These are representative sample records from Science.gov related to your search topic.
For comprehensive and current results, perform a real-time search at Science.gov.
1

Gravitational Waves From Low Mass Neutron Stars  

E-print Network

Low mass neutron stars may be uniquely strong sources of gravitational waves (GW). The neutron star crust can support large deformations for low mass stars. This is because of the star's weaker gravity. We find maximum ellipticities $\\epsilon$ (fractional difference in moments of inertia) that are 1000 times larger, and maximum quadrupole moments $Q_{22}$ over 100 times larger, for low mass stars than for 1.4 $M_\\odot$ neutron stars. Indeed, we calculate that the crust can support an $\\epsilon$ as large as 0.01 for a minimum mass neutron star. A 0.12 $M_\\odot$ star, that is maximally strained and rotating at 100 Hz, will produce a characteristic gravitational wave strain of $h_0=2.1\\times 10^{-24}$ at a distance of 1 kpc. The GW detector Advanced LIGO should be sensitive to such objects through out the Milky Way Galaxy.

C. J. Horowitz

2009-12-08

2

Gravitational waves from low mass neutron stars  

SciTech Connect

Low mass neutron stars may be uniquely strong sources of gravitational waves. The neutron star crust can support large deformations for low mass stars. This is because of the star's weaker gravity. We find maximum ellipticities {epsilon} (fractional difference in moments of inertia) that are 1000 times larger, and maximum quadrupole moments Q{sub 22} over 100 times larger, for low mass stars than for 1.4M{sub {center_dot}}neutron stars. Indeed, we calculate that the crust can support an {epsilon} as large as 0.005 for a minimum mass neutron star. A 0.12M{sub {center_dot}}star, that is maximally strained and rotating at 100 Hz, will produce a characteristic gravitational wave strain of h{sub 0}=2.1x10{sup -24} at a distance of 1 kpc. The gravitational wave detector Advanced LIGO should be sensitive to such objects through out the Milky Way Galaxy. A low mass neutron star could be uniquely identified from a large observed spin down rate and its discovery would have important implications for general relativity, supernova mechanisms, and possibly nucleosynthesis.

Horowitz, C. J. [Department of Physics and Nuclear Theory Center, Indiana University, Bloomington, Indiana 47405 (United States)

2010-05-15

3

Spectroscopic Observations of Nearby Low Mass Stars  

NASA Astrophysics Data System (ADS)

Young low-mass stars are known to be bright in X-ray and UV due to a high level of magnetic activity. By cross-correlating the GALEX Catalog with the WISE and 2MASS Point Source Catalogs, we have identified more than 2,000 stars whose UV excesses suggest ages in the 10-100 Myr range. We used the Shane 3-m telescope at Lick Observatory on Mount Hamilton, California to observe some of these 2,000 stars spectroscopically. We measured the equivalent width of lithium at 6708 A absorption and H-alpha emission lines. Out of a total of 122 stars observed with the Kast grating spectrometer, we find that roughly 10% have strong lithium absorption features. The high percentage of stars with lithium present is further evidence of the importance of UV emission as a youth indicator for low-mass stars. In addition, we used high-resolution spectra obtained with the Hamilton echelle spectrograph to determine radial velocities for several UV-bright stars. These radial velocities will be useful for the calculation of Galactic UVW space velocities for determination of possible moving group membership. This work is supported by NASA Astrophysics Data Analysis Program award NNX12AH37G to RIT and UCLA and Chilean FONDECYT grant 3130520 to Universidad de Chile. This submission presents work for the GALNYSS project and should be linked to abstracts submitted by David Rodriguez, Laura Vican, and Joel Kastner.

Vican, Laura; Zuckerman, B. M.; Rodriguez, D.

2014-01-01

4

Mass and radius formulas for low-mass neutron stars  

E-print Network

Neutron stars, produced at the death of massive stars, are often regarded as giant neutron-rich nuclei. This picture is especially relevant for low-mass (below about solar mass) neutron stars, where non-nucleonic components are not expected to occur. Due to the saturation property of nucleonic matter, leading to the celebrated liquid-drop picture of atomic nuclei, empirical nuclear masses and radii can be approximately expressed as function of atomic mass number. It is, however, not straightforward to express masses and radii of neutron stars even in the low-mass range where the structure is determined by a balance between the pressure of neutron-rich nucleonic matter and the gravity. Such expressions would be of great use given possible simultaneous mass and radius measurements. Here we successfully construct theoretical formulas for the masses and radii of low-mass neutron stars from various models that are consistent with empirical masses and radii of stable nuclei. In this process, we discover a new equation-of-state parameter that characterizes the structure of low-mass neutron stars. This parameter, which plays a key role in connecting the mass-radius relation of the laboratory nuclei to that of the celestial objects, could be constrained from future observations of low-mass neutron stars.

Hajime Sotani; Kei Iida; Kazuhiro Oyamatsu; Akira Ohnishi

2013-12-31

5

Do Magnetic Fields Actually Inflate Low-Mass Stars?  

NASA Astrophysics Data System (ADS)

Magnetic fields have been hypothesized to inflate the radii of low-mass stars-defined as less than 0.8 M ?-in detached eclipsing binaries (DEBs). We evaluate this hypothesis using the magnetic Dartmouth stellar evolution code. Results suggest that magnetic suppression of thermal convection can inflate low-mass stars that possess a radiative core and convective outer envelope. A scaling relation between X-ray luminosity and surface magnetic flux indicates that model surface magnetic field strength predictions are consistent with observations. This supports the notion that magnetic fields may be inflating these stars. However, magnetic models are unable to reproduce radii of fully convective stars in DEBs. Instead, we propose that model discrepancies below the fully convective boundary are related to metallicity.

Feiden, Gregory A.; Chaboyer, Brian

2014-08-01

6

Young stars of low mass in the Gum nebula  

NASA Technical Reports Server (NTRS)

Observations are presented for four recently formed stars in the vicinity of the Gum nebula which are heavily obscured by surrounding dust and are associated with small reflection nebulae. HH46 is the only currently active star of the sample, and it is found to have a spectral type in the range of late G-early K, with superimposed emission lines of H-alpha, Ca II, Fe I, Fe II, and weak He I at near zero velocities. It is suggested that the observed scenario of low-mass stars in an older massive star environment may be analogous to the circumstances surrounding the birth of the sun.

Graham, J. A.; Heyer, Mark H.

1989-01-01

7

Young stars of low mass in the Gum nebula  

SciTech Connect

Observations are presented for four recently formed stars in the vicinity of the Gum nebula which are heavily obscured by surrounding dust and are associated with small reflection nebulae. HH46 is the only currently active star of the sample, and it is found to have a spectral type in the range of late G-early K, with superimposed emission lines of H-alpha, Ca II, Fe I, Fe II, and weak He I at near zero velocities. It is suggested that the observed scenario of low-mass stars in an older massive star environment may be analogous to the circumstances surrounding the birth of the sun. 53 refs.

Graham, J.A.; Heyer, M.H. (Carnegie Institution of Washington, Washington, DC (USA))

1989-06-01

8

Metallicity of low-mass stars in Orion  

NASA Astrophysics Data System (ADS)

Context: Determining the metal content of low-mass members of young associations provides a tool that addresses different issues, such as triggered star formation or the link between the metal-rich nature of planet-host stars and the early phases of planet formation. The Orion complex is a well known example of possible triggered star formation and is known to host a rich variety of proto-planetary disks around its low-mass stars. Available metallicity measurements yield discrepant results. Aims: We analyzed FLAMES/UVES and Giraffe spectra of low-mass members of three groups/clusters belonging to the Orion association. Our goal is the homogeneous determination of the metallicity of the sample stars, which allows us to look for [Fe/H] differences between the three regions and for the possible presence of metal-rich stars. Methods: Nine members of the ONC and one star each in the ? Ori cluster and OB1b subgroup were analyzed. After the veiling determination, we retrieved the metallicity by means of equivalent widths and/or spectral synthesis using MOOG. Results: We obtain an average metallicity for the ONC [{Fe/H}] =-0.01± 0.04. No metal-rich stars were detected and the dispersion within our sample is consistent with measurement uncertainties. The metallicity of the ? Ori member is also solar, while the OB1b star has an [Fe/H] significantly below the ONC average. If confirmed by additional [Fe/H] determinations in the OB1b subgroup, this result would support the triggered star formation and the self-enrichment scenario for the Orion complex. Based on observations collected at Paranal Observatory, ESO (Chile). Programs 072.D-0019, 074.C-0757, 076.D-0136, 076.C-0145.

D'Orazi, V.; Randich, S.; Flaccomio, E.; Palla, F.; Sacco, G. G.; Pallavicini, R.

2009-07-01

9

Luminosity functions for very low mass stars and brown dwarfs  

NASA Technical Reports Server (NTRS)

A theoretical investigation of the luminosity function for low-mass objects to constrain the stellar initial mass function at the low-mass end is reported. The ways in which luminosity functions for low-mass stars are affected by star formation histories, brown dwarf and premain-sequence cooling rates and main-sequence mass luminosity relations, and the IMF are examined. Cooling rates and the mass-luminosity relation are determined through a new series of evolutionary calculations for very low mass stars and brown dwarfs in the range 0.05-0.50 solar mass. Model luminosity functions are constructed for specific comparison with the results of four recent observational surveys. The likelihood that the stellar mass function in the solar neighborhood is increasing at masses near the bottom of the main sequence and perhaps at lower masses is confirmed. In the most optimistic case, brown dwarfs contribute half of the local missing disk mass. The actual contribution is likely to be considerably less.

Laughlin, Gregory; Bodenheimer, Peter

1993-01-01

10

Resolving the Discrepancy of Low-Mass Stars with IGRINS  

NASA Astrophysics Data System (ADS)

Observed properties of low-mass stars (M < 0.8 solar msses) have been found to be in disagreement with stellar models, the observed radii being inflated and the observed temperatures being too low. To study this discrepancy, we are observing a sample of low-mass eclipsing binaries using the 2.7-m Harlan J. Smith telescope at McDonald Observatory as well as the LCOGT network to increase the number of well-characterized systems. We are also using IGRINS, a new high resolution (R=40,000) IR (H+K) spectrograph on the 2.7-m HJST, to measure the fundamental stellar parameters (Teff, R, M, abundances, activity) of a sample of eclipsing binaries consisting of two low-mass components. Finally, to calibrate these eclipsing binaries, we are observing a temperature calibration sample of single M dwarfs with precise temperature measurements from interferometry and a metallicity calibration sample of M dwarfs in wide binaries with solar-type stars. Relationships between these parameters will help us better understand the discrepancy between models and observed properties of low-mass stars.

Riddle, Andrew; Kraus, Adam L.

2015-01-01

11

Mass and radius formulas for low-mass neutron stars  

E-print Network

Neutron stars, produced at the death of massive stars, are often regarded as giant neutron-rich nuclei. This picture is especially relevant for low-mass (below about solar mass) neutron stars, where non-nucleonic components are not expected to occur. Due to the saturation property of nucleonic matter, leading to the celebrated liquid-drop picture of atomic nuclei, empirical nuclear masses and radii can be approximately expressed as function of atomic mass number. It is, however, not straightforward to express masses and radii of neutron stars even in the low-mass range where the structure is determined by a balance between the pressure of neutron-rich nucleonic matter and the gravity. Such expressions would be of great use given possible simultaneous mass and radius measurements. Here we successfully construct theoretical formulas for the masses and radii of low-mass neutron stars from various models that are consistent with empirical masses and radii of stable nuclei. In this process, we discover a new equat...

Sotani, Hajime; Oyamatsu, Kazuhiro; Ohnishi, Akira

2014-01-01

12

COUNTING LOW-MASS STARS IN INTEGRATED LIGHT  

SciTech Connect

Low-mass stars (M {approx}< 0.4 M{sub Sun }) are thought to comprise the bulk of the stellar mass of galaxies but they constitute only of order 1 percent of the bolometric luminosity of an old stellar population. Directly estimating the number of low-mass stars from integrated flux measurements of old stellar systems is therefore possible but very challenging, given the numerous variables that can affect the light at the percent level. Here we present a new population synthesis model created specifically for the purpose of measuring the low-mass initial mass function (IMF) down to {approx}0.1 M{sub Sun} for metal-rich stellar populations with ages in the range 3-13.5 Gyr. Our fiducial model is based on the synthesis of three separate isochrones, and a combination of optical and near-IR empirical stellar libraries in order to produce integrated light spectra over the wavelength interval 0.35 {mu}m < {lambda} < 2.4 {mu}m at a resolving power of R Almost-Equal-To 2000. New synthetic stellar atmospheres and spectra have been computed in order to model the spectral variations due to changes in individual elemental abundances including C, N, Na, Mg, Si, Ca, Ti, Fe, and generic {alpha} elements. We demonstrate the power of combining blue spectral features with surface gravity-sensitive near-IR features in order to simultaneously constrain the low-mass IMF, stellar population age, metallicity, and abundance pattern from integrated light measurements. Finally, we show that the shape of the low-mass IMF can also be directly constrained by employing a suite of surface gravity-sensitive spectral features, each of which is most sensitive to a particular mass interval.

Conroy, Charlie [Harvard-Smithsonian Center for Astrophysics, Cambridge, MA (United States); Van Dokkum, Pieter [Department of Astrophysical Sciences, Yale University, New Haven, CT (United States)

2012-03-01

13

The APOGEE Low-Mass Star Ancillary Project  

NASA Astrophysics Data System (ADS)

As a high-resolution, near-infrared, fiber-fed instrument, APOGEE presents a unique opportunity to obtain multi-epoch radial velocity measurements of a large number of low-mass stars. These observations will reveal unseen companions, improving our understanding of stellar multiplicity at the bottom of the Main Sequence, and may even identify candidate sub-stellar companions. These same data contains an unprecedented wealth of information about the kinematics, rotation, and metallicities of these stars. I will describe the status of our Ancillary Science program, and ongoing efforts to get the best possible radial velocity precision from the APOGEE data.

Blake, Cullen; Mahadevan, Suvrath; Deshpande, Rohit; Bender, Chad F.; Terrien, Ryan; Crepp, Justin R.; Carlberg, Joleen K.; Nidever, David L.; Stassun, Keivan; Hawley, Suzanne L.; Hearty, Fred; Allende-Prieto, Carlos

2015-01-01

14

Angular Momentum Transport within Evolved Low-mass Stars  

NASA Astrophysics Data System (ADS)

Asteroseismology of 1.0-2.0 M ? red giants by the Kepler satellite has enabled the first definitive measurements of interior rotation in both first ascent red giant branch (RGB) stars and those on the helium burning clump. The inferred rotation rates are 10-30 days for the ?0.2 M ? He degenerate cores on the RGB and 30-100 days for the He burning core in a clump star. Using the Modules for Experiments in Stellar Evolution code, we calculate state-of-the-art stellar evolution models of low mass rotating stars from the zero-age main sequence to the cooling white dwarf (WD) stage. We include transport of angular momentum due to rotationally induced instabilities and circulations, as well as magnetic fields in radiative zones (generated by the Tayler-Spruit dynamo). We find that all models fail to predict core rotation as slow as observed on the RGB and during core He burning, implying that an unmodeled angular momentum transport process must be operating on the early RGB of low mass stars. Later evolution of the star from the He burning clump to the cooling WD phase appears to be at nearly constant core angular momentum. We also incorporate the adiabatic pulsation code, ADIPLS, to explicitly highlight this shortfall when applied to a specific Kepler asteroseismic target, KIC8366239.

Cantiello, Matteo; Mankovich, Christopher; Bildsten, Lars; Christensen-Dalsgaard, Jørgen; Paxton, Bill

2014-06-01

15

The formation of the first galaxies and the transition to low-mass star formation  

E-print Network

The formation of the first galaxies at redshifts z ~ 10-15 signaled the transition from the simple initial state of the universe to one of ever increasing complexity. We here review recent progress in understanding their assembly process with numerical simulations, starting with cosmological initial conditions and modelling the detailed physics of star formation. In this context we emphasize the importance and influence of selecting appropriate initial conditions for the star formation process. We revisit the notion of a critical metallicity resulting in the transition from primordial to present-day initial mass functions and highlight its dependence on additional cooling mechanisms and the exact initial conditions. We also review recent work on the ability of dust cooling to provide the transition to present-day low-mass star formation. In particular, we highlight the extreme conditions under which this transition mechanism occurs, with violent fragmentation in dense gas resulting in tightly packed clusters.

Thomas H. Greif; Dominik R. G. Schleicher; Jarrett L. Johnson; Anne-Katharina Jappsen; Ralf S. Klessen; Paul C. Clark; Simon C. O. Glover; Athena Stacy; Volker Bromm

2009-10-20

16

Review on low-mass stars and brown dwarfs  

E-print Network

In this review, we examine the successes and weaknesses of modern low-mass star and brown dwarf theory.(1) We first focus on the mechanical (equation of state) and thermal (atmosphere) properties and on the evolution. We then examine the current shortcomings of the theory and we discuss recent observational analysis which have suggested discrepancies between models and observations.(2) We then examine the stellar and brown dwarf IMF and suggest that a power-law above the average thermal Jeans mass (about 1 Msol) rolling over a lognormal form below this limit adequately reproduces the observations of field and young cluster stellar and brown dwarf distributions. This yields a reasonably accurate estimate of the stellar and brown dwarf Galactic census. Finally (3) we argue that the combination of turbulence driven fragmentation at large scale and gravity at small scales provides an appealing solution for the general star and brown dwarf formation mechanism. It also provides a physical ground for the aforementioned power-law + lognormal form for the IMF, whereas a series of different power laws lacks such a physical motivation. At last, we argue that the deuterium-burning limit as the distinction between stars and planets has no physical foundation in this modern star formation scheme. Opacity limited fragmentation extending down to a few (exoplanets claimed recently in the literature are most likely regular low-mass brown dwarfs and the direct detection of an extrasolar planet remains for now elusive.

G. Chabrier; I. Baraffe; F. Allard; P. H. Hauschildt

2005-09-27

17

Low-mass relics of early star formation.  

PubMed

The earliest stars to form in the Universe were the first sources of light, heat and metals after the Big Bang. The products of their evolution will have had a profound impact on subsequent generations of stars. Recent studies of primordial star formation have shown that, in the absence of metals (elements heavier than helium), the formation of stars with masses 100 times that of the Sun would have been strongly favoured, and that low-mass stars could not have formed before a minimum level of metal enrichment had been reached. The value of this minimum level is very uncertain, but is likely to be between 10(-6) and 10(-4) that of the Sun. Here we show that the recent discovery of the most iron-poor star known indicates the presence of dust in extremely low-metallicity gas, and that this dust is crucial for the formation of lower-mass second-generation stars that could survive until today. The dust provides a pathway for cooling the gas that leads to fragmentation of the precursor molecular cloud into smaller clumps, which become the lower-mass stars. PMID:12712198

Schneider, R; Ferrara, A; Salvaterra, R; Omukai, K; Bromm, V

2003-04-24

18

FEEDBACK EFFECTS ON LOW-MASS STAR FORMATION  

SciTech Connect

Protostellar feedback, both radiation and bipolar outflows, dramatically affects the fragmentation and mass accretion from star-forming cores. We use ORION, an adaptive mesh refinement gravito-radiation-hydrodynamics code, to simulate low-mass star formation in a turbulent molecular cloud in the presence of protostellar feedback. We present results of the first simulations of a star-forming cluster that include both radiative transfer and protostellar outflows. We run four simulations to isolate the individual effects of radiation feedback and outflow feedback as well as the combination of the two. We find that outflows reduce protostellar masses and accretion rates each by a factor of three and therefore reduce protostellar luminosities by an order of magnitude. This means that, while radiation feedback suppresses fragmentation, outflows render protostellar radiation largely irrelevant for low-mass star formation above a mass scale of 0.05 M{sub Sun }. We find initial fragmentation of our cloud at half the global Jeans length, around 0.1 pc. With insufficient protostellar radiation to stop it, these 0.1 pc cores fragment repeatedly, forming typically 10 stars each. The accretion rate in these stars scales with mass as predicted from core accretion models that include both thermal and turbulent motions; the accretion rate does not appear to be consistent with either competitive accretion or accretion from an isothermal sphere. We find that protostellar outflows do not significantly affect the overall cloud dynamics, in the absence of magnetic fields, due to their small opening angles and poor coupling to the dense gas. The outflows reduce the mass from the cores by 2/3, giving a core to star efficiency, {epsilon}{sub core} {approx_equal} 1/3. The simulations are also able to reproduce many observation of local star-forming regions. Our simulation with radiation and outflows reproduces the observed protostellar luminosity function. All of the simulations can reproduce observed core mass functions, though we find they are sensitive to telescope resolution. We also reproduce the two-point correlation function of these observed cores. Lastly, we reproduce the initial mass function itself, including the low-mass end, when outflows are included.

Hansen, Charles E.; Klein, Richard I.; McKee, Christopher F. [Astronomy Department, University of California, Berkeley, CA 94720 (United States); Fisher, Robert T. [Physics Department, University of Massachusetts, Dartmouth, MA (United States)

2012-03-01

19

Low Mass Stellar Companions to Nearby A and B Stars  

NASA Astrophysics Data System (ADS)

Recent discoveries of planets orbiting retired A-stars on close orbits and young A-stars on very wide orbits have renewed interest in the properties of nearby intermediate-mass stars. Especially interesting are the young stars, because directly-imaged planets orbiting them may be bright enough for characterization (e.g. HR 8799, Beta Pictoris, etc). However, intermediate-mass stars and especially young intermediate mass stars are part of multiple systems more often than not. Close stellar companions may affect the formation and orbital evolution of any planets, and the properties of the companions can help constrain the binary formation mechanism. The mass ratio distribution of a population of stars, especially if it is significantly different from the distribution for wide companions, is helpful to distinguish companions that were born in or affected by the circumprimary disk from those which formed through fragmentation of the molecular core. We have conducted a spectroscopic survey of 400 nearby A- and B-type stars, aimed at detecting stellar companions as late as M4 for all orbital separations <100 AU. We have searched for companions to the stars by cross-correlating the spectra against model templates for F-M type stars; a significant peak in the cross-correlation function indicates a detection. Our cross-correlation technique can detect low-mass companions with orbits that are too wide to detect with radial velocity monitoring and too small to detect with imaging techniques, making it complementary to work already done. We present initial results from our survey and present the distribution of mass ratios for inner companions.

Gullikson, Kevin; Kraus, Adam L.

2015-01-01

20

Efficiencies of Low-Mass Star and Star Cluster Formation  

Microsoft Academic Search

Using a quantitative model for bipolar outflows driven by hydromagnetic\\u000aprotostellar winds, we calculate the efficiency of star formation assuming that\\u000aavailable gas is either converted into stars or ejected in outflows. We\\u000aestimate the efficiency of a single star formation event in a protostellar\\u000acore, finding 25%-70% for cores with various possible degrees of flattening.\\u000aThe core mass function

Christopher D. Matzner; Christopher F. McKee

2000-01-01

21

Low mass stellar companions around four giant stars  

NASA Astrophysics Data System (ADS)

We present three low-mass and one solar mass companions found around four intermediate-mass giants HD1695, HD120235, HD145316 and HD200004 from precise radial velocity measurements using the 1.5 m Russian-Turkish Telescope (RTT150) at the TÜB?TAK National Observatory of Turkey (TUG). The stellar parameters, which are effective temperature (Teff), surface gravity (logg) and metallicity ([Fe/H]), as well as rotational velocity (vsini) are obtained from spectral analysis. From the estimated stellar masses, the orbital parameters of the companions are also derived. We find two types of Keplerian solutions for the companion of HD120235: (1) periods 5522 days and eccentricity of e?0.93, and (2) periods 1566 days and eccentricity of e?0.83. From the abundances analysis HD1695 is found to be a metal-rich star with [Fe/H]>0.1, while HD200004 is a metal poor star with [Fe/H]<-0.2. The other two stars, HD120235 and HD145316, have solar-like abundances with [Fe/H]?0.0. Our stellar parameters and orbital solutions show that all of these stars are evolved intermediate-mass giants.

Y?lmaz, M.; Bikmaev, I.; Sato, B.; Selam, S. O.; Galeev, A. I.; Keskin, V.; Izumiura, H.; Irtuganov, E. N.; Kambe, E.; Özavc?, ?.; Melnikov, S. S.; Zhuchkov, R. Ya.; Okada, N.

2015-01-01

22

Tidal Alignment of Exoplanets Around Low Mass Stars  

NASA Astrophysics Data System (ADS)

Using the Rossiter-McLaughlin effect, the projected spin-orbit angle for many exoplanetary systems has now been measured. Thanks to this rapidly increasing sample it is becoming clear that stars with surface convective zones appear to be well aligned while those without span a wide range of inclinations. The explanation proposed (Winn et al 2010) is that perhaps only the convective zones align with the planet, while the cores remain misaligned. This explanation suffers from two problems however: the core-envelope coupling in low mass stars appears to be strong enough to prevent long-lived differential rotation and even if only the convective zone is aligned, the planet generally does not survive for long after that. Since tides due to a planet on a misaligned orbit have a component at the rotational frequency of the star, and for an aligned planet the only frequency is the difference between the generally fast planet and the slowly rotating star, it is conceivable that misaligned systems are subject to much enhanced dissipation, acting for example on resonantly excited inertial waves in the star. However, Rogers & Lin (2013) point out that under inertial mode dissipation, in addition to aligned orbits one would expect a pile-up on polar and/or exactly counter-rotating orbits. We propose that the extra equilibrium solutions disappear if one includes in the evolution the fact that stars evolve and shed angular momentum throughout their lifetime. We have built a model including all those effects and will show results exploring this explanation.

Penev, Kaloyan; Jackson, Brian K.

2014-06-01

23

Multiplicity among Young Brown Dwarfs and Very Low Mass Stars  

NASA Astrophysics Data System (ADS)

We report on a near-infrared adaptive optics imaging survey of 31 young brown dwarfs and very low mass (VLM) stars, 28 of which are in the Chamaeleon I star-forming region, using the ESO Very Large Telescope. We resolve the suspected 0.16'' (~26 AU) binary Cha H? 2 and present two new binaries, Hn 13 and CHXR 15, with separations of 0.13'' (~20 AU) and 0.30'' (~50 AU), respectively; the latter is one of the widest VLM systems known. We find a binary frequency of 11+9-6%, thus confirming the trend for a lower binary frequency with decreasing mass. By combining our work with previous surveys, we arrive at the largest sample of young VLM objects (72) with high angular resolution imaging to date. Its multiplicity fraction is in statistical agreement with that for VLM objects in the field. Furthermore, we note that many field stellar binaries with lower binding energies and/or wider cross sections have survived dynamical evolution and that statistical models suggest tidal disruption by passing stars is unlikely to affect the binary properties of our systems. Thus, we argue that there is no significant evolution of multiplicity with age among brown dwarfs and VLM stars in OB and T associations between a few megayears to several gigayears. Instead, the observations so far suggest that VLM objects are either less likely to be born in fragile multiple systems than solar-mass stars or such systems are disrupted very early. We dedicate this paper to the memory of our coauthor, Eduardo Delgado-Donate, who died in a hiking accident in Tenerife earlier this year.

Ahmic, Mirza; Jayawardhana, Ray; Brandeker, Alexis; Scholz, Alexander; van Kerkwijk, Marten H.; Delgado-Donate, Eduardo; Froebrich, Dirk

2007-12-01

24

RV SURVEY FOR PLANETS OF BROWN DWARFS AND VERY LOW-MASS STARS IN CHA I  

E-print Network

1 RV SURVEY FOR PLANETS OF BROWN DWARFS AND VERY LOW-MASS STARS IN CHA I Viki Joergens1 and Ralph, Germany ABSTRACT We have carried out a radial velocity (RV) search for planets and brown dwarf companions to very young (1-10 Myr) brown dwarfs and very low-mass stars in the Cha I star forming region

Joergens, Viki

25

Warm cores around regions of low-mass star formation  

NASA Astrophysics Data System (ADS)

Warm cores (or hot corinos) around low-mass protostellar objects show a rich chemistry with strong spatial variations. This chemistry is generally attributed to the sublimation of icy mantles on dust grains initiated by the warming effect of the stellar radiation. We have used a model of the chemistry in warm cores in which the sublimation process is based on extensive laboratory data; these data indicate that sublimation from mixed ices occurs in several well-defined temperature bands. We have determined the position of these bands for the slow warming by a solar-mass star. The resulting chemistry is dominated by the sublimation process and by subsequent gas-phase reactions; strong spatial and temporal variations in certain molecular species are found to occur, and our results are, in general, consistent with observational results for the well-studied source IRAS 16293-2422. The model used is similar to the one that describes the chemistry of hot cores. We infer that the chemistry of both hot cores and warm cores may be described by the same model (suitably adjusted for different physical parameters).

Awad, Zainab; Viti, Serena; Collings, Mark P.; Williams, David A.

2010-10-01

26

Flows, Fragmentation, and Star Formation. I. Low-mass Stars in Taurus  

E-print Network

The remarkably filamentary spatial distribution of young stars in the Taurus molecular cloud has significant implications for understanding low-mass star formation in relatively quiescent conditions. The large scale and regular spacing of the filaments suggests that small-scale turbulence is of limited importance, which could be consistent with driving on large scales by flows which produced the cloud. The small spatial dispersion of stars from gaseous filaments indicates that the low-mass stars are generally born with small velocity dispersions relative to their natal gas, of order the sound speed or less. The spatial distribution of the stars exhibits a mean separation of about 0.25 pc, comparable to the estimated Jeans length in the densest gaseous filaments, and is consistent with roughly uniform density along the filaments. The efficiency of star formation in filaments is much higher than elsewhere, with an associated higher frequency of protostars and accreting T Tauri stars. The protostellar cores generally are aligned with the filaments, suggesting that they are produced by gravitational fragmentation, resulting in initially quasi-prolate cores. Given the absence of massive stars which could strongly dominate cloud dynamics, Taurus provides important tests of theories of dispersed low-mass star formation and numerical simulations of molecular cloud structure and evolution.

Lee Hartmann

2002-07-10

27

Neutron star formation in theoretical supernovae. Low mass stars and white dwarfs  

SciTech Connect

The presupernova evolution of stars that form semi-degenerate or strongly degenerate O + Ne + Mg cores is discussed. For the 10 to 13 Msub solar stars, behavior of off-center neon flashes is crucial. The 8 to 10 m/sub solar stars do not ignite neon and eventually collapse due to electron captures. Properties of supernova explosions and neutron stars expected from these low mass progenitors are compared with the Crab nebula. The conditions for which neutron stars form from accretion-induced collapse of white dwarfs in clsoe binary systems is also examined.

Nomoto, K.

1986-01-01

28

Very low mass stars and white dwarfs in NGC 6397  

NASA Technical Reports Server (NTRS)

Deep Wide Field/Planetary Camera 2 (WFPC2) images in wide bands centered at 606 and 802 nm were taken with the Hubble Space Telescope (HST) 4.6 min from the center of the galactic globular cluster NGC 6397. The images were used to accurately position approximately 2120 stars detected in the field on a color magnitude diagram down to a limiting magnitude m(sub 814) approximately = m(sub I) approximately = 26 determined reliably and solely by counting statistics. A white dwarf sequence and a rich, narrow cluster main sequence are detected for the first time, the latter stretching from m(sub 814) = 18.5 to m(sub 814) = 24.0 where it becomes indistinguishable from the field population. Two changes of slope of the main sequence at m(sub 814) approximately = 20 and m(sub 814) approximately = 22.5 are evident. The corresponding luminosity function increases slowly from M(sub 814) approximately = 6.5 to 8.5 are expected from ground-based observations but then drops sharply from there dwon to the measurement limit. The corresponding mass function obtained bu using the only presently available mass-luminosity function for the cluster's metallicity rises to a plateau between approximately 0.25 and approximately 0.15 solar mass, but drops toward the expected mass limit of the normal hydrogen burning main sequence at approximately 0.1 solar mass. This result is in clear contrast to that obtained from the ground and implies either a substantial modification of the cluster's initial mass function due to dynamical evolution in its lifetime, or that very low mass stars are not produced in any dynamically significant amount by clusters of this type. The white dwarf sequence is in reasonable agreement with a cooling sequence of models of mass 0.5 solar mass at the canonical distance of NGC 6397 with a scatter that is most likely due to photometric errors, but may also reflect real differences in mass or chemical composition. Contamination from unresolved galaxies, which cannot be reliably identified with our filters, makes it difficult to meaningfully compare the observed white dwarf luminosity function with its theoretical counterpart.

Paresce, Francesco; De Marchi, Guido; Romaniello, Martino

1995-01-01

29

An optical spectroscopic HR diagram for low-mass stars and brown dwarfs in Orion  

Microsoft Academic Search

The masses and temperatures of young low mass stars and brown dwarfs in star-\\u000aforming regions are not yet well established because of uncertainties in the\\u000aage of individual objects and the spectral type vs. temperature scale\\u000aappropriate for objects with ages of only a few Myr. Using multi-object optical\\u000aspectroscopy, 45 low-mass stars and brown dwarfs in the Trapezium

F. C. Riddick; P. F. Roche; P. W. Lucas

2007-01-01

30

Revisiting XENON100's constraints (and signals?) for low-mass dark matter  

SciTech Connect

Although observations made with the CoGeNT and CDMS experiments have been interpreted as possible signals of low-mass ( ? 7–10 GeV) dark matter particles, constraints from the XENON100 collaboration appear to be incompatible with this hypothesis, at least at face value. In this paper, we revisit XENON100's constraint on dark matter in this mass range, and consider how various uncertainties and assumptions made might alter this conclusion. We also note that while XENON100's two nuclear recoil candidates each exhibit very low ratios of ionization-to-scintillation signals, making them difficult to attribute to known electronic or neutron backgrounds, they are consistent with originating from dark matter particles in the mass range favored by CoGeNT and CDMS. We argue that with lower, but not implausible, values for the relative scintillation efficiency of liquid xenon (L{sub eff}), and the suppression of the scintillation signal in liquid xenon at XENON100's electric field (S{sub nr}), these two events could consistently arise from dark matter particles with a mass and cross section in the range favored by CoGeNT and CDMS. If this interpretation is correct, we predict that the LUX experiment, with a significantly higher light yield than XENON100, should observe dark matter induced events at an observable rate of ? 3–24 per month.

Hooper, Dan, E-mail: dhooper@fnal.gov [Center for Particle Astrophysics, Fermi National Accelerator Laboratory, Batavia, IL 60510 (United States)

2013-09-01

31

A quest for activity cycles in low-mass stars  

NASA Astrophysics Data System (ADS)

Long-term photometric measurements in a sample of ultrashort-period (P?0.5 days or less) single and binary stars of different interior structures are analysed. A loose correlation exists between the rotational rate and cycle lengths of active stars, regardless of their evolutionary state and the corresponding physical parameters. The shortest cycles are expected for the fastest rotators of the order of 1-2 years, which is reported in this paper.

Vida, K.; Kriskovics, L.; Oláh, K.

2013-11-01

32

Finding X-ray Coronal Cycles in Low Mass Stars  

NASA Astrophysics Data System (ADS)

We seek to increase the number of stars known to have an X-ray coronal cycle. Four stars (including the Sun) are known to experience periodic long-term coronal flux variability but the statistics are not superb. In this analysis, we analyze four stellar sources that have been observed frequently by Chandra and XMM-Newton over the last ~11 years. These four sources were the brightest among numerous stellar point sources within the Chandra Deep Field South. Solar flares can dramatically increase the flux measured for our stars on short time intervals and, in observations with insufficient time coverage, can be confused for the maximum of the stars' magnetic cycles (if they have one). We have discarded times where solar proton flares are detected in the data. We utilize an APEC model, which represents the coronal plasma, to fit our stellar spectra. As our sources are very faint, we do not subtract the background, but instead we fit the background and source spectra simultaneously. We use the chi-squared statistic to evaluate the confidence of our fits. We present four light curves which suggest that a long-term X-ray flux variability similar to our Sun (the solar X-ray flux can vary by a factor of 10 over ~11 years) is not present in these stellar sources. None of our stars experienced a flux variability exceeding a factor of 3 over an 11 year time scale but one of the four stars in our sample exhibits short term variability over a one year period. However, our stellar sources are too faint to conclusively state that the flux remains constant throughout all epochs.This work is supported by the National Science Foundation REU and Department of Defense AS-SURE programs under NSF Grant no. 1262851 and by the Smithsonian Institution.

Wilson, Maurice; Guenther, Hans Moritz; Auchettl, Katie

2015-01-01

33

Numerical Results on Low Mass Star and Brown Dwarf Multiplicity  

E-print Network

We have undertaken a series of hydrodynamic + N-body simulations in order to explore the properties of young stars. Our results suggest that the IMF may be sensitive to environment in its substellar region, with more brown dwarfs being formed where clusters are denser or more compact. We find that multiple stars are a natural outcome of collapsing turbulent flows, with a high incidence of N > 2 multiples. We find a positive correlation of multiplicity with primary mass but a companion frequency that decreases with age. Binary brown dwarfs are rarely formed, in conflict with observations. Brown dwarfs as companions are predominantly found orbiting binaries or triples at large separations.

Eduardo Delgado-Donate; Cathie Clarke

2004-12-17

34

Destruction of 3He in Low Mass Stars and Implications for Chemical Evolution  

E-print Network

Recent observations of 3He/H in different environments tend to prove that 3He is not produced in low mass stars, contrary to the standard predictions of stellar evolution theory. We show how a simple consistent mechanism, namely rotation-induced mixing, can lead to the destruction of 3He in low mass stars and simultaneously account for the low 12C/13C ratios and low lithium abundances observed in giant stars of different populations. This process should both naturally account for the recent measurements of 3He/H in galactic HII regions and allow for high values of 3He observed in some planetary nebulae.

C. Charbonnel

1995-12-22

35

Membership and Multiplicity among Very Low Mass Stars and Brown Dwarfs in the Pleiades Cluster  

Microsoft Academic Search

We present near-infrared photometry and optical spectroscopy of very low mass stars and brown dwarf candidates in the Pleiades open cluster. The membership status of these objects is assessed using color-magnitude diagrams, lithium and spectral types. Eight objects out of 45 appear to be nonmembers. A search for companions among 34 very low mass Pleiades members (M<=0.09 Msolar) in high

E. L. Martín; W. Brandner; J. Bouvier; K. L. Luhman; J. Stauffer; G. Basri; M. R. Zapatero Osorio; D. Barrado y Navascués

2000-01-01

36

Low Mass Star Formation in the Gum Nebula: The CG~30/31/38 complex  

E-print Network

We present photometric and spectroscopic results for the low mass pre-main sequence (PMS) stars with spectral types K - M in the cometary globule (CG) 30/31/38 complex. We obtained multi-object high resolution spectra for the targets selected as possible PMS stars from multi-wavelength photometry. We identified 11 PMS stars brighter than V = 16.5 with ages < 5 Myr at a distance of approximately 200 pc. The spatial distribution of the PMS stars, CG clouds, and ionizing sources (O stars and supernova remnants) suggests a possible triggered origin of the star formation in this region. We confirm the youth of the photometrically selected PMS stars using the lithium abundances. The radial velocities of the low mass PMS stars are consistent with those of the cometary globules. Most of the PMS stars show weak Halpha emission with W(Halpha) < 10 A. Only 1 out of the 11 PMS stars shows a moderate near-IR excess, which suggests a short survival time (t < 5 Myr) of circumstellar disks in this star forming environment. In addition, we find five young late type stars and one Ae star which have no obvious relation to the CG 30/31/38 complex. We also discuss a possible scenario of star formation history in the CG 30/31/38 region.

Jinyoung Serena Kim; Frederick M. Walter; Scott J. Wolk

2005-02-15

37

An age-activity calibration for old low-mass stars  

NASA Astrophysics Data System (ADS)

Low-mass stars are highly interesting targets: we are able to detect planets in their habitable zones, and upcoming searches for biomarkers in exoplanet atmospheres will focus on low-mass star systems due to their ubiquity and proximity. We aim to develop an age-activity calibration for old low-mass stars, using wide binary systems consisting of an M or K dwarf and a white dwarf. The age of the system is determined by the WD cooling time plus its progenitor lifetime, yielding reliable ages in the regime >1 Gyr. For an exploratory sample of 7 systems where we have already derived ages, we propose to perform Chandra ACIS-S observations to determine the X-ray luminosities of the M dwarfs and correlate their stellar activity with age. We ask for a total observing time of 110 ks.

Poppenhaeger, Katja

2014-09-01

38

RELATIVISTIC IRON EMISSION LINES IN NEUTRON STAR LOW-MASS X-RAY BINARIES AS PROBES OF NEUTRON STAR RADII  

E-print Network

RELATIVISTIC IRON EMISSION LINES IN NEUTRON STAR LOW-MASS X-RAY BINARIES AS PROBES OF NEUTRON STAR redshift, but these lines appear to be extremely rare and difficult to observe, and the only detection for placing upper limits on neutron star radii using relativistic iron emission lines (see Miller 2007

Miller, Cole

39

Proton-capture Nucleosynthesis In Low Mass Stars: Effects of New Reaction Rates  

SciTech Connect

We present computations of nucleosynthesis in low-mass asymptotic-giant-branch stars of solar metallicity experiencing deep mixing. In this framework, we discuss the effects of recent improvements in relevant reaction rates for proton captures on intermediate-mass nuclei. The calculations are then performed on the basis of a parameterized circulation, where the effects of the new nuclear inputs are best compared to previous works. We find that especially the new reaction rate for the {sup 14}N(p,{gamma}){sup 15}O reaction implies considerable modifications in the composition of low mass red giant stars.

Palmerini, S.; Busso, M. [Dipartimento di Fisica, Universita degli Studi di Perugia and INFN Sezione di Perugia (Italy); La Cognata, M. [DMFCI - Universita di Catania and Laboratori Nazionali del Sud - INFN, Catania (Italy); Cristallo, S. [Departamento de Fisica Teorica y del Cosmos, Universidad de Granada (Spain); INAF Osservatorio Astronomico di Collurania, Teramo (Italy)

2011-10-28

40

GLOBAL STAR FORMATION REVISITED  

SciTech Connect

A general treatment of disk star formation is developed from a dissipative multiphase model, with the dominant dissipation due to cloud collisions. The Schmidt-Kennicutt (SK) law emerges naturally for star-forming disks and starbursts. We predict that there should be an inverse correlation between Tully-Fisher law and SK law residuals. The model is extended to include a multiphase treatment of supernova feedback that leads to a turbulent pressure-regulated generalization of the star formation law and is applicable to gas-rich starbursts. Enhanced pressure, as expected in merger-induced star formation, enhances star formation efficiency. An upper limit is derived for the disk star formation rate in starbursts that depends on the ratio of global ISM to cloud pressures. We extend these considerations to the case where the interstellar gas pressure in the inner galaxy is dominated by outflows from a central active galactic nucleus (AGN). During massive spheroid formation, AGN-driven winds trigger star formation, resulting in enhanced supernova feedback and outflows. The outflows are comparable to the AGN-boosted star formation rate and saturate in the super-Eddington limit. Downsizing of both SMBH and spheroids is a consequence of AGN-driven positive feedback. Bondi accretion feeds the central black hole with a specific accretion rate that is proportional to the black hole mass. AGN-enhanced star formation is mediated by turbulent pressure and relates spheroid star formation rate to black hole accretion rate. The relation between black hole mass and spheroid velocity dispersion has a coefficient (Salpeter time to gas consumption time ratio) that provides an arrow of time. Highly efficient, AGN-boosted star formation can occur at high redshift.

Silk, Joseph [Physics Department, University of Oxford, 1 Keble Road, Oxford OX1 3RH (United Kingdom); Norman, Colin [Physics Department, Johns Hopkins University, 2400 North Charles Street, Baltimore, MD 21218 (United States)], E-mail: silk@astro.ox.ac.uk, E-mail: norman@stsci.edu

2009-07-20

41

Probing the Formation of the First Low-Mass Stars with Stellar Archaeology  

E-print Network

We investigate the conditions under which the first low-mass stars formed in the universe by confronting theoretical predictions governing the transition from massive Population III to low-mass Population II stars with recent observational C and/or O abundance data of metal-poor Galactic stars. Based on an existing theory, we introduce a new "observer-friendly" function, the transition discriminant D_trans, which provides empirical constraints as well as a powerful comparison between the currently available data of metal-poor halo stars and theoretical predictions of the formation of the first low-mass stars (<1 M_sun). Specifically, we compare the empirical stellar results with the theory that fine-structure lines of C and O dominate the transition from Pop III to Pop II in the early universe. We find the observational data for halo objects as well as for dSph galaxies and globular clusters to be consistent with this theory. An explanation for the observed lack of metal-poor stars in dSph galaxies and globular clusters is also suggested. Finally, we predict that any star to be found with [Fe/H]<-4 should have enhanced C and/or O abundances. The high C and O abundances of the two most iron-poor stars are in line with our prediction.

Anna Frebel; Jarrett L. Johnson; Volker Bromm

2007-06-04

42

The History of Low-Mass Star Formation in the Upper Scorpius OB Association  

NASA Astrophysics Data System (ADS)

We use a large sample of about 100 low-mass pre-main-sequence (PMS) stars in the Upper Scorpius OB association to explore the star formation history and the initial mass function of this association. Upper Scorpius is an ideal target for such a study, because the star formation process there is finished. The PMS stars have recently been found in a spatially unbiased wide-field survey of X-ray-selected stars in a 160 deg^2 area, covering the Upper Scorpius association nearly completely. Following the optical characterization of these PMS stars, we present a new HR diagram for this association. We perform a detailed analysis of the HR diagram, taking proper account of the uncertainties and the effects of unresolved binaries, and derive ages and masses for the PMS stars. We find that the low-mass PMS stars have a mean age of about 5 Myr and show no evidence for a large age dispersion. This agrees very well with the age of 5-6 Myr previously found for the massive stars and shows that low-mass and high-mass stars are coeval and cospatial and thus have formed together. We conclude that the star formation process in Upper Scorpius was probably triggered by the shock wave of a supernova explosion in the nearby Upper Centaurus-Lupus association. After a short burst of very high star formation activity, which lasted only for a few Myr, star formation in Upper Scorpius was halted, probably by the strong winds and the ionizing radiation of the numerous massive stars that dispersed the molecular cloud.

Preibisch, Thomas; Zinnecker, Hans

1999-05-01

43

Low Mass Star Formation in the Gum Nebula: The CG~30/31/38 complex  

E-print Network

We present photometric and spectroscopic results for the low mass pre-main sequence (PMS) stars with spectral types K - M in the cometary globule (CG) 30/31/38 complex. We obtained multi-object high resolution spectra for the targets selected as possible PMS stars from multi-wavelength photometry. We identified 11 PMS stars brighter than V = 16.5 with ages < 5 Myr at a distance of approximately 200 pc. The spatial distribution of the PMS stars, CG clouds, and ionizing sources (O stars and supernova remnants) suggests a possible triggered origin of the star formation in this region. We confirm the youth of the photometrically selected PMS stars using the lithium abundances. The radial velocities of the low mass PMS stars are consistent with those of the cometary globules. Most of the PMS stars show weak Halpha emission with W(Halpha) < 10 A. Only 1 out of the 11 PMS stars shows a moderate near-IR excess, which suggests a short survival time (t < 5 Myr) of circumstellar disks in this star forming envir...

Kim, J S; Wolk, S J; Kim, Jinyoung Serena; Walter, Frederick M.

2005-01-01

44

Optical and Infrared Photometry of Low-Mass Stars in Eclipsing Binaries  

NASA Astrophysics Data System (ADS)

We report eclipse timings and optical/infrared photometry of a sample of binary stars that include low-mass nondegenerate stars, some of which are post-common envelope (PCE) secondary companions to hot subdwarf B (sdB) stars. We model the reflection effect in the latter systems to characterize the temperature of the heated hemisphere of the secondaries, first from blackbody fits and then by comparison to libraries of the spectral energy distribution of dwarf M stars. We explore how consistent our phenomenological results are with the properties inferred from analysis of light curve shapes, and discuss the prospects for turning these reflection effect systems into double-lined spectroscopic binaries for characterization of the mass-radius relationship of low-mass PCE secondaries.

Hartman, Zachary; Terndrup, Donald M.

2015-01-01

45

New results on neutron star low mass transients in the quiescence  

E-print Network

We review the main observational properties of neutron star low mass transients in quiescence. We first survey the discoveries of BeppoSAX. We then focus on recent discoveries by Chandra and XMM-Newton, with special emphsasis on the detection of the quiescent counterpart of SAX J1808.4-3658 and the study of variability in the quiescent state.

Sergio Campana; Luigi Stella

2003-09-30

46

S-process nucleosynthesis - Classical approach and asymptotic giant branch models for low-mass stars  

Microsoft Academic Search

A critical comparison is made between the results of s-process nucleosynthesis obtained with the phenomenological classical approach and a stellar model for helium shell burning in low-mass stars. For the first time, close agreement is found between the abundances determined by the classical analysis and the results of a stellar model. The calculated abundances are found in good agreement with

F. Kaeppeler; R. Gallino; M. Busso; G. Picchio; C. M. Raiteri

1990-01-01

47

TITANIUM AND VANADIUM CHEMISTRY IN LOW-MASS DWARF STARS Katharina Lodders  

E-print Network

TITANIUM AND VANADIUM CHEMISTRY IN LOW-MASS DWARF STARS Katharina Lodders Planetary Chemistry The equilibrium gas and condensation chemistry of titanium and vanadium in M, L, and T dwarf atmos- pheres are stable. Vanadium condenses into solid solution with Ti-bearing condensates (as observed in meteorites

Fegley Jr., Bruce

48

ROTATIONAL PERIODS OF VERY YOUNG BROWN DWARFS AND VERY LOW MASS STARS IN CHAMAELEON I1  

E-print Network

ROTATIONAL PERIODS OF VERY YOUNG BROWN DWARFS AND VERY LOW MASS STARS IN CHAMAELEON I1 V. Joergens May 20 ABSTRACT We have studied the photometric variability of very young brown dwarfs and very low periods in the Gunn i and R bands for the three M6.5­M7 type brown dwarf candidates Cha H 2, Cha H 3

Joergens, Viki

49

Evolution of low-mass star and brown dwarf eclipsing binaries  

Microsoft Academic Search

Context: We examine the evolution of low-mass star and brown dwarf eclipsing binaries. These objects are rapid rotators and are believed to shelter large magnetic fields. Aims: We suggest that reduced convective efficiency, due to fast rotation and large field strengths, and\\/or to magnetic spot coverage of the radiating surface significantly affect their evolution, leading to a reduced heat flux

Gilles Chabrier; Jose Gallardo; Isabelle Baraffe

2007-01-01

50

Relativistic Astrophysics in Black Hole and Low-Mass Neutron Star X-ray Binaries  

NASA Technical Reports Server (NTRS)

During the five-year period, our study of "Relativistic Astrophysics in Black Hole and Low-Mass Neutron Star X-ray Binaries" has been focused on the following aspects: observations, data analysis, Monte-Carlo simulations, numerical calculations, and theoretical modeling. Most of the results of our study have been published in refereed journals and conference presentations.

2000-01-01

51

X-ray Census of Young Low-Mass Stars Candidates Associated with Gomez's Hamburger  

NASA Astrophysics Data System (ADS)

Gomez's Hamburger is a bipolar nebula discovered in 1985 in front of the Galactic bulge that was believed to be a proto-planetary nebula, i.e., illuminated by a post-AGB star. This interpretation was recently challenged by millimetric observations that unveiled a spectacular CO disk in keplerian rotation around an intermediate-mass star. Gomez's Hamburger is a pre main-sequence intermediate-mass star with a circumstellar disk seen edge-on, located at about 300 pc. We propose a 17 ks observation with Chandra ACIS-I to select in X-rays the young low-mass star candidates associated with this object.

Grosso, Nicolas

2009-09-01

52

Very Low Mass Stars and Brown Dwarfs in Taurus-Auriga  

Microsoft Academic Search

We present high-resolution optical spectra obtained with the HIRES spectrograph on the W. M. Keck I Telescope of seven low-mass T Tauri stars (LMTTs) and brown dwarfs in Taurus-Auriga. The observed Li I 6708 Å absorption, low surface gravity signatures, and radial velocities confirm that all are members of the Taurus star-forming region; no new spectroscopic binaries are identified. Four

Russel J. White; Gibor Basri

2003-01-01

53

Large-scale ? 2 -dynamo in low-mass stars and brown dwarfs  

Microsoft Academic Search

We develop a model based on three dimensional mean-field magnetohydrodynamics computations for the gen- eration of large scale magnetic fields in fully convective objects like low-mass stars, brown dwarfs and possibly gaseous plan- ets. The dynamo process is of type and thus differs from the shell-dynamo at work in more massive stars. The dynamo is found to become supercritical for

Gilles Chabrier; M. Küker

2006-01-01

54

Marginally stable orbits around neutron stars and QPOs in low-mass X-ray binaries  

Microsoft Academic Search

A variety of quasi-periodic intensity oscillations (QPOs) have been observed in low-mass X-ray binaries. To explain some of these QPOs, scenarios have been proposed that require the magnetic field of the neutron star to be negligible, the radius Rms of the marginally stable orbit to be greater than the equatorial radius R_eq of the star, and the character of the

M. C. Miller; F. K. Lamb

1992-01-01

55

The low-mass star and sub-stellar populations of the 25 Orionis group  

NASA Astrophysics Data System (ADS)

We present the results of a survey of the low-mass star and brown dwarf population of the 25 Orionis group. Using optical photometry from the CIDA (Centro de Investigaciones de Astronomía `Francisco J. Duarte', Mérida, Venezuela) Deep Survey of Orion, near-IR photometry from the Visible and Infrared Survey Telescope for Astronomy and low-resolution spectroscopy obtained with Hectospec at the MMT telescope, we selected 1246 photometric candidates to low-mass stars and brown dwarfs with estimated masses within 0.02 ? M/M? ? 0.8 and spectroscopically confirmed a sample of 77 low-mass stars as new members of the cluster with a mean age of ˜7 Myr. We have obtained a system initial mass function of the group that can be well described by either a Kroupa power-law function with indices ?3 = -1.73 ± 0.31 and ?2 = 0.68 ± 0.41 in the mass ranges 0.03 ? M/M? ? 0.08 and 0.08 ? M/M? ? 0.5, respectively, or a Scalo lognormal function with coefficients m_c=0.21^{+0.02}_{-0.02} and ? = 0.36 ± 0.03 in the mass range 0.03 ? M/M? ? 0.8. From the analysis of the spatial distribution of this numerous candidate sample, we have confirmed the east-west elongation of the 25 Orionis group observed in previous works, and rule out a possible southern extension of the group. We find that the spatial distributions of low-mass stars and brown dwarfs in 25 Orionis are statistically indistinguishable. Finally, we found that the fraction of brown dwarfs showing IR excesses is higher than for low-mass stars, supporting the scenario in which the evolution of circumstellar discs around the least massive objects could be more prolonged.

Downes, Juan José; Briceño, César; Mateu, Cecilia; Hernández, Jesús; Vivas, Anna Katherina; Calvet, Nuria; Hartmann, Lee; Petr-Gotzens, Monika G.; Allen, Lori

2014-10-01

56

Fundamental properties of low-mass stars in eclipsing binary systems  

NASA Astrophysics Data System (ADS)

Eclipsing binary systems offer a unique opportunity to accurately measure the fundamental properties of stars. In the past years, masses and radii of low-mass stars in these systems have been obtained with uncertainties well below the 3% level, imposing stringent constrains on theoretical models. Several results revealed that there is a discrepancy between the radius predicted by models and that from the observations of about 5-10% depending on the mass range. On the other hand, effective temperatures are overestimated by models by about a 10%. These discrepancies have been usually attributed to stellar magnetic activity, however, the correct scenario is not yet clear. The progress in the understanding of stellar activity on low-mass stars and its relation with the radius discrepancies with models is summarized here both from the empirical and the theoretical points of view.

Morales, J. C.; Ribas, I.; Jordi, C.

2013-02-01

57

Probing the circumstellar environments of very young low-mass stars using water masers  

NASA Technical Reports Server (NTRS)

The VLA is used to search nearby very young low-mass stars for water maser emission. The sample consists of 26 low-luminosity IRAS sources embedded in dense molecular cores, a class of sources suspected to be newly forming low-mass stars on the order of a few hundred thousand years old. Three sources were detected. High spatial resolution maps show the region of maser emission is generally confined to an area smaller than about 0.5 arcsec near the star, and the velocities of individual components span intervals ranging from 20 to 40 km/s. It is inferred from the fact that the maser velocities are too large to be due to gravitational motions in at least two of the sources that the masers are associated with the winds from the young low-mass stars. A comparison of the high spatial resolution maser data to lower-resolution CO data shows no evidence for higher collimation close to the star; the stellar wind cavity appears to have similar collimation at 10 exp 15 cm as at 10 exp 7 to 10 exp 18 cm.

Terebey, S.; Vogel, S. N.; Myers, P. C.

1992-01-01

58

X-ray sources in regions of star formation. 5: The low mass stars of the Upper Scorpius association  

NASA Technical Reports Server (NTRS)

We report followup investigations of Einstein x-ray observations of the Upper Scorpius OB association. We identify 28 low mass pre-main sequence stars as counterparts of x-ray sources in the approximately = 7 square degrees of the OB association observed. Based on kinematics and lithium abundances, these stars are low mass members of the OB association. We use optical spectra and optical and near-IR photometry to determine the stellar luminosities, effective temperatures, masses, and ages. We show that the bolometric corrections and effective temperatures of the G and K stars are consistent with those of subgiants. The low mass stars have isochronal ages of 1-2 Myr, depending on the choice of evolutionary models, with very small dispersion (sigma approximately = 1 Myr). This age is significantly younger than the 5-6 Myr found for the more massive B stars. The small dispersion in stellar ages, less than 10% the sound-crossing time of the association, suggests that star formation was triggered. We present two scenarios for star formation in this association. In the two-episode scenario, formation of the low mass stars was triggered by a supernova explosion, and the low mass stars form quickly, with high efficiency. Alternatively, high and low mass star formation was all initiated at the same time, some 5-6 Myr ago, and the apparent systematic age difference is an artifact of how the isochrones are dated. The effect of the supernova is to terminate mass accretion and yield an apparently coeval population. We show that the incompleteness in the x-ray sampling is about 65%, and is strongly dependent on stellar mass. After correction for incompleteness, we estimate there are about 2000 low mass members (stellar mass less than 2 solar mass) of this association. The mass function in this association is indistinguishable from that of the field. The ratio of naked to classical T Tauri stars is much larger than in Tau-Aur, and may be attributable to the local environment. We also present observations of eight ROX sources associated with the rho Oph cloud, and observations of non-pre-main sequence (PMS) stars in our fields.

Walter, Frederick M.; Vrba, Frederick J.; Mathieu, Robert D.; Brown, Alexander; Myers, Philip C.

1994-01-01

59

A HYBRID SCENARIO FOR THE FORMATION OF BROWN DWARFS AND VERY LOW MASS STARS  

SciTech Connect

We present a calculation of protostellar disk formation and evolution in which gaseous clumps (essentially, the first Larson cores formed via disk fragmentation) are ejected from the disk during the early stage of evolution. This is a universal process related to the phenomenon of ejection in multiple systems of point masses. However, it occurs in our model entirely due to the interaction of compact, gravitationally bound gaseous clumps and is free from the smoothing-length uncertainty that is characteristic of models using sink particles. Clumps that survive ejection span a mass range of 0.08-0.35 M{sub Sun }, and have ejection velocities 0.8 {+-} 0.35 km s{sup -1}, which are several times greater than the escape speed. We suggest that, upon contraction, these clumps can form substellar or low-mass stellar objects with notable disks, or even close-separation very low mass binaries. In this hybrid scenario, allowing for ejection of clumps rather than finished protostars/proto-brown-dwarfs, disk formation and the low velocity dispersion of low-mass objects are naturally explained, while it is also consistent with the observation of isolated low-mass clumps that are ejection products. We conclude that clump ejection and the formation of isolated low-mass stellar and substellar objects is a common occurrence, with important implications for understanding the initial mass function, the brown dwarf desert, and the formation of stars in all environments and epochs.

Basu, Shantanu [Department of Physics and Astronomy, University of Western Ontario, London, Ontario, N6A 3K7 (Canada); Vorobyov, Eduard I., E-mail: basu@uwo.ca, E-mail: eduard.vorobiev@univie.ac.at [Institute of Astrophysics, The University of Vienna, Vienna, 1180 (Austria)

2012-05-01

60

Investigating the Properties of Low-Mass Stars Using Spectra of Wide Binaries  

NASA Astrophysics Data System (ADS)

We present results from a study designed to characterize wide, low-mass (< 0.5 M_Sun) binaries identified in the Sloan Digital Sky Survey (SDSS). We examine the SDSS database level completeness (for identifying visual binaries) and analyze the pairs that both have individual SDSS spectra. A comprehensive by-eye examination reveals that a significant fraction of the sources within 1" of the primary stellar source are misclassified as duplicate detections and, hence, are omitted from the photometric primary catalog in the SDSS database. This discrepancy has a noticeable effect on estimates of the binary fraction, mass function, luminosity function, and other studies that rely on large, photometric samples of low-mass stars. Due to their coeval nature, binaries with at least one low-mass component are important for calibrating the age-activity relation and the relative metallicity scales. Better defined stellar ages and metallicities allow for a proper analysis of stellar and Galactic evolution using ubiquitous low-mass stars. We constructed a spectroscopic sample of wide binaries, for which there is at least one low-mass component and an individual spectrum for each star. Each binary was verified using measurements of their common proper motions and a chance alignment probability calculated from a six-dimensional model of the Milky Way. The orbital separation of the binary components provides an extra age constraint due to mechanisms that destroy wide binaries during thin-disk dynamical heating. We evaluate the behavior of the magnetic activity in coeval systems, with a specific focus on the dependence of activity on orbital separation and location in the Galactic disk. The preliminary results of our analysis will help calibrate the age-activity relation in M dwarfs. In addition, we calibrate the relative metallicity scale for metal poor K and M dwarfs using a modified index based on TiO and CaH molecular band features.

Schluns, Kyle; West, A. A.; Dhital, S.; Massey, A. P.

2013-01-01

61

Asymmetric Dark Matter May Alter the Evolution of Low-mass Stars and Brown Dwarfs  

E-print Network

We study energy transport by asymmetric dark matter in the interiors of very low-mass stars and brown dwarfs. Our motivation is to explore astrophysical signatures of asymmetric dark matter, which otherwise may not be amenable to conventional indirect dark matter searches. In viable models, the additional cooling of very-low mass stellar cores can alter stellar properties. Asymmetric dark matter with mass 4 brown dwarfs. Such light dark matter is of particular interest given results from the DAMA, CoGeNT, and CRESST dark matter searches. We discuss possibilities for observing dark matter effects in stars in the solar neighborhood, globular clusters, and, of particular promise, local dwarf galaxies, among other environments, as well as exploiting these effects to constrain dark matter properties.

