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Sample records for low-mass stars revisited

  1. Mass-Radius relation of low-mass stars revisited with the VLTI

    NASA Astrophysics Data System (ADS)

    Demory, B.-O.; Sgransan, D.; Forveille, T.; Queloz, D.; Delfosse, X.; Perrier, C.

    2009-02-01

    We report the measurements of 5 single, low-mass and very low-mass stars angular diameter obtained with VINCI (VLT Interferometer Commissioning Instrument) in 2002 and AMBER (Astronomical Multi-BEam Recombiner) since 2007 on the VLTI array. We determined radii with accuracies of 1 to 5% for low-mass and very low mass stars ranging from M5.5V to K0.5V, thus encompassing a good fraction of the M-R relation for low-mass stars. Those results allow to revisit the current state of mass-radius relation for those objects from which a good agreement with models is shown up to about 0.6-0.7 solar masses. We explore remaining discrepancies in the upper part of the mass-radius relation and point out effects that may be due to stellar metallicity.

  2. Mass-radius relation of low and very low-mass stars revisited with the VLTI

    NASA Astrophysics Data System (ADS)

    Demory, B.-O.; Sgransan, D.; Forveille, T.; Queloz, D.; Beuzit, J.-L.; Delfosse, X.; di Folco, E.; Kervella, P.; Le Bouquin, J.-B.; Perrier, C.; Benisty, M.; Duvert, G.; Hofmann, K.-H.; Lopez, B.; Petrov, R.

    2009-10-01

    We measured the radii of 7 low and very low-mass stars using long baseline interferometry with the VLTI interferometer and its VINCI and AMBER near-infrared recombiners. We use these new data, together with literature measurements, to examine the luminosity-radius and mass-radius relations for K and M dwarfs. The precision of the new interferometric radii now competes with what can be obtained for double-lined eclipsing binaries. Interferometry provides access to much less active stars, as well as to stars with much better measured distances and luminosities, and therefore complements the information obtained from eclipsing systems. The radii of magnetically quiet late-K to M dwarfs match the predictions of stellar evolution models very well, providing direct confirmation that magnetic activity explains the discrepancy that was recently found for magnetically active eclipsing systems. The radii of the early K dwarfs are reproduced well for a mixing length parameter that approaches the solar value, as qualitatively expected. Based on data collected with the VLTI/VINCI and VLTI/AMBER instruments at ESO Paranal Observatory, programmes ID 60.A-9220, 080.D-0653 and 082.D-0196.

  3. Low-mass star and planet formation

    NASA Technical Reports Server (NTRS)

    Boss, Alan P.

    1989-01-01

    Low-mass star and planet formation is reviewed through a brief comparison of the results of cosmogonical models with observations ranging from studies of star-forming regions to searches for planetary companions to low-mass stars. Five key phases are described, starting from the dense, interstellar cloud cores that form low-mass stars, through the protostellar collapse and fragmentation phase, to the formation of a protostellar object accreting gas from the surrounding protostellar disk and cloud envelope. Descriptions are given for the phase where planets are formed in the protostellar disk, and the dissipation of the bulk of the protostellar disk and the appearance of an optically visible, premain-sequence star.

  4. Gravitational waves from low mass neutron stars

    SciTech Connect

    Horowitz, C. J.

    2010-05-15

    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.

  5. Olivier Chesneau's Work on Low Mass Stars

    NASA Astrophysics Data System (ADS)

    Lagadec, E.

    2015-12-01

    During his too short career, Olivier Chesneau pioneered the study of the circumstellar environments of low mass evolved stars using very high angular resolution techniques. He applied state of the art high angular resolution techniques, such as optical interferometry and adaptive optics imaging, to the the study of a variety of objects, from AGB stars to Planetary Nebulae, via e.g. Born Again stars, RCB stars and Novae. I present here an overview of this work and most important results by focusing on the paths he followed and key encounters he made to reach these results. Olivier liked to work in teams and was very strong at linking people with complementary expertises to whom he would communicate his enthusiasm and sharp ideas. His legacy will live on through the many people he inspired.

  6. Space motions of low-mass stars.

    NASA Astrophysics Data System (ADS)

    Upgren, A. R.; Caruso, J. R.

    1988-08-01

    Radial-velocity measures are presented for 225 stars, most of which are dwarf K and M stars. The data were obtained with the CfA digital speedometer, whose uncertainty is less than 1 km/s. Calibrations of three earlier radial-velocity studies and comparisons with three other contemporary ones lead to the evaluation of the standard error for an individual star as determined in each investigation. The data from two masks, matching solar type and M type stars, form a ratio that measures stellar surface temperature quite closely and appear to be useful in detecting the presence of unseen companions. A few previously unrecognized binaries have been detected; those with most certainty include stars nos. 366B, 453, and 46A of the McCormick lists of dwarf stars.

  7. Aging jets from low-mass stars

    NASA Technical Reports Server (NTRS)

    Graham, J. A.; Chen, W. P.

    1994-01-01

    An extended faint optical jet is associated with the compact emission region plus faint star known as HH 55. HH 55 is located in the Lupus 2 cloud 2 min SW of the well studied T Tauri star RU Lupi. The HH 55 jet extends 55 sec N and 35 sec S in PA 160 deg. The HH 55 star is an emission line star of spectral type M3.5. Its image in the emission lines of H-alpha and (S II) is slightly elongated by 2 sec - 3 sec to the S but in continuum light is symmetrical and pointlike ((full width at half maximum) (FWHM) = 1.7 sec). The star and jet have several features in common with the star and jet known as Sz 102 = Th 28 in the nearby Lupus 3 cloud. We suggest that these objects are representative of the late evolutionary stage of the HH jet-outflow phenomenon and point out that such objects may be quite common although difficult to detect. With L(sub bol) approximately = 0.005 solar luminosity, and log T(sub e) approximately = 3.5, the HH 55 star is close to the main sequence and evolutionary tracks suggest an age of 3 x 10(exp 7) yr.

  8. The evolution of very low mass stars

    NASA Technical Reports Server (NTRS)

    Nelson, L. A.; Rappaport, S. A.; Joss, P. C.

    1986-01-01

    The results of numerical evolutionary calculations for stars with masses in the range of 0.01-0.10 solar mass are presented. The stellar models by which these stars are followed from the early stages of contraction through deuterium burning to the very late stages of degenerate cooling at ages comparable to that of the Galaxy are described, and the methodology used to investigate the major uncertainties in the input physics are discussed. It is found that, for brown dwarfs with masses substantially below the end of the hydrogen-burning main sequence, the evolution of the effective temperature and bolometric luminosity are fairly well determined, despite the residual uncertainties in the input physics. In particular, the evolution is remarkably insensitive to the choice of the atmospheric opacity law at low temperatures and to the amount of mismatch in specific entropy across the stellar envelope. The minimum mass for a star to attain main-sequence status is moderately sensitive to the assumed input physics, particularly the surface boundary conditions.

  9. Young stars of low mass in the Gum nebula

    NASA Technical Reports Server (NTRS)

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

    1989-01-01

    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.

  10. Fundamental Properties of Low-Mass Stars and Brown Dwarfs

    SciTech Connect

    Liu, Michael C.; Dupuy, Trent J.; Stassun, Keivan G.; Allard, France; Blake, Cullen H.; Bonnefoy, M.; Cody, Ann Marie; Kraus, Adam; Day-Jones, A. C.; Lopez-Morales, Mercedes

    2009-02-16

    Precise measurements of the fundamental properties of low-mass stars and brown dwarfs are key to understanding the physics underlying their formation and evolution. While there has been great progress over the last decade in studying the bulk spectrophotometric properties of low-mass objects, direct determination of their masses, radii, and temperatures have been very sparse. Thus, theoretical predictions of low-mass evolution and ultracool atmospheres remain to be rigorously tested. The situation is alarming given that such models are widely used, from the determination of the low-mass end of the initial mass function to the characterization of exoplanets.An increasing number of mass, radius, and age determinations are placing critical constraints on the physics of low-mass objects. A wide variety of approaches are being pursued, including eclipsing binary studies, astrometric-spectroscopic orbital solutions, interferometry, and characterization of benchmark systems. In parallel, many more systems suitable for concerted study are now being found, thanks to new capabilities spanning both the very widest (all-sky surveys) and very narrowest (diffraction-limited adaptive optics) areas of the sky. This Cool Stars 15 splinter session highlighted the current successes and limitations of this rapidly growing area of precision astrophysics.

  11. The rotational velocities of low-mass stars

    NASA Technical Reports Server (NTRS)

    Stauffer, John B.; Hartmann, Lee W.

    1986-01-01

    The rotational velocities of stars provide important clues to how stars form and evolve. Yet until recently, studies of stellar rotation were limited to stars more massive than the sun. This is beginning to change, and an observational outline of the rotational velocity evolution of stars less massive than the sun can now be provided. Low-mass stars rotate slowly during the early stages of premain-sequence evolution, and spin up as they contract to the main sequence. This spin-up culminates in a brief period of very rapid rotation at an age of order 50 million years. Physical interpretation of this increase in rotation and the subsequent main-sequence spin-down are complicated by the possibility of differential internal rotation. The observed rapidity of spin-down among G dwarfs suggests that initially only the outer convective envelopes of these stars are slowed. The data suggest an intrinsic spread in angular momentum among young stars of the same mass and age, a spread which is apparently minimized by the angular-momentum loss mechanism in old low-mass stars.

  12. Luminosity functions for very low mass stars and brown dwarfs

    NASA Technical Reports Server (NTRS)

    Laughlin, Gregory; Bodenheimer, Peter

    1993-01-01

    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.

  13. Resolving the Discrepancy of Low-Mass Stars with IGRINS

    NASA Astrophysics Data System (ADS)

    Riddle, Andrew; Kraus, Adam L.

    2015-01-01

    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.

  14. Accurate masses of very low mass stars. II. The very low mass triple system GL 866

    NASA Astrophysics Data System (ADS)

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

    1999-10-01

    We present very accurate orbital parameters and mass measurements (2.4% accuracy) for the well known very low mass triple system Gl 866. We obtain first orbital elements for the short-period orbit and greatly improve the long period orbit. All three stars have masses close to 0.1 M_sun, and the system thus provides the strongest constraints to date on the mass-luminosity relation close to the brown dwarf limit. Based on observations made at the Observatoire de Haute Provence (CNRS), and at the CFH Telescope, operated by the NRCC, the CNRS and the University of Hawaii. Table 3 is only avaliable electronically with the On-Line publication at http://link.springer.de/link/service/journals/00230/

  15. Angular momentum transport within evolved low-mass stars

    SciTech Connect

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

    2014-06-10

    Asteroseismology of 1.0-2.0 M {sub ☉} 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 {sub ☉} 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.

  16. COUNTING LOW-MASS STARS IN INTEGRATED LIGHT

    SciTech Connect

    Conroy, Charlie; Van Dokkum, Pieter

    2012-03-01

    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.

  17. Low Mass Stellar Companions to Nearby A and B Stars

    NASA Astrophysics Data System (ADS)

    Gullikson, Kevin; Kraus, Adam L.

    2015-01-01

    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.

  18. Dust discs around low-mass main-sequence stars

    NASA Technical Reports Server (NTRS)

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

    1988-01-01

    The current understanding of the formation of circumstellar disks as a natural accompaniment to the process of low-mass star formation is examined. Models of the thermal emission from the dust disks around the prototype stars Alpha Lyr, Alpha PsA, Beta Pic, and Epsilon Eri are discussed, which indicate that the central regions of three of these disks are almost devoid of dust within radii ranging between 17 and 26 AU, with the temperature of the hottest zone lying between about 115 and 210 K. One possible explanation of the dust-free zones is the presence of a planet at the inner boundary of each cloud which sweeps up grains crossing its orbit.

  19. Low mass stellar companions around four giant stars

    NASA Astrophysics Data System (ADS)

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

    2015-01-01

    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 TB?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.

  20. Empirical Isochrones for Low Mass Stars in Nearby Young Associations

    NASA Astrophysics Data System (ADS)

    Herczeg, Gregory J.; Hillenbrand, Lynne A.

    2015-07-01

    Absolute ages of young stars are important for many issues in pre-main-sequence stellar and circumstellar evolution but long have been recognized as difficult to derive and calibrate. In this paper, we use literature spectral types and photometry to construct empirical isochrones in Hertzsprung-Russell diagrams for low mass stars and brown dwarfs in the η Cha, ɛ Cha, and TW Hya Associations and the β Pic and Tuc-Hor Moving Groups. A successful theory of pre-main-sequence evolution should match the shapes of the stellar loci for these groups of young stars. However, when comparing the combined empirical isochrones to isochrones predicted from evolutionary models, discrepancies lead to a spectral type (mass) dependence in stellar age estimates. Improved prescriptions for convection and boundary conditions in the latest models of pre-main-sequence evolution lead to a significantly improved correspondence between empirical and model isochrones, with small offsets at low temperatures that may be explained by observational uncertainties or by model limitations. Independent of model predictions, linear fits to combined stellar loci of these regions provide a simple empirical method to order clusters by luminosity with a reduced dependence on spectral type. Age estimates calculated from various sets of modern models that reproduce Li depletion boundary ages of the β Pic Moving Group also imply a ˜4 Myr age for the low mass members of the Upper Sco OB Association, which is younger than the 11 Myr age that has been recently estimated for intermediate and high mass members.

  1. Tidal Alignment of Exoplanets Around Low Mass Stars

    NASA Astrophysics Data System (ADS)

    Penev, Kaloyan; Jackson, Brian K.

    2014-06-01

    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.

  2. Multiplicity among Young Brown Dwarfs and Very Low Mass Stars

    NASA Astrophysics Data System (ADS)

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

    2007-12-01

    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.

  3. The naked T Tauri stars - The low-mass pre-main sequence unveiled

    NASA Astrophysics Data System (ADS)

    Walter, Frederick M.

    1987-01-01

    The author discusses the search for low-mass pre-main-sequence (PMS) stars associated with X-ray sources in regions of star formation. The survey to date has revealed at least 30 low-mass PMS stars in the Taurus-Auriga region, and a comparable number in Ophiuchus. These stars are the naked T Tauri stars, unveiled versions of the well-known classical T Tauri stars. The author discusses the properties of these newly discovered PMS stars and their relation to the classical T Tauri stars. He concludes that the naked T Tauri stars are the true low-mass PMS stars, and that the observable characteristics defining the classical T Tauri stars are due to the interaction of an underlying, fairly normal star with a dominant circumstellar environment. He discusses the impact the naked T Tauri stars are likely to have on our understanding of the PMS evolution of low-mass stars.

  4. The naked T Tauri stars - The low-mass pre-main sequence unveiled

    NASA Technical Reports Server (NTRS)

    Walter, Frederick M.

    1987-01-01

    The search for low-mass premain-sequence (PMS) stars associated with X-ray sources in regions of star formation is discussed. The survey to date has revealed at least 30 low-mass PMS stars in the Tau-Aur region, and a comparable number in Oph. These stars are the naked T Tau stars, unveiled versions of the well-known classical T Tau stars. The properties of these newly discovered PMS stars and their relation to the classical T Tau stars are discussed, and it is concluded that the naked T Tau stars are the true low-mass PMS stars, and that the observable characteristics defining the classical T Tau stars are due to the interaction of an underlying, fairly normal star with a dominant circumstellar environment. The impact the naked T Tau stars are likely to have on models of the PMS evolution of low-mass stars is considered.

  5. Angular momentum loss in low-mass stars

    NASA Technical Reports Server (NTRS)

    Kawaler, Steven D.

    1988-01-01

    The wind models discussed by Mestel (1984) are used here to formulate a general expression for the rate of angular momentum loss by magnetic stellar winds as a function of magnetic field configuration, rotation rate, and stellar model properties. The sensitivity of the rotation velocity to the various wind model parameters, the initial angular momenta, and the time dependence of the angular velocity for each mass is shown. The theoretical results are compared with observational ones, and it is found that the existence of very rapidly rotating stars in young clusters implies that low-mass stars are formed with a large spread of angular momentum. The high efficiency of angular momentum loss through magnetic stellar winds causes the rotation velocity to become less dependent on initial angular momentum J0 with time; by 300 million yur, the rotation velocity becomes independent of J0. This results in a decrease with time in the spread of rotation velocities as a function of stellar mass in young clusters.

  6. H2, CO and Dust Emission Around Low Mass Stars

    NASA Technical Reports Server (NTRS)

    Hollenbach, David; DeVincenzi, Donald (Technical Monitor)

    2002-01-01

    We model the thermal balance, the chemistry, and the radiative transfer in dusty disks orbiting young, low mass stars. These models are motivated by observations of infrared and ultraviolet transitions of H2 from protoplanetary disks, as well as millimeter and submillimeter observations of other molecules such as CO, and infrared continuum observations of the dust. The dust grains are heated primarily by the stellar radiation and the infrared radiation field produced by the dust itself. The gas is heated by collisions with warmer dust grains, X-rays from the region close to the stellar surface, UV pumping of hydrogen molecules, and the grain photoelectric heating mechanism initiated by UV photons from the central star. We treat cases where the gas to dust ratio is high, because the dust has settled to the midplane and coagulated into relatively large objects. We discuss situations in which the infrared emission from H2 can be detected, and how the comparison of the observations with our models can deduce physical parameters such as the mass and the density and temperature distribution of the gas.

  7. H2, CO and Dust Emission Around Low Mass Stars

    NASA Technical Reports Server (NTRS)

    Hollenbach, David; DeVincenzi, D. (Technical Monitor)

    2002-01-01

    We model the thermal balance, the chemistry, and the radiative transfer in dusty disks orbiting young, low mass stars. These models are motivated by observations of infrared and ultraviolet transitions of H2 from protoplanetary disks, as well as millimeter and submillimeter observations of other molecules such as CO, and infrared continuum observations of the dust. The dust grains are heated primarily by the stellar radiation and the infrared radiation field produced by the dust itself. The gas is heated by collisions with warmer dust grains, X-rays from the region close to the stellar surface, UV (ultraviolet) pumping of hydrogen molecules, and the grain photoelectric heating mechanism initiated by UV photons from the central star. We treat cases where the gas to dust ratio is high, because the dust has settled to the midplane and coagulated into relatively large objects. We discuss situations in which the infrared emission from H2 can be detected, and how the comparison of the observations with our models can deduce physical parameters such as the mass and the density and temperature distribution of the gas.

  8. Atmospheres of Quiescent Low-Mass Neutron Stars

    NASA Astrophysics Data System (ADS)

    Karpov, Platon; Medin, Zachary; Calder, Alan; Lattimer, James M.

    2016-01-01

    Observations of the neutron stars in quiescent low-mass X-ray binaries are important for determining their masses and radii which can lead to powerful constraints on the dense matter nuclear equation of state. The interpretation of these sources is complex and their spectra differ appreciably from blackbodies. Further progress hinges on reducing the uncertainties stemming from models of neutron star atmospheres. We present a suite of low-temperature neutron star atmospheres of different chemical compositions (pure H and He). Our models are constructed over a range of temperatures [log(T/1 K)=5.3, 5.6, 5.9, 6.2, 6.5] and surface gravities [log(g/1 cm/s2)=14.0, 14.2, 14.4, 14.6]. We generated model atmospheres using zcode - a radiation transfer code developed at Los Alamos National Laboratory. In order to facilitate analytic studies, we developed three-parameter fits to our models, and also compared them to diluted blackbodies in the energy range of 0.4-5 keV (CXO/MGE). From the latter, we extract color-correction factors (fc), which represent the shift of the spectra as compared to a blackbody with the same effective temperature. These diluted blackbodies are also useful for studies of photspheric expansion X-ray bursts. We provide a comparison of our models to previous calculations using the McGill Planar Hydrogen Atmosphere Code (McPHAC). These results enhance our ability to interpret thermal emission from neutron stars and to constrain the mass-radius relationship of these exotic objects.This research was supported in part by the U.S. Department of Energy under grant DE-FG02-87ER40317 and by resources at the Institute for Advanced Computational Science at Stony Brook University. This research was carried out in part under the auspices of the National Nuclear Security Administration of the U.S. Department of Energy at Los Alamos National Laboratory and supported by Contract No. DE-AC52-06NA25396.

  9. ROSAT observations of Cepheus OB3: the discovery of low-mass stars

    NASA Astrophysics Data System (ADS)

    Naylor, Tim; Fabian, A. C.

