-regulated cooling flows in elliptical galaxies and in cluster cores - Is exclusively low mass star formation really necessary?

NASA Technical Reports Server (NTRS)

Silk, J.; Djorgovski, S.; Wyse, R. F. G.; Bruzual A., G.

1986-01-01

A self-consistent treatment of the heating by supernovae associated with star formation in a spherically symmetric cooling flow in a cluster core or elliptical galaxy is presented. An initial stellar mass function similar to that in the solar neighborhood is adopted. Inferred star-formation rates, within the cooling region - typically the inner 100 kpc around dominant galaxies at the centers of cooling flows in XD clusters - are reduced by about a factor of 2, relative to rates inferred when the heat input from star formation is ignored. Truncated initial mass functions (IMFs) are also considered, in which massive star formation is suppressed in accordance with previous treatments, and colors are predicted for star formation in cooling flows associated with central dominant elliptical galaxies and with isolated elliptical galaxies surrounded by gaseous coronae. The low inferred cooling-flow rates around isolated elliptical galaxies are found to be insensitive to the upper mass cutoff in the IMF, provided that the upper mass cutoff exceeds 2 M solar mass. Comparison with observed colors favors a cutoff in the IMF above 1 M solar mass in at least two well-studied cluster cooling flows, but a normal IMF cannot be excluded definitively. Models for NGC 1275 support a young (less than about 3 Gyr) cooling flow. As for the isolated elliptical galaxies, the spread in colors is consistent with a normal IMF. A definitive test of the IMF arising via star formation in cooling flows requires either UV spectral data or supernova searches in the cooling-flow-centered galaxies.

MUSE discovers perpendicular arcs in the inner filament of Centaurus A

NASA Astrophysics Data System (ADS)

Hamer, S.; Salomé, P.; Combes, F.; Salomé, Q.

2015-03-01

Context. Evidence of active galactic nuclei (AGN) interaction with the intergalactic medium is observed in some galaxies and many cool core clusters. Radio jets are suspected to dig large cavities into the surrounding gas. In most cases, very large optical filaments (several kpc) are also seen all around the central galaxy. The origin of these filaments is still not understood. Star-forming regions are sometimes observed inside the filaments and are interpreted as evidence of positive feedback (AGN-triggered star formation). Aims: Centaurus A is a very nearby galaxy with huge optical filaments aligned with the AGN radio-jet direction. Here, we searched for line ratio variations along the filaments, kinematic evidence of shock-broadend line widths, and large-scale dynamical structures. Methods: We observed a 1' × 1' region around the so-called inner filament of Cen A with the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope (VLT) during the Science Verification period. Results: (i) The brightest lines detected are the Hαλ6562.8, [NII]λ6583, [OIII]λ4959+5007 and [SII]λ6716+6731. MUSE shows that the filaments are made of clumpy structures inside a more diffuse medium aligned with the radio-jet axis. We find evidence of shocked shells surrounding the star-forming clumps from the line profiles, suggesting that the star formation is induced by shocks. The clump line ratios are best explained by a composite of shocks and star formation illuminated by a radiation cone from the AGN. (ii) We also report a previously undetected large arc-like structure: three streams running perpendicular to the main filament; they are kinematically, morphologically, and excitationally distinct. The clear difference in the excitation of the arcs and clumps suggests that the arcs are very likely located outside of the radiation cone and match the position of the filament only in projection. The three arcs are thus most consistent with neutral material swept along by a backflow of the jet plasma from the AGN outburst that is ionised through a difuse radiation field with a low-ionisation parameter that continues to excite gas away from the radiation cone. Appendices are available in electronic form at http://www.aanda.org

REVIEWS OF TOPICAL PROBLEMS: Cooling of neutron stars and superfluidity in their cores

NASA Astrophysics Data System (ADS)

Yakovlev, Dmitrii G.; Levenfish, Kseniya P.; Shibanov, Yurii A.

1999-08-01

We study the heat capacity and neutrino emission reactions (direct and modified Urca processes, nucleon-nucleon bremsstrahlung, Cooper pairing of nucleons) in the supranuclear density matter of neutron star cores with superfluid neutrons and protons. Various superfluidity types are analysed (singlet-state pairing and two types of triplet-state pairing, without and with gap nodes at the nucleon Fermi surface). The results are used for cooling simulations of isolated neutron stars. Both the standard cooling and the cooling enhanced by the direct Urca process are strongly affected by nucleon superfluidity. Comparison of the cooling theory of isolated neutron stars with observations of their thermal radiation may give stringent constraints on the critical temperatures of the neutron and proton superfluidities in the neutron star cores.

ORBITAL CIRCULARIZATION OF HOT AND COOL KEPLER ECLIPSING BINARIES

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

Eylen, Vincent Van; Albrecht, Simon; Winn, Joshua N., E-mail: vincent@phys.au.dk

The rate of tidal circularization is predicted to be faster for relatively cool stars with convective outer layers, compared to hotter stars with radiative outer layers. Observing this effect is challenging because it requires large and well-characterized samples that include both hot and cool stars. Here we seek evidence of the predicted dependence of circularization upon stellar type, using a sample of 945 eclipsing binaries observed by Kepler . This sample complements earlier studies of this effect, which employed smaller samples of better-characterized stars. For each Kepler binary we measure e cos ω based on the relative timing of themore » primary and secondary eclipses. We examine the distribution of e cos ω as a function of period for binaries composed of hot stars, cool stars, and mixtures of the two types. At the shortest periods, hot–hot binaries are most likely to be eccentric; for periods shorter than four days, significant eccentricities occur frequently for hot–hot binaries, but not for hot–cool or cool–cool binaries. This is in qualitative agreement with theoretical expectations based on the slower dissipation rates of hot stars. However, the interpretation of our results is complicated by the largely unknown ages and evolutionary states of the stars in our sample.« less

Exploring the Dynamics of Exoplanetary Systems in a Young Stellar Cluster

NASA Astrophysics Data System (ADS)

Thornton, Jonathan Daniel; Glaser, Joseph Paul; Wall, Joshua Edward

2018-01-01

I describe a dynamical simulation of planetary systems in a young star cluster. One rather arbitrary aspect of cluster simulations is the choice of initial conditions. These are typically chosen from some standard model, such as Plummer or King, or from a “fractal” distribution to try to model young clumpy systems. Here I adopt the approach of realizing an initial cluster model directly from a detailed magnetohydrodynamical model of cluster formation from a 1000-solar-mass interstellar gas cloud, with magnetic fields and radiative and wind feedback from massive stars included self-consistently. The N-body simulation of the stars and planets starts once star formation is largely over and feedback has cleared much of the gas from the region where the newborn stars reside. It continues until the cluster dissolves in the galactic field. Of particular interest is what would happen to the free-floating planets created in the gas cloud simulation. Are they captured by a star or are they ejected from the cluster? This method of building a dynamical cluster simulation directly from the results of a cluster formation model allows us to better understand the evolution of young star clusters and enriches our understanding of extrasolar planet development in them. These simulations were performed within the AMUSE simulation framework, and combine N-body, multiples and background potential code.

NGC 7538 IRS. 1. Interaction of a Polarized Dust Spiral and a Molecular Outflow

NASA Astrophysics Data System (ADS)

Wright, M. C. H.; Hull, Charles L. H.; Pillai, Thushara; Zhao, Jun-Hui; Sandell, Göran

2014-12-01

We present dust polarization and CO molecular line images of NGC 7538 IRS 1. We combined data from the Submillimeter Array, the Combined Array for Research in Millimeter-wave Astronomy, and the James Clerk Maxwell Telescope to make images with ~2.''5 resolution at 230 and 345 GHz. The images show a remarkable spiral pattern in both the dust polarization and molecular outflow. These data dramatically illustrate the interplay between a high infall rate onto IRS 1 and a powerful outflow disrupting the dense, clumpy medium surrounding the star. The images of the dust polarization and the CO outflow presented here provide observational evidence for the exchange of energy and angular momentum between the infall and the outflow. The spiral dust pattern, which rotates through over 180° from IRS 1, may be a clumpy filament wound up by conservation of angular momentum in the infalling material. The redshifted CO emission ridge traces the dust spiral closely through the MM dust cores, several of which may contain protostars. We propose that the CO maps the boundary layer where the outflow is ablating gas from the dense gas in the spiral.

INTEGRAL Long-Term Monitoring of the Supergiant Fast X-Ray Transient XTE J1739-302

NASA Technical Reports Server (NTRS)

Blay, P.; Martinez-Nunez, S.; Negueruela, I.; Pottschmidt, K.; Smith, D. M.; Torrejon, J. M.; Reig, P.; Kretschmar, P.; Kreykenbohm, I.

2008-01-01

Context. In the past few years, a new class of High Mass X-Ray Binaries (HMXRB) has been claimed to exist, the Supergiant Fast X-ray Transients (SFXT). These are X-ray binary systems with a compact companion orbiting a supergiant star which show very short and bright outbursts in a series of activity periods overimposed on longer quiescent periods. Only very recently the first attempts to model the behaviour of these sources have been published, some of them within the framework of accretion from clumpy stellar winds. Aims. Our goal is to analyze the properties of XTE J1739-302/IGR J17391-3021 within the context of the clumpy structure of the supergiant wind. Methods. We have used INTEGRAL and RXTE/PCA observations in order to obtain broad band (1 - 200 keV) spectra and light curves of XTE J1739-302 and investigate its X-ray spectrum and temporal variability. Results. We have found that XTE J1739-302 follows a much more complex behaviour than expected. Far from presenting a regular variability pattern, XTE J1739-302 shows periods of high, intermediate, and low flaring activity.

Cooling of Accretion-Heated Neutron Stars

NASA Astrophysics Data System (ADS)

Wijnands, Rudy; Degenaar, Nathalie; Page, Dany

2017-09-01

We present a brief, observational review about the study of the cooling behaviour of accretion-heated neutron stars and the inferences about the neutron-star crust and core that have been obtained from these studies. Accretion of matter during outbursts can heat the crust out of thermal equilibrium with the core and after the accretion episodes are over, the crust will cool down until crust-core equilibrium is restored. We discuss the observed properties of the crust cooling sources and what has been learned about the physics of neutron-star crusts. We also briefly discuss those systems that have been observed long after their outbursts were over, i.e, during times when the crust and core are expected to be in thermal equilibrium. The surface temperature is then a direct probe for the core temperature. By comparing the expected temperatures based on estimates of the accretion history of the targets with the observed ones, the physics of neutron-star cores can be investigated. Finally, we discuss similar studies performed for strongly magnetized neutron stars in which the magnetic field might play an important role in the heating and cooling of the neutron stars.

MYStIX: Dynamical evolution of young clusters

NASA Astrophysics Data System (ADS)

Kuhn, Michael A.

2014-08-01

The spatial structure of young stellar clusters in Galactic star-forming regions provides insight into these clusters’ dynamical evolution---a topic with implications for open questions in star-formation and cluster survival. The Massive Young Star-Forming Complex Study in Infrared and X-ray (MYStIX) provides a sample of >30,000 young stars in star-forming regions (d<3.6 kpc) that contain at least one O-type star. We use the finite mixture model analysis to identify subclusters of stars and determine their properties: including subcluster radii, intrinsic numbers of stars, central density, ellipticity, obscuration, and age. In 17 MYStIX regions we find 142 subclusters, with a diverse radii and densities and age spreads of up to ~1 Myr in a region. There is a strong negative correlation between subcluster radius and density, which indicates that embedded subclusters expand but also gain stars as they age. Subcluster expansion is also shown by a positive radius--age correlation, which indicates that subclusters are expanding at <1 km/s. The subcluster ellipticity distribution and number--density relation show signs of a hierarchical merger scenario, whereby young stellar clusters are built up through mergers of smaller clumps, causing evolution from a clumpy spatial distribution of stars (seen in some regions) to a simpler distribution of stars (seen in other regions). Many of the simple young stellar clusters show signs of dynamically relaxation, even though they are not old enough for this to have occurred through two-body interactions. However, this apparent contradiction might be explained if small subcluster, which have shorter dynamical relaxation times, can produce dynamically relaxed clusters through hierarchical mergers.

The formation process of the He I lambda 10830 line in cool giant stars

NASA Technical Reports Server (NTRS)

Luttermoser, Donald G.

1993-01-01

The Final Report on the formation process of the He I lambda 10830 line in cool giant stars is presented. The research involves observing a sample of cool giant stars with ROSAT. These stars were selected from the list of bright stars which display He I lambda 10830 in absorption or emission and lie on the cool side of the coronal dividing line. With measured x ray fluxes or upper limits measured by the Position Sensitive Proportional Counter (PSPC), the role x rays play in the formation of this important line was investigated using the non-LTE radiative transfer code PANDORA. Hydrodynamic calculations were performed to investigate the contributions of acoustic wave heating in the formation of this line as well.

Cooling of neutron stars

NASA Technical Reports Server (NTRS)

Pethick, C. J.

1992-01-01

It is at present impossible to predict the interior constitution of neutron stars based on theory and results from laboratory studies. It has been proposed that it is possible to obtain information on neutron star interiors by studying thermal radiation from their surfaces, because neutrino emission rates, and hence the temperature of the central part of a neutron star, depend on the properties of dense matter. The theory predicts that neutron stars cool relatively slowly if their cores are made up of nucleons, and cool faster if the matter is in an exotic state, such as a pion condensate, a kaon condensate, or quark matter. This view has recently been questioned by the discovery of a number of other processes that could lead to copious neutrino emission and rapid cooling.

Cooling of Compact Stars with Nucleon Superfluidity and Quark Superconductivity

NASA Astrophysics Data System (ADS)

Noda, Tsuneo; Hashimoto, Masa-aki; Yasutake, Nobutoshi; Maruyama, Toshiki; Tatsumi, Toshitaka

We show a cooling scenario of compact stars to satisfy recent observations of compact stars. The central density of compact stars can exceed the nuclear density, and it is considered that many hadronic phases appear at such a density. It is discussed that neutron superfluidity (1S0 for lower density, and 3P2 for higher density) and proton superfluidity/superconductivity (1S0) appears in all compact stars. And some "Exotic" states are considered to appear in compact stars, such as meson condensation, hyperon mixing, deconfinement of quarks and quark colour superconductivity. These exotic states appear at the density region above the threshold densities of each state. We demonstrate the thermal evolution of isolated compact stars, adopting the effects of nucleon superfluidity and quark colour superconductivity. We assume large gap energy (Δ > 10 MeV) for colour superconducting quark phase, and include the effects of nucleon superfluidity with parametrised models. We simulate the cooling history of compact stars, and shows that the heavier star does not always cool faster than lighter one, which is determined by the parameters of neutron 3P2 superfluidity.

Radiation hydrodynamical instabilities in cosmological and galactic ionization fronts

NASA Astrophysics Data System (ADS)

Whalen, Daniel J.; Norman, Michael L.

2011-11-01

Ionization fronts, the sharp radiation fronts behind which H/He ionizing photons from massive stars and galaxies propagate through space, were ubiquitous in the universe from its earliest times. The cosmic dark ages ended with the formation of the first primeval stars and galaxies a few hundred Myr after the Big Bang. Numerical simulations suggest that stars in this era were very massive, 25-500 solar masses, with H(II) regions of up to 30,000 light-years in diameter. We present three-dimensional radiation hydrodynamical calculations that reveal that the I-fronts of the first stars and galaxies were prone to violent instabilities, enhancing the escape of UV photons into the early intergalactic medium (IGM) and forming clumpy media in which supernovae later exploded. The enrichment of such clumps with metals by the first supernovae may have led to the prompt formation of a second generation of low-mass stars, profoundly transforming the nature of the first protogalaxies. Cosmological radiation hydrodynamics is unique because ionizing photons coupled strongly to both gas flows and primordial chemistry at early epochs, introducing a hierarchy of disparate characteristic timescales whose relative magnitudes can vary greatly throughout a given calculation. We describe the adaptive multistep integration scheme we have developed for the self-consistent transport of both cosmological and galactic ionization fronts.

The transiting dust clumps in the evolved disc of the Sun-like UXor RZ Psc

PubMed Central

Kenworthy, Matthew A.; Pepper, Joshua; Rodriguez, Joseph E.; Siverd, Robert J.; Stassun, Keivan G.; Wyatt, Mark C.

2017-01-01

RZ Psc is a young Sun-like star, long associated with the UXor class of variable stars, which is partially or wholly dimmed by dust clumps several times each year. The system has a bright and variable infrared excess, which has been interpreted as evidence that the dimming events are the passage of asteroidal fragments in front of the host star. Here, we present a decade of optical photometry of RZ Psc and take a critical look at the asteroid belt interpretation. We show that the distribution of light curve gradients is non-uniform for deep events, which we interpret as possible evidence for an asteroidal fragment-like clump structure. However, the clumps are very likely seen above a high optical depth midplane, so the disc’s bulk clumpiness is not revealed. While circumstantial evidence suggests an asteroid belt is more plausible than a gas-rich transition disc, the evolutionary status remains uncertain. We suggest that the rarity of Sun-like stars showing disc-related variability may arise because (i) any accretion streams are transparent and/or (ii) turbulence above the inner rim is normally shadowed by a flared outer disc. PMID:28280566

The transiting dust clumps in the evolved disc of the Sun-like UXor RZ Psc.

PubMed

Kennedy, Grant M; Kenworthy, Matthew A; Pepper, Joshua; Rodriguez, Joseph E; Siverd, Robert J; Stassun, Keivan G; Wyatt, Mark C

2017-01-01

RZ Psc is a young Sun-like star, long associated with the UXor class of variable stars, which is partially or wholly dimmed by dust clumps several times each year. The system has a bright and variable infrared excess, which has been interpreted as evidence that the dimming events are the passage of asteroidal fragments in front of the host star. Here, we present a decade of optical photometry of RZ Psc and take a critical look at the asteroid belt interpretation. We show that the distribution of light curve gradients is non-uniform for deep events, which we interpret as possible evidence for an asteroidal fragment-like clump structure. However, the clumps are very likely seen above a high optical depth midplane, so the disc's bulk clumpiness is not revealed. While circumstantial evidence suggests an asteroid belt is more plausible than a gas-rich transition disc, the evolutionary status remains uncertain. We suggest that the rarity of Sun-like stars showing disc-related variability may arise because (i) any accretion streams are transparent and/or (ii) turbulence above the inner rim is normally shadowed by a flared outer disc.

The transiting dust clumps in the evolved disc of the Sun-like UXor RZ Psc

NASA Astrophysics Data System (ADS)

Kennedy, Grant M.; Kenworthy, Matthew A.; Pepper, Joshua; Rodriguez, Joseph E.; Siverd, Robert J.; Stassun, Keivan G.; Wyatt, Mark C.

2017-01-01

RZ Psc is a young Sun-like star, long associated with the UXor class of variable stars, which is partially or wholly dimmed by dust clumps several times each year. The system has a bright and variable infrared excess, which has been interpreted as evidence that the dimming events are the passage of asteroidal fragments in front of the host star. Here, we present a decade of optical photometry of RZ Psc and take a critical look at the asteroid belt interpretation. We show that the distribution of light curve gradients is non-uniform for deep events, which we interpret as possible evidence for an asteroidal fragment-like clump structure. However, the clumps are very likely seen above a high optical depth midplane, so the disc's bulk clumpiness is not revealed. While circumstantial evidence suggests an asteroid belt is more plausible than a gas-rich transition disc, the evolutionary status remains uncertain. We suggest that the rarity of Sun-like stars showing disc-related variability may arise because (i) any accretion streams are transparent and/or (ii) turbulence above the inner rim is normally shadowed by a flared outer disc.

Unveiling the nature of bright z ≃ 7 galaxies with the Hubble Space Telescope

NASA Astrophysics Data System (ADS)

Bowler, R. A. A.; Dunlop, J. S.; McLure, R. J.; McLeod, D. J.

2017-04-01

We present new Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3) imaging of 25 extremely luminous (-23.2 ≤ MUV ≲ -21.2) Lyman-break galaxies (LBGs) at z ≃ 7. The sample was initially selected from 1.65 deg2 of ground-based imaging in the UltraVISTA/COSMOS and UDS/SXDS fields, and includes the extreme Lyman α emitters, 'Himiko' and 'CR7'. A deconfusion analysis of the deep Spitzer photometry available suggests that these galaxies exhibit strong rest-frame optical nebular emission lines (EW0(Hβ + [O III]) > 600 Å). We find that irregular, multiple-component morphologies suggestive of clumpy or merging systems are common (fmulti > 0.4) in bright z ≃ 7 galaxies, and ubiquitous at the very bright end (MUV < -22.5). The galaxies have half-light radii in the range r1/2 ˜ 0.5-3 kpc. The size measurements provide the first determination of the size-luminosity relation at z ≃ 7 that extends to MUV ˜ -23. We find the relation to be steep with r1/2 ∝ L1/2. Excluding clumpy, multicomponent galaxies however, we find a shallower relation that implies an increased star formation rate surface density in bright LBGs. Using the new, independent, HST/WFC3 data we confirm that the rest-frame UV luminosity function at z ≃ 7 favours a power-law decline at the bright end, compared to an exponential Schechter function drop-off. Finally, these results have important implications for the Euclid mission, which we predict will detect >1000 similarly bright galaxies at z ≃ 7. Our new HST imaging suggests that the vast majority of these galaxies will be spatially resolved by Euclid, mitigating concerns over dwarf star contamination.

Hubble Space Telescope Hx Imaging of Star-forming Galaxies at z approximately equal to 1-1.5: Evolution in the Size and Luminosity of Giant H II Regions

NASA Technical Reports Server (NTRS)

Livermore, R. C.; Jones, T.; Richard, J.; Bower, R. G.; Ellis, R. S.; Swinbank, A. M.; Rigby, J. R.; Smail, Ian; Arribas, S.; Rodriguez-Zaurin, J.;

Hot Subdwarf Stars Among the Objects Rejected from the PG Catalog: a First Assessment Using GALEX Photometry

NASA Technical Reports Server (NTRS)

Wade, Richard A.; Stark, M. A.; Green, Richard F.; Durrell, Patrick R.

2009-01-01

The hot subdwarf (sd) stars in the Palomar Green (PG) catalog of ultraviolet excess (UVX) objects play a key role in investigations of the frequency and types of binary companions and the distribution of orbital periods. These are important for establishing whether and by which channels the sd stars arise from interactions in close binary systems. It has been suggested that the list of PG sd stars is biased by the exclusion of many stars in binaries, whose spectra show the Ca I1 K line in absorption. A total of 1125 objects that were photometrically selected as candidates were ultimately rejected from the final PG catalog using this K-line criterion. We study 88 of these 'PG-Rejects' (PGRs), to assess whether there are significant numbers of unrecognized sd stars in binaries among the PGR objects. The presence of a sd should cause a large UVX, compared with the cool K-line star. We assemble GALEX, Johnson V, and 2MASS photometry and compare the colors of these PGR objects with those of known sd stars, cool single stars, and hot+cool binaries. Sixteen PGRs were detected in both the far- and near-ultraviolet GALEX passbands. Eleven of these, plus the 72 cases with only an upper limit in the far-ultraviolet band, are interpreted as single cool stars, appropriately rejected by the PG spectroscopy. Of the remaining five stars, three are consistent with being sd stars paired with a cool main sequence companion, while two may be single stars or composite systems of another type. We discuss the implications of these findings for the 1125 PGR objects as a whole. An enlarged study is desirable to increase confidence in these first results and to identify individual sd+cool binaries or other composites for follow-up study. The GALEX AIS data have sufficient sensitivity to carry out this larger study.

High-energy astrophysics: A theoretical analysis of thermal radiation from neutron stars

NASA Technical Reports Server (NTRS)

Applegate, James H.

1994-01-01

The unambiguous detection of thermal radiation from the surface of a cooling neutron star was one of the most anxiously awaited results in neutron star physics. This particular Holy Grail was found by Halpern and Holt, who used ROSAT to detect pulsed X-rays from the gamma-ray source Geminga and demonstrate that it was a neutron star, probably a radio pulsar beamed away from us. At an age of approximately 3.4 x 10(exp 5) years, Geminga is in the photon cooling era. Its surface temperature of 5.2 x 10(exp 5) K can be explained within the contexts of both the slow and fast cooling scenarios. In the slow cooling scenario, the surface temperature is too high unless the specific heat of the interior is reduced by extensive baryon pairing. In the fast cooling scenario, the surface temperature will be much too low unless the fast neutrino cooling is shut off by baryon pairing. Two other pulsars, PSR 0656+14 and PSR 1055-52, have also been detected in thermal X-rays by ROSAT. They are also in the photon cooling era. All of this research's neutron star cooling models to date have used the unmagnetized effective temperature-interior temperature relation for the outer boundary condition. Models are being improved by using published magnetic envelope calculations and assumed geometried for the surface magnetic field to determine local interior temperature-emitted flux relations for the surface of the star.

INTEGRAL and XMM-Newton observations of the puzzling binary system LSI +61 303

NASA Astrophysics Data System (ADS)

Chernyakova, Masha; Neronov, A.; Walter, R.

LSI +61° 303 is one of the few X-ray binaries with Be star companion from which both radio and high-energy gamma-ray emission have been observed. We present XMM-Newton and INTE- GRAL observations which reveal variability of the X-ray spectral index of the system. The X-ray spectrum is hard (photon index Γ ≃ 1.5) during the orbital phases of both high and low X-ray flux. However, the spectrum softens at the moment of transition from high to low X-ray state. The spectrum of the system in the hard X-ray band does not reveal the presence of a cut-off (or, at least a spectral break) at 10-60 keV energies, expected if the compact object is an accreting neu- tron star. The observed spectrum and spectral variability can be explained if the compact object in the system is a rotation powered pulsar. In this case the recently found X-ray spectral variability of the system on the several kiloseconds time scale can be explained by the clumpy structure of the Be star disk.

Mass Loss from Stars: Prospects with ALMA and Other Radio Interferometers

NASA Astrophysics Data System (ADS)

Richards, Anita

2018-04-01

We can now fully resolve a small sample of stars, in general spotty and/or aspherical, with radii larger (as a function of observing wavelength) than the optical or NIR photosphere R*, requiring the full capabilities of ALMA, e-MERLIN, the NG-VLA or SKA with long baselines. ALMA results has confirmed the presence of continuum hot-spots as well as molecular absorption, against surpisingly large stellar diameters. These studies can be used to investigate the transport of mass and energy through the layers above the photosphere, timescales depending on whether radiative, ionisation/recombination effects, or bulk transport dominate. Maser properties can be measured with an order of magnitude higher resolutiong than thermal lines. The clumpiness of the wind could be related to local ejection of mass from the stellar surface. Models now provide the tools to reconstruct physical conditions from multiple maser lines, and could reveal changes associated with the formation of dust and the transition from complicated infall and outflow near the star, to the radially accelerating wind. I will concentrate on practical aspects of current and potential high-resolution observations to these ends.

ζ Oph and the weak-wind problem

NASA Astrophysics Data System (ADS)

Gvaramadze, V. V.; Langer, N.; Mackey, J.

2012-11-01

Mass-loss rate, ?, is one of the key parameters affecting evolution and observational manifestations of massive stars and their impact on the ambient medium. Despite its importance, there is a factor of ˜100 discrepancy between empirical and theoretical ? of late-type O dwarfs, the so-called weak-wind problem. In this Letter, we propose a simple novel method to constrain ? of runaway massive stars through observation of their bow shocks and Strömgren spheres, which might be of decisive importance for resolving the weak-wind problem. Using this method, we found that ? of the well-known runaway O9.5 V star ζ Oph is more than an order of magnitude higher than that derived from ultraviolet (UV) line fitting and is by a factor of 6-7 lower than those based on the theoretical recipe by Vink et al. and the Hα line. The discrepancy between ? derived by our method and that based on UV lines would be even more severe if the stellar wind is clumpy. At the same time, our estimate of ? agrees with that predicted by the moving reversing layer theory by Lucy.

Prospects of detecting baryon and quark superfluidity from cooling neutron stars

PubMed

Page; Prakash; Lattimer; Steiner

2000-09-04

Baryon and quark superfluidity in the cooling of neutron stars are investigated. Future observations will allow us to constrain combinations of the neutron or Lambda-hyperon pairing gaps and the star's mass. However, in a hybrid star with a mixed phase of hadrons and quarks, quark gaps larger than a few tenths of an MeV render quark matter virtually invisible for cooling. If the quark gap is smaller, quark superfluidity could be important, but its effects will be nearly impossible to distinguish from those of other baryonic constituents.

The Radio-X-ray Relation in Cool Stars: Are We Headed Toward a Divorce?

NASA Astrophysics Data System (ADS)

Forbrich, J.; Wolk, S. J.; Güdel, M.; Benz, A.; Osten, R.; Linsky, J. L.; McLean, M.; Loinard, L.; Berger, E.

2011-12-01

This splinter session was devoted to reviewing our current knowledge of correlated X-ray and radio emission from cool stars in order to prepare for new large radio observatories such as the EVLA. A key interest was to discuss why the X-ray and radio luminosities of some cool stars are in clear breach of a correlation that holds for other active stars, the so-called Güdel-Benz relation. This article summarizes the contributions whereas the actual presentations can be accessed on the splinter website.

The spectral energy distribution of the redshift 7.1 quasar ULAS J1120+0641

NASA Astrophysics Data System (ADS)

Barnett, R.; Warren, S. J.; Banerji, M.; McMahon, R. G.; Hewett, P. C.; Mortlock, D. J.; Simpson, C.; Venemans, B. P.; Ota, K.; Shibuya, T.

2015-03-01

We present new observations of the highest-redshift quasar known, ULAS J1120+0641, redshift z = 7.084, obtained in the optical, at near-, mid-, and far-infrared wavelengths, and in the sub-mm. We combine these results with published X-ray and radio observations to create the multiwavelength spectral energy distribution (SED), with the goals of measuring the bolometric luminosity Lbol, and quantifying the respective contributions from the AGN and star formation. We find three components are needed to fit the data over the wavelength range 0.12-1000 μm: the unobscured quasar accretion disk and broad-line region, a dusty clumpy AGN torus, and a cool 47K modified black body to characterise star formation. Despite the low signal-to-noise ratio of the new long-wavelength data, the normalisation of any dusty torus model is constrained within ±40%. We measure a bolometric luminosity Lbol = 2.6 ± 0.6 × 1047 erg s-1 = 6.7 ± 1.6 × 1013 L⊙, to which the three components contribute 31%,32%,3%, respectively, with the remainder provided by the extreme UV < 0.12 μm. We tabulate the best-fit model SED. We use local scaling relations to estimate a star formation rate (SFR) in the range 60-270 M⊙/yr from the [C ii] line luminosity and the 158 μm continuum luminosity. An analysis of the equivalent widths of the [C ii] line in a sample of z> 5.7 quasars suggests that these indicators are promising tools for estimating the SFR in high-redshift quasars in general. At the time observed the black hole was growing in mass more than 100 times faster than the stellar bulge, relative to the mass ratio measured in the local universe, i.e. compared to MBH/Mbulge ≃ 1.4 × 10-3, for ULAS J1120+0641 we measure ṀBH/Ṁbulge ≃ 0.2. Full Table 3 is 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/A31

Effects of neutrino emissivity on the cooling of neutron stars in the presence of a strong magnetic field

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

Coelho, Eduardo Lenho, E-mail: eduardo.coelho@uva.br; Chiapparini, Marcelo; Negreiros, Rodrigo Picanço

One of the most interesting kind of neutron stars are the pulsars, which are highly magnetized neutron stars with fields up to 10{sup 14} G at the surface. The strength of magnetic field in the center of a neutron star remains unknown. According to the scalar virial theorem, magnetic field in the core could be as large as 10{sup 18} G. In this work we study the influence of strong magnetic fields on the cooling of neutron stars coming from direct Urca process. Direct Urca process is an extremely efficient mechanism for cooling a neutron star after its formation. Themore » matter is described using a relativistic mean-field model at zero temperature with eight baryons (baryon octet), electrons and muons. We obtain the relative population of each species of particles as function of baryon density for different magnetic fields. We calculate numerically the cooling of neutron stars for a parametrized magnetic field and compare the results for the case without a magnetic field.« less

Orbital misalignment of the Neptune-mass exoplanet GJ 436b with the spin of its cool star

NASA Astrophysics Data System (ADS)

Bourrier, Vincent; Lovis, Christophe; Beust, Hervé; Ehrenreich, David; Henry, Gregory W.; Astudillo-Defru, Nicola; Allart, Romain; Bonfils, Xavier; Ségransan, Damien; Delfosse, Xavier; Cegla, Heather M.; Wyttenbach, Aurélien; Heng, Kevin; Lavie, Baptiste; Pepe, Francesco

2018-01-01

The angle between the spin of a star and the orbital planes of its planets traces the history of the planetary system. Exoplanets orbiting close to cool stars are expected to be on circular, aligned orbits because of strong tidal interactions with the stellar convective envelope. Spin–orbit alignment can be measured when the planet transits its star, but such ground-based spectroscopic measurements are challenging for cool, slowly rotating stars. Here we report the three-dimensional characterization of the trajectory of an exoplanet around an M dwarf star, derived by mapping the spectrum of the stellar photosphere along the chord transited by the planet. We find that the eccentric orbit of the Neptune-mass exoplanet GJ 436b is nearly perpendicular to the stellar equator. Both eccentricity and misalignment, surprising around a cool star, can result from dynamical interactions (via Kozai migration) with a yet-undetected outer companion. This inward migration of GJ 436b could have triggered the atmospheric escape that now sustains its giant exosphere.

The connection between the peaks in velocity dispersion and star-forming clumps of turbulent galaxies

NASA Astrophysics Data System (ADS)

Oliva-Altamirano, P.; Fisher, D. B.; Glazebrook, K.; Wisnioski, E.; Bekiaris, G.; Bassett, R.; Obreschkow, D.; Abraham, R.

2018-02-01

We present Keck/OSIRIS adaptive optics observations with 150-400 pc spatial sampling of 7 turbulent, clumpy disc galaxies from the DYNAMO sample ($0.07

A COMPARATIVE STUDY OF KNOTS OF STAR FORMATION IN INTERACTING VERSUS SPIRAL GALAXIES

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

Smith, Beverly J.; Olmsted, Susan; Jones, Keith

2016-03-15

Interacting galaxies are known to have higher global rates of star formation on average than normal galaxies, relative to their stellar masses. Using UV and IR photometry combined with new and published Hα images, we have compared the star formation rates (SFRs) of ∼700 star forming complexes in 46 nearby interacting galaxy pairs with those of regions in 39 normal spiral galaxies. The interacting galaxies have proportionally more regions with high SFRs than the spirals. The most extreme regions in the interacting systems lie at the intersections of spiral/tidal structures, where gas is expected to pile up and trigger starmore » formation. Published Hubble Space Telescope images show unusually large and luminous star clusters in the highest luminosity regions. The SFRs of the clumps correlate with measures of the dust attenuation, consistent with the idea that regions with more interstellar gas have more star formation. For the clumps with the highest SFRs, the apparent dust attenuation is consistent with the Calzetti starburst dust attenuation law. This suggests that the high luminosity regions are dominated by a central group of young stars surrounded by a shell of clumpy interstellar gas. In contrast, the lower luminosity clumps are bright in the UV relative to Hα, suggesting either a high differential attenuation between the ionized gas and the stars, or a post-starburst population bright in the UV but faded in Hα. The fraction of the global light of the galaxies in the clumps is higher on average for the interacting galaxies than for the spirals. Thus either star formation in interacting galaxies is “clumpier” on average, or the star forming regions in interacting galaxies are more luminous, dustier, or younger on average.« less

Winds from cool stars

NASA Technical Reports Server (NTRS)

Dupree, A. K.

1995-01-01

Spectral observations of cool stars enable study of the presence and character of winds and the mass loss process in objects with effective temperatures, gravities, and atmospheric compositions which differ from that of the Sun. A wealth of recent spectroscopic measurements from the Hubble Space Telescope, and the Extreme Ultraviolet Explorer complement high resolution ground-based measures in the optical and infrared spectral regions. Such observations when combined with realistic semi-empirical atmospheric modeling allow us to estimate the physical conditions in the atmospheres and winds of many classes of cool stars. Line profiles support turbulent heating and mass motions. In low gravity stars, evidence is found for relatively fast (approximately 200 km s(exp -1)), warm winds with rapid acceleration occurring in the chromosphere. In some cases outflows commensurate with stellar escape velocities are present. Our current understanding of cool star winds will be reviewed including the implications of stellar observations for identification of atmospheric heating and acceleration processes.

Spectroscopic observations of cool degenerate star candidates

NASA Technical Reports Server (NTRS)

Hintzen, P.

1986-01-01

Spectroscopic observations are reported for 23 Luyten Half-Second degenerate star candidates and for 13 Luyten-Palomar common proper-motion pairs containing possible degenerate star components. Twenty-five degenerate stars are identified, 20 of which lack previous spectroscopy. Most of these stars are cool - Luyten color class g or later. One star, LP 77-57, shows broad continuum depressions similar to those in LHS 1126, which Liebert and Dahn attributed to pressure-shifted C2. A second degenerate star, LHS 290, exhibits apparent strong Swan bands which are blueshifted about 75 A. Further observations, including polarimetry and photometry, are required to appraise the spectroscopic peculiarities of these stars. Finally, five cool, sharp-lined DA white dwarfs have been observed to detect lines of metals and to determine line strengths. None of these DAs show signs of Mg b or the G band, and four show no evidence of Ca II K. The attempt to detect Ca MI in the fifth star, G199-71, was inconclusive.

The circumstellar environment of evolved stars as traced by molecules and dust. The diagnostic power of Herschel

NASA Astrophysics Data System (ADS)

Lombaert, Robin

2013-12-01

Low-to-intermediate mass stars end their life on the asymptotic giant branch (AGB), an evolutionary phase in which the star sheds most of its mantle into the circumstellar environment through a stellar wind. This stellar wind expands at relatively low velocities and enriches the interstellar medium with elements newly made in the stellar interior. The physical processes controlling the gas and dust chemistry in the outflow, as well as the driving mechanism of the wind itself, are poorly understood and constitute the broader context of this thesis work. In a first chapter, we consider the thermodynamics of the high-density wind of the oxygen-rich star oh, using observations obtained with the PACS instrument onboard the Herschel Space Telescope. Being one of the most abundant molecules, water vapor can be dominant in the energy balance of the inner wind of these types of stars, but to date, its cooling contribution is poorly understood. We aim to improve the constraints on water properties by careful combination of both dust and gas radiative-transfer models. This unified treatment is needed due to the high sensitivity of water excitation to dust properties. A combination of three types of diagnostics reveals a positive radial gradient of the dust-to-gas ratio in oh. The second chapter deals with the dust chemistry of carbon-rich winds. The 30-mic dust emission feature is commonly identified as due to magnesium sulfide (MgS). However, the lack of short-wavelength measurements of the optical properties of this dust species prohibits the determination of the temperature profile of MgS, and hence its feature strength and shape, questioning whether this species is responsible for the 30-mic feature. By considering the very optically thick wind of the extreme carbon star LL Peg, this problem can be circumvented because in this case the short-wavelength optical properties are not important for the radial temperature distribution. We attribute the 30-mic feature to MgS, but require that the dust species is embedded in a heterogeneous composite grain structure together with carbonaceous compounds. The final chapter considers the circumstellar gas chemistry of carbon-rich AGB stars. The recent discovery of warm water vapor in carbon-rich winds challenges our understanding of chemical processes ongoing in the wind. Two mechanisms for producing warm water were proposed: water formation induced by interstellar ultraviolet photons penetrating into the inner region of a clumpy wind, and water formation induced by shocks passing through the atmospheric and inner-wind molecular gas. A sample of eighteen carbon-rich AGB stars has been observed with the Herschel Space Telescope and offers insights into the dependence of water properties on the stellar and circumstellar conditions. We suggest that both proposed water formation mechanisms must be at work to account for the following findings: 1) warm water is present in all observed carbon stars; 2) water formation efficiency decreases with higher circumstellar column density; 3) water properties strongly depend on the variability characteristics of the AGB stars; and 4) a positive water abundance gradient is present up to at most ˜ 50 rstar in individual stars.

Crustal Cooling in the Neutron Star Low-Mass X-Ray Binary KS 1731-260

NASA Astrophysics Data System (ADS)

Merritt, Rachael L.

Neutron stars in binary systems can undergo periods of accretion (outburst), where in- falling material heats the crust of the star out of thermal equilibrium with the core. When accretion stops (quiescence), we can directly observe the thermal relaxation of the crust. Crustal cooling of accretion-heated neutron stars provides insight into the stellar interior of neutron stars. The neutron star X-ray transient, KS 1731-260, was in outburst for 12.5 years before returning to quiescence in 2001. Here, we present a 150 ks Chandra observation of KS 1731-260 taken in August 2015, about 14.5 years into quiescence. We find that the neutron star surface temperature is consistent with the previous observation, suggesting the crust has reached thermal equilibrium with the core. Using a theoretical thermal evolution code, we fit the observed cooling curves and constrain the core temperature, composition, and the required level of extra shallow heating.

Clumpy cold dark matter

NASA Technical Reports Server (NTRS)

Silk, Joseph; Stebbins, Albert

1993-01-01

A study is conducted of cold dark matter (CDM) models in which clumpiness will inhere, using cosmic strings and textures suited to galaxy formation. CDM clumps of 10 million solar mass/cu pc density are generated at about z(eq) redshift, with a sizable fraction surviving. Observable implications encompass dark matter cores in globular clusters and in galactic nuclei. Results from terrestrial dark matter detection experiments may be affected by clumpiness in the Galactic halo.

IUE observations of the atmospheric eclipsing binary system Zeta Aurigae

NASA Technical Reports Server (NTRS)

Champman, R. D.

1980-01-01

IUE observations of the eclipsing binary system Zeta Aurigae made prior to and during the eclipse of the relatively small B8 V star by the cool supergiant star (spectral type K2 II) are reported. Spectral lines produced by the absorption of B star radiation in the atmosphere of the K star during eclipse can be used as a probe of the extended K star atmosphere, due to the negligible cool star continuum in the 1200-3200 A region. Spectra taken prior to eclipse are found to be similar to those of the single B8 V star 64 Ori, with the exception of very strong multi-component absorption lines of Si II, Si IV, C IV and the Mg resonance doublet with strong P Cygni profiles, indicating a double shell. Absorption lines including those corresponding to Al II, Al III, Cr II, Mn II, Fe II, Ni II and Ca II are observed to increase in strength and number as the eclipse progresses, with high-ionization-potential lines formed far from the K star, possibly in a shock wave, and low-ionization potential lines, formed in cool plasma, probably a cool wind, nearer to the K star. Finally, an emission-line spectra with lines corresponding to those previously observed in absorption is noted at the time the B-star continuum had disappeared.

THE OCCURRENCE RATE OF SMALL PLANETS AROUND SMALL STARS

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

Dressing, Courtney D.; Charbonneau, David, E-mail: cdressing@cfa.harvard.edu

We use the optical and near-infrared photometry from the Kepler Input Catalog to provide improved estimates of the stellar characteristics of the smallest stars in the Kepler target list. We find 3897 dwarfs with temperatures below 4000 K, including 64 planet candidate host stars orbited by 95 transiting planet candidates. We refit the transit events in the Kepler light curves for these planet candidates and combine the revised planet/star radius ratios with our improved stellar radii to revise the radii of the planet candidates orbiting the cool target stars. We then compare the number of observed planet candidates to themore » number of stars around which such planets could have been detected in order to estimate the planet occurrence rate around cool stars. We find that the occurrence rate of 0.5-4 R{sub Circled-Plus} planets with orbital periods shorter than 50 days is 0.90{sup +0.04}{sub -0.03} planets per star. The occurrence rate of Earth-size (0.5-1.4 R{sub Circled-Plus }) planets is constant across the temperature range of our sample at 0.51{sub -0.05}{sup +0.06} Earth-size planets per star, but the occurrence of 1.4-4 R{sub Circled-Plus} planets decreases significantly at cooler temperatures. Our sample includes two Earth-size planet candidates in the habitable zone, allowing us to estimate that the mean number of Earth-size planets in the habitable zone is 0.15{sup +0.13}{sub -0.06} planets per cool star. Our 95% confidence lower limit on the occurrence rate of Earth-size planets in the habitable zones of cool stars is 0.04 planets per star. With 95% confidence, the nearest transiting Earth-size planet in the habitable zone of a cool star is within 21 pc. Moreover, the nearest non-transiting planet in the habitable zone is within 5 pc with 95% confidence.« less

Magnetic neutron star cooling and microphysics

NASA Astrophysics Data System (ADS)

Potekhin, A. Y.; Chabrier, G.

2018-01-01

Aims: We study the relative importance of several recent updates of microphysics input to the neutron star cooling theory and the effects brought about by superstrong magnetic fields of magnetars, including the effects of the Landau quantization in their crusts. Methods: We use a finite-difference code for simulation of neutron-star thermal evolution on timescales from hours to megayears with an updated microphysics input. The consideration of short timescales (≲1 yr) is made possible by a treatment of the heat-blanketing envelope without the quasistationary approximation inherent to its treatment in traditional neutron-star cooling codes. For the strongly magnetized neutron stars, we take into account the effects of Landau quantization on thermodynamic functions and thermal conductivities. We simulate cooling of ordinary neutron stars and magnetars with non-accreted and accreted crusts and compare the results with observations. Results: Suppression of radiative and conductive opacities in strongly quantizing magnetic fields and formation of a condensed radiating surface substantially enhance the photon luminosity at early ages, making the life of magnetars brighter but shorter. These effects together with the effect of strong proton superfluidity, which slows down the cooling of kiloyear-aged neutron stars, can explain thermal luminosities of about a half of magnetars without invoking heating mechanisms. Observed thermal luminosities of other magnetars are still higher than theoretical predictions, which implies heating, but the effects of quantizing magnetic fields and baryon superfluidity help to reduce the discrepancy.

Physics of Cool Stars: Densities, Sizes, and Energetics

NASA Technical Reports Server (NTRS)

Dupree, Andrea K.

2001-01-01

The ORFEUS 1 (Orbiting and Retrievable Far and Extreme Ultraviolet Spectrometer) telescope obtained far ultraviolet spectra (lambda-lambda 912-1218) of luminous cool stars as a part of our observing program. Two classes of objects were measured: luminous single stars beta Dra (HD 159181) and two hybrid stars alpha Aqr (HD 209750) and alpha TrA (HD 150798) and two active binary systems: 44i Boo and UX Ari.

Infrared spectrum of an extremely cool white-dwarf star

PubMed

Hodgkin; Oppenheimer; Hambly; Jameson; Smartt; Steele

2000-01-06

White dwarfs are the remnant cores of stars that initially had masses of less than 8 solar masses. They cool gradually over billions of years, and have been suggested to make up much of the 'dark matter' in the halo of the Milky Way. But extremely cool white dwarfs have proved difficult to detect, owing to both their faintness and their anticipated similarity in colour to other classes of dwarf stars. Recent improved models indicate that white dwarfs are much more blue than previously supposed, suggesting that the earlier searches may have been looking for the wrong kinds of objects. Here we report an infrared spectrum of an extremely cool white dwarf that is consistent with the new models. We determine the star's temperature to be 3,500 +/- 200 K, making it the coolest known white dwarf. The kinematics of this star indicate that it is in the halo of the Milky Way, and the density of such objects implied by the serendipitous discovery of this star is consistent with white dwarfs dominating the dark matter in the halo.

Cool Star Marked by Long-Lived Storm Artist Concept

NASA Image and Video Library

2015-12-10

This illustration shows a cool star, called W1906+40, marked by a raging storm near one of its poles. The storm is thought to be similar to the Great Red Spot on Jupiter. Scientists discovered it using NASA's Kepler and Spitzer space telescopes. The location of the storm is estimated to be near the north pole of the star based on computer models of the data. The telescopes cannot see the storm itself, but learned of its presence after observing how the star's light changes over time. The storm travels around with the star, making a full lap about every 9 hours. When it passes into a telescope's field of view, it causes light of particular infrared and visible wavelengths to dip in brightness. The storm has persisted for at least two years. Astronomers aren't sure why it has lasted so long. While planets are known to have cloudy storms, this is the best evidence yet for a star with the same type of storm. The star, W1906+40, belongs to a thermally cool class of objects called L-dwarfs. Some L-dwarfs are considered stars because they fuse atoms and generate light, as our sun does, while others, called brown dwarfs, are known as "failed stars" for their lack of atomic fusion. The L-dwarf W1906+40 is thought to be a star based on estimates of its age (the older the L-dwarf, the more likely it is a star). Its temperature is about 2,200 Kelvin (3,500 degrees Fahrenheit). That may sound scorching hot, but as far as stars go, it is relatively cool. Cool enough, in fact, for clouds to form in its atmosphere. W1906+40 is located 53 light-years away in the constellation Lyra. http://photojournal.jpl.nasa.gov/catalog/PIA20055

The Not So Simple Globular Cluster ω Cen. I. Spatial Distribution of the Multiple Stellar Populations

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

Calamida, A.; Saha, A.; Strampelli, G.

2017-04-01

We present a multi-band photometric catalog of ≈1.7 million cluster members for a field of view of ≈2° × 2° across ω Cen. Photometry is based on images collected with the Dark Energy Camera on the 4 m Blanco telescope and the Advanced Camera for Surveys on the Hubble Space Telescope . The unprecedented photometric accuracy and field coverage allowed us, for the first time, to investigate the spatial distribution of ω Cen multiple populations from the core to the tidal radius, confirming its very complex structure. We found that the frequency of blue main-sequence stars is increasing compared to red main-sequencemore » stars starting from a distance of ≈25′ from the cluster center. Blue main-sequence stars also show a clumpy spatial distribution, with an excess in the northeast quadrant of the cluster pointing toward the direction of the Galactic center. Stars belonging to the reddest and faintest red-giant branch also show a more extended spatial distribution in the outskirts of ω Cen, a region never explored before. Both these stellar sub-populations, according to spectroscopic measurements, are more metal-rich compared to the cluster main stellar population. These findings, once confirmed, make ω Cen the only stellar system currently known where metal-rich stars have a more extended spatial distribution compared to metal-poor stars. Kinematic and chemical abundance measurements are now needed for stars in the external regions of ω Cen to better characterize the properties of these sub-populations.« less

Constraints on Bygone Nucleosynthesis of Accreting Neutron Stars

DOE PAGES

Meisel, Zach; Deibel, Alex

2017-03-06

Nuclear burning near the surface of an accreting neutron star produces ashes that, when compressed deeper by further accretion, alter the star’s thermal and compositional structure. Bygone nucleosynthesis can be constrained by the impact of compressed ashes on the thermal relaxation of quiescent neutron star transients. In particular, Urca cooling nuclei pairs in nuclear burning ashes that cool the neutron star crust via neutrino emission from e --capture/β --decay cycles and provide signatures of prior nuclear burning over the ~century timescales it takes to accrete to the e --capture depth of the strongest cooling pairs. By using crust cooling modelsmore » of the accreting neutron star transient MAXI J0556-332, we show that this source likely lacked Type I X-ray bursts and superbursts ≳120 years ago. Reduced nuclear physics uncertainties in rp-process reaction rates and e --capture weak transition strengths for low-lying transitions will improve nucleosynthesis constraints using this technique.« less

Geminga: A cooling superfluid neutron star

NASA Technical Reports Server (NTRS)

Page, Dany

1994-01-01

We compare the recent temperature estimate for Geminga with neutron star cooling models. Because of its age (approximately 3.4 x 10(exp 5) yr), Geminga is in the photon cooling era. We show that its surface temperature (approximately 5.2 x 10(exp 5) K) can be understood by both types of neutrino cooling scenarios, i.e., slow neutrino cooling by the modified Urca process or fast neutrino cooling by the direct Urca process or by some exotic matter, and thus does not allow us to discriminate between these two competing schemes. However, for both types of scenarios, agreement with the observed temperature can only be obtained if baryon pairing is present in most, if not all, of the core of the star. Within the slow neutrino cooling scenario, early neutrino cooling is not sufficient to explain the observed low temperature, and extensive pairing in the core is necessary to reduce the specific heat and increase the cooling rate in the present photon cooling era. Within all the fast neutrino cooling scenarios, pairing is necessary throughout the whole core to control the enormous early neutrino emission which, without pairing suppression, would result in a surface temperature at the present time much lower than observed. We also comment on the recent temperature estimates for PSR 0656+14 and PSR 1055-52, which pertain to the same photon cooling era. If one assumes that all neutron stars undergo fast neutrino cooling, then these two objects also provide evidence for extensive baryon pairing in their core; but observational uncertainties also permit a more conservative interpretation, with slow neutrino emission and no pairing at all. We argue though that observational evidence for the slow neutrino cooling model (the 'standard' model) is in fact very dim and that the interpretation of the surface temperature of all neutron stars could be done with a reasonable theoretical a priori within the fast neutrino cooling scenarios only. In this case, Geminga, PSR 0656+14, and PSR 1055-52 all show evidence of baryon pairing down to their very centers.

Supergiants and their shells in young globular clusters

NASA Astrophysics Data System (ADS)

Szécsi, Dorottya; Mackey, Jonathan; Langer, Norbert

2018-04-01

Context. Anomalous surface abundances are observed in a fraction of the low-mass stars of Galactic globular clusters, that may originate from hot-hydrogen-burning products ejected by a previous generation of massive stars. Aims: We aim to present and investigate a scenario in which the second generation of polluted low-mass stars can form in shells around cool supergiant stars within a young globular cluster. Methods: Simulations of low-metallicity massive stars (Mi 150-600 M⊙) show that both core-hydrogen-burning cool supergiants and hot ionizing stellar sources are expected to be present simulaneously in young globular clusters. Under these conditions, photoionization-confined shells form around the supergiants. We have simulated such a shell, investigated its stability and analysed its composition. Results: We find that the shell is gravitationally unstable on a timescale that is shorter than the lifetime of the supergiant, and the Bonnor-Ebert mass of the overdense regions is low enough to allow star formation. Since the low-mass stellar generation formed in this shell is made up of the material lost from the supergiant, its composition necessarily reflects the composition of the supergiant wind. We show that the wind contains hot-hydrogen-burning products, and that the shell-stars therefore have very similar abundance anomalies that are observed in the second generation stars of globular clusters. Considering the mass-budget required for the second generation star-formation, we offer two solutions. Either a top-heavy initial mass function is needed with an index of -1.71 to -2.07. Alternatively, we suggest the shell-stars to have a truncated mass distribution, and solve the mass budget problem by justifiably accounting for only a fraction of the first generation. Conclusions: Star-forming shells around cool supergiants could form the second generation of low-mass stars in Galactic globular clusters. Even without forming a photoionizaton-confined shell, the cool supergiant stars predicted at low-metallicity could contribute to the pollution of the interstellar medium of the cluster from which the second generation was born. Thus, the cool supergiant stars should be regarded as important contributors to the evolution of globular clusters.

On bipolar ejection. [of matter in astronomical systems

NASA Technical Reports Server (NTRS)

Cameron, A. G. W.

1985-01-01

Observations of bipolar outflows, including jets often with clumpy concentrations of matter, have been made for a wide variety of astronomical systems. In most but not all of the systems, an accretion disk is present. It is proposed that the general process responsible for bipolar ejection involves the conversion of rotational energy into magnetic energy, usually in the form of a polar magnetic torus, deep in the interiors of the systems involved. If the buoyancy of the torus resullts in draining the field lines of most of the matter which they thread, then the acceleration of the remaining matter in the toroidal bubble may produce velocities in excess of the escape velocity from the surface of the system. It is contemplated that this process will be repeated many times in most systems. A discussion is given of the application of these ideas to protostars, to stars evolved beyond the main sequence, to neutron stars, and to black holes on both stellar and galactic scales.

The role of orbital dynamics and cloud-cloud collisions in the formation of giant molecular clouds in global spiral structures

NASA Technical Reports Server (NTRS)

Roberts, William W., Jr.; Stewart, Glen R.

1987-01-01

The role of orbit crowding and cloud-cloud collisions in the formation of GMCs and their organization in global spiral structure is investigated. Both N-body simulations of the cloud system and a detailed analysis of individual particle orbits are used to develop a conceptual understanding of how individual clouds participate in the collective density response. Detailed comparisons are made between a representative cloud-particle simulation in which the cloud particles collide inelastically with one another and give birth to and subsequently interact with young star associations and stripped down simulations in which the cloud particles are allowed to follow ballistic orbits in the absence of cloud-cloud collisions or any star formation processes. Orbit crowding is then related to the behavior of individual particle trajectories in the galactic potential field. The conceptual picture of how GMCs are formed in the clumpy ISMs of spiral galaxies is formulated, and the results are compared in detail with those published by other authors.

EVIDENCE FOR MASS EJECTION ASSOCIATED WITH LONG SECONDARY PERIODS IN RED GIANTS

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

Wood, P. R.; Nicholls, C. P., E-mail: wood@mso.anu.edu.a, E-mail: nicholls@mso.anu.edu.a

2009-12-10

Approximately 30% of luminous red giants exhibit a long secondary period (LSP) of variation in their light curves in addition to a shorter primary period of oscillation. The cause of the LSP has so far defied explanation: leading possibilities are binarity and a nonradial mode of oscillation. Here, large samples of red giants in the Large Magellanic Cloud both with and without LSPs are examined for evidence of an 8 or 24 mum mid-IR excess caused by circumstellar dust. It is found that stars with LSPs show a significant mid-IR excess compared to stars without LSPs. Furthermore, the near-IR Jmore » - K color seems unaffected by the presence of the 24 mum excess. These findings indicate that LSPs cause mass ejection from red giants and that the lost mass and circumstellar dust is most likely in either a clumpy or a disk-like configuration. The underlying cause of the LSP and the mass ejection remains unknown.« less

Hubble Space Telescope Imaging of the Mass-losing Supergiant VY Canis Majoris

NASA Astrophysics Data System (ADS)

Kastner, Joel H.; Weintraub, David A.

1998-04-01

The highly luminous M supergiant VY CMa is a massive star that appears to be in its final death throes, losing mass at high rate en route to exploding as a supernova. Subarcsecond-resolution optical images of VY CMa, obtained with the Faint Object Camera (FOC) aboard the Hubble Space Telescope, vividly demonstrate that mass loss from VY CMa is highly anisotropic. In the FOC images, the optical ``star'' VY CMa constitutes the bright, well-resolved core of an elongated reflection nebula. The imaged nebula is ~3" (~4500 AU) in extent and is clumpy and highly asymmetric. The images indicate that the bright core, which lies near one edge of the nebula, is pure scattered starlight. We conclude that at optical wavelengths VY CMa is obscured from view along our line of sight by its own dusty envelope. The presence of the extended reflection nebula then suggests that this envelope is highly flattened and/or that the star is surrounded by a massive circumstellar disk. Such axisymmetric circumstellar density structure should have profound effects on post-red supergiant mass loss from VY CMa and, ultimately, on the shaping of the remnant of the supernova that will terminate its post-main-sequence evolution.

IGR J16318-4848: 7 Years of INTEGRAL Observations

NASA Technical Reports Server (NTRS)

Barragan, Laura; Wilms, Joern; kreykenbohm, Ingo; Hanke, manfred; Fuerst, Felix; Pottschmidt, Katja; Rothschild, Richard

2011-01-01

Since the discovery of IGR 116318-4848 in 2003 January, INTEGRAL has accumulated more than 5.8 Ms in IBIS/ISGRI. We present the first extensive analysis of the archival INTEGRAL data (IBIS/ISGRI, and JEM-X when available) for this source, together with the observations carried out by XMM-Newton (twice in 2003, and twice in 2004) and Suzaku (2006). The source is very variable in the long-term, with periods of low activity, where the source is almost not detected, and flares with a luminosity approximately 10 times greater than its average value (5.4 cts/s). IGR 116318-4848 is a HMXB containing a sgB[e] star and a compact object (most probably a neutron star) deeply embedded in the stellar wind of the mass donor. The variability of the source (also in the short-term) can be ascribed to the wind of the optical star being very clumpy. We study the variation of the spectral parameters in time scales of INTEGRAL revolutions. The photoelectric absorption is, with NH around 10(exp 24)/ square cm, unusually high. During brighter phases the strong K-alpha iron line known from XMM-Newton and Suzaku observations is also detectable with the JEM-X instrument.

Classification of extremely metal-poor stars: absent region in A(C)-[Fe/H] plane and the role of dust cooling

NASA Astrophysics Data System (ADS)

Chiaki, Gen; Tominaga, Nozomu; Nozawa, Takaya

2017-11-01

Extremely metal-poor (EMP) stars are the living fossils with records of chemical enrichment history at the early epoch of galaxy formation. By the recent large observation campaigns, statistical samples of EMP stars have been obtained. This motivates us to reconsider their classification and formation conditions. From the observed lower limits of carbon and iron abundances of Acr(C) ∼ 6 and [Fe/H]cr ∼ -5 for C-enhanced EMP (CE-EMP) and C-normal EMP (CN-EMP) stars, we confirm that gas cooling by dust thermal emission is indispensable for the fragmentation of their parent clouds to form such low mass, i.e. long-lived stars, and that the dominant grain species are carbon and silicate, respectively. We constrain the grain radius r_i^cool of a species i and condensation efficiency fij of a key element j as r_C^cool / f_C,C = 10 {μ m} and r_Sil^cool / f_Sil,Mg = 0.1 {μ m} to reproduce Acr(C) and [Fe/H]cr, which give a universal condition 10[C/H] - 2.30 + 10[Fe/H] > 10-5.07 for the formation of every EMP star. Instead of the conventional boundary [C/Fe] = 0.7 between CE-EMP and CN-EMP stars, this condition suggests a physically meaningful boundary [C/Fe]b = 2.30 above and below which carbon and silicate grains are dominant coolants, respectively.

The Geminga neutron star: Evidence for nucleon superfluidity at very high density

NASA Technical Reports Server (NTRS)

Page, Dany

1993-01-01

A comparison of the recent age and temperature estimates of the Geminga neutron star with cooling models is presented. This star is already in the photon cooling era and it is shown that its temperature can be understood within both the slow and fast neutrino emission scenarios and consequently will not allow discrimination between these two scenarios. However in both cases agreement of the theoretical cooling curves with the observed temperature depends crucially on the presence of nucleon pairing in most, if not all, of the core.

The influence of H2O line blanketing on the spectra of cool dwarf stars

NASA Technical Reports Server (NTRS)

Allard, F.; Hauschildt, P. H.; Miller, S.; Tennyson, J.

1994-01-01

We present our initial results of model atmosphere calculations for cool M dwarfs using an opacity sampling method and a new list of H2O lines. We obtain significantly improved fits to the infrared spectrum of the M dwarf VB10 when compared to earlier models. H2O is by far the dominant opacity source in cool stars. To illustrate this, we show the Rosseland mean of the total extinction under various assumptions. Our calculations demonstrate the importance of a good treatment of the water opacities in cool stars and the improvements possible by using up-to-date data for the water line absorption.

Three very cool degenerate stars in Luyten common proper motion binaries - Implications for the age of the galactic disk

NASA Technical Reports Server (NTRS)

Hintzen, Paul; Oswalt, Terry D.; Liebert, James; Sion, Edward M.

1989-01-01

During the course of a spectroscopic study of Luyten common proper motion (CPM) stars, spectrophotometric observations have been obtained of three binaries containing degenerate stars with estimated absolute magnitudes M(V) of about 16. Each of the three pairs consists of a yellow degenerate star primary and a DC 13 + secondary 1.4-2.3 mag fainter. One of the primary stars is spectral class DC 7, another is a sharp-lined DA 8, and the third shows peculiar broad absorption features which we interpret as pressure-shifted C2 Swan bands. The LP 701 - 69/70 system has survived for over 8 billion years without disruption by passing stars, despite its 1500 a.u. orbital major axis. The three cool degenerate companions nearly double the available sample of stars at the low-luminosity terminus of the white dwarf cooling sequence. These findings appear consistent with the conclusion that degenerate stars in the old disk population have not had time to evolve to a luminosity fainter than M(V) about 16.2.

Legacy ExtraGalactic UV Survey (LEGUS): The HST View of Star Formation in Nearby Galaxies

NASA Astrophysics Data System (ADS)

Calzetti, Daniela; Lee, J. C.; Adamo, A.; Aloisi, A.; Andrews, J. E.; Brown, T. M.; Chandar, R.; Christian, C. A.; Cignoni, M.; Clayton, G. C.; Da Silva, R. L.; de Mink, S. E.; Dobbs, C.; Elmegreen, B.; Elmegreen, D. M.; Evans, A. S.; Fumagalli, M.; Gallagher, J. S.; Gouliermis, D.; Grebel, E.; Herrero-Davo`, A.; Hilbert, B.; Hunter, D. A.; Johnson, K. E.; Kennicutt, R.; Kim, H.; Krumholz, M. R.; Lennon, D. J.; Martin, C. D.; Nair, P.; Nota, A.; Pellerin, A.; Prieto, J.; Regan, M. W.; Sabbi, E.; Schaerer, D.; Schiminovich, D.; Smith, L. J.; Thilker, D. A.; Tosi, M.; Van Dyk, S. D.; Walterbos, R. A.; Whitmore, B. C.; Wofford, A.

2014-01-01

The Treasury program LEGUS (HST/GO-13364) is the first HST UV Atlas of nearby galaxies, and is aimed at the thorough investigation of star formation and its relation with galaxy environment, from the scales of individual stars to those of ~kpc clustered structures. The 154-orbits program is obtaining NUV,U,B,V,I images of 50 star-forming galaxies in the distance range 4-12 Mpc, covering the full range of morphology, star formation rate (SFR), mass, metallicity, internal structure, and interaction state found in the local Universe. The imaging survey will yield accurate recent (<50 Myr) star formation histories (SFHs) from resolved massive stars, and the extinction-corrected ages and masses of star clusters and associations. These extensive inventories of massive stars, clustered systems, and SFHs will be used to: (1) quantify how the clustering of star formation evolves both in space and in time; (2) discriminate among models of star cluster evolution; (3) investigate the effects of SFH on the UV SFR calibrations; (4) explore the impact of environment on star formation and cluster evolution across the full range of galactic and ISM properties. LEGUS observations will inform theories of star formation and galaxy evolution, and improve the understanding of the physical underpinning of the gas-star formation relation and the nature of the clumpy star formation at high redshift. LEGUS will generate the most homogeneous high-resolution, wide-field UV dataset to date, building and expanding on the GALEX legacy. Data products that will be delivered to the community include: catalogs of massive stars and star clusters, catalogs of star cluster properties (ages, masses, extinction), and a one-stop shop for all the ancillary data available for this well-studied galaxy sample. LEGUS will provide the reference survey and the foundation for future observations with JWST and with ALMA. This abstract accompanies another one from the same project, and presents the status of the project, its structure, and the data products that will be delivered to the community; the other abstract presents the science goals of LEGUS and how these will be addressed by the HST observations.

Star Formation Rates in Cooling Flow Clusters: A UV Pilot Study with Archival XMM-Newton Optical Monitor Data

NASA Technical Reports Server (NTRS)

Hicks, A. K.; Mushotzky, R.

2006-01-01

We have analyzed XMM-Newton Optical Monitor (OM) UV (180-400 nm) data for a sample of 33 galaxies. 30 are cluster member galaxies, and nine of these are central cluster galaxies (CCGs) in cooling flow clusters having mass deposition rates which span a range of 8 - 525 Solar Mass/yr. By comparing the ratio of UV to 2MASS J band fluxes, we find a significant UV excess in many, but not all, cooling flow CCGs, a finding consistent with the outcome of previous studies based on optical imaging data (McNamara & O'Connell 1989; Cardiel, Gorgas, & Aragon-Salamanca 1998; Crawford et al. 1999). This UV excess is a direct indication of the presence of young massive stars, and therefore recent star formation, in these galaxies. Using the Starburst99 spectral energy distribution (SED) model of continuous star formation over a 900 Myr period, we derive star formation rates of 0.2 - 219 solar Mass/yr for the cooling flow sample. For 2/3 of this sample it is possible to equate Chandra/XMM cooling flow mass deposition rates with UV inferred star formation rates, for a combination of starburst lifetime and IMF slope. This is a pilot study of the well populated XMM UV cluster archive and a more extensive follow up study is currently underway.

Heating and cooling of the neutral ISM in the NGC 4736 circumnuclear ring

NASA Astrophysics Data System (ADS)

van der Laan, T. P. R.; Armus, L.; Beirao, P.; Sandstrom, K.; Groves, B.; Schinnerer, E.; Draine, B. T.; Smith, J. D.; Galametz, M.; Wolfire, M.; Croxall, K.; Dale, D.; Herrera Camus, R.; Calzetti, D.; Kennicutt, R. C.

2015-03-01

The manner in which gas accretes and orbits within circumnuclear rings has direct implications for the star formation process. In particular, gas may be compressed and shocked at the inflow points, resulting in bursts of star formation at these locations. Afterwards the gas and young stars move together through the ring. In addition, star formation may occur throughout the ring, if and when the gas reaches sufficient density to collapse under gravity. These two scenarios for star formation in rings are often referred to as the "pearls-on-a-string" and "popcorn" paradigms. In this paper, we use new Herschel/PACS observations, obtained as part of the KINGFISH open time key program, along with archival Spitzer and ground-based observations from the SINGS Legacy project, to investigate the heating and cooling of the interstellar medium in the nearby star-forming ring galaxy, NGC 4736. By comparing spatially resolved estimates of the stellar far-ultraviolet flux available for heating, with the gas and dust cooling derived from the far-infrared continuum and line emission, we show that while star formation is indeed dominant at the inflow points in NGC 4736, additional star formation is needed to balance the gas heating and cooling throughout the ring. This additional component most likely arises from the general increase in gas density in the ring over its lifetime. Our data provide strong evidence, therefore, for a combination of the two paradigms for star formation in the ring in NGC 4736.

Mid-Infrared Silicate Dust Features in Seyfert 1 Spectra

NASA Astrophysics Data System (ADS)

Thompson, Grant D.; Levenson, N. A.; Sirocky, M. M.; Uddin, S.

2007-12-01

Silicate dust emission dominates the mid-infrared spectra of galaxies, and the dust produces two spectral features, at 10 and 18 μm. These features' strengths (in emission or absorption) and peak wavelengths reveal the geometry of the dust distribution, and they are sensitive to the dust composition. We examine mid-infrared spectra of 32 Seyfert 1 active galactic nuclei (AGN), observed with the Infrared Spectrograph aboard the Spitzer Space Telescope. In the spectra, we typically find the shorter-wavelength feature in emission, at an average peak wavelength of 10.0 μm, although it is known historically as the "9.7 μm" feature. In addition, peak wavelength increases with feature strength. The 10 and 18 μm feature strengths together are sensitive to the dust geometry surrounding the central heating engine. Numerical calculations of radiative transfer distinguish between clumpy and smooth distributions, and we find that the surroundings of these AGN (the obscuring "tori" of unified AGN schemes) are clumpy. Polycyclic aromatic hydrocarbon (PAH) features are associated with star formation, and we find strong PAH emission (luminosity ≥ 1042 erg/s) in only four sources, three of which show independent evidence for starbursts. We will explore the effects of luminosity on dust geometry and chemistry in a comparison sample of quasars. We acknowledge work supported by the NSF under grant number 0237291.

SUSTAINING STAR FORMATION RATES IN SPIRAL GALAXIES: SUPERNOVA-DRIVEN TURBULENT ACCRETION DISK MODELS APPLIED TO THINGS GALAXIES

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

Vollmer, Bernd; Leroy, Adam K., E-mail: bvollmer@astro.u-strasbg.fr

2011-01-15

Gas disks of spiral galaxies can be described as clumpy accretion disks without a coupling of viscosity to the actual thermal state of the gas. The model description of a turbulent disk consisting of emerging and spreading clumps contains free parameters, which can be constrained by observations of molecular gas, atomic gas, and the star formation rate for individual galaxies. Radial profiles of 18 nearby spiral galaxies from THINGS, HERACLES, SINGS, and GALEX data are used to compare the observed star formation efficiency, molecular fraction, and velocity dispersion to the model. The observed radially decreasing velocity dispersion can be reproducedmore » by the model. In the framework of this model, the decrease in the inner disk is due to the stellar mass distribution which dominates the gravitational potential. Introducing a radial break in the star formation efficiency into the model improves the fits significantly. This change in the star formation regime is realized by replacing the free-fall time in the prescription of the star formation rate with the molecule formation timescale. Depending on the star formation prescription, the break radius is located near the transition region between the molecular-gas-dominated and atomic-gas-dominated parts of the galactic disk or closer to the optical radius. It is found that only less massive galaxies (log M(M{sub sun}) {approx}< 10) can balance gas loss via star formation by radial gas accretion within the disk. These galaxies can thus access their gas reservoirs with large angular momentum. On the other hand, the star formation of massive galaxies is determined by the external gas mass accretion rate from a putative spherical halo of ionized gas or from satellite accretion. In the absence of this external accretion, star formation slowly exhausts the gas within the optical disk within the star formation timescale.« less

Sustaining Star Formation Rates in Spiral Galaxies Supernova-driven Turbulent Accretion Disk Models Applied to THINGS Galaxies

NASA Astrophysics Data System (ADS)

Vollmer, Bernd; Leroy, Adam K.

2011-01-01

Gas disks of spiral galaxies can be described as clumpy accretion disks without a coupling of viscosity to the actual thermal state of the gas. The model description of a turbulent disk consisting of emerging and spreading clumps contains free parameters, which can be constrained by observations of molecular gas, atomic gas, and the star formation rate for individual galaxies. Radial profiles of 18 nearby spiral galaxies from THINGS, HERACLES, SINGS, and GALEX data are used to compare the observed star formation efficiency, molecular fraction, and velocity dispersion to the model. The observed radially decreasing velocity dispersion can be reproduced by the model. In the framework of this model, the decrease in the inner disk is due to the stellar mass distribution which dominates the gravitational potential. Introducing a radial break in the star formation efficiency into the model improves the fits significantly. This change in the star formation regime is realized by replacing the free-fall time in the prescription of the star formation rate with the molecule formation timescale. Depending on the star formation prescription, the break radius is located near the transition region between the molecular-gas-dominated and atomic-gas-dominated parts of the galactic disk or closer to the optical radius. It is found that only less massive galaxies (log M(M ⊙) <~ 10) can balance gas loss via star formation by radial gas accretion within the disk. These galaxies can thus access their gas reservoirs with large angular momentum. On the other hand, the star formation of massive galaxies is determined by the external gas mass accretion rate from a putative spherical halo of ionized gas or from satellite accretion. In the absence of this external accretion, star formation slowly exhausts the gas within the optical disk within the star formation timescale.