Andrew R. Zentner; Andrew P. Hearin

2011-11-18

62

Abundant Crystalline Silicates in the Disk of a Very Low Mass Star  

Microsoft Academic Search

We announce the discovery of SST-Lup3-1, a very low mass star close to the brown dwarf boundary in Lupus III with a circum(sub)stellar disk, discovered by the ``Cores to Disks'' Spitzer Legacy Program from mid-infrared, with very conspicuous crystalline silicate features in its spectrum. It is the first of such objects with a full 5-35 mum spectrum taken with the

B. Merín; J.-C. Augereau; E. F. van Dishoeck; J. Kessler-Silacci; C. P. Dullemond; G. A. Blake; F. Lahuis; J. M. Brown; V. C. Geers; K. M. Pontoppidan; F. Comerón; A. Frasca; S. Guieu; J. M. Alcalá; A. C. A. Boogert; N. J. Evans II; P. D'Alessio; L. G. Mundy; N. Chapman

2007-01-01

63

Evolutionary Models for Very Low-Mass Stars and Brown Dwarfs with Dusty Atmospheres  

Microsoft Academic Search

We present evolutionary calculations for very low-mass stars and brown dwarfs based on synthetic spectra and nongray atmosphere models which include dust formation and opacity, i.e., objects with Teff<~2800 K. The interior of the most massive brown dwarfs is shown to develop a conductive core after ~2 Gyr which slows down their cooling. Comparison is made in optical and infrared

G. Chabrier; I. Baraffe; F. Allard; P. Hauschildt

2000-01-01

64

Accurate masses of very low mass stars. III. 16 new or improved masses  

NASA Astrophysics Data System (ADS)

We have obtained adaptive optics images and accurate radial velocities for 7 very low mass objects, In the course of a long term effort to determine accurate masses for very low mass stars (M<0.6 M_sun). We use the new data, together with measurements from the litterature for some stars, to determine new or improved orbits for these 7 systems. They provide masses for 16 very low mass stars with accuracies that range between 0.2% and 5%, and in some cases a very accurate distance as well. This information is used in a companion paper to discuss the Mass-Luminosity relation for the V, J, H and K photometric bands. Observations collected at the European Southern Observatory -- ESO, Chile, Laboratório Nacional de Astrofísica -- LNA, Brazil and Observatoire de Haute Provence -- OHP, France Table 7 is available only in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strsbg.fr/Abstract.html

Ségransan, D.; Delfosse, X.; Forveille, T.; Beuzit, J.-L.; Udry, S.; Perrier, C.; Mayor, M.

2000-12-01

65

THE RADIUS DISCREPANCY IN LOW-MASS STARS: SINGLE VERSUS BINARIES  

SciTech Connect

A long-standing issue in the theory of low-mass stars is the discrepancy between predicted and observed radii and effective temperatures. In spite of the increasing availability of very precise radius determinations from eclipsing binaries and interferometric measurements of radii of single stars, there is no unanimous consensus on the extent (or even the existence) of the discrepancy and on its connection with other stellar properties (e.g., metallicity, magnetic activity). We investigate the radius discrepancy phenomenon using the best data currently available (accuracy ?< 5%). We have constructed a grid of stellar models covering the entire range of low-mass stars (0.1-1.25 M{sub ?}) and various choices of the metallicity and mixing length parameter, ?. We used an improved version of the Yale Rotational stellar Evolution Code, implementing surface boundary conditions based on the most up-to-date PHOENIX atmosphere models. Our models are in good agreement with others in the literature and improve and extend the low mass end of the Yale-Yonsei isochrones. Our calculations include rotation-related quantities, such as moments of inertia and convective turnover timescales, useful in studies of magnetic activity and rotational evolution of solar-like stars. Consistent with previous works, we find that both binaries and single stars have radii inflated by about 3% with respect to the theoretical models; among binaries, the components of short orbital period systems are found to be the most deviant. We conclude that both binaries and single stars are comparably affected by the radius discrepancy phenomenon.

Spada, F. [Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, D-14482, Potsdam (Germany); Demarque, P. [Department of Astronomy, Yale University, New Haven, CT 06520-8101 (United States); Kim, Y.-C. [Yonsei University Observatory and Astronomy Department, Yonsei University, Seoul 120-749 (Korea, Republic of); Sills, A., E-mail: fspada@aip.de [Department of Physics and Astronomy, McMaster University, Hamilton, ON, L8S 4M1 (Canada)

2013-10-20

66

Brown dwarfs and low-mass stars in the Pleiades and Praesepe:- Membership and binarity  

Microsoft Academic Search

We present near infrared J-, H- and K-band photometry and optical\\u000aspectroscopy of low-mass star and brown dwarf (BD) candidates in the Pleiades\\u000aand Praesepe open clusters. We find that Pleiades stars with K=10.5--13 are\\u000arather redder than the NextGen isochrones. We also identify this effect amongst\\u000aalphaPer sources from the literature, but find no evidence of it for field

D. J. Pinfield; P. D. Dobbie; R. F. Jameson; I. A. Steele; H. R. A. Jones; A. C. Katsiyannis

2003-01-01

67

The formation of brown dwarfs and low-mass stars by disc fragmentation  

E-print Network

We suggest that a high proportion of brown dwarfs are formed by gravitational fragmentation of massive, extended discs around Sun-like stars. We argue that such discs should arise frequently, but should be observed infrequently, precisely because they fragment rapidly. By performing an ensemble of radiation-hydrodynamic simulations, we show that such discs typically fragment within a few thousand years to produce mainly brown dwarfs (including planetary-mass brown dwarfs) and low-mass hydrogen-burning stars. Subsequently most of the brown dwarfs are ejected by mutual interactions. We analyse the properties of these objects that form by disc fragmentation, and compare them with observations.

Dimitris Stamatellos; Anthony P. Whitworth

2008-09-29

68

Chemical abundances and star-formation histories of low-mass dwarf galaxies  

NASA Astrophysics Data System (ADS)

The study of the nature and evolution of the lowest-mass galaxies is important to our overall understanding of the formation and evolution of galaxies over time. Low-mass systems are a key to many lingering issues concerning galaxy formation including the apparent lack of very low-mass dwarf galaxies in the local universe. Additionally, low-mass dwarf galaxies are very metal-poor and thus provide possible analogs to processes of star-formation and evolution in the early universe. Low-mass galaxies are very low-luminosity and thus observing the lowest-mass systems is a diffcult task. Until recently, nearly all studies of low-mass galaxies were preformed with optically selected samples. These samples have an unavoidable bias toward higher surface brightness systems and it is unclear whether they can accurately characterize the dwarf galaxy population. In this work, we present studies performed on samples of gas-rich low-mass dwarf irregular galaxies chosen from the catalogs of two blind HI-surveys, the Arecibo Dual Beam Survey and Arecibo Legacy Fast ALFA Survey. These catalogs are free of optical biases and thus may provide a more comprehensive view of the gas-rich dwarf galaxy population. We find that optical properties of our samples are broadly consistent with other samples of dwarf irregulars, however our sample tends toward extremely low surface brightness in some cases. A small number of our galaxies have properties which indicate they may be nearly undetectable in typical optical surveys if in a more face-on orientation. The star-formation rates for our sample are roughly consistent with expectations for low surface brightness dwarf irregulars and distinctly lower than that seen in blue compact dwarf galaxies. We perform a spectral analysis of both samples and discover two new extremely-metal deficient (XMD) galaxies; this is particularly important as XMD galaxies have been largely elusive despite significant effort to find them. Our spectral analysis provides evidence that HI-selected samples may probe a chemically distinct population and that the luminosity-metallicity relationship may not be universal for low-mass systems. Understanding the deviations from this trend may help reveal the underlying physical mechanisms that cause the relationship to exist.

Haurberg, Nathalie C.

69

The massive neutron star or low-mass black hole in 2S0921-630  

E-print Network

We report on optical spectroscopy of the eclipsing Halo LMXB 2S0921-630, that reveals the absorption line radial velocity curve of the K0III secondary star with a semi-amplitude K_2=92.89 +/- 3.84 km/s, a systemic velocity $\\gamma$=34.9 +/- 3.3 \\kms and an orbital period P_orb of 9.0035 +/- 0.0029 day (1-sigma). Given the quality of the data, we find no evidence for the effects of X-ray irradiation. Using the previously determined rotational broadening of the mass donor, and applying conservative limits on the orbital inclination, we constrain the compact object mass to be 2.0-4.3 Msolar (1-sigma), ruling out a canonical neutron star at the 99% level. Since the nature of the compact object is unclear, this mass range implies that the compact object is either a low-mass black hole with a mass slightly higher than the maximum neutron star mass (2.9 Msolar) or a massive neutron star. If the compact object is a black hole, it confirms the prediction of the existence of low-mass black holes, while if the object is a massive neutron star its high mass severely constrains the equation of state of nuclear matter.

T. Shahbaz; J. Casares; C. Watson; P. A. Charles; R. I. Hynes; S. C. Shih; D. Steeghs

2004-09-30

70

Evolution and Nucleosynthesis in Low-Mass Asymptotic Giant Branch Stars. I. Formation of Population I Carbon Stars  

Microsoft Academic Search

New models of thermally pulsing asymptotic giant branch (TP-AGB) stars of low mass and solar chemical composition are presented, namely, 1 <= M\\/M&sun; = 1.5 M&sun; after about 24-26 thermal pulses and 15-17 TDU episodes. At C-star formation, the core mass is less than 0.7 M&sun;, and the luminosity is of the order of 104 L&sun;. The dredged-up mass increases

Oscar Straniero; Alessandro Chieffi; Marco Limongi; Maurizio Busso; Roberto Gallino; Claudio Arlandini

1997-01-01

71

Discovery of an outflow of the very low-mass star ISO 143  

NASA Astrophysics Data System (ADS)

We discover that the very young very low-mass star ISO 143 (M5) is driving an outflow based on spectro-astrometry of forbidden [S II] emission lines at 6716 Å and 6731 Å observed in UVES/VLT spectra. This adds another object to the handful of brown dwarfs and very low-mass stars (M5-M8) for which an outflow has been confirmed and which show that the T Tauri phase continues at very low masses. We find the outflow of ISO 143 to be intrinsically asymmetric and the accretion disk to not obscure the outflow, as only the red outflow component is visible in the [S II] lines. ISO 143 is only the third T Tauri object showing a stronger red outflow component in spectro-astrometry, after RW Aur (G5) and ISO 217 (M6.25). We show here that, including ISO 143, two out of seven outflows confirmed in the very low-mass regime (M5-M8) are intrinsically asymmetric. We measure a spatial extension of the outflow in [S II] of up to 200-300 mas (about 30-50 AU) and velocities of up to 50-70 km s-1. We furthermore detect line emission of ISO 143 in Ca II (8498 Å), O I (8446 Å), He I (7065 Å), and weakly in [Fe II] (7155 Å). Based on a line profile analysis and decomposition we demonstrate that (i) the Ca II emission can be attributed to chromospheric activity, a variable wind, and the magnetospheric infall zone, (ii) the O I emission mainly to accretion-related processes but also a wind, and (iii) the He I emission to chromospheric or coronal activity. We estimate a mass outflow rate of ISO 143 of ~10-10 M? yr-1 and a mass accretion rate in the range of ~10-8 to ~10-9 M? yr-1. These values are consistent with those of other brown dwarfs and very low-mass stars. The derived ?out/?acc ratio of 1-20% does not support previous findings of this number being very large (>40%) for very low-mass objects. Based on observations obtained at the Very Large Telescope of the European Southern Observatory at Paranal, Chile in program 080.C- 0904(A) 082.C-0023(A+B).

Joergens, V.; Kopytova, T.; Pohl, A.

2012-12-01

72

Oscillations of red dwarfs in evolved low-mass binaries with neutron stars  

NASA Technical Reports Server (NTRS)

We investigate a novel aspect of a problem related to the properties of low-mass binaries (LMBs) with millisecond pulsars: the pulsations of the red dwarf (donor) companion of the neutron star (NS). The illumination of the donor star by the pulsar's high-energy nonthermal radiation and relativistic wind may substantially affect its structure. We present a quantitative analysis of the oscillation spectrum of a red dwarf which has evolved in an LMB and has undergone the stage of evaporation. We calculate the p- and g-modes for red dwarfs with masses in the interval (0.2-0.6) stellar mass. For comparison, similar calculations are presented for zero age main-sequence (ZAMS) stars of the same masses. For less massive donor stars (approximately 0.2 stellar mass) the oscillation spectrum becomes quantitatively different from that of their ZAMS counterparts. The differnce is due to the fact that a ZAMS star of 0.2 stellar mass is fully convective, while the donor star in an LMB is expected to be far from thermal equilibrium and not fully convective. As a result, in contrast to a low-mass ZAMS star, a red dwarf of the same mass in an LMB allows the existence of g-modes. We also consider tidally forced g-modes, and perform a linear analysis of these oscillations for different degrees of nonsynchronism between the orbital and spin rotation of the red dwarf component. We demonstrate the existence of a series of reasonances for the low-order g-modes which may occur in LMBs at a late stage of their evolution. We discuss the possibility that these oscillations may trigger Roche lobe overflow and sudden mass loss by the donor star. Further implications of this effect for gamma- and X-ray burst phenomena are outlined.

Sarna, Marek J.; Lee, Umin; Muslimov, Alexander G.

1994-01-01

73

Using HST Globular Clusters to Derive an Empirical Photometric Metallicity Relation for Low-mass Stars  

NASA Astrophysics Data System (ADS)

We present an analysis of low-mass stars in the ACS Survey of Galactic Globular Clusters (AGC). The AGC obtained exquisite V and I-band photometry of 65 globular clusters, measuring their main sequences down to ~0.2 solar masses. We have used these data to derive an empirical color-[Fe/H]-Mv relation for low-mass dwarfs, employing photometry and photometric [Fe/H] as measured by the AGC team. The resulting relation has two major applications to stellar and Galactic astronomy. First, photometric metallicities can be calculated for any M dwarf with a trigonometric parallax and optical photometry. Second, this relation can be used to improve current photometric parallax relations, resulting in more precise distances for millions of low-mass stars. With large surveys such as LSST and GAIA looming, this relation will facilitate a deeper understanding of the structure and chemical history of the Milky Way. We gratefully acknowledge the financial support of NSF AST-1151462 in this work.

Fuchs, Miriam; Bochanski, J. J.; Willman, B.; Dotter, A. L.; West, A. A.

2013-01-01

74

KOI-126: a triply eclipsing hierarchical triple with two low-mass stars.  

PubMed

The Kepler spacecraft has been monitoring the light from 150,000 stars in its primary quest to detect transiting exoplanets. Here, we report on the detection of an eclipsing stellar hierarchical triple, identified in the Kepler photometry. KOI-126 [A, (B, C)], is composed of a low-mass binary [masses M(B) = 0.2413 ± 0.0030 solar mass (M(?)), M(C) = 0.2127 ± 0.0026 M(?); radii R(B) = 0.2543 ± 0.0014 solar radius (R(?)), R(C) = 0.2318 ± 0.0013 R(?); orbital period P(1) = 1.76713 ± 0.00019 days] on an eccentric orbit about a third star (mass M(A) = 1.347 ± 0.032 M(?); radius R(A) = 2.0254 ± 0.0098 R(?); period of orbit around the low-mass binary P(2) = 33.9214 ± 0.0013 days; eccentricity of that orbit e(2) = 0.3043 ± 0.0024). The low-mass pair probe the poorly sampled fully convective stellar domain offering a crucial benchmark for theoretical stellar models. PMID:21224439

Carter, Joshua A; Fabrycky, Daniel C; Ragozzine, Darin; Holman, Matthew J; Quinn, Samuel N; Latham, David W; Buchhave, Lars A; Van Cleve, Jeffrey; Cochran, William D; Cote, Miles T; Endl, Michael; Ford, Eric B; Haas, Michael R; Jenkins, Jon M; Koch, David G; Li, Jie; Lissauer, Jack J; MacQueen, Phillip J; Middour, Christopher K; Orosz, Jerome A; Rowe, Jason F; Steffen, Jason H; Welsh, William F

2011-02-01

75

Characterizing the Star Formation of the Low-Mass SHIELD Galaxies from Hubble Space Telescope Imaging  

E-print Network

The Survey of HI in Extremely Low-mass Dwarfs (SHIELD) is an on-going multi-wavelength program to characterize the gas, star formation, and evolution in gas-rich, very low-mass galaxies that populate the faint end of the galaxy luminosity function. The galaxies were selected from the first ~10% of the HI ALFALFA survey based on their low HI mass and low baryonic mass. Here, we measure the star-formation properties from optically resolved stellar populations for 12 galaxies using a color-magnitude diagram fitting technique. We derive lifetime average star-formation rates (SFRs), recent SFRs, stellar masses, and gas fractions. Overall, the recent SFRs are comparable to the lifetime SFRs with mean birthrate parameter of 1.4, with a surprisingly narrow standard deviation of 0.7. Two galaxies are classified as dwarf transition galaxies (dTrans). These dTrans systems have star-formation and gas properties consistent with the rest of the sample, in agreement with previous results that some dTrans galaxies may simply...

McQuinn, Kristen B W; Dolphin, Andrew E; Skillman, Evan D; Haynes, Martha P; Simones, Jacob E; Salzer, John J; Adams, Elizabeth A K; Elson, Ed C; Giovanelli, Riccardo; Ott, Jürgen

2015-01-01

76

FLAMES spectroscopy of low-mass stars in the young clusters ? Ori and ? Ori  

NASA Astrophysics Data System (ADS)

Aims: We performed a detailed membership selection and studied the accretion properties of low-mass stars in the two apparently very similar young (1-10 Myr) clusters ? Ori and ? Ori. Methods: We observed 98 and 49 low-mass (0.2-1.0 M?) stars in ? Ori and ? Ori respectively, using the multi-object optical spectrograph FLAMES at the VLT, with the high-resolution (R˜ 17 000) HR15N grating (6470-6790 Å). We used radial velocities, Li and H? to establish cluster membership and H? and other optical emission lines to analyze the accretion properties of members. Results: We identified 65 and 45 members of the ? Ori and ? Ori clusters, respectively, and discovered 16 new candidate binary systems. We also measured rotational broadening for 20 stars and estimated the mass accretion rates in 25 stars of the ? Ori cluster, finding values between 10-11 and 10-7.7~M? yr-1 and in 4 stars of the ? Ori cluster, finding values between 10-11 and 10-10.1~M? yr-1. Comparing our results with the infrared photometry obtained by the Spitzer satellite, we find that the fraction of stars with disks and the fraction of active disks is larger in the ? Ori cluster (52±9% and 78±16%) than in ? Ori (28±8% and 40±20%). Conclusions: The different disk and accretion properties of the two clusters could be due either to the effect of the high-mass stars and the supernova explosion in the ? Ori cluster or to different ages of the cluster populations. Further observations are required to draw a definitive conclusion. Based on Data collected at the ESO Very Large Telescope, Paranal Observatory, Chile [programs 074.D-0136(A) and 076.C-0125(A)]. Tables 1, 2 and 4-7 are only available in electronic form at http://www.aanda.org

Sacco, G. G.; Franciosini, E.; Randich, S.; Pallavicini, R.

2008-09-01

77

DETECTING PLANETS AROUND VERY LOW MASS STARS WITH THE RADIAL VELOCITY METHOD  

SciTech Connect

The detection of planets around very low-mass stars with the radial velocity (RV) method is hampered by the fact that these stars are very faint at optical wavelengths where the most high-precision spectrometers operate. We investigate the precision that can be achieved in RV measurements of low mass stars in the near-infrared (NIR) Y-, J-, and H-bands, and we compare it to the precision achievable in the optical assuming comparable telescope and instrument efficiencies. For early-M stars, RV measurements in the NIR offer no or only marginal advantage in comparison with optical measurements. Although they emit more flux in the NIR, the richness of spectral features in the optical outweighs the flux difference. We find that NIR measurement can be as precise as optical measurements in stars of spectral type {approx}M4, and from there the NIR gains in precision toward cooler objects. We studied potential calibration strategies in the NIR finding that a stable spectrograph with a ThAr calibration can offer enough wavelength stability for m s{sup -1} precision. Furthermore, we simulate the wavelength-dependent influence of activity (cool spots) on RV measurements from optical to NIR wavelengths. Our spot simulations reveal that the RV jitter does not decrease as dramatically toward longer wavelengths as often thought. The jitter strongly depends on the details of the spots, i.e., on spot temperature and the spectral appearance of the spot. At low temperature contrast ({approx}200 K), the jitter shows a decrease toward the NIR up to a factor of 10, but it decreases substantially less for larger temperature contrasts. Forthcoming NIR spectrographs will allow the search for planets with a particular advantage in mid- and late-M stars. Activity will remain an issue, but simultaneous observations at optical and NIR wavelengths can provide strong constraints on spot properties in active stars.

Reiners, A.; Bean, J. L.; Dreizler, S.; Seifahrt, A. [Institut fuer Astrophysik, Georg-August-Universitaet, Friedrich-Hund-Platz 1, 37077 Goettingen (Germany); Huber, K. F.; Czesla, S., E-mail: ansgar.reiners@phys.uni-goettingen.d [Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg (Germany)

2010-02-10

78

A M2FS Spectroscopic Study of Low-mass Young Stars in Orion OB1  

NASA Astrophysics Data System (ADS)

Surveys of pre-main sequence stars in the ~4-10 Myr range provide a window into the decline of the accretion phase of stars and the formation of planets. Nearby star clusters and stellar associations allow for the study of these young stellar populations all the way down to the lowest mass members. One of the best examples of nearby 4-10 Myr old stellar populations is the Orion OB1 association. The CIDA Variability Survey of Orion OB1 (CVSO - Briceño et al. 2001) has used the variability properties of low-mass pre-main-sequence (PMS) stars to identify hundreds of K and M-type stellar members of the Orion OB1 association, a number of them displaying IR-excess emission and thought to be representative of more evolved disk-bearing young stars. Characterizing these young, low-mass objects using spectroscopy is integral to understanding the accretion phase in young stars. We present preliminary results of a spectroscopic survey of candidate and confirmed Orion OB1 low-mass members taken during November 2014 and February 2014 using the Michigan/Magellan Fiber Spectrograph (M2FS), a PI instrument on the Magellan Clay Telescope (PI: M. Matteo). Target fields located in the off-cloud regions of Orion were identified in the CVSO, and observed using the low and high-resolution modes of M2FS. Both low and high-resolution spectra are needed in order to confirm membership and derive masses, ages, kinematics and accretion properties. Initial analysis of these spectra reveal many new K and M-type members of the Orion OB1 association in these low extinction, off-cloud areas. These are the more evolved siblings of the youngest stars still embedded in the molecular clouds, like those in the Orion Nebula Cluster. With membership and spectroscopic indicators of accretion we are building the most comprehensive stellar census of this association, enabling us to derive a robust estimate of the fraction of young stars still accreting at a various ages, a key constraint for the end of accretion and the formation of giant planets.

Kaleida, Catherine C.; Briceno, Cesar; Calvet, Nuria; Mateo, Mario L.; Hernandez, Jesus

2015-01-01

79

Revealing the Chamaeleon: Young, low-mass stars surrounding eta and epsilon Chamaeleontis  

NASA Astrophysics Data System (ADS)

The deep southern sky surrounding the Chamaeleon dark clouds is abundant with pre-main sequence stars of various ages. Because of their youth (5-10 Myr) and proximity (d~100 pc), members of the open cluster eta Chamaeleontis and the nearby epsilon Chamaeleontis Association are ideal laboratories to study the formation and evolution of extrasolar planetary systems. To better understand their role as potential planet hosts, this thesis explores the formation, dynamical evolution, accretion and disk properties of both groups' low-mass members. The notable lack of low-mass stars in the young open cluster eta Cha has long been puzzling. Two possible explanations have been suggested; a top-heavy initial mass function or dynamical evolution, which preferentially ejected the low-mass members. Previous efforts to find these stars several degrees from the cluster core have been unsuccessful. By undertaking a wider (95 sq deg) photometric and proper motion survey with extensive follow-up spectroscopy, we have identified eight low-mass stars that were ejected from eta Cha over the past 5-10 Myr. Comparison with recent simulations shows our results are consistent with a dynamical origin for the current configuration of the cluster, without the need to invoke an initial mass function deficient in low-mass objects. Two of the dispersed members exhibited strong, variable H-alpha emission during our observations, including a star which had an event suggestive of accretion from a circumstellar disk. New infrared photometry confirms the presence of the disk. This star demonstrates that infrequent, episodic accretion can continue at low levels long after most disks around `old' pre-main sequence stars have dissipated. Another two confirmed non-members are slightly older than the cluster, but are only 42 arcseconds apart and share similar kinematics and distances. We show that they almost certainly form a wide (4000-6000 AU) ~10 Myr-old binary at 100-150 pc. The system is one of the widest pre-main sequence binaries known. Its isolation and dynamical fragility put strong constraints on any birthplace and mode of formation, which we propose was in a turbulent gas filament in the vicinity of the Scorpius-Centaurus OB Association. In addition to eta Cha, we have also examined membership of the unbound epsilon Cha Association, which lies some 10 degrees to the east and has a similar age, distance and kinematics. The two groups were almost certainly born in the outer regions of Sco-Cen only a few million years apart. Many members of epsilon Cha have been proposed in the decade since its discovery. After considering the kinematics of candidates from the literature, we have confirmed 11 further stars as likely members. Many new members possess infrared spectral energy distributions attributable to circumstellar disks, including four stars with strong H-alpha and forbidden emission which are actively accreting material.

Murphy, S. J.

2012-01-01

80

Age-luminosity relations for low-mass metal-poor stars  

E-print Network

We present a grid of evolutionary calculations for metal-poor low-mass stars for a variety of initial helium and metal abundances. The intention is mainly to provide a database for deriving directly stellar ages of halo and globular cluster stars for which basic stellar parameters are known, but the tracks can also be used for isochrone or luminosity function construction, since they extend to the tip of the red giant branch. Fitting formulae for age-luminosity relations are provided as well. The uncertainties of the evolutionary ages due to inherent shortcomings in the models and due to the unclear effectiveness of diffusion are discussed. A first application to field single stars is presented.

Achim Weiss; Helmut Schlattl

1999-12-15

81

Dust Heating by Low-mass Stars in Massive Galaxies at z<1  

E-print Network

Using the Hubble Space Telescope/Wide Field Camera 3 imaging data and multi-wavelength photometric catalog, we investigated the dust temperature of passively evolving and star-forming galaxies at 0.2 10^{10} Msun have a relatively high dust temperature of Tdust > 20 K, for which the formation efficiency of molecular hydrogen on the surface of dust grains in the diffuse ISM is expected to be very low from the laboratory experiments. The fraction of passively evolving galaxies strongly depends on the expected dust temperature at all redshifts and increases rapidly with increasing the temperature around Tdust ~ 20 K. These results suggest that the dust heating by low-mass stars in massive galaxies plays an important role for the continuation of their passive evolution, because the lack of the shielding effect of the molecular hydrogen on the UV radiation can prevent the gas cooling and formation of new stars.

Kajisawa, M; Taniguchi, Y; Kobayashi, M A R; Ichikawa, T; Fukui, Y

2015-01-01

82

Investigating Low-Mass Binary Stars And Brown Dwarfs with Near-Infrared Spectroscopy  

NASA Astrophysics Data System (ADS)

The mass of a star at formation determines its subsequent evolution and demise. Low-mass stars are the most common products of star formation and their long main-sequence lifetimes cause them to accumulate over time. Star formation also produces many substellar-mass objects known as brown dwarfs, which emerge from their natal molecular clouds and continually cool as they age, pervading the Milky Way. Low-mass stars and brown dwarfs exhibit a wide range of physical characteristics and their abundance make them ideal subjects for testing formation and evolution models. I have examined a pair of pre-main sequence spectroscopic binaries and used radial velocity variations to determine orbital solutions and mass ratios. Additionally, I have employed synthetic spectra to estimate their effective temperatures and place them on theoretical Hertzsprung-Russell diagrams. From this analysis I discuss the formation and evolution of young binary systems and place bounds on absolute masses and radii. I have also studied the late-type T dwarfs revealed by the Wide-field Infrared Survey Explorer (WISE). This includes the exemplar T8 subdwarf Wolf 1130C, which has the lowest inferred metallicity in the literature and spectroscopic traits consistent with old age. Comparison to synthetic spectra implies that the dispersion in near-infrared colors of late-type T dwarfs is a result of age and/or thin sulfide clouds. With the updated census of the L, T, and Y dwarfs we can now study specific brown dwarf subpopulations. Finally, I present a number of future studies that would develop our understanding of the physical qualities of T dwarf color outliers and disentangle the tracers of age and atmospheric properties.

Mace, Gregory Nathan

83

Very Low Mass Stars and Brown Dwarfs in Taurus-Auriga  

E-print Network

We present high resolution optical spectra obtained with the HIRES spectrograph on the Keck I telescope of low mass T Tauri stars and brown dwarfs (LMTTs) in Taurus-Auriga. Of particular interest is the previously classified "continuum T Tauri star" GM Tau, which has a spectral type of M6.5 and a mass just below the stellar/substellar boundary. None of the LMTTs in Taurus are rapidly rotating (vsini 270 km/s are classical T Tauri stars (i.e. accreting), independent of stellar spectral type. Although LMTTs can have accretion rates comparable to that of more typical, higher-mass T Tauri stars (e.g. K7-M0), the average mass accretion rate appears to decrease with decreasing mass. The diminished frequency of accretion disks for LMTTs, in conjunction with their lower, on average, mass accretion rates, implies that they are formed with less massive disks than higher-mass T Tauri stars. The radial velocities, circumstellar properties and known binaries do not support the suggestion that many of the lowest mass members of Taurus have been ejected from higher stellar density regions within the cloud. Instead, LMTTs appear to have formed and are evolving in the same way as higher-mass T Tauri stars, but with smaller disks and shorter disk lifetimes.

Russel White; Gibor Basri

2002-09-09

84

Marginally stable orbits around neutron stars and QPOs in low-mass X-ray binaries  

NASA Astrophysics Data System (ADS)

A variety of quasi-periodic intensity oscillations (QPOs) have been observed in low-mass X-ray binaries. To explain some of these QPOs, scenarios have been proposed that require the magnetic field of the neutron star to be negligible, the radius Rms of the marginally stable orbit to be greater than the equatorial radius R_eq of the star, and the character of the flow between Rms and R_eq to be significantly different from that outside Rms. Motivated in part by these scenarios, we have investigated further the physics of disk accretion by rotating nonmagnetic neutron stars. Among the important effects that must be included in any accurate treatment are the effect of rotation on the shape of the star, the effects of stellar rotation and oblateness on the metric, gravitational lensing, and the effects of radiation stresses on the accreting plasma. Although previous studies have considered some of these effects, ours is the first to consider all of them. We find that the character of the accretion flow near the neutron star is generally qualitatively different from that discussed by previous authors. We discuss the implications of our results for scenarios in which QPOs are produced in the flow near the neutron star. This work was supported in part by NSF grant PHY 91-00283 and NASA grant NAGW 1583.

Miller, M. C.; Lamb, F. K.

1992-12-01

85

An X-ray survey of low-mass stars in Trumpler 16 with Chandra  

NASA Astrophysics Data System (ADS)

Aims: We identify and characterize low-mass stars in the 3 Myr old Trumpler 16 region by means of a deep Chandra X-ray observation, and study their optical and near-IR properties. We compare the X-ray activity of Trumpler 16 stars with the known characteristics of Orion and Cygnus OB2 stars. Methods: We analyzed a 88.4 ks Chandra ACIS-I observation pointed at the center of Trumpler 16. Because of diffuse X-ray emission, source detection was performed using the PWDetect code for two different energy ranges: 0.5-8.0 keV and 0.9-8.0 keV. Results were merged into a single final list. We positionally correlated X-ray sources with optical and 2MASS catalogs. Source events were extracted with the IDL-based routine ACIS-Extract. X-ray variability was characterized using the Kolmogorov-Smirnov test and spectra were fitted by using XSPEC. The X-ray spectra of early-type, massive stars were analyzed individually. Results: Our list of X-ray sources consists of 1035 entries, 660 of which have near-IR counterparts and are probably associated with Trumpler 16 members. From near-IR, color-color, and color-magnitude diagrams we compute individual masses of stars and their Av values. The cluster median extinction is Av = 3.6 mag, while OB-type stars appear less absorbed, having Av = 2.0 mag. About 15% of the near-IR counterparts show disk-induced excesses. X-ray variability is found in 77 sources, and typical X-ray spectral parameters are N_H˜ 5.37 × 1021 cm-2 and kT˜ 1.95 keV. The OB stars appear, softer with a median kT˜ 0.65 keV. The median X-ray luminosity is 6.3 × 1030 {erg s-1}, while variable sources show a larger median Lx value of 13 × 1030 {erg s-1}. OB-stars have an even higher median Lx of 80 × 1030 {erg s-1}, about 10 times that of the low-mass stars. Conclusions: The Trumpler 16 region has a very rich population of low-mass X-ray emitting stars. A large fraction of its circumstellar disks have survived the intense radiation field of its massive stars. Stars with masses 1.5-2.5 M_? display X-ray activity similar to the Cyg OB2 stars, but much less intense than observed for Orion nebula cluster members. Full Tables [see full textsee full text]-[see full textsee full text] are only available in electronic form at http://www.aanda.org

Albacete-Colombo, J. F.; Damiani, F.; Micela, G.; Sciortino, S.; Harnden, F. R., Jr.

2008-11-01

86

Old Stars in Young Clusters: Lithium-depleted Low-Mass Stars of the Orion Nebula Cluster  

Microsoft Academic Search

We measured lithium in a sample of low-mass stars (~0.1-0.3 Msolar) of the Orion Nebula cluster. We find evidence for significant Li depletion in four high-probability members, corresponding to nuclear ages between ~15 and 30 Myr. In two cases, there is excellent agreement between the mass and age based on models of Li burning and those derived from the H-R

F. Palla; S. Randich; Ya. V. Pavlenko; E. Flaccomio; R. Pallavicini

2007-01-01

87

Precise Radial Velocity Measurements: Key to Discover Low-mass Companions and Exoplanets Around Stars  

NASA Astrophysics Data System (ADS)

The search for planets around stars with empirical methods turn out to be as one of the most remarkable astronomical topics since late 90’s. New observational techniques and instrumentations which are introduced in last decade made possible the discoveries of very low-mass and planetary companions around distant stars. More than 340 exoplanets have been discovered so far (see http://exoplanet.eu) and their numbers are still increasing. Most of them are discovered with Precise Radial Velocity Measurement Technique. In this contribution, we shall give a summary on our exoplanet search project started at TÜBITAK - Turkish National Observatory in the framework of an international collaboration between Turkish, Japanese and Russian colleagues and present the achieved radial velocity precision after the 2 years of test observations.

Selam, S. O.; Yilmaz, M.; Izumiura, H.; Bikmaev, I.; Sato, B.; Kambe, E.; Keskin, V.

2010-12-01

88

High and Low Metallicity Models of Extremely Low-Mass Stars and Brown Dwarfs  

NASA Astrophysics Data System (ADS)

We present our latest generation of evolutionary models for very low-luminosity stars (near the ends of their respective Main Sequences) and for Brown Dwarfs. These results have been computed for a wide range of metallicities which includes extremely old subdwarfs (0.0001 < Z < 0.02). Using sophisticated atmospheric models from the Allard-Hauschildt library, we precompute an extensive grid of outer boundary conditions and then interpolate this grid as the models are being calculated. Using very sophisticated input physics such as the OPAL opacities, the Alexander and Ferguson low-temperature opacities, and our own equation of state which is largely derived from that of the SCVH low-temperature EOS, we have calculated the evolution of low-mass stars and Brown Dwarfs ranging from 0.001 to 0.5 solar masses. The physical properties of these models will be presented and the observational implications will be discussed briefly.

Nelson, Lorne A.

2011-05-01

89

Angular Momentum Transfer in Star-Discs Encounters: The Case of Low-Mass Discs  

E-print Network

A prerequisite for the formation of stars and planetary systems is that angular momentum is transported in some way from the inner regions of the accretion disc. Tidal effects may play an important part in this angular momentum transport. Here the angular momentum transfer in an star-disc encounter is investigated numerically for a variety of encounter parameters in the case of low mass discs. Although good agreement is found with analytical results for the entire disc, the loss {\\it inside} the disc can be up to an order of magnitude higher than previously assumed. The differences in angular momentum transport by secondaries on a hyperbolic, parabolic and elliptical path are shown, and it is found that a succession of distant encounters might be equally, if not more, successful in removing angular momentum than single close encounter.

S. Pfalzner

2003-10-27

90

Spatially resolved atomic and molecular emission from the very low-mass star IRS54.  

NASA Astrophysics Data System (ADS)

Molecular outflows from very low-mass stars and brown dwarfs have been studied very little, and only a few objects have been directly imaged. Using VLT SINFONI K-band observations, we spatially resolved, for the first time, the H_2 emission around IRS54, a ˜0.1-0.2 M_? Class I source. The molecular emission shows a complex structure delineating a large outflow cavity and an asymmetric molecular jet. In addition, new [Fe II] VLT ISAAC observations at 1.644 mu m allowed us to discover the atomic jet counterpart which extends down to the central source. The outflow structure is similar to those found in low-mass Class I young stellar objects and Classical T Tauri stars. However, its L_acc/L_bol ratio is very high (˜80%), and the derived mass accretion rate is about one order of magnitude higher than in objects with similar mass, pointing to the young nature of the investigated source.

Garcia Lopez, R.; Caratti o Garatti, A.; Weigelt, G.; Nisini, B.; Antoniucci, S.

91

On the effects of Cosmions upon the structure and evolution of very low mass stars  

NASA Technical Reports Server (NTRS)

A number of recent studies have suggested that cosmions, or WIMPS, may play an important role in the energetics of the solar interior; in particular, it has been argued that these hypothetical particles may transport sufficient energy within the nuclear-burning solar core so as to depress the solar core temperature to the point of resolving the solar neutrino problem. Solutions to the solar neutrino problem have proven themselves to be quite nonunique, so that it is of some interest whether the cosmion solution can be tested in some independent manner. It is argued that if cosmions solve the solar neutrino problem, then they must also play an important role in the evolution of low mass main sequence stars; and, second, that if they do so, then a simple (long mean free path) model for the interaction of cosmions with baryons leads to changes in the structure of the nuclear-burning core which may be in principal observable. Such changes include suppression of a fully-convective core in very low mass main sequence stars; and a possible thermal runaway in the core of the nuclear burning region. Some of these changes may be directly observable, and hence may provide independent constraints on the properties of the cosmions required to solve the solar neutrino problem, perhaps even ruling them out.

Deluca, E. E.; Griest, K.; Rosner, R.; Wang, J.

1989-01-01

92

Masses and Gravities of Blue Horizontal Branch Stars Revisited  

E-print Network

Previous spectroscopic analyses of Blue Horizontal Branch (BHB) stars in six globular clusters revealed too low masses in four clusters when compared to canonical evolutionary theory, while the masses of the BHB stars in NGC 6752 and M 5 are found to be consistent with theory. We recalculated BHB star masses using new cluster distances derived by Reid (1997a,b) from HIPPARCOS parallaxes of local subdwarfs by main sequence fitting. The new distances are larger than previous estimates resulting in larger masses for the BHB stars. Since the increase in distance is small for NGC 6752 and M 5, the agreement with predicted masses persists. For M 15 and M 92 the masses now come into good agreement with theoretical predictions, while for NGC 288 and NGC 6397 the mass deficit is reduced but the BHB star masses remain slightly too low. Previous spectroscopic analyses also highlighted the problem of too low gravities for some BHB stars. The gravities and absolute magnitudes of BHB stars are revisited in the light of new evolutionary Horizontal Branch models.

U. Heber; S. Moehler; I. N. Reid

1997-09-11

93

New Low-Mass Stars and Brown Dwarfs with Disks in Lupus  

E-print Network

Using the Infrared Array Camera and the Multiband Imaging Photometer aboard the {\\it Spitzer Space Telescope}, we have obtained images of the Lupus 3 star-forming cloud at 3.6, 4.5, 5.8, 8.0, and 24 \\micron. We present photometry in these bands for the 41 previously known members that are within our images. In addition, we have identified 19 possible new members of the cloud based on red 3.6-8.0 \\micron colors that are indicative of circumstellar disks. We have performed optical spectroscopy on 6 of these candidates, all of which are confirmed as young low-mass members of Lupus 3. The spectral types of these new members range from M4.75 to M8, corresponding to masses of 0.2-0.03 $M_\\odot$ for ages of $\\sim1$ Myr according to theoretical evolutionary models. We also present optical spectroscopy of a candidate disk-bearing object in the vicinity of the Lupus 1 cloud, 2M 1541-3345, which Jayawardhana & Ivanov recently classified as a young brown dwarf ($M\\sim0.03$ $M_\\odot$) with a spectral type of M8. In contrast to their results, we measure an earlier spectral type of M5.75$\\pm$0.25 for this object, indicating that it is probably a low-mass star ($M\\sim0.1$ $M_\\odot$). In fact, according to its gravity-sensitive absorption lines and its luminosity, 2M 1541-3345 is older than members of the Lupus clouds ($\\tau\\sim1$ Myr) and instead is probably a more evolved pre-main-sequence star that is not directly related to the current generation of star formation in Lupus.

P. R. Allen; K. L. Luhman; P. C. Myers; S. T. Megeath; L. E. Allen; L. Hartmann; G. G. Fazio

2007-02-16

94

VERY LOW MASS STELLAR AND SUBSTELLAR COMPANIONS TO SOLAR-LIKE STARS FROM MARVELS. IV. A CANDIDATE BROWN DWARF OR LOW-MASS STELLAR COMPANION TO HIP 67526  

SciTech Connect

We report the discovery of a candidate brown dwarf (BD) or a very low mass stellar companion (MARVELS-5b) to the star HIP 67526 from the Multi-object Apache point observatory Radial Velocity Exoplanet Large-area Survey (MARVELS). The radial velocity curve for this object contains 31 epochs spread over 2.5 yr. Our Keplerian fit, using a Markov Chain Monte Carlo approach, reveals that the companion has an orbital period of 90.2695{sup +0.0188}{sub -0.0187} days, an eccentricity of 0.4375 {+-} 0.0040, and a semi-amplitude of 2948.14{sup +16.65}{sub -16.55} m s{sup -1}. Using additional high-resolution spectroscopy, we find the host star has an effective temperature T{sub eff} = 6004 {+-} 34 K, a surface gravity log g (cgs) =4.55 {+-} 0.17, and a metallicity [Fe/H] =+0.04 {+-} 0.06. The stellar mass and radius determined through the empirical relationship of Torres et al. yields 1.10 {+-} 0.09 M{sub Sun} and 0.92 {+-} 0.19 R{sub Sun }. The minimum mass of MARVELS-5b is 65.0 {+-} 2.9M{sub Jup}, indicating that it is likely to be either a BD or a very low mass star, thus occupying a relatively sparsely populated region of the mass function of companions to solar-type stars. The distance to this system is 101 {+-} 10 pc from the astrometric measurements of Hipparcos. No stellar tertiary is detected in the high-contrast images taken by either FastCam lucky imaging or Keck adaptive optics imaging, ruling out any star with mass greater than 0.2 M{sub Sun} at a separation larger than 40 AU.

Jiang Peng; Ge Jian; De Lee, Nathan; Fleming, Scott W.; Lee, Brian L.; Ma Bo; Wang, Ji [Astronomy Department, University of Florida, 211 Bryant Space Science Center, P.O. Box 112055, Gainesville, FL 32611 (United States); Cargile, Phillip; Hebb, Leslie; Stassun, Keivan G. [Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235 (United States); Crepp, Justin R. [Department of Physics, University of Notre Dame, 225 Nieuwland Science Hall, Notre Dame, IN 46556 (United States); Porto de Mello, Gustavo F.; Ferreira, Leticia D. [Observatorio do Valongo, Universidade Federal do Rio de Janeiro, Ladeira do Pedro Antonio, 43, CEP: 20080-090, Rio de Janeiro, RJ (Brazil); Esposito, Massimiliano; Femenia, Bruno; Gonzalez Hernandez, Jonay I. [Instituto de Astrofisica de Canarias, C/Via Lactea S/N, E-38200 La Laguna (Spain); Gaudi, B. Scott [Department of Astronomy, The Ohio State University, 140 West 18th Avenue, Columbus, OH 43210 (United States); Ghezzi, Luan [Laboratorio Interinstitucional de e-Astronomia (LIneA), Rio de Janeiro, RJ 20921-400 (Brazil); Wisniewski, John P. [Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 West Brooks Street, Norman, OK 73019 (United States); Agol, Eric, E-mail: jpaty@mail.ustc.edu.cn [Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195-1580 (United States); and others

2013-09-15

95

Radial velocities of very low mass stars and candidate brown dwarf members of the Hyades and Pleiades  

NASA Technical Reports Server (NTRS)

We have determined H alpha equivalent widths and radial velocities with 1 sigma accuracies of approximately 5 km s(exp -1) for approximately 20 candidate very low mass members of the Hyades and Pleiades clusters. The radial velocities for the Hyades sample suggest that nearly all of these stars are indeed highly probable members of the Hyades. The faintest stars in the Hyades sample have masses of order 0.1 solar mass. We also obtained radial velocities for four candidate very low mass members of the Pleiades and two objects that are candidate BD Pleiads. All of these stars have apparent V magnitudes fainter than the Hyades stars we observed, and the resultant radial velocity accuracy is worse. We believe that the three brighter stars are indeed likely very low mass stellar members of the Pleiades, whereas the status of the two brown dwarf candidates is uncertain. The Hyades stars we have observed and the three Pleiades very low mass stars are the lowest mass members of any open cluster whose membership has been confirmed by radial velocities and whose chromospheric activity has been measured. We see no change in chromospheric activity at the boundary where stars are expected to become fully convective (M approximately equals 0.3 solar mass) in either cluster. In the Pleiades, however, there may be a decrease in chromospheric activity for stars with (V-I)(sub K) greater than 3.5 (M less than or equal to 0.1 solar mass).

Stauffer, John R.; Liebert, James; Giampapa, Mark; Macintosh, Bruce; Reid, Neill; Hamilton, Donald

1994-01-01

96

THE LOW-MASS INTERACTING BINARY SYSTEM OO Aql REVISITED: A NEW QUADRUPLE SYSTEM  

SciTech Connect

In this study we present photometric and spectroscopic variation analysis and an orbital period study of a low-mass interacting system OO Aql. Simultaneous solution of the light and radial velocity curves provides us with the determination of a new set of stellar physical parameters for the primary and the secondary companion, M{sub 1} = 1.05(2) M{sub Sun }, M{sub 2} = 0.89(2) M{sub Sun }, R{sub 1} = 1.38(2) R{sub Sun }, R{sub 2} = 1.28(2) R{sub Sun }, log (L{sub 1}/L{sub Sun }) = 0.258, and log (L{sub 2}/L{sub Sun }) = 0.117, and the separation of the components was determined to be a = 3.333(16) R{sub Sun }. Newly obtained parameters yield the distance of the system as 136(8) pc. Analyses of the mid-eclipse times indicate a period increase of (P/ P-dot )=4 Multiplication-Sign 10{sup 7} yr that can be interpreted in terms of the mass transfer (dM/dt) = 5 Multiplication-Sign 10{sup -8} M{sub Sun} yr{sup -1} from the less massive component to the more massive component. Our new solution confirmed that OO Aql is a multiple system in the form of AB + C + D. We found initial astrophysical parameters for the component of the system and its current age to be 8.6 Gyr using a non-conservative stellar evolution model (EV-TWIN code).

Icli, T.; Kocak, D.; Boz, G. C.; Yakut, K. [Department of Astronomy and Space Sciences, University of Ege, 35100, Bornova-Izmir (Turkey)

2013-05-15

97

The temperature and chronology of heavy-element synthesis in low-mass stars  

NASA Astrophysics Data System (ADS)

Roughly half of the heavy elements (atomic mass greater than that of iron) are believed to be synthesized in the late evolutionary stages of stars with masses between 0.8 and 8 solar masses. Deep inside the star, nuclei (mainly iron) capture neutrons and progressively build up (through the slow-neutron-capture process, or s-process) heavier elements that are subsequently brought to the stellar surface by convection. Two neutron sources, activated at distinct temperatures, have been proposed: 13C and 22Ne, each releasing one neutron per ?-particle (4He) captured. To explain the measured stellar abundances, stellar evolution models invoking the 13C neutron source (which operates at temperatures of about one hundred million kelvin) are favoured. Isotopic ratios in primitive meteorites, however, reflecting nucleosynthesis in the previous generations of stars that contributed material to the Solar System, point to higher temperatures (more than three hundred million kelvin), requiring at least a late activation of 22Ne (ref. 1). Here we report a determination of the s-process temperature directly in evolved low-mass giant stars, using zirconium and niobium abundances, independently of stellar evolution models. The derived temperature supports 13C as the s-process neutron source. The radioactive pair 93Zr-93Nb used to estimate the s-process temperature also provides, together with the pair 99Tc-99Ru, chronometric information on the time elapsed since the start of the s-process, which we determine to be one million to three million years.

Neyskens, P.; van Eck, S.; Jorissen, A.; Goriely, S.; Siess, L.; Plez, B.

2015-01-01

98

TP-AGB evolution with overshoot for low-mass stars as a function of metallicity  

E-print Network

We give a brief review on the properties of asymptotic giant branch models with overshoot. Then we describe new model calculations with overshoot. Initial masses are ranging from 1 to 3Msun and metallicities are Z=0.02, 0.01 and 0.001. Third dredge-up occurs efficiently for low masses and carbon stars are formed, with some at core masses as low as 0.58Msun. After the thermal pulse at which stars become C-rich the luminosities are in the range of the observed C-star luminosity function during the whole interpulse phase and for all C-star models. The dredge-up evolution depends mainly on the core mass at the first thermal pulse and on the metallicity. The Z=0.001 models of the 2 and 3Msun sequence become C-rich almost instantaneously after the onset of the first thermal pulses. For the 2Msun case the C/O ratio initially exceeds 4. During following dredge-up episodes the C/O ratio decreases.

Falk Herwig; Thomas Bloecker; Thomas Driebe

1999-12-16

99

Photometric and spectroscopic study of low mass embedded star clusters in reflection nebulae  

E-print Network

An analysis of the candidate embedded stellar systems in the reflection nebulae vdBH-RN26, vdBH-RN38, vdBH-RN53a, GGD20, ESO95-RN18 and NGC6595 is presented. Optical spectroscopic data from CASLEO (Argentina) in conjunction with near infrared photometry from the 2MASS Point Source Catalogue were employed. The analysis is based on source surface density, colour-colour and colour-magnitude diagrams together with theoretical pre-main sequence isochrones. We take into account the field population affecting the analysis by carrying out a statistical subtraction. The fundamental parameters for the stellar systems were derived. The resulting ages are in the range 1-4 Myr and the objects are dominated by pre-main sequence stars. The observed masses locked in the clusters are less than 25 solar masses. The studied systems have no stars of spectral types earlier than B, indicating that star clusters do not necessarily evolve through an HII region phase. The relatively small locked mass combined with the fact that they are not numerous in catalogues suggests that these low mass clusters are not important donors of stars to the field populations.

J. B. Soares; E. Bica; A. V. Ahumada; J. J. Claria

2004-11-07

100

A Pulsation Search Among Young Brown Dwarfs and Very Low Mass Stars  

E-print Network

In 2005, Palla & Baraffe proposed that brown dwarfs (BDs) and very low mass stars (VLMSs; <0.1 solar masses) may be unstable to radial oscillations during the pre-main-sequence deuterium burning phase. With associated periods of 1-4 hours, this potentially new class of pulsation offers unprecedented opportunities to probe the interiors and evolution of low-mass objects in the 1-15 million year age range. Following up on reports of short-period variability in young clusters, we designed a high-cadence photometric monitoring campaign to search for deuterium-burning pulsation among a sample of 348 BDs and VLMSs in the four young clusters $\\sigma$ Orionis, Chamaeleon I, IC 348, and Upper Scorpius. In the resulting light curves we achieved sensitivity to periodic signals of amplitude several millimagnitudes, on timescales from 15 minutes to two weeks. Despite the exquisite data quality, we failed to detect any periodicities below seven hours. We conclude that D-burning pulsations are not able to grow to obs...

Cody, Ann Marie

2014-01-01

101

A Pulsation Search among Young Brown Dwarfs and Very-low-mass Stars  

NASA Astrophysics Data System (ADS)

In 2005, Palla & Baraffe proposed that brown dwarfs (BDs) and very-low-mass stars (VLMSs; < 0.1 solar masses) may be unstable to radial oscillations during the pre-main-sequence deuterium burning phase. With associated periods of one to four hours, this potentially new class of pulsation offers unprecedented opportunities to probe the interiors and evolution of low-mass objects in the 1-15 million year age range. Following up on reports of short-period variability in young clusters, we designed a high-cadence photometric monitoring campaign to search for deuterium-burning pulsation among a sample of 348 BDs and VLMSs in the four young clusters ? Orionis, Chamaeleon I, IC 348, and Upper Scorpius. In the resulting light curves we achieved sensitivity to periodic signals of amplitude several millimagnitudes, on timescales from 15 minutes to two weeks. Despite the exquisite data quality, we failed to detect any periodicities below seven hours. We conclude that D-burning pulsations are not able to grow to observable amplitudes in the early pre-main sequence. In spite of the nondetection, we did uncover a rich set of variability behavior—both periodic and aperiodic—on day to week timescales. We present new compilations of variable sources from our sample, as well as three new candidate cluster members in Chamaeleon I.

Cody, Ann Marie; Hillenbrand, Lynne A.

2014-12-01

102

THE SEARCH FOR LOW-MASS COMPANIONS OF B STARS IN THE CARINA NEBULA CLUSTER TRUMPLER 16  

SciTech Connect

We have developed lists of likely B3-A0 stars (called 'late B' stars) in the young cluster Trumpler 16. The following criteria were used: location within 3' of {eta} Car, an appropriate V and B - V combination, and proper motion (where available). Color and magnitude cuts have been made assuming an E(B - V) = 0.55 mag {+-} 0.1, which is a good approximation close to the center of Trumpler 16. These lists have been cross-correlated with X-ray sources found in the Chandra Carina Complex Project. Previous studies have shown that only very rarely (if at all) do late main-sequence B stars produce X-rays. We present evidence that the X-ray-detected sources are binaries with low-mass companions, since stars less massive than 1.4 M{sub sun} are strong X-ray sources at the age of the cluster. Both the median X-ray energies and X-ray luminosities of these sources are in good agreement with values for typical low-mass coronal X-ray sources. We find that 39% of the late B stars based on a list with proper motions have low-mass companions. Similarly, 32% of a sample without proper motions have low-mass companions. We discuss the X-ray detection completeness. These results on low-mass companions of intermediate-mass stars are complementary to spectroscopic and interferometric results and probe new parameter space of low-mass companions at all separations. They do not support a steeply rising distribution of mass ratios to low masses for intermediate-mass (5 M{sub sun}) primaries, such as would be found by random pairing from the initial mass function.

Remage Evans, Nancy [Smithsonian Astrophysical Observatory, MS 4, 60 Garden Street, Cambridge, MA 02138 (United States); DeGioia-Eastwood, Kathleen [Department of Physics and Astronomy, Northern Arizona University, Box 6010, Flagstaff, AZ 86011-6010 (United States); Gagne, Marc [Department of Geology and Astronomy, West Chester University, West Chester, PA 19883 (United States); Townsley, Leisa; Broos, Patrick [Department of Astronomy and Astrophysics, 525 Davey Laboratory, Pennsylvania State University, University Park, PA 16802 (United States); Wolk, Scott [Smithsonian Astrophysical Observatory, MS 70, 60 Garden Street, Cambridge, MA 02138 (United States); Naze, Yael [GAPHE Departement AGO, Universite de Liege, Allee du 6 Aout 17, Bat. B5C, B4000-Liege (Belgium); Corcoran, Michael [NASA's Goddard Space Flight Center, Code 662, Greenbelt, MD 20771 (United States); Oskinova, Lida [Institute for Physics and Astronomy, University Potsdam, 14476 Potsdam (Germany); Moffat, Anthony F. J. [Dept. de Physique, Univ. de Montreal, CP 6128 Succ. A. Centre-Ville, Montreal, QC H3C 3J7 (Canada); Wang Junfeng [Smithsonian Astrophysical Observatory, MS 06, 60 Garden Street, Cambridge, MA 02138 (United States); Walborn, Nolan R., E-mail: nevans@cfa.harvard.edu [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States)

2011-05-01

103

A HERSCHEL SURVEY OF COLD DUST IN DISKS AROUND BROWN DWARFS AND LOW-MASS STARS  

SciTech Connect

We report the complete photometric results from our Herschel study which is the first comprehensive program to search for far-infrared emission from cold dust around young brown dwarfs (BDs). We surveyed 50 fields containing 51 known or suspected BDs and very low mass stars that have evidence of circumstellar disks based on Spitzer photometry and/or spectroscopy. The objects with known spectral types range from M3 to M9.5. Four of the candidates were subsequently identified as extragalactic objects. Of the remaining 47 we have successfully detected 36 at 70 {mu}m and 14 at 160 {mu}m with signal-to-noise ratio (S/N) greater than 3, as well as several additional possible detections with low S/N. The objects exhibit a range of [24]-[70] {mu}m colors suggesting a range in mass and/or structure of the outer disk. We present modeling of the spectral energy distributions of the sample and discuss trends visible in the data. Using two Monte Carlo radiative transfer codes we investigate disk masses and geometry. We find a very wide range in modeled total disk masses from less than 10{sup -6} M{sub Sun} up to 10{sup -3} M{sub Sun} with a median disk mass of the order of 3 Multiplication-Sign 10{sup -5} M{sub Sun }, suggesting that the median ratio of disk mass to central object mass may be lower than for T Tauri stars. The disk scale heights and flaring angles, however, cover a range consistent with those seen around T Tauri stars. The host clouds in which the young BDs and low-mass stars are located span a range in estimated age from {approx}1-3 Myr to {approx}10 Myr and represent a variety of star-forming environments. No obvious dependence on cloud location or age is seen in the disk properties, though the statistical significance of this conclusion is not strong.

Harvey, Paul M.; Evans, Neal J. II [Astronomy Department, University of Texas at Austin, 1 University Station C1400, Austin, TX 78712-0259 (United States); Henning, Thomas [Max Planck Institut for Astronomy, Koenigstuhl 17, 69117 Heidelberg (Germany); Liu Yao; Wolf, Sebastian [Institute of Theoretical Physics and Astrophysics, University of Kiel, Leibnizstr. 15, 24098 Kiel (Germany); Menard, Francois; Pinte, Christophe [UJF-Grenoble 1/CNRS-INSU, Institut de Planetologie et d'Astrophysique (IPAG) UMR 5274, BP 53, 38041 Grenoble cedex 9 (France); Cieza, Lucas A. [Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822 (United States); Pascucci, Ilaria, E-mail: pmh@astro.as.utexas.edu, E-mail: nje@astro.as.utexas.edu, E-mail: henning@mpia.de, E-mail: wolf@astrophysik.uni-kiel.de, E-mail: yliu@pmo.ac.cn, E-mail: yliu@pmo.ac.cn, E-mail: menard@obs.ujf-grenoble.fr, E-mail: christophe.pinte@obs.ujf-grenoble.fr, E-mail: lcieza@ifa.hawaii.edu, E-mail: pascucci@lpl.arizona.edu [Lunar and Planetary Laboratory, Department of Planetary Sciences, University of Arizona, 1629 E. University Boulevard, Tucson, AZ 85721 (United States)

2012-08-10

104

Quest for quiescent neutron star low mass x-ray binaries in the Small Magellanic Cloud  

NASA Astrophysics Data System (ADS)

We present the first spectral search for neutron stars (NSs) in low-mass X-ray binaries (LMXBs) between outbursts in the Small Magellanic Cloud (SMC). We identify and discuss candidate LMXBs in quiescence in the SMC using deep Chandra X-ray observations of two portions of the SMC. We produce X-ray color-magnitude-diagrams of XRSs of these two fields and identify 10 candidates for quiescent NS LMXBs. Spectral fitting and searches for optical counterparts rule out five, leaving five candidate quiescent NS LMXBs. We estimate that we are sensitive to ˜ 10% of quiescent NS LMXBs in our fields. Our fields include 4.4 × 10^7 M? of stellar mass, giving an upper limit of 10^-6 LMXBs per M? in the SMC. We place a lower limit on the average duty cycle of NS LMXBs as ˜ 0.003.

Chowdhury, Mizanul Huq

105

A Search for Low Mass Stars and Brown Dwarfs in the Upper Scorpius OB Association  

E-print Network

We are conducting a U,B,R,I monitoring program to identify pre-main sequence stars and brown dwarfs in a 12.5x12.2 square degree region of the Upper Scorpius OB association (5-10 Myr). We will use these data in combination with a follow-up spectroscopy survey to derive the low mass IMF in Upper Sco and to explore the prevalence of 5-10 Myr circumstellar disks. We will also analyze the spatial distribution of association members as a function of stellar mass, from which we will be able to place constraints on brown dwarf formation scenarios. We expect to identify 700-1800 previously unknown pre-main sequence objects in Upper Sco with M brown dwarfs at 5-10 Myr.

C. L. Slesnick; J. M. Carpenter; L. A. Hillenbrand

2004-11-19

106

Magnetic fields and star formation in low-mass Magellanic-type and peculiar galaxies  

NASA Astrophysics Data System (ADS)

Aims: We investigate how magnetic properties of Magellanic-type and perturbed objects are related to star-forming activity, galactic type, and mass. Methods: We present radio and magnetic properties of five Magellanic-type and two peculiar low-mass galaxies observed at 4.85 and/or 8.35 GHz with the Effelsberg 100 m telescope. The sample is extended to 17 objects by including five Magellanic-type galaxies and five dwarf ones. Results: The distribution of the observed radio emission of low-mass galaxies at 4.85/8.35 GHz is closely connected with the galactic optical discs, which are independent for unperturbed galaxies and those which show signs of tidal interactions. The strengths of total magnetic field are within 5-9 ?G, while the ordered fields reach 1-2 ?G, and both these values are larger than in typical dwarf galaxies and lower than in spirals. The magnetic field strengths in the extended sample of 17 low-mass galaxies are well correlated with the surface density of star formation rate (correlation coefficient of 0.87) and manifest a power-law relation with an exponent of 0.25 ± 0.02 extending a similar relation found for dwarf galaxies. We claim that the production of magnetic energy per supernova event is very similar for all the various galaxies. It constitutes about 3% (1049 erg) of the individual supernovae energy release. We show that the total magnetic field energy in galaxies is almost linearly related to the galactic gas mass, which indicates equipartition of the magnetic energy and the turbulent kinetic energy of the interstellar medium. The Magellanic-type galaxies fit very well with the radio-infrared relation constructed for surface brightness of galaxies of various types, including bright spirals and interacting objects (with a slope of 0.96 ± 0.03 and correlation coefficient of 0.95). We found that the typical far-infrared relation based on luminosity of galaxies is tighter and steeper but more likely to inherit a partial correlation from a tendency that larger objects are also more luminous. Conclusions: The estimated values of thermal fractions, radio spectral indices, and magnetic field strengths of the Magellanic-type galaxies are between the values determined for grand-design spirals and dwarf galaxies. The confirmed magnetic field-star formation and radio-infrared relations for low-mass galaxies point to similar physical processes that must be at work in all galaxies. More massive, larger galaxies have usually stronger magnetic fields and larger global star formation rates, but we show that their values of magnetic energy release per supernova explosion are still similar to those of dwarf galaxies. Based on observations with the 100-m telescope at Effelsberg operated by the Max-Planck-Institut für Radioastronomie (MPIfR) on behalf of the Max-Planck-Gesellschaft.