    1999-02-01

    We present X-ray images of the high-mass star-forming region Cepheus OB3, in which we discover over 50 point sources, the majority of which are T Tauri stars. We use the ratio of high-mass to low-mass stars to constrain the initial mass function, and find that it is consistent with that for field stars. This supports the picture that there is little difference between high-mass and low-mass star-forming regions. The presence of low-mass stars between the high-mass stars and the molecular cloud they are ionizing argues against mechanisms whereby heating of the cloud inhibits low-mass star formation. Finally, the efficiency of star formation in this region is shown to be of the order of a few per cent.

  10. Star Formation in Low-Mass Star-Forming Galaxies at intermediate redshifts

    NASA Astrophysics Data System (ADS)

    Gallego, Jesus; Rodriguez-Muoz, Luca; Pacifici, Camilla; Tresse, Laurence; Charlot, Stphane; Gil de Paz, Armando; Gomez-Guijarro, Carlos

    2015-08-01

    Dwarf galaxies play a key role in galaxy formation and evolution: (1) hierarchical models predict that low-mass systems merged to form massive galaxies (building block paradigm; Dekel & Silk 1986); (2) dwarf systems might have been responsible for the reionization of the Universe (Wyithe & Loeb 2006); (3) theoretical models are particularly sensitive to the density of low-mass systems at diferent redshifts (Mamon et al. 2011), being one of the key science cases for the future E-ELT (Evans et al. 2013). While the history of low-mass dark matter halos is relatively well understood, the formation history of dwarf galaxies is still poorly reproduced by the models due to the distinct evolution of baryonic and dark matter.We present Star Formation properties of a sample of low-mass Star-Forming Galaxies (LMSFGs; 7.3 < log M?/Mo < 8.0, at 0.3 < zspec < 0.9) selected by photometric stellar mass and apparent magnitude. The SFRs and overall properties were obtained through the analysis of their spectral energy distributions based on (1) HST and ground-based multi-broadband photometry and (2) deep spectroscopy from VLT and GTC telescopes.The SFRs and stellar masses derived for both samples place our targets on the standard main sequence of star-forming galaxies, but extending the sequence at least one dex to low mass systems.

  11. Model Atmospheres From Very Low Mass Stars to Brown Dwarfs

    NASA Astrophysics Data System (ADS)

    Allard, F.; Homeier, D.; Freytag, B.

    2011-12-01

    Since the discovery of brown dwarfs in 1994, and the discovery of dust cloud formation in the latest Very Low Mass Stars (VLMs) and Brown Dwarfs (BDs) in 1996, the most important challenge in modeling their atmospheres as become the understanding of cloud formation and advective mixing. For this purpose, we have developed radiation hydrodynamic 2D model atmosphere simulations to study the formation of forsterite dust in presence of advection, condensation, and sedimentation across the M-L-T VLMs to BDs sequence (Teff = 2800 K to 900 K, Freytag et al. 2010). We discovered the formation of gravity waves as a driving mechanism for the formation of clouds in these atmospheres, and derived a rule for the velocity field versus atmospheric depth and Teff, which is relatively insensitive to gravity. This rule has been used in the construction of the new model atmosphere grid, BT-Settl, to determine the micro-turbulence velocity, the diffusion coefficient, and the advective mixing of molecules as a function of depth. This new model grid of atmospheres and synthetic spectra has been computed for 100,000 K > Teff > 400 K, 5.5 > logg > -0.5, and [M/H]= +0.5 to -1.5, and the reference solar abundances of Asplund et al. (2009). We found that the new solar abundances allow an improved (close to perfect) reproduction of the photometric and spectroscopic VLMs properties, and, for the first time, a smooth transition between stellar and substellar regimes -- unlike the transition between the NextGen models from Hauschildt et al. 1999a,b, and the AMES-Dusty models from Allard et al. 2001. In the BDs regime, the BT-Settl models propose an improved explanation for the M-L-T spectral transition. In this paper, we therefore present the new BT-Settl model atmosphere grid, which explains the entire transition from the stellar to planetary mass regimes.

  12. Very low mass stars and white dwarfs in NGC 6397

    NASA Technical Reports Server (NTRS)

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

    1995-01-01

    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.

  13. Terrestrial Planet Formation around Low-Mass Stars: Effect of the Mass of Central Stars

    NASA Astrophysics Data System (ADS)

    Oshino, Shoichi; Matsumoto, Yuji; Kokubo, Eiichiro

    2015-12-01

    The Kepler space telescope has detected several thousand planets and candidates.Their central stars are mainly FGK-type stars.It is difficult to observe M-stars by using visible light since M-stars have their peak radiation in the infrared region.However, recently there are several survey projects for planets around M-stars such as the InfraRed Doppler (IRD) survey of the Subaru telescope.Therefore it is expected that the number of planets around M-stars will increase in the near future.The habitable zone of M-stars is closer to the stars than that of G-stars.For this reason, the possibility of finding habitable planets is expected to be higher.Here we study the formation of close-in terrestrial planets by giant impacts of protoplanets around low-mass stars by using N-body simulations.An important parameter that controls formation processes is the ratio between the physical radius of a planet and its Hill radius, which decreases with the stellar mass.We systematically change the mass of the central stars and investigate its effects on terrestrial planet formation.We find that the mass of the maximum planet decreases with the mass of central stars, while the number of planets in the system increases.We also find that the orbital separation of adjacent planets normalized by their Hill radius increases with the stellar mass.

  14. RADIATIVE AND KINETIC FEEDBACK BY LOW-MASS PRIMORDIAL STARS

    SciTech Connect

    Whalen, Daniel; Hueckstaedt, Robert M.; McConkie, Thomas O.

    2010-03-20

    Ionizing UV radiation and supernova (SN) flows amidst clustered minihalos at high redshift regulated the rise of the first stellar populations in the universe. Previous studies have addressed the effects of very massive primordial stars on the collapse of nearby halos into new stars, but the absence of the odd-even nucleosynthetic signature of pair-instability SNe in ancient metal-poor stars suggests that Population III stars may have been less than 100 M{sub sun}. We extend our earlier survey of local UV feedback on star formation to 25-80 M{sub sun} stars and include kinetic feedback by SNe for 25-40 M{sub sun} stars. We find radiative feedback to be relatively uniform over this mass range, primarily because the larger fluxes of more massive stars are offset by their shorter lifetimes. Our models demonstrate that prior to the rise of global UV backgrounds, Lyman-Werner (LW) photons from nearby stars cannot prevent halos from forming new stars. These calculations also reveal that violent dynamical instabilities can erupt in the UV radiation front enveloping a primordial halo, but that they ultimately have no effect on the formation of a star. Finally, our simulations suggest that relic H II regions surrounding partially evaporated halos may expel LW backgrounds at lower redshifts, allowing stars to form that were previously suppressed. We provide fits to radiative and kinetic feedback on star formation for use in both semianalytic models and numerical simulations.

  15. Nucleosynthesis of Binary low mass zero-metallicity stars

    NASA Astrophysics Data System (ADS)

    Lau, Ho Bun Herbert; Stancliffe, R. J.; Tout, C. A.

    The Cambridge STARS code is used to model the evolution and nucleosynthesis of binary zero- metallicity low to intermediate mass stars. The surfaces of these stars are enriched in CNO ele- ments after second dredge up. During binary interaction metals can be released from these stars and the secondary enriched in CNO. The observed abundances of HE 0107-5240 can be repro- duced from enhanced wind accretion from a 7 M after second dredge up. HE 1327-2326, richer in nitrogen and Sr, can similarly be formed by wind accretion in a later AGB phase after third dredge up.

  16. A Review in Mixed Chemistry of Low Mass Evolved Stars

    NASA Astrophysics Data System (ADS)

    Guzman-Ramirez, L.

    2015-12-01

    During the late stages of their evolution, Sun-like stars bring the products of nuclear burning to the surface. Although there is a chemical dichotomy between oxygen-rich and carbon-rich evolved stars, the dredge-up itself has never been directly observed. In the last three decades, however, a few stars have been shown to display both carbon- and oxygen-rich material in their circumstellar envelopes. These phenomena is seen in both Galactic Disk and Bulge planetary nebulae. For the Galactic Disk objects the mixed chemistry phenomenon is best explained through a recent dredge-up of carbon produced by nucleosynthesis inside the star during the Asymptotic Giant Branch that changed the surface chemistry of the star. On the contrary, we conclude that the mixed chemistry phenomenon occurring in the Galactic Bulge planetary nebulae is best explained through hydrocarbon chemistry in an ultraviolet (UV)-irradiated, dense torus.

  17. Preserving chemical signatures of primordial star formation in the first low-mass stars

    NASA Astrophysics Data System (ADS)

    Ji, Alexander P.; Frebel, Anna; Bromm, Volker

    2015-11-01

    We model early star-forming regions and their chemical enrichment by Population III (Pop III) supernovae with nucleosynthetic yields featuring high [C/Fe] ratios and pair-instability supernova (PISN) signatures. We aim to test how well these chemical abundance signatures are preserved in the gas prior to forming the first long-lived low-mass stars (or second-generation stars). Our results show that second-generation stars can retain the nucleosynthetic signature of their Pop III progenitors, even in the presence of nucleosynthetically normal Pop III core-collapse supernovae. We find that carbon-enhanced metal-poor stars are likely second-generation stars that form in minihaloes. Furthermore, it is likely that the majority of Pop III supernovae produce high [C/Fe] yields. In contrast, metals ejected by a PISN are not concentrated in the first star-forming haloes, which may explain the absence of observed PISN signatures in metal-poor stars. We also find that unique Pop III abundance signatures in the gas are quickly wiped out by the emergence of Pop II supernovae. We caution that the observed fractions of stars with Pop III signatures cannot be directly interpreted as the fraction of Pop III stars producing that signature. Such interpretations require modelling the metal enrichment process prior to the second-generation stars' formation, including results from simulations of metal mixing. The full potential of stellar archaeology can likely be reached in ultrafaint dwarf galaxies, where the simple formation history may allow for straightforward identification of second-generation stars.

  18. A rocky planet transiting a nearby low-mass star.

    PubMed

    Berta-Thompson, Zachory K; Irwin, Jonathan; Charbonneau, David; Newton, Elisabeth R; Dittmann, Jason A; Astudillo-Defru, Nicola; Bonfils, Xavier; Gillon, Michaël; Jehin, Emmanuël; Stark, Antony A; Stalder, Brian; Bouchy, Francois; Delfosse, Xavier; Forveille, Thierry; Lovis, Christophe; Mayor, Michel; Neves, Vasco; Pepe, Francesco; Santos, Nuno C; Udry, Stéphane; Wünsche, Anaël

    2015-11-12

    M-dwarf stars--hydrogen-burning stars that are smaller than 60 per cent of the size of the Sun--are the most common class of star in our Galaxy and outnumber Sun-like stars by a ratio of 12:1. Recent results have shown that M dwarfs host Earth-sized planets in great numbers: the average number of M-dwarf planets that are between 0.5 to 1.5 times the size of Earth is at least 1.4 per star. The nearest such planets known to transit their star are 39 parsecs away, too distant for detailed follow-up observations to measure the planetary masses or to study their atmospheres. Here we report observations of GJ 1132b, a planet with a size of 1.2 Earth radii that is transiting a small star 12 parsecs away. Our Doppler mass measurement of GJ 1132b yields a density consistent with an Earth-like bulk composition, similar to the compositions of the six known exoplanets with masses less than six times that of the Earth and precisely measured densities. Receiving 19 times more stellar radiation than the Earth, the planet is too hot to be habitable but is cool enough to support a substantial atmosphere, one that has probably been considerably depleted of hydrogen. Because the host star is nearby and only 21 per cent the radius of the Sun, existing and upcoming telescopes will be able to observe the composition and dynamics of the planetary atmosphere. PMID:26560298

  19. A rocky planet transiting a nearby low-mass star

    NASA Astrophysics Data System (ADS)

    Berta-Thompson, Zachory K.; Irwin, Jonathan; Charbonneau, David; Newton, Elisabeth R.; Dittmann, Jason A.; Astudillo-Defru, Nicola; Bonfils, Xavier; Gillon, Michal; Jehin, Emmanul; Stark, Antony A.; Stalder, Brian; Bouchy, Francois; Delfosse, Xavier; Forveille, Thierry; Lovis, Christophe; Mayor, Michel; Neves, Vasco; Pepe, Francesco; Santos, Nuno C.; Udry, Stphane; Wnsche, Anal

    2015-11-01

    M-dwarf starshydrogen-burning stars that are smaller than 60 per cent of the size of the Sunare the most common class of star in our Galaxy and outnumber Sun-like stars by a ratio of 12:1. Recent results have shown that M dwarfs host Earth-sized planets in great numbers: the average number of M-dwarf planets that are between 0.5 to 1.5 times the size of Earth is at least 1.4 per star. The nearest such planets known to transit their star are 39 parsecs away, too distant for detailed follow-up observations to measure the planetary masses or to study their atmospheres. Here we report observations of GJ 1132b, a planet with a size of 1.2 Earth radii that is transiting a small star 12 parsecs away. Our Doppler mass measurement of GJ 1132b yields a density consistent with an Earth-like bulk composition, similar to the compositions of the six known exoplanets with masses less than six times that of the Earth and precisely measured densities. Receiving 19 times more stellar radiation than the Earth, the planet is too hot to be habitable but is cool enough to support a substantial atmosphere, one that has probably been considerably depleted of hydrogen. Because the host star is nearby and only 21 per cent the radius of the Sun, existing and upcoming telescopes will be able to observe the composition and dynamics of the planetary atmosphere.

  20. Finding X-ray Coronal Cycles in Low Mass Stars

    NASA Astrophysics Data System (ADS)

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

    2015-01-01

    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.

  1. Global Star Formation Revisited

    NASA Astrophysics Data System (ADS)

    Silk, Joseph; Norman, Colin

    2009-07-01

    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.

  2. GLOBAL STAR FORMATION REVISITED

    SciTech Connect

    Silk, Joseph; Norman, Colin E-mail: norman@stsci.edu

    2009-07-20

    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.

  3. Low Mass Stars and Brown Dwarfs in Cepheus OB3

    NASA Astrophysics Data System (ADS)

    Thompson, A. A.; Jeffries, R. D.; Naylor, T.

    2003-10-01

    An initial analysis suggests a surprising dearth of stars below the hydrogen burning threshold in the young unbound OB association, Cepheus OB3b. Evidence has been collected from two deep imaging surveys, covering 1.12 square degrees in the optical (RIcZ) bands, and 0.13 square degrees in the near infra-red (JHKs). The limiting magnitudes are approximately 23.5 in I and 20.0 in J. Suspected cluster members have been selected initially from an I v I-Z colour magnitude diagram. By matching the positions of these sources with the corresponding NIR counterparts, we have completed a catalogue of optical and NIR magnitudes for candidate members of the association. Following individual de-reddening of these stars, masses have been derived from the J magnitudes using model isochrones. Our conclusions are complicated by both incompleteness due to the crowded nature of the region and an apparent age spread.

  4. The First Stars: A Low-Mass Formation Mode

    NASA Technical Reports Server (NTRS)

    Stacy, Athena; Bromm, Volker

    2014-01-01

    We perform numerical simulations of the growth of a Population III stellar system under photodissociating feedback. We start from cosmological initial conditions at z = 100, self-consistently following the formation of a minihalo at z = 15 and the subsequent collapse of its central gas to high densities. The simulations resolve scales as small as approx. 1 AU, corresponding to gas densities of 10(exp 16)/cu cm. Using sink particles to represent the growing protostars, we evolve the stellar system for the next 5000 yr. We find that this emerging stellar group accretes at an unusually low rate compared with minihalos which form at earlier times (z = 20-30), or with lower baryonic angular momentum. The stars in this unusual system will likely reach masses ranging from <1Stellar Mass to approx. 5 Stellar Mass by the end of their main-sequence lifetimes, placing them in the mass range for which stars will undergo an asymptotic giant branch (AGB) phase. Based upon the simulation, we predict the rare existence of Population III stars that have survived to the present day and have been enriched by mass overflow from a previous AGB companion.

  5. The first stars: A low-mass formation mode

    SciTech Connect

    Stacy, Athena; Bromm, Volker

    2014-04-10

    We perform numerical simulations of the growth of a Population III stellar system under photodissociating feedback. We start from cosmological initial conditions at z = 100, self-consistently following the formation of a minihalo at z = 15 and the subsequent collapse of its central gas to high densities. The simulations resolve scales as small as ∼1 AU, corresponding to gas densities of 10{sup 16} cm{sup –3}. Using sink particles to represent the growing protostars, we evolve the stellar system for the next 5000 yr. We find that this emerging stellar group accretes at an unusually low rate compared with minihalos which form at earlier times (z = 20-30), or with lower baryonic angular momentum. The stars in this unusual system will likely reach masses ranging from <1 M {sub ☉} to ∼5 M {sub ☉} by the end of their main-sequence lifetimes, placing them in the mass range for which stars will undergo an asymptotic giant branch (AGB) phase. Based upon the simulation, we predict the rare existence of Population III stars that have survived to the present day and have been enriched by mass overflow from a previous AGB companion.

  6. Spectroscopy and photometry for low-mass stars in Praesepe

    NASA Technical Reports Server (NTRS)

    Williams, Scott D.; Stauffer, John R.; Prosser, Charles F.; Herter, Terry

    1994-01-01

    We have obtained spectral types, H alpha equivalent widths, and optical photometry for a small sample of late K and M dwarf candidate members of the Praesepe open cluster. At least for the small sample of stars we have observed, all of the Paesepe members later than M2 have H alpha in emission. The chromospheric activity of the Praesepe satrs is essentially the same as that for Hyades members of the same mass, as expected since the two clusters are thought to be the same age.

  7. A rocky planet transiting a nearby low-mass star

    NASA Astrophysics Data System (ADS)

    Berta-Thompson, Zachory K.; Irwin, Jonathan; Charbonneau, David; Newton, Elisabeth R.; Dittmann, Jason; Astudillo-Defru, Nicola; Bonfils, Xavier; Gillon, Michael; Jehin, Emmanuel; Stark, Antony; Stalder, Brian; Bouchy, Francois; Delfosse, Xavier; Forveille, Thierry; Lovis, Christoph; Mayor, Michel; Neves, Vasco; Pepe, Francesco; Santos, Nuno; Udry, Stphane; Wunsche, Anael

    2015-12-01

    Results from Kepler indicate that M dwarfs host, on average, at least 1.4 planets between 0.5 and 1.5 Earth radii per star. Yet, the closest small planets known to transit M dwarfs have been too distant to allow Doppler measurements of their masses or spectroscopic studies of their atmospheres. Here, we announce a new planet discovered by the MEarth-South observatory, an Earth-size planet transiting an M dwarf that is only 12 pc away. The density of the planet, determined from radial velocity observations with HARPS, is consistent with an Earth-like rock/iron composition. With an equilibrium temperature of 530K (assuming a Bond albedo of 0.3), this planet is cooler than most other rocky planets with measured densities. Although too hot to be habitable, it is cool enough that it may have retained a substantial atmosphere over its lifetime. Thanks to the star's proximity and its diminutive size of only 1/5th the radius of the Sun, this new world likely provides the first opportunity for our community to spectroscopically examine the atmosphere of a terrestrial exoplanet. We estimate that JWST could secure high signal-to-noise spectra of the planet's atmosphere, both in transmission during transit and in emission at secondary eclipse.

  8. The Nitrogen Constraint on the Habitability of Planets around Low Mass M-stars

    NASA Astrophysics Data System (ADS)

    Tian, Feng

    2011-09-01

    The traditional habitable zones around stars are defined based on the stability of liquid water over geological timescales. Being too far away from the stars, the planet would be incapable of maintaining a warm surface and thus no liquid water. Being too close to the star, the planet would experience a 'runaway' greenhouse phase, during which its oceans could be lost quickly, and end up similar to our sister planet, Venus. The definition of tranditional habitable zones does not consider the availability of other elements important for life. All life as we know it needs nitrogen. Our calculations of upper planetary atmospheres show that nitrogen could be lost rapidly from planetary atmospheres with CO2 concentrations lower than certain threshold. This suggests that life on planets around low mass M-stars may be self-limiting, and planets of low mass M-stars are less favorable places to search for life than G- or K-type stars.

  9. The Nitrogen Constraint on Habitability of Planets of Low Mass M-stars

    NASA Astrophysics Data System (ADS)

    Tian, F.

    2011-12-01

    The traditional habitable zones around stars are defined based on the stability of liquid water over geological timescales. Being too far away from the stars, the planet would be incapable of maintaining a warm surface and thus no liquid water. Being too close to the star, the planet would experience a 'runaway' greenhouse phase, during which its oceans could be lost quickly, and end up similar to our sister planet, Venus. The definition of tranditional habitable zones does not consider the availability of other elements important for life. All life as we know it needs nitrogen. Our calculations of upper planetary atmospheres show that nitrogen could be lost rapidly from planetary atmospheres with CO2 concentrations lower than certain threshold. This suggests that life on planets around low mass M-stars may be selflimiting, and planets of low mass M-stars are less favorable places to search for life than G- or K-type stars.