APEX/SABOCA observations of small-scale structure of infrared-dark clouds . I. Early evolutionary stages of star-forming cores

NASA Astrophysics Data System (ADS)

Ragan, Sarah E.; Henning, Thomas; Beuther, Henrik

2013-11-01

Infrared-dark clouds (IRDCs) harbor the early phases of cluster and high-mass star formation and are comprised of cold (~20 K), dense (n > 104 cm-3) gas. The spectral energy distribution (SED) of IRDCs is dominated by the far-infrared and millimeter wavelength regime, and our initial Herschel study examined IRDCs at the peak of the SED with high angular resolution. Here we present a follow-up study using the SABOCA instrument on APEX which delivers 7.8″ angular resolution at 350 μm, matching the resolution we achieved with Herschel/PACS, and allowing us to characterize substructure on ~0.1 pc scales. Our sample of 11 nearby IRDCs are a mix of filamentary and clumpy morphologies, and the filamentary clouds show significant hierarchical structure, while the clumpy IRDCs exhibit little hierarchical structure. All IRDCs, regardless of morphology, have about 14% of their total mass in small scale core-like structures which roughly follow a trend of constant volume density over all size scales. Out of the 89 protostellar cores we identified in this sample with Herschel, we recover 40 of the brightest and re-fit their SEDs and find their properties agree fairly well with our previous estimates (⟨ T ⟩ ~ 19 K). We detect a new population of "cold cores" which have no 70 μm counterpart, but are 100 and 160 μm-bright, with colder temperatures (⟨ T ⟩ ~ 16 K). This latter population, along with SABOCA-only detections, are predominantly low-mass objects, but their evolutionary diagnostics are consistent with the earliest starless or prestellar phase of cores in IRDCs. Based on observations carried out with the Atacama Pathfinder Experiment (APEX). APEX is a collaboration between Max Planck Institut für Radioastronomie (MPIfR), Onsala Space Observatory (OSO), and the European Southern Observatory (ESO).Appendices are available in electronic form at http://www.aanda.org

Role of strangeness to the neutron star mass and cooling

NASA Astrophysics Data System (ADS)

Lee, Chang-Hwan; Lim, Yeunhwan; Hyun, Chang Ho; Kwak, Kyujin

2018-01-01

Neutron star provides unique environments for the investigation of the physics of extreme dense matter beyond normal nuclear saturation density. In such high density environments, hadrons with strange quarks are expected to play very important role in stabilizing the system. Kaons and hyperons are the lowest mass states with strangeness among meson and bayron families, respectively. In this work, we investigate the role of kaons and hyperons to the neutron star mass, and discuss their role in the neutron star cooling.

PHIBSS: MOLECULAR GAS, EXTINCTION, STAR FORMATION, AND KINEMATICS IN THE z = 1.5 STAR-FORMING GALAXY EGS13011166

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

Genzel, R.; Tacconi, L. J.; Kurk, J.

We report matched resolution imaging spectroscopy of the CO 3-2 line (with the IRAM Plateau de Bure millimeter interferometer) and of the H{alpha} line (with LUCI at the Large Binocular Telescope) in the massive z = 1.53 main-sequence galaxy EGS 13011166, as part of the ''Plateau de Bure high-z, blue-sequence survey'' (PHIBSS: Tacconi et al.). We combine these data with Hubble Space Telescope V-I-J-H-band maps to derive spatially resolved distributions of stellar surface density, star formation rate, molecular gas surface density, optical extinction, and gas kinematics. The spatial distribution and kinematics of the ionized and molecular gas are remarkably similarmore » and are well modeled by a turbulent, globally Toomre unstable, rotating disk. The stellar surface density distribution is smoother than the clumpy rest-frame UV/optical light distribution and peaks in an obscured, star-forming massive bulge near the dynamical center. The molecular gas surface density and the effective optical screen extinction track each other and are well modeled by a ''mixed'' extinction model. The inferred slope of the spatially resolved molecular gas to star formation rate relation, N = dlog{Sigma}{sub starform}/dlog{Sigma}{sub molgas}, depends strongly on the adopted extinction model, and can vary from 0.8 to 1.7. For the preferred mixed dust-gas model, we find N = 1.14 {+-} 0.1.« less

Wide- and contact-binary formation in substructured young stellar clusters

NASA Astrophysics Data System (ADS)

Dorval, J.; Boily, C. M.; Moraux, E.; Roos, O.

2017-02-01

We explore with collisional gravitational N-body models the evolution of binary stars in initially fragmented and globally subvirial clusters of stars. Binaries are inserted in the (initially) clumpy configurations so as to match the observed distributions of the field-binary-stars' semimajor axes a and binary fraction versus primary mass. The dissolution rate of wide binaries is very high at the start of the simulations, and is much reduced once the clumps are eroded by the global infall. The transition between the two regimes is sharper as the number of stars N is increased, from N = 1.5 k up to 80 k. The fraction of dissolved binary stars increases only mildly with N, from ≈15 per cent to ≈25 per cent for the same range in N. We repeated the calculation for two initial system mean number densities of 6 per pc3 (low) and 400 per pc3 (high). We found that the longer free-fall time of the low-density runs allows for prolonged binary-binary interactions inside clumps and the formation of very tight (a ≈ 0.01 au) binaries by exchange collisions. This is an indication that the statistics of such compact binaries bear a direct link to their environment at birth. We also explore the formation of wide (a ≳ 5 × 104 au) binaries and find a low (≈0.01 per cent) fraction mildly bound to the central star cluster. The high-precision astrometric mission Gaia could identify them as outflowing shells or streams.

The Eagle Nebula: a spectral template for star forming regions

NASA Astrophysics Data System (ADS)

Flagey, Nicolas; Boulanger, Francois; Carey, Sean; Compiegne, Mathieu; Dwek, Eli; Habart, Emilie; Indebetouw, Remy; Montmerle, Thierry; Noriega-Crespo, Alberto

2008-03-01

IRAC and MIPS have revealed spectacular images of massive star forming regions in the Galaxy. These vivid illustrations of the interaction between the stars, through their winds and radiation, and their environment, made of gas and dust, still needs to be explained. The large scale picture of layered shells of gas components, is affected by the small scale interaction of stars with the clumpy medium that surrounds them. To understand spatial variations of physical conditions and dust properties on small scales, spectroscopic imaging observations are required on a nearby object. The iconic Eagle Nebula (M16) is one of the nearest and most observed star forming region of our Galaxy and as such, is a well suited template to obtain this missing data set. We thus propose a complete spectral map of the Eagle Nebula (M16) with the IRS/Long Low module (15-38 microns) and MIPS/SED mode (55-95 microns). Analysis of the dust emission, spectral features and continuum, and of the H2 and fine-structure gas lines within our models will provide us with constraints on the physical conditions (gas ionization state, pressure, radiation field) and dust properties (temperature, size distribution) at each position within the nebula. Only such a spatially and spectrally complete map will allow us to characterize small scale structure and dust evolution within the global context and understand the impact of small scale structure on the evolution of dusty star forming regions. This project takes advantage of the unique ability of IRS at obtaining sensitive spectral maps covering large areas.

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.

Heavy Elements and Cool Stars

NASA Technical Reports Server (NTRS)

Wahlgren, Glenn M.; Carpenter, Kenneth G.; Norris, Ryan P.

2008-01-01

We report on progress in the analysis of high-resolution near-IR spectra of alpha Orionis (M2 Iab) and other cool, luminous stars. Using synthetic spectrum techniques, we search for atomic absorption lines in the stellar spectra and evaluate the available line parameter data for use in our abundance analyses. Our study concentrates on the post iron-group elements copper through zirconium as a means of investigating the slow neutron-capture process of nucleosynthesis in massive stars and the mechanisms that transport recently processed material up into the photospheric region. We discuss problems with the atomic data and model atmospheres that need to be addressed before theoretically derived elemental abundances from pre-supernova nucleosynthesis calculations can be tested by comparison with abundances determined from observations of cool, massive stars.

Neutrino Processes in Neutron Stars

NASA Astrophysics Data System (ADS)

Kolomeitsev, E. E.; Voskresensky, D. N.

2010-10-01

The aim of these lectures is to introduce basic processes responsible for cooling of neutron stars and to show how to calculate the neutrino production rate in dense strongly interacting nuclear medium. The formalism is presented that treats on equal footing one-nucleon and multiple-nucleon processes and reactions with virtual bosonic modes and condensates. We demonstrate that neutrino emission from dense hadronic component in neutron stars is subject of strong modifications due to collective effects in the nuclear matter. With the most important in-medium processes incorporated in the cooling code an overall agreement with available soft X ray data can be easily achieved. With these findings the so-called “standard” and “non-standard” cooling scenarios are replaced by one general “nuclear medium cooling scenario” which relates slow and rapid neutron star coolings to the star masses (interior densities). The lectures are split in four parts. Part I: After short introduction to the neutron star cooling problem we show how to calculate neutrino reaction rates of the most efficient one-nucleon and two-nucleon processes. No medium effects are taken into account in this instance. The effects of a possible nucleon pairing are discussed. We demonstrate that the data on neutron star cooling cannot be described without inclusion of medium effects. It motivates an assumption that masses of the neutron stars are different and that neutrino reaction rates should be strongly density dependent. Part II: We introduce the Green’s function diagram technique for systems in and out of equilibrium and the optical theorem formalism. The latter allows to perform calculations of production rates with full Green’s functions including all off-mass-shell effects. We demonstrate how this formalism works within the quasiparticle approximation. Part III: The basic concepts of the nuclear Fermi liquid approach are introduced. We show how strong interaction effects can be included within the Green’s function formalism. Softening of the pion mode with an baryon density increase is explicitly incorporated. We show examples of inconsistencies in calculations without inclusion of medium effects. Then we demonstrate calculations of different reaction rates in non-superfluid nuclear matter with taking into account medium effects. Many new reaction channels are open up in the medium and should be analyzed. Part IV: We discuss the neutrino production reactions in superfluid nuclear systems. The reaction rates of processes associated with the pair breaking and formation are calculated. Special attention is focused on the gauge invariance and the exact fulfillment of the Ward identities for the vector current. Finally we present comparison of calculations of neutron star cooling performed within nuclear medium cooling scenario with the available data.

Prospects for Chemically Tagging Stars in the Galaxy

NASA Astrophysics Data System (ADS)

Ting, Yuan-Sen; Conroy, Charlie; Goodman, Alyssa

2015-07-01

It is now well-established that the elemental abundance patterns of stars hold key clues not only to their formation, but also to the assembly histories of galaxies. One of the most exciting possibilities is the use of stellar abundance patterns as “chemical tags” to identify stars that were born in the same molecular cloud. In this paper, we assess the prospects of chemical tagging as a function of several key underlying parameters. We show that in the fiducial case of 104 distinct cells in chemical space and {10}5-{10}6 stars in the survey, one can expect to detect ∼ {10}2-{10}3 groups that are ≥slant 5σ overdensities in the chemical space. However, we find that even very large overdensities in chemical space do not guarantee that the overdensity is due to a single set of stars from a common birth cloud. In fact, for our fiducial model parameters, the typical 5σ overdensity is comprised of stars from a wide range of clusters with the most dominant cluster contributing only 25% of the stars. The most important factors limiting the identification of disrupted clusters via chemical tagging are the number of chemical cells in the chemical space and the survey sampling rate of the underlying stellar population. Both of these factors can be improved through strategic observational plans. While recovering individual clusters through chemical tagging may prove challenging, we show, in agreement with previous work, that different CMFs imprint different degrees of clumpiness in chemical space. These differences provide the opportunity to statistically reconstruct the slope and high-mass cutoff of CMF and its evolution through cosmic time.

The initial mass function and star formation law in the outer disc of NGC 2915

NASA Astrophysics Data System (ADS)

Bruzzese, S. M.; Meurer, G. R.; Lagos, C. D. P.; Elson, E. C.; Werk, J. K.; Blakeslee, John P.; Ford, H.

2015-02-01

Using Hubble Space Telescope (HST) Advanced Camera for Surveys/Wide Field Camera data we present the photometry and spatial distribution of resolved stellar populations in the outskirts of NGC 2915, a blue compact dwarf with an extended H I disc. These observations reveal an elliptical distribution of red giant branch stars, and a clumpy distribution of main-sequence stars that correlate with the H I gas distribution. We constrain the upper-end initial mass function (IMF) and determine the star formation law (SFL) in this field, using the observed main-sequence stars and an assumed constant star formation rate. Previously published Hα observations of the field, which show one faint H II region, are used to provide further constraints on the IMF. We find that the main-sequence luminosity function analysis alone results in a best-fitting IMF with a power-law slope α = -2.85 and upper-mass limit M_u = 60 M_{⊙}. However, if we assume that all Hα emission is confined to H II regions then the upper-mass limit is restricted to M_u ≲ 20 M_{⊙}. For the luminosity function fit to be correct, we have to discount the Hα observations implying significant diffuse ionized gas or escaping ionizing photons. Combining the HST photometry with H I imaging, we find the SFL has a power-law index N = 1.53 ± 0.21. Applying these results to the entire outer H I disc indicates that it contributes 11-28 per cent of the total recent star formation in NGC 2915, depending on whether the IMF is constant within the disc or varies from the centre to the outer region.

High resolution radio and optical observations of the central starburst in the low-metallicity dwarf galaxy II Zw 40

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

Kepley, Amanda A.; Reines, Amy E.; Johnson, Kelsey E.

2014-02-01

The extent to which star formation varies in galaxies with low masses, low metallicities, and high star formation rate surface densities is not well constrained. To gain insight into star formation under these physical conditions, this paper estimates the ionizing photon fluxes, masses, and ages for young massive clusters in the central region of II Zw 40—the prototypical low-metallicity dwarf starburst galaxy—from radio continuum and optical observations. Discrete, cluster-sized sources only account for half the total radio continuum emission; the remainder is diffuse. The young (≲ 5 Myr) central burst has a star formation rate surface density that significantly exceedsmore » that of the Milky Way. Three of the 13 sources have ionizing photon fluxes (and thus masses) greater than R136 in 30 Doradus. Although isolating the effects of galaxy mass and metallicity is difficult, the H II region luminosity function and the internal extinction in the center of II Zw 40 appear to be primarily driven by a merger-related starburst. The relatively flat H II region luminosity function may be the result of an increase in interstellar medium pressure during the merger and the internal extinction is similar to that generated by the clumpy and porous dust in other starburst galaxies.« less

Nearly simultaneous observations of chromospheric and coronal radiative losses of cool stars

NASA Technical Reports Server (NTRS)

Schrijver, C. J.; Dobson, A. K.; Radick, R. R.

1992-01-01

The flux-flux relationships of cool stars are studied on the basis of nearly simultaneous measurements of Ca II H+K, Mg II h+k, and soft X-ray fluxes. A linear relationship is derived between IUE Mg II h+k fluxes and Mount Wilson Ca II H+K fluxes which were obtained within 36 hr of each other for a sample of 26 F5-K3 main-sequence stars. Nearly simultaneous EXOSAT soft X-ray fluxes are compared with Ca II H+K fluxes for a sample of 20 dwarfs and gaints with spectral types ranging from F6 to K2, and 72 additional cool stars for which noncontemporaneous Ca II H+K and EINSTEIN soft X-ray fluxes are available are compared. It is confirmed that a nonradiatively heated chromosphere exists on even the least active main-sequence stars. This basal chromosphere is probably independent of stellar magnetic activity.

Imaging the cool stars in the interacting binaries AE Aqr, BV Cen and V426 Oph

NASA Astrophysics Data System (ADS)

Watson, C. A.; Steeghs, D.; Dhillon, V. S.; Shahbaz, T.

2007-10-01

It is well known that magnetic activity in late-type stars increases with increasing rotation rate. Using inversion techniques akin to medical imaging, the rotationally broadened profiles from such stars can be used to reconstruct `Doppler images' of the distribution of cool, dark starspots on their stellar surfaces. Interacting binaries, however, contain some of the most rapidly rotating late-type stars known and thus provide important tests of stellar dynamo models. Furthermore, magnetic activity is thought to play a key role in their evolution, behaviour and accretion dynamics. Despite this, we know comparatively little about the magnetic activity and its influence on such binaries. In this review we summarise the concepts behind indirect imaging of these systems, and present movies of the starspot distributions on the cool stars in some interacting binaries. We conclude with a look at the future opportunities that such studies may provide.

The impact of heat blanketing envelopes on neutron stars cooling

NASA Astrophysics Data System (ADS)

Beznogov, M. V.; Yakovlev, D. G.; Fortin, M.; Haensel, P.; Zdunik, J. L.

2017-11-01

The goal of this work is to investigate the effects of chemical composition of heat blanketing envelopes of neutron stars on their thermal states and thermal evolution. To this purpose, we employ newly constructed models of the envelopes composed of binary ion mixtures (H-He, He-C, C-Fe) varying the mass of lighter ions (H, He or C) in the envelope. The results are compared with those calculated using the standard “onion-like” envelope. For illustration, we apply these results to estimate the internal temperature of the Vela pulsar and to study cooling of neutron stars. We show that uncertainties in the chemical composition of the envelopes can lead up to ~ 2.5 times uncertainty of the internal temperature of the star which significantly complicates theoretical reconstruction of the internal structure of cooling neutron stars from observations of their thermal surface emission.

Jupiter Analogs Orbit Stars with an Average Metallicity Close to That of the Sun

NASA Astrophysics Data System (ADS)

Buchhave, Lars A.; Bitsch, Bertram; Johansen, Anders; Latham, David W.; Bizzarro, Martin; Bieryla, Allyson; Kipping, David M.

2018-03-01

Jupiter played an important role in determining the structure and configuration of the Solar System. Whereas hot-Jupiter type exoplanets preferentially form around metal-rich stars, the conditions required for the formation of planets with masses, orbits, and eccentricities comparable to Jupiter (Jupiter analogs) are unknown. Using spectroscopic metallicities, we show that stars hosting Jupiter analogs have an average metallicity close to solar, in contrast to their hot-Jupiter and eccentric cool-Jupiter counterparts, which orbit stars with super-solar metallicities. Furthermore, the eccentricities of Jupiter analogs increase with host-star metallicity, suggesting that planet–planet scatterings producing highly eccentric cool Jupiters could be more common in metal-rich environments. To investigate a possible explanation for these metallicity trends, we compare the observations to numerical simulations, which indicate that metal-rich stars typically form multiple Jupiters, leading to planet–planet interactions and, hence, a prevalence of either eccentric cool Jupiters or hot Jupiters with circularized orbits. Although the samples are small and exhibit variations in their metallicities, suggesting that numerous processes other than metallicity affect the formation of planetary systems, the data in hand suggests that Jupiter analogs and terrestrial-sized planets form around stars with average metallicities close to solar, whereas high-metallicity systems preferentially host eccentric cool Jupiter or hot Jupiters, indicating that higher metallicity systems may not be favorable for the formation of planetary systems akin to the Solar System.

A Massive, Cooling-Flow-Induced Starburst in the Core of a Highly Luminous Galaxy Cluster

NASA Technical Reports Server (NTRS)

McDonald, M.; Bayliss, M.; Benson, B. A.; Foley, R. J.; Ruel, J.; Sullivan, P.; Veilleux, S.; Aird, K. A.; Ashby, M. L. N.; Bautz, M.;

On the structure of the outer layers of cool carbon stars

NASA Technical Reports Server (NTRS)

Querci, F.; Querci, M.; Wing, R. F.; Cassatella, A.; Heck, A.

1982-01-01

Exposures on the spectra of four late C-type stars have been made with the IUE satellite in the wavelength range of the LWR camera (1900-3200 A). Two Mira variables near maximum light and two semiregular variables were observed. Although the exposure times used, which range up to 240 min in the low-resolution mode, were more than sufficient to record the continuum and emission lines of Mg II, Fe II, and Al II in normal M stars of similar magnitude and temperature, no light was recorded. It is concluded that the far-ultraviolet continuum is strongly depressed in these cool carbon stars. The absence of UV emission lines implies either that the chromospheric lines observed in M stars require an ultraviolet flux for their excitation, or that cool carbon stars have no chromosphere at all or that the opacity source is located above even the emission-line-forming region. This opacity source, which is probably some carbon condensate since it is weak or absent in M stars while absorbing strongly in C stars, is discussed both in terms of the chromospheric interpretation of the emission lines and in terms of their shock-wave interpretation.

The coolest extremely low-mass white dwarfs

NASA Astrophysics Data System (ADS)

Calcaferro, Leila M.; Althaus, Leandro G.; Córsico, Alejandro H.

2018-06-01

Context. Extremely low-mass white dwarf (ELM WD; M⋆ ≲ 0.18-0.20 M⊙) stars are thought to be formed in binary systems via stable or unstable mass transfer. Although stable mass transfer predicts the formation of ELM WDs with thick hydrogen (H) envelopes that are characterized by dominant residual nuclear burning along the cooling branch, the formation of ELM WDs with thinner H envelopes from unstable mass loss cannot be discarded. Aims: We compute new evolutionary sequences for helium (He) core WD stars with thin H envelopes with the main aim of assessing the lowest Teff that could be reached by this type of stars. Methods: We generate a new grid of evolutionary sequences of He-core WD stars with thin H envelopes in the mass range from 0.1554 to 0.2025 M⊙, and assess the changes in both the cooling times and surface gravity induced by a reduction of the H envelope. We also determine, taking into account the predictions of progenitor evolution, the lowest Teff reached by the resulting ELM WDs. Results: We find that a slight reduction in the H envelope yields a significant increase in the cooling rate of ELM WDs. Because of this, ELM WDs with thin H envelopes could cool down to 2500 K, in contrast to their canonical counterparts that cool down to 7000 K. In addition, we find that a reduction of the thickness of the H envelope markedly increases the surface gravity (g) of these stars. Conclusions: If ELM WDs are formed with thin H envelopes, they could be detected at very low Teff. The detection of such cool ELM WDs would be indicative that they were formed with thin H envelopes, thus opening the possibility of placing constraints on the possible mechanisms of formation of this type of star. Last but not least, the increase in g due to the reduction of the H envelope leads to consequences in the spectroscopic determinations of these stars.

Spiraling Out of Control: Three-dimensional Hydrodynamical Modeling of the Colliding Winds in η Carinae

NASA Astrophysics Data System (ADS)

Parkin, E. R.; Pittard, J. M.; Corcoran, M. F.; Hamaguchi, K.

2011-01-01

Three-dimensional adaptive mesh refinement hydrodynamical simulations of the wind-wind collision between the enigmatic supermassive star η Car and its mysterious companion star are presented which include radiative driving of the stellar winds, gravity, optically thin radiative cooling, and orbital motion. Simulations with static stars with a periastron passage separation reveal that the preshock companion star's wind speed is sufficiently reduced so that radiative cooling in the postshock gas becomes important, permitting the runaway growth of nonlinear thin-shell instabilities (NTSIs) which massively distort the wind-wind collision region (WCR). However, large-scale simulations, which include the orbital motion of the stars, show that orbital motion reduces the impact of radiative inhibition and thus increases the acquired preshock velocities. As such, the postshock gas temperature and cooling time see a commensurate increase, and sufficient gas pressure is preserved to stabilize the WCR against catastrophic instability growth. We then compute synthetic X-ray spectra and light curves and find that, compared to previous models, the X-ray spectra agree much better with XMM-Newton observations just prior to periastron. The narrow width of the 2009 X-ray minimum can also be reproduced. However, the models fail to reproduce the extended X-ray minimum from previous cycles. We conclude that the key to explaining the extended X-ray minimum is the rate of cooling of the companion star's postshock wind. If cooling is rapid then powerful NTSIs will heavily disrupt the WCR. Radiative inhibition of the companion star's preshock wind, albeit with a stronger radiation-wind coupling than explored in this work, could be an effective trigger.

The luminosities of cool supergiants in the Magellanic Clouds, and the Humphreys-Davidson limit revisited

NASA Astrophysics Data System (ADS)

Davies, Ben; Crowther, Paul A.; Beasor, Emma R.

2018-05-01

The empirical upper luminosity boundary Lmax of cool supergiants, often referred to as the Humphreys-Davidson limit, is thought to encode information on the general mass-loss behaviour of massive stars. Further, it delineates the boundary at which single stars will end their lives stripped of their hydrogen-rich envelope, which in turn is a key factor in the relative rates of Type-II to Type-Ibc supernovae from single star channels. In this paper we have revisited the issue of Lmax by studying the luminosity distributions of cool supergiants (SGs) in the Large and Small Magellanic Clouds (LMC/SMC). We assemble samples of cool SGs in each galaxy which are highly-complete above log L/L⊙=5.0, and determine their spectral energy distributions from the optical to the mid-infrared using modern multi-wavelength survey data. We show that in both cases Lmax appears to be lower than previously quoted, and is in the region of log L/L⊙=5.5. There is no evidence for Lmax being higher in the SMC than in the LMC, as would be expected if metallicity-dependent winds were the dominant factor in the stripping of stellar envelopes. We also show that Lmax aligns with the lowest luminosity of single nitrogen-rich Wolf-Rayet stars, indicating of a change in evolutionary sequence for stars above a critical mass. From population synthesis analysis we show that the Geneva evolutionary models greatly over-predict the numbers of cool SGs in the SMC. We also argue that the trend of earlier average spectral types of cool SGs in lower metallicity environments represents a genuine shift to hotter temperatures. Finally, we use our new bolometric luminosity measurements to provide updated bolometric corrections for cool supergiants.

Search for white dwarf companions of cool stars with peculiar element abundances

NASA Technical Reports Server (NTRS)

Boehm-Vitense, E.

1984-01-01

A search for a white dwarf companions of cool stars with peculiar element abundances was undertaken. One additional star the xi Cet, was found with a white dwarf companion. It was found that HR 1016, 56Uma, 16 Ser, have high excitation emission lines which indicate a high temperature object in the system. It is suggested that since these indications for high temperature companions were seen for all nearby Ba stars, it is highly probable that all Ba stars have white dwarf companions, and that the peculiar element abundances seen in the Ba stars are due to mass transfer. Observations, arguments and conclusions are presented. White dwarf companions were not found. Together with the Li and Be abundances and the chromospheric emission line spectra in these stars were studied. No white dwarf companions were seen for subgiant CH stars.

The Thermal State of KS 1731-260 after 14.5 years in Quiescence

NASA Astrophysics Data System (ADS)

Merritt, Rachael L.; Cackett, Edward M.; Brown, Edward F.; Page, Dany; Cumming, Andrew; Degenaar, Nathalie; Deibel, Alex; Homan, Jeroen; Miller, Jon M.; Wijnands, Rudy

2016-12-01

Crustal cooling of accretion-heated neutron stars provides insight into the stellar interior of neutron stars. The neutron star X-ray transient, KS 1731-260, was in outburst for 12.5 years before returning to quiescence in 2001. We have monitored the cooling of this source since then through Chandra and XMM-Newton observations. Here we present a 150 ks Chandra observation of KS 1731-260 taken in 2015 August, about 14.5 years into quiescence and 6 years after the previous observation. We find that the neutron star surface temperature is consistent with the previous observation, suggesting that crustal cooling has likely stopped and the crust has reached thermal equilibrium with the core. Using a theoretical crust thermal evolution code, we fit the observed cooling curves and constrain the core temperature (T c = 9.35 ± 0.25 × 107 K), composition (Q {}{imp}={4.4}-0.5+2.2), and level of extra shallow heating required (Q sh = 1.36 ± 0.18 MeV/nucleon). We find that the presence of a low thermal conductivity layer, as expected from nuclear pasta, is not required to fit the cooling curve well, but cannot be excluded either.

The thermal state of KS 1731-260 after 14.5 years in quiescence

NASA Astrophysics Data System (ADS)

Merritt, R.; Cackett, E.; Brown, E.; Page, D.; Cumming, A.; Degenaar, N.; Deibel, A.; Homan, J.; Miller, J.; Wijnands, R.

2017-10-01

Crustal cooling of accretion-heated neutron stars provides insight into the stellar interior of neutron stars. The neutron star X-ray transient, KS 1731-260, was in outburst for 12.5 years before returning to quiescence in 2001. We have monitored the cooling of this source since then through Chandra and XMM-Newton observations. Here, we present a 150 ks Chandra observation of KS 1731-260 taken in August 2015, about 14.5 years into quiescence, and 6 years after the previous observation. We find that the neutron star surface temperature is consistent with the previous observation, suggesting that crustal cooling has likely stopped and the crust has reached thermal equilibrium with the core. Using a theoretical crust thermal evolution code, we fit the observed cooling curves and constrain the core temperature (T_c = 9.35±0.25×10^7 K), composition (Q_{imp} = 4.4^{+2.2}_{-0.5}) and level of extra shallow heating required (Q_{sh} = 1.36±0.18 MeV/nucleon). We find that the presence of a low thermal conductivity layer, as expected from nuclear pasta, is not required to fit the cooling curve well, but cannot be excluded either.

Hot-Jupiter Breakfasts Realign Stars

NASA Astrophysics Data System (ADS)

Kohler, Susanna

2015-08-01

Two researchers at the University of Chicago have recently developed a new theory to explain an apparent dichotomy in the orbits of planets around cool vs. hot stars. Their model proposes that the spins of cool stars are affected when they ingest hot Jupiters (HJs) early in their stellar lifetimes. A Puzzling Dichotomy: In exoplanet studies, there is a puzzling difference observed between planet orbits around cool and hot (those with Teff ≥ 6250 K) stars: the orbital planes of planets around cool stars are primarily aligned with the host star's spin, whereas the orbital planes of planets around hot stars seem to be randomly distributed. Previous attempts to explain this dichotomy have focused on tidal interactions between the host star and the planets observed in the system. Now Titos Matsakos and Arieh Königl have taken these models a step further — by including in their calculations not only the effects of observed planets, but also those of HJs that may have been swallowed by the star long before we observed the systems. Modeling Meals: Plots of the distribution of the obliquity λ for hot Jupiters around cool hosts (upper plot) and hot hosts (lower plot). The dashed line shows the initial distribution, the bins show the model prediction for the final distribution after the systems evolve, and the black dots show the current observational data. [Matsakos & Königl, 2015]" class="size-thumbnail wp-image-223" height="386" src="http://aasnova.org/wp-content/uploads/2015/08/fig22-260x386.png" width="260" /> Plots of the distribution of the obliquity λ for hot Jupiters around cool hosts (upper plot) and hot hosts (lower plot). The dashed line shows the initial distribution, the bins show the model prediction for the final distribution after the systems evolve, and the black dots show the current observational data. [Matsakos & Königl, 2015] The authors' model assumes that as HJs are formed and migrate inward through the protoplanetary disk, they stall out near the star (where they have periods of ~2 days) and get stranded as the gas disk evaporates around them. Tidal interactions can cause these planets to become ingested by the host star within 1 Gyr. Using Monte Carlo simulations, the authors model these star-planet tidal interactions and evolve a total of 10^6 systems: half with hot (Teff = 6400 K), main-sequence hosts, and half with cool (Teff = 5500 K), solar-type hosts. The initial obliquities — the angle between the stellar spin and the planets' orbital angular momentum vectors — are randomly distributed between 0° and 180°. The authors find that early stellar ingestion of planets might be very common: to match observations, roughly half of all stellar hosts must ingest an HJ early in their lifetimes! This scenario results in a good match with observational data: about 50% of cool hosts' spins become roughly aligned with the orbital plane of their planets after they absorb the orbital angular momentum of the HJ they ingest. Hot stars, on the other hand, generally retain their random distributions of obliquity, because their angular momentum is typically higher than the orbital angular momentum of the ingested planet. Citation: Titos Matsakos and Arieh Königl 2015, ApJ, 809, L20. doi: 10.1088/2041-8205/809/2/L20

The structure, energy balance, and winds of cool stars

NASA Technical Reports Server (NTRS)

Linsky, J. L.

1982-01-01

The phenomena associated with magnetic fields in the Sun are summarized and it is shown that similar phenomena occur in cool stars. High dispersion spectra are providing unique information concerning densities, atmospheric extension, and emission line widths. A recent unanticipated discovery is that the transition lines are redshifted (an antiwind) in beta Dra (G2 Ib) and perhaps other stars. This is interpreted as indicating downflows in closed magnetic flux tubes as are seen in the solar flux tubes above sunspots. The G and K giants and supergiants are classified as active stars, quiet stars, or hybrid stars depending on whether their atmospheres are dominated by closed magnetic flux tubes, open field geometries, or a predominately open geometry with a few closed flux tubes embedded.

Clumpy wind accretion in Supergiant X-ray Binaries

NASA Astrophysics Data System (ADS)

El Mellah, I.; Sundqvist, J. O.; Keppens, R.

2017-12-01

Supergiant X-ray binaries (\\sgx) contain a neutron star (NS) orbiting a Supergiant O/B star. The fraction of the dense and fast line-driven wind from the stellar companion which is accreted by the NS is responsible for most of the X-ray emission from those system. Classic \\sgx display photometric variability of their hard X-ray emission, typically from a few 10^{35} to a few 10^{37}erg\\cdots^{-1}. Inhomogeneities (\\aka clumps) in the wind from the star are expected to play a role in this time variability. We run 3D hydrodynamical (HD) finite volume simulations to follow the accretion of the inhomogeneous stellar wind by the NS over almost 3 orders of magnitude. To model the unperturbed wind far upstream the NS, we use recent simulations which managed to resolve its micro-structure. We observe the formation of a Bondi-Hoyle-Lyttleton (BHL) like bow shock around the accretor and follow the clumps as they cross it, down to the NS magnetosphere. Compared to previous estimations discarding the HD effects, we measure lower time variability due to both the damping effect of the shock and the necessity to evacuate angular momentum to enable accretion. We also compute the associated time-variable column density and compare it to recent observations in Vela X-1.

A cooling neutron star crust after recurrent outbursts: modelling the accretion outburst history of Aql X-1

NASA Astrophysics Data System (ADS)

Ootes, Laura S.; Wijnands, Rudy; Page, Dany; Degenaar, Nathalie

2018-07-01

With our neutron star crust cooling code NSCOOL, we track the thermal evolution of the neutron star in Aql X-1 over the full accretion outburst history from 1996 until 2015. For the first time, we model many outbursts (23 outbursts were detected) collectively and in great detail. This allows us to investigate the influence of previous outbursts on the internal temperature evolution and to test different neutron star crust cooling scenarios. Aql X-1 is an ideal test source for this purpose, because it shows frequent, short outbursts and thermally dominated quiescence spectra. The source goes into outburst roughly once a year for a few months. Assuming that the quiescent Swift/X-Ray Telescope observations of Aql X-1 can be explained within the crust cooling scenario, we find three main conclusions. First, the data are well reproduced by our model if the envelope composition and shallow heating parameters are allowed to change between outbursts. This is not the case if both shallow heating parameters (strength and depth) are tied throughout all accretion episodes, supporting earlier results that the properties of the shallow heating mechanism are not constant between outbursts. Secondly, from our models, shallow heating could not be connected to one specific spectral state during outburst. Thirdly, and most importantly, we find that the neutron star in Aql X-1 does not have enough time between outbursts to cool down to crust-core equilibrium and that heating during one outburst influences the cooling curves of the next.

Starquakes, Heating Anomalies, and Nuclear Reactions in the Neutron Star Crust

NASA Astrophysics Data System (ADS)

Deibel, Alex Thomas

When the most massive stars perish, their cores may remain intact in the form of extremely dense and compact stars. These stellar remnants, called neutron stars, are on the cusp of becoming black holes and reach mass densities greater than an atomic nucleus in their centers. Although the interiors of neutron stars were difficult to investigate at the time of their discovery, the advent of modern space-based telescopes (e.g., Chandra X-ray Observatory) has pushed our understanding of the neutron star interior into exciting new realms. It has been shown that the neutron star interior spans an enormous range of densities and contains many phases of matter, and further theoretical progress must rely on numerical calculations of neutron star phenomena built with detailed nuclear physics input. To further investigate the properties of the neutron star interior, this dissertation constructs numerical models of neutron stars, applies models to various observations of neutron star high-energy phenomena, and draws new conclusions about the neutron star interior from these analyses. In particular, we model the neutron star's outermost ? 1 km that encompasses the neutron star's envelope, ocean, and crust. The model must implement detailed nuclear physics to properly simulate the hydrostatic and thermal structure of the neutron star. We then apply our model to phenomena that occur in these layers, such as: thermonuclear bursts in the envelope, g-modes in the ocean, torsional oscillations of the crust, and crust cooling of neutron star transients. A comparison of models to observations provides new insights on the properties of dense matter that are often difficult to probe through terrestrial experiments. For example, models of the quiescent cooling of neutron stars, such as the accreting transient MAXI J0556-332, at late times into quiescence probe the thermal transport properties of the deep neutron star crust. This modeling provides independent data from astronomical observations on the nature of neutron superfluidity and the thermal conductivity of nuclear pasta. Our neutron star modeling efforts also pose new questions. For instance, reaction networks find that neutrino emission from cycling nuclear reactions is present in the neutron star ocean and crust, and potentially cools an accreting neutron star. This is a theory we attempt to verify using observations of neutron star transients and thermonuclear bursts, although it remains unclear if this cooling occurs. Furthermore, on some accreting neutron stars, more heat than supplied by nuclear reactions is needed to explain their high temperatures at the outset of quiescence. Although the presence of heating anomalies seems common, the source of extra heating is difficult to determine.

Late-time Cooling of Neutron Star Transients and the Physics of the Inner Crust

NASA Astrophysics Data System (ADS)

Deibel, Alex; Cumming, Andrew; Brown, Edward F.; Reddy, Sanjay

2017-04-01

An accretion outburst onto a neutron star transient heats the neutron star’s crust out of thermal equilibrium with the core. After the outburst, the crust thermally relaxes toward equilibrium with the neutron star core, and the surface thermal emission powers the quiescent X-ray light curve. Crust cooling models predict that thermal equilibrium of the crust will be established ≈ 1000 {days} into quiescence. Recent observations of the cooling neutron star transient MXB 1659-29, however, suggest that the crust did not reach thermal equilibrium with the core on the predicted timescale and continued to cool after ≈ 2500 {days} into quiescence. Because the quiescent light curve reveals successively deeper layers of the crust, the observed late-time cooling of MXB 1659-29 depends on the thermal transport in the inner crust. In particular, the observed late-time cooling is consistent with a low thermal conductivity layer near the depth predicted for nuclear pasta that maintains a temperature gradient between the neutron star’s inner crust and core for thousands of days into quiescence. As a result, the temperature near the crust-core boundary remains above the critical temperature for neutron superfluidity, and a layer of normal neutrons forms in the inner crust. We find that the late-time cooling of MXB 1659-29 is consistent with heat release from a normal neutron layer near the crust-core boundary with a long thermal time. We also investigate the effect of inner crust physics on the predicted cooling curves of the accreting transient KS 1731-260 and the magnetar SGR 1627-41.

HST/COS Observations of the UV-Bright Star Y453 in the Globular Cluster M4 (NGC 6121)

NASA Astrophysics Data System (ADS)

Dixon, William V.; Chayer, Pierre; Benjamin, Robert A.

2016-01-01

Post-AGB stars represent a short-lived phase of stellar evolution during which stars cross the optical color-magnitude diagram from the cool, red tip of the assymptotic giant branch (AGB) to the hot, blue tip of the white-dwarf cooling curve. Their surface chemistry reflects the nuclear-shell burning, mixing, and mass-loss processes characteristic of AGB stars, and their high effective temperatures allow the detection of elements that are unobservable in cool giants. Post-AGB stars in globular clusters offer the additional advantages of known distance, age, and initial chemistry. To better understand the AGB evolution of low-mass stars, we have observed the post-AGB star Y453 in the globular cluster M4 (NGC 6121) with the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope. The star, which has an effective temperature of at least 60,000 K, shows absorption from He, C, N, O, Ne, Si, S, Ti, Cr, Mn, Fe, Co, Ni, and Ga. While the star's C and O abundances are consistent with those measured in a sample of nitrogen-poor RGB stars in M4, its N abundance is considerably enhanced. The star's low C abundance suggests that it left the AGB before the onset of third dredge-up.This work was supported by NASA grant HST-GO-13721.001-A to the University of Wisconsin, Whitewater. P.C. is supported by the Canadian Space Agency under a contract with NRC Herzberg Astronomy and Astrophysics.

A Unified Equation of State on a Microscopic Basis : Implications for Neutron Stars Structure and Cooling

NASA Astrophysics Data System (ADS)

Burgio, G. F.

2018-03-01

We discuss the structure of Neutron Stars by modelling the homogeneous nuclear matter of the core by a suitable microscopic Equation of State, based on the Brueckner-Hartree-Fock many-body theory, and the crust, including the pasta phase, by the BCPM energy density functional which is based on the same Equation of State. This allows for a uni ed description of the Neutron Star matter over a wide density range. A comparison with other uni ed approaches is discussed. With the same Equation of State, which features strong direct Urca processes and using consistent nuclear pairing gaps as well as effective masses, we model neutron star cooling, in particular the current rapid cooldown of the neutron star Cas A. We nd that several scenarios are possible to explain the features of Cas A, but only large and extended proton 1 S 0 gaps and small neutron 3 PF 2 gaps can accommodate also the major part of the complete current cooling data.

Clumpy wind accretion in supergiant neutron star high mass X-ray binaries

NASA Astrophysics Data System (ADS)

Bozzo, E.; Oskinova, L.; Feldmeier, A.; Falanga, M.

2016-05-01

The accretion of the stellar wind material by a compact object represents the main mechanism powering the X-ray emission in classical supergiant high mass X-ray binaries and supergiant fast X-ray transients. In this work we present the first attempt to simulate the accretion process of a fast and dense massive star wind onto a neutron star, taking into account the effects of the centrifugal and magnetic inhibition of accretion ("gating") due to the spin and magnetic field of the compact object. We made use of a radiative hydrodynamical code to model the nonstationary radiatively driven wind of an O-B supergiant star and then place a neutron star characterized by a fixed magnetic field and spin period at a certain distance from the massive companion. Our calculations follow, as a function of time (on a total timescale of several hours), the transitions of the system through all different accretion regimes that are triggered by the intrinsic variations in the density and velocity of the nonstationary wind. The X-ray luminosity released by the system is computed at each time step by taking into account the relevant physical processes occurring in the different accretion regimes. Synthetic lightcurves are derived and qualitatively compared with those observed from classical supergiant high mass X-ray binaries and supergiant fast X-ray transients. Although a number of simplifications are assumed in these calculations, we show that taking into account the effects of the centrifugal and magnetic inhibition of accretion significantly reduces the average X-ray luminosity expected for any neutron star wind-fed binary. The present model calculations suggest that long spin periods and stronger magnetic fields are favored in order to reproduce the peculiar behavior of supergiant fast X-ray transients in the X-ray domain.

The Chandra Delta Ori Large Project: Occultation Measurements of the Shocked Gas tn the Nearest Eclipsing O-Star Binary

NASA Technical Reports Server (NTRS)

Corcoran, Michael F.; Nichols, Joy; Naze, Yael; Rauw, Gregor; Pollock, Andrew; Moffat, Anthony; Richardson, Noel; Evans, Nancy; Hamaguchi, Kenji; Oskinova, Lida;

X-ray imaging and spectroscopic study of the SNR Kes 73 hosting the magnetar 1E 1841-045

NASA Astrophysics Data System (ADS)

Kumar, H. S.; Safi-Harb, S.; Slane, P. O.; Gotthelf, E. V.

2014-01-01

We present the first detailed Chandra and XMM-Newton study of the young Galactic supernova remnant (SNR) Kes 73 associated with the anomalous X-ray pulsar (AXP) 1E 1841-045. Images of the remnant in the radio (20 cm), infrared (24 μm), and X-rays (0.5-7 keV) reveal a spherical morphology with a bright western limb. High-resolution Chandra images show bright diffuse emission across the remnant, with several small-scale clumpy and knotty structures filling the SNR interior. The overall Chandra and XMM-Newton spectrum of the SNR is best described by a two-component thermal model with the hard component characterized by a low ionization timescale, suggesting that the hot plasma has not yet reached ionization equilibrium. The soft component is characterized by enhanced metal abundances from Mg, Si, and S, suggesting the presence of metal-rich supernova ejecta. We discuss the explosion properties of the supernova and infer the mass of its progenitor star. Such studies shed light on our understanding of SNRs associated with highly magnetized neutron stars.

Suppression of cooling by strong magnetic fields in white dwarf stars.

PubMed

Valyavin, G; Shulyak, D; Wade, G A; Antonyuk, K; Zharikov, S V; Galazutdinov, G A; Plachinda, S; Bagnulo, S; Machado, L Fox; Alvarez, M; Clark, D M; Lopez, J M; Hiriart, D; Han, Inwoo; Jeon, Young-Beom; Zurita, C; Mujica, R; Burlakova, T; Szeifert, T; Burenkov, A

2014-11-06

Isolated cool white dwarf stars more often have strong magnetic fields than young, hotter white dwarfs, which has been a puzzle because magnetic fields are expected to decay with time but a cool surface suggests that the star is old. In addition, some white dwarfs with strong fields vary in brightness as they rotate, which has been variously attributed to surface brightness inhomogeneities similar to sunspots, chemical inhomogeneities and other magneto-optical effects. Here we describe optical observations of the brightness and magnetic field of the cool white dwarf WD 1953-011 taken over about eight years, and the results of an analysis of its surface temperature and magnetic field distribution. We find that the magnetic field suppresses atmospheric convection, leading to dark spots in the most magnetized areas. We also find that strong fields are sufficient to suppress convection over the entire surface in cool magnetic white dwarfs, which inhibits their cooling evolution relative to weakly magnetic and non-magnetic white dwarfs, making them appear younger than they truly are. This explains the long-standing mystery of why magnetic fields are more common amongst cool white dwarfs, and implies that the currently accepted ages of strongly magnetic white dwarfs are systematically too young.

SG1120-1202: Mass-quenching as Tracked by UV Emission in the Group Environment at z=0.37

NASA Astrophysics Data System (ADS)

Monroe, Jonathan T.; Tran, Kim-Vy H.; Gonzalez, Anthony H.

2017-02-01

We use the Hubble Space Telescope to obtain WFC3/F390W imaging of the supergroup SG1120-1202 at z=0.37, mapping the UV emission of 138 spectroscopically confirmed members. We measure total (F390W-F814W) colors and visually classify the UV morphology of individual galaxies as “clumpy” or “smooth.” Approximately 30% of the members have pockets of UV emission (clumpy) and we identify for the first time in the group environment galaxies with UV morphologies similar to the “jellyfish” galaxies observed in massive clusters. We stack the clumpy UV members and measure a shallow internal color gradient, which indicates that unobscured star formation is occurring throughout these galaxies. We also stack the four galaxy groups and measure a strong trend of decreasing UV emission with decreasing projected group distance ({R}{proj}). We find that the strong correlation between decreasing UV emission and increasing stellar mass can fully account for the observed trend in (F390W-F814W)-{R}{proj}, I.e., mass-quenching is the dominant mechanism for extinguishing UV emission in group galaxies. Our extensive multi-wavelength analysis of SG1120-1202 indicates that stellar mass is the primary predictor of UV emission, but that the increasing fraction of massive (red/smooth) galaxies at {R}{proj} ≲ 2 R 200 and existence of jellyfish candidates is due to the group environment.

Model atmospheres for cool stars. [varying chemical composition

NASA Technical Reports Server (NTRS)

Johnson, H. R.

1974-01-01

This report contains an extensive series of model atmospheres for cool stars having a wide range in chemical composition. Model atmospheres (temperature, pressure, density, etc.) are tabulated, along with emergent energy flux distributions, limb darkening, and information on convection for selected models. The models are calculated under the usual assumptions of hydrostatic equilibrium, constancy of total energy flux (including transport both by radiation and convection) and local thermodynamic equilibrium. Some molecular and atomic line opacity is accounted for as a straight mean. While cool star atmospheres are regimes of complicated physical conditions, and these atmospheres are necessarily approximate, they should be useful for a number of kinds of spectral and atmospheric analysis.

Stellar Magnetism, Winds and their Effects on Planetary Environments

NASA Astrophysics Data System (ADS)

Vidotto, A. A.

2016-08-01

Here, I review some recent works on magnetism of cool, main-sequence stars, their winds and potential impact on surrounding exoplanets. The winds of these stars are very tenuous and persist during their lifetime. Although carrying just a small fraction of the stellar mass, these magnetic winds carry away angular momentum, thus regulating the rotation of the star. Since cool stars are likely to be surrounded by planets, understanding the host star winds and magnetism is a key step towards characterisation of exoplanetary environments. As rotation and activity are intimately related, the spin down of stars leads to a decrease in stellar activity with age. As a consequence, as stars age, a decrease in high-energy (X-ray, extreme ultraviolet) irradiation is observed, which can a ect the evaporation of exoplanetary atmospheres and, thus, also altering exoplanetary evolution.

Regulation of star formation in giant galaxies by precipitation, feedback and conduction.

PubMed

Voit, G M; Donahue, M; Bryan, G L; McDonald, M

2015-03-12

The Universe's largest galaxies reside at the centres of galaxy clusters and are embedded in hot gas that, if left undisturbed, would cool quickly and create many more new stars than are actually observed. Cooling can be regulated by feedback from accretion of cooling gas onto the central black hole, but requires an accretion rate finely tuned to the thermodynamic state of the hot gas. Theoretical models in which cold clouds precipitate out of the hot gas via thermal instability and accrete onto the black hole exhibit the necessary tuning. Recent observational evidence shows that the abundance of cold gas in the centres of clusters increases rapidly near the predicted threshold for instability. Here we report observations showing that this precipitation threshold extends over a large range in cluster radius, cluster mass and cosmic time. We incorporate the precipitation threshold into a framework of theoretical models for the thermodynamic state of hot gas in galaxy clusters. According to that framework, precipitation regulates star formation in some giant galaxies, while thermal conduction prevents star formation in others if it can compensate for radiative cooling and shut off precipitation.

Chandra Observations of MS0440.5+0204 & MS0839.9+2938: Cooling Flow Clusters in Formation?

NASA Astrophysics Data System (ADS)

McNamara, Brian

2000-09-01

We propose to observe two redshift z~0.2 clusters, MS0839.9+2938 and MS0440+0204, discovered as bright X-ray sources in the Einstein Medium Sensitivity Survey. The cluster cores are structured in the X-ray and optical bands, and they harbor large cooling flows. Their central cluster galaxies contain luminous nebular emission systems, active star formation, and strong radio sources. Using the Chandra data, we will determine whether the large discrepancies between the X-ray cooling rates and optical star formation rates can be reconciled, and we will test the hypothesis that cooling flows form as cool, dense groups accrete into massive clusters.

ON THE TIDAL ORIGIN OF HOT JUPITER STELLAR OBLIQUITY TRENDS

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

Dawson, Rebekah I., E-mail: rdawson@berkeley.edu

It is debated whether the two hot Jupiter populations—those on orbits misaligned from their host star's spin axis and those well-aligned—result from two migration channels or from two tidal realignment regimes. Here I demonstrate that equilibrium tides raised by a planet on its star can account for three observed spin-orbit alignment trends: the aligned orbits of hot Jupiters orbiting cool stars, the planetary mass cut-off for retrograde planets, and the stratification by planet mass of cool host stars' rotation frequencies. The first trend can be caused by strong versus weak magnetic braking (the Kraft break), rather than realignment of themore » star's convective envelope versus the entire star. The second trend can result from a small effective stellar moment of inertia participating in the tidal realignment in hot stars, enabling massive retrograde planets to partially realign to become prograde. The third trend is attributable to higher-mass planets more effectively counteracting braking to spin up their stars. Both hot and cool stars require a small effective stellar moment of inertia participating in the tidal realignment, e.g., an outer layer weakly coupled to the interior. I demonstrate via Monte Carlo that this model can match the observed trends and distributions of sky-projected misalignments and stellar rotation frequencies. I discuss implications for inferring hot Jupiter migration mechanisms from obliquities, emphasizing that even hot stars do not constitute a pristine sample.« less

Towards new generation spectroscopic models of cool stars

NASA Astrophysics Data System (ADS)

Bergemann, Maria

2018-06-01

Abstract: Spectroscopy is a unique tool to determine the physical parameters of stars. Knowledge of stellar chemical abundances, masses, and ages is the key to understanding the evolution of their host populations. I will focus on the current outstanding problems in spectroscopy of cool stars, which are the most useful objects in studies of our local Galactic neighborhood but also very distant systems, like faint dwarf Spheroidal galaxies. Among the most debated issues is to what extent can we trust the techniques, which rely on the classical assumptions of local thermodynamic equilibrium and hydrostatic balance. I will summarise the ongoing efforts to improve the models of cool stars, with the emphasis on NLTE and 3D modelling. I will then discuss how these exciting observations impact our knowledge of abundances in the Milky Way and in dSph systems, and present outlook for the future studies.

Unexpectedly large mass loss during the thermal pulse cycle of the red giant star R Sculptoris.

PubMed

Maercker, M; Mohamed, S; Vlemmings, W H T; Ramstedt, S; Groenewegen, M A T; Humphreys, E; Kerschbaum, F; Lindqvist, M; Olofsson, H; Paladini, C; Wittkowski, M; de Gregorio-Monsalvo, I; Nyman, L-A

2012-10-11

The asymptotic-giant-branch star R Sculptoris is surrounded by a detached shell of dust and gas. The shell originates from a thermal pulse during which the star underwent a brief period of increased mass loss. It has hitherto been impossible to constrain observationally the timescales and mass-loss properties during and after a thermal pulse--parameters that determine the lifetime of the asymptotic giant branch and the amount of elements returned by the star. Here we report observations of CO emission from the circumstellar envelope and shell around R Sculptoris with an angular resolution of 1.3″. What was previously thought to be only a thin, spherical shell with a clumpy structure is revealed to also contain a spiral structure. Spiral structures associated with circumstellar envelopes have been previously seen, leading to the conclusion that the systems must be binaries. Combining the observational data with hydrodynamic simulations, we conclude that R Sculptoris is a binary system that underwent a thermal pulse about 1,800 years ago, lasting approximately 200 years. About 3 × 10(-3) solar masses of material were ejected at a velocity of 14.3 km s(-1) and at a rate around 30 times higher than the pre-pulse mass-loss rate. This shows that about three times more mass was returned to the interstellar medium during and immediately after the pulse than previously thought.

Neutron star cooling and pion condensation

NASA Technical Reports Server (NTRS)

Umeda, Hideyuki; Nomoto, Ken'ichi; Tsuruta, Sachiko; Muto, Takumi; Tatsumi, Toshitaka

1994-01-01

The nonstandard cooling of a neutron star with the central pion core is explored. By adopting the latest results from the pion condensation theory, neutrino emissivity is calulated for both pure charged pions and a mixture of charged and neutral pions, and the equations of state are constructed for the pion condensate. The effect of superfluidity on cooling is investigated, adopting methods more realistic than in previous studies. Our theoretical models are compared with the currently updated observational data, and possible implications are explored.

FIRST ZEEMAN DOPPLER IMAGING OF A COOL STAR USING ALL FOUR STOKES PARAMETERS

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

Rosén, L.; Kochukhov, O.; Wade, G. A.

Magnetic fields are ubiquitous in active cool stars, but they are in general complex and weak. Current Zeeman Doppler imaging (ZDI) studies of cool star magnetic fields chiefly employ circular polarization observations because linear polarization is difficult to detect and requires a more sophisticated radiative transfer modeling to interpret. But it has been shown in previous theoretical studies, and in the observational analyses of magnetic Ap stars, that including linear polarization in the magnetic inversion process makes it possible to correctly recover many otherwise lost or misinterpreted magnetic features. We have obtained phase-resolved observations in all four Stokes parameters ofmore » the RS CVn star II Peg at two separate epochs. Here we present temperature and magnetic field maps reconstructed for this star using all four Stokes parameters. This is the very first such ZDI study of a cool active star. Our magnetic inversions reveal a highly structured magnetic field topology for both epochs. The strength of some surface features is doubled or even quadrupled when linear polarization is taken into account. The total magnetic energy of the reconstructed field map also becomes about 2.1–3.5 times higher. The overall complexity is also increased as the field energy is shifted toward higher harmonic modes when four Stokes parameters are used. As a consequence, the potential field extrapolation of the four Stokes parameter ZDI results indicates that magnetic field becomes weaker at a distance of several stellar radii due to a decrease of the large-scale field component.« less

Understanding Emission From The Ism In The Milky Way And Other Galaxies Using [Nii] And Other Tracers

NASA Astrophysics Data System (ADS)

Goldsmith, Paul

We propose to combine recently-obtained data on the far-infrared fine structure lines of ionized nitrogen ([NII]) with studies of radio continuum and other far-infrared lines, to address a number of key questions about the structure of the interstellar medium, star formation, and to bootstrap our detailed understanding of processes in the Milky Way to more distant galaxies. The [CII] 158 μm line is the most powerful single far-infrared line emitted by galaxies, but since carbon can be ionized by photons having wavelengths longer than required to ionized hydrogen, [CII] emission can originate in atomic, molecular, and ionized regions. Due to its ionization potential being higher than that of hydrogen, nitrogen is a more selective tracer than carbon, and can serve as a critical ingredient to determine how much of the [CII] emission is produced in HII regions powered by massive young stars. We propose to take advantage of a newly-completed Herschel survey of the Galactic plane in both of the [NII] lines at 205 μm and 122 μm wavelength, complementing the GOT C+ survey of [CII]. The relative intensity of the two [NII] lines is a direct indicator of the electron density, and thus allows determination of the N+ column density. With the two data sets we can then determine what fraction of the [CII] emission is produced in HII regions, and what may be from the more diffuse components, thus enabling reconciliation of the Herschel and COBE measurements of this critical tracer. We also propose to use Planck and WMAP all-sky maps of the radio free-free emission to determine the emission measure. Comparison with the electron density and ionized nitrogen column density enables a novel probe of clumpiness in this component of the interstellar medium in addition to giving an improved method of determining the overall Galactic electron density distribution. A second aspect of our investigation of clumpiness will be to utilize the fact that each [NII] observation with the PACS instrument on Herschel consists of 25 spectra on a 5x5 grid. Using these, we will compare derived column densities and electron densities over these small regions to assess the degree of clumpiness. A third input to our analysis of inhomogeneities will be to compare the Planck & WMAP images of selected regions with much higher resolution ground-based data from survey such as CORNISH and GALFACTS. We also will use the improved understanding of [CII] and [NII] emission to develop a template for use in interpreting emission of these (and possibly other) tracers from other galaxies. We will be able to calculate the relative luminosities of these two tracers as a function of Galactocentric distance in the Milky Way, which will be available for comparison with existing and future data on other galaxies. This very active field suffers from an inability to distinguish the multiple sources of [CII] emission, and the understanding of how this spectral line works as a tracer of star formation is of very great interest to a large community of extragalactic astronomers. Together these studies will significantly improve our understanding of [CII] emission and its role as a coolant of the ISM, and the role of HII regions produced by massive young stars as sources of far-infrared line emission. In addition to a better appreciation of the life cycle of the interstellar medium, this work will also set the stage for better understanding of far-infrared emission from other galaxies though development of an improved Milky Way template. All of the data have been taken and are available including our own Herschel [NII] data, which have been reduced. As indicated in the proposal, we have made a limited demonstration of the required techniques, so that we are confident that this proposal will yield the desired new information and astrophysical results.

Molecular rotational line profiles from oxygen-rich red giant winds

NASA Technical Reports Server (NTRS)

Justtanont, K.; Skinner, C. J.; Tielens, A. G. G. M.

1994-01-01

We have developed a radiative transfer model of the dust and gas envelopes around late-type stars. The gas kinetic temperature for each star is calculated by solving equations of motion and the energy balance simultaneously. The main processes include viscous heating and adiabatic and radiative cooling. Heating is dominated by viscosity as the grains stream outward through the gas, with some contribution in oxygen-rich stars by near-IR pumping of H2O followed by collisional de-excitation in the inner envelope. For O-rich stars, rotational H2O cooling is a dominant mechanism in the middle part of the envelope, with CO cooling being less significant. We have applied our model to three well-studied oxygen-rich red giant stars. The three stars cover a wide range of mass-loss rates, and hence they have different temperature structures. The derived temperature structures are used in calculating CO line profiles for these objects. Comparison of the dust and gas mass-loss rates suggests that mass-loss rates are not constant during the asymptotic giant branch phase. In particular, the results show that the low CO 1-0 antenna temperatures of OH/IR stars reflect an earlier phase of much lower mass-loss rate.

Gaia Confirms that SDSS J102915+172927 is a Dwarf Star

NASA Astrophysics Data System (ADS)

Bonifacio, P.; Caffau, E.; Spite, M.; Spite, F.; François, P.; Zaggia, S.; Arenou, F.; Haigron, R.; Leclerc, N.; Marchal, O.; Panuzzo, P.; Plum, G.; Sartoretti, P.

2018-05-01

The Gaia Data Release 2 provides a parallax of 0.734+/-0.073 mas for SDSS J102915+172927, currently the most metal-poor known object. This parallax implies that it is dwarf star, ruling out the scenario that it is a subgiant. The subgiant scenario had as a corollary that the star had been formed in a medium highly enriched in C, thus making line cooling efficient during the collapse, that was also highly enriched in Fe by Type Ia SNe. This scenario can also now be ruled out for this star, reinforcing the need of dust cooling and fragmentation to explain its formation.

Cooling of hypernuclear compact stars

NASA Astrophysics Data System (ADS)

Raduta, Adriana R.; Sedrakian, Armen; Weber, Fridolin

2018-04-01

We study the thermal evolution of hypernuclear compact stars constructed from covariant density functional theory of hypernuclear matter and parametrizations which produce sequences of stars containing two-solar-mass objects. For the input in the simulations, we solve the Bardeen-Cooper-Schrieffer gap equations in the hyperonic sector and obtain the gaps in the spectra of Λ, Ξ0, and Ξ- hyperons. For the models with masses M/M⊙ ≥ 1.5 the neutrino cooling is dominated by hyperonic direct Urca processes in general. In the low-mass stars the (Λp) plus leptons channel is the dominant direct Urca process, whereas for more massive stars the purely hyperonic channels (Σ-Λ) and (Ξ-Λ) are dominant. Hyperonic pairing strongly suppresses the processes on Ξ-s and to a lesser degree on Λs. We find that intermediate-mass 1.5 ≤ M/M⊙ ≤ 1.8 models have surface temperatures which lie within the range inferred from thermally emitting neutron stars, if the hyperonic pairing is taken into account. Most massive models with M/M⊙ ≃ 2 may cool very fast via the direct Urca process through the (Λp) channel because they develop inner cores where the S-wave pairing of Λs and proton is absent.

Dark energy and the quietness of the local Hubble flow

NASA Astrophysics Data System (ADS)

Axenides, M.; Perivolaropoulos, L.

2002-06-01

The linearity and quietness of the local (<10 Mpc) Hubble flow (LHF) in view of the very clumpy local universe is a long standing puzzle in standard and in open CDM (cold dark matter) cosmogony. The question addressed in this paper is whether the antigravity component of the recently discovered dark energy can cool the velocity flow enough to provide a solution to this puzzle. We calculate the growth of matter fluctuations in a flat universe containing a fraction ΩX(t0) of dark energy obeying the time independent equation of state pX=wρX. We find that dark energy can indeed cool the LHF. However the dark energy parameter values required to make the predicted velocity dispersion consistent with the observed value vrms~=40 km/s have been ruled out by other observational tests constraining the dark energy parameters w and ΩX. Therefore despite the claims of recent qualitative studies, dark energy with time independent equation of state cannot by itself explain the quietness and linearity of the local Hubble flow.

The first high resolution image of coronal gas in a starbursting cool core cluster

NASA Astrophysics Data System (ADS)

Johnson, Sean

2017-08-01

Galaxy clusters represent a unique laboratory for directly observing gas cooling and feedback due to their high masses and correspondingly high gas densities and temperatures. Cooling of X-ray gas observed in 1/3 of clusters, known as cool-core clusters, should fuel star formation at prodigious rates, but such high levels of star formation are rarely observed. Feedback from active galactic nuclei (AGN) is a leading explanation for the lack of star formation in most cool clusters, and AGN power is sufficient to offset gas cooling on average. Nevertheless, some cool core clusters exhibit massive starbursts indicating that our understanding of cooling and feedback is incomplete. Observations of 10^5 K coronal gas in cool core clusters through OVI emission offers a sensitive means of testing our understanding of cooling and feedback because OVI emission is a dominant coolant and sensitive tracer of shocked gas. Recently, Hayes et al. 2016 demonstrated that synthetic narrow-band imaging of OVI emission is possible through subtraction of long-pass filters with the ACS+SBC for targets at z=0.23-0.29. Here, we propose to use this exciting new technique to directly image coronal OVI emitting gas at high resolution in Abell 1835, a prototypical starbursting cool-core cluster at z=0.252. Abell 1835 hosts a strong cooling core, massive starburst, radio AGN, and at z=0.252, it offers a unique opportunity to directly image OVI at hi-res in the UV with ACS+SBC. With just 15 orbits of ACS+SBC imaging, the proposed observations will complete the existing rich multi-wavelength dataset available for Abell 1835 to provide new insights into cooling and feedback in clusters.

An IUE's eye view of cool-star outer atmospheres

NASA Technical Reports Server (NTRS)

Ayres, T. R.

1981-01-01

Three topics are discussed which together demonstrate the power of the IUE to probe the occurrences of chromospheres and coronas in the cool half of the HR diagram. These are: (1) the complementary low dispersion and echelle observing modes; (2) Mg II h and k: chromospheric cooling and width luminosity correlation; and (3) empirical correlations among chromospheric, transition region, and coronal emission. The spectra of alpha Centauri (G2 V + K1 V) and Capella (G6 III + F9 III) are compared with that of the Sun and recent low dispersion surveys of cool star emission in the 1150 A to 2000 A short wavelength region are summarized.

The Infrared Telescope Facility (IRTF) Spectral Library: Cool Stars

NASA Astrophysics Data System (ADS)

Rayner, John T.; Cushing, Michael C.; Vacca, William D.

2009-12-01

We present a 0.8-5 μm spectral library of 210 cool stars observed at a resolving power of R ≡ λ/Δλ ~ 2000 with the medium-resolution infrared spectrograph, SpeX, at the 3.0 m NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. The stars have well-established MK spectral classifications and are mostly restricted to near-solar metallicities. The sample not only contains the F, G, K, and M spectral types with luminosity classes between I and V, but also includes some AGB, carbon, and S stars. In contrast to some other spectral libraries, the continuum shape of the spectra is measured and preserved in the data reduction process. The spectra are absolutely flux calibrated using the Two Micron All Sky Survey photometry. Potential uses of the library include studying the physics of cool stars, classifying and studying embedded young clusters and optically obscured regions of the Galaxy, evolutionary population synthesis to study unresolved stellar populations in optically obscured regions of galaxies and synthetic photometry. The library is available in digital form from the IRTF Web site.

Nanolensed Fast Radio Bursts

NASA Astrophysics Data System (ADS)

Eichler, David

2017-12-01

It is suggested that fast radio bursts can probe gravitational lensing by clumpy dark matter objects that range in mass from 10-3 M ⊙-102 M ⊙. They may provide a more sensitive probe than observations of lensings of objects in the Magellanic Clouds, and could find or rule out clumpy dark matter with an extended mass spectrum.

Theory of winds in late-type evolved and pre-main-sequence stars

NASA Technical Reports Server (NTRS)

Macgregor, K. B.

1983-01-01

Recent observational results confirm that many of the physical processes which are known to occur in the Sun also occur among late-type stars in general. One such process is the continuous loss of mass from a star in the form of a wind. There now exists an abundance of either direct or circumstantial evidence which suggests that most (if not all) stars in the cool portion of the HR diagram possess winds. An attempt is made to assess the current state of theoretical understanding of mass loss from two distinctly different classes of late-type stars: the post-main-sequence giant/supergiant stars and the pre-main-sequence T Tauri stars. Toward this end, the observationally inferred properties of the wind associated with each of the two stellar classes under consideration are summarized and compared against the predictions of existing theoretical models. Although considerable progress has been made in attempting to identify the mechanisms responsible for mass loss from cool stars, many fundamental problems remain to be solved.

Cool Stars May Have Different Prebiotic Chemical Mix

NASA Technical Reports Server (NTRS)

2009-01-01

NASA's Spitzer Space Telescope detected a prebiotic, or potentially life-forming, molecule called hydrogen cyanide (HCN) in the planet-forming disks around yellow stars like our sun, but not in the disks around cooler, reddish stars.

The observations are plotted in this graph, called a spectrum, in which light from the gas in the disks around the stars has been split up into its basic components, or wavelengths. Data from stars like our sun are yellow, and data from cool stars are orange. Light wavelengths are shown on the X-axis, and the relative brightness of disk emission is shown on the Y-axis. The signature of a baseline molecule, called acetylene (C₂H₂), was seen for both types of stars, but hydrogen cyanide was seen only around stars like our sun.

Hydrogen cyanide is an organic, nitrogen-containing molecule. Five hydrogen cyanide molecules can link up to form adenine, one of the four chemical bases of DNA.

Weighing Ultra-Cool Stars

NASA Astrophysics Data System (ADS)

2004-05-01

Large Ground-Based Telescopes and Hubble Team-Up to Perform First Direct Brown Dwarf Mass Measurement [1] Summary Using ESO's Very Large Telescope at Paranal and a suite of ground- and space-based telescopes in a four-year long study, an international team of astronomers has measured for the first time the mass of an ultra-cool star and its companion brown dwarf. The two stars form a binary system and orbit each other in about 10 years. The team obtained high-resolution near-infrared images; on the ground, they defeated the blurring effect of the terrestrial atmosphere by means of adaptive optics techniques. By precisely determining the orbit projected on the sky, the astronomers were able to measure the total mass of the stars. Additional data and comparison with stellar models then yield the mass of each of the components. The heavier of the two stars has a mass around 8.5% of the mass of the Sun and its brown dwarf companion is even lighter, only 6% of the solar mass. Both objects are relatively young with an age of about 500-1,000 million years. These observations represent a decisive step towards the still missing calibration of stellar evolution models for very-low mass stars. PR Photo 19a/04: Orbit of the ultra-cool stars in 2MASSW J0746425+2000321. PR Photo 19b/04: Animated Gif of the orbital motion. Telephone number star Even though astronomers have found several hundreds of very low mass stars and brown dwarfs, the fundamental properties of these extreme objects, such as masses and surface temperatures, are still not well known. Within the cosmic zoo, these ultra-cool stars represent a class of "intermediate" objects between giant planets - like Jupiter - and "normal" stars less massive than our Sun, and to understand them well is therefore crucial to the field of stellar astrophysics. The problem with these ultra-cool stars is that contrary to normal stars that burn hydrogen in their central core, no unique relation exists between the luminosity of the star and its mass. Indeed, luminosities and surface temperatures of ultra-cool dwarf stars depend both on their age and their mass. An older, somewhat more massive ultra-cool dwarf can thus have exactly the same temperature as a younger, less massive one. It is therefore a basic goal of modern astrophysics to obtain independently the masses of an ultra-cool dwarf star. This is in principle possible by studying such objects that are members in a binary system. This is precisely what an international team of astronomers [2] has now done in a four-year long study of a binary stellar system with an ultra-cool dwarf star, using a plethora of top telescopic facilities, including ESO's Very Large Telescope, as well as Keck I and Gemini North in Hawaii and also the Hubble Space Telescope. This system - with the telephone number name of 2MASSW J0746425+2000321 [3]- is located at a distance of 40 light-years. Beating the seeing ESO PR Photo 19a/04 ESO PR Photo 19a/04 Orbit of the ultra-cool stars in 2MASSW J0746425+2000321 [Preview - JPEG: 400 x 548 pix - 121k] [Normal - JPEG: 800 x 1095 pix - 320k] [Hires - JPEG: 2591 x 3546 pix - 1.8M] [Hires - TIFF: 2591 x 3546 pix - 36.8M] ESO PR Photo 19b/04 ESO PR Photo 19b/04 Animated GIF showing the orbital motion (size: 416 kb) Caption: ESO PR Photo 19a/04 shows the orbit of the brown dwarf around the ultra-cool dwarf. Each red dot on the orbit corresponds to one observation made with a ground- or space-based telescope. The observations cover 60% of the whole orbit. ESO PR Photo 19b/04 is an animated Gif showing the motion of the brown dwarf and the various high-resolution images obtained by the astronomers. The astronomers used high-angular-resolution imaging to see both stars in the binary system and to measure their motion over a four-year period. However, this is more easily said than done, as the separation on the sky between the two stars is quite small: between 0.13 and 0.22 arcsec. This corresponds to the size of a 1-Euro coin, seen at a distance of about 25 km. This separation is so small that it is normally not possible to differentiate the two stars due to the blurring effect of atmospheric turbulence (the "seeing"). It is therefore necessary to use the technique of adaptive optics. This wonderful method is based on the measurement of the image quality in real-time and sending corresponding corrective signals up to 100 times every second to a small deformable mirror, located in front of the detector. As the mirror continuously modifies its shape, the disturbing effect of the turbulence is neutralised. Applied at the VLT, this technique has resulted in images which are at least ten times sharper than the "seeing" and which therefore show many more details in the observed objects. At the Very Large Telescope, the astronomers used the state-of-the-art adaptive optics NACO instrument [4]. Says Hervé Bouy, principal author of the paper presenting the results described here: "NACO offers the possibility to work in the infrared and is therefore ideally suited for the study of ultra-cool stars, which emit most of their light in this wavelength range. With the combination of the high efficiency of NACO and the VLT, and the excellent atmospheric conditions prevailing at Paranal, we were able to achieve very sharp images of this binary stellar system, almost as good as if the telescope were located in space." Ultra-cool and on diet During their four-year long study, seven different relative positions of the two components of the binary system were measured and Hervé Bouy and his co-workers were able to determine with good precision the stellar orbits. They find that the two stars revolve around each other once every 10 years and that their physical separation is only 2.5 times the distance of the Earth to the Sun - as astronomers say, 2.5 Astronomical Units. Using Kepler's laws, it is then straightforward to derive the total mass of the system. The obtained value is less than 15 % of the mass of the Sun. The astronomers then used the photometric data of each star obtained in several wavebands, as well as spectra obtained with the Hubble Space Telescope to study the two objects in more detail. Using the latest stellar models of the group of the Ecole Normale Supérieure de Lyon, they found that both stars have roughly the same surface temperature, around 1500 °C (1800 K). For a star, this is ultra-cool indeed - by comparison, the surface temperature of the Sun is more than three times higher. Using theoretical models, the team also found that the two stars are rather young (in astrophysical terms) - their age is between 500 and 1,000 million years only. The more massive of the two has a mass between 7.5 and 9.5% the mass of the Sun, while its companion has a mass between 5 and 7% of the solar mass. Objects weighing less than about 7% of our Sun have been variously called "Brown Dwarfs", "Failed Stars" or "Super Planets". Indeed, since they have no sustained energy generation by thermal nuclear reactions in their interior, many of their properties are more similar to those of giant gas planets in our own solar system such as Jupiter, than to stars like the Sun. The system 2MASSW J0746425+2000321 is thus apparently made up of a brown dwarf orbiting a slightly more massive ultra-cool dwarf star. It is a true "Rosetta stone" in the new field of low-mass stellar astrophysics and further studies will surely provide more valuable information about these objects in the transitional zone between stars and planets. More information The research described in this press release is published in the research journal Astronomy & Astrophysics ("First determination of the dynamical mass of a binary L1.5 dwarf" by H. Bouy et al.). The paper is available in PDF format on the publisher web site.