Jurusik, W.; Drzazga, R. T.; Jableka, M.; Chy?y, K. T.; Beck, R.; Klein, U.; We?gowiec, M.

2014-07-01

107

Energy generation in convective shells of low mass, low metallicity stars  

SciTech Connect

We report on the non-negligible energy generation from the {sup 13}C neutron source and neutron capture reactions in low mass, low metallicity AGB stars. About 10{sup 4} L{sub {circle dot}} are generated within the thermal pulse convective shell by the combination of the {sup 13}C({alpha}, n){sup 16}O rate and the sum of the Y(Z,A)(n,{gamma})Y(Z,A + 1) reactions and beta decays. The inclusion of this energy source in an AGB thermal pulse evolution is shown to alter the evolution of the convective shell boundaries, and, hence, how the {sup 13}C is ingested into the convective shell. Also, the duration of the pulse itself is reduced by the additional energy input. The nucleosynthetic consequences are discussed for these evolutionary changes. 17 refs., 5 figs.

Bazan, G. (Illinois Univ., Urbana, IL (USA). Dept. of Astronomy); Lattanzio, J.C. (Lawrence Livermore National Lab., CA (USA))

1989-11-10

108

Colliding Winds in Low-Mass Binary Star Systems: wind interactions and implications for habitable planets  

E-print Network

Context. In binary star systems, the winds from the two components impact each other, leading to strong shocks and regions of enhanced density and temperature. Potentially habitable circumbinary planets must continually be exposed to these interactions regions. Aims. We study, for the first time, the interactions between winds from low-mass stars in a binary system, to show the wind conditions seen by potentially habitable circumbinary planets. Methods. We use the advanced 3D numerical hydrodynamic code Nurgush to model the wind interactions of two identical winds from two solar mass stars with circular orbits and a binary separation of 0.5 AU. As input into this model, we use a 1D hydrodynamic simulation of the solar wind, run using the Versatile Advection Code. We derive the locations of stable and habitable orbits in this system to explore what wind conditions potentially habitable planets will be exposed to during their orbits. Results. Our wind interaction simulations result in the formation of two stron...

Johnstone, C P; Pilat-Lohinger, E; Bisikalo, D; Güdel, M; Eggl, S

2015-01-01

109

A method to constrain the neutron star magnetic field in Low Mass X-ray Binaries  

NASA Astrophysics Data System (ADS)

We describe here a method to put an upper limit to the strength of the magnetic field of neutron stars in low mass X-ray binaries for which the spin period and the X-ray luminosity during X-ray quiescent periods are known. This is obtained using simple considerations about the position of the magnetospheric radius during quiescent periods. We applied this method to the accreting millisecond pulsar SAX J1808.4-3658, which shows coherent X-ray pulsations at a frequency of ~ 400 Hz and a quiescent X-ray luminosity of ~ 5 × 1031 ergs/s, and found that B <= 5 × 108 Gauss in this source. Combined with the lower limit inferred from the presence of X-ray pulsations, this constrains the SAX J1808.4-3658 neutron star magnetic field in the quite narrow range (1 - 5) × 108 Gauss. Similar considerations applied to the case of Aql X-1 and KS 1731-260 give neutron star magnetic fields lower than ~ 109 Gauss.

di Salvo, T.; Robba, N. R.; Iaria, R.; Lavagetto, G.; Burderi, L.

2005-09-01

110

Direct imaging search for planets around low-mass stars and spectroscopic characterization of young exoplanets  

NASA Astrophysics Data System (ADS)

Low--mass stars between 0.1--0.6 M? are the most abundant members our galaxy and may be the most common sites of planet formation, but little is known about the outer architecture of their planetary systems. We have carried out a high-contrast adaptive imaging search for gas giant planets between 1--13 MJup around 122 newly identified young M dwarfs in the solar neighborhood ( ? 35 pc). Half of our targets are younger than 145 Myr, and 90% are younger than 580 Myr. After removing 39 resolved stellar binaries, our homogeneous sample of 83 single young M dwarfs makes it the largest imaging search for planets around low--mass stars to date. Our H- and K- band coronagraphic observations with Subaru/HiCIAO and Keck/NIRC2 achieve typical contrasts of 9--13 mag and 12--14 mag at 100, respectively, which corresponds to limiting masses of ˜1--10 M Jup at 10--30 AU for most of our sample. We discovered four brown dwarfs with masses between 25--60 MJup at projected separations of 4--190 AU. Over 100 candidate planets were discovered, nearly all of which were found to be background stars from follow-up second epoch imaging. Our null detection of planets nevertheless provides strong statistical constraints on the occurrence rate of giant planets around M dwarfs. Assuming circular orbits and a logarithmically-flat power law distribution in planet mass and semi--major axis of the form d 2N=(dloga dlogm) infinity m0 a0, we measure an upper limit (at the 95% confidence level) of 8.8% and 12.6% for 1--13 MJup companions between 10--100 AU for hot start and cold start evolutionary models, respectively. For massive gas giant planets in the 5--13 M Jup range like those orbiting HR 8799, GJ 504, and beta Pictoris, we find that fewer than 5.3% (7.8%) of M dwarfs harbor these planets between 10--100 AU for a hot start (cold start) formation scenario. Our best constraints are for brown dwarf companions; the frequency of 13--75 MJup companions between (de--projected) physical separations of 10--100 AU is 2.1+2.1-1.2 %. Altogether, our results show that gas giant planets, especially massive ones, are rare in the outskirts of M dwarf planetary systems. If disk instability is a viable way to form planets, our constraints for the most common type of star imply that overall it is an inefficient mechanism.

Bowler, Brendan Peter

111

Spitzer: Accretion in Low-Mass Stars and Brown Dwarfs in the ? Orionis Cluster  

NASA Astrophysics Data System (ADS)

We present multiwavelength optical and IR photometry of 170 previously known low-mass stars and brown dwarfs of the 5 Myr Collinder 69 cluster (? Orionis). The new photometry supports cluster membership for most of them, with less than 15% of the previous candidates identified as probable nonmembers. The near-IR photometry allows us to identify stars with IR excesses, and we find that the Class II population is very large, around 25% for stars (in the spectral range M0-M6.5) and 40% for brown dwarfs, down to 0.04 Msolar, despite the fact that the H? equivalent width is low for a significant fraction of them. In addition, there are a number of substellar objects, classified as Class III, that have optically thin disks. The Class II members are distributed in an inhomogeneous way, lying preferentially in a filament running toward the southeast. The IR excesses for the Collinder 69 members range from pure Class II (flat or nearly flat spectra longward of 1 ?m), to transition disks with no near-IR excess but excesses beginning within the IRAC wavelength range, to two stars with excess only detected at 24 ?m. Collinder 69 thus appears to be at an age where it provides a natural laboratory for the study of primordial disks and their dissipation. Based on observations collected by the Spitzer Space Telescope, at the German-Spanish Astronomical Center of Calar Alto jointly operated by the Max-Planck-Institut für Astronomie Heidelberg and the Instituto de Astrofísica de Andalucía (CSIC), and at the WHT operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias.

Barrado y Navascués, David; Stauffer, John R.; Morales-Calderón, María; Bayo, Amelia; Fazzio, Giovanni; Megeath, Tom; Allen, Lori; Hartmann, Lee W.; Calvet, Nuria

2007-07-01

112

The temperature and chronology of heavy-element synthesis in low-mass stars.  

PubMed

Roughly half of the heavy elements (atomic mass greater than that of iron) are believed to be synthesized in the late evolutionary stages of stars with masses between 0.8 and 8 solar masses. Deep inside the star, nuclei (mainly iron) capture neutrons and progressively build up (through the slow-neutron-capture process, or s-process) heavier elements that are subsequently brought to the stellar surface by convection. Two neutron sources, activated at distinct temperatures, have been proposed: (13)C and (22)Ne, each releasing one neutron per ?-particle ((4)He) captured. To explain the measured stellar abundances, stellar evolution models invoking the (13)C neutron source (which operates at temperatures of about one hundred million kelvin) are favoured. Isotopic ratios in primitive meteorites, however, reflecting nucleosynthesis in the previous generations of stars that contributed material to the Solar System, point to higher temperatures (more than three hundred million kelvin), requiring at least a late activation of (22)Ne (ref. 1). Here we report a determination of the s-process temperature directly in evolved low-mass giant stars, using zirconium and niobium abundances, independently of stellar evolution models. The derived temperature supports (13)C as the s-process neutron source. The radioactive pair (93)Zr-(93)Nb used to estimate the s-process temperature also provides, together with the pair (99)Tc-(99)Ru, chronometric information on the time elapsed since the start of the s-process, which we determine to be one million to three million years. PMID:25567282

Neyskens, P; Van Eck, S; Jorissen, A; Goriely, S; Siess, L; Plez, B

2015-01-01

113

Thermohaline mixing and the photospheric composition of low-mass giant stars  

NASA Astrophysics Data System (ADS)

Aims: By means of numerical simulations and different recipes, we test the efficiency of thermohaline mixing as a process to alter the surface abundances in low-mass giant stars. Methods: We compute full evolutionary sequences of red giant branch stars close to the luminosity bump by including state-of-the-art composition transport prescriptions for the thermohaline mixing regimes. In particular, we adopt a self-consistent double-diffusive convection theory that allows handling both instabilities that arise when thermal and composition gradients compete against each other and a very recent empirically motivated and parameter-free asymptotic scaling law for thermohaline composition transport. Results: In agreement with previous works, we find that, during the red giant stage, a thermohaline instability sets in shortly after the hydrogen burning shell (HBS) encounters the chemical discontinuity left behind by the first dredge-up. We also find that the thermohaline unstable region, which initially appears on the exterior wing of the HBS, is unable to reach the outer convective envelope, with the consequence that no mixing of elements occurs that produces a noncanonical modification of the stellar surface abundances. Also in agreement with previous works, we find that artificially increasing the mixing efficiency of thermohaline regions makes it possible to connect both unstable regions, thus affecting the photospheric composition. However, we find that to reproduce the observed abundances of red giant branch stars close to the luminosity bump, thermohaline mixing efficiency has to be artificially increased by about four orders of magnitude from what is predicted by recent 3D numerical simulations of thermohaline convection close to astrophysical environments. From this we conclude that the chemical abundance anomalies of red giant stars cannot be explained on the basis of thermohaline mixing alone.

Wachlin, F. C.; Miller Bertolami, M. M.; Althaus, L. G.

2011-09-01

114

Coronal Activity in Low-Mass Pre-Main Sequence Stars: NGC 2264  

NASA Technical Reports Server (NTRS)

We present the preliminary results of an analysis of ROSAT images in the region of the populous young (age approx. 3 Myr) star-forming region NGC 2264. The cluster was imaged with the ROSAT HRI in two sets of pointings -- one set near the central region of the cluster, centered on the star LW Mon, and the other set in the southern part of the cluster, centered near the star V428 Mon, just south of the Cone Nebula. In total 113 unique X-ray sources have been identified in the ROSAT images with signal-to-noise ratios greater than 3. The limiting luminosities (log Lx(ergs/sec)) for 3-sigma detections are estimated to be 30.18, 30.23, and 30.08 for the northern field, southern field, and overlap region between the two fields respectively. Extensive optical photometry, classification spectroscopy, and proper motions, obtained from recent ground-based surveys of this region, were used to identify the most likely optical counterpart to each X-ray source. Although most of our X-ray selected sample appears to be associated with NGC 2264 members, we find that the vast majority of the cluster membership was undetected in the ROSAT HRI survey. The X-ray cumulative luminosity function for solar-mass stars in NGC 2264 shows that most of the low-mass members probably have X-ray luminosities similar to those seen for the X-ray brightest members of older clusters such as IC 2391/IC 2602 (age approx. 50 Myr) and the Pleiades (age approx. 100 Myr). This research was funded in part by the SAO Summer Intern Program and NASA grant NAG5-8120.

Tebbe, H. J.; Patten, B. M.

2000-01-01

115

The effect of feedback and reionization on star formation in low-mass dwarf galaxy haloes  

NASA Astrophysics Data System (ADS)

We simulate the evolution of a 109 M? dark matter halo in a cosmological setting with an adaptive mesh refinement code as an analogue to local low-luminosity dwarf irregular and dwarf spheroidal galaxies. The primary goal of our study is to investigate the roles of reionization and supernova feedback in determining the star-formation histories of low-mass dwarf galaxies. We include a wide range of physical effects, including metal cooling, molecular hydrogen formation and cooling, photoionization and photodissociation from a metagalactic (but not local) background, a simple prescription for self-shielding, star formation and a simple model for supernova-driven energetic feedback. To better understand the impact of each physical effect, we carry out simulations excluding each major effect in turn. We find that reionization is primarily responsible for expelling most of the gas in our simulations, but that supernova feedback is required to disperse the dense, cold gas in the core of the halo. Moreover, we show that the timing of reionization can produce an order-of-magnitude difference in the final stellar mass of the system. For our full physics run with reionization at z = 9, we find a stellar mass of about 105 M? at z = 0 and a mass-to-light ratio within the half-light radius of approximately 130 M?/L?, consistent with observed low-luminosity dwarfs. However, the resulting median stellar metallicity is 0.06 Z?, considerably larger than observed systems. In addition, we find that star formation is truncated between redshifts 4 and 7, at odds with the observed late-time star formation in isolated dwarf systems but in agreement with Milky Way ultrafaint dwarf spheroidals. We investigate the efficacy of energetic feedback in our simple thermal-energy-driven feedback scheme, and suggest that it may still suffer from excessive radiative losses, despite reaching stellar particle masses of about 100 M? and a comoving spatial resolution of 11 pc.

Simpson, Christine M.; Bryan, Greg L.; Johnston, Kathryn V.; Smith, Britton D.; Mac Low, Mordecai-Mark; Sharma, Sanjib; Tumlinson, Jason

2013-07-01

116

The first low-mass stars: critical metallicity or dust-to-gas ratio?  

E-print Network

We explore the minimal conditions which enable the formation of metal-enriched solar and sub-solar mass stars. We find that in the absence of dust grains, gas fragmentation occurs at densities nH ~ [10^4-10^5]cm^{-3} when the metallicity exceeds Z ~ 10^{-4} Zsun. The resulting fragmentation masses are > 10 Msun. The inclusion of Fe and Si cooling does not affect the thermal evolution as this is dominated by molecular cooling even for metallicities as large as Z = 10^{-2} Zsun. The presence of dust is the key driver for the formation of low-mass stars. We focus on three representative core-collapse supernova (SN) progenitors, and consider the effects of reverse shocks of increasing strength: these reduce the depletion factors, fdep = Mdust/(Mdust+Mmet), alter the shape of the grain size distribution function and modify the relative abundances of grain species and of metal species in the gas phase. We find that the lowest metallicity at which fragmentation occurs is Z=10^{-6} Zsun for gas pre-enriched by the ex...

Schneider, Raffaella; Bianchi, Simone; Valiante, Rosa

2011-01-01

117

The effects of rotation on the surface composition and yields of low mass AGB stars.  

NASA Astrophysics Data System (ADS)

Over the past 20 years, stellar evolutionary models have been strongly improved in order to reproduce with reasonable accuracy both photometric and spectroscopic observations. Notwithstanding, the majority of these models do not take into account macroscopic phenomena, like rotation and/or magnetic fields. Their explicit treatment could modify stellar physical and chemical properties. One of the most interesting problems related to stellar nucleosynthesis is the behavior of the s-process spectroscopic indexes ([hs/ls] and [Pb/hs]) in Asymptotic Giant Branch (AGB) stars. In this contribution we show that, for a fixed metallicity, rotation can lead to a spread in the [hs/ls] and [Pb/hs] in low-mass AGB stars. In particular, we demonstrate that the Eddington-Sweet and the Goldreich-Schubert-Fricke instabilities may have enough time to smear the 13C-pocket (the major neutron source) and the 14N-pocket (the major neutron poison). In fact, a different overlap between these pockets leads to a different neutrons-to-seeds ratio, with important consequences on the corresponding s-process distributions. Possible consequences on the chemical evolution of Galactic globular clusters are discussed.

Cristallo, S.; Piersanti, L.; Straniero, O.

118

Miniature MMIC Low Mass/Power Radiometer Modules for the 180 GHz GeoSTAR Array  

NASA Technical Reports Server (NTRS)

We have developed and demonstrated miniature 180 GHz Monolithic Microwave Integrated Circuit (MMIC) radiometer modules that have low noise temperature, low mass and low power consumption. These modules will enable the Geostationary Synthetic Thinned Aperture Radiometer (GeoSTAR) of the Precipitation and All-weather Temperature and Humidity (PATH) Mission for atmospheric temperature and humidity profiling. The GeoSTAR instrument has an array of hundreds of receivers. Technology that was developed included Indium Phosphide (InP) MMIC Low Noise Amplifiers (LNAs) and second harmonic MMIC mixers and I-Q mixers, surface mount Multi-Chip Module (MCM) packages at 180 GHz, and interferometric array at 180 GHz. A complete MMIC chip set for the 180 GHz receiver modules (LNAs and I-Q Second harmonic mixer) was developed. The MMIC LNAs had more than 50% lower noise temperature (NT=300K) than previous state-of-art and MMIC I-Q mixers demonstrated low LO power (3 dBm). Two lots of MMIC wafers were processed with very high DC transconductance of up to 2800 mS/mm for the 35 nm gate length devices. Based on these MMICs a 180 GHz Multichip Module was developed that had a factor of 100 lower mass/volume (16x18x4.5 mm3, 3g) than previous generation 180 GHz receivers.

Kangaslahti, Pekka; Tanner, Alan; Pukala, David; Lambrigtsen, Bjorn; Lim, Boon; Mei, Xiaobing; Lai, Richard

2010-01-01

119

Identifying low-mass members of nearby star clusters using proper motion & color selection  

NASA Astrophysics Data System (ADS)

I present a combined kinematic and photometric search for new, low-mass (m ? 0.2 M? ) members of nearby (d < 300 pc) star clusters. Using both proper motion and color criteria, a total of 33 low-mass objects have been newly recognized as members of the Taurus, Praesepe, and Pleiades clusters. In addition, 18 potential cluster members are noted, and 4 members are recovered from previous member searches. Multi-epoch imaging was performed using i-band Megacam observations unique to this study, combined with archival CFH telescope data in the optical I and Z bands. Near-infrared detections were also acquired from the 2MASS survey. The imaging data were processed using the Pan-STARRS IPP data pipeline software in order to provide high-precision relative astrometry, from which proper motions were extracted. Low-resolution, near-infrared spectroscopy from the IRTF telescope gives confirmation on the membership status of the selected candidates. The addition of proper motion criteria to complement the often-used color selection allows for a more effective identification of low-mass cluster members whose broadband spectral features are similar to the bulk of galactic field objects lying along the line-of-sight. Culling the candidates using proper motion also significantly reduces the amount of candidates that require spectroscopic follow-up, even in the NIR color-space with the highest levels of field contamination. Comparison of the search results to a galactic field model by Robin et al. (2003) provides strong evidence that brighter member candidates in Taurus (i < 17) found to be of mid-M spectral types are highly likely to be clusters members rather than field dwarfs. While the addition of new members to the Praesepe and Pleiades clusters are minor compared to the current known population, there is suggestive evidence that the mass function of Taurus is significantly lacking in mid-M dwarfs, and in fact may actually resemble the mass functions of other similarly-aged clusters. The successful application of this search technique using the IPP software is significant, as the Pan-STARRS survey program is conducting regular imaging of 75% of the total sky over the next 3-4 years.

Pitts, Mark A.

120

Low-mass galaxy assembly in simulations: regulation of early star formation by radiation from massive stars  

NASA Astrophysics Data System (ADS)

Despite recent success in forming realistic present-day galaxies, simulations still form the bulk of their stars earlier than observations indicate. We investigate the process of stellar mass assembly in low-mass field galaxies, a dwarf and a typical spiral, focusing on the effects of radiation from young stellar clusters on the star formation (SF) histories. We implement a novel model of SF with a deterministic low efficiency per free-fall time, as observed in molecular clouds. Stellar feedback is based on observations of star-forming regions, and includes radiation pressure from massive stars, photoheating in H II regions, supernovae and stellar winds. We find that stellar radiation has a strong effect on the formation of low-mass galaxies, especially at z > 1, where it efficiently suppresses SF by dispersing cold and dense gas, preventing runaway growth of the stellar component. This behaviour is evident in a variety of observations but had so far eluded analytical and numerical models without radiation feedback. Compared to supernovae alone, radiation feedback reduces the SF rate by a factor of ˜100 at z ? 2, yielding rising SF histories which reproduce recent observations of Local Group dwarfs. Stellar radiation also produces bulgeless spiral galaxies and may be responsible for excess thickening of the stellar disc. The galaxies also feature rotation curves and baryon fractions in excellent agreement with current data. Lastly, the dwarf galaxy shows a very slow reduction of the central dark matter density caused by radiation feedback over the last ˜7 Gyr of cosmic evolution.

Trujillo-Gomez, Sebastian; Klypin, Anatoly; Colín, Pedro; Ceverino, Daniel; Arraki, Kenza S.; Primack, Joel

2015-01-01

121

Externally Induced or Internally Produced: What is the Source of the Extreme Magnetic Activity Observed in Very Low Mass Stars?  

NASA Astrophysics Data System (ADS)

Recent observations of the very low mass donor stars in short period interacting binaries have revealed the presence of active chromospheres. Our group has obtained initial spectra for five such systems to date and found evidence of stellar activity in all of them, including a brown dwarf mass (0.055 M-sun) donor star. The Washington group has obtained a SDSS sample of a number of close, but non-interacting white dwarf + red dwarf binaries and find H(alpha) emission related to activity. We plan to perform the first detailed phase-resolved spectroscopic study of the active chromospheres in this set of rapidly rotating (<0.3 day), low mass (<0.25 M-sun), fully convective late-type stars and brown dwarfs. Our observational goals are to study the extent, nature and short-term changes of these active chromospheres on the low mass stars of close binaries. Is the extreme activity caused by the low mass star itself or externally driven by tidal or magnetic forces? The long term goals are to complete a large enough sample (~10 systems) to provide statistically useful measurements and to use the extended time allocation to obtain long term ``coverage" spectra of each system to monitor, measure, and understand the solar-type cycles likely to be present.

Howell, Steve B.; Giampapa, Mark; Harrison, Thomas; Hawley, Suzanne; Hill, Frank; Honeycutt, Kent; Kafka, Stella; Silvestri, Nicole; Szkody, Paula; Walter, Fred; West, Andrew

2008-02-01

122

A census of very-low-mass stars and brown dwarfs in the ? Orionis cluster  

NASA Astrophysics Data System (ADS)

Context: The knowledge of the initial mass function (IMF) in open clusters constitutes one way of constraining the formation of low-mass stars and brown dwarfs, along with the frequency of multiple systems and the properties of disks. Aims: The aim of the project is to determine the shape of the mass function in the low-mass and substellar regimes in the ? Orionis cluster (~3 Myr, ~352 pc, solar metallicity) as accurately as possible and compare it with the results in other clusters. Methods: We have analysed the near-infrared photometric data from the fourth data release (DR4) of the UKIRT Infrared Deep Sky Suvey (UKIDSS) Galactic clusters survey (GCS) to derive the cluster luminosity and mass functions, evaluate the extent of the cluster, and study the distribution and variability of low-mass stars and brown dwarfs down to the deuterium-burning limit. Results: We have recovered most of the previously published members and found a total of 287 candidate members within the central 30 arcmin in the 0.5-0.009 M? mass range, including new objects not previously reported in the literature. This new catalogue represents a homogeneous dataset of brown dwarf member candidates over the central 30 arcmin of the cluster. The expected photometric contamination by field objects with similar magnitudes and colours to ? Orionis members is ~15%. We present evidence of variability at the 99.5% confidence level over ~yearly timescales in 10 member candidates that exhibit signs of youth and the presence of disks. The level of variability is low (?0.3 mag) and does not impact the derivation of the cluster luminosity and mass functions. Furthermore, we find a possible dearth of brown dwarfs within the central five arcmin of the cluster, which is not caused by a lower level of photometric sensitivity around the massive, O-type multiple star ? Ori in the GCS survey. Using state-of-the-art theoretical models, we derived the luminosity and mass functions within the central 30 arcmin from the cluster centre, with completeness down to J = 19 mag, corresponding to masses ranging from 0.5 M? down to the deuterium-burning mass boundary (~0.013 M?). Conclusions: The mass function of ? Orionis in this mass interval shows a power law index ? = 0.5±0.2 (when expressed as dN/dM ? M-?), which agrees with the one derived for the 3-5 Myr cluster Upper Sco (based on similar data obtained with the GCS) in the same mass range. This work is based in part on data obtained as part of the UKIRT Infrared Deep Sky Survey. The United Kingdom Infrared Telescope is operated by the Joint Astronomy Centre on behalf of the Science and Technology Facilities Council of the UK. Appendices are only available at http://www.aanda.org

Lodieu, N.; Zapatero Osorio, M. R.; Rebolo, R.; Martín, E. L.; Hambly, N. C.

2009-10-01

123

The Planets Around Low-Mass Stars (PALMS) Direct Imaging Survey  

NASA Astrophysics Data System (ADS)

Direct imaging is the only method to study the outer architecture (>10 AU) of extrasolar planetary systems in a targeted fashion. Previous imaging surveys have primarily focused on intermediate- and high-mass stars because of the relative dearth of known nearby young M dwarfs. As a result, even though M dwarfs make up 70% of stars in our galaxy, there are few constraints on the population of giant planets at moderate separations (10-100 AU) in this stellar mass regime. We present results from an ongoing high-contrast adaptive optics imaging survey targeting newly identified nearby (<35 pc) young (<300 Myr) M dwarfs with Keck-2/NIRC2 and Subaru/HiCIAO. We have already discovered four young brown dwarf companions with masses between 30-70 Mjup; two of these are members of the ~120 Myr AB Dor moving group, and another one will yield a dynamical mass in the near future. Follow-up optical and near-infrared spectroscopy of these companions reveal spectral types of late-M to early-L and spectroscopic indicators of youth such as angular H-band morphologies, weak J-band alkali lines, and Li absorption and Halpha emission in one target. Altogether our survey is sensitive to planet masses a few times that of Jupiter at separations down to ~10 AU. With a sample size of roughly 80 single M dwarfs, this program represents the deepest and most extensive imaging search for planets around young low-mass stars to date.

Bowler, Brendan P.; Liu, M. C.; Shkolnik, E.; Mann, A.; Tamura, M.

2013-01-01

124

Low-mass Companions to van Maanen 2 and Other Nearby Stars  

NASA Astrophysics Data System (ADS)

Most of the presently known low-mass companions and planets in extrasolar systems have been discovered via a periodic variation of radial velocity of the primary star. The astrometric method, although currently less accurate, is an alternative and independent way to look for brown dwarf and planetary companions. It is based on the reflex stellar motion caused by the orbital motion in the system. The astrometric method may provide important information on the physical size and inclination of the orbit, and, subsequently, a dynamical estimation of the total and secandary masses. It works fine for objects that may be difficult for the radial velocity search, e.g., very hot primary stars or almost face-on orbits. Using the Hipparcos Intermediate Astrometry Data, we are conducting a systematic screening of nearby (distances less than 20-30 pc) stars, paying special attention to astrometric binaries with considerable accelerations or discrepant long-term and short-term proper motions. Several new systems have been discovered with probable brown dwarf or giant planet companions, including van Maanen 2 (GJ 35), HD 219571, GJ 914A (85 Peg A), GJ 533, GJ 9616 and GJ 9387. Preliminary astrometric solutions are obtained, statistical confidence levels are computed, and secondary masses are estimated. The companion to van Maanen 2 has a mass of about 0.08 Msun, and may be the nearest boundary object between the classes of brown dwarfs and super-giant planets. It orbits the nearest cool white dwarf 3.67 Byr of age, at a distance of just 4.4 pc from the Sun. The estimated period is 1.57 yr, and the predicted semi-amplitude of radial velocity is 0.48 km/s. The maximum separation between the primary white dwarf and the secondary substellar object is approximately 0.3 arcsec, which leaves the possibility of direct imaging of the latter with existing facilities.

Makarov, V.

2003-12-01

125

UM 625 Revisited: Multiwavelength Study of a Seyfert 1 Galaxy with a Low-mass Black Hole  

NASA Astrophysics Data System (ADS)

UM 625, previously identified as a narrow-line active galactic nucleus (AGN), actually exhibits broad H? and H? lines whose width and luminosity indicate a low black hole (BH) mass of 1.6 × 106 M ?. We present a detailed multiwavelength study of the nuclear and host galaxy properties of UM 625. Analysis of Chandra and XMM-Newton observations suggests that this system contains a heavily absorbed and intrinsically X-ray weak (?ox = -1.72) nucleus. Although not strong enough to qualify as radio loud, UM 625 does belong to a minority of low-mass AGNs detected in the radio. The broadband spectral energy distribution constrains the bolometric luminosity to L bol ? (0.5-3) × 1043 erg s-1 and L bol/L Edd ? 0.02-0.15. A comprehensive analysis of Sloan Digital Sky Survey and Hubble Space Telescope images shows that UM 625 is a nearly face-on S0 galaxy with a prominent, relatively blue pseudobulge (Sérsic index n = 1.60) that accounts for ~60% of the total light in the R band. The extended disk is featureless, but the central ~150-400 pc contains a conspicuous semi-ring of bright, blue star-forming knots, whose integrated ultraviolet luminosity suggests a star formation rate of ~0.3 M ? yr-1. The mass of the central BH roughly agrees with the value predicted from its bulge velocity dispersion but is significantly lower than that expected from its bulge luminosity.

Jiang, Ning; Ho, Luis C.; Dong, Xiao-Bo; Yang, Huan; Wang, Junxian

2013-06-01

126

Low-Mass Stars in the Sloan Digital Sky Survey: Galactic Structure, Kinematics, and the Luminosity Function  

E-print Network

Modern sky surveys, such as the Sloan Digital Sky Survey and the Two-Micron All Sky Survey, have revolutionized the study of low-mass stars. With millions of photometric and spectroscopic observations, intrinsic stellar properties can be studied with unprecedented statistical significance. Low-mass stars dominate the local Milky Way and are ideal tracers of the Galactic potential and the thin and thick disks. Recent efforts, driven by SDSS observations, have sought to place the local low-mass stellar population in a broader Galactic context. I highlight a recent measurement of the luminosity and mass functions of M dwarfs, using a new technique optimized for large surveys. Starting with SDSS photometry, the field luminosity function and local Galactic structure are measured simultaneously. The sample size used to estimate the LF is nearly three orders of magnitude larger than any previous study, offering a definitive measurement of this quantity. The observed LF is transformed into a mass function and compare...

Bochanski, John J

2010-01-01

127

ATMOSPHERIC CHEMISTRY IN GIANT PLANETS, BROWN DWARFS, AND LOW-MASS DWARF STARS. II. SULFUR AND PHOSPHORUS  

E-print Network

ATMOSPHERIC CHEMISTRY IN GIANT PLANETS, BROWN DWARFS, AND LOW-MASS DWARF STARS. II. SULFUR to model sulfur and phosphorus chemistry in giant planets, brown dwarfs, and extrasolar giant planets (EGPs, brown dwarfs, and extrasolar giant planets (EGPs)--are ideal environments for the formation of molecules

Fegley Jr., Bruce

128

Evolution and Nucleosynthesis in Low-Mass Asymptotic Giant Branch Stars. II. Neutron Capture and the s-Process  

Microsoft Academic Search

We present a new analysis of neutron capture occurring in low-mass asymptotic giant branch (AGB) stars suffering recurrent thermal pulses. We use dedicated evolutionary models for stars of initial mass in the range 1 to 3 M&sun; and metallicity from solar to half solar. Mass loss is taken into account with the Reimers parameterization. The third dredge-up mechanism is self-consistently

Roberto Gallino; Claudio Arlandini; Maurizio Busso; Maria Lugaro; Claudia Travaglio; Oscar Straniero; Alessandro Chieffi; Marco Limongi

1998-01-01

129

THE Na 8200 Angstrom-Sign DOUBLET AS AN AGE INDICATOR IN LOW-MASS STARS  

SciTech Connect

We investigate the use of the gravity sensitive neutral sodium (Na I) doublet at 8183 Angstrom-Sign and 8195 Angstrom-Sign (Na 8200 Angstrom-Sign doublet) as an age indicator for M dwarfs. We measured the Na doublet equivalent width (EW) in giants, old dwarfs, young dwarfs, and candidate members of the {beta} Pic moving group using medium-resolution spectra. Our Na 8200 A doublet EW analysis shows that the feature is useful as an approximate age indicator in M-type dwarfs with (V - K{sub s}) {>=} 5.0, reliably distinguishing stars older and younger than 100 Myr. A simple derivation of the dependence of the Na EW on temperature and gravity supports the observational results. An analysis of the effects of metallicity shows that this youth indicator is best used on samples with similar metallicity. The age estimation technique presented here becomes useful in a mass regime where traditional youth indicators are increasingly less reliable, is applicable to other alkali lines, and will help identify new low-mass members in other young clusters and associations.

Schlieder, Joshua E.; Simon, Michal [Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794 (United States); Lepine, Sebastien; Rice, Emily [Department of Astrophysics, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024 (United States); Fielding, Drummond [Department of Physics and Astronomy, Johns Hopkins University, 366 Bloomberg Center, 3400 North Charles Street, Baltimore, MD 21218 (United States); Tomasino, Rachael, E-mail: michal.simon@stonybrook.edu, E-mail: schlieder@mpia-hd.mpg.de, E-mail: lepine@amnh.org, E-mail: erice@amnh.org, E-mail: dfieldi1@jhu.edu, E-mail: tomas1r@cmich.edu [Department of Physics, Central Michigan University, Mount Pleasant, MI 48859 (United States)

2012-05-15

130

New Low-Mass Stars and Brown Dwarfs with Disks in Lupus  

E-print Network

Using the Infrared Array Camera and the Multiband Imaging Photometer aboard the {\\it Spitzer Space Telescope}, we have obtained images of the Lupus 3 star-forming cloud at 3.6, 4.5, 5.8, 8.0, and 24 \\micron. We present photometry in these bands for the 41 previously known members that are within our images. In addition, we have identified 19 possible new members of the cloud based on red 3.6-8.0 \\micron colors that are indicative of circumstellar disks. We have performed optical spectroscopy on 6 of these candidates, all of which are confirmed as young low-mass members of Lupus 3. The spectral types of these new members range from M4.75 to M8, corresponding to masses of 0.2-0.03 $M_\\odot$ for ages of $\\sim1$ Myr according to theoretical evolutionary models. We also present optical spectroscopy of a candidate disk-bearing object in the vicinity of the Lupus 1 cloud, 2M 1541-3345, which Jayawardhana & Ivanov recently classified as a young brown dwarf ($M\\sim0.03$ $M_\\odot$) with a spectral type of M8. In co...

Allen, P R; Myers, P C; Megeath, S T; Allen, L E; Hartmann, L; Fazio, G G

2007-01-01

131

Statistics of Low-Mass Companions to Stars: Implications for Their Origin  

NASA Technical Reports Server (NTRS)

One of the more significant results from observational astronomy over the past few years has been the detection, primarily via radial velocity studies, of low-mass companions (LMCs) to solar-like stars. The commonly held interpretation of these is that the majority are "extrasolar planets" whereas the rest are brown dwarfs, the distinction made on the basis of apparent discontinuity in the distribution of M sin i for LMCs as revealed by a histogram. We report here results from statistical analysis of M sin i, as well as of the orbital elements data for available LMCs, to rest the assertion that the LMCs population is heterogeneous. The outcome is mixed. Solely on the basis of the distribution of M sin i a heterogeneous model is preferable. Overall, we find that a definitive statement asserting that LMCs population is heterogeneous is, at present, unjustified. In addition we compare statistics of LMCs with a comparable sample of stellar binaries. We find a remarkable statistical similarity between these two populations. This similarity coupled with marked populational dissimilarity between LMCs and acknowledged planets motivates us to suggest a common origin hypothesis for LMCs and stellar binaries as an alternative to the prevailing interpretation. We discuss merits of such a hypothesis and indicate a possible scenario for the formation of LMCs.

Stepinski, T. F.; Black, D. C.

2001-01-01

132

First radius measurements of very low mass stars with the VLTI  

E-print Network

e present 4 very low mass stars radii measured with the VLTI using the 2.2 microns VINCI test instrument. The observations were carried out during the commissioning of the 104-meter-baseline with two 8-meter-telescopes. We measure angular diameters of 0.7-1.5 mas with accuracies of 0.04-0.11 mas, and for spectral type ranging from M0V to M5.5V. We determine an empirical mass-radius relation for M dwarfs based on all available radius measurements. The observed relation agrees well with theoretical models at the present accuracy level, with possible discrepancy around 0.5-0.8 Msolar that needs to be confirmed. In the near future, dozens of M dwarfs radii will be measured with 0.1-1% accuracy, with the VLTI, thanks to the improvements expected from the near infrared instrument AMBER. This will bring strong observational constraints on both atmosphere and interior physics.

Ségransan, D; Forveille, T; Queloz, D

2003-01-01

133

PERIODIC VARIABILITY OF LOW-MASS STARS IN SLOAN DIGITAL SKY SURVEY STRIPE 82  

SciTech Connect

We present a catalog of periodic stellar variability in the 'Stripe 82' region of the Sloan Digital Sky Survey. After aggregating and re-calibrating catalog-level data from the survey, we ran a period-finding algorithm (Supersmoother) on all point-source light curves. We used color selection to identify systems that are likely to contain low-mass stars, in particular M dwarfs and white dwarfs. In total, we found 207 candidates, the vast majority of which appear to be in eclipsing binary systems. The catalog described in this paper includes 42 candidate M dwarf/white dwarf pairs, four white dwarf pairs, 59 systems whose colors indicate they are composed of two M dwarfs and whose light-curve shapes suggest they are in detached eclipsing binaries, and 28 M dwarf systems whose light-curve shapes suggest they are in contact binaries. We find no detached systems with periods longer than 3 days, thus the majority of our sources are likely to have experienced orbital spin-up and enhanced magnetic activity. Indeed, 26 of 27 M dwarf systems that we have spectra for show signs of chromospheric magnetic activity, far higher than the 24% seen in field stars of the same spectral type. We also find binaries composed of stars that bracket the expected boundary between partially and fully convective interiors, which will allow the measurement of the stellar mass-radius relationship across this transition. The majority of our contact systems have short orbital periods, with small variance (0.02 days) in the sample near the observed cutoff of 0.22 days. The accumulation of these stars at short orbital period suggests that the process of angular momentum loss, leading to period evolution, becomes less efficient at short periods. These short-period systems are in a novel regime for studying the effects of orbital spin-up and enhanced magnetic activity, which are thought to be the source of discrepancies between mass-radius predictions and measurements of these properties in eclipsing binaries.

Becker, A. C.; Hawley, S. L.; Ivezic, Z.; Kowalski, A. F.; Sesar, B. [Astronomy Department, University of Washington, Seattle, WA 98195 (United States); Bochanski, J. J. [Astronomy and Astrophysics Department, Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802 (United States); West, A. A. [Department of Astronomy, Boston University, 725 Commonwealth Avenue, Boston, MA 02215 (United States)

2011-04-10

134

Hot Organic Molecules Toward a Young Low-Mass Star: A Look at Inner Disk Chemistry  

E-print Network

Spitzer Space Telescope spectra of the low mass young stellar object (YSO) IRS 46 (L_bol ~ 0.6 L_sun) in Ophiuchus reveal strong vibration-rotation absorption bands of gaseous C2H2, HCN, and CO2. This is the only source out of a sample of ~100 YSO's that shows these features and the first time they are seen in the spectrum of a solar-mass YSO. Analysis of the Spitzer data combined with Keck L- and M-band spectra gives excitation temperatures of > 350 K and abundances of 10(-6)-10(-5) with respect to H2, orders of magnitude higher than those found in cold clouds. In spite of this high abundance, the HCN J=4-3 line is barely detected with the James Clerk Maxwell Telescope, indicating a source diameter less than 13 AU. The (sub)millimeter continuum emission and the absence of scattered light in near-infrared images limits the mass and temperature of any remnant collapse envelope to less than 0.01 M_sun and 100 K, respectively. This excludes a hot-core type region as found in high-mass YSO's. The most plausible origin of this hot gas rich in organic molecules is in the inner (<6 AU radius) region of the disk around IRS 46, either the disk itself or a disk wind. A nearly edge-on 2-D disk model fits the spectral energy distribution (SED) and gives a column of dense warm gas along the line of sight that is consistent with the absorption data. These data illustrate the unique potential of high-resolution infrared spectroscopy to probe organic chemistry, gas temperatures and kinematics in the planet-forming zones close to a young star.

F. Lahuis; E. F. van Dishoeck; A. C. A. Boogert; K. M. Pontoppidan; G. A. Blake; C. P. Dullemond; N. J. Evans II; M. R. Hogerheijde; J. K. Joergensen; J. E. Kessler-Silacci; C. Knez

2005-11-29

135

Forecasting life: a study of activity cycles in low-mass stars: lessons from long-term stellar light curves.  

PubMed

Magnetic activity cycles are indirect traces of magnetic fields and can provide an insight on the nature and action of stellar dynamos and stellar magnetic activity. This, in turn, can determine local space weather and activity effects on stellar habitable zones. Using photometric monitoring of low-mass stars, we study the presence and properties of their magnetic activity cycles. We introduce long-term light curves of our sample stars, and discuss the properties of the observed trends, especially at spectral types where stars are fully convective (later than M3). PMID:22729352

Kafka, Stella

2012-06-01

136

Dynamics and multiplicity of brown dwarfs and young, low mass stars  

NASA Astrophysics Data System (ADS)

This thesis addresses the formation and evolution of very low mass (VLM) stellar objects using their multiplicity and dynamics. First, we surveyed thirteen VLM [Special characters omitted.] objects in the Taurus star-forming region using near-infrared speckle imaging techniques on the W.M. Keck 10 m telescope. Of these thirteen, five were found to be binary. These new systems have properties that differ significantly from older field VLM binaries in that the young systems have wider separations and lower mass ratios, supporting the idea that VLM binaries undergo significant dynamical evolution ~5-10 Myr after their formation. Second, we present a pilot study for our dynamics work in which we completed a five year monitoring campaign of the close binary TWA 5Aab in the TW Hydrae association, using speckle and adaptive optics on the Keck telescopes. Our observations allowed us to determine this system's astrometric orbit. We calculate a total mass of 0.71 ± 0.14 [Special characters omitted.] (D/44 pc) 3 for this system. Finally, we present the results of a 3 year monitoring program of a sample of 26 VLM field binaries using both astrometric and spectroscopic data obtained in conjunction with the laser guide star adaptive optics system on the Keck telescope. By combining astrometry and spectroscopy, degeneracies in orbital parameters can be resolved and individual component masses can be obtained. We present relative orbits for 15 of these systems, which allow us to derive the total system mass. In addition, we find the absolute orbits for 6 systems in our sample, which allows us to derive individual masses. We compare our dynamical mass measurements to the predictions of theoretical evolutionary models and find that there are systematic discrepancies, where both models considered either underpredict or overpredict the correct mass. The discrepancies are a function of spectral type, with late M through mid L systems tending to have their masses underpredicted, while one T type system has its mass overpredicted. These discrepancies imply that either the temperatures predicted by evolutionary and atmosphere models are inconsistent for an object of a given mass, or the mass-radius relationship or cooling timescales predicted by the evolutionary models are incorrect.

Konopacky, Quinn Morgan

137

A Search for Low Mass Stars and Substellar Companions and A Study of Circumbinary Gas and Dust Disks  

NASA Astrophysics Data System (ADS)

We have searched for nearby low-mass stars and brown dwarfs and have studied the planet-forming environment of binary stars. We have carried out a search for young, low-mass stars in nearby stellar associations using X-ray and UV source catalogs. We discovered a new technique to identify 10-100 Myr-old low-mass stars within 100 pc of the Earth using GALEX-optical/near-IR data. We present candidate young stars found by applying this new method in the 10 Myr old TW Hydrae and Scorpius-Centaurus associations. In addition, we have searched for the coolest brown dwarf class: Y-dwarfs, expected to appear at temperatures <500 K. Using wide-field near infrared imaging with ground (CTIO, Palomar, KPNO) and space (Spitzer, AKARI) observatories, we have looked for companions to nearby, old (2 Gyr or older), high proper motion white dwarfs. We present results for Southern Hemisphere white dwarfs. Additionally, we have characterized how likely planet formation occurs in binary star systems. While 20% of planets have been discovered around one member of a binary system, these binaries have semi-major axes larger than 20 AU. We have performed an AO and spectroscopic search for binary stars among a sample of known debris disk stars, which allows us to indirectly study planet formation and evolution in binary systems. As a case study, we examined the gas and dust present in the circumbinary disk around V4046 Sagittarii, a 2.4-day spectroscopic binary. Our results demonstrate it is unlikely that planets can form in binaries with stellar semi-major axes of 10s of AU. This research has been funded by a NASA ADA grant to UCLA and RIT.

Rodriguez, David R.

2011-01-01

138

RV survey for planets of brown dwarfs and very low-mass stars in ChaI  

E-print Network

We have carried out a radial velocity (RV) search for planets and brown dwarf companions to very young (1-10Myr) brown dwarfs and very low-mass stars in the ChaI star forming region. This survey has been carried out with the high-resolution Echelle spectrograph UVES at the VLT. It is sensitive down to Jupiter mass planets. Out of the twelve monitored very low-mass stars and brown dwarfs, ten have constant RVs in the presented RV survey. This hints at a small multiplicity fraction of the studied population of brown dwarfs and very low-mass stars in ChaI at small separations. Upper limits for the mass Msini of possible companions have been estimated to range between 0.1 and 1.5 Jupiter masses. However, two very low-mass stars in ChaI show significant RV variations. The nature of these variations is still unclear. If caused by orbiting objects the recorded variability amplitudes would correspond to planets of the order of a few Jupiter masses. Furthermore, as a by-product of the RV survey for companions, we have studied the kinematics of the brown dwarfs in ChaI. Precise kinematic studies of young brown dwarfs are interesting in the context of the question if brown dwarfs are formed by the recently proposed ejection scenario. We have found that the RV dispersion of brown dwarfs in ChaI is only 2.2 km\\s giving a first empirical upper limit for possible ejection velocities.

Viki Joergens; Ralph Neuhäuser

2003-06-23

139

Herschel/PACS view of disks around low-mass stars and brown dwarfs in the TW Hydrae association  

NASA Astrophysics Data System (ADS)

We conducted Herschel/PACS observations of five very low-mass stars or brown dwarfs located in the TW Hya association with the goal of characterizing the properties of disks in the low stellar mass regime. We detected all five targets at 70 ?m and 100 ?m and three targets at 160 ?m. Our observations, combined with previous photometry from 2MASS, WISE, and SCUBA-2, enabled us to construct spectral energy distributions (SEDs) with extended wavelength coverage. Using sophisticated radiative transfer models, we analyzed the observed SEDs of the five detected objects with a hybrid fitting strategy that combines the model grids and the simulated annealing algorithm and evaluated the constraints on the disk properties via the Bayesian inference method. The modeling suggests that disks around low-mass stars and brown dwarfs are generally flatter than their higher mass counterparts, but the range of disk mass extends to well below the value found in T Tauri stars, and the disk scale heights are comparable in both groups. The inferred disk properties (i.e., disk mass, flaring, and scale height) in the low stellar mass regime are consistent with previous findings from large samples of brown dwarfs and very low-mass stars. We discuss the dependence of disk properties on their host stellar parameters and find a significant correlation between the Herschel far-IR fluxes and the stellar effective temperatures, probably indicating that the scaling between the stellar and disk masses (i.e., Mdisk ? M?) observed mainly in low-mass stars may extend down to the brown dwarf regime. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendix A is available in electronic form at http://www.aanda.org

Liu, Yao; Herczeg, Gregory J.; Gong, Munan; Allers, Katelyn N.; Brown, Joanna M.; Kraus, Adam L.; Liu, Michael C.; Shkolnik, Evgenya L.; van Dishoeck, Ewine F.

2015-01-01

140

Planets around Low-mass Stars (PALMS). IV. The Outer Architecture of M Dwarf Planetary Systems  

NASA Astrophysics Data System (ADS)

We present results from a high-contrast adaptive optics imaging search for giant planets and brown dwarfs (gsim1 M Jup) around 122 newly identified nearby (lsim40 pc) young M dwarfs. Half of our targets are younger than 135 Myr and 90% are younger than the Hyades (620 Myr). After removing 44 close stellar binaries (implying a stellar companion fraction of >35.4% ± 4.3% within 100 AU), 27 of which are new or spatially resolved for the first time, our remaining sample of 78 single M dwarfs makes this the largest imaging search for planets around young low-mass stars (0.1-0.6 M ?) to date. Our H- and K-band coronagraphic observations with Keck/NIRC2 and Subaru/HiCIAO achieve typical contrasts of 12-14 mag and 9-13 mag at 1'', respectively, which correspond to limiting planet masses of 0.5-10 M Jup at 5-33 AU for 85% of our sample. We discovered four young brown dwarf companions: 1RXS J235133.3+312720 B (32 ± 6 M Jup; L0+2-1; 120 ± 20 AU), GJ 3629 B (64+30-23 M Jup; M7.5 ± 0.5; 6.5 ± 0.5 AU), 1RXS J034231.8+121622 B (35 ± 8 M Jup; L0 ± 1; 19.8 ± 0.9 AU), and 2MASS J15594729+4403595 B (43 ± 9 M Jup; M8.0 ± 0.5; 190 ± 20 AU). Over 150 candidate planets were identified; we obtained follow-up imaging for 56% of these but all are consistent with background stars. Our null detection of planets enables strong statistical constraints on the occurrence rate of long-period giant planets around single M dwarfs. We infer an upper limit (at the 95% confidence level) of 10.3% and 16.0% for 1-13 M Jup planets between 10-100 AU for hot-start and cold-start (Fortney) evolutionary models, respectively. Fewer than 6.0% (9.9%) of M dwarfs harbor massive gas giants in the 5-13 M Jup range like those orbiting HR 8799 and ? Pictoris between 10-100 AU for a hot-start (cold-start) formation scenario. The frequency of brown dwarf (13-75 M Jup) companions to single M dwarfs between 10-100 AU is 2.8+2.4-1.5%. Altogether we find that giant planets, especially massive ones, are rare in the outskirts of M dwarf planetary systems. Although the first directly imaged planets were found around massive stars, there is currently no statistical evidence for a trend of giant planet frequency with stellar host mass at large separations as predicted by the disk instability model of giant planet formation. Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. This work was also based on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.

Bowler, Brendan P.; Liu, Michael C.; Shkolnik, Evgenya L.; Tamura, Motohide

2015-01-01

141

Evolutionary models for low-mass stars and brown dwarfs: Uncertainties and limits at very young ages  

Microsoft Academic Search

We analyse pre-Main Sequence evolutionary tracks for low mass stars with masses m <= 1.4 Msun based on the Baraffe et al. (\\\\cite{Bar98}) input physics. We also extend the recent Chabrier et al. (2000) evolutionary models based on dusty atmosphere to young brown dwarfs down to one mass of Jupiter. We analyse current theoretical uncertainties due to molecular line lists,

I. Baraffe; G. Chabrier; F. Allard; P. H. Hauschildt

2002-01-01

142

The T Tauri Phase Down to Nearly Planetary Masses: Echelle Spectra of 82 Very Low Mass Stars and Brown Dwarfs  

Microsoft Academic Search

Using the largest high-resolution spectroscopic sample to date of young, very low mass stars and brown dwarfs, we investigate disk accretion in objects ranging from just above the hydrogen-burning limit all the way to nearly planetary masses. Our 82 targets span spectral types from M5 to M9.5, or masses from 0.15 Msolar down to about 15 jupiters. They are confirmed

Subhanjoy Mohanty; Ray Jayawardhana; Gibor Basri

2005-01-01

143

A survey for low mass spectroscopic binary stars in the young clusters around sigma Orionis and lambda Orionis  

E-print Network

We have obtained multi-epoch, high-resolution spectroscopy of 218 candidate low-mass stars and brown dwarfs in the young clusters around sigma Ori and lambda Ori. We find that 196 targets are cluster members based on their radial velocity, the equivalent width of their NaI 8200 lines and the spectral type from their TiO band strength. We have identified 11 new binary stars among the cluster members based on their variable radial velocity and an additional binary from the variation in its line width and shape. The sample covers the magnitude range Ic=14-18.9 (mass =~ 0.55-0.03 Msun), but all of the binary stars are brighter than Ic=16.6 (mass =~ 0.12Msun) and 10 are brighter than Ic=15.5 (mass =~ 0.23Msun). There is a significant lack of spectroscopic binaries in our sample at faint magnitudes even when we account for the decrease in sensitivity with increasing magnitude. We can reject the hypothesis that the fraction of spectroscopic binaries is a uniform function of Ic magnitude with more than 99% confidence. The spectroscopic binary fraction for stars more massive than about 0.1Msun (Ic spectroscopic binary fraction for very low mass (VLM) stars (mass spectroscopic binaries among young VLM stars and brown dwarfs when compared to more massive stars in the same star-forming region. This implies a difference in the total binary fraction between VLM stars and BDs compared to more massive stars or a difference in the distribution of semi-major axes, or both. (Abridged)

P. F. L. Maxted; R. D. Jeffries; J. M. Oliveira; T. Naylor; R. J. Jackson

2008-01-23

144

Can the Growth of Dust Grains in Low-Metallicity Star-Forming Clouds Affect the Formation of Metal-Poor Low-Mass Stars?  

E-print Network

The discovery of a low-mass star with such low metallicity as ~ 1 um) or the sticking probability is too small (<~ 0.01). We find that even if the initial dust-to-gas mass ratio is well below the minimum value required for the dust-induced fragmentation, the grain growth increases the dust mass high enough to cause the gas fragmentation into sub-solar mass clumps. We suggest that as long as the critical metal abundance is satisfied, the grain growth could play an important role in the formation of low-mass stars with metallicity as low as 10^{-5} Z_sun.

Nozawa, Takaya; Nomoto, Ken'ichi

2012-01-01

145

Evolutionary and pulsational properties of low-mass white dwarf stars with oxygen cores resulting from close binary evolution  

E-print Network

The present work is designed to explore the evolutionary and pulsational properties of low-mass white dwarfs with carbon/oxygen cores. In particular, we follow the evolution of a 0.33 Msun white dwarf remnant in a self-consistent way with the predictions of nuclear burning, element diffusion and the history of the white dwarf progenitor. Attention is focused on the occurrence of hydrogen shell flashes induced by diffusion processes during cooling phases. The evolutionary stages prior to the white dwarf formation are also fully accounted for by computing the conservative binary evolution of an initially 2.5-Msun Pop. I star with a 1.25 Msun companion, and period P_i= 3 days. Evolution is followed down to the domain of the ZZ Ceti stars on the white dwarf cooling branch. We find that chemical diffusion induces the occurrence of an additional hydrogen thermonuclear flash which leads to stellar models with thin hydrogen envelopes. As a result, a fast cooling is encountered at advanced stages of evolution. In addition, we explore the adiabatic pulsational properties of the resulting white dwarf models. As compared with their helium-core counterparts, low-mass oxygen-core white dwarfs are characterized by a pulsational spectrum much more featured, an aspect which could eventually be used for distinguishing both types of stars if low-mass white dwarfs were in fact found to pulsate as ZZ Ceti-type variables. Finally, we perform a non-adiabatic pulsational analysis on the resulting carbon/oxygen low-mass white dwarf models.

L. G. Althaus; A. H. Corsico; A. Gautschy; Z. Han; A. M. Serenelli; J. A. Panei

2003-09-09

146

Rotational mixing in low-mass stars II. Self-consistent models of Pop II RGB stars  

E-print Network

In this paper we study the effects of rotation in low-mass, low-metallicity RGB stars. We present the first evolutionary models taking into account self-consistently the latest prescriptions for the transport of angular momentum by meridional circulation and shear turbulence in stellar interiors as well as the associated mixing processes for chemicals computed from the ZAMS to the upper RGB. We discuss in details the uncertainties associated with the physical description of the rotational mixing and study carefully their effects on the rotation profile, diffusion coefficients, structural evolution, lifetimes and chemical signatures at the stellar surface. We focus in particular on the various assumptions concerning the rotation law in the convective envelope, the initial rotation velocity distribution, the presence of mean molecular weight gradients and the treatment of the horizontal and vertical turbulence. This exploration leads to two main conclusions : (1) After the completion of the first dredge-up, the degree of differential rotation (and hence mixing) is maximised in the case of a differentially rotating convective envelope (i.e., j_CE(r) = cst), as anticipated in previous studies. (2) Even with this assumption, and contrary to some previous claims, the present treatment for the evolution of the rotation profile and associated meridional circulation and shear turbulence does not lead to enough mixing of chemicals to explain the abundance anomalies in low-metallicity field and globular cluster RGB stars observed around the bump luminosity. This study raises questions that need to be addressed in a near future. These include for example the interaction between rotation and convection and the trigger of additional hydrodynamical instabilities.

A. Palacios; C. Charbonnel; S. Talon; L. Siess

2006-02-17

147

Metallicities of Low Mass Inefficient Star Forming Dwarfs in S4G: Testing the Closed Box Paradigm  

NASA Astrophysics Data System (ADS)

Low mass dwarf galaxies are the most numerous extragalactic population in the Local Universe. Many gas-rich dwarfs appear to be forming stars less efficiently than normal, massive disk galaxies and are therefore important laboratories for the study of star formation. Here we present new observations using the Palomar Double Spectrograph for 19 dwarf galaxies from the S4G Survey with the lowest stellar to HI mass ratios. Preliminary analysis of the data indicate a wide range of metallicities which vary by as much as 0.5 dex in a single galaxy in different star forming regions. Such a dispersion in metallicities favors an open box model and the results suggest a varied star formation history, possibly induced via minor mergers and accretion.

McKay, Myles; Stierwalt, Sabrina; Sheth, Kartik; de Swardt, Bonita, , Dr.; Walter, Donald K.

2015-01-01

148

Photometric monitoring of open clusters: Low-mass eclipsing binary stars and the stellar mass-luminosity-radius relation  

NASA Astrophysics Data System (ADS)

This thesis describes a photometric monitoring survey of Galactic star clusters designed to detect low-mass eclipsing binary star systems through variations in their relative lightcurves. The aim is to use cluster eclipsing binaries to measure the masses and radii of M-dwarf stars with ages and metallicities known from studies of brighter cluster stars. This information will provide an improved calibration of the mass-luminosity-radius relation for low-mass stars, be used to test stellar structure and evolution models, and help quantify the contribution of low-mass stars to the global mass census in the Galaxy. The survey is designed to detect eclipse events in stars of ~0.3 M_sun and consists of 600 Gbytes of raw imaging data on six open clusters with a range of ages (~ 0.15 - 4 Gyr) and metallicites (~ -0.2 - 0.0 dex). The clusters NGC 1647 and M 35 contain excellent candidate systems showing eclipse like variations in brightness and photometry consistent with cluster membership. The analysis of these clusters and the eclipsing M-dwarf stars detected in them are presented. Analysis of the candidate system in NGC 1647 confirms the object as a newly discovered M-dwarf eclipsing binary in the cluster with compenent masses of M 1 = 0.47 ± 0.05[Special characters omitted.] and M 2 = 0.19 ± 0.02[Special characters omitted.] . The small mass ratio ( M 2 / M 1 ) and low secondary mass of this object provide an unprecedented opportunity to test stellar models. We find that no stellar evolution models are consistent with all the properties of both M-dwarf stars in the eclipsing binary. The candidate in M 35 has been confirmed as an M-dwarf eclipsing binary, and the masses of the individual components are estimated to be M 1 ~ 0.25 M_sun and M 2 ~ 0.15 M_sun . Additional high resolution spectroscopic and photometric observations, for which we have applied and been awarded time, are necessary to accurately derive the intrinsic properties of the individual stellar components. The completion of our analysis will yield values for the intrinsic properties of four M-dwarf stars of known age and metallicity with masses of ~ 0.5 - 0.1 M_sun.

Hebb, Leslie

2006-06-01

149

MINERVA-Red: A Census of Planets Orbiting the Nearest Low-mass Stars to the Sun  

NASA Astrophysics Data System (ADS)

Recent results from Kepler and ground-based exoplanet surveys suggest that low-mass stars host numerous small planets. Since low-mass stars are intrinsically faint at optical wavelengths, obtaining the Doppler precision necessary to detect these companions remains a challenge for existing instruments. We describe MINERVA-Red, a project to use a dedicated, robotic, near-infrared optimized 0.7 meter telescope and a specialized Doppler spectrometer to carry out an intensive, multi-year campaign designed to reveal the planetary systems orbiting some of the closest stars to the Sun. The MINERVA-Red cross-dispersed echelle spectrograph is optimized for the 'deep red', between 800 nm and 900 nm, where these stars are relatively bright. The instrument is very compact and designed for the ultimate in Doppler precision by using single-mode fiber input. We describe the spectrometer and the status of the MINERVA-Red project, which is expected to begin routine operations at Whipple Observatory on Mt Hopkins, Arizona, in 2015.