  10. The Nitrogen Constraint on Habitability of Planets around Low Mass M-stars

    NASA Astrophysics Data System (ADS)

    Tian, F.

    2011-10-01

    The traditional habitable zones around stars are defined based on the stability of liquid water over geological timescales. Being too far away from the stars, the planet would be incapable of maintaining a warm surface and thus no liquid water. Being too close to the star, the planet would experience a 'runaway' greenhouse phase, during which its oceans could be lost quickly, and end up similar to our sister planet, Venus. The definition of tranditional habitable zones does not consider the availability of other elements important for life. All life as we know it needs nitrogen. Our calculations of upper planetary atmospheres show that nitrogen could be lost rapidly from planetary atmospheres with CO2 concentrations lower than certain threshold. This suggests that life on planets around low mass M-stars may be selflimiting, and planets of low mass M-stars are less favorable places to search for life than G- or K-type stars.

  11. A Vanishing Star Revisited

    NASA Astrophysics Data System (ADS)

    1999-07-01

    VLT Observations of an Unusual Stellar System Reinhold Hfner of the Munich University Observatory (Germany) is a happy astronomer. In 1988, when he was working at a telescope at the ESO La Silla observatory, he came across a strange star that suddenly vanished off the computer screen. He had to wait for more than a decade to get the full explanation of this unusual event. On June 10-11, 1999, he observed the same star with the first VLT 8.2-m Unit Telescope (ANTU) and the FORS1 astronomical instrument at Paranal [1]. With the vast power of this new research facility, he was now able to determine the physical properties of a very strange stellar system in which two planet-size stars orbit each other. One is an exceedingly hot white dwarf star , weighing half as much as the Sun, but only twice as big as the Earth. The other is a much cooler and less massive red dwarf star , one-and-a-half times the size of planet Jupiter. Once every three hours, the hot star disappears behind the other, as seen from the Earth. For a few minutes, the brightness of the system drops by a factor of more than 250 and it "vanishes" from view in telescopes smaller than the VLT. A variable star named NN Serpentis ESO PR Photo 30a/99 ESO PR Photo 30a/99 [Preview - JPEG: 400 x 468 pix - 152k] [Normal - JPEG: 800 x 936 pix - 576k] [High-Res - JPEG: 2304 x 2695 pix - 4.4M] Caption to ESO PR Photo 30a/99 : The sky field around the 17-mag variable stellar system NN Serpentis , as seen in a 5 sec exposure through a V(isual) filter with VLT ANTU and FORS1. It was obtained just before the observation of an eclipse of this unsual object and served to centre the telescope on the corresponding sky position. The field shown here measures 4.5 x 4.5 armin 2 (1365 x 1365 pix 2 ; 0.20 arcsec/pix). The field is somewhat larger than that shown in Photo 30b/99 and has the same orientation to allow comparison: North is about 20 anticlockwise from the top and East is 90 clockwise from that direction. The unsual star in question is designated NN Serpentis , or just NN Ser . As the name indicates, it is located in the constellation of Serpens (The Serpent), about 12 north of the celestial equator. A double letter, here "NN", is used to denote variable stars [2]. It is a rather faint object of magnitude 17, about 25,000 times fainter than what can be perceived with the unaided eye. The distance is about 600 light-years (180 pc). In July 1988, Reinhold Hfner performed observations of NN Ser (at that time still known by its earlier name PG 1550+131 ) with the Danish 1.54-m telescope at La Silla. He was surprised, but also very pleased to discover that it underwent a very deep eclipse every 187 minutes. Within less than 2 minutes, the brightness dropped by a factor of more than 100 (5 magnitudes). During the next 9 minutes, the star completely disappeared from view - it was too faint to be observed with this telescope. It then again reappeared and the entire event was over after just 11 minutes. Why eclipses are so important for stellar studies An eclipse occurs when one of the stars in a binary stellar system moves in front of the other, as seen by the observer. The effect is similar to what happens during a solar eclipse when the Moon moves in front of the Sun. In both cases, the eclipse may be partial or total , depending on whether or not the eclipsed star (or the Sun) is completely hidden from view. The occurence of eclipses in stellar systems, as seen from the Earth, depends on the spatial orientation of the orbital plane and the sizes of the two stars. Two eclipses take place during one orbital revolution, but they may not both be observable. The physical properties of the two stars in a binary system (e.g., the sizes of the stars, the size and shape of the orbit, the distribution of the light on the surfaces of the stars, their temperatures etc.) can be determined from the measured "light-curve" of the system (a plot of brightness vrs. time). The stars are always too close to each other to be seen as anything but a point of light. The light-curve thus describes the way the total brightness of the two stars changes during one orbital revolution, including the variation of the combined light of the two components as they cover each other during the eclipses. Already in 1988, it was concluded that the eclipse observed in NN Ser must be caused by a bright and hot star (a white dwarf ) being hidden by another body, most probably a red dwarf star . Because of the dramatic effect, this object soon became known as the "Vanishing Star" , cf. ESO Press Release 09/88 (8 December 1988). Critical information missing for NN Ser One particularly critical piece of information is needed for a light-curve study to succeed, that is whether the eclipse is "total" or "partial" . If during the eclipse one star is entirely hidden by the other, we only see the light of the star in front. In that case, the measured amount of light does not change during the phase of totality. The light-curve is "flat" at the bottom of the minimum and the measured brightness indicates the intrinsic luminosity of the eclipsing star. Moreover, for a given orbit, the duration of the totality is proportional to the size of that star. This crucial information was not available for NN Ser . The brightness at minimum was simply too faint to allow any measurements of the system with available telescopes during this phase. For this reason, the properties of the eclipsing star could only be guessed. Reaching for the bottom The new VLT observations have overcome this. Thanks to the powerful combination of the 8.2-m ANTU telescope and the multi-mode FORS1 instrument, it was possible to measure the complete lightcurve of NN Ser , also during the darkest phase of the eclipse. This extreme observation demanded most careful preparation. Since there is very little light available, the longest possible integration time must be used in order to collect a sufficient number of photons and to achieve a reasonable photometric accuracy. However, the eclipse only lasts a few minutes and it would only be possible to exposure and read-out a few, normal exposures from the CCD camera, not enough to fully characterize the light curve at minimum. Reinhold Hfner decided to use another method. By having the telescope perform a controlled change of position on the sky ("drift") during the exposure, the light from NN Ser before, during and after the eclipse will not be registered on the same spot of the camera detector, but rather along a line. He carefully chose a direction in which this line would not cross those of other stars in the neighbourhood of NN Ser . This was ensured by rotating FORS1 to a predetermined position angle. The drift rate was fixed as one pixel (0.20 arcsec) per 3 seconds of time, a compromise between the necessary integration time and desired time resolution that would give the best chance to document the exact shape of the light-curve . In theory, this would then allow the measurement of the intensity along the recorded trail of NN Ser and hence its brightness at any given time during the eclipse. But how deep would the eclipse be? Would the resulting exposure on each pixel at minimum light be long enough to register a measurable signal? Seeing the light from the cool star! ESO PR Photo 30b/99 ESO PR Photo 30b/99 [Preview - JPEG: 400 x 464 pix - 156k] [Normal - JPEG: 800 x 927 pix - 584k] [High-Res - JPEG: 2292 x 2662 pix - 4.1M] ESO PR Photo 30c/99 ESO PR Photo 30c/99 [Preview - JPEG: 472 x 400 pix - 48k] [Normal - JPEG: 943 x 800 pix - 96k] Caption to ESO PR Photo 30b/99 : 18.5-min "drift" exposure with VLT ANTU and FORS1 of the sky field around the variable stellar system NN Ser (indicated with an arrow). The telescope moved 1 pixel (0.20 arcsec) every 3 seconds so that the images of the stars in the field are trailed from left to right. After some minutes, the very deep eclipse of NN Ser begins when the brightness drops dramatically during the first partial phase. The star is clearly visible at a constant level all through the total phase at minimum light. It then brightens during the second partial phase and is back to the former level after approximately 10.5 min. The FORS1 instrument was rotated by about 70 to ensure that the trail of NN Ser would not overlap those of the neighbouring stellar images during this special exposure. The field shown measures 2.7 x 2.7 armin 2 and may be compared with that shown in Photo 30a/99; it has the same orientation. Caption to ESO PR Photo 30c/99 : The light-curve of the variable stellar system NN Ser , as extracted from the drift exposure shown in Photo 30b/99 . The count rate is proportional to the brightness of the object; it is about 18,000 counts/pix outside the eclipse and decreases to about 70 counts during the total eclipse (since the full range of the eclipse is shown here, this low level is almost indistinguishable from 0 in this figure). Various properties of the two stars in the NN Ser system may be determined from the shape of the light-curve. The fact that the light-curve is "flat" at the bottom is a clear sign that the eclipse is total , i.e. the hot white dwarf star is completely hidden behind the cool red dwarf star. As ESO PR Photo 30b/99 shows, ANTU and FORS1 did manage this difficult observation! Aided by an excellent seeing of 0.5 arcsec, i.e. a good concentration of the light on each pixel, the recorded signal from NN Ser - although very faint - is well measurable at all times during the eclipse . In the mean, about 70 counts/pixel were registered at the minimum, down from about 18,000 outside the eclipse ( Photo 30c/99 ). The ratio is then about 250, corresponding to just over 6 magnitudes. The measured magnitude during eclipse is 23.0 in the V-band (green-yellow; wavelength 550 nm). Of even greater importance is the fact that the light-curve is found to be perfectly flat at the bottom, i.e. the eclipse is most certainly total . The white dwarf star is therefore being completely hidden as it moves behind the cooler and larger star, and we see only the latter during the eclipse. As explained above, this then allows to determine many of its properties. For instance, the fact that the light-curve has no obvious "soft shoulders" at the beginning and end of the total phase indicates that the white dwarf abruptly disappears from view. Thus the faint star cannot have a very extended atmosphere, otherwise the brightness change would have been more gradual. The total phase was found to last 7 m 37 s and each of the partial phases only 1 m 26 s. This shows that the orbit must be nearly perpendicular to the plane of the sky. This angle is referred to as the orbital inclination ; for NN Ser , it must be in the interval between 84 - 90. A preliminary analysis indicates that the diameter of the cool star is between 200,000 and 245,000 km, i.e. about 1.5 times that of planet Jupiter. The white dwarf is even smaller; its diameter is between 25,000 and 31,000 km, or about twice the size of the Earth. The distance between the two stars is 660,000 km, or half the size of the Sun. Thus NN Ser is really a very small system - it would easily fit into our central star! The surface temperatures are widely different, about 55,000 and 2,800 degrees, respectively. By adding to this analysis earlier measurements of the orbital velocity of the white dwarf star, it is possible to estimate the mass of the cool star as between 0.10 and 0.14 solar masses. The white dwarf is significantly heavier, about 0.57 solar masses. Stellar objects with masses below approx. 0.08 solar mass are believed to be brown dwarfs , i.e. "still-born" stars in which nuclear fusion did not ignite. Since the mass of the cool star in NN Ser is near this limit, could it perhaps be such an object? A spectrum of the cool star ESO PR Photo 30d/99 ESO PR Photo 30d/99 [Preview - JPEG: 480 x 400 pix - 60k] [Normal - JPEG: 960 x 800 pix - 136k] Caption to ESO PR Photo 30d/99 : The spectrum of the cool dwarf star in the variable stellar system NN Ser . The 5 min exposure was obtained during the total phase of the eclipse, when the magnitude of the system was V = 23.0. Several TiO bands are clearly visible in this slightly smoothed tracing. A few deep and narrow "absorption" features are residuals from sky subtraction. The original resolution is 0.55 nm/pix. A spectral type of M6 or later is deduced for NN Ser . The spectrum of a more nearby (and hence much brighter) M6.5 dwarf star (temperature approx. 2600 degrees) is shown below for comparison. The VLT has already delivered the answer: it turns out to be no . The cool component of NN Ser may be a very small and faint object, but it is a real star that harbours nuclear processes in its interior. The temperature is on the high side for a brown dwarf, but the definite proof can only be obtained from the spectrum. ANTU and FORS1 were able to obtain a spectrum of NN Ser during the total eclipse, i.e. at a time when the visual magnitude was 23.0, cf. Photo 30d/99 . The exposure had to be limited to 5 min only, in order to ensure that there would be no contamination by extra light from the much brighter white dwarf companion star, as this is the case during the partial phases of the eclipse. Despite the difficult circumstances, it was possible to record a faint spectrum in the 600 - 900 nm (red - near-IR) wavelength interval. Although it is quite noisy, several molecular bands of TiO (titanium oxide) are well visible; VO (vanadium oxide) bands may also be present. They allow the classification of the spectrum as that of a very-late-type star, of spectral type M6 or later . This is in reasonable agreement with the mentioned temperature around 2800 degrees. In any case, this spectrum is quite unlike that of a brown dwarf, thus confirming that the cool companion star in NN Ser is a normal hydrogen-burning red dwarf star . NN Ser: a "missing link" in stellar theory The binary system NN Ser is now in an evolutionary stage that is referred to as the pre-cataclysmic phase. It will be followed by the cataclysmic phase , during which a gas stream will flow from the larger star to the smaller one. This phenomenon is characterized by frequent and abrupt increase in brightness. While many stars are known that are now in that unstable phase, only a few stars have ever been found to be in the preceding, transitory phase. Of these, NN Ser is the only one that has such a deep eclipse and for which it has now become possible to determine quite well the properties of the two components. NN Ser thus represents a most welcome example of a "missing link" in the theory of stellar evolution. It is therefore of great interest to perform further observations of such a rare object. They will include attempts to obtain more spectra to define the spectral type of the cool star very accurately. This will allow a critical check of current theories of atmospheres and evolutionary computations for the smallest and lightest stars. But for now, Reinhold Hfner looks forward to further nights at Paranal with the ESO astronomers there. "We worked together in a wonderful way during these demanding observations", he said, "and without their great support all of this would have been next to impossible!" Notes [1] These observations were carried out during "guaranteed observing time", allocated to the three German institutes that built the FORS instrument. More details about this instrument and related issues are available in ESO Press Release 14/98. [2] Astronomers designate variable stars according to the constellation in which they are seen in the sky and the order in which they are recognized as having variable brightness. For historical reasons, the first variable star in a given constellation (that is not already known by a greek letter, e.g. "Delta Cephei") is designated as "R" (e.g. "R Coronae Borealis"), the second as "S", etc. until "Z". Then follow "RR", "RS",..."RZ", "SS"..."SZ" until "ZZ" and only then from the beginning of the alphabet, "AA"..."AZ", "BA".. etc. until "QZ". How to obtain ESO Press Information ESO Press Information is made available on the World-Wide Web (URL: http://www.eso.org../ ). ESO Press Photos may be reproduced, if credit is given to the European Southern Observatory.

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

    SciTech Connect

    Hooper, Dan

    2013-09-01

    Although observations made with the CoGeNT and CDMS experiments have been interpreted as possible signals of low-mass ( ? 710 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 ? 324 per month.

  13. R-mode Instability of Low-mass Bare Strange Stars

    NASA Astrophysics Data System (ADS)

    Pi, C. M.; Yang, S. H.

    2015-07-01

    The r-mode instability window of low-mass strange stars is studied using the modified bag model of strange quark matter and reasonable sets of parameters. The results show that the spin frequency limit of strange stars increases with the decrease of their mass, and the highest observed spin frequency (716 Hz) of pulsars can be explained if the strange star mass is lower than about 0.1-0.2 M_{?}, depending on the parameters of the equation of state.

  14. Herschel photometry of disks around low-mass stars in the R CrA cloud

    SciTech Connect

    Harvey, Paul M.; Henning, Thomas; Liu, Yao; Wolf, Sebastian E-mail: nje@astro.as.utexas.edu E-mail: yliu@pmo.ac.cn E-mail: yliu@pmo.ac.cn

    2014-11-01

    We report photometric results from a subset of a Herschel-PACS program to observe cool dust in disks around low-mass stars as a complement to our earlier program to measure far-infrared emission from brown dwarfs. In this latest study we observed five low-mass objects in the nearby R Corona Australis region and detected at least three at 70 μm. Using a Monte Carlo radiative transfer code we have investigated the disk masses and geometry based on detailed spectral energy distribution (SED) modeling, and we compare these new results to those from our earlier larger sample of brown dwarfs. In particular, our SED analysis for these five objects shows again that disk geometries of brown dwarfs or low-mass stars are generally similar to their higher mass counterparts like T Tauri disks, but the range of disk mass extends to well below the value found in T Tauri stars.

  15. A Chandra Search for Low-mass Companions of Late B Stars in Tr 16

    NASA Astrophysics Data System (ADS)

    Remage Evans, Nancy; DeGioia-Eastwood, K.; Gagne, M.; Townsley, L.; Wolk, S.; Naze, Y.; Broos, P.; Corcoran, M.; Oskinova, L.; Moffat, A. F. J.; Wang, J.; Walborn, N.

    2011-01-01

    The cluster Tr 16 is included within the area of the large survey of the Carina region with Chandra (PI: Townsley). Stars later than B3 are not known to produce X-rays. On the other hand, low mass stars (later than mid-F spectral type) produce copious X-rays when they are young. We have developed a list of B3 to A0 stars in the young cluster Tr 16 which: 1.) are within 3' of Eta Car, 2.) have an appropriate V and B-V combination (including a range of +/- 0.1 in E(B-V), and 3.) have proper motions consistent with cluster membership. We have identified stars from this list which are X-ray sources on a 90 ksec Chandra image of Tr 16. Presumably the X-rays are produced by a low mass companion, at least in nearly all cases. This attribution is reinforced by the fact that the X-ray sources have higher median temperatures than O and early B sources. In addition, the spectral fits to 4 strongest sources produce temperatures typical of low-mass coronal sources. On this basis, 39% of the late B stars have low mass companions. Interpretation of this number depends on the completeness of the X-ray detections, however discussion of the low mass stars in Tr 16 indicates that stars which will be M stars on the main sequence are detected. N. Evans acknowledges support from the Chandra X-ray Center NASA Contract NAS8-03060

  16. Star formation histories of low-mass star forming galaxies at intermediate redshifts

    NASA Astrophysics Data System (ADS)

    Rodrguez-Muoz, L.; Gallego, J.; Pacifici, C.; Tresse, L.; Charlot, S.; Gil de Paz, A.; Barro, G.; Villar, V.

    2015-05-01

    In this work, we present constraints on the star formation histories (SFHs) of a sample of 74 spectroscopically confirmed low-mass star forming galaxies (LMSFGs) with stellar masses 7.3star formation (SF) and chemical enrichment histories, and state of the art modeling of the stellar and nebular emission, and dust attenuation. Our work extends the SF Main Sequence over 2 dex toward lower stellar masses. The median SFH of the sample of LMSFGs shows that more than 90% of the stellar mass estimated for the targets is formed in the 0.7-3.0 Gyr period prior to their observation.

  17. The multiplicity of planet host stars - new low-mass companions to planet host stars

    NASA Astrophysics Data System (ADS)

    Mugrauer, M.; Seifahrt, A.; Neuhuser, R.

    2007-07-01

    We present new results from our ongoing multiplicity study of exoplanet host stars, carried out with the infrared camera SofI (Son of ISAAC) at European Southern Observatory-New Technology Telescope (ESO-NTT). We have identified new low-mass companions to the planet host stars HD101930 and HD65216. HD101930AB is a wide binary system composed of the planet host star HD101930A and its companion HD101930B which is a M0 to M1 dwarf with a mass of about 0.7 Msolar separated from the primary by ~73arcsec (2200au projected separation). HD65216 forms a hierarchical triple system, with a projected separation of 253au (angular separation of about 7arcsec) between the planet host star HD65216A and its close binary companion HD65216BC, whose two components are separated by only ~0.17arcsec (6au of projected separation). Two VLT-NACO images separated by 3yr confirm that this system is comoving to the planet host star. The infrared photometry of HD65216B and C is consistent with a M7 to M8 (0.089 Msolar) and a L2 to L3 dwarf (0.078 Msolar), respectively, both close to the substellar limit. An infrared spectrum with VLT-Infrared Spectrometer and Array Camera (VLT-ISAAC) of the pair HD65216BC, even though not resolved spatially, confirms this late spectral type. Furthermore, we present H- and K-band ISAAC infrared spectra of HD16141B, the recently detected comoving companion of the planet host star HD16141A. The infrared spectroscopy as well as the apparent infrared photometry of HD16141B are both fully consistent with a M2 to M3 dwarf located at the distance of the planet host star. Based on observations obtained on La Silla in ESO programs 075.C-0098(A), 077.C-0572(A) and 078.C-0376(A), as well as on Paranal in ESO programs 070.C-0557(A), 076.C-0057(A) and 078.C-0376(B). E-mail: markus@astro.uni-jena.de

  18. The Sodium Doublets as Youth Indicators for Low-Mass Stars

    NASA Astrophysics Data System (ADS)

    Schlieder, J. E.; Fielding, D.; Lepine, S.; Rice, E.; Tomasino, R.; Simon, M.; Shara, M. M.