CONNECTING STAR FORMATION QUENCHING WITH GALAXY STRUCTURE AND SUPERMASSIVE BLACK HOLES THROUGH GRAVITATIONAL HEATING OF COOLING FLOWS

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

Guo, Fulai, E-mail: fulai.guo@phys.ethz.ch

2014-12-20

Recent observations suggested that star formation quenching in galaxies is related to galaxy structure. Here we propose a new mechanism to explain the physical origin of this correlation. We assume that while quenching is maintained in quiescent galaxies by a feedback mechanism, cooling flows in the hot halo gas can still develop intermittently. We study cooling flows in a large suite of around 90 hydrodynamic simulations of an isolated galaxy group, and find that the flow development depends significantly on the gravitational potential well in the central galaxy. If the galaxy's gravity is not strong enough, cooling flows result inmore » a central cooling catastrophe, supplying cold gas and feeding star formation to galactic bulges. When the bulge grows prominent enough, compressional heating starts to offset radiative cooling and maintains cooling flows in a long-term hot mode without producing a cooling catastrophe. Our model thus describes a self-limited growth channel for galaxy bulges and naturally explains the connection between quenching and bulge prominence. In particular, we explicitly demonstrate that M{sub ∗}/R{sub eff}{sup 1.5} is a good structural predictor of quenching. We further find that the gravity from the central supermassive black hole also affects the bimodal fate of cooling flows, and we predict a more general quenching predictor to be M{sub bh}{sup 1.6}M{sub ∗}/R{sub eff}{sup 1.5}, which may be tested in future observational studies.« less

Cool circumstellar matter around nearby main-sequence stars

NASA Technical Reports Server (NTRS)

Walker, H. J.; Wolstencroft, R. D.

1988-01-01

Stars are presented which have characteristics similar to Vega and other main-sequence stars with cool dust disks, based on the IRAS Point Source Catalog fluxes. The objects are selected to have a 60-micron/100-micron ratio similar to Vega, Beta Pic, Alpha PsA, and Epsilon Eri, and they are also required to show evidence of extension in the IRAS Working Survey Database. The fluxes are modeled using a blackbody energy distribution. The temperatures derived range from 50 to 650 K. The diameters of the dust disks observed by IRAS are estimated.

Identification of new fluorescence processes in the UV spectra of cool stars from new energy levels of Fe II and Cr II

NASA Technical Reports Server (NTRS)

Johansson, Sveneric; Carpenter, Kenneth G.

1988-01-01

Two fluorescence processes operating in atmospheres of cool stars, symbiotic stars, and the Sun are presented. Two emission lines, at 1347.03 and 1360.17 A, are identified as fluorescence lines of Cr II and Fe II. The lines are due to transitions from highly excited levels, which are populated radiatively by the hydrogen Lyman alpha line due to accidental wavelength coincidences. Three energy levels, one in Cr II and two in Fe II, are reported.

Turbulence modeling needs of commercial CFD codes: Complex flows in the aerospace and automotive industries

NASA Technical Reports Server (NTRS)

Befrui, Bizhan A.

1995-01-01

This viewgraph presentation discusses the following: STAR-CD computational features; STAR-CD turbulence models; common features of industrial complex flows; industry-specific CFD development requirements; applications and experiences of industrial complex flows, including flow in rotating disc cavities, diffusion hole film cooling, internal blade cooling, and external car aerodynamics; and conclusions on turbulence modeling needs.

The Not So Simple Globular Cluster ω Cen. I. Spatial Distribution of the Multiple Stellar Populations

NASA Astrophysics Data System (ADS)

Calamida, A.; Strampelli, G.; Rest, A.; Bono, G.; Ferraro, I.; Saha, A.; Iannicola, G.; Scolnic, D.; James, D.; Smith, C.; Zenteno, A.

2017-04-01

We present a multi-band photometric catalog of ≈1.7 million cluster members for a field of view of ≈2° × 2° across ω Cen. Photometry is based on images collected with the Dark Energy Camera on the 4 m Blanco telescope and the Advanced Camera for Surveys on the Hubble Space Telescope. The unprecedented photometric accuracy and field coverage allowed us, for the first time, to investigate the spatial distribution of ω Cen multiple populations from the core to the tidal radius, confirming its very complex structure. We found that the frequency of blue main-sequence stars is increasing compared to red main-sequence stars starting from a distance of ≈25‧ from the cluster center. Blue main-sequence stars also show a clumpy spatial distribution, with an excess in the northeast quadrant of the cluster pointing toward the direction of the Galactic center. Stars belonging to the reddest and faintest red-giant branch also show a more extended spatial distribution in the outskirts of ω Cen, a region never explored before. Both these stellar sub-populations, according to spectroscopic measurements, are more metal-rich compared to the cluster main stellar population. These findings, once confirmed, make ω Cen the only stellar system currently known where metal-rich stars have a more extended spatial distribution compared to metal-poor stars. Kinematic and chemical abundance measurements are now needed for stars in the external regions of ω Cen to better characterize the properties of these sub-populations. Based on observations made with the Dark Energy Camera (DECam) on the 4 m Blanco telescope (NOAO) under programs 2014A-0327, 2015A-0151, 2016A-0189, PIs: A. Calamida, A. Rest, and on observations made with the NASA/ESA Hubble Space Telescope, obtained by the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

The Origin of Nonradiative Heating/momentum in Hot Stars

NASA Technical Reports Server (NTRS)

Underhill, A. B. (Editor); Michalitsianos, A. G. (Editor)

1985-01-01

The origin of nonradiative heating and momentum in the atmospheres of stars is studied. The similarities and differences between what occurs in the hot stars and what occurs in cool stars are emphasized. Key points in the theory are reviewed. Areas requiring new study are indicated.

Gas expulsion vs gas retention in young stellar clusters II: effects of cooling and mass segregation

NASA Astrophysics Data System (ADS)

Silich, Sergiy; Tenorio-Tagle, Guillermo

2018-05-01

Gas expulsion or gas retention is a central issue in most of the models for multiple stellar populations and light element anti-correlations in globular clusters. The success of the residual matter expulsion or its retention within young stellar clusters has also a fundamental importance in order to understand how star formation proceeds in present-day and ancient star-forming galaxies and if proto-globular clusters with multiple stellar populations are formed in the present epoch. It is usually suggested that either the residual gas is rapidly ejected from star-forming clouds by stellar winds and supernova explosions, or that the enrichment of the residual gas and the formation of the second stellar generation occur so rapidly, that the negative stellar feedback is not significant. Here we continue our study of the early development of star clusters in the extreme environments and discuss the restrictions that strong radiative cooling and stellar mass segregation provide on the gas expulsion from dense star-forming clouds. A large range of physical initial conditions in star-forming clouds which include the star-forming cloud mass, compactness, gas metallicity, star formation efficiency and effects of massive stars segregation are discussed. It is shown that in sufficiently massive and compact clusters hot shocked winds around individual massive stars may cool before merging with their neighbors. This dramatically reduces the negative stellar feedback, prevents the development of the global star cluster wind and expulsion of the residual and the processed matter into the ambient interstellar medium. The critical lines which separate the gas expulsion and the gas retention regimes are obtained.

Multiplicity of the Galactic Senior Citizens: A High-resolution Search for Cool Subdwarf Companions

NASA Astrophysics Data System (ADS)

Ziegler, Carl; Law, Nicholas M.; Baranec, Christoph; Riddle, Reed L.; Fuchs, Joshua T.

2015-05-01

Cool subdwarfs are the oldest members of the low-mass stellar population. Mostly present in the galactic halo, subdwarfs are characterized by their low-metallicity. Measuring their binary fraction and comparing it to solar-metallicity stars could give key insights into the star formation process early in the Milky Way’s history. However, because of their low luminosity and relative rarity in the solar neighborhood, binarity surveys of cool subdwarfs have suffered from small sample sizes and incompleteness. Previous surveys have suggested that the binary fraction of red subdwarfs is much lower than for their main-sequence cousins. Using the highly efficient Robo-AO system, we present the largest high-resolution survey of subdwarfs, sensitive to angular separations (ρ ≥slant 0.″ 15) and contrast ratios ({Δ }{{m}i} ≤slant 6) invisible in past surveys. Of 344 target cool subdwarfs, 43 are in multiple systems, 19 of which are newly discovered, for a binary fraction of 12.5 ± 1.9%. We also discovered seven triple star systems for a triplet fraction of 2.0 ± 0.8%. Comparisons to similar surveys of solar-metallicity dwarf stars gives a ∼3σ disparity in luminosity between companion stars, with subdwarfs displaying a shortage of low-contrast companions. We also observe a lack of close subdwarf companions in comparison to similar-mass dwarf multiple systems.

The Relationship Between Brightest Cluster Galaxy Star Formation and the Intracluster Medium in CLASH

NASA Astrophysics Data System (ADS)

Fogarty, Kevin; Postman, Marc; Larson, Rebecca; Donahue, Megan; Moustakas, John

2017-09-01

We study the nature of feedback mechanisms in the 11 CLASH brightest cluster galaxies (BCGs) that exhibit extended ultraviolet and nebular line emission features. We estimate star formation rates (SFRs), dust masses, and starburst durations using a Bayesian photometry-fitting technique that accounts for both stellar and dust emission from the UV through far-IR. By comparing these quantities to intracluster medium (ICM) cooling times and freefall times derived from X-ray observations and lensing estimates of the cluster mass distribution, we discover a tight relationship between the BCG SFR and the ICM cooling time to freefall time ratio, {t}{cool}/{t}{ff}, with an upper limit on the intrinsic scatter of 0.15 dex. Furthermore, starburst durations may correlate with ICM cooling times at a radius of 0.025 {R}500, and the two quantities converge upon reaching the gigayear regime. Our results provide a direct observational link between the thermodynamical state of the ICM and the intensity and duration of BCG star formation activity, and appear consistent with a scenario where active galactic nuclei induce condensation of thermally unstable ICM overdensities that fuel long-duration (>1 Gyr) BCG starbursts. This scenario can explain (a) how gas with a low cooling time is depleted without causing a cooling flow and (b) the scaling relationship between SFR and {t}{cool}/{t}{ff}. We also find that the scaling relation between SFR and dust mass in BCGs with SFRs < 100 {M}⊙ yr-1 is similar to that in star-forming field galaxies; BCGs with large (> 100 {M}⊙ yr-1) SFRs have dust masses comparable to extreme starbursts.

Baryons Matter: Why Luminous Satellite Galaxies have Reduced Central Masses

NASA Astrophysics Data System (ADS)

Zolotov, Adi; Brooks, Alyson M.; Willman, Beth; Governato, Fabio; Pontzen, Andrew; Christensen, Charlotte; Dekel, Avishai; Quinn, Tom; Shen, Sijing; Wadsley, James

2012-12-01

Using high-resolution cosmological hydrodynamical simulations of Milky Way-massed disk galaxies, we demonstrate that supernovae feedback and tidal stripping lower the central masses of bright (-15 < MV < -8) satellite galaxies. These simulations resolve high-density regions, comparable to giant molecular clouds, where stars form. This resolution allows us to adopt a prescription for H2 formation and destruction that ties star formation to the presence of shielded, molecular gas. Before infall, supernova feedback from the clumpy, bursty star formation captured by this physically motivated model leads to reduced dark matter (DM) densities and shallower inner density profiles in the massive satellite progenitors (M vir >= 109 M ⊙, M * >= 107 M ⊙) compared with DM-only simulations. The progenitors of the lower mass satellites are unable to maintain bursty star formation histories, due to both heating at reionization and gas loss from initial star-forming events, preserving the steep inner density profile predicted by DM-only simulations. After infall, gas stripping from satellites reduces the total central masses of satellites simulated with DM+baryons relative to DM-only satellites. Additionally, enhanced tidal stripping after infall due to the baryonic disk acts to further reduce the central DM densities of the luminous satellites. Satellites that enter with cored DM halos are particularly vulnerable to the tidal effects of the disk, exacerbating the discrepancy in the central masses predicted by baryon+DM and DM-only simulations. We show that DM-only simulations, which neglect the highly non-adiabatic evolution of baryons described in this work, produce denser satellites with larger central velocities. We provide a simple correction to the central DM mass predicted for satellites by DM-only simulations. We conclude that DM-only simulations should be used with great caution when interpreting kinematic observations of the Milky Way's dwarf satellites.

Star Formation at z = 2.481 in the Lensed Galaxy SDSS J1110 = 6459. I. Lens Modeling and Source Reconstruction

NASA Astrophysics Data System (ADS)

Johnson, Traci L.; Sharon, Keren; Gladders, Michael D.; Rigby, Jane R.; Bayliss, Matthew B.; Wuyts, Eva; Whitaker, Katherine E.; Florian, Michael; Murray, Katherine T.

2017-07-01

Using the combined resolving power of the Hubble Space Telescope and gravitational lensing, we resolve star-forming structures in a z˜ 2.5 galaxy on scales much smaller than the usual kiloparsec diffraction limit of HST. SGAS J111020.0+645950.8 is a clumpy, star-forming galaxy lensed by the galaxy cluster SDSS J1110+6459 at z=0.659, with a total magnification ˜ 30× across the entire arc. We use a hybrid parametric/non-parametric strong lensing mass model to compute the deflection and magnification of this giant arc, reconstruct the light distribution of the lensed galaxy in the source plane, and resolve the star formation into two dozen clumps. We develop a forward-modeling technique to model each clump in the source plane. We ray-trace the model to the image plane, convolve with the instrumental point-spread function (PSF), and compare with the GALFIT model of the clumps in the image plane, which decomposes clump structure from more extended emission. This technique has the advantage, over ray-tracing, of accounting for the asymmetric lensing shear of the galaxy in the image plane and the instrument PSF. At this resolution, we can begin to study star formation on a clump-by-clump basis, toward the goal of understanding feedback mechanisms and the buildup of exponential disks at high redshift. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained 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 # 13003.

Disordered Nuclear Pasta, Magnetic Field Decay, and Crust Cooling in Neutron Stars

NASA Astrophysics Data System (ADS)

Horowitz, C. J.; Berry, D. K.; Briggs, C. M.; Caplan, M. E.; Cumming, A.; Schneider, A. S.

2015-01-01

Nuclear pasta, with nonspherical shapes, is expected near the base of the crust in neutron stars. Large-scale molecular dynamics simulations of pasta show long lived topological defects that could increase electron scattering and reduce both the thermal and electrical conductivities. We model a possible low-conductivity pasta layer by increasing an impurity parameter Qimp . Predictions of light curves for the low-mass x-ray binary MXB 1659-29, assuming a large Qimp, find continued late time cooling that is consistent with Chandra observations. The electrical and thermal conductivities are likely related. Therefore, observations of late time crust cooling can provide insight on the electrical conductivity and the possible decay of neutron star magnetic fields (assuming these are supported by currents in the crust).

Disordered nuclear pasta, magnetic field decay, and crust cooling in neutron stars.

PubMed

Horowitz, C J; Berry, D K; Briggs, C M; Caplan, M E; Cumming, A; Schneider, A S

2015-01-23

Nuclear pasta, with nonspherical shapes, is expected near the base of the crust in neutron stars. Large-scale molecular dynamics simulations of pasta show long lived topological defects that could increase electron scattering and reduce both the thermal and electrical conductivities. We model a possible low-conductivity pasta layer by increasing an impurity parameter Q_{imp}. Predictions of light curves for the low-mass x-ray binary MXB 1659-29, assuming a large Q_{imp}, find continued late time cooling that is consistent with Chandra observations. The electrical and thermal conductivities are likely related. Therefore, observations of late time crust cooling can provide insight on the electrical conductivity and the possible decay of neutron star magnetic fields (assuming these are supported by currents in the crust).

An evolving trio of hybrid stars: C111

NASA Technical Reports Server (NTRS)

Sonneborn, George (Technical Monitor); Dupree, Andrea K.

2005-01-01

Hybrid stars are a class of cool, luminous single stars originally identified based on the appearance of their ultraviolet IUE spectra. C IV emission is present (signifying temperatures of at least lo5 K), and asymmetric emission cores of Mg I1 are found, accompanied by absorption features at low and high velocities, indicating a massive stellar wind and circumstellar material. Many members of this class have been identified and X-rays have been detected from most hybrids. They represent the critical evolutionary state between coronal-like objects and the Alpha Ori-like objects and assume a pivotal role in the definition of coronal evolution, atmospheric heating processes, and mechanisms to drive winds of cool stars.

Cool Stars May Have Different Prebiotic Chemical Mix

NASA Image and Video Library

2009-04-07

NASA Spitzer Space Telescope detected a prebiotic, or potentially life-forming, molecule called hydrogen cyanide HCN in the planet-forming disks around yellow stars like our sun, but not in the disks around cooler, reddish stars.

PREFACE: Stellar Atmospheres in the Gaia Era - Preface

NASA Astrophysics Data System (ADS)

Lobel, Alex; De Greve, Jean-Pierre; Van Rensbergen, Walter

2011-12-01

Volume 328 (2011) of the Journal of Physics: Conference Series provides a record of the invited and contributed talks, and of the posters presented at the GREAT-ESF workshop entitled `Stellar Atmospheres in the Gaia Era: Quantitative Spectroscopy and Comparative Spectrum Modelling' (http://great-esf.oma.be and mirrored at http://spectri.freeshell.org/great-esf). The conference was held on 23-24 June 2011 at the Vrije Universiteit Brussel, Belgium. 47 scientists from 11 countries around the world attended the workshop. The ESA-Gaia satellite (launch mid 2013) will observe a billion stellar objects in the Galaxy and provide spectrophotometric and high-resolution spectra of an unprecedented number of stars observed with a space-based instrument. The confrontation of these data with theoretical models will significantly advance our understanding of the physics of stellar atmospheres. New stellar populations such as previously unknown emission line stars will be discovered, and fundamental questions such as the basic scenarios of stellar evolution will be addressed with Gaia data. The 33 presentations and 4 main discussion sessions at the workshop addressed important topics in spectrum synthesis methods and detailed line profile calculations urgently needed for accurate modelling of stellar spectra. It brought together leading scientists and students of the stellar physics communities investigating hot and cool star spectra. The scientific programme of the workshop consisted of 23 oral (6 invited) and 10 poster presentations about cool stars (first day; Comparative Spectrum Modelling and Quantitative Spectroscopy of Cool Stars), and hot stars (second day; Quantitative Spectroscopy of Hot Stars). The hot and cool stars communities use different spectrum modelling codes for determining basic parameters such as the effective temperature, surface gravity, iron abundance, and the chemical composition of stellar atmospheres. The chaired sessions of the first day highlighted new research results with spectral synthesis codes developed for cool stars, while the second day focused on codes applied for modeling the spectra of hot stars. The workshop addressed five major topics in stellar atmospheres research: Spectrum synthesis codes Radiation hydrodynamics codes Atmospheric parameters, abundance, metallicity, and chemical tagging studies Large spectroscopic surveys New atomic database The workshop presentations discussed various important scientific issues by comparing detailed model spectra to identify differences that can influence and bias the resulting atmospheric parameters. Theoretical line-blanketed model spectra were compared in detail to high-resolution spectroscopic observations. Stellar spectra computed (i.e., in the Gaia Radial Velocity Spectrometer wavelength range) with 1-D model atmosphere structures were mutually compared, but also to 3-D models from advanced radiation hydrodynamics codes. Atmospheric parameters derived from spectrum synthesis calculations assuming Local Thermodynamic Equilibrium (LTE) were evaluated against more sophisticated non-LTE models of metal-poor stars and the extended atmospheres of giants and supergiants. The workshop presented an overview of high-resolution synthetic spectral libraries of model spectra computed with the synthesis codes. The spectral model grids will be utilized to derive stellar parameters with the Discrete Source Classifier Algorithms currently under development in the Gaia DPAC consortium (http://www.rssd.esa.int/index.php?project=GAIA&page=DPAC_Introduction). They are implemented for training Gaia data analysis algorithms for the classification of a wide variety of hot and cool star types; FGK and M stars, OB stars, white dwarfs, red supergiants, peculiar A and B stars, carbon stars, ultra cool dwarfs, various types of emission line stars, Be stars, Wolf-Rayet stars, etc. A substantial number of oral and poster presentations discussed different techniques for measuring the abundance of various chemical elements from stellar spectra. The presented methods utilize spectra observed with large spectral dispersion, for example for accurately measuring iron, carbon, and nitrogen abundances. These methods are important for ongoing development and testing of automated and supervised algorithms for determining detailed chemical composition in tagging studies of large (chemo-dynamical) spectroscopic surveys planned to complement the Gaia (astrometric and kinematic) census of the Galaxy. The complete scientific programme is available here. The workshop website also offers the presentation viewgraphs (in PDF format) and some nice photographs of the talks and poster breaks http://great-esf.oma.be/program.php.

On the extended stellar structure around NGC 288

NASA Astrophysics Data System (ADS)

Piatti, Andrés E.

2018-01-01

We report on observational evidence of an extra-tidal clumpy structure around NGC 288 from homogeneous coverage of a large area with the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) PS1 data base. The extra-tidal star population has been disentangled from that of the Milky Way (MW) field by using a cleaning technique that successfully reproduces the stellar density, luminosity function and colour distributions of MW field stars. We have produced the cluster stellar density radial profile and a stellar density map from independent approaches, and we found the results to be in excellent agreement - the feature extends up to 3.5 times further than the cluster tidal radius. Previous works based on shallower photometric data sets have speculated on the existence of several long tidal tails, similar to that found in Pal 5. The present outcome shows that NGC 288 could hardly have such tails, but it favours the notion that the use of interactions with the MW tidal field has been a relatively inefficient process for stripping stars off the cluster. These results point to the need for a renewed overall study of the external regions of Galactic globular clusters (GGCs) in order to reliably characterize them. It will then be possible to investigate whether there is any connection between detected tidal tails, extra-tidal stellar populations and extended diffuse halo-like structures, and the dynamical histories of GGCs in the Galaxy.

A MODEL FOR (QUASI-)PERIODIC MULTIWAVELENGTH PHOTOMETRIC VARIABILITY IN YOUNG STELLAR OBJECTS

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

Kesseli, Aurora Y.; Petkova, Maya A.; Wood, Kenneth

We present radiation transfer models of rotating young stellar objects (YSOs) with hot spots in their atmospheres, inner disk warps, and other three-dimensional effects in the nearby circumstellar environment. Our models are based on the geometry expected from magneto-accretion theory, where material moving inward in the disk flows along magnetic field lines to the star and creates stellar hot spots upon impact. Due to rotation of the star and magnetosphere, the disk is variably illuminated. We compare our model light curves to data from the Spitzer YSOVAR project to determine if these processes can explain the variability observed at opticalmore » and mid-infrared wavelengths in young stars. We focus on those variables exhibiting “dipper” behavior that may be periodic, quasi-periodic, or aperiodic. We find that the stellar hot-spot size and temperature affects the optical and near-infrared light curves, while the shape and vertical extent of the inner disk warp affects the mid-IR light curve variations. Clumpy disk distributions with non-uniform fractal density structure produce more stochastic light curves. We conclude that magneto-accretion theory is consistent with certain aspects of the multiwavelength photometric variability exhibited by low-mass YSOs. More detailed modeling of individual sources can be used to better determine the stellar hot-spot and inner disk geometries of particular sources.« less

A Suzaku, NuSTAR and XMMNewton} view on variable absorption and relativistic reflection in NGC 4151

NASA Astrophysics Data System (ADS)

Beuchert, T.; Markowitz, A.; Dauser, T.; Garcia, J.; Keck, M.; Wilms, J.; Kadler, M.; Brenneman, L.; Zdziarski, A.

2017-10-01

We disentangle X-ray disk reflection from complex line-of-sight absorption in NGC 4151 using Suzaku, NuSTAR, and XMMNewton}. Extending upon Keck et al. (2015), we develop a physically-motivated baseline model using the latest lamp-post reflection code relxillCp_lp, which includes a Comptonization continuum. We identify two components at heights of 1.2 and 15.0 gravitational radii using a long-look simultaneous Suzaku/NuSTAR observation but argue for a vertically extended corona as opposed to distinct primary sources. We also find two neutral absorbers (one full-covering and one partial-covering), an ionized absorber (log ξ=2.8), and a highly-ionized ultra-fast outflow, all reported previously. All analyzed spectra are well described by this baseline model. The bulk of the spectral variability on time-scales from days to years can be attributed to changes of both neutral absorbers, which are inversely correlated with the hard X-ray continuum flux. The observed evolution is either consistent with changes in the absorber structure (clumpy absorber in the outer BLR or a dusty radiatively driven wind) or a geometrically stable neutral absorber that becomes increasingly ionized at a rising flux level. The soft X-rays below 1 keV are dominated by photoionized emission from extended gas, which may act as a warm mirror for the nuclear radiation.

Momentum and energy balance in late-type stellar winds

NASA Technical Reports Server (NTRS)

Macgregor, K. B.

1981-01-01

Observations at ultraviolet and X-ray wavelengths indicate that the classical picture of a static stellar atmosphere containing a radiative equilibrium temperature distribution is inapplicable to the majority of late type stars. Mass loss and the presence of atmospheric regions characterized by gas temperatures in excess of the stellar effective temperature appear to be almost ubiquitous throughout the HR diagram. Evidence pertaining to the thermal and dynamical structure of the outer envelopes of cool stars is summarized. These results are compared with the predictions of several theoretical models which were proposed to account for mass loss from latetype stars. Models in which the outflow is thermally radiatively, or wave driven are considered for identification of the physical processes responsible for the observed wind properties. The observed variation of both the wind, thermal and dynamical structure as one proceeds from the supergiant branch toward the main sequence in the cool portion of the HR diagram give consideration to potential mechanisms for heating and cooling the flow from low gravity stars.

Revisiting the Cooling Flow Problem in Galaxies, Groups, and Clusters of Galaxies

NASA Astrophysics Data System (ADS)

McDonald, M.; Gaspari, M.; McNamara, B. R.; Tremblay, G. R.

2018-05-01

We present a study of 107 galaxies, groups, and clusters spanning ∼3 orders of magnitude in mass, ∼5 orders of magnitude in central galaxy star formation rate (SFR), ∼4 orders of magnitude in the classical cooling rate ({\\dot{M}}cool}\\equiv {M}gas}(r< {r}cool})/{t}cool}) of the intracluster medium (ICM), and ∼5 orders of magnitude in the central black hole accretion rate. For each system in this sample, we measure the ICM cooling rate, {\\dot{M}}cool}, using archival Chandra X-ray data and acquire the SFR and systematic uncertainty in the SFR by combining over 330 estimates from dozens of literature sources. With these data, we estimate the efficiency with which the ICM cools and forms stars, finding {ε }cool}\\equiv {SFR}/{\\dot{M}}cool}=1.4 % +/- 0.4% for systems with {\\dot{M}}cool}> 30 M ⊙ yr‑1. For these systems, we measure a slope in the SFR–{\\dot{M}}cool} relation greater than unity, suggesting that the systems with the strongest cool cores are also cooling more efficiently. We propose that this may be related to, on average, higher black hole accretion rates in the strongest cool cores, which could influence the total amount (saturating near the Eddington rate) and dominant mode (mechanical versus radiative) of feedback. For systems with {\\dot{M}}cool}< 30 M ⊙ yr‑1, we find that the SFR and {\\dot{M}}cool} are uncorrelated and show that this is consistent with star formation being fueled at a low (but dominant) level by recycled ISM gas in these systems. We find an intrinsic log-normal scatter in SFR at a fixed {\\dot{M}}cool} of 0.52 ± 0.06 dex (1σ rms), suggesting that cooling is tightly self-regulated over very long timescales but can vary dramatically on short timescales. There is weak evidence that this scatter may be related to the feedback mechanism, with the scatter being minimized (∼0.4 dex) for systems for which the mechanical feedback power is within a factor of two of the cooling luminosity.

Net Reaction Rate and Neutrino Cooling Rate for the Urca Process in Departure from Chemical Equilibrium in the Crust of Fast-accreting Neutron Stars

NASA Astrophysics Data System (ADS)

Wang, Wei-Hua; Huang, Xi; Zheng, Xiao-Ping

We discuss the effect of compression on Urca shells in the ocean and crust of accreting neutron stars, especially in superbursting sources. We find that Urca shells may be deviated from chemical equilibrium in neutron stars which accrete at several tenths of the local Eddington accretion rate. The deviation depends on the energy threshold of the parent and daughter nuclei, the transition strength, the temperature, and the local accretion rate. In a typical crust model of accreting neutron stars, the chemical departures range from a few tenths of kBT to tens of kBT for various Urca pairs. If the Urca shell can exist in crusts of accreting neutron stars, compression may enhance the net neutrino cooling rate by a factor of about 1-2 relative to the neutrino emissivity in chemical equilibrium. For some cases, such as Urca pairs with small energy thresholds and/or weak transition strength, the large chemical departure may result in net heating rather than cooling, although the released heat can be small. Strong Urca pairs in the deep crust are hard to be deviated even in neutron stars accreting at the local Eddington accretion rate.

Mining the HST Treasury: The ASTRAL Reference Spectra for Evolved M Stars

NASA Technical Reports Server (NTRS)

Carpenter, K. G.; Ayres, T.; Harper, G.; Kober, G.; Wahlgren, G. M.

2012-01-01

The "Advanced Spectral Library (ASTRAL) Project: Cool Stars" (PI = T. Ayres) is an HST Cycle 18 Treasury Program designed to collect a definitive set of representative, high-resolution (R greater than 100,000) and high signal/noise (S/N greater than 100) UV spectra of eight F-M evolved cool stars. These extremely high-quality STIS UV echelle spectra are available from the HST archive and through the University of Colorado (http://casa.colorado.edu/ayres/ASTRAL/) portal and will enable investigations of a broad range of problems -- stellar, interstellar. and beyond -- for many years. In this current paper, we concentrate on producing a roadrnap to the very rich spectra of the two evolved M stars in the sample, the M3.4 giant Gamma Crucis (GaCrux) and the M2Iab supergiant Alpha Orionis (Betelgeuse) and illustrate the huge increase in coverage and quality that these spectra provide over that previously available from IUE and earlier HST observations. These roadmaps will facilitate the study of the spectra, outer atmospheres, and winds of not only these stars. but also numerous other cool, low-gravity stars and make a very interesting comparison to the already-available atlases of the K2III giant Arcturus.

Chemical Soups Around Cool Stars

NASA Technical Reports Server (NTRS)

2009-01-01

This artist's conception shows a young, hypothetical planet around a cool star. A soupy mix of potentially life-forming chemicals can be seen pooling around the base of the jagged rocks. Observations from NASA's Spitzer Space Telescope hint that planets around cool stars the so-called M-dwarfs and brown dwarfs that are widespread throughout our galaxy might possess a different mix of life-forming, or prebiotic, chemicals than our young Earth.

Life on our planet is thought to have arisen out of a pond-scum-like mix of chemicals. Some of these chemicals are thought to have come from a planet-forming disk of gas and dust that swirled around our young sun. Meteorites carrying the chemicals might have crash-landed on Earth.

Astronomers don't know if these same life-generating processes are taking place around stars that are cooler than our sun, but the Spitzer observations show their disk chemistry is different. Spitzer detected a prebiotic molecule, called hydrogen cyanide, in the disks around yellow stars like our sun, but found none around cooler, less massive, reddish stars. Hydrogen cyanide is a carbon-containing, or organic compound. Five hydrogen cyanide molecules can join up to make adenine a chemical element of the DNA molecule found in all living organisms on Earth.

Red Supergiants as Potential Type IIn Supernova Progenitors: Spatially Resolved 4.6 μm CO Emission Around VY CMa and Betelgeuse

NASA Astrophysics Data System (ADS)

Smith, Nathan; Hinkle, Kenneth H.; Ryde, Nils

2009-03-01

We present high-resolution 4.6 μm CO spectra of the circumstellar environments of two red supergiants (RSGs) that are potential supernova (SN) progenitors: Betelgeuse and VY Canis Majoris (VY CMa). Around Betelgeuse, 12CO emission within ±3'' (±12 km s-1) follows a mildly clumpy but otherwise spherical shell, smaller than its ~55'' shell in K I λ7699. In stark contrast, 4.6 μm CO emission around VY CMa is coincident with bright K I in its clumpy asymmetric reflection nebula, within ±5'' (±40 km s-1) of the star. Our CO data reveal redshifted features not seen in K I spectra of VY CMa, indicating a more isotropic distribution of gas punctuated by randomly distributed asymmetric clumps. The relative CO and K I distribution in Betelgeuse arises from ionization effects within a steady wind, whereas in VY CMa, K I is emitted from skins of CO cloudlets resulting from episodic mass ejections 500-1000 yr ago. In both cases, CO and K I trace potential pre-SN circumstellar matter: we conclude that an extreme RSG like VY CMa might produce a Type IIn event like SN 1988Z if it were to explode in its current state, but Betelgeuse will not. VY CMa demonstrates that luminous blue variables are not necessarily the only progenitors of SNe IIn, but it underscores the requirement that SNe IIn suffer enhanced episodic mass loss shortly before exploding. Based on observations obtained at the Gemini Observatory.

Thick Disks in the Hubble Space Telescope Frontier Fields

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

Elmegreen, Bruce G.; Elmegreen, Debra Meloy; Tompkins, Brittany

Thick disk evolution is studied using edge-on galaxies in two Hubble Space Telescope Frontier Field Parallels. The galaxies were separated into 72 clumpy types and 35 spiral types with bulges. Perpendicular light profiles in F435W, F606W, and F814W ( B , V , and I ) passbands were measured at 1 pixel intervals along the major axes and fitted to sech{sup 2} functions convolved with the instrument line spread function (LSF). The LSF was determined from the average point spread function of ∼20 stars in each passband and field, convolved with a line of uniform brightness to simulate disk blurring.more » A spread function for a clumpy disk was also used for comparison. The resulting scale heights were found to be proportional to galactic mass, with the average height for a 10{sup 10±0.5} M {sub ⊙} galaxy at z = 2 ± 0.5 equal to 0.63 ± 0.24 kpc. This value is probably the result of a blend between thin and thick disk components that cannot be resolved. Evidence for such two-component structure is present in an inverse correlation between height and midplane surface brightness. Models suggest that the thick disk is observed best between the clumps, and there the average scale height is 1.06 ± 0.43 kpc for the same mass and redshift. A 0.63 ± 0.68 mag V − I color differential with height is also evidence for a mixture of thin and thick components.« less

A clumpy and anisotropic galaxy halo at redshift 1 from gravitational-arc tomography

NASA Astrophysics Data System (ADS)

Lopez, Sebastian; Tejos, Nicolas; Ledoux, Cédric; Barrientos, L. Felipe; Sharon, Keren; Rigby, Jane R.; Gladders, Michael D.; Bayliss, Matthew B.; Pessa, Ismael

2018-02-01

Every star-forming galaxy has a halo of metal-enriched gas that extends out to at least 100 kiloparsecs, as revealed by the absorption lines that this gas imprints on the spectra of background quasars. However, quasars are sparse and typically probe only one narrow beam of emission through the intervening galaxy. Close quasar pairs and gravitationally lensed quasars have been used to circumvent this inherently one-dimensional technique, but these objects are rare and the structure of the circumgalactic medium remains poorly constrained. As a result, our understanding of the physical processes that drive the recycling of baryons across the lifetime of a galaxy is limited. Here we report integral-field (tomographic) spectroscopy of an extended background source—a bright, giant gravitational arc. We can thus coherently map the spatial and kinematic distribution of Mg ɪɪ absorption—a standard tracer of enriched gas—in an intervening galaxy system at redshift 0.98 (around 8 billion years ago). Our gravitational-arc tomography unveils a clumpy medium in which the absorption strength decreases with increasing distance from the galaxy system, in good agreement with results for quasars. Furthermore, we find strong evidence that the gas is not distributed isotropically. Interestingly, we detect little kinematic variation over a projected area of approximately 600 square kiloparsecs, with all line-of-sight velocities confined to within a few tens of kilometres per second of each other. These results suggest that the detected absorption originates from entrained recycled material, rather than in a galactic outflow.

A clumpy and anisotropic galaxy halo at redshift 1 from gravitational-arc tomography.

PubMed

Lopez, Sebastian; Tejos, Nicolas; Ledoux, Cédric; Barrientos, L Felipe; Sharon, Keren; Rigby, Jane R; Gladders, Michael D; Bayliss, Matthew B; Pessa, Ismael

2018-02-22

Every star-forming galaxy has a halo of metal-enriched gas that extends out to at least 100 kiloparsecs, as revealed by the absorption lines that this gas imprints on the spectra of background quasars. However, quasars are sparse and typically probe only one narrow beam of emission through the intervening galaxy. Close quasar pairs and gravitationally lensed quasars have been used to circumvent this inherently one-dimensional technique, but these objects are rare and the structure of the circumgalactic medium remains poorly constrained. As a result, our understanding of the physical processes that drive the recycling of baryons across the lifetime of a galaxy is limited. Here we report integral-field (tomographic) spectroscopy of an extended background source-a bright, giant gravitational arc. We can thus coherently map the spatial and kinematic distribution of Mg ɪɪ absorption-a standard tracer of enriched gas-in an intervening galaxy system at redshift 0.98 (around 8 billion years ago). Our gravitational-arc tomography unveils a clumpy medium in which the absorption strength decreases with increasing distance from the galaxy system, in good agreement with results for quasars. Furthermore, we find strong evidence that the gas is not distributed isotropically. Interestingly, we detect little kinematic variation over a projected area of approximately 600 square kiloparsecs, with all line-of-sight velocities confined to within a few tens of kilometres per second of each other. These results suggest that the detected absorption originates from entrained recycled material, rather than in a galactic outflow.

Cool stars, stellar systems, and the sun; Proceedings of the 7th Cambridge Workshop, Tucson, AZ, Oct. 9-12, 1991

NASA Technical Reports Server (NTRS)

Giampapa, Mark S. (Editor); Bookbinder, Jay A. (Editor)

1992-01-01

Consideration is given to HST observations of late-type stars, molecular absorption in the UV spectrum of Alpha Ori, EUV emission from late-type stars, Rosat observations of the Pleiades cluster, a deep ROSAT observation of the Hyades cluster, optical spectroscopy detected by EXOSAT, stellar photospheric convection, a structure of the solar X-ray corona, magnetic surface images of the BY Dra Star HD 82558, a Zebra interpretatin of BY Dra stars, optical flares on II Peg, a low-resolution spectroscopic survey of post-T tauri candidates, millimeter and sub-millimeter emission from flare stars, and activity in tidally interacting binaries. Attention is also given to modeling stellar angular momentum evolution, extended 60-micron emission from nearby Mira variables, the PANDORA atmosphere program, the global properties of active regions, oscillations in a stratified atmosphere, lithium abundances in northern RS CVn binaries, a new catalog of cool dwarf stars, the Far UV Spectrograph Explorer, and development of reflecting coronagraphs.

White Dwarfs in Star Clusters: The Initial-Final Mass Relation for Stars from 0.85 to 8 M$_\\odot$

NASA Astrophysics Data System (ADS)

Cummings, Jeffrey; Kalirai, Jason; Tremblay, P.-E.; Ramírez-Ruiz, Enrico

2018-01-01

The spectroscopic study of white dwarfs provides both their mass, cooling age, and intrinsic photometric properties. For white dwarfs in the field of well-studied star clusters, this intrinsic photometry can be used to determine if they are members of that star cluster. Comparison of a member white dwarf's cooling age to its total cluster's age provides the evolutionary timescale of its progenitor star, and hence the mass. This is the initial-final mass relation (IFMR) for stars, which gives critical information on how a progenitor star evolves and loses mass throughout its lifetime, and how this changes with progenitor mass. Our work, for the first time, presents a uniform analysis of 85 white dwarf cluster members spanning from progenitor masses of 0.85 to 8 M$_\\odot$. Comparison of our work to theoretical IFMRs shows remarkable consistency in their shape but differences remain. We will discuss possible explanations for these differences, including the effects of stellar rotation.

Outer atmospheres of cool stars. XII - A survey of IUE ultraviolet emission line spectra of cool dwarf stars

NASA Technical Reports Server (NTRS)

Linsky, J. L.; Bornmann, P. L.; Carpenter, K. G.; Hege, E. K.; Wing, R. F.; Giampapa, M. S.; Worden, S. P.

1982-01-01

Quantitative information is obtained on the chromospheres and transition regions of M dwarf stars, in order to determine how the outer atmospheres of dMe stars differ from dM stars and how they compare with the outer atmospheres of quiet and active G and K type dwarfs. IUE spectra of six dMe and four dM stars, together with ground-based photometry and spectroscopy of the Balmer and Ca II H and K lines, show no evidence of flares. It is concluded, regarding the quiescent behavior of these stars, that emission-line spectra resemble that of the sun and contain emission lines formed in regions with 4000-20,000 K temperatures that are presumably analogous to the solar chromosphere, as well as regions with temperatures of 20,000-200,000 K that are presumably analogous to the solar transition region. Emission-line surface fluxes are proportional to the emission measure over the range of temperatures at which the lines are formed.

Models of symbiotic stars

NASA Technical Reports Server (NTRS)

Friedjung, Michael

1993-01-01

One of the most important features of symbiotic stars is the coexistence of a cool spectral component that is apparently very similar to the spectrum of a cool giant, with at least one hot continuum, and emission lines from very different stages of ionization. The cool component dominates the infrared spectrum of S-type symbiotics; it tends to be veiled in this wavelength range by what appears to be excess emission in D-type symbiotics, this excess usually being attributed to circumstellar dust. The hot continuum (or continua) dominates the ultraviolet. X-rays have sometimes also been observed. Another important feature of symbiotic stars that needs to be explained is the variability. Different forms occur, some variability being periodic. This type of variability can, in a few cases, strongly suggest the presence of eclipses of a binary system. One of the most characteristic forms of variability is that characterizing the active phases. This basic form of variation is traditionally associated in the optical with the veiling of the cool spectrum and the disappearance of high-ionization emission lines, the latter progressively appearing (in classical cases, reappearing) later. Such spectral changes recall those of novae, but spectroscopic signatures of the high-ejection velocities observed for novae are not usually detected in symbiotic stars. However, the light curves of the 'symbiotic nova' subclass recall those of novae. We may also mention in this connection that radio observations (or, in a few cases, optical observations) of nebulae indicate ejection from symbiotic stars, with deviations from spherical symmetry. We shall give a historical overview of the proposed models for symbiotic stars and make a critical analysis in the light of the observations of symbiotic stars. We describe the empirical approach to models and use the observational data to diagnose the physical conditions in the symbiotics stars. Finally, we compare the results of this empirical approach with existing models and discuss unresolved problems requiring new observational and theoretical work.

Broadband Spectroscopy Using Two Suzaku Observations of the HMXB GX 301-2

NASA Technical Reports Server (NTRS)

Suchy, Slawomir; Fuerst, Felix; Pottschmidt, Katja; Caballero, Isabel; Kreykenbohm, Ingo; Wilms, Joern; Markowitz, Alex; Rothschild, Richard E.

2012-01-01

We present the analysis of two Suzaku observations of GX 301-2 at two orbital phases after the periastron passage. Variations in the column density of the line-of-sight absorber are observed, consistent with accretion from a clumpy wind. In addition to a CRSF, multiple fluorescence emission lines were detected in both observations. The variations in the pulse profiles and the CRSF throughout the pulse phase have a signature of a magnetic dipole field. Using a simple dipole model we calculated the expected magnetic field values for different pulse phases and were able to extract a set of geometrical angles, loosely constraining the dipole geometry in the neutron star. From the variation of the CRSF width and energy, we found a geometrical solution for the dipole, making the inclination consistent with previously published values.

Broadband Spectroscopy Using Two Suzaku Observations of the HMXB GX 301-2

NASA Astrophysics Data System (ADS)

Suchy, Slawomir; Fürst, Felix; Pottschmidt, Katja; Caballero, Isabel; Kreykenbohm, Ingo; Wilms, Jörn; Markowitz, Alex; Rothschild, Richard E.

2012-02-01

We present the analysis of two Suzaku observations of GX 301-2 at two orbital phases after the periastron passage. Variations in the column density of the line-of-sight absorber are observed, consistent with accretion from a clumpy wind. In addition to a cyclotron resonance scattering feature (CRSF), multiple fluorescence emission lines were detected in both observations. The variations in the pulse profiles and the CRSF throughout the pulse phase have a signature of a magnetic dipole field. Using a simple dipole model we calculated the expected magnetic field values for different pulse phases and were able to extract a set of geometrical angles, loosely constraining the dipole geometry in the neutron star. From the variation of the CRSF width and energy, we found a geometrical solution for the dipole, making the inclination consistent with previously published values.

An X-Ray Spectral Model for Clumpy Tori in Active Galactic Nuclei

NASA Astrophysics Data System (ADS)

Liu, Yuan; Li, Xiaobo

2014-05-01

We construct an X-ray spectral model for the clumpy torus in an active galactic nucleus (AGN) using Geant4, which includes the physical processes of the photoelectric effect, Compton scattering, Rayleigh scattering, γ conversion, fluorescence line, and Auger process. Since the electrons in the torus are expected to be bounded instead of free, the deviation of the scattering cross section from the Klein-Nishina cross section has also been included, which changes the X-ray spectra by up to 25% below 10 keV. We have investigated the effect of the clumpiness parameters on the reflection spectra and the strength of the fluorescent line Fe Kα. The volume filling factor of the clouds in the clumpy torus only slightly influences the reflection spectra, however, the total column density and the number of clouds along the line of sight significantly change the shapes and amplitudes of the reflection spectra. The effect of column density is similar to the case of a smooth torus, while a small number of clouds along the line of sight will smooth out the anisotropy of the reflection spectra and the fluorescent line Fe Kα. The smoothing effect is mild in the low column density case (N H = 1023 cm-2), whereas it is much more evident in the high column density case (N H = 1025 cm-2). Our model provides a quantitative tool for the spectral analysis of the clumpy torus. We suggest that the joint fits of the broad band spectral energy distributions of AGNs (from X-ray to infrared) should better constrain the structure of the torus.

Deep Chandra , HST-COS, and megacam observations of the Phoenix cluster: Extreme star formation and AGN feedback on hundred kiloparsec scales

DOE PAGES

McDonald, Michael; McNamara, Brian R.; Perimeter Institute for Theoretical Physics, Waterloo; ...

2015-09-28

In this study, we present new ultraviolet, optical, and X-ray data on the Phoenix galaxy cluster (SPT-CLJ2344-4243). Deep optical imaging reveals previously undetected filaments of star formation, extending to radii of ~50–100 kpc in multiple directions. Combined UV-optical spectroscopy of the central galaxy reveals a massive (2 × 10 9 M ⊙), young (~4.5 Myr) population of stars, consistent with a time-averaged star formation rate of 610 ± 50 M ⊙ yr –1. We report a strong detection of O vi λλ1032,1038, which appears to originate primarily in shock-heated gas, but may contain a substantial contribution (>1000 M ⊙ yrmore » –1) from the cooling intracluster medium (ICM). We confirm the presence of deep X-ray cavities in the inner ~10 kpc, which are among the most extreme examples of radio-mode feedback detected to date, implying jet powers of 2 – 7 × 10 45 erg s –1. We provide evidence that the active galactic nucleus inflating these cavities may have only recently transitioned from "quasar-mode" to "radio-mode," and may currently be insufficient to completely offset cooling. A model-subtracted residual X-ray image reveals evidence for prior episodes of strong radio-mode feedback at radii of ~100 kpc, with extended "ghost" cavities indicating a prior epoch of feedback roughly 100 Myr ago. This residual image also exhibits significant asymmetry in the inner ~200 kpc (0.15R 500), reminiscent of infalling cool clouds, either due to minor mergers or fragmentation of the cooling ICM. Taken together, these data reveal a rapidly evolving cool core which is rich with structure (both spatially and in temperature), is subject to a variety of highly energetic processes, and yet is cooling rapidly and forming stars along thin, narrow filaments.« less

Dynamical onset of superconductivity and retention of magnetic fields in cooling neutron stars

NASA Astrophysics Data System (ADS)

Ho, Wynn C. G.; Andersson, Nils; Graber, Vanessa

2017-12-01

A superconductor of paired protons is thought to form in the core of neutron stars soon after their birth. Minimum energy conditions suggest magnetic flux is expelled from the superconducting region due to the Meissner effect, such that the neutron star core is largely devoid of magnetic fields for some nuclear equation of state and proton pairing models. We show via neutron star cooling simulations that the superconducting region expands faster than flux is expected to be expelled because cooling timescales are much shorter than timescales of magnetic field diffusion. Thus magnetic fields remain in the bulk of the neutron star core for at least 106-107yr . We estimate the size of flux free regions at 107yr to be ≲100 m for a magnetic field of 1011G and possibly smaller for stronger field strengths. For proton pairing models that are narrow, magnetic flux may be completely expelled from a thin shell of approximately the above size after 105yr . This shell may insulate lower conductivity outer layers, where magnetic fields can diffuse and decay faster, from fields maintained in the highly conducting deep core.

A Game of Hide and Seek: Expectations of Clumpy Resources Influence Hiding and Searching Patterns

PubMed Central

Wilke, Andreas; Minich, Steven; Panis, Megane; Langen, Tom A.; Skufca, Joseph D.; Todd, Peter M.

2015-01-01

Resources are often distributed in clumps or patches in space, unless an agent is trying to protect them from discovery and theft using a dispersed distribution. We uncover human expectations of such spatial resource patterns in collaborative and competitive settings via a sequential multi-person game in which participants hid resources for the next participant to seek. When collaborating, resources were mostly hidden in clumpy distributions, but when competing, resources were hidden in more dispersed (random or hyperdispersed) patterns to increase the searching difficulty for the other player. More dispersed resource distributions came at the cost of higher overall hiding (as well as searching) times, decreased payoffs, and an increased difficulty when the hider had to recall earlier hiding locations at the end of the experiment. Participants’ search strategies were also affected by their underlying expectations, using a win-stay lose-shift strategy appropriate for clumpy resources when searching for collaboratively-hidden items, but moving equally far after finding or not finding an item in competitive settings, as appropriate for dispersed resources. Thus participants showed expectations for clumpy versus dispersed spatial resources that matched the distributions commonly found in collaborative versus competitive foraging settings. PMID:26154661

ARC-1989-A89-7024

NASA Image and Video Library

1989-08-23

P-34679 Range : 2 million km. ( 1.2 million miles ) In this Voyager 2, wide-angle image, the two main rings of Neptune can be clearly seen. In the lower part of the frame, the originally-announced ring arc, consisting of three distinct features, is visible. This feature covers about 35 degrees of longitude and has yet to be radially resolved in Voyager Images. from higher resolution images it is known that this region contains much more material than the diffuse belts seen elsewhere in its orbit, which seem to encircle the planet. This is consistent with the fact that ground-based observations of stellar occultations by the rings show them to be very broken and clumpy. The more sensitive, wide-angle camera is revealing more widely distributed but fainter material. Each of these rings of material lies just outside of the orbit of a newly discovered moon. One of these moons, 1989N2, may be seen in the upper right corner. The moon is streaked by its orbital motion, whereas the stars in the frame are less smeared. the dark area around the bright moon and star are artifacts of the processing required to bring out the faint rings.

Wide-field SCUBA-2 observations of NGC 2264: submillimetre clumps and filaments

NASA Astrophysics Data System (ADS)

Buckle, J. V.; Richer, J. S.

2015-10-01

We present wide-field observations of the NGC 2264 molecular cloud in the dust continuum at 850 and 450 μm using SCUBA-2 on the James Clerk Maxwell Telescope. Using 12CO 3 → 2 molecular line data, we determine that emission from CO contaminates the 850 μm emission at levels ˜30 per cent in localized regions associated with high-velocity molecular outflows. Much higher contamination levels of 60 per cent are seen in shocked regions near the massive star S Mon. If not removed, the levels of CO contamination would contribute an extra 13 per cent to the dust mass in NGC 2264. We use the FELLWALKER routine to decompose the dust into clumpy structures, and a Hessian-based routine to decompose the dust into filamentary structures. The filaments can be described as a hub-filament structure, with lower column density filaments radiating from the NGC 2264 C protocluster hub. Above mean filament column densities of 2.4 × 1022 cm-2, star formation proceeds with the formation of two or more protostars. Below these column densities, filaments are starless, or contain only a single protostar.

Feeding supermassive black holes by collisional cascades

NASA Astrophysics Data System (ADS)

Faber, Christian; Dehnen, Walter

2018-05-01

The processes driving gas accretion on to supermassive black holes (SMBHs) are still poorly understood. Angular momentum conservation prevents gas within ˜10 pc of the black hole from reaching radii ˜10-3 pc where viscous accretion becomes efficient. Here we present simulations of the collapse of a clumpy shell of swept-up isothermal gas, which is assumed to have formed as a result of feedback from a previous episode of AGN activity. The gas falls towards the SMBH forming clumps and streams, which intersect, collide, and often form a disc. These collisions promote partial cancellations of angular momenta, resulting in further infall and more collisions. This continued collisional cascade generates a tail of gas with sufficiently small angular momenta and provides a viable route for gas inflow to sub-parsec scales. The efficiency of this process hardly depends on details, such as gas temperature, initial virial ratio and power spectrum of the gas distribution, as long as it is not strongly rotating. Adding star formation to this picture might explain the near-simultaneous formation of the S-stars (from tidally disrupted binaries formed in plunging gas streams) and the sub-parsec young stellar disc around Sgr A⋆.

Feeding supermassive black holes by collisional cascades

NASA Astrophysics Data System (ADS)

Faber, Christian; Dehnen, Walter

2018-07-01

The processes driving gas accretion on to supermassive black holes (SMBHs) are still poorly understood. Angular momentum conservation prevents gas within ˜10 pc of the black hole from reaching radii ˜10-3pc where viscous accretion becomes efficient. Here we present simulations of the collapse of a clumpy shell of swept-up isothermal gas, which is assumed to have formed as a result of feedback from a previous episode of AGN activity. The gas falls towards the SMBH forming clumps and streams, which intersect, collide and often form a disc. These collisions promote partial cancellations of angular momenta, resulting in further infall and more collisions. This continued collisional cascade generates a tail of gas with sufficiently small angular momenta and provides a viable route for gas inflow to sub-parsec scales. The efficiency of this process hardly depends on details, such as gas temperature, initial virial ratio and power spectrum of the gas distribution, as long as it is not strongly rotating. Adding star formation to this picture might explain the near-simultaneous formation of the S-stars (from tidally disrupted binaries formed in plunging gas streams) and the sub-parsec young stellar disc around Sgr A⋆.

VLTI-Pionier Imaging of the Carbon AGB Star R Sculptoris and the Supergiant V766 Centauri

NASA Astrophysics Data System (ADS)

Wittkowski, Markus

2018-04-01

I will present reconstructed images of the carbon-rich AGB star R Scl and of the supergiant V766 Cen, both recently obtained from VLTI-PIONIER data. The images are compared to state-of-the art atmosphere and wind models. The images of R Scl exhibit a complex structure within the stellar disk. This structure is most likely caused by giant convection cells, resulting in large-scale shock fronts, and their effects on clumpy molecule and dust formation seen against the photosphere. Images of V 766 Cen were obtained at three epochs. The first epoch shows a complex elongated structure within the photospheric disk, consistent with a red supergiant harboring giant photospheric convection cells. The second and third epochs show a qualitatively and quantitatively different structure with significantly increased contrast, which is not compatible with current models of convection. Instead we interpret the 2016 and 2017 epochs as showing a previously suggested close eclipsing companion in front of the primary, which was located behind the primary at the 2014 epoch. Finally, I will show preliminary reconstructed images of a small sample of further red supergiants.

Very Deep inside the SN 1987A Core Ejecta: Molecular Structures Seen in 3D

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

Abellán, F. J.; Marcaide, J. M.; Indebetouw, R.

2017-06-20

Most massive stars end their lives in core-collapse supernova explosions and enrich the interstellar medium with explosively nucleosynthesized elements. Following core collapse, the explosion is subject to instabilities as the shock propagates outward through the progenitor star. Observations of the composition and structure of the innermost regions of a core-collapse supernova provide a direct probe of the instabilities and nucleosynthetic products. SN 1987A in the Large Magellanic Cloud is one of very few supernovae for which the inner ejecta can be spatially resolved but are not yet strongly affected by interaction with the surroundings. Our observations of SN 1987A withmore » the Atacama Large Millimeter/submillimeter Array are of the highest resolution to date and reveal the detailed morphology of cold molecular gas in the innermost regions of the remnant. The 3D distributions of carbon and silicon monoxide (CO and SiO) emission differ, but both have a central deficit, or torus-like distribution, possibly a result of radioactive heating during the first weeks (“nickel heating”). The size scales of the clumpy distribution are compared quantitatively to models, demonstrating how progenitor and explosion physics can be constrained.« less

Gas Content and Kinematics in Clumpy, Turbulent Star-forming Disks

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

White, Heidi A.; Abraham, Roberto G.; Fisher, David B.

2017-09-01

We present molecular gas-mass estimates for a sample of 13 local galaxies whose kinematic and star-forming properties closely resemble those observed in z ≈ 1.5 main-sequence galaxies. Plateau de Bure observations of the CO[1-0] emission line and Herschel Space Observatory observations of the dust emission both suggest molecular gas-mass fractions of ∼20%. Moreover, dust emission modeling finds T {sub dust} < 30 K, suggesting a cold dust distribution compared to their high infrared luminosity. The gas-mass estimates argue that z ∼ 0.1 DYNAMO galaxies not only share similar kinematic properties with high- z disks, but they are also similarly richmore » in molecular material. Pairing the gas-mass fractions with existing kinematics reveals a linear relationship between f {sub gas} and σ / v {sub c}, consistent with predictions from stability theory of a self-gravitating disk. It thus follows that high gas-velocity dispersions are a natural consequence of large gas fractions. We also find that the systems with the lowest t {sub dep} (∼0.5 Gyr) have the highest ratios of σ / v{sub c} and more pronounced clumps, even at the same high molecular gas fraction.« less

Density Determinations of the Coronae of Cool Stars Using a Newly Assigned Pair of Fe Xiv Lines in the Spectra of α Canis Minor, α Centauri, and the Sun [Density determinations of the coronae of cool stars using a newly assigned pair of Fe XIV lines in the spectra of alpha Canis Minor, alpha Centauri, and the Sun.

DOE PAGES

Beiersdorfer, P.; Hell, N.; Lepson, J. K.; ...

2015-12-02

We identified a previously unassigned pair of lines between 169 and 170 Å in the coronae of cool stars. Here, we attribute these lines to Fe xiv and show that their intensity ratio is sensitive to the electron density. Using observations taken with the Low Energy Transmission Grating Spectrometer of the Chandra X-ray Observatory we infer a density of log (n e/cm -3) = 10.2 ± 0.7 and 10.3 ± 0.8 from the newly identified line pair in the coronae of Procyon and α Cen A, respectively.

Prominence formation and ejection in cool stars

NASA Astrophysics Data System (ADS)

Villarreal D'Angelo, Carolina; Jardine, Moira; See, Victor

2018-03-01

The observational signatures of prominences have been detected in single and binary G and K type stars for many years now, but recently this has been extended to the M dwarf regime. Prominences carry away both mass and angular momentum when they are ejected and the impact of this mass on any orbiting planets may be important for the evolution of exoplanetary atmospheres. By means of the classification used in the massive star community, that involves knowledge of two parameters (the co-rotation and Alfvén radii, rK and rA), we have determined which cool stars could support prominences. From a model of mechanical support, we have determined that the prominence mass mp/M⋆ = (EM/EG)(r⋆/rK)2F where E_MB_\\star ^2r_\\star ^3 and E_G = GM_\\star ^2/r_\\star are magnetic and gravitational energies and F is a geometric factor. Our calculated masses and ejection frequencies (typically 1016 - 1017 g and 0.4 d, respectively) are consistent with observations and are sufficient to ensure that an exoplanet orbiting in the habitable zone of an M dwarf could suffer frequent impacts.

Thermal structure and cooling of neutron stars with magnetized envelopes

NASA Astrophysics Data System (ADS)

Potekhin, A. Y.; Yakovlev, D. G.

2001-07-01

The thermal structure of neutron stars with magnetized envelopes is studied using modern physics input. The relation between the internal (Tint) and local surface temperatures is calculated and fitted by analytic expressions for magnetic field strengths B from 0 to 1016 G and arbitrary inclination of the field lines to the surface. The luminosity of a neutron star with dipole magnetic field is calculated and fitted as a function of B, Tint, stellar mass and radius. In addition, we simulate cooling of neutron stars with magnetized envelopes. In particular, we analyse ultramagnetized envelopes of magnetars and also the effects of the magnetic field of the Vela pulsar on the determination of critical temperatures of neutron and proton superfluids in its core.

On the outer atmospheres of hybrid stars

NASA Technical Reports Server (NTRS)

Hartmann, L.; Dupree, A. K.; Jordan, C.; Brown, A.

1985-01-01

Deep exposures with the IUE satellite have been obtained in order to search for high-temperature emission from stars with cool winds. Iota Aur and Theta Her are confirmed as hybrid stars, and an additional hybrid object, Gamma Aql, has been discovered. The emission line fluxes of the hybrid stars are analyzed to establish the emission measure distribution and, as far as possible, the electron density. The discovery of variable, high-velocity Mg II circumstellar absorption in Gamma Aql, Theta Her, and Alpha TrA is reported. Very long exposure of the latter show that cool material is being accelerated to velocities of at least 180 km/s. These observations suggest that high-velocity mass loss is more common than previously thought.

VizieR Online Data Catalog: CANDELS z~2 galaxy properties (Trump+, 2014)

NASA Astrophysics Data System (ADS)

Trump, J. R.; Barro, G.; Juneau, S.; Weiner, B. J.; Luo, B.; Brammer, G. B.; Bell, E. F.; Brandt, W. N.; Dekel, A.; Guo, Y.; Hopkins, P. F.; Koo, D. C.; Kocevski, D. D.; McIntosh, D. H.; Momcheva, I.; Faber, S. M.; Ferguson, H. C.; Grogin, N. A.; Kartaltepe, J.; Koekemoer, A. M.; Lotz, J.; Maseda, M.; Mozena, M.; Nandra, K.; Rosario, D. J.; Zeimann, G. R.

2017-04-01

We select a sample of 44 clumpy galaxies from the Great Observatories Origins Deep Survey South (GOODS-S; Giavalisco et al. 2004ApJ...600L..93G) region of CANDELS. For comparison, we also construct mass-matched samples of 41 smooth (non-clumpy) and 35 intermediate galaxies. All galaxies have H<24 (to ensure reliable classification of clumpiness) and have [O III] detected at the 3σ level (for reliable AGN line ratio diagnostics) in the redshift range 1.3

On the Spectral Evolution of Helium-atmosphere White Dwarfs Showing Traces of Hydrogen

NASA Astrophysics Data System (ADS)

Rolland, B.; Bergeron, P.; Fontaine, G.

2018-04-01

We present a detailed spectroscopic analysis of 115 helium-line (DB) and 28 cool, He-rich hydrogen-line (DA) white dwarfs based on atmosphere fits to optical spectroscopy and photometry. We find that 63% of our DB population show hydrogen lines, making them DBA stars. We also demonstrate the persistence of pure DB white dwarfs with no detectable hydrogen feature at low effective temperatures. Using state-of-the art envelope models, we next compute the total quantity of hydrogen, M H, that is contained in the outer convection zone as a function of effective temperature and atmospheric H/He ratio. We find that some (T eff, M H) pairs cannot physically exist as a homogeneously mixed structure; such a combination can only occur as stratified objects of the DA spectral type. On that basis, we show that the values of M H inferred for the bulk of the DBA stars are too large and incompatible with the convective dilution scenario. We also present evidence that the hydrogen abundances measured in DBA and cool, helium-rich white dwarfs cannot be globally accounted for by any kind of accretion mechanism onto a pure DB star. We suggest that cool, He-rich DA white dwarfs are most likely created by the convective mixing of a DA star with a thin hydrogen envelope; they are not cooled down DBAs. We finally explore several scenarios that could account for the presence of hydrogen in DBA stars.

Synthetic Absorption Lines for a Clumpy Medium: A Spectral Signature for Cloud Acceleration in AGN?

NASA Technical Reports Server (NTRS)

Waters, Tim; Proga, Daniel; Dannen, Randall; Kallman, Timothy R.

2017-01-01

There is increasing evidence that the highly ionized multiphase components of AGN disc winds may be due to thermal instability. The ions responsible for forming the observed X-ray absorption lines may only exist in relatively cool clumps that can be identified with the so-called warm absorbers. Here we calculate synthetic absorption lines for such warm absorbers from first principles by combining 2D hydrodynamic solutions of a two-phase medium with a dense grid of photoionization models to determine the detailed ionization structure of the gas. Our calculations reveal that cloud disruption, which leads to a highly complicated velocity field (i.e. a clumpy flow), will only mildly affect line shapes and strengths when the warm gas becomes highly mixed but not depleted. Prior to complete disruption, clouds that are optically thin to the driving UV resonance lines will cause absorption at an increasingly blueshifted line-of-sight velocity as they are accelerated. This behavior will imprint an identifiable signature on the line profile if warm absorbers are enshrouded in an even broader absorption line produced by a high column of intercloud gas. Interestingly, we show that it is possible to develop a spectral diagnostic for cloud acceleration by differencing the absorption components of a doublet line, a result that can be qualitatively understood using a simple partial covering model. Our calculations also permit us to comment on the spectral differences between cloud disruption and ionization changes driven by flux variability. Notably, cloud disruption offers another possibility for explaining absorption line variability.

Self-Consistent Hydrodynamical Models For Stellar Winds

NASA Astrophysics Data System (ADS)

Boulangier, Jels; Homan, Ward; van Marle, Allard Jan; Decin, Leen; de Koter, Alex

2016-07-01

The physical and chemical conditions in the atmosphere of pulsating AGB stars are not well understood. In order to properly model this region, which is packed with shocks arisen from the pulsational behaviour of the star, we aim to understand the interplay between spatial and temporal changes in both the chemical composition and the hydro/thermodynamical behaviour inside these regions. Ideal models require the coupling of hydrodynamics, chemistry and radiative transfer, in three dimensions. As this is computationally not yet feasible, we aim to model this zone via a bottom-up approach. At first, we build correct 3D hydrodynamical set-up without any cooling or heating. Omitting cooling hampers the mass-loss of the AGB star within the reasonable confines of a realistic parameter space. Introducing cooling will decrease the temperature gradients in the atmosphere, counteracting the mass-loss even more. However, cooling also ensures the existence of regions where the temperature is low enough for the formation of dust to take place. This dust will absorb the momentum of the impacting photons from the AGB photosphere, accelerate outward and collide with the obstructing gas, dragging it along. Moreover, since chemistry, nucleation and dust formation depend critically on the temperature structure of the circumstellar environment, it is of utmost importance to include all relevant heating/cooling sources. Efforts to include cooling have been undertaken in the last decades, making use of different radiative cooling mechanisms for several chemical species, with some simplified radiative transfer. However, often the chemical composition of these 1D atmosphere models is fixed, implying the very strong assumption of chemical equilibrium, which is not at all true for a pulsating AGB atmosphere. We wish to model these atmospheres making as few assumptions as possible on equilibrium conditions. Therefore, as a first step, we introduce H2 dissociative cooling to the hydrodynamical model, arguing this is the dominant cooling factor. Using dissociative H2 cooling allows the ratio of the H-H2 gas mixture to vary, making the cooling efficiency time and space dependent. This will affect local cooling, in turn affecting the hydrodynamics and chemical composition, hereby introducing a feedback loop. Secondly, most significant radiative heating/cooling sources will be introduced to obtain the most realistic temperature structure. Next, dust acceleration will be introduced in the regions cool enough for dust condensation to exists. Hereby laying the basis of our hydrodynamical chemistry model for stellar winds of evolved stars.

Future of Ultraviolet Astronomy Based on Six Years of IUE Research

NASA Technical Reports Server (NTRS)

Mead, J. M. (Editor); Chapman, R. D. (Editor); Kondo, Y. (Editor)

1984-01-01

Physical insights into the various astronomical objects which were studied using the International Ultraviolet Explorer (IUE) satellite. Topics covered included galaxies, cool stars, hot stars, close binaries, variable stars, the interstellar medium, the solar system, and IUE follow-on missions.

Star Formation and Gas Dynamics in Galactic Disks: Physical Processes and Numerical Models

NASA Astrophysics Data System (ADS)

Ostriker, Eve C.

2011-04-01

Star formation depends on the available gaseous ``fuel'' as well as galactic environment, with higher specific star formation rates where gas is predominantly molecular and where stellar (and dark matter) densities are higher. The partition of gas into different thermal components must itself depend on the star formation rate, since a steady state distribution requires a balance between heating (largely from stellar UV for the atomic component) and cooling. In this presentation, I discuss a simple thermal and dynamical equilibrium model for the star formation rate in disk galaxies, where the basic inputs are the total surface density of gas and the volume density of stars and dark matter, averaged over ~kpc scales. Galactic environment is important because the vertical gravity of the stars and dark matter compress gas toward the midplane, helping to establish the pressure, and hence the cooling rate. In equilibrium, the star formation rate must evolve until the gas heating rate is high enough to balance this cooling rate and maintain the pressure imposed by the local gravitational field. In addition to discussing the formulation of this equilibrium model, I review the current status of numerical simulations of multiphase disks, focusing on measurements of quantities that characterize the mean properties of the diffuse ISM. Based on simulations, turbulence levels in the diffuse ISM appear relatively insensitive to local disk conditions and energetic driving rates, consistent with observations. It remains to be determined, both from observations and simulations, how mass exchange processes control the ratio of cold-to-warm gas in the atomic ISM.

Formation of the First Star Clusters and Massive Star Binaries by Fragmentation of Filamentary Primordial Gas Clouds

NASA Astrophysics Data System (ADS)

Hirano, Shingo; Yoshida, Naoki; Sakurai, Yuya; Fujii, Michiko S.

2018-03-01

We perform a set of cosmological simulations of early structure formation incorporating baryonic streaming motions. We present a case where a significantly elongated gas cloud with ∼104 solar mass (M ⊙) is formed in a pre-galactic (∼107 M ⊙) dark halo. The gas streaming into the halo compresses and heats the massive filamentary cloud to a temperature of ∼10,000 Kelvin. The gas cloud cools rapidly by atomic hydrogen cooling, and then by molecular hydrogen cooling down to ∼400 Kelvin. The rapid decrease of the temperature and hence of the Jeans mass triggers fragmentation of the filament to yield multiple gas clumps with a few hundred solar masses. We estimate the mass of the primordial star formed in each fragment by adopting an analytic model based on a large set of radiation hydrodynamics simulations of protostellar evolution. The resulting stellar masses are in the range of ∼50–120 M ⊙. The massive stars gravitationally attract each other and form a compact star cluster. We follow the dynamics of the star cluster using a hybrid N-body simulation. We show that massive star binaries are formed in a few million years through multi-body interactions at the cluster center. The eventual formation of the remnant black holes will leave a massive black hole binary, which can be a progenitor of strong gravitational wave sources similar to those recently detected by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO).

The Productivity of Oxygenic Photosynthesis around Cool, M Dwarf Stars

NASA Astrophysics Data System (ADS)

Lehmer, Owen R.; Catling, David C.; Parenteau, Mary N.; Hoehler, Tori M.

2018-06-01

In the search for life around cool stars, the presence of atmospheric oxygen is a prominent biosignature, as it may indicate oxygenic photosynthesis (OP) on the planetary surface. On Earth, most oxygenic photosynthesizing organisms (OPOs) use photons between 400 and 750 nm, which have sufficient energy to drive the photosynthetic reaction that generates O2 from H2O and CO2. OPOs around cool stars may evolve similar biological machinery capable of producing oxygen from water. However, in the habitable zones (HZs) of the coolest M dwarf stars, the flux of 400–750 nm photons may be just a few percent that of Earth’s. We show that the reduced flux of 400–750 nm photons around M dwarf stars could result in Earth-like planets being growth limited by light, unlike the terrestrial biosphere, which is limited by nutrient availability. We consider stars with photospheric temperatures between 2300 and 4200 K and show that such light-limited worlds could occur at the outer edge of the HZ around TRAPPIST-1-like stars. We find that even if OP can use photons longer than 750 nm, there would still be insufficient energy to sustain the Earth’s extant biosphere throughout the HZ of the coolest stars. This is because such stars emit largely in the infrared and near-infrared, which provide sufficient energy to make the planet habitable, but limits the energy available for OP. TRAPPIST-1f and g may fall into this category. Biospheres on such planets, potentially limited by photon availability, may generate small biogenic signals, which could be difficult for future observations to detect.

Chemical fingerprints of hot Jupiter planet formation

NASA Astrophysics Data System (ADS)

Maldonado, J.; Villaver, E.; Eiroa, C.

2018-05-01

Context. The current paradigm to explain the presence of Jupiter-like planets with small orbital periods (P < 10 days; hot Jupiters), which involves their formation beyond the snow line following inward migration, has been challenged by recent works that explore the possibility of in situ formation. Aims: We aim to test whether stars harbouring hot Jupiters and stars with more distant gas-giant planets show any chemical peculiarity that could be related to different formation processes. Methods: Our methodology is based on the analysis of high-resolution échelle spectra. Stellar parameters and abundances of C, O, Na, Mg, Al, Si, S, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, and Zn for a sample of 88 planet hosts are derived. The sample is divided into stars hosting hot (a < 0.1 au) and cool (a > 0.1 au) Jupiter-like planets. The metallicity and abundance trends of the two sub-samples are compared and set in the context of current models of planet formation and migration. Results: Our results show that stars with hot Jupiters have higher metallicities than stars with cool distant gas-giant planets in the metallicity range +0.00/+0.20 dex. The data also shows a tendency of stars with cool Jupiters to show larger abundances of α elements. No abundance differences between stars with cool and hot Jupiters are found when considering iron peak, volatile elements or the C/O, and Mg/Si ratios. The corresponding p-values from the statistical tests comparing the cumulative distributions of cool and hot planet hosts are 0.20, <0.01, 0.81, and 0.16 for metallicity, α, iron-peak, and volatile elements, respectively. We confirm previous works suggesting that more distant planets show higher planetary masses as well as larger eccentricities. We note differences in age and spectral type between the hot and cool planet host samples that might affect the abundance comparison. Conclusions: The differences in the distribution of planetary mass, period, eccentricity, and stellar host metallicity suggest a different formation mechanism for hot and cool Jupiters. The slightly larger α abundances found in stars harbouring cool Jupiters might compensate their lower metallicities allowing the formation of gas-giant planets. Based on data products from observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 072.C-0033(A), 072.C-0488(E), 074.B-0455(A), 075.C-0202(A), 077.C-0192(A), 077.D-0525(A), 078.C-0378(A), 078.C-0378(B), 080.A-9021(A), 082.C-0312(A) 082.C-0446(A), 083.A-9003(A), 083.A-9011(A), 083.A-9011(B), 083.A-9013(A), 083.C-0794(A), 084.A-9003(A), 084.A-9004(B), 085.A-9027(A), 085.C-0743(A), 087.A-9008(A), 088.C-0892(A), 089.C-0440(A), 089.C-0444(A), 089.C-0732(A), 090.C-0345(A), 092.A-9002(A), 192.C-0852(A), 60.A-9036(A), 60.A-9120(B), and 60.A-9700(A); and on data products from the SOPHIE archive.