Blake, Cullen; Johnson, John; Plavchan, Peter; Sliski, David; Wittenmyer, Robert A.; Eastman, Jason D.; Barnes, Stuart

2015-01-01

150

DISCOVERY OF A LOW-MASS COMPANION TO A METAL-RICH F STAR WITH THE MARVELS PILOT PROJECT  

SciTech Connect

We report the discovery of a low-mass companion orbiting the metal-rich, main sequence F star TYC 2949-00557-1 during the Multi-object APO Radial Velocity Exoplanet Large-area Survey (MARVELS) pilot project. The host star has an effective temperature T{sub eff} = 6135 {+-} 40 K, logg = 4.4 {+-} 0.1, and [Fe/H] = 0.32 {+-} 0.01, indicating a mass of M = 1.25 {+-} 0.09 M{sub sun} and R = 1.15 {+-} 0.15 R{sub sun}. The companion has an orbital period of 5.69449 {+-} 0.00023 days and straddles the hydrogen burning limit with a minimum mass of 64 M{sub J} , and thus may be an example of the rare class of brown dwarfs orbiting at distances comparable to those of 'Hot Jupiters'. We present relative photometry that demonstrates that the host star is photometrically stable at the few millimagnitude level on time scales of hours to years, and rules out transits for a companion of radius {approx}>0.8 R{sub J} at the 95% confidence level. Tidal analysis of the system suggests that the star and companion are likely in a double synchronous state where both rotational and orbital synchronization have been achieved. This is the first low-mass companion detected with a multi-object, dispersed, fixed-delay interferometer.

Fleming, Scott W.; Ge Jian; Mahadevan, Suvrath; Lee, Brian; Cuong Nguyen, Duy; Morehead, Robert C.; Wan Xiaoke; Zhao Bo; Liu Jian; Guo Pengcheng; Kane, Stephen R. [Department of Astronomy, University of Florida, 211 Bryant Space Science Center, Gainesville, FL 326711-2055 (United States); Eastman, Jason D.; Siverd, Robert J.; Scott Gaudi, B. [Department of Astronomy, Ohio State University, 140 West 18th Avenue, Columbus, OH 43210 (United States); Niedzielski, Andrzej [Torun Center for Astronomy, Nicolaus Copernicus University, ul. Gagarina 11, 87-100, Torun (Poland); Sivarani, Thirupathi [Indian Institute of Astrophysics, Bangalore 560034 (India); Stassun, Keivan G.; Gary, Bruce [Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235 (United States); Wolszczan, Alex [Department of Astronomy and Astrophysics, The Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802 (United States); Barnes, Rory, E-mail: scfleming@astro.ufl.ed [Department of Astronomy, University of Washington, P.O. Box 351580, Seattle, WA 98195 (United States)

2010-08-01

151

Radial velocities of very low mass stars and candidate brown dwarf members of the Hyades and Pleiades, 2  

NASA Technical Reports Server (NTRS)

We have determined H alpha equivalent widths and radial velocities with 1 sigma accuracies of approximately 5 km/s for approximately 20 candidate very low mass members of the Pleiades cluster and for a few proposed very low mass members of the Hyades. Most of the Pleiades targets were selected from the recent Hambly, Hawkins, and Jameson proper motion survey, where they were identified as probable Pleiades brown dwarfs with an age spread from 3 to 70 Myr. Our spectroscopic data and a reinterpretation of the photometric data confirm that these objects are indeed likely Pleiades members; however, we believe that they more likely have masses slightly above the hydrogen burning mass limit and that there is no firm evidence for an age spread amongst these stars. All of the very low mass Pleiades and Hyades members show H alpha in emission. However, the ratio of H alpha flux to biometric flux in the Pleiades shows a maximum near M(sub Bol) approximately equal to 9.5 (M approximately equal to 0.3 solar mass) and a sharp decrease to lower masses. This break occurs at the approximate mass where low mass stars are expected to become fully convective, and it is tempting to assume that the decrease in H alpha flux is caused by some change in the behavior of stellar dynamos at this mass. We do not see a similar break in activity at this mass in the Hyades. We discuss possible evolutionary explanations for this difference in the H alpha activity between the two clusters.

Stauffer, John R.; Liebert, James; Giampapa, Mark

1995-01-01

152

Collaborating with "professional" amateurs: low-mass stars in fragile multiple system  

E-print Network

The boundary between professional and amateur astronomers gets narrower and narrower. We present several real examples, most of them published in refereed journals, of works resulting from fruitful collaborations between key amateur astronomers in Spain and professional colleagues. The common denominator of these works is the search for binaries, mostly nearby, wide, common proper-motion pairs with low-mass stellar components, including some of the most fragile systems ever found.

Caballero, J A; Tobal, T; Miret, F X; Rica, F M; Cairol, J; Miret, N; Novalbos, I; Montes, D; Klutsch, A

2012-01-01

153

Measurement of neutron star parameters: a review of methods for low-mass X-ray binaries  

E-print Network

Measurement of at least three independent parameters, for example, mass, radius and spin frequency, of a neutron star is probably the only way to understand the nature of its supranuclear core matter. Such a measurement is extremely difficult because of various systematic uncertainties. The lack of knowledge of several system parameter values gives rise to such systematics. Low-mass X-ray binaries, which contain neutron stars, provide a number of methods to constrain the stellar parameters. Joint application of these methods has a great potential to significantly reduce the systematic uncertainties, and hence to measure three independent neutron star parameters accurately. Here we review the methods based on (1) thermonuclear X-ray bursts; (2) accretion-powered millisecond-period pulsations; (3) kilohertz quasi-periodic oscillations; (4) broad relativistic iron lines; (5) quiescent emissions; and (6) binary orbital motions.

Sudip Bhattacharyya

2010-01-11

154

SPITZER OBSERVATIONS OF THE {lambda} ORIONIS CLUSTER. II. DISKS AROUND SOLAR-TYPE AND LOW-MASS STARS  

SciTech Connect

We present IRAC/MIPS Spitzer Space Telescope observations of the solar-type and the low-mass stellar population of the young ({approx}5 Myr) {lambda} Orionis cluster. Combining optical and Two Micron All Sky Survey photometry, we identify 436 stars as probable members of the cluster. Given the distance (450 pc) and the age of the cluster, our sample ranges in mass from 2 M{sub sun} to objects below the substellar limit. With the addition of the Spitzer mid-infrared data, we have identified 49 stars bearing disks in the stellar cluster. Using spectral energy distribution slopes, we place objects in several classes: non-excess stars (diskless), stars with optically thick disks, stars with 'evolved disks' (with smaller excesses than optically thick disk systems), and 'transitional disk' candidates (in which the inner disk is partially or fully cleared). The disk fraction depends on the stellar mass, ranging from {approx}6% for K-type stars (R{sub C} - J < 2) to {approx}27% for stars with spectral-type M5 or later (R{sub C} - J>4). We confirm the dependence of disk fraction on stellar mass in this age range found in other studies. Regarding clustering levels, the overall fraction of disks in the {lambda} Orionis cluster is similar to those reported in other stellar groups with ages normally quoted as {approx}5 Myr.

Hernandez, Jesus [Centro de Investigaciones de AstronomIa, Apdo. Postal 264, Merida 5101-A (Venezuela, Bolivarian Republic of); Morales-Calderon, Maria [Laboratorio de Astrofisica Estelar y Exoplanetas (LAEX), Centro de Astrobiologia (CAB, INTA-CSIC), LAEFF, P.O. Box 78, E-28691, Villanueva de la canada, Madrid (Spain); Calvet, Nuria; Hartmann, L. [Department of Astronomy, University of Michigan, 830 Dennison Building, 500 Church Street, Ann Arbor, MI 48109 (United States); Muzerolle, J. [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States); Gutermuth, R. [Five Colleges Astronomy Department, Smith College, Northampton, MA 01027 (United States); Luhman, K. L. [Center for Exoplanets and Habitable Worlds, The Pennsylvania State University, University Park, PA 16802 (United States); Stauffer, J., E-mail: hernandj@cida.v, E-mail: muzerol@stsci.ed [Spitzer Science Center, Caltech M/S 220-6, 1200 East California Boulevard, Pasadena, CA 91125 (United States)

2010-10-20

155

Spitzer Observations of the ? Orionis Cluster. II. Disks Around Solar-type and Low-mass Stars  

NASA Astrophysics Data System (ADS)

We present IRAC/MIPS Spitzer Space Telescope observations of the solar-type and the low-mass stellar population of the young (~5 Myr) ? Orionis cluster. Combining optical and Two Micron All Sky Survey photometry, we identify 436 stars as probable members of the cluster. Given the distance (450 pc) and the age of the cluster, our sample ranges in mass from 2 M sun to objects below the substellar limit. With the addition of the Spitzer mid-infrared data, we have identified 49 stars bearing disks in the stellar cluster. Using spectral energy distribution slopes, we place objects in several classes: non-excess stars (diskless), stars with optically thick disks, stars with "evolved disks" (with smaller excesses than optically thick disk systems), and "transitional disk" candidates (in which the inner disk is partially or fully cleared). The disk fraction depends on the stellar mass, ranging from ~6% for K-type stars (RC - J < 2) to ~27% for stars with spectral-type M5 or later (RC - J>4). We confirm the dependence of disk fraction on stellar mass in this age range found in other studies. Regarding clustering levels, the overall fraction of disks in the ? Orionis cluster is similar to those reported in other stellar groups with ages normally quoted as ~5 Myr.

Hernández, Jesús; Morales-Calderon, Maria; Calvet, Nuria; Hartmann, L.; Muzerolle, J.; Gutermuth, R.; Luhman, K. L.; Stauffer, J.

2010-10-01

156

Observations of suspected low-mass post-T Tauri stars and their evolutionary status  

NASA Technical Reports Server (NTRS)

The results of a study of five X-ray discovered weak emission pre-main-sequence stars in the Taurus-Auriga star formation complex are presented. All are of spectral type K7-M0, and about 1-2 mag above the main sequence. One is a double-lined spectroscopic binary, the first spectroscopic binary PMS star to be confirmed. The ages, masses, and radii of these stars as determined by photometry and spectroscopy are discussed. The difference in emission strength between these and the T Tauri stars is investigated, and it is concluded that these 'post-T Tauri' stars do indeed appear more evolved than the T Tauri stars, although there is no evidence of any significant difference in ages.

Mundt, R.; Walter, F. M.; Feigelson, E. D.; Finkenzeller, U.; Herbig, G. H.; Odell, A. P.

1983-01-01

157

Herschel/PACS view of disks around low-mass stars and brown dwarfs in the TW Hya association  

E-print Network

We conducted Herschel/PACS observations of five very low-mass stars or brown dwarfs located in the TW Hya association with the goal of characterizing the properties of disks in the low stellar mass regime. We detected all five targets at $70\\,\\mu{\\rm{m}}$ and $100\\,\\mu{\\rm{m}}$ and three targets at $160\\,\\mu{\\rm{m}}$. Our observations, combined with previous photometry from 2MASS, WISE, and SCUBA-2, enabled us to construct SEDs with extended wavelength coverage. Using sophisticated radiative transfer models, we analyzed the observed SEDs of the five detected objects with a hybrid fitting strategy that combines the model grids and the simulated annealing algorithm and evaluated the constraints on the disk properties via the Bayesian inference method. The modelling suggests that disks around low-mass stars and brown dwarfs are generally flatter than their higher mass counterparts, but the range of disk mass extends to well below the value found in T Tauri stars, and the disk scale heights are comparable in both...

Liu, Yao; Gong, Munan; Allers, Katelyn N; Brown, Joanna M; Kraus, Adam L; Liu, Michael C; Shkolnik, Evgenya L; van Dishoeck, Ewine F

2014-01-01

158

Supernova dust formation and the grain growth in the early universe: The critical metallicity for low-mass star formation  

E-print Network

We investigate the condition for the formation of low-mass second-generation stars in the early universe. It has been proposed that gas cooling by dust thermal emission can trigger fragmentation of a low-metallicity star-forming gas cloud. In order to determine the critical condition in which dust cooling induces the formation of low-mass stars, we follow the thermal evolution of a collapsing cloud by a one-zone semi-analytic collapse model. Earlier studies assume the dust amount in the local universe, where all refractory elements are depleted onto grains, and/or assume the constant dust amount during gas collapse. In this paper, we employ the models of dust formation and destruction in early supernovae to derive the realistic dust compositions and size distributions for multiple species as the initial conditions of our collapse calculations. We also follow accretion of heavy elements in the gas phase onto dust grains, i.e., grain growth, during gas contraction. We find that grain growth well alters the frag...

Chiaki, Gen; Nozawa, Takaya; Yoshida, Naoki; Schneider, Raffaella; Omukai, Kazuyuki; Limongi, Marco; Chieffi, Alessandro

2014-01-01

159

From Stars to Super-Planets: The Low-Mass IMF in the Young Cluster IC348  

NASA Technical Reports Server (NTRS)

We investigate the low-mass population of the young cluster IC348 down to the deuterium-burning limit, a fiducial boundary between brown dwarf and planetary mass objects, using a new and innovative method for the spectral classification of late-type objects. Using photometric indices, constructed from HST/NICMOS narrow-band imaging, that measure the strength of the 1.9 micron water band, we determine the spectral type and reddening for every M-type star in the field, thereby separating cluster members from the interloper population. Due to the efficiency of our spectral classification technique, our study is complete from approximately 0.7 solar mass to 0.015 solar mass. The mass function derived for the cluster in this interval, dN/d log M alpha M(sup 0.5), is similar to that obtained for the Pleiades, but appears significantly more abundant in brown dwarfs than the mass function for companions to nearby sun-like stars. This provides compelling observational evidence for different formation and evolutionary histories for substellar objects formed in isolation vs. as companions. Because our determination of the IMF is complete to very low masses, we can place interesting constraints on the role of physical processes such as fragmentation in the star and planet formation process and the fraction of dark matter in the Galactic halo that resides in substellar objects.

Najita, Joan R.; Tiede, Glenn P.; Carr, John S.

2000-01-01

160

Testing Stellar Evolution Models: Absolute Dimensions of the Low-Mass Eclipsing Binary Star V651 Cassiopeiae  

NASA Astrophysics Data System (ADS)

We report accurate values of several key quantities for the low-mass, 0.9968096 day period, double-lined eclipsing binary V651 Cas. We determine accurate values for the masses, radii and temperatures of the primary and secondary as follows: M = 0.8553(81) solar masses, R = 0.957(17) solar radii, and effective temperature = 5733(100) K for the primary component, and M = 0.7564(48) solar masses, R = 0.771(15) solar radii, and effective temperature = 5113(105) for the secondary component, with formal uncertainties shown in parentheses. A comparison with the stellar evolution models from the Dartmouth Stellar Evolution Program suggests an age of 11(1) Gyr for a best-fit metallicity of [Fe/H] = -0.2. While the isochrone mentioned correctly reproduces the measured radii and temperatures of the stars within the current uncertainties, we note that the secondary radius appears marginally larger and the temperature marginally cooler than models would predict. This is consistent with similar discrepancies found for other low-mass stars, generally accredited to surface activity. With further improvement in the measurement errors, and a spectroscopic measure of the metallicity, V651 Cas should be a valuable system for understanding the effects of magnetic activity on the global structure of low-mass stars and for providing guidance to improve stellar evolution models.This work was supported in part by the NSF REU and DoD ASSURE programs under NSF grant no. 1262851 and by the Smithsonian Institution.

Matthews, Allison; Torres, Guillermo

2015-01-01

161

VizieR Online Data Catalog: I photometry of very low mass ONC stars (Rodriguez-Ledesma+, 2009)  

NASA Astrophysics Data System (ADS)

Table 1 lists the 2908 very low mass objects in the Orion Nebula Cluster with magnitudes between 14-21mag in I band for which accurate photometry was performed. It contains the position, brightness and type of variability of these stars, as well as 2MASS counterparts when available. Table 2 lists the 487 objects found to be periodic in this study, for which their brightness, period and peak-to-peak amplitudes a re given, as well as periods measured in Herbst et al. 2002 (Cat. J/A+A/396/513) when available. (2 data files).

Rodriguez-Ledesma, M. V.; Mundt, R.; Eisloeffel, J.

2009-06-01

162

Mon. Not. R. Astron. Soc. 356, 359370 (2005) doi:10.1111/j.1365-2966.2004.08437.x 1E 1207.4-5209: a low-mass bare strange star?  

E-print Network

.4-5209: a low-mass bare strange star? R. X. Xu School of Physics, Peking University, Beijing 100871, China rotation- and accretion-powered low-mass bare strange stars are studied, with particular regard to their astrophysical appearance. It is suggested that low-mass bare strange stars, with weaker ferromagnetic fields

Xu, Ren-Xin

163

Young Low-Mass Stars and Brown Dwarfs in IC 348  

Microsoft Academic Search

I present new results from a continuing program to identify and characterize\\u000athe low-mass stellar and substellar populations in the young cluster IC 348\\u000a(1-10~Myr). Optical spectroscopy has revealed young objects with spectral types\\u000aas late as M8.25. The intrinsic J-H and H-K colors of these sources are\\u000adwarf-like, whereas the R-I and I-J colors appear intermediate between the\\u000acolors

K. L. Luhman

1999-01-01

164

HABITABLE ZONES AROUND LOW MASS STARS AND THE SEARCH FOR EXTRATERRESTRIAL LIFE  

Microsoft Academic Search

. Habitable planets are likely to exist around stars not too different from the Sun ifcurrent theories about terrestrial climate evolution are correct. Some of these planets may haveevolved life, and some of the inhabited planets may have evolved O 2 -rich atmospheres. Suchatmospheres could be detected spectroscopically on planets around nearby stars using a spacebasedinterferometer to search for the

James F. Kasting

1997-01-01

165

PHYSICAL PROPERTIES OF YOUNG BROWN DWARFS AND VERY LOW MASS STARS INFERRED FROM HIGH-RESOLUTION MODEL SPECTRA  

SciTech Connect

By comparing near-infrared spectra with atmospheric models, we infer the effective temperature, surface gravity, projected rotational velocity, and radial velocity for 21 very low mass stars and brown dwarfs. The unique sample consists of two sequences in spectral type from M6-M9, one of 5-10 Myr objects and one of >1 Gyr field objects. A third sequence is comprised of only {approx}M6 objects with ages ranging from <1 Myr to >1 Gyr. Spectra were obtained in the J band at medium (R {approx} 2000) and high (R {approx} 20,000) resolutions with NIRSPEC on the Keck II telescope. Synthetic spectra were generated from atmospheric structures calculated with the PHOENIX model atmosphere code. Using multi-dimensional least-squares fitting and Monte Carlo routines we determine the best-fit model parameters for each observed spectrum and note which spectral regions provide consistent results. We identify successes in the reproduction of observed features by atmospheric models, including pressure-broadened K I lines, and investigate deficiencies in the models, particularly missing FeH opacity, that will need to be addressed in order to extend our analysis to cooler objects. The precision that can be obtained for each parameter using medium- and high-resolution near-infrared spectra is estimated and the implications for future studies of very low mass stars and brown dwarfs are discussed.

Rice, Emily L.; Mclean, Ian S. [Department of Physics and Astronomy, UCLA, Los Angeles, CA 90095-1562 (United States); Barman, T.; Prato, L. [Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001 (United States); Kirkpatrick, J. Davy [Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125 (United States)], E-mail: erice@amnh.org

2010-01-01

166

A LIKELY CLOSE-IN LOW-MASS STELLAR COMPANION TO THE TRANSITIONAL DISK STAR HD 142527  

SciTech Connect

With the uniquely high contrast within 0.''1 ({Delta}mag(L') = 5-6.5 mag) available using Sparse Aperture Masking with NACO at Very Large Telescope, we detected asymmetry in the flux from the Herbig Fe star HD 142527 with a barycenter emission situated at a projected separation of 88 {+-} 5 mas (12.8 {+-} 1.5 AU at 145 pc) and flux ratios in H, K, and L' of 0.016 {+-} 0.007, 0.012 {+-} 0.008, and 0.0086 {+-} 0.0011, respectively (3{sigma} errors), relative to the primary star and disk. After extensive closure-phase modeling, we interpret this detection as a close-in, low-mass stellar companion with an estimated mass of {approx}0.1-0.4 M{sub Sun }. HD 142527 has a complex disk structure, with an inner gap imaged in both the near and mid-IR as well as a spiral feature in the outer disk in the near-IR. This newly detected low-mass stellar companion may provide a critical explanation of the observed disk structure.

Biller, Beth; Benisty, Myriam; Chauvin, Gael; Olofsson, Johan; Pott, Joerg-Uwe; Mueller, Andre; Bonnefoy, Mickaeel; Henning, Thomas [Max-Planck-Institut fuer Astronomie, Koenigstuhl 17, 69117 Heidelberg (Germany); Lacour, Sylvestre; Thebault, Philippe [LESIA, CNRS/UMR-8109, Observatoire de Paris, UPMC, Universite Paris Diderot, 5 place Jules Janssen, 92195 Meudon (France); Juhasz, Attila [Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden (Netherlands); Sicilia-Aguilar, Aurora [Departamento de Fisica Teorica, Facultad de Ciencias, Universidad Autonoma de Madrid, 28049 Cantoblanco, Madrid (Spain); Tuthill, Peter [School of Physics, University of Sydney, NSW 2006 (Australia); Crida, Aurelien, E-mail: biller@mpia.de [Universite de Nice - Sophia antipolis/C.N.R.S./Observatoire de la Cote d'Azur, Laboratoire Lagrange (UMR 7293), Boulevard de l'Observatoire, B.P. 4229 06304 NICE cedex 04 (France)

2012-07-10

167

Measuring the X-ray Emission Impacting the Planets Orbiting Nearby Low-mass Stars  

NASA Astrophysics Data System (ADS)

M dwarf planetary systems present a truly exciting opportunity to discover and study the first habitable extrasolar planets in the next 5-10 years. As part of our larger HST MUSCLES project, we propose ACIS-S observations of 4 low-mass exoplanet hosts (3 M dwarfs -- GJ581, GJ1214, GJ849, and 1 K dwarf -- HD97658) that have no existing measurements of their coronal X-ray emission. We will measure their X-ray luminosities and coronal temperatures, and derive the high energy radiation field to facilitate exoplanet atmospheric modeling. These planetary systems allow study of exoplanet atmospheric chemistry and evolution under a wide diversity of physical situations. X-ray heating enhances evaporation and atmospheric escape, which can impact the long-term stability of exoplanetary atmospheres.

Brown, Alexander

2013-09-01

168

Constraining the Low-mass Slope of the Star Formation Sequence at 0.5 < z < 2.5  

NASA Astrophysics Data System (ADS)

We constrain the slope of the star formation rate (SFR; log ?) to stellar mass (log M sstarf) relation down to log (M sstarf/M ?) = 8.4 (log (M sstarf/M ?) = 9.2) at z = 0.5 (z = 2.5) with a mass-complete sample of 39,106 star-forming galaxies selected from the 3D-HST photometric catalogs, using deep photometry in the CANDELS fields. For the first time, we find that the slope is dependent on stellar mass, such that it is steeper at low masses (log ?vproplog M sstarf) than at high masses (log ?vprop(0.3-0.6)log M sstarf). These steeper low-mass slopes are found for three different star formation indicators: the combination of the ultraviolet (UV) and infrared (IR), calibrated from a stacking analysis of Spitzer/MIPS 24 ?m imaging; ?-corrected UV SFRs; and H? SFRs. The normalization of the sequence evolves differently in distinct mass regimes as well: for galaxies less massive than log (M sstarf/M ?) < 10 the specific SFR (?/M sstarf) is observed to be roughly self-similar with ?/M sstarfvprop(1 + z)1.9, whereas more massive galaxies show a stronger evolution with ?/M sstarfvprop(1 + z)2.2-3.5 for log (M sstarf/M ?) = 10.2-11.2. The fact that we find a steep slope of the star formation sequence for the lower mass galaxies will help reconcile theoretical galaxy formation models with the observations.

Whitaker, Katherine E.; Franx, Marijn; Leja, Joel; van Dokkum, Pieter G.; Henry, Alaina; Skelton, Rosalind E.; Fumagalli, Mattia; Momcheva, Ivelina G.; Brammer, Gabriel B.; Labbé, Ivo; Nelson, Erica J.; Rigby, Jane R.

2014-11-01

169

Constraining the Low-Mass Slope of the Star Formation Sequence at 0.5?z?2.5  

NASA Astrophysics Data System (ADS)

A wealth of data from deep extragalactic surveys have revealed a picture where star-forming galaxies follow a tight relation between star formation rate and stellar mass. This observed star formation sequence encapsulates information about feedback, gas density and gas accretion rates over cosmic time. I will present a self-consistent empirical study measuring the slope of this relation for a complete sample of galaxies selected from the 3D-HST photometric catalogs at 0.5?z?2.5, using deep photometry in the CANDELS fields. Probing a factor of ten lower in stellar mass than previous high-redshift studies, we show that the slope of the star formation rate - stellar mass relation is mass-dependent; we measure a steep slope of order unity out to z=2.5 for low mass galaxies, and a slope that becomes increasingly flatter with time at the highest masses. These observations of the star formation sequence help reconcile existing tensions with theoretical galaxy formation models.

Whitaker, Katherine E.; Franx, Marijn; Leja, Joel; Van Dokkum, Pieter G.; Henry, Alaina L.; Skelton, Rosalind; Fumagalli, Mattia; Momcheva, Ivelina G.; Brammer, Gabriel; Labbe, Ivo; Nelson, Erica; Rigby, Jane R.; 3D-HST Collaboration

2015-01-01

170

A NEW METHOD TO IDENTIFY NEARBY, YOUNG, LOW-MASS STARS  

SciTech Connect

We describe a new method to identify young, late-type stars within {approx}150 pc of the Earth that employs visual or near-infrared (NIR) data and the GALEX GR4/5 database. For spectral types later than K5, we demonstrate that the ratio of GALEX near-ultraviolet to visual and NIR emission is larger for stars with ages between 10 and 100 Myr than for older, main-sequence stars. A search in regions of the sky encompassing the TW Hya and Scorpius-Centaurus Associations has returned 54 high-quality candidates for follow up. Spectroscopic observations of 24 of these M1-M5 objects reveal Li 6708 A absorption in at least 17 systems. Because GALEX surveys have covered a significant fraction of the sky, this methodology should prove valuable for future young star studies.

Rodriguez, David R.; Zuckerman, B. [Department of Physics and Astronomy, University of California, Los Angeles, CA 90095 (United States); Bessell, M. S. [Australian National University, Cotter Road, Weston Creek, ACT 2611 (Australia); Kastner, Joel H., E-mail: drodrigu@astro.ucla.edu [Center for Imaging Science, Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester, NY 14623 (United States)

2011-02-01

171

Detection of X-ray emission from the young low-mass star Rossiter 137B  

NASA Technical Reports Server (NTRS)

Rst 137B, a close M-dwarf companion to the active K-star HD 36705, has been detected in a High Resolution Image in the Einstein Observatory Archive. The X-ray surface fluxes (0.2-4 keV) from both stars are close to the empirical saturation level, F(x)/F(bol) of about 0.001, defined by rapid rotators and very young stars. This supports the earlier results of the youthfulness of the system. This young couple is an excellent subject for studies of dependence of early evolution on stellar mass. Rst 137B is one of the latest spectral types and thus lowest-mass premain-sequence stars yet detected as an X-ray source.

Vilhu, O.; Linsky, J. L.

1987-01-01

172

Elemental abundances of low-mass stars in the young clusters 25 Orionis and ? Orionis  

NASA Astrophysics Data System (ADS)

Aims: We aim to derive the chemical pattern of the young clusters 25 Orionis and ? Orionis through homogeneous and accurate measurements of elemental abundances. Methods: We present flames/uves observations of a sample of 14 K-type targets in the 25 Ori and ? Ori clusters. We measured their radial velocities to confirm cluster membership. We derived stellar parameters and abundances of Fe, Na, Al, Si, Ca, Ti, and Ni using the code MOOG. Results: All 25 Ori stars are confirmed cluster members without evidence of binarity; in ? Ori we identify one non member and one possible single-lined binary star. We find an average metallicity [Fe/H] = -0.05 ± 0.05 for 25 Ori, where the error is the 1? standard deviation from the average. The ? Ori members have a mean Fe abundance value of 0.01 ± 0.01. The other elements show close-to-solar ratios and no star-to-star dispersion. Conclusions: Our results, along with previous metallicity determinations in the Orion complex, show a small but detectable dispersion in the [Fe/H] distribution of the complex. This appears to be compatible with large-scale star formation episodes and initial nonuniformity in the precloud medium. We show that, as expected, the abundance distribution of star-forming regions is consistent with the chemical pattern of the Galactic thin disk. Based on flames observations collected at the Paranal Observatory (Chile). Program 082.D-0796(A).

Biazzo, K.; Randich, S.; Palla, F.; Briceño, C.

2011-06-01

173

Complex magnetic topology and strong differential rotation on the low-mass T Tauri star V2247 Oph  

NASA Astrophysics Data System (ADS)

From observations collected with the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope, we report the detection of Zeeman signatures on the low-mass classical T Tauri star (cTTS) V2247 Oph. Profile distortions and circular polarization signatures detected in photospheric lines can be interpreted as caused by cool spots and magnetic regions at the surface of the star. The large-scale field is of moderate strength and highly complex; moreover, both the spot distribution and the magnetic field show significant variability on a time-scale of only 1 week, as a likely result of strong differential rotation. Both properties make V2247 Oph very different from the (more massive) prototypical cTTS BP Tau; we speculate that this difference reflects the lower mass of V2247 Oph. During our observations, V2247 Oph was in a low-accretion state, with emission lines showing only weak levels of circular polarization; we nevertheless find that excess emission apparently concentrates in a mid-latitude region of a strong radial field, suggesting that it is the footpoint of an accretion funnel. The weaker and more complex field that we report on V2247 Oph may share similarities with those of very-low-mass late-M dwarfs and potentially explain why low-mass cTTSs rotate on average faster than intermediate-mass ones. These surprising results need confirmation from new independent data sets on V2247 Oph and other similar low-mass cTTSs. Based on observations obtained at the Canada-France-Hawaii Telescope, operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France and the University of Hawaii. E-mail: donati@ast.obs-mip.fr (J-FD); mskelly@ast.obs-mip.fr (MBS); jerome.bouvier@obs.ujf-grenoble.fr (JB); mmj@st-andrews.ac.uk (MMJ); sg64@st-andrews.ac.uk (SGG); jmorin@ast.obs-mip.fr (JM); ghussain@eso.org (GAJH); catherine.dougados@obs.ujf-grenoble.fr (CD); francois.menard@obs.ujf-grenoble.fr (FM); y.unruh@imperial.ac.uk (YU)

Donati, J.-F.; Skelly, M. B.; Bouvier, J.; Jardine, M. M.; Gregory, S. G.; Morin, J.; Hussain, G. A. J.; Dougados, C.; Ménard, F.; Unruh, Y.

2010-03-01

174

Angular momentum regulation in low-mass young stars surrounded by accretion disks  

NASA Technical Reports Server (NTRS)

From study of a sample of 34 T Tauri stars with photometrically derived rotation periods and spectral types later than KS, we find that the observed periods appear to be related to the presence or absence of an accretion disk. Those stars which we infer to be surrounded by accretion disks have rotation periods P(rot) over 4 days with a most probable P(rot) of about 8.5 days, while those stars which lack accretion disk signatures cover a wide range of P(rot) from 1.5 to 16 days, including a significant number of objects with P(rot) less than 4 days. This suggests the possibility that the 'initial' angular momentum of a star is not established until it dissipates its circumstellar accretion disk. During the disk accretion phase, the stellar angular velocity appears to be regulated at a low value, countering the tendency of the star to spin up both from contraction toward the main sequence and from the accretion of inner disk material of high specific angular momentum. When the accretion disk is dissipated, this regulation mechanism will cease to function. At this point, the star is no longer maintained at a low angular velocity, but is 'free' to conserve its angular momentum, and thus to increase its angular velocity in response to contraction and changes in moment of inertia. This hypothesis, combined with a spread in disk dispersal time scales, provides a context for explaining the observed distribution of stellar rotational velocities for stars on the ZAMS in young clusters.

Edwards, Suzan; Strom, Stephen E.; Hartigan, Patrick; Strom, Karen M.; Hillenbrand, Lynne A.; Herbst, William; Attridge, Joanne; Merrill, K. M.; Probst, Ron; Gatley, Ian

1993-01-01

175

Long duration X-ray flash from low mass population III stars  

NASA Astrophysics Data System (ADS)

Recent numerical simulations suggest that the typical mass of a Population III (Pop III) star become ~ 40Msolar. We investigate if a Pop III star can raise a gamma ray burst (GRB) by considering the jet propagation in the stellar envelope for a 40Msolar Pop III model. We find that a relativistic jet launched from the central black hole pierces the stellar envelope, although a Pop III star has a large envelope since the mass loss is not expected for zero metallicity stars. Thus, a Pop III star can be the progenitor of a GRB. We predict that GRBs from 40Msolar Pop III progenitors have much longer duration ~ 105 sec, slightly lower luminosity ~6×1050 erg/sec and much softer spectrum peak energy ~ 5 keV than the local long GRBs. Therefore, observationally, we regard a Pop III GRB as a long duration X-ray flash. We can observe such Pop III GRBs using the future detector Lobster.

Nakauchi, Daisuke; Suwa, Yudai; Kashiyama, Kazumi; Nakamura, Takashi

2012-09-01

176

Predicting Lyman-alpha and Mg II Fluxes from Low-Mass Stars  

NASA Astrophysics Data System (ADS)

A star’s UV emission can greatly affect the atmospheric chemistry and physical properties of closely orbiting planets with the potential for severe mass loss. In particular, the Lyman-alpha emission line at 1216 A, which dominates the far-ultraviolet (FUV) spectrum, is a major source of photodissociation of important atmospheric molecules such as water and methane. The intrinsic flux of Lyman-alpha, however, cannot be directly measured due to the absorption of neutral hydrogen in the interstellar medium and contamination by geocoronal emission. To date, reconstruction of the intrinsic Lyman-alpha line based on Hubble Space Telescope spectra has been accomplished for a few dozen nearby stars, 28 of which have also been observed by the Galaxy Evolution Explorer (GALEX). Our investigation provides a correlation between published intrinsic Lyman-alpha and GALEX FUV and near-ultraviolet (NUV) chromospheric fluxes for K and M stars. The negative correlations between the ratio of the Lyman-alpha to the GALEX fluxes reveal how the relative strength of Lyman-alpha compared to the broadband fluxes weakens as the FUV and NUV excess flux increase. We also correlate GALEX fluxes with the strong near-ultraviolet Mg II h+k spectral emission lines formed at lower chromospheric temperatures than Lyman-alpha. The reported correlations provide estimates of intrinsic Lyman-alpha and Mg II fluxes for the thousands of K and M stars in the GALEX all-sky surveys. These will constrain new stellar upper-atmosphere models for cool stars and provide realistic inputs to models describing exoplanetary photochemistry and atmospheric evolution in the absence of ultraviolet spectroscopy.

Shkolnik, Evgenya; Rolph, Kristina; Peacock, Sarah; Barman, Travis

2015-01-01

177

Supernova dust formation and the grain growth in the early universe: the critical metallicity for low-mass star formation  

NASA Astrophysics Data System (ADS)

We investigate the condition for the formation of low-mass second-generation stars in the early Universe. It has been proposed that gas cooling by dust thermal emission can trigger fragmentation of a low-metallicity star-forming gas cloud. In order to determine the critical condition in which dust cooling induces the formation of low-mass stars, we follow the thermal evolution of a collapsing cloud by a one-zone semi-analytic collapse model. Earlier studies assume the dust amount in the local Universe, where all refractory elements are depleted on to grains, and/or assume the constant dust amount during gas collapse. In this paper, we employ the models of dust formation and destruction in early supernovae to derive the realistic dust compositions and size distributions for multiple species as the initial conditions of our collapse calculations. We also follow accretion of heavy elements in the gas phase on to dust grains, i.e. grain growth, during gas contraction. We find that grain growth well alters the fragmentation property of the clouds. The critical conditions can be written by the gas metallicity Zcr and the initial depletion efficiency fdep,0 of gas-phase metal on to grains, or dust-to-metal mass ratio, as (Zcr/10-5.5 Z?) = (fdep,0/0.18)-0.44 with small scatters in the range of Zcr = [0.06-3.2] × 10-5 Z?. We also show that the initial dust composition and size distribution are important to determine Zcr.

Chiaki, Gen; Marassi, Stefania; Nozawa, Takaya; Yoshida, Naoki; Schneider, Raffaella; Omukai, Kazuyuki; Limongi, Marco; Chieffi, Alessandro

2015-01-01

178

An ecological case study of low-mass star formation in Taurus  

NASA Astrophysics Data System (ADS)

This thesis presents an extensive observational study of the Lynds dark cloud L1551 with the goal of understanding the past, present, and future star formation in the region. This is achieved by considering the connection between the diverse phenomena observed within the cloud and the cloud environment. Roughly 140 [Special characters omitted.] of total mass at ~15 K lies within a 0.9 pc projected radius, and the dispersion in molecular line emission is close to what is expected from a virialized cloud. Roughly 35 pre-main sequence stars comprise the L1551 association which has a spread in mass ~0.1--1.5 [Special characters omitted.] and a spread in age of more than 6 times the dynamical time of the present day cloud. The older stars are more spatially dispersed and may be slightly more massive than the recently formed stars. A pre-stellar core discovered in L1551 is evidence that there will be at least one more star or stellar system to form in the cloud. This ~2 [Special characters omitted.] cold core, named L1551-MC, is likely to collapse in [Special characters omitted.] 1 Myr. Asymmetric molecular line profiles toward L1551-MC suggest that it is building up mass via accretion and no sign of gravitational collapse is seen. L1551-MC also appears chemically young through the comparison of emission from several molecular species. A high-resolution, wide-field mosaic of L1551 using the BIMA interferometer is combined with single-dish data to construct fully sampled maps in both 13 CO(1 - 0) and C 18 O(1 - 0). The 13 CO emission shows clear evidence for the destruction of the cloud by the embedded stars while the line wings trace the boundaries between outflows and ambient gas. The C 18 O emission is optically thin and is used to characterize the turbulence in L1551 which may be due to stellar feedback. No other significant input of energy is seen. The derived quantities in this thesis along with auxiliary information from the literature are used to construct a comprehensive picture of star formation in L1551. The timeline of events is inconsistent with purely dynamical models and is most consistent with a scenario where stellar feedback mediates star formation. Magnetic field measurements within L1551 are needed to quantify their role in the evolution of the cloud.

Swift, Jonathan Jay

179

A survey for low-mass spectroscopic binary stars in the young clusters around ? Orionis and ? Orionis  

NASA Astrophysics Data System (ADS)

We have obtained multi-epoch, high-resolution spectroscopy of 218 candidate low-mass stars and brown dwarfs (BDs) in the young clusters around ? Ori and ? Ori. We find that 196 targets are cluster members based on their radial velocity, the equivalent width of their NaI8200 lines and the spectral type from their TiO band strength. We have identified 11 new binary stars among the cluster members based on their variable radial velocity and an additional binary from the variation in its linewidth and shape. Of these, six are double-lined spectroscopic binaries (SB2) where the components of the binary are of comparable brightness. The others are single-lined binaries (SB1) in which the companion is faint or the spectra of the stars are blended. There are three narrow-lined SB1 binaries in our sample for which the companion is more than 2.5 mag fainter than the primary. This suggests that the mass ratio distribution for the spectroscopic binaries in our sample is broad but that there may be a peak in the distribution near q = 1. The sample covers the magnitude range IC = 14-18.9 (mass ~0.55-0.03Msolar), but all of the binary stars are brighter than IC = 16.6 (mass ~0.12Msolar) and 10 are brighter than IC = 15.5 (mass ~0.23Msolar). There is a significant lack of spectroscopic binaries in our sample at faint magnitudes even when we account for the decrease in sensitivity with increasing magnitude. We can reject the hypothesis that the fraction of spectroscopic binaries is a uniform function of IC magnitude with more than 99 per cent confidence. The spectroscopic binary fraction for stars more massive than about 0.1Msolar (IC < 16.9) is fbright = 0.095+0.012-0.028. The 90 per cent confidence upper limit to the spectroscopic binary fraction for very low-mass (VLM) stars (mass <0.1Msolar) and BDs is ffaint < 7.5 per cent. The hypothesis that fbright and ffaint are equal can be rejected with 90 per cent confidence. The average detection probability for our survey is 50 per cent or more for binaries with separations up to 0.28au for stars with IC < 16.9 and 0.033au for the fainter stars in our sample. We conclude that we have found strong evidence for a change in the fraction of spectroscopic binaries among young VLM stars and BDs when compared to more massive stars in the same star-forming region. This implies a difference in the total binary fraction between VLM stars and BDs compared to more massive stars or a difference in the distribution of semimajor axes, or both.

Maxted, P. F. L.; Jeffries, R. D.; Oliveira, J. M.; Naylor, T.; Jackson, R. J.

2008-04-01

180

Hot Organic Molecules Toward a Young Low-Mass Star: A Look at Inner Disk Chemistry  

E-print Network

Spitzer Space Telescope spectra of the low mass young stellar object (YSO) IRS 46 (L_bol ~ 0.6 L_sun) in Ophiuchus reveal strong vibration-rotation absorption bands of gaseous C2H2, HCN, and CO2. This is the only source out of a sample of ~100 YSO's that shows these features and the first time they are seen in the spectrum of a solar-mass YSO. Analysis of the Spitzer data combined with Keck L- and M-band spectra gives excitation temperatures of > 350 K and abundances of 10(-6)-10(-5) with respect to H2, orders of magnitude higher than those found in cold clouds. In spite of this high abundance, the HCN J=4-3 line is barely detected with the James Clerk Maxwell Telescope, indicating a source diameter less than 13 AU. The (sub)millimeter continuum emission and the absence of scattered light in near-infrared images limits the mass and temperature of any remnant collapse envelope to less than 0.01 M_sun and 100 K, respectively. This excludes a hot-core type region as found in high-mass YSO's. The most plausible o...

Lahuis, F; Boogert, A C A; Dullemond, C P; Hogerheijde, M R; Jørgensen, J K; Kessler-Silacci, J E; Knez, C; Pontoppidan, K M; Van Dishoeck, E F

2006-01-01

181

Planetary protection in the extreme environments of low-mass stars  

NASA Astrophysics Data System (ADS)

Recent results showed that the magnetic field of M-dwarf (dM) stars, currently the main targets in searches for terrestrial planets, is very different from the solar one, both in topology as well as in intensity. In particular, the magnetised environment surrounding a planet orbiting in the habitable zone (HZ) of dM stars can differ substantially to the one encountered around the Earth. These extreme magnetic fields can compress planetary magnetospheres to such an extent that a significant fraction of the planet's atmosphere may be exposed to erosion by the stellar wind. Using observed surface magnetic maps for a sample of 15 dM stars, we investigate the minimum degree of planetary magnetospheric compression caused by the intense stellar magnetic fields. We show that hypothetical Earth-like planets with similar terrestrial magnetisation (~1 G) orbiting at the inner (outer) edge of the HZ of these stars would present magnetospheres that extend at most up to 6.1 (11.7) planetary radii. To be able to sustain an Earth-sized magnetosphere, the terrestrial planet would either need to orbit significantly farther out than the traditional limits of the HZ; or else, if it were orbiting within the life-bearing region, it would require a minimum magnetic field ranging from a few G to up to a few thousand G.

Vidotto, A. A.; Jardine, M.; Morin, J.; Donati, J.-F.; Lang, P.; Russell, A. J. B.

2014-08-01

182

Radiative feedback by low-mass stars in the first generation  

SciTech Connect

The survival of cosmological minihalos in both ionizing and Lyman-Werner (LW) UV fields from nearby and distant sources has attracted recent attention for its role in regulating the rise of stellar populations at high red-shifts. Numerical models suggest that the first stars form in isolation in small dark matter halos of {approx} 10{sup 5}-10{sup 7} M{sub {circle_dot}} at z {approx} 20-30 and that they are very massive, 25-500 M{sub {circle_dot}}. These stars form large H II regions 2.5-5 kpc in radius capable of engulfing nearby halos. With the rise of Population III stars throughout the cosmos also comes a global LW background that sterilizes mini-halos of H{sub 2}, delaying or preventing new star formation in them. At high redshifts, ionizaing radiation is therefore relatively local while LW photons can originate from many megaparsects away because their energies lie below the ionization limit of H.

Whalen, Daniel James [Los Alamos National Laboratory; Hueckstaedt, Robert [Los Alamos National Laboratory; Mcconkie, Thomas [Los Alamos National Laboratory

2009-01-01

183

EVOLUTION, NUCLEOSYNTHESIS, AND YIELDS OF LOW-MASS ASYMPTOTIC GIANT BRANCH STARS AT DIFFERENT METALLICITIES. II. THE FRUITY DATABASE  

SciTech Connect

By using updated stellar low-mass stars models, we systematically investigate the nucleosynthesis processes occurring in asymptotic giant branch (AGB) stars. In this paper, we present a database dedicated to the nucleosynthesis of AGB stars: FRANEC Repository of Updated Isotopic Tables and Yields (FRUITY). An interactive Web-based interface allows users to freely download the full (from H to Bi) isotopic composition, as it changes after each third dredge-up (TDU) episode and the stellar yields the models produce. A first set of AGB models, having masses in the range 1.5 {<=}M/M{sub Sun} {<=} 3.0 and metallicities 1 Multiplication-Sign 10{sup -3} {<=} Z {<=} 2 Multiplication-Sign 10{sup -2}, is discussed. For each model, a detailed description of the physical and the chemical evolution is provided. In particular, we illustrate the details of the s-process and we evaluate the theoretical uncertainties due to the parameterization adopted to model convection and mass loss. The resulting nucleosynthesis scenario is checked by comparing the theoretical [hs/ls] and [Pb/hs] ratios to those obtained from the available abundance analysis of s-enhanced stars. On the average, the variation with the metallicity of these spectroscopic indexes is well reproduced by theoretical models, although the predicted spread at a given metallicity is substantially smaller than the observed one. Possible explanations for such a difference are briefly discussed. An independent check of the TDU efficiency is provided by the C-stars luminosity function. Consequently, theoretical C-stars luminosity functions for the Galactic disk and the Magellanic Clouds have been derived. We generally find good agreement with observations.

Cristallo, S.; Dominguez, I.; Abia, C. [Departamento de Fisica Teorica y del Cosmos, Universidad de Granada, 18071 Granada (Spain); Piersanti, L.; Straniero, O.; Gallino, R.; Di Rico, G.; Quintini, M. [INAF-Osservatorio Astronomico di Collurania, 64100 Teramo (Italy); Bisterzo, S. [Dipartimento di Fisica Generale, Universita di Torino, 10125 Torino (Italy)

2011-12-01

184

The ultraviolet radiation environment in the habitable zones around low-mass exoplanet host stars  

NASA Astrophysics Data System (ADS)

The EUV (200-911 Å), FUV (912-1750 Å), and NUV (1750-3200 Å) spectral energy distribution of exoplanet host stars has a profound influence on the atmospheres of Earth-like planets in the habitable zone. The stellar EUV radiation drives atmospheric heating, while the FUV (in particular, Ly ?) and NUV radiation fields regulate the atmospheric chemistry: the dissociation of H2O and CO2, the production of O2 and O3, and may determine the ultimate habitability of these worlds. Despite the importance of this information for atmospheric modeling of exoplanetary systems, the EUV/FUV/NUV radiation fields of cool (K and M dwarf) exoplanet host stars are almost completely unconstrained by observation or theory. We present observational results from a Hubble Space Telescope survey of M dwarf exoplanet host stars, highlighting the importance of realistic UV radiation fields for the formation of potential biomarker molecules, O2 and O3. We conclude by describing preliminary results on the characterization of the UV time variability of these sources.

France, Kevin; Linsky, Jeffrey L.; Parke Loyd, R. O.

2014-11-01

185

Photometric and spectroscopic study of low mass embedded star clusters in reflection nebulae  

E-print Network

An analysis of the candidate embedded stellar systems in the reflection nebulae vdBH-RN26, vdBH-RN38, vdBH-RN53a, GGD20, ESO95-RN18 and NGC6595 is presented. Optical spectroscopic data from CASLEO (Argentina) in conjunction with near infrared photometry from the 2MASS Point Source Catalogue were employed. The analysis is based on source surface density, colour-colour and colour-magnitude diagrams together with theoretical pre-main sequence isochrones. We take into account the field population affecting the analysis by carrying out a statistical subtraction. The fundamental parameters for the stellar systems were derived. The resulting ages are in the range 1-4 Myr and the objects are dominated by pre-main sequence stars. The observed masses locked in the clusters are less than 25 solar masses. The studied systems have no stars of spectral types earlier than B, indicating that star clusters do not necessarily evolve through an HII region phase. The relatively small locked mass combined with the fact that they ...

Soares, J B; Ahumada, A V; Claria, J J

2004-01-01

186

A VLA Search for Water Masers in Six HII Regions: Tracers of Triggered Low-Mass Star Formation  

E-print Network

We present a search for water maser emission at 22 GHz associated with young low-mass protostars in six HII regions -- M16, M20, NGC 2264, NGC 6357, S125, and S140. The survey was conducted with the NRAO Very Large Array from 2000 to 2002. For several of these HII regions, ours are the first high-resolution observations of water masers. We detected 16 water masers: eight in M16, four in M20, three in S140, and one in NGC 2264. All but one of these were previously undetected. No maser emission was detected from NGC 6357 or S125. There are two principle results to our study. (1) The distribution of water masers in M16 and M20 does not appear to be random but instead is concentrated in a layer of compressed gas within a few tenths of a parsec of the ionization front. (2) Significantly fewer masers are seen in the observed fields than expected based on other indications of ongoing star formation, indicating that the maser-exciting lifetime of protostars is much shorter in HII regions than in regions of isolated star formation. Both of these results confirm predictions of a scenario in which star formation is first triggered by shocks driven in advance of ionization fronts, and then truncated approximately 10^5 years later when the region is overrun by the ionization front.

Kevin R. Healy; J. Jeff Hester; Mark J. Claussen

2004-04-19

187

Accurate Alpha Abundance and C/O of Low-mass Stars  

NASA Astrophysics Data System (ADS)

We investigate methods for measuring elemental abundances in M dwarf stars from high resolution (R>25,000), near-infrared spectra. With synthetic spectra from the BT-Settl model atmosphere library, we identify NIR features sensitive to enhancement of alpha elements (C, O, Mg, Si, S, Ca, and Ti) in M dwarfs. We also describe a method for measuring the amount of oxygen not bound in CO from molecular and atomic features in the NIR Y-band that, when combined with recently published methods of measuring carbon abundance in M dwarfs from K-band spectra, provides the ratio of carbon to oxygen (C/O). The ratio of carbon to oxygen is an important parameter for determining interior structures of exoplanets and processes that drive planet formation as evidenced by recent studies of the super Earth 55 Cnc e. We outline a potential observing program to empirically calibrate these methods via a sample of M dwarfs with widely-separated (5''-1500''), but associated, F, G or K-type binary companions. Once calibrated, we will apply these methods to a survey of nearby M dwarfs, including many stars that will be observed by NASA's Transiting Exoplanet Survey Satellite (TESS).

Veyette, Mark; Muirhead, Philip; Mann, Andrew

2015-01-01

188

Angular momentum in stars - The Kraft curve revisited  

NASA Technical Reports Server (NTRS)

The relationship between angular momentum and stellar mass for stars earlier than F0 has been rederived using current stellar models and rotational velocities. The form of the relationship is the same as found by Kraft (1970), but that the exponent of the power-law fit is higher. It is shown that the derived relation is consistent with the mean rotational velocity being the same fraction of the equatorial breakup velocity for all masses. By extending this correlation to low masses, the mean initial angular momentum for low-mass stars is estimated.

Kawaler, Steven D.

1987-01-01

189

Relativistic Astrophysics in Black Hole and Low-Mass Neutron Star Binaries LTSA98  

NASA Technical Reports Server (NTRS)

My group, in close collaboration with Dr. Zhang's group at University of Alabama-Huntsville, have been systematically analyzing and re-analyzing a substantial amount of archival data from previous and ongoing X-ray missions, in order to study possible relativistic effects around stellar-mass black holes and neutron stars. Our effort has been focused primarily on the data from the Rossi X-ray Timing Explorer. We carefully studied interesting quasi-periodic X-ray variability in newly discovered black hole candidates (XTE J1859+226 and XTE J1550-564), which, as we had proposed earlier, could be caused by general relativistic process (e.g., frame dragging) around the central black hole. We also discovered an intriguing temporal correlation between X-ray photons at different energies that is associated with the quasi-periodic signals of interest. The results provided new insights into the physical origin of the phenomena. Furthermore, we studied the spectral lines of black hole candidates which provide another avenue for studying general relativistic processes around black holes. The lines-may originate in the relativistic jets (which could be powered by the spin of the black hole) or in the disk around the black hole, as in the cases of 4U 1630-47 and GX 339-4 (two well-known black hole candidates), and may thus be distorted or shifted due to relativistic effects. Of course, neutron star systems were not forgotten either. After examining the properties of newly discovered fast quasi-periodic variability (at kiloHertz) associated with such systems, we proposed a relativistic model to explain the origin of the signals. We have also started to use new great observatories in orbit (such as Chandra and XMM-Newton) to observe the sources that are of interest to us. Finally, interesting results were also been obtained from our collaborations with other groups who are interested in some of the same objects. Such collaborative efforts have greatly enhanced the project and will likely continue in the future.

Cui, Wei

2000-01-01

190

Astrometric confirmation of young low-mass binaries and multiple systems in the Chamaeleon star-forming regions  

NASA Astrophysics Data System (ADS)

Context. The star-forming regions in Chamaeleon (Cha) are one of the nearest (distance ~ 165 pc) and youngest (age ~ 2 Myr) conglomerates of recently formed stars and the ideal target for population studies of star formation. Aims: We investigate a total of 16 Cha targets that have been suggested, but not confirmed, to be binaries or multiple systems in previous literature. Methods: We used the adaptive optics instrument Naos-Conica (NACO) at the Very Large Telescope Unit Telescope (UT) 4 / YEPUN of the Paranal Observatory, at 2-5 different epochs, in order to obtain relative and absolute astrometric measurements, as well as differential photometry in the J, H, and K band. On the basis of known proper motions and these observations, we analyse the astrometric results in our proper motion diagram (PMD: angular separation / position angle versus time), to eliminate possible (non-moving) background stars, establish co-moving binaries and multiples, and search for curvature as indications for orbital motion. Results: All previously suggested close components are co-moving and no background stars are found. The angular separations range between 0.07 and 9 arcsec, corresponding to projected distances between the components of 6-845 AU. Thirteen stars are at least binaries and the remaining three (RX J0919.4-7738, RX J0952.7-7933, VW Cha) are confirmed high-order multiple systems with up to four components. In 13 cases, we found significant slopes in the PMDs, which are compatible with orbital motion whose periods (estimated from the observed gradients in the position angles) range from 60 to 550 years. However, in only four cases there are indications of a curved orbit, the ultimate proof of a gravitational bond. Conclusions: A statistical study based on the 2MASS catalogue confirms the high probability of all 16 stellar systems being gravitationally bound. Most of the secondary components are well above the mass limit of hydrogen burning stars (0.08 M?), and have masses twice as high as this value or more. Massive primary components appear to avoid the simultaneous formation of equal-mass secondary components, while extremely low-mass secondary components are hard to find for both high and low mass primaries owing to the much higher dynamic range and the faintness of the secondaries. Based on observations made with ESO telescopes at the Paranal Observatory under program IDs 076.C-0292(A), 078.C-0535(A), 080.C-0424(A), 082.C-0489(A), 084.C-0364(B), 086.C-0638(A) & 086.C-0600(B), the Hubble Space Telescope under program ID GO-8716 and data obtained from the ESO/ST-ECF Science Archive Facility from the Paranal Observatory under program IDs 075.C-0042(A), 076.C-0579(A), 278.C-5070(A) and from the Hubble Space Telescope under programme IDs SNAP-7387, GO-11164. Appendix A is available in electronic form at http://www.aanda.org

Vogt, N.; Schmidt, T. O. B.; Neuhäuser, R.; Bedalov, A.; Roell, T.; Seifahrt, A.; Mugrauer, M.

2012-10-01

191

Can the Growth of Dust Grains in Low-metallicity Star-forming Clouds Affect the Formation of Metal-poor Low-mass Stars?  

NASA Astrophysics Data System (ADS)

The discovery of a low-mass star with such a low metallicity as <=4.5 × 10-5 Z ? reveals the critical role of dust in the formation of extremely metal-poor stars. In this Letter, we explore the effect of the growth of dust grains through accretion of gaseous refractory elements in very low metallicity pre-stellar cores on cloud fragmentation induced by dust emission cooling. Employing a simple model of grain growth in a gravitationally collapsing gas, we show that Fe and Si grains can grow efficiently at hydrogen densities of ~= 1010-1014 cm-3 in the clouds with metal abundances of -5 <~ [Fe, Si/H] <~ -3. The critical metal number abundances, above which the grain growth could induce the fragmentation of the gas clouds, are estimated to be A crit ~= 10-9-10-8, unless the initial grain radius is too large (gsim1 ?m) or the sticking probability is too small (lsim0.01). We find that even if the initial dust-to-gas mass ratio is well below the minimum value required for the dust-induced fragmentation, the grain growth increases the dust mass high enough to cause the gas to fragment into sub-solar mass clumps. We suggest that as long as the critical metal abundance is satisfied, grain growth could play an important role in the formation of low-mass stars with metallicity as low as 10-5 Z ?.

Nozawa, Takaya; Kozasa, Takashi; Nomoto, Ken'ichi

2012-09-01

192

Magnetic Inhibition of Convection and the Fundamental Properties of Low-mass Stars. II. Fully Convective Main-sequence Stars  

NASA Astrophysics Data System (ADS)

We examine the hypothesis that magnetic fields are inflating the radii of fully convective main-sequence stars in detached eclipsing binaries (DEBs). The magnetic Dartmouth stellar evolution code is used to analyze two systems in particular: Kepler-16 and CM Draconis. Magneto-convection is treated assuming stabilization of convection and also by assuming reductions in convective efficiency due to a turbulent dynamo. We find that magnetic stellar models are unable to reproduce the properties of inflated fully convective main-sequence stars, unless strong interior magnetic fields in excess of 10 MG are present. Validation of the magnetic field hypothesis given the current generation of magnetic stellar evolution models therefore depends critically on whether the generation and maintenance of strong interior magnetic fields is physically possible. An examination of this requirement is provided. Additionally, an analysis of previous studies invoking the influence of star spots is presented to assess the suggestion that star spots are inflating stars and biasing light curve analyses toward larger radii. From our analysis, we find that there is not yet sufficient evidence to definitively support the hypothesis that magnetic fields are responsible for the observed inflation among fully convective main-sequence stars in DEBs.

Feiden, Gregory A.; Chaboyer, Brian

2014-07-01

193

TIME-SERIES PHOTOMETRY OF STARS IN AND AROUND THE LAGOON NEBULA. I. ROTATION PERIODS OF 290 LOW-MASS PRE-MAIN-SEQUENCE STARS IN NGC 6530  

SciTech Connect

We have conducted a long-term, wide-field, high-cadence photometric monitoring survey of {approx}50,000 stars in the Lagoon Nebula H II region. This first paper presents rotation periods for 290 low-mass stars in NGC 6530, the young cluster illuminating the nebula, and for which we assemble a catalog of infrared and spectroscopic disk indicators, estimated masses and ages, and X-ray luminosities. The distribution of rotation periods we measure is broadly uniform for 0.5 days < P < 10 days; the short-period cutoff corresponds to breakup. We observe no obvious bimodality in the period distribution, but we do find that stars with disk signatures rotate more slowly on average. The stars' X-ray luminosities are roughly flat with rotation period, at the saturation level (log L{sub X} /L{sub bol} Almost-Equal-To -3.3). However, we find a significant positive correlation between L{sub X} /L{sub bol} and corotation radius, suggesting that the observed X-ray luminosities are regulated by centrifugal stripping of the stellar coronae. The period-mass relationship in NGC 6530 is broadly similar to that of the Orion Nebula Cluster (ONC), but the slope of the relationship among the slowest rotators differs from that in the ONC and other young clusters. We show that the slope of the period-mass relationship for the slowest rotators can be used as a proxy for the age of a young cluster, and we argue that NGC 6530 may be slightly younger than the ONC, making it a particularly important touchstone for models of angular momentum evolution in young, low-mass stars.

Henderson, Calen B. [Department of Astronomy, Ohio State University, 140 W. 18th Ave., Columbus, OH 43210 (United States); Stassun, Keivan G., E-mail: henderson@astronomy.ohio-state.edu [Department of Physics and Astronomy, Vanderbilt University, VU Station B 1807, Nashville, TN 37235 (United States)

2012-03-01

194

Binaries discovered by the SPY survey. VI. Discovery of a low mass companion to the hot subluminous planetary nebula central star EGB 5 - a recently ejected common envelope?  

Microsoft Academic Search

Hot subdwarf B stars (sdBs) in close binary systems are assumed to be formed via common envelope ejection. According to theoretical models, the amount of energy and angular momentum deposited in the common envelope scales with the mass of the companion. That low mass companions near or below the core hydrogen-burning limit are able to trigger the ejection of this

S. Geier; R. Napiwotzki; U. Heber; G. A. Nelemans

2011-01-01

195

Scientist and observational astronomer with expertise/interests in: Low-mass stars and brown dwarfs Search for extra-solar planets  

E-print Network

Scientist and observational astronomer with expertise/interests in: Low-mass stars and brown dwarfs project. "A systematic search for nearby white dwarfs in the GALEX archive: positions and motions of 100,000 disk and halo white dwarfs". $90,000- National Aeronautics and Space Administration (NASA), Hubble

196

ATMOSPHERIC CHEMISTRY IN GIANT PLANETS, BROWN DWARFS, AND LOW-MASS DWARF STARS. III. IRON, MAGNESIUM, AND SILICON  

SciTech Connect

We use thermochemical equilibrium calculations to model iron, magnesium, and silicon chemistry in the atmospheres of giant planets, brown dwarfs, extrasolar giant planets (EGPs), and low-mass stars. The behavior of individual Fe-, Mg-, and Si-bearing gases and condensates is determined as a function of temperature, pressure, and metallicity. Our equilibrium results are thus independent of any particular model atmosphere. The condensation of Fe metal strongly affects iron chemistry by efficiently removing Fe-bearing species from the gas phase. Monatomic Fe is the most abundant Fe-bearing gas throughout the atmospheres of EGPs and L dwarfs, and in the deep atmospheres of giant planets and T dwarfs. Mg- and Si-bearing gases are effectively removed from the atmosphere by forsterite (Mg{sub 2}SiO{sub 4}) and enstatite (MgSiO{sub 3}) cloud formation. Monatomic Mg is the dominant magnesium gas throughout the atmospheres of EGPs and L dwarfs and in the deep atmospheres of giant planets and T dwarfs. Silicon monoxide (SiO) is the most abundant Si-bearing gas in the deep atmospheres of brown dwarfs and EGPs, whereas SiH{sub 4} is dominant in the deep atmosphere of Jupiter and other gas giant planets. Several other Fe-, Mg-, and Si-bearing gases become increasingly important with decreasing effective temperature. In principle, a number of Fe, Mg, and Si gases are potential tracers of weather or diagnostic of temperature in substellar atmospheres.