    2011-12-01

    We investigate the use of the Na I doublets at 5890 and 5896 (the Fraunhofer D lines) and 8183 and 8195 as gravity indicators for stars of late K and M spectral type. As is well known, the equivalent widths (EWs) of these doublets increase with photospheric log(g). We show that the EWs of members of the ? Pictoris moving group (BPMG) (age 10-20 Myr) lie between the EWs of giants and main sequence stars based on the analysis of approx. 200 spectra collected with the MDM 1.3-meter McGraw-Hill telescope and the SMARTS 1.5-meter telescope. We find the Na D lines are useful age indicators for low mass BPMG candidates earlier than M2 and the 8200 doublet becomes useful for stars later than M4. The EWs of the Na doublets may therefore be used to establish low gravity, hence youth, among low mass stars in general.

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

    NASA Technical Reports Server (NTRS)

    2000-01-01

    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.

  20. Structure and Evolution of Low-mass Stars: Where Do Magnetic stars Become Completely Convective?

    NASA Astrophysics Data System (ADS)

    Mullan, D. J.; MacDonald, J.

    2003-10-01

    Stars on the main sequence are expected to be completely convective if their mass lies below a certain value, Mcc. Standard stellar structure codes suggest that Mcc is in the range (0.3-0.4)M⊙. However, certain physical effects that are not incorporated in standard models may alter the value of Mcc significantly. Here we quantify the alterations that are brought about in Mcc when we include magnetic field effects. In particular, we modify the criterion for convective stability in the manner prescribed by Gough and Tayler (1966). We find that magnetic M dwarfs tend to have radii that are larger than expected for their Teff values, or Teff values that are too low for their radii. Available observational data provide quantitative support for these structural findings. Moreover, we find that, given the magnetic fields which are allowed to exist stably in low-mass stars, Mcc may fall to values that are as small as 0.1M⊙. We suggest that this result is pertinent to understanding why coronae and chromospheres in active M dwarfs fail to exhibit detectable alterations at spectral class M3-M4.

  1. The Motion Verified Red Stars (MoVeRS) Catalog and Low-Mass Field Stars with Warm Dust

    NASA Astrophysics Data System (ADS)

    Theissen, Christopher; West, Andrew A.; Dhital, Saurav

    2016-01-01

    We present the Motion Verified Red Stars (MoVeRS) catalog of proper motion selected low-mass stars from SDSS, 2MASS, and WISE. These surveys provide a time baseline of ~12 years for sources found in all three surveys, and a precision better than 10 mas/year.The MoVeRS catalog is augmented with proper motions from SDSS+USNO-B and the full sample contains 8,735,004 photometric point-sources selected based on colors and their significant (2?) proper motions. This catalog will be useful for finding new low-mass common proper motion systems, along with providing a large input catalog for numerous studies of low-mass stars. In addition, we use the MoVeRS catalog to present a preliminary sample of low-mass field stars exhibiting signatures of warm dust (mid-infrared excesses). Such systems are thought to originate from collisions of terrestrial planets, raising even more questions about the habitability of planetary systems around low-mass stars.

  2. Hard X-ray flux from low-mass stars in the Cygnus OB2 association

    NASA Astrophysics Data System (ADS)

    Caramazza, M.; Drake, J. J.; Micela, G.; Flaccomio, E.

    2009-08-01

    Context: The Cygnus OB2 association, the central engine of the Cygnus X star-forming region, is the subject of an extensive INTEGRAL Key Project that will accumulate 6Ms of observations. Analysis of 2Ms of observations by De Becker and co-workers provides the most sensitive limit yet obtained on hard X-ray emission from the cluster. Aims: We investigate the X-ray emission in the 20-40 keV band expected from the flaring low-mass stellar population in Cygnus OB2. We discuss whether such emission needs to be considered in the interpretation of existing and future X-ray observations of the region, and whether such observations might provide insight into the high-energy processes on low-mass pre-main sequence stars. Methods: The total hard X-ray flux from low-mass stars is estimated by assuming the observed soft X-ray emission stems from a superposition of flares. We further assume the ratio of hard X-ray to soft X-ray emission is described by a scaling found for solar flares by Isola and co-workers. Results: We estimate the low-mass stellar hard X-ray flux in the 20-40 keV band to lie in the range ~21031-61032 erg s-1 and discuss some potential biases that might affect this result. Conclusions: Hard X-ray emission could lie at a level not much below the current observed flux upper limits for Cygnus OB2. If this emission could be detected, it would provide insight into the hard X-ray production of large flares on pre-main sequence stars. We highlight the penetrating power of hard X-rays from low-mass stellar populations as a possible pointer to our Galaxy's hidden star-forming clusters and super-clusters using more sensitive observations from future missions.

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

    NASA Astrophysics Data System (ADS)

    Downes, Juan Jos; Briceo, Csar; Mateu, Cecilia; Hernndez, Jess; Vivas, Anna Katherina; Calvet, Nuria; Hartmann, Lee; Petr-Gotzens, Monika G.; Allen, Lori

    2014-10-01

    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 Astronoma `Francisco J. Duarte', Mrida, 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.

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

    NASA Technical Reports Server (NTRS)

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

    1994-01-01

    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.

  5. The effect of starspots on the radii of low-mass pre-main-sequence stars

    NASA Astrophysics Data System (ADS)

    Jackson, R. J.; Jeffries, R. D.

    2014-07-01

    A polytropic model is used to investigate the effects of dark photospheric spots on the evolution and radii of magnetically active, low-mass (M < 0.5 M?), pre-main-sequence (PMS) stars. Spots slow the contraction along Hayashi tracks and inflate the radii of PMS stars by a factor of (1 - ?)-N compared to unspotted stars of the same luminosity, where ? is the equivalent covering fraction of dark starspots and N ? 0.45 0.05. This is a much stronger inflation than predicted by Spruit & Weiss for main-sequence stars with the same ?, where N 0.2-0.3. These models have been compared to radii determined for very magnetically active K- and M-dwarfs in the young Pleiades and NGC 2516 clusters, and the radii of tidally locked, low-mass eclipsing binary components. The binary components and zero-age main-sequence K-dwarfs have radii inflated by 10 per cent compared to an empirical radius-luminosity relation that is defined by magnetically inactive field dwarfs with interferometrically measured radii; low-mass M-type PMS stars, that are still on their Hayashi tracks, are inflated by up to 40 per cent. If this were attributable to starspots alone, we estimate that an effective spot coverage of 0.35 < ? < 0.51 is required. Alternatively, global inhibition of convective flux transport by dynamo-generated fields may play a role. However, we find greater consistency with the starspot models when comparing the loci of active young stars and inactive field stars in colour-magnitude diagrams, particularly for the highly inflated PMS stars, where the large, uniform temperature reduction required in globally inhibited convection models would cause the stars to be much redder than observed.

  6. Characterizing the Star Formation of the Low-mass Shield Galaxies from Hubble Space Telescope Imaging

    NASA Astrophysics Data System (ADS)

    McQuinn, Kristen. B. W.; Cannon, John M.; 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, Jrgen

    2015-03-01

    The Survey of Hi in Extremely Low-mass Dwarfs 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 Arecibo Legacy Fast ALFA 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 be low-luminosity dwarf irregulars. We do not find a correlation between the recent star formation activity and the distance to the nearest neighboring galaxy, suggesting that the star formation process is not driven by gravitational interactions, but regulated internally. Further, we find a broadening in the star formation and gas properties (i.e., specific SFRs, stellar masses, and gas fractions) compared to the generally tight correlation found in more massive galaxies. Overall, the star formation and gas properties indicate these very low-mass galaxies host a fluctuating, non-deterministic, and inefficient star formation process. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

  7. Studies of low-mass star formation with the large deployable reflector

    NASA Technical Reports Server (NTRS)

    Hollenbach, D. J.; Tielens, Alexander G. G. M.

    1984-01-01

    Estimates are made of the far-infrared and submillimeter continuum and line emission from regions of low mass star formation. The intensity of this emission is compared with the sensitivity of the large deployable reflector (LDR), a large space telescope designed for this wavelength range. The proposed LDR is designed to probe the temperature, density, chemical structure, and the velocity field of the collapsing envelopes of these protostars. The LDR is also designed to study the accretion shocks on the cores and circumstellar disks of low-mass protostars, and to detect shock waves driven by protostellar winds.

  8. Spectroscopic Observations of Low-Mass Stars in the GALNYSS Survey

    NASA Astrophysics Data System (ADS)

    Vican, Laura; Zuckerman, Ben M.; Rodriguez, David

    2016-01-01

    Young, low-mass stars are known to be bright in X-ray and UV due to their high levels of magnetic activity. By cross-correlating the GALEX Catalog with the 2MASS Point Source Catalog, we have identified a list of over 2,000 stars whose UV excesses suggest that they are in the 10-100 Myr age range. We used several medium and high-resolution spectrometers in the Northern and Southern hemisphere to obtain optical spectra of ~500 of these stars. By measuring their lithium equivalent widths and H? emission, we have been able to confirm the youth of many stars in our catalog. Furthermore, we were able to measure radial velocities and UVW galactic space velocities for stars with high-resolution spectra, and were able to place some of these stars in nearby young moving groups.

  9. A substantial population of low-mass stars in luminous elliptical galaxies.

    PubMed

    van Dokkum, Pieter G; Conroy, Charlie

    2010-12-16

    The stellar initial mass function (IMF) describes the mass distribution of stars at the time of their formation and is of fundamental importance for many areas of astrophysics. The IMF is reasonably well constrained in the disk of the Milky Way but we have very little direct information on the form of the IMF in other galaxies and at earlier cosmic epochs. Here we report observations of the Na?(I) doublet and the Wing-Ford molecular FeH band in the spectra of elliptical galaxies. These lines are strong in stars with masses less than 0.3M(?) (where M(?) is the mass of the Sun) and are weak or absent in all other types of stars. We unambiguously detect both signatures, consistent with previous studies that were based on data of lower signal-to-noise ratio. The direct detection of the light of low-mass stars implies that they are very abundant in elliptical galaxies, making up over 80% of the total number of stars and contributing more than 60% of the total stellar mass. We infer that the IMF in massive star-forming galaxies in the early Universe produced many more low-mass stars than the IMF in the Milky Way disk, and was probably slightly steeper than the Salpeter form in the mass range 0.1M(?) to 1M(?). PMID:21124316

  10. The coronal temperatures of low-mass main-sequence stars

    NASA Astrophysics Data System (ADS)

    Johnstone, C. P.; Gdel, M.

    2015-06-01

    Aims: We study the X-ray emission of low-mass main-sequence stars to derive a reliable general scaling law between coronal temperature and the level of X-ray activity. Methods: We collect ROSAT measurements of hardness ratios and X-ray luminosities for a large sample of stars to derive which stellar X-ray emission parameter is most closely correlated with coronal temperature. We calculate average coronal temperatures for a sample of 24 low-mass main-sequence stars with measured emission measure distributions (EMDs) collected from the literature. These EMDs are based on high-resolution X-ray spectra measured by XMM-Newton and Chandra. Results: We confirm that there is one universal scaling relation between coronal average temperature and surface X-ray flux, FX, that applies to all low-mass main-sequence stars. We find that coronal temperature is related to FX by T?cor = 0.11 FX0.26, where T?cor is in MK and FX is in erg s-1 cm-2.

  11. Suppression of accretion on to low-mass Population III stars

    NASA Astrophysics Data System (ADS)

    Johnson, Jarrett L.; Khochfar, Sadegh

    2011-05-01

    Motivated by recent theoretical work suggesting that a substantial fraction of Population (Pop) III stars may have had masses low enough for them to survive to the present day, we consider the role that the accretion of metal-enriched gas may have had in altering their surface composition, thereby disguising them as Pop II stars. We demonstrate that if weak, solar-like winds are launched from low-mass Pop III stars formed in the progenitors of the dark matter halo of the Galaxy, then such stars are likely to avoid significant enrichment via accretion of material from the interstellar medium. We find that at early times accretion is easily prevented if the stars are ejected from the central regions of the haloes in which they form, either by dynamical interactions with more massive Pop III stars or by violent relaxation during halo mergers. While accretion may still take place during passage through sufficiently dense molecular clouds at later times, we find that the probability of such a passage is generally low (?0.1), assuming that stars have velocities of the order of the maximum circular velocity of their host haloes and accounting for the orbital decay of merging haloes. In turn, due to the higher gas density required for accretion on to stars with higher velocities, we find an even lower probability of accretion (10-2) for the subset of Pop III stars formed at z > 10, which are more quickly incorporated into massive haloes than stars formed at lower redshift. While there is no a priori reason to assume that low-mass Pop III stars do not have solar-like winds, without them surface enrichment via accretion is likely to be inevitable. We briefly discuss the implications that our results hold for stellar archaeology.

  12. A Hybrid Scenario for the Formation of Brown Dwarfs and Very Low Mass Stars

    NASA Astrophysics Data System (ADS)

    Basu, Shantanu; Vorobyov, Eduard I.

    2012-05-01

    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 ?, and have ejection velocities 0.8 0.35 km s-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.

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

    SciTech Connect

    Basu, Shantanu; Vorobyov, Eduard I. E-mail: eduard.vorobiev@univie.ac.at

    2012-05-01

    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.

  14. The discovery of low-mass pre-main-sequence stars in Cepheus OB3b

    NASA Astrophysics Data System (ADS)

    Pozzo, M.; Naylor, T.; Jeffries, R. D.; Drew, J. E.

    2003-05-01

    We report the discovery of a low-mass pre-main-sequence (PMS) stellar population in the younger subgroup of the Cepheus OB3 association, Cep OB3b, using UBVI CCD photometry and follow-up spectroscopy. The optical survey covers approximately 1300 arcmin2 on the sky and gives a global photometric and astrometric catalogue for more than 7000 objects. The location of a PMS population is well defined in a V versus (V-I) colour-magnitude diagram. Multifibre spectroscopic results for optically selected PMS candidates confirm the T Tauri nature for 10 objects, with equal numbers of classical TTS (CTTS) and weak-line TTS (WTTS). There are six other objects that we classify as possible PMS stars. The newly discovered TTS stars have masses in the range ~0.9-3.0 Msolar and ages from <1 to nearly 10 Myr, based on the Siess, Dufour & Forestini isochrones. Their location close to the O and B stars of the association (especially the O7n star) demonstrates that low-mass star formation is indeed possible in such an apparently hostile environment dominated by early-type stars and that the latter must have been less effective in eroding the circumstellar discs of their lower-mass siblings compared with other OB associations (e.g. ?-Ori). We attribute this to the nature of the local environment, speculating that the bulk of molecular material, which shielded low-mass stars from the ionizing radiation of their early-type siblings, has only recently been removed.

  15. Characterizing the mass accretion rates in young low-mass stars at low metallicity

    NASA Astrophysics Data System (ADS)

    Da Rio, Nicola

    2012-10-01

    Mass accretion from circumstellar disks onto young pre-main sequence {PMS} stars is a key aspect in the evolution of both the central star and the surrounding material.Over the past few years, new observational studies have improved dramatically our knowledge on these accretion processes, in particular thanks to the Hubble Space Telescope. A recent work in the Orion Nebula Cluster - the prototypical few Myr old Galactic star formaing region - allowed us to obtain very accurate measurements of mass accretion rates {Mdot} for 700 PMS stars. This enabled the analysis of the dependence of Mdot with stellar mass and age with unprecedented accuracy.Moreover, several recent works based on HST Halpha photometry have investigated accretion rates in metal-poor young regions of the Magellanic Clouds {MCs}. These studies demonstrate a clear anti-correlation of Mdot with metallicity, but results are limited to intermediate mass stars {M 1Msun}.We aim at completing the parameter space of these studies, i.e., study Mdot for low mass stars {down to 0.3 Msun, the peak of the initial mass function} in the Magellanic Clouds.This can be achieved through deep Halpha imaging of a particular region of the LMC, LH 95, whose low-mass PMS population has been already well characterized by us down to 0.2Msun. The methods we will use to derive Mdot from the proposed observations, together with the data already in our hands, have been fully tested in other regions of the MCs. The proposed observations will enable us to probe the role of metallicity in low-mass star formation, in order to set additional constraints on PMS theory.

  16. 4U 1746-37: An ultra-low-mass compact star candidate

    NASA Astrophysics Data System (ADS)

    Li, Zhao-Sheng

    2015-11-01

    the Rossi X-ray Timing Explorer observed three photospheric radius expansion bursts in 4U 1746-37, all with low touchdown fluxes. We discuss the possibility of a low-mass neutron star in 4U 1746-37 if the touchdown flux corresponds to its Eddington limit. 4U 1746-37 is a dipping binary system which has a high inclination angle. Two geometric effects, the reflection of the far side accretion disc and the obscuration of the near side accretion disc have also been included. We apply a Monte-Carlo simulation, with typical values of hydrogen mass fraction and color correction factor, to constrain the mass and radius of the neutron star in 4U 1746-37. We found that the mass of 4U 1746-37 is 0.41+0.70-0.30M_? at 99.7 % confidence which is an ultra-low-mass compact star candidate. It could be reproduced by a self-bound compact star, i.e., quark star or quark-cluster star, from an ccretion-induced collapse process.

  17. Variation of tidal dissipation in the convective envelope of low-mass stars along their evolution

    NASA Astrophysics Data System (ADS)

    Mathis, S.

    2015-08-01

    Context. Since 1995, more than 1500 exoplanets have been discovered around a wide variety of host stars (from M- to A-type stars). Tidal dissipation in stellar convective envelopes is an important factor that shapes the orbital architecture of short-period systems. Aims: Our objective is to understand and evaluate how tidal dissipation in the convective envelope of low-mass stars (from M to F types) depends on their mass, evolutionary stage, and rotation. Methods: Using a simplified two-layer assumption, we analytically compute the frequency-averaged tidal dissipation in the convective envelope. This dissipation is due to the conversion into heat of the kinetic energy of tidal non-wavelike/equilibrium flow and inertial waves because of the viscous friction applied by turbulent convection. Using grids of stellar models allows us to study the variation of the dissipation as a function of stellar mass and age on the pre-main sequence and on the main sequence for stars with masses ranging from 0.4 to 1.4 M⊙. Results: During their pre-main sequence, all low-mass stars have an increase in the frequency-averaged tidal dissipation for a fixed angular velocity in their convective envelope until they reach a critical aspect and mass ratios (respectively α = Rc/Rs and β = Mc/Ms, where Rs,Ms,Rc, and Mc are the star's radius and mass and its radiative core's radius and mass). Next, the dissipation evolves on the main sequence to an asymptotic value that is highest for 0.6 M⊙ K-type stars and that then decreases by several orders of magnitude with increasing stellar mass. Finally, the rotational evolution of low-mass stars strengthens the importance of tidal dissipation during the pre-main sequence for star-planet and multiple star systems. Conclusions: As shown by observations, tidal dissipation in stars' convection zones varies over several orders of magnitude as a function of stellar mass, age, and rotation. We demonstrate that i) it reaches a maximum value on the pre-main sequence for all stellar masses and ii) on the main sequence and at fixed angular velocity, it is at a maximum for 0.6 M⊙ K-type stars and decreases with increasing mass. Appendix A is available in electronic form at http://www.aanda.org

  18. Activity and rotation of low mass stars in young open clusters

    SciTech Connect

    Seifahrt, Andreas; Reiners, Ansgar; Scholz, Aleks; Basri, Gibor

    2009-02-16

    We present first results from a multi-object spectroscopy campaign in IC2602, the Hyades, the Pleiades, and the Coma cluster using VLT/FLAMES. We analysed the data for radial velocity, rotational velocity (v sin i), and H{alpha}-activity. Here, we highlight three aspects of this study in the context of rotational braking and the rotation-activity relationship among low mass stars. Finally we discuss the cluster membership of sources in IC2602.

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

    SciTech Connect

    Spada, F.; Demarque, P.; Kim, Y.-C.; Sills, A.

    2013-10-20

    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.