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

McDonald, Michael; McNamara, Brian R.; Perimeter Institute for Theoretical Physics, Waterloo

In this study, we present new ultraviolet, optical, and X-ray data on the Phoenix galaxy cluster (SPT-CLJ2344-4243). Deep optical imaging reveals previously undetected filaments of star formation, extending to radii of ~50–100 kpc in multiple directions. Combined UV-optical spectroscopy of the central galaxy reveals a massive (2 × 10 9 M ⊙), young (~4.5 Myr) population of stars, consistent with a time-averaged star formation rate of 610 ± 50 M ⊙ yr –1. We report a strong detection of O vi λλ1032,1038, which appears to originate primarily in shock-heated gas, but may contain a substantial contribution (>1000 M ⊙ yrmore » –1) from the cooling intracluster medium (ICM). We confirm the presence of deep X-ray cavities in the inner ~10 kpc, which are among the most extreme examples of radio-mode feedback detected to date, implying jet powers of 2 – 7 × 10 45 erg s –1. We provide evidence that the active galactic nucleus inflating these cavities may have only recently transitioned from "quasar-mode" to "radio-mode," and may currently be insufficient to completely offset cooling. A model-subtracted residual X-ray image reveals evidence for prior episodes of strong radio-mode feedback at radii of ~100 kpc, with extended "ghost" cavities indicating a prior epoch of feedback roughly 100 Myr ago. This residual image also exhibits significant asymmetry in the inner ~200 kpc (0.15R 500), reminiscent of infalling cool clouds, either due to minor mergers or fragmentation of the cooling ICM. Taken together, these data reveal a rapidly evolving cool core which is rich with structure (both spatially and in temperature), is subject to a variety of highly energetic processes, and yet is cooling rapidly and forming stars along thin, narrow filaments.« less

Evidence for a warm wind from the red star in symbiotic binaries

NASA Technical Reports Server (NTRS)

Friedjung, M.; Stencel, R. E.; Viotti, R.

1983-01-01

A systematic redshift of the high ionization resonance emission lines with respect to the intercombination lines is found from an examination of the ultraviolet spectra of symbiotic stars obtained with IUE. After consideration of other possibilities, this is most probably explained by photon scattering in an expanding envelope optically thick to the resonance lines. Line formation in a wind, or at the base of a wind is therefore suggested. Reasons are also given indicating line formation of the most ionized species in a region with an electron temperature of the order of 100,000 K, probably around the cool star. The behavior of the emission line width with ionization energy seems to support this model. The cool components of symbiotic stars appear to differ from normal red giants, which do not have winds of this temperature. An explanation in terms of a higher rotation velocity due to the binary nature of these stars is suggested.

A possible brown dwarf companion to Gliese 569

NASA Technical Reports Server (NTRS)

Forrest, W. J.; Shure, Mark; Skrutskie, M. F.

1988-01-01

A faint cool companion to Gliese 569, discovered during an IR imaging survey of nearby stars, may be the lowest-mass stellar object yet found. The companion is somewhat cooler in its 1.65-3.75-micron energy distribution than the coolest known main-sequence stars, indicating a low mass. Despite its lower temperature, it is more luminous than similar extremely low-mass stars, suggesting that it is either a young low-mass star evolving toward the main sequence or a cooling substellar brown dwarf. The primary star has emission lines and a low space velocity and exhibits flaring, all of which imply youth for this system. Observations of Gliese 569 and its companion over a period of 2 yr confirm the common proper motion expected of a true binary. The 5-arcsec apparent separation (50 AU) implies an orbital period of roughly 500 yr, which will permit an eventual direct determination of the mass of the companion.

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.

Coronal Structures in Cool Stars: XMM-NEWTON Hybrid Stars and Coronal Evolution

NASA Technical Reports Server (NTRS)

Dupree, Andrea K.; Mushotzky, Richard (Technical Monitor)

2003-01-01

This program addresses the evolution of stellar coronas by comparing a solar-like corona in the supergiant Beta Dra (G2 Ib-IIa) to the corona in the allegedly more evolved state of a hybrid star, alpha TrA (K2 II-III). Because the hybrid star has a massive wind, it appears likely that the corona will be cooler and less dense as the magnetic loop structures are no longer closed. By analogy with solar coronal holes, when the topology of the magnetic field is configured with open magnetic structures, both the coronal temperature and density are lower than in atmospheres dominated by closed loops. The hybrid stars assume a pivotal role in the definition of coronal evolution, atmospheric heating processes and mechanisms to drive winds of cool stars. We are attempting to determine if this model of coronal evolution is correct by using XMM-NEWTON RGS spectra for the 2 targets we were allocated through the Guest Observer program.

A Search for Variability in Warm and Cool C-rich DQ White Dwarfs

NASA Astrophysics Data System (ADS)

Dupuis, Christopher Michael; Williams, Kurtis A.

2018-01-01

Hot DQ white dwarfs are a rare class of white dwarfs that have atmospheres dominated by carbon with little to no hydrogen or helium. Recently it has been found that the majority of these stars are photometrically variable likely due to rapid rotation with star spots. The cool progeny of the hot DQs are expected to also be rapidly rotating as no strong braking mechanisms should be present. We present the time-series photometry of multiple warm and cool C-rich DQ white dwarfs as part of an ongoing search for variability in hot DQ white dwarfs and their progeny. This program will permit us to confirm rotation as the source of variability, compare the distribution of rotation rates to those of more common white dwarf spectral types, and constrain the evolutionary rates of hot DQ rotation. These data are one way to better understand the formation scenarios of these stars.

Disordered nuclear pasta, magnetic field decay, and crust cooling in neutron stars

NASA Astrophysics Data System (ADS)

Horowitz, C. J.; Berry, D. K.; Briggs, C. M.; Caplan, M. E.; Cumming, A.; Schneider, A. S.

2015-04-01

Nuclear pasta, with non-spherical shapes, is expected near the base of the crust in neutron stars. Large scale molecular dynamics simulations of pasta show long lived topological defects that could increase electron scattering and reduce both the thermal and electrical conductivities. We model a possible low conductivity pasta layer by increasing an impurity parameter Qimp. Predictions of light curves for the low mass X-ray binary MXB 1659-29, assuming a large Qimp, find continued late time cooling that is consistent with Chandra observations. The electrical and thermal conductivities are likely related. Therefore observations of late time crust cooling can provide insight on the electrical conductivity and the possible decay of neutron star magnetic fields (assuming these are supported by currents in the crust). This research was supported in part by DOE Grants DE-FG02-87ER40365 (Indiana University) and DE-SC0008808 (NUCLEI SciDAC Collaboration).

Absorption line profiles in a companion spectrum of a mass losing cool supergiant

NASA Technical Reports Server (NTRS)

Rodrigues, Liliya L.; Boehm-Vitense, Erika

1990-01-01

Cool star winds can best be observed in resonance absorption lines seen in the spectrum of a hot companion, due to the wind passing in front of the blue star. We calculated absorption line profiles that would be seen in the ultraviolet part of the blue companion spectrum. Line profiles are derived for different radial dependences of the cool star wind and for different orbital phases of the binary. Bowen and Wilson find theoretically that stellar pulsations drive mass loss. We therefore apply our calculations to the Cepheid binary S Muscae which has a B5V companion. We find an upper limit for the Cepheid mass loss of M less than or equal to 7 x 10(exp -10) solar mass per year provided that the stellar wind of the companion does not influence the Cepheid wind at large distances.

Cool carbon stars in the halo and in dwarf galaxies: Hα, colours, and variability

NASA Astrophysics Data System (ADS)

Mauron, N.; Gigoyan, K. S.; Berlioz-Arthaud, P.; Klotz, A.

2014-02-01

The population of cool carbon (C) stars located far from the galactic plane is probably made of debris of small galaxies such as the Sagittarius dwarf spheroidal galaxy (Sgr), which are disrupted by the gravitational field of the Galaxy. We aim to know this population better through spectroscopy, 2MASS photometric colours, and variability data. When possible, we compared the halo results to C star populations in the Fornax dwarf spheroidal galaxy, Sgr, and the solar neighbourhood. We first present a few new discoveries of C stars in the halo and in Fornax. The number of spectra of halo C stars is now 125. Forty percent show Hα in emission. The narrow location in the JHK diagram of the halo C stars is found to differ from that of similar C stars in the above galaxies. The light curves of the Catalina and LINEAR variability databases were exploited to derive the pulsation periods of 66 halo C stars. A few supplementary periods were obtained with the TAROT telescopes. We confirm that the period distribution of the halo strongly resembles that of Fornax, and we found that it is very different from the C stars in the solar neighbourhood. There is a larger proportion of short-period Mira/SRa variables in the halo than in Sgr, but the survey for C stars in this dwarf galaxy is not complete, and the study of their variability needs to be continued to investigate the link between Sgr and the cool halo C stars. Based on observations made with the NTT and 3.6 m telescope at the European Southern Observatory (La Silla, Chile; programs 084.D-0302 and 070.D-0203), with the TAROT telescopes at La Silla and at Observatoire de la Côte d'Azur (France), and on the exploitation of the Catalina Sky Survey and the LINEAR variability databases.Appendix A is available in electronic form at http://www.aanda.org

Star-forming brightest cluster galaxies at 0.25

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

McDonald, M.; Stalder, B.; Bayliss, M.

2016-01-22

We present a multiwavelength study of the 90 brightest cluster galaxies (BCGs) in a sample of galaxy clusters selected via the Sunyaev Zel'dovich effect by the South Pole Telescope, utilizing data from various ground- and space-based facilities. We infer the star-formation rate (SFR) for the BCG in each cluster—based on the UV and IR continuum luminosity, as well as the [O ii]λλ3726,3729 emission line luminosity in cases where spectroscopy is available—and find seven systems with SFR > 100 M⊙ yr-1. We find that the BCG SFR exceeds 10 M⊙ yr-1 in 31 of 90 (34%) cases at 0.25 < z < 1.25, compared to ~1%–5% at z ~ 0 from the literature. At z gsim 1, this fraction increases tomore » $${92}_{-31}^{+6}$$%, implying a steady decrease in the BCG SFR over the past ~9 Gyr. At low-z, we find that the specific SFR in BCGs is declining more slowly with time than for field or cluster galaxies, which is most likely due to the replenishing fuel from the cooling ICM in relaxed, cool core clusters. At z gsim 0.6, the correlation between the cluster central entropy and BCG star formation—which is well established at z ~ 0—is not present. Instead, we find that the most star-forming BCGs at high-z are found in the cores of dynamically unrelaxed clusters. We use data from the Hubble Space Telescope to investigate the rest-frame near-UV morphology of a subsample of the most star-forming BCGs, and find complex, highly asymmetric UV morphologies on scales as large as ~50–60 kpc. The high fraction of star-forming BCGs hosted in unrelaxed, non-cool core clusters at early times suggests that the dominant mode of fueling star formation in BCGs may have recently transitioned from galaxy–galaxy interactions to ICM cooling.« less

Star-forming brightest cluster galaxies at 0.25 < z < 1.25: A transitioning fuel supply

DOE PAGES

McDonald, M.; Stalder, B.; Bayliss, M.; ...

2016-01-22

In this paper, we present a multiwavelength study of the 90 brightest cluster galaxies (BCGs) in a sample of galaxy clusters selected via the Sunyaev Zel'dovich effect by the South Pole Telescope, utilizing data from various ground- and space-based facilities. We infer the star-formation rate (SFR) for the BCG in each cluster—based on the UV and IR continuum luminosity, as well as the [O ii]λλ3726,3729 emission line luminosity in cases where spectroscopy is available—and find seven systems with SFR > 100 M ⊙ yr -1. We find that the BCG SFR exceeds 10 M ⊙ yr -1 in 31 of 90 (34%) cases at 0.25 < z < 1.25, compared to ~1%–5% at z ~ 0 from the literature. At z ≳ 1, this fraction increases tomore » $${92}_{-31}^{+6}$$%, implying a steady decrease in the BCG SFR over the past ~9 Gyr. At low-z, we find that the specific SFR in BCGs is declining more slowly with time than for field or cluster galaxies, which is most likely due to the replenishing fuel from the cooling ICM in relaxed, cool core clusters. At z ≳ 0.6, the correlation between the cluster central entropy and BCG star formation—which is well established at z ~ 0—is not present. Instead, we find that the most star-forming BCGs at high-z are found in the cores of dynamically unrelaxed clusters. We use data from the Hubble Space Telescope to investigate the rest-frame near-UV morphology of a subsample of the most star-forming BCGs, and find complex, highly asymmetric UV morphologies on scales as large as ~50–60 kpc. Finally, the high fraction of star-forming BCGs hosted in unrelaxed, non-cool core clusters at early times suggests that the dominant mode of fueling star formation in BCGs may have recently transitioned from galaxy–galaxy interactions to ICM cooling.« less

Does Radiative Feedback by the First Stars Promote or Prevent Second Generation Star Formation?

NASA Astrophysics Data System (ADS)

Ahn, Kyungjin; Shapiro, Paul R.

2008-03-01

We present a self-consistent study of formation of Pop III stars in the early stage of cosmic reionization. We study the effect of starlight from the first stars on the ability of other minihalos in their neighborhood to form additional stars. We show that the ionization front (I-front) is trapped by the neighboring minihalos, after it is transformed from R-type to D-type and preceded by a shock front. The fate of the core of nearby minihalos is mostly determined by the response of the core to this shock front, which leads to molecular cooling and collapse that, when compared to the same halo without external radiation, is (a) expedited, (b) delayed, (c) unaltered, or (d) reversed and prevented, depending upon the flux and halo mass and evolutionary stage. Roughly speaking, most halos that were destined to cool, collapse and form stars in the absence of external radiation are found to do so even when exposed to the first Pop III star in their neighborhood, while those that would not have done so are still not able to.

2MASS J06562998+3002455: Not a Cool White Dwarf Candidate, but a Population II Halo Star

NASA Astrophysics Data System (ADS)

de la Fuente Marcos, Raúl; de la Fuente Marcos, Carlos

2018-06-01

2MASS J06562998+3002455 or PSS 309-6 is a high proper-motion star that was discovered during a survey with the 2.1 m telescope at Kitt Peak National Observatory. Here, we reevaluate the status of this interesting star using Gaia DR2. Our results strongly suggest that PSS 309-6 could be a Population II star as the value of its V component is close to -220 km/s, which is typical for halo stars in the immediate solar neighborhood. Kapteyn's star is the nearest known halo star and PSS 309-6 exhibits similar kinematic and photometric signatures. Its properties also resemble those of 2MASS J15484023-3544254, which was once thought to be the nearest cool white dwarf but was later reclassified as K-type subdwarf. Although it is virtually certain that PSS 309-6 is not a nearby white dwarf but a more distant Population II subdwarf, further spectroscopic information, including radial velocity measurements, is necessary to fully characterize this probable member of the Galactic halo.

Overview of the observations of symbiotic stars

NASA Technical Reports Server (NTRS)

Viotti, Roberto

1993-01-01

The term Symbiotic stars commonly denotes variable stars whose optical spectra simultaneously present a cool absorption spectrum (typically TiO absorption bands) and emission lines of high ionization energy. This term is now used for the category of variable stars with composite spectrum. The main spectral features of these objects are: (1) the presence of the red continuum typical of a cool star, (2) the rich emission line spectrum, and (3) the UV excess, frequently with the Balmer continuum in emission. In addition to the peculiar spectrum, the very irregular photometric and spectroscopic variability is the major feature of the symbiotic stars. Moreover, the light curve is basic to identify the different phases of activity in a symbiotic star. The physical mechanisms that cause the symbiotic phenomenon and its variety are the focus of this paper. An astronomical phenomenon characterized by a composite stellar spectrum with two apparently conflicting features, and large variability has been observed. Our research set out to find the origin of this behavior and, in particular, to identify and measure the physical mechanism(s) responsible for the observed phenomena.

Dark matter heating in strange stars

NASA Astrophysics Data System (ADS)

Huang, Xi; Wang, Wen; Zheng, XiaoPing

2014-04-01

We study the effect of dark matter heating on the temperature of typical strange star (SS hereafter) ( M = 1.4 M⊙, R = 10 km) in normal phase (NSS hereafter) and in a possible existing colour-flavour locked (CFL)phase (CSS hereafter). For NSS, the influence of dark matter heating is ignored until roughly 107 yr. After 107 yr, the dark matter heating is dominant that significantly delays the star cooling, which maintains a temperature much higher than that predicted by standard cooling model for old stars. Especially for CSS, the emissivity of dark matter will play a leading role after roughly 104 yr, which causes the temperature to rise. This leads to the plateau of surface temperature appearing in ˜106.5 yr which is earlier than that of NSS (˜107 yr).

Lyman alpha initiated winds in late-type stars

NASA Technical Reports Server (NTRS)

Haisch, B. M.; Linsky, J. L.; Vanderhucht, K. A.

1979-01-01

The IUE survey of late-type stars revealed a sharp division in the HR diagram between stars with solar type spectra (chromosphere and transition region lines) and those with non-solar type spectra (only chromosphere lines). Models of both hot coronae and cool wind flows were calculated using stellar model chromospheres as starting points for stellar wind calculations in order to investigate the possibility of having a supersonic transition locus in the HR diagram dividing hot coronae from cool winds. From these models, it is concluded that the Lyman alpha flux may play an important role in determining the location of a stellar wind critical point. The interaction of Lyman alpha radiation pressure with Alfven waves in producing strong, low temperature stellar winds in the star Arcturus is examined.

APOKASC 2.0: Asteroseismology and Spectroscopy for Cool Stars

NASA Astrophysics Data System (ADS)

Pinsonneault, Marc H.; Elsworth, Yvonne P.; APOKASC

2017-01-01

The APOGEE survey has obtained and analyzed high resolution H band spectra of more than 10,000 cool dwarfs and giants in the original Kepler fields. The APOKASC effort combines this data with asteroseismology and star spot studies, resulting in more than 7,000 stellar mass estimates for dwarfs and giants with high quality abundances, temperatures, and surface gravities. We highlight the main results from this effort so far, which include a tight correlation between surface abundances in giants and stellar mass, precise absolute gravity calibrations, and the discovery of unexpected stellar populations, such as young alpha-enhanced stars. We discuss grid modeling estimates for stellar masses and compare the absolute asteroseismic mass scale to calibrators in star clusters and the halo Directions for future efforts are discussed.

NASA'S Chandra Finds Superfluid in Neutron Star's Core

NASA Astrophysics Data System (ADS)

2011-02-01

NASA's Chandra X-ray Observatory has discovered the first direct evidence for a superfluid, a bizarre, friction-free state of matter, at the core of a neutron star. Superfluids created in laboratories on Earth exhibit remarkable properties, such as the ability to climb upward and escape airtight containers. The finding has important implications for understanding nuclear interactions in matter at the highest known densities. Neutron stars contain the densest known matter that is directly observable. One teaspoon of neutron star material weighs six billion tons. The pressure in the star's core is so high that most of the charged particles, electrons and protons, merge resulting in a star composed mostly of uncharged particles called neutrons. Two independent research teams studied the supernova remnant Cassiopeia A, or Cas A for short, the remains of a massive star 11,000 light years away that would have appeared to explode about 330 years ago as observed from Earth. Chandra data found a rapid decline in the temperature of the ultra-dense neutron star that remained after the supernova, showing that it had cooled by about four percent over a 10-year period. "This drop in temperature, although it sounds small, was really dramatic and surprising to see," said Dany Page of the National Autonomous University in Mexico, leader of a team with a paper published in the February 25, 2011 issue of the journal Physical Review Letters. "This means that something unusual is happening within this neutron star." Superfluids containing charged particles are also superconductors, meaning they act as perfect electrical conductors and never lose energy. The new results strongly suggest that the remaining protons in the star's core are in a superfluid state and, because they carry a charge, also form a superconductor. "The rapid cooling in Cas A's neutron star, seen with Chandra, is the first direct evidence that the cores of these neutron stars are, in fact, made of superfluid and superconducting material," said Peter Shternin of the Ioffe Institute in St Petersburg, Russia, leader of a team with a paper accepted in the journal Monthly Notices of the Royal Astronomical Society. Both teams show that this rapid cooling is explained by the formation of a neutron superfluid in the core of the neutron star within about the last 100 years as seen from Earth. The rapid cooling is expected to continue for a few decades and then it should slow down. "It turns out that Cas A may be a gift from the Universe because we would have to catch a very young neutron star at just the right point in time," said Page's co-author Madappa Prakash, from Ohio University. "Sometimes a little good fortune can go a long way in science." The onset of superfluidity in materials on Earth occurs at extremely low temperatures near absolute zero, but in neutron stars, it can occur at temperatures near a billion degrees Celsius. Until now there was a very large uncertainty in estimates of this critical temperature. This new research constrains the critical temperature to between one half a billion to just under a billion degrees. Cas A will allow researchers to test models of how the strong nuclear force, which binds subatomic particles, behaves in ultradense matter. These results are also important for understanding a range of behavior in neutron stars, including "glitches," neutron star precession and pulsation, magnetar outbursts and the evolution of neutron star magnetic fields. Small sudden changes in the spin rate of rotating neutron stars, called glitches, have previously given evidence for superfluid neutrons in the crust of a neutron star, where densities are much lower than seen in the core of the star. This latest news from Cas A unveils new information about the ultra-dense inner region of the neutron star. "Previously we had no idea how extended superconductivity of protons was in a neutron star," said Shternin's co-author Dmitry Yakovlev, also from the Ioffe Institute. The cooling in the Cas A neutron star was first discovered by co-author Craig Heinke, from the University of Alberta, Canada, and Wynn Ho from the University of Southampton, UK, in 2010. It was the first time that astronomers have measured the rate of cooling of a young neutron star. Page's co-authors were Prakash, James Lattimer (State University of New York at Stony Brook), and Andrew Steiner (Michigan State University.) Shternin's co-authors were Yakovlev, Heinke, Ho, and Daniel Patnaude (Harvard-Smithsonian Center for Astrophysics.) More information, including images and other multimedia, can be found at: http://chandra.harvard.edu and http://chandra.nasa.gov

The problem of the barium stars

NASA Technical Reports Server (NTRS)

Bohm-Vitense, E.; Nemec, J.; Proffitt, C.

1984-01-01

Ultraviolet observations of barium stars and other cool stars with peculiar element abundances are reported. Those observations attempted to find hot white dwarf companions. Among six real barium stars studied, only Zeta Cap was found to have a white dwarf companion. Among seven mild, or marginal, barium stars studied, at least three were found to have hot subluminous companions. It is likely that all of them have white dwarf companions.

Radio Jets Clearing the Way Through a Galaxy: Watching Feedback in Action in the Seyfert Galaxy IC 5063

NASA Astrophysics Data System (ADS)

Morganti, R.; Oosterloo, T. A.; Oonk, J. B. R.; Frieswijk, W.; Tadhunter, C. N.

2015-12-01

High-resolution (0.5 arcsec) CO(2-1) observations performed with the Atacama Large Millimetre/submillimetre Array have been used to trace the kinematics of the molecular gas in the Seyfert 2 galaxy{IC 5063}. Although one of the most radio-loud Seyfert galaxy, IC 5063 is a relatively weak radio source (P1.4GHz=3 ×1023 W Hz-1). The data reveal that the kinematics of the gas is very complex. A fast outflow of molecular gas extends along the entire radio jet (˜ 1 kpc), with the highest outflow velocities about 0.5 kpc from the nucleus, at the location of the brighter hot-spot in the W lobe. All the observed characteristics can be described by a scenario of a radio plasma jet expanding into a clumpy medium, interacting directly with the clouds and inflating a cocoon that drives a lateral outflow into the interstellar medium. This suggests that most of the observed cold molecular outflow is due to fast cooling of the gas after the passage of a shock and that it is the end product of the cooling process.

Hubble Space Telescope observations of cool white dwarf stars: Detection of new species of heavy elements

NASA Technical Reports Server (NTRS)

Shipman, Harry; Barnhill, Maurice; Provencal, Judi; Roby, Scott; Bues, Irmela; Cordova, France; Hammond, Gordon; Hintzen, Paul; Koester, Detlev; Liebert, James

1995-01-01

Observations of cool white dwarf stars with the Hubble Space Telescope (HST) has uncovered a number of spectral features from previouslly unobserved species. In this paper we present the data on four cool white dwarfs. We present identifications, equivalent width measurements, and brief summaries of the significance of our findings. The four stars observed are GD 40 (DBZ3, G 74-7 (DAZ), L 745-46A (DZ), and LDS 749B (DBA). Many additional species of heavey elements were detected in GD 40 and G 74-7. In L 745-46A, while the detections are limited to Fe 1, Fe II, and Mg II, the quality of the Mg II h and K line profiles should permit a test of the line broadening theories, which are so crucial to abundance determinations. The clear detection of Mg II h and k in LDS 749 B should, once an abundance determination is made, provide a clear test of the hypothesis that the DBA stars are the result of accretion from the interstellar medium. This star contains no other clear features other than a tantalizing hint of C II 1335 with a P Cygni profile, and some expected He 1 lines.

Mass loss from red giants - Results from ultraviolet spectroscopy

NASA Technical Reports Server (NTRS)

Linsky, J. L.

1985-01-01

New instrumentation in space, primarily the IUE spacecraft, has enabled the application of ultraviolet spectroscopic techniques to the determination of physical properties and reliable mass loss rates for red giant winds. One important result is the determination of where in the H-R diagram are found stars with hot outer atmospheres and with cool winds. So far it appears that single cool stars, except perhaps the so-called hybrid stars, have either hot outer atmospheres or cool winds but not both. The C II resonance (1335 A) and intersystem (2325 A) multiplets have been used to derive temperatures, densities, and geometrical extents for the chromospheric portions of red giant winds, with the result that the red giants and the earlier giants with hot coronae have qualitatively different chromospheres. Mass loss rates can now be derived accurately from the analysis of asymmetric emission lines, such as the Mg II resonance lines, and from P Cygni profile lines of atoms in the dominant ionization stage when a hot star is available to probe the wind of a red giant. The Zeta Aur systems, consisting of a K-M supergiant and a main sequence B star are important systems for reliable mass loss rates for the red supergiant components are becoming available.

What is the maximum mass of a Population III galaxy?

NASA Astrophysics Data System (ADS)

Visbal, Eli; Bryan, Greg L.; Haiman, Zoltán

2017-08-01

We utilize cosmological hydrodynamic simulations to study the formation of Population III (Pop III) stars in dark matter haloes exposed to strong ionizing radiation. We simulate the formation of three haloes subjected to a wide range of ionizing fluxes, and find that for high flux, ionization and photoheating can delay gas collapse and star formation up to halo masses significantly larger than the atomic cooling threshold. The threshold halo mass at which gas first collapses and cools increases with ionizing flux for intermediate values, and saturates at a value approximately an order of magnitude above the atomic cooling threshold for extremely high flux (e.g. ≈5 × 108 M⊙ at z ≈ 6). This behaviour can be understood in terms of photoheating, ionization/recombination and Ly α cooling in the pressure-supported, self-shielded gas core at the centre of the growing dark matter halo. We examine the spherically averaged radial velocity profiles of collapsing gas and find that a gas mass of up to ≈106 M⊙ can reach the central regions within 3 Myr, providing an upper limit on the amount of massive Pop III stars that can form. The ionizing radiation increases this limit by a factor of a few compared to strong Lyman-Werner radiation alone. We conclude that the bright He II 1640 Å emission recently observed from the high-redshift galaxy CR7 cannot be explained by Pop III stars alone. However, in some haloes, a sufficient number of Pop III stars may form to be detectable with future telescopes such as the James Webb Space Telescope.

X-ray mapping of the stellar wind in the binary PSR J2032+4127/MT91 213

NASA Astrophysics Data System (ADS)

Petropoulou, M.; Vasilopoulos, G.; Christie, I. M.; Giannios, D.; Coe, M. J.

2018-02-01

PSR J2032+4127 is a young and rapidly rotating pulsar on a highly eccentric orbit around the high-mass Be star MT91 213. X-ray monitoring of the binary system over an ˜4000 d period with Swift has revealed an increase of the X-ray luminosity which we attribute to the synchrotron emission of the shocked pulsar wind. We use Swift X-ray observations to infer a clumpy stellar wind with r-2 density profile and constrain the Lorentz factor of the pulsar wind to 105 < γw < 106. We investigate the effects of an axisymmetric stellar wind with polar gradient on the X-ray emission. Comparison of the X-ray light curve hundreds of days before and after the periastron can be used to explore the polar structure of the wind.

BROADBAND SPECTROSCOPY USING TWO SUZAKU OBSERVATIONS OF THE HMXB GX 301-2

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

Suchy, Slawomir; Markowitz, Alex; Rothschild, Richard E.

2012-02-01

We present the analysis of two Suzaku observations of GX 301-2 at two orbital phases after the periastron passage. Variations in the column density of the line-of-sight absorber are observed, consistent with accretion from a clumpy wind. In addition to a cyclotron resonance scattering feature (CRSF), multiple fluorescence emission lines were detected in both observations. The variations in the pulse profiles and the CRSF throughout the pulse phase have a signature of a magnetic dipole field. Using a simple dipole model we calculated the expected magnetic field values for different pulse phases and were able to extract a set ofmore » geometrical angles, loosely constraining the dipole geometry in the neutron star. From the variation of the CRSF width and energy, we found a geometrical solution for the dipole, making the inclination consistent with previously published values.« less

Fragmentation inside atomic cooling haloes exposed to Lyman-Werner radiation

NASA Astrophysics Data System (ADS)

Regan, John A.; Downes, Turlough P.

2018-04-01

Supermassive stars born in pristine environments in the early Universe hold the promise of being the seeds for the supermassive black holes observed as high redshift quasars shortly after the epoch of reionisation. H2 suppression is thought to be crucial in order to negate normal Population III star formation and allow high accretion rates to drive the formation of supermassive stars. Only in the cases where vigorous fragmentation is avoided will a monolithic collapse be successful, giving rise to a single massive central object. We investigate the number of fragmentation sites formed in collapsing atomic cooling haloes subject to various levels of background Lyman-Werner flux. The background Lyman-Werner flux manipulates the chemical properties of the gas in the collapsing halo by destroying H2. We find that only when the collapsing gas cloud shifts from the molecular to the atomic cooling regime is the degree of fragmentation suppressed. In our particular case, we find that this occurs above a critical Lyman-Werner background of J ˜ 10 J21. The important criterion being the transition to the atomic cooling regime rather than the actual value of J, which will vary locally. Once the temperature of the gas exceeds T ≳ 104 K and the gas transitions to atomic line cooling, then vigorous fragmentation is strongly suppressed.

A temperate rocky super-Earth transiting a nearby cool star

NASA Astrophysics Data System (ADS)

Dittmann, Jason A.; Irwin, Jonathan M.; Charbonneau, David; Bonfils, Xavier; Astudillo-Defru, Nicola; Haywood, Raphaëlle D.; Berta-Thompson, Zachory K.; Newton, Elisabeth R.; Rodriguez, Joseph E.; Winters, Jennifer G.; Tan, Thiam-Guan; Almenara, Jose-Manuel; Bouchy, François; Delfosse, Xavier; Forveille, Thierry; Lovis, Christophe; Murgas, Felipe; Pepe, Francesco; Santos, Nuno C.; Udry, Stephane; Wünsche, Anaël; Esquerdo, Gilbert A.; Latham, David W.; Dressing, Courtney D.

2017-04-01

M dwarf stars, which have masses less than 60 per cent that of the Sun, make up 75 per cent of the population of the stars in the Galaxy. The atmospheres of orbiting Earth-sized planets are observationally accessible via transmission spectroscopy when the planets pass in front of these stars. Statistical results suggest that the nearest transiting Earth-sized planet in the liquid-water, habitable zone of an M dwarf star is probably around 10.5 parsecs away. A temperate planet has been discovered orbiting Proxima Centauri, the closest M dwarf, but it probably does not transit and its true mass is unknown. Seven Earth-sized planets transit the very low-mass star TRAPPIST-1, which is 12 parsecs away, but their masses and, particularly, their densities are poorly constrained. Here we report observations of LHS 1140b, a planet with a radius of 1.4 Earth radii transiting a small, cool star (LHS 1140) 12 parsecs away. We measure the mass of the planet to be 6.6 times that of Earth, consistent with a rocky bulk composition. LHS 1140b receives an insolation of 0.46 times that of Earth, placing it within the liquid-water, habitable zone. With 90 per cent confidence, we place an upper limit on the orbital eccentricity of 0.29. The circular orbit is unlikely to be the result of tides and therefore was probably present at formation. Given its large surface gravity and cool insolation, the planet may have retained its atmosphere despite the greater luminosity (compared to the present-day) of its host star in its youth. Because LHS 1140 is nearby, telescopes currently under construction might be able to search for specific atmospheric gases in the future.

A temperate rocky super-Earth transiting a nearby cool star.

PubMed

Dittmann, Jason A; Irwin, Jonathan M; Charbonneau, David; Bonfils, Xavier; Astudillo-Defru, Nicola; Haywood, Raphaëlle D; Berta-Thompson, Zachory K; Newton, Elisabeth R; Rodriguez, Joseph E; Winters, Jennifer G; Tan, Thiam-Guan; Almenara, Jose-Manuel; Bouchy, François; Delfosse, Xavier; Forveille, Thierry; Lovis, Christophe; Murgas, Felipe; Pepe, Francesco; Santos, Nuno C; Udry, Stephane; Wünsche, Anaël; Esquerdo, Gilbert A; Latham, David W; Dressing, Courtney D

2017-04-19

M dwarf stars, which have masses less than 60 per cent that of the Sun, make up 75 per cent of the population of the stars in the Galaxy. The atmospheres of orbiting Earth-sized planets are observationally accessible via transmission spectroscopy when the planets pass in front of these stars. Statistical results suggest that the nearest transiting Earth-sized planet in the liquid-water, habitable zone of an M dwarf star is probably around 10.5 parsecs away. A temperate planet has been discovered orbiting Proxima Centauri, the closest M dwarf, but it probably does not transit and its true mass is unknown. Seven Earth-sized planets transit the very low-mass star TRAPPIST-1, which is 12 parsecs away, but their masses and, particularly, their densities are poorly constrained. Here we report observations of LHS 1140b, a planet with a radius of 1.4 Earth radii transiting a small, cool star (LHS 1140) 12 parsecs away. We measure the mass of the planet to be 6.6 times that of Earth, consistent with a rocky bulk composition. LHS 1140b receives an insolation of 0.46 times that of Earth, placing it within the liquid-water, habitable zone. With 90 per cent confidence, we place an upper limit on the orbital eccentricity of 0.29. The circular orbit is unlikely to be the result of tides and therefore was probably present at formation. Given its large surface gravity and cool insolation, the planet may have retained its atmosphere despite the greater luminosity (compared to the present-day) of its host star in its youth. Because LHS 1140 is nearby, telescopes currently under construction might be able to search for specific atmospheric gases in the future.

A Post-AGB Star in the Small Magellanic Cloud Observed with the Spitzer Infrared Spectrograph

DTIC Science & Technology

2006-10-23

spectral features, MSX SMC 029, in the Small Magellanic Cloud (SMC) usimg the low-resolution modules of the Infrared Spectrograph on the Spitzer Space ...029, in the Small Magellanic Cloud (SMC) using the low-resolution modules of the Infrared Spectrograph on the Spitzer Space Telescope. A cool dust... outer atmosphere expands and pulsates, pushing gas away from the star where it can cool and condense into dust grains. The resulting circumstellar dust

Brown Dwarf Weather (Artist's Concept)

NASA Image and Video Library

2017-06-06

This artist's concept shows what the weather might look like on cool star-like bodies known as brown dwarfs. These giant balls of gas start out life like stars, but lack the mass to sustain nuclear fusion at their cores, and instead, fade and cool with time. Observations from NASA's Spitzer Space Telescope suggest that most brown dwarfs are roiling with one or more planet-size storms akin to Jupiter's "Great Red Spot." https://photojournal.jpl.nasa.gov/catalog/PIA21475

NGC 346: Looking in the Cradle of a Massive Star Cluster

NASA Astrophysics Data System (ADS)

Gouliermis, Dimitrios A.; Hony, Sacha

2017-03-01

How does a star cluster of more than few 10,000 solar masses form? We present the case of the cluster NGC 346 in the Small Magellanic Cloud, still embedded in its natal star-forming region N66, and we propose a scenario for its formation, based on observations of the rich stellar populations in the region. Young massive clusters host a high fraction of early-type stars, indicating an extremely high star formation efficiency. The Milky Way galaxy hosts several young massive clusters that fill the gap between young low-mass open clusters and old massive globular clusters. Only a handful, though, are young enough to study their formation. Moreover, the investigation of their gaseous natal environments suffers from contamination by the Galactic disk. Young massive clusters are very abundant in distant starburst and interacting galaxies, but the distance of their hosting galaxies do not also allow a detailed analysis of their formation. The Magellanic Clouds, on the other hand, host young massive clusters in a wide range of ages with the youngest being still embedded in their giant HII regions. Hubble Space Telescope imaging of such star-forming complexes provide a stellar sampling with a high dynamic range in stellar masses, allowing the detailed study of star formation at scales typical for molecular clouds. Our cluster analysis on the distribution of newly-born stars in N66 shows that star formation in the region proceeds in a clumpy hierarchical fashion, leading to the formation of both a dominant young massive cluster, hosting about half of the observed pre-main-sequence population, and a self-similar dispersed distribution of the remaining stars. We investigate the correlation between stellar surface density (and star formation rate derived from star-counts) and molecular gas surface density (derived from dust column density) in order to unravel the physical conditions that gave birth to NGC 346. A power law fit to the data yields a steep correlation between these two parameters with a considerable scatter. The fraction of stellar over the total (gas plus young stars) mass is found to be systematically higher within the central 15 pc (where the young massive cluster is located) than outside, which suggests variations in the star formation efficiency within the same star-forming complex. This trend possibly reflects a change of star formation efficiency in N66 between clustered and non-clustered star formation. Our findings suggest that the formation of NGC 346 is the combined result of star formation regulated by turbulence and of early dynamical evolution induced by the gravitational potential of the dense interstellar medium.

Further constraints on neutron star crustal properties in the low-mass X-ray binary 1RXS J180408.9-342058

NASA Astrophysics Data System (ADS)

Parikh, A. S.; Wijnands, R.; Degenaar, N.; Ootes, L.; Page, D.

2018-05-01

We report on two new quiescent XMM-Newton observations (in addition to the earlier Swift/XRT and XMM-Newton coverage) of the cooling neutron star crust in the low-mass X-ray binary 1RXS J180408.9-342058. Its crust was heated during the ˜4.5 month accretion outburst of the source. From our quiescent observations, fitting the spectra with a neutron star atmosphere model, we found that the crust had cooled from ˜100 to ˜73 eV from ˜8 to ˜479 d after the end of its outburst. However, during the most recent observation, taken ˜860 d after the end of the outburst, we found that the crust appeared not to have cooled further. This suggested that the crust had returned to thermal equilibrium with the neutron star core. We model the quiescent thermal evolution with the theoretical crustal cooling code NSCool and find that the source requires a shallow heat source, in addition to the standard deep crustal heating processes, contributing ˜0.9 MeV per accreted nucleon during outburst to explain its observed temperature decay. Our high quality XMM-Newton data required an additional hard component to adequately fit the spectra. This slightly complicates our interpretation of the quiescent data of 1RXS J180408.9-342058. The origin of this component is not fully understood.

Dynamical Model Calculations of AGB Star Winds Including Time Dependent Dust Formation and Non-LTE Radiative Cooling

NASA Astrophysics Data System (ADS)

Schirrmacher, V.; Woitke, P.; Sedlmayr, E.

Stars on the Asymptotic Giant Branch (AGB) are pulsating objects in a late evolutionary stage. The stellar pulsation creates sound waves which steepen up to shock waves in the upper atmosphere and lead to a time dependent levitation of the outer atmosphere. Thereby, the stellar pulsation triggers and facilitates the formation of dust close to the star. The dust is accelerated by radiation pressure and drags the gas outwards due to frictional forces which is identified to provide the basic mass loss mechanism. A longstanding problem concerning the modelling of these physical processes is the influence of the propagating shock waves on the temperature structure of the wind, which strongly influences the dust formation. We have therefore improved our numerical models of AGB-star envelopes by including (i) a detailed calculation of non-LTE radiative heating and cooling rates, predominantly arising from atomic and molecular lines and (ii) atomic and molecular exitation aswell as ionisation and dissociation in the equation of state. First results, presented here, show that the cooling time scales behind the shock waves are usually rather short, but the binding energies of molecular hydrogen provide an important energy buffer capable to delay the radiative heating or cooling. Thus considerable deviations from radiative equilibrium may occur in the important inner dust forming layers.

Discovery of Low-ionization Envelopes in the Planetary Nebula NGC 5189: Spatially-resolved Diagnostics from HST Observations

NASA Astrophysics Data System (ADS)

Danehkar, Ashkbiz; Karovska, Margarita; Maksym, Walter Peter; Montez, Rodolfo

2018-01-01

The planetary nebula NGC 5189 shows one of the most spectacular morphological structures among planetary nebulae with [WR]-type central stars. Using high-angular resolution HST/WFC3 imaging, we discovered inner, low-ionization structures within a region of 0.3 parsec × 0.2 parsec around the central binary system. We used Hα, [O III], and [S II] emission line images to construct line-ratio diagnostic maps, which allowed us to spatially resolve two distinct low-ionization envelopes within the inner, ionized gaseous environment, extending over a distance of 0.15 pc from the central binary. Both the low-ionization envelopes appear to be expanding along a NE to SW symmetric axis. The SW envelope appears smaller than its NE counterpart. Our diagnostic maps show that highly-ionized gas surrounds these low-ionization envelopes, which also include filamentary and clumpy structures. These envelopes could be a result of a powerful outburst from the central interacting binary, when one of the companions (now a [WR] star) was in its AGB evolutionary stage, with a strong mass-loss generating dense circumstellar shells. Dense material ejected from the progenitor AGB star is likely heated up as it propagates along a symmetric axis into the previously expelled low-density material. Our new diagnostic methodology is a powerful tool for high-angular resolution mapping of low-ionization structures in other planetary nebulae with complex structures possibly caused by past outbursts from their progenitors.

SHARDS: constraints on the dust attenuation law of star-forming galaxies at z ˜ 2

NASA Astrophysics Data System (ADS)

Tress, Mónica; Mármol-Queraltó, Esther; Ferreras, Ignacio; Pérez-González, Pablo G.; Barro, Guillermo; Pampliega, Belén Alcalde; Cava, Antonio; Domínguez-Sánchez, Helena; Eliche-Moral, Carmen; Espino-Briones, Néstor; Esquej, Pilar; Hernán-Caballero, Antonio; Rodighiero, Giulia; Rodriguez-Muñoz, Lucía

2018-04-01

We make use of the Survey of High-z Absorption Red and Dead Sources, an ultradeep (<26.5AB) galaxy survey that provides optical photospectra at resolution R ˜ 50, via medium-band filters (FWHM ˜ 150 Å). This data set is combined with ancillary optical and NIR fluxes to constrain the dust attenuation law in the rest-frame NUV region of star-forming galaxies within the redshift window 1.5 < z < 3. We focus on the NUV bump strength (B) and the total-to-selective extinction ratio (RV), targeting a sample of 1753 galaxies. By comparing the data with a set of population synthesis models coupled to a parametric dust attenuation law, we constrain RV and B, as well as the colour excess, E(B - V). We find a correlation between RV and B, which can be interpreted either as a result of the grain size distribution, or a variation of the dust geometry among galaxies. According to the former, small dust grains are associated with a stronger NUV bump. The latter would lead to a range of clumpiness in the distribution of dust within the interstellar medium of star-forming galaxies. The observed wide range of NUV bump strengths can lead to a systematic in the interpretation of the UV slope β typically used to characterize the dust content. In this study, we quantify these variations, concluding that the effects are Δβ ˜ 0.4.

X-Ray Burst Oscillations: From Flame Spreading to the Cooling Wake

NASA Technical Reports Server (NTRS)

Mahmoodifar, Simin; Strohmayer, Tod

2016-01-01

Type I X-ray bursts are thermonuclear flashes observed from the surfaces of accreting neutron stars (NSs) in low mass X-ray binaries. Oscillations have been observed during the rise and/or decay of some of these X-ray bursts. Those seen during the rise can be well explained by a spreading hot spot model, but large amplitude oscillations in the decay phase remain mysterious because of the absence of a clear-cut source of asymmetry. To date there have not been any quantitative studies that consistently track the oscillation amplitude both during the rise and decay (cooling tail) of bursts. Here we compute the light curves and amplitudes of oscillations in X-ray burst models that realistically account for both flame spreading and subsequent cooling. We present results for several such "cooling wake" models, a "canonical" cooling model where each patch on the NS surface heats and cools identically, or with a latitude-dependent cooling timescale set by the local effective gravity, and an "asymmetric" model where parts of the star cool at significantly different rates. We show that while the canonical cooling models can generate oscillations in the tails of bursts, they cannot easily produce the highest observed modulation amplitudes. Alternatively, a simple phenomenological model with asymmetric cooling can achieve higher amplitudes consistent with the observations.

Emission-Line Galaxies from the PEARS Hubble Ultra Deep Field: A 2-D Detection Method and First Results

NASA Technical Reports Server (NTRS)

Gardner, J. P.; Straughn, Amber N.; Meurer, Gerhardt R.; Pirzkal, Norbert; Cohen, Seth H.; Malhotra, Sangeeta; Rhoads, james; Windhorst, Rogier A.; Gardner, Jonathan P.; Hathi, Nimish P.;

Frontiers of stellar evolution

NASA Technical Reports Server (NTRS)

Lambert, David L. (Editor)

1991-01-01

The present conference discusses theoretical and observational views of star formation, spectroscopic constraints on the evolution of massive stars, very low mass stars and brown dwarfs, asteroseismology, globular clusters as tests of stellar evolution, observational tests of stellar evolution, and mass loss from cool evolved giant stars. Also discussed are white dwarfs and hot subdwarfs, neutron stars and black holes, supernovae from single stars, close binaries with evolved components, accretion disks in interacting binaries, supernovae in binary systems, stellar evolution and galactic chemical evolution, and interacting binaries containing compact components.

The HETDEX pilot survey. V. The physical origin of Lyα emitters probed by near-infrared spectroscopy

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

Song, Mimi; Finkelstein, Steven L.; Gebhardt, Karl

2014-08-10

We present the results from a Very Large Telescope/SINFONI and Keck/NIRSPEC near-infrared spectroscopic survey of 16 Lyα emitters (LAEs) at z = 2.1-2.5 in the COSMOS and GOODS-N fields discovered from the Hobby Eberly Telescope Dark Energy Experiment Pilot Survey. We detect rest-frame optical nebular lines (Hα and/or [O III] λ5007) for 10 of the LAEs and measure physical properties, including the star formation rate (SFR), gas-phase metallicity, gas mass fraction, and Lyα velocity offset. We find that LAEs may lie below the mass-metallicity relation for continuum-selected star-forming galaxies at the same redshift. The LAEs all show velocity shifts ofmore » Lyα relative to the systemic redshift ranging between +85 and +296 km s{sup –1} with a mean of +180 km s{sup –1}. This value is smaller than measured for continuum-selected star-forming galaxies at similar redshifts. The Lyα velocity offsets show a moderate correlation with the measured SFR (2.5σ), but no significant correlations are seen with the SFR surface density, specific SFR, stellar mass, or dynamical mass (≲1.5σ). Exploring the role of dust, kinematics of the interstellar medium (ISM), and geometry on the escape of Lyα photons, we find no signature of selective quenching of resonantly scattered Lyα photons. However, we also find no evidence that a clumpy ISM is enhancing the Lyα equivalent width. Our results suggest that the low metallicity in LAEs may be responsible for yielding an environment with a low neutral hydrogen column density and less dust, easing the escape of Lyα photons over that in continuum-selected star-forming galaxies.« less

Accretion from a clumpy massive-star wind in supergiant X-ray binaries

NASA Astrophysics Data System (ADS)

El Mellah, I.; Sundqvist, J. O.; Keppens, R.

2018-04-01

Supergiant X-ray binaries (SgXB) host a compact object, often a neutron star (NS), orbiting an evolved O/B star. Mass transfer proceeds through the intense line-driven wind of the stellar donor, a fraction of which is captured by the gravitational field of the NS. The subsequent accretion process on to the NS is responsible for the abundant X-ray emission from SgXB. They also display peak-to-peak variability of the X-ray flux by a factor of a few 10-100, along with changes in the hardness ratios possibly due to varying absorption along the line of sight. We use recent radiation-hydrodynamic simulations of inhomogeneities (a.k.a. clumps) in the non-stationary wind of massive hot stars to evaluate their impact on the time-variable accretion process. For this, we run 3D hydrodynamic simulations of the wind in the vicinity of the accretor to investigate the formation of the bow shock and follow the inhomogeneous flow over several spatial orders of magnitude, down to the NS magnetosphere. In particular, we show that the impact of the wind clumps on the time variability of the intrinsic mass accretion rate is severely tempered by the crossing of the shock, compared to the purely ballistic Bondi-Hoyle-Lyttleton estimation. We also account for the variable absorption due to clumps passing by the line of sight and estimate the final effective variability of the column density and mass accretion rate for different orbital separations. Finally, we compare our results to the most recent analysis of the X-ray flux and the hardness ratio in Vela X-1.

Rhapsody-G simulations I: the cool cores, hot gas and stellar content of massive galaxy clusters

DOE PAGES

Hahn, Oliver; Martizzi, Davide; Wu, Hao -Yi; ...

2017-01-25

We present the rhapsody-g suite of cosmological hydrodynamic zoom simulations of 10 massive galaxy clusters at the M vir ~10 15 M ⊙ scale. These simulations include cooling and subresolution models for star formation and stellar and supermassive black hole feedback. The sample is selected to capture the whole gamut of assembly histories that produce clusters of similar final mass. We present an overview of the successes and shortcomings of such simulations in reproducing both the stellar properties of galaxies as well as properties of the hot plasma in clusters. In our simulations, a long-lived cool-core/non-cool-core dichotomy arises naturally, andmore » the emergence of non-cool cores is related to low angular momentum major mergers. Nevertheless, the cool-core clusters exhibit a low central entropy compared to observations, which cannot be alleviated by thermal active galactic nuclei feedback. For cluster scaling relations, we find that the simulations match well the M 500–Y 500 scaling of Planck Sunyaev–Zeldovich clusters but deviate somewhat from the observed X-ray luminosity and temperature scaling relations in the sense of being slightly too bright and too cool at fixed mass, respectively. Stars are produced at an efficiency consistent with abundance-matching constraints and central galaxies have star formation rates consistent with recent observations. In conclusion, while our simulations thus match various key properties remarkably well, we conclude that the shortcomings strongly suggest an important role for non-thermal processes (through feedback or otherwise) or thermal conduction in shaping the intracluster medium.« less

rhapsody-g simulations - I. The cool cores, hot gas and stellar content of massive galaxy clusters

NASA Astrophysics Data System (ADS)

Hahn, Oliver; Martizzi, Davide; Wu, Hao-Yi; Evrard, August E.; Teyssier, Romain; Wechsler, Risa H.

2017-09-01

We present the rhapsody-g suite of cosmological hydrodynamic zoom simulations of 10 massive galaxy clusters at the Mvir ˜ 1015 M⊙ scale. These simulations include cooling and subresolution models for star formation and stellar and supermassive black hole feedback. The sample is selected to capture the whole gamut of assembly histories that produce clusters of similar final mass. We present an overview of the successes and shortcomings of such simulations in reproducing both the stellar properties of galaxies as well as properties of the hot plasma in clusters. In our simulations, a long-lived cool-core/non-cool-core dichotomy arises naturally, and the emergence of non-cool cores is related to low angular momentum major mergers. Nevertheless, the cool-core clusters exhibit a low central entropy compared to observations, which cannot be alleviated by thermal active galactic nuclei feedback. For cluster scaling relations, we find that the simulations match well the M500-Y500 scaling of Planck Sunyaev-Zeldovich clusters but deviate somewhat from the observed X-ray luminosity and temperature scaling relations in the sense of being slightly too bright and too cool at fixed mass, respectively. Stars are produced at an efficiency consistent with abundance-matching constraints and central galaxies have star formation rates consistent with recent observations. While our simulations thus match various key properties remarkably well, we conclude that the shortcomings strongly suggest an important role for non-thermal processes (through feedback or otherwise) or thermal conduction in shaping the intracluster medium.

Color gradients in cooling flow cluster central galaxies and the ionization of cluster emission line systems

NASA Technical Reports Server (NTRS)

Romanishin, W.

1988-01-01

Preliminary results are given for a program to measure color gradients in the central galaxies in clusters with a variety of cooling flow rates. The objectives are to search for extended blue continuum regions indicative of star formation, to study the spatial distribution of star formation, and to make a quantitative measure of the amount of light from young stars, which can lead to a measure of the star formation rate (for an assumed initial mass function). Four clusters with large masses and large cluster H-alpha emission fluxes are found to have an excess of blue light concentrated to the centers of the cluster central galaxy. Assumption of a disk IMF leads to the conclusion that the starlight might play a major role in ionizing the emission line gas in these clusters.

Study of magnetized accretion flow with variable Γ equation of state

NASA Astrophysics Data System (ADS)

Singh, Kuldeep; Chattopadhyay, Indranil

2018-05-01

We present here the solutions of magnetized accretion flow on to a compact object with hard surface such as neutron stars. The magnetic field of the central star is assumed dipolar and the magnetic axis is assumed to be aligned with the rotation axis of the star. We have used an equation of state for the accreting fluid in which the adiabatic index is dependent on temperature and composition of the flow. We have also included cooling processes like bremsstrahlung and cyclotron processes in the accretion flow. We found all possible accretion solutions. All accretion solutions terminate with a shock very near to the star surface and the height of this primary shock does not vary much with either the spin period or the Bernoulli parameter of the flow, although the strength of the shock may vary with the period. For moderately rotating central star, there is possible formation of multiple sonic points in the flow and therefore, a second shock far away from the star surface may also form. However, the second shock is much weaker than the primary one near the surface. We found that if rotation period is below a certain value (P*), then multiple critical points or multiple shocks are not possible and P* depends upon the composition of the flow. We also found that cooling effect dominates after the shock and that the cyclotron and the bremsstrahlung cooling processes should be considered to obtain a consistent accretion solution.

VO-compliant libraries of high resolution spectra of cool stars

NASA Astrophysics Data System (ADS)

Montes, D.

2008-10-01

In this contribution we describe a Virtual Observatory (VO) compliant version of the libraries of high resolution spectra of cool stars described by Montes et al. (1997; 1998; and 1999). Since their publication the fully reduced spectra in FITS format have been available via ftp and in the World Wide Web. However, in the VO all the spectra will be accessible using a common web interface following the standards of the International Virtual Observatory Alliance (IVOA). These libraries include F, G, K and M field stars, from dwarfs to giants. The spectral coverage is from 3800 to 10000 Å, with spectral resolution ranging from 0.09 to 3.0 Å.

Was the Milky Way Bulge Formed from the Buckling Disk Instability, Hierarchical Collapse, Accretion of Clumps, or All of the Above?

NASA Astrophysics Data System (ADS)

Nataf, David M.

2017-09-01

The assembly of the Milky Way bulge is an old topic in astronomy, one now in a period of renewed and rapid development. That is due to tremendous advances in observations of bulge stars, motivating observations of both local and high-redshift galaxies, and increasingly sophisticated simulations. The dominant scenario for bulge formation is that of the Milky Way as a nearly pure disk galaxy, with the inner disk having formed a bar and buckled. This can potentially explain virtually all bulge stars with [Fe/H] ≳ -1.0, which comprise 95% of the stellar population. The evidence is the incredible success in N-body models of this type in making non-trivial, non-generic predictions, such as the rotation curve and velocity dispersion measured from radial velocities, and the spatial morphologies of the peanut/X-shape and the long bar. The classical bulge scenario, whereby the bulge formed from early dissipative collapse and mergers, remains viable for stars with [Fe/H] ≲ -1.0 and potentially a minority of the other stars. A classical bulge is expected from Λ-CDM cosmological simulations, can accentuate the properties of an existing bar in a hybrid system, and is most consistent with the bulge abundance trends such as [Mg/Fe], which are elevated relative to both the thin and thick disks. Finally, the clumpy-galaxy scenario is considered, as it is the correct description of most Milky Way precursors given observations of high-redshift galaxies. Simulations predict that these star-forming clumps will sometimes migrate to the centres of galaxies where they may form a bulge, and galaxies often include a bulge clump as well. They will possibly form a bar with properties consistent with those of the Milky Way, such as the exponential profile and metallicity gradient. Given the relative successes of these scenarios, the Milky Way bulge is plausibly of composite origin, with a classical bulge and/or inner halo numerically dominant for stars with [Fe/H] ≲ -1.0, a buckling thick disk for stars with - 1.0 ≲ [Fe/H

High angular resolution observations of the cool giant V Hya

NASA Astrophysics Data System (ADS)

Pedretti, E.; Monnier, J. D.; Millan Gabet, R.; Traub, W. A.; Tuthill, P.; Danchi, W.; Berger, J.; Schloerb, F. P.; Thureau, N. D.; Carleton, N. P.; Lacasse, M. G.; Schuller, P. A.; Ragland, S.; Brewer, M.

2005-12-01

We present the preliminary interferometric observations of the cool giant star V Hya. V Hya, which is known to have mass-loss and to be surrounded by a dust shell,was observed in three narrow-band filters in the H bandpass at the infrared optical telescope array (IOTA), using the IONIC three-telescope beam combiner. The star was also observed at the Keck telescope using an aperture mask. We discuss the results and try to fit simple models to the observed data.

Microwave continuum measurements and estimates of mass loss rates for cool giants and supergiants

NASA Technical Reports Server (NTRS)

Drake, S. A.; Linsky, J. L.

1986-01-01

Attention is given to the results of a sensitive, 6-cm radio continuum survey conducted with the NRAO VLA of 39 of the nearest single cool giants and supergiants of G0-M5 spectral types; the survey was conducted in order to obtain accurate measurements of the mass loss rates of ionized gas for a representative sample of such stars, in order to furnish constraints for, and a better understanding of, the total mass loss rates. The inferred angular diameters for the cool giant sources are noted to be twice as large as photospheric angular diameters, implying that these stars are surrounded by extended chromospheres containing warm partially ionized gas.

X-ray Emission from Pre-Main-Sequence Stars - Testing the Solar Analogy

NASA Technical Reports Server (NTRS)

Skinner, Stephen L.

2000-01-01

This LTSA award funded my research on the origin of stellar X-ray emission and the validity of the solar-stellar analogy. This research broadly addresses the relevance of our current understanding of solar X-ray physics to the interpretation of X-ray emission from stars in general. During the past five years the emphasis has been on space-based X-ray observations of very young stars in star-forming regions (T Tauri stars and protostars), cool solar-like G stars, and evolved high-mass Wolf-Rayet (WR) stars. These observations were carried out primarily with the ASCA and ROSAT space-based observatories (and most recently with Chandra), supplemented by ground-based observations. This research has focused on the identification of physical processes that are responsible for the high levels of X-ray emission seen in pre-main-sequence (PMS) stars, active cool stars, and WR stars. A related issue is how the X-ray emission of such stars changes over time, both on short timescales of days to years and on evolutionary timescales of millions of years. In the case of the Sun it is known that magnetic fields play a key role in the production of X-rays by confining the coronal plasma in loop-like structures where it is heated to temperatures of several million K. The extent to which the magnetically-confined corona interpretation can be applied to other X-ray emitting stars is the key issue that drives the research summarized here.

Evidence for extended chromospheres surrounding red giant stars

NASA Technical Reports Server (NTRS)

Stencel, R. E.

1982-01-01

Observational evidence and theoretical arguments are summarized which indicate that regions of partially ionized hydrogen extending several stellar radii are an important feature of red giant and supergiant stars. The implications of the existence of extended chromospheres are examined in terms of the nature of the other atmospheres of, and mass loss from cool stars.

Characterizing K2 Candidate Planetary Systems Orbiting Low-Mass Stars. I. Classifying Low-Mass Host Stars Observed During Campaigns 1-7

NASA Technical Reports Server (NTRS)

Dressing, Courtney D.; Newton, Elisabeth R.; Schlieder, Joshua E.; Charbomeau, David; Krutson, Heather A.; Vanderburg, Andrew; Sinukoff, Evan

2017-01-01

We present near-infrared spectra for 144 candidate planetary systems identified during Campaigns 1-7 of the NASA K2 Mission. The goal of the survey was to characterize planets orbiting low-mass stars, but our Infrared Telescope Facility/SpeX and Palomar/TripleSpec spectroscopic observations revealed that 49% of our targets were actually giant stars or hotter dwarfs reddened by interstellar extinction. For the 72 stars with spectra consistent with classification as cool dwarfs (spectral types K3-M4), we refined their stellar properties by applying empirical relations based on stars with interferometric radius measurements. Although our revised temperatures are generally consistent with those reported in the Ecliptic Plane Input Catalog (EPIC), our revised stellar radii are typically 0.13 solar radius (39%) larger than the EPIC values, which were based on model isochrones that have been shown to underestimate the radii of cool dwarfs. Our improved stellar characterizations will enable more efficient prioritization of K2 targets for follow-up studies.

Thermal Evolution of Neutron Stars

NASA Astrophysics Data System (ADS)

Geppert, Ulrich R. M. E.

The thermal evolution of neutron stars is a subject of intense research, both theoretical and observational. The evolution depends very sensitively on the state of dense matter at supranuclear densities, which essentially controls the neutrino emission. The evolution depends, too, on the structure of the stellar outer layers which control the photon emission. Various internal heating processes and the magnetic field strength and structure will influence the thermal evolution. Of great importance for the cooling processes is also whether, when, and where superfluidity and superconductivity appear within the neutron star. This article describes and discusses these issues and presents neutron star cooling calculations based on a broad collection of equations of state for neutron star matter and internal magnetic field geometries. X-ray observations provide reliable data, which allow conclusions about the surface temperatures of neutron stars. To verify the thermal evolution models, the results of model calculations are compared with the body of observed surface temperatures and their distribution. Through these comparisons, a better understanding can be obtained of the physical processes that take place under extreme conditions in the interior of neutron

Chemical Enrichment and Physical Conditions in IZw18*

NASA Technical Reports Server (NTRS)

Lebouteiller, V.; Heap, S.; Hubeny, I.; Kunth, D.

2013-01-01

Low-metallicity star-forming dwarf galaxies are prime targets to understand the chemical enrichment of the interstellar medium. The H I region contains the bulk of the mass in blue compact dwarfs, and it provides important constraints on the dispersal and mixing of heavy elements released by successive star-formation episodes. The metallicity of the H I region is also a critical parameter to investigate the future star-formation history, as metals provide most of the gas cooling that will facilitate and sustain star formation. Aims. Our primary objective is to study the enrichment of the H I region and the interplay between star-formation history and metallicity evolution. Our secondary obje ctive is to constrain the spatial- and time-scales over which the HI and H II regions are enriched, and the mass range of stars responsible for the heavy element production. Finally, we aim to examine the gas heating and cooling mechanisms in the H I region. Methods. We observed the most metal-poor star-forming galaxy in the Local Universe, I Zw 18, with the Cosmic Origin Spectrograph onboard Hubble. The abundances in the neutral gas are derived from far-ultraviolet absorption-lines (H I, CIII, CIIi*, N I, OI,...) and are compared to the abundances in the H II region. Models are constructed to calculate the ionization structure and the thermal processes. We investigate the gas cooling in the HIi region through physical diagnostics drawn from the fine-structure level of C+. Results. We find that H I region abundances are lower by a factor of approx 2 as compared to the H II region. There is no differential depletion on dust between the H I and H II region. Using sulfur as a metallicity tracer, we calculate a metallicity of 1/46 Z(solar) (vs. 1/31 Z(solar) in the H II region). From the study of the C/O, [O/Fe], and N/O abundance ratios, we propose that C, N, O, and Fe are mainly produced in massive stars. We argue that the H I envelope may contain pockets of pristine gas with a metallicity essentially null. Finally, we derive the physical conditions in the H I region by investigating the C II* absorption line. The cooling rate derived from C II* is consistent with collisions with H(sup 0) atoms in the diffuse neutral gas. We calculate the star-formation rate from the CII* cooling rate assuming that photoelectric effect on dust is the dominant gas heating mechanism. Our determination is in good agreement with the values in the literature if we assume a low dust-to-gas ratio (approx 2000 times lower than the Milky Way value).

The structure and energy balance of cool star atmospheres

NASA Technical Reports Server (NTRS)

Linsky, J. L.

1982-01-01

The atmospheric structure and energy balance phenomena associated with magnetic fields in the Sun are reviewed and it is shown that similar phenomena occur in cool stars. The evidence for the weakening or disappearance of transition regions and coronae is discussed together with the appearance of extended cool chromospheres with large mass loss, near V-R = 0.80 in the H-R diagram. Like the solar atmosphere, these atmospheres are not homogeneous and there is considerable evidence for plage regions with bright TR emission lines that overlie dark (presumably magnetic) star spots. The IUE observations are providing important information on the energy balance in these atmospheres that should guide theoretical calculations of the nonradiative heating rate. Recent high dispersion spectra are providing unique information concerning which components of close binary systems are the dominant contributors to the observed emission. A recent unanticipated discovery is that the transition lines are redshifted (an antiwind) in DRa (G2 Ib) and perhaps other stars. Finally, the G and K giants and supergiants are classified into three groups depending on whether their atmospheres are dominated by closed magnetic flux tubes, open field geometries, or a predominately open geometry with a few closed flux tubes embedded.

The multi-line slope method for measuring the effective magnetic field of cool stars: an application to the solar-like cycle of ɛ Eri

NASA Astrophysics Data System (ADS)

Scalia, C.; Leone, F.; Gangi, M.; Giarrusso, M.; Stift, M. J.

2017-12-01

One method for the determination of integrated longitudinal stellar fields from low-resolution spectra is the so-called slope method, which is based on the regression of the Stokes V signal against the first derivative of Stokes I. Here we investigate the possibility of extending this technique to measure the magnetic fields of cool stars from high-resolution spectra. For this purpose we developed a multi-line modification to the slope method, called the multi-line slope method. We tested this technique by analysing synthetic spectra computed with the COSSAM code and real observations obtained with the high-resolution spectropolarimeters Narval, HARPSpol and the Catania Astrophysical Observatory Spectropolarimeter (CAOS). We show that the multi-line slope method is a fast alternative to the least squares deconvolution technique for the measurement of the effective magnetic fields of cool stars. Using a Fourier transform on the effective magnetic field variations of the star ε Eri, we find that the long-term periodicity of the field corresponds to the 2.95-yr period of the stellar dynamo, revealed by the variation of the activity index.

Advanced Spectral Library (ASTRAL): Atomic Fluorescence in Cool, Evolved Stars

NASA Astrophysics Data System (ADS)

Carpenter, Ken G.; Nielsen, Krister E.; Kober, Gladys V.; Rau, Gioia

2018-01-01

The "Advanced Spectral Library (ASTRAL) Project: Cool Stars" (PI = T. Ayres) collected a definitive set of representative, high-resolution (R~46,000 in the FUV up to ~1700 Å, R~30,000 for 1700-2150 Å, and R~114,000 >2150 Å) and high signal/noise (S/N>100) UV spectra of eight F-M evolved cool stars. These extremely high-quality STIS UV echelle spectra are available from the HST archive and from the Univ. of Colorado (http://casa.colorado.edu/~ayres/ASTRAL/) and will enable investigations of a broad range of problems -- stellar, interstellar, and beyond -- for many years. In this paper, we extend our study of the very rich emission-line spectra of the four evolved K-M stars in the sample, Beta Gem (K0 IIIb), Gamma Dra (K5 III), Gamma Cru (M3.4 III), and Alpha Ori (M2 Iab), to study the atomic fluorescence processes operating in their outer atmospheres. We summarize the pumping transitions and fluorescent line products known on the basis of previous work (e.g. Carpenter 1988, etc.) and newly identified in our current, on-going analysis of these extraordinary ASTRAL STIS spectra.

Short-Wavelength Infrared Views of Messier 81

NASA Technical Reports Server (NTRS)

2003-01-01

The magnificent spiral arms of the nearby galaxy Messier 81 are highlighted in this NASA Spitzer Space Telescope image. Located in the northern constellation of Ursa Major (which also includes the Big Dipper), this galaxy is easily visible through binoculars or a small telescope. M81 is located at a distance of 12 million light-years from Earth.

Because of its proximity, M81 provides astronomers with an enticing opportunity to study the anatomy of a spiral galaxy in detail. The unprecedented spatial resolution and sensitivity of Spitzer at infrared wavelengths show a clear separation between the several key constituents of the galaxy: the old stars, the interstellar dust heated by star formation activity, and the embedded sites of massive star formation. The infrared images also permit quantitative measurements of the galaxy's overall dust content, as well as the rate at which new stars are being formed.

The infrared image was obtained by Spitzer's infrared array camera. It is a four-color composite of invisible light, showing emissions from wavelengths of 3.6 microns (blue), 4.5 microns (green), 5.8 microns (yellow) and 8.0 microns (red). Winding outward from the bluish-white central bulge of the galaxy, where old stars predominate and there is little dust, the grand spiral arms are dominated by infrared emission from dust. Dust in the galaxy is bathed by ultraviolet and visible light from the surrounding stars. Upon absorbing an ultraviolet or visible-light photon, a dust grain is heated and re-emits the energy at longer infrared wavelengths. The dust particles, composed of silicates (which are chemically similar to beach sand) and polycyclic aromatic hydrocarbons, trace the gas distribution in the galaxy. The well-mixed gas (which is best detected at radio wavelengths) and dust provide a reservoir of raw materials for future star formation.

The infrared-bright clumpy knots within the spiral arms denote where massive stars are being born in giant H II (ionized hydrogen) regions. The 8-micron emission traces the regions of active star formation in the galaxy. Studying the locations of these regions with respect to the overall mass distribution and other constituents of the galaxy (e.g., gas) will help identify the conditions and processes needed for star formation. With the Spitzer observations, this information comes to us without complications from absorption by cold dust in the galaxy, which makes interpretation of visible-light features uncertain.

The white stars scattered throughout the field of view are foreground stars within our own Milky Way galaxy.

Black-hole-regulated star formation in massive galaxies.

PubMed

Martín-Navarro, Ignacio; Brodie, Jean P; Romanowsky, Aaron J; Ruiz-Lara, Tomás; van de Ven, Glenn

2018-01-18

Supermassive black holes, with masses more than a million times that of the Sun, seem to inhabit the centres of all massive galaxies. Cosmologically motivated theories of galaxy formation require feedback from these supermassive black holes to regulate star formation. In the absence of such feedback, state-of-the-art numerical simulations fail to reproduce the number density and properties of massive galaxies in the local Universe. There is, however, no observational evidence of this strongly coupled coevolution between supermassive black holes and star formation, impeding our understanding of baryonic processes within galaxies. Here we report that the star formation histories of nearby massive galaxies, as measured from their integrated optical spectra, depend on the mass of the central supermassive black hole. Our results indicate that the black-hole mass scales with the gas cooling rate in the early Universe. The subsequent quenching of star formation takes place earlier and more efficiently in galaxies that host higher-mass central black holes. The observed relation between black-hole mass and star formation efficiency applies to all generations of stars formed throughout the life of a galaxy, revealing a continuous interplay between black-hole activity and baryon cooling.

Black-hole-regulated star formation in massive galaxies

NASA Astrophysics Data System (ADS)

Martín-Navarro, Ignacio; Brodie, Jean P.; Romanowsky, Aaron J.; Ruiz-Lara, Tomás; van de Ven, Glenn

2018-01-01

Supermassive black holes, with masses more than a million times that of the Sun, seem to inhabit the centres of all massive galaxies. Cosmologically motivated theories of galaxy formation require feedback from these supermassive black holes to regulate star formation. In the absence of such feedback, state-of-the-art numerical simulations fail to reproduce the number density and properties of massive galaxies in the local Universe. There is, however, no observational evidence of this strongly coupled coevolution between supermassive black holes and star formation, impeding our understanding of baryonic processes within galaxies. Here we report that the star formation histories of nearby massive galaxies, as measured from their integrated optical spectra, depend on the mass of the central supermassive black hole. Our results indicate that the black-hole mass scales with the gas cooling rate in the early Universe. The subsequent quenching of star formation takes place earlier and more efficiently in galaxies that host higher-mass central black holes. The observed relation between black-hole mass and star formation efficiency applies to all generations of stars formed throughout the life of a galaxy, revealing a continuous interplay between black-hole activity and baryon cooling.

Multi-periodic pulsations of a stripped red-giant star in an eclipsing binary system.

PubMed

Maxted, Pierre F L; Serenelli, Aldo M; Miglio, Andrea; Marsh, Thomas R; Heber, Ulrich; Dhillon, Vikram S; Littlefair, Stuart; Copperwheat, Chris; Smalley, Barry; Breedt, Elmé; Schaffenroth, Veronika

2013-06-27

Low-mass white-dwarf stars are the remnants of disrupted red-giant stars in binary millisecond pulsars and other exotic binary star systems. Some low-mass white dwarfs cool rapidly, whereas others stay bright for millions of years because of stable fusion in thick surface hydrogen layers. This dichotomy is not well understood, so the potential use of low-mass white dwarfs as independent clocks with which to test the spin-down ages of pulsars or as probes of the extreme environments in which low-mass white dwarfs form cannot fully be exploited. Here we report precise mass and radius measurements for the precursor to a low-mass white dwarf. We find that only models in which this disrupted red-giant star has a thick hydrogen envelope can match the strong constraints provided by our data. Very cool low-mass white dwarfs must therefore have lost their thick hydrogen envelopes by irradiation from pulsar companions or by episodes of unstable hydrogen fusion (shell flashes). We also find that this low-mass white-dwarf precursor is a type of pulsating star not hitherto seen. The observed pulsation frequencies are sensitive to internal processes that determine whether this star will undergo shell flashes.