Visscher, Channon [Current address: Lunar and Planetary Institute, USRA, Houston, TX 77058-1113 (United States); Lodders, Katharina; Fegley, Bruce, E-mail: visscher@lpi.usra.ed, E-mail: lodders@wustl.ed, E-mail: bfegley@wustl.ed [Planetary Chemistry Laboratory, Department of Earth and Planetary Sciences, McDonnell Center for the Space Sciences, Washington University, St. Louis, MO 63130-4899 (United States)

2010-06-20

197

UPPER BOUNDS ON r-MODE AMPLITUDES FROM OBSERVATIONS OF LOW-MASS X-RAY BINARY NEUTRON STARS  

SciTech Connect

We present upper limits on the amplitude of r-mode oscillations and gravitational-radiation-induced spin-down rates in low-mass X-ray binary neutron stars, under the assumption that the quiescent neutron star luminosity is powered by dissipation from a steady-state r-mode. For masses <2 M{sub Sun} we find dimensionless r-mode amplitudes in the range from about 1 Multiplication-Sign 10{sup -8} to 1.5 Multiplication-Sign 10{sup -6}. For the accreting millisecond X-ray pulsar sources with known quiescent spin-down rates, these limits suggest that {approx}< 1% of the observed rate can be due to an unstable r-mode. Interestingly, the source with the highest amplitude limit, NGC 6440, could have an r-mode spin-down rate comparable to the observed, quiescent rate for SAX J1808-3658. Thus, quiescent spin-down measurements for this source would be particularly interesting. For all sources considered here, our amplitude limits suggest that gravitational wave signals are likely too weak for detection with Advanced LIGO. Our highest mass model (2.21 M{sub Sun }) can support enhanced, direct Urca neutrino emission in the core and thus can have higher r-mode amplitudes. Indeed, the inferred r-mode spin-down rates at these higher amplitudes are inconsistent with the observed spin-down rates for some of the sources, such as IGR J00291+5934 and XTE J1751-305. In the absence of other significant sources of internal heat, these results could be used to place an upper limit on the masses of these sources if they were made of hadronic matter, or alternatively it could be used to probe the existence of exotic matter in them if their masses were known.

Mahmoodifar, Simin [Department of Physics, University of Maryland College Park, MD 20742 (United States); Strohmayer, Tod [Astrophysics Science Division, NASA's Goddard Space Flight Center, Greenbelt, MD 20771 (United States)

2013-08-20

198

Upper Bounds on r-Mode Amplitudes from Observations of Low-Mass X-Ray Binary Neutron Stars  

NASA Technical Reports Server (NTRS)

We present upper limits on the amplitude of r-mode oscillations and gravitational-radiation-induced spin-down rates in low-mass X-ray binary neutron stars, under the assumption that the quiescent neutron star luminosity is powered by dissipation from a steady-state r-mode. For masses <2M solar mass we find dimensionless r-mode amplitudes in the range from about 1×10(exp-8) to 1.5×10(exp-6). For the accreting millisecond X-ray pulsar sources with known quiescent spin-down rates, these limits suggest that approx. less than 1% of the observed rate can be due to an unstable r-mode. Interestingly, the source with the highest amplitude limit, NGC 6440, could have an r-mode spin-down rate comparable to the observed, quiescent rate for SAX J1808-3658. Thus, quiescent spin-down measurements for this source would be particularly interesting. For all sources considered here, our amplitude limits suggest that gravitational wave signals are likely too weak for detection with Advanced LIGO. Our highest mass model (2.21M solar mass) can support enhanced, direct Urca neutrino emission in the core and thus can have higher r-mode amplitudes. Indeed, the inferred r-mode spin-down rates at these higher amplitudes are inconsistent with the observed spin-down rates for some of the sources, such as IGR J00291+5934 and XTE J1751-305. In the absence of other significant sources of internal heat, these results could be used to place an upper limit on the masses of these sources if they were made of hadronic matter, or alternatively it could be used to probe the existence of exotic matter in them if their masses were known.

Mahmoodifar, Simin; Strohmayer, Tod

2013-01-01

199

KEPLER CYCLE 1 OBSERVATIONS OF LOW-MASS STARS: NEW ECLIPSING BINARIES, SINGLE STAR ROTATION RATES, AND THE NATURE AND FREQUENCY OF STARSPOTS  

SciTech Connect

We have analyzed Kepler light curves for 849 stars with T{sub eff} {<=} 5200 K from our Cycle 1 Guest Observer program. We identify six new eclipsing binaries, one of which has an orbital period of 29.91 days and two of which are probably W UMa variables. In addition, we identify a candidate 'warm Jupiter' exoplanet. We further examine a subset of 670 sources for variability. Of these objects, 265 stars clearly show periodic variability that we assign to rotation of the low-mass star. At the photometric precision level provided by Kepler, 251 of our objects showed no evidence for variability. We were unable to determine periods for 154 variable objects. We find that 79% of stars with T{sub eff} {<=} 5200 K are variable. The rotation periods we derive for the periodic variables span the range 0.31 days {<=} P{sub rot} {<=} 126.5 days. A considerable number of stars with rotation periods similar to the solar value show activity levels that are 100 times higher than the Sun. This is consistent with results for solar-like field stars. As has been found in previous studies, stars with shorter rotation periods generally exhibit larger modulations. This trend flattens beyond P{sub rot} = 25 days, demonstrating that even long-period binaries may still have components with high levels of activity and investigating whether the masses and radii of the stellar components in these systems are consistent with stellar models could remain problematic. Surprisingly, our modeling of the light curves suggests that the active regions on these cool stars are either preferentially located near the rotational poles, or that there are two spot groups located at lower latitudes, but in opposing hemispheres.

Harrison, T. E.; Coughlin, J. L.; Ule, N. M. [Department of Astronomy, New Mexico State University, Box 30001, MSC 4500, Las Cruces, NM 88003-8001 (United States); Lopez-Morales, M., E-mail: tharriso@nmsu.edu, E-mail: jlcough@nmsu.edu, E-mail: nmule@nmsu.edu, E-mail: mlopez@ieec.uab.es [Institut de Ciencies de L'Espai (CSIC-IEEC), Campus UAB, Fac. Ciencies. Torre C5 parell 2, 08193 Bellaterra, Barcelona (Spain)

2012-01-15

200

CAN THE GROWTH OF DUST GRAINS IN LOW-METALLICITY STAR-FORMING CLOUDS AFFECT THE FORMATION OF METAL-POOR LOW-MASS STARS?  

SciTech Connect

The discovery of a low-mass star with such a low metallicity as {<=}4.5 Multiplication-Sign 10{sup -5} Z{sub Sun} reveals the critical role of dust in the formation of extremely metal-poor stars. In this Letter, we explore the effect of the growth of dust grains through accretion of gaseous refractory elements in very low metallicity pre-stellar cores on cloud fragmentation induced by dust emission cooling. Employing a simple model of grain growth in a gravitationally collapsing gas, we show that Fe and Si grains can grow efficiently at hydrogen densities of {approx_equal} 10{sup 10}-10{sup 14} cm{sup -3} in the clouds with metal abundances of -5 {approx}< [Fe, Si/H] {approx}< -3. The critical metal number abundances, above which the grain growth could induce the fragmentation of the gas clouds, are estimated to be A{sub crit} {approx_equal} 10{sup -9}-10{sup -8}, unless the initial grain radius is too large ({approx}>1 {mu}m) or the sticking probability is too small ({approx}<0.01). We find that even if the initial dust-to-gas mass ratio is well below the minimum value required for the dust-induced fragmentation, the grain growth increases the dust mass high enough to cause the gas to fragment into sub-solar mass clumps. We suggest that as long as the critical metal abundance is satisfied, grain growth could play an important role in the formation of low-mass stars with metallicity as low as 10{sup -5} Z{sub Sun }.

Nozawa, Takaya; Nomoto, Ken'ichi [Kavli Institute for the Physics and Mathematics of the Universe (WPI), Todai Institutes for Advanced Study, University of Tokyo, Kashiwa, Chiba 277-8583 (Japan); Kozasa, Takashi, E-mail: takaya.nozawa@ipmu.jp [Department of Cosmosciences, Graduate School of Science, Hokkaido University, Sapporo 060-0810 (Japan)

2012-09-10

201

VERY LOW MASS STELLAR AND SUBSTELLAR COMPANIONS TO SOLAR-LIKE STARS FROM MARVELS. I. A LOW-MASS RATIO STELLAR COMPANION TO TYC 4110-01037-1 IN A 79 DAY ORBIT  

SciTech Connect

TYC 4110-01037-1 has a low-mass stellar companion, whose small mass ratio and short orbital period are atypical among binary systems with solar-like (T{sub eff} {approx}< 6000 K) primary stars. Our analysis of TYC 4110-01037-1 reveals it to be a moderately aged ({approx}<5 Gyr) solar-like star having a mass of 1.07 {+-} 0.08 M{sub Sun} and radius of 0.99 {+-} 0.18 R{sub Sun }. We analyze 32 radial velocity (RV) measurements from the SDSS-III MARVELS survey as well as 6 supporting RV measurements from the SARG spectrograph on the 3.6 m Telescopio Nazionale Galileo telescope obtained over a period of {approx}2 years. The best Keplerian orbital fit parameters were found to have a period of 78.994 {+-} 0.012 days, an eccentricity of 0.1095 {+-} 0.0023, and a semi-amplitude of 4199 {+-} 11 m s{sup -1}. We determine the minimum companion mass (if sin i = 1) to be 97.7 {+-} 5.8 M{sub Jup}. The system's companion to host star mass ratio, {>=}0.087 {+-} 0.003, places it at the lowest end of observed values for short period stellar companions to solar-like (T{sub eff} {approx}< 6000 K) stars. One possible way to create such a system would be if a triple-component stellar multiple broke up into a short period, low q binary during the cluster dispersal phase of its lifetime. A candidate tertiary body has been identified in the system via single-epoch, high contrast imagery. If this object is confirmed to be comoving, we estimate it would be a dM4 star. We present these results in the context of our larger-scale effort to constrain the statistics of low-mass stellar and brown dwarf companions to FGK-type stars via the MARVELS survey.

Wisniewski, John P.; Agol, Eric; Barnes, Rory [Astronomy Department, University of Washington, Box 351580, Seattle, WA 98195 (United States); Ge, Jian; De Lee, Nathan; Fleming, Scott W.; Lee, Brian L.; Chang, Liang [Department of Astronomy, University of Florida, 211 Bryant Space Science Center, Gainesville, FL 32611-2055 (United States); Crepp, Justin R. [Department of Astrophysics, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125 (United States); Eastman, Jason; Gaudi, B. Scott [Department of Astronomy, Ohio State University, 140 West 18th Avenue, Columbus, OH 43210 (United States); Esposito, Massimiliano; Gonzalez Hernandez, Jonay I.; Prieto, Carlos Allende [Instituto de Astrofisica de Canarias (IAC), E-38205 La Laguna, Tenerife (Spain); Ghezzi, Luan; Da Costa, Luiz N.; Porto De Mello, G. F. [Laboratorio Interinstitucional de e-Astronomia-LIneA, Rio de Janeiro, RJ 20921-400 (Brazil); Stassun, Keivan G.; Cargile, Phillip [Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235 (United States); Bizyaev, Dmitry, E-mail: jwisnie@u.washington.edu [Apache Point Observatory, P.O. Box 59, Sunspot, NM 88349-0059 (United States); and others

2012-05-15

202

A New Comptonization Model for Weakly Magnetized, Accreting Neutron Stars in Low-Mass X-Ray Binaries  

NASA Astrophysics Data System (ADS)

We have developed a new model for the X-ray spectral fitting package XSPEC that takes into account the effects of both thermal and dynamical (i.e., bulk) Comptonization. The model consists of two components: one is the direct blackbody-like emission due to seed photons that are not subjected to effective Compton scattering, while the other is a convolution of the Green's function of the energy operator with a blackbody-like seed photon spectrum. When combined thermal and bulk effects are considered, the analytical form of the Green's function may be obtained as a solution of the diffusion equation describing Comptonization. Using data from the BeppoSAX, INTEGRAL, and RXTE satellites, we test our model on the spectra of a sample of six bright neutron star low-mass X-ray binaries with low magnetic fields, covering three different spectral states. Particular attention is given to the transient power-law-like hard X-ray (>~30 keV) tails, which we interpret in the framework of the bulk motion Comptonization process. We show that the values of the best-fit ?-parameter, which represents the importance of bulk with respect to thermal Comptonization, can be physically meaningful and can at least qualitatively describe the physical conditions of the environment in the innermost part of the system. Moreover, we show that in fitting the thermal Comptonization spectra to the X-ray spectra of these systems, the best-fit parameters of our model are in excellent agreement with those from compTT, a broadly used and well-established XSPEC model.

Farinelli, Ruben; Titarchuk, Lev; Paizis, Ada; Frontera, Filippo

2008-06-01

203

Neutron Star Radius Measurement with the Quiescent Low-mass X-ray Binary U24 in NGC 6397  

NASA Astrophysics Data System (ADS)

This paper reports the spectral and timing analyses of the quiescent low-mass X-ray binary (qLMXB) U24 observed during five archived Chandra/ACIS exposures of the nearby globular cluster NGC 6397, for a total of 350 ks. We find that the X-ray flux and the parameters of the hydrogen atmosphere spectral model are consistent with those previously published for this source. On short timescales, we find no evidence of aperiodic intensity variability, with 90% confidence upper limits during five observations ranging between <8.6% rms and <19% rms, in the 0.0001-0.1 Hz frequency range (0.5-8.0 keV); and no evidence of periodic variability, with maximum observed powers in this frequency range having a chance probability of occurrence from a Poisson-deviated light curve in excess of 10%. We also report the improved neutron star (NS) physical radius measurement, with statistical accuracy of the order of ~10%: R NS = 8.9+0.9 -0.6 km for M NS = 1.4 M sun. Alternatively, we provide the confidence regions in mass-radius space as well as the best-fit projected radius R ? = 11.9+1.0 -0.8 km, as seen by an observer at infinity. The best-fit effective temperature, kT eff = 80+4 -5 eV, is used to estimate the NS core temperature which falls in the range T core = (3.0-9.8) × 107 K, depending on the atmosphere model considered. This makes U24 the third most precisely measured NS radius among qLMXBs, after those in ? Cen and M13.

Guillot, Sebastien; Rutledge, Robert E.; Brown, Edward F.

2011-05-01

204

Organic Chemistry of Low-Mass Star-Forming Cores. I. 7 mm Spectroscopy of Chamaeleon MMSl  

NASA Technical Reports Server (NTRS)

Observations are presented of emission lines from organic molecules at frequencies 32-50 GHz in the vicinity of Chamaeleon MMS1. This chemically rich dense cloud core harbors an extremely young, very low luminosity protostellar object and is a candidate first hydrostatic core. Column densities are derived and emission maps are presented for species including polyynes, cyanopolyynes, sulphuretted carbon chains, and methanol. The polyyne emission peak lies about 5000 AU from the protostar, whereas methanol peaks about 15,000 AU away. Averaged over the telescope beam, the molecular hydrogen number density is calculated to be 10(exp 6) / cubic cm and the gas kinetic temperature is in the range 5-7 K. The abundances of long carbon chains are very large and are indicative of a nonequilibrium carbon chemistry; C6H and HC7N column densities are 5.9(sup +2.9) (sub -1.3) x 10(exp 11) /cubic cm and 3.3 (sup +8.0)(sub -1.5) x 10(exp 12)/sq cm, respectively, which are similar to the values found in the most carbon-chain-rich protostars and prestellar cores known, and are unusually large for star-forming gas. Column density upper limits were obtained for the carbon chain anions C4H(-) and C6H(-), with anion-to-neutral ratios [C4H(-)]/[C4H] < 0.02% and [C6H(-l)]/[C6H] < 10%, consistent with previous observations in interstellar clouds and low-mass protostars. Deuterated HC,3 and c-C3H2 were detected. The [DC3N]/[HC,N] ratio of approximately 4% is consistent with the value typically found in cold interstellar gas.

Cordiner, Martn A.; Charnley, Steven B.; Wirtstroem, Eva S.; Smith, Robert G.

2012-01-01

205

Episodic Accretion at Early Stages of Evolution of Low-Mass Stars and Brown Dwarfs: A Solution for the Observed Luminosity Spread in HR Diagrams?  

Microsoft Academic Search

We present evolutionary models for young low-mass stars and brown dwarfs taking into account episodic phases of accretion at early stages of the evolution, a scenario supported by recent large surveys of embedded protostars. An evolution including short episodes of vigorous accretion (\\\\dot{M}>= 10^{-4} M_&sun; yr^{-1}) followed by longer quiescent phases (\\\\dot{M}< 10^{-6} M_&sun; yr^{-1}) can explain the observed luminosity

I. Baraffe; G. Chabrier; J. Gallardo

2009-01-01

206

Searching for Proto-clusters in a Quiescent GIant Molecular Cloud: A Survey for Low Mass Stars and Protostars in Maddalena's Cloud  

NASA Astrophysics Data System (ADS)

We propose an IRAC and MIPS survey toward recently identified dense cores in Maddalena's cloud, a unique, quiescent giant molecular cloud. To date, no sites of star formation have been identified in this 100,000 solar mass cloud; however, our detection of massive (~ 500 solar mass) dense cores suggests that the formation of entire clusters of stars is imminent. At a distance of 2 kpc, it is possible that isolated, low mass pre-main sequence stars and protostars remain undetected this cloud. IRAC and MIPS will be able to detect any T-Tauri star down to the hydrogen burning limit, as well as one solar luminosity protostars too faint to have been detected with MSX and IRAS. We will also observe the cool dust in the dense cores using the MIPS 70 and 160 micron bands. With these data, we can map the dust temperature and column density in the dense cores with twice the angular resolution of our millimeter-wave maps. We will use the dust map to study the density structure of the dense cores and compare the density structure to active cluster forming cores. With this data, we will determine whether low mass star formation has started toward the dense cores, giving us a rare glimpse at what appears to be the first stages of cluster formation.

Megeath, Tom; Ashby, Matthew; Wilson, Thomas

2004-09-01

207

Effects of a Close Low-mass Companion on the Spectrum and Light Curve of a Mira-type Star  

Microsoft Academic Search

The case of a close binary system, containing a red giant and a compact low-mass object (`planet'), is considered. The `planet' is orbiting within the inner layers of the red-giant circumstellar envelope. Its supersonic motion engenders a conical shock wave, which ionises gas and creates a small HII region within the envelope. This hot spot, too, is moving across the

G. M. Rudnitskij

1998-01-01

208

2M1155-79 (= T CHAMAELEONTIS B): A LOW-MASS, WIDE-SEPARATION COMPANION TO THE NEARBY, 'OLD' T TAURI STAR T CHAMAELEONTIS  

SciTech Connect

The early-K star T Cha, a member of the nearby (D Almost-Equal-To 100 pc) {epsilon} Cha Association, is a relatively 'old' (age {approx} 7 Myr) T Tauri star that is still sporadically accreting from an orbiting disk whose inner regions are now evidently being cleared by a close, substellar companion. We report the identification, via analysis of proper motions, serendipitous X-ray imaging spectroscopy, and follow-up optical spectroscopy, of a new member of the {epsilon} Cha Association that is very likely a low-mass companion to T Cha at a projected separation of {approx}38 kAU. The combined X-ray and optical spectroscopy data indicate that the companion, T Cha B (= 2M1155-79), is a weak-lined T Tauri star (wTTS) of spectral type M3 and age {approx}< 10 Myr. The serendipitous X-ray (XMM-Newton) observation of T Cha B, which targeted T Cha, also yields serendipitous detections of two background wTTS in the Chamaeleon cloud complex, including one newly discovered, low-mass member of the Cha cloud pre-main-sequence (pre-MS) population. T Cha becomes the third prominent example of a nearby, 'old' yet still actively accreting, K-type pre-MS star/disk system (the others being TW Hya and V4046 Sgr) to feature a low-mass companion at very large (12-40 kAU) separation, suggesting that such wide-separation companions may affect the conditions and timescales for planet formation around solar-mass stars.

Kastner, Joel H.; Thompson, Emily A.; Montez, Rodolfo; Sacco, Giuseppe Germano [Center for Imaging Science and Laboratory for Multiwavelength Astrophysics, Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester, NY 14623 (United States); Murphy, Simon J.; Bessell, Michael S., E-mail: jhk@cis.rit.edu [RSAA, Mount Stromlo Observatory, Australian National University, ACT 2611 (Australia)

2012-03-10

209

2M1155-79 (= T Chamaeleontis B): A Low-mass, Wide-separation Companion to the nearby, "Old" T Tauri Star T Chamaeleontis  

NASA Astrophysics Data System (ADS)

The early-K star T Cha, a member of the nearby (D ? 100 pc) epsilon Cha Association, is a relatively "old" (age ~ 7 Myr) T Tauri star that is still sporadically accreting from an orbiting disk whose inner regions are now evidently being cleared by a close, substellar companion. We report the identification, via analysis of proper motions, serendipitous X-ray imaging spectroscopy, and follow-up optical spectroscopy, of a new member of the epsilon Cha Association that is very likely a low-mass companion to T Cha at a projected separation of ~38 kAU. The combined X-ray and optical spectroscopy data indicate that the companion, T Cha B (= 2M1155-79), is a weak-lined T Tauri star (wTTS) of spectral type M3 and age <~ 10 Myr. The serendipitous X-ray (XMM-Newton) observation of T Cha B, which targeted T Cha, also yields serendipitous detections of two background wTTS in the Chamaeleon cloud complex, including one newly discovered, low-mass member of the Cha cloud pre-main-sequence (pre-MS) population. T Cha becomes the third prominent example of a nearby, "old" yet still actively accreting, K-type pre-MS star/disk system (the others being TW Hya and V4046 Sgr) to feature a low-mass companion at very large (12-40 kAU) separation, suggesting that such wide-separation companions may affect the conditions and timescales for planet formation around solar-mass stars.

Kastner, Joel H.; Thompson, Emily A.; Montez, Rodolfo; Murphy, Simon J.; Bessell, Michael S.; Sacco, Giuseppe Germano

2012-03-01

210

Discovery of Par 1802 as a Low-Mass, Pre-Main-Sequence Eclipsing Binary in the Orion Star-Forming Region  

E-print Network

We report the discovery of a pre-main-sequence, low-mass, double-lined, spectroscopic, eclipsing binary in the Orion star-forming region. We present our observations including radial velocities derived from optical high-resolution spectroscopy, and present an orbit solution that permits the determination of precise empirical masses for both components of the system. We measure that Par 1802 is composed of two equal mass (0.39+-0.03, 0.40+-0.03 Msun) stars in a circular, 4.7 day orbit. There is strong evidence, such as the system exhibiting strong Li lines and a center-of-mass velocity consistent with cluster membership, that this system is a member of the Orion star-forming region and quite possibly the Orion Nebula Cluster, and therefore has an age of only a few million years. As there are currently only a few empirical mass and radius measurements for low-mass, PMS stars, this system presents an interesting test for the predictions of current theoretical models of pre-main sequence stellar evolution.

P. A. Cargile; K. G. Stassun; R. D. Mathieu

2007-09-21

211

BANYAN. IV. Fundamental Parameters of Low-mass Star Candidates in Nearby Young Stellar Kinematic Groups—Isochronal Age Determination using Magnetic Evolutionary Models  

NASA Astrophysics Data System (ADS)

Based on high-resolution optical spectra obtained with ESPaDOnS at Canada-France-Hawaii Telescope, we determine fundamental parameters (T eff, R, L bol, log g, and metallicity) for 59 candidate members of nearby young kinematic groups. The candidates were identified through the BANYAN Bayesian inference method of Malo et al., which takes into account the position, proper motion, magnitude, color, radial velocity, and parallax (when available) to establish a membership probability. The derived parameters are compared to Dartmouth magnetic evolutionary models and field stars with the goal of constraining the age of our candidates. We find that, in general, low-mass stars in our sample are more luminous and have inflated radii compared to older stars, a trend expected for pre-main-sequence stars. The Dartmouth magnetic evolutionary models show a good fit to observations of field K and M stars, assuming a magnetic field strength of a few kG, as typically observed for cool stars. Using the low-mass members of the ? Pictoris moving group, we have re-examined the age inconsistency problem between lithium depletion age and isochronal age (Hertzspring-Russell diagram). We find that the inclusion of the magnetic field in evolutionary models increases the isochronal age estimates for the K5V-M5V stars. Using these models and field strengths, we derive an average isochronal age between 15 and 28 Myr and we confirm a clear lithium depletion boundary from which an age of 26 ± 3 Myr is derived, consistent with previous age estimates based on this method.

Malo, Lison; Doyon, René; Feiden, Gregory A.; Albert, Loïc; Lafrenière, David; Artigau, Étienne; Gagné, Jonathan; Riedel, Adric

2014-09-01

212

Evolutionary and pulsational properties of low-mass white dwarf stars with oxygen cores resulting from close binary evolution  

Microsoft Academic Search

The present work is designed to explore the evolutionary and pulsational properties of low-mass white dwarfs with carbon\\/oxygen cores. In particular, we follow the evolution of a 0.33-Msolar white dwarf remnant in a self-consistent way with the predictions of nuclear burning, element diffusion and the history of the white dwarf progenitor. Attention is focused on the occurrence of hydrogen shell

L. G. Althaus; A. H. Córsico; A. Gautschy; Z. Han; A. M. Serenelli; J. A. Panei

2004-01-01

213

Evolutionary and pulsational properties of low-mass white dwarf stars with oxygen cores resulting from close binary evolution  

Microsoft Academic Search

The present work is designed to explore the evolutionary and pulsational\\u000aproperties of low-mass white dwarfs with carbon\\/oxygen cores. In particular, we\\u000afollow the evolution of a 0.33 Msun white dwarf remnant in a self-consistent\\u000away with the predictions of nuclear burning, element diffusion and the history\\u000aof the white dwarf progenitor. Attention is focused on the occurrence of\\u000ahydrogen

L. G. Althaus; A. H. Corsico; A. Gautschy; Z. Han; A. M. Serenelli; J. A. Panei

2003-01-01

214

A Catalog of Low-Mass Star-Forming Cores Observed with SHARC-II at 350 ?m  

NASA Astrophysics Data System (ADS)

This paper presents a catalog of low-mass protostellar cores observed with the SHARC-II instrument at 350 ?m. The catalog includes 82 maps covering a total of 166 detected sources. For each source, we include basic source properties such as position, flux density, and radius. We also present quantification of the sensitivity of SHARC-II 350 ?m observations to extended emission and to the observing mode used on the telescope.

Suresh, Akshaya; Arce, Hector G.; Dunham, Michael

2015-01-01

215

Binaries discovered by the SPY survey. VI. Discovery of a low mass companion to the hot subluminous planetary nebula central star EGB 5 - a recently ejected common envelope?  

NASA Astrophysics Data System (ADS)

Hot subdwarf B stars (sdBs) in close binary systems are assumed to be formed via common envelope ejection. According to theoretical models, the amount of energy and angular momentum deposited in the common envelope scales with the mass of the companion. That low mass companions near or below the core hydrogen-burning limit are able to trigger the ejection of this envelope is well known. The currently known systems have very short periods ?0.1-0.3 d. Here we report the discovery of a low mass companion (M2 > 0.14 M?) orbiting the sdB star and central star of a planetary nebula EGB 5 with an orbital period of 16.5 d at a minimum separation of 23 R?. Its long period is only just consistent with the energy balance prescription of the common envelope. The marked difference between the short and long period systems will provide strong constraints on the common envelope phase, in particular if the masses of the sdB stars can be measured accurately. Due to selection effects, the fraction of sdBs with low mass companions and similar or longer periods may be quite high. Low mass stellar and substellar companions may therefore play a significant role for the still unclear formation of hot subdwarf stars. Furthermore, the nebula around EGB 5 may be the remnant of the ejected common envelope making this binary a unique system to study this short und poorly understood phase of binary evolution. Based on observations at the Paranal Observatory of the European Southern Observatory for programmes No. 167.H-0407(A) and 71.D-0383(A). Based on observations collected at the Centro Astronómico Hispano Alemán (CAHA) at Calar Alto, operated jointly by the Max-Planck Institut für Astronomie and the Instituto de Astrofísica de Andalucía (CSIC). Some of the data used in this work were obtained at the William Herschel Telescope (WHT) operated by the Isaac Newton Group of Telescopes (ING).

Geier, S.; Napiwotzki, R.; Heber, U.; Nelemans, G.

2011-04-01

216

Detection of Nine M8.0-L0.5 Binaries: The Very Low Mass Binary Population and its Implications for Brown Dwarf and VLM Star Formation  

Microsoft Academic Search

Use of the highly sensitive Hokupa'a\\/Gemini curvature wavefront sensor has\\u000aallowed direct adaptive optics (AO) guiding on very low mass (VLM) stars with\\u000aSpT=M8.0-L0.5. A survey of 39 such objects detected 9 VLM binaries. Most of\\u000athese systems are tight (separation <5 AU) and have similar masses (Delta\\u000aKs<0.8 mag; 0.85

Laird M. Close; Nick Siegler; Melanie Freed; Beth Biller

2003-01-01

217

A search for low mass companions and a new determination of effective temperatures for T-Tauri stars  

NASA Astrophysics Data System (ADS)

I present an analysis of over 300 spectra of the photospheres of young stars in order to address three outstanding questions of current interest: (1) whether or not the youngest stars are accompanied by massive substellar companions, (2) the degree to which starspots influence radial velocity measurements in young stars, and (3) establishment of a scale to convert spectral observations to effective temperatures and determining effective temperature for a large sample of stars. The first question is essential for understanding the star and planet formation process, the second question sets fundamental limits on ability of radial velocity surveys to detect young planets, and the third is a key for getting accurate mass and age estimates. Results from a new radial velocity survey of T Tauri stars focus on three specific stars---BP Tau, DN Tau, and LkCa19---with the largest number of observations. All the objects show far greater radial velocity variability than our radial velocity standards. Using a new simulation of the effect of star spots on the photometric and radial velocity variability of T Tauri star, I show the data for LkCa19 is fully consistent with variations caused by large star spots present on this rapidly rotating young star. In the case of BP Tau and DN Tau, the origin of the velocity variability is currently not well established. I also present the results of a new method to determine the effective temperature for weak-lined T Tauri stars, from the direct fitting of TiO bands near 7000 to NextGen synthetic spectra. The new temperatures are consistently warmer (by as much as 400K) than previous determinations based on spectral type. For the lowest mass young stars, the revised effective temperature doubles the mass inferred from evolutionary models.

Huerta, Marcos

218

A Search for Low Mass Companions and a New Determination of Effective Temperature for T-Tauri Stars  

NASA Astrophysics Data System (ADS)

We present an analysis of over 300 spectra of the photospheres of young stars in order to address three outstanding questions of current interest: (1) whether or not the youngest stars are accompanied by massive substellar companions, (2), the degree to which starspots influence radial velocity measurements in young stars, and (3) establishment of a scale to convert spectral observations to effective temperatures and determining effective temperature for a large sample of stars. The first question is essential for understanding the star and planet formation process, the second question sets fundamental limits on ability of radial velocity surveys to detect young planets, and the third is a key for getting accurate mass and age estimates. Results from a new radial velocity survey of T Tauri stars focus on three specific stars _ BP Tau, DN Tau, and LkCa19 _ with the largest number of observations. All the objects show far greater radial velocity variability than our radial velocity standards. Using a new simulation of the effect of star spots on the photometric and radial velocity variability of T Tauri star, we show the data for LkCa19 is fully consistent with variations caused by large star spots present on this rapidly rotating young star. In the case of BP Tau and DN Tau, the origin of the velocity variability is currently not well established. We also present the results of a new method to determine the effective temperature for weak-lined T Tauri stars, from the direct fitting of TiO bands near 7000Å to NextGen synthetic spectra. The new temperatures are consistently warmer (by as much as 400K) than previous determinations based on spectral type. For the lowest mass young stars, the revised effective temperature doubles the mass inferred from evolutionary models.

Huerta, Marcos; Hartigan, P.; Prato, L.; Johns-Krull, C. M.; Jaffe, D.

2007-05-01

219

Membership, binarity and accretion among very low-mass stars and brown dwarfs of the Sigma Orionis cluster  

Microsoft Academic Search

Intermediate resolution (R=7000) spectroscopy is presented for 76\\u000aphotometrically selected very low mass (0.04

M. J. Kenyon; R. D. Jeffries; Tim Naylor; J. M. Oliveira; P. F. L. Maxted

2004-01-01

220

THE MASS AND RADIUS OF THE NEUTRON STAR IN THE BULGE LOW-MASS X-RAY BINARY KS 1731-260  

SciTech Connect

Measurements of neutron star masses and radii are instrumental in determining the equation of state of their interiors, understanding the dividing line between neutron stars and black holes, and obtaining accurate statistics of source populations in the Galaxy. We report here on the measurement of the mass and radius of the neutron star in the low-mass X-ray binary KS 1731-260. The analysis of the spectroscopic data on multiple thermonuclear bursts yields well-constrained values for the apparent angular area and the Eddington flux of the source, both of which depend in a distinct way on the mass and radius of the neutron star. The binary KS 1731-260 is in the direction of the Galactic bulge, allowing a distance estimate based on the density of stars in that direction. Making use of the Han and Gould model, we determine the probability distribution over the distance to the source, which is approximately flat between 7 and 9 kpc. Combining these measurements, we place a strong upper bound on the radius of the neutron star, R {<=} 12.5 km, while confining its mass to M {<=} 2.1 M{sub Sun }.

Oezel, Feryal; Guever, Tolga [Department of Astronomy, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721 (United States); Gould, Andrew, E-mail: fozel@email.arizona.edu, E-mail: tguver@email.arizona.edu, E-mail: gould@astronomy.ohio-state.edu [Department of Astronomy, Ohio State University, 140 West 18th Avenue, Columbus, OH 43210 (United States)

2012-03-20

221

On the Identification of High-Mass Star Forming Regions Using IRAS: Contamination by Low-Mass Protostars  

NASA Astrophysics Data System (ADS)

We present the results of a survey of a small sample (14) of low-mass protostars (LIR<103 Lsolar) for 6.7 GHz methanol maser emission performed using the ATNF Parkes radio telescope. No new masers were discovered. We find that the lower luminosity limit for maser emission is near 103 Lsolar by comparison of the sources in our sample with previously detected methanol maser sources. We examine the IRAS properties of our sample and compare them with sources previously observed for methanol maser emission, almost all of which satisfy the Wood & Churchwell criterion for selecting candidate UC H II regions. We find that about half of our sample satisfy this criterion, and in addition, almost all of this subgroup have integrated fluxes between 25 and 60 ?m that are similar to sources with detectable methanol maser emission. By identifying a number of low-mass protostars in this work and from the literature that satisfy the Wood & Churchwell criterion for candidate UC H II regions, we show conclusively for the first time that the fainter flux end of their sample is contaminated by lower mass nonionizing sources, confirming the suggestion by van der Walt and Ramesh & Sridharan.

Bourke, Tyler L.; Hyland, A. R.; Robinson, Garry

2005-06-01

222

Revisiting Forbidden Lines in T Tauri stars  

NASA Astrophysics Data System (ADS)

Low excitation forbidden lines of [O I], [S II], and [N II] in the spectra of accreting young stars have long been recognized as mass outflow tracers due to their primarily blueshifted emission. The profiles often possess two kinematic components, a high velocity component (centroids from -50 to -200 km/s) arising in an extended collimated jet and a low velocity component (centroids from -5 to -10 km/s) possibly arising in some form of disk wind. Moreover, a recent paper by Rigliaco et al. (2013) explores the possibility that the low velocity component may itself be comprised of distinct broad and narrow kinematic contributions. Using high-resolution spectra acquired with the Keck I HIRES spectrograph, at a velocity resolution of 5 km/s, we aim to separate the various kinematic components in T Tauri forbidden lines. Observed profiles from lines of [O I] 6300, [0 I] 5577, and [S II] 6731 are decomposed via Gaussian fits into components that share kinematic features across multiple lines. For the high velocity components, we modernize the relation between mass ejection in the jets and mass accretion rates onto the star, originally found by Hartigan, Edwards, and Ghandor (1995). For the low velocity components, we confirm that a combination of broad and narrow components is commonly observed, and line ratios of each component are compared to those expected from models of slow photo-evaporative flows from the disk.

Feng, Wanda; Edwards, Suzan; Pascucci, Ilaria; Rigliaco, Elisabetta

2015-01-01

223

A THERMAL INFRARED IMAGING STUDY OF VERY LOW MASS, WIDE-SEPARATION BROWN DWARF COMPANIONS TO UPPER SCORPIUS STARS: CONSTRAINING CIRCUMSTELLAR ENVIRONMENTS  

SciTech Connect

We present a 3-5 {mu}m LBT/MMT adaptive optics imaging study of three Upper Scorpius stars with brown dwarf (BD) companions with very low masses/mass ratios (M{sub BD} <25 M{sub Jup}; M{sub BD}/M{sub *} Almost-Equal-To 1%-2%) and wide separations (300-700 AU): GSC 06214, 1RXS 1609, and HIP 78530. We combine these new thermal IR data with existing 1-4 {mu}m and 24 {mu}m photometry to constrain the properties of the BDs and identify evidence for circumprimary/circumsecondary disks in these unusual systems. We confirm that GSC 06214B is surrounded by a disk, further showing that this disk produces a broadband IR excess due to small dust near the dust sublimation radius. An unresolved 24 {mu}m excess in the system may be explained by the contribution from this disk. 1RXS 1609B exhibits no 3-4 {mu}m excess, nor does its primary; however, the system as a whole has a modest 24 {mu}m excess, which may come from warm dust around the primary and/or BD. Neither object in the HIP 78530 system exhibits near- to mid-IR excesses. We additionally find that the 1-4 {mu}m colors of HIP 78530B match a spectral type of M3 {+-} 2, inconsistent with the M8 spectral type assigned based on its near-IR spectrum, indicating that it may be a low-mass star rather than a BD. We present new upper limits on additional low-mass companions in the system (<5 M{sub Jup} beyond 175 AU). Finally, we examine the utility of circumsecondary disks as probes of the formation histories of wide BD companions, finding that the presence of a disk may disfavor BD formation near the primary with subsequent outward scattering.

Bailey, Vanessa; Hinz, Philip M.; Su, Kate Y. L.; Hoffmann, William F.; Rieke, George; Rodigas, Timothy; Skemer, Andrew; Vaitheeswaran, Vidhya [Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721 (United States)] [Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721 (United States); Currie, Thayne [Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4 (Canada)] [Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4 (Canada); Esposito, Simone; Pinna, Enrico; Puglisi, Alfio [Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, I-50125 Firenze (Italy)] [Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, I-50125 Firenze (Italy); Hill, John M. [Large Binocular Telescope Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721 (United States)] [Large Binocular Telescope Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721 (United States); Jones, Terry [School of Physics and Astronomy, University of Minnesota, 116 Church St. SE, Minneapolis, MN 55455 (United States)] [School of Physics and Astronomy, University of Minnesota, 116 Church St. SE, Minneapolis, MN 55455 (United States); Kim, Jihun [College of Optical Sciences, University of Arizona, 1630 E. University Blvd., Tucson, AZ 85721 (United States)] [College of Optical Sciences, University of Arizona, 1630 E. University Blvd., Tucson, AZ 85721 (United States); Leisenring, Jarron; Meyer, Michael [Institut fuer Angewandte Physik, Eidgenoessische Technische Hochschule-Zuerich, CH-8093 (Switzerland)] [Institut fuer Angewandte Physik, Eidgenoessische Technische Hochschule-Zuerich, CH-8093 (Switzerland); Murray-Clay, Ruth; Skrutskie, Michael F. [Harvard-Smithsonian Center for Astrophysics, Harvard University, 60 Garden St., Cambridge, MA 02138 (United States)] [Harvard-Smithsonian Center for Astrophysics, Harvard University, 60 Garden St., Cambridge, MA 02138 (United States); Nelson, Matthew J., E-mail: vbailey@as.arizona.edu [Department of Astronomy, University of Virginia, Charlottesville, VA 22904 (United States); and others

2013-04-10

224

The G+M Eclipsing Binary V530 Orionis: A Stringent Test of Magnetic Stellar Evolution Models for Low-mass Stars  

NASA Astrophysics Data System (ADS)

We report extensive photometric and spectroscopic observations of the 6.1 day period, G+M-type detached double-lined eclipsing binary V530 Ori, an important new benchmark system for testing stellar evolution models for low-mass stars. We determine accurate masses and radii for the components with errors of 0.7% and 1.3%, as follows: M A = 1.0038 ± 0.0066 M ?, M B = 0.5955 ± 0.0022 M ?, R A = 0.980 ± 0.013 R ?, and R B = 0.5873 ± 0.0067 R ?. The effective temperatures are 5890 ± 100 K (G1 V) and 3880 ± 120 K (M1 V), respectively. A detailed chemical analysis probing more than 20 elements in the primary spectrum shows the system to have a slightly subsolar abundance, with [Fe/H] = -0.12 ± 0.08. A comparison with theory reveals that standard models underpredict the radius and overpredict the temperature of the secondary, as has been found previously for other M dwarfs. On the other hand, models from the Dartmouth series incorporating magnetic fields are able to match the observations of the secondary star at the same age as the primary (~3 Gyr) with a surface field strength of 2.1 ± 0.4 kG when using a rotational dynamo prescription, or 1.3 ± 0.4 kG with a turbulent dynamo approach, not far from our empirical estimate for this star of 0.83 ± 0.65 kG. The observations are most consistent with magnetic fields playing only a small role in changing the global properties of the primary. The V530 Ori system thus provides an important demonstration that recent advances in modeling appear to be on the right track to explain the long-standing problem of radius inflation and temperature suppression in low-mass stars.

Torres, Guillermo; Sandberg Lacy, Claud H.; Pavlovski, Krešimir; Feiden, Gregory A.; Sabby, Jeffrey A.; Bruntt, Hans; Viggo Clausen, Jens

2014-12-01

225

Contributions of the Pulkovo and Kharkiv Scientific Schools to the search for exoplanets and low-mass dark satellites of stars  

NASA Astrophysics Data System (ADS)

This article is devoted to the Pulkovo astronomer, Prof. Aleksandr Nikolaevich Deich (Deutsch) (1899-1986), on the 110-th anniversary of his birth. Deich is known as the founder of the Pulkovo program for observing stars with invisible companions, as well as for his research on the star 61 Cyg, which was suspected, in his time, of having invisible companions with the masses of planets. Astrometric observations on the long focus astrograph and searches for exoplanets of nearby stars are reviewed. Modern methods of searching for exoplanets are summarized briefly. Instrument designs proposed by astronomers at Kharkiv (Scientific Research Institute of Astronomy at Kharkiv National University, NIIA KhNU) and Kazan (Institute of Astronomy, Kazan State University, AO KGU) for use in the search for low-mass dark components of stars are discussed. Examples are given of confirmations of invisible companions of stars which were first discovered by observation. A number of theoretical results on this topic from Kharkiv National University (Scientific Research Institute of Astronomy at Kharkiv and the Dept. of Astronomy) are noted.

Zakhozhay, V. A.; Gnedin, Yu. N.; Shakht, N. A.

2010-12-01

226

SHORT-DURATION LENSING EVENTS. I. WIDE-ORBIT PLANETS? FREE-FLOATING LOW-MASS OBJECTS? OR HIGH-VELOCITY STARS?  

SciTech Connect

Short-duration lensing events tend to be generated by low-mass lenses or by lenses with high transverse velocities. Furthermore, for any given lens mass and speed, events of short duration are preferentially caused by nearby lenses (mesolenses) that can be studied in detail, or else by lenses so close to the source star that finite-source-size effects may be detected, yielding information about both the Einstein ring radius and the surface of the lensed star. Planets causing short-duration events may be in orbits with any orientation, and may have semimajor axes smaller than 1 AU, or they may reach the outer limits of their planetary systems, in the region corresponding to the solar system's Oort Cloud. They can have masses larger than Jupiter's or smaller than Pluto's. Lensing therefore has a unique potential to expand our understanding of planetary systems. A particular advantage of lensing is that it can provide precision measurements of system parameters, including the masses of and projected separation between star and planet. We demonstrate how the parameters can be extracted and show that a great deal can be learned. For example, it is remarkable that the gravitational mass of nearby free-floating planet-mass lenses can be measured by complementing observations of a photometric event with deep images that detect the planet itself. A fraction of short events may be caused by high-velocity stars located within a kiloparsec. Many high-velocity lenses are likely to be neutron stars that received large natal kicks. Other high-speed stars may be members of the halo population. Still others may be hypervelocity stars that have been ejected from the Galactic center, or runaway stars escaped from close binaries, possibly including the progenitor binaries of Type Ia supernovae.

Di Stefano, Rosanne [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)

2012-08-01

227

A WIDELY SEPARATED, HIGHLY OCCLUDED COMPANION TO THE NEARBY LOW-MASS T TAURI STAR TWA 30  

E-print Network

We report the discovery of TWA 30B, a wide (~3400 AU), co-moving M dwarf companion to the nearby (~42 pc) young star TWA 30. Companionship is confirmed from their statistically consistent proper motions and radial velocities ...

Looper, Dagny L.

228

Lithium abundances along the red giant branch: FLAMES-GIRAFFE spectra of a large sample of low-mass bulge stars  

NASA Astrophysics Data System (ADS)

Context. A small number of K-type giants on the red giant branch (RGB) is known to be very rich in lithium (Li). This fact is not accounted for by standard stellar evolution theory. The exact phase and mechanism of Li enrichment is still a matter of debate. Aims: Our goal is to probe the abundance of Li along the RGB, from its base to the tip, to confine Li-rich phases that are supposed to occur on the RGB. Methods: For this end, we obtained medium-resolution spectra with the FLAMES spectrograph at the VLT in GIRAFFE mode for a large sample of 401 low-mass RGB stars located in the Galactic bulge. The Li abundance was measured in the stars with a detectable Li 670.8 nm line by means of spectral synthesis with COMARCS model atmospheres. A new 2MASS (J - KS) - Teff calibration from COMARCS models is presented in the Appendix. Results: Thirty-one stars with a detectable Li line were identified, three of which are Li-rich according to the usual criterion (log ?(Li) > 1.5). The stars are distributed all along the RGB, not concentrated in any particular phase of the red giant evolution (e.g. the luminosity bump or the red clump). The three Li-rich stars are clearly brighter than the luminosity bump and red clump, and do not show any signs of enhanced mass loss. Conclusions: We conclude that the Li enrichment mechanism cannot be restricted to a clearly defined phase of the RGB evolution of low-mass stars (M ~ 1 M?), contrary to earlier suggestions from disk field stars. Based on observations at the Very Large Telescope of the European Southern Observatory, Cerro Paranal/Chile under Programme 083.D-0046(A).Table 1 is only available 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/538/A36

Lebzelter, T.; Uttenthaler, S.; Busso, M.; Schultheis, M.; Aringer, B.

2012-02-01

229

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

NASA Technical Reports Server (NTRS)

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.

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

1986-01-01

230

VizieR Online Data Catalog: lam Ori and sig Ori low-mass stars spectroscopy (Maxted+, 2008)  

NASA Astrophysics Data System (ADS)

Radial velocities and spectral indices for 218 stars and brown dwarfs around {sigma} and {lambda} Orionis. Observations were obtained with the FLAMES/GIRAFFE multi-object spectrograph on ESO's VLT UT2 (Kueyen) telescope (programme ID 076.C_145). The equivalent width of the NaI 8200{AA} and the strength of the TiO 8442{AA} spectral features were measured from the median average spectrum. Radial velocities were measured by cross-correlation against a template spectrum of the brown dwarf star USco CTIO 055 at multiple epochs. (3 data files).

Maxted, P. F. L.; Jeffries, R. D.; Oliveira, J. M.; Naylor, T.; Jackson, R. J.

2008-05-01

231

A search for iron emission lines in the Chandra X-ray spectra of neutron star low-mass X-ray binaries  

E-print Network

While iron emission lines are well studied in black hole systems, both in X-ray binaries and Active Galactic Nuclei, there has been less of a focus on these lines in neutron star low-mass X-ray binaries (LMXBs). However, recent observations with Suzaku and XMM-Newton have revealed broad asymmetric iron line profiles in 4 neutron star LMXBs, confirming an inner disk origin for these lines in neutron star systems. Here, we present a search for iron lines in 6 neutron star LMXBs. For each object we have simultaneous Chandra and RXTE observations at 2 separate epochs, allowing for both a high resolution spectrum, as well as broadband spectral coverage. Out of the six objects in the survey, we only find significant iron lines in two of the objects, GX 17+2 and GX 349+2. However, we cannot rule out that there are weak, broad lines present in the other sources. The equivalent width of the line in GX 17+2 is consistent between the 2 epochs, while in GX 349+2 the line equivalent width increases by a factor of ~3 between epochs as the source flux decreases by a factor of 1.3. This suggests that the disk is highly ionized, and the line is dominated by recombination emission. We find that there appears to be no specific locations in the long-term hardness-intensity diagrams where iron emission lines are formed, though more sources and further observations are required.

E. M. Cackett; J. M. Miller; J. Homan; M. van der Klis; W. H. G. Lewin; M. Mendez; J. Raymond; D. Steeghs; R. Wijnands

2008-09-18

232

A SURVEY OF H{sub 2}O, CO{sub 2}, AND CO ICE FEATURES TOWARD BACKGROUND STARS AND LOW-MASS YOUNG STELLAR OBJECTS USING AKARI  

SciTech Connect

We present near-infrared spectroscopic observations of 19 molecular clouds made using the AKARI satellite, and the data reduction pipeline written to analyze those observations. The 2.5-5 ?m spectra of 30 objects—22 field stars behind quiescent molecular clouds and 8 low-mass young stellar objects in cores—were successfully extracted using the pipeline. Those spectra are further analyzed to calculate the column densities of key solid phase molecular species, including H{sub 2}O, CO{sub 2}, CO, and OCN{sup –}. The profile of the H{sub 2}O ice band is seen to vary across the objects observed and we suggest that the extended red wing may be an evolutionary indicator of both dust and ice mantle properties. The observation of 22 spectra with fluxes as low as < 5 mJy toward background stars, including 15 where the column densities of H{sub 2}O, CO, and CO{sub 2} were calculated, provides valuable data that could help to benchmark the initial conditions in star-forming regions prior to the onset of star formation.

Noble, J. A. [Aix-Marseille Université, PIIM UMR 7345, F-13397 Marseille (France); Fraser, H. J. [Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA (United Kingdom); Aikawa, Y. [Department of Earth and Planetary Sciences, Kobe University, Kobe 657-8501 (Japan); Pontoppidan, K. M. [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States); Sakon, I., E-mail: helen.fraser@open.ac.uk [Department of Astronomy, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0003 (Japan)

2013-10-01

233

On the origin of the IMF: First detection of a low-mass star ejected from a triple stellar system  

NASA Astrophysics Data System (ADS)

Using high-resolution, multi-epoch VLA observations, we have detected orbital motions in several low-luminosity protobinary systems in the Taurus and rho-Ophiuchus molecular complexes. The masses obtained from Kepler's third law are of the order of 0.5 to 1 Msun, as would have been expected for such low-mass protostars. In addition, in one of the sources studied (a triple system in Taurus), one of the three component appears to have been recently ejected from the system. During the first 15 of the 20 years covered by the observations, this component has been on a closed elliptical orbit with a velocity of a few km/s, but in the last 5 years, it has started to spiral out at high speed (20 km/s). Such an ejection is not unexpected in a triple system, because such systems are thought to exhibit chaotic behaviours. However, this is the first time that it is detected directly. The implications for the IMF will be discussed

Loinard, L.; Rodriguez, L. F.; Rodriguez, M.

2002-12-01

234

WASP 1628+10 - an EL CVn-type binary with a very low mass stripped red giant star and multiperiodic pulsations  

NASA Astrophysics Data System (ADS)

The star 1SWASP J162842.31+101416.7 (WASP 1628+10) is one of several EL CVn-type stars recently identified using the Wide Angle Search for Planets (WASP) data base, i.e. an eclipsing binary star in which an A-type dwarf star (WASP 1628+10 A) eclipses the remnant of a disrupted red giant star (WASP 1628+10 B). We have measured the masses, radii and luminosities of the stars in WASP 1628+10 using photometry obtained in three bands (u', g', r') with the ULTRACAM instrument and medium-resolution spectroscopy. The properties of the remnant are well matched by models for stars in a rarely observed state evolving to higher effective temperatures at nearly constant luminosity prior to becoming a very low mass white dwarf composed almost entirely of helium, i.e. we confirm that WASP 1628+10 B is a precursor of a helium white dwarf (pre-He-WD). WASP 1628+10 A appears to be a normal A2 V star with a mass of 1.36 ± 0.05 M?. By fitting models to the spectrum of this star around the H? line we find that it has an effective temperature Teff, A = 7500 ± 200 K and a metallicity [Fe/H] = -0.3 ± 0.3. The mass of WASP 1628+10 B is only 0.135 ± 0.02 M?. The effective temperature of this pre-He-WD is approximately 9200 K. The ULTRACAM photometry of WASP 1628+10 shows variability at several frequencies around 40 cycles d-1, which is typical for ? Sct-type pulsations often observed in early A-type stars like WASP 1628+10 A. We also observe frequencies near 114 and 129 cycles d-1, much higher than the frequencies normally seen in ? Sct stars. Additional photometry through the primary eclipse will be required to confirm that these higher frequencies are due to pulsations in WASP 1628+10 B. If confirmed, this would be only the second known example of a pre-He-WD showing high-frequency pulsations.

Maxted, P. F. L.; Serenelli, A. M.; Marsh, T. R.; Catalán, S.; Mahtani, D. P.; Dhillon, V. S.

2014-10-01

235

Photometric study to understand the ambiguity between accretion and chromospheric activity present in low-mass stars  

NASA Astrophysics Data System (ADS)

We conducted a photometric study of a sample of pre-main sequence and TTauri stars in a spectral range between G-K and some early M of masses from 0.5 to 3 M_?, in associations near the Sun (20-200 pc). We measured the excess of the UV band and found that for stars of spectral type early M and late K, the UV band increases considerably. There are two possible scenarios for such an increase: (1) Chromospheric activity, because the stars have radiative and convective cores, giving rise to magnetic activity which generates this excess present in the continuum and (2) the processes of accretion present in stars that have a disk of gas and dust, which they accrete to them by means of the magnetic lines field. Because of the ambiguity in the two effects (since they affect the same emission lines) it is difficult to distinguish which dominates. We propose that an excess of 0.2 magnitudes in the U band for our sample allows us to determine if both processes are at work, or if chromospheric activity dominates, since accretion stops at a very early age, 10 million years.

Restrepo, O. A.

2014-06-01

236

Astrometric confirmation of young low-mass binaries and multiple systems in the Chamaeleon star-forming regions  

E-print Network

The star-forming regions in Chamaeleon are one of the nearest (distance ~165 pc) and youngest (age ~2 Myrs) conglomerates of recently formed stars and the ideal target for population studies of star formation. We investigate a total of 16 Cha targets, which have been suggested, but not confirmed as binaries or multiple systems in previous literature. We used the adaptive optics instrument Naos-Conica (NACO) at the Very Large Telescope Unit Telescope 4 of the Paranal Observatory, at 2-5 different epochs, in order to obtain relative and absolute astrometric measurements, as well as differential photometry in the J, H, and K band. On the basis of known proper motions and these observations, we analyse the astrometric results in our "Proper Motion Diagram" (PMD: angular separation / position angle versus time), to eliminate possible (non-moving) background stars, establish co-moving binaries and multiples, and search for curvature as indications for orbital motion. All previously suggested close components are co...

Vogt, N; Neuhäuser, R; Bedalov, A; Roell, T; Seifahrt, A; Mugrauer, M

2012-01-01

237

Companions to white dwarfs - Very low-mass stars and the brown dwarf candidate GD 165B  

Microsoft Academic Search

The initial mass function (IMF) for star formation at the bottom of the main sequence is estimated by measuring the number of low-luminosity companions to white dwarfs. A histogram of the number of companions versus luminosity indicates that the IMF is flat or increasing with decreasing stellar mass down to, at least, 0.1 solar mass. It is shown that GD

B. Zuckerman; E. E. Becklin

1992-01-01

238

On the bolometric quiescent luminosity and luminosity swing of black hole candidate and neutron star low mass X-ray transients  

E-print Network

Low mass X-ray transients (LMXRTs) hosting black hole candidates (BHCs) display on average a factor of ~100 larger swing in the minimum (quiescent) to maximum (outburst) X-ray luminosity than neutron stars (NSs), despite the fact that the swing in the mass inflow rate is likely in the same range. Advection dominated accretion flows (ADAFs) were proposed to interpret such a difference. The residual optical/UV emission of quiescent LMXRTs, after subtraction of the companion star spectrum, is produced by synchrotron radiation in the (latest version) of ADAF and therefore is part of the ADAF's luminosity budget. We demonstrate that, once the residual optical/UV emission is taken into account, the bolometric luminosity swing of BHCs is consistent with that of NSs. We explore here an alternative scenario to ADAFs in which very little mass accretion onto the collapsed star takes place in the quiescence intervals. The residual optical/UV emission of BHCs are expected to derive from the energy released by the matter transferred from the companion star at radii comparable to the circularisation radius. The quiescent X-ray luminosity originates either from accretion onto the BH at very low rates and/or from coronal activity in the companion star or in the outer disk. For comparably small mass inflow rates, the NSs in these systems are likely in the radio pulsar regime. In the interaction of the radio pulsar relativistic wind with matter transferred from the companion star, a shock forms, the power law-like emission of which powers both the harder X-ray emission and most of the residual optical/UV. The soft, thermal-like X-ray component may arise from the cooling of the NS surface. This scenario matches well both the X-ray and bolometric luminosity swing of LMXRTs. (ABRIDGED).

Sergio Campana; Luigi Stella

2000-05-07

239

BANYAN. V. A Systematic All-sky Survey for New Very Late-type Low-mass Stars and Brown Dwarfs in Nearby Young Moving Groups  

NASA Astrophysics Data System (ADS)

We present the BANYAN All-Sky Survey (BASS) catalog, consisting of 228 new late-type (M4-L6) candidate members of nearby young moving groups (YMGs) with an expected false-positive rate of ~13%. This sample includes 79 new candidate young brown dwarfs and 22 planetary-mass objects. These candidates were identified through the first systematic all-sky survey for late-type low-mass stars and brown dwarfs in YMGs. We cross-matched the Two Micron All Sky Survey and AllWISE catalogs outside of the galactic plane to build a sample of 98,970 potential >=M5 dwarfs in the solar neighborhood and calculated their proper motions with typical precisions of 5-15 mas yr–1. We selected highly probable candidate members of several YMGs from this sample using the Bayesian Analysis for Nearby Young AssociatioNs II tool (BANYAN II). We used the most probable statistical distances inferred from BANYAN II to estimate the spectral type and mass of these candidate YMG members. We used this unique sample to show tentative signs of mass segregation in the AB Doradus moving group and the Tucana-Horologium and Columba associations. The BASS sample has already been successful in identifying several new young brown dwarfs in earlier publications, and will be of great interest in studying the initial mass function of YMGs and for the search of exoplanets by direct imaging; the input sample of potential close-by >=M5 dwarfs will be useful to study the kinematics of low-mass stars and brown dwarfs and search for new proper motion pairs.

Gagné, Jonathan; Lafrenière, David; Doyon, René; Malo, Lison; Artigau, Étienne

2015-01-01

240

Kilohertz quasi-periodic oscillations in low mass X-ray binary sources and their relation with the neutron star magnetic field  

E-print Network

Starting from the observation that kilohertz Quasi Period Oscillations (kHz QPO) occur in a very narrow range of X-ray luminosities in neutron star low mass X-ray binaries, we try to link the kHz QPO observability to variations of the neutron star magnetospheric radius, in response to changing mass inflow rate. At low luminosities, the drop off of kHz QPOs activity may be explained by the onset of the centrifugal barrier, when the magnetospheric radius reaches the corotation radius. At the opposite side, at higher luminosities, the magnetospheric radius may reach the neutron star and the vanishing of the magnetosphere may led to the stopping of the kHz QPOs activity. If we apply these constraints, the magnetic fields of atoll (B ~ 0.3-1 10^8 G for Aql X-1) and Z (B ~ 1-8 10^8 G for Cyg X-2) sources can be derived. These limits naturally apply in the framework of beat frequency models but can also work in the case of general relativistic models.

Sergio Campana

2000-03-15

241

Long-duration X-Ray Flash and X-Ray-rich Gamma-Ray Bursts from Low-mass Population III Stars  

NASA Astrophysics Data System (ADS)

Recent numerical simulations suggest that Population III (Pop III) stars were born with masses not larger than ~100 M ? and typically ~40 M ?. By self-consistently considering the jet generation and propagation in the envelope of these low-mass Pop III stars, we find that a Pop III blue supergiant star has the possibility of giving rise to a gamma-ray burst (GRB) even though it keeps a massive hydrogen envelope. We evaluate observational characteristics of Pop III GRBs and predict that Pop III GRBs have a duration of ~105 s in the observer frame and a peak luminosity of ~5 × 1050 erg s-1. Assuming that the E p-L p (or E p-E ?, iso) correlation holds for Pop III GRBs, we find that the spectrum peak energy falls at approximately a few keV (or ~100 keV) in the observer frame. We discuss the detectability of Pop III GRBs by future satellite missions such as EXIST and Lobster. If the E p-E ?, iso correlation holds, we have the possibility to detect Pop III GRBs at z ~ 9 as long-duration X-ray-rich GRBs by EXIST. Conversely, if the E p-L p correlation holds, we have the possibility to detect Pop III GRBs up to z ~ 19 as long-duration X-ray flashes by Lobster.