  20. Candidate Very-Low-Mass Companions to Nearby Stars Found in the WISE Survey

    NASA Astrophysics Data System (ADS)

    Mennen, Anne; Dutcher, D.; Lepine, S.; Faherty, J.

    2012-01-01

    We report the identification in the Wide-Field Survey Explorer (WISE) preliminary release of 36 probable very-low-mass companions to nearby stars from the SUPERBLINK proper motion catalogue. We examined all WISE sources within one arcminute of a subset of 156,000 SUPERBLINK stars with proper motions between 0.040 and 0.015 seconds of arc per year, photometric distances within 100 parsecs, and positions at least seven degrees from the galactic plane. Using proper motions calculated by comparing the WISE positions of the sources to those of their counterparts in the 2MASS Catalogue, we identified all WISE sources sharing a common proper motion with the SUPERBLINK star. We eliminated all sources detected in the Palomar Sky Survey blue plates, keeping only those red enough to be low-mass or brown dwarf companions. We used WISE and 2MASS colors to select only objects consistent with being M, L, or T dwarfs, leaving only 36 likely companions. Based on their color and assumed distances, we estimate the 36 low-mass companions to be either late M or early L dwarfs. Follow-up spectroscopic observations will be required for confirmation and formal spectral classification of the companions. We acknowledge the American Museum of Natural History and the National Science Foundation for their support.

  1. Hydrogen Burning in Low Mass Stars Constrains Scalar-Tensor Theories of Gravity.

    PubMed

    Sakstein, Jeremy

    2015-11-13

    The most general scalar-tensor theories of gravity predict a weakening of the gravitational force inside astrophysical bodies. There is a minimum mass for hydrogen burning in stars that is set by the interplay of plasma physics and the theory of gravity. We calculate this for alternative theories of gravity and find that it is always significantly larger than the general relativity prediction. The observation of several low mass red dwarf stars therefore rules out a large class of scalar-tensor gravity theories and places strong constraints on the cosmological parameters appearing in the effective field theory of dark energy. PMID:26613428

  2. Hydrogen Burning in Low Mass Stars Constrains Scalar-Tensor Theories of Gravity

    NASA Astrophysics Data System (ADS)

    Sakstein, Jeremy

    2015-11-01

    The most general scalar-tensor theories of gravity predict a weakening of the gravitational force inside astrophysical bodies. There is a minimum mass for hydrogen burning in stars that is set by the interplay of plasma physics and the theory of gravity. We calculate this for alternative theories of gravity and find that it is always significantly larger than the general relativity prediction. The observation of several low mass red dwarf stars therefore rules out a large class of scalar-tensor gravity theories and places strong constraints on the cosmological parameters appearing in the effective field theory of dark energy.

  3. A Deep HRI Survey of Low-Mass PMS Stars in NGC 2264

    NASA Technical Reports Server (NTRS)

    Patten, Brian

    1999-01-01

    Brian Patten is the Principal Investigator of the NASA ADP project 'A Deep HRI Survey of Low-Mass PMS Stars in NGC 2264'. This project was funded to support primarily the data reduction and analysis for new ROSAT data to be acquired in ROSAT AO8. For AO8 we were awarded two deep (100 ks) exposures with the ROSAT HRI instrument of a rotation and proper-motion selected sample of young (3 Myr - 15 Myr), low-mass, PMS stars in the populous star-forming region NGC 2264. These X-ray data were to be combined with an extensive rotation database for members of this cluster to allow us, for the first time, to probe the early evolution of magnetic dynamo activity for both fully convective stars and those stars found lower on the Hayashi tracks which have developed radiative cores. This database would have been used to study the interrelationship between coronal activity level, interior structure, and rotation rate as a function of mass and age.in the PMS and to define empirical constraints for theoretical models of angular momentum and magnetic dynamo evolution.

  4. 19 low mass hypervelocity star candidates from the first data release of the LAMOST survey

    NASA Astrophysics Data System (ADS)

    Li, Yin-Bi; Luo, A.-Li; Zhao, Gang; Lu, You-Jun; Wei, Peng; Du, Bing; Li, Xiang; Zhao, Yong-Heng; Han, Zhan-Wen; Wang, Bo; Wu, Yue; Zhang, Yong; Hou, Yong-Hui; Wang, Yue-Fei; Yang, Ming

    2015-08-01

    Hypervelocity stars are believed to be ejected out from the Galactic center through dynamical interactions between (binary) stars and the central supermassive black hole(s). In this paper, we report 19 low mass F/G/K type hypervelocity star candidates from over one million stars found in the first data release of the LAMOST regular survey. We determine the unbound probability for each candidate using a Monte-Carlo simulation by assuming a non-Gaussian proper-motion error distribution, and Gaussian heliocentric distance and radial velocity error distributions. The simulation results show that all the candidates have unbound possibilities over 50% as expected, and one of them may even exceed escape velocity with over 90% probability. In addition, we compare the metallicities of our candidates with the metallicity distribution functions of the Galactic bulge, disk, halo and globular clusters, and conclude that the Galactic bulge or disk is likely the birth place for our candidates.

  5. Modeling Low-mass Stars in the Presence of Magnetic Fields

    NASA Astrophysics Data System (ADS)

    Mullan, D. J.; MacDonald, J.

    2011-12-01

    In the presence of a vertical magnetic field, the onset of convection in a star is inhibited relative to its onset in a non-magnetic star. Following early work of Gough and Tayler for a perfectly conducting medium, and including subsequent modifications due to finite magnetic resistivity, we have computed models of low-mass stars using a magnetically-altered criterion for the onset of convection. Our models have larger radii and cooler effective temperatures than non-magnetic models with the same mass and age. Our magnetic models provide a unified framework for interpreting a range of observational data, including radii which are observed to be "bloated" by as much as 10% relative to standard models, and reduced effective temperatures, for stars ranging in spectral types from solar to brown dwarfs.

  6. A Deep X-Ray Survey of Low Mass PMS Stars in NGC 2264

    NASA Technical Reports Server (NTRS)

    Patten, Brian M.; Mushotzky, Richard (Technical Monitor)

    2001-01-01

    We have proposed to make a deep X-ray survey of a rotation- and proper-motion selected sample of young (3-15 Myr old), low-mass, pre-main sequence (PMS) stars in the populous star-forming region NGC 2264. These X-ray Multimirror Mission (XMM) data will be combined with an extensive set of rotation data for members of this cluster to allow us, for the first time, to probe the early evolution of magnetic dynamo activity for both fully convective stars and those stars found lower on their Hayashi tracks, which are developing radiative cores. We will use these data to study the interrelationship between rotation, interior structure, and coronal activity as a function of mass and age in the PMS and to define empirical constraints for theoretical models of angular momentum/dynamo evolution.

  7. Two new pulsating low-mass pre-white dwarfs or SX Phoenicis stars?

    NASA Astrophysics Data System (ADS)

    Corti, M. A.; Kanaan, A.; Córsico, A. H.; Kepler, S. O.; Althaus, L. G.; Koester, D.; Sánchez Arias, J. P.

    2016-03-01

    Context. The discovery of pulsations in low-mass stars opens an opportunity to probe their interiors and determine their evolution by employing the tools of asteroseismology. Aims: We aim to analyse high-speed photometry of SDSS J145847.02+070754.46 and SDSS J173001.94+070600.25 and discover brightness variabilities. In order to locate these stars in the Teff - log g diagram, we fit optical spectra (SDSS) with synthetic non-magnetic spectra derived from model atmospheres. Methods: To carry out this study, we used the photometric data we obtained for these stars with the 2.15 m telescope at CASLEO, Argentina. We analysed their light curves and applied the discrete Fourier transform (FT) to determine the pulsation frequencies. Finally, we compare both stars in the Teff - log g diagram, with two known pre-white dwarfs and seven pulsating pre-ELM white dwarf stars, δ Scuti, and SX Phe stars Results: We report the discovery of pulsations in SDSS J145847.02+070754.46 and SDSS J173001.94+070600.25. We determine their effective temperature and surface gravity to be Teff = 7972 ± 200 K, log g = 4.25 ± 0.5 and Teff = 7925 ± 200 K, log g = 4.25 ± 0.5, respectively. With these parameters, these new pulsating low-mass stars can be identified with either ELM white dwarfs (with ~0.17 M⊙) or more massive SX Phe stars. We identified pulsation periods of 3278.7 and 1633.9 s for SDSS J145847.02+070754.46 and a pulsation period of 3367.1 s for SDSS J173001.94+070600.25. These two new objects, together with those of Maxted et al. (2013, 2014), indicate the possible existence of a new instability domain towards the late stages of evolution of low-mass white dwarf stars, although their identification with SX Phe stars cannot be discarded. Visiting Astronomer, Complejo Astronómico El Leoncito operated under agreement between the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina and the National Universities of La Plata, Córdoba, and San Juan.

  8. Age-dating Low-Mass Star-Forming Galaxies at intermediate redshifts

    NASA Astrophysics Data System (ADS)

    Gallego, Jesus; Rodriguez-Muñoz, Lucía; Pacifici, Camilla; Tresse, Laurence; Charlot, Stéphane; Gil de Paz, Armando; Barro, Guillermo; Gomez-Guijarro, Carlos; Villar, Víctor

    2015-08-01

    Dwarf galaxies play a key role in galaxy formation and evolution: (1) hierarchical models predict that low-mass systems merged to form massive galaxies (building block paradigm; Dekel & Silk 1986); (2) dwarf systems might have been responsible for the reionization of the Universe (Wyithe & Loeb 2006); (3) theoretical models are particularly sensitive to the density of low-mass systems at diferent redshifts (Mamon et al. 2011), being one of the key science cases for the future E-ELT (Evans et al. 2013). While the history of low-mass dark matter halos is relatively well understood, the formation history of dwarf galaxies is still poorly reproduced by the models due to the distinct evolution of baryonic and dark matter.We present physical properties and constraints on the star formation histories (SFHs) of a sample of low-mass Star-Forming Galaxies (LMSFGs; 7.3 < log M∗/Mo < 8.0, at 0.3 < zspec < 0.9) selected by photometric stellar mass and apparent magnitude. The SFHs were obtained through the analysis of their spectral energy distributions using a novel approach (Pacifici et al. 2012) that (1) consistently combines photometric (HST and ground-based multi-broadband) and spectroscopic (equivalent widths of emission lines from VLT and GTC spectroscopy) data, and (2) uses physically motivated SFHs with non-uniform variations of the star formation rate (SFR) as a function of time.The median SFH of our LMSFGs 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 dwarf SFGs, consistent with the cosmological downsizing trends. We find similar median SFH timescales for a slightly more massive secondary sample 8.0 < log M∗/Mo < 9.1).

  9. On the formation redshift of Low-Mass Star-Forming Galaxies at intermediate redshifts

    NASA Astrophysics Data System (ADS)

    Gallego, Jesus; Rodriguez-Muoz, Luca; Pacifici, Camilla; Tresse, Laurence; Charlot, Stphane; Gil de Paz, Armando; Barro, Guillermo; Gomez-Guijarro, Carlos; Villar, Vctor

    2015-08-01

    Dwarf galaxies play a key role in galaxy formation and evolution: (1) hierarchical models predict that low-mass systems merged to form massive galaxies (building block paradigm; Dekel & Silk 1986); (2) dwarf systems might have been responsible for the reionization of the Universe (Wyithe & Loeb 2006); (3) theoretical models are particularly sensitive to the density of low-mass systems at diferent redshifts (Mamon et al. 2011), being one of the key science cases for the future E-ELT (Evans et al. 2013). While the history of low-mass dark matter halos is relatively well understood, the formation history of dwarf galaxies is still poorly reproduced by the models due to the distinct evolution of baryonic and dark matter.We present constraints on the star formation histories (SFHs) of a sample of low-mass Star-Forming Galaxies (LMSFGs; 7.3 < log M?/Mo < 8.0, at 0.3 < zspec < 0.9) selected by photometric stellar mass and apparent magnitude. The SFHs were obtained through the analysis of their spectral energy distributions using a novel approach (Pacifici et al. 2012) that (1) consistently combines photometric (HST and ground-based multi-broadband) and spectroscopic (equivalent widths of emission lines from VLT and GTC spectroscopy) data, and (2) uses physically motivated SFHs with non-uniform variations of the star formation rate (SFR) as a function of time.The median SFH of our LMSFGs 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 dwarf SFGs, consistent with the cosmological downsizing trends. We find similar median SFH timescales for a slightly more massive secondary sample 8.0 < log M?/Mo < 9.1).This is a pilot study for future surveys on dwarf galaxies at high redshift.

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

    NASA Astrophysics Data System (ADS)

    Haurberg, Nathalie C.

    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.

  11. Distributed low-mass star formation in the IRDC G34.43+00.24

    SciTech Connect

    Foster, Jonathan B.; Arce, Hctor G.; Offner, Stella; Kassis, Marc; Sanhueza, Patricio; Jackson, James M.; Finn, Susanna C.; Sakai, Takeshi; Sakai, Nami; Yamamoto, Satoshi; Guzmn, Andrs E.; Rathborne, Jill M.

    2014-08-20

    We have used deep near-infrared observations with adaptive optics to discover a distributed population of low-mass protostars within the filamentary Infrared Dark Cloud G34.43+00.24. We use maps of dust emission at multiple wavelengths to determine the column density structure of the cloud. In combination with an empirically verified model of the magnitude distribution of background stars, this column density map allows us to reliably determine overdensities of red sources that are due to embedded protostars in the cloud. We also identify protostars through their extended emission in the K band, which comes from excited H{sub 2} in protostellar outflows or reflection nebulosity. We find a population of distributed low-mass protostars, suggesting that low-mass protostars may form earlier than, or contemporaneously with, high-mass protostars in such a filament. The low-mass protostellar population may also produce the narrow line-width SiO emission observed in some clouds without high-mass protostars. Finally, we use a molecular line map of the cloud to determine the virial parameter per unit length along the filament and find that the highest mass protostars form in the most bound portion of the filament, as suggested by theoretical models.

  12. The seismic properties of low-mass He-core white dwarf stars

    NASA Astrophysics Data System (ADS)

    Crsico, A. H.; Romero, A. D.; Althaus, L. G.; Hermes, J. J.

    2012-11-01

    Context. In recent years, many low-mass (? 0.45 M?) white dwarf stars expected to harbor He cores have been detected in the field of the Milky Way and in several galactic globular and open clusters. Until recently, no objects of this kind showed pulsations. This situation has changed recently with the exciting discovery of SDSS J184037.78+642312.3, the first pulsating low-mass white dwarf star. Aims: Motivated by this extremely important finding, and in view of the very valuable asteroseismological potential of these objects, we present here a detailed pulsational study applied to low-mass He-core white dwarfs with masses ranging from 0.17 to 0.46 M?, based on full evolutionary models representative of these objects. This study is aimed to provide a theoretical basis from which to interpret future observations of variable low-mass white dwarfs. Methods: The background stellar models on which our pulsational analysis was carried out were derived by taking into account the complete evolutionary history of the progenitor stars, with special emphasis on the diffusion processes acting during the white dwarf cooling phase. We computed nonradial g-modes to assess the dependence of the pulsational properties of these objects with stellar parameters such as the stellar mass and the effective temperature, and also with element diffusion processes. We also performed a g- and p-mode pulsational stability analysis on our models and found well-defined blue edges of the instability domain, where these stars should start to exhibit pulsations. Results: We found substantial differences in the seismic properties of white dwarfs with M? ? 0.20 M? and the extremely low-mass (ELM) white dwarfs (M? ? 0.20 M?). Specifically, g-mode pulsation modes in ELM white dwarfs mainly probe the core regions and are not dramatically affected by mode-trapping effects by the He/H interface, whereas the opposite is true for more massive He-core white dwarfs. We found that element diffusion processes substantially affects the shape of the He/H chemical transition region, leading to non-negligible changes in the period spectrum of low-mass white dwarfs, in particular in the range of stellar masses characteristic of ELM objects. Finally, our stability analysis successfully predicts the pulsations of the only known variable low-mass white dwarf (SDSS J184037.78+642312.3) at the right effective temperature, stellar mass and range of periods. Conclusions: Our computations predict both g- and p-mode pulsational instabilities in a significant number of known low-mass and ELM white dwarfs. It is worth observing these stars in order to discover if they pulsate. Stellar models and tabulated pulsation periods are 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/547/A96

  13. SHIELD: The Star Formation Law in Extremely Low-mass Galaxies

    NASA Astrophysics Data System (ADS)

    Teich, Yaron; McNichols, Andrew; Cannon, John M.; SHIELD Team

    2016-01-01

    The "Survey of HI in Extremely Low-mass Dwarfs" (SHIELD) is a multiwavelength, legacy-class observational study of 12 low-mass dwarf galaxies discovered in Arecibo Legacy Fast ALFA (ALFALFA) survey data products. Here we analyze the relationships between HI and star formation in these systems using multi-configuration, high spatial (~300 pc) and spectral (0.82 - 2.46 km s-1 ch-1) resolution HI observations from the Karl G. Jansky Very Large Array, Hα imaging from the WIYN 3.5m telescope, and archival GALEX far-ultraviolet imaging. We compare the locations and intensities of star formation with the properties of the neutral ISM. We quantify the degree of local co-spatiality between star forming regions and regions of high HI column densities using the Kennicutt-Schmidt (K-S) relation. The values of the K-S index N vary considerably from system to system; because no single galaxy is representative of the sample, we instead focus on the narratives of the individual galaxies and their complex distribution of gaseous and stellar components. At the extremely faint end of the HI mass function, these systems are dominated by stochastic fluctuations in their interstellar media, which governs whether or not they show signs of recent star formation.Support for this work was provided by NSF grant AST-1211683 to JMC at Macalester College.

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

    PubMed

    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

    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

  15. The outburst and nature of young eruptive low mass stars in dark clouds

    NASA Astrophysics Data System (ADS)

    Ninan, J. P.; Ojha, D. K.; Bhatt, B. C.; Mallick, K. K.; Tej, A.; Sahu, D. K.; Ghosh, S. K.; Mohan, V.

    The FU Orionis (FUor) or EX Orionis (EXor) phenomenon has attracted increasing attention in recent years and is now accepted as a crucial element in the early evolution of low-mass stars. FUor and EXor eruptions of young stellar objects (YSOs) are caused by strongly enhanced accretion from the surrounding disk. FUors display optical outbursts of 4 mag or more and last for several decades, whereas EXors show smaller outbursts (?m 2 - 3 mag) that last from a few months to a few years and may occur repeatedly. Therefore, FUor/EXor eruptions represent a rare but very important phenomenon in early stellar evolution, during which a young low-mass YSO brightens by up to several optical magnitudes. Hence, long-term observations of this class of eruptive variables are important to design theoretical models of low-mass star formation. In this paper, we present recent results from our long-term monitoring observations of three rare types of eruptive young variables with the 2-m Himalayan Chandra Telescope (HCT) and the 2-m IUCAA Girawali Observatory (IGO) telescope.

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

    NASA Astrophysics Data System (ADS)

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

    2013-01-01

    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.

  17. Complex organic molecules during low-mass star formation: Pilot survey results

    SciTech Connect

    berg, Karin I.; Graninger, Dawn; Lauck, Trish

    2014-06-10

    Complex organic molecules (COMs) are known to be abundant toward some low-mass young stellar objects (YSOs), but how these detections relate to typical COM abundance are not yet understood. We aim to constrain the frequency distribution of COMs during low-mass star formation, beginning with this pilot survey of COM lines toward six embedded YSOs using the IRAM 30 m Telescope. The sample was selected from the Spitzer c2d ice sample and covers a range of ice abundances. We detect multiple COMs, including CH{sub 3}CN, toward two of the YSOs, and tentatively toward a third. Abundances with respect to CH{sub 3}OH vary between 0.7% and 10%. This sample is combined with previous COM observations and upper limits to obtain a frequency distributions of CH{sub 3}CN, HCOOCH{sub 3}, CH{sub 3}OCH{sub 3}, and CH{sub 3}CHO. We find that for all molecules more than 50% of the sample have detections or upper limits of 1%-10% with respect to CH{sub 3}OH. Moderate abundances of COMs thus appear common during the early stages of low-mass star formation. A larger sample is required, however, to quantify the COM distributions, as well as to constrain the origins of observed variations across the sample.

  18. Pathways towards Neptune-mass Planets around Very Low-mass Stars

    NASA Astrophysics Data System (ADS)

    Dreizler, S.; Bean, J.; Seifahrt, A.; Hartman, H.; Nilsson, H.; Wiedemann, G.; Reiners, A.; Henry, T. J.