Stellar feedback strongly alters the amplification and morphology of galactic magnetic fields

NASA Astrophysics Data System (ADS)

Su, Kung-Yi; Hayward, Christopher C.; Hopkins, Philip F.; Quataert, Eliot; Faucher-Giguère, Claude-André; Kereš, Dušan

2018-01-01

Using high-resolution magnetohydrodynamic simulations of idealized, non-cosmological galaxies, we investigate how cooling, star formation and stellar feedback affect galactic magnetic fields. We find that the amplification histories, saturation values and morphologies of the magnetic fields vary considerably depending on the baryonic physics employed, primarily because of differences in the gas density distribution. In particular, adiabatic runs and runs with a subgrid (effective equation of state) stellar feedback model yield lower saturation values and morphologies that exhibit greater large-scale order compared with runs that adopt explicit stellar feedback and runs with cooling and star formation but no feedback. The discrepancies mostly lie in gas denser than the galactic average, which requires cooling and explicit fragmentation to capture. Independent of the baryonic physics included, the magnetic field strength scales with gas density as B ∝ n2/3, suggesting isotropic flux freezing or equipartition between the magnetic and gravitational energies during the field amplification. We conclude that accurate treatments of cooling, star formation and stellar feedback are crucial for obtaining the correct magnetic field strength and morphology in dense gas, which, in turn, is essential for properly modelling other physical processes that depend on the magnetic field, such as cosmic ray feedback.

Potential cooling of an accretion-heated neutron star crust in the low-mass X-ray binary 1RXS J180408.9-342058

NASA Astrophysics Data System (ADS)

Parikh, A. S.; Wijnands, R.; Degenaar, N.; Ootes, L. S.; Page, D.; Altamirano, D.; Cackett, E. M.; Deller, A. T.; Gusinskaia, N.; Hessels, J. W. T.; Homan, J.; Linares, M.; Miller, J. M.; Miller-Jones, J. C. A.

2017-04-01

We have monitored the transient neutron star low-mass X-ray binary 1RXS J180408.9-342058 in quiescence after its ˜4.5 month outburst in 2015. The source has been observed using Swift and XMM-Newton. Its X-ray spectra were dominated by a thermal component. The thermal evolution showed a gradual X-ray luminosity decay from ˜18 × 1032 to ˜4 × 1032 (D/5.8 kpc)2 erg s-1 between ˜8 and ˜379 d in quiescence, and the inferred neutron star surface temperature (for an observer at infinity; using a neutron star atmosphere model) decreased from ˜100 to ˜71 eV. This can be interpreted as cooling of an accretion-heated neutron star crust. Modelling the observed temperature curve (using nscool) indicated that the source required ˜1.9 MeV per accreted nucleon of shallow heating in addition to the standard deep crustal heating to explain its thermal evolution. Alternatively, the decay could also be modelled without the presence of deep crustal heating, only having a shallow heat source (again ˜1.9 MeV per accreted nucleon was required). However, the XMM-Newton data statistically required an additional power-law component. This component contributed ˜30 per cent of the total unabsorbed flux in 0.5-10 keV energy range. The physical origin of this component is unknown. One possibility is that it arises from low-level accretion. The presence of this component in the spectrum complicates our cooling crust interpretation because it might indicate that the smooth luminosity and temperature decay curves we observed may not be due to crust cooling but due to some other process.

Superfluidity in the Core of Neutron Stars

NASA Astrophysics Data System (ADS)

Page, Dany

2013-04-01

The year (1958) after the publication of the BCS theory, Bohr, Mottelson & Pines showed that nuclei should also contain superfluid neutrons and superconducting protons. In 1959, A. Migdal proposed that neutron superfluidity should also occur in the interior of neutron stars. Pairing in nuclei forms Cooper pairs with zero spin, but the relevant component of the nuclear interaction becomes repulsive at densities larger than the nuclear matter density. It has been proposed that neutron-neutron interaction in the spin-triplet state, and L=1 orbital angular momentum, that is known to be attractive from laboratory experiments, may result in a new form of neutron superfluidity in the neutron star interior. I will review our present understanding of the structure of neutron stars and describe how superfluidity strongly affects their thermal evolution. I will show how a ``Minimal Model'' that excludes the presence of ``exotic'' matter (Bose condensates, quarks, etc.) is compatible with most observations of the surface temperatures of young isolated neutron stars in the case this neutron superfluid exists. Compared to the case of isotropic spin-zero Cooper pairs, the formation of anisotropic spin-one Cooper pairs results in a strong neutrino emission that leads to an enhanced cooling of neutron stars after the onset of the pairing phase transition and allows the Minimal Cooling scenario to be compatible with most observations. In the case the pairing critical temperature Tc is less than about 6 x10^8 K, the resulting rapid cooling of the neutron star may be observable. It was recently reported that 10 years of Chandra observations of the 333 year young neutron star in the Cassiopeia A supernova remnant revealed that its temperature has dropped by about 5%. This result indicates that neutrons in this star are presently becoming superfluid and, if confirmed, provides us with the first direct observational evidence for neutron superfluidity at supra-nuclear densities.

Testing AGN unification via inference from large catalogs

NASA Astrophysics Data System (ADS)

Nikutta, Robert; Ivezic, Zeljko; Elitzur, Moshe; Nenkova, Maia

2018-01-01

Source orientation and clumpiness of the central dust are the main factors in AGN classification. Type-1 QSOs are easy to observe and large samples are available (e.g. in SDSS), but obscured type-2 AGN are dimmer and redder as our line of sight is more obscured, making it difficult to obtain a complete sample. WISE has found up to a million QSOs. With only 4 bands and a relatively small aperture the analysis of individual sources is challenging, but the large sample allows inference of bulk properties at a very significant level.CLUMPY (www.clumpy.org) is arguably the most popular database of AGN torus SEDs. We model the ensemble properties of the entire WISE AGN content using regularized linear regression, with orientation-dependent CLUMPY color-color-magnitude (CCM) tracks as basis functions. We can reproduce the observed number counts per CCM bin with percent-level accuracy, and simultaneously infer the probability distributions of all torus parameters, redshifts, additional SED components, and identify type-1/2 AGN populations through their IR properties alone. We increase the statistical power of our AGN unification tests even further, by adding other datasets as axes in the regression problem. To this end, we make use of the NOAO Data Lab (datalab.noao.edu), which hosts several high-level large datasets and provides very powerful tools for handling large data, e.g. cross-matched catalogs, fast remote queries, etc.

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

McDonald, M.; Stalder, B.; Bayliss, M.

In this paper, we present a multiwavelength study of the 90 brightest cluster galaxies (BCGs) in a sample of galaxy clusters selected via the Sunyaev Zel'dovich effect by the South Pole Telescope, utilizing data from various ground- and space-based facilities. We infer the star-formation rate (SFR) for the BCG in each cluster—based on the UV and IR continuum luminosity, as well as the [O ii]λλ3726,3729 emission line luminosity in cases where spectroscopy is available—and find seven systems with SFR > 100 M ⊙ yr -1. We find that the BCG SFR exceeds 10 M ⊙ yr -1 in 31 of 90 (34%) cases at 0.25 < z < 1.25, compared to ~1%–5% at z ~ 0 from the literature. At z ≳ 1, this fraction increases tomore » $${92}_{-31}^{+6}$$%, implying a steady decrease in the BCG SFR over the past ~9 Gyr. At low-z, we find that the specific SFR in BCGs is declining more slowly with time than for field or cluster galaxies, which is most likely due to the replenishing fuel from the cooling ICM in relaxed, cool core clusters. At z ≳ 0.6, the correlation between the cluster central entropy and BCG star formation—which is well established at z ~ 0—is not present. Instead, we find that the most star-forming BCGs at high-z are found in the cores of dynamically unrelaxed clusters. We use data from the Hubble Space Telescope to investigate the rest-frame near-UV morphology of a subsample of the most star-forming BCGs, and find complex, highly asymmetric UV morphologies on scales as large as ~50–60 kpc. Finally, the high fraction of star-forming BCGs hosted in unrelaxed, non-cool core clusters at early times suggests that the dominant mode of fueling star formation in BCGs may have recently transitioned from galaxy–galaxy interactions to ICM cooling.« less

Quasi-spherical accretion in High Mass X-ray Binaries

NASA Astrophysics Data System (ADS)

Postnov, Konstantin

2016-07-01

Quasi-spherical accreion onto magnetized neutron stars from stellar winds in high-mass X-ray binaries is discussed. Depending on the X-ray luminosity of the neutron star, the accretion can proceed in two regimes (modes): at L_x ≳ 4× 10^{36} erg/s, Compton cooling of accreting matter near magnetosphere leads to a supersonic (Bondi) accretion, while at smaller X-ray luminosity the Compton cooling is ineffective, and subsonic settling accretion regime sets in. In this regime, a hot convective shell is formed around the magnetosphere, and the plasma entry rate into magnetosphere is controlled by less effective radiative plasma cooling. The shell mediates the angular momentum transfer from/to the neutron star magnetosphere. Observational evidences for the different accretion regimes in slowly rotating X-ray pulsars with moderate and low X-ray luminosity, as well as possible manifestations of non-stationary quasi-spherical settling accretion due to the magnetospheric shell instability in Supergiant Fast X-ray Transients will be presented.

An Ultraviolet/Optical Atlas of Bright Galaxies

NASA Astrophysics Data System (ADS)

Marcum, Pamela M.; O'Connell, Robert W.; Fanelli, Michael N.; Cornett, Robert H.; Waller, William H.; Bohlin, Ralph C.; Neff, Susan G.; Roberts, Morton S.; Smith, Andrew M.; Cheng, K.-P.; Collins, Nicholas R.; Hennessy, Gregory S.; Hill, Jesse K.; Hill, Robert S.; Hintzen, Paul; Landsman, Wayne B.; Ohl, Raymond G.; Parise, Ronald A.; Smith, Eric P.; Freedman, Wendy L.; Kuchinski, Leslie E.; Madore, Barry; Angione, Ronald; Palma, Christopher; Talbert, Freddie; Stecher, Theodore P.

2001-02-01

We present wide-field imagery and photometry of 43 selected nearby galaxies of all morphological types at ultraviolet and optical wavelengths. The ultraviolet (UV) images, in two broad bands at 1500 and 2500 Å, were obtained using the Ultraviolet Imaging Telescope (UIT) during the Astro-1 Spacelab mission. The UV images have ~3" resolution, and the comparison sets of ground-based CCD images (in one or more of B, V, R, and Hα) have pixel scales and fields of view closely matching the UV frames. The atlas consists of multiband images and plots of UV/optical surface brightness and color profiles. Other associated parameters, such as integrated photometry and half-light radii, are tabulated. In an appendix, we discuss the sensitivity of different wavebands to a galaxy's star formation history in the form of ``history weighting functions'' and emphasize the importance of UV observations as probes of evolution during the past 10-1000 Myr. We find that UV galaxy morphologies are usually significantly different from visible band morphologies as a consequence of spatially inhomogeneous stellar populations. Differences are quite pronounced for systems in the middle range of Hubble types, Sa through Sc, but less so for ellipticals or late-type disks. Normal ellipticals and large spiral bulges are fainter and more compact in the UV. However, they typically exhibit smooth UV profiles with far-UV/optical color gradients which are larger than any at optical/IR wavelengths. The far-UV light in these cases is probably produced by extreme horizontal branch stars and their descendants in the dominant, low-mass, metal-rich population. The cool stars in the large bulges of Sa and Sb spirals fade in the UV while hot OB stars in their disks brighten, such that their Hubble classifications become significantly later. In the far-UV, early-type spirals often appear as peculiar, ringlike systems. In some spiral disks, UV-bright structures closely outline the spiral pattern; in others, the disks can be much more fragmented and chaotic than at optical wavelengths. Contributions by bright active galactic nuclei (AGNs) to the integrated UV light in our sample range from less than 10% to nearly 100%. A number of systems have unusual UV-bright structures in their inner disks, including rings, compact knots, and starburst nuclei, which could easily dominate the UV light in high-redshift analogs. A significant but variable fraction of the far-UV light in spiral disks is diffuse rather than closely concentrated to star-forming regions. Dust in normal spiral disks does not control UV morphologies, even in some highly inclined disk systems. The heaviest extinction is apparently confined to thin layers and the immediate vicinity of young H II complexes; the UV light emerges from thicker star distributions, regions evacuated of dust by photodestruction or winds, or by virtue of strong dust clumpiness. Only in cases where the dust layers are disturbed does dust appear to be a major factor in UV morphology. The UV-bright plume of M82 indicates that dust scattering of UV photons can be important in some cases. In a companion paper, we discuss far-UV data from the Astro-2 mission and optical comparisons for another 35 galaxies, emphasizing face-on spirals.

VizieR Online Data Catalog: BCool survey of solar-type stars (Marsden+ 2014)

NASA Astrophysics Data System (ADS)

Marsden, S. C.; Petit, P.; Jeffers, S. V.; Morin, J.; Fares, R.; Reiners, A.; Do Nascimento, J.-D., Jr.; Auriere, M.; Bouvier, J.; Carter, B. D.; Catala, C.; Dintrans, B.; Donati, J.-F.; Gastine, T.; Jardine, M.; Konstantinova-Antova, R.; Lanoux, J.; Lignieres, F.; Morgenthaler, A.; Ramirez-Velez, J. C.; Theado, S.; Van Grootel, V.; BCool Collaboration

2015-04-01

The goal of the BCool spectropolarimetric survey is to observe as many of the bright (V<~9.0) solar-type stars as possible to further our understanding of the magnetic activity of cool stars. In this first paper, we present the spectropolarimetric snapshots of 170 solar-type stars that we have observed starting in 2006 until 2013 as part of the BCool survey. (5 data files).

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

McDonald, Michael; Bautz, Marshall W.; Miller, Eric D.

We present new ultraviolet, optical, and X-ray data on the Phoenix galaxy cluster (SPT-CLJ2344-4243). Deep optical imaging reveals previously undetected filaments of star formation, extending to radii of ∼50–100 kpc in multiple directions. Combined UV-optical spectroscopy of the central galaxy reveals a massive (2 × 10{sup 9} M{sub ⊙}), young (∼4.5 Myr) population of stars, consistent with a time-averaged star formation rate of 610 ± 50 M{sub ⊙} yr{sup −1}. We report a strong detection of O vi λλ1032,1038, which appears to originate primarily in shock-heated gas, but may contain a substantial contribution (>1000 M{sub ⊙} yr{sup −1}) from themore » cooling intracluster medium (ICM). We confirm the presence of deep X-ray cavities in the inner ∼10 kpc, which are among the most extreme examples of radio-mode feedback detected to date, implying jet powers of 2–7 × 10{sup 45} erg s{sup −1}. We provide evidence that the active galactic nucleus inflating these cavities may have only recently transitioned from “quasar-mode” to “radio-mode,” and may currently be insufficient to completely offset cooling. A model-subtracted residual X-ray image reveals evidence for prior episodes of strong radio-mode feedback at radii of ∼100 kpc, with extended “ghost” cavities indicating a prior epoch of feedback roughly 100 Myr ago. This residual image also exhibits significant asymmetry in the inner ∼200 kpc (0.15R{sub 500}), reminiscent of infalling cool clouds, either due to minor mergers or fragmentation of the cooling ICM. Taken together, these data reveal a rapidly evolving cool core which is rich with structure (both spatially and in temperature), is subject to a variety of highly energetic processes, and yet is cooling rapidly and forming stars along thin, narrow filaments.« less

SOFIA Observations of S106: Dynamics of the Warm Gas

NASA Technical Reports Server (NTRS)

Simon, R.; Schneider, N.; Stutzki, J.; Gusten, R.; Graf, U. U.; Hartogh, P.; Guan, X.; Staguhn, J. G.; Benford, D. J.

2012-01-01

Context The H II region/PDR/molecular cloud complex S106 is excited by a single O-star. The full extent of the warm and dense gas close to the star has not been mapped in spectrally resolved high-J CO or [C II] lines, so the kinematics of the warm. partially ionized gas, are unknown. Whether the prominent dark lane bisecting the hourglass-shaped nebula is due solely to the shadow cast by a small disk around the exciting star or also to extinction in high column foreground gas was an open question until now. Aims. To disentangle the morphology and kinematics of warm neutral and ionized gas close to the star, study their relation to the bulk of the molecular gas. and to investigate the nature of the dark lane. Methods. We use the heterodyne receiver GREAT on board SOFIA to observe velocity resolved spectral lines of [C II] and CO 11 yields 10 in comparison with so far unpublished submm continuum data at 350 micron (8HARC-Il) and complementary molecular line data. Results. The high angular and spectral resolution observations show a very complex morphology and kinematics of the inner S106 region, with many different components at different excitation conditions contributing to the observed emission. The [C II] lines are found to be bright and very broad. tracing high velocity gas close to the interface of molecular cloud and H II region. CO 11 yields 10 emission is more confined.. both spatially and in velocity, to the immediate surroundings of S 106 IR showing the presence of warm, high density (clumpy) gas. Our high angular resolution submm continuum observations rule out the scenario where the dark lane separating the two lobes is due solely to the shadow cast by a small disk close to the star. The lane is clearly seen also as warm, high column density gas at the boundary of the molecular cloud and H II region.

The Effect of Starspots on Detectability of Exoplanet Atmospheres

NASA Astrophysics Data System (ADS)

Hofmann, Ryan; Berta-Thompson, Zachory

2018-01-01

Transmission spectroscopy is an effective tool for detecting and characterizing the atmospheres of transiting extrasolar planets. However, the presence of cool spots on a planet’s host star can be a source of uncertainty that is difficult to account for. Cool starspots introduce wavelength-dependent features and noise into the transmission spectrum of an orbiting exoplanet. For sufficiently cool stars, especially M dwarfs, this could cause false detections of water and other species in the planet’s atmosphere. To understand the extent of this problem, we use a combination of PHOENIX model spectra and the starspot simulation code MACULA to simulate the effects of starspots on observed transmission spectra for a wide variety of stars and spot configurations. By comparing the simulated DoTV (Depth of Transit Variation) due to starspots with models of the expected DoTV from exoplanet atmospheres with a given composition, we can estimate the level of effect the starspots have on the detectability of various atmospheres. For example, our results indicate for TRAPPIST-1’s planets that while the large amplitude absorption features from a H/He-rich atmosphere should be easily detectable, a pure water atmosphere would be much harder to distinguish from starspot noise. Consequently, proper characterization of exoplanet atmospheres, especially around cool, active host stars, requires a proper understanding of the star’s spot properties and suitable methods for reducing or removing spot-induced brightness fluctuations as a source of noise.

THE BIGGEST EXPLOSIONS IN THE UNIVERSE. II

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

Whalen, Daniel J.; Smidt, Joseph; Johnson, Jarrett L.

2013-11-10

One of the leading contenders for the origin of supermassive black holes (SMBHs) at z ∼> 7 is catastrophic baryon collapse in atomically cooled halos at z ∼ 15. In this scenario, a few protogalaxies form in the presence of strong Lyman-Werner UV backgrounds that quench H{sub 2} formation in their constituent halos, preventing them from forming stars or blowing heavy elements into the intergalactic medium prior to formation. At masses of 10{sup 8} M{sub ☉} and virial temperatures of 10{sup 4} K, gas in these halos rapidly cools by H lines, in some cases forming 10{sup 4}-10{sup 6} M{submore » ☉} Population III stars and, a short time later, the seeds of SMBHs. Instead of collapsing directly to black holes (BHs), some of these stars died in the most energetic thermonuclear explosions in the universe. We have modeled the explosions of such stars in the dense cores of line-cooled protogalaxies in the presence of cosmological flows. In stark contrast to the explosions in diffuse regions in previous simulations, these supernovae briefly engulf the protogalaxy, but then collapse back into its dark matter potential. Fallback drives turbulence that efficiently distributes metals throughout the interior of the halo and fuels the rapid growth of nascent BHs at its center. The accompanying starburst and X-ray emission from these line-cooled galaxies easily distinguish them from more slowly evolving neighbors and might reveal the birthplaces of SMBHs on the sky.« less

Direct Urca Processes Involving Proton 1 S 0 Superfluidity in Neutron Star Cooling

NASA Astrophysics Data System (ADS)

Xu, Yan; Yu, Zi; Zhang, Xiao-Jun; Fan, Cun-Bo; Liu, Guang-Zhou; Zhao, En-Guang; Huang, Xiu-Lin; Liu, Cheng-Zhi

2018-04-01

A detailed description of the baryon direct Urca processes A: n\\to p+e+{\\bar{ν }}e, B: Λ \\to p+e+{\\bar{ν }}e and C: {\\Xi }-\\to Λ +e+{\\bar{ν }}e related to the neutron star cooling is given in the relativistic mean field approximation. The contributions of the reactions B and C on the neutrino luminosity are calculated by means of the relativistic expressions of the neutrino energy losses. Our results show that the total neutrino luminosities of the reactions A, B and C within the mass range (1.603–2.067) M⊙ ((1.515–1.840) M⊙ for TM1 model) for GM1 model are larger than the corresponding values for neutron star without hyperons. Furthermore, although the neutrino emissivity of the reaction A is suppressed with the appearance of the proton 1 S 0 superfluid, the contribution of the reactions B and C can still quicken a massive neutron star cooling. In particular, the reaction C in PSR J1614-2230 and J0348+0432 is not suppressed by the proton 1 S 0 superfluid due to the higher threshold density of the reaction C, which will further speed up the two pulsars cooling. Supported by the National Natural Science Foundation of China under Grant Nos. 11447165, 11373047, 11404336 and U1731240, Youth Innovation Promotion Association, CAS under Grant No. 2016056, and the Development Project of Science and Technology of Jilin Province under Grant No. 20180520077JH

Bipolar nebulae and mass loss from red giant stars

NASA Technical Reports Server (NTRS)

Cohen, M.

1985-01-01

Observations of several bipolar nebulae are used to learn something of the nature of mass loss from the probable red-giant progenitors of these nebulae. Phenomena discussed are: (1) probable GL 2688's optical molecular emissions; (2) newly discovered very high velocity knots along the axis of OH 0739 - 14, which reveal evidence for mass ejections of + or 300 km/s from the M9 III star embedded in this nebula; (3) the bipolar structure of three extreme carbon stars, and the evidence for periodic mass ejection in IRC + 30219, also at high speed (about 80 km/s); and (4) the curious cool TiO-rich region above Parsamian 13, which may represent the very recent shedding of photospheric material from a cool, oxygen-rich giant. Several general key questions about bipolar nebulae that relate to the process of mass loss from their progenitor stars are raised.

New candidates for carbon stars with silicate features

NASA Technical Reports Server (NTRS)

Chan, S. J.; Kwok, Sun

1991-01-01

All stars in the General Catalog of Cool Galactic Carbon Stars with IRAS 12-micron fluxes greater than 10 Jy were searched for Low-Resolution-Spectrometer (LRS) spectra in the IRAS LRS data base. Out of the 532 spectra examined, 11 were found to show the 9.7-micron silicate emission feature. Four of these are identified for the first time. This group of carbon stars may represent transition objects between oxygen-rich and carbon-rich stars on the asymptotic giant branch.

New Observational Evidence of Flash Mixing on the White Dwarf Cooling Curve

NASA Technical Reports Server (NTRS)

Brown, T. M.; Lanz, T.; Sweigart, A. V.; Cracraft, Misty; Hubeny, Ivan; Landsman, W. B.

2011-01-01

Blue hook stars are a class of subluminous extreme horizontal branch stars that were discovered in UV images of the massive globular clusters w Cen and NGC 2808. These stars occupy a region of the HR diagram that is unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that the blue hook stars are very likely the progeny of stars that undergo extensive internal mixing during a late helium-core flash on the white dwarf cooling curve. This "flash mixing" produces hotter-than-normal EHB stars with atmospheres significantly enhanced in helium and carbon. The larger bolometric correction, combined with the decrease in hydrogen opacity, makes these stars appear sub luminous in the optical and UV. Flash mixing is more likely to occur in stars born with a high helium abundance, due to their lower mass at the main sequence turnoff. For this reason, the phenomenon is more common in those massive globular clusters that show evidence for secondary populations enhanced in helium. However, a high helium abundance does not, by itself, explain the presence of blue hook stars in massive globular clusters. Here, we present new observational evidence for flash mixing, using recent HST observations. These include UV color-magnitude diagrams of six massive globular clusters and far-UV spectroscopy of hot subdwarfs in one of these clusters (NGC 2808).

Some aspects of cool main sequence star ages derived from stellar rotation (gyrochronology)

NASA Astrophysics Data System (ADS)

Barnes, S. A.; Spada, F.; Weingrill, J.

2016-09-01

Rotation periods for cool stars can be measured with good precision by monitoring starspot light modulation. Observations have shown that the rotation periods of dwarf stars of roughly solar metallicity have such systematic dependencies on stellar age and mass that they can be used to derive reliable ages, a procedure called gyrochronology. We review the method and show illustrative cases, including recent ground- and space-based data. The age uncertainties approach 10 % in the best cases, making them a valuable complement to, and constraint on, asteroseismic or other ages. Edited, updated, and refereed version of a presentation at the WE-Heraeus-Seminar in Bad Honnef, Germany: Reconstructing the Milky Way's History: Spectroscopic Surveys, Asteroseismology and Chemodynamical Models

The dusty atmosphere of the brown dwarf Gliese 229B.

PubMed

Griffith, C A; Yelle, R V; Marley, M S

1998-12-11

The brown dwarf Gliese 229B has an observable atmosphere too warm to contain ice clouds like those on Jupiter and too cool to contain silicate clouds like those on low-mass stars. These unique conditions permit visibility to higher pressures than possible in cool stars or planets. Gliese 229B's 0.85- to 1.0-micrometer spectrum indicates particulates deep in the atmosphere (10 to 50 bars) having optical properties of neither ice nor silicates. Their reddish color suggests an organic composition characteristic of aerosols in planetary stratospheres. The particles' mass fraction (10(-7)) agrees with a photochemical origin caused by incident radiation from the primary star and suggests the occurrence of processes native to planetary stratospheres.

The Birth of a Galaxy: Primordial Metal Enrichment and Stellar Populations

NASA Astrophysics Data System (ADS)

Wise, John H.; Turk, Matthew J.; Norman, Michael L.; Abel, Tom

2012-01-01

By definition, Population III stars are metal-free, and their protostellar collapse is driven by molecular hydrogen cooling in the gas phase, leading to large characteristic masses. Population II stars with lower characteristic masses form when the star-forming gas reaches a critical metallicity of 10-6-10-3.5 Z ⊙. We present an adaptive mesh refinement radiation hydrodynamics simulation that follows the transition from Population III to Population II star formation. The maximum spatial resolution of 1 comoving parsec allows for individual molecular clouds to be well resolved and their stellar associations to be studied in detail. We model stellar radiative feedback with adaptive ray tracing. A top-heavy initial mass function for the Population III stars is considered, resulting in a plausible distribution of pair-instability supernovae and associated metal enrichment. We find that the gas fraction recovers from 5% to nearly the cosmic fraction in halos with merger histories rich in halos above 107 M ⊙. A single pair-instability supernova is sufficient to enrich the host halo to a metallicity floor of 10-3 Z ⊙ and to transition to Population II star formation. This provides a natural explanation for the observed floor on damped Lyα systems metallicities reported in the literature, which is of this order. We find that stellar metallicities do not necessarily trace stellar ages, as mergers of halos with established stellar populations can create superpositions of t-Z evolutionary tracks. A bimodal metallicity distribution is created after a starburst occurs when the halo can cool efficiently through atomic line cooling.

From Cool to Hot F-stars: The Influence of Two Ionization Regions in the Acoustic Oscillations

NASA Astrophysics Data System (ADS)

Brito, Ana; Lopes, Ilídio

2018-02-01

The high-precision data available from the Kepler satellite allows us to study the complex outer convective envelopes of solar-type stars. We use a seismic diagnostic, specialized for investigating the outer layers of solar-type stars, to infer the impact of the ionization processes on the oscillation spectrum, for a sample of Kepler stars. These stars, of spectral type F, cover all of the observational seismic domain of the acoustic oscillation spectrum in solar-type stars. They also cover the range between a cool F-dwarf (∼6000 K) and a hotter F-star (∼6400 K). Our study reveals the existence of two relevant ionization regions. One of these regions, which is located closer to the surface of the star, is commonly associated with the second ionization of helium, although other chemical species also contribute to ionization. The second region, located deeper in the envelope, is linked with the ionization of heavy elements. Specifically, in this study, we analyze the elements carbon, nitrogen, oxygen, neon, and iron. Both regions can be related to the K electronic shell. We show that, while for cooler stars like the Sun, the influence of this second region on the oscillation frequencies is small; in hotter stars, its influence becomes comparable to the influence of the region of the second ionization of helium. This can guide us in the study of the outer layers of F-stars, specifically with the understanding of phenomena related to rotation and magnetic activity in these stars.

ON THE TIDAL DISSIPATION OF OBLIQUITY

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

Rogers, T. M.; Lin, D. N. C., E-mail: tami@lpl.arizona.edu, E-mail: lin@ucolick.org

2013-05-20

We investigate tidal dissipation of obliquity in hot Jupiters. Assuming an initial random orientation of obliquity and parameters relevant to the observed population, the obliquity of hot Jupiters does not evolve to purely aligned systems. In fact, the obliquity evolves to either prograde, retrograde, or 90 Degree-Sign orbits where the torque due to tidal perturbations vanishes. This distribution is incompatible with observations which show that hot Jupiters around cool stars are generally aligned. This calls into question the viability of tidal dissipation as the mechanism for obliquity alignment of hot Jupiters around cool stars.

Observations of the Hot Horizontal Branch Stars in the Metal-Rich Bulge Globular Cluster NGC 6388

NASA Technical Reports Server (NTRS)

Moehler, S.; Sweigart, A. V.

2006-01-01

The metal-rich bulge globular cluster NGC 6388 shows a distinct blue horizontal-branch tail in its colour-magnitude diagram (Rich et al. 1997) and is thus a strong case of the well-known 2nd Parameter Problem. In addition, its horizontal branch (HB) shows an upward tilt toward bluer colours, which cannot be explained by canonical evolutionary models. Several non-canonical scenarios have been proposed to explain these puzzling observations. In order to test the predictions of these scenarios, we have obtained medium resolution spectra to determine the atmospheric parameters of a sample of the blue HB stars in NGC 6388.Using the medium resolution spectra, we determine effective temperatures, surface gravities and helium abundances by fitting the observed Balmer and helium lines with appropriate theoretical stellar spectra. As we know the distance to the cluster, we can verify our results by determining masses for the stars. During the data reduction we took special care to correctly subtract the background, which is dominated by the overlapping spectra of cool stars. The cool blue tail stars in our sample with T(sub eff) approximately 10000 K have lower than canonical surface gravities, suggesting that these stars are, on average, approximately equal to 0.4 mag brighter than canonical HB stars in agreement with the observed upward slope of the HB in NGC 6388. Moreover, the mean mass of these stars agrees well with theoretical predictions. In contrast, the hot blue tail stars in our sample with T(sub eff) greater than or equal to 12000 K show significantly lower surface gravities than predicted by any scenario, which can reproduce the photometric observations. Their masses are also too low by about a factor of 2 compared to theoretical predictions. The physical parameters of the blue HB stars at about 10,000 K support the helium pollution scenario. The low gravities and masses of the hot blue tail stars, however, are probably caused by problems with the data reduction, most likely due to remaining background light in the spectra, which would affect the fainter hot blue tail stars much more strongly than the brighter cool blue tail stars. Our study of the hot blue tail stars in NGC 6388 illustrates the obstacles which are encountered when attempting to determine the atmospheric parameters of hot HB stars in very crowded fields using ground-based observations. We discuss these obstacles and offer possible solutions for future projects.

Herschel Cool Universe Artist Concept

NASA Image and Video Library

2013-03-05

Artist impression of Herschel is set against an image captured by the observatory, showing baby stars forming in the Rosette nebula. The bright spots are dusty cocoons containing massive forming stars, each one up to ten times the mass of our own sun.

Rapid Neutrino Cooling in the Neutron Star MXB 1659-29.

PubMed

Brown, Edward F; Cumming, Andrew; Fattoyev, Farrukh J; Horowitz, C J; Page, Dany; Reddy, Sanjay

2018-05-04

We show that the neutron star in the transient system MXB 1659-29 has a core neutrino luminosity that substantially exceeds that of the modified Urca reactions (i.e., n+n→n+p+e^{-}+ν[over ¯]_{e} and inverse) and is consistent with the direct Urca (n→p+e^{-}+ν[over ¯]_{e} and inverse) reaction occurring in a small fraction of the core. Observations of the thermal relaxation of the neutron star crust following 2.5 yr of accretion allow us to measure the energy deposited into the core during accretion, which is then reradiated as neutrinos, and infer the core temperature. For a nucleonic core, this requires that the nucleons are unpaired and that the proton fraction exceeds a critical value to allow the direct Urca reaction to proceed. The neutron star in MXB 1659-29 is the first with a firmly detected thermal component in its x-ray spectrum that needs a fast neutrino-cooling process. Measurements of the temperature variation of the neutron star core during quiescence would place an upper limit on the core specific heat and serve as a check on the fraction of the neutron star core in which nucleons are unpaired.

Rapid Neutrino Cooling in the Neutron Star MXB 1659-29

NASA Astrophysics Data System (ADS)

Brown, Edward F.; Cumming, Andrew; Fattoyev, Farrukh J.; Horowitz, C. J.; Page, Dany; Reddy, Sanjay

2018-05-01

We show that the neutron star in the transient system MXB 1659-29 has a core neutrino luminosity that substantially exceeds that of the modified Urca reactions (i.e., n +n →n +p +e-+ν¯ e and inverse) and is consistent with the direct Urca (n →p +e-+ν¯e and inverse) reaction occurring in a small fraction of the core. Observations of the thermal relaxation of the neutron star crust following 2.5 yr of accretion allow us to measure the energy deposited into the core during accretion, which is then reradiated as neutrinos, and infer the core temperature. For a nucleonic core, this requires that the nucleons are unpaired and that the proton fraction exceeds a critical value to allow the direct Urca reaction to proceed. The neutron star in MXB 1659-29 is the first with a firmly detected thermal component in its x-ray spectrum that needs a fast neutrino-cooling process. Measurements of the temperature variation of the neutron star core during quiescence would place an upper limit on the core specific heat and serve as a check on the fraction of the neutron star core in which nucleons are unpaired.

Evidence for halo kinematics among cool carbon-rich dwarfs

NASA Astrophysics Data System (ADS)

Farihi, J.; Arendt, A. R.; Machado, H. S.; Whitehouse, L. J.

2018-07-01

This paper reports preliminary, yet compelling, kinematical inferences for N≳ 600 carbon-rich dwarf stars that demonstrate around 30-60 per cent are members of the Galactic halo. The study uses a spectroscopically and non-kinematically selected sample of stars from the SDSS, and cross-correlates these data with three proper motion catalogues based on Gaia DR1 astrometry to generate estimates of their 3D space velocities. The fraction of stars with halo-like kinematics is roughly 30 per cent for distances based on a limited number of parallax measurements, with the remainder dominated by the thick disc, but close to 60 per cent of the sample lies below an old, metal-poor disc isochrone in reduced proper motion. An ancient population is consistent with an extrinsic origin for C/O >1 in cool dwarfs, where a fixed mass of carbon pollution more readily surmounts lower oxygen abundance and with a lack of detectable ultraviolet-blue flux from younger white dwarf companions. For an initial stellar mass function that favours low-mass stars as in the Galactic disc, the dC stars are likely to be the dominant source of carbon-enhanced, metal-poor stars in the Galaxy.

Evidence for halo kinematics among cool carbon-rich dwarfs

NASA Astrophysics Data System (ADS)

Farihi, J.; Arendt, A. R.; Machado, H. S.; Whitehouse, L. J.

2018-04-01

This paper reports preliminary yet compelling kinematical inferences for N ≳ 600 carbon-rich dwarf stars that demonstrate around 30% to 60% are members of the Galactic halo. The study uses a spectroscopically and non-kinematically selected sample of stars from the SDSS, and cross-correlates these data with three proper motion catalogs based on Gaia DR1 astrometry to generate estimates of their 3-D space velocities. The fraction of stars with halo-like kinematics is roughly 30% for distances based on a limited number of parallax measurements, with the remainder dominated by the thick disk, but close to 60% of the sample lie below an old, metal-poor disk isochrone in reduced proper motion. An ancient population is consistent with an extrinsic origin for C/O >1 in cool dwarfs, where a fixed mass of carbon pollution more readily surmounts lower oxygen abundances, and with a lack of detectable ultraviolet-blue flux from younger white dwarf companions. For an initial stellar mass function that favors low-mass stars as in the Galactic disk, the dC stars are likely to be the dominant source of carbon-enhanced, metal-poor stars in the Galaxy.

Condition for dust evacuation from the first galaxies

NASA Astrophysics Data System (ADS)

Fukushima, Hajime; Yajima, Hidenobu; Omukai, Kazuyuki

2018-06-01

Dust enables low-mass stars to form from low-metallicity gas by inducing fragmentation of clouds via cooling by thermal emission. Dust may, however, be evacuated from star-forming clouds due to the radiation force from massive stars. We study here the condition for dust evacuation by comparing the dust evacuation time with the time of cloud destruction due to either expansion of H II regions or supernovae. The cloud destruction time has a weak dependence on cloud radius, while the dust evacuation time is shorter for a cloud with a smaller radius. Dust evacuation, thus, occurs in compact star-forming clouds whose column density is NH ≃ 1024-1026 cm-2. The critical halo mass above which dust evacuation occurs is lower for higher formation red shift, e.g. ˜109 M⊙ at red shift z ˜ 3 and ˜107 M⊙ at z ˜ 9. In addition, the metallicity of the gas should be less than ˜10-2 Z⊙, otherwise attenuation by dust reduces the radiation force significantly. From the dust-evacuated gas, massive stars are likely to form, even with a metallicity above ˜10-5 Z⊙, the critical value for low-mass star formation due to dust cooling. This can explain the dearth of ultra-metal-poor stars with a metallicity lower than ˜10-4 Z⊙.

Phibss: Molecular Gas, Extinction, Star Formation, and Kinematics in the z = 1.5 Star-forming Galaxy EGS13011166

NASA Astrophysics Data System (ADS)

Genzel, R.; Tacconi, L. J.; Kurk, J.; Wuyts, S.; Combes, F.; Freundlich, J.; Bolatto, A.; Cooper, M. C.; Neri, R.; Nordon, R.; Bournaud, F.; Burkert, A.; Comerford, J.; Cox, P.; Davis, M.; Förster Schreiber, N. M.; García-Burillo, S.; Gracia-Carpio, J.; Lutz, D.; Naab, T.; Newman, S.; Saintonge, A.; Shapiro Griffin, K.; Shapley, A.; Sternberg, A.; Weiner, B.

2013-08-01

We report matched resolution imaging spectroscopy of the CO 3-2 line (with the IRAM Plateau de Bure millimeter interferometer) and of the Hα line (with LUCI at the Large Binocular Telescope) in the massive z = 1.53 main-sequence galaxy EGS 13011166, as part of the "Plateau de Bure high-z, blue-sequence survey" (PHIBSS: Tacconi et al.). We combine these data with Hubble Space Telescope V-I-J-H-band maps to derive spatially resolved distributions of stellar surface density, star formation rate, molecular gas surface density, optical extinction, and gas kinematics. The spatial distribution and kinematics of the ionized and molecular gas are remarkably similar and are well modeled by a turbulent, globally Toomre unstable, rotating disk. The stellar surface density distribution is smoother than the clumpy rest-frame UV/optical light distribution and peaks in an obscured, star-forming massive bulge near the dynamical center. The molecular gas surface density and the effective optical screen extinction track each other and are well modeled by a "mixed" extinction model. The inferred slope of the spatially resolved molecular gas to star formation rate relation, N = dlogΣstar form/dlogΣmol gas, depends strongly on the adopted extinction model, and can vary from 0.8 to 1.7. For the preferred mixed dust-gas model, we find N = 1.14 ± 0.1. Based on observations with the Plateau de Bure millimetre interferometer, operated by the Institute for Radio Astronomy in the Millimetre Range (IRAM), which is funded by a partnership of INSU/CNRS (France), MPG (Germany), and IGN (Spain). Based also on data acquired with the Large Binocular Telescope (LBT). The LBT is an international collaboration among institutions in Germany, Italy, and the United States. LBT Corporation partners are LBT Beteiligungsgesellschaft, Germany, representing the Max-Planck Society, the Astrophysical Institute Potsdam, and Heidelberg University; Istituto Nazionale di Astrofisica, Italy; The University of Arizona on behalf of the Arizona University system; The Ohio State University, and The Research Corporation, on behalf of the University of Notre Dame, University of Minnesota, and University of Virginia.

Magnetic Inclination E Ects In Star-Planet Magnetic Interactions

NASA Astrophysics Data System (ADS)

Strugarek, Antoine

2017-10-01

A large fraction of the exoplanets discovered today are in a close-in orbit around their host star. This proximity allows them to be magnetically connected to their host, which lead to e cient energy and angular momentum exchanges between the star and the planet. We carry out three-dimensional magneto-hydrodynamic simulations of close-in star-planet systems to characterize the e ect of the inclination of the planetary magnetic eld on the star-planet magnetic interaction. We parametrize this e ect in scaling laws depending on the star, planet, and stellar wind properties that can be applied to any exoplanetary systems around cool stars.

Characterizing K2 Planetary Systems Orbiting Cool Dwarfs

NASA Astrophysics Data System (ADS)

Dressing, Courtney D.; Newton, Elisabeth R.; Schlieder, Joshua; Vanderburg, Andrew; Charbonneau, David; Knutson, Heather; K2C2

2017-01-01

The NASA K2 mission is using the repurposed Kepler spacecraft to search for transiting planets in multiple fields along the ecliptic plane. K2 observes 10,000 - 30,000 stars in each field for roughly 80 days, which is too short to observe multiple transits of planets in the habitable zones of Sun-like stars, but long enough to detect potentially habitable planets orbiting low-mass dwarfs. Accordingly, M and K dwarfs are frequently nominated as K2 Guest Observer targets and K2 has already observed significantly more low-mass stars than the original Kepler mission. While the K2 data are therefore an enticing resource for studying the properties and frequency of planetary systems orbiting low-mass stars, many K2 cool dwarfs are not well-characterized. We are refining the properties of K2 planetary systems orbiting cool dwarfs by acquiring medium-resolution NIR spectra with SpeX on the IRTF and TripleSpec on the Palomar 200". In our initial sample of 144 potential cool dwarfs hosting candidate planetary systems detected by K2, we noted a high contamination rate from giants (16%) and reddened hotter dwarfs (31%). After employing empirically-based relations to determine the temperatures, radii, masses, luminosities, and metallicities of K2 planet candidate host stars, we found that our new cool dwarf radius estimates were 10-40% larger than the initial values, indicating that the radii of the associated planet candidates were also underestimated. Refining the stellar parameters allows us to identify astrophysical false positives and better constrain the radii and insolation flux environments of bona fide transiting planets. I will present our resulting catalog of system properties and highlight the most attractive K2 planets for radial velocity mass measurement and atmospheric characterization with Spitzer, HST, JWST, and the next generation of extremely large ground- and space-based telescopes. We gratefully acknowledge funding from the NASA Sagan Fellowship Program, the NASA K2 Guest Observer Program, the NASA XRP Program, the John Templeton Foundation, the National Science Foundation Astronomy & Astrophysics Postdoctoral Program, and the National Science Foundation Graduate Research Fellowship Program.

A system of three transiting super-Earths in a cool dwarf star

NASA Astrophysics Data System (ADS)

Díez Alonso, E.; Suárez& Gómez, S. L.; González Hernández, J. I.; Suárez Mascareño, A.; González Gutiérrez, C.; Velasco, S.; Toledo-Padrón, B.; de Cos Juez, F. J.; Rebolo, R.

2018-05-01

We present the detection of three super-Earths transiting the cool star LP415-17, monitored by K2 mission in its 13th campaign. High-resolution spectra obtained with High Accuracy Radial velocity Planet Searcher-North/Telescopio Nazionale Galileo (HARPS-N/TNG) showed that the star is a mid-late K dwarf. Using spectral synthesis models, we infer its effective temperature, surface gravity, and metallicity, and subsequently determined from evolutionary models a stellar radius of 0.58 R⊙. The planets have radii of 1.8, 2.6, and 1.9 R⊕ and orbital periods of 6.34, 13.85, and 40.72 d. High-resolution images discard any significant contamination by an intervening star in the line of sight. The orbit of the furthest planet has radius of 0.18 au, close to the inner edge of the habitable zone. The system is suitable to improve our understanding of formation and dynamical evolution of super-Earth systems in the rocky-gaseous threshold, their atmospheres, internal structure, composition, and interactions with host stars.

Intrinsic Lyα Profile Reconstructions of the MUSCLES Low-Mass Exoplanet Host Stars

NASA Astrophysics Data System (ADS)

Youngblood, Allison A.; France, Kevin; Loyd, R. O. Parke

2015-12-01

UV stellar radiation can significantly impact planetary atmospheres through heating and photochemistry, even regulating production of potential biomarkers. Cool stars emit the majority of their UV radiation in the form of emission lines, and the incident UV radiation on close-in habitable-zone planets is significant. Lyα (1215.67 Å) dominates the 912 - 3200 Å spectrum of cool stars, but strong absorption from the interstellar medium (ISM) makes direct observations of the intrinsic Lyα emission of even nearby stars challenging. The MUSCLES Hubble Space Telescope Treasury Survey (Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems) has completed observations of 7 M and 4 K stars hosting exoplanets (d < 22 pc). We have reconstructed the intrinsic Lyα profiles using an MCMC technique and used the results to estimate the extreme ultraviolet (100 - 911 Å) spectrum. We also present empirical relations between Lyα and chromospheric UV metal lines, e.g., Mg II, for use when ISM absorption prevents direct measurement of Lyα. The spectra presented here will be made publicly available through MAST to support exoplanet atmosphere modeling.

Spectrophotometry of Symbiotic Stars (Abstract)

NASA Astrophysics Data System (ADS)

Boyd, D.

2017-12-01

(Abstract only) Symbiotic stars are fascinating objects - complex binary systems comprising a cool red giant star and a small hot object, often a white dwarf, both embedded in a nebula formed by a wind from the giant star. UV radiation from the hot star ionizes the nebula, producing a range of emission lines. These objects have composite spectra with contributions from both stars plus the nebula and these spectra can change on many timescales. Being moderately bright, they lend themselves well to amateur spectroscopy. This paper describes the symbiotic star phenomenon, shows how spectrophotometry can be used to extract astrophysically useful information about the nature of these systems, and gives results for three symbiotic stars based on the author's observations.

Spectrophotometry of Symbiotic Stars

NASA Astrophysics Data System (ADS)

Boyd, David

2017-06-01

Symbiotic stars are fascinating objects - complex binary systems comprising a cool red giant star and a small hot object, often a white dwarf, both embedded in a nebula formed by a wind from the giant star. UV radiation from the hot star ionises the nebula producing a range of emission lines. These objects have composite spectra with contributions from both stars plus the nebula and these spectra can change on many timescales. Being moderately bright, they lend themselves well to amateur spectroscopy. This paper describes the symbiotic star phenomenon, shows how spectrophotometry can be used to extract astrophysically useful information about the nature of these systems, and gives results for three symbiotic stars based on the author's observations.

Copernicus observations of Betelgeuse and Antares

NASA Technical Reports Server (NTRS)

Bernat, A. P.; Lambert, D. L.

1975-01-01

Copernicus observations of the M-supergiants, alpha Ori and alpha Sco, are presented. The MgII h and k resonance lines are strongly in emission in both stars. The k line is highly asymmetric in both stars but the h line is symmetric. Upper limits for several other resonance lines are given for alpha Ori. The possibility is explored that the k line asymmetry is caused by overlying resonance lines of MnI and FeI formed in the cool circumstellar gas shells around these stars. Observations of the MnI 4030-4033 A lines are used to show that circumstellar shell absorption is too weak to explain the asymmetry. It is suggested that the absorption occurs in a cool turbulent region between the base of the circumstellar shell and the top of the chromosphere.

The Effects of Metallicity and Grain Size on Gravitational Instabilities in Protoplanetary Disks

NASA Astrophysics Data System (ADS)

Cai, Kai; Durisen, Richard H.; Michael, Scott; Boley, Aaron C.; Mejía, Annie C.; Pickett, Megan K.; D'Alessio, Paola

2006-01-01

Observational studies show that the probability of finding gas giant planets around a star increases with the star's metallicity. Our latest simulations of disks undergoing gravitational instabilities (GIs) with realistic radiative cooling indicate that protoplanetary disks with lower metallicity generally cool faster and thus show stronger overall GI activity. More importantly, the global cooling times in our simulations are too long for disk fragmentation to occur, and the disks do not fragment into dense protoplanetary clumps. Our results suggest that direct gas giant planet formation via disk instabilities is unlikely to be the mechanism that produced most observed planets. Nevertheless, GIs may still play an important role in a hybrid scenario, compatible with the observed metallicity trend, where structure created by GIs accelerates planet formation by core accretion.

The Effects of Metallicity and Grain Size on Gravitational Instabilities in Protoplanetary Disks

NASA Astrophysics Data System (ADS)

Cai, K.; Durisen, R. H.; Michael, S.; Boley, A. C.; Mejía, A. C.; Pickett, M. K.; D'Alessio, P.

Observational studies show that the probability of finding gas giant planets around a star increases with the star's metallicity. Our latest simulations of disks undergoing gravitational instabilities (GIs) with realistic radiative cooling indicate that protoplanetary disks with lower metallicity generally cool faster and thus show stronger overall GI-activity. More importantly, the global cooling times in our simulations are too long for disk fragmentation to occur, and the disks do not fragment into dense protoplanetary clumps. Our results suggest that direct gas giant planet formation via disk instabilities is unlikely to be the mechanism that produced most observed planets. Nevertheless, GIs may still play an important role in a hybrid scenario, compatible with the observed metallicity trend, where structure created by GIs accelerates planet formation by core accretion.

An analytic resolution to the competition between Lyman-Werner radiation and metal winds in direct collapse black hole hosts

NASA Astrophysics Data System (ADS)

Agarwal, Bhaskar; Regan, John; Klessen, Ralf S.; Downes, Turlough P.; Zackrisson, Erik

2017-10-01

A near pristine atomic cooling halo close to a star forming galaxy offers a natural pathway for forming massive direct collapse black hole (DCBH) seeds, which could be the progenitors of the z > 6 redshift quasars. The close proximity of the haloes enables a sufficient Lyman-Werner flux to effectively dissociate H2 in the core of the atomic cooling halo. A mild background may also be required to delay star formation in the atomic cooling halo, often attributed to distant background galaxies. In this paper, we investigate the impact of metal pollution from both the background galaxies and the close star forming galaxy under extremely unfavourable conditions such as instantaneous metal mixing. We find that within the time window of DCBH formation, the level of pollution never exceeds the critical threshold (Zcr ˜ 1 × 10-5 Z⊙) and attains a maximum metallicity of Z ˜ 2 × 10- 6 Z⊙. As the system evolves, the metallicity eventually exceeds the critical threshold, long after the DCBH has formed.

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

McDonald, Michael; Bautz, Marshall W.; Benson, Bradford

We present Hubble Space Telescope Wide Field Camera 3 observations of the core of the Phoenix cluster (SPT-CLJ2344-4243) in five broadband filters spanning rest-frame 1000-5500 A. These observations reveal complex, filamentary blue emission, extending for >40 kpc from the brightest cluster galaxy. We observe an underlying, diffuse population of old stars, following an r {sup 1/4} distribution, confirming that this system is somewhat relaxed. The spectral energy distribution in the inner part of the galaxy, as well as along the extended filaments, is a smooth continuum and is consistent with that of a star-forming galaxy, suggesting that the extended, filamentarymore » emission is not due to the central active galactic nucleus, either from a large-scale ionized outflow or scattered polarized UV emission, but rather a massive population of young stars. We estimate an extinction-corrected star formation rate of 798 {+-} 42 M{sub Sun} yr{sup -1}, consistent with our earlier work based on low spatial resolution ultraviolet, optical, and infrared imaging. The lack of tidal features and multiple bulges, combine with the need for an exceptionally massive (>10{sup 11} M{sub Sun }) cold gas reservoir, suggest that this star formation is not the result of a merger of gas-rich galaxies. Instead, we propose that the high X-ray cooling rate of {approx}2700 M{sub Sun} yr{sup -1} is the origin of the cold gas reservoir. The combination of such a high cooling rate and the relatively weak radio source in the cluster core suggests that feedback has been unable to halt cooling in this system, leading to this tremendous burst of star formation.« less

Radiative Hydrodynamic Simulations of In Situ Star Formation in the Galactic Center

NASA Astrophysics Data System (ADS)

Frazer, Chris; Heitsch, Fabian

2018-01-01

Many stars observed in the Galactic Center (GC) orbit the supermassive black hole (SMBH), Sagittarius A*, in a region where the extreme gravitational field is expected to inhibit star formation. Yet, many of these stars are young which favors an in situ formation scenario. Previous numerical work on this topic has focused on two possible solutions. First, the tidal capture of a > 10^4 Msun infalling molecular cloud by an SMBH may result in the formation of a surrounding gas disk which then rapidly cools and forms stars. This process results in stellar populations that are consistent with the observed stellar disk in the GC. Second, dense gas clumps of approximately 100 Msun on highly eccentric orbits about an SMBH can experience sparks of star formation via orbital compressions occurring during pericenter passage. In my dissertation, I build upon these models using a series of grid-based radiative hydrodynamic simulations, including the effects of both ionizing ultraviolet light from existing stars as well as X-ray radiation emanating from the central black hole. Radiation is treated with an adaptive ray-tracing routine, including appropriate heating and cooling for both neutral and ionized gas. These models show that ultraviolet radiation is sufficiently strong to heat low mass gas clouds, thus suppressing star formation from clump compression. Gas disks that form from cloud capture become sufficiently dense to provide shielding from the radiation of existing central stars, thus allowing star formation to continue. Conversely, X-rays easily penetrate and heat the potentially star forming gas. For sufficiently high radiation fields, this provides a mechanism to disrupt star formation for both scenarios considered above.

Gamma rays from clumpy wind-jet interactions in high-mass microquasars

NASA Astrophysics Data System (ADS)

de la Cita, V. M.; del Palacio, S.; Bosch-Ramon, V.; Paredes-Fortuny, X.; Romero, G. E.; Khangulyan, D.

2017-07-01

Context. The stellar winds of the massive stars in high-mass microquasars are thought to be inhomogeneous. The interaction of these inhomogeneities, or clumps, with the jets of these objects may be a major factor in gamma-ray production. Aims: Our goal is to characterize a typical scenario of clump-jet interaction, and calculate the contribution of these interactions to the gamma-ray emission from these systems. Methods: We use axisymmetric, relativistic hydrodynamical simulations to model the emitting flow in a typical clump-jet interaction. Using the simulation results we perform a numerical calculation of the high-energy emission from one of these interactions. The radiative calculations are performed for relativistic electrons locally accelerated at the jet shock, and the synchrotron and inverse Compton radiation spectra are computed for different stages of the shocked clump evolution. We also explore different parameter values, such as viewing angle and magnetic field strength. The results derived from one clump-jet interaction are generalized phenomenologically to multiple interactions under different wind models, estimating the clump-jet interaction rates, and the resulting luminosities in the GeV range. Results: If particles are efficiently accelerated in clump-jet interactions, the apparent gamma-ray luminosity through inverse Compton scattering with the stellar photons can be significant even for rather strong magnetic fields and thus efficient synchrotron cooling. Moreover, despite the standing nature or slow motion of the jet shocks for most of the interaction stage, Doppler boosting in the postshock flow is relevant even for mildly relativistic jets. Conclusions: For clump-to-average wind density contrasts greater than or equal to ten, clump-jet interactions could be bright enough to match the observed GeV luminosity in Cyg X-1 and Cyg X-3 when a jet is present in these sources, with required non-thermal-to-total available power fractions greater than 0.01 and 0.1, respectively.

Evidence for extended chromospheres surrounding red giant stars

NASA Technical Reports Server (NTRS)

Stencel, R. E.

1981-01-01

There is now an increasing amount of both observational evidence and theoretical arguments that regions of partially ionized hydrogen extending several stellar radii are an important feature of red giant and supergiant stars. This evidence is discussed and the implications of the existence of extended chromospheres in terms of the nature of the outer atmospheres of, and mass loss from, cool stars are examined.

Couleurs, etoiles, temperatures.

NASA Astrophysics Data System (ADS)

Spite, F.

The eye is able to distinguish very tiny color differences of contiguous objects (at high light level, cones vision), but it is not a reliable colorimeter. Hot objects (a heated iron rod) emits some red light, a hotter object would provide a yellow-orange light (the filament of a bulb) and a still hotter one a white or even bluish light : this may be at reverse of common life codes, where "red" means hot water and/or danger, and "blue" cool water or cool air. Stars are a good illustration of the link between temperatures and colors. A heated iron rod has a temperature of about 800 K. Let us recall that K is a temperature unit (Kelvin) such that the Kelvin temperature is the Celsius temperature +273).The so called red stars (or cool stars) have temperature around 3000 K, higher than "white-hot iron". The Sun has a still higher temperature (5800 K) and its color is white : the solar light is by definition the "white light", and includes violet, blue, green, yellow, orange and red colors in balanced proportions (the maximum in the yellow-green). It is often said that the Sun is a yellow star. Admittedly, a brief glimpse at the Sun (take care ! never more than a VERY brief glimpse !) provides a perception of yellow light, but such a vision, with the eye overwhelmed by a fierce light, is not able to provide a good evaluation of the solar color : prefer a white sheet of paper illuminated by the Sun at noon and conclude that "the Sun is a white star". It is sometimes asked why red, white and bluish stars are seen in the sky, but no green stars : the solar light has its maximum intensity in the green, but such a dominant green light, equilibrated by some blue and some red light, is what we call "white", so that stars similar to the Sun, with a maximum in the green, are seen as white stars. Faint stars (rods vision of the eye) are also seen as white stars. Spots on the Sun (never look at the Sun ! let us say spots on "projected images of the Sun") appear as black spots : they are in fact bright areas, only slightly less luminous than the undisturbed surface of the Sun, but the eye has a particular of enhancing enormously the contrasts.

Searching for 300, 000 Degree Gas in the Core of the Phoenix Cluster with HST-COS

NASA Astrophysics Data System (ADS)

McDonald, Michael

2013-10-01

The high central density of the intracluster medium in some galaxy clusters suggests that the hot 10,000,000K gas should cool completely in less than a Hubble time. In these clusters, simple cooling models predict 100-1000 solar masses per year of cooling gas should fuel massive starbursts in the central galaxy. The fact that the typical central cluster galaxy is a massive, "red and dead" elliptical galaxy, with little evidence for a cool ISM, has led to the realization of the "cooling flow problem". It is now thought that mechanical feedback from the central supermassive blackhole, in the form of radio-blown bubbles, is offsetting cooling, leading to an exceptionally precise {residuals of less than 10 percent} balance between cooling and feedback in nearly every galaxy cluster in the local Universe. In the recently-discovered Phoenix cluster, where z=0.596, we observe an 800 solar mass per year starburst within the central galaxy which accounts for about 30 percent of the classical cooling prediction for this system. We speculate that this may represent the first "true" cooling flow, with the factor of 3 difference between cooling and star formation being attributed to star formation efficiency, rather than a problem with cooling. In order to test these predictions, we propose far-UV spectroscopic observations of the OVI 1032A emission line, which probes 10^5.5K gas, in the central galaxy of the Phoenix cluster. If detected at the expected levels, this would provide compelling evidence that the starburst is, indeed, fueled by runaway cooling of the intracluster medium, confirming the presence of the first, bonafide cooling flow.

Two peculiar subclasses of cool DB stars - The DBA's and the DBZ's

NASA Technical Reports Server (NTRS)

Shipman, Harry L.

1987-01-01

This paper reviews several recent observational investigations which have demonstrated that a significant number of white dwarfs with He-dominated photospheres contain trace hydrogen. In addition, follow-up work on the Case Blue Star survey has uncovered an analog of GD 40 - another DBZ star. Various scenarios for their origin are discussed. The paper concludes with a brief synopsis of a discussion of how many more subclasses of white dwarf stars might exist and what it would take to discover them.

Models of the circumstellar medium of evolving, massive runaway stars moving through the Galactic plane

NASA Astrophysics Data System (ADS)

Meyer, D. M.-A.; Mackey, J.; Langer, N.; Gvaramadze, V. V.; Mignone, A.; Izzard, R. G.; Kaper, L.

2014-11-01

At least 5 per cent of the massive stars are moving supersonically through the interstellar medium (ISM) and are expected to produce a stellar wind bow shock. We explore how the mass-loss and space velocity of massive runaway stars affect the morphology of their bow shocks. We run two-dimensional axisymmetric hydrodynamical simulations following the evolution of the circumstellar medium of these stars in the Galactic plane from the main sequence to the red supergiant phase. We find that thermal conduction is an important process governing the shape, size and structure of the bow shocks around hot stars, and that they have an optical luminosity mainly produced by forbidden lines, e.g. [O III]. The Hα emission of the bow shocks around hot stars originates from near their contact discontinuity. The Hα emission of bow shocks around cool stars originates from their forward shock, and is too faint to be observed for the bow shocks that we simulate. The emission of optically thin radiation mainly comes from the shocked ISM material. All bow shock models are brighter in the infrared, i.e. the infrared is the most appropriate waveband to search for bow shocks. Our study suggests that the infrared emission comes from near the contact discontinuity for bow shocks of hot stars and from the inner region of shocked wind for bow shocks around cool stars. We predict that, in the Galactic plane, the brightest, i.e. the most easily detectable bow shocks are produced by high-mass stars moving with small space velocities.

Mass Measurement of 56Sc Reveals a Small A=56 Odd-Even Mass Staggering, Implying a Cooler Accreted Neutron Star Crust

DOE PAGES

Meisel, Z.; George, S.; Ahn, S.; ...

2015-10-16

We present the mass excesses of 52-57Sc, obtained from recent time-of-flight nuclear mass measurements at the National Superconducting Cyclotron Laboratory at Michigan State University. The masses of 56Sc and 57Sc were determined for the first time with atomic mass excesses of -24.85(59)((+0)(-54)) MeV and -21.0(1.3) MeV, respectively, where the asymmetric uncertainty for 56Sc was included due to possible contamination from a long-lived isomer. The 56Sc mass indicates a small odd-even mass staggering in the A = 56 mass chain towards the neutron drip line, significantly deviating from trends predicted by the global FRDM mass model and favoring trends predicted bymore » the UNEDF0 and UNEDF1 density functional calculations. Together with new shell-model calculations of the electron-capture strength function of 56Sc, our results strongly reduce uncertainties in model calculations of the heating and cooling at the 56Ti electron-capture layer in the outer crust of accreting neutron stars. We find that, in contrast to previous studies, neither strong neutrino cooling nor strong heating occurs in this layer. We conclude that Urca cooling in the outer crusts of accreting neutron stars that exhibit superbursts or high temperature steady-state burning, which are predicted to be rich in A approximate to 56 nuclei, is considerably weaker than predicted. Urca cooling must instead be dominated by electron capture on the small amounts of adjacent odd-A nuclei contained in the superburst and high temperature steady-state burning ashes. This may explain the absence of strong crust Urca cooling inferred from the observed cooling light curve of the transiently accreting x-ray source MAXI J0556-332.« less

Mass Measurement of 56Sc Reveals a Small A =56 Odd-Even Mass Staggering, Implying a Cooler Accreted Neutron Star Crust

NASA Astrophysics Data System (ADS)

Meisel, Z.; George, S.; Ahn, S.; Bazin, D.; Brown, B. A.; Browne, J.; Carpino, J. F.; Chung, H.; Cole, A. L.; Cyburt, R. H.; Estradé, A.; Famiano, M.; Gade, A.; Langer, C.; Matoš, M.; Mittig, W.; Montes, F.; Morrissey, D. J.; Pereira, J.; Schatz, H.; Schatz, J.; Scott, M.; Shapira, D.; Smith, K.; Stevens, J.; Tan, W.; Tarasov, O.; Towers, S.; Wimmer, K.; Winkelbauer, J. R.; Yurkon, J.; Zegers, R. G. T.

2015-10-01

We present the mass excesses of 52-57Sc, obtained from recent time-of-flight nuclear mass measurements at the National Superconducting Cyclotron Laboratory at Michigan State University. The masses of 56Sc and 57Sc were determined for the first time with atomic mass excesses of -24.85 (59 )(-54+0) MeV and -21.0 (1.3 ) MeV , respectively, where the asymmetric uncertainty for 56Sc was included due to possible contamination from a long-lived isomer. The 56Sc mass indicates a small odd-even mass staggering in the A =56 mass chain towards the neutron drip line, significantly deviating from trends predicted by the global FRDM mass model and favoring trends predicted by the UNEDF0 and UNEDF1 density functional calculations. Together with new shell-model calculations of the electron-capture strength function of 56Sc, our results strongly reduce uncertainties in model calculations of the heating and cooling at the 56Ti electron-capture layer in the outer crust of accreting neutron stars. We find that, in contrast to previous studies, neither strong neutrino cooling nor strong heating occurs in this layer. We conclude that Urca cooling in the outer crusts of accreting neutron stars that exhibit superbursts or high temperature steady-state burning, which are predicted to be rich in A ≈56 nuclei, is considerably weaker than predicted. Urca cooling must instead be dominated by electron capture on the small amounts of adjacent odd-A nuclei contained in the superburst and high temperature steady-state burning ashes. This may explain the absence of strong crust Urca cooling inferred from the observed cooling light curve of the transiently accreting x-ray source MAXI J0556-332.

Atmospheric Modeling of Cool Giant and Supergiant Stars

NASA Astrophysics Data System (ADS)

Linsky, Jeffrey L.

1984-07-01

We propose to continue our collaborative program of obtaining and analysing high dispersion SWP spectra of cool stars. We request high dispersion, short wavelength IUE spectra of the stars alpha Tau (K5III), gamma Cru (M3III), epsilon Peg (K2Ib) and beta Cam (G0Ib) with exposure times of 16 hours or more. These spectra will provide measurements of line profiles, widths and Doppler shifts in addition to density-sensitive and opacity-sensitive line ratios. Models of chromospheric and transition region (where present) structure will be calculated by a combination of emission measure analysis, line opacity/probability of escape methods and model atmosphere calculations for optically thick resonance lines such as MgII h and k, including partial redistribution radiative transfer. These models will be used to investigate the atmospheric energy balance and the nature of energy transport and nonradiative energy deposition processes. The results will be considered in relation to stellar evolution and compared with the chromospheric properties of other stars previously studied by the authors and their collaborators.

Line Profile Variations of Solar Analog Stars: Chromospheric Indexes vs. Li Abundance. The Host Star Search.

NASA Astrophysics Data System (ADS)

Amazo-Gómez, E. M.; Harutyunyan, G.; Alvarado-Gómez, J. D.; Strassmeier, K. G.; Weber, M.; Carroll, T. A.

2015-10-01

PolarBase contains stellar spectropolarimetric data collected with the NARVAL & ESPaDOnS instruments (Petit et al. 2014). Their respective spectral resolutions are 65 000 and 68 000, in spectropolarimetric mode. As the first part of this work, we use the NARVAL spectropolarimetric repositories. We selected spectra from a sample of cool stars with effective Temperature (T eff) ranging between 4900 to 6000 K. This sample contains stellar systems with and without reported exoplanets. We exploit the full wavelength range from 380 to 900 nm in order to obtain chromospheric indexes such as the Ca ii H&K S-Index, and a Ca ii IRT and Hα index. We calibrated our measurements using the Mount Wilson S-Index values. Furthermore, we employ lithium (Li) abundance measurements from the literature (Gonzalez et al. 2010; Delgado Mena et al. 2014; Israelian et al. 2004), investigating in this way a possible correlation between the chromospheric activity measurements and the Li abundance in 32 selected cool stars.

The Secular Evolution Of Disc Galaxies And The Origin Of Exponential And Double Exponential Surface Density Profiles

NASA Astrophysics Data System (ADS)

Elmegreen, Bruce G.

2016-10-01

Exponential radial profiles are ubiquitous in spiral and dwarf Irregular galaxies, but the origin of this structural form is not understood. This talk will review the observations of exponential and double exponential disks, considering both the light and the mass profiles, and the contributions from stars and gas. Several theories for this structure will also be reviewed, including primordial collapse, bar and spiral torques, clump torques, galaxy interactions, disk viscosity and other internal processes of angular momentum exchange, and stellar scattering off of clumpy structure. The only process currently known that can account for this structure in the most theoretically difficult case is stellar scattering off disks clumps. Stellar orbit models suggest that such scattering can produce exponentials even in isolated dwarf irregulars that have no bars or spirals, little shear or viscosity, and profiles that go out too far for the classical Mestel case of primordial collapse with specific angular momentum conservation.

Mapping out the origins of compact stellar systems

NASA Astrophysics Data System (ADS)

Romanowsky, Aaron J.; Brodie, Jean P.; SAGES Collaboration

2017-03-01

We present a suite of extragalactic explorations of the origins and nature of globular clusters (GCs) and ultra-compact dwarfs (UCDs), and the connections between them. An example of GC metallicity bimodality is shown to reflect underlying, distinct metal-poor and metal-rich stellar halo populations. Metallicity-matching methods are used to trace the birth sites and epochs of GCs in giant E/S0s, pointing to clumpy disk galaxies at z ~ 3 for the metal-rich GCs, and to a combination of accreted and in-situ formation modes at z ~ 5-6 for the metal-poor GCs. An increasingly diverse zoo of compact stellar systems is being discovered, including objects that bridge the gaps between UCDs and faint fuzzies, and between UCDs and compact ellipticals. Many of these have properties pointing to origins as the stripped nuclei of larger galaxies, and a smoking-gun example is presented of an ω Cen-like star cluster embedded in a tidal stream.

First Images of R Aquarii and Its Asymmetric H2O Shell

NASA Astrophysics Data System (ADS)

Ragland, S.; Le Coroller, H.; Pluzhnik, E.; Cotton, W. D.; Danchi, W. C.; Monnier, J. D.; Traub, W. A.; Willson, L. A.; Berger, J.-P.; Lacasse, M. G.

2008-05-01

We report imaging observations of the symbiotic long-period Mira variable R Aquarii (R Aqr) at near-infrared and radio wavelengths. The near-infrared observations were made with the IOTA imaging interferometer in three narrowband filters centered at 1.51, 1.64, and 1.78 μm, which sample mainly water, continuum, and water features, respectively. Our near-infrared fringe visibility and closure phase data are analyzed using three models. (1) A uniform disk model with wavelength-dependent sizes fails to fit the visibility data, and is inconsistent with the closure phase data. (2) A three-component model, consisting of a Mira star, water shell, and an off-axis point source, provide a good fit to all data. (3) A model generated by a constrained image reconstruction analysis provides more insight, suggesting that the water shell is highly nonuniform, i.e., clumpy. The VLBA observations of SiO masers in the outer molecular envelope show evidence of turbulence, with jetlike features containing velocity gradients.

The structure and nature of NGC 2017 IRS. 1: High-resolution radio continuum maps

NASA Technical Reports Server (NTRS)

Smith, Howard A.; Beck, Sara C.

1994-01-01

We have observed the star formation cluster NGC 2071 IRS 1, 2, and 3, with 0.14 sec spatial resolution at 2 cm. The strong source IRS 1 breaks up into a bright peak sitting on a narrow line emission extending over about 400 AU, with three much weaker peaks. This ridge, which has a p.a. = 100 deg, is not aligned with any of the other structures that have previously been seen around IRS 1: its orientation is about 55 deg from the CO outflow direction, and 35 deg from a hypothetical disk direction. The spectral and spatial results, combined with earlier radio and infrared observations, indicate that most likely the radio and infrared emission from the exciting source, IRS 1, is produced by a dense wind hidden by at least 100 visual magnitudes of extinction; the extended ridge of emission comes from an optically thin H II region with characteristic dimensions of approximately AU and which may result from a clumpy distribution of local gas and dust.

Cool Andromeda

NASA Image and Video Library

2013-01-28

In this new view of the Andromeda, also known as M31, galaxy from the Herschel space observatory, cool lanes of forming stars are revealed in the finest detail yet. M31 is the nearest major galaxy to our own Milky Way at a distance of 2.5 million light-ye

Heavy nuclei as thermal insulation for protoneutron stars

NASA Astrophysics Data System (ADS)

Nakazato, Ken'ichiro; Suzuki, Hideyuki; Togashi, Hajime

2018-03-01

A protoneutron star (PNS) is a newly formed compact object in a core collapse supernova. In this paper, the neutrino emission from the cooling process of a PNS is investigated using two types of nuclear equation of state (EOS). It is found that the neutrino signal is mainly determined by the high-density EOS. The neutrino luminosity and mean energy are higher and the cooling time scale is longer for the softer EOS. Meanwhile, the neutrino mean energy and the cooling time scale are also affected by the low-density EOS because of the difference in the population of heavy nuclei. Heavy nuclei have a large scattering cross section with neutrinos owing to the coherent effects and act as thermal insulation near the surface of a PNS. The neutrino mean energy is higher and the cooling time scale is longer for an EOS with a large symmetry energy at low densities, namely a small density derivative coefficient of the symmetry energy, L .

PROBING THE DEEP END OF THE MILKY WAY WITH KEPLER : ASTEROSEISMIC ANALYSIS OF 854 FAINT RED GIANTS MISCLASSIFIED AS COOL DWARFS

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

Mathur, S.; García, R. A.; Beck, P. G.

Asteroseismology has proven to be an excellent tool to determine not only global stellar properties with good precision, but also to infer the stellar structure, dynamics, and evolution for a large sample of Kepler stars. Prior to the launch of the mission, the properties of Kepler targets were inferred from broadband photometry, leading to the Kepler Input Catalog (KIC). The KIC was later revised in the Kepler Star Properties Catalog, based on literature values and an asteroseismic analysis of stars that were unclassified in the KIC. Here, we present an asteroseismic analysis of 45,400 stars that were classified as dwarfsmore » in the Kepler Star Properties Catalog. We found that around 2% of the sample shows acoustic modes in the typical frequency range that put them in the red-giant category rather than the cool dwarf category. We analyze the asteroseismic properties of these stars, derive their surface gravities, masses, and radii, and present updated effective temperatures and distances. We show that the sample is significantly fainter than the previously known oscillating giants in the Kepler field, with the faintest stars reaching down to a Kepler magnitude of Kp ∼ 16. We demonstrate that 404 stars are at distances beyond 5 kpc and that the stars are significantly less massive than for the original Kepler red-giant sample, consistent with a population of distant halo giants. A comparison with a galactic population model shows that up to 40 stars might be genuine halo giants, which would increase the number of known asteroseismic halo stars by a factor of 4. The detections presented here will provide a valuable sample for galactic archeology studies.« less

Three-dimensional radiative transfer models of clumpy tori in Seyfert galaxies

NASA Astrophysics Data System (ADS)

Schartmann, M.; Meisenheimer, K.; Camenzind, M.; Wolf, S.; Tristram, K. R. W.; Henning, T.