Nakauchi, Daisuke; Suwa, Yudai; Sakamoto, Takanori; Kashiyama, Kazumi; Nakamura, Takashi

2012-11-01

242

A Search for Very Low-mass Stars and Brown Dwarfs in the Young sigma Orionis Cluster  

Microsoft Academic Search

We present a CCD-based photometric survey covering 870 sq. arcmin in a young\\u000astellar cluster around the young multiple star sigma Orionis. Our survey\\u000alimiting R, I, and Z magnitudes are 23.2, 21.8, and 21.0, respectively. From\\u000aour colour-magnitude diagrams, we have selected 49 faint objects, which\\u000asmoothly extrapolate the photometric sequence defined by more massive known\\u000amembers. Adopting the

V. J. S. Bejar; M. R. Zapatero Osorio; R. Rebolo

1999-01-01

243

Quark-Novae in Low-Mass X-ray Binaries with massive neutron stars: A universal model for short-hard Gamma-Ray Bursts  

E-print Network

We show that several features reminiscent of short-hard Gamma-ray Bursts (GRBs) arise naturally when Quark-Novae occur in low-mass X-ray binaries born with massive neutron stars (> 1.6M_sun) and harboring a circumbinary disk. Near the end of the first accretion phase, conditions are just right for the explosive conversion of the neutron star to a quark star (Quark-Nova). In our model, the subsequent interaction of material from the neutron star's ejected crust with the circumbinary disk explains the duration, variability and near-universal nature of the prompt emission in short-hard GRBs. We also describe a statistical approach to ejecta break-up and collision to obtain the photon spectrum in our model, which turns out remarkably similar to the empirical Band function (Band 1993). We apply the model to the fluence and spectrum of GRB 000727, GRB 000218, and GRB980706A obtaining excellent fits. Extended emission (spectrum and duration) is explained by shock-heating and ablation of the white dwarf by the highly energetic ejecta. Depending on the orbital separation when the Quark-Nova occurs, we isolate interesting regimes within our model when both prompt and extended emission can occur. We find that the spectrum can carry signatures typical of Type Ib/c SNe although these should appear less luminous than normal type Ib/c SNe. Late X-ray activity is due to accretion onto the quark star as well as its spin-down luminosity. Afterglow activity arise from the expanding shell of material from the shock-heated expanding circumbinary disk. We find a correlation between the duration and spectrum of short-hard GRBs as well as modest hard-to-soft time evolution of the peak energy.

Rachid Ouyed; Jan E. Staff; Prashanth Jaikumar

2011-01-16

244

A CHANGE IN THE QUIESCENT X-RAY SPECTRUM OF THE NEUTRON STAR LOW-MASS X-RAY BINARY MXB 1659-29  

SciTech Connect

The quasi-persistent neutron star low-mass X-ray binary MXB 1659-29 went into quiescence in 2001, and we have followed its quiescent X-ray evolution since. Observations over the first 4 yr showed a rapid drop in flux and temperature of the neutron star atmosphere, interpreted as cooling of the neutron star crust which had been heated during the 2.5 yr outburst. However, observations taken approximately 1400 and 2400 days into quiescence were consistent with each other, suggesting the crust had reached thermal equilibrium with the core. Here we present a new Chandra observation of MXB 1659-29 taken 11 yr into quiescence and 4 yr since the last Chandra observation. This new observation shows an unexpected factor of {approx}3 drop in count rate and change in spectral shape since the last observation, which cannot be explained simply by continued cooling. Two possible scenarios are that either the neutron star temperature has remained unchanged and there has been an increase in the column density, or, alternatively the neutron star temperature has dropped precipitously and the spectrum is now dominated by a power-law component. The first scenario may be possible given that MXB 1659-29 is a near edge-on system, and an increase in column density could be due to build-up of material in, and a thickening of, a truncated accretion disk during quiescence. But, a large change in disk height may not be plausible if standard accretion disk theory holds during quiescence. Alternatively, the disk may be precessing, leading to a higher column density during this latest observation.

Cackett, E. M. [Department of Physics and Astronomy, Wayne State University, 666 W. Hancock Street, Detroit, MI 48201 (United States); Brown, E. F. [Department of Physics and Astronomy, National Superconducting Cyclotron Laboratory, and the Joint Institute for Nuclear Astrophysics, Michigan State University, East Lansing, MI 48824 (United States); Cumming, A. [Department of Physics, McGill University, 3600 rue University, Montreal, QC H3A 2T8 (Canada); Degenaar, N.; Miller, J. M. [Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042 (United States); Fridriksson, J. K.; Wijnands, R. [Astronomical Institute ''Anton Pannekoek'', University of Amsterdam, Science Park 904, 1098-XH Amsterdam (Netherlands); Homan, J., E-mail: ecackett@wayne.edu [Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 70 Vassar Street, Cambridge, MA 02139 (United States)

2013-09-10

245

A Thermal Infrared Imaging Study of Very Low Mass, Wide-separation Brown Dwarf Companions to Upper Scorpius Stars: Constraining Circumstellar Environments  

NASA Astrophysics Data System (ADS)

We present a 3-5 ?m LBT/MMT adaptive optics imaging study of three Upper Scorpius stars with brown dwarf (BD) companions with very low masses/mass ratios (M BD <25 M Jup; M BD/M sstarf ? 1%-2%) and wide separations (300-700 AU): GSC 06214, 1RXS 1609, and HIP 78530. We combine these new thermal IR data with existing 1-4 ?m and 24 ?m photometry to constrain the properties of the BDs and identify evidence for circumprimary/circumsecondary disks in these unusual systems. We confirm that GSC 06214B is surrounded by a disk, further showing that this disk produces a broadband IR excess due to small dust near the dust sublimation radius. An unresolved 24 ?m excess in the system may be explained by the contribution from this disk. 1RXS 1609B exhibits no 3-4 ?m excess, nor does its primary; however, the system as a whole has a modest 24 ?m excess, which may come from warm dust around the primary and/or BD. Neither object in the HIP 78530 system exhibits near- to mid-IR excesses. We additionally find that the 1-4 ?m colors of HIP 78530B match a spectral type of M3 ± 2, inconsistent with the M8 spectral type assigned based on its near-IR spectrum, indicating that it may be a low-mass star rather than a BD. We present new upper limits on additional low-mass companions in the system (<5 M Jup beyond 175 AU). Finally, we examine the utility of circumsecondary disks as probes of the formation histories of wide BD companions, finding that the presence of a disk may disfavor BD formation near the primary with subsequent outward scattering. Observations reported here were obtained at the LBT and MMT Observatories. The MMT Observatory is a joint facility of the University of Arizona and the Smithsonian Institution. The LBT is an international collaboration among institutions in the United States, Italy, and Germany. LBT Corporation partners are: The University of Arizona on behalf of the Arizona university system; Istituto Nazionale di Astrofisica, Italy; LBT Beteiligungsgesellschaft, Germany, representing the Max-Planck Society, the Astrophysical Institute Potsdam, and Heidelberg University; The Ohio State University; and The Research Corporation, on behalf of The University of Notre Dame, University of Minnesota, and University of Virginia.

Bailey, Vanessa; Hinz, Philip M.; Currie, Thayne; Su, Kate Y. L.; Esposito, Simone; Hill, John M.; Hoffmann, William F.; Jones, Terry; Kim, Jihun; Leisenring, Jarron; Meyer, Michael; Murray-Clay, Ruth; Nelson, Matthew J.; Pinna, Enrico; Puglisi, Alfio; Rieke, George; Rodigas, Timothy; Skemer, Andrew; Skrutskie, Michael F.; Vaitheeswaran, Vidhya; Wilson, John C.

2013-04-01

246

INFRARED AND OPTICAL POLARIMETRY AROUND THE LOW-MASS STAR-FORMING REGION NGC 1333 IRAS 4A  

SciTech Connect

We performed J- and R-band linear polarimetry with the 4.2 m William Herschel Telescope at the Observatorio del Roque de los Muchachos and with the 1.6 m telescope at the Observatorio do Pico dos Dias, respectively, to derive the magnetic field geometry of the diffuse molecular cloud surrounding the embedded protostellar system NGC 1333 IRAS 4A. We obtained interstellar polarization data for about three dozen stars. The distribution of polarization position angles has low dispersion and suggests the existence of an ordered magnetic field component at physical scales larger than the protostar. Some of the observed stars present intrinsic polarization and evidence of being young stellar objects. The estimated mean orientation of the interstellar magnetic field as derived from these data is almost perpendicular to the main direction of the magnetic field associated with the dense molecular envelope around IRAS 4A. Since the distribution of the CO emission in NGC 1333 indicates that the diffuse molecular gas has a multi-layered structure, we suggest that the observed polarization position angles are caused by the superposed projection of different magnetic field components along the line of sight.

Alves, Felipe O.; Girart, Josep M. [Institut de Ciencies de l'Espai (IEEC-CSIC), Campus UAB, Facultat de Ciencies, 08193 Bellaterra, Catalunya (Spain); Acosta-Pulido, Jose A. [Instituto de Astrofisica de Canarias, E-38200 La Laguna, Tenerife (Spain); Franco, Gabriel A. P. [Departamento de Fisica-ICEx-UFMG, 30.123-970 Belo Horizonte (Brazil); Lopez, Rosario, E-mail: oliveira@ice.cat, E-mail: girart@ice.cat, E-mail: jap@iac.es, E-mail: franco@fisica.ufmg.br, E-mail: rosario@am.ub.es, E-mail: falves@astro.uni-bonn.de [Departament d'Astronomia i Meteorologia (IEEC-UB), Institut de Ciencies del Cosmos, Universitat de Barcelona, MartI i Franques 1, E-08028 Barcelona (Spain)

2011-07-15

247

Dark-matter halo mergers as a fertile environment for low-mass Population III star formation  

E-print Network

While Population III stars are typically thought to be massive, pathways towards lower-mass Pop III stars may exist when the cooling of the gas is particularly enhanced. A possible route is enhanced HD cooling during the merging of dark-matter halos. The mergers can lead to a high ionization degree catalysing the formation of HD molecules and may cool the gas down to the cosmic microwave background (CMB) temperature. In this paper, we investigate the merging of mini-halos with masses of a few 10$^5$~M$_\\odot$ and explore the feasibility of this scenario. We have performed three-dimensional cosmological hydrodynamics calculations with the ENZO code, solving the thermal and chemical evolution of the gas by employing the astrochemistry package KROME. Our results show that the HD abundance is increased by two orders of magnitude compared to the no-merging case and the halo cools down to $\\sim$60 K triggering fragmentation. Based on Jeans estimates the expected stellar masses are about 10 M$_\\odot$. Our findings s...

Bovino, S; Grassi, T; Schleicher, D R G

2014-01-01

248

NLTE in a Hot Hydrogen Star: Auer & Mihalas Revisited  

NASA Astrophysics Data System (ADS)

We pay tribute to two landmark papers published by Auer & Mihalas in 1969. They modeled hot-star NLTE-RE hydrogen-only atmospheres, using two simplified hydrogen atoms: ApJ 156, 157: H I levels 1, 2 and c, Lyman ? the only line ApJ 156, 681: H I levels 1, 2, 3 and c, Balmer ? the only line and computed LTE and NLTE models with the single line turned on and off. The results were extensively analyzed in the two papers. Any student of stellar line formation should take these beautiful papers to heart. The final exercise in Rutten's lecture notes ``Radiative Transfer in Stellar Atmospheres'' asks the student to work through five pages of questions concerning diagrams from the first paper alone! That exercise led to the present work in which we recompute the Auer-Mihalas hot-hydrogen-star models with TLUSTY, adding results from a complete hydrogen atom for comparison. Our motivation for this Auer-Mihalas re-visitation is twofold: 1. to add diagnostic diagrams to the ones published by Auer & Mihalas, in particular B?, J?, S? graphs to illustrate the role of the radiation field, and radiative heating & cooling graphs to illustrate the radiative energy budget, 2. to see the effect of adding the rest of the hydrogen atom.

Wiersma, J.; Rutten, R. J.; Lanz, T.

2003-01-01

249

Age-Related Observations of Low Mass Pre-Main and Young Main Sequence Stars (Invited Review)  

E-print Network

This overview summarizes the age dating methods available for young sub-solar mass stars. Pre-main sequence age diagnostics include the Hertzsprung-Russell (HR) diagram, spectroscopic surface gravity indicators, and lithium depletion; asteroseismology is also showing recent promise. Near and beyond the zero-age main sequence, rotation period or vsini and activity (coronal and chromospheric) diagnostics along with lithium depletion serve as age proxies. Other authors in this volume present more detail in each of the aforementioned areas. Herein, I focus on pre-main sequence HR diagrams and address the questions: Do empirical young cluster isochrones match theoretical isochrones? Do isochrones predict stellar ages consistent with those derived via other independent techniques? Do the observed apparent luminosity spreads at constant effective temperature correspond to true age spreads? While definitive answers to these questions are not provided, some methods of progression are outlined.

Lynne A. Hillenbrand

2008-12-06

250

Recent Stellar Mass Assembly of Low-mass Star-forming Galaxies at Redshifts 0.3 < z < 0.9  

NASA Astrophysics Data System (ADS)

The epoch when low-mass star-forming galaxies (LMSFGs) form the bulk of their stellar mass is uncertain. While some models predict an early formation, others favor a delayed scenario until later ages of the universe. We present constraints on the star formation histories (SFHs) of a sample of LMSFGs obtained through the analysis of their spectral energy distributions using a novel approach that (1) consistently combines photometric (broadband) and spectroscopic (equivalent widths of emission lines) data, and (2) uses physically motivated SFHs with non-uniform variations of the star formation rate (SFR) as a function of time. The sample includes 31 spectroscopically confirmed LMSFGs (7.3 <= log M */M ? <= 8.0), at 0.3 < z spec < 0.9, in the Extended-Chandra Deep Field-South field. Among them, 24 were selected with photometric stellar mass log M */M ? < 8.0, 0.3 < z phot < 1.0, and m NB816, AB < 26 mag; the remaining 7 were selected as blue compact dwarfs within the same photometric redshift and magnitude ranges. We also study a secondary sample of 43 more massive spectroscopically confirmed galaxies (8.0 < log M */M ? <= 9.1), selected with the same criteria. The SFRs and stellar masses derived for both samples place our targets on the standard main sequence of star-forming galaxies. The median SFH of LMSFGs at intermediate redshifts appears to form 90% of the median stellar mass inferred for the sample in the ~0.5-1.8 Gyr immediately preceding the observation. These results suggest a recent stellar mass assembly for LMSFGs, consistent with the cosmological downsizing trends. We find similar median SFH timescales for the more massive secondary sample. Based on observations carried out with the European Southern Observatory (ESO) Very Large Telescope (VLT) at the La Silla Paranal Observatory under programs 088.A-0321 and 090.A-0858.

Rodríguez-Muñoz, Lucía; Gallego, Jesús; Pacifici, Camilla; Tresse, Laurence; Charlot, Stéphane; Gil de Paz, Armando; Barro, Guillermo; Villar, Víctor

2015-01-01

251

Testing Model Atmospheres for Young Very Low Mass Stars and Brown Dwarfs in the Infrared: Evidence for Significantly Underestimated Dust Opacities  

E-print Network

We test state-of-the-art model atmospheres for young very low-mass stars and brown dwarfs in the infrared, by comparing the predicted synthetic photometry over 1.2-24 {\\mu}m to the observed photometry of M-type spectral templates in star-forming regions. We find that (1) in both early and late young M types, the model atmospheres imply effective temperatures (Teff) several hundred Kelvin lower than predicted by the standard Pre-Main Sequence spectral type-Teff conversion scale (where the latter is based on theoretical evolutionary models). It is only in the mid-M types that the two temperature estimates agree. (2) The Teff discrepancy in the early M types (corresponding to stellar masses above 0.6 Msol at a few Myr) probably arises from remaining uncertainties in the treatment of atmospheric convection. The agreement in the mid-M types implies a reasonably good atmospheric modeling of H2O opacities, which dominate in the infrared at these spectral types. Conversely, the Teff discrepancy in the late M types is...

Tottle, Jonathan

2014-01-01

252

Spectral Softening Between Outburst and Quiescence In The Neutron Star Low-Mass X-Ray Binary SAX J1750.8-2900  

E-print Network

Tracking the spectral evolution of transiently accreting neutron stars between outburst and quiescence probes relatively poorly understood accretion regimes. Such studies are challenging because they require frequent monitoring of sources with luminosities below the thresholds of current all-sky X-ray monitors. We present the analysis of over 30 observations of the neutron star low-mass X-ray binary SAX J1750.8-2900 taken across four years with the X-ray telescope aboard Swift. We find spectral softening with decreasing luminosity both on long ($\\sim$1 year) and short ($\\sim$days to week) timescales. As the luminosity decreases from $4\\times10^{36}$ erg s$^{-1}$ to $ \\sim1\\times10^{35} $ erg s$^{-1}$ (0.5-10 keV), the power law photon index increases from from 1.4 to 2.9. Although not statistically required, our spectral fits allow an additional soft component that displays a decreasing temperature as the luminosity decreases from $4 \\times 10^{36} $ to $6 \\times 10^{34}$ erg s$^{-1}$. Spectral softening exhi...

Allen, Jessamyn L; Homan, Jeroen; Chakrabarty, Deepto

2015-01-01

253

VizieR Online Data Catalog: RJHKs photometry of sigma Ori low-mass stars (Scholz+, 2004)  

NASA Astrophysics Data System (ADS)

Coordinates and photometry of probable cluster members in the sigma Ori cluster. Time series observations of this cluster were carried out using the CCD cameras at the 2m Schmidt telescope of the Thuringer Landessternwarte Tautenburg (TLS) and the 1.23m telescope on Calar Alto (CA). From these images, we derived coordinates (columns 2 and 3) as well as photometry in the R- and I-band (columns 5 and 6). Additional near- infrared photometry in J, H, K comes from the 2MASS database (columns 7-9). The cluster member selection is based on (I,I-J) colour magnitude diagrams. The R-I and J-H colours were used to reject contaminating field stars. The column 4 indicates whether the object was identified with the CA photometry (c), the TLS photometry (t), or both. The masses given in column 10 were estimated by comparing the near-infrared photometry with the evolutionary tracks of Baraffe et al. (1998A&A...337..403). These tracks cover the mass range from 0.02 to 1.4 solar masses, for some objects with lower or higher masses we give upper or lower mass limits. The last two columns contain information about the results of the time series analysis. Column 10 gives the RMS of the CA lightcurve for targets No 1-52 and the RMS of the TLS lightcurve for all others. The last column indicates whether the target is classified as variable (v) and/or periodically variable (p). For a few objects, time series analysis was not possible, because they are either too faint or too bright in most of the images. (1 data file).

Scholz, A.; Eisloeffel, J.

2004-03-01

254

SOPHIE velocimetry of Kepler transit candidates. XIII. KOI-189 b and KOI-686 b: two very low-mass stars in long-period orbits  

NASA Astrophysics Data System (ADS)

We present the radial-velocity follow-up of two Kepler planetary transiting candidates (KOI-189 and KOI-686) carried out with the SOPHIE spectrograph at the Observatoire de Haute Provence. These data promptly discard these objects as viable planet candidates and show that the transiting objects are in the regime of very low-mass stars, where a strong discrepancy between observations and models persists for the mass and radius parameters. By combining the SOPHIE spectra with the Kepler light curve and photometric measurements found in the literature, we obtain a full characterization of the transiting companions, their orbits, and their host stars. The two companions are in significantly eccentric orbits with relatively long periods (30 days and 52.5 days), which makes them suitable objects for a comparison with theoretical models, since the effects invoked to understand the discrepancy with observations are weaker for these orbital distances. KOI-189 b has a mass M = 0.0745 ± 0.0033 M? and a radius R = 0.1025 ± 0.0024 R?. The density of KOI-189 b is significantly lower than expected from theoretical models for a system of its age. We explore possible explanations for this difference. KOI-189 b is the smallest hydrogen-burning star with such a precise determination of its fundamental parameters. KOI-686 b is larger and more massive (M = 0.0915 ± 0.0043 M?; R = 0.1201 ± 0.0033 R?), and its position in the mass-radius diagram agrees well with theoretical expectations. Based on observations collected with the SOPHIE spectrograph on the 1.93 m telescope at Observatoire de Haute-Provence (CNRS), France (programs 11A.PNP.MOUT and 11B.PNP.MOUT).Tables 1, 2, and 6 are available in electronic form at http://www.aanda.org

Díaz, R. F.; Montagnier, G.; Leconte, J.; Bonomo, A. S.; Deleuil, M.; Almenara, J. M.; Barros, S. C. C.; Bouchy, F.; Bruno, G.; Damiani, C.; Hébrard, G.; Moutou, C.; Santerne, A.

2014-12-01

255

THE RELATIONSHIP BETWEEN {nu}{sub max} AND AGE t FROM ZAMS TO RGB-TIP FOR LOW-MASS STARS  

SciTech Connect

Stellar age is an important quantity in astrophysics, which is useful for many fields both in the universe and galaxies. It cannot be determined by direct measurements, but can only be estimated or inferred. We attempt to find a useful indicator of stellar age, which is accurate from the zero-age main sequence to the tip of red giant branch for low-mass stars. Using the Yale Rotation and Evolution Code (YREC), a grid of stellar models has been constructed. Meanwhile, the frequency of maximum oscillations' power {nu}{sub max} and the large frequency separation {Delta}{nu} are calculated using the scaling relations. For the stars, the masses of which are from 0.8 M{sub Sun} to 2.8 M{sub Sun }, we can obtain the {nu}{sub max} and stellar age by combing the scaling relations with the four sets of grid models (YREC, Dotter et al., Marigo et al., and YY isochrones). We find that {nu}{sub max} is tightly correlated and decreases monotonically with the age of the star from the main sequence to the red giant evolutionary stages. Moreover, we find that the line shapes of the curves in the Age versus {nu}{sub max} diagram, which is plotted by the four sets of grid models, are consistent for red giants with masses from 1.1 M{sub Sun} to 2.8 M{sub Sun }. For red giants, the differences of correlation coefficients between Age and {nu}{sub max} for different grid models are minor and can be ignored. Interestingly, we find two peaks that correspond to the subgiants and bump of red giants in the Age versus {nu}{sub max} diagram. By general linear least-squares, we make the polynomial fitting and deduce the relationship between log(Age) and log({nu}{sub max}) in red giants' evolutionary state.

Tang, Y. K.; Gai, N., E-mail: tyk450@163.com, E-mail: ning.gai@hotmail.com [Department of Physics, Dezhou University, Dezhou 253023 (China)

2013-07-10

256

Thirty New Low-mass Spectroscopic Binaries  

Microsoft Academic Search

As part of our search for young M dwarfs within 25 pc, we acquired high-resolution spectra of 185 low-mass stars compiled by the NStars project that have strong X-ray emission. By cross-correlating these spectra with radial velocity standard stars, we are sensitive to finding multi-lined spectroscopic binaries. We find a low-mass spectroscopic binary fraction of 16% consisting of 27 SB2s,

Evgenya L. Shkolnik; Leslie Hebb; Michael C. Liu; I. Neill Reid; Andrew Collier Cameron

2010-01-01

257

Spectral Energy Distributions of Young Stars in IC 348: The Role of Disks in Angular Momentum Evolution of Young, Low-mass Stars  

NASA Astrophysics Data System (ADS)

Theoretical work suggests that a young star's angular momentum content and rotation rate may be strongly influenced by magnetic interactions with its circumstellar disk. A generic prediction of these "disk-locking" theories is that a disk-locked star will be forced to co-rotate with the Keplerian angular velocity of the inner edge of the disk; that is, the disk's inner-truncation radius should equal its co-rotation radius. These theories have also been interpreted to suggest a gross correlation between young stars' rotation periods and the structural properties of their circumstellar disks, such that slowly rotating stars possess close-in disks that enforce the star's slow rotation, whereas rapidly rotating stars possess anemic or evacuated inner disks that are unable to brake the stars and instead the stars spin up as they contract. To test these expectations, we model the spectral energy distributions (SEDs) of 33 young stars in IC 348 with known rotation periods and infrared excesses indicating the presence of circumstellar disks. For each star, we match the observed SED, typically sampling 0.6-8.0 ?m, to a grid of 200,000 pre-computed star+disk radiative transfer models, from which we infer the disk's inner-truncation radius. We then compare this truncation radius to the disk's co-rotation radius, calculated from the star's measured rotation period. We do not find obvious differences in the disk truncation radii of slow rotators versus rapid rotators. This holds true both at the level of whether close-in disk material is present at all, and in analyzing the precise location of the inner disk edge relative to the co-rotation radius among the subset of stars with close-in disk material. One interpretation is that disk locking is unimportant for the IC 348 stars in our sample. Alternatively, if disk locking does operate, then it must operate on both the slow and rapid rotators, potentially producing both spin-up and spin-down torques, and the transition from the disk-locked state to the disk-released state must occur more rapidly than the stellar contraction timescale.

Le Blanc, Thompson S.; Covey, Kevin R.; Stassun, Keivan G.

2011-08-01

258

Spectroscopy of very low-mass stars and brown dwarfs in the Lambda Orionis star-forming region. II. Rotation, activity and other properties of spectroscopically confirmed members of Collinder 69  

NASA Astrophysics Data System (ADS)

Context. Most observational studies conducted so far point toward brown dwarfs sharing a similar formation mechanism as the one that is accepted for low-mass stars. However, larger databases and more systematic studies are needed before strong conclusions can be reached. Aims: In this second paper of a series devoted to studying the spectroscopic properties of the Lambda Orionis star-forming region members, we study accretion, activity and rotation for a wide set of spectroscopically confirmed members of the central star cluster Collinder 69 to assess analogies and/or differences between the brown-dwarf and stellar populations of this cluster. Moreover, we present comparisons with other star-forming regions of similar and different ages to address environmental effects on our conclusions. Methods: We studied prominent photospheric lines to derive rotational velocities and emission lines to distinguish between accretion processes and chromospheric activity. In addition, we include information about disk presence and X-ray emission. Results: We report very strong differences in the disk fractions of low-mass stars and brown dwarfs (~58%) when compared to higher mass stars (26-3+4%), with 0.6 M? being the critical mass we find for this dichotomy. As a byproduct, we address the implications of the spatial distribution of disk and diskless members in the formation scenario of the cluster itself. We used the H? emission to distinguish among accreting and non-accreting sources, finding that 38-7+8% of sources harboring disks undergo active accretion and that his percentage stays similar in the substellar regime. For these sources we have estimated accretion rates. Finally, regarding rotational velocities, we find a high dispersion in vsin(i) that is even higher among the diskless population. Based on the ESO observing programs 080.C-0592 and 078.C-0124; and observing programs from Calar Alto, Keck, Subaru, and Magellan.Appendices A and Table 6 are available in electronic form at http://www.aanda.org

Bayo, A.; Barrado, D.; Huélamo, N.; Morales-Calderón, M.; Melo, C.; Stauffer, J.; Stelzer, B.

2012-11-01

259

X-Shooter spectroscopy of young stellar objects: IV -- Accretion in low-mass stars and sub-stellar objects in Lupus  

E-print Network

We present X-Shooter/VLT observations of a sample of 36 accreting low-mass stellar and sub-stellar objects (YSOs) in the Lupus star forming region, spanning a range in mass from ~0.03 to ~1.2Msun, but mostly with 0.1Msun < Mstar < 0.5Msun. Our aim is twofold: firstly, analyse the relationship between excess-continuum and line emission accretion diagnostics, and, secondly, to investigate the accretion properties in terms of the physical properties of the central object. The accretion luminosity (Lacc), and from it the accretion rate (Macc), is derived by modelling the excess emission, from the UV to the near-IR, as the continuum emission of a slab of hydrogen. The flux and luminosity (Ll) of a large number of emission lines of H, He, CaII, etc., observed simultaneously in the range from ~330nm to 2500nm, were computed. The luminosity of all the lines is well correlated with Lacc. We provide empirical relationships between Lacc and the luminosity of 39 emission lines, which have a lower dispersion as comp...

Alcalá, J M; Manara, C F; Spezzi, L; Stelzer, B; Frasca, A; Biazzo, K; Covino, E; Randich, S; Rigliaco, E; Testi, L; Comerón, F; Cupani, G; D'Elia, V

2013-01-01

260

Angular momentum evolution of very low mass stars and brown dwarfs in the Orion Nebula Cluster: Spectroscopic determinations of masses and mass accretion rates.  

NASA Astrophysics Data System (ADS)

With the help of an extensive photometric monitoring program in the young (1 Myr) Orion Nebular Cluster (ONC, distance ~ 450pc) we derived rotational periods of nearly 500 objects, 120 of which are potential brown dwarfs (BDs) (M?75M_jup). The faintest of these variable objects has I=20.7 and corresponds to a BD of only 15 M_jup in ONC if an average A_v=1.5 is assumed. This sample extends three magnitudes deeper in I than the one of Herbst et al. 2002. This is by far the most extensive and complete rotational periods data set in the very low mass (VLM) star and BD regime and it will represent the most important reference sample for all future angular momentum evolution study in this mass regime for many years to come. In order to exploit the great scientific potential of this sample we propose here medium resolution (R ~ 2200) multiobject spectroscopy with HYDRA to derive spectral types, A_v values, masses and mass accretion rates of these pre-main sequence (PMS) objects.

Rodriguez-Ledesma, M. V.; Herbst, William; Mundt, Reinhard

2008-08-01

261

BANYAN. V. A Systematic All-Sky Survey for New Very Late-Type Low-Mass Stars and Brown Dwarfs in Nearby Young Moving Groups  

E-print Network

We present the BANYAN All-Sky Survey (BASS) catalog, consisting of 228 new late-type (M4-L6) candidate members of nearby young moving groups (YMGs) with an expected false-positive rate of ~ 13%. This sample includes 79 new candidate young brown dwarfs and 22 planetary-mass objects. These candidates were identified through the first systematic all-sky survey for late-type low-mass stars and brown dwarfs in YMGs. We cross-matched the 2MASS and ALLWISE catalogs outside of the galactic plane to build a sample of 98 970 potential $\\geq$ M5 dwarfs in the solar neighborhood and calculated their proper motions with typical precisions of 5-15 mas yr$^{-1}$. We selected highly probable candidate members of several YMGs from this sample using the Bayesian Analysis for Nearby Young AssociatioNs II tool (BANYAN II). We used the most probable statistical distances inferred from BANYAN II to estimate the spectral type and mass of these candidate YMG members. We used this unique sample to show tentative signs of mass segrega...

Gagné, Jonathan; Doyon, René; Malo, Lison; Artigau, Étienne

2014-01-01

262

The G+M eclipsing binary V530 Orionis: A stringent test of magnetic stellar evolution models for low-mass stars  

E-print Network

We report extensive photometric and spectroscopic observations of the 6.1-day period, G+M-type detached double-lined eclipsing binary V530 Ori, an important new benchmark system for testing stellar evolution models for low-mass stars. We determine accurate masses and radii for the components with errors of 0.7% and 1.3%, as follows: M(A) = 1.0038 +/- 0.0066 M(sun), M(B) = 0.5955 +/- 0.0022 M(sun), R(A) = 0.980 +/- 0.013 R(sun), and R(B) = 0.5873 +/- 0.0067 R(sun). The effective temperatures are 5890 +/- 100 K (G1V) and 3880 +/- 120 K (M1V), respectively. A detailed chemical analysis probing more than 20 elements in the primary spectrum shows the system to have a slightly subsolar abundance, with [Fe/H] = -0.12 +/- 0.08. A comparison with theory reveals that standard models underpredict the radius and overpredict the temperature of the secondary, as has been found previously for other M dwarfs. On the other hand, models from the Dartmouth series incorporating magnetic fields are able to match the observations ...

Torres, G; Pavlovski, K; Feiden, G A; Sabby, J A; Bruntt, H; Clausen, J V

2014-01-01

263

PLANETS AROUND LOW-MASS STARS (PALMS). I. A SUBSTELLAR COMPANION TO THE YOUNG M DWARF 1RXS J235133.3+312720  

SciTech Connect

We report the discovery of a brown dwarf companion to the young M dwarf 1RXS J235133.3+312720 as part of a high contrast imaging search for planets around nearby young low-mass stars with Keck-II/NIRC2 and Subaru/HiCIAO. The 2.''4 ({approx}120 AU) pair is confirmed to be comoving from two epochs of high-resolution imaging. Follow-up low- and moderate-resolution near-infrared spectroscopy of 1RXS J2351+3127 B with IRTF/SpeX and Keck-II/OSIRIS reveals a spectral type of L0{sup +2}{sub -1}. The M2 primary star 1RXS J2351+3127 A exhibits X-ray and UV activity levels comparable to young moving group members with ages of {approx}10-100 Myr. UVW kinematics based the measured radial velocity of the primary and the system's photometric distance (50 {+-} 10 pc) indicate it is likely a member of the {approx}50-150 Myr AB Dor moving group. The near-infrared spectrum of 1RXS J2351+3127 B does not exhibit obvious signs of youth, but its H-band morphology shows subtle hints of intermediate surface gravity. The spectrum is also an excellent match to the {approx}200 Myr M9 brown dwarf LP 944-20. Assuming an age of 50-150 Myr, evolutionary models imply a mass of 32 {+-} 6 M{sub Jup} for the companion, making 1RXS J2351+3127 B the second lowest-mass member of the AB Dor moving group after the L4 companion CD-35 2722 B and one of the few benchmark brown dwarfs known at young ages.

Bowler, Brendan P.; Liu, Michael C.; Cieza, Lucas A.; Kraus, Adam L. [Institute for Astronomy, University of Hawai'i, 2680 Woodlawn Drive, Honolulu, HI 96822 (United States); Shkolnik, Evgenya L. [Lowell Observatory, 1400 W. Mars Hill Road, Flagstaff, AZ 86001 (United States); Dupuy, Trent J. [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States); Tamura, Motohide, E-mail: bpbowler@ifa.hawaii.edu [National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588 (Japan)

2012-07-10

264

X-Ray Emission from Young Stellar Objects in the ? Chamaeleontis Group: The Herbig Ae Star HD 104237 and Associated Low-Mass Stars  

NASA Astrophysics Data System (ADS)

We present Chandra HETGS observations of the Herbig Ae star HD 104237 and the associated young stars comprising lower mass stars, in the 0.15-1.75 Msolar mass range, in their pre-main-sequence phase. The brightest X-ray source in the association is the central system harboring the Herbig Ae primary and a K3 companion. Its X-ray variability indicates modulation possibly on timescales of the rotation period of the Herbig Ae star, and this would imply that the primary significantly contributes to the overall emission. The spectrum of the Herbig Ae+K3 system shows a soft component significantly more pronounced than in other K-type young stars. This soft emission is reminiscent of the unusually soft spectra observed for the single Herbig Ae stars HD 163296 and AB Aur, and therefore we tentatively attribute it to the Herbig Ae of the binary system. The HETGS spectrum shows strong emission lines corresponding to a wide range of plasma temperatures. The He-like triplet of Mg XI and Ne IX suggests the presence of plasma at densities of about 1012 cm-3, possibly indicating an accretion-related X-ray production mechanism. The analysis of the zeroth-order spectra of the other sources indicates X-ray emission characteristics typical of pre-main-sequence stars of similar spectral type, with the exception of the T Tauri HD 104237-D, whose extremely soft emission is very similar to the emission of the classical T Tauri star TW Hya and suggests X-ray production by shocked accreting plasma.

Testa, Paola; Huenemoerder, David P.; Schulz, Norbert S.; Ishibashi, Kazunori

2008-11-01

265

Measurement of the Radius of Neutron Stars with High Signal-to-noise Quiescent Low-mass X-Ray Binaries in Globular Clusters  

NASA Astrophysics Data System (ADS)

This paper presents the measurement of the neutron star (NS) radius using the thermal spectra from quiescent low-mass X-ray binaries (qLMXBs) inside globular clusters (GCs). Recent observations of NSs have presented evidence that cold ultra dense matter—present in the core of NSs—is best described by "normal matter" equations of state (EoSs). Such EoSs predict that the radii of NSs, R NS, are quasi-constant (within measurement errors, of ~10%) for astrophysically relevant masses (M NS>0.5 M ?). The present work adopts this theoretical prediction as an assumption, and uses it to constrain a single R NS value from five qLMXB targets with available high signal-to-noise X-ray spectroscopic data. Employing a Markov chain Monte-Carlo approach, we produce the marginalized posterior distribution for R NS, constrained to be the same value for all five NSs in the sample. An effort was made to include all quantifiable sources of uncertainty into the uncertainty of the quoted radius measurement. These include the uncertainties in the distances to the GCs, the uncertainties due to the Galactic absorption in the direction of the GCs, and the possibility of a hard power-law spectral component for count excesses at high photon energy, which are observed in some qLMXBs in the Galactic plane. Using conservative assumptions, we found that the radius, common to the five qLMXBs and constant for a wide range of masses, lies in the low range of possible NS radii, R_NS =9.1^{+ 1.3}_{- 1.5} \\,km (90%-confidence). Such a value is consistent with low-R NS equations of state. We compare this result with previous radius measurements of NSs from various analyses of different types of systems. In addition, we compare the spectral analyses of individual qLMXBs to previous works.

Guillot, Sebastien; Servillat, Mathieu; Webb, Natalie A.; Rutledge, Robert E.

2013-07-01

266

IC 348-SMM2E: a Class 0 proto-brown dwarf candidate forming as a scaled-down version of low-mass stars  

NASA Astrophysics Data System (ADS)

We report on Submillimeter Array observations of the 870 ?m continuum and CO (3-2), 13CO (2-1), and C18O (2-1) line emission of a faint object, SMM2E, near the driving source of the HH 797 outflow in the IC 348 cluster. The continuum emission shows an unresolved source for which we estimate a mass of gas and dust of 30 MJup, and the CO (3-2) line reveals a compact bipolar outflow centred on SMM2E, and barely seen also in 13CO (2-1). In addition, C18O (2-1) emission reveals hints of a possible rotating envelope/disc perpendicular to the outflow, for which we infer a dynamical mass of ˜16 MJup. In order to further constrain the accreted mass of the object, we gathered data from Spitzer, Herschel, and new and archive submillimetre observations, and built the spectral energy distribution (SED). The SED can be fitted with one single-modified blackbody from 70 ?m down to 2.1 cm, using a dust temperature of ˜24 K, a dust emissivity index of 0.8, and an envelope mass of ˜35 MJup. The bolometric luminosity is 0.10 L?, and the bolometric temperature is 35 K. Thus, SMM2E is comparable to the known Class 0 objects in the stellar domain. An estimate of the final mass indicates that SMM2E will most likely remain substellar, and the SMM2E outflow force matches the trend with luminosity known for young stellar objects. Thus, SMM2E constitutes an excellent example of a Class 0 proto-brown dwarf candidate which forms as a scaled-down version of low-mass stars. Finally, SMM2E seems to be part of a wide (˜2400 au) multiple system of Class 0 sources.

Palau, Aina; Zapata, Luis A.; Rodríguez, Luis F.; Bouy, Hervé; Barrado, David; Morales-Calderón, María; Myers, Philip C.; Chapman, Nicholas; Juárez, Carmen; Li, Di

2014-10-01

267

Spectroscopy of very low mass stars and brown dwarfs in the Lambda Orionis star forming region. I. Enlarging the census down to the planetary mass domain in Collinder 69  

NASA Astrophysics Data System (ADS)

Context. Whilst there is a generally accepted evolutionary scheme for the formation of low-mass stars, the analogous processes when moving down in mass to the brown dwarf regime are not yet well understood. Aims: In this first paper, we try to compile the most complete and unbiased spectroscopically confirmed census of the population of Collinder 69, the central cluster of the Lambda Orionis star forming region, as a first step in addressing the question of how brown dwarfs and planetary mass objects form. Methods: We study age dependent features in optical and near-infrared spectra of candidate members to the cluster (such as alkali lines and accretion-associated indicators). In addition, we complement that study with the analysis of other youth indicators, such as X-ray emission or mid-infrared excess. Results: We confirm the membership to Collinder 69 of ~90 photometric candidate members. As a byproduct, we determine a temperature scale for young M, very low-mass stars, and brown dwarfs. We assemble one of the most complete initial mass functions from 0.016 to 20 M?. Finally, we study the implications of the spatial distribution of the confirmed members for the proposed mechanisms of brown dwarf formation. Appendix A is available in electronic form at http://www.aanda.orgTable 6 is also 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/536/A63

Bayo, A.; Barrado, D.; Stauffer, J.; Morales-Calderón, M.; Melo, C.; Huélamo, N.; Bouy, H.; Stelzer, B.; Tamura, M.; Jayawardhana, R.

2011-12-01

268

MEASUREMENT OF THE RADIUS OF NEUTRON STARS WITH HIGH SIGNAL-TO-NOISE QUIESCENT LOW-MASS X-RAY BINARIES IN GLOBULAR CLUSTERS  

SciTech Connect

This paper presents the measurement of the neutron star (NS) radius using the thermal spectra from quiescent low-mass X-ray binaries (qLMXBs) inside globular clusters (GCs). Recent observations of NSs have presented evidence that cold ultra dense matter-present in the core of NSs-is best described by ''normal matter'' equations of state (EoSs). Such EoSs predict that the radii of NSs, R{sub NS}, are quasi-constant (within measurement errors, of {approx}10%) for astrophysically relevant masses (M{sub NS}>0.5 M{sub Sun }). The present work adopts this theoretical prediction as an assumption, and uses it to constrain a single R{sub NS} value from five qLMXB targets with available high signal-to-noise X-ray spectroscopic data. Employing a Markov chain Monte-Carlo approach, we produce the marginalized posterior distribution for R{sub NS}, constrained to be the same value for all five NSs in the sample. An effort was made to include all quantifiable sources of uncertainty into the uncertainty of the quoted radius measurement. These include the uncertainties in the distances to the GCs, the uncertainties due to the Galactic absorption in the direction of the GCs, and the possibility of a hard power-law spectral component for count excesses at high photon energy, which are observed in some qLMXBs in the Galactic plane. Using conservative assumptions, we found that the radius, common to the five qLMXBs and constant for a wide range of masses, lies in the low range of possible NS radii, R{sub NS}=9.1{sup +1.3}{sub -1.5} km (90%-confidence). Such a value is consistent with low-R{sub NS} equations of state. We compare this result with previous radius measurements of NSs from various analyses of different types of systems. In addition, we compare the spectral analyses of individual qLMXBs to previous works.

Guillot, Sebastien; Rutledge, Robert E. [Department of Physics, McGill University, 3600 rue University, Montreal, QC, H2X-3R4 (Canada); Servillat, Mathieu [Laboratoire AIM (CEA/DSM/IRFU/SAp, CNRS, Universite Paris Diderot), CEA Saclay, Bat. 709, F-91191 Gif-sur-Yvette (France); Webb, Natalie A., E-mail: guillots@physics.mcgill.ca, E-mail: rutledge@physics.mcgill.ca [Universite de Toulouse, UPS-OMP, IRAP, Toulouse (France)

2013-07-20

269

The Earliest Phases of Star Formation (EPoS): a Herschel key project. The thermal structure of low-mass molecular cloud cores  

NASA Astrophysics Data System (ADS)

Context. The temperature and density structure of molecular cloud cores are the most important physical quantities that determine the course of the protostellar collapse and the properties of the stars they form. Nevertheless, density profiles often rely either on the simplifying assumption of isothermality or on observationally poorly constrained model temperature profiles. The instruments of the Herschel satellite provide us for the first time with both the spectral coverage and the spatial resolution that is needed to directly measure the dust temperature structure of nearby molecular cloud cores. Aims: With the aim of better constraining the initial physical conditions in molecular cloud cores at the onset of protostellar collapse, in particular of measuring their temperature structure, we initiated the guaranteed time key project (GTKP) "The Earliest Phases of Star Formation" (EPoS) with the Herschel satellite. This paper gives an overview of the low-mass sources in the EPoS project, the Herschel and complementary ground-based observations, our analysis method, and the initial results of the survey. Methods: We study the thermal dust emission of 12 previously well-characterized, isolated, nearby globules using FIR and submm continuum maps at up to eight wavelengths between 100 ?m and 1.2 mm. Our sample contains both globules with starless cores and embedded protostars at different early evolutionary stages. The dust emission maps are used to extract spatially resolved SEDs, which are then fit independently with modified blackbody curves to obtain line-of-sight-averaged dust temperature and column density maps. Results: We find that the thermal structure of all globules (mean mass 7 M?) is dominated by external heating from the interstellar radiation field and moderate shielding by thin extended halos. All globules have warm outer envelopes (14-20 K) and colder dense interiors (8-12 K) with column densities of a few 1022 cm-2. The protostars embedded in some of the globules raise the local temperature of the dense cores only within radii out to about 5000 AU, but do not significantly affect the overall thermal balance of the globules. Five out of the six starless cores in the sample are gravitationally bound and approximately thermally stabilized. The starless core in CB 244 is found to be supercritical and is speculated to be on the verge of collapse. For the first time, we can now also include externally heated starless cores in the Lsmm/Lbol vs. Tbol diagram and find that Tbol < 25 K seems to be a robust criterion to distinguish starless from protostellar cores, including those that only have an embedded very low-luminosity object. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Partially based on observations carried out with the IRAM 30 m Telescope, with the Atacama Pathfinder Experiment (APEX), and with the James Clerk Maxwell Telescope (JCMT). IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). APEX is a collaboration between Max Planck Institut für Radioastronomie (MPIfR), Onsala Space Observatory (OSO), and the European Southern Observatory (ESO). The JCMT is operated by the Joint Astronomy Centre on behalf of the Particle Physics and Astronomy Research Council of the United Kingdom, the Netherlands Association for Scientific Research, and the National Research Council of Canada.Appendices A, B and C are available in electronic form at http://www.aanda.org

Launhardt, R.; Stutz, A. M.; Schmiedeke, A.; Henning, Th.; Krause, O.; Balog, Z.; Beuther, H.; Birkmann, S.; Hennemann, M.; Kainulainen, J.; Khanzadyan, T.; Linz, H.; Lippok, N.; Nielbock, M.; Pitann, J.; Ragan, S.; Risacher, C.; Schmalzl, M.; Shirley, Y. L.; Stecklum, B.; Steinacker, J.; Tackenberg, J.

2013-03-01

270

Observa(onal Constraints on Brown Dwarf Forma(on Revisited  

E-print Network

Observa(onal Constraints on Brown Dwarf Forma(on Revisited Kevin Luhman (Penn State) #12;If a large frac(on of low-mass stars and brown dwarfs form (10x) IC348 Chamaeleon #12;Taurus Width of IMF and abundance of brown dwarfs

Joergens, Viki

271

STATISTICAL PROPERTIES OF GALACTIC {delta} SCUTI STARS: REVISITED  

SciTech Connect

We present statistical characteristics of 1578 {delta} Scuti stars including nearby field stars and cluster member stars within the Milky Way. We obtained 46% of these stars (718 stars) from work by Rodriguez and collected the remaining 54% of stars (860 stars) from other literature. We updated the entries with the latest information of sky coordinates, color, rotational velocity, spectral type, period, amplitude, and binarity. The majority of our sample is well characterized in terms of typical period range (0.02-0.25 days), pulsation amplitudes (<0.5 mag), and spectral types (A-F type). Given this list of {delta} Scuti stars, we examined relations between their physical properties (i.e., periods, amplitudes, spectral types, and rotational velocities) for field stars and cluster members, and confirmed that the correlations of properties are not significantly different from those reported in Rodriguez's work. All the {delta} Scuti stars are cross-matched with several X-ray and UV catalogs, resulting in 27 X-ray and 41 UV-only counterparts. These counterparts are interesting targets for further study because of their uniqueness in showing {delta} Scuti-type variability and X-ray/UV emission at the same time. The compiled catalog can be accessed through the Web interface http://stardb.yonsei.ac.kr/DeltaScuti.

Chang, S.-W.; Kim, D.-W.; Byun, Y.-I. [Department of Astronomy, Yonsei University, Seoul 120-749 (Korea, Republic of); Protopapas, P., E-mail: seowony@galaxy.yonsei.ac.kr, E-mail: kim@mpia-hd.mpg.de [Harvard Smithsonian Center for Astrophysics, Cambridge, MA 02138 (United States)

2013-05-15

272

REVISITING THE FIRST GALAXIES: THE EPOCH OF POPULATION III STARS  

SciTech Connect

We investigate the transition from primordial Population III (Pop III) star formation to normal Pop II star formation in the first galaxies using new cosmological hydrodynamic simulations. We find that while the first stars seed their host galaxies with metals, they cannot sustain significant outflows to enrich the intergalactic medium, even assuming a top-heavy initial mass function. This means that Pop III star formation could potentially continue until z Almost-Equal-To 6 in different unenriched regions of the universe, before being ultimately shut off by cosmic reionization. Within an individual galaxy, the metal production and stellar feedback from Pop II stars overtake Pop III stars in 20-200 Myr, depending on galaxy mass.

Muratov, Alexander L.; Gnedin, Oleg Y.; Zemp, Marcel [Department of Astronomy, University of Michigan, Ann Arbor, MI 48109 (United States); Gnedin, Nickolay Y., E-mail: muratov@umich.edu [Particle Astrophysics Center, Fermi National Accelerator Laboratory, Batavia, IL 60510 (United States)

2013-08-10

273

THIRTY NEW LOW-MASS SPECTROSCOPIC BINARIES  

SciTech Connect

As part of our search for young M dwarfs within 25 pc, we acquired high-resolution spectra of 185 low-mass stars compiled by the NStars project that have strong X-ray emission. By cross-correlating these spectra with radial velocity standard stars, we are sensitive to finding multi-lined spectroscopic binaries. We find a low-mass spectroscopic binary fraction of 16% consisting of 27 SB2s, 2 SB3s, and 1 SB4, increasing the number of known low-mass spectroscopic binaries (SBs) by 50% and proving that strong X-ray emission is an extremely efficient way to find M-dwarf SBs. WASP photometry of 23 of these systems revealed two low-mass eclipsing binaries (EBs), bringing the count of known M-dwarf EBs to 15. BD-22 5866, the ESB4, was fully described in 2008 by Shkolnik et al. and CCDM J04404+3127 B consists of two mid-M stars orbiting each other every 2.048 days. WASP also provided rotation periods for 12 systems, and in the cases where the synchronization time scales are short, we used P{sub rot} to determine the true orbital parameters. For those with no P{sub rot}, we used differential radial velocities to set upper limits on orbital periods and semimajor axes. More than half of our sample has near-equal-mass components (q > 0.8). This is expected since our sample is biased toward tight orbits where saturated X-ray emission is due to tidal spin-up rather than stellar youth. Increasing the samples of M-dwarf SBs and EBs is extremely valuable in setting constraints on current theories of stellar multiplicity and evolution scenarios for low-mass multiple systems.

Shkolnik, Evgenya L. [Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, DC 20015 (United States); Hebb, Leslie; Cameron, Andrew C. [School of Physics and Astronomy, University of St. Andrews, North Haugh, St Andrews, Fife KY16 9SS (United Kingdom); Liu, Michael C. [Institute for Astronomy, University of Hawaii at Manoa, 2680 Woodlawn Drive, Honolulu, HI 96822 (United States); Neill Reid, I., E-mail: shkolnik@dtm.ciw.ed, E-mail: Andrew.Cameron@st-and.ac.u, E-mail: leslie.hebb@vanderbilt.ed, E-mail: mliu@ifa.hawaii.ed, E-mail: inr@stsci.ed [Space Telescope Science Institute, Baltimore, MD 21218 (United States)

2010-06-20

274

Discovery of a correlation between the frequency of the mHz quasi-periodic oscillations and the neutron-star temperature in the low-mass X-ray binary 4U 1636-53  

NASA Astrophysics Data System (ADS)

We detected millihertz quasi-periodic oscillations (QPOs) in an XMM-Newton observation of the neutron-star low-mass X-ray binary 4U 1636-53. These QPOs have been interpreted as marginally stable burning on the neutron-star surface. At the beginning of the observation the QPO was at around 8 mHz, together with a possible second harmonic. About 12 ks into the observation a type I X-ray burst occurred and the QPO disappeared; the QPO reappeared ˜25 ks after the burst and it was present until the end of the observation. We divided the observation into four segments to study the evolution of the spectral properties of the source during intervals with and without mHz QPO. We find that the temperature of the neutron-star surface increases from the QPO segment to the non-QPO segment, and vice versa. We also find a strong correlation between the frequency of the mHz QPO and the temperature of a blackbody component in the energy spectrum representing the temperature of neutron-star surface. Our results are consistent with previous results that the frequency of the mHz QPO depends on the variation of the heat flux from the neutron-star crust, and therefore supports the suggestion that the observed QPO frequency drifts could be caused by the cooling of deeper layers.

Lyu, Ming; Méndez, Mariano; Altamirano, Diego

2014-12-01

275

Discovery of a Brown Dwarf Very Close to the Sun: A Methane-rich Brown Dwarf Companion to the Low-Mass Star SCR 1845-6357  

Microsoft Academic Search

We present VLT\\/NACO SDI images of the very nearby star SCR 1845-6357 (hereafter SCR 1845). SCR 1845 is a recently discovered M8.5 star just 3.85 pc from the Sun. Using the capabilities of the unique SDI device, we discovered a substellar companion to SCR 1845 at a separation of 4.5 AU (1.170\\

B. A. Biller; M. Kasper; L. M. Close; W. Brandner; S. Kellner

2006-01-01

276

Revisiting Hipparcos data for pre-main sequence stars  

E-print Network

We cross-correlate the Herbig & Bell and Hipparcos Catalogues in order to extract the results for young stellar objects (YSOs). We compare the distances of individual young stars and the distance of their presumably associated molecular clouds, taking into account post-Hipparcos distances to the relevant associations and using Hipparcos intermediate astrometric data to derive new parallaxes of the pre-main sequence stars based on their grouping. We confirm that YSOs are located in their associated clouds, as anticipated by a large body of work, and discuss reasons which make the individual parallaxes of some YSOs doubtful. We find in particular that the distance of Taurus YSOs as a group is entirely consistent with the molecular cloud distance, although Hipparcos distances of some faint Taurus-Auriga stars must be viewed with caution. We then improve some of the solutions for the binary and multiple pre-main sequence stars. In particular, we confirm three new astrometric young binaries discovered by Hipparcos: RY Tau, UX Ori, and IX Oph.

Claude Bertout; Noel Robichon; Frederic Arenou

1999-09-27

277

Sulphur and zinc abundances in Galactic halo stars revisited  

E-print Network

High resolution UVES spectra of 40 main-sequence stars with -3.3 < [Fe/H] < -1.0 are used to derive S, Fe and Zn abundances from lines in the 400 - 950 nm region. For one star we also present novel observations of the SI triplet at 1.046 micron carried out with the ESO VLT CRIRES spectrograph. Comparison of sulphur abundances from the weak and strong SI lines provides important constraints on non-LTE effects. The high sulphur abundances reported by others for some metal-poor stars are not confirmed; instead, when taking non-LTE effects into account, the Galactic halo stars distribute around a plateau at [S/Fe] = +0.2 dex with a scatter of 0.07 dex only. This indicates that sulphur in Galactic halo stars has been made by alpha-capture processes in massive SNe. The observed scatter in S/Fe is, however, much smaller than predicted from current stochastic models of the chemical evolution of the early Galaxy, suggesting that either the models or the calculated yields of massive SNe should be revised. [Zn/Fe] is close to zero for metallicities in the range -2.0 < [Fe/H] < -1.0 but increases to a level of [Zn/Fe] = +0.15 dex in the range -2.7 < [Fe/H] < -2.0. At still lower metallicities [Zn/Fe] rises steeply to a value around [Zn/Fe] = +0.5 dex at [Fe/H] = -3.2. We also examine the behaviour of S/Zn and find that departures from the solar ratio are significantly reduced at all metallicities if non-LTE corrections to the abundances of these two elements are adopted. This effect, if confirmed, would reduce the usefulness of the S/Zn ratio as a diagnostic of past star-formation activity, but would bring closer together the values measured in damped Lyman-alpha systems and in Galactic stars.

P. E. Nissen; C. Akerman; M. Asplund; D. Fabbian; F. Kerber; H. U. Kaufl; M. Pettini

2007-02-26

278

Very low-luminosity Class I/Flat outflow sources in sigma Orionis: Clues to alternative formation mechanisms for very low-mass stars  

NASA Astrophysics Data System (ADS)

We present an optical through sub-millimetre multi-wavelength study of two very low-luminosity Class I/Flat systems, Mayrit 1701117 and Mayrit 1082188, in the sigma Orionis cluster. We performed moderate resolution (R 1000) optical ( 0.4-0.9mu) spectroscopy with the TWIN spectrograph at the Calar Alto 3.5-m telescope. The spectra for both sources show prominent emission in accretion- and outflow-associated lines. The mean accretion rate measured from multiple line diagnostics is 6.4x10^{-10} Msun/yr for Mayrit 1701117, and 2.5x10^{-10} Msun/yr for Mayrit 1082188. The outflow mass loss rates for the two systems are similar and estimated to be 1x10^{-9} Msun/yr. The activity rates are within the range observed for low-mass Class I protostars. We obtained sub-millimetre continuum observations with the Submillimetre Common-User Bolometer Array (SCUBA-2) bolometer at the James Clerk Maxwell Telescope. Both objects are detected at a >5-sigma level in the SCUBA-2 850mu band. The bolometric luminosity of the targets as measured from the observed spectral energy distribution over 0.8-850mu is 0.18+/-0.04 Lsun for Mayrit 1701117, and 0.16+/-0.03 Lsun for Mayrit 1082188, and is in the very low-mass range. The total dust+gas mass derived from sub-millimetre fluxes is 36 M_Jup and 22 M_Jup for Mayrit 1701117 and Mayrit 1082188, respectively. There is the possibility that some of the envelope material might be dissipated by the strong outflows driven by these sources, resulting in a final mass of the system close to or below the sub-stellar limit. Given the membership of these objects in a relatively evolved cluster of 3 Myr of age, we consider an alternate formation mechanism in the context of the `hybrid' model of disk fragmentation, followed by ejection of a gaseous clump.

Riaz, Basmah; Whelan, E.; Thompson, M.; Vorobyov, E.; Lodieu, N.

2015-01-01

279

THE MASS AND THE RADIUS OF THE NEUTRON STAR IN THE TRANSIENT LOW-MASS X-RAY BINARY SAX J1748.9-2021  

SciTech Connect

We use time-resolved spectroscopy of thermonuclear X-ray bursts observed from SAX J1748.9-2021 to infer the mass and the radius of the neutron star in the binary. Four X-ray bursts observed from the source with the Rossi X-ray Timing Explorer enable us to measure the angular size and the Eddington limit on the neutron star surface. Combined with a distance measurement to the globular cluster NGC 6440, in which SAX J1748.9-2021 resides, we obtain two solutions for the neutron star radius and mass, R = 8.18 {+-} 1.62 km and M = 1.78 {+-} 0.3 M{sub Sun} or R = 10.93 {+-} 2.09 km and M = 1.33 {+-} 0.33 M{sub Sun }.

Guever, Tolga [Faculty of Engineering and Natural Sciences, Sabanc Latin-Small-Letter-Dotless-I University, Orhanl Latin-Small-Letter-Dotless-I , Tuzla, Istanbul 34956 (Turkey); Oezel, Feryal [Department of Astronomy, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721 (United States)

2013-03-01

280

REVISITING THE FIRST GALAXIES: THE EFFECTS OF POPULATION III STARS ON THEIR HOST GALAXIES  

SciTech Connect

We revisit the formation and evolution of the first galaxies using new hydrodynamic cosmological simulations with the adaptive refinement tree code. Our simulations feature a recently developed model for H{sub 2} formation and dissociation, and a star formation recipe that is based on molecular rather than atomic gas. Here, we develop and implement a recipe for the formation of metal-free Population III (Pop III) stars in galaxy-scale simulations that resolve primordial clouds with sufficiently high density. We base our recipe on the results of prior zoom-in simulations that resolved the protostellar collapse in pre-galactic objects. We find the epoch during which Pop III stars dominated the energy and metal budget of the first galaxies to be short-lived. Galaxies that host Pop III stars do not retain dynamical signatures of their thermal and radiative feedback for more than 10{sup 8} years after the lives of the stars end in pair-instability supernovae, even when we consider the maximum reasonable efficiency of the feedback. Though metals ejected by the supernovae can travel well beyond the virial radius of the host galaxy, they typically begin to fall back quickly, and do not enrich a large fraction of the intergalactic medium. Galaxies with a total mass in excess of 3 Multiplication-Sign 10{sup 6} M{sub Sun} re-accrete most of their baryons and transition to metal-enriched Pop II star formation.

Muratov, Alexander L.; Gnedin, Oleg Y.; Zemp, Marcel [Department of Astronomy, University of Michigan, Ann Arbor, MI 48109 (United States); Gnedin, Nickolay Y., E-mail: muratov@umich.edu [Particle Astrophysics Center, Fermi National Accelerator Laboratory, Batavia, IL 60510 (United States)

2013-08-01

281

Effects of Rotationally Induced Mixing in Compact Binary Systems with Low-mass Secondaries and in Single Solar-type Stars  

NASA Astrophysics Data System (ADS)

Many population synthesis and stellar evolution studies have addressed the evolution of close binary systems in which the primary is a compact remnant and the secondary is filling its Roche lobe, thus triggering mass transfer. Although tidal locking is expected in such systems, most studies have neglected the rotationally induced mixing that may occur. Here we study the possible effects of mixing in mass-losing stars for a range of secondary star masses and metallicities. We find that tidal locking can induce rotational mixing prior to contact and thus affect the evolution of the secondary star if the effects of the Spruit-Tayler dynamo are included both for angular momentum and chemical transport. Once contact is made, the effect of mass transfer tends to be more rapid than the evolutionary timescale, so the effects of mixing are no longer directly important, but the mass-transfer strips matter to inner layers that may have been affected by the mixing. These effects are enhanced for secondaries of 1-1.2 M ? and for lower metallicities. We discuss the possible implications for the paucity of carbon in the secondaries of the cataclysmic variable SS Cyg and the black hole candidate XTE J1118+480 and for the progenitor evolution of Type Ia supernovae. We also address the issue of the origin of blue straggler stars in globular and open clusters. We find that for models that include rotation consistent with that observed for some blue straggler stars, evolution is chemically homogeneous. This leads to tracks in the H-R diagram that are brighter and bluer than the non-rotating main-sequence turn-off point. Rotational mixing could thus be one of the factors that contribute to the formation of blue stragglers.

Chatzopoulos, E.; Robinson, Edward L.; Wheeler, J. Craig

2012-08-01

282

Period analysis of two non-Blazhko RRab stars, FN Lyr and V894 Cyg, based on Kepler photometry: evidence of low-mass companions on wider orbits  

NASA Astrophysics Data System (ADS)

Long-cadence-corrected pre-search data conditioning (PDC) fluxes of FN Lyr (KIC 6936115) and V894 Cyg (KIC 9591503), observed continuously by the Kepler mission, spanning over 1470 d, are used to determine hundreds of times of maximum and minimum for the analysis of O - C residuals. The interpretation of the clear variations in the O - C diagrams is that these are caused by the light-travel-time effect as a result of additional companions. The mass functions of the companions are f (M) = (3.94 ± 0.82) × 10-6 and (2.01 ± 0.22) × 10-4 M?. Assuming that the orbital plane inclination follows a random distribution, the companions to both stars can be constrained to be substellar objects (brown dwarf or giant planet), with 89.4 and 59.4 per cent probability, respectively. Under the assumption that the orbital inclination equals 90°, the distances between the companions and the central RR Lyrae stars at periastron should be 1.03 and 0.50 au, respectively. In addition, the orbital periods are 794.8 and 1084.4 d for FN Lyr and V894 Cyg, respectively. By comparing these orbital parameters with those of B subdwarf stars in binary systems, there are strong hints that horizontal branch stars might have different evolution histories. The long-term pulsation period changes are also discussed. Based on our studies, RR Lyrae stars in binary systems are not rare, at least among binary systems with wider separations.