    2010-10-01

    Radial velocities measured from near-infrared spectra are a potentially powerful tool to search for planets around low-mass stars. The radial velocity precision routinely obtained in the visible can, however, not be achieved in the NIR with existing techniques. In this paper, we describe a method for measuring high-precision radial velocities of a sample of the lowest-mass M dwarfs using CRIRES on the VLT. Our project makes use of a gas cell filled with ammonia to calibrate the instrument response similar to the iodine cell technique that has been used so successfully in the visible. Tests of the method based on the analysis of hundreds of spectra obtained for late M dwarfs over six months demonstrate that precisions of ˜5 m s-1 are obtainable over long timescales, and precisions better than 3 m s-1 can be obtained over timescales up to a week. This allows to search for low-mass planets, i.e., Neptune-mass or even Super-Earth planets around very low-mass stars or sub-stellar objects.

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

    NASA Astrophysics Data System (ADS)

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

    2008-09-01

    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 ). 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 (529% and 7816%) than in ? Ori (288% and 4020%). 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

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

    SciTech Connect

    Reiners, A.; Bean, J. L.; Dreizler, S.; Seifahrt, A.; Huber, K. F.; Czesla, S.

    2010-02-10

    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.

  1. Detecting Planets Around Very Low Mass Stars with the Radial Velocity Method

    NASA Astrophysics Data System (ADS)

    Reiners, A.; Bean, J. L.; Huber, K. F.; Dreizler, S.; Seifahrt, A.; Czesla, S.

    2010-02-01

    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 ~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-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 (~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.

  2. Theoretical Limits on Magnetic Field Strengths in Low-mass Stars

    NASA Astrophysics Data System (ADS)

    Browning, Matthew K.; Weber, Maria A.; Chabrier, Gilles; Massey, Angela P.

    2016-02-01

    Observations have suggested that some low-mass stars have larger radii than predicted by 1D structure models. Some theoretical models have invoked very strong interior magnetic fields (of order 1 MG or more) as a possible cause of such large radii. Whether fields of that strength could in principle be generated by dynamo action in these objects is unclear, and we do not address the matter directly. Instead, we examine whether such fields could remain in the interior of a low-mass object for a significant amount of time, and whether they would have any other obvious signatures. First, we estimate the timescales for the loss of strong fields by magnetic buoyancy instabilities. We consider a range of field strengths and simple morphologies, including both idealized flux tubes and smooth layers of field. We confirm some of our analytical estimates using thin flux tube magnetohydrodynamic simulations of the rise of buoyant fields in a fully convective M-dwarf. Separately, we consider the Ohmic dissipation of such fields. We find that dissipation provides a complementary constraint to buoyancy: while small-scale, fibril fields might be regenerated faster than they rise, the dissipative heating associated with such fields would in some cases greatly exceed the luminosity of the star. We show how these constraints combine to yield limits on the internal field strength and morphology in low-mass stars. In particular, we find that for stars of 0.3 solar masses, no fields in flux tubes stronger than about 800 kG are simultaneously consistent with both constraints.

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

    NASA Astrophysics Data System (ADS)

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

    2015-01-01

    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.

  4. Reevaluating the Mass-Radius Relation for Low-mass, Main-sequence Stars

    NASA Astrophysics Data System (ADS)

    Feiden, Gregory A.; Chaboyer, Brian

    2012-09-01

    We examine the agreement between the observed and theoretical low-mass (<0.8 M ?) stellar main-sequence mass-radius relationship by comparing detached eclipsing binary (DEB) data with a new, large grid of stellar evolution models. The new grid allows for a realistic variation in the age and metallicity of the DEB population, characteristic of the local galactic neighborhood. Overall, our models do a reasonable job of reproducing the observational data. A large majority of the models match the observed stellar radii to within 4%, with a mean absolute error of 2.3%. These results represent a factor of two improvement compared to previous examinations of the low-mass mass-radius relationship. The improved agreement between models and observations brings the radius deviations within the limits imposed by potential starspot-related uncertainties for 92% of the stars in our DEB sample.

  5. Hydrodynamics of winds from irradiated companion stars in low-mass X-ray binaries

    NASA Technical Reports Server (NTRS)

    Tavani, Marco; London, Richard

    1993-01-01

    We study the hydrodynamics of evaporative winds driven by X-rays and/or soft gamma-rays irradiating the outer layers of companion stars in low-mass X-ray binaries (LMXBs). We consider several irradiating fluxes and spectra for LMXBs with white dwarf and main-sequence companion stars. The thermal structure of the base of the coronal region, the position of the sonic point, and the value of the mass-loss rate are calculated in the case of spherical geometry. We consider photospheric and coronal heating from both X-ray photoionization and Compton scattering of X-rays and soft gamma-rays with energy about 1 MeV possibly irradiating the companion star in LMXBs. Evaporative winds may play a relevant role for the evolution of a special class of radiation-driven LMXBs, and this study is a step toward a quantitative understanding of the mechanism driving LMXB evolution.

  6. The effect of star-spots on the ages of low-mass stars determined from the lithium depletion boundary

    NASA Astrophysics Data System (ADS)

    Jackson, R. J.; Jeffries, R. D.

    2014-12-01

    In a coeval group of low-mass stars, the luminosity of the sharp transition between stars that retain their initial lithium and those at slightly higher masses in which Li has been depleted by nuclear reactions, the lithium depletion boundary (LDB), has been advanced as an almost model-independent means of establishing an age scale for young stars. Here, we construct polytropic models of contracting pre-main sequence stars (PMS) that have cool, magnetic star-spots blocking a fraction β of their photospheric flux. Star-spots slow the descent along Hayashi tracks, leading to lower core temperatures and less Li destruction at a given mass and age. The age, τLDB, determined from the luminosity of the LDB, LLDB, is increased by a factor of (1 - β)-E compared to that inferred from unspotted models, where E ≃ 1 + dlog τLDB/dlog LLDB and has a value ˜0.5 at ages <80 Myr, decreasing to ˜0.3 for older stars. Spotted stars have virtually the same relationship between K-band bolometric correction and colour as unspotted stars, so this relationship applies equally to ages inferred from the absolute K magnitude of the LDB. Low-mass PMS stars do have star-spots, but the appropriate value of β is highly uncertain with a probable range of 0.1 < β < 0.4. For the smaller β values, our result suggests a modest systematic increase in LDB ages that is comparable with the maximum levels of theoretical uncertainty previously claimed for the technique. The largest β values would however increase LDB ages by 20-30 per cent and demand a re-evaluation of other age estimation techniques calibrated using LDB ages.

  7. Discovery of a low-mass companion to the F7V star HD 984

    NASA Astrophysics Data System (ADS)

    Meshkat, T.; Bonnefoy, M.; Mamajek, E. E.; Quanz, S. P.; Chauvin, G.; Kenworthy, M. A.; Rameau, J.; Meyer, M. R.; Lagrange, A.-M.; Lannier, J.; Delorme, P.

    2015-11-01

    We report the discovery of a low-mass companion to the nearby (d = 47 pc) F7V star HD 984. The companion is detected 0.19 arcsec away from its host star in the L' band with the Apodized Phase Plate on NaCo/Very Large Telescope and was recovered by L'-band non-coronagraphic imaging data taken a few days later. We confirm the companion is comoving with the star with SINFONI integral field spectrograph H + K data. We present the first published data obtained with SINFONI in pupil-tracking mode. HD 984 has been argued to be a kinematic member of the 30 Myr-old Columba group, and its HR diagram position is not altogether inconsistent with being a zero-age main sequence star of this age. By consolidating different age indicators, including isochronal age, coronal X-ray emission, and stellar rotation, we independently estimate a main-sequence age of 115 ± 85 Myr (95 per cent CL) which does not rely on this kinematic association. The mass of directly imaged companions are usually inferred from theoretical evolutionary tracks, which are highly dependent on the age of the star. Based on the age extrema, we demonstrate that with our photometric data alone, the companion's mass is highly uncertain: between 33 and 96 MJup (0.03-0.09 M⊙) using the COND evolutionary models. We compare the companion's SINFONI spectrum with field dwarf spectra to break this degeneracy. Based on the slope and shape of the spectrum in the H band, we conclude that the companion is an M6.0 ± 0.5 dwarf. The age of the system is not further constrained by the companion, as M dwarfs are poorly fit on low-mass evolutionary tracks. This discovery emphasizes the importance of obtaining a spectrum to spectral type companions around F-stars.

  8. Fundmental Parameters of Low-Mass Stars, Brown Dwarfs, and Planets

    NASA Astrophysics Data System (ADS)

    Montet, Benjamin; Johnson, John A.; Bowler, Brendan; Shkolnik, Evgenya

    2016-01-01

    Despite advances in evolutionary models of low-mass stars and brown dwarfs, these models remain poorly constrained by observations. In order to test these predictions directly, masses of individual stars must be measured and combined with broadband photometry and medium-resolution spectroscopy to probe stellar atmospheres. I will present results from an astrometric and spectroscopic survey of low-mass pre-main sequence binary stars to measure individual dynamical masses and compare to model predictions. This is the first systematic test of a large number of stellar systems of intermediate age between young star-forming regions and old field stars. Stars in our sample are members of the Tuc-Hor, AB Doradus, and beta Pictoris moving groups, the last of which includes GJ 3305 AB, the wide binary companion to the imaged exoplanet host 51 Eri. I will also present results of Spitzer observations of secondary eclipses of LHS 6343 C, a T dwarf transiting one member of an M+M binary in the Kepler field. By combining these data with Kepler photometry and radial velocity observations, we can measure the luminosity, mass, and radius of the brown dwarf. This is the first non-inflated brown dwarf for which all three of these parameters have been measured, providing the first benchmark to test model predictions of the masses and radii of field T dwarfs. I will discuss these results in the context of K2 and TESS, which will find additional benchmark transiting brown dwarfs over the course of their missions, including a description of the first planet catalog developed from K2 data and a program to search for transiting planets around mid-M dwarfs.

  9. Very Low Mass Stars with Extremely Low Metallicity in the Milky Way's Halo

    NASA Astrophysics Data System (ADS)

    Aoki, Wako; Beers, Timothy C.; Takuma, Suda; Honda, Satoshi; Lee, Young Sun

    2015-08-01

    Large surveys and follow-up spectroscopic studies in the past few decades have been providing chemical abundance data for a growing number of very metal-poor ([Fe/H] <-2) stars. Most of them are red giants or main-sequence turn-off stars having masses near 0.8 solar masses. Lower mass stars with extremely low metallicity ([Fe/H] <-3) have yet to be well explored. Our high-resolution spectroscopic study for very metal-poor stars found with SDSS has identified four cool main-sequence stars with [Fe/H] <-2.5 among 137 objects (Aoki et al. 2013, AJ, 145, 13). The effective temperatures of these stars are 4500--5000 K, corresponding to a mass of around 0.5 solar masses. Our standard analysis of the high-resolution spectra based on 1D-LTE model atmospheres have obtained self-consistent chemical abundances for these objects, assuming small values of micro-turbulent velocities compared with giants and turn-off stars. The low temperature of the atmospheres of these objects enables us to measure their detailed chemical abundances. Interestingly, two of the four stars have extreme chemical abundance patterns: one has the largest excesses of heavy neutron-capture elements associated with the r-process abundance pattern known to date (Aoki et al. 2010, ApJL 723, L201), and the other exhibits low abundances of the alpha-elements and odd-Z elements, suggested to be the signatures of the yields of very massive stars ( >100 solar masses; Aoki et al. 2014, Science 345, 912). Although the sample size is still small, these results indicate the potential of very low-mass stars as probes to study the early stages of the Milky Way's halo formation.

  10. A Search for Close, Low-Mass Companions to Nearby A and B stars

    NASA Astrophysics Data System (ADS)

    Gullikson, Kevin; Kraus, Adam

    2014-02-01

    A stars have become high-priority targets for direct-imaging planet searches following the recent discoveries of planets orbiting e.g. HR 8799 and (beta) Pictoris. Close stellar companions to these stars can affect the formation and orbital evolution of any planets, and so a census of the multiplicity properties of nearby intermediate mass stars is needed. To this end, we propose to observe a sample of southern main sequence A- and B-type stars with high signal-to-noise ratio, high- resolution spectroscopy, in order to search for close low-mass companions. We will cross-correlate the spectra we observe against model spectra for F- to M-type secondary stars; a detected companion will appear as a peak in the cross-correlation function. The cross- correlation method is sensitive to mass-ratios q? M_s/M_p ? 0.06-0.15, and to orbital separations less then a few tens of AU. We will use the binary population we reveal to measure the mass-ratio distribution for intermediate mass stars, which can help distinguish between companions formed through disk fragmentation and molecular core fragmentation.

  11. New circumstellar disk candidates around young low mass stars and brown dwarfs

    NASA Astrophysics Data System (ADS)

    Boucher, Anne; Lafrenire, David; Gagn, Jonathan; Malo, Lison; Doyon, Rene

    2015-12-01

    It is now common knowledge that circumstellar disks are signposts of past or ongoing planetary system formation. Their presence and their properties, in relation to those of their host star, also bear valuable information about the process of star formation itself. To address these questions, we started a project to uncover new circumstellar disks around newly identified low mass star and brown dwarf candidates in nearby young kinematic associations. Being near the stellar/substellar mass boundary, these hosts - and their potential disks - are particularly interesting to study both star and planet formation. We used a least squares approach to fit synthetic spectra to the observed photometric data of each star, covering from 0.8 m up to 22 m, and then identified candidates showing a significant excess compared to the best fits. We then carefully looked at the data for these candidates to filter out those biased by contaminants or other artefacts. We ended up with a list of 4 young stars and brown dwarfs strongly suspected of being surrounded by a disk. Here we will present our search method and some properties of our newly identified disk-bearing candidates.

  12. A Unified Analysis of Time Variability in Chandra Grating Observations of Low-Mass Stars

    NASA Astrophysics Data System (ADS)

    Posson-Brown, Jennifer; Kashyap, V.; Saar, S.; Drake, J.

    2011-09-01

    We are carrying out a unified study of archival Chandra grating observations of active low-mass coronal stars and present preliminary results here. We include ACIS-S/HETG, ACIS-S/LETG, and HRC-S/LETG observations. Gratings data are optimal for timing analysis since they are free from pile-up and allow for joint spectro-temporal analysis. We discuss techniques for timing analysis of gratings data and explore the distribution of stellar flare energies and the time variability of individual lines fluxes. This work is supported by CXC NASA contract NAS8-39073 and Chandra grant AR0-11001X.

  13. Dust Heating By Low-mass Stars in Massive Galaxies at z< 1

    NASA Astrophysics Data System (ADS)

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

    2015-03-01

    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\\lt z\\lt 1.0 in the CANDELS fields. We estimated the stellar radiation field by low-mass stars from the stellar mass and surface brightness profile of these galaxies and then calculated their steady-state dust temperature. At first, we tested our method using nearby early-type galaxies with the deep far-IR data by the Herschel Virgo cluster survey and confirmed that the estimated dust temperatures are consistent with the observed temperatures within the uncertainty. We then applied the method to galaxies at 0.2\\lt z\\lt 1.0, and found that most passively evolving galaxies with {{M}star}\\gt {{10}10} {{M}? } have relatively high dust temperatures of {{T}dust}\\gt 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 increasing temperature around {{T}dust} 20 K. These results suggest that the dust heating by low-mass stars in massive galaxies plays an important role in 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.

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

    NASA Astrophysics Data System (ADS)

    Mace, Gregory Nathan

    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.

  15. Angular momentum redistribution by mixed modes in evolved low-mass stars. I. Theoretical formalism

    NASA Astrophysics Data System (ADS)

    Belkacem, K.; Marques, J. P.; Goupil, M. J.; Sonoi, T.; Ouazzani, R. M.; Dupret, M. A.; Mathis, S.; Mosser, B.; Grosjean, M.

    2015-07-01

    Seismic observations by the space-borne mission Kepler have shown that the core of red giant stars slows down while evolving, requiring an efficient physical mechanism to extract angular momentum from the inner layers. Current stellar evolution codes fail to reproduce the observed rotation rates by several orders of magnitude and instead predict a drastic spin-up of red giant cores. New efficient mechanisms of angular momentum transport are thus required. In this framework, our aim is to investigate the possibility that mixed modes extract angular momentum from the inner radiative regions of evolved low-mass stars. To this end, we consider the transformed Eulerian mean (TEM) formalism, which allows us to consider the combined effect of both the wave momentum flux in the mean angular momentum equation and the wave heat flux in the mean entropy equation as well as their interplay with the meridional circulation. In radiative layers of evolved low-mass stars, the quasi-adiabatic approximation, the limit of slow rotation, and the asymptotic regime can be applied for mixed modes and enable us to establish a prescription for the wave fluxes in the mean equations. The formalism is finally applied to a 1.3 M⊙ benchmark model, representative of observed CoRoT and Kepler oscillating evolved stars. We show that the influence of the wave heat flux on the mean angular momentum is not negligible and that the overall effect of mixed modes is to extract angular momentum from the innermost region of the star. A quantitative and accurate estimate requires realistic values of mode amplitudes. This is provided in a companion paper. Appendix A is available in electronic form at http://www.aanda.org

  16. Mapping the circumstellar environment of a young very low mass star

    NASA Astrophysics Data System (ADS)

    Bozhinova, Inna; Scholz, Alexander; Wood, Kenneth; Starkey, David; Horne, Keith

    2015-10-01

    Young stellar objects exhibit variability due to surface features on the star, star-disk interaction, and inhomogenities in the inner disk. Over recent years, multi-band monitoring campaigns have proven to be an effective tool to map the complex environment of young stars and to investigate the physical processes associated with the formation of planets. Here we propose to use Spitzer, combined with ground-based telescopes, to monitor a young very low mass star simultaneous in the mid-infrared and optical. Our target has shown persistent high-level variability over more than a decade of optical monitoring. Our aim is to map the geometry of the inner disk and the accretion flow, for the first time for an object with a mass of only 0.1 Msol. There are clear indications that accretion and disk evolution are dependent on the mass of the central object. By targeting a very low mass star we can explore the physical processes in the inner disk in an extreme parameter regime. We plan to apply two different strategies to obtain spatial constraints. We will monitor over the rotational timescale of several days, to obtain azimuthal information about hot spots on the stellar surface and structures in the inner disk material. In addition, we will derive the inner radius of the disk by measuring the delay between optical and mid-infrared variations ('light echos') over timescales of one hour. In total, we ask for 10 hours of Spitzer/IRAC 4.5mu observing, spread over ten days. Guaranteed time at ground-based telescope will provide the simultaneous optical data. Our team combines the expertise for monitoring campaigns, radiative transfer modeling, and light echo modeling.