2008-04-01

Context: Tori of Active Galactic Nuclei (AGN) are made up of a mixture of hot and cold gas, as well as dust. In order to protect the dust grains from destruction by the surrounding hot gas as well as by the energetic (UV/optical) radiation from the accretion disk, the dust is often assumed to be distributed in clouds. Aims: A new three-dimensional model of AGN dust tori is extensively investigated. The torus is modelled as a wedge-shaped disk within which dusty clouds are randomly distributed throughout the volume, by taking the dust density distribution of the corresponding continuous model into account. We especially concentrate on the differences between clumpy and continuous models in terms of the temperature distributions, the surface brightness distributions and interferometric visibilities, as well as spectral energy distributions. Methods: Radiative transfer calculations with the help of the three-dimensional Monte Carlo radiative transfer code MC3D are used in order to simulate spectral energy distributions as well as surface brightness distributions at various wavelengths. In a second step, interferometric visibilities for various inclination as well as position angles and baselines are calculated, which can be used to directly compare our models to interferometric observations with the MIDI instrument. Results: We find that the radial temperature distributions of clumpy models possess significantly enhanced scatter compared to the continuous cases. Even at large distances, clouds can be heated directly by the central accretion disk. The existence of the silicate 10 μm-feature in absorption or in emission depends sensitively on the distribution, the size and optical depth of clouds in the innermost part of the dust distribution. With this explanation, failure and success of previous modelling efforts of clumpy tori can be understood. The main reason for this outcome are shadowing effects of clouds within the central region. We underline this result with the help of several parameter variations. After adapting the parameters of our clumpy standard model to the circumstances of the Seyfert 2 Circinus galaxy, it can qualitatively explain recent mid-infrared interferometric observations performed with MIDI, as well as high resolution spectral data.

Pressure of the hot gas in simulations of galaxy clusters

NASA Astrophysics Data System (ADS)

Planelles, S.; Fabjan, D.; Borgani, S.; Murante, G.; Rasia, E.; Biffi, V.; Truong, N.; Ragone-Figueroa, C.; Granato, G. L.; Dolag, K.; Pierpaoli, E.; Beck, A. M.; Steinborn, Lisa K.; Gaspari, M.

2017-06-01

We analyse the radial pressure profiles, the intracluster medium (ICM) clumping factor and the Sunyaev-Zel'dovich (SZ) scaling relations of a sample of simulated galaxy clusters and groups identified in a set of hydrodynamical simulations based on an updated version of the treepm-SPH GADGET-3 code. Three different sets of simulations are performed: the first assumes non-radiative physics, the others include, among other processes, active galactic nucleus (AGN) and/or stellar feedback. Our results are analysed as a function of redshift, ICM physics, cluster mass and cluster cool-coreness or dynamical state. In general, the mean pressure profiles obtained for our sample of groups and clusters show a good agreement with X-ray and SZ observations. Simulated cool-core (CC) and non-cool-core (NCC) clusters also show a good match with real data. We obtain in all cases a small (if any) redshift evolution of the pressure profiles of massive clusters, at least back to z = 1. We find that the clumpiness of gas density and pressure increases with the distance from the cluster centre and with the dynamical activity. The inclusion of AGN feedback in our simulations generates values for the gas clumping (√{C}_{ρ }˜ 1.2 at R200) in good agreement with recent observational estimates. The simulated YSZ-M scaling relations are in good accordance with several observed samples, especially for massive clusters. As for the scatter of these relations, we obtain a clear dependence on the cluster dynamical state, whereas this distinction is not so evident when looking at the subsamples of CC and NCC clusters.

Get Ready for Gaia: Cool White Dwarfs in Common Proper Motion with Tycho Stars

NASA Astrophysics Data System (ADS)

Hambly, N.; Rowell, N.; Lam, M.

2017-03-01

We discuss the Gaia Data Release 1 (September 2016) and preliminary work on maximising the benefit for cool white dwarf (WD) science in advance of the full parallax catalogue which will appear around one year later in DR2. The Tycho catalogue is used in conjunction with the all-sky ground based astrometric/ photometric SuperCOSMOS Sky Survey in order to identify candidate faint common proper motion objects to the Tycho stars. Gaia DR1 is supplemented by the Tycho-Gaia Astrometric Solution catalogue containing some 2 million parallaxes with Hipparcos-like precision for Tycho stars. While hotter, brighter WDs are present in Tycho, cooler examples are much rarer (if present at all) and CPM offers one method to infer precision distances for a statistically useful sample of these very faint WDs.

Copernicus observations of Betelgeuse and Antares

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

Bernat, A.P.; Lambert, D.L.

1975-01-01

Copernicus observations of the M-supergiants, ..cap alpha.. Ori and ..cap alpha.. Sco, are presented. The Mg II H and K resonance lines are strongly in emission in both stars. The K line is highly asymmetric in both stars but the H line is symmetric. Upper limits for several other resonance lines are given for ..cap alpha.. Ori. The possibility is explored that the K line asymmetry is caused by overlying resonance lines of Mn I and Fe I formed in the cool circumstellar gas shells around these stars. Observations of the Mn I 4030--4033 A lines are used to showmore » that circumstellar shell absorption is too weak to explain the asymmetry. It is suggested that the absorption occurs in a cool turbulent region between the base of the circumstellar shell and the top of the chromosphere. (auth)« less

Hα3: an Hα imaging survey of HI selected galaxies from ALFALFA. IV. Structure of galaxies in the Local and Coma superclusters

NASA Astrophysics Data System (ADS)

Fossati, M.; Gavazzi, G.; Savorgnan, G.; Fumagalli, M.; Boselli, A.; Gutiérrez, L.; Hernández Toledo, H.; Giovanelli, R.; Haynes, M. P.

2013-05-01

Context. We present the analysis of the galaxy structural parameters from Hα3, an Hα narrow-band imaging follow-up survey of ~800 galaxies selected from the HI Arecibo Legacy Fast ALFA Survey (ALFALFA) in the Local supercluster, including the Virgo cluster, and in the Coma supercluster. Aims: Taking advantage of Hα3, which provides the complete census of the recent star-forming, HI-rich galaxies in the local universe, we aim to investigate the structural parameters of the young (<10 Myr) and the old (>1 Gyr) stellar populations. By comparing the sizes of these stellar components, we investigated the spatial scale on which galaxies are growing at the present cosmological epoch and the role of the environment in quenching the star-formation activity. Methods: We computed the concentration, asymmetry, and clumpiness (CAS) structural parameters for recently born and old stars. To quantify the sizes we computed half-light radii and a new parameter dubbed EW/r based on the half-light radius of the Hα equivalent width map. To highlight the environmental perturbation, we adopt an updated calibration of the HI-deficiency parameter (DefHI) that we use to divide the sample in unperturbed galaxies (DefHI ≤ 0.3) and perturbed galaxies (DefHI > 0.3). Results: The concentration index computed in the r band depends on the stellar mass and on the Hubble type these variables are related because most massive galaxies are bulge dominated therefore highly concentrated. Going toward later spirals and irregulars the concentration index and the mass decrease along with the bulge-to-disk ratio. Blue compact dwarfs (BCDs) are an exception because they have similar mass, but they are more concentrated than dwarf irregulars. The asymmetry and the clumpiness increase along the spiral sequence up to Sc-Sd, but they decrease going in the dwarf regime, where the light distribution is smooth and more symmetric. When measured on Hα images, the CAS parameters show no obvious correlations with Hubble type. Irrespective of whether we used the ratio between effective radii or the EW/r parameter, we found that the concentration index is the main parameter that describes the current growth of isolated galaxies but, for a fixed concentration, the stellar mass plays a second-order role. At the present epoch, massive galaxies are growing inside-out, conversely, the dwarfs are growing on the scale of their already assembled mass. Observations taken at the observatory of San Pedro Martir (Baja California, Mexico), belonging to the Mexican Observatorio Astronómico Nacional.Tables A.1 and A.2 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/553/A91

The MUSCLES Treasury Survey: Intrinsic Lyα Profile Reconstructions and UV, X-ray, and Optical Correlations of Low-mass Exoplanet Host Stars

NASA Astrophysics Data System (ADS)

Youngblood, Allison; France, Kevin; Loyd, R. O. Parke

2016-01-01

UV stellar radiation can significantly impact planetary atmospheres through heating and photochemistry, even regulating production of potential biomarkers. Cool stars emit the majority of their UV radiation in the form of emission lines, and the incident UV radiation on close-in habitable-zone planets is significant. Lyα (1215.67 Å) dominates the 912 - 3200 Å spectrum of cool stars, but strong absorption from the interstellar medium (ISM) makes direct observations of the intrinsic Lyα emission of even nearby stars challenging. The MUSCLES Hubble Space Telescope Treasury Survey (Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems) has completed observations of 7 M and 4 K stars hosting exoplanets (d < 22 pc) with simultaneous X-ray and ground-based optical spectroscopy for many of the targets. We have reconstructed the intrinsic Lyα profiles using an MCMC technique and used the results to estimate the extreme ultraviolet (100 - 911 Å) spectrum. We also present empirical relations between chromospheric UV and optical lines, e.g., Lyα, Mg II, Ca II H & K, and Hα, for use when direct UV observations of low-mass exoplanet host stars are not possible. The spectra presented here will be made publicly available through MAST to support exoplanet atmosphere modeling.

Comparative Modelling of the Spectra of Cool Giants

NASA Technical Reports Server (NTRS)

Lebzelter, T.; Heiter, U.; Abia, C.; Eriksson, K.; Ireland, M.; Neilson, H.; Nowotny, W; Maldonado, J; Merle, T.; Peterson, R.;

Coronal Structures in Cool Stars

NASA Technical Reports Server (NTRS)

Oliversen, Ronald (Technical Monitor); Dupree, Andrea K.

2004-01-01

Many papers have been published that further elucidate the structure of coronas in cool stars as determined from EUVE, HST, FUSE, Chandra, and XMM-Newton observations. In addition we are exploring the effects of coronas on the He I 1083081 transition that is observed in the infrared. Highlights of these are summarized below including publications during this reporting period and presentations. Ground-based magnetic Doppler imaging of cool stars suggests that active stars have active regions located at high latitudes on their surface. We have performed similar imaging in X-ray to locate the sites of enhanced activity using Chandra spectra. Chandra HETG observations of the bright eclipsing contact binary 44i Boo and Chandra LETG observations for the eclipsing binary VW Cep show X-ray line profiles that are Doppler-shifted by orbital motion. After careful analysis of the spectrum of each binary, a composite line-profile is constructed by adding the individual spectral lines. This high signal-to-noise ratio composite line-profile yields orbital velocities for these binaries that are accurate to 30 km/sec and allows their orbital motion to be studied at higher time resolutions. In conjunction with X-ray lightcurves, the phase-binned composite line-profiles constrain coronal structures to be small and located at high latitudes. These observations and techniques show the power of the Doppler Imaging Technique applied to X-ray line emission.

Forming Hot Jupiters: Observational Constraints on Gas Giant Formation and migration

NASA Astrophysics Data System (ADS)

Becker, Juliette; Vanderburg, Andrew; Adams, Fred C.; Khain, Tali; Bryan, Marta

2018-04-01

Since the first extrasolar planets were detected, the existence of hot Jupiters has challenged prevailing theories of planet formation. The three commonly considered pathways for hot Jupiter formation are in situ formation, runaway accretion in the outer disk followed by disk migration, and tidal migration (occurring after the disk has dissipated). None of these explains the entire observed sample of hot Jupiters, suggesting that different selections of systems form via different pathways. The way forward is to use observational data to constrain the migration pathways of particular classes of systems, and subsequently assemble these results into a coherent picture of hot Jupiter formation. We present constraints on the migratory pathway for one particular type of system: hot Jupiters orbiting cool stars (T< 6200 K). Using the full observational sample, we find that the orbits of most wide planetary companions to hot Jupiters around these cool stars must be well aligned with the orbits of the hot Jupiters and the spins of the host stars. The population of systems containing both a hot Jupiter and an exterior companion around a cool star thus generally exist in roughly coplanar configurations, consistent with the idea that disk-driven migratory mechanisms have assembled most of this class of systems. We then discuss the overall applicability of this result to a wider range of systems and the broader implications on planet formation.

Strong variable linear polarization in the cool active star II Peg

NASA Astrophysics Data System (ADS)

Rosén, Lisa; Kochukhov, Oleg; Wade, Gregg A.

2014-08-01

Magnetic fields of cool active stars are currently studied polarimetrically using only circular polarization observations. This provides limited information about the magnetic field geometry since circular polarization is only sensitive to the line-of-sight component of the magnetic field. Reconstructions of the magnetic field topology will therefore not be completely trustworthy when only circular polarization is used. On the other hand, linear polarization is sensitive to the transverse component of the magnetic field. By including linear polarization in the reconstruction the quality of the reconstructed magnetic map is dramatically improved. For that reason, we wanted to identify cool stars for which linear polarization could be detected at a level sufficient for magnetic imaging. Four active RS CVn binaries, II Peg, HR 1099, IM Peg, and σ Gem were observed with the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope. Mean polarization profiles in all four Stokes parameters were derived using the multi-line technique of least-squares deconvolution (LSD). Not only was linear polarization successfully detected in all four stars in at least one observation, but also, II Peg showed an extraordinarily strong linear polarization signature throughout all observations. This qualifies II Peg as the first promising target for magnetic Doppler imaging in all four Stokes parameters and, at the same time, suggests that other such targets can possibly be identified.

Analysis of stratification of Cr with depth in the atmospheres of normal and CP stars.

NASA Astrophysics Data System (ADS)

Savanov, I. S.

We present the results of analysis of stratification of Cr with depth in the atmospheres of normal, Am, HgMn and Ap stars. On the base of high resolution CCD-spectrograms obtained with spectrograph of 2.6-meter Shajn reflector using synthetic spectrum technique analysis of eight Cr II lines (30 multiplet) located in the wings of Hb Hydrogen line. For all investigated Am stars as well as for HgMn components of 46 Dra we suspect the similar small increase of Cr abundance in the upper atmosphere. For the hot spotted Ap stars α2 CVn and 17 Com we do not find evidence for the inhomogeneity distribution of Chromium while for the group of cool Ap stars (β CrB, HR 7575, 10 Aql and γ Equ) the increase of Cr abundance with depth was found. Detailed modelling of Cr II line profiles was performed in order to obtain vertical distributions of this element in the atmospheres of several Ap and Am stars. Our results for cool Ap stars were compared with vertical Cr distribution calculated by J.Babel using the diffusion theory and magnetically confined wind for Ap star 53 Cam (e.g. Astronomy and Astrophysics, v.258, p.449-463, 1992). It was shown that our observational material cannot be interpreted using hypothesis of depth-dependent microturbulent velocity. While the investigation of β CrB we demonstrate that obtained results insignificantly depend upon the uncertainties of the parameters of models of atmospheres.

ALMA observations of TiO2 around VY Canis Majoris

NASA Astrophysics Data System (ADS)

De Beck, E.; Vlemmings, W.; Muller, S.; Black, J. H.; O'Gorman, E.; Richards, A. M. S.; Baudry, A.; Maercker, M.; Decin, L.; Humphreys, E. M.

2015-08-01

Context. Titanium dioxide, TiO2, is a refractory species that could play a crucial role in the dust-condensation sequence around oxygen-rich evolved stars. To date, gas phase TiO2 has been detected only in the complex environment of the red supergiant VY CMa. Aims: We aim to constrain the distribution and excitation of TiO2 around VY CMa in order to clarify its role in dust formation. Methods: We analyse spectra and channel maps for TiO2 extracted from ALMA science verification data. Results: We detect 15 transitions of TiO2, and spatially resolve the emission for the first time. The maps demonstrate a highly clumpy, anisotropic outflow in which the TiO2 emission likely traces gas exposed to the stellar radiation field. An accelerating bipolar-like structure is found, oriented roughly east-west, of which the blue component runs into and breaks up around a solid continuum component. A distinct tail to the south-west is seen for some transitions, consistent with features seen in the optical and near-infrared. Conclusions: We find that a significant fraction of TiO2 remains in the gas phase outside the dust-formation zone and suggest that this species might play only a minor role in the dust-condensation process around extreme oxygen-rich evolved stars like VY CMa. Appendix A is available in electronic form at http://www.aanda.org

Neptune Rings and 1989N2

NASA Image and Video Library

1996-01-29

In this image from NASA's Voyager wide-angle taken on Aug. 23 1989, the two main rings of Neptune can be clearly seen. In the lower part of the frame the originally announced ring arc, consisting of three distinct features, is visible. This feature covers about 35 degrees of longitude and has yet to be radially resolved in Voyager images. From higher resolution images it is known that this region contains much more material than the diffuse belts seen elsewhere in its orbit, which seem to encircle the planet. This is consistent with the fact that ground-based observations of stellar occultations by the rings show them to be very broken and clumpy. The more sensitive wide-angle camera is revealing more widely distributed but fainter material. Each of these rings of material lies just outside of the orbit of a newly discovered moon. One of these moons, 1989N2, may be seen in the upper right corner. The moon is streaked by its orbital motion, whereas the stars in the frame are less smeared. The dark area around the bright moon and star are artifacts of the processing required to bring out the faint rings. This wide-angle image was taken from a range of 2 million kilometers (1.2 million miles), through the clear filter. http://photojournal.jpl.nasa.gov/catalog/PIA00053

Physical Properties of Sub-galactic Clumps at 0.5 ≤ Z ≤ 1.5 in the UVUDF

NASA Astrophysics Data System (ADS)

Soto, Emmaris; de Mello, Duilia F.; Rafelski, Marc; Gardner, Jonathan P.; Teplitz, Harry I.; Koekemoer, Anton M.; Ravindranath, Swara; Grogin, Norman A.; Scarlata, Claudia; Kurczynski, Peter; Gawiser, Eric

2017-03-01

We present an investigation of clumpy galaxies in the Hubble Ultra Deep Field at 0.5≤slant z≤slant 1.5 in the rest-frame far-ultraviolet (FUV) using Hubble Space Telescope Wide Field Camera 3 broadband imaging in F225W, F275W, and F336W. An analysis of 1404 galaxies yields 209 galaxies that host 403 kpc scale clumps. These host galaxies appear to be typical star-forming galaxies, with an average of 2 clumps per galaxy and reaching a maximum of 8 clumps. We measure the photometry of the clumps and determine the mass, age, and star formation rates (SFR) using the spectral energy distribution fitting code FAST. We find that clumps make an average contribution of 19% to the total rest-frame FUV flux of their host galaxy. Individually, clumps contribute a median of 5% to the host galaxy SFR and an average of ˜4% to the host galaxy mass, with total clump contributions to the host galaxy stellar mass ranging widely from lower than 1% up to 93%. Clumps in the outskirts of galaxies are typically younger, with higher SFRs, than clumps in the inner regions. The results are consistent with clump migration theories in which clumps form through violent gravitational instabilities in gas-rich turbulent disks, eventually migrate toward the center of the galaxies, and coalesce into the bulge.

The Multi-Layer Variable Absorbers in NGC 1365 Revealed by XMM-Newton and NuSTAR

NASA Technical Reports Server (NTRS)

Rivers, E.; Risaliti, G.; Walton, D. J.; Harrison, F.; Arevalo, P.; Baur, F. E.; Boggs, S. E.; Brenneman, L. W.; Brightman, M.; Zhang, W. W.

2015-01-01

Between 2012 July and 2013 February, NuSTAR and XMM-Newton performed four long-look joint observations of the type 1.8 Seyfert, NGC 1365. We have analyzed the variable absorption seen in these observations in order to characterize the geometry of the absorbing material. Two of the observations caught NGC 1365 in an unusually low absorption state, revealing complexity in the multi-layer absorber that had previously been hidden. We find the need for three distinct zones of neutral absorption in addition to the two zones of ionized absorption and the Compton-thick torus previously seen in this source. The most prominent absorber is likely associated with broad-line region clouds with column densities of around approximately 10 (sup 23) per square centimeter and a highly clumpy nature as evidenced by an occultation event in 2013 February. We also find evidence of a patchy absorber with a variable column around approximately 10 (sup 22) per square centimeter and a line-of-sight covering fraction of 0.3-0.9, which responds directly to the intrinsic source flux, possibly due to a wind geometry. A full-covering, constant absorber with a low column density of approximately 1 by 10 (sup 22) per square centimeter is also present, though the location of this low density haze is unknown.

Hypercat - Hypercube of AGN tori

NASA Astrophysics Data System (ADS)

Nikutta, Robert; Lopez-Rodriguez, Enrique; Ichikawa, Kohei; Levenson, Nancy A.; Packham, Christopher C.

2018-06-01

AGN unification and observations hold that a dusty torus obscures the central accretion engine along some lines of sight. SEDs of dust tori have been modeled for a long time, but resolved emission morphologies have not been studied in much detail, because resolved observations are only possible recently (VLTI,ALMA) and in the near future (TMT,ELT,GMT). Some observations challenge a simple torus model, because in several objects most of MIR emission appears to emanate from polar regions high above the equatorial plane, i.e. not where the dust supposedly resides.We introduce our software framework and hypercube of AGN tori (Hypercat) made with CLUMPY (www.clumpy.org), a large set of images (6 model parameters + wavelength) to facilitate studies of emission and dust morphologies. We make use of Hypercat to study the morphological properties of the emission and dust distributions as function of model parameters. We find that a simple clumpy torus can indeed produce 10-micron emission patterns extended in polar directions, with extension ratios compatible with those found in observations. We are able to constrain the range of parameters that produce such morphologies.

Not-So-Bright Bulbs

NASA Technical Reports Server (NTRS)

2008-01-01

This artist's concept shows the dimmest star-like bodies currently known -- twin brown dwarfs referred to as 2M 0939. The twins, which are about the same size, are drawn as if they were viewed close to one of the bodies.

Brown dwarfs are neither planets nor stars. They form like stars out of collapsing clouds of gas and dust, but they don't have enough mass to ignite nuclear burning in their cores and become full-blown stars. They are similar to Jupiter in that they are cool balls of gas, but they are warmer and heavier. Astronomers say that the universe is littered with these cosmic misfits, but because they are so dim, they are hard to find.

NASA's Spitzer Space Telescope is fitted with heat-seeking infrared eyes, which allow it to detect the minute glow of cool objects like brown dwarfs. Data from Spitzer and the Anglo-Australian Observatory in Australia together reveal that both of the brown dwarfs making up 2M 0939 share the title of dimmest known brown dwarfs. Their atmospheres are also among the coolest known for any brown dwarf (565 to 635 Kelvin or 560 to 680 degrees Fahrenheit).

The term 'brown dwarf' comes from the fact that these objects cool and change over time, and therefore do not have a definitive color. The 2M 0939 brown dwarfs, if we could see them directly, would have a dark magenta hue due to their cool temperatures and the presence of water, methane and ammonia gases in their atmospheres.

2M 0939's full name is 2MASS J09393548-2448279 after the partly NASA-funded infrared mission, the Two Micron All Sky Survey, or '2MASS,' which first detected the object in 1999.

Simulations of Early Structure Formation: Primordial Gas Clouds

NASA Astrophysics Data System (ADS)

Yoshida, Naoki; Abel, Tom; Hernquist, Lars; Sugiyama, Naoshi

2003-08-01

We use cosmological simulations to study the origin of primordial star-forming clouds in a ΛCDM universe, by following the formation of dark matter halos and the cooling of gas within them. To model the physics of chemically pristine gas, we employ a nonequilibrium treatment of the chemistry of nine species (e-, H, H+, He, He+, He++, H2, H+2, H-) and include cooling by molecular hydrogen. By considering cosmological volumes, we are able to study the statistical properties of primordial halos, and the high resolution of our simulations enables us to examine these objects in detail. In particular, we explore the hierarchical growth of bound structures forming at redshifts z~25-30 with total masses in the range ~105-106Msolar. We find that when the amount of molecular hydrogen in these objects reaches a critical level, cooling by rotational line emission is efficient, and dense clumps of cold gas form. We identify these ``gas clouds'' as sites for primordial star formation. In our simulations, the threshold for gas cloud formation by molecular cooling corresponds to a critical halo mass of ~5×105h-1Msolar, in agreement with earlier estimates, but with a weak dependence on redshift in the range z>16. The complex interplay between the gravitational formation of dark halos and the thermodynamic and chemical evolution of the gas clouds compromises analytic estimates of the critical H2 fraction. Dynamical heating from mass accretion and mergers opposes relatively inefficient cooling by molecular hydrogen, delaying the production of star-forming clouds in rapidly growing halos. We also investigate the effect of photodissociating ultraviolet radiation on the formation of primordial gas clouds. We consider two extreme cases, first by including a uniform radiation field in the optically thin limit and second by accounting for the maximum effect of gas self-shielding in virialized regions. For radiation with Lyman-Werner band flux J>10-23 ergs s-1 cm-2 Hz-1 sr-1, hydrogen molecules are rapidly dissociated, rendering gas cooling inefficient. In both the cases we consider, the overall effect can be described by computing an equilibrium H2 abundance for the radiation flux and defining an effective shielding factor. Based on our numerical results, we develop a semianalytic model of the formation of the first stars and demonstrate how it can be coupled with large N-body simulations to predict the star formation rate in the early universe.

Star in Deep Freeze Chills Theory, MIT Researchers Report

NASA Astrophysics Data System (ADS)

2001-09-01

CAMBRIDGE, Mass. -- Like a frozen turkey that just won't thaw, a strange star near the center of the Milky Way is surprising MIT experts and colleagues with its remarkably low temperature. The odd behavior is chilling current theories of stellar physics. A famously battered neutron star named KS 1731-260 appears no hotter than some of its tranquil brethren, despite enduring the heat of constant thermonuclear explosions with the force of billions of hydrogen bombs every second across a region only a few miles wide for the past 12 years. Dr. Rudi Wijnands, an astrophysicist at MIT's Center for Space Research, used the Chandra X-ray Observatory to measure the temperature of the neutron star at a very opportune moment, only months after the nuclear war apparently ended and the smoke cleared. He presented his team's findings September 5 in Washington, D.C. at a scientific conference entitled "Two Years of Science with Chandra." "Twelve years of constant thermonuclear explosions: One would think that would heat things up," said Wijnands. "This leaves us wondering whether some neutron stars are in the freezer for a much longer time than previously thought and consequently take a long time to heat up, or whether they cool down incredibly fast. Either explanation has profound implications for our field." Neutron stars are the dense, core remains of stars once many times more massive than our Sun. They are created in dazzling supernovas, in which the outer shell of the star explodes into space, and the core, containing about as much mass as the Sun, implodes and collapses into a sphere no wider than Cambridge, Massachusetts. Despite their tiny size, neutron stars are visible in several ways. One is through accretion. Neutron stars are a strong source of gravity. When they exist in binary star systems, such as KS 1731-260, they can attract the gas from what is often a "healthy" hydrogen-burning companion star (although the nature of KS 1731-260's companion is not clear.) Gas spirals around the neutron star and comes crashing down onto its surface, leading to nuclear explosions. The fury glows predominantly in X-ray radiation. Neutron star binaries can experience varying periods of active accretion. KS 1731-260's period was particularly prolonged, lasting from mid-1988 to the end of 2000. Other systems flare from week to week or year to year. When the stars are in quiescence, they are harder to detect. Chandra has the resolution and photon-collecting power and is in the right energy range to detect neutron stars glowing dimly in quiescence. KS 1731-260 is the first neutron star enduring such prolonged accretion to enter into quiescence during Chandra's reign. Other systems only dump matter onto the neutron star for a short period at a time, so no one expects much heating. The Chandra observation provided the first chance to test the theory of neutron star heating and cooling for a system with such prolonged episodes of accretion. The team found KS 1731-260 to be at least 10 times cooler than expected, a mere 3.5 million degrees. This is about the same temperature as neutron stars that get only a week's or month's worth of dumping. KS 1731-260 therefore appears to be much too cool, assuming that this and the other neutron stars have accretion episodes at roughly similar time intervals. So what happened to the heat? There are complicated models of neutrino cooling to explain rapid temperature decreases. But maybe KS 1731-260 was in a relative deep freeze before 1988 and took 12 years just to get to the temperature it is today. According to the popular model of neutron star heating and cooling, Wijnands calculated that KS 1731-260 would need to have been dormant (that is, no accretion) for over a thousand years to get so cold that 12 years of fury would only raise it to its current temperature. "Neutron stars in such close binaries are not known to stop accreting for periods longer that about 100 years," said Walter Lewin, a professor of physics also at MIT. "We may have identified a new type of neutron star system that can lie dormant for thousands of years. If so, there could be hundreds of these systems in our Galaxy." The next step is to use Chandra to take the temperature of scores of other neutron stars experiencing various phases of accretion and quiescence. Other members of the observation team include Jon Miller of MIT, Craig Markwardt from NASA Goddard Space Flight Center, and Michiel van der Klis from the University of Amsterdam. The observation was made with Chandra's Advanced CCD Imaging Spectrometer, which was conceived and developed for NASA by Pennsylvania State University and MIT.

Gone with the Wind: Watching Galaxy Transformation in Abell 2125

NASA Astrophysics Data System (ADS)

Keel, W.; Owen, F.; Ledlow, M.; Wang, D.

2003-12-01

Dense environments clearly foster the transformation of galaxies, but it has proven difficult to untangle the roles of various processes in cluster environments. We have found a uniquely strong case for ongoing stripping of gas from the galaxy C153 in Abell 2125. The cluster, at z=0.25, includes merging subsystems with a relative line-of-sight velocity near 2000 km/s. C153, identified using the VLA as a strong radio source powered by star formation, is the brightest cluster member with activity of this kind, and part of the less populous blueshifted grouping. Several lines of evidence indicate that it is being swept by a stripping event. (1) A tail of ionized gas is seen in [O II] emission, which extends at least 70 kpc toward the cluster core, coinciding with a soft X-ray feature seen in the Chandra observations reported by Wang et al. (2) HST WFPC2 images reveal disturbed and clumpy morphology, including luminous star-forming complexes and chaotic dust features. (3) The spectral energy distribution and Gemini GMOS absorption-line spectrum indicate a massive burst of star formation ≈ 108 years ago superimposed on an older and much fainter population. (4) The stellar and gas kinematics are decoupled, with multiple gas velocity systems including counter-rotating components. The large velocity difference between the galaxy and (most of the) intracluster medium may contribute to the signatures being more prominent than hitherto seen. The starburst age is consistent with estimates of the time since the closest encounter of the major subsystems during the cluster-level merger. We continue to explore whether a starburst outflow or tidal damage has added to the role of stripping by the ICM, and how star formation has proceeded in the gas after leaving the galaxy disk. This work was supported by NASA through HST grant GO-07279.01-96A, and by the NSF through facilities at NRAO, Kitt Peak, and Gemini-North.

Spatially Resolved Hα Maps and Sizes of 57 Strongly Star-forming Galaxies at z ~ 1 from 3D-HST: Evidence for Rapid Inside-out Assembly of Disk Galaxies

NASA Astrophysics Data System (ADS)

Nelson, Erica June; van Dokkum, Pieter G.; Brammer, Gabriel; Förster Schreiber, Natascha; Franx, Marijn; Fumagalli, Mattia; Patel, Shannon; Rix, Hans-Walter; Skelton, Rosalind E.; Bezanson, Rachel; Da Cunha, Elisabete; Kriek, Mariska; Labbe, Ivo; Lundgren, Britt; Quadri, Ryan; Schmidt, Kasper B.

2012-03-01

We investigate the buildup of galaxies at z ~ 1 using maps of Hα and stellar continuum emission for a sample of 57 galaxies with rest-frame Hα equivalent widths >100 Å in the 3D-HST grism survey. We find that the Hα emission broadly follows the rest-frame R-band light but that it is typically somewhat more extended and clumpy. We quantify the spatial distribution with the half-light radius. The median Hα effective radius re (Hα) is 4.2 ± 0.1 kpc but the sizes span a large range, from compact objects with re (Hα) ~ 1.0 kpc to extended disks with re (Hα) ~ 15 kpc. Comparing Hα sizes to continuum sizes, we find =1.3 ± 0.1 for the full sample. That is, star formation, as traced by Hα, typically occurs out to larger radii than the rest-frame R-band stellar continuum; galaxies are growing their radii and building up from the inside out. This effect appears to be somewhat more pronounced for the largest galaxies. Using the measured Hα sizes, we derive star formation rate surface densities, ΣSFR. We find that ΣSFR ranges from ~0.05 M ⊙ yr-1 kpc-2 for the largest galaxies to ~5 M ⊙ yr-1 kpc-2 for the smallest galaxies, implying a large range in physical conditions in rapidly star-forming z ~ 1 galaxies. Finally, we infer that all galaxies in the sample have very high gas mass fractions and stellar mass doubling times <500 Myr. Although other explanations are also possible, a straightforward interpretation is that we are simultaneously witnessing the rapid formation of compact bulges and large disks at z ~ 1.

NuSTAR View of the Black Hole Wind in the Galaxy Merger IRAS F11119+3257

NASA Astrophysics Data System (ADS)

Tombesi, F.; Veilleux, S.; Meléndez, M.; Lohfink, A.; Reeves, J. N.; Piconcelli, E.; Fiore, F.; Feruglio, C.

2017-12-01

Galactic winds driven by active galactic nuclei (AGNs) have been invoked to play a fundamental role in the co-evolution between supermassive black holes and their host galaxies. Finding observational evidence of such feedback mechanisms is of crucial importance and it requires a multi-wavelength approach in order to compare winds at different scales and phases. In Tombesi et al., we reported the detection of a powerful ultra-fast outflow (UFO) in the Suzaku X-ray spectrum of the ultra-luminous infrared galaxy IRAS F11119+3257. The comparison with a galaxy-scale OH molecular outflow observed with Herschel in the same source supported the energy-conserving scenario for AGN feedback. The main objective of this work is to perform an independent check of the Suzaku results using the higher sensitivity and wider X-ray continuum coverage of NuSTAR. We clearly detect a highly ionized Fe K UFO in the 100 ks NuSTAR spectrum with parameters N H = (3.2 ± 1.5) × 1024 cm-2, log ξ = {4.0}-0.3+1.2 erg s-1 cm, and {v}{out}={0.253}-0.118+0.061c. The launching radius is likely at a distance of r ≥ 16r s from the black hole. The mass outflow rate is in the range of {\\dot{M}}{out} ≃ 0.5-2 M ⊙ yr-1. The UFO momentum rate and power are {\\dot{P}}{out} ≃ 0.5-2 L AGN/c and {\\dot{E}}{out} ≃ 7%-27% L AGN, respectively. The UFO parameters are consistent between the 2013 Suzaku and the 2015 NuSTAR observations. Only the column density is found to be variable, possibly suggesting a clumpy wind. The comparison with the energetics of molecular outflows estimated in infrared and millimeter wavelengths support a connection between the nuclear and galaxy-scale winds in luminous AGNs.

Galaxy And Mass Assembly: resolving the role of environment in galaxy evolution

NASA Astrophysics Data System (ADS)

Brough, S.; Croom, S.; Sharp, R.; Hopkins, A. M.; Taylor, E. N.; Baldry, I. K.; Gunawardhana, M. L. P.; Liske, J.; Norberg, P.; Robotham, A. S. G.; Bauer, A. E.; Bland-Hawthorn, J.; Colless, M.; Foster, C.; Kelvin, L. S.; Lara-Lopez, M. A.; López-Sánchez, Á. R.; Loveday, J.; Owers, M.; Pimbblet, K. A.; Prescott, M.

2013-11-01

We present observations of 18 galaxies from the Galaxy And Mass Assembly (GAMA) survey made with the SPIRAL optical integral field unit (IFU) on the Anglo-Australian Telescope. The galaxies are selected to have a narrow range in stellar mass (6 × 109 < M* < 2 × 1010 M⊙) in order to focus on the effects of environment. Local galaxy environments are measured quantitatively using fifth nearest neighbour surface densities. We find that the total star formation rates (SFR) measured from the IFU data are consistent with total SFRs measured from aperture correcting either GAMA or Sloan Digital Sky Survey single-fibre observations. The mean differences are SFRGAMA/SFRIFU = 1.26 ± 0.23, σ = 0.90 and for the Sloan Digital Sky Survey we similarly find SFRBrinchmann/SFRIFU = 1.34 ± 0.17, σ = 0.67. Examining the relationships with environment, we find that off-centre and clumpy Hα emission is not significantly dependent on environment, being present in 2/7 (29^{+20}_{-11} per cent) galaxies in high-density environments (>0.77 Mpc-2), and 5/11 (45^{+15}_{-13} per cent) galaxies in low-density environments (<0.77 Mpc-2). We find a weak but not significant relationship of the total SFRs of star-forming galaxies with environment. Due to the size of our sample and the scatter observed we do not draw a definitive conclusion about a possible SFR dependence on environment. Examining the spatial distribution of the Hα emission, we find no evidence for a change in shape or amplitude of the radial profile of star-forming galaxies with environment. If these observations are borne out in larger samples, this would infer that any environment-driven star formation suppression must either act very rapidly (the `infall-and-quench' model) or that galaxies must evolve in a density-dependent manner (an `in situ evolution' model).

Cool Dwarfs 1o-7

NASA Astrophysics Data System (ADS)

Ambruster, Carol W.

Most of the cool dwarfs in the interesting age range 10^7-10^8 yr are too faint for IUE, yet such stars are critically important from the viewpoint of stellar evolution. Among stars of this age are the Pleiades K dwarfs, some of which appear to be on the main sequence, and some of which are still arriving there. Up until last year, only 2 stars in this age range had been observed by IUE, both recently: HD 36705 (AB Dor) and HD 17433. Three more stars were identified by the present investigators and observed with IUE during the past (11th) year: HD 129333, a single, nearby solar-type GOV star; HD 82558, a rapidly rotating, single, K2V star; and Ross 137B, the M dwarf common proper motion companion to AB Dor. We have since identified 5 more stars between 10^7 and 10^8 years old that are bright enough to be observed by IUE. They are physically associated, but distant, companions to main sequence O and B stars, identified in the survey of Lindroos (1986). Their ages are thus determined by the short main sequence lifetimes of the hot primaries. Rotational velocities are not yet known for our 5 proposed targets; we will be obtaining these and other data in the coming year. We therefore request time for basic IUE observations of these stars, an LWP-lo, LWP-hi and SWP-lo, for each star. This will ensure that crucial basic fluxes are in the IUE archives, should the satellite die in the coming year. Furthermore these data are immediately useful in filling the gap in the exhaustive study by Simon, Herbig and Boesgaard (1985) of the evolution of TR and chromospheric activity with age. More in-depth coverage will be proposed next year.

Chromospherically Active Stars in the RAVE Survey. II. Young Dwarfs in the Solar Neighborhood

NASA Astrophysics Data System (ADS)

Žerjal, M.; Zwitter, T.; Matijevič, G.; Grebel, E. K.; Kordopatis, G.; Munari, U.; Seabroke, G.; Steinmetz, M.; Wojno, J.; Bienaymé, O.; Bland-Hawthorn, J.; Conrad, C.; Freeman, K. C.; Gibson, B. K.; Gilmore, G.; Kunder, A.; Navarro, J.; Parker, Q. A.; Reid, W.; Siviero, A.; Watson, F. G.; Wyse, R. F. G.

2017-01-01

A large sample of over 38,000 chromospherically active candidate solar-like stars and cooler dwarfs from the RAVE survey is addressed in this paper. An improved activity identification with respect to the previous study was introduced to build a catalog of field stars in the solar neighborhood with an excess emission flux in the calcium infrared triplet wavelength region. The central result of this work is the calibration of the age-activity relation for main-sequence dwarfs in a range from a few 10 {Myr} up to a few Gyr. It enabled an order of magnitude age estimation of the entire active sample. Almost 15,000 stars are shown to be younger than 1 {Gyr} and ˜2000 younger than 100 {Myr}. The young age of the most active stars is confirmed by their position off the main sequence in the J - K versus {N}{UV}-V diagram showing strong ultraviolet excess, mid-infrared excess in the J - K versus {W}1-{W}2 diagram, and very cool temperatures (J-K> 0.7). They overlap with the reference pre-main-sequence RAVE stars often displaying X-ray emission. The activity level increasing with the color reveals their different nature from the solar-like stars and probably represents an underlying dynamo-generating magnetic fields in cool stars. Of the RAVE objects from DR5, 50% are found in the TGAS catalog and supplemented with accurate parallaxes and proper motions by Gaia. This makes the database of a large number of young stars in a combination with RAVE’s radial velocities directly useful as a tracer of the very recent large-scale star formation history in the solar neighborhood. The data are available online in the Vizier database.

Strange stars

NASA Technical Reports Server (NTRS)

Alcock, Charles; Farhi, Edward; Olinto, Angela

1986-01-01

Strange matter, a form of quark matter that is postulated to be absolute stable, may be the true ground stage of the hadrons. If this hypothesis is correct, neutron stars may convert to 'strange stars'. The mass-radius relation for strange stars is very different from that of neutron stars; there is no minimum mass, and for mass of 1 solar mass or less, mass is proportional to the cube of the radius. For masses between 1 solar mass and 2 solar masses, the radii of strange stars are about 10 km, as for neutron stars. Strange stars may have an exposed quark surface, which is capable of radiating at rates greatly exceeding the Eddington limit, but has a low emissivity for X-ray photons. The stars may have a thin crust with the same composition as the preneutron drip outer layer of a conventional neutron star crust. Strange stars cool efficiently via neutrino emission.

The Emission and Distribution of Dust of the Torus of NGC 1068

NASA Astrophysics Data System (ADS)

Lopez-Rodriguez, Enrique; Fuller, Lindsay; Alonso-Herrero, Almudena; Efstathiou, Andreas; Ichikawa, Kohei; Levenson, Nancy A.; Packham, Chris; Radomski, James; Ramos Almeida, Cristina; Benford, Dominic J.; Berthoud, Marc; Hamilton, Ryan; Harper, Doyal; Kovávcs, Attila; Santos, Fabio P.; Staguhn, J.; Herter, Terry

2018-06-01

We present observations of NGC 1068 covering the 19.7–53.0 μm wavelength range using FORCAST and HAWC+ on board SOFIA. Using these observations, high-angular-resolution infrared (IR) and submillimeter observations, we find an observational turnover of the torus emission in the 30–40 μm wavelength range with a characteristic temperature of 70–100 K. This component is clearly different from the diffuse extended emission in the narrow line and star formation regions at 10–100 μm within the central 700 pc. We compute 2.2–432 μm 2D images using the best inferred CLUMPY torus model based on several nuclear spectral energy distribution (SED) coverages. We find that when 1–20 μm SED is used, the inferred result gives a small torus size (<4 pc radius) and a steep radial dust distribution. The computed torus using the 1–432 μm SED provides comparable torus sizes, {5.1}-0.4+0.4 pc radius, and morphology to the recently resolved 432 μm Atacama Large Millimeter Array observations. This result indicates that the 1–20 μm wavelength range is not able to probe the full extent of the torus. The characterization of the turnover emission of the torus using the 30–60 μm wavelength range is sensitive to the detection of cold dust in the torus. The morphology of the dust emission in our 2D image at 432 μm is spatially coincident with the cloud distribution, while the morphology of the emission in the 1–20 μm wavelength range shows an elongated morphology perpendicular to the cloud distribution. We find that our 2D CLUMPY torus image at 12 μm can produce comparable results to those observed using IR interferometry.

Clumpy filaments of the Chamaeleon I cloud: C18O mapping with the SEST

NASA Astrophysics Data System (ADS)

Haikala, L. K.; Harju, J.; Mattila, K.; Toriseva, M.

2005-02-01

The Chamaeleon I dark cloud (Cha I) has been mapped in C18O with an angular resolution of 1 arcmin using the SEST telescope. The large scale structures previously observed with lower spatial resolution in the cloud turn into a network of clumpy filaments. The automatic Clumpfind routine developed by \\cite{williams1994} is used to identify individual clumps in a consistent way. Altogether 71 clumps were found and the total mass of these clumps is 230 M⊙. The dense ``cores'' detected with the NANTEN telescope (\\cite{mizuno1999}) and the very cold cores detected in the ISOPHOT serendipity survey (\\cite{toth2000}) form parts of these filaments but decompose into numerous ``clumps''. The filaments are preferentially oriented at right angles to the large-scale magnetic field in the region. We discuss the cloud structure, the physical characteristics of the clumps and the distribution of young stars. The observed clump mass spectrum is compared with the predictions of the turbulent fragmentation model of \\cite{padoan2002}. Agreement is found if fragmentation has been driven by very large-scale hypersonic turbulence, and if by now it has had time to dissipate into modestly supersonic turbulence in the interclump gas. According to numerical simulations, large-scale turbulence should have resulted in filamentary structures as seen in Cha I. The well-oriented magnetic field does not, however, support this picture, but suggests magnetically steered large-scale collapse. The origin of filaments and clumps in Cha I is thus controversial. A possible solution is that the characterization of the driving turbulence fails and that in fact different processes have been effective on small and large scales in this cloud. Based on observations collected at the European Southern Observatory, La Silla, Chile. FITS files are only available in electronic form at http://www.edpsciences.org

Wing galaxies: A formation mechanism of the clumpy irregular galaxy Markarian 297

NASA Technical Reports Server (NTRS)

Taniguchi, Yoshiaki; Noguchi, Masafumi

1990-01-01

In order to contribute to an understanding of collision-induced starburst activities, the authors present a detailed case study on the starburst galaxy Markarian 297 (= NGC 6052 = Arp 209; hereafter Mrk 297). This galaxy is classified as a clumpy irregular galaxy (CIG) according to its morphological properties (cf. Heidmann, 1987). Two major clumps and many small clumps are observed in the entire region of Mrk 297 (Hecquet, Coupinot, and Maucherat 1987). The overall morphology of Mrk 297 is highly chaotic and thus it seems difficult to determine possible orbits of galaxy-galaxy collision. However, the authors have serendipitously found a possible orbit during a course of numerical simulations for a radial-penetration collision between galaxies. The radial-penetration collision means that an intruder penetrates a target galaxy radially passing by its nucleus. This kind of collision is known to explain a formation mechanism of ripples around disk galaxies (Wallin and Struck-Marcell 1988). Here, the authors show that the radial-penetration collision between galaxies successfully explains both overall morphological and kinematical properties of Mrk 297. The authors made two kinds of numerical simulations for Mrk 297. One is N-body (1x10(exp 4) particles) simulations in which effects of self gravity of the stellar disk are taken into account. These simulations are used to study detailed morphological feature of Mrk 297. The response of gas clouds are also investigated in order to estimate star formation rates in such collisions. The other is test-particle simulations, which are utilized to obtain a rough picture of Mrk 297 and to analyze the velocity field of Mrk 297. The techniques of the numerical simulations are the same as those in Noguchi (1988) and Noguchi and Ishibashi (1986). In the present model, an intruding galaxy with the same mass of a target galaxy moves on a rectilinear orbit which passes the center of the target.

Direct URCA process in neutron stars

NASA Technical Reports Server (NTRS)

Lattimer, James M.; Prakash, Madappa; Pethick, C. J.; Haensel, Pawel

1991-01-01

It is shown that the direct URCA process can occur in neutron stars if the proton concentration exceeds some critical value in the range 11-15 percent. The proton concentration, which is determined by the poorly known symmetry energy of matter above nuclear density, exceeds the critical value in many current calculations. If it occurs, the direct URCA process enhances neutrino emission and neutron star cooling rates by a large factor compared to any process considered previously.

The Planck Dusty Gravitationally Enhanced subMillimeter Sources (GEMS)

NASA Astrophysics Data System (ADS)

Frye, Brenda

2015-10-01

The brightest, strongly lensed high-redshift galaxies are veritable gems to study intense star formation in the early Universe. How do the high and irregular clumpy stellar and gas mass surface densities, strong radiation fields, and high turbulence regulate the rapid growth of these galaxies? We will use HST/WFC3 to investigate the stellar component of 6 of the brightest high-redshift sub-millimeter galaxies on the sky, which were recently discovered with the Planck all-sky survey. All are giant arcs or partial Einstein rings with angular sizes up to 17 in shallow CFHT K-band or Spitzer/IRAC imaging. FIR luminosities are 10^13-14 L_sun, with dust SEDs strongly dominated by intense star formation near the maximum possible rates (maximal starbursts). All have spectroscopic redshifts z=2.2-3.6 and magnification factors >/=20. We already have multiwavelength data sets to characterize their gas and dust column densities and kinematics, and propose here to acquire deep, high-resolution rest-frame optical imaging to study the stellar populations and morphologies. With WFC3 imaging in F110W & F160W we will: (1) constrain the stellar morphologies, ages, and mass-to-light ratios (2) Identify clumps and measure their properties to test several clump formation scenarios(3) Enhance our on-going lens modeling through the most accurate positions, morphologies and colorsOnly the brightest of the arc clumps are visible from the ground in the NIR. To register the flux along the full extent of the arcs, and importantly to probe individual star forming regions of the size of 30 Dor at z 2-3 in the brightest high-z sub-mm galaxies requires HST.

SHORT-LIVED STAR-FORMING GIANT CLUMPS IN COSMOLOGICAL SIMULATIONS OF z Almost-Equal-To 2 DISKS

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

Genel, Shy; Genzel, Reinhard; Foerster Schreiber, Natascha M.

Many observed massive star-forming z Almost-Equal-To 2 galaxies are large disks that exhibit irregular morphologies, with Almost-Equal-To 1 kpc, Almost-Equal-To 10{sup 8}-10{sup 10}M{sub o-dot} clumps. We present the largest sample to date of high-resolution cosmological smoothed particle hydrodynamics simulations that zoom-in on the formation of individual M{sub *} Almost-Equal-To 10{sup 10.5}M{sub o-dot} galaxies in Almost-Equal-To 10{sup 12}M{sub o-dot} halos at z Almost-Equal-To 2. Our code includes strong stellar feedback parameterized as momentum-driven galactic winds. This model reproduces many characteristic features of this observed class of galaxies, such as their clumpy morphologies, smooth and monotonic velocity gradients, high gas fractions (f{submore » g} Almost-Equal-To 50%), and high specific star formation rates ({approx}>1 Gyr{sup -1}). In accord with recent models, giant clumps (M{sub clump} Almost-Equal-To (5 Multiplication-Sign 10{sup 8}-10{sup 9})M{sub o-dot}) form in situ via gravitational instabilities. However, the galactic winds are critical for their subsequent evolution. The giant clumps we obtain are short-lived and are disrupted by wind-driven mass loss. They do not virialize or migrate to the galaxy centers as suggested in recent work neglecting strong winds. By phenomenologically implementing the winds that are observed from high-redshift galaxies and in particular from individual clumps, our simulations reproduce well new observational constraints on clump kinematics and clump ages. In particular, the observation that older clumps appear closer to their galaxy centers is reproduced in our simulations, as a result of inside-out formation of the disks rather than inward clump migration.« less

Neutral ISM, Ly α , and Lyman-continuum in the Nearby Starburst Haro 11

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

Rivera-Thorsen, T. Emil; Östlin, Göran; Hayes, Matthew

2017-03-01

Star-forming galaxies are believed to be a major source of Lyman continuum (LyC) radiation responsible for reionizing the early universe. Direct observations of escaping ionizing radiation have however been sparse and with low escape fractions. In the local universe, only 10 emitters have been observed, with typical escape fractions of a few percent. The mechanisms regulating this escape need to be strongly evolving with redshift in order to account for the epoch of reionization. Gas content and star formation feedback are among the main suspects, known to both regulate neutral gas coverage and evolve with cosmic time. In this paper,more » we reanalyze Hubble Space Telescope ( HST )-Cosmic Origins Spectrograph (COS) spectrocopy of the first detected local LyC leaker, Haro 11. We examine the connection between LyC leakage and Ly α line shape, and feedback-influenced neutral interstellar medium (ISM) properties like kinematics and gas distribution. We discuss the two extremes of an optically thin, density bounded ISM and a riddled, optically thick, ionization bounded ISM, and how Haro 11 fits into theoretical predictions. We find that the most likely ISM model is a clumpy neutral medium embedded in a highly ionized medium with a combined covering fraction of unity and a residual neutral gas column density in the ionized medium high enough to be optically thick to Ly α , but low enough to be at least partly transparent to LyC and undetected in Si ii. This suggests that star formation feedback and galaxy-scale interaction events play a major role in opening passageways for ionizing radiation through the neutral medium.« less

Multiwavelength observations of NaSt1 (WR 122): equatorial mass loss and X-rays from an interacting Wolf-Rayet binary

NASA Astrophysics Data System (ADS)

Mauerhan, Jon; Smith, Nathan; Van Dyk, Schuyler D.; Morzinski, Katie M.; Close, Laird M.; Hinz, Philip M.; Males, Jared R.; Rodigas, Timothy J.

2015-07-01

NaSt1 (aka Wolf-Rayet 122) is a peculiar emission-line star embedded in an extended nebula of [N II] emission with a compact dusty core. The object was previously characterized as a Wolf-Rayet (WR) star cloaked in an opaque nebula of CNO-processed material, perhaps analogous to η Car and its Homunculus nebula, albeit with a hotter central source. To discern the morphology of the [N II] nebula we performed narrow-band imaging using the Hubble Space Telescope and Wide-field Camera 3. The images reveal that the nebula has a disc-like geometry tilted ≈12° from edge-on, composed of a bright central ellipsoid surrounded by a larger clumpy ring. Ground-based spectroscopy reveals radial velocity structure (±10 km s-1) near the outer portions of the nebula's major axis, which is likely to be the imprint of outflowing gas. Near-infrared adaptive-optics imaging with Magellan AO has resolved a compact ellipsoid of Ks-band emission aligned with the larger [N II] nebula, which we suspect is the result of scattered He I line emission (λ2.06 μm). Observations with the Chandra X-ray Observatory have revealed an X-ray point source at the core of the nebula that is heavily absorbed at energies <1 keV and has properties consistent with WR stars and colliding-wind binaries. We suggest that NaSt1 is a WR binary embedded in an equatorial outflow that formed as the result of non-conservative mass transfer. NaSt1 thus appears to be a rare and important example of a stripped-envelope WR forming through binary interaction, caught in the brief Roche lobe overflow phase.

Inner Disk Structure and Transport Mechanisms in the Transitional Disk around T Cha

NASA Astrophysics Data System (ADS)

Brown, Alexander

2017-08-01

To better understand how Earth-like planets form around low-mass stars, we propose to study the UV (HST), X-ray (XMM), and optical (LCOGT) variability of the young star T Cha. This variability is caused by obscuration of the star by clumpy material in the rim of its inner disk. Changing sight lines through the disk allow measurement of the temperature and column density of both molecular and atomic gas and the physical properties of the dust grains in the well-mixed inner disk, as well as determining the gas-to-dust ratio. The gas-to-dust ratio affects planetesimal growth and disk stability but is difficult to measure in local regions of disks. Three 5 orbit visits, separated by 3-7 days, are required for use of analysis techniques comprising both differential pair-method comparison of spectra with differing A_v (particularly important for determining the dust extinction curve, A_lambda, where removal of the foreground extinction requires multiple epochs) and detailed spectral fitting of gas absorption features at each epoch. The inner disk of T Cha is particularly interesting, because T Cha has a transitional disk with a large gap at 0.2-15 AU in the dust disk and allows study of the gas and dust structure in the terrestrial planet formation zone during this important rapid phase of protoplanetary disk evolution. Characterizing the high energy (UV/X-ray) radiation field is also essential for in-depth studies of the disk in other spectral regions. Results from these observations will have wide relevance to the modeling and understanding of protoplanetary disk structure and evolution, and the complex gas and dust physics and chemistry in disk surface layers.

Spitzer Spies Spectacular Sombrero

NASA Image and Video Library

2005-05-04

NASA's Spitzer Space Telescope set its infrared eyes on one of the most famous objects in the sky, Messier 104, also called the Sombrero galaxy. In this striking infrared picture, Spitzer sees an exciting new view of a galaxy that in visible light has been likened to a "sombrero," but here looks more like a "bulls-eye." Recent observations using Spitzer's infrared array camera uncovered the bright, smooth ring of dust circling the galaxy, seen in red. In visible light, because this galaxy is seen nearly edge-on, only the near rim of dust can be clearly seen in silhouette. Spitzer's full view shows the disk is warped, which is often the result of a gravitational encounter with another galaxy, and clumpy areas spotted in the far edges of the ring indicate young star-forming regions. Spitzer's infrared view of the starlight from this galaxy, seen in blue, can pierce through obscuring murky dust that dominates in visible light. As a result, the full extent of the bulge of stars and an otherwise hidden disk of stars within the dust ring are easily seen. The Sombrero galaxy is located some 28 million light years away. Viewed from Earth, it is just six degrees south of its equatorial plane. Spitzer detected infrared emission not only from the ring, but from the center of the galaxy too, where there is a huge black hole, believed to be a billion times more massive than our Sun. This picture is composed of four images taken at 3.6 (blue), 4.5 (green), 5.8 (orange), and 8.0 (red) microns. The contribution from starlight (measured at 3.6 microns) has been subtracted from the 5.8 and 8-micron images to enhance the visibility of the dust features. http://photojournal.jpl.nasa.gov/catalog/PIA07899

IC 751: A New Changing Look AGN Discovered by NuSTAR

NASA Technical Reports Server (NTRS)

Ricci, C.; Bauer, F. E.; Arevalo, P.; Boggs, S.; Brandt, W. N.; Christensen, F. E.; Craig, W. W.; Ghandi, P.; Hailey, C. J.; Harrison, F. A.;

Spectroscopy and nonthermal processes

NASA Technical Reports Server (NTRS)

Querci, Monique

1987-01-01

Stellar spectra are analyzed to determine nonthermal processes for cool stars. A shock wave crossing model is supported by a study of the behavior of absorption and emission spectra. The shock waves are attributed to atmospheric kinetics. Circumstellar spectral lines are studied for information about gaseous circumstellar layers. The description of stellar envelopes is carried on through circumstellar dust. Characteristic properties of polarization in the dust are described in the case of specific stars, emphasizing narrowband observations in Mira, semiregular, and supergiant stars. Finally, the direct approach to measuring the angular diameters of stars and mapping the distribution of circumstellar dust and gas by lunar occultation or interferometry is discussed, using two prototype stars, an M supergiant and a dusty carbon star.

Comments on stellar boundary cooling and the reality of supermetallicity

NASA Technical Reports Server (NTRS)

Deming, D.

1980-01-01

The paper discusses the 'super-metal-rich' (SMR) stars and reexamines Peterson's analysis of the SMR prototype mu Leo (1978) with regard to a postulated error in continuum error. Model atmospheres are used to compute theoretical equivalent widths and to explore the sensitivity of these widths to metallicity, temperature, surface gravity, and microturbulence. It is shown that Peterson's results are sensitive to continuum placement, and that her data does not indicate that the temperature gradient is steeper in mu Leo than in normal giants. It is concluded that the SMR stars are very metal rich and are also somewhat boundary cooled, possibly due to high metallicity.

TIME-DEPENDENT, COMPOSITIONALLY DRIVEN CONVECTION IN THE OCEANS OF ACCRETING NEUTRON STARS

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

Medin, Zach; Cumming, Andrew, E-mail: zmedin@lanl.gov, E-mail: cumming@physics.mcgill.ca

2015-03-20

We discuss the effect of convection driven by chemical separation at the ocean-crust boundary of accreting neutron stars. We extend the steady-state results of Medin and Cumming to transient accretors, by considering the time-dependent cases of heating during accretion outbursts and cooling during quiescence. During accretion outbursts, inward heat transport has only a small effect on the temperature profile in the outer layers until the ocean is strongly enriched in light elements, a process that takes hundreds of years to complete. During quiescence, however, inward heat transport rapidly cools the outer layers of the ocean while keeping the inner layersmore » hot. We find that this leads to a sharp drop in surface emission at around a week followed by a gradual recovery as cooling becomes dominated by the crust. Such a dip should be observable in the light curves of these neutron star transients, if enough data is taken at a few days to a month after the end of accretion. If such a dip is definitively observed, it will provide strong constraints on the chemical composition of the ocean and outer crust.« less

Mining the HST "Advanced Spectral Library (ASTRAL)": The Evolution of Winds from non-coronal to hybrid giant stars

NASA Astrophysics Data System (ADS)

Nielsen, Krister E.; Carpenter, Ken G.; Kober, Gladys V.; Rau, Gioia

2018-01-01

The HST/STIS treasury program ASTRAL enables investigations of the character and dynamics of the wind and chromosphere of cool stars, using high quality spectral data. This paper shows how the wind features change with spectral class by comparing the non-coronal objects (Alpha Ori, Gamma Cru) with the hybrid stars (Gamma Dra, Beta Gem). In particular we study the intrinsic strength variation of the numerous FeII profiles observed in the near-ultraviolet HST spectrum that are sensitive to the wind opacity, turbulence and flow velocity. The FeII relative emission strength and wavelengths shifts between the absorption and emission components reflects the acceleration of the wind from the base of the chromosphere. We present the analysis of the outflowing wind characteristics when transitioning from the cool non-coronal objects toward the warmer objects with chromospheric emission from significantly hotter environments.

The outer atmospheres of cool M giants: High-dispersion ultraviolet spectra of Rho Per, 2 Cen, and g Her

NASA Technical Reports Server (NTRS)

Eaton, Joel A.; Johnson, Hollis R.

1986-01-01

Long duration IUE spectra were obtained to extend coverage of cool giants studied in the ultraviolet at high dispersion to M6. The chromospheric spectra of the three stars, which consist of a profusion of Fe II lines and a few lines of Mg II, Mg I, Al II, C II, C I, Cr II, and Fe I, are remarkably similar, both among themselves and with respect to stars of earlier spectral type. These lines present a picture of a warm chromosphere that is static in the average but may be far from uniform in density and ionization. The Mg II emission lines of 2 Cen show 2 unresolved absorption components, the shorter at the velocity of the local interstellar medium. The longer is blueshifted from the star by 12 to 18 km/sec and must be one of very few observed shell lines uncontaminated by interstellar absorption.

The Molecular and Dust Envelope of HD 56126

NASA Astrophysics Data System (ADS)

Meixner, M.; Zalucha, A.; Ueta, T.; Fong, D.; Justtanont, K.

2004-10-01

We present millimeter interferometry images of the CO J=1-0 line emission arising in the circumstellar envelope of HD 56126 (=IRAS 07134+1005), which is one of the best-studied 21 μm proto-planetary nebulae (PPNs). The CO emission extends from 1.2" to 7" in radius from the central star and appears consistent with a simple expanding envelope, as expected for a post-AGB star. The CO envelope is very clumpy with no apparent fast wind to explain these microstructures that must have arisen during the AGB mass loss. We quantitatively model the molecular envelope using a radiative transfer code that we have modified for detached shells. Our best-fit model reveals that two sequential winds created the circumstellar envelope of HD 56126: an AGB wind that lasted 6500 yr with a mass-loss rate of 5.1×10-6 Msolar yr-1 and a more intense superwind that lasted 840 yr with a mass-loss rate of 3×10-5 Msolar yr-1 and that ended the star's life on the AGB 1240 yr ago. The total mass of this envelope is 0.059 Msolar, which indicates a lower limit progenitor mass for the system of 0.66 Msolar, quite reasonable for this low-metallicity star that probably resides in the thick disk of the Galaxy. Comparison with images of the dust emission reveals a structure similar to that of the gas in the inner regions. Using 2-D UST, we model the dust emission of this source so that the model is consistent with the CO emission model and find a total dust mass of 7.8×10-4 Msolar, a superwind-dust mass-loss rate of 1.9×10-7 Msolar yr-1 and an AGB-dust mass-loss rate of 9.6×10-8 Msolar yr-1. We derive an average gas-to-dust mass ratio of 75, which is consistent with ISM values but low for what most consider for carbon stars. Our results indicate that TiC nanocrystals are probably not the carrier of the 21 μm feature.

(C-12)O emission from the envelopes of cool stars in the solar neighborhood

NASA Technical Reports Server (NTRS)

Margulis, M.; Van Blerkom, D. J.; Snell, R. L.; Kleinmann, S. G.

1990-01-01

Results are presented on observations of the CO J = 1-0 line emission from all M giants, S stars, and C stars listed in the Two-Micron Sky Survey having strong FIR emission and lying north of delta = -10 deg. The data from this survey and other data for C and S stars show that the line profiles of these stars look like flattened parabolas and have roughly the same shape for different stars. In contrast, the shapes of the spectral lines from giant M stars are diverse, ranging from triangular to spiked and asymmetric, suggesting that the envelopes of M stars have complex kinematics and structure. The outflow velocities inferred from the line profiles of the stars surveyed span a range of more than an order of magnitude, with the velocities of C stars correlating with IR color.

Synchrotron cooling and annihilation of an e/+/-e/-/ plasma - The radiation mechanism for the 5 March, 1979 transient

NASA Technical Reports Server (NTRS)

Ramaty, R.; Bussard, R. W.; Lingenfelter, R. E.

1981-01-01

Positron-electron pair radiation is examined as a mechanism that could be responsible for the impulsive phase emission of the 5 March, 1979 transient. Synchrotron cooling and subsequent annihilation of the pairs can account for the energy spectrum, the very high brightness, and the 0.4 MeV feature observed from this transient, whose source is likely to be a neutron star in the supernova remnant N49 in the Large Magellanic Cloud. In this model, the observed radiation is produced in the skin layer of a hot, radiation-dominated pair atmosphere, probably confined to the vicinity of the neutron star by a strong magnetic field. In this layer, about 10 to the 12th generations of pairs are formed (by photon-photon collisions), cooled and annihilated during the 0.15 s duration of the impulsive phase.

Extreme emission-line galaxies out to z ~ 1 in zCOSMOS. I. Sample and characterization of global properties

NASA Astrophysics Data System (ADS)

Amorín, R.; Pérez-Montero, E.; Contini, T.; Vílchez, J. M.; Bolzonella, M.; Tasca, L. A. M.; Lamareille, F.; Zamorani, G.; Maier, C.; Carollo, C. M.; Kneib, J.-P.; Le Fèvre, O.; Lilly, S.; Mainieri, V.; Renzini, A.; Scodeggio, M.; Bardelli, S.; Bongiorno, A.; Caputi, K.; Cucciati, O.; de la Torre, S.; de Ravel, L.; Franzetti, P.; Garilli, B.; Iovino, A.; Kampczyk, P.; Knobel, C.; Kovač, K.; Le Borgne, J.-F.; Le Brun, V.; Mignoli, M.; Pellò, R.; Peng, Y.; Presotto, V.; Ricciardelli, E.; Silverman, J. D.; Tanaka, M.; Tresse, L.; Vergani, D.; Zucca, E.

2015-06-01

Context. The study of large and representative samples of low-metallicity star-forming galaxies at different cosmic epochs is of great interest to the detailed understanding of the assembly history and evolution of low-mass galaxies. Aims: We present a thorough characterization of a large sample of 183 extreme emission-line galaxies (EELGs) at redshift 0.11 ≤ z ≤ 0.93 selected from the 20k zCOSMOS bright survey because of their unusually large emission line equivalent widths. Methods: We use multiwavelength COSMOS photometry, HST-ACS I-band imaging, and optical zCOSMOS spectroscopy to derive the main global properties of star-forming EELGs, such as sizes, stellar masses, star formation rates (SFR), and reliable oxygen abundances using both "direct" and "strong-line" methods. Results: The EELGs are extremely compact (r50 ~ 1.3 kpc), low-mass (M∗ ~ 107-1010 M⊙) galaxies forming stars at unusually high specific star formation rates (sSFR ≡ SFR/M⋆ up to 10-7 yr-1) compared to main sequence star-forming galaxies of the same stellar mass and redshift. At rest-frame UV wavelengths, the EELGs are luminous and show high surface brightness and include strong Lyα emitters, as revealed by GALEX spectroscopy. We show that zCOSMOS EELGs are high-ionization, low-metallicity systems, with median 12+log (O/H) = 8.16 ± 0.21 (0.2 Z⊙) including a handful of extremely metal-deficient (<0.1 Z⊙) EELGs. While ~80% of the EELGs show non-axisymmetric morphologies, including clumpy and cometary or tadpole galaxies, we find that ~29% of them show additional low-surface-brightness features, which strongly suggests recent or ongoing interactions. As star-forming dwarfs in the local Universe, EELGs are most often found in relative isolation. While only very few EELGs belong to compact groups, almost one third of them are found in spectroscopically confirmed loose pairs or triplets. Conclusions: The zCOSMOS EELGs are galaxies caught in a transient and probably early period of their evolution, where they are efficiently building up a significant fraction of their present-day stellar mass in an ongoing, galaxy-wide starburst. Therefore, the EELGs constitute an ideal benchmark for comparison studies between low- and high-redshift low-mass star-forming galaxies. Full Tables 1 and 2 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/578/A105

Multi-Wavelength Views of Messier 81

NASA Technical Reports Server (NTRS)

2003-01-01

[figure removed for brevity, see original site] Click on individual images below for larger view

[figure removed for brevity, see original site]

[figure removed for brevity, see original site]

[figure removed for brevity, see original site]

[figure removed for brevity, see original site]

The magnificent spiral arms of the nearby galaxy Messier 81 are highlighted in this image from NASA's Spitzer Space Telescope. Located in the northern constellation of Ursa Major (which also includes the Big Dipper), this galaxy is easily visible through binoculars or a small telescope. M81 is located at a distance of 12 million light-years.

The main image is a composite mosaic obtained with the multiband imaging photometer for Spitzer and the infrared array camera. Thermal infrared emission at 24 microns detected by the photometer (red, bottom left inset) is combined with camera data at 8.0 microns (green, bottom center inset) and 3.6 microns (blue, bottom right inset).

A visible-light image of Messier 81, obtained at Kitt Peak National Observatory, a ground-based telescope, is shown in the upper right inset. Both the visible-light picture and the 3.6-micron near-infrared image trace the distribution of stars, although the Spitzer image is virtually unaffected by obscuring dust. Both images reveal a very smooth stellar mass distribution, with the spiral arms relatively subdued.

As one moves to longer wavelengths, the spiral arms become the dominant feature of the galaxy. The 8-micron emission is dominated by infrared light radiated by hot dust that has been heated by nearby luminous stars. Dust in the galaxy is bathed by ultraviolet and visible light from nearby stars. Upon absorbing an ultraviolet or visible-light photon, a dust grain is heated and re-emits the energy at longer infrared wavelengths. The dust particles are composed of silicates (chemically similar to beach sand), carbonaceous grains and polycyclic aromatic hydrocarbons and trace the gas distribution in the galaxy. The well-mixed gas (which is best detected at radio wavelengths) and dust provide a reservoir of raw materials for future star formation.

The 24-micron multiband imaging photometer image shows emission from warm dust heated by the most luminous young stars. The infrared-bright clumpy knots within the spiral arms show where massive stars are being born in giant H II (ionized hydrogen) regions. Studying the locations of these star forming regions with respect to the overall mass distribution and other constituents of the galaxy (e.g., gas) will help identify the conditions and processes needed for star formation.

The Origin of Molecular Clouds in Central Galaxies

NASA Astrophysics Data System (ADS)

Pulido, F. A.; McNamara, B. R.; Edge, A. C.; Hogan, M. T.; Vantyghem, A. N.; Russell, H. R.; Nulsen, P. E. J.; Babyk, I.; Salomé, P.

2018-02-01

We present an analysis of 55 central galaxies in clusters and groups with molecular gas masses and star formation rates lying between {10}8 {and} {10}11 {M}ȯ and 0.5 and 270 {M}ȯ {{yr}}-1, respectively. Molecular gas mass is correlated with star formation rate, Hα line luminosity, and central atmospheric gas density. Molecular gas is detected only when the central cooling time or entropy index of the hot atmosphere falls below ∼1 Gyr or ∼35 keV cm2, respectively, at a (resolved) radius of 10 kpc. These correlations indicate that the molecular gas condensed from hot atmospheres surrounding the central galaxies. We explore the origins of thermally unstable cooling by evaluating whether molecular gas becomes prevalent when the minimum of the cooling to free-fall time ratio ({t}{cool}/{t}{ff}) falls below ∼10. We find that (1) molecular gas-rich systems instead lie between 10< \\min ({t}{cool}/{t}{ff})< 25, where {t}{cool}/{t}{ff}=25 corresponds approximately to cooling time and entropy thresholds of 1 Gyr and 35 {keV} {{cm}}2, respectively; (2) \\min ({t}{cool}/{t}{ff}) is uncorrelated with molecular gas mass and jet power; and (3) the narrow range 10< \\min ({t}{cool}/{t}{ff})< 25 can be explained by an observational selection effect, although a real physical effect cannot be excluded. These results and the absence of isentropic cores in cluster atmospheres are in tension with models that assume thermal instability ensues from linear density perturbations in hot atmospheres when {t}{cool}/{t}{ff}≲ 10. Some of the molecular gas may instead have condensed from atmospheric gas lifted outward by buoyantly rising X-ray bubbles or by dynamically induced uplift (e.g., mergers, sloshing).

Interaction of clumpy dark matter with interstellar medium in astrophysical systems

NASA Astrophysics Data System (ADS)

Baushev, A. N.

2012-02-01

Contemporary cosmological conceptions suggest that the dark matter in haloes of galaxies and galaxy clusters has most likely a clumpy structure. If a stream of gas penetrates through it, a small-scale gravitational field created by the clumps disturbs the flow resulting in momentum exchange between the stream and the dark matter. In this article, we perform an analysis of this effect, based on the hierarchical halo model of the dark matter structure and Navarro-Frenk-White density profiles. We consider the clumps of various masses, from the smallest up to the highest ones M≥ 109 M⊙. It has been found that in any event the effect grows with the mass of the clump: not only the drag force ? acting on the clump but also its acceleration ? increases. We discuss various astrophysical systems. The mechanism proved to be ineffective in the case of galaxy or galaxy cluster collisions. On the other hand, it played an important role during the process of galaxy formation. As a result, the dark matter should have formed a more compact, oblate and faster rotating substructure in the halo of our Galaxy. We have shown that this thick disc should be more clumpy than the halo. This fact is very important for the indirect detection experiments since it is the clumps that give the main contribution to the annihilation signal. Our calculations show that the mechanism of momentum exchange between the dark and baryon matter is ineffective on the outskirts of the galactic halo. It means that the clumps from there were not transported to the thick disc, and this region should be more clumpy than the halo on the average.

Is the SDSS ZZ Ceti instability strip really pure?

NASA Astrophysics Data System (ADS)

de Souza Oliveira, Kepler

2006-08-01

We propose to obtain SNR > 60 optical spectra of the DA white dwarf stars for which the Sloan Digital Sky Survey spectra indicated temperatures inside de ZZ Ceti instability strip, but time series photometry show they are not variables. The Sloan spectra have insufficient SNR, specially below 4000A, where there are hydrogen lines whose strength can be used to measure surface gravity accurately. Theoretically and observationally, the location of the instability strip depends both on temperature and mass. To use the properties derived from the pulsating stars as applying to all white dwarf stars, and their progenitors, we must demonstrate pulsation is a normal evolutionary state. As the instability strip is only 1200K wide, accurate temperatures and log g must be obtained and therefore the spectra must include the log g sensitive lines Hgamma to H9. White dwarf stars, the objects of this proposal, are the end point of evolution of around 97% of all stars born. As they cool, they pass through instability strips, where they are seen as multi-periodic pulsators. Each pulsation is an independent measurement, placing another constraint on the stellar properties. Pulsations allow the determination of the stellar compositional layers, including the core, crucial to understand the progenitor's evolution, from AGB to planetary nebulae nuclei, "born again" phase, and their possible evolution to SNIa through accretion. As white dwarf progenitors lose at least half of their masses before turning into white dwarfs, they contribute to the interstellar medium enrichment, and measuring their structure in detail will allow us to decode nuclear reaction rates and convection, which determine their evolution. Pulsating white dwarf stars are also laboratories for physics at high densities as crystallization, neutrino cooling, and axion emission. White dwarf cooling, also measured through pulsations, allows an independent measurement of the age of the galactic components and was the first to indicate an age of 13 Gyr to the Universe, back in 1987. Now that we have observed white dwarf stars in all the components of our galaxy, possible differences in component ages are detectable. Our goal is to determine if the instalibity strip is pure, implying the information we obtain on the variables applies to white dwarf stars in general. As these stars are on average fainter than g=18.2, we require blue sensitive 8m class telescope.