Li, L.-J.; Qian, S.-B.

2014-10-01

283

Discovery of the Near-infrared Counterpart to the Luminous Neutron-star Low-mass X-Ray Binary GX 3+1  

NASA Astrophysics Data System (ADS)

Using the High Resolution Camera on board the Chandra X-ray Observatory, we have measured an accurate position for the bright persistent neutron star X-ray binary and atoll source GX 3+1. At a location that is consistent with this new position, we have discovered the near-infrared (NIR) counterpart to GX 3+1 in images taken with the PANIC and FourStar cameras on the Magellan Baade Telescope. The identification of this Ks = 15.8 ± 0.1 mag star as the counterpart is based on the presence of a Br ? emission line in an NIR spectrum taken with the Folded-port InfraRed Echelette spectrograph on the Baade Telescope. The absolute magnitude derived from the best available distance estimate to GX 3+1 indicates that the mass donor in the system is not a late-type giant. We find that the NIR light in GX 3+1 is likely dominated by the contribution from a heated outer accretion disk. This is similar to what has been found for the NIR flux from the brighter class of Z sources, but unlike the behavior of atolls fainter (LX ? 1036-1037 erg s-1) than GX 3+1, where optically thin synchrotron emission from a jet probably dominates the NIR flux. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

van den Berg, Maureen; Homan, Jeroen; Fridriksson, Joel K.; Linares, Manuel

2014-10-01

284

Chemistry of Low Mass Substellar Objects  

E-print Network

"Brown dwarfs" is the collective name for objects more massive than giant planets such as Jupiter but less massive than M dwarf stars. This review gives a brief description of the classification and chemistry of low mass dwarfs. The current spectral classification of stars includes L and T dwarfs that encompass the coolest known stars and substellar objects. The relatively low atmospheric temperatures and high total pressures in substellar dwarfs lead to molecular gas and condensate chemistry. The chemistry of elements such as C, N, O, Ti, V, Fe, Cr, and the alkali elements play a dominant role in shaping the optical and infrared spectra of the "failed" stars. Chemical diagnostics for the subclassifications are described.

Katharina Lodders; Bruce Fegley, Jr

2006-01-17

285

BLACK HOLE-NEUTRON STAR MERGERS WITH A HOT NUCLEAR EQUATION OF STATE: OUTFLOW AND NEUTRINO-COOLED DISK FOR A LOW-MASS, HIGH-SPIN CASE  

SciTech Connect

Neutrino emission significantly affects the evolution of the accretion tori formed in black hole-neutron star mergers. It removes energy from the disk, alters its composition, and provides a potential power source for a gamma-ray burst. To study these effects, simulations in general relativity with a hot microphysical equation of state (EOS) and neutrino feedback are needed. We present the first such simulation, using a neutrino leakage scheme for cooling to capture the most essential effects and considering a moderate mass (1.4 M{sub ?} neutron star, 5.6 M{sub ?} black hole), high-spin (black hole J/M {sup 2} = 0.9) system with the K{sub 0} = 220 MeV Lattimer-Swesty EOS. We find that about 0.08 M{sub ?} of nuclear matter is ejected from the system, while another 0.3 M{sub ?} forms a hot, compact accretion disk. The primary effects of the escaping neutrinos are (1) to make the disk much denser and more compact, (2) to cause the average electron fraction Y{sub e} of the disk to rise to about 0.2 and then gradually decrease again, and (3) to gradually cool the disk. The disk is initially hot (T ? 6 MeV) and luminous in neutrinos (L{sub ?} ? 10{sup 54} erg s{sup –1}), but the neutrino luminosity decreases by an order of magnitude over 50 ms of post-merger evolution.

Deaton, M. Brett; Duez, Matthew D. [Department of Physics and Astronomy, Washington State University, Pullman, WA 99164 (United States); Foucart, Francois; O'Connor, Evan [Canadian Institute for Theoretical Astrophysics, University of Toronto, Toronto, Ontario M5S 3H8 (Canada); Ott, Christian D.; Scheel, Mark A.; Szilagyi, Bela [TAPIR, MC 350-17, California Institute of Technology, Pasadena, CA 91125 (United States); Kidder, Lawrence E.; Muhlberger, Curran D., E-mail: mbdeaton@wsu.edu, E-mail: m.duez@wsu.edu [Center for Radiophysics and Space Research, Cornell University, Ithaca, NY 14853 (United States)

2013-10-10

286

THE INFRARED EYE OF THE WIDE-FIELD CAMERA 3 ON THE HUBBLE SPACE TELESCOPE REVEALS MULTIPLE MAIN SEQUENCES OF VERY LOW MASS STARS IN NGC 2808  

SciTech Connect

We use images taken with the infrared channel of the Wide Field Camera 3 on the Hubble Space Telescope to study the multiple main sequences (MSs) of NGC 2808. Below the turnoff, the red, the middle, and the blue MS, previously detected from visual-band photometry, are visible over an interval of about 3.5 F160W magnitudes. The three MSs merge together at the level of the MS bend. At fainter magnitudes, the MS again splits into two components containing {approx}65% and {approx}35% of stars, with the most-populated MS being the bluest one. Theoretical isochrones suggest that the latter is connected to the red MS discovered in the optical color-magnitude diagram (CMD) and hence corresponds to the first stellar generation, having primordial helium and enhanced carbon and oxygen abundances. The less-populated MS in the faint part of the near-IR CMD is helium-rich and poor in carbon and oxygen, and it can be associated with the middle and the blue MS of the optical CMD. The finding that the photometric signature of abundance anti-correlation is also present in fully convective MS stars reinforces the inference that they have a primordial origin.

Milone, A. P.; Aparicio, A.; Monelli, M., E-mail: milone@iac.es, E-mail: aparicio@iac.es, E-mail: monelli@iac.es [Instituto de Astrofisica de Canarias, E-38200 La Laguna, Tenerife, Canary Islands (Spain); and others

2012-08-01

287

Discovery of low mass objects in Taurus  

NASA Technical Reports Server (NTRS)

In infrared (2.2 micron, K-band) search of small regions (25 in square) near 26 members of the Taurus star-forming association has revealed 20 dim (K = 13-16 mag) stellar objects near 13 of them. Of these 20 objects, 9 are exceptionally red. It is argued that these 9 are probably also Taurus members. From the luminosities (0.4 to 4 times 10 the -3 power luminosity) and ages (estimated at 10(exp 6) years), masses can be determined by reference to theoretical low-mass cooling curves. The masses are in the range 0.005 to 0.015 solar mass, i.e., low-mass brown dwarfs. Proper motion studies of 7 of the objects visible on the POSS plates conducted by Burton Jones establish that 4 are highly probable Taurus members while 1 is a possible member.

Forrest, W. J.; Ninkov, Z.; Garnett, J. D.; Skrutskie, M. F.; Shure, M.

1989-01-01

288

Water in star-forming regions with Herschel (WISH) V. The physical conditions in low-mass protostellar outflows revealed by multi-transition water observations  

E-print Network

Context: Outflows are an important part of the star formation process as both the result of ongoing active accretion and one of the main sources of mechanical feedback on small scales. Water is the ideal tracer of these effects because it is present in high abundance in various parts of the protostar. Method: We present \\textit{Herschel} HIFI spectra of multiple water-transitions towards 29 nearby Class 0/I protostars as part of the WISH Survey. These are decomposed into different Gaussian components, with each related to one of three parts of the protostellar system; quiescent envelope, cavity shock and spot shocks in the jet and at the base of the outflow. We then constrain the excitation conditions present in the two outflow-related components. Results: Water emission is optically thick but effectively thin, with line ratios that do not vary with velocity, in contrast to CO. The physical conditions of the cavity and spot shocks are similar, with post-shock H$_{2}$ densities of order 10$^{5}-$10$^{8}$\\,cm$^...

Mottram, J C; van Dishoeck, E F; Bruderer, S; José-García, I San; Karska, A; Visser, R; Santangelo, G; Benz, A O; Bergin, E A; Caselli, P; Herpin, F; Hogerheijde, M R; Johnstone, D; van Kempen, T A; Liseau, R; Nisini, B; Tafalla, M; van der Tak, F F S; Wyrowski, F

2014-01-01

289

Thirty New Low-mass Spectroscopic Binaries  

NASA Astrophysics Data System (ADS)

As part of our search for young M dwarfs within 25 pc, we acquired high-resolution spectra of 185 low-mass stars compiled by the NStars project that have strong X-ray emission. By cross-correlating these spectra with radial velocity standard stars, we are sensitive to finding multi-lined spectroscopic binaries. We find a low-mass spectroscopic binary fraction of 16% consisting of 27 SB2s, 2 SB3s, and 1 SB4, increasing the number of known low-mass spectroscopic binaries (SBs) by 50% and proving that strong X-ray emission is an extremely efficient way to find M-dwarf SBs. WASP photometry of 23 of these systems revealed two low-mass eclipsing binaries (EBs), bringing the count of known M-dwarf EBs to 15. BD-22 5866, the ESB4, was fully described in 2008 by Shkolnik et al. and CCDM J04404+3127 B consists of two mid-M stars orbiting each other every 2.048 days. WASP also provided rotation periods for 12 systems, and in the cases where the synchronization time scales are short, we used P rot to determine the true orbital parameters. For those with no P rot, we used differential radial velocities to set upper limits on orbital periods and semimajor axes. More than half of our sample has near-equal-mass components (q > 0.8). This is expected since our sample is biased toward tight orbits where saturated X-ray emission is due to tidal spin-up rather than stellar youth. Increasing the samples of M-dwarf SBs and EBs is extremely valuable in setting constraints on current theories of stellar multiplicity and evolution scenarios for low-mass multiple systems. Based on observations collected at the W. M. Keck Observatory, the Canada-France-Hawaii Telescope and by the WASP Consortium. The Keck Observatory is operated as a scientific partnership between the California Institute of Technology, the University of California, and NASA, and was made possible by the generous financial support of the W. M. Keck Foundation. The CFHT is operated by the National Research Council of Canada, the Centre National de la Recherche Scientifique of France, and the University of Hawaii. The WASP Consortium consists of astronomers primarily from the Queen's University Belfast, St Andrews, Keele, Leicester, The Open University, Isaac Newton Group La Palma, and Instituto de Astrofísica de Canarias. The SuperWASP Cameras were constructed and operated with funds made available from Consortium Universities and the UK's Science and Technology Facilities Council.

Shkolnik, Evgenya L.; Hebb, Leslie; Liu, Michael C.; Reid, I. Neill; Collier Cameron, Andrew

2010-06-01

290

THE BINARY FRACTION OF LOW-MASS WHITE DWARFS  

SciTech Connect

We describe spectroscopic observations of 21 low-mass ({<=}0.45 M{sub sun}) white dwarfs (WDs) from the Palomar-Green survey obtained over four years. We use both radial velocities and infrared photometry to identify binary systems, and find that the fraction of single, low-mass WDs is {<=}30%. We discuss the potential formation channels for these single stars including binary mergers of lower-mass objects. However, binary mergers are not likely to explain the observed number of single low-mass WDs. Thus, additional formation channels, such as enhanced mass loss due to winds or interactions with substellar companions, are likely.

Brown, Justin M. [Franklin and Marshall College, 415 Harrisburg Avenue, Lancaster, PA 17604 (United States); Kilic, Mukremin; Brown, Warren R.; Kenyon, Scott J., E-mail: brown.justin.michael@gmail.com [Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138 (United States)

2011-04-01

291

Contribution of Low Mass Galaxies to Reionization  

NASA Astrophysics Data System (ADS)

Here we quantify the abundance of faint galaxies at high redshifts and their ionizing photon budget using a complete sample of ~ 10,000 simulated galaxies from the Vulcan simulation, a new state of the art cosmological simulation of a 25Mpc per side volume, with unprecedented spatial resolution. Their predicted population is consistent with the most recent observations of the UV luminosity function up to z ~ 8. In addition, we measure the fraction of photons that can escape their host halo and ionize neutral hydrogen in the IGM. With the uniform resolution of our simulation we have ~ 100 systems with reliable morphologies, and therefore realistic distributions of young stars (sources) relative to the neutral hydrogen (absorbers). With this model, we predict the contribution of low mass galaxies to reionization, in line with the theory that star forming galaxies are a major source of ionizing photons, especially if there is an underlying faint population undetectable by current surveys.

Anderson, Lauren M.; Quinn, Thomas R.; Governato, Fabio; Brooks, Alyson; Pontzen, Andrew

2015-01-01

292

A Steep Faint-End Slope of the UV Luminosity Function at z~2-3: Implications for the Global Stellar Mass Density and Star Formation in Low Mass Halos  

E-print Network

We use the deep ground-based optical photometry of the Lyman Break Galaxy (LBG) Survey to derive robust measurements of the faint-end slope (alpha) of the UV LF at redshifts 1.92000 spectroscopic redshifts and ~31000 LBGs in 31 spatially-independent fields over a total area of 3261 arcmin^2. These data allow us to select galaxies to 0.07L* and 0.10L* at z~2 and z~3, respectively. A maximum likelihood analysis indicates steep values of alpha(z=2)=-1.73+/-0.07 and alpha(z=3)=-1.73+/-0.13. This result is robust to luminosity dependent systematics in the Ly-alpha equivalent width and reddening distributions, is similar to the steep values advocated at z>4, and implies that ~93% of the unobscured UV luminosity density at z~2-3 arises from sub-L* galaxies. With a realistic luminosity dependent reddening distribution, faint to moderately luminous galaxies account for >70% and >25% of the bolometric luminosity density and present-day stellar mass density, respectively, when integrated over 1.92 contrasts with the shallower value inferred locally, suggesting that the evolution in the faint-end slope may be dictated simply by the availability of low mass halos capable of supporting star formation at z<2. [Abridged

Naveen A. Reddy; Charles C. Steidel

2008-10-15

293

FUV Spectra of Evolved Late-K and M Stars: Mass Loss Revisited and Stellar Activity  

NASA Technical Reports Server (NTRS)

This is the final report for the FUSE Cycle 1 program A100: FUV Spectra of Evolved Late-K and M Stars: Mass Loss revisited and Stellar Activity. Targets alpha TrA (K3 II) and gamma Cru (M3 III) were originally assigned 25 ksec each, to be observed in the medium aperture. Once the in-flight performance and telescope alignment problems were known, the observations were reprogrammed to optimized the scientific return of the program. Alpha TrA was scheduled for 25 ksec observations in both the medium and large apertures. The principle aim of this program was to measure the stellar FUV line and continuum emission, in order to estimate the photoionization radiation field and to determine the level of stellar activity through the fluxes in the collisionally excited high temperature diagnostics: C III 977Angstroms and O VI 1032,1038Angstrom doublet. The medium aperture observations were obtained successfully while the large aperture observations were thought by Johns Hopkins University (JHU)to be lost to satellite problems. There was insufficient signal-to- noise in the medium aperture short wavelength Sic channels to do quantitative science.

Harper, Graham M.

2002-01-01

294

ACCURATE LOW-MASS STELLAR MODELS OF KOI-126  

SciTech Connect

The recent discovery of an eclipsing hierarchical triple system with two low-mass stars in a close orbit (KOI-126) by Carter et al. appeared to reinforce the evidence that theoretical stellar evolution models are not able to reproduce the observational mass-radius relation for low-mass stars. We present a set of stellar models for the three stars in the KOI-126 system that show excellent agreement with the observed radii. This agreement appears to be due to the equation of state implemented by our code. A significant dispersion in the observed mass-radius relation for fully convective stars is demonstrated; indicative of the influence of physics currently not incorporated in standard stellar evolution models. We also predict apsidal motion constants for the two M dwarf companions. These values should be observationally determined to within 1% by the end of the Kepler mission.

Feiden, Gregory A.; Chaboyer, Brian [Department of Physics and Astronomy, Dartmouth College, 6127 Wilder Laboratory, Hanover, NH 03755 (United States); Dotter, Aaron, E-mail: gregory.a.feiden@dartmouth.edu [Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218 (United States)

2011-10-10

295

Low-mass X-ray binaries  

NASA Astrophysics Data System (ADS)

A review is given of current understanding of low-mass X-ray binaries (LMXBs), which are luminous X-ray sources composed of a late-type optical companion (mass less than about 1 solar mass) and a neutron star (or possibly a black hole). Thirty-two LMXBs have been identified with optical counterparts in the Galaxy and one in the Large Magellanic Cloud (Brad and McClintock, 1983). It is unlikely that there are more than about 100 active LMXBs in the Galaxy, compared with about 200,000 cataclysmic variables. Topics covered in the review are: typical X-ray and optical properties; orbital periods; the nature of the compact source; accretion disks; formation; mass transfer mechanisms; and globular clusters and bright bulge X-ray sources.

McClintock, J. E.; Rappaport, S. A.

296

THE ENIGMATIC YOUNG, LOW-MASS VARIABLE TWA 30  

E-print Network

TWA 30 is a remarkable young (7 ± 3 Myr), low-mass (0.12 ± 0.04 M [subscript ?]), late-type star (M5 ± 1) residing 42 ± 2 pc away from the Sun in the TW Hydrae Association (TWA). It shows strong outflow spectral signatures ...

Looper, Dagny L.

297

NA62 Low Mass Spectrometer  

NASA Astrophysics Data System (ADS)

The NA62 experiment at CERN aims at a precision measurement of the ultra-rare decay K^+ rightarrow ?^+?bar?. A low mass (˜ 1.8%X0) spectrometer, whose construction is ongoing, has been designed to track charged kaon decays products. The system operates in vacuum, and will be operative in October 2014, when the first physics run is scheduled. The straw detector is made of 4 stations, each equipped with 1792 straws, arranged in 4 views (X, Y, U and V). A high aperture magnet (MNP33), placed between the second and the third chamber, provides a 0.36T dipole vertical B-field, required to measure the momentum of the charged particles. A 64-straws prototype was constructed in 2010. It was used as test bench for electronics commissioning and detector characterization. Time resolution and space-time relation were measured. A first test with a full chamber and final beam setup was performed in November 2012.

Palladino, V.

2014-06-01

298

Low-mass companions to Bright Giants  

NASA Astrophysics Data System (ADS)

Asymmetric Planetary Nebulae (APN) are formed by bipolar outflows through various mechanism like fast rotation (Blackman et al. 2001), magnetic field (Regos & Tout 1995) or binarity (Harpaz & Soker 1994; Soker 1996; Livio & Soker 2002). The binary scenario seems currently to be best supported by observations as the most efficient in producing the observed APN (De Marco et al. 2004; Soker 2006). Detailed studies of disk formation in binaries leading to APN were presented for instance in Reyes-Ruiz & Lopez (1999), Blackman et al. (2001) and Nordhaus & Blackman (2006). To estimate relative efficiently of the various channels of APN production properties of the population of stars to become AGB stars have to be known. Here our RV search for planets around evolved stars the Penn State-Torun Centre for Astronomy Planet Search (PTPS), whose primary, long-term goal is to improve our understanding of the evolution of planetary systems around aging stars (Niedzielski et al. 2007; Niedzielski & Wolszczan 2008) may be of some help. 1036 stars are monitored within PTPS with the Hobby-Eberly Telescope (HET, Ramsey et al. 1998) and its High Resolution Spectrograph (HRS, Tull et al. 1998) for RV variations using the high precision iodine-cell technique since 2004. The sample is mainly composed of evolved low- and intermediate- mass single or SB1 stars: 449 giants (including 343 clump giants) and 297 subgiants but it also contains 151 slightly evolved dwarfs. All SB1 and SB2 stellar-mass binaries have been identified in the sample. Detailed spectroscopic analysis of 348 stars, mostly giants has been completed by Zieli?Ñski et al. (2012). Similar analyses for 403 giants and subgiants (Niedzielski et. al. in prep.) and 146 dwarf (Deka et al. in prep.) are in preparation. In addition to stellar atmospheric parameters the spectroscopic studies deliver masses and luminosities (through fits to evolutionary tracks) as well as ages required for further considerations on planetary systems evolution - the main goal of PTPS. The sample was optimized for HET and HRS. It contains relatively bright stars with V in the range of 9-12 mag, randomly distributed over the northern hemisphere. After 2-3 epochs or precise RV HET observations all stars with amplitudes exceeding the HET/HRS PSF FWHM - 5 km s-1 (SB1) or below 5?ERV - ˜ 20-50 m s (single) were rejected from further monitoring. Stars with significant cross-correlation profile variations were identified as SB2 and also excluded. All remaining 300 stars are systematically monitored in search for low-mass companions. Over a dozen stars with planetary-mass companions have already been discovered (Niedzielski et al. 2007, 2009a, b; Gettel et al. 2012a, b; Nowak et al. 2013). Here I will present our new results concerning the most luminous giants with log(L/LSun)> 2, presumably post Horizontal Branch stars.

Niedzielski, A.; Wolszczan, A.; Nowak, G.; Adamów, M.; Deka, B.; Górecka, M.; Kowalik, K.

2014-04-01

299

VERY LOW MASS STELLAR AND SUBSTELLAR COMPANIONS TO SOLAR-LIKE STARS FROM MARVELS. II. A SHORT-PERIOD COMPANION ORBITING AN F STAR WITH EVIDENCE OF A STELLAR TERTIARY AND SIGNIFICANT MUTUAL INCLINATION  

SciTech Connect

We report the discovery via radial velocity (RV) measurements of a short-period (P = 2.430420 {+-} 0.000006 days) companion to the F-type main-sequence star TYC 2930-00872-1. A long-term trend in the RV data also suggests the presence of a tertiary stellar companion with P > 2000 days. High-resolution spectroscopy of the host star yields T{sub eff} = 6427 {+-} 33 K, log g = 4.52 {+-} 0.14, and [Fe/H] = -0.04 {+-} 0.05. These parameters, combined with the broadband spectral energy distribution (SED) and a parallax, allow us to infer a mass and radius of the host star of M{sub 1} = 1.21 {+-} 0.08 M{sub Sun} and R{sub 1} = 1.09{sup +0.15}{sub -0.13} R{sub Sun }. The minimum mass of the inner companion is below the hydrogen-burning limit; however, the true mass is likely to be substantially higher. We are able to exclude transits of the inner companion with high confidence. Further, the host star spectrum exhibits a clear signature of Ca H and K core emission, indicating stellar activity, but a lack of photometric variability and small vsin I suggest that the primary's spin axis is oriented in a pole-on configuration. The rotational period of the primary estimated through an activity-rotation relation matches the orbital period of the inner companion to within 1.5 {sigma}, suggesting that the primary and inner companion are tidally locked. If the inner companion's orbital angular momentum vector is aligned with the stellar spin axis as expected through tidal evolution, then it has a stellar mass of {approx}0.3-0.4 M{sub Sun }. Direct imaging limits the existence of stellar companions to projected separations <30 AU. No set of spectral lines and no significant flux contribution to the SED from either companion are detected, which places individual upper mass limits of M{sub {l_brace}2,3{r_brace}} {approx}< 1.0 M{sub Sun }, provided they are not stellar remnants. If the tertiary is not a stellar remnant, then it likely has a mass of {approx}0.5-0.6 M{sub Sun }, and its orbit is likely significantly inclined from that of the secondary, suggesting that the Kozai-Lidov mechanism may have driven the dynamical evolution of this system.

Fleming, Scott W.; Ge Jian; De Lee, Nathan; Jiang Peng; Lee, Brian; Nelson, Ben [Department of Astronomy, University of Florida, 211 Bryant Space Science Center, Gainesville, FL 2611-2055 (United States); Barnes, Rory [Department of Astronomy, University of Washington, P.O. Box 351580, Seattle, WA 98195 (United States); Beatty, Thomas G.; Gaudi, B. Scott; Shappee, Benjamin J. [Department of Astronomy, Ohio State University, 140 West 18th Avenue, Columbus, OH 43210 (United States); Crepp, Justin R. [Department of Astronomy, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125 (United States); Esposito, Massimiliano; Femenia, Bruno; Gonzalez Hernandez, Jonay I. [Instituto de Astrofisica de Canarias (IAC), E-38205 La Laguna, Tenerife (Spain); Ferreira, Leticia; Porto de Mello, Gustavo F. [Observatorio do Valongo, Universidade Federal do Rio de Janeiro, Ladeira do Pedro Antonio, 43, CEP: 20080-090, Rio de Janeiro, RJ (Brazil); Gary, Bruce; Hebb, Leslie; Stassun, Keivan [Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235 (United States); Ghezzi, Luan, E-mail: scfleming@psu.edu [Laboratorio Interinstitucional de e-Astronomia, LIneA, Rua Gal. Jose Cristino 77, Rio de Janeiro, RJ-20921-400 (Brazil); and others

2012-09-15

300

Testing the correlation between low mass planets and debris disks  

NASA Astrophysics Data System (ADS)

The number of dusty debris disks has increased across all spectral types through recent infrared surveys. This has provided greater overlap with stars known to host extrasolar planets via RV surveys. New studies have therefore investigated how the different properties of host stars, exoplanets, and debris disks may be correlated, with the objective of giving empirical support to competing theories of planet formation and evolution. One such emerging correlation is that stars with only low mass planets are more likely to host prominent debris disks than stars that have at least one giant planet. If true, then M dwarfs should have abundant debris disks given that they more frequently have low mass planetary systems. However, the information needed to critically test these ideas is lacking. For most systems, the presence of an outer planet with >30 Earth masses has not been observationally tested, nor are there many M dwarf debris disks available for detailed scrutiny. Here we propose to use STIS coronagraphy to image for the first time the debris disks around three nearby stars in optical scattered light. Searching for sharp dust belt structures indirectly tests for the existence of outer planets that are otherwise undetectable by RV or adaptive optics planet searches. Moreover, two of our target stars are the most recently discovered M dwarf debris disks, both closer to the Sun than AU Mic. The scattered light observations of these two targets would present a major advance in characterizing how M dwarf debris disks co-evolve with planets under different stellar environments.

Kalas, Paul

2014-10-01

301

Very low-mass members of the Lupus 3 cloud  

E-print Network

We report on a multi-band survey for very low-mass stars and brown dwarfs in the Lupus 3 cloud with the Wide Field Imager (WFI) at the ESO/MPG 2.2m telescope on La Silla Observatory (Chile). Our multiband optical photometry is combined with available 2MASS JHK photometry to identify 19 new young stars and 3 brown dwarf candidates as probable members of this star forming region. Our objects are mostly clustered around the cloud core. Stars and brown dwarfs have similar levels of H-alpha emission, probably a signature of accretion. One object, a brown dwarf candidate, exhibits a near-infrared excess, which may indicate the presence of a disk, but its H-alpha emission cannot be confirmed due to its faintness in the optical passbands. We also find two visual pairs of probable Lupus 3 members that may be wide binaries.

Belén López Martí; Jochen Eislöffel; Reinhard Mundt

2005-08-24

302

Population models of low-mass binaries  

NASA Astrophysics Data System (ADS)

I review the current understanding of the formation and evolutionary history of binaries with low-mass companions, focusing on systems that contain a white dwarf, or a component that will evolve into a white dwarf. I will highlight the potential and pitfalls of binary catalogues emerging from exoplanet transit searches, and review the state of the art of population models for low-mass binaries.

Kolb, Ulrich

2014-09-01

303

New Low-Mass Members of Nearby Young Moving Groups  

NASA Astrophysics Data System (ADS)

We are now ready to expand our program to identify new low-mass members of nearby young moving groups (NYMGs) to stars of mass ?0.3 M_?. This is important to: (1) complete the census of low-mass stars near the Sun, (2) provide high priority targets for disk and exoplanet studies by direct imaging, and (3) provide a well- characterized sample of nearby, young stars for detailed study of their physical and kinematic properties. Our proven technique starts with a proper motion selection algorithm, proceeds to vet the sample for indicators of youth, and requires as its last step the measurement of candidate member radial velocities (RVs). So far, we have measured more than 100 candidate RVs using CSHELL on the NASA-IRTF and PHOENIX on Gemini-South, yielding more than 50 likely new moving group members. Here we propose to continue our RV follow-up of candidate NYMG members using PHOENIX on the KPNO 4m. We aim to measure RVs and determine spectral types of 23 faint (V?15, H?9), late-type (?M4) candidates of the (beta) Pic (10 Myrs), AB Dor (70 Myrs), Tuc/Hor (30 Myrs), and TW Hydrae (8 Myrs) moving groups.

Schlieder, Joshua; Simon, Michal; Rice, Emily; Lepine, Sebastien

2012-08-01

304

The Chemical Composition Contrast between M3 and M13 Revisited: New Abundances for 28 Giant Stars in M3  

NASA Astrophysics Data System (ADS)

We report new chemical abundances of 23 bright red giant members of the globular cluster M3, based on high-resolution (R~45,000) spectra obtained with the Keck I telescope. The observations, which involve the use of multislits in the HIRES Keck I spectrograph, are described in detail. Combining these data with a previously reported small sample of M3 giants obtained with the Lick 3 m telescope, we compare metallicities and [X/Fe] ratios for 28 M3 giants with a 35-star sample in the similar-metallicity cluster M13, and with Galactic halo field stars having [Fe/H]<-1. For elements having atomic number A>=A(Si), we derive little difference in [X/Fe] ratios in the M3, M13, or halo field samples. All three groups exhibit C depletion with advancing evolutionary state beginning at the level of the red giant branch ``bump,'' but the overall depletion of about 0.7-0.9 dex seen in the clusters is larger than that associated with the field stars. The behaviors of O, Na, Mg, and Al are distinctively different among the three stellar samples. Field halo giants and subdwarfs have a positive correlation of Na with Mg, as predicted from explosive or hydrostatic carbon burning in Type II supernova sites. Both M3 and M13 show evidence of high-temperature proton-capture synthesis from the ON, NeNa, and MgAl cycles, while there is no evidence for such synthesis among halo field stars. But the degree of such extreme proton-capture synthesis in M3 is smaller than it is in M13: the M3 giants exhibit only modest deficiencies of O and corresponding enhancements of Na, less extreme overabundances of Al, fewer stars with low Mg and correspondingly high Na, and no indication that O depletions are a function of advancing evolutionary state, as has been claimed for M13. We have also considered NGC 6752, for which Mg isotopic abundances have been reported by Yong et al. Giants in NGC 6752 and M13 satisfy the same anticorrelation of O abundances with the ratio (25Mg+26Mg)/24Mg, which measures the relative contribution of rare to abundant isotopes of Mg. This points to a scenario in which these abundance ratios arose in the ejected material of 3-6 Msolar cluster stars, material that was then used to form the atmospheres of the presently evolving low-mass cluster stars. It also suggests that the low oxygen abundance seen among the most evolved M13 giants arose in hot bottom O-to-N processing in these same intermediate-mass cluster stars. Thus, mixing is required by the dependence of some abundance ratios on luminosity, but an earlier nucleosynthesis process in a hotter environment than giants or main-sequence stars is required by the variations previously seen in stars near the main sequence. The nature and the site of the earlier process is constrained but not pinpointed by the observed Mg isotopic ratio. Based on data obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

Sneden, Christopher; Kraft, Robert P.; Guhathakurta, Puragra; Peterson, Ruth C.; Fulbright, Jon P.

2004-04-01

305

Low mass rolling element for bearings  

NASA Technical Reports Server (NTRS)

Low mass rolling elements for bearings having a high fatigue strength and high resistance to flexure fatigue are reported. The elements have a lightweight core with a hollow center or is made of a low density material. The core is plated to provide a hard surface.

Parker, R. J. (inventor)

1973-01-01

306

Methanol abundance in low mass protostars  

E-print Network

Methanol lines observations of a sample of low mass Class 0 protostars are presented. Using a 1D radiative transfer model, I show that several protostars have large abundance jumps in the inner hot and dense region of envelopes, probably because of thermal grain mantle evaporation. These abundances are compared with a grain surface chemistry model.

S. Maret

2004-11-12

307

SEARCH FOR VERY LOW MASS OBJECTS IN THE GALACTIC HALO  

E-print Network

We present results from a search for gravitational microlensing of stars in the Large Magellanic Cloud by low mass objects in the Galactic Halo. The search uses the CCD light curves of about 82,000 stars with up to 46 measurements per night over a period of 10 months. No light curve exhibits a form that is consistent with a microlensing event of maximum amplification greater than 1.2. This null result makes it unlikely that the Halo is dominated by objects in the mass range $5 10^{-8}M_{\\odot} < M <5 10^{-4}M_{\\odot} $. keywords{Galaxy : Halo, kinematics and dynamics, stellar content -- Cosmology : dark matter, gravitational lensing}

EROS collaboration; E. Aubourg; P. Bareyre; S. Brehin; M. Gros; M. Lachieze-Rey; B. Laurent; E. Lesquoy; C. Magneville; A. Milsztajn; L. Moscoso; F. Queinnec; J. Rich; M. Spiro; L. Vigroux; S. Zylberajch; R. Ansari; F. Cavalier; M. Moniez; J. P. Beaulieu; R. Ferlet; Ph. Grison; A. Vidal-Madjar; E. Maurice; L. Prevot; C. Gry; J. Guibert; O. Moreau; F. Tajahmady

1995-03-05

308

Our Nearest 15 Million Neighbors: The Field Low-Mass Stellar Luminosity and Mass Functions  

NASA Astrophysics Data System (ADS)

We report on a new measurement of the luminosity function (LF) and mass function (MF) of field low-mass dwarfs using Sloan Digital Sky Survey (SDSS) photometry. The final catalog is composed of ~15 million low-mass stars (0.1 Msolarstars are estimated using new photometric parallax relations, constructed from ugriz photometry of nearby low-mass stars with trigonometric parallaxes. The LF is measured with a novel technique, which simultaneously measures Galactic structure and the stellar LF. The resulting LF is compared to previous studies and converted to a MF. The MF is well-described by a log-normal distribution, with M° = 0.27 Msolar.

Bochanski, John J.; Hawley, Suzanne L.; Reid, I. Neill; Covey, Kevin R.; West, Andrew A.; Golimowski, David A.; Ivezi?, Željko

2009-02-01

309

VERY LOW MASS STELLAR AND SUBSTELLAR COMPANIONS TO SOLAR-LIKE STARS FROM MARVELS. V. A LOW ECCENTRICITY BROWN DWARF FROM THE DRIEST PART OF THE DESERT, MARVELS-6b  

SciTech Connect

We describe the discovery of a likely brown dwarf (BD) companion with a minimum mass of 31.7 {+-} 2.0 M{sub Jup} to GSC 03546-01452 from the MARVELS radial velocity survey, which we designate as MARVELS-6b. For reasonable priors, our analysis gives a probability of 72% that MARVELS-6b has a mass below the hydrogen-burning limit of 0.072 M{sub Sun }, and thus it is a high-confidence BD companion. It has a moderately long orbital period of 47.8929{sup +0.0063}{sub -0.0062} days with a low eccentricity of 0.1442{sup +0.0078}{sub -0.0073}, and a semi-amplitude of 1644{sup +12}{sub -13} m s{sup -1}. Moderate resolution spectroscopy of the host star has determined the following parameters: T{sub eff} = 5598 {+-} 63, log g = 4.44 {+-} 0.17, and [Fe/H] = +0.40 {+-} 0.09. Based upon these measurements, GSC 03546-01452 has a probable mass and radius of M{sub *} = 1.11 {+-} 0.11 M{sub Sun} and R{sub *} = 1.06 {+-} 0.23 R{sub Sun} with an age consistent with less than {approx}6 Gyr at a distance of 219 {+-} 21 pc from the Sun. Although MARVELS-6b is not observed to transit, we cannot definitively rule out a transiting configuration based on our observations. There is a visual companion detected with Lucky Imaging at 7.''7 from the host star, but our analysis shows that it is not bound to this system. The minimum mass of MARVELS-6b exists at the minimum of the mass functions for both stars and planets, making this a rare object even compared to other BDs. It also exists in an underdense region in both period/eccentricity and metallicity/eccentricity space.

De Lee, Nathan; Stassun, Keivan G.; Cargile, Phillip [Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235 (United States); Ge, Jian; Fleming, Scott W.; Lee, Brian L.; Chang Liang [Department of Astronomy, University of Florida, 211 Bryant Space Science Center, Gainesville, FL 32611-2055 (United States); Crepp, Justin R. [Department of Physics, University of Notre Dame, 225 Nieuwland Science Hall, Notre Dame, IN 46556 (United States); Eastman, Jason; Gaudi, B. Scott [Department of Astronomy, Ohio State University, 140 West 18th Avenue, Columbus, OH 43210 (United States); Esposito, Massimiliano; Femenia, Bruno; Gonzalez Hernandez, Jonay I.; Allende Prieto, Carlos [Instituto de Astrofisica de Canarias (IAC), E-38205 La Laguna, Tenerife (Spain); Ghezzi, Luan [Observatorio Nacional, Rua Gal. Jose Cristino 77, Rio de Janeiro, RJ 20921-400 (Brazil); Wisniewski, John P. [H L Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W Brooks St Norman, OK 73019 (United States); Wood-Vasey, W. Michael [Pittsburgh Particle physics, Astrophysics, and Cosmology Center (PITT PACC), Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260 (United States); Agol, Eric; Barnes, Rory [Astronomy Department, University of Washington, Box 351580, Seattle, WA 98195 (United States); Bizyaev, Dmitry, E-mail: nathan.delee@vanderbilt.edu [Apache Point Observatory, P.O. Box 59, Sunspot, NM 88349-0059 (United States); and others

2013-06-15

310

Low-mass dileptons from nonequilibrium QGP  

E-print Network

The rate of the emission of the high energy low-mass dileptons from the QGP is found in the first nonvanishing order with respect to strong coupling. We base on the real-time kinetic approach [2] without an explicit assumption about a complete thermal equilibrium in the emitting system. For the class of the partons distributions which may simulate that of the "hot glue scenario"[1] the rate of emission is found analytically . ( Figures can be obtained from the author )

A. Makhlin

1993-06-25

311

Calorimetry of low mass Pu239 items  

SciTech Connect

Calorimetric assay has the reputation of providing the highest precision and accuracy of all nondestructive assay measurements. Unfortunately, non-destructive assay practitioners and measurement consumers often extend, inappropriately, the high precision and accuracy of calorimetric assay to very low mass items. One purpose of this document is to present more realistic expectations for the random uncertainties associated with calorimetric assay for weapons grade plutonium items with masses of 200 grams or less.

Cremers, Teresa L [Los Alamos National Laboratory; Sampson, Thomas E [Los Alamos National Laboratory

2010-01-01

312

Ribosomal RNA genes and deuterostome phylogeny revisited: More cyclostomes, elasmobranchs, reptiles, and a brittle star  

Microsoft Academic Search

This is an expanded study of the relationships among the deuterostome animals based on combined, nearly complete 28S and 18S rRNA genes (>3925 nt.). It adds sequences from 20 more taxa to the ?45 sequences used in past studies. Seven of the new taxa were sequenced here (brittle star Ophiomyxa, lizard Anolis, turtle Chrysemys, sixgill shark Hexanchus, electric ray Narcine,

Jon Mallatt; Christopher J. Winchell

2007-01-01

313

OGLE-2005-BLG-153: MICROLENSING DISCOVERY AND CHARACTERIZATION OF A VERY LOW MASS BINARY  

SciTech Connect

The mass function and statistics of binaries provide important diagnostics of the star formation process. Despite this importance, the mass function at low masses remains poorly known due to observational difficulties caused by the faintness of the objects. Here we report the microlensing discovery and characterization of a binary lens composed of very low mass stars just above the hydrogen-burning limit. From the combined measurements of the Einstein radius and microlens parallax, we measure the masses of the binary components of 0.10 {+-} 0.01 M{sub sun} and 0.09 {+-} 0.01 M{sub sun}. This discovery demonstrates that microlensing will provide a method to measure the mass function of all Galactic populations of very low mass binaries that is independent of the biases caused by the luminosity of the population.

Hwang, K.-H.; Han, C.; Ryu, Y.-H. [Department of Physics, Chungbuk National University, Cheongju 361-763 (Korea, Republic of); Udalski, A.; Kubiak, M.; Szymanski, M. K.; Pietrzynski, G.; Soszynski, I.; Szewczyk, O.; Ulaczyk, K.; Wyrzykowski, L. [Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warszawa (Poland); Bond, I. A. [Institute of Information and Mathematical Sciences, Massey University, Private Bag 102-904, North Shore Mail Centre, Auckland (New Zealand); Beaulieu, J.-P. [Institut d'Astrophysique de Paris, CNRS, Universite Pierre et Marie Curie UMR7095, 98bis Boulevard Arago, 75014 Paris (France); Dominik, M.; Horne, K. [Scottish Universities Physics Alliance, University of St. Andrews, School of Physics and Astronomy, North Haugh, St. Andrews KY16 9SS (United Kingdom); Gould, A.; Gaudi, B. S. [Department of Astronomy, Ohio State University, 140 W. 18th Ave., Columbus, OH 43210 (United States); Abe, F. [Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya 464-8601 (Japan); Botzler, C. S. [Department of Physics, University of Auckland, Private Bag 92019, Auckland (New Zealand); Hearnshaw, J. B. [Department of Physics and Astronomy, University of Canterbury, Private Bag 4800, Christchurch 8020 (New Zealand)

2010-11-01

314

Updated Electron-Conduction Opacities: The Impact on Low-Mass Stellar Models  

E-print Network

We review the theory of electron-conduction opacity, a fundamental ingredient in the computation of low-mass stellar models; shortcomings and limitations of the existing calculations used in stellar evolution are discussed. We then present new determinations of the electron-conduction opacity in stellar conditions for an arbitrary chemical composition, that improve over previous works and, most importantly, cover the whole parameter space relevant to stellar evolution models (i.e., both the regime of partial and high electron degeneracy). A detailed comparison with the currently used tabulations is also performed. The impact of our new opacities on the evolution of low-mass stars is assessed by computing stellar models along both the H- and He-burning evolutionary phases, as well as Main Sequence models of very low-mass stars and white dwarf cooling tracks.

S. Cassisi; A. Y. Potekhin; A. Pietrinferni; M. Catelan; M. Salaris

2007-03-01

315

THE r-PROCESS IN PROTO-NEUTRON-STAR WIND REVISITED  

SciTech Connect

We examine the r-process in the neutrino-driven proto-neutron-star (PNS) wind of core-collapse supernovae in light of the recent findings of massive neutron stars in binaries as well as of an indication of neutron-richness in the PNS ejecta because of the nucleon potential corrections on neutrino opacities. To this end, a spherically symmetric, general relativistic, steady-state wind model is applied for a wide range of PNS masses between 1.2 M{sub Sun} and 2.4 M{sub Sun} with the latter reaching the causality limit. Nucleosynthesis calculations with these PNS models are performed by assuming a time evolution of electron fraction with its minimal value of Y{sub e} = 0.4, which mimics recent hydrodynamical results. The fundamental nucleosynthetic aspect of the PNS wind is found to be the production of Sr, Y, and Zr in quasi-equilibrium and of the elements with A Almost-Equal-To 90-110 by a weak r-process, which can be an explanation for the abundance signatures in r-process-poor Galactic halo stars. PNSs more massive than 2.0 M{sub Sun} can eject heavy r-process elements, however, with substantially smaller amount than what is needed to account for the solar content. PNS winds can be thus the major origin of light trans-iron elements but no more than 10% of those heavier than A {approx} 110, although they may be the sources of the low-level abundances of Sr and Ba found in numerous metal-poor stars if the maximum mass of PNSs exceeds 2.0 M{sub Sun }.

Wanajo, Shinya, E-mail: shinya.wanajo@nao.ac.jp [National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588 (Japan)

2013-06-20

316

Identification of Very Low Mass Brown Dwarfs in IC348  

NASA Astrophysics Data System (ADS)

We propose to perform near-infrared low resolution spectroscopic follow-up observations with NIRI/Gemini North of 15 planetary mass brown dwarf (BD) candidates in IC 348 (300pc, 3Myr). The sample was identified from a wide-field deep photometric survey using MegaCam and WIRCam/CFHT. The proposed observations will allow us to confirm the membership of the candidates via the study of spectral youth indicators, to derive the spectral types and reddening, and assess the low mass nature of the confirmed sources by placing them in a H-R diagram. This will represent an unprecendent sample to eventually achieving a complete census of the low-mass population in IC 348, ideally suited to constrain the stellar, substellar and planet formation processes. Over the last decade, a number of large deep surveys (UKIDSS, GDPS, C2d, GOODS, PanSTARR, and more modestly the one we present here) have lead to some of the most important discoveries in the field of substellar star formation. The outcoming samples usually require follow-up time on 8m-class telescopes, without which the time, money and expertise invested in these large programs are not returned. An observatory like Gemini provides a unique opportunity to obtain these follow-up observations and achieve the final scientific goals of these surveys.

Duchene, Gaspard; Alves de Oliveira, Catarina; Moraux, Estelle; Bouvier, Jerome; Bouy, Herve

2010-08-01

317

Low mass dilepton production at RHIC energies  

Microsoft Academic Search

.  Recent results on low mass dilepton measurements from the PHENIX experiment are reported. Invariant mass spectra of \\u000a \\u000a are measured for the first time in Au-Au collisions at \\u000a \\u000a GeV in Run2. In d-Au collisions, the yields and MT slopes of both \\u000a \\u000a and \\u000a \\u000a are measured. Both results are consistent with each other within errors. In the future, a Hadron Blind Detector will

K. Ozawa; S. Afanasiev; C. Aidala; Y. Akiba; A. Al-Jamel; J. Alexander; G. Alley; R. Amirikas; K. Aoki; L. Aphecetche; R. Armendariz; V. Armijo; R. Averbeck; R. Azmoun; V. Babintsev; A. Baldisseri; B. Bassalleck; S. Bathe; S. Batsouli; V. Baublis; F. Bauer; A. Bazilevsky; S. Belikov; Y. Berdnikov; S. Bhagavatula; M. Bobrek; H. Borel; S. Borenstein; N. Brun; N. Bruner; D. Bucher; H. Buesching; V. Bumazhnov; G. Bunce; S. Butsyk; X. Camard; P. Chand; S. Chernichenko; J. Chiba; M. Chiu; J. Choi; S. Chollet; T. Chujo; V. Cianciolo; D. Clark; Y. Cobigo; P. Constantin; M. Csanad; T. Csorgo; H. Cunitz; F. Deak; A. Debraine; H. Delagrange; A. Denisov; A. Deshpande; A. Devismes; O. Dietzsch; O. Drapier; A. Drees; A. Durum; D. Dutta; V. Dzhordzhadze; K. ElChenawi; A. Enokizono; B. Espagnon; S. Esumi; V. Evseev; C. Finck; F. Fleuret; Z. Fraenkel; A. Franz; J. Fried; S. Gadrat; J. Gannon; S. Garpman; F. Gastaldi; M. Germain; P. Giannotti; A. Glenn; G. Gogiberidze; M. Gonin; J. Gosset; Y. Goto; N. Grau; M. GrossePerdekamp; W. Guryn; T. Hachiya; H. Hamagaki; J. Harder; M. Harvey; K. Hasuko; N. Hayashi; M. Heffner; N. Heine; M. Hibino; P. Hidas; H. Hiejima; R. Hobbs; W. Holzmann; K. Homma; B. Hong; T. Horaguchi; R. Hutter; T. Ichihara; K. Imai; M. Inaba; M. Inuzuka; D. Isenhower; L. Isenhower; M. Ishihara; M. Issah; A. Isupov; Y. Jeong; O. Jinnouchi; D. Jouan; S. Kahn; F. Kajihara; S. Kametani; N. Kamihara; A. Kandasamy; M. Kaneta; M. Kann; A. Karar; S. Kato; K. Katou; T. Kawabata; A. Kazantsev

2005-01-01

318

Windows Through the Dusty Disks Surrounding the Youngest Low-Mass Protostellar Objects  

Microsoft Academic Search

The formation and evolution of young low-mass stars are characterized by important processes of mass loss and accretion occurring in the innermost regions of their placentary circumstellar disks. Because of the large obscuration of these disks at optical and infrared wavelengths in the early protostellar stages (class 0 sources), they were previously detected only at radio wavelengths using interferometric techniques.

J. Cernicharo; A. Noriega-Crespo; D. Cesarsky; B. Lefloch; E. González-Alfonso; F. Najarro; E. Dartois; S. Cabrit

2000-01-01

319

Solid and gas phase inventory of the low mass protostar Elias 29 (ae Oph)  

E-print Network

CHAPTER 7 Solid and gas phase inventory of the low mass protostar Elias 29 (ae Oph) To be submitted ) Class I protostellar object Elias 29 in the ae Ophiuchi molecular cloud. It provides a unique amount outflow. In many aspects, Elias 29 resembles the Herbig Ae star AB Aur, although Elias 29 is less evolved

Boogert, Adwin

320

Chemistry in low-mass protostellar and protoplanetary regions  

PubMed Central

When interstellar clouds collapse to form new stars and planets, the surrounding gas and dust become part of the infalling envelopes and rotating disks, thus providing the basic material from which new solar systems are formed. Instrumentation to probe the chemistry in low-mass star-forming regions has only recently become available. The results of a systematic program to study the abundances in solar-mass protostellar and protoplanetary regions are presented. Surveys at submillimeter and infrared wavelengths reveal a rich chemistry, including simple and complex (organic) gases, ices, polycyclic aromatic hydrocarbons, and silicates. Each of these species traces different aspects of the physical and chemical state of the objects as they evolve from deeply embedded protostars to pre-main sequence stars with planet-forming disks. Quantitative information on temperatures, densities, and abundances is obtained through molecular excitation and radiative transfer models as well as from analysis of solid-state line profiles. The chemical characteristics are dominated by freeze-out in the coldest regions and ice evaporation in the warmer zones. In the surface layers of disks, UV radiation controls the chemistry. The importance of complementary laboratory experiments and calculations to obtain basic molecular data is emphasized. PMID:16894165

van Dishoeck, Ewine F.

2006-01-01

321

Chemistry in low-mass protostellar and protoplanetary regions.  

PubMed

When interstellar clouds collapse to form new stars and planets, the surrounding gas and dust become part of the infalling envelopes and rotating disks, thus providing the basic material from which new solar systems are formed. Instrumentation to probe the chemistry in low-mass star-forming regions has only recently become available. The results of a systematic program to study the abundances in solar-mass protostellar and protoplanetary regions are presented. Surveys at submillimeter and infrared wavelengths reveal a rich chemistry, including simple and complex (organic) gases, ices, polycyclic aromatic hydrocarbons, and silicates. Each of these species traces different aspects of the physical and chemical state of the objects as they evolve from deeply embedded protostars to pre-main sequence stars with planet-forming disks. Quantitative information on temperatures, densities, and abundances is obtained through molecular excitation and radiative transfer models as well as from analysis of solid-state line profiles. The chemical characteristics are dominated by freeze-out in the coldest regions and ice evaporation in the warmer zones. In the surface layers of disks, UV radiation controls the chemistry. The importance of complementary laboratory experiments and calculations to obtain basic molecular data is emphasized. PMID:16894165

van Dishoeck, Ewine F

2006-08-15

322

Ribosomal RNA genes and deuterostome phylogeny revisited: more cyclostomes, elasmobranchs, reptiles, and a brittle star.  

PubMed

This is an expanded study of the relationships among the deuterostome animals based on combined, nearly complete 28S and 18S rRNA genes (>3925 nt.). It adds sequences from 20 more taxa to the approximately 45 sequences used in past studies. Seven of the new taxa were sequenced here (brittle star Ophiomyxa, lizard Anolis, turtle Chrysemys, sixgill shark Hexanchus, electric ray Narcine, Southern Hemisphere lamprey Geotria, and Atlantic hagfish Myxine for 28S), and the other 13 were from GenBank and the literature (from a chicken, dog, rat, human, three lungfishes, and several ray-finned fishes, or Actinopterygii). As before, our alignments were based on secondary structure but did not account for base pairing in the stems of rRNA. The new findings, derived from likelihood-based tree-reconstruction methods and by testing hypotheses with parametric bootstrapping, include: (1) brittle star joins with sea star in the echinoderm clade, Asterozoa; (2) with two hagfishes and two lampreys now available, the cyclostome (jawless) fishes remain monophyletic; (3) Hexanchiform sharks are monophyletic, as Hexanchus groups with the frilled shark, Chlamydoselachus; (4) turtle is the sister taxon of all other amniotes; (5) bird is closer to the lizard than to the mammals; (6) the bichir Polypterus is in a monophyletic Actinopterygii; (7) Zebrafish Danio is the sister taxon of the other two teleosts we examined (trout and perch); (8) the South American and African lungfishes group together to the exclusion of the Australian lungfish. Other findings either upheld those of the previous rRNA-based studies (e.g., echinoderms and hemichordates group as Ambulacraria; orbitostylic sharks; batoids are not derived from any living lineage of sharks) or were obvious (monophyly of mammals, gnathostomes, vertebrates, echinoderms, etc.). Despite all these findings, the rRNA data still fail to resolve the relations among the major groups of deuterostomes (tunicates, Ambulacraria, cephalochordates and vertebrates) and of gnathostomes (chondrichthyans, lungfishes, coelacanth, actinopterygians, amphibians, and amniotes), partly because tunicates and lungfishes are rogue taxa that disrupt the tree. Nonetheless, parametric bootstrapping showed our RNA-gene data are only consistent with these dominant hypotheses: (1) deuterostomes consist of Ambulacraria plus Chordata, with Chordata consisting of tunicates and 'vertebrates plus cephalochordates'; and (2) lungfishes are the closest living relatives of tetrapods. PMID:17276090

Mallatt, Jon; Winchell, Christopher J

2007-06-01

323

Brief search for low-mass objects  

NASA Astrophysics Data System (ADS)

Observations of the nearby white dwarfs EG 5 (van Maanen 2) and EG 290 obtained in the J, H, K, L, L-prime, and M bands using an RC1 photometer at the Cassegrain focus of the 3.3-m IR telescope at Mauna Kea on Dec. 2, 1983 are reported. The L-band fluxes are analyzed to search for low-mass companion objects, but none is found. Upper limits of Te = 850 K and L = 5 x 10 to the -6th solar luminosity are established for the putative companions of both dwarfs, corresponding to a mass of 0.027 solar mass or less for an age of 3 Gyr.

Krishna Kumar, C.

1985-03-01

324

Low-mass Objects in Moving Groups  

NASA Astrophysics Data System (ADS)

We present here the kinematic study of part of a 132 target sample of low-mass objects, previously selected by photometric and astrometric criteria as possible members to five young moving groups (MGs): Local Association (Pleiades moving group, 20 - 150 Myr), Ursa Mayor group (Sirius supercluster, 300 Myr), Hyades supercluster (600 Myr), IC 2391 supercluster (35 Myr) and Castor moving group (200 Myr). We calculated their kinematic galactic components (U,V,W) and apply simple kinematic criteria of membership. The confirmed members will provide a new and substantial population of age and metallicity benchmark ultra cool dwarfs (UCDs). This will give a valuable set to test atmosphere and evolutionary models of ages below 1 Gyr and suitable targets for use in adaptive optic (AO) imaging to search for sub-stellar companions/exoplanets.

Gálvez-Ortiz, M. C.; Clarke, J. R. A.; Pinfield, D. J.; Folkes, S. L.; Jenkins, J. S.; García Pérez, A. E.; Burningham, B.; Day-Jones, A. C.; Jones, H. R. A.

2010-10-01

325

A SPITZER SEARCH FOR SUBSTELLAR COMPANIONS TO LOW-MASS WHITE DWARFS  

SciTech Connect

The formation scenarios for single low-mass (M < 0.45 M{sub sun}) white dwarfs (WDs) include enhanced mass loss from a metal-rich progenitor star or a common envelope phase of a solar-like star with a close-in massive planet or a brown dwarf. Both scenarios suggest that low-mass WDs may have planets. Here, we present a Spitzer IRAC search for substellar and planetary mass companions to 14 low-mass WDs. One of our targets, HS 1653+7753, displays near- and mid-infrared flux excess. However, follow-up MMT observations show that this excess is due to a nearby resolved source, which is mostly likely a background object. Another target, PG 2257+162, shows flux excess compatible with a late-type stellar companion. We do not detect substellar companions to any of the remaining targets. In addition, eight of these stars do not show any radial velocity variations, ruling out stellar mass companions including other WDs. We conclude that a significant fraction of the low-mass WDs in our sample do not have stellar or massive brown dwarf companions.

Kilic, Mukremin; Brown, Warren R.; McLeod, B., E-mail: mkilic@cfa.harvard.ed [Smithsonian Astrophysical Observatory, 60 Garden St., Cambridge, MA 02138 (United States)

2010-01-01

326

MODEL ATMOSPHERES OF VERY LOW MASS STARS AND BROWN DWARFS  

Microsoft Academic Search

As progressively cooler stellar and substellar objects are discovered, the presence first of molecules and then of condensed particulates greatly complicates the understanding of their physical properties. Accurate model atmospheres that include these processes are the key to establishing their atmospheric parameters. They play a crucial role in determining structural characteristics by setting the surface conditions of model interiors and

France Allard; Peter H. Hauschildt; David R. Alexander; Sumner Starrfield

1997-01-01

327

A search for very low mass companions of nearby stars  

E-print Network

of discoveries, the census of substel- lar objects in the solar neighborhood remains incomplete. Present generation imaging surveys conducted in the near and mid-infrared are expected to unveil numerous ultra- cool the brown dwarf-planet transition zone. Figure 1: VISTA ­ the Visible and Infrared Survey Tele- scope

Joergens, Viki

328

Observations of young low-mass stars in dense cores  

NASA Technical Reports Server (NTRS)

In this period the first maps were made during two flights of the Kuipper Airborne Observatory (KAO) in January 1986. One of these shows extended emission at 100 and 160 microns from the dense core B35 containing the IRAS point source 05417+0907. The 160 micron emission has approximately the same extent as the NH3 (1,1) line emission at 1.4 cm, indicating close correspondence between the warm dust and the dense gas. The 160 micron map shows a previously unknown secondary maximum about 90 arcsec north of the IRAS source.

Myers, Philip C.

1987-01-01

329

Binary Star Orbits. III. Revisiting the Remarkable Case of Tweedledum and Tweedledee  

NASA Astrophysics Data System (ADS)

Two of the most challenging objects for optical interferometry in the middle of the last century were the close components (FIN 332) of the wide visual binary STF2375 (= WDS 18455+0530 = HIP 92027 = ADS 11640). Each component of the wide pair was found to have subcomponents of approximately the same magnitude, position angle, and separation and, hence, were designated by the tongue-in-cheek monikers "Tweedledum and Tweedledee" by the great visual interferometrist William S. Finsen in 1953. They were later included in a list of "Double Stars that Vex the Observer" by W.H. van den Bos in 1958. While speckle interferometry has reaped a rich harvest investigating the close inteferometric binaries of Finsen, the "Tweedles" have continued to both fascinate and exasperate due to both the great similarity of the close pairs and the inherent 180° ambiguity associated with interferometry. Detailed analysis of all published observations of the system has revealed several errors which are here corrected, allowing for determination of these orbital elements which resolve the quadrant ambiguity. A unique software filter was developed which allowed subarrays from archival ICCD speckle data from 1982 to be re-reduced. Those data, combined with new and unpublished observations obtained in 2001-2009 from NOAO 4 m telescopes, the Mount Wilson 100 inch telescope and the Naval Observatory Flagstaff Station 61 inch telescope as well as high-quality unresolved measures all allow for the correct orbits to be determined. Co-planarity of the multiple system is also investigated.

Mason, Brian D.; Hartkopf, William I.; McAlister, Harold A.

2010-07-01

330

MINERVA: A Dedicated Observatory for Detection of Nearby Low-Mass Exoplanets  

NASA Astrophysics Data System (ADS)

Detection of low-mass planets around GKM stars requires sub-meter-per-second radial velocity precision. Stellar noise sources (starspots, oscillations, and granulation) necessitate high cadence observations. MINERVA is a dedicated observatory for velocimetric detection of low mass exoplanets orbiting nearby stars. Our array of four robotic 0.7-meter PlaneWave telescopes feeds a purpose-built, temperature-stabilized, iodine cell spectrometer from Callaghan Innovation. We will monitor bright, sun-like stars within 100 pc every clear night from Whipple Observatory on Mt Hopkins, Arizona. Each telescope is also equipped with an Andor CCD for followup photometry and education use. Commissioning is underway on the site and science observations will begin in early 2015.

McCrady, Nate; Johnson, John; Wright, Jason; Wittenmyer, Robert A.; Blake, Cullen; Swift, Jonathan; Eastman, Jason D.; Plavchan, Peter; Riddle, Reed L.; Muirhead, Philip Steven; Bottom, Michael; Zhao, Ming; Beatty, Thomas G.

2015-01-01

331

Ursa Major: a missing low-mass CDM halo?  

E-print Network

The recently discovered Ursa Major dwarf spheroidal (dSph) galaxy candidate is about five to eight times less luminous than the faintest previously known dSphs And IX, Draco, and UMi. In this Letter, we present velocity measurements of seven color-magnitude selected UMajor candidate stars. Two of them are apparent non-members based on metallicity and velocity, and the remaining five stars yield a systemic heliocentric velocity of v=-52.45 +/-4.27 km/s and a central line of sight velocity dispersion of 9.3 (+11.7 -1.2) km/s, with 95% confidence that the dispersion is >6.5 km/s. Assuming that UMajor is in dynamical equilibrium, it is clearly dark matter dominated, and cannot be a purely stellar system like a globular cluster. It has an inferred central mass-to-light ratio of M/L~500 and, based on our studies of other dSphs, may possess a much larger total mass to light ratio. UMajor is unexpectedly massive for its low luminosity -- indeed, UMajor appears to be the most dark-matter dominated galaxy yet discovered. The presence of so much dark matter in UMajor immediately suggests that it may be a member of the missing population of low-mass galaxies predicted by the Cold Dark Matter (CDM) paradigm. Given the weak correlation between dSph mass and luminosity, it is entirely likely that a population of dark dwarfs surrounds our Galaxy.