  17. The Interstellar Medium and Star Formation of Nearby, Low-Mass Galaxies

    NASA Astrophysics Data System (ADS)

    Warren, Steven Ray

    This thesis presents four different studies of the interstellar medium (ISM) and stellar content of 40 nearby (D ? 4 Mpc), low-mass galaxies. We aim to address two fundamental questions: "How do stellar processes effect the ISM in low-mass galaxies?" and "What are the local gas conditions which lead to molecular cloud formation?". Much of the data presented here come from our survey the "Very Large Array - Advanced Camera for Surveys Nearby Galaxy Survey Treasury" (VLA-ANGST). VLA-ANGST is a targeted atomic hydrogen (H I) emission line survey directed towards 35 low-mass galaxies selected from the ANGST Hubble Space Telescope (HST) galaxy sample of the nearby universe. The VLA-ANGST project is the largest survey of its kind, demanding nearly 600 hours of VLA observing time. This unprecedented amount of observing time gives us data which has long lasting legacy value for its wealth of high resolution and high sensitivity information on the H I gas content and dynamics in a large sample of nearby, low-mass galaxies. H I data from the VLA-ANGST project will be used to explore the interactions between the gas and stellar content as well as trace the underlying dark matter distribution. Combining the H I and HST data with other tracers of recent star formation (e.g., emission processes from far ultraviolet star light, dust in the infrared, and carbon monoxide in the submillimeter) provides a comprehensive census of each galaxy, useful for understanding their evolution. We investigate the role of multiple generations of star formation in the formation of large, kiloparsec scale cavities observed in the global H I distributions of five nearby, low mass galaxies. The small gravitational potential wells of some low-mass galaxies allow the outflow of energy from stellar processes (e.g., winds, supernovae, etc.) to help shape their gas distributions. We find that stellar processes produce ample energy (at least an order of magnitude or more) to have been the dominant creation source for the observed cavities. The molecular gas responsible for the formation of stars remains elusive in many of the low-mass galaxies. We present a novel new technique to trace the immediate precursor of the molecular gas: cold H I. We apply our technique to a large sample of 31 nearby, low-mass galaxies and detect cold H I in 85% of the final sample (23/27) after quality control cuts are applied. The cold H I discoveries presented here represent a significant step forward in our ability to study the precursory gas to star formation where standard techniques fail. We find that the cold H I occupies only a small fraction of the total H I content in each galaxy, consistent with both theory and other observational techniques in the literature. The cold H I is typically found in higher density gas, but is markedly absent from the highest density peaks where current star formation is presumably heating the gas. Observations targeting the areas rich in cold H I gas may be the only way to study the conditions of star formation in some low-mass galaxies. Finally, we present direct observations of the molecular hydrogen content in one of the only low-mass galaxies with a molecular gas detection, NGC 4214. We use the Infrared Spectrograph onboard the Spitzer Space Telescope to measure pure rotational lines of the ground state of molecular hydrogen (H2). These observa

  18. IDENTIFYING THE YOUNG LOW-MASS STARS WITHIN 25 pc. II. DISTANCES, KINEMATICS, AND GROUP MEMBERSHIP

    SciTech Connect

    Shkolnik, Evgenya L.; Anglada-Escude, Guillem; Liu, Michael C.; Bowler, Brendan P.; Weinberger, Alycia J.; Boss, Alan P.; Reid, I. Neill; Tamura, Motohide

    2012-10-10

    We have conducted a kinematic study of 165 young M dwarfs with ages of {approx}<300 Myr. Our sample is composed of stars and brown dwarfs with spectral types ranging from K7 to L0, detected by ROSAT and with photometric distances of {approx}<25 pc assuming that the stars are single and on the main sequence. In order to find stars kinematically linked to known young moving groups (YMGs), we measured radial velocities for the complete sample with Keck and CFHT optical spectroscopy and trigonometric parallaxes for 75 of the M dwarfs with the CAPSCam instrument on the du Pont 2.5 m Telescope. Due to their youthful overluminosity and unresolved binarity, the original photometric distances for our sample underestimated the distances by 70% on average, excluding two extremely young ({approx}<3 Myr) objects found to have distances beyond a few hundred parsecs. We searched for kinematic matches to 14 reported YMGs and identified 10 new members of the AB Dor YMG and 2 of the Ursa Majoris group. Additional possible candidates include six Castor, four Ursa Majoris, two AB Dor members, and one member each of the Her-Lyr and {beta} Pic groups. Our sample also contains 27 young low-mass stars and 4 brown dwarfs with ages {approx}<150 Myr that are not associated with any known YMG. We identified an additional 15 stars that are kinematic matches to one of the YMGs, but the ages from spectroscopic diagnostics and/or the positions on the sky do not match. These warn against grouping stars together based only on kinematics and that a confluence of evidence is required to claim that a group of stars originated from the same star-forming event.

  19. Neutron star masses and radii from quiescent low-mass x-ray binaries

    SciTech Connect

    Lattimer, James M.; Steiner, Andrew W. E-mail: steiner3@uw.edu

    2014-04-01

    We perform a systematic analysis of neutron star radius constraints from five quiescent low-mass X-ray binaries and examine how they depend on measurements of their distances and amounts of intervening absorbing material, as well as their assumed atmospheric compositions. We construct and calibrate to published results a semi-analytic model of the neutron star atmosphere which approximates these effects for the predicted masses and radii. Starting from mass and radius probability distributions established from hydrogen-atmosphere spectral fits of quiescent sources, we apply this model to compute alternate sets of probability distributions. We perform Bayesian analyses to estimate neutron star mass-radius curves and equation of state (EOS) parameters that best-fit each set of distributions, assuming the existence of a known low-density neutron star crustal EOS, a simple model for the high-density EOS, causality, and the observation that the neutron star maximum mass exceeds 2 M {sub ?}. We compute the posterior probabilities for each set of distance measurements and assumptions about absorption and composition. We find that, within the context of our assumptions and our parameterized EOS models, some absorption models are disfavored. We find that neutron stars composed of hadrons are favored relative to those with exotic matter with strong phase transitions. In addition, models in which all five stars have hydrogen atmospheres are found to be weakly disfavored. Our most likely models predict neutron star radii that are consistent with current experimental results concerning the nature of the nucleon-nucleon interaction near the nuclear saturation density.

  20. Ultraviolet and X-ray Activity and Flaring on Low-Mass Exoplanet Host Stars

    NASA Astrophysics Data System (ADS)

    France, Kevin; Parke Loyd, R. O.; Brown, Alexander

    2015-08-01

    The spectral and temporal behavior of exoplanet host stars is a critical input to models of the chemistry and evolution of planetary atmospheres. High-energy photons (X-ray to NUV) from these stars regulate the atmospheric temperature profiles and photochemistry on orbiting planets, influencing the production of potential “biomarker” gases. We present results from the MUSCLES Treasury Survey, an ongoing study of time-resolved UV and X-ray spectroscopy of nearby M and K dwarf exoplanet host stars. This program uses contemporaneous Hubble Space Telescope and Chandra (or XMM) observations to characterize the time variability of the energetic radiation field incident on the habitable zones planetary systems at d < 15 pc. We find that all exoplanet host stars observed to date exhibit significant levels of chromospheric and transition region UV emission. M dwarf exoplanet host stars display 30 - 2000% UV emission line amplitude variations on timescales of minutes-to-hours. The relative flare/quiescent UV flux amplitudes on old (age > 1 Gyr) planet-hosting M dwarfs are comparable to active flare stars (e.g., AD Leo), despite their lack of flare activity at visible wavelengths. We also detect similar UV flare behavior on a subset of our K dwarf exoplanet host stars. We conclude that strong flares and stochastic variability are common, even on “optically inactive” M dwarfs hosting planetary systems. These results argue that the traditional assumption of weak UV fields and low flare rates on older low-mass stars needs to be revised.

  1. The Water Content of Exo-earths in the Habitable Zone around Low-mass Stars

    NASA Astrophysics Data System (ADS)

    Mulders, Gijs Dirk; Ciesla, Fred; Pascucci, Ilaria; apai, Daniel

    2015-08-01

    Terrestrial planets in the habitable zones of low-mass M dwarf stars have become the focus of many astronomical studies: they are more easily accessible to detection and characterization than their counterparts around sunlike stars. The habitability of these planets, however, faces a number of challenges, including inefficient or negligible water delivery during accretion. To understand the water content of planets in and around the habitable zone, simulations of the final stages of planet formation are necessary.We present detailed accretion simulations of wet and dry planetary embryos around a range of stellar masses. We focus on different pathways of delivering water from beyond the snow line to terrestrial planets in the habitable zone. We explore the impact of using either asteroid-like or comet-like bodies, and the effects of a dispersion in snow line locations. We derive the probability distribution of water abundances for terrestrial sized planets in the habitable zone.While these models predict that the bulk of terrestrial planets in the habitable zones of M stars will be dry, a small fraction receives earth-like amounts of water. Given their larger numbers and higher planet occurrence rates, this population of water-enriched worlds in the habitable zone of M stars may equal that around sun-like stars in numbers.References:Ciesla, Mulders et al. 2015Mulders et al. ApJ subm.

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

    NASA Astrophysics Data System (ADS)

    Nelson, Lorne A.

    2011-05-01

    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.

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

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

    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.

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

    NASA Technical Reports Server (NTRS)

    Simon, Theodore

    2005-01-01

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

  5. Models of very-low-mass stars, brown dwarfs and exoplanets

    PubMed Central

    Allard, F.; Homeier, D.; Freytag, B.

    2012-01-01

    Within the next few years, GAIA and several instruments aiming to image extrasolar planets will be ready. In parallel, low-mass planets are being sought around red dwarfs, which offer more favourable conditions, for both radial velocity detection and transit studies, than solar-type stars. In this paper, the authors of a model atmosphere code that has allowed the detection of water vapour in the atmosphere of hot Jupiters review recent advances in modelling the stellar to substellar transition. The revised solar oxygen abundances and cloud model allow the photometric and spectroscopic properties of this transition to be reproduced for the first time. Also presented are highlight results of a model atmosphere grid for stars, brown dwarfs and extrasolar planets. PMID:22547243

  6. Models of very-low-mass stars, brown dwarfs and exoplanets.

    PubMed

    Allard, F; Homeier, D; Freytag, B

    2012-06-13

    Within the next few years, GAIA and several instruments aiming to image extrasolar planets will be ready. In parallel, low-mass planets are being sought around red dwarfs, which offer more favourable conditions, for both radial velocity detection and transit studies, than solar-type stars. In this paper, the authors of a model atmosphere code that has allowed the detection of water vapour in the atmosphere of hot Jupiters review recent advances in modelling the stellar to substellar transition. The revised solar oxygen abundances and cloud model allow the photometric and spectroscopic properties of this transition to be reproduced for the first time. Also presented are highlight results of a model atmosphere grid for stars, brown dwarfs and extrasolar planets. PMID:22547243

  7. The Herschel/PACS view of disks around low-mass stars in Chamaleon-I

    NASA Astrophysics Data System (ADS)

    Olofsson, J.; Sz?cs, L.; Henning, Th.; Linz, H.; Pascucci, I.; Joergens, V.

    2013-12-01

    Context. Circumstellar disks are expected to be planet birthplaces. The potential for forming one or more planets of various masses is essentially driven by the initial mass of the disks, a crucial parameter for any planet formation theory. Constraining the masses of disks is of great interest for low-mass stars, which are expected to harbor less massive disks. Aims: We present and analyze Herschel/PACS observations of disk-bearing M-type stars that belong to the young ~2 Myr old Chamaleon-I star-forming region, to better constrain the properties of the circumstellar material and the stellar mass dependence of these parameters. Methods: We used the radiative transfer code RADMC to successfully model the spectral energy distributions (SEDs) of 17 M-type stars detected at PACS wavelengths. Our modeling strategy is carefully designed so that we search for the most probable disks parameters among a large grid of models, via Bayesian inference, which is an approach that has already proven successful. Results: Based on the modeling results, we first discuss the relatively low detection rates of M5 and later spectral type stars with respect to the PACS sensitivity, and argue that their disks masses, or flaring indices, are likely to be low (Mdisk ~ 10-5 M?, ? ~ 1.1). For M0 to M3 stars, we find a relatively broad range of disk masses (10-4-10-3 M?), scale heights, and flaring indices. Via a parametrization of dust stratification, we can reproduce the peak fluxes of the 10 ?m emission feature observed with Spitzer/IRS, and find that disks around M-type stars may display signs of dust sedimentation. We discuss a tentative correlation between the strength of the 10 ?m emission feature and the parametrized stratification. Conclusions: The Herschel/PACS observations of low-mass stars in Cha-I provide new constraints on their disk properties, overall suggesting that disk parameters for early M-type stars are comparable to those for more massive stars (e.g., comparable scale height and flaring angles). However, regions of the disks emitting at about 100 ?m may still be in the optically thick regime, preventing direct determination of disk masses. Thus the modeled disk masses should be considered as lower limits. Nevertheless, we are able to extend the wavelength coverage of SED models and start characterizing effects, such as dust sedimentation, an effort leading the way towards ALMA observations of these low-mass stars. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Tables 1, 2, and 5 are available in electronic form at http://www.aanda.org

  8. Tracing the origin of warm water emission through the stages of low-mass star formation

    NASA Astrophysics Data System (ADS)

    Vilhelm Persson, Magnus; Jorgensen, Jes K.; Coutens, Audrey; van Dishoeck, Ewine

    2015-08-01

    Water is a crucial molecule in the physics and chemistry of star- and planet formation, but its evolution from cold cores to disks is still poorly constrained. The gas-phase abundance of water varies between cold and warm regions up to a factor of 105 and this abundance variation makes water an excellent diagnostic of the physical structure in these sources.The origin of the warm water emission in deeply-embedded low-mass protostars is still debated, however. Current options include the innermost envelope (‘hot corino’), heated by the luminosity from the central protostar; a young disk heated by shocks related to ongoing accretion or the warm disk surface layers heated radiatively by the young star. Determining the location and kinematics of the warm water is important because it provides insights into whether water, and the locked up complex organics, actually moves from the outer envelope into the disk, and if so, whether it enters the disk mostly as gas or ice. Evolutionary models suggest that water and complex species enter the disk mostly as ice but this is so far unconfirmed observationally.Thus, in our collaboration we are undertaking a study of warm water in low-mass protostars. So far we have obtained interferometric maps of several isotopologues of water toward four deeply-embedded (i.e. Class 0) low-mass protostars with PdBI and ALMA. The detected water emission is compact toward the Class 0 sources, and a significant source of uncertainty in determining the abundances is the poorly constrained physical structure in the inner regions. Thus we try to constrain this physical structure by fitting simple disk models to the dust continuum visibilities that are left after subtracting a model of the spherical envelope. Furthermore we estimate upper limits to the warm water content toward the Class I protostars TMC-1A and L1527 from observations with PdBI.In this talk I will summarize our ongoing work in tracing the warm water emission through the various stages of low-mass star formation.

  9. Direct Exoplanet Imaging with JWST NIRCam: Low-Mass Stars, Low-Mass Planets, and Critical Constraints on Planet Formation

    NASA Astrophysics Data System (ADS)

    Schlieder, Joshua E.; Meyer, Michael; Reggiani, Maddalena; Quanz, Sascha; Beichman, Charles A.; Greene, Thomas P.; Burrows, Adam Seth

    2016-01-01

    As next generation exoplanet imagers are making their first discoveries, the largest population of stars in the Galaxy, the M dwarfs, are largely unaccounted for in their surveys. However, RV trends and micro lensing have revealed that M dwarfs host a substantial population of Neptune to Jupiter mass planets between ~1-10 AU. The unprecedented sensitivity of NIRCam on the JWST provides direct access to this population of gas-giants. A NIRCam 3 - 5 ?m survey for such planets will place critical constraints on planet formation by: 1) measuring the luminosities of young, sub-Jupiter mass planets, 2) providing constraints on the peak in the companion surface density vs. separation distribution, and 3) measuring the frequency of ?Jupiter mass giants in the outskirts of these systems (>10 AU). We have carefully constructed a sample of nearby, young, late-type stars, performed NIRCam survey simulations, and will report on the expected yield and advantages of JWST compared to current ground based capabilities.

  10. An Astrometric Companion to the Nearby Metal-Poor, Low-Mass Star LHS 1589

    NASA Astrophysics Data System (ADS)

    Lpine, Sbastien; Rich, R. Michael; Shara, Michael M.; Cruz, Kelle L.; Skemer, Andrew

    2007-10-01

    We report the discovery of a companion to the high proper motion star LHS 1589, a nearby high-velocity, low-mass subdwarf. The companion (LHS 1589B) is located 0.224''+/-0.004'' to the southwest of the primary (LHS 1589A), and is 0.5 mag fainter than the primary in the Ks band. The pair was resolved with the IRCAL infrared camera at Lick Observatory, operating with the Laser Guide Star Adaptive Optics system. A low-resolution spectrum of the unresolved pair obtained at the MDM observatory shows the source to be consistent with a cool subdwarf of spectral subtype sdK7.5. A photometric distance estimate places the metal-poor system at a distance d=81+/-18 pc from the Sun. We also measure a radial velocity Vrad=67+/-8 km s-1, which, together with the proper motion and estimated distance, suggests that the pair is roaming the inner Galactic halo on a highly eccentric orbit. With a projected orbital separation s=18.1+/-4.8 AU, and a crude estimate of the system's total mass, we estimate the orbital period of the system to be in the range 75 yr low-mass stars. Based on observations performed with the Laser Guide Star Adaptive Optics system at the Lick Observatory, operated by the University of California system. Based on observations conducted at the MDM observatory, operated jointly by the University of Michigan, Dartmouth College, the Ohio State University, Columbia University, and the University of Ohio.

  11. The High-Energy Radiation Environment of Planets around Low-Mass Stars

    NASA Astrophysics Data System (ADS)

    Shkolnik, Evgenya; Miles, Brittany; Barman, Travis; Peacock, Sarah

    2015-12-01

    Low-mass stars are the dominant planet hosts averaging about one planet per star. Many of these planets orbit in the canonical habitable zone (HZ) of the star where, if other conditions allowed, liquid water may exist on the surface.A planet’s habitability, including atmospheric retention, is strongly dependent on the star’s ultraviolet (UV) emission, which chemically modifies, ionizes, and even erodes the atmosphere over time including the photodissociation of important diagnostic molecules, e.g. H2O, CH4, and CO2. The UV spectral slope of a low-mass star can enhance atmospheric lifetimes, and increase the detectability of biologically generated gases. But, a different slope may lead to the formation of abiotic oxygen and ozone producing a false-positive biosignature for oxygenic photosynthesis. Realistic constraints on the incident UV flux over a planet’s lifetime are necessary to explore the cumulative effects on the evolution, composition, and fate of a HZ planetary atmosphere.NASA’s Galaxy Evolution Explorer (GALEX) provides a unique data set with which to study the broadband UV emission from many hundreds of M dwarfs. The GALEX satellite has imaged nearly 3/4 of the sky simultaneously in two UV bands: near-UV (NUV; 175-275 nm) and far-UV (FUV; 135-175 nm). With these data these, we are able to calculate the mean UV emission and its level of variability at these wavelengths over critical planet formation and evolution time scales to better understand the probable conditions in HZ planetary atmospheres.In the near future, dedicated CubeSats (miniaturized satellites for space research) to monitor M dwarf hosts of transiting exoplanets will provide the best opportunity to measure their UV variability, constrain the probabilities of detecting habitable (and inhabited) planets, and provide the correct context within which to interpret IR transmission and emission spectroscopy of transiting exoplanets.

  12. The atomic and molecular content of disks around very low-mass stars and brown dwarfs

    SciTech Connect

    Pascucci, I.; Herczeg, G.; Carr, J. S.; Bruderer, S.

    2013-12-20

    There is growing observational evidence that disk evolution is stellar-mass-dependent. Here, we show that these dependencies extend to the atomic and molecular content of disk atmospheres. We analyze a unique dataset of high-resolution Spitzer/IRS spectra from eight very low mass star and brown dwarf disks. We report the first detections of Ne{sup +}, H{sub 2}, CO{sub 2}, and tentative detections of H{sub 2}O toward these faint and low-mass disks. Two of our [Ne II] 12.81 ?m emission lines likely trace the hot (?5000 K) disk surface irradiated by X-ray photons from the central stellar/sub-stellar object. The H{sub 2} S(2) and S(1) fluxes are consistent with arising below the fully or partially ionized surface traced by the [Ne II] emission in gas at ?600 K. We confirm the higher C{sub 2}H{sub 2}/HCN flux and column density ratio in brown dwarf disks previously noted from low-resolution IRS spectra. Our high-resolution spectra also show that the HCN/H{sub 2}O fluxes of brown dwarf disks are on average higher than those of T Tauri disks. Our LTE modeling hints that this difference extends to column density ratios if H{sub 2}O lines trace warm ?600 K disk gas. These trends suggest that the inner regions of brown dwarf disks have a lower O/C ratio than those of T Tauri disks, which may result from a more efficient formation of non-migrating icy planetesimals. An O/C = 1, as inferred from our analysis, would have profound implications on the bulk composition of rocky planets that can form around very low mass stars and brown dwarfs.

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

    NASA Technical Reports Server (NTRS)

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

    1989-01-01

    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.

  14. The structure of accretion discs around low-mass young stars

    NASA Astrophysics Data System (ADS)

    Lachaume, R.

    2003-07-01

    It is nowadays widely accepted that low mass star formation initiated from a molecular cloud undergoes a phase where the central object is surrounded by a disc, in which planets may form later. The study of such a disc mainly aims at the understanding of the evolutionary sequence of star formation and of planet formation. Radiative transfer in this disc is of high importance, for it is heated by viscous dissipation or stellar irradiation for instance. The production and transportation of thermal energy in this disc therefore conditions its chemical and physical properties inside. It has countless consequences in terms of structure. I shall present a study of the discs based on a deep analytical description of the radiative transfer and a delay of the numerical implementation. This method allows a better grasp on processes and physical conditions in these objects. I shall constrain model parameters and establish a new diagnosis combining the spectral energy distribution and the visibilities obtained with optical long-baseline interferometers. The former observable is a well known technique, and the latter new and promising: it provides information at the scale of one astronomical unit for the closest star forming regions. I shall start a generalisation of the analytical studies of the radiative transfer in stellar atmospheres, with two striking discrepancies: viscous heating occurs everywhere in the disc and the surface is illuminated by the star. I apply this formalism in a numerical simulation of a disc heated by viscosity alone. I then develop a simplified version of this transfer in a two-layer disc: the surface is heated by the interior and by the star, and the interior by viscosity and by the surface. This simplification allow to derive a simplified analytical description of the physical conditions in a disc heated by the two processes mentioned above. I shall tackle the interpretation of observations in the end. I shall of the possibilities brought by interferometry concerning marginally resolved objects, as most young star will be. I then shall present best fits of the two-layer models for low mass young stars already observed by interferometers. The LaTeX and PDF files of the manuscript (in French with English appendices) are made available by the National Centre for Scientific Reasearch (CNRS) archive site (http://tel.ccsd.cnrs.fr/documents/archives0/00/00/64/74).