Observation and modelling of main-sequence star chromospheres - X. Radiative budgets on Gl 867A and AU Mic (dM1e), and a two-component model chromosphere for Gl 205 (dM1)

NASA Astrophysics Data System (ADS)

Houdebine, E. R.

2010-04-01

We report on high-resolution observations of two dM1 stars: Gl 867A, an active dM1e star, and Gl 205, a less active dM1 star. The wavelength coverage is from 3890 to 6820Å with a resolving power of about 45000. The difference spectrum of these two stars allows us to make a survey of spectral lines sensitive to magnetic activity. We chose these two stars because, to within measurement errors, they have very close properties: Gl 867A has R = 0.726Rsolar, [M/H] = 0.080 dex and Teff = 3416 K, and Gl 205 has R = 0.758Rsolar, [M/H] = 0.101 dex and Teff = 3493 K. We find that besides traditional chromospheric lines, many photospheric lines are `filled-in' in the active star spectrum. These differences are, most of the time, weak in absolute fluxes but can be large in terms of differences in the spectral-line equivalent widths. We calculate the differences in surface fluxes between these two stars for many spectral lines. We derive the radiative budgets for two dM1e stars: Gl 867A and AU Mic. We show that the sum of the numerous spectral lines represents a significant fraction of the radiative cooling of the outer atmosphere. We also re-investigate the cooling from the continuum from the visible to the extreme ultraviolet; we find that earlier predictions of the calculations of Houdebine et al. (Paper V) are in good agreement with observations. We emphasize that if this radiative cooling is chromospheric in character, then in chromospheric model calculations, we should include the radiative losses in CaI, CrI, VI, TiI and FeI. From simple constraints, we derive model chromospheres for quiescent and active regions on Gl 205. We show that the quiescent regions have a strong absorption Hα profile. The plage regions show a filled-in intermediate activity Hα profile. We also present possible spectral line profiles of quiescent and active regions on Gl 867A. Based on observations collected at Observatoire de Haute Provence, France. E-mail: eric_houdebine@yahoo.fr

The heating of dust in starburst galaxies: The contribution of the nonionizing radiation

NASA Technical Reports Server (NTRS)

Calzetti, D.; Bohlin, R. C.; Kinney, Anne L.; Storchi-Bergmann, T.; Heckman, Timothy M.

1995-01-01

The IUE UV and optical spectra and the far-infrared (FIR) IRAS flux densities of a sample of starburst and blue compact galaxies are used to investigate the relationship between dust obscuration and dust emission. The amount of dust obscuration at UV wavelengths correlates with the FIR-to-blue ratio; and an analysis of the correlation indicates that not only the ionizing but also the nonionizing radiation contribute to the FIR emission. The amount of UV and optical energy lost to dust obscuration accounts for most of the cool dust FIUR emission and for about 70% of the warm dust FIR emission. The remaining 30% of the warm dust FIR flux is probably due to dust emission from regions of star formation which are embedded in opaque giant molecular clouds and do not contribute to the integrated UV and optical spectrum. The use of the FIR emission as an indicator of high-mass star formation rate in star-forming galaxies can be problematic, since the contribution to the FIR flux from cool dust emission heated by relatively old stars is nonnegligible.

Spots and activity of Pleiades stars from observations with the Kepler Space Telescope (K2)

NASA Astrophysics Data System (ADS)

Savanov, I. S.; Dmitrienko, E. S.

2017-11-01

Observations of the K2 continuation of Kepler Space Telescope program are used to estimate the spot coverage S (the fractional spotted area on the surface of an active star) for stars of the Pleiades cluster. The analysis is based on data on photometric variations of 759 confirmed clustermembers, together with their atmospheric parameters, masses, and rotation periods. The relationship between the activity ( S) of these Pleiades stars and their effective temperatures shows considerable change in S for stars with temperatures T eff less than 6100 K (this can be considered the limiting value for which spot formation activity begins) and a monotonic increase in S for cooler objects (a change in the slope for stars with Teff 3700 K). The scatter in this parameter ΔS about its mean dependence on the (V -Ks)0 color index remains approximately the same over the entire ( V- K s )0 range, including cool, fully convective dwarfs. The computated S values do not indicate differences between slowly rotating and rapidly rotating stars with color indices 1.1 < ( V- K s )0 < 3.7. The main results of this study include measurements of the activity of a large number of stars having the same age (759 members of the Pleiades cluster), resulting in the first determination of the relationship between the spot-forming activity and masses of stars. For 27 stars with masses differing from the solarmass by nomore than 0.1 M⊙, themean spot coverage is S = 0.031±0.003, suggesting that the activity of candidate young Suns is more pronounced than that of the present-day Sun. These stars rotate considerably faster than the Sun, with an average rotation period of 4.3d. The results of this study of cool, low-mass dwarfs of the Pleiades cluster are compared to results from an earlier study of 1570 M stars.

A radio-pulsing white dwarf binary star.

PubMed

Marsh, T R; Gänsicke, B T; Hümmerich, S; Hambsch, F-J; Bernhard, K; Lloyd, C; Breedt, E; Stanway, E R; Steeghs, D T; Parsons, S G; Toloza, O; Schreiber, M R; Jonker, P G; van Roestel, J; Kupfer, T; Pala, A F; Dhillon, V S; Hardy, L K; Littlefair, S P; Aungwerojwit, A; Arjyotha, S; Koester, D; Bochinski, J J; Haswell, C A; Frank, P; Wheatley, P J

2016-09-15

White dwarfs are compact stars, similar in size to Earth but approximately 200,000 times more massive. Isolated white dwarfs emit most of their power from ultraviolet to near-infrared wavelengths, but when in close orbits with less dense stars, white dwarfs can strip material from their companions and the resulting mass transfer can generate atomic line and X-ray emission, as well as near- and mid-infrared radiation if the white dwarf is magnetic. However, even in binaries, white dwarfs are rarely detected at far-infrared or radio frequencies. Here we report the discovery of a white dwarf/cool star binary that emits from X-ray to radio wavelengths. The star, AR Scorpii (henceforth AR Sco), was classified in the early 1970s as a δ-Scuti star, a common variety of periodic variable star. Our observations reveal instead a 3.56-hour period close binary, pulsing in brightness on a period of 1.97 minutes. The pulses are so intense that AR Sco's optical flux can increase by a factor of four within 30 seconds, and they are also detectable at radio frequencies. They reflect the spin of a magnetic white dwarf, which we find to be slowing down on a 10 7 -year timescale. The spin-down power is an order of magnitude larger than that seen in electromagnetic radiation, which, together with an absence of obvious signs of accretion, suggests that AR Sco is primarily spin-powered. Although the pulsations are driven by the white dwarf's spin, they mainly originate from the cool star. AR Sco's broadband spectrum is characteristic of synchrotron radiation, requiring relativistic electrons. These must either originate from near the white dwarf or be generated in situ at the M star through direct interaction with the white dwarf's magnetosphere.

Weak metal lines in optical high-resolution Very Large Telescope and Keck spectra of "cool" PG 1159 stars

NASA Astrophysics Data System (ADS)

Werner, K.; Rauch, T.

2014-09-01

PG 1159 stars are very hot (effective temperatures Teff = 75 000-200 000 K), hydrogen-deficient (pre-) white dwarfs. They probably are the result of a late helium-shell flash that laid bare the He, C, and O rich intershell matter of the progenitor Asymptotic Giant Branch (AGB) star. Their chemical surface composition thus allows to conclude on details of AGB-star nucleosynthesis. Due to their very high effective temperatures, detailed spectral analyses are usually completely reliant on ultraviolet observations, except for some species in the hottest PG 1159 stars (Teff ≳ 130 000 K), which do exhibit highly excited lines from the CNO elements and neon (C iv, N v, O vi, Ne vii-viii) in optical spectra. Particularly problematic are, however, the coolest members of the PG 1159 class that exclusively show C iv lines in the optical. Access to the nitrogen abundance is important to decide which of the late-thermal pulse evolutionary scenarios was experienced by a particular star, while a high oxygen abundance is an important marker that the star could pulsate. In the present paper, we investigate high-resolution high signal-to-noise optical spectra of three "cool" PG 1159 stars (PG 0122+200, PG 2131+066, MCT 0130-1937, Teff = 80 000-95 000 K). With the help of non-LTE model atmospheres and synthetic spectra, we are able to identify a large number of weak CNO lines (C iii, N iv, O iii-v) that were not detected before in these stars. They allow abundance determinations and enable us to constrain the effective temperature to high precision through ionization equilibria without the requirement to access the ultraviolet spectral range.

Amazing Andromeda Galaxy

NASA Technical Reports Server (NTRS)

2006-01-01

The many 'personalities' of our great galactic neighbor, the Andromeda galaxy, are exposed in this new composite image from NASA's Galaxy Evolution Explorer and the Spitzer Space Telescope.

The wide, ultraviolet eyes of Galaxy Evolution Explorer reveal Andromeda's 'fiery' nature -- hotter regions brimming with young and old stars. In contrast, Spitzer's super-sensitive infrared eyes show Andromeda's relatively 'cool' side, which includes embryonic stars hidden in their dusty cocoons.

Galaxy Evolution Explorer detected young, hot, high-mass stars, which are represented in blue, while populations of relatively older stars are shown as green dots. The bright yellow spot at the galaxy's center depicts a particularly dense population of old stars.

Swaths of red in the galaxy's disk indicate areas where Spitzer found cool, dusty regions where stars are forming. These stars are still shrouded by the cosmic clouds of dust and gas that collapsed to form them.

Together, Galaxy Evolution Explorer and Spitzer complete the picture of Andromeda's swirling spiral arms. Hints of pinkish purple depict regions where the galaxy's populations of hot, high-mass stars and cooler, dust-enshrouded stars co-exist.

Located 2.5 million light-years away, the Andromeda is our largest nearby galactic neighbor. The galaxy's entire disk spans about 260,000 light-years, which means that a light beam would take 260,000 years to travel from one end of the galaxy to the other. By comparison, our Milky Way galaxy's disk is about 100,000 light-years across.

This image is a false color composite comprised of data from Galaxy Evolution Explorer's far-ultraviolet detector (blue), near-ultraviolet detector (green), and Spitzer's multiband imaging photometer at 24 microns (red).

SPECTROPOLARIMETRY OF THE CLASSICAL T TAURI STAR BP TAU

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

Chen, Wei; Johns-Krull, Christopher M., E-mail: wc2@rice.edu, E-mail: cmj@rice.edu

We implement a least-squares deconvolution (LSD) code to study magnetic fields on cool stars. We first apply our code to high-resolution optical echelle spectra of 53 Cam (a magnetic Ap star) and three well-studied cool stars (Arcturus, 61 Cyg A, and ξ Boo A) as well as the Sun (by observing the asteroid Vesta) as tests of the code and the instrumentation. Our analysis is based on several hundred photospheric lines spanning the wavelength range 5000 Å to 9000 Å. We then apply our LSD code to six nights of data on the Classical T Tauri Star BP Tau. Amore » maximum longitudinal field of 370 ± 80 G is detected from the photospheric lines on BP Tau. A 1.8 kG dipole tilted at 129° with respect to the rotation axis and a 1.4 kG octupole tilted at 104° with respect to the rotation axis, both with a filling factor of 0.25, best fit our LSD Stokes V profiles. Measurements of several emission lines (He I 5876 Å, Ca II 8498 Å, and 8542 Å) show the presence of strong magnetic fields in the line formation regions of these lines, which are believed to be the base of the accretion footpoints. The field strength measured from these lines shows night-to-night variability consistent with rotation of the star.« less

Definition and empirical structure of the range of stellar chromospheres-coronae across the H-R diagram: Cool stars

NASA Technical Reports Server (NTRS)

Linsky, J. L.

1986-01-01

Major advances in our understanding of non-radiative heating and other activity in stars cooler than T sub eff = 10,000K has occured in the last few years. This observational evidence is reviewed and the trends that are now becoming apparent are discussed. The evidence for non-radiatively heated outer atmospheric layers (chromospheres, transition regions, and coronae) in dwarf stars cooler than spectral type A7, in F and G giants, pre-main sequence stars, and close bindary systems is unambiguous, as is the evidence for chromospheres in the K and M giants and supergiants. The existence of non-radiative heating in the outer layers of the A stars remains undetermined despite repeated searches at all wavelengths. Two important trends in the data are the decrease in plasma emission measure with age on the main sequence and decreasing rotational velocity. Variability and atmospheric inhomogeneity are commonly seen, and there is considerable evidence that magnetic fields define the geometry and control the energy balance in the outer atmospheric layers. In addition, the microwave observations imply that non-thermal electrons are confined in coronal magnetic flux tubes in at least the cool dwarfs and RS CVn systems. The chromospheres in the K and M giants and supergiants are geometrically extended, as are the coronae in the RS CVn systems and probably also in other stars.

Contemporaneous Ultraviolet and Optical Observations of Direct and Raman-scattered O VI Lines in Symbiotic Stars

NASA Astrophysics Data System (ADS)

Birriel, Jennifer J.; Espey, Brian R.; Schulte-Ladbeck, Regina E.

2000-12-01

Symbiotic stars are binary systems consisting of a hot star, typically a white dwarf, and a cool giant companion. The wind from the cool star is ionized by the radiation from the hot star, resulting in the characteristic combination of sharp nebular emission lines and stellar molecular absorption bands in the optical spectrum. Most of the emission lines are readily identifiable with common ions. However, two strong, broad emission lines at 6825 and 7082 Å defied identification with known atoms and ions. In 1989 Schmid made the case that these long unidentified emission lines resulted from the Raman scattering of the O VI resonance photons at 1032, 1038 Å by neutral hydrogen. We present contemporaneous far-UV and optical observations of direct and Raman-scattered O VI lines for nine symbiotic stars obtained with the Hopkins Ultraviolet Telescope (Astro-2) and various ground-based optical telescopes. The O VI emission lines are present in every instance in which the λλ6825, 7082 lines are present, in support of the Schmid Raman-scattering model. We calculate the scattering efficiencies and discuss the results in terms of the Raman-scattering model. Additionally, we measure the flux of the Fe II fluorescence line at 1776 Å, which is excited by the O VI line at 1032 Å, and calculate the first estimates of the conversion efficiencies for this process.

Dark-ages Reionization and Galaxy Formation Simulation - XIV. Gas accretion, cooling, and star formation in dwarf galaxies at high redshift

NASA Astrophysics Data System (ADS)

Qin, Yuxiang; Duffy, Alan R.; Mutch, Simon J.; Poole, Gregory B.; Geil, Paul M.; Mesinger, Andrei; Wyithe, J. Stuart B.

2018-06-01

We study dwarf galaxy formation at high redshift (z ≥ 5) using a suite of high-resolution, cosmological hydrodynamic simulations and a semi-analytic model (SAM). We focus on gas accretion, cooling, and star formation in this work by isolating the relevant process from reionization and supernova feedback, which will be further discussed in a companion paper. We apply the SAM to halo merger trees constructed from a collisionless N-body simulation sharing identical initial conditions to the hydrodynamic suite, and calibrate the free parameters against the stellar mass function predicted by the hydrodynamic simulations at z = 5. By making comparisons of the star formation history and gas components calculated by the two modelling techniques, we find that semi-analytic prescriptions that are commonly adopted in the literature of low-redshift galaxy formation do not accurately represent dwarf galaxy properties in the hydrodynamic simulation at earlier times. We propose three modifications to SAMs that will provide more accurate high-redshift simulations. These include (1) the halo mass and baryon fraction which are overestimated by collisionless N-body simulations; (2) the star formation efficiency which follows a different cosmic evolutionary path from the hydrodynamic simulation; and (3) the cooling rate which is not well defined for dwarf galaxies at high redshift. Accurate semi-analytic modelling of dwarf galaxy formation informed by detailed hydrodynamical modelling will facilitate reliable semi-analytic predictions over the large volumes needed for the study of reionization.

The complex magnetic field topology of the cool Ap star 49 Cam

NASA Astrophysics Data System (ADS)

Silvester, J.; Kochukhov, O.; Rusomarov, N.; Wade, G. A.

2017-10-01

49 Cam is a cool magnetic chemically peculiar star that has been noted for showing strong, complex Zeeman linear polarization signatures. This paper describes magnetic and chemical surface maps obtained for 49 Cam using the Invers10 magnetic Doppler imaging code and high-resolution spectropolarimetric data in all four Stokes parameters collected with the ESPaDOnS and Narval spectropolarimeters at the Canada-France-Hawaii Telescope and Pic du Midi Observatory. The reconstructed magnetic field maps of 49 Cam show a relatively complex structure. Describing the magnetic field topology in terms of spherical harmonics, we find significant contributions of modes up to ℓ = 3, including toroidal components. Observations cannot be reproduced using a simple low-order multipolar magnetic field structure. 49 Cam exhibits a level of field complexity that has not been seen in magnetic maps of other cool Ap stars. Hence, we concluded that relatively complex magnetic fields are observed in Ap stars at both low and high effective temperatures. In addition to mapping the magnetic field, we also derive surface abundance distributions of nine chemical elements, including Ca, Sc, Ti, Cr, Fe, Ce, Pr, Nd and Eu. Comparing these abundance maps with the reconstructed magnetic field geometry, we find no clear relationship of the abundance distributions with the magnetic field for some elements. However, for other elements some distinct patterns are found. We discuss these results in the context of other recent magnetic mapping studies and theoretical predictions of radiative diffusion.

Cool Bottom Processing on the AGB and Presolar Grain Compositions

NASA Technical Reports Server (NTRS)

Nollett, Kenneth M.; Busso, M.; Wasserburg, G. J.

2002-01-01

We describe results from a model of cool bottom processing (CBP) in AGB (asymptotic giant branch) stars. We predict O, Al, C and N isotopic compositions of circumstellar grains. Measured compositions of mainstream SiC grains and many oxide grains are consistent with CBP. Additional information is contained in the original extended abstract.

The "Cool Algol" BD+05 706 : Photometric observations of a new eclipsing double-lined spectroscopic binary

NASA Astrophysics Data System (ADS)

Marschall, L. A.; Torres, G.; Neuhauser, R.

1998-05-01

BVRI Observations of the star BD+05 706, carried out between January, 1997, and April 1998 using the 0.4m reflector and Photometrics CCD camera at the Gettysburg College Observatory, show that the star is an eclipsing binary system with a light curve characteristic of a class of semi-detached binaries known as the "cool Algols". These results are in good agreement with the previous report of BD+05 706 as a cool Algol by Torres, Neuhauser, and Wichmann,(Astron. J., 115, May 1998) who based their classification on the strong X-ray emission detected by Rosat and on a series of spectroscopic observations of the radial velocities of both components of the system obtained at the Oak Ridge Observatory, the Fred L. Whipple Observatory, and the Multiple Mirror Telescope. Only 10 other examples of cool Algols are known, and the current photometric light curve, together with the radial velocity curves obtained previously, allows us to derive a complete solution for the physical parameters of each component, providing important constraints on models for these interesting systems.

Search for cold gas in clusters with and without cooling flows

NASA Technical Reports Server (NTRS)

Grabelsky, D. A.; Ulmer, M. P.

1990-01-01

The dominant galaxy in each of approx. 40 clusters was studied using co-added Infrared Astronomy Satellite (IRAS) survey data, and 11 of these galaxies were observed for CO (J=1 to 0) emission with the 12 m telescope at Kitt Peak. Half of the galaxies in the sample are in clusters reported to have cooling flows while the other half are not. Six of the galaxies appear to have been detected by IRAS at fairly low flux levels, in addition to one previously known strong detection; all seven have reported cooling flows. No detectable CO emission (to 2 to 3 mK) was found in any of the 11 galaxies observed. Assuming accretion rates of approx. 100 Solar Mass yr(-1), the star formation rates and efficiencies in these galaxies must be quite high in order to render the CO undetectable. At the same time, the infrared luminosities of these galaxies is unremarkable, suggesting that the correlation between star formation efficiency and infrared luminosity found for spirals may not hold for cooling flows.

Tidal Heating of Earth-like Exoplanets around M Stars: Thermal, Magnetic, and Orbital Evolutions

PubMed Central

Barnes, R.

2015-01-01

Abstract The internal thermal and magnetic evolution of rocky exoplanets is critical to their habitability. We focus on the thermal-orbital evolution of Earth-mass planets around low-mass M stars whose radiative habitable zone overlaps with the “tidal zone,” where tidal dissipation is expected to be a significant heat source in the interior. We develop a thermal-orbital evolution model calibrated to Earth that couples tidal dissipation, with a temperature-dependent Maxwell rheology, to orbital circularization and migration. We illustrate thermal-orbital steady states where surface heat flow is balanced by tidal dissipation and cooling can be stalled for billions of years until circularization occurs. Orbital energy dissipated as tidal heat in the interior drives both inward migration and circularization, with a circularization time that is inversely proportional to the dissipation rate. We identify a peak in the internal dissipation rate as the mantle passes through a viscoelastic state at mantle temperatures near 1800 K. Planets orbiting a 0.1 solar-mass star within 0.07 AU circularize before 10 Gyr, independent of initial eccentricity. Once circular, these planets cool monotonically and maintain dynamos similar to that of Earth. Planets forced into eccentric orbits can experience a super-cooling of the core and rapid core solidification, inhibiting dynamo action for planets in the habitable zone. We find that tidal heating is insignificant in the habitable zone around 0.45 (or larger) solar-mass stars because tidal dissipation is a stronger function of orbital distance than stellar mass, and the habitable zone is farther from larger stars. Suppression of the planetary magnetic field exposes the atmosphere to stellar wind erosion and the surface to harmful radiation. In addition to weak magnetic fields, massive melt eruption rates and prolonged magma oceans may render eccentric planets in the habitable zone of low-mass stars inhospitable for life. Key Words: Tidal dissipation—Thermal history—Planetary interiors—Magnetic field. Astrobiology 15, 739–760. PMID:26393398

Tidal Heating of Earth-like Exoplanets around M Stars: Thermal, Magnetic, and Orbital Evolutions.

PubMed

Driscoll, P E; Barnes, R

2015-09-01

The internal thermal and magnetic evolution of rocky exoplanets is critical to their habitability. We focus on the thermal-orbital evolution of Earth-mass planets around low-mass M stars whose radiative habitable zone overlaps with the "tidal zone," where tidal dissipation is expected to be a significant heat source in the interior. We develop a thermal-orbital evolution model calibrated to Earth that couples tidal dissipation, with a temperature-dependent Maxwell rheology, to orbital circularization and migration. We illustrate thermal-orbital steady states where surface heat flow is balanced by tidal dissipation and cooling can be stalled for billions of years until circularization occurs. Orbital energy dissipated as tidal heat in the interior drives both inward migration and circularization, with a circularization time that is inversely proportional to the dissipation rate. We identify a peak in the internal dissipation rate as the mantle passes through a viscoelastic state at mantle temperatures near 1800 K. Planets orbiting a 0.1 solar-mass star within 0.07 AU circularize before 10 Gyr, independent of initial eccentricity. Once circular, these planets cool monotonically and maintain dynamos similar to that of Earth. Planets forced into eccentric orbits can experience a super-cooling of the core and rapid core solidification, inhibiting dynamo action for planets in the habitable zone. We find that tidal heating is insignificant in the habitable zone around 0.45 (or larger) solar-mass stars because tidal dissipation is a stronger function of orbital distance than stellar mass, and the habitable zone is farther from larger stars. Suppression of the planetary magnetic field exposes the atmosphere to stellar wind erosion and the surface to harmful radiation. In addition to weak magnetic fields, massive melt eruption rates and prolonged magma oceans may render eccentric planets in the habitable zone of low-mass stars inhospitable for life.

The Star, the Dwarf and the Planet

NASA Astrophysics Data System (ADS)

2006-10-01

Astronomers have detected a new faint companion to the star HD 3651, already known to host a planet. This companion, a brown dwarf, is the faintest known companion of an exoplanet host star imaged directly and one of the faintest T dwarfs detected in the Solar neighbourhood so far. The detection yields important information on the conditions under which planets form. "Such a system is an interesting example that might prove that planets and brown dwarfs can form around the same star", said Markus Mugrauer, lead author of the paper presenting the discovery. ESO PR Photo 39a/06 ESO PR Photo 39a/06 The Companion to HD 3651 HD 3651 is a star slightly less massive than the Sun, located 36 light-years away in the constellation Pisces (the "Fish"). For several years, it has been known to harbour a planet less massive than Saturn, sitting closer to its parent star than Mercury is from the Sun: the planet accomplishes a full orbit in 62 days. Mugrauer and his colleagues first spotted the faint companion in 2003 on images from the 3.8-m United Kingdom Infrared Telescope (UKIRT) in Hawaii. Observations in 2004 and 2006 using ESO's 3.6 m New Technology Telescope (NTT) at La Silla provided the crucial confirmation that the speck of light is not a spurious background star, but indeed a true companion. The newly found companion, HD 3651B, is 16 times further away from HD 3651 than Neptune is from the Sun. HD 3651B is the dimmest directly imaged companion of an exoplanet host star. Furthermore, as it is not detected on the photographic plates of the Palomar All Sky Survey, the companion must be even fainter in the visible spectral range than in the infrared, meaning it is a very cool low-mass sub-stellar object. Comparing its characteristics with theoretical models, the astronomers infer that the object has a mass between 20 and 60 Jupiter masses, and a temperature between 500 and 600 degrees Celsius. It is thus ten times colder and 300 000 less luminous than the Sun. These properties place it in the category of cool T-type brown dwarfs. ESO PR Photo 38b/06 ESO PR Photo 39b/06 The Relative Position of the Companion to HD 3651 "Due to their faintness even in the infrared, these cool T dwarfs are very difficult to find", said Mugrauer. "Only two other brown dwarfs with similar brightness are presently known. Their study will provide important insights into the atmospheric properties of cool sub-stellar objects." More than 170 stars are currently known to host exoplanets. In some cases, these stars were also found to have one or several stellar companions, showing that planet formation can also take place in a dynamically more complex environment than our own Solar System where planet formation occurred around an isolated single star. In 2001, Mugrauer and his colleagues started an observational programme to find out whether exoplanet host stars are single or married. In this programme, known exoplanet host stars are systematically imaged at two different epochs, at least several months apart. True companions can be distinguished from coincidental background objects as only they move together with the stars over time. With this effective search strategy several new companions of exoplanet host stars have been detected. Most of the detected companions are low-mass stars in the same evolutionary state as the Sun. In two cases, however, the astronomers found the companions to be white dwarfs, that is, stars at the end of their life. These intriguing systems bear evidence that planets can even survive the troubled last moments in the life of a nearby star. The planet host star HD 3651 is thus surrounded by two sub-stellar objects. The planet, HD 3651b, is very close, while the newly found brown dwarf companion revolves around the star 1500 times farther away than the planet. This system is the first imaged example that planets and brown dwarfs can form around the same star.

Zooming in on the Starburst at the Core of the Phoenix Cluster

NASA Astrophysics Data System (ADS)

McDonald, Michael

2011-10-01

In a recently published letter to Nature, we report the discovery of the most X-ray luminous galaxy cluster in the known Universe, within which the intracluster medium is cooling at an unprecedented rate. In the core of this cluster, the brightest cluster galaxy is forming stars at an unmatched rate of 740 Msun/yr, which is highly unusual for this class of galaxy which are typically referred to as "red and dead". We suspect that the extreme cooling and star formation rates are intimately linked: the cooling intracluster gas is most likely providing fuel for the starburst. We request 2 orbits of near-UV and optical broadband WFC3-UVIS imaging in order to morphologically classify this starburst as a result of i} cooling, infalling gas {filamentary UV emission}; ii} a recent merger {tidal tails with both UV and optical emission}; or iii} a starburst- or AGN-driven wind {wide opening angle}. These data will also allow us to determine the stellar populations of both the starburst and the underlying, older stellar populations, and will provide a much sharper view of the central AGN, allowing us to more carefully extract the contribution to the extended UV emission from young stars. Our early results have already received substantial attention from the international press, and we expect that a dramatically improved picture of the heart of this cluster would stir up as much, if not more, interest from the public.

Thermal states of neutron stars with a consistent model of interior

NASA Astrophysics Data System (ADS)

Fortin, M.; Taranto, G.; Burgio, G. F.; Haensel, P.; Schulze, H.-J.; Zdunik, J. L.

2018-04-01

We model the thermal states of both isolated neutron stars and accreting neutron stars in X-ray transients in quiescence and confront them with observations. We use an equation of state calculated using realistic two-body and three-body nucleon interactions, and superfluid nucleon gaps obtained using the same microscopic approach in the BCS approximation. Consistency with low-luminosity accreting neutron stars is obtained, as the direct Urca process is operating in neutron stars with mass larger than 1.1 M⊙ for the employed equation of state. In addition, proton superfluidity and sufficiently weak neutron superfluidity, obtained using a scaling factor for the gaps, are necessary to explain the cooling of middle-aged neutron stars and to obtain a realistic distribution of neutron star masses.

Ponderable soliton stars

NASA Technical Reports Server (NTRS)

Chiu, Hong-Yee

1990-01-01

The theory of Lee and Pang (1987), who obtained solutions for soliton stars composed of zero-temperature fermions and bosons, is applied here to quark soliton stars. Model soliton stars based on a simple physical model of the proton are computed, and the properties of the solitons are discussed, including the important problem of the existence of a limiting mass and thus the possible formation of black holes of primordial origin. It is shown that there is a definite mass limit for ponderable soliton stars, so that during cooling a soliton star might reach a stage beyond which no equilibrium configuration exists and the soliton star probably will collapse to become a black hole. The radiation of ponderable soliton stars may alter the short-wavelength character of the cosmic background radiation, and may be observed as highly redshifted objects at z of about 100,000.

Debris Disks Among the Shell Stars: Insights from Spitzer

NASA Technical Reports Server (NTRS)

Roberge, Aki; Weinberger, Alycia; Teske, Johanna

2008-01-01

Shell stars are a class of early-type stars that show narrow absorption lines in their spectra that appear to arise from circumstellar class. This observationally defined class contains a variety of objects, including evolved stars and classical Be stars. However, some of the main sequence shell stars harbor debris disks and younger protoplanetary disks, though this aspect of the class has been largely overlooked. We surveyed a set of main sequence stars for cool dust using Spitzer MIPS and found four additional systems with IR excesses at both 24 and 70 microns. This indicates that the stars have both circumstellar gas and dust, and are likely to be edge-on debris disks. Our estimate of the disk fraction among nearby main sequence shell stars is 48% +/- 14%. We discuss here the nature of the shell stars and present preliminary results from ground-based optical spectra of the survey target stars. We will also outline our planned studies aimed at further characterization of the shell star class.

X-ray-emitting filaments in the cooling flow cluster A2029

NASA Technical Reports Server (NTRS)

Sarazin, Craig L.; O'Connell, Robert W.; Mcnamara, Brian R.

1992-01-01

High-resolution X-ray observations of the cluster A2029 are presented which confirm the presence of a cooling flow, despite the lack of optical line emission or evidence for recent star formation. The cooling rate and radius are about 370 solar mass/yr and 230 kpc, respectively. Emission from the inner cooling flow is dominated by a number of X-ray-emitting filaments. This may be the first case where such inhomogeneities are clearly resolved. The filaments are theorized to be supported in part by magnetic fields and may be connected with the filaments of very strong Faraday rotation seen in several nearly cooling flows.

The direct cooling tail method for X-ray burst analysis to constrain neutron star masses and radii

NASA Astrophysics Data System (ADS)

Suleimanov, Valery F.; Poutanen, Juri; Nättilä, Joonas; Kajava, Jari J. E.; Revnivtsev, Mikhail G.; Werner, Klaus

2017-04-01

Determining neutron star (NS) radii and masses can help to understand the properties of matter at supra-nuclear densities. Thermal emission during thermonuclear X-ray bursts from NSs in low-mass X-ray binaries provides a unique opportunity to study NS parameters, because of the high fluxes, large luminosity variations and the related changes in the spectral properties. The standard cooling tail method uses hot NS atmosphere models to convert the observed spectral evolution during cooling stages of X-ray bursts to the Eddington flux FEdd and the stellar angular size Ω. These are then translated to the constraints on the NS mass M and radius R. Here we present the improved, direct cooling tail method that generalizes the standard approach. First, we adjust the cooling tail method to account for the bolometric correction to the flux. Then, we fit the observed dependence of the blackbody normalization on flux with a theoretical model directly on the M-R plane by interpolating theoretical dependences to a given gravity, hence ensuring only weakly informative priors for M and R instead of FEdd and Ω. The direct cooling method is demonstrated using a photospheric radius expansion burst from SAX J1810.8-2609, which has happened when the system was in the hard state. Comparing to the standard cooling tail method, the confidence regions are shifted by 1σ towards larger radii, giving R = 11.5-13.0 km at M = 1.3-1.8 M⊙ for this NS.

Detection of a Red Supergiant Progenitor Star of a Type II-Plateau Supernova

NASA Astrophysics Data System (ADS)

Smartt, Stephen J.; Maund, Justyn R.; Hendry, Margaret A.; Tout, Christopher A.; Gilmore, Gerard F.; Mattila, Seppo; Benn, Chris R.

2004-01-01

We present the discovery of a red supergiant star that exploded as supernova 2003gd in the nearby spiral galaxy M74. The Hubble Space Telescope (HST) and the Gemini Telescope imaged this galaxy 6 to 9 months before the supernova explosion, and subsequent HST images confirm the positional coincidence of the supernova with a single resolved star that is a red supergiant of 8+4-2 solar masses. This confirms both stellar evolution models and supernova theories predicting that cool red supergiants are the immediate progenitor stars of type II-plateau supernovae.

VizieR Online Data Catalog: Hot subdwarf stars in LAMOST DR1 (Luo+, 2016)

NASA Astrophysics Data System (ADS)

Luo, Y.-P.; Nemeth, P.; Liu, C.; Deng, L.-C.; Han, Z.-W.

2018-01-01

We present a catalog of 166 spectroscopically identified hot subdwarf stars from LAMOST DR1, 44 of which show the characteristics of cool companions in their optical spectra. Atmospheric parameters of 122 subdwarf stars with non-composite spectra were measured by fitting the profiles of hydrogen (H) and helium (He) lines with synthetic spectra from non-LTE model atmospheres. A unique property of our sample is that it covers a large range in apparent magnitude and galactic latitude, therefore it contains a mix of stars from different populations and galactic environments. (3 data files).

Time-dependent corona models - Scaling laws

NASA Technical Reports Server (NTRS)

Korevaar, P.; Martens, P. C. H.

1989-01-01

Scaling laws are derived for the one-dimensional time-dependent Euler equations that describe the evolution of a spherically symmetric stellar atmosphere. With these scaling laws the results of the time-dependent calculations by Korevaar (1989) obtained for one star are applicable over the whole Hertzsprung-Russell diagram and even to elliptic galaxies. The scaling is exact for stars with the same M/R-ratio and a good approximation for stars with a different M/R-ratio. The global relaxation oscillation found by Korevaar (1989) is scaled to main sequence stars, a solar coronal hole, cool giants and elliptic galaxies.

Selected highlights from the Extreme Ultraviolet Explorer

NASA Technical Reports Server (NTRS)

Bowyer, S.; Malina, R. F.

1995-01-01

We present a few scientific highlights from the Extreme Ultraviolet Explorer (EUVE) all-sky and deep surveys, from the EUVE Righ Angle Program, and from the EUVE Guest Observer Program. The First EUVE Source Catalog includes 410 extreme ultraviolet (EUV) sources detected in the initial processing of the EUVE all-sky data. A program of optical identification indicates that counterparts include cool star coronae, flare stars, hot white dwarfs, central stars of planetary nebulae, B star photospheres and winds, an X-ray binary, extragalactic objects (active galactic nuclei, BL Lacertae), solar system objects (Moon, Mars, Io,), supernova remnants, and two novae.

Flash Mixing of the White Dwarf Cooling Curve Spectroscopic Confirmation in NGC 2808

NASA Technical Reports Server (NTRS)

Brown, Thomas M.; Lanz, Thierry; Sweigart, Allen V.; Cracraft, Misty; Hubeny, Ivan; Landsman, Wayne B.

2009-01-01

We present new HST far-UV spectroscopy of two dozen hot evolved stars in NGC 2808, a massive globular cluster with a large population of "blue-hook" stars. The blue-hook stars are found in ultraviolet color-magnitude diagrams of the most massive globular clusters, where they fall at luminosities immediately below the hot end of the horizontal branch (HB), in a region of the HR diagram unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that these subluminous HB stars are very likely the progeny of stars that undergo extensive internal mixing during a late He-core flash on the white dwarf cooling curve. This flash mixing leads to hotter temperatures and an enormous enhancement of the surface He and C abundances; the hotter temperatures and associated decrease in the hydrogen opacity shortward of the Lyman limit makes the stars brighter in the extreme UV but appear sub luminous in the UV and optical. Our far-UV spectroscopy demonstrates that, relative to normal HB stars at the same color, the blue-hook stars of NGC 2808 are hotter and greatly enhanced in He and C, thus providing unambiguous evidence of flash mixing in the subluminous population. Although the C abundance in the blue-hook stars is orders of magnitude larger than that in the normal HB stars, the atmospheric C abundance in both the blue-hook and normal HB stars appears to be affected by gravitational settling. The abundance variations seen in C, Si, and the Fe-peak elements indicate that atmospheric diffusion is at play in our sample, with all of our hot subdwarfs at 25,000 K to 50,000 K exhibiting large enhancements of the iron-peak elements. The hottest subdwarfs in our blue-hook sample may be pulsators, given that they fall in the temperature range of newly-discovered pulsating subdwarfs in omega Cen.

Asteroseismology of White Dwarf Stars

NASA Technical Reports Server (NTRS)

Hansen, Carl J.

1997-01-01

The primary purpose of this investigation has been to study various aspects of multimode pulsations in variable white dwarfs. In particular, nonlinear interactions among pulsation modes in white dwarfs (and, to some extent, in other variable stars), analysis of recent observations where such interactions are important, and preliminary work on the effects of crystallization in cool white dwarfs are reported.

Theory of Radiation Transfer in Neutron Star Atmospheres

NASA Technical Reports Server (NTRS)

Zavlin, Vyacheslav

2006-01-01

The possibility for direct investigation of thermal emission from isolated neutron stars opened about a quarter of century ago with the launch of the first X-ray observatories Einstein and EXOSAT stimulated developing models of the neutron star surface radiation which began at the end of 80's. Confronting observational data with theoretical models of thermal emission allows one to infer the surface temperatures, magnetic fields, chemical composition, and neutron star masses and radii. This information, supplemented with the model equations of state and neutron star cooling models, provides an opportunity to understand the fundamental properties of the superdense matter in the stars' interiors. Almost all available models are based on the assumption that thermal radiation emitted by a neutron star is formed in the superficial star's layers--atmosphere. The neutron star atmospheres are very different from those of usual stars due to the immense gravity and huge magnetic fields. In this presentation we review the current status of the neutron star atmosphere modeling, present most important results, discuss problems and possible future developments.

The Mass and Absorption Columns of Galactic Gaseous Halos

NASA Astrophysics Data System (ADS)

Qu, Zhijie; Bregman, Joel N.

2018-01-01

The gaseous halo surrounding the galaxy is a reservoir for the gas on the galaxy disk, supplying materials for the star formation. We developed a gaseous halo model connecting the galactic disk and the gaseous halo by assuming the star formation rate is equal to the radiative cooling rate. Besides the single-phase collisional gaseous halo, we also consider the photoionization effect and a time-independent cooling model that assumes the mass cooling rate is constant over all temperatures. The photoionization dominates the low mass galaxy and the outskirts of the massive galaxy due to the low-temperature or low-density nature. The multi-phase cooling model dominates the denser region within the cooling radius, where the efficient radiative cooling must be included. Applying these two improvements, our model can reproduce the most of observed high ionization state ions (i.e., O VI, O VII, Ne VIII and Mg X). Our models show that the O VI column density is almost a constant of around 10^14 cm^-2 over a wide stellar mass from M_\\star ~10^8 M_Sun to 10^11 M_Sun, which is constant with current observations. This model also implies the O VI is photoionized for the galaxy with a halo mass <~ 3 * 10^11 M_Sun, while for more massive galaxies, the O VI is from the cooling-down medium from higher temperature materials (collisional ionized). As higher ionization states, Mg X and Ne VIII are also consistent with observations with the column density of 10^13.5 - 10^14.0 cm^-2, however, the absorber-galaxy pair sample is few to constrain the connection with the galaxy. Based on our calculation, such a gaseous halo cannot close the census of baryonic materials in the galaxy, which shows the same tendency as the baryonic fraction function of the EAGLE simulation. Finally, our model predicts plateaus of the Ne VIII and the Mg X column densities above the sub-L^* galaxy, and the possibly detectable O VII and O VIII column densities for low-mass galaxies, which help to determine the required detection limit for the future observations and missions.

Effect of lithium hydride on the cooling of primordial gas

NASA Astrophysics Data System (ADS)

Liu, Boyuan; Bromm, Volker

2018-05-01

We complete the formulation of the standard model of first star formation by exploring the possible impact of LiH cooling, which has been neglected in previous simulations of non-linear collapse. Specifically, we find that at redshift z ≳ 5, the cooling by LiH has no effect on the thermal evolution of shocked primordial gas, and of collapsing primordial gas into minihaloes or relic H II regions, even if the primordial lithium abundance were enhanced by one order of magnitude. Adding the most important lithium species to a minimum network of primordial chemistry, we demonstrate that insufficient LiH is produced in all cases considered, about [LiH/Li] ˜ 10-9 for T ≲ 100 K. Indeed, LiH cooling would only be marginally significant in shocked primordial gas for the highly unlikely case that the LiH abundance were increased by nine orders of magnitude, implying that all lithium would have to be converted into LiH. In this study, photodestruction processes are not considered, and the collisional disassociation rate of LiH is possibly underestimated, rendering our results an extreme upper limit. Therefore, the cooling by LiH can safely be neglected for the thermal evolution of Population III star-forming gas.

Simulating the impact of dust cooling on the statistical properties of the intra-cluster medium

NASA Astrophysics Data System (ADS)

Pointecouteau, Etienne; da Silva, Antonio; Catalano, Andrea; Montier, Ludovic; Lanoux, Joseph; Roncarelli, Mauro; Giard, Martin

2009-08-01

From the first stages of star and galaxy formation, non-gravitational processes such as ram pressure stripping, SNs, galactic winds, AGNs, galaxy-galaxy mergers, etc. lead to the enrichment of the IGM in stars, metals as well as dust, via the ejection of galactic material into the IGM. We know now that these processes shape, side by side with gravitation, the formation and the evolution of structures. We present here hydrodynamic simulations of structure formation implementing the effect of the cooling by dust on large scale structure formation. We focus on the scale of galaxy clusters and study the statistical properties of clusters. Here, we present our results on the TX-M and the LX-M scaling relations which exhibit changes on both the slope and normalization when adding cooling by dust to the standard radiative cooling model. For example, the normalization of the TX-M relation changes only by a maximum of 2% at M=1014M⊙ whereas the normalization of the LX-TX changes by as much as 10% at TX=1keV for models that including dust cooling. Our study shows that the dust is an added non-gravitational process that contributes shaping the thermodynamical state of the hot ICM gas.

Does radiative feedback by the first stars promote or prevent second generation star formation?

NASA Astrophysics Data System (ADS)

Ahn, Kyungjin; Shapiro, Paul R.

2007-03-01

We study the effect of starlight from the first stars on the ability of other minihaloes in their neighbourhood to form additional stars. The first stars in the Λ cold dark matter (ΛCDM) universe are believed to have formed in minihaloes of total mass ~105-6 Msolar at redshifts z >~ 20, when molecular hydrogen (H2) formed and cooled the dense gas at their centres, leading to gravitational collapse. Simulations suggest that the Population III (Pop III) stars thus formed were massive (~100 Msolar) and luminous enough in ionizing radiation to cause an ionization front (I-front) to sweep outward, through their host minihalo and beyond, into the intergalactic medium. Our previous work suggested that this I-front was trapped when it encountered other, nearby minihaloes, and that it failed to penetrate the dense gas at their centres within the lifetime of the Pop III stars (<~3 Myr). The question of what the dynamical consequences were for these target minihaloes, of their exposure to the ionizing and dissociating starlight from the Pop III star requires further study, however. Towards this end, we have performed a series of detailed, one-dimensional (1D), radiation-hydrodynamical simulations to answer the question of whether star formation in these surrounding minihaloes was triggered or suppressed by radiation from the first stars. We have varied the distance to the source (and, hence, the flux) and the mass and evolutionary stage of the target haloes to quantify this effect. We find (1) trapping of the I-front and its transformation from R-type to D-type, preceded by a shock front; (2) photoevaporation of the ionized gas (i.e. all gas originally located outside the trapping radius); (3) formation of an H2 precursor shell which leads the I-front, stimulated by partial photoionization; and (4) the shock-induced formation of H2 in the minihalo neutral core when the shock speeds up and partially ionizes the gas. The fate of the neutral core is mostly determined by the response of the core to this shock front, which leads to molecular cooling and collapse that, when compared to the same halo without external radiation, is (a) expedited, or (b) delayed, or (c) unaltered, or (d) reversed or prevented, depending upon the flux (i.e. distance to the source) and the halo mass and evolutionary stage. When collapse is expedited, star formation in neighbouring minihaloes or in merging subhaloes within the host minihalo sometimes occurs within the lifetime of the first star. Roughly speaking, most haloes that were destined to cool, collapse and form stars in the absence of external radiation are found to do so even when exposed to the first Pop III star in their neighbourhood, while those that would not have done so are still not able to. A widely held view that the first Pop III stars must exert either positive or negative feedback on the formation of the stars in neighbouring minihaloes should, therefore, be revisited.

Stellar motion induced by gravitational instabilities in protoplanetary discs

NASA Astrophysics Data System (ADS)

Michael, Scott; Durisen, R. H.

2010-07-01

We test the effect of assumptions about stellar motion on the behaviour of gravitational instabilities (GIs) in protoplanetary discs around solar-type stars by performing two simulations that are identical in all respects except the treatment of the star. In one simulation, the star is assumed to remain fixed at the centre of the inertial reference frame. In the other, stellar motion is handled properly by including an indirect potential in the hydrodynamic equations to model the star's reference frame as one which is accelerated by star/disc interactions. The discs in both simulations orbit a solar mass star, initially extend from 2.3 to 40 au with a ϖ-1/2 surface density profile, and have a total mass of 0.14 Msolar. The γ = 5/3 ideal gas is assumed to cool everywhere with a constant cooling time of two outer rotation periods. The overall behaviour of the disc evolution is similar, except for weakening in various measures of GI activity by about at most tens of per cent for the indirect potential case. Overall conclusions about disc evolution in earlier papers by our group, where the star was always assumed to be fixed in an inertial frame, remain valid. There is no evidence for independent one-armed instabilities, like the Stimulation by the Long-range Interaction of Newtonian Gravity (SLING), in either simulation. On the other hand, the stellar motion about the system centre of mass (COM) in the simulation with the indirect potential is substantial, up to 0.25 au during the burst phase, as GIs initiate, and averaging about 0.9 au during the asymptotic phase, when the GIs reach an overall balance of heating and cooling. These motions appear to be a stellar response to non-linear interactions between discrete global spiral modes in both the burst and asymptotic phases of the evolution, and the star's orbital motion about the COM reflects the orbit periods of disc material near the corotation radii of the dominant spiral waves. This motion is, in principle, large enough to be observable and could be confused with stellar wobble due to the presence of one or more super-Jupiter mass protoplanets orbiting at 10's au. We discuss why the excursions in our simulation are so much larger than those seen in simulations by Rice et al.

High rotational CO lines in post-AGB stars and PNe

NASA Technical Reports Server (NTRS)

Justtanont, K.; Tielens, Alexander G. G. M.; Skinner, C. J.; Haas, Michael R.

1995-01-01

A significant fraction of a star's initial mass is lost while it is on the Asymptotic Giant Branch (AGB). Mass loss rates range from 10(exp -7) solar mass/yr for early AGB stars to a few 10(exp -4) solar mass/yr for stars at the tip of the AGB. Dust grains condense from the outflow as the gas expands and form a dust shell around the central star. A superwind (approximately 10(exp -4) to 10(exp -3) solar mass/yr) is thought to terminate the AGB phase. In the post-AGB phase, the star evolves to a higher effective temperature, the mass loss decreases (approximately 10(exp -8) solar mass/yr), but the wind velocity increases (approximately 1000 km/s). During this evolution, dust and gas are exposed to an increasingly harsher radiation field and when T(sub eff) reaches about 30,000 K, the nebula is ionized and becomes a planetary nebula (PN). Photons from the central star can create a photodissociation region (PDR) in the expanding superwind. Gas can be heated through the photoelectric effect working on small grains and polycyclic aromatic hydrocarbons (PAH's). This gas can cool via the atomic fine structure lines of O I (63 microns and 145 microns) and C II (158 microns), as well as the rotational lines of CO. In the post-AGB phase, the fast wind from the central star will interact with the material ejected during the AGB phase. The shock caused by this interaction will dissociate and heat the gas. This warm gas will cool through atomic fine structure lines of O I and the rotational lines of (newly formed) CO.

Dissertation Award in Nuclear Physics Recipient: Astromaterials in Neutron Stars

NASA Astrophysics Data System (ADS)

Caplan, Matthew E.

2017-09-01

Stars freeze. As they age and cool white dwarfs and neutron stars crystallize, with remarkable materials forming in their interiors. These `astromaterials' have structures similar to terrestrial crystalline solids and liquid crystals, though they are over a trillion times denser. Notably, because their material properties affect the observable properties of the star, astromaterials must be understood to interpret observations of neutron stars. Thus, astromaterial science can be thought of as an interdisciplinary field, using techniques from material science to study nuclear physics systems with astrophysical relevance. In this talk, I will discuss recent results from simulations of astromaterials and how we use these results to interpret observations of neutron stars in X-ray binaries. In addition, I will discuss how nuclear pasta, in neutron stars, forms structures remarkably similar to biophysical membranes seen in living organisms.

Multiplicity among chemically peculiar stars. II. Cool magnetic Ap stars

NASA Astrophysics Data System (ADS)

Carrier, F.; North, P.; Udry, S.; Babel, J.

2002-10-01

We present new orbits for sixteen Ap spectroscopic binaries, four of which might in fact be Am stars, and give their orbital elements. Four of them are SB2 systems: HD 5550, HD 22128, HD 56495 and HD 98088. The twelve other stars are: HD 9996, HD 12288, HD 40711, HD 54908, HD 65339, HD 73709, HD 105680, HD 138426, HD 184471, HD 188854, HD 200405 and HD 216533. Rough estimates of the individual masses of the components of HD 65339 (53 Cam) are given, combining our radial velocities with the results of speckle interferometry and with Hipparcos parallaxes. Considering the mass functions of 74 spectroscopic binaries from this work and from the literature, we conclude that the distribution of the mass ratio is the same for cool Ap stars and for normal G dwarfs. Therefore, the only differences between binaries with normal stars and those hosting an Ap star lie in the period distribution: except for the case of HD 200405, all orbital periods are longer than (or equal to) 3 days. A consequence of this peculiar distribution is a deficit of null eccentricities. There is no indication that the secondary has a special nature, like e.g. a white dwarf. Based on observations collected at the Observatoire de Haute-Provence (CNRS), France. Tables 1 to 3 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/394/151 Appendix B is only available in electronic form at http://www.edpsciences.org

Inefficient jet-induced star formation in Centaurus A. High resolution ALMA observations of the northern filaments

NASA Astrophysics Data System (ADS)

Salomé, Q.; Salomé, P.; Miville-Deschênes, M.-A.; Combes, F.; Hamer, S.

2017-12-01

NGC 5128 (Centaurus A) is one of the best targets to study AGN feedback in the local Universe. At 13.5 kpc from the galaxy, optical filaments with recent star formation lie along the radio jet direction. This region is a testbed for positive feedback, here through jet-induced star formation. Atacama Pathfinder EXperiment (APEX) observations have revealed strong CO emission in star-forming regions and in regions with no detected tracers of star formation activity. In cases where star formation is observed, this activity appears to be inefficient compared to the Kennicutt-Schmidt relation. We used the Atacama Large Millimeter/submillimeter Array (ALMA) to map the 12CO(1-0) emission all along the filaments of NGC 5128 at a resolution of 1.3'' 23.8pc. We find that the CO emission is clumpy and is distributed in two main structures: (i) the Horseshoe complex, located outside the HI cloud, where gas is mostly excited by shocks and where no star formation is observed, and (ii) the Vertical filament, located at the edge of the HI shell, which is a region of moderate star formation. We identified 140 molecular clouds using a clustering method applied to the CO data cube. A statistical study reveals that these clouds have very similar physical properties, such as size, velocity dispersion, and mass, as in the inner Milky Way. However, the range of radius available with the present ALMA observations does not enable us to investigate whether or not the clouds follow the Larson relation. The large virial parameter αvir of the clouds suggests that gravity is not dominant and clouds are not gravitationally unstable. Finally, the total energy injection in the northern filaments of Centaurus A is of the same order as in the inner part of the Milky Way. The strong CO emission detected in the northern filaments is an indication that the energy injected by the jet acts positively in the formation of dense molecular gas. The relatively high virial parameter of the molecular clouds suggests that the injected kinetic energy is too strong for star formation to be efficient. This is particularly the case in the horseshoe complex, where the virial parameter is the largest and where strong CO is detected with no associated star formation. This is the first evidence of AGN positive feedback in the sense of forming molecular gas through shocks, associated with low star formation efficiency due to turbulence injection by the interaction with the radio jet. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2015.1.01019.S.The full Table A.1 and a catalogue of the molecular clouds are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/608/A98

Imaging the Hot Stellar Content of Early Type Galaxies

NASA Astrophysics Data System (ADS)

Bertola, Francesco

1991-07-01

WE PROPOSE TO IMAGE WITH THE FOC IN THE F/96 CONFIGURATION FIVE EARLY TYPE GALAXIES IN FOUR PASSBANDS CENTERED AT 1500 A, 2200 A, 2800 A AND 3400 A. WHEN COUPLED WITH PHOTOMETRY OBTAINED FROM THE GROUND OUR OBSERVATIONS WILL ALLOW US TO DERIVE COMPLETE SED OF THESE GALAXIES AS A FUNCTION OF THE DISTANCE FROM THE CENTER. THIS IS A KEY STEP TOWARDS THE UNDERSTANDING OF STELLAR POPULATIONS - IN PARTICULAR THE ONE RESPONSIBLE FOR THE UV EMISSION - IN EARLY TYPE GALAXIES AND WILL PROVIDE IMPORTANT INSIGHT IN THEIR FORMATION AND EVOLUTION. WE PLAN TO OBSERVE NGC 1399, NGC 2681, NGC 4552, NGC 5018 AND NGC 4627 WHICH SAMPLE A WIDE RANGE OF INTRINSIC PROPERTIES AS INDICATED BY PREVIOUS IUE OBSERVATIONS. FOR NGC 4627 THERE IS EVIDENCE OF ONGOING STAR FORMATION AND THE HST WILL BE ABLE TO SHOW THE CHARACTERISTIC CLUMPINESS. NGC 2681 HAD A STARBUST OF AGE GREATER THAN 1 GYR. NGC 4552 IS ONE OF THE MOST METAL RICH GALAXY KNOWN. NGC 1399 HAS THE SAME METALLICITY AND LUMINOSITY OF THE PREVIOUS GALAXY BUT IS A MUCH STRONGER X-RAY EMITTER. NGC 5018 IS A VERY GOOD CANDIDATE FOR ONGOING STAR FORMATION. WE BELIEVE IN THIS WAY WE CAN OBTAIN SED FOR THE TWO-DIMENSIONAL IMAGES OF EARLY TYPE GALAXIES FROM BROAD BAND IMAGING ALONE. THE CALIBRATION OF OUR FILTER SYSTEM WILL ALLOW US TO APPLY IT TO THE BIDIMENSIONAL ANALYSIS OF THE GENERAL SAMPLE OF EARLY TYPE GALAXIES.

Stellar winds and coronae of low-mass Population II/III stars

NASA Astrophysics Data System (ADS)

Suzuki, Takeru K.

2018-06-01

We investigated stellar winds from zero-/low-metallicity low-mass stars by magnetohydrodynamical simulations for stellar winds driven by Alfvén waves from stars with mass M = (0.6-0.8) M⊙ and metallicity Z = (0-1) Z⊙, where M⊙ and Z⊙ are the solar mass and metallicity, respectively. Alfvénic waves, which are excited by the surface convection, travel upward from the photosphere and heat up the corona by their dissipation. For lower Z, denser gas can be heated up to the coronal temperature because of the inefficient radiation cooling. The coronal density of Population II/III stars with Z ≤ 0.01 Z⊙ is one to two orders of magnitude larger than that of a solar-metallicity star with the same mass, and as a result, the mass loss rate, \\dot{M}, is 4.5-20 times larger. This indicates that metal accretion on low-mass Pop. III stars is negligible. The soft X-ray flux of the Pop. II/III stars is also expected to be ˜1-30 times larger than that of a solar-metallicity counterpart owing to the larger coronal density, even though the radiation cooling efficiency is smaller. A larger fraction of the input Alfvénic wave energy is transmitted to the corona in low-Z stars because they avoid severe reflection owing to the smaller density difference between the photosphere and the corona. Therefore, a larger fraction is converted to the thermal energy of the corona and the kinetic energy of the stellar wind. From this energetics argument, we finally derived a scaling of \\dot{M} as \\dot{M}∝ L R_{\\star }^{11/9} M_{\\star }^{-10/9} T_eff^{11/2}[\\max (Z/Z_{⊙},0.01)]^{-1/5}, where L, R⋆, and Teff are the stellar luminosity, radius, and effective temperature, respectively.

Stellar winds and coronae of low-mass Population II/III stars

NASA Astrophysics Data System (ADS)

Suzuki, Takeru K.

2018-04-01

We investigated stellar winds from zero-/low-metallicity low-mass stars by magnetohydrodynamical simulations for stellar winds driven by Alfvén waves from stars with mass M = (0.6-0.8) M⊙ and metallicity Z = (0-1) Z⊙, where M⊙ and Z⊙ are the solar mass and metallicity, respectively. Alfvénic waves, which are excited by the surface convection, travel upward from the photosphere and heat up the corona by their dissipation. For lower Z, denser gas can be heated up to the coronal temperature because of the inefficient radiation cooling. The coronal density of Population II/III stars with Z ≤ 0.01 Z⊙ is one to two orders of magnitude larger than that of a solar-metallicity star with the same mass, and as a result, the mass loss rate, \\dot{M}, is 4.5-20 times larger. This indicates that metal accretion on low-mass Pop. III stars is negligible. The soft X-ray flux of the Pop. II/III stars is also expected to be ˜1-30 times larger than that of a solar-metallicity counterpart owing to the larger coronal density, even though the radiation cooling efficiency is smaller. A larger fraction of the input Alfvénic wave energy is transmitted to the corona in low-Z stars because they avoid severe reflection owing to the smaller density difference between the photosphere and the corona. Therefore, a larger fraction is converted to the thermal energy of the corona and the kinetic energy of the stellar wind. From this energetics argument, we finally derived a scaling of \\dot{M} as \\dot{M}∝ L R_{\\star }^{11/9} M_{\\star }^{-10/9} T_eff^{11/2}[\\max (Z/Z_{⊙},0.01)]^{-1/5}, where L, R⋆, and Teff are the stellar luminosity, radius, and effective temperature, respectively.

A Revised Estimate of the Occurrence Rate of Terrestrial Planets in the Habitable Zones around Kepler M-dwarfs

NASA Astrophysics Data System (ADS)

Kopparapu, Ravi Kumar

2013-04-01

Because of their large numbers, low-mass stars may be the most abundant planet hosts in our Galaxy. Furthermore, terrestrial planets in the habitable zones (HZs) around M-dwarfs can potentially be characterized in the near future and hence may be the first such planets to be studied. Recently, Dressing & Charbonneau used Kepler data and calculated the frequency of terrestrial planets in the HZ of cool stars to be 0.15^{+0.13}_{-0.06} per star for Earth-size planets (0.5-1.4 R ⊕). However, this estimate was derived using the Kasting et al. HZ limits, which were not valid for stars with effective temperatures lower than 3700 K. Here we update their result using new HZ limits from Kopparapu et al. for stars with effective temperatures between 2600 K and 7200 K, which includes the cool M stars in the Kepler target list. The new HZ boundaries increase the number of planet candidates in the HZ. Assuming Earth-size planets as 0.5-1.4 R ⊕, when we reanalyze their results, we obtain a terrestrial planet frequency of 0.48^{+0.12}_{-0.24} and 0.53^{+0.08}_{-0.17} planets per M-dwarf star for conservative and optimistic limits of the HZ boundaries, respectively. Assuming Earth-size planets as 0.5-2 R ⊕, the frequency increases to 0.51^{+0.10}_{-0.20} per star for the conservative estimate and to 0.61^{+0.07}_{-0.15} per star for the optimistic estimate. Within uncertainties, our optimistic estimates are in agreement with a similar optimistic estimate from the radial velocity survey of M-dwarfs (0.41^{+0.54}_{-0.13}). So, the potential for finding Earth-like planets around M stars may be higher than previously reported.

A REVISED ESTIMATE OF THE OCCURRENCE RATE OF TERRESTRIAL PLANETS IN THE HABITABLE ZONES AROUND KEPLER M-DWARFS

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

Kopparapu, Ravi Kumar

Because of their large numbers, low-mass stars may be the most abundant planet hosts in our Galaxy. Furthermore, terrestrial planets in the habitable zones (HZs) around M-dwarfs can potentially be characterized in the near future and hence may be the first such planets to be studied. Recently, Dressing and Charbonneau used Kepler data and calculated the frequency of terrestrial planets in the HZ of cool stars to be 0.15{sup +0.13}{sub -0.06} per star for Earth-size planets (0.5-1.4 R{sub Circled-Plus }). However, this estimate was derived using the Kasting et al. HZ limits, which were not valid for stars with effectivemore » temperatures lower than 3700 K. Here we update their result using new HZ limits from Kopparapu et al. for stars with effective temperatures between 2600 K and 7200 K, which includes the cool M stars in the Kepler target list. The new HZ boundaries increase the number of planet candidates in the HZ. Assuming Earth-size planets as 0.5-1.4 R{sub Circled-Plus }, when we reanalyze their results, we obtain a terrestrial planet frequency of 0.48{sup +0.12}{sub -0.24} and 0.53{sup +0.08}{sub -0.17} planets per M-dwarf star for conservative and optimistic limits of the HZ boundaries, respectively. Assuming Earth-size planets as 0.5-2 R{sub Circled-Plus }, the frequency increases to 0.51{sup +0.10}{sub -0.20} per star for the conservative estimate and to 0.61{sup +0.07}{sub -0.15} per star for the optimistic estimate. Within uncertainties, our optimistic estimates are in agreement with a similar optimistic estimate from the radial velocity survey of M-dwarfs (0.41{sup +0.54}{sub -0.13}). So, the potential for finding Earth-like planets around M stars may be higher than previously reported.« less

Space Shuttle Projects

NASA Image and Video Library

1996-11-01

This STS-80 onboard photograph shows the Orbiting Retrievable Far and Extreme Ultraviolet Spectrometer-Shuttle Pallet Satellite II (ORFEUS-SPAS II), photographed during approach by the Space Shuttle Orbiter Columbia for retrieval. Built by the German Space Agency, DARA, the ORFEUS-SPAS II, a free-flying satellite, was dedicated to astronomical observations at very short wavelengths to: investigate the nature of hot stellar atmospheres, investigate the cooling mechanisms of white dwarf stars, determine the nature of accretion disks around collapsed stars, investigate supernova remnants, and investigate the interstellar medium and potential star-forming regions. Some 422 observations of almost 150 astronomical objects were completed, including the Moon, nearby stars, distant Milky Way stars, stars in other galaxies, active galaxies, and quasar 3C273. The STS-80 mission was launched November 19, 1996.

The TESS Input Catalog and Selection of Targets for the TESS Transit Search

NASA Astrophysics Data System (ADS)

Pepper, Joshua; Stassun, Keivan G.; Paegert, Martin; Oelkers, Ryan; De Lee, Nathan Michael; Torres, Guillermo; TESS Target Selection Working Group

2018-01-01

The TESS mission will photometrically survey millions of the brightest stars over almost the entire the sky to detect transiting exoplanets. A key step to enable that search is the creation of the TESS Input Catalog (TIC), a compiled catalog of 700 million stars and galaxies with observed and calculated parameters. From the TIC we derive the Candidate Target List (CTL) to identify target stars for the 2-minute TESS postage stamps. The CTL is designed to identify the best stars for the detection of small planets, which includes all bright cool dwarf stars in the sky. I will describe the target selection strategy, the distribution of stars in the current CTL, and how both the TIC and CTL will expand and improve going forward.

XMM-Newton X-ray Observatory Guest Observer program (AO-1) at CASA

NASA Technical Reports Server (NTRS)

Skinner, Stephen L.

2003-01-01

In this research program, we obtained and analyzed X-ray observations of the Wolf-Rayet (WR) star WR 110 (HD 165688) using the XMM-Newton space-based observatory. Radio observations were also obtained using the Very Large Array (VLA) radio telescope located in New Mexico and operated by the Natl. Radio Astronomy Observatory (NRAO). This star was targeted for observations primarily because it is believed to be a single WR star without a companion. Single WR stars are thought to emit X-rays from cool plasma in shocks distributed throughout their powerful stellar winds. However, there has been little observational work done to test this idea since single WR stars are relatively weak X-ray sources and have been difficult to detect with previous generation telescopes. The launch of XMM-Newton provides a new telescope that is much more sensitive than its predecessors, allowing single WR stars to be studied in detail for the first time. X-ray emission was clearly detected from WR 110. Analysis of its spectrum yields a surprising result. Its X-ray emitting plasma is distributed over a range of temperatures and is dominated by relatively cool plasma with a characteristic temperature T is approximately 6 million K. Such plasma can be explained by existing theoretical wind shock models. However, the spectrum also shows hotter plasma whose temperature is uncertain but is thought to be in excess of T approximately 30 million K. The origin of this hotter plasma is yet unknown, but possible mechanisms are identified

A Strong Shallow Heat Source in the Accreting Neutron Star MAXI J0556-332

NASA Astrophysics Data System (ADS)

Deibel, Alex; Cumming, Andrew; Brown, Edward F.; Page, Dany

2015-08-01

An accretion outburst in an X-ray transient deposits material onto the neutron star primary; this accumulation of matter induces reactions in the neutron star’s crust. During the accretion outburst these reactions heat the crust out of thermal equilibrium with the core. When accretion halts, the crust cools to its long-term equilibrium temperature on observable timescales. Here we examine the accreting neutron star transient MAXI J0556-332, which is the hottest transient, at the start of quiescence, observed to date. Models of the quiescent light curve require a large deposition of heat in the shallow outer crust from an unknown source. The additional heat injected is ≈4-10 MeV per accreted nucleon; when the observed decline in accretion rate at the end of the outburst is accounted for, the required heating increases to ≈6-16 MeV. This shallow heating is still required to fit the light curve even after taking into account a second accretion episode, uncertainties in distance, and different surface gravities. The amount of shallow heating is larger than that inferred for other neutron star transients and is larger than can be supplied by nuclear reactions or compositionally driven convection; but it is consistent with stored mechanical energy in the accretion disk. The high crust temperature ({T}b≳ {10}9 {{K}}) makes its cooling behavior in quiescence largely independent of the crust composition and envelope properties, so that future observations will probe the gravity of the source. Fits to the light curve disfavor the presence of Urca cooling pairs in the crust.

A disrupted molecular torus around Eta Carinae as seen in 12CO with ALMA

NASA Astrophysics Data System (ADS)

Smith, Nathan; Ginsburg, Adam; Bally, John

2018-03-01

We present Atacama Large Millimeter Array (ALMA) observations of 12CO 2-1 emission from circumstellar material around the massive star Eta Carinae (η Car). These observations reveal new structural details about the cool equatorial torus located ˜4000 au from the star. The CO torus is not a complete azimuthal loop, but rather, is missing its near side, which appears to have been cleared away. The missing material matches the direction of apastron in the eccentric binary system, making it likely that η Car's companion played an important role in disrupting portions of the torus soon after ejection. Molecular gas seen in ALMA data aligns well with the cool dust around η Car previously observed in mid-infrared (IR) maps, whereas hot dust resides at the inner surface of the molecular torus. The CO also coincides with the spatial and velocity structure of near-IR H2 emission. Together, these suggest that the CO torus seen by ALMA is actually the pinched waist of the Homunculus polar lobes, which glows brightly because it is close to the star and warmer than the poles. The near side of the torus appears to be a blowout, associated with fragmented equatorial ejecta. We discuss implications for the origin of various features north-west of the star. CO emission from the main torus implies a total gas mass in the range of 0.2-1 M⊙ (possibly up to 5 M⊙ or more, although with questionable assumptions). Deeper observations are needed to constrain CO emission from the cool polar lobes.

Convection in stars and heating of coronae

NASA Technical Reports Server (NTRS)

Mullan, D. J.

1991-01-01

The properties of convection in the sun and other cool stars are summarized. Recent studies of convection which have involved the use of supercomputers to model the flow of compressible gas in three dimensions are discussed. It is shown how the results of these computations may eventualy provide an understanding of how nonthermal processes heat coronal gas to temperatures of millions of degrees.

A Limit on the Warm Dark Matter Particle Mass from the Redshifted 21 cm Absorption Line

NASA Astrophysics Data System (ADS)

Safarzadeh, Mohammadtaher; Scannapieco, Evan; Babul, Arif

2018-06-01

The recent Experiment to Detect the Global Epoch of Reionization Signature (EDGES) collaboration detection of an absorption signal at a central frequency of ν = 78 ± 1 MHz points to the presence of a significant Lyα background by a redshift of z = 18. The timing of this signal constrains the dark matter particle mass (m χ ) in the warm dark matter (WDM) cosmological model. WDM delays the formation of small-scale structures, and therefore a stringent lower limit can be placed on m χ based on the presence of a sufficiently strong Lyα background due to star formation at z = 18. Our results show that coupling the spin temperature to the gas through Lyα pumping requires a minimum mass of m χ > 3 keV if atomic cooling halos dominate the star formation rate at z = 18, and m χ > 2 keV if {{{H}}}2 cooling halos also form stars efficiently at this redshift. These limits match or exceed the most stringent limits cited to date in the literature, even in the face of the many uncertainties regarding star formation at high redshift.

Spitzer SAGE-Spec: Near infrared spectroscopy, dust shells, and cool envelopes in extreme Large Magellanic Cloud asymptotic giant branch stars

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

Blum, R. D.; Srinivasan, S.; Kemper, F.

2014-11-01

K-band spectra are presented for a sample of 39 Spitzer Infrared Spectrograph (IRS) SAGE-Spec sources in the Large Magellanic Cloud. The spectra exhibit characteristics in very good agreement with their positions in the near-infrared—Spitzer color-magnitude diagrams and their properties as deduced from the Spitzer IRS spectra. Specifically, the near-infrared spectra show strong atomic and molecular features representative of oxygen-rich and carbon-rich asymptotic giant branch stars, respectively. A small subset of stars was chosen from the luminous and red extreme ''tip'' of the color-magnitude diagram. These objects have properties consistent with dusty envelopes but also cool, carbon-rich ''stellar'' cores. Modest amountsmore » of dust mass loss combine with the stellar spectral energy distribution to make these objects appear extreme in their near-infrared and mid-infrared colors. One object in our sample, HV 915, a known post-asymptotic giant branch star of the RV Tau type, exhibits CO 2.3 μm band head emission consistent with previous work that demonstrates that the object has a circumstellar disk.« less

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.

Evidence for a Major Merger Origin of High-Redshift Submillimeter Galaxies

NASA Astrophysics Data System (ADS)

Conselice, Christopher J.; Chapman, Scott C.; Windhorst, Rogier A.

2003-10-01

Submillimeter-detected galaxies located at redshifts z>1 host a major fraction of the bolometric luminosity at high redshifts due to thermal emission from heated dust grains, yet the nature of these objects remains a mystery. The major problem in understanding their origin is whether the dust-heating mechanism is predominantly caused by star formation or active galactic nuclei and what triggered this activity. We address this issue by examining the structures of 11 submillimeter galaxies imaged with STIS on the Hubble Space Telescope. We argue that ~61%+/-21% of these submillimeter sources are undergoing an active major merger using the CAS (concentration, asymmetry, clumpiness) quantitative morphological system. We rule out at ~5 σ confidence that these submillimeter galaxies are normal Hubble types at high redshift. This merger fraction appears to be higher than for Lyman break galaxies undergoing mergers at similar redshifts. Using reasonable constraints on the stellar masses of Lyman break galaxies and these submillimeter sources, we further argue that at redshifts z~2-3, systems with high stellar masses are more likely than lower mass galaxies to be involved in major mergers.

High temperature structure in cool binary stars

NASA Technical Reports Server (NTRS)

Dupree, A. K.; Brickhouse, Nancy S.; Hanson, G. J.

1995-01-01

Strong high temperature emission lines in the EUVE spectra of binary stars containing cool components (Alpha Aur (Capella), 44 iota Boo, Lambda And, and VY Ari) provide the basis to define reliably the differential emission measure of hot plasma. The emission measure distributions for the short-period (P less than or equal to 13 d) binary systems show a high temperature enhancement over a relatively narrow temperature region similar to that originally found in Capella (Dupree et al. 1993). The emission measure distributions of rapidly rotating single stars 31 Com and AB Dor also contain a local enhancement of the emission measure although at different temperatures and width from Capella, suggesting that the enhancement in these objects may be characteristic of rapid rotation of a stellar corona. This feature might be identified with a (polar) active region, although its density and absolute size are unknown; in the binaries Capella and VY Ari, the feature is narrow and it may arise from an interaction region between the components.