J. T. Kleyna; M. I. Wilkinson; N. W. Evans; G. Gilmore

2005-07-07

332

The Low-Mass Content of the Hyades stellar cluster  

NASA Astrophysics Data System (ADS)

The Hyades cluster is an ideal target to study the dynamical evolution of a star cluster over the entire mass range due to its intermediate age and proximity to the Sun. We extend the Hyades mass function towards lower masses into the brown dwarf regime and use the full three-dimensional spatial information to characterize the dynamical evolution of the cluster. In addition, once binaries are identified, the new members will allow us to characterize the evolution of ultra-cool dwarfs at an age of 650 Myr. We perform a kinematic and photometric selection using the PPMXL and Pan-STARRS1 sky surveys, to search for cluster members up to 30 pc from the cluster centre. We determine our detection efficiency and field star contamination rate to derive the cluster luminosity and mass function. A minimum spanning tree algorithm is used to quantify the mass segregation. As likeliest candidates, we discover 43 new Hyades member candidates with mass estimates below 0.43 Msun, and double the number of low-mass member candidates. The cluster is significantly mass segregated. The extension of the mass function towards lower masses provides an even clearer signature than estimated before.

Goldman, Bertrand; Roeser, Siegfried; Schilbach, Elena; Henning, Thomas; Olczak, Christoph; Magnier, Eugene A.

2013-07-01

333

Low Mass Members in Nearby Young Moving Groups Revealed  

NASA Astrophysics Data System (ADS)

We are now ready to expand our program that identifies highly probable low-mass members of the nearby young moving groups (NYMGs) to stars of mass ~ 0.1 Msun. This is important 1) To provide high priority targets for exoplanet searches by direct imaging, 2) To complete the census of the membership in the NYMGs, and 3) To provide a well-characterized sample of nearby young stars for detailed study of their physical properties and multiplicity (the median distances of the (beta) Pic and AB Dor groups are ~ 35 pc with ages ~ 12 and 50 Myr respectively). Our proven technique starts with a proper motion selection algorithm, proceeds to vet the sample for indicators of youth, and requires as its last step the measurement of candidate member radial velocities (RVs). So far, we have obtained all RV measurements with the high resolution IR spectrometer at the NASA-IRTF and have reached the limits of its applicability. To identify probable new members in the south, and also those of the lowest mass, we need the sensitivity of PHOENIX at Gemini-S and NIRSPEC at Keck-II.

Schlieder, Joshua; Simon, Michal; Rice, Emily; Lepine, Sebastien

2010-08-01

334

Modeling water emission from low-mass protostellar envelopes  

E-print Network

Within low-mass star formation, water vapor plays a key role in the chemistry and energy balance of the circumstellar material. The Herschel Space Observatory will open up the possibility to observe water lines originating from a wide range of excitation energies.Our aim is to simulate the emission of rotational water lines from envelopes characteristic of embedded low-mass protostars. A large number of parameters that influence the water line emission are explored: luminosity, density,density slope and water abundances.Both dust and water emission are modelled using full radiative transfer in spherical symmetry. The temperature profile is calculated for a given density profile. The H2O level populations and emission profiles are in turn computed with a non-LTE line code. The results are analyzed to determine the diagnostic value of different lines, and are compared with existing observations. Lines can be categorized in: (i) optically thick lines, including ground-state lines, mostly sensitive to the cold outer part; (ii) highly excited (E_u>200-250 K) optically thin lines sensitive to the abundance in the hot inner part; and (iii) lines which vary from optically thick to thin depending on the abundances. Dust influences the emission of water significantly by becoming optically thick at the higher frequencies, and by pumping optically thin lines. A good physical model of a source, including a correct treatment of dust, is a prerequisite to infer the water abundance structure and possible jumps at the evaporation temperature from observations. The inner warm (T>100 K) envelope can be probed byhighly-excited lines, while a combination of excited and spectrally resolved ground state lines probes the outer envelope. Observations of H218O lines, although weak, provide even stronger constraints on abundances.

T. A. van Kempen; S. D. Doty; E. F. van Dishoeck; M. R. Hogerheijde; J. K. Joergensen

2008-05-06

335

ROTATIONAL VELOCITIES OF INDIVIDUAL COMPONENTS IN VERY LOW MASS BINARIES  

SciTech Connect

We present rotational velocities for individual components of 11 very low mass (VLM) binaries with spectral types between M7 and L7.5. These results are based on observations taken with the near-infrared spectrograph, NIRSPEC, and the Keck II laser guide star adaptive optics system. We find that the observed sources tend to be rapid rotators (v sin i > 10 km s{sup -1}), consistent with previous seeing-limited measurements of VLM objects. The two sources with the largest v sin i, LP 349-25B and HD 130948C, are rotating at {approx}30% of their break-up speed, and are among the most rapidly rotating VLM objects known. Furthermore, five binary systems, all with orbital semimajor axes {approx}<3.5 AU, have component v sin i values that differ by greater than 3{sigma}. To bring the binary components with discrepant rotational velocities into agreement would require the rotational axes to be inclined with respect to each other, and that at least one component is inclined with respect to the orbital plane. Alternatively, each component could be rotating at a different rate, even though they have similar spectral types. Both differing rotational velocities and inclinations have implications for binary star formation and evolution. We also investigate possible dynamical evolution in the triple system HD 130948A-BC. The close binary brown dwarfs B and C have significantly different v sin i values. We demonstrate that components B and C could have been torqued into misalignment by the primary star, A, via orbital precession. Such a scenario can also be applied to another triple system in our sample, GJ 569A-Bab. Interactions such as these may play an important role in the dynamical evolution of VLM binaries. Finally, we note that two of the binaries with large differences in component v sin i, LP 349-25AB and 2MASS 0746+20AB, are also known radio sources.

Konopacky, Q. M.; Macintosh, B. A. [Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550 (United States); Ghez, A. M. [UCLA Division of Astronomy and Astrophysics, Los Angeles, CA 90095-1562 (United States); Fabrycky, D. C. [Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064 (United States); White, R. J. [Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30303 (United States); Barman, T. S. [Lowell Observatory, 1400 W. Mars Hill Rd., Flagstaff, AZ 86001 (United States); Rice, E. L. [American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024-5192 (United States); Hallinan, G. [Department of Astrophysics, California Institute of Technology, MC 249-17, Pasadena, CA 91125 (United States); Duchene, G., E-mail: macintosh1@llnl.gov, E-mail: konopacky@di.utoronto.ca, E-mail: ghez@astro.ucla.edu, E-mail: fabrycky@ucolick.org, E-mail: white@chara.gsu.edu, E-mail: barman@lowell.edu, E-mail: erice@amnh.org, E-mail: gh@astro.caltech.edu, E-mail: gduchene@berkeley.edu [Astronomy Department, University of California, Berkeley, Hearst Field Annex B-20, CA 94720-3411 (United States)

2012-05-01

336

The MUSCLES Treasury Survey: Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems  

NASA Astrophysics Data System (ADS)

This proposal aims to acquire critical UV observations of low-mass host stars, providing a treasury database for studies of exemplary nearby exoplanetary systems and potentially habitable worlds not yet discovered. We propose the MUSCLES Treasury Survey: a UV survey of nearby low-mass exoplanetary host stars. Using HST-COS and STIS, we will observe the 1150 - 5700A fluxes, reconstruct the important Ly-alpha emission lines, and use these data to estimate the extreme-UV (200 - 912A) irradiances incident upon exoplanetary atmospheres. The UV data will be complemented with contemporaneous X-ray and ground-based observations as well as new M dwarf atmosphere models to constrain atmospheric heating rates and provide a baseline for long-term ground-based studies of these systems.

France, Kevin

2014-09-01

337

CHANDRA OBSERVATION OF POLARIS: CENSUS OF LOW-MASS COMPANIONS  

SciTech Connect

We have observed Cepheid Polaris ({alpha} UMi A: F7 Ib [Aa] + F6 V [Ab]) with Chandra ACIS-I for 10 ks. An X-ray source was found at the location of Polaris with log L{sub X} = 28.89 erg s{sup -1} (0.3-8 keV) and kT = 0.6 keV. A spectrum this soft could come from either the supergiant or the dwarf, as shown by comparable coronal stars. Two resolved low-mass visual companions, 'C' and 'D', are not physical members of the system based on the lack of X-rays (indicating an age older than the Cepheid) and inconsistent proper motions. Polaris B is not an X-ray source, consistent with its early F spectral type, and probably does not have a lower mass companion itself. A possible more distant member is identified, and an additional less plausible one. This provides a complete census of companions out to 0.1 pc covering a mass ratio range of an order of magnitude and a {Delta}V of nearly 15 mag.

Remage Evans, Nancy; Wolk, Scott J.; Karovska, Margarita; Spitzbart, Bradley [Smithsonian Astrophysical Observatory, MS 4, 60 Garden Street, Cambridge, MA 02138 (United States); Guinan, Edward; Engle, Scott [Department of Astronomy, Villanova University, Villanova, PA 19085 (United States); Schlegel, Eric [Department of Physics and Astronomy, University of Texas, San Antonio, 1 UTSA Circle, San Antonio, TX 78249-0697 (United States); Mason, Brian D., E-mail: nevans@cfa.harvard.ed [US Naval Observatory, 3450 Massachusetts Avenue, NW, Washington, DC 20392-5420 (United States)

2010-05-15

338

A suppressed contribution of low-mass galaxies to reionization due to supernova feedback  

NASA Astrophysics Data System (ADS)

Motivated by recent observations of the star formation rate density function out to z ˜ 7, we describe a simple model for the star formation rate density function at high redshift based on the extended Press-Schechter formalism. This model postulates a starburst following each major merger, lasting for a time tSF and converting at most f?, max of galactic gas into stars. We include a simple physical prescription for supernova feedback that suppresses star formation in low-mass galaxies. Constraining tSF and f?, max to describe the observed star formation rate density at high redshifts, we find that individual starbursts were terminated after a time of tSF ˜ 107 yr. This is comparable to the main-sequence lifetimes of supernova progenitors, indicating that high-redshift starbursts are quenched once supernova feedback had time to develop. High-redshift galaxies convert ˜10 per cent of their mass into stars for galaxies with star formation rates above ˜1 M? yr-1, but a smaller fraction for lower luminosity galaxies. Our best-fitting model successfully predicts the observed relation between star formation rate and stellar mass at z ? 4, while our deduced relation between stellar mass and halo mass is also consistent with data on the dwarf satellites of the Milky Way. We find that supernova feedback lowers the efficiency of star formation in the lowest mass galaxies and makes their contribution to reionization small. As a result, photoionization feedback on low-mass galaxy formation does not significantly affect the reionization history. Using a semi-analytic model for the reionization history, we infer that approximately half of the ionizing photons needed to complete reionization have already been observed in star-forming galaxies.

Wyithe, J. Stuart B.; Loeb, Abraham

2013-01-01

339

Twins Among the Low Mass Spectroscopic Binaries  

E-print Network

We report an analysis of twins of spectral types F or later in the 9th Catalog of Spectroscopic Binaries (SB9). Twins, the components of binaries with mass ratio within 2% of 1.0, are found among the binaries with primaries of F and G spectral type. They are most prominent among the binaries with periods less than 43 days, a cutoff first identified by Lucy. Within the subsample of binaries with Ptwins do not differ from the other binaries in their distributions of periods (median P~7d), masses, or orbital eccentricities. Combining the mass ratio distribution in the SB9 in the mass range 0.6 to 0.85 Msun with that measured by Mazeh et al. for binaries in the Carney-Latham high proper motion survey, we estimate that the frequency of twins in a large sample of spectroscopic binaries is about 3%. Current theoretical understanding indicates that accretion of high specific angular momentum material by a protobinary tends to equalize its masses. We speculate that the excess of twins is produced in those star forming regions where the accretion processes were able to proceed to completion for a minority of protobinaries. This predicts that the components of a young twin may appear to differ in age and that, in a sample of spectroscopic binaries in a star formation region, the twins are, on average, older than the binaries with mass ratios much smaller than 1.

M. Simon; R. C. Obbie

2008-12-12

340

The Mass Distribution of Companions to Low-mass White Dwarfs  

NASA Astrophysics Data System (ADS)

Measuring the masses of companions to single-line spectroscopic binary stars is (in general) not possible because of the unknown orbital plane inclination. Even when the mass of the visible star can be measured, only a lower limit can be placed on the mass of the unseen companion. However, since these inclination angles should be isotropically distributed, for a large enough, unbiased sample, the companion mass distribution can be deconvolved from the distribution of observables. In this work, we construct a hierarchical probabilistic model to infer properties of unseen companion stars given observations of the orbital period and projected radial velocity of the primary star. We apply this model to three mock samples of low-mass white dwarfs (LMWDs; M <~ 0.45 M ?) and a sample of post-common-envelope binaries. We use a mixture of two Gaussians to model the WD and neutron star (NS) companion mass distributions. Our model successfully recovers the initial parameters of these test data sets. We then apply our model to 55 WDs in the extremely low-mass (ELM) WD Survey. Our maximum a posteriori model for the WD companion population has a mean mass ?WD = 0.74 M ?, with a standard deviation ?WD = 0.24 M ?. Our model constrains the NS companion fraction f NS to be <16% at 68% confidence. We make samples from the posterior distribution publicly available so that future observational efforts may compute the NS probability for newly discovered LMWDs.

Andrews, Jeff J.; Price-Whelan, Adrian M.; Agüeros, Marcel A.

2014-12-01

341

FORMATION OF MILLISECOND PULSARS FROM INTERMEDIATE- AND LOW-MASS X-RAY BINARIES  

SciTech Connect

We present a systematic study of the evolution of intermediate- and low-mass X-ray binaries consisting of an accreting neutron star of mass 1.0-1.8 M{sub Sun} and a donor star of mass 1.0-6.0 M{sub Sun }. In our calculations we take into account physical processes such as unstable disk accretion, radio ejection, bump-induced detachment, and outflow from the L{sub 2} point. Comparing the calculated results with the observations of binary radio pulsars, we report the following results. (1) The allowed parameter space for forming binary pulsars in the initial orbital period-donor mass plane increases with increasing neutron star mass. This may help explain why some millisecond pulsars with orbital periods longer than {approx}60 days seem to have less massive white dwarfs than expected. Alternatively, some of these wide binary pulsars may be formed through mass transfer driven by planet/brown-dwarf-involved common envelope evolution. (2) Some of the pulsars in compact binaries might have evolved from intermediate-mass X-ray binaries with anomalous magnetic braking. (3) The equilibrium spin periods of neutron stars in low-mass X-ray binaries are in general shorter than the observed spin periods of binary pulsars by more than one order of magnitude, suggesting that either the simple equilibrium spin model does not apply or there are other mechanisms/processes spinning down the neutron stars.

Shao Yong [Department of Astronomy, Nanjing University, Nanjing 210093 (China); Li Xiangdong, E-mail: lixd@nju.edu.cn [Key Laboratory of Modern Astronomy and Astrophysics, Nanjing University, Ministry of Education, Nanjing 210093 (China)

2012-09-01

342

VizieR Online Data Catalog: Properties of Collinder 69 low-mass members (Bayo+, 2012)  

NASA Astrophysics Data System (ADS)

Most observational studies conducted so far point toward brown dwarfs sharing a similar formation mechanism as the one that is accepted for low-mass stars. However, larger databases and more systematic studies are needed before strong conclusions can be reached. In this second paper of a series devoted to studying the spectroscopic properties of the Lambda Orionis star-forming region members, we study accretion, activity and rotation for a wide set of spectroscopically confirmed members of the central star cluster Collinder 69 to assess analogies and/or differences between the brown-dwarf and stellar populations of this cluster. Moreover, we present comparisons with other star-forming regions of similar and different ages to address environmental effects on our conclusions. We studied prominent photospheric lines to derive rotational velocities and emission lines to distinguish between accretion processes and chromospheric activity. In addition, we include information about disk presence and X-ray emission. (1 data file).

Bayo, A.; Barrado, D.; Huelamo, N.; Morales-Calderon, M.; Melo, C.; Stauffer, J.; Stelzer, B.

2013-01-01

343

Photometric Studies of Twelve Deep, Low-mass Ratio Overcontact Binary Systems  

NASA Astrophysics Data System (ADS)

The formations of the blue straggler stars and the FK Com-type stars are unsolved problems in stellar astrophysics. One of the possibilities for their formations is from the coalescence of W UMa-type overcontact binary systems. Therefore, deep (f > 50%), low-mass ratio (q < 0.25) overcontact binary stars are a very important source to understand the phenomena of Blue Straggler/FK Com-type stars. Recently, 12 W UMa-type binary stars, FG Hya, GR Vir, IK Per, TV Mus, CU Tau, V857 Her, V410 Aur, XY Boo, SX CrV, QX And, GSC 619-232, and AH Cnc, were investigated photometrically. Apart from TV Mus, XY boo, and GSC 619-232, new observations of the other 9 binaries were obtained. Complete light curves of the 10 systems, FG Hya, GR Vir, IK Per, TV Mus, CU Tau, V857 Her, GSC 619-232, V410 Aur, XY Boo, and AH Cnc, were analyzed with the 2003 version of the W-D code. It is shown that all of those systems are deep (f > 50%), low-mass ratio (q < 0.25) overcontact binary stars. We found that the system GSC 619-232 has the highest degree of overcontact (f = 93.4%). The derived photometric mass ratio of V857 Her, q = 0.0653, indicates that it is the lowest-mass ratio system among W UMa-type binaries. Of the 12 sample stars, long-term period changes of 11 systems were found. About 58% (seven) of the sample binaries show cyclic period oscillation. No cyclic period changes were discovered for the other 5 systems, which may be caused by the short observational time interval or by insufficient observations. Therefore, we think that all W UMa-type binary stars may contain cyclic period variations. By considering the long-term period changes (both increase and decrease) of those binary stars, we proposed two evolutionary scenarios evolving from deep, low-mass ratio overcontact binaries into Blue Straggler/FK Com-type stars.

Qian, Shengbang; Yang, Yuangui; Zhu, Liying; He, Jiajia; Yuan, Jingzhao

2006-08-01

344

SDSS J184037.78+642312.3: THE FIRST PULSATING EXTREMELY LOW MASS WHITE DWARF  

SciTech Connect

We report the discovery of the first pulsating extremely low mass (ELM) white dwarf (WD), SDSS J184037.78+642312.3 (hereafter J1840). This DA (hydrogen-atmosphere) WD is by far the coolest and the lowest-mass pulsating WD, with T{sub eff} = 9100 {+-} 170 K and log g = 6.22 {+-} 0.06, which corresponds to a mass of {approx}0.17 M{sub Sun }. This low-mass pulsating WD greatly extends the DAV (or ZZ Ceti) instability strip, effectively bridging the log g gap between WDs and main-sequence stars. We detect high-amplitude variability in J1840 on timescales exceeding 4000 s, with a non-sinusoidal pulse shape. Our observations also suggest that the variability is multi-periodic. The star is in a 4.6 hr binary with another compact object, most likely another WD. Future, more extensive time-series photometry of this ELM WD offers the first opportunity to probe the interior of a low-mass, presumably He-core WD using the tools of asteroseismology.

Hermes, J. J.; Montgomery, M. H.; Winget, D. E. [Department of Astronomy, University of Texas at Austin, Austin, TX 78712 (United States); Brown, Warren R.; Kenyon, Scott J. [Smithsonian Astrophysical Observatory, 60 Garden St, Cambridge, MA 02138 (United States); Kilic, Mukremin, E-mail: jjhermes@astro.as.utexas.edu [Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks St., Norman, OK 73019 (United States)

2012-05-10

345

Extremely Low Mass: The Circumstellar Envelope of a Potential Proto-Brown Dwarf  

NASA Technical Reports Server (NTRS)

What is the environment for planet formation around extremely low mass stars? Is the environment around brown dwarfs and extremely low mass stars conducive and sufficiently massive for planet production? The determining conditions may be set very early in the process of the host object's formation. IRAS 16253-2429, the source of the Wasp-Waist Nebula seen in Spitzer IRAC images, is an isolated, very low luminosity ("VeLLO") Class 0 protostar in the nearby rho Ophiuchi cloud. We present VLA ammonia mapping observations of the dense gas envelope feeding the central core accreting system. We find a flattened envelope perpendicular to the outflow axis, and gas cavities that appear to cradle the outflow lobes as though carved out by the flow and associated (apparently precessing) jet, indicating environmental disruption. Based on the NH3 (1,1) and (2,2) emission distribution, we derive the mass, velocity fields and temperature distribution for the envelope. We discuss the combined evidence for this source to be one of the youngest and lowest mass sources in formation yet known, and discuss the ramifications for planet formation potential in this extremely low mass system.

Wiseman, Jennifer

2011-01-01

346

Sensitive Limits on the Water Abundance in Cold Low Mass Molecular Cores  

E-print Network

We present SWAS observations of water vapor in two cold star-less clouds, B68 and Core D in rho Ophiuchus. Sensitive non-detections of the 1(10)-1(01) transition of o-H2O are reported for each source. Both molecular cores have been previously examined by detailed observations that have characterized the physical structure. Using these rather well defined physical properties and a Monte-Carlo radiation transfer model we have removed one of the largest uncertainties from the abundance calculation and set the lowest water abundance limit to date in cold low-mass molecular cores. These limits are water vapor in cold (T water is not near zero, these limits are well below theoretical predictions and appear to support the suggestion that most of the water in dense low-mass cores is frozen onto the surfaces of cold dust grains.

Edwin A. Bergin; Ronald L. Snell

2002-11-22

347

The origin of low-mass white dwarfs  

SciTech Connect

We present white dwarf mass distributions of a large sample of post common-envelope binaries and wide white dwarf main sequence binaries and demonstrate that these distributions are statistically independent. While the former contains a much larger fraction of low-mass white dwarfs, the latter is similar to single white dwarf mass distributions. Taking into account observational biases we also show that the majority of low-mass white dwarfs are formed in close binaries.

Rebassa-Mansergas, A.; Schreiber, M. R. [Departamento de Fisica y Astronomia, Universidad de Valparaiso, Avenida Gran Bretana 1111, Valparaiso (Chile); Gaensicke, B. T.; Girven, J. [Department of Physics, University of Warwick, Coventry CV4 7AL (United Kingdom); Gomez-Moran, A. Nebot [CNRS, Universite de Strasbourg, Observatoire Astronomique, 11 rue de l'Universite, F-67000 Strasbourg (France)

2010-11-23

348

Direct Search for Low Mass Dark Matter Particles with CCDs  

A direct dark matter search is performed using fully-depleted high-resistivity CCD detectors. Due to their low electronic readout noise (RMS ~7 eV) these devices operate with a very low detection threshold of 40 eV, making the search for dark matter particles with low masses (~5 GeV) possible. The results of an engineering run performed in a shallow underground site are presented, demonstrating the potential of this technology in the low mass region.

Barreto, J [Rio de Janeiro Federal U.; Cease, H.; Diehl, H.T.; Estrada, J.; Flaugher, B.; Harrison, N.; Jones, J.; Kilminster, B [Fermilab; Molina, J [Asuncion Natl. U.; Smith, J.; Sonnenschein, A [Fermilab

2012-05-15

349

An Ultra-low-mass and Small-radius Compact Object in 4U 1746-37?  

NASA Astrophysics Data System (ADS)

Photospheric radius expansion (PRE) bursts have already been used to constrain the masses and radii of neutron stars. RXTE observed three PRE bursts in 4U 1746-37, all with low touchdown fluxes. We discuss here the possibility of a low-mass neutron star in 4U 1746-37 because the Eddington luminosity depends on stellar mass. With typical values of hydrogen mass fraction and color correction factor, a Monte Carlo simulation was applied to constrain the mass and radius of a neutron star in 4U 1746-37. 4U 1746-37 has a high inclination angle. Two geometric effects, the reflection of the far-side accretion disk and the obscuration of the near-side accretion disk, have also been included in the mass and radius constraints of 4U 1746-37. If the reflection of the far-side accretion disk is accounted for, a low-mass compact object (mass of 0.41 ± 0.14 M ? and radius of 8.73 ± 1.54 km at 68% confidence) exists in 4U 1746-37. If another effect operated, 4U 1746-37 may contain an ultra-low-mass and small-radius object (M = 0.21 ± 0.06 M ?, R = 6.26 ± 0.99 km at 68% confidence). Combining all possibilities, the mass of 4U 1746-37 is 0.41+0.70-0.30 M_? at 99.7% confidence. For such low-mass neutron stars, it could be reproduced by a self-bound compact star, i.e., a quark star or quark-cluster star.

Li, Zhaosheng; Qu, Zhijie; Chen, Li; Guo, Yanjun; Qu, Jinlu; Xu, Renxin

2015-01-01

350

The evolutionary link between low-mass X-ray binaries and millisecond radio pulsars  

NASA Astrophysics Data System (ADS)

Low-mass X-ray binaries (LMXBs) and millisecond radio pulsars (MSRPs) are two different manifestations of neutron stars in binary systems. They are thought to be evolutionary linked, but many questions about their connection remain. Recent discoveries have opened up a new vista to investigate the LMXB/MSRP link. The neutron star XSS J12270-4859 was recently observed to switch between the two different manifestations. Here, we propose to exploit the unique UV capabilities of the HST to search for the presence of a quiescent accretion disk and to test if the neutron star is hot. This will give insight into its accretion history and the mechanism driving its metamorphosis, which will have direct implications for our understanding of the LMXB/MSRP evolutionary link.

Degenaar, Nathalie

2014-10-01

351

Revealing the Physics of r Modes in Low-Mass X-Ray Binaries  

SciTech Connect

We consider the astrophysical constraints on the gravitational-wave-driven r-mode instability in accreting neutron stars in low-mass x-ray binaries. We use recent results on superfluid and superconducting properties to infer the core temperature in these neutron stars and show the diversity of the observed population. Simple theoretical models indicate that many of these systems reside inside the r-mode instability region. However, this is in clear disagreement with expectations, especially for the systems containing the most rapidly rotating neutron stars. The inconsistency highlights the need to reevaluate our understanding of the many areas of physics relevant to the r-mode instability. We summarize the current status of our understanding, and we discuss directions for future research which could resolve this dilemma.

Ho, Wynn C. G.; Andersson, Nils; Haskell, Brynmor [School of Mathematics, University of Southampton, Southampton, SO17 1BJ (United Kingdom); Astronomical Institute ''Anton Pannekoek'', University of Amsterdam, 1098XH Amsterdam (Netherlands)

2011-09-02

352

Revisiting CoRoT RR Lyrae stars: detection of period doubling and temporal variation of additional frequencies  

NASA Astrophysics Data System (ADS)

Context. High-precision, space-based photometric missions like CoRoT and Kepler have revealed new and surprising phenomena in classical variable stars. Such discoveries were the period doubling in RR Lyrae stars and the frequent occurrence of additional periodicities some of which can be explained by radial overtone modes, but others are discordant with the radial eigenfrequency spectrum. Aims: We search for signs of period doubling in CoRoT RR Lyrae stars. The occurrence of this dynamical effect in modulated RR Lyrae stars might help us to gain more information about the mysterious Blazhko effect. The temporal variability of the additional frequencies in representatives of all subtypes of RR Lyrae stars is also investigated. Methods: We preprocess CoRoT light curves by applying trend and jump correction and outlier removal. Standard Fourier technique is used to analyze the frequency content of our targets and follow the time-dependent phenomena. Results: The most comprehensive collection of CoRoT RR Lyrae stars, including new discoveries is presented and analyzed. We found alternating maxima and in some cases half-integer frequencies in four CoRoT Blazhko RR Lyrae stars, as clear signs of the presence of period doubling. This reinforces that period doubling is an important ingredient for understanding the Blazhko effect - a premise we derived previously from the Kepler RR Lyrae sample. As expected, period doubling is detectable only for short time intervals in most modulated RRab stars. Our results show that the temporal variability of the additional frequencies in all RR Lyrae subtypes is ubiquitous. The ephemeral nature and the highly variable amplitude of these variations suggest a complex underlying dynamics of and an intricate interplay between radial and possibly nonradial modes in RR Lyrae stars. The omnipresence of additional modes in all types of RR Lyrae - except in non-modulated RRab stars - implies that asteroseismology of these objects should be feasible in the near future. The CoRoT space mission was developed and operated by the French space agency CNES, with participation of ESA's RSSD and Science Programs, Austria, Belgium, Brazil, Germany, and Spain.Follow-up observations were obtained at Piszkés-tet?, the Mountain Station of Konkoly Observatory.Appendices are available in electronic form at http://www.aanda.orgFull 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/570/A100

Szabó, R.; Benk?, J. M.; Paparó, M.; Chapellier, E.; Poretti, E.; Baglin, A.; Weiss, W. W.; Kolenberg, K.; Guggenberger, E.; Le Borgne, J.-F.

2014-10-01

353

The Role of Dwarf-Dwarf Interactions in the Evolution of Low Mass Galaxies  

NASA Astrophysics Data System (ADS)

We present the initial results from TiNy Titans, the first systematic study of a sample of isolated interacting dwarf galaxies and the mechanisms governing their star formation. Mergers of massive galaxies provide a significant mode of galaxy evolution; they are observed to trigger intense starbursts and significantly rearrage the gas, dust, and stars. Large volume simulations of structure formation, based in LambdaCDM cosmology, predict that mergers between low mass galaxies should occur more frequently than those between massive galaxies at all redshifts. However, the merger sequence for low mass galaxies is relatively unexplored. A few intriguing examples of dwarf-dwarf interactions exist in the literature, but the efficiency of gas removal and the enhancement of star formation in dwarfs via pre-processing (i.e. dwarf-dwarf interactions occuring before the accretion by a massive host) have never been studied for a sample of dwarfs covering a range of interaction stages. Our multiwavelength approach gathers high resolution optical, UV, and radio imaging to probe the effects of interactions on the star formation and ISM in a sample of dwarfs selected from the Sloan Digital Sky Survey. We find that: 1) star formation is enhanced in paired dwarfs over their unpaired analogs, 2) the enhancement in star formation is more pronounced as a function of pair separation than that observed in massive galaxy pairs, 3) the dwarf-dwarf interactions contribute significantly to the population of starbursting dwarfs, and 4) the paired dwarfs still have large gas reservoirs and exhibit no signs of quenching.

Stierwalt, Sabrina; Besla, Gurtina; Patton, David R.; Johnson, Kelsey E.; Kallivayalil, Nitya; Putman, Mary E.; Privon, George C.; Ross, Glen

2015-01-01

354

Modeling the luminosity function of galactic low-mass X-ray binaries  

NASA Astrophysics Data System (ADS)

The evolution of the family of binaries with a low-mass star and a compact neutron star companion (low-mass X-ray binaries (LMXBs) with neutron stars) ismodeled by the method of population synthesis. Continuous Roche-lobe filling by the optical star in LMXBs is assumed to be maintained by the removal of orbital angular momentum from the binary by a magnetic stellar wind from the optical star and the radiation of gravitational waves by the binary. The developed model of LMXB evolution has the following significant distinctions: (1) allowance for the effect of the rotational evolution of a magnetized compact remnant on themass transfer scenario in the binary, (2) amore accurate allowance for the response of the donor star to mass loss at the Roche-lobe filling stage. The results of theoretical calculations are shown to be in good agreement with the observed orbital period-X-ray luminosity diagrams for persistent Galactic LMXBs and their X-ray luminosity function. This suggests that the main elements of binary evolution, on the whole, are correctly reflected in the developed code. It is shown that most of the Galactic bulge LMXBs at luminosities L x > 1037 erg s-1 should have a post-main-sequence Roche-lobe-filling secondary component (low-mass giants). Almost all of the models considered predict a deficit of LMXBs at X-ray luminosities near ˜1036.5 erg s-1 due to the transition of the binary from the regime of angular momentum removal by a magnetic stellar wind to the regime of gravitational waves (analogous to the widely known period gap in cataclysmic variables, accreting white dwarfs). At low luminosities, the shape of the model luminosity function for LMXBs is affected significantly by their transient behavior-the accretion rate onto the compact companion is not always equal to the mass transfer rate due to instabilities in the accretion disk around the compact object. The best agreement with observed binaries is achieved in the models suggesting that heavy neutron stars with masses 1.4-1.9 M ? can be born.

Kuranov, A. G.; Postnov, K. A.; Revnivtsev, M. G.

2014-01-01

355

Revisiting CoRoT RR Lyrae stars: detection of period doubling and temporal variation of additional frequencies  

E-print Network

We search for signs of period doubling in CoRoT RR Lyrae stars. The occurrence of this dynamical effect in modulated RR Lyrae stars might help us to gain more information about the mysterious Blazhko effect. The temporal variability of the additional frequencies in representatives of all subtypes of RR Lyrae stars is also investigated. We pre-process CoRoT light curves by applying trend and jump correction and outlier removal. Standard Fourier technique is used to analyze the frequency content of our targets and follow the time dependent phenomena. The most comprehensive collection of CoRoT RR Lyrae stars, including new discoveries is presented and analyzed. We found alternating maxima and in some cases half-integer frequencies in four CoRoT Blazhko RR Lyrae stars, as clear signs of the presence of period doubling. This reinforces that period doubling is an important ingredient to understand the Blazhko effect - a premise we derived previously from the Kepler RR Lyrae sample. As expected, period doubling is d...

Szabó, R; Paparó, M; Chapellier, E; Poretti, E; Baglin, A; Weiss, W W; Kolenberg, K; Guggenberger, E; Borgne, J -F Le

2014-01-01

356

The MUSCLES Treasury Survey: Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems  

NASA Astrophysics Data System (ADS)

It has recently been discovered that 10 - 50% of M dwarfs host Earth-size planets in their habitable zones. Furthermore, the nearest potentially habitable super-Earths orbit M dwarfs, meaning that these systems likely represent our best chance to discover habitable worlds in the coming decade. The ultraviolet (UV) spectrum incident upon Earth-like planets drives the dissociation of water and CO2, the production of O2 and ozone, and may determine their ultimate habitability. At present, we lack the observational and theoretical basis to predict the energetic radiation spectrum (X-ray through UV) of an M dwarf. UV variability of low-mass exoplanet host stars, in particular the possibly sterilizing effect of flare activity, is almost completely unexplored observationally. This proposal aims to acquire the critical UV observations of low-mass host stars now, providing a treasury database for studies of exemplary nearby exoplanetary systems and potentially habitable worlds not yet discovered. Building on our successful pilot program of spectrally and temporally resolved UV radiation fields, we propose the MUSCLES Treasury Survey: a UV survey of nearby low-mass exoplanetary host stars. Using HST-COS and STIS, we will observe the 1150 - 5700A fluxes, reconstruct the important Ly-alpha emission lines, and use these data to estimate the extreme-UV (200 - 912A) irradiances incident upon exoplanetary atmospheres. The UV data will be complemented with contemporaneous X-ray and ground-based observations as well as new M dwarf atmosphere models to constrain atmospheric heating rates and provide a baseline for long-term ground-based studies of these systems.

France, Kevin

2014-10-01

357

Photometry of Two Intense Low Mass X-Ray Binaries  

NASA Astrophysics Data System (ADS)

The intense galactic X-ray source GX349+2 (Sco X-2) belongs to the class of persistently bright low-mass X-ray binaries called Z-sources. GX349+2 has only recently been optically identified with a 19th mag star. Of the six known Z-sources, only two (Sco X-1 and Cyg X-2) have been studied in the optical. It has been suggested that Z-sources as a group are characterized by evolved companions and correspondingly long orbital periods (Sco X-1, P=0.8d; Cyg X-2, P=9.8d). Recently Southwell et al. have presented spectroscopic observations of GX349+2 suggesting a 14d orbital period. We have obtained broadband photometry of the system on six consecutive nights in May 1995, and find evidence for a 21.7 +/- 0.3hr period of 0.14 mag half-amplitude, superposed on erratic flickering typical of Sco X-1 type objects. As with other Z-sources, caution will be needed to insure that the variations are truly periodic, and not simply due to chaotic variability observed over a relatively short time span. If our period is confirmed, then the nature of the 14d spectroscopic variation found by Southwell et al. is unclear. GX13+1 is a bright X-ray burst source, located in the galactic bulge. Due to heavy obscuration, no optical counterpart brighter than R ~ 22 has been detected, but an infrared counterpart (K=12) has recently been identified by Naylor et al. (1991) based on spatial coincidence with an accurate radio position. GX13+1 is unusual as there is a disagreement over its classification. Studies of the X-ray time variability place it among the Atoll-sources. However, there is some evidence that the system contains a giant companion (Garcia et al. 1992) which would place it among the Z-sources. In an attempt to determine the period of the system, we observed GX13+1 for 9 days in May -- July 1995. Preliminary photometry confirms variability of ~ 0.4 mag on a timescale of several days, as previously discovered by Charles & Naylor (1992). If GX13+1 is found to have a large orbital period, it would be indicative of a giant companion, and thus challenge the distinction between Atoll- and Z-sources on the basis of orbital and evolutionary characteristics.

Wachter, S.; Margon, B.; Anderson, S.

1995-12-01

358

Creation by stellar ablation of the low-mass companion to pulsar 1829 - 10  

NASA Technical Reports Server (NTRS)

It is argued that the low-mass companion of PSR1829 - 10 began its life as a star and has been ablated down to its present mass by absorbing a portion of the pulsar's spindown energy. Similar phenomena have already been seen in two other binary pulsars, PSR1957 + 20 and PSR1744 + 24A. The final mass of the remnant is determined by the interplay between decreasing spindown luminosity, recession of the companion from the pulsar as a result of its mass loss, and most important, shrinkage of the companion due to convective cooling of its interior.

Krolik, Julian H.

1991-01-01

359

Star Clusters Sterrenstelsels & Kosmos  

E-print Network

Star Clusters Sterrenstelsels & Kosmos deel 2 1 #12;Types of star clusters 2 #12;Open or Galactic Clusters · "Open" or Galactic clusters are low mass, relatively small (~10 pc diameter) clusters of stars in the Galactic disk containing stars · The Pleiades cluster is a good example of an open cluster

Weijgaert, Rien van de

360

The formation of low-mass helium white dwarfs orbiting pulsars . Evolution of low-mass X-ray binaries below the bifurcation period  

NASA Astrophysics Data System (ADS)

Context. Millisecond pulsars (MSPs) are generally believed to be old neutron stars (NSs) that have been spun up to high rotation rates via accretion of matter from a companion star in a low-mass X-ray binary (LMXB). This scenario has been strongly supported by various pieces of observational evidence. However, many details of this recycling scenario remain to be understood. Aims: Here we investigate binary evolution in close LMXBs to study the formation of radio MSPs with low-mass helium white dwarf companions (He WDs) in tight binaries with orbital periods Porb ? 2-9h. In particular, we examine i) if the observed systems can be reproduced by theoretical modelling using standard prescriptions of orbital angular momentum losses (i.e. with respect to the nature and the strength of magnetic braking), ii) if our computations of the Roche-lobe detachments can match the observed orbital periods, and iii) if the correlation between WD mass and orbital period (MWD, Porb) is valid for systems with Porb< 2 days. Methods: Numerical calculations with a detailed stellar evolution code were used to trace the mass-transfer phase in ~400 close LMXB systems with different initial values of donor star mass, NS mass, orbital period, and the so-called ?-index of magnetic braking. Subsequently, we followed the orbital and the interior evolution of the detached low-mass (proto) He WDs, including stages with residual shell hydrogen burning. Results: We find that severe fine-tuning is necessary to reproduce the observed MSPs in tight binaries with He WD companions of mass <0.20 M?, which suggests that something needs to be modified or is missing in the standard input physics of LMXB modelling. Results from previous independent studies support this conclusion. We demonstrate that the theoretically calculated (MWD, Porb)-relation is in general also valid for systems with Porb< 2 days, although with a large scatter in He WD masses between 0.15-0.20 M?. The results of the thermal evolution of the (proto) He WDs are reported in a follow-up paper (Paper II).

Istrate, A. G.; Tauris, T. M.; Langer, N.

2014-11-01

361

UNVEILING A POPULATION OF GALAXIES HARBORING LOW-MASS BLACK HOLES WITH X-RAYS  

SciTech Connect

We report the discovery of three low-mass black hole (BH) candidates residing in the centers of low-mass galaxies at z < 0.3 in the Chandra Deep Field-South Survey. These BHs are initially identified as candidate active galactic nuclei based on their X-ray emission in deep Chandra observations. Multi-wavelength observations are used to strengthen our claim that such emission is powered by an accreting supermassive BH. While the X-ray luminosities are low at L{sub X} {approx} 10{sup 40} erg s{sup -1} (and variable in one case), we argue that they are unlikely to be attributed to star formation based on H{alpha} or UV fluxes. Optical spectroscopy from Keck and the VLT allows us to (1) measure accurate redshifts, (2) confirm their low stellar host mass, (3) investigate the source(s) of photo-ionization, and (4) estimate extinction. With stellar masses of M{sub *} < 3 Multiplication-Sign 10{sup 9} M{sub Sun} determined from Hubble Space Telescope/Advanced Camera for Surveys imaging, the host galaxies are among the lowest mass systems known to host actively accreting BHs. We estimate BH masses M{sub BH} {approx} 2 Multiplication-Sign 10{sup 5} M{sub Sun} based on scaling relations between BH mass and host properties for more luminous systems. In one case, a broad component of the H{alpha} emission-line profile is detected, thus providing a virial mass estimate. BHs in such low-mass galaxies are of considerable interest as the low-redshift analogs to the seeds of the most massive BHs at high redshift which have remained largely elusive to date. Our study highlights the power of deep X-ray surveys to uncover such low-mass systems.

Schramm, M.; Silverman, J. D. [Kavli Institute for the Physics and Mathematics of the Universe (WPI), Todai Institutes for Advanced Study, the University of Tokyo, Kashiwanoha 5-1-5, Kashiwa-shi, Chiba 277-8568 (Japan); Greene, J. E. [Department of Astrophysical Science, Princeton University, Princeton, NJ 08544 (United States); Brandt, W. N.; Luo, B. [Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802 (United States); Xue, Y. Q. [Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Chinese Academy of Sciences, Hefei, Anhui 230026 (China); Capak, P. [Spitzer Science Center, 314-6 California Institute of Technology, 1201 East California Boulevard, Pasadena, CA 91125 (United States); Kakazu, Y. [Department of Astronomy, California Institute of Technology, Pasadena, CA 91125 (United States); Kartaltepe, J. [National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ 85719 (United States); Mainieri, V., E-mail: malte.schramm@ipmu.jp [European Southern Observatory, Karl-Schwarzschild-Strasse 2, Garching, D-85748 (Germany)

2013-08-20

362

Bayesian search for low-mass planets around nearby M dwarfs - estimates for occurrence rate based on global detectability statistics  

NASA Astrophysics Data System (ADS)

Due to their higher planet-star mass ratios, M dwarfs are the easiest targets for detection of low-mass planets orbiting nearby stars using Doppler spectroscopy. Furthermore, because of their low masses and luminosities, Doppler measurements enable the detection of low-mass planets in their habitable zones that correspond to closer orbits than for solar-type stars. We re-analyse literature Ultraviolet and Visual Echelle Spectrograph (UVES) radial velocities of 41 nearby M dwarfs in a combination with new velocities obtained from publicly available spectra from the HARPS-ESO spectrograph of these stars in an attempt to constrain any low-amplitude Keplerian signals. We apply Bayesian signal detection criteria, together with posterior sampling techniques, in combination with noise models that take into account correlations in the data and obtain estimates for the number of planet candidates in the sample. More generally, we use the estimated detection probability function to calculate the occurrence rate of low-mass planets around nearby M dwarfs. We report eight new planet candidates in the sample (orbiting GJ 27.1, GJ 160.2, GJ 180, GJ 229, GJ 422, and GJ 682), including two new multiplanet systems, and confirm two previously known candidates in the GJ 433 system based on detections of Keplerian signals in the combined UVES and High Accuracy Radial velocity Planet Searcher (HARPS) radial velocity data that cannot be explained by periodic and/or quasi-periodic phenomena related to stellar activities. Finally, we use the estimated detection probability function to calculate the occurrence rate of low-mass planets around nearby M dwarfs. According to our results, M dwarfs are hosts to an abundance of low-mass planets and the occurrence rate of planets less massive than 10 M? is of the order of one planet per star, possibly even greater. Our results also indicate that planets with masses between 3 and 10 M? are common in the stellar habitable zones of M dwarfs with an estimated occurrence rate of 0.21^{+0.03}_{-0.05} planets per star.

Tuomi, Mikko; Jones, Hugh R. A.; Barnes, John R.; Anglada-Escudé, Guillem; Jenkins, James S.

2014-06-01

363

Stars : the end of a star  

NSDL National Science Digital Library

What happens during the death of a star? This activity page, part of an interactive laboratory series for grades 8-12, introduces students to the final processes of stars. Here students read about low-mass, medium-mass, and massive stars. Low-mass stars produce white dwarfs. A pop-up window describes how white dwarfs form. Medium-mass stars produce neutron stars and supernova. Pop-up information explains the supernova process. Massive stars undergo carbon burning. An interactive lab activity presents students the opportunity to predict temperature, pressure, and gravity changes that occur during carbon fusion. In a final lab activity, students compare initial star size with the type of death that occurs. Activity questions about star death are provided for each star size and are recordable and printable. Copyright 2005 Eisenhower National Clearinghouse

University of Utah. Astrophysics Science Project Integrating Research and Education (ASPIRE)

2003-01-01

364

Mon. Not. R. Astron. Soc. 382, 18041808 (2007) doi:10.1111/j.1365-2966.2007.12481.x Near-infrared spectroscopy of the very low mass companion  

E-print Network

: spectroscopic ­ stars: low-mass, brown dwarfs ­ white dwarfs. 1 INTRODUCTION Observations of substellar-infrared spectroscopy of the very low mass companion to the hot DA white dwarf PG 1234+482 P. R. Steele,1 M. R. Burleigh We present a near-infrared spectrum of the hot (Teff 55 000 K) hydrogen atmosphere (DA) white dwarf

Burleigh, Matt

365

DN Tau - a young low-mass CTTS in X-rays  

NASA Astrophysics Data System (ADS)

We present a deep XMM-Newton observation of DN Tau, a M0 type classical T Tauri star (CTTS) and extend the sample of young accreting stars studied with high-resolution X-ray spectroscopy to lower masses. We detect X-ray emission from magnetic activity and accretion shocks. DN Tau's X-ray properties link it to more massive and older CTTS. The strong hot corona makes DN Tau one of the X-ray brightest CTTS in its mass range, while the low mass and large radius result in a very cool accretion component and thus reduces its imprint in the observed X-ray spectrum and emission line diagnostics.

Robrade, Jan; Guedel, Manuel; Guenther, Moritz; Schmitt, Juergen

2013-07-01

366

Project MUSCLES: Measuring the Ultraviolet Spectral Characteristics in Low-mass Exoplanetary Systems  

NASA Astrophysics Data System (ADS)

Extrasolar planets orbiting M-stars may represent our best chance to discover habitable worlds in the coming decade. The ultraviolet {UV} spectrum incident upon the habitable zone of these planets drives the dissociation of water, the production of ozone, and may determine their ultimate habitability. Despite the importance of the incident UV radiation for understanding planetary atmospheres in the habitable zone, existing model calculations have not used measured, local UV fluxes. At present, we lack the observational and theoretical basis to predict the far- and near-UV spectrum of an M-dwarf. We propose Project MUSCLES {Measuring the Ultraviolet Spectral Characteristics in Low-mass Exoplanetary Systems} to remedy this situation by building the first spectrally and temporally resolved catalog of local M-stars hosting exoplanetary systems.

France, Kevin

2011-10-01

367

Investigating the Low-Mass Stellar Initial Mass Function in Draco  

NASA Astrophysics Data System (ADS)

We investigate the low-mass (?0.5 to 0.8 M?) stellar initial mass function (IMF) in three different regions of the Draco dwarf spheroidal galaxy using archival imaging of resolved stars taken with HST/ACS and WFC3. We model the optical color-magnitude diagrams of each field assuming two different IMF models (power-law, log-normal), three different stellar evolution libraries (Padova, BaSTI, Dartmouth), and a binary star model. For the power-law model, we find that the best-fit IMF slope varies per field, and that none are in good agreement with a Salpeter IMF. All fields show more consistent log-normal parameters, which are also in reasonable agreement with values for a standard Chabrier IMF. However, there are large degeneracies between the characteristic mass and dispersion of the log-normal, that can only be reduced with data that extends to lower stellar masses. Finally, we note that application of different stellar models can lead to drastically different IMF results, particularly in the case of the power-law fit. We therefore caution that uncertainties in stellar evolution models may be the dominant sources of uncertainty in studies of the low-mass IMF.

Sotoudeh, Soroush; Weisz, Daniel R.; Dolphin, Andrew E.; Skillman, Evan D.

2015-01-01

368

Measuring the orbital periods of low mass X-ray binaries in the X-ray band  

NASA Astrophysics Data System (ADS)

A low mass X-ray binary (LMXB) contains either a neutron star or a black hole accreting materials from its low mass companion star. It is one of the primary astrophysical sources for studying stellar-mass compact objects and accreting phenomena. As with other binary systems, the most important parameter of an LMXB is the orbital period, which allows us to learn about the nature of the binary system and constrain the properties of the system's components, including the compact object. As a result, measuring the orbital periods of LMXBs is essential for investigating these systems even though fewer than half of them have known orbital periods. This article introduces the different methods for measuring the orbital periods in the X-ray band and reviews their application to various types of LMXBs, such as eclipsing and dipping sources, as well as pulsar LMXBs.

Chou, Yi

2014-11-01

369

Line-Driven Winds Revisited in the Context of Be Stars: ?-slow Solutions with High k Values  

NASA Astrophysics Data System (ADS)

The standard, or fast, solutions of m-CAK line-driven wind theory cannot account for slowly outflowing disks like the ones that surround Be stars. It has been previously shown that there exists another family of solutions—the ?-slow solutions—that is characterized by much slower terminal velocities and higher mass-loss rates. We have solved the one-dimensional m-CAK hydrodynamical equation of rotating radiation-driven winds for this latter solution, starting from standard values of the line force parameters (?, k, and ?), and then systematically varying the values of ? and k. Terminal velocities and mass-loss rates that are in good agreement with those found in Be stars are obtained from the solutions with lower ? and higher k values. Furthermore, the equatorial densities of such solutions are comparable to those that are typically assumed in ad hoc models. For very high values of k, we find that the wind solutions exhibit a new kind of behavior.

Silaj, J.; Curé, M.; Jones, C. E.

2014-11-01

370

Line-Driven Winds Revisited in the Context of Be Stars: $\\Omega$-slow Solutions with High $k$ Values  

E-print Network

The standard, or fast, solutions of m-CAK line-driven wind theory cannot account for slowly outflowing disks like the ones that surround Be stars. It has been previously shown that there exists another family of solutions --- the $\\Omega$-slow solutions --- that is characterized by much slower terminal velocities and higher mass-loss rates. We have solved the one-dimensional m-CAK hydrodynamical equation of rotating radiation-driven winds for this latter solution, starting from standard values of the line force parameters ($\\alpha$, $k$, and $\\delta$), and then systematically varying the values of $\\alpha$ and $k$. Terminal velocities and mass-loss rates that are in good agreement with those found in Be stars are obtained from the solutions with lower $\\alpha$ and higher $k$ values. Furthermore, the equatorial densities of such solutions are comparable to those that are typically assumed in ad hoc models. For very high values of $k$, we find that the wind solutions exhibit a new kind of behavior.

Silaj, J; Jones, C E

2014-01-01

371

Mosquitoes survive raindrop collisions by virtue of their low mass  

PubMed Central

In the study of insect flight, adaptations to complex flight conditions such as wind and rain are poorly understood. Mosquitoes thrive in areas of high humidity and rainfall, in which raindrops can weigh more than 50 times a mosquito. In this combined experimental and theoretical study, we here show that free-flying mosquitoes can survive the high-speed impact of falling raindrops. High-speed videography of those impacts reveals a mechanism for survival: A mosquito’s strong exoskeleton and low mass renders it impervious to falling drops. The mosquito’s low mass causes raindrops to lose little momentum upon impact and so impart correspondingly low forces to the mosquitoes. Our findings demonstrate that small fliers are robust to in-flight perturbations. PMID:22665779

Dickerson, Andrew K.; Shankles, Peter G.; Madhavan, Nihar M.; Hu, David L.

2012-01-01

372

Ten Low Mass Companions from the Keck Precision Velocity Survey  

E-print Network

Ten new low mass companions have emerged from the Keck precision Doppler velocity survey, with minimum (msini) masses ranging from 0.8 mjup to 0.34 msun. Five of these are planet candidates with msini < 12 mjup, two are brown dwarf candidates with msini ~30 mjup, and three are low mass stellar companions. Hipparcos astrometry reveals the orbital inclinations and masses for three of the (more massive) companions, and it provides upper limits to the masses for the rest. A new class of extrasolar planet is emerging, characterized by nearly circular orbits and orbital radii greater than 1 AU. The planet HD 4208b appears to be a member of this new class. The mass distribution of extrasolar planets continues to exhibit a rapid rise from 10 mjup toward the lowest detectable masses near 1 msat.

Steven S. Vogt; R. Paul Butler; Geoffrey W. Marcy; Debra A. Fischer; Dimitri Pourbaix; Kevin Apps; Gregory Laughlin

2001-10-16

373

Dark matter in low mass surface density galaxies  

E-print Network

Low mass surface density spiral and irregular galaxies like low surface brightness (LSB) and dwarf galaxies are unique laboratories to study the dynamical properties of Dark Matter halos because their mass is generally dominated by dark matter at all galactocentric radii. We present results from the largest sample ever assembled of high resolution Halpha velocity fields of LSB and dwarf galaxies in order to study their mass distributions.

Laurent Chemin; Claude Carignan; Philippe Amram

2007-07-10

374

COMPLEX MOLECULES TOWARD LOW-MASS PROTOSTARS: THE SERPENS CORE  

SciTech Connect

Gas-phase complex organic molecules are commonly detected toward high-mass protostellar hot cores. Detections toward low-mass protostars and outflows are comparatively rare, and a larger sample is the key to investigate how the chemistry responds to its environment. Guided by the prediction that complex organic molecules form in CH{sub 3}OH-rich ices and thermally or non-thermally evaporate with CH{sub 3}OH, we have identified three sight lines in the Serpens core-SMM1, SMM4, and SMM4-W-which are likely to be rich in complex organics. Using the IRAM 30 m telescope, narrow lines (FWHM of 1-2 km s{sup -1}) of CH{sub 3}CHO and CH{sub 3}OCH{sub 3} are detected toward all sources, HCOOCH{sub 3} toward SMM1 and SMM4-W, and C{sub 2}H{sub 5}OH not at all. Beam-averaged abundances of individual complex organics range between 0.6% and 10% with respect to CH{sub 3}OH when the CH{sub 3}OH rotational temperature is applied. The summed complex organic abundances also vary by an order of magnitude, with the richest chemistry toward the most luminous protostar SMM1. The range of abundances compare well with other beam-averaged observations of low-mass sources. Complex organic abundances are of the same order of magnitude toward low-mass protostars and high-mass hot cores, but HCOOCH{sub 3} is relatively more important toward low-mass protostars. This is consistent with a sequential ice photochemistry, dominated by CHO-containing products at low temperatures and early times.

Oeberg, Karin I. [Harvard-Smithsonian Center for Astrophysics, MS 42, 60 Garden Street, Cambridge, MA 02138 (United States); Van der Marel, Nienke; Kristensen, Lars E.; Van Dishoeck, Ewine F. [Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden (Netherlands)

2011-10-10

375

Low-mass dilepton production in $pp$ and $AA$ collisions  

E-print Network

We adopt a factorized QCD formalism to describe the transverse momentum distribution of low-mass lepton pairs produced in $pp$ collisions, when the pair transverse momentum $Q_T \\gg Q$, with the pair's invariant mass $Q$ as low as $Q \\sim \\Lambda_{\\mathrm{QCD}}$. We extend this formalism to dilepton production in $AA$ collisions by including the nuclear-dependent power correction due to parton multiple scattering.

Zhong-Bo Kang; Jian-Wei Qiu; Werner Vogelsang

2009-07-26

376

Low-mass dilepton production in pp and AA collisions  

SciTech Connect

We adopt a factorized QCD formalism to describe the transverse momentum distribution of low mass lepton pairs produced in pp collisions, when the pair transverse momentum QT4 >> Q, with the pair's invariant mass Q as low as Q {approx} {Lambda}QCD. We extend this formalism to dilepton production in AA collisions by including the nuclear-dependent power correction due to parton multiple scattering.

Kang, Z.B.; Vogelsang, W.; J.W. Qiu

2009-10-05

377

A Quadrupole mass spectrometer for resolution of low mass isotopes  

Microsoft Academic Search

The qualitative and quantitative identification of low mass isotopes in the mass range 1–6 u poses certain difficulties when\\u000a attempting to achieve the required resolution with an instrument suitable for deployment within a process environment. Certain\\u000a adjacent species present in the process sample (HT and D2) require a resolution greater than 930 to achieve an accurate measurement. We demonstrate here

Jeyan Sreekumar; Thomas J. Hogan; Stephen Taylor; Phillip Turner; Christopher Knott

2010-01-01

378

Life Cycles of Stars  

NSDL National Science Digital Library

This Powerpoint presentation inroduces younger students to the life cycles of stars. Topics include stellar nurseries, types of stars, supernovae, the fates of stars of either high or low mass, and the creation of heavier elements by continued fusion of successively heavier elements.

379

THE FORMATION OF SUPERMASSIVE BLACK HOLES FROM LOW-MASS POP III SEEDS  

SciTech Connect

The existence of 10{sup 9} M{sub Sun} black holes (BHs) in massive galaxies by z {approx} 7 is one of the great unsolved mysteries in cosmological structure formation. One theory argues that they originate from the BHs of Pop III stars at z {approx} 20 and then accrete at the Eddington limit down to the epoch of reionization, which requires that they have constant access to rich supplies of fuel. Because early numerical simulations suggested that Pop III stars were {approx}>100 M{sub Sun }, the supermassive black hole (SMBH) seeds considered up to now were 100-300 M{sub Sun }. However, there is a growing numerical and observational consensus that some Pop III stars were tens of solar masses, not hundreds, and that 20-40 M{sub Sun} BHs may have been much more plentiful at high redshift. However, we find that natal kicks imparted to 20-40 M{sub Sun} Pop III BHs during formation eject them from their halos and hence their fuel supply, precluding them from Eddington-limit growth. Consequently, SMBHs are far less likely to form from low-mass Pop III stars than from very massive ones.

Whalen, Daniel J. [Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213 (United States); Fryer, Chris L. [CCS-2, Los Alamos National Laboratory, Los Alamos, NM 87545 (United States)

2012-09-01

380

Hot subdwarfs in (eclipsing) binaries with brown dwarf or low-mass main-sequence companions  

NASA Astrophysics Data System (ADS)

The formation of hot subdwarf stars (sdBs), which are core helium-burning stars located on the extended horizontal branch, is not yet understood. Many of the known hot subdwarf stars reside in close binary systems with short orbital periods of between a few hours and a few days, with either M-star or white-dwarf companions. Common-envelope ejection is the most probable formation channel. Among these, eclipsing systems are of special importance because it is possible to constrain the parameters of both components tightly by combining spectroscopic and light-curve analyses. They are called HW Virginis systems. Soker (1998) proposed that planetary or brown-dwarf companions could cause the mass loss necessary to form an sdB. Substellar objects with masses greater than >10 M_J were predicted to survive the common-envelope phase and end up in a close orbit around the stellar remnant, while planets with lower masses would entirely evaporate. This raises the question if planets can affect stellar evolution. Here we report on newly discovered eclipsing or not eclipsing hot subdwarf binaries with brown-dwarf or low-mass main-sequence companions and their spectral and photometric analysis to determine the fundamental parameters of both components.

Schaffenroth, Veronika; Geier, Stephan; Heber, Uli

2014-09-01

381

ORBITAL SOLUTIONS FOR TWO YOUNG, LOW-MASS SPECTROSCOPIC BINARIES IN OPHIUCHUS  

SciTech Connect

We report the orbital parameters for ROXR1 14 and RX J1622.7-2325Nw, two young, low-mass, and double-lined spectroscopic binaries recently discovered in the Ophiuchus star-forming region. Accurate orbital solutions were determined from over a dozen high-resolution spectra taken with the Keck II and Gemini South telescopes. These objects are T Tauri stars with mass ratios close to unity and periods of {approx}5 and {approx}3 days, respectively. In particular, RX J1622.7-2325Nw shows a non-circularized orbit with an eccentricity of 0.30, higher than any other short-period pre-main-sequence (PMS) spectroscopic binary known to date. We speculate that the orbit of RX J1622.7-2325Nw has not yet circularized because of the perturbing action of a {approx}1'' companion, itself a close visual pair. A comparison of known young spectroscopic binaries (SBs) and main-sequence (MS) SBs in the eccentricity-period plane shows an indistinguishable distribution of the two populations, implying that orbital circularization occurs in the first 1 Myr of a star's lifetime. With the results presented in this paper we increase by {approx}4% the small sample of PMS spectroscopic binary stars with known orbital elements.

Rosero, V.; Prato, L.; Wasserman, L. H. [Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001 (United States); Rodgers, B., E-mail: viviana@lowell.edu, E-mail: lprato@lowell.edu, E-mail: lhw@lowell.edu, E-mail: brodgers@gemini.edu [Gemini Observatory, Gemini South, AURA/Chile, P.O. Box 26732, Tucson, AZ 85726 (United States)

2011-01-15

382

LOW-MASS ECLIPSING BINARIES IN THE INITIAL KEPLER DATA RELEASE  

SciTech Connect

We identify 231 objects in the newly released Cycle 0 data set from the Kepler Mission as double-eclipse, detached eclipsing binary systems with T{sub eff} < 5500 K and orbital periods shorter than {approx}32 days. We model each light curve using the JKTEBOP code with a genetic algorithm to obtain precise values for each system. We identify 95 new systems with both components below 1.0 M{sub sun} and eclipses of at least 0.1 mag, suitable for ground-based follow-up. Of these, 14 have periods less than 1.0 day, 52 have periods between 1.0 and 10.0 days, and 29 have periods greater than 10.0 days. This new sample of main-sequence, low-mass, double-eclipse, detached eclipsing binary candidates more than doubles the number of previously known systems and extends the sample into the completely heretofore unexplored P > 10.0 day period regime. We find preliminary evidence from these systems that the radii of low-mass stars in binary systems decrease with period. This supports the theory that binary spin-up is the primary cause of inflated radii in low-mass binary systems, although a full analysis of each system with radial-velocity and multi-color light curves is needed to fully explore this hypothesis. Also, we present seven new transiting planet candidates that do not appear among the list of 706 candidates recently released by the Kepler team, or in the Kepler False Positive Catalog, along with several other new and interesting systems. We also present novel techniques for the identification, period analysis, and modeling of eclipsing binaries.

Coughlin, J. L.; Harrison, T. E.; Ule, N. [Department of Astronomy, New Mexico State University, P.O. Box 30001, MSC 4500, Las Cruces, NM 88003-8001 (United States); Lopez-Morales, M. [Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 Broad Branch Road NW, Washington, DC 20015 (United States); Hoffman, D. I., E-mail: jlcough@nmsu.edu [California Institute of Technology, MC 249-17, 1200 East California B