Photometric Calibrations of Gemini Images of NGC 6253

NASA Astrophysics Data System (ADS)

Pearce, Sean; Jeffery, Elizabeth

2017-01-01

We present preliminary results of our analysis of the metal-rich open cluster NGC 6253 using imaging data from GMOS on the Gemini-South Observatory. These data are part of a larger project to observe the effects of high metallicity on white dwarf cooling processes, especially the white dwarf cooling age, which have important implications on the processes of stellar evolution. To standardize the Gemini photometry, we have also secured imaging data of both the cluster and standard star fields using the 0.6-m SARA Observatory at CTIO. By analyzing and comparing the standard star fields of both the SARA data and the published Gemini zero-points of the standard star fields, we will calibrate the data obtained for the cluster. These calibrations are an important part of the project to obtain a standardized deep color-magnitude diagram to analyze the cluster. We present the process of verifying our standardization process. With a standardized CMD, we also present an analysis of the cluster's main sequence turn off age.

Early spectra of the gravitational wave source GW170817: Evolution of a neutron star merger.

PubMed

Shappee, B J; Simon, J D; Drout, M R; Piro, A L; Morrell, N; Prieto, J L; Kasen, D; Holoien, T W-S; Kollmeier, J A; Kelson, D D; Coulter, D A; Foley, R J; Kilpatrick, C D; Siebert, M R; Madore, B F; Murguia-Berthier, A; Pan, Y-C; Prochaska, J X; Ramirez-Ruiz, E; Rest, A; Adams, C; Alatalo, K; Bañados, E; Baughman, J; Bernstein, R A; Bitsakis, T; Boutsia, K; Bravo, J R; Di Mille, F; Higgs, C R; Ji, A P; Maravelias, G; Marshall, J L; Placco, V M; Prieto, G; Wan, Z

2017-12-22

On 17 August 2017, Swope Supernova Survey 2017a (SSS17a) was discovered as the optical counterpart of the binary neutron star gravitational wave event GW170817. We report time-series spectroscopy of SSS17a from 11.75 hours until 8.5 days after the merger. Over the first hour of observations, the ejecta rapidly expanded and cooled. Applying blackbody fits to the spectra, we measured the photosphere cooling from [Formula: see text] to [Formula: see text] kelvin, and determined a photospheric velocity of roughly 30% of the speed of light. The spectra of SSS17a began displaying broad features after 1.46 days and evolved qualitatively over each subsequent day, with distinct blue (early-time) and red (late-time) components. The late-time component is consistent with theoretical models of r-process-enriched neutron star ejecta, whereas the blue component requires high-velocity, lanthanide-free material. Copyright © 2017, American Association for the Advancement of Science.

STARING INTO THE WINDS OF DESTRUCTION: HST/NICMOS IMAGES OF THE PLANETARY NEBULA NGC 7027

NASA Technical Reports Server (NTRS)

2002-01-01

The Hubble Space Telescope's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) has captured a glimpse of a brief stage in the burnout of NGC 7027, a medium-mass star like our sun. The infrared image (on the left) shows a young planetary nebula in a state of rapid transition. This image alone reveals important new information. When astronomers combine this photo with an earlier image taken in visible light, they have a more complete picture of the final stages of star life. NGC 7027 is going through spectacular death throes as it evolves into what astronomers call a 'planetary nebula.' The term planetary nebula came about not because of any real association with planets, but because in early telescopes these objects resembled the disks of planets. A star can become a planetary nebula after it depletes its nuclear fuel - hydrogen and helium - and begins puffing away layers of material. The material settles into a wind of gas and dust blowing away from the dying star. This NICMOS image captures the young planetary nebula in the middle of a very short evolutionary phase, lasting perhaps less than 1,000 years. During this phase, intense ultraviolet radiation from the central star lights up a region of gas surrounding it. (This gas is glowing brightly because it has been made very hot by the star's intense ultraviolet radiation.) Encircling this hot gas is a cloud of dust and cool molecular hydrogen gas that can only be seen by an infrared camera. The molecular gas is being destroyed by ultraviolet light from the central star. THE INFRARED VIEW -- The composite color image of NGC 7027 (on the left) is among the first data of a planetary nebula taken with NICMOS. This picture is actually composed of three separate images taken at different wavelengths. The red color represents cool molecular hydrogen gas, the most abundant gas in the universe. The image reveals the central star, which is difficult to see in images taken with visible light. Surrounding it is an elongated region of gas and dust cast off by the star. This gas (appearing as white) has a temperature of several tens of thousands of degrees Fahrenheit. The object has two 'cones' of cool molecular hydrogen gas (the red material) glowing in the infrared. The gas has been energized by ultraviolet light from the star - a process known as fluorescence. Most of the material shed by the star remains outside of the bright regions. It is invisible in this image because the layers of material in and near the bright regions are still shielding it from the central star's intense radiation. NGC 7027 is one of the smallest objects of its kind to be imaged by the Hubble telescope. However, the region seen here is approximately 14,000 times the average distance between Earth and the sun. THE INFRARED AND VISIBLE LIGHT VIEW -- This visible and infrared light picture of NGC 7027 (on the right) provides a more complete view of how this planetary nebula is being shaped, revealing steps in its evolution. This image is composed of three exposures, one from the Wide Field and Planetary Camera 2 (WFPC2) and two from NICMOS. The blue represents the WFPC2 image; the green and red, NICMOS exposures. The white is emission from the hot gas surrounding the central star; the red and pink represent emission from cool molecular hydrogen gas. In effect, the colors represent the three layers in the material ejected by the dying star. Each layer depicts a change in temperature, beginning with a hot, bright central region, continuing with a thin boundary zone where molecular hydrogen gas is glowing and being destroyed, and ending with a cool, blue outer region of molecular gas and dust. NICMOS has allowed astronomers to clearly see the transition layer from hot, glowing atomic gas to cold molecular gas. The origin of the newly seen filamentary structures is not yet understood. The transition region is clearly seen as the pink- and red-colored cool molecular hydrogen gas. An understanding of the atomic and chemical processes taking place in this transition region are of importance to other areas of astronomy as well, including star formation regions. WFPC2 is best used to study the hot, glowing gas, which is the bright, oval-shaped region surrounding the central star. With WFPC2 we also see material beyond this core with light from the central star that is reflecting off dust in the cold gas surrounding the nebula. Combining exposures from the two cameras allows astronomers to clearly see the way the nebula is being shaped by winds and radiation. This information will help astronomers understand the complexities of stellar evolution. NGC 7027 is located about 3,000 light-years from the sun in the direction of the constellation Cygnus the Swan. Credits: William B. Latter (SIRTF Science Center/Caltech) and NASA Other team investigators are: J. L. Hora (Smithsonian Astrophysical Observatory), J. H. Bieging (Steward Observatory), D. M. Kelly (University of Wyoming), A. Dayal (JPL/Caltech), A.G.G.M. Tielens (University of Groningen), and S. Trammell (University of North Carolina at Charlotte).

A surprise at the bottom of the main sequence: Rapid rotation and NO H-alpha emission

NASA Technical Reports Server (NTRS)

Basri, Gibor; Marcy, Geoffrey W.

1995-01-01

We report Kech Observatory high-resolution echelle spectra from 640-850 nm for eight stars near the faint end of the main sequence. These spectra are the highest resolution spectra of such late-type stars, and clearly resolve the TiO, VO, and atomic lines. The sample includes the field brown-dwarf candidate, BRI 0021-0214 (M9.5+). Very unexpectedly, it shows the most rapid rotation in the entire samples, v sin i approximately 40 km/s, which is 20x faster than typical field nonemission M stars. Equally surprising is that BRI 0021 exhibits no emission or absorptionat H-alpha. We argue that this absence is not simply due to its cool photosphere, but that stellar activity declines in a fundamental way at the end of the main sequence. As it is the first very late M dwarf observed at high spectral resolution, BRI 0021 may be signaling a qualitative change in the angular momentum loss rate among the lowest mass stars. Conventionally, its rapid rotation would have marked BRI 0021 as very young, consistent with the selection effect which arises if the latest-type dwarfs are really brown dwarfs on cooling curves. In any case, it is unprecedented to find no sign of stellar activity in such a rapidly rotating convective star. We also discuss the possible conflict between this observation and the extremely strong H-alpha seen in another very cool star, PC 0025+0447. Extrapolation of M-L relations for BRI 0021 yields M approximately 0.065 solar mass, and the other sample objects have expected masses near the H-burning limit. These include two Pleiades brown-dwarf candidates, four field M6 dwarfs and one late-type T Tauri star. The two Pleiades M6 dwarfs have v sin i of 26 and 37 km/s, H-alpha in emission, and radial velocities consistent with Pleiades M6 dwarfs have v sin i of 26 and 37 km/s, H-alpha in emission, and radial velocities consistent with Pleiades membership. Similarly, the late-type T Tauri star has v sin i approximately 30 km/s and H alpha emission indicate of its youth. Two of the four late-type field dMe star also exhibit rotation above 5 km/s, consistent with expectations. BRI 0021 has no measurable absoprtion due to lithium, indicating that it is likely to be more massive than 0.065 solar mass.

Near-infrared spectroscopic observations of massive young stellar object candidates in the central molecular zone

NASA Astrophysics Data System (ADS)

Nandakumar, G.; Schultheis, M.; Feldmeier-Krause, A.; Schödel, R.; Neumayer, N.; Matteucci, F.; Ryde, N.; Rojas-Arriagada, A.; Tej, A.

2018-01-01

Context. The central molecular zone (CMZ) is a 200 pc region around the Galactic centre. The study of star formation in the central part of the Milky Way is of great interest as it provides a template for the closest galactic nuclei. Aims: We present a spectroscopic follow-up of photometrically selected young stellar object (YSO) candidates in the CMZ of the Galactic centre. Our goal is to quantify the contamination of this YSO sample by reddened giant stars with circumstellar envelopes and to determine the star formation rate (SFR) in the CMZ. Methods: We obtained KMOS low-resolution near-infrared spectra (R 4000) between 2.0 and 2.5 μm of sources, many of which have been previously identified by mid-infrared photometric criteria as massive YSOs in the Galactic centre. Our final sample consists of 91 stars with good signal-to-noise ratio. We separated YSOs from cool late-type stars based on spectral features of CO and Brγ at 2.3 μm and 2.16 μm, respectively. We made use of spectral energy distribution (SED) model fits to the observed photometric data points from 1.25 to 24 μm to estimate approximate masses for the YSOs. Results: Using the spectroscopically identified YSOs in our sample, we confirm that existing colour-colour diagrams and colour-magnitude diagrams are unable to efficiently separate YSOs and cool late-type stars. In addition, we define a new colour-colour criterion that separates YSOs from cool late-type stars in the H-KS vs. H -[8.0] diagram. We use this new criterion to identify YSO candidates in the |l| < 1.̊5, |b| < 0.̊5, region and use model SED fits to estimate their approximate masses. By assuming an appropriate initial mass function (IMF) and extrapolating the stellar IMF down to lower masses, we determine a SFR of 0.046 ± 0.026 M⊙ yr-1 assuming an average age of 0.75 ± 0.25 Myr for the YSOs. This value is lower than estimates found using the YSO counting method in the literature. Conclusions: Our SFR estimate in the CMZ agrees with the previous estimates from various methods and reaffirms that star formation in the CMZ is proceeding at a lower rate than predicted by various star forming models. Based on observations collected at the European Southern Observatory, Chile, programme number 097.C-0208(A).

Constraining the axion-photon coupling with massive stars.

PubMed

Friedland, Alexander; Giannotti, Maurizio; Wise, Michael

2013-02-08

We point out that stars in the mass window ~8-12M([circumpunct]) can serve as sensitive probes of the axion-photon interaction, g(Aγγ). Specifically, for these stars axion energy losses from the helium-burning core would shorten and eventually eliminate the blue loop phase of the evolution. This would contradict observational data, since the blue loops are required, e.g., to account for the existence of Cepheid stars. Using the MESA stellar evolution code, modified to include the extra cooling, we conservatively find g(Aγγ)

Ground Vehicle CFD at TARDEC

DTIC Science & Technology

2012-05-21

Cooling Sytem: StarCCM+ Blast / Crew Safety: LS- Dyna Fatigue & FEA: Abaqus / NCode Each code run with ~40-80 CPUs on TARDEC HPC Models...suppression, blast solid modeling have particular scaling problems because of the use of Lagrangian particles Example: Dust modeling for engine...for technology demonstrator vehicles UNCLASSIFIED 9 Example CFD Interest Areas • Underbody mine blast • HVAC design / interior cooling

Semi-empirical models of the wind in cool supergiant stars

NASA Technical Reports Server (NTRS)

Kuin, N. P. M.; Ahmad, Imad A.

1988-01-01

A self-consistent semi-empirical model for the wind of the supergiant in zeta Aurigae type systems is proposed. The damping of the Alfven waves which are assumed to drive the wind is derived from the observed velocity profile. Solution of the ionization balance and energy equation gives the temperature structure for given stellar magnetic field and wave flux. Physically acceptable solutions of the temperature structure place limits on the stellar magnetic field. A crude formula for a critical mass loss rate is derived. For a mass loss rate below the critical value the wind cannot be cool. Comparison between the observed and the critical mass loss rate suggests that the proposed theory may provide an explanation for the coronal dividing line in the Hertzsprung-Russell diagram. The physical explanation may be that the atmosphere has a cool wind, unless it is physically impossible to have one. Stars which cannot have a cool wind release their nonthermal energy in an outer atmosphere at coronal temperatures. It is possible that in the absence of a substantial stellar wind the magnetic field has less incentive to extend radially outward, and coronal loop structures may become more dominant.

Hybrid Stars and Coronal Evolution

NASA Technical Reports Server (NTRS)

Mushotzky, Richard (Technical Monitor); Dupree, Andrea K.

2004-01-01

This program addresses the evolution of stellar coronas by comparing a solar-like corona in the supergiant Dra (G2 Ib-IIa) to the corona in the allegedly more evolved state of a hybrid star, TrA (K2 11-111). Because the hybrid star has a massive wind, it appears likely that the corona will be cooler and less dense as the magnetic loop structures are no longer closed. By analogy with solar coronal holes, when the topology of the magnetic field is configured with open magnetic structures, both the coronal temperature and density are lower than in atmospheres dominated by closed loops. The hybrid stars assume a pivotal role in the definition of coronal evolution, atmospheric heating processes and mechanisms to drive winds of cool stars.

NASA’s Spitzer Reveals Largest Batch of Earth-Size, Habitable-Zone Planets Around a Single Star

NASA Image and Video Library

2017-02-22

NASA held a news conference Feb. 22 at the agency’s headquarters to discuss the finding by the Spitzer Space Telescope of seven Earth-sized planets around a tiny, nearby, ultra-cool dwarf star. Three of these planets are in the habitable zone, the region around the star in which liquid water is most likely to thrive on a rocky planet. This is the first time so many planets have been found in a single star's habitable zone, and the first time so many Earth-sized planets have been found around the same star. The finding of this planetary system, called TRAPPIST-1, is the best target yet for studying the atmospheres of potentially habitable, Earth-sized worlds

A seven-year northern sky survey of Ap stars for rapid variability

NASA Technical Reports Server (NTRS)

Nelson, Matthew J.; Kreidl, Tobias J.

1993-01-01

A high-speed photometric survey of 120 Ap stars in the northern sky, has been conducted, between 1985 and 1991, in order to search for rapid variability. Stars of spectral types, namely from B8 to F4, have been selected for the survey. The selected pulsational variable stars occupy the hotter regions of the instability strip of the Hertzsprung-Russel diagram. Noted is the absence of pulsations in the hotter B8-A3 Ap stars; this does not, however, preclude the existence of pulsations, since HD 218495 was recently discovered to be a rapidly oscillating Ap (roAp) star. The primary result of this study is that various combinations of photometric indices, while pointing towards roAp stars having the characteristic signatures of cool, SrCrEu stars, still fail to isolate the roAp phenomenon from similar nonpulsating Ap stars. Color-magnitude and color-color diagrams are presented in order to complete this survey.

Modelling the spectral energy distribution of galaxies. V. The dust and PAH emission SEDs of disk galaxies

NASA Astrophysics Data System (ADS)

Popescu, C. C.; Tuffs, R. J.; Dopita, M. A.; Fischera, J.; Kylafis, N. D.; Madore, B. F.

2011-03-01

We present a self-consistent model of the spectral energy distributions (SEDs) of spiral galaxies from the ultraviolet (UV) to the mid-infrared (MIR)/far-infrared (FIR)/submillimeter (submm) based on a full radiative transfer calculation of the propagation of starlight in galaxy disks. This model predicts not only the total integrated energy absorbed in the UV/optical and re-emitted in the infrared/submm, but also the colours of the dust emission based on an explicit calculation of the strength and colour of the UV/optical radiation fields heating the dust, and incorporating a full calculation of the stochastic heating of small dust grains and PAH molecules. The geometry of the translucent components of the model is empirically constrained using the results from the radiation transfer analysis of Xilouris et al. on spirals in the middle range of the Hubble sequence, while the geometry of the optically thick components is constrained from physical considerations with a posteriori checks of the model predictions with observational data. Following the observational constraints, the model has both a distribution of diffuse dust associated with the old and young disk stellar populations as well as a clumpy component arising from dust in the parent molecular clouds in star forming regions. In accordance with the fragmented nature of dense molecular gas in typical star-forming regions, UV light from massive stars is allowed to either freely stream away into the diffuse medium in some fraction of directions or be geometrically blocked and locally absorbed in clumps. These geometrical constraints enable the dust emission to be predicted in terms of a minimum set of free parameters: the central face-on dust opacity in the B-band τ^f_B, a clumpiness factor F for the star-forming regions, the star-formation rate SFR, the normalised luminosity of the old stellar population old and the bulge-to-disk ratio B/D. We show that these parameters are almost orthogonal in their predicted effect on the colours of the dust/PAH emission. In most practical applications B/D will actually not be a free parameter but (together with the angular size θgal and inclination i of the disk) act as a constraint derived from morphological decomposition of higher resolution optical images. This also extends the range of applicability of the model along the Hubble sequence. We further show that the dependence of the dust emission SED on the colour of the stellar photon field depends primarily on the ratio between the luminosities of the young and old stellar populations (as specified by the parameters SFR and old) rather than on the detailed colour of the emissions from either of these populations. The model is thereby independent of a priori assumptions of the detailed mathematical form of the dependence of SFR on time, allowing UV/optical SEDs to be dereddened without recourse to population synthesis models. Utilising these findings, we show how the predictive power of radiative transfer calculations can be combined with measurements of θgal, i and B/D from optical images to self-consistently fit UV/optical-MIR/FIR/submm SEDs observed in large statistical surveys in a fast and flexible way, deriving physical parameters on an object-by-object basis. We also identify a non-parametric test of the fidelity of the model in practical applications through comparison of the model predictions for FIR colour and surface brightness with the corresponding observed quantities. This should be effective in identifying objects such as AGNs or star-forming galaxies with markedly different geometries to those of the calibrators of Xilouris et al. The results of the calculations are made available in the form of a large library of simulated dust emission SEDs spanning the whole parameter space of our model, together with the corresponding library of dust attenuation calculated using the same model. We dedicate this paper to the memory of Angelos Misiriotis, sorely missed as a friend, collaborator and exceptional scientist.Appendices are only available in electronic form at http://www.aanda.orgThe data are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/527/A109

Effects of stellar evolution and ionizing radiation on the environments of massive stars

NASA Astrophysics Data System (ADS)

Mackey, J.; Langer, N.; Mohamed, S.; Gvaramadze, V. V.; Neilson, H. R.; Meyer, D. M.-A.

2014-09-01

We discuss two important effects for the astrospheres of runaway stars: the propagation of ionizing photons far beyond the astropause, and the rapid evolution of massive stars (and their winds) near the end of their lives. Hot stars emit ionizing photons with associated photoheating that has a significant dynamical effect on their surroundings. 3-D simulations show that H ii regions around runaway O stars drive expanding conical shells and leave underdense wakes in the medium they pass through. For late O stars this feedback to the interstellar medium is more important than that from stellar winds. Late in life, O stars evolve to cool red supergiants more rapidly than their environment can react, producing transient circumstellar structures such as double bow shocks. This provides an explanation for the bow shock and linear bar-shaped structure observed around Betelgeuse.

QUIESCENCE CORRELATES STRONGLY WITH DIRECTLY MEASURED BLACK HOLE MASS IN CENTRAL GALAXIES

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

Terrazas, Bryan A.; Bell, Eric F.; Henriques, Bruno M. B.

Roughly half of all stars reside in galaxies without significant ongoing star formation. However, galaxy formation models indicate that it is energetically challenging to suppress the cooling of gas and the formation of stars in galaxies that lie at the centers of their dark matter halos. In this Letter, we show that the dependence of quiescence on black hole and stellar mass is a powerful discriminant between differing models for the mechanisms that suppress star formation. Using observations of 91 star-forming and quiescent central galaxies with directly measured black hole masses, we find that quiescent galaxies host more massive blackmore » holes than star-forming galaxies with similar stellar masses. This observational result is in qualitative agreement with models that assume that effective, more-or-less continuous active galactic nucleus feedback suppresses star formation, strongly suggesting the importance of the black hole in producing quiescence in central galaxies.« less

Star Formation in Nearby Clusters (SFiNCs)

NASA Astrophysics Data System (ADS)

Getman, Konstantin

Most stars form in clusters that rapidly disperse, yet we have a poor understanding of the processes of cluster formation and early evolution. Do clusters form `top-down', rapidly in a dense molecular cloud core? Or, since clouds are turbulent, do clusters form `bottomup' by merging subclusters produced in small kinematically-distinct molecular structures? Do clusters principally form in elongated molecular structures such as Infrared Dark Clouds and Herschel filaments? One of the central reasons for slow progress in resolving these questions is the lack of homogeneous and reliable census of stellar members (both disk-bearing and disk-free) for a wide range of star forming environments. To address these issues we are now completing our major effort, called MYStIX (Massive Young Star-Forming Complex Study in Infrared and X-ray). It combines the Chandra archive with UKIRT+2MASS near-infrared and Spitzer mid-infrared surveys to identify young stellar objects in a wide range of evolutionary stages, from protostars to disk-free pre-main sequence stars, in 20 star forming regions at distances from 0.4 to 3.6 kpc. Each MYStIX region was chosen to have a rich OB-dominated cluster. Started in 2009 with NASA/ADAP and NSF funding, MYStIX has emerged with 8 technical/catalog and the first 4 of a series of science papers (http://astro.psu.edu/mystix). Early MYStIX results include: demonstration of diverse morphologies of young clusters from simple ellipsoids to elongated, clumpy substructures; demonstration of spatio-age gradients across star formation regions; the discovery of core-halo age gradients within two rich nearby MYStIX clusters; and the discovery of important astrophysically empirical correlations among different subcluster properties such as age, absorption, core radius, central stellar density, and total intrinsic population. The early MYStIX result provide new observational evidence for subcluster merging and cluster expansion following gas dissipation. We propose here to extend the MYStIX effort to an archive study of 19 nearer and smaller star forming regions where the stellar clusters are dominated by a single late-OB star rather than numerous O stars as in the MYStIX fields. We call this project `Star Formation in Nearby Clusters' or SFiNCs (homophonic with `sphinx'). With a homogeneous analysis of the Chandra, 2MASS, Spitzer and Herschel archives, we expect to identify and characterize over 50 SFiNCs subclusters. The inferred empirical correlations among different cluster properties for nearly 200 SFiNCs+MYStIX subclusters with 30-3000 detected stars on scales of 0.1-20 pc will allow, for the first time, direct comparison with the results of theoretical simulations of cluster formation to seek deeper answers to the fundamental questions posed above. It is possible, for example, that smaller molecular clouds have less turbulence and thus produce small clusters in a single event rather than through subcluster mergers. Models based on meteoritic isotopes suggest that our Solar System formed in a complex of SFiNCs/MYStIX-like clusters (Gounelle & Meynet 2012, A&A, 545, 4). This project addresses NASA SMD Strategic Subgoals 3C (Advance scientific knowledge of the origin and history of the solar system) and 3D.3 (Understand how individual stars form and how those processes ultimately affect the formation of planetary systems). It lies in the `Star formation and pre-main sequence stars' Research Area of the Astrophysics Data Analysis program.

First X-ray Statistical Tests for Clumpy-Torus Models: Constraints from RXTEmonitoring of Seyfert AGN

NASA Astrophysics Data System (ADS)

Markowitz, Alex; Krumpe, Mirko; Nikutta, R.

2016-06-01

In two papers (Markowitz, Krumpe, & Nikutta 2014, and Nikutta et al., in prep.), we derive the first X-ray statistical constraints for clumpy-torus models in Seyfert AGN by quantifying multi-timescale variability in line of-sight X-ray absorbing gas as a function of optical classification.We systematically search for discrete absorption events in the vast archive of RXTE monitoring of 55 nearby type Is and Compton-thin type IIs. We are sensitive to discrete absorption events due to clouds of full-covering, neutral/mildly ionized gas transiting the line of sight. Our results apply to both dusty and non-dusty clumpy media, and probe model parameter space complementary to that for eclipses observed with XMM-Newton, Suzaku, and Chandra.We detect twelve eclipse events in eight Seyferts, roughly tripling the number previously published from this archive. Event durations span hours to years. Most of our detected clouds are Compton-thin, and most clouds' distances from the black hole are inferred to be commensurate with the outer portions of the BLR or the inner regions of infrared-emitting dusty tori.We present the density profiles of the highest-quality eclipse events; the column density profile for an eclipsing cloud in NGC 3783 is doubly spiked, possibly indicating a cloud that is being tidallysheared. We discuss implications for cloud distributions in the context of clumpy-torus models. We calculate eclipse probabilities for orientation-dependent Type I/II unification schemes.We present constraints on cloud sizes, stability, and radial distribution. We infer that clouds' small angular sizes as seen from the SMBH imply 107 clouds required across the BLR + torus. Cloud size is roughly proportional to distance from the black hole, hinting at the formation processes (e.g., disk fragmentation). All observed clouds are sub-critical with respect to tidal disruption; self-gravity alone cannot contain them. External forces, such as magnetic fields or ambient pressure, are needed to contain them; otherwise, clouds must be short-lived.

Chromospheric Activity in Cool Luminous Stars

NASA Astrophysics Data System (ADS)

Dupree, Andrea

2018-04-01

Spatially unresolved spectra of giant and supergiant stars demonstrate ubiquitous signatures of chromospheric activity, variable outflows, and winds. The advent of imaging techniques and spatially resolved spectra reveal complex structures in these extended stellar atmospheres that we do not understand. The presence and behavior of these atmospheres is wide ranging and impacts stellar activity, magnetic fields, angular momentum loss, abundance determinations, and the understanding of stellar cluster populations.

Constraints on Grain Formation around Carbon Stars from Laboratory Studies of Presolar Graphite

NASA Astrophysics Data System (ADS)

Bernatowicz, Thomas J.; Akande, Onaolapo Wali; Croat, Thomas K.; Cowsik, Ramanath

2005-10-01

We report the results of an investigation into the physical conditions in the mass outflows of asymptotic giant branch (AGB) carbon stars that are required for the formation of micron-sized presolar graphite grains, with and without previously formed internal crystals of titanium carbide (TiC). A lower mass limit of 1.1 Msolar for stars capable of contributing grains to the solar nebula is derived. This mass limit, in conjunction with a mass-luminosity relation for carbon stars, identifies the region of the H-R diagram relevant to the production of presolar graphite. Detailed dynamical models of AGB outflows, along with constraints provided by kinetics and equilibrium thermodynamics, indicate that grain formation occurs at radii from 2.3 to 3.7 AU for AGB carbon stars in the 1.1-5 Msolar range. This analysis also yields time intervals available for graphite growth that are on the order of a few years. By considering the luminosity variations of carbon stars, we show that grains formed during minima in the luminosity are likely to be evaporated subsequently, while those formed at luminosity maxima will survive. We calculate strict upper limits on grain sizes for graphite and TiC in spherically symmetric AGB outflows. Graphite grains can reach diameters in the observed micron size range (1-2 μm) only under ideal growth conditions (perfect sticking efficiency, no evaporation, no depletion of gas species contributing to grain growth), and then only in outflows from carbon stars with masses <~2.5 Msolar. The same is true for TiC grains that are found within presolar graphite, which have mean diameters of 24+/-14 nm. In general, the mass-loss rates that would be required to produce the observed grain sizes in spherically symmetric outflows are at least an order of magnitude larger than the maximum observed AGB carbon star mass-loss rates. These results, as well as pressure constraints derived from equilibrium thermodynamics, force us to conclude that presolar graphite and TiC must form in regions of enhanced density (clumps, jets) in AGB outflows having small angular scales. As shown in the companion paper by Croat et al., the enrichment of 12C in many AGB graphites, and the overabundances of the s-process elements Mo, Zr, and Ru in the carbides found within them, often greatly exceed the values observed astronomically in AGB outflows. These observations not only lend further support to the idea that the outflows are clumpy, but also imply that the outflowing matter is not well mixed in the circumstellar envelope out to the radii where grain condensation takes place.

Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars. VI. First chromosphere model of a late-type giant

NASA Astrophysics Data System (ADS)

Wedemeyer, Sven; Kučinskas, Arūnas; Klevas, Jonas; Ludwig, Hans-Günter

2017-10-01

Aims: Although observational data unequivocally point to the presence of chromospheres in red giant stars, no attempts have been made so far to model them using 3D hydrodynamical model atmospheres. We therefore compute an exploratory 3D hydrodynamical model atmosphere for a cool red giant in order to study the dynamical and thermodynamic properties of its chromosphere, as well as the influence of the chromosphere on its observable properties. Methods: Three-dimensional radiation hydrodynamics simulations are carried out with the CO5BOLD model atmosphere code for a star with the atmospheric parameters (Teff ≈ 4010 K, log g = 1.5, [ M / H ] = 0.0), which are similar to those of the K-type giant star Aldebaran (α Tau). The computational domain extends from the upper convection zone into the chromosphere (7.4 ≥ log τRoss ≥ - 12.8) and covers several granules in each horizontal direction. Using this model atmosphere, we compute the emergent continuum intensity maps at different wavelengths, spectral line profiles of Ca II K, the Ca II infrared triplet line at 854.2 nm, and Hα, as well as the spectral energy distribution (SED) of the emergent radiative flux. Results: The initial model quickly develops a dynamical chromosphere that is characterised by propagating and interacting shock waves. The peak temperatures in the chromospheric shock fronts reach values of up to 5000 K, although the shock fronts remain quite narrow. Similar to the Sun, the gas temperature distribution in the upper layers of red giant stars is composed of a cool component due to adiabatic cooling in the expanding post-shock regions and a hot component due to shock waves. For this red giant model, the hot component is a rather flat high-temperature tail, which nevertheless affects the resulting average temperatures significantly. Conclusions: The simulations show that the atmospheres of red giant stars are dynamic and intermittent. Consequently, many observable properties cannot be reproduced with static 1D models, but require advanced 3D hydrodynamical modelling. Furthermore, including a chromosphere in the models might produce significant contributions to the emergent UV flux.

Multi-Spectral Stereo Atmospheric Remote Sensing (STARS) for Retrieval of Cloud Properties and Cloud-Motion Vectors

NASA Astrophysics Data System (ADS)

Kelly, M. A.; Boldt, J.; Wilson, J. P.; Yee, J. H.; Stoffler, R.

2017-12-01

The multi-spectral STereo Atmospheric Remote Sensing (STARS) concept has the objective to provide high-spatial and -temporal-resolution observations of 3D cloud structures related to hurricane development and other severe weather events. The rapid evolution of severe weather demonstrates a critical need for mesoscale observations of severe weather dynamics, but such observations are rare, particularly over the ocean where extratropical and tropical cyclones can undergo explosive development. Coincident space-based measurements of wind velocity and cloud properties at the mesoscale remain a great challenge, but are critically needed to improve the understanding and prediction of severe weather and cyclogenesis. STARS employs a mature stereoscopic imaging technique on two satellites (e.g. two CubeSats, two hosted payloads) to simultaneously retrieve cloud motion vectors (CMVs), cloud-top temperatures (CTTs), and cloud geometric heights (CGHs) from multi-angle, multi-spectral observations of cloud features. STARS is a pushbroom system based on separate wide-field-of-view co-boresighted multi-spectral cameras in the visible, midwave infrared (MWIR), and longwave infrared (LWIR) with high spatial resolution (better than 1 km). The visible system is based on a pan-chromatic, low-light imager to resolve cloud structures under nighttime illumination down to ¼ moon. The MWIR instrument, which is being developed as a NASA ESTO Instrument Incubator Program (IIP) project, is based on recent advances in MWIR detector technology that requires only modest cooling. The STARS payload provides flexible options for spaceflight due to its low size, weight, power (SWaP) and very modest cooling requirements. STARS also meets AF operational requirements for cloud characterization and theater weather imagery. In this paper, an overview of the STARS concept, including the high-level sensor design, the concept of operations, and measurement capability will be presented.

Flash Mixing on the White-Dwarf Cooling Curve: Understanding Hot Horizontal Branch Anomalies in NGC 2808

NASA Technical Reports Server (NTRS)

Brown, Thomas M.; Sweigart, Allen V.; Lanz, Thierry; Landsman, Wayne B.; Hubeny, Ivan; Fisher, Richard R. (Technical Monitor)

2001-01-01

We present an ultraviolet color-magnitude diagram (CMD) spanning the hot horizontal branch (HB), blue straggler, and white dwarf populations of the globular cluster NGC 2808. These data, obtained with the Space Telescope Imaging Spectrograph (STIS), demonstrate that NGC 2808 harbors a significant population of hot subluminous HB stars, an anomaly only previously reported for the globular cluster omega Cen. Our theoretical modeling indicates that the location of these subluminous stars in the CMD, as well as the high temperature gap along the HB of NGC 2808, can be explained if these stars underwent a late helium-core flash while descending the white dwarf cooling curve. We show that the convective zone produced by such a late helium flash will penetrate into the hydrogen envelope, thereby mixing hydrogen into the hot helium-burning interior, where it is rapidly consumed. This phenomenon is analogous to the "born again" scenario for producing hydrogen-deficient stars following a late helium-shell flash. The flash mixing of the envelope greatly enhances the envelope helium and carbon abundances that, in turn, leads to a discontinuous increase in the HB effective temperatures. We argue that the hot HB gap is associated with this theoretically predicted dichotomy in the HB properties. Moreover, the changes in the emergent spectral energy distribution caused by these abundance changes are primarily responsible for explaining the hot subluminous HB stars. Although further evidence is needed to confirm that a late helium-core flash can account for the subluminous HB stars and the hot HB gap, we demonstrate that an understanding of these stars requires the use of appropriate theoretical models for their evolution, atmospheres, and spectra.

Performance and stability analysis of gas-injection enhanced natural circulation in heavy-liquid-metal-cooled systems

NASA Astrophysics Data System (ADS)

Yoo, Yeon-Jong

The purpose of this study is to investigate the performance and stability of the gas-injection enhanced natural circulation in heavy-liquid-metal-cooled systems. The target system is STAR-LM, which is a 400-MWt-class advanced lead-cooled fast reactor under development by Argonne National Laboratory and Oregon State University. The primary loop of STAR-LM relies on natural circulation to eliminate main circulation pumps for enhancement of passive safety. To significantly increase the natural circulation flow rate for the incorporation of potential future power uprates, the injection of noncondensable gas into the coolant above the core is envisioned ("gas lift pump"). Reliance upon gas-injection enhanced natural circulation raises the concern of flow instability due to the relatively high temperature change in the reactor core and the two-phase flow condition in the riser. For this study, the one-dimensional flow field equations were applied to each flow section and the mixture models of two-phase flow, i.e., both the homogeneous and drift-flux equilibrium models were used in the two-phase region of the riser. For the stability analysis, the linear perturbation technique based on the frequency-domain approach was used by employing the Nyquist stability criterion and a numerical root search method. It has been shown that the thermal power of the STAR-LM natural circulation system could be increased from 400 up to 1152 MW with gas injection under the limiting void fraction of 0.30 and limiting coolant velocity of 2.0 m/s from the steady-state performance analysis. As the result of the linear stability analysis, it has turned out that the STAR-LM natural circulation system would be stable even with gas injection. In addition, through the parametric study, it has been found that the thermal inertia effects of solid structures such as fuel rod and heat exchanger tube should be considered in the stability analysis model. The results of this study will be a part of the optimized stable design of the gas-injection enhanced natural circulation of STAR-LM with substantially improved power level and economical competitiveness. Furthermore, combined with the parametric study, this research could contribute a guideline for the design of other similar heavy-liquid-metal-cooled natural circulation systems with gas injection.

The Inhomogeneous Centers of Cooling Flows in Galaxy Clusters

NASA Astrophysics Data System (ADS)

Sharma, Mangala

2004-04-01

The intracluster medium (ICM) in the centers of galaxy clusters is cool, dense and may be imhomogeneous. We present Chandra X-ray Observatory imaging spectroscopic data on two galaxy clusters, Abell 1991 and MS 0839.8+2938, that have cooling flows in their central few hundred kpc. Their cD galaxies show current star formation, and host compact radio sources. The hot ICM at both their centers has nonhomogeneities on kiloparsec scales. These finer structures are likely to be signatures of the formation of clusters through infall of smaller, cooler subclusters.

Synchrotron cooling and annihilation of an E(+)-E(-) plasma: The radiation mechanism for the March 5, 1979 transient

NASA Technical Reports Server (NTRS)

Ramaty, R.; Lingenfelter, R. E.; Bussard, R. W.

1980-01-01

Positron-electron pair radiation is examined as a mechanism that could be responsible for the impulsive phase emission of the March 5, 1979 transient. Synchrotron cooling and subsequent annihilation of the pairs can account for the energy spectrum, the very high brightness, and the approximately 0.4 MeV feature observed from this transient, whose source is likely to be a neutron star in the supernova remnant N49 in the Large Magellanic Cloud. In this model, the observed radiation is produced in the skin layer of a hot, radiation dominated pair atmosphere, probably confined to the vicinity of the neutron star by a strong magnetic field. The width of this layer is only about 0.1 mm. In this layer, approximately 10 to the 12th power generations of pairs are formed (by photon-photon collisions), cooled and annihilated during the approximately 0.15 sec duration of the impulsive phase. The very large burst energy implied by the distance of the Large Magellanic Cloud, and its very rapid release, are unsolved problems. Nonetheless, the possibility of neutron star vibrations, which could transport the energy coherently to the surface, heat the atmosphere mechanically to a hot, pair-producing temperature, and have a characteristic damping time roughly equal to the duration of the impulsive phase are addressed.

A Multi-Wavelength Survey of Intermediate-Mass Star-Forming Regions

NASA Astrophysics Data System (ADS)

Lundquist, Michael J.; Kobulnicky, Henry A.; Kerton, Charles R.

2015-01-01

Current research into Galactic star formation has focused on either massive star-forming regions or nearby low-mass regions. We present results from a survey of Galactic intermediate-mass star-forming regions (IM SFRs). These regions were selected from IRAS colors that specify cool dust and large PAH contribution, suggesting that they produce stars up to but not exceeding about 8 solar masses. Using WISE data we have classified 984 candidate IM SFRs as star-like objects, galaxies, filamentary structures, or blobs/shells based on their mid-infrared morphologies. Focusing on the blobs/shells, we combined follow-up observations of deep near-infrared (NIR) imaging with optical and NIR spectroscopy to study the stellar content, confirming the intermediate-mass nature of these regions. We also gathered CO data from OSO and APEX to study the molecular content and dynamics of these regions. We compare these results to those of high-mass star formation in order to better understand their role in the star-formation paradigm.

Stellar feedback in galaxies and the origin of galaxy-scale winds

NASA Astrophysics Data System (ADS)

Hopkins, Philip F.; Quataert, Eliot; Murray, Norman

2012-04-01

Feedback from massive stars is believed to play a critical role in driving galactic super-winds that enrich the intergalactic medium and shape the galaxy mass function, mass-metallicity relation and other global galaxy properties. In previous papers, we have introduced new numerical methods for implementing stellar feedback on sub-giant molecular cloud (sub-GMC) through galactic scales in numerical simulations of galaxies; the key physical processes include radiation pressure in the ultraviolet through infrared, supernovae (Type I and Type II), stellar winds ('fast' O star through 'slow' asymptotic giant branch winds), and H II photoionization. Here, we show that these feedback mechanisms drive galactic winds with outflow rates as high as ˜10-20 times the galaxy star formation rate. The mass-loading efficiency (wind mass-loss rate divided by the star formation rate) scales roughly as ? (where Vc is the galaxy circular velocity), consistent with simple momentum-conservation expectations. We use our suite of simulations to study the relative contribution of each feedback mechanism to the generation of galactic winds in a range of galaxy models, from Small Magellanic Cloud like dwarfs and Milky Way (MW) analogues to z˜ 2 clumpy discs. In massive, gas-rich systems (local starbursts and high-z galaxies), radiation pressure dominates the wind generation. By contrast, for MW-like spirals and dwarf galaxies the gas densities are much lower and sources of shock-heated gas such as supernovae and stellar winds dominate the production of large-scale outflows. In all of our models, however, the winds have a complex multiphase structure that depends on the interaction between multiple feedback mechanisms operating on different spatial scales and time-scales: any single feedback mechanism fails to reproduce the winds observed. We use our simulations to provide fitting functions to the wind mass loading and velocities as a function of galaxy properties, for use in cosmological simulations and semi-analytic models. These differ from typically adopted formulae with an explicit dependence on the gas surface density that can be very important in both low-density dwarf galaxies and high-density gas-rich galaxies.

First image of the L1157 molecular jet by the CALYPSO IRAM-PdBI survey

NASA Astrophysics Data System (ADS)

Podio, L.; Codella, C.; Gueth, F.; Cabrit, S.; Maury, A.; Tabone, B.; Lefèvre, C.; Anderl, S.; André, P.; Belloche, A.; Bontemps, S.; Hennebelle, P.; Lefloch, B.; Maret, S.; Testi, L.

2016-09-01

Context. Fast jets are thought to be a crucial ingredient of star formation because they might extract angular momentum from the disk and thus allow mass accretion onto the star. However, it is unclear whether jets are ubiquitous, and likewise, their contribution to mass and angular momentum extraction during protostar formation remains an open question. Aims: Our aim is to investigate the ejection process in the low-mass Class 0 protostar L1157. This source is associated with a spectacular bipolar outflow, and the recent detection of high-velocity SiO suggests the occurrence of a jet. Methods: Observations of CO 2 -1 and SiO 5 - 4 at ~0.8 arcsec resolution were obtained with the IRAM Plateau de Bure Interferometer (PdBI) as part of the CALYPSO large program. The jet and outflow structure were fit with a precession model. We derived the column density of CO and SiO, as well as the jet mass-loss rate and mechanical luminosity. Results: High-velocity CO and SiO emission resolve for the first time the first 200 au of the outflow-driving molecular jet. The jet is strongly asymmetric, with the blue lobe ~0.65 times slower than the red lobe. This suggests that the large-scale asymmetry of the outflow is directly linked to the jet velocity and that the asymmetry in the launching mechanism has been at work for the past 1800 yr. Velocity asymmetries are common in T Tauri stars, which suggests that the jet formation mechanism from Class 0 to Class II stages might be similar. Our model simultaneously fits the properties of the inner jet and of the clumpy 0.2 pc scale outflow by assuming that the jet precesses counter-clockwise on a cone inclined by 73° to the line of sight with an opening angle of 8° on a period of ~1640 yr. The estimated jet mass flux and mechanical luminosity are Ṁjet ~ 7.7 × 10-7M⊙ yr-1 and Ljet ~ 0.9L⊙, indicating that the jet could extract at least 25% of the gravitational energy released by the forming star.

Lithium in lower-main-sequence stars of the Alpha Persei cluster

NASA Technical Reports Server (NTRS)

Balachandran, Suchitra; Lambert, David L.; Stauffer, John R.

1988-01-01

Lithium abundances are presented for main-sequence stars of spectral types F, G, and K in the young open cluster Alpha Per. For 46 cluster members, a correlation between Li abundance and projected rotational velocity v sin i is found: all of the Li-poor stars are slow rotators. Two explanations are proposed to account for the correlation: (1) that the Li depletion is introduced following a rapid spin-down phase experienced by young low-mass stars, and that this episode of Li depletion may be the dominant one determining the spread of Li abundances among young low-mass main-sequence stars, and (2) that star formation has occurred over a finite period such that the older stars have undergone a spin-down and depletion of Li by a means that may or may not depend on rotation. The Li abundance in the warm and rapidly rotating stars appears to be undepleted, as is predicted by recent models of pre-main-sequence stars. The depletion observed in the cool stars exceeds the level predicted by these models.

Modeling for Stellar Feedback in Galaxy Formation Simulations

NASA Astrophysics Data System (ADS)

Núñez, Alejandro; Ostriker, Jeremiah P.; Naab, Thorsten; Oser, Ludwig; Hu, Chia-Yu; Choi, Ena

2017-02-01

Various heuristic approaches to model unresolved supernova (SN) feedback in galaxy formation simulations exist to reproduce the formation of spiral galaxies and the overall inefficient conversion of gas into stars. Some models, however, require resolution-dependent scalings. We present a subresolution model representing the three major phases of supernova blast wave evolution—free expansion, energy-conserving Sedov-Taylor, and momentum-conserving snowplow—with energy scalings adopted from high-resolution interstellar-medium simulations in both uniform and multiphase media. We allow for the effects of significantly enhanced SN remnant propagation in a multiphase medium with the cooling radius scaling with the hot volume fraction, {f}{hot}, as {(1-{f}{hot})}-4/5. We also include winds from young massive stars and AGB stars, Strömgren sphere gas heating by massive stars, and a mechanism that limits gas cooling that is driven by radiative recombination of dense H II regions. We present initial tests for isolated Milky Way-like systems simulated with the Gadget-based code SPHgal with improved SPH prescription. Compared to pure thermal SN input, the model significantly suppresses star formation at early epochs, with star formation extended both in time and space in better accord with observations. Compared to models with pure thermal SN feedback, the age at which half the stellar mass is assembled increases by a factor of 2.4, and the mass-loading parameter and gas outflow rate from the galactic disk increase by a factor of 2. Simulation results are converged for a variation of two orders of magnitude in particle mass in the range (1.3-130) × 104 solar masses.

The lithium-rotation connection in the 125 Myr-old Pleiades cluster

NASA Astrophysics Data System (ADS)

Bouvier, J.; Barrado, D.; Moraux, E.; Stauffer, J.; Rebull, L.; Hillenbrand, L.; Bayo, A.; Boisse, I.; Bouy, H.; DiFolco, E.; Lillo-Box, J.; Calderón, M. Morales

2018-06-01

Context. The evolution of lithium abundance over a star's lifetime is indicative of transport processes operating in the stellar interior. Aims: We revisit the relationship between lithium content and rotation rate previously reported for cool dwarfs in the Pleiades cluster. Methods: We derive new LiI 670.8 nm equivalent width measurements from high-resolution spectra obtained for low-mass Pleiades members. We combine these new measurements with previously published ones, and use the Kepler K2 rotational periods recently derived for Pleiades cool dwarfs to investigate the lithium-rotation connection in this 125 Myr-old cluster. Results: The new data confirm the correlation between lithium equivalent width and stellar spin rate for a sample of 51 early K-type members of the cluster, where fast rotating stars are systematically lithium-rich compared to slowly rotating ones. The correlation is valid for all stars over the (J-Ks) color range 0.50-0.70 mag, corresponding to a mass range from about 0.75 to 0.90 M⊙, and may extend down to lower masses. Conclusions: We argue that the dispersion in lithium equivalent widths observed for cool dwarfs in the Pleiades cluster reflects an intrinsic scatter in lithium abundances, and suggest that the physical origin of the lithium dispersion pattern is to be found in the pre-main sequence rotational history of solar-type stars. Based on observations made at Observatoire de Haute Provence (CNRS), France, at the Nordic Optical Telescope (IAC), Spain, and at the W. M. Keck Observatory, Hawaii, USA.Full Table B.1 is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/613/A63

Three-Dimensional General-Relativistic Magnetohydrodynamic Simulations of Remnant Accretion Disks from Neutron Star Mergers: Outflows and r -Process Nucleosynthesis

NASA Astrophysics Data System (ADS)

Siegel, Daniel M.; Metzger, Brian D.

2017-12-01

The merger of binary neutron stars, or of a neutron star and a stellar-mass black hole, can result in the formation of a massive rotating torus around a spinning black hole. In addition to providing collimating media for γ -ray burst jets, unbound outflows from these disks are an important source of mass ejection and rapid neutron capture (r -process) nucleosynthesis. We present the first three-dimensional general-relativistic magnetohydrodynamic (GRMHD) simulations of neutrino-cooled accretion disks in neutron star mergers, including a realistic equation of state valid at low densities and temperatures, self-consistent evolution of the electron fraction, and neutrino cooling through an approximate leakage scheme. After initial magnetic field amplification by magnetic winding, we witness the vigorous onset of turbulence driven by the magnetorotational instability (MRI). The disk quickly reaches a balance between heating from MRI-driven turbulence and neutrino cooling, which regulates the midplane electron fraction to a low equilibrium value Ye≈0.1 . Over the 380-ms duration of the simulation, we find that a fraction ≈20 % of the initial torus mass is unbound in powerful outflows with asymptotic velocities v ≈0.1 c and electron fractions Ye≈0.1 - 0.25 . Postprocessing the outflows through a nuclear reaction network shows the production of a robust second- and third-peak r process. Though broadly consistent with the results of previous axisymmetric hydrodynamical simulations, extrapolation of our results to late times suggests that the total ejecta mass from GRMHD disks is significantly higher. Our results provide strong evidence that postmerger disk outflows are an important site for the r process.

Three-Dimensional General-Relativistic Magnetohydrodynamic Simulations of Remnant Accretion Disks from Neutron Star Mergers: Outflows and r-Process Nucleosynthesis.

PubMed

Siegel, Daniel M; Metzger, Brian D

2017-12-08

The merger of binary neutron stars, or of a neutron star and a stellar-mass black hole, can result in the formation of a massive rotating torus around a spinning black hole. In addition to providing collimating media for γ-ray burst jets, unbound outflows from these disks are an important source of mass ejection and rapid neutron capture (r-process) nucleosynthesis. We present the first three-dimensional general-relativistic magnetohydrodynamic (GRMHD) simulations of neutrino-cooled accretion disks in neutron star mergers, including a realistic equation of state valid at low densities and temperatures, self-consistent evolution of the electron fraction, and neutrino cooling through an approximate leakage scheme. After initial magnetic field amplification by magnetic winding, we witness the vigorous onset of turbulence driven by the magnetorotational instability (MRI). The disk quickly reaches a balance between heating from MRI-driven turbulence and neutrino cooling, which regulates the midplane electron fraction to a low equilibrium value Y_{e}≈0.1. Over the 380-ms duration of the simulation, we find that a fraction ≈20% of the initial torus mass is unbound in powerful outflows with asymptotic velocities v≈0.1c and electron fractions Y_{e}≈0.1-0.25. Postprocessing the outflows through a nuclear reaction network shows the production of a robust second- and third-peak r process. Though broadly consistent with the results of previous axisymmetric hydrodynamical simulations, extrapolation of our results to late times suggests that the total ejecta mass from GRMHD disks is significantly higher. Our results provide strong evidence that postmerger disk outflows are an important site for the r process.

The CGM of Massive Galaxies: Where Cold Gas Goes to Die?

NASA Astrophysics Data System (ADS)

Howk, Jay

2017-08-01

We propose to survey the cold HI content and metallicity of the circumgalactic medium (CGM) around 50 (45 new, 5 archival) z 0.5 Luminous Red Galaxies (LRGs) to directly test a fundamental prediction of galaxy assembly models: that cold, metal-poor accretion does not survive to the inner halos of very massive galaxies. Accretion and feedback through the CGM play key roles in our models of the star formation dichotomy in galaxies. Low mass galaxies are thought to accrete gas in cold streams, while high mass galaxies host hot, dense halos that heat incoming gas and prevent its cooling, thereby quenching star formation. HST/COS has provided evidence for cold, metal-poor streams in the halos of star-forming galaxies (consistent with cold accretion). Observations have also demonstrated the presence of cool gas in the halos of passive galaxies, a potential challenge to the cold/hot accretion model. Our proposed observations will target the most massive galaxies and address the origin of the cool CGM gas by measuring the metallicity. This experiment is enabled by our novel approach to deriving metallicities, allowing the use of much fainter QSOs. It cannot be done with archival data, as these rare systems are not often probed along random sight lines. The H I column density (and metallicity) measurements require access to the UV. The large size of our survey is crucial to robustly assess whether the CGM in these galaxies is unique from that of star-forming systems, a comparison that provides the most stringent test of cold-mode accretion/quenching models to date. Conversely, widespread detections of metal-poor gas in these halos will seriously challenge the prevailing theory.

Net Zero Energy Manufactured Homes May Be on their Way

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

Gilbride, Theresa L.; Dentz, Jordan

This article, published in Home Energy Magazine, describes a research project sponsored by the U.S. Department of Energy's Building America program, to construct and test the first manufactured home in the United States built to the performance criteria of DOE's Zero Energy Ready Home program. A 15-month study was conducted to compare the real-world performance of the DOE Zero Energy Ready home and two other manufactured homes - one built to just above industry standard construction and one built to the ENERGY STAR Certified Home criteria. The homes were built by Clayton Homes' Southern Energy Division and testing was sponsoredmore » by DOE's Building America program and conducted by the Levy Partnership. The DOE ZERH had increased initial construction costs of $6,607 compared to the standard home versus $4,340 for the ENERGY STAR home but reduced energy bills by $50 per month compared to a $33/month savings for the ENERGY STAR home, and monthly savings will continue for the life of the home. Savings were especially noticeable in the summer in this cooling-dominated test location. The DOE ZERH cut cooling costs in half compared to the ENERGY STAR home which performed only slightly better than the standard home in summer, while winter savings between the two advanced homes were more similar. Two technology advances were tested in the DOE ZERH home. Instead of the typical ducted heating and cooling system, the DOE ZERH home was equipped with a ductless heat pump; to condition the bedroom, holes were cut into bedroom walls and small fans were installed to pull air into those rooms, while door undercuts and transfer grilles provide return paths. A novel dense-pack attic insulation was also implemented.« less

CANDELS+3D-HST: Compact SFGs at z ∼ 2-3, the progenitors of the first quiescent galaxies

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

Barro, G.; Faber, S. M.; Koo, D. C.

We analyze the star-forming and structural properties of 45 massive (log(M/M{sub ☉}) >10) compact star-forming galaxies (SFGs) at 2 < z < 3 to explore whether they are progenitors of compact quiescent galaxies at z ∼ 2. The optical/NIR and far-IR Spitzer/Herschel colors indicate that most compact SFGs are heavily obscured. Nearly half (47%) host an X-ray-bright active galactic nucleus (AGN). In contrast, only about 10% of other massive galaxies at that time host AGNs. Compact SFGs have centrally concentrated light profiles and spheroidal morphologies similar to quiescent galaxies and are thus strikingly different from other SFGs, which typically aremore » disk-like and sometimes clumpy or irregular. Most compact SFGs lie either within the star formation rate (SFR)-mass main sequence (65%) or below it (30%), on the expected evolutionary path toward quiescent galaxies. These results show conclusively that galaxies become more compact before they lose their gas and dust, quenching star formation. Using extensive HST photometry from CANDELS and grism spectroscopy from the 3D-HST survey, we model their stellar populations with either exponentially declining (τ) star formation histories (SFHs) or physically motivated SFHs drawn from semianalytic models (SAMs). SAMs predict longer formation timescales and older ages ∼2 Gyr, which are nearly twice as old as the estimates of the τ models. Both models yield good spectral energy distribution fits, indicating that the systematic uncertainty in the age due to degeneracies in the SFH is of that order of magnitude. However, SAM SFHs better match the observed slope and zero point of the SFR-mass main sequence. Contrary to expectations, some low-mass compact SFGs (log(M/M{sub ☉}) =10-10.6) have younger ages but lower specific SFRs than that of more massive galaxies, suggesting that the low-mass galaxies reach the red sequence faster. If the progenitors of compact SFGs are extended SFGs, state-of-the-art SAMs show that mergers and disk instabilities (DIs) are both able to shrink galaxies, but DIs are more frequent (60% versus 40%) and form more concentrated galaxies. We confirm this result via high-resolution hydrodynamic simulations.« less

The NGC 281 west cluster. I. Star formation in photoevaporating clumps.

NASA Astrophysics Data System (ADS)

Megeath, S. T.; Wilson, T. L.

1997-09-01

The NGC281 West molecular cloud is an excellent test case for studying star formation in the clumpy interface between a \\hii region and a giant molecular cloud. We present here a study based on new high resolution radio and near-infrared data. Using the IRAM 30-meter telescope, we have mapped the interface in the \\cotwo, \\coone, and \\cs transitions with FWHP beamwidths <= 22''. We have imaged the same region with the VLA in the 20, 6 and 2 cm continuum bands to obtain complementary maps of the ionized gas distribution with angular resolutions <= 13''. In addition, we have obtained near-infrared J and K'-band images to detect young stars in the interface. The 30-meter data shows the molecular gas is concentrated into three clumps with masses of 570, > 210, and 300 \\msun and average volume densities of 1.4, >1, and 2 x 10(4) \\cm. We detect \\cs emission in two of the clumps, indicating peak densities in excess of 5x 10(5) \\cm are attained in the clumps. A comparison of the \\co line data with the 20 cm continuum image suggests that the molecular clumps are being photoevaporated through their direct exposure to the UV radiation from neighboring OB stars. The luminosity and extent of the observed 20 cm emission is in good agreement with theoretical predictions. We use models of photoevaporative flows to estimate the pressure exerted on the clumps by the ionized gas and find that it exceeds the internal, turbulent pressure of the clumps by a factor of a 2.5. Although a pressure equilibrium is not excluded given the uncertainties inherent in determining the pressures of the ionized and molecular gases, our best estimates of the clumps and flow parameters favor the the existence of low velocity shocks (1.5 \\kms) in the clumps. The clumps exhibit broad, non-gaussian lineshapes and complex kinematical structures suggestive of shocks. Further evidence for shocks is found in a comparison of position-velocity diagrams with published numerical simulations of imploding spherical clumps. We discuss the possibility that the knots of \\cs emission may trace gas compressed by converging shock waves. The K'-band observations show a rich cluster of primarily low mass stars in the \\hii/molecular interface, which we argue is divided into two distinct sub-clusters. We associate one sub-cluster with the two clumps nearest the OB stars, and the second sub-cluster with the third clump. The two clumps nearest the OB stars contain an embedded population, suggesting that star formation is ongoing. We discuss the impact photoevaporation is having on star formation in these two clumps. We find that photoevaporation is dispersing the molecular gas from which the cluster is forming and estimate that the molecular gas will be completely evaporated in 5 Myr. Deep K'-band imaging of the two clumps show that the stars are detected primarily on the sides of the clumps facing the OB stars and in the adjoining \\hii region. We examine three explanations for this asymmetry: displacement of the clump centers from the cluster center by the acceleration of the molecular gas through photoevaporation (i.e. the rocket effect), unveiling of young, embedded stars by ionization-shock fronts, and the triggered formation of stars by shocks advancing into the clumps. If shock compression is indeed ongoing in the clumps, then we argue that there is a good case for shock triggered star formation.

The IACOB project. V. Spectroscopic parameters of the O-type stars in the modern grid of standards for spectral classification

NASA Astrophysics Data System (ADS)

Holgado, G.; Simón-Díaz, S.; Barbá, R. H.; Puls, J.; Herrero, A.; Castro, N.; Garcia, M.; Maíz Apellániz, J.; Negueruela, I.; Sabín-Sanjulián, C.

2018-06-01

Context. The IACOB and OWN surveys are two ambitious, complementary observational projects which have made available a large multi-epoch spectroscopic database of optical high resolution spectra of Galactic massive O-type stars. Aims: Our aim is to study the full sample of (more than 350) O stars surveyed by the IACOB and OWN projects. As a first step towards this aim, we have performed the quantitative spectroscopic analysis of a subsample of 128 stars included in the modern grid of O-type standards for spectral classification. The sample comprises stars with spectral types in the range O3-O9.7 and covers all luminosity classes. Methods: We used the semi-automatized IACOB-BROAD and IACOB-GBAT/FASTWIND tools to determine the complete set of spectroscopic parameters that can be obtained from the optical spectrum of O-type stars. A quality flag was assigned to the outcome of the IACOB-GBAT/FASTWIND analysis for each star, based on a visual evaluation of how the synthetic spectrum of the best fitting FASTWIND model reproduces the observed spectrum. We also benefitted from the multi-epoch character of the IACOB and OWN surveys to perform a spectroscopic variability study of the complete sample, providing two different flags for each star accounting for spectroscopic binarity as well as variability of the main wind diagnostic lines. Results: We obtain - for the first time in a homogeneous and complete manner - the full set of spectroscopic parameters of the "anchors" of the spectral classification system in the O star domain. We provide a general overview of the stellar and wind parameters of this reference sample, as well as updated recipes for the SpT-Teff and SpT-log g calibrations for Galactic O-type stars. We also propose a distance-independent test for the wind-momentum luminosity relationship. We evaluate the reliability of our semi-automatized analysis strategy using a subsample of 40 stars extensively studied in the literature, and find a fairly good agreement between our derived effective temperatures and gravities and those obtained by means of more traditional "by-eye" techniques and different stellar atmosphere codes. The overall agreement between the synthetic spectra associated with the IACOB-GBAT/FASTWIND best fitting models and the observed spectra is good for most of the analyzed targets, but 46 stars out of the 128 present a particular behavior of the wind diagnostic lines that cannot be reproduced by our grid of spherically symmetric unclumped models. These are potential targets of interest for more detailed investigations of clumpy winds and/or the existence of additional circumstellar emitting components contaminating the wind diagnostic lines (e.g., disks, magnetospheres). Last, our spectroscopic variability study has led to the detection of clear or likely signatures of spectroscopic binarity in 27% of the stars and small amplitude radial velocity variations in the photospheric lines of another 30%. Additionally, 31% of the investigated stars show variability in the wind diagnostic lines. Tables D.1 and D.2 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/613/A65

Herschel observations of the circumstellar environments of the Herbig Be stars R Mon and PDS 27

NASA Astrophysics Data System (ADS)

Jiménez-Donaire, M. J.; Meeus, G.; Karska, A.; Montesinos, B.; Bouwman, J.; Eiroa, C.; Henning, T.

2017-09-01

Context. The circumstellar environments of Herbig Be stars in the far-infrared are poorly characterised, mainly because they are often embedded and rather distant. The analysis of far-infrared spectroscopy allows us to make a major step forward by covering multiple rotational lines of molecules, e.g. CO, that are useful probes of the physical conditions of the gas. Aims: We characterise the gas and dust in the discs and environments of Herbig Be stars, and we compare the results with those of their lower-mass counterparts, the Herbig Ae stars. Methods: We report and analyse far-infrared observations of two Herbig Be stars, R Mon and PDS 27, obtained with the Herschel instruments PACS and SPIRE. We construct spectral energy distributions and derive the infrared excess. We extract line fluxes from the PACS and SPIRE spectra and construct rotational diagrams in order to estimate the excitation temperature of the gas. We derive CO, [O I] and [C I] luminosities to determine the physical conditions of the gas, and the dominant cooling mechanism. Results: We confirm that the Herbig Be stars are surrounded by remnants from their parental clouds, with an IR excess that mainly originates in a disc. In R Mon we detect [O I], [C I], [C II], CO (26 transitions), water and OH, while in PDS 27 we only detect [C I] and CO (8 transitions). We attribute the absence of OH and water in PDS 27 to UV photo-dissociation and photo-evaporation. From the rotational diagrams, we find several components for CO; we derive Trot949 ± 90 K, 358 ± 20 K and 77 ± 12 K for R Mon; 96 ± 12 K and 31 ± 4 K for PDS 27; and 25 ± 8 K and 27 ± 6 K for their respective compact neighbours. The forsterite feature at 69 μm was not detected in either of the sources, probably due to the lack of (warm) crystalline dust in a flat disc. We find that cooling by molecules is dominant in the Herbig Be stars, while this is not the case in Herbig Ae stars where cooling by [O I] dominates. Moreover, we show that in the Herbig Be star R Mon, outflow shocks are the dominant gas heating mechanism, while in Herbig Ae stars it is stellar. Conclusions: The outflow of R Mon contributes to the observed line emission by heating the gas in the central spaxel/beam covering the disc and in the immediate surroundings, as well as in those spaxels/beams covering the parabolic shell around it. PDS 27, a B2 star, has dispersed a large part of its gas content and/or destroyed molecules; this is likely given its intense UV field. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

Coronal Structures in Cool Stars

NASA Technical Reports Server (NTRS)

Oliversen, Ronald (Technical Monitor); Dupree, Andrea K.

2005-01-01

We have extended our study of the structure of coronas in cool stars to very young stars still accreting from their surrounding disks. In addition we are pursing the connection between coronal X-rays and a powerful diagnostic line in the infrared, the He I 10830Angstrom transition of helium. Highlights of these are summarized below including publications during this reporting period and presentations. Spectroscopy of the infrared He I (lambda10830) line with KECK/NIRSPEC and IRTF/CSHELL and of the ultraviolet C III (lambda977) and O VI (lambda1032) emission with FUSE reveals that the classical T Tauri star TW Hydrae exhibits P Cygni profiles, line asymmetries, and absorption indicative of a continuous, fast (approximately 400 kilometers per second), hot (approximately 300,000 K) accelerating outflow with a mass loss rate approximately 10(exp -11)-10(exp -12) solar mass yr(sup -1) or larger. Spectra of T Tauri N appear consistent with such a wind. The source of the emission and outflow seems restricted to the stars themselves. Although the mass accretion rate is an order of magnitude less for TW Hya than for T Tau, the outflow reaches higher velocities at chromospheric temperatures in TW Hya. Winds from young stellar objects may be substantially hotter and faster than previously thought. The ultraviolet emission lines, when corrected for absorption are broad. Emission associated with the accretion flow and shock is likely to show turbulent broadening. We note that the UV line widths are significantly larger than the X-ray line widths. If the X-rays from TW Hya are generated at the accretion shock, the UV lines may not be directly associated with the shock. On the other hand, studies of X-ray emission in young star clusters, suggest that the strength of the X-ray emission is correlated with stellar rotation, thus casting doubt on an accretion origin for the X-rays. We are beginning to access the infrared spectral region where the He I 108308Angstroms transition occurs. This line is particularly useful as a diagnostic of coronal radiation since it is formed by recombination following photoionization of neutral helium by coronal X-rays. Because the lower level of the transition is metastable, infrared radiation from the stellar photosphere is absorbed which provides a diagnostic of atmospheric dynamics. This transition is useful both in young stars in the T Tauri phase and in active cool star binaries. We will investigate the influence of coronal x-rays on the strength of this transition.

Spectral Line Polarisation Atlases for 53 Cam (A4p) and alpha 2 CVn (A0p)

NASA Astrophysics Data System (ADS)

Wade, G. A.

2002-08-01

Wade, Donati & Landstreet (2000) presented a atlas of the R=35,000 Stokes IQUV spectrum of the cool magnetic Ap star beta CrB in the spectral range 450-660 nm. In this report we present analogous atlases for the well-studied magnetic Ap stars 53 Cam (HD 65339, A4p) and alpha 2 CVn (HD 112413, A0p).

Chromospheric dust formation, stellar masers and mass loss

NASA Technical Reports Server (NTRS)

Stencel, R. E.

1986-01-01

A multistep scenario which describes a plausible mass loss mechanism associated with red giant and related stars is outlined. The process involves triggering a condensation instability in an extended chromosphere, leading to the formation of cool, dense clouds which are conducive to the formation of molecules and dust grains. Once formed, the dust can be driven away from the star by radiation pressure. Consistency with various observed phenomena is discussed.

Lighting the universe with filaments.

PubMed

Gao, Liang; Theuns, Tom

2007-09-14

The first stars in the universe form when chemically pristine gas heats as it falls into dark-matter potential wells, cools radiatively because of the formation of molecular hydrogen, and becomes self-gravitating. Using supercomputer simulations, we demonstrated that the stars' properties depend critically on the currently unknown nature of the dark matter. If the dark-matter particles have intrinsic velocities that wipe out small-scale structure, then the first stars form in filaments with lengths on the order of the free-streaming scale, which can be approximately 10(20) meters (approximately 3 kiloparsecs, corresponding to a baryonic mass of approximately 10(7) solar masses) for realistic "warm dark matter" candidates. Fragmentation of the filaments forms stars with a range of masses, which may explain the observed peculiar element abundance pattern of extremely metal-poor stars, whereas coalescence of fragments and stars during the filament's ultimate collapse may seed the supermassive black holes that lurk in the centers of most massive galaxies.

Basic properties and variability

NASA Technical Reports Server (NTRS)

Querci, Francois R.

1987-01-01

Giant and supergiant M, S, and C stars are discussed in this survey of research. Basic properties as determined by spectra, chemical composition, photometry, or variability type are discussed. Space motions and space distributions of cool giants are described. Distribution of these stars in our galaxy and those nearby is discussed. Mira variables in particular are surveyed with emphasis on the following topics: (1) phase lag phenomenon; (2) Mira light curves; (3) variations in color indices; (4) determination of multiple periods; (5) correlations between quantities such as period length, light-curve shape, infrared (IR) excess, and visible and IR color diagram; (6) semiregular (SR) variables and different time scales in SR light variations; (7) irregular variable Lb and Lc stars; (8) different time-scale light variations; (9) hydrogen-deficient carbon (HdC) stars, in particular RCB stars; and (10) irreversible changes and rapid evolution in red variable stars.

Chandra Observations of Magnetic White Dwarfs and Their Theoretical Implications

NASA Technical Reports Server (NTRS)

Musielak, Z. E.; Noble, M.; Porter, J. G.; Winget, D. E.; Six, N. Frank (Technical Monitor)

2002-01-01

Observations of cool DA and DB white dwarfs have not yet been successful in detecting coronal X-ray emission but observations of late-type dwarfs and giants show that coronae are common for these stars. To produce coronal X-rays, a star must have dynamo-generated surface magnetic fields and a well-developed convection zone. There is strong observational evidence that the DA star LHS 1038 and the DB star GD 358 have weak and variable surface magnetic fields. Since these fields are likely to be generated by dynamo action and since both stars have well-developed convection zones, theory predicts detectable levels of coronal X-rays from these white dwarfs. However, we present analysis of Chandra observations of both stars showing no detectable X-ray emission. The derived upper limits for the X-ray fluxes provide strong constraints on theories of formation of coronae around magnetic white dwarfs.

13CO Survey of Northern Intermediate-Mass Star-Forming Regions

NASA Astrophysics Data System (ADS)

Lundquist, Michael J.; Kobulnicky, H. A.; Kerton, C. R.

2014-01-01

We conducted a survey of 13CO with the OSO 20-m telescope toward 68 intermediate-mass star-forming regions (IM SFRs) visible in the northern hemisphere. These regions have mostly been excluded from previous CO surveys and were selected from IRAS colors that specify cool dust and large PAH contribution. These regions are known to host stars up to, but not exceeding, about 8 solar masses. We detect 13CO in 57 of the 68 IM SFRs down to a typical RMS of ~50 mK. We present kinematic distances, minimum column densities, and minimum masses for these IM SFRs.

HR 7098: a new cool HgMN star ?

NASA Astrophysics Data System (ADS)

Monier, R.; Gebran, M.; Royer, F.; Kılıcoǧlu, T.

2017-12-01

Using one archival high dispersion high quality spectrum of HR 7098 (A0V) obtained with the échelle spectrograph SOPHIE at Observatoire de Haute Provence, we show that this star is not a superficially normal A0V star as hitherto thought. The model atmosphere and spectrum synthesis modeling of the spectrum of HR 7098 reveals real departures of its abundances from the solar composition. We report here on our first determinations of the elemental abundances of 35 elements in the atmosphere of HR 7098. Helium and Carbon are underabundant whereas the very heavy elements are overabundant in HR 7098.

Life beyond the solar system.

NASA Technical Reports Server (NTRS)

Sagan, C.

1972-01-01

Review of some of the highlights and more recent developments in the search for extraterrestrial intelligence. The first major problem is one of the generality of the formation of planetary systems. Observations of the nearest stars which are not members of binary or multiple stars indicates that fully half have companions of planetary mass. The presence of organic compounds in meteorites, probably in Jovian planets, in comets, in the interstellar medium, and in cool stars implies that the production of organic compounds essential for the origin of life should be pervasive throughout the universe. Possibilities of interstellar communication are discussed.

The formation of direct collapse black holes under the influence of streaming velocities

NASA Astrophysics Data System (ADS)

Schauer, Anna T. P.; Regan, John; Glover, Simon C. O.; Klessen, Ralf S.

2017-11-01

We study the influence of a high baryonic streaming velocity on the formation of direct collapse black holes (DCBHs) with the help of cosmological simulations carried out using the moving mesh code arepo. We show that a streaming velocity that is as large as three times the root-mean-squared value is effective at suppressing the formation of H2-cooled minihaloes, while still allowing larger atomic cooling haloes (ACHs) to form. We find that enough H2 forms in the centre of these ACHs to effectively cool the gas, demonstrating that a high streaming velocity by itself cannot produce the conditions required for DCBH formation. However, we argue that high streaming velocity regions do provide an ideal environment for the formation of DCBHs in close pairs of ACHs (the `synchronized halo' model). Due to the absence of star formation in minihaloes, the gas remains chemically pristine until the ACHs form. If two such haloes form with only a small separation in time and space, then the one forming stars earlier can provide enough ultraviolet radiation to suppress H2 cooling in the other, allowing it to collapse to form a DCBH. Baryonic streaming may therefore play a crucial role in the formation of the seeds of the highest redshift quasars.

Chandra Observations of Hydra A

NASA Technical Reports Server (NTRS)

McNamara, Brian; Lavoie, Anthony R. (Technical Monitor)

2000-01-01

We present Chandra X-ray Observations of the Hydra A cluster of galaxies, and we report the discovery of structure in the central 80 kpc of the cluster's X-ray-emitting gas. The most remarkable structures are depressions in the X-ray surface brightness, approx. 25 - 35 kpc diameter, that are coincident with Hydra A's radio lobes. The depressions are nearly devoid of X-ray-emitting gas, and there is no evidence for shock-heated gas surrounding the radio lobes. We suggest the gas within the surface brightness depressions was displaced as the radio lobes expanded subsonically, leaving cavities in the hot atmosphere. The gas temperature declines from 4 keV at 70 kpc to 3 keV in the inner 20 kpc of the brightest cluster galaxy (BCG), and the cooling time of the gas is approx. 600 Myr in the inner 10 kpc. These properties are consistent with the presence of a approx. 34 solar mass/yr cooling flow within a 70 kpc radius. Bright X-ray emission is present in the BCG surrounding a recently-accreted disk of nebular emission and young stars. The star formation rate is commensurate with the cooling rate of the hot gas within the volume of the disk, although the sink for the material that may be cooling at larger radii remains elusive.