  15. Tides, planetary companions, and habitability: habitability in the habitable zone of low-mass stars

    NASA Astrophysics Data System (ADS)

    Van Laerhoven, C.; Barnes, R.; Greenberg, R.

    2014-07-01

    Earth-scale planets in the classical habitable zone (HZ) are more likely to be habitable if they possess active geophysics. Without a constant internal energy source, planets cool as they age, eventually terminating tectonic activity. Planets orbiting low-mass stars can be very old, due to the longevity of such stars, so they may be rendered sterile to life in this way. However, the presence of an outer companion could generate enough tidal heat in the HZ planet to prevent such cooling. The range of mass and orbital parameters for the companion that give adequate long-term heating of the inner HZ planet, while avoiding very early total desiccation, is probably substantial. We locate the ideal location for the outer of a pair of planets, under the assumption that the inner planet has the same incident flux as Earth, orbiting example stars: a generic late M dwarf (Teff = 2670 K) and the M9V/L0 dwarf DEN1048. Thus discoveries of Earth-scale planets in the HZ zone of old small stars should be followed by searches for outer companion planets that might be essential for current habitability.

  16. An IRAS Hires study of low mass star formation in the Taurus molecular ring

    NASA Technical Reports Server (NTRS)

    Terebey, Susan; Surace, Jason A.

    1994-01-01

    The Taurus molecular cloud supposedly has no star clusters but only isolated star formation. However, the Infrared Astronomical Satellite (IRAS) shows us that a small star cluster is currently forming in Taurus. Most of the sources are deeply embedded and are probably low-mass protostars. We use High Resolution (HiRes) images of the IRAS data from the Infrared Processing and Analysis Center (IPAC) to look for additional infrared members of the cluster. We also investigate the question of whether the infrared emission matches predictions for protostellar sources by examining whether the dust emission is resolved on scales of one arcminute (approx. 10(exp 17) cm). With the exception of a luminous visible star, HD 29647, we find that the sources L1527, TMC1A, TMC1, TMC1C, tMR1, and IC2087 are unresolved in the HiRes images at 60 microns. Further analysis of IC2087 shows that it is unresolved at all four IRAS wavelengths.

  17. Variability in young very low mass stars: Two surprises from spectrophotometric monitoring

    NASA Astrophysics Data System (ADS)

    Bozhinova, I.; Scholz, A.; Eislöffel, J.

    2016-03-01

    We present simultaneous photometric and spectroscopic observations of seven young and highly variable M dwarfs in star forming regions in Orion, conducted in 4 observing nights with FORS2 at ESO/VLT. All seven targets show significant photometric variability in the I-band, with amplitudes between 0.1-0.8 mag, The spectra, however, remain remarkably constant, with spectral type changes less than 0.5 subtypes. Thus, the brightness changes are not caused by veiling that `fills in' absorption features. Three objects in the σ Ori cluster (age ˜3 Myr) exhibit strong Hα emission and Hα variability, in addition to the continuum variations. Their behaviour is mostly consistent with the presence of spots with temperature of ˜300 K above the photosphere and filling factors between 0.2-0.4, in contrast to typical hot spots observed in more massive stars. The remaining targets near ɛ Ori, likely to be older, show eclipse-like lightcurves, no significant Hα activity and are better represented by variable extinction due to circumstellar material. Interestingly, two of them show no evidence of infrared excess emission. Our study shows that high-amplitude variability in young very low mass stars can be caused by different phenomena than in more massive T Tauri stars and can persist when the disk has disappeared and accretion has ceased.

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  19. Modeling the Submillimeter Dust Continuum Emission from Nearby Low Mass Star Forming Cores

    NASA Astrophysics Data System (ADS)

    Shirley, Y. L.; Young, C. H.; Evans, N. J., II; Rawlings, J. M. C.

    2001-12-01

    Current theories of a low mass star formation predict the evolution of the density distribution, n({r}, t), temperature distribution, T({r}, t), and the velocity field ,{v}({r}, t), of the envelope of protostellar cores with time. Optically thin dust emission at submillimeter wavelengths provides a powerful diagnostic to constrain the envelope density and temperature structure. In this study, thirty-nine low mass cores were mapped with SCUBA at 850 and 450 μ m on the JCMT during sixteen nights between January 1998 and February 2000. The sources were selected from the earliest phases (pre-T Tauri) in the proposed evolutionary scheme for low mass protostars (6 Pre-protostellar Cores (PPCs), 15 Class 0, 18 Class I) with luminosities indicative of low mass star formation (Lbol < 50 Lsun) and with distances less than 450 pc. High signal-to-noise maps allowed azimuthally averaged radial profiling out to 60 arcseconds from the continuum centroid. The similarities and differences in the submillimeter continuum emission properties of the envelopes of PPCs, Class 0, and Class I sources on 103 to 104 AU scales are summarized. We have modeled the normalized radial intensity distributions and spectral energy distributions (SED) for sixteen sources from the SCUBA survey (3 PPCs, 7 Class 0, and 6 Class I) using a one dimensional radiative transfer code (Egan, Leung, & Spagna 1988) with internal heating from a central protostar (Class 0 and I objects) and external heating from the interstellar radiation field (all objects) to calculate the dust temperature distribution. Power law, Bonnor-Ebert, Shu inside-out collapse, and Plummer density distributions were tested to match the observed normalized radial profiles and observed SED simultaneously. Realistic beam profiles and chopping were used to simulate the observations. We find Bonnor-Ebert spheres with central densities of 105 to 106 cm-3 reproduce the PPC radial profiles while power law models (n(r) ~ r-p, p = 1.1 - 2.3) reproduce the Class 0 and Class I profiles. Properties of the best fit models and future modeling efforts are discussed. Egan, M. P., Leung, C. M., & Spagna, G. R. 1988, Comput. Phys. Comm., 48, 271

  20. 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

    NASA Astrophysics Data System (ADS)

    Jiang, Peng; Ge, Jian; Cargile, Phillip; Crepp, Justin R.; De Lee, Nathan; Porto de Mello, Gustavo F.; Esposito, Massimiliano; Ferreira, Letícia D.; Femenia, Bruno; Fleming, Scott W.; Gaudi, B. Scott; Ghezzi, Luan; González Hernández, Jonay I.; Hebb, Leslie; Lee, Brian L.; Ma, Bo; Stassun, Keivan G.; Wang, Ji; Wisniewski, John P.; Agol, Eric; Bizyaev, Dmitry; Brewington, Howard; Chang, Liang; Nicolaci da Costa, Luiz; Eastman, Jason D.; Ebelke, Garrett; Gary, Bruce; Kane, Stephen R.; Li, Rui; Liu, Jian; Mahadevan, Suvrath; Maia, Marcio A. G.; Malanushenko, Viktor; Malanushenko, Elena; Muna, Demitri; Nguyen, Duy Cuong; Ogando, Ricardo L. C.; Oravetz, Audrey; Oravetz, Daniel; Pan, Kaike; Pepper, Joshua; Paegert, Martin; Allende Prieto, Carlos; Rebolo, Rafael; Santiago, Basilio X.; Schneider, Donald P.; Shelden Bradley, Alaina C.; Sivarani, Thirupathi; Snedden, Stephanie; van Eyken, J. C.; Wan, Xiaoke; Weaver, Benjamin A.; Zhao, Bo

    2013-09-01

    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^{+0.0188}_{-0.0187} days, an eccentricity of 0.4375 ± 0.0040, and a semi-amplitude of 2948.14^{+16.65}_{-16.55} m s-1. Using additional high-resolution spectroscopy, we find the host star has an effective temperature T 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 ⊙ and 0.92 ± 0.19 R ⊙. The minimum mass of MARVELS-5b is 65.0 ± 2.9M 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 ⊙ at a separation larger than 40 AU.

  1. 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

    Jiang Peng; Ge Jian; De Lee, Nathan; Fleming, Scott W.; Lee, Brian L.; Ma Bo; Wang, Ji; Cargile, Phillip; Hebb, Leslie; Stassun, Keivan G.; Crepp, Justin R.; Porto de Mello, Gustavo F.; Ferreira, Leticia D.; Esposito, Massimiliano; Femenia, Bruno; Gonzalez Hernandez, Jonay I.; Ghezzi, Luan; Wisniewski, John P.; Agol, Eric; and others

    2013-09-15

    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.

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

    NASA Technical Reports Server (NTRS)

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

    1994-01-01

    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).

  3. ON THE NEED FOR DEEP-MIXING IN ASYMPTOTIC GIANT BRANCH STARS OF LOW MASS

    SciTech Connect

    Busso, M.; Palmerini, S.; Maiorca, E.; Cristallo, S.; Abia, C.; Straniero, O.; Gallino, R.; Cognata, M. La

    2010-07-01

    The photospheres of low-mass red giants show CNO isotopic abundances that are not satisfactorily accounted for by canonical stellar models. The same is true for the measurements of these isotopes and of the {sup 26}Al/{sup 27}Al ratio in presolar grains of circumstellar origin. Non-convective mixing, occurring during both red giant branch (RGB) and asymptotic giant branch (AGB) stages, is the explanation commonly invoked to account for the above evidence. Recently, the need for such mixing phenomena on the AGB was questioned, and chemical anomalies usually attributed to them were suggested to be formed in earlier phases. We have therefore re-calculated extra-mixing effects in low-mass stars for both the RGB and AGB stages, in order to verify the above claims. Our results contradict them; we actually confirm that slow transport below the convective envelope occurs also on the AGB. This is required primarily by the oxygen isotopic mix and the {sup 26}Al content of presolar oxide grains. Other pieces of evidence exist, in particular from the isotopic ratios of carbon stars of type N, or C(N), in the Galaxy and in the LMC, as well as of SiC grains of AGB origin. We further show that, when extra-mixing occurs in the RGB phases of Population I stars above about 1.2 M{sub sun}, this consumes {sup 3}He in the envelope, probably preventing the occurrence of thermohaline diffusion on the AGB. Therefore, we argue that other extra-mixing mechanisms should be active in those final evolutionary phases.

  4. Constraining the neutron star equation of state using quiescent low-mass X-ray binaries

    SciTech Connect

    Jonker, P. G.

    2008-02-27

    Chandra or XMM-Newton observations of quiescent low-mass X-ray binaries can provide important constraints on the equation of state of neutron stars. The mass and radius of the neutron star can potentially be determined from fitting a neutron star atmosphere model to the observed X-ray spectrum. For a radius measurement it is of critical importance that the distance to the source is well constrained since the fractional uncertainty in the radius is at least as large as the fractional uncertainty in the distance. Uncertainties in modelling the neutron star atmosphere remain. At this stage it is not yet clear if the soft thermal component in the spectra of many quiescent X-ray binaries is variable on timescales too short to be accommodated by the cooling neutron star scenario. This can be tested with a long XMM-Newton observation of the neutron star X-ray transient Cen X-4 in quiescence. With such an observation one can use the Reflection Grating Spectrometer spectrum to constrain the interstellar extinction to the source. This removes this parameter from the X-ray spectral fitting of the EPIC pn and MOS spectra and allows one to investigate whether the variability observed in the quiescent X-ray spectrum of this source is due to variations in the soft thermal spectral component or variations in the power law spectral component coupled with variations in N{sub H}. This will test whether the soft thermal component can indeed be due to the hot thermal glow of the neutron star. Irrespective of the outcome of such a study, the observed cooling in quiescence in sources for which the crust is significantly out of thermal equilibrium with the core due to a prolonged outburst, such as KS 1731-260, seem excellent candidates for mass and radius determinations through modelling the observed X-rays with a neutron star atmosphere model (the caveats about the source distance and atmosphere modelling do also apply here obviously and presently prevent one from obtaining such constraints). Finally, the fact that the soft thermal glow in sources such as SAX J1808.4-3658 and 1H 1905+000 has not been detected in quiescence means that the neutron star cores of these sources must be cold. The most plausible explanation seems to be that the neutron stars are more massive than 1.4 M{sub {center_dot}} and cool via the direct URCA process.

  5. Feedback Effects in the High Mass and Low Mass Star Formation

    NASA Astrophysics Data System (ADS)

    Klein, R. I.

    2010-09-01

    The formation of massive stars remains one of the most significant unsolved problems in astrophysics, with implications for the formation of the elements and the structure and evolution of galaxies. It is these stars, with masses greater than 8-10 solar masses, that eventually explode as supernovae and produce most of the heavy elements in the universe, dominate the energy injection into the interstellar medium of galaxies and by injecting both heavy elements and energy into the surrounding medium, shape the evolution of galaxies. Despite the importance of massive star formation, relatively little is known about them theoretically as they pose a major theoretical challenge: How is it possible to sustain a sufficiently high mass accretion rate into a protostellar core despite the radiation pressure on the accreting envelope? I discuss our work on the first 3D simulations of massive star formation. Using our high resolution 3D radiation-hydrodynamic adaptive mesh refinement code ORION with a v/c correct treatment of the radiation transport, we have investigated the formation of high mass stars from both smooth and turbulent initial conditions in the collapsing massive core. I discuss our work on identifying 2 new mechanisms that efficiently solve the problem of the Eddington barrier to high mass star formation; the presence of 3D Rayleigh Taylor instabilities in radiation driven bubbles present in the accreting envelope and the feedback due to protostellar outflows providing radiation an escape mechanism from the accreting envelope in addition to the feedback from protostellar radiation and its affect on stellar multiplicity. I also discuss the effects of radiative transfer on low mass star formation in a turbulent molecular cloud. I compare the distribution of stellar masses, accretion rates, and temperatures in the cases with and without radiative transfer, and demonstrate that radiative feedback has profound effect on accretion, multiplicity, and mass by reducing the number of stars formed and the total rate at which gas turns into stars. Calculations that omit radiative feedback from protostars significantly underestimate the gas temperature and the strength of this effect.

  6. Low mass binary neutron star mergers: Gravitational waves and neutrino emission

    NASA Astrophysics Data System (ADS)

    Foucart, Francois; Haas, Roland; Duez, Matthew D.; O'Connor, Evan; Ott, Christian D.; Roberts, Luke; Kidder, Lawrence E.; Lippuner, Jonas; Pfeiffer, Harald P.; Scheel, Mark A.

    2016-02-01

    Neutron star mergers are among the most promising sources of gravitational waves for advanced ground-based detectors. These mergers are also expected to power bright electromagnetic signals, in the form of short gamma-ray bursts, infrared/optical transients powered by r-process nucleosynthesis in neutron-rich material ejected by the merger, and radio emission from the interaction of that ejecta with the interstellar medium. Simulations of these mergers with fully general relativistic codes are critical to understand the merger and postmerger gravitational wave signals and their neutrinos and electromagnetic counterparts. In this paper, we employ the Spectral Einstein Code to simulate the merger of low mass neutron star binaries (two 1.2 M⊙ neutron stars) for a set of three nuclear-theory-based, finite temperature equations of state. We show that the frequency peaks of the postmerger gravitational wave signal are in good agreement with predictions obtained from recent simulations using a simpler treatment of gravity. We find, however, that only the fundamental mode of the remnant is excited for long periods of time: emission at the secondary peaks is damped on a millisecond time scale in the simulated binaries. For such low mass systems, the remnant is a massive neutron star which, depending on the equation of state, is either permanently stable or long lived (i.e. rapid uniform rotation is sufficient to prevent its collapse). We observe strong excitations of l =2 , m =2 modes, both in the massive neutron star and in the form of hot, shocked tidal arms in the surrounding accretion torus. We estimate the neutrino emission of the remnant using a neutrino leakage scheme and, in one case, compare these results with a gray two-moment neutrino transport scheme. We confirm the complex geometry of the neutrino emission, also observed in previous simulations with neutrino leakage, and show explicitly the presence of important differences in the neutrino luminosity, disk composition, and outflow properties between the neutrino leakage and transport schemes.

  7. Angular Momentum Evolution of Low-Mass Stars in the Pleiades

    NASA Astrophysics Data System (ADS)

    Krishnamurthi, Anita

    This dissertation presents a study of the angular momentum evolution in low-mass stars, concentrating on the Pleiades. I start with a theoretical study of the angular momentum evolution of 0.5-1.2 Msolar stars. Stellar models are constructed including the effects of internal angular momentum transport, angular momentum loss, and accretion disk lifetimes in the pre-main-sequence phase of evolution. I systematically and consistently explore the effects of varying these parameters. The angular momentum loss is prescribed to occur through a magnetic wind and to saturate at some value of the rotation. The resulting rotation rates are found to be a strong function of the value of this saturation threshold. I use rotation data from open clusters as constraints on the models. I find that a mass dependent saturation threshold is necessary to reproduce the distribution of rotation rates seen in open clusters. The inferred life-time of the circumstellar accretion disk is found to depend on the assumptions made about core-envelope decoupling. The solid body models require longer disk lifetimes (~10-20 Myr) while the differential rotation models require shorter disk lifetimes (~3-10 Myr). This is consistent with the lifetime of the infra-red excess observed in pre-main-sequence stars, believed to be a signature of accretion disks. I then present the results of a monitoring program to determine the photometric rotational periods of low mass stars in the Pleiades. I have determined rotation periods for 18 stars in the Pleiades, increasing the number of known rotation periods in the Pleiades to 51. I report the discovery of cool, slow rotators with high amplitudes of variation. This contradicts previous conclusions about the use of amplitudes as an alternate diagnostic of the saturation of angular momentum loss. I discuss the correlation between X-ray activity and rotation rates. I find that the X-ray data can be observational indicators of mass-dependent saturation in the angular momentum loss proposed on theoretical grounds. Finally, I discuss the correlation of surface Li abundances with rotation and activity. I find that there is a wide range in abundances with rotation. This disputes the strong correlation claimed between rotation and Li abundance.

  8. X-Shooter study of accretion in ?-Ophiucus: very low-mass stars and brown dwarfs

    NASA Astrophysics Data System (ADS)

    Manara, C. F.; Testi, L.; Natta, A.; Alcal, J. M.

    2015-07-01

    We present new VLT/X-Shooter optical and near-infrared spectra of a sample of 17 candidate young low-mass stars and brown dwarfs located in the ?-Ophiucus cluster. We derived the spectral type and extinction for all the targets, and then we determined their physical parameters. All the objects but one have M??0.6 M?, and eight have mass below or close to the hydrogen-burning limit. Using the intensity of various permitted emission lines present in their spectra, we determined the accretion luminosity and mass accretion rates (?acc) for all the objects. When compared with previous works targeting the same sample, we find that, in general, these objects are not as strongly accreting as previously reported, and we suggest that the reason is our more accurate estimate of the photospheric parameters. We also compare our findings with recent works in other slightly older star-forming regions, such as Lupus, to investigate possible differences in the accretion properties, but we find that the accretion properties for our targets have the same dependence on the stellar and substellar parameters as in the other regions. This leads us to conclude that we do not find evidence for a different dependence of ?acc with M? when comparing low-mass stars and brown dwarfs. Moreover, we find a similar small (?1 dex) scatter in the ?acc-M? relation as in some of our recent works in other star-forming regions, and no significant differences in ?acc due to different ages or properties of the regions. The latter result suffers, however, from low statistics and sample selection biases in the current studies. The small scatter in the ?acc-M? correlation confirms that mass accretion rate measurements in the literature based on uncertain photospheric parameters and single accretion indicators, such as the H? width, can lead to a scatter that is unphysically large. Our studies show that only broadband spectroscopic surveys coupled with a detailed analysis of the photospheric and accretion properties allows us to properly study the evolution of disk accretion rates in star-forming regions. This work is based on observations made with ESO Telescopes at the Paranal Observatory under programme ID 085.C-0876.Appendices are available in electronic form at http://www.aanda.org

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

    SciTech Connect

    Harvey, Paul M.; Evans, Neal J. II; Henning, Thomas; Liu Yao; Wolf, Sebastian; Menard, Francois; Pinte, Christophe; Pascucci, Ilaria E-mail: nje@astro.as.utexas.edu E-mail: wolf@astrophysik.uni-kiel.de E-mail: yliu@pmo.ac.cn E-mail: christophe.pinte@obs.ujf-grenoble.fr E-mail: pascucci@lpl.arizona.edu

    2012-08-10

    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.

  10. Hard X-ray Flux from Low-Mass Stars in the Cygnus OB2 Association

    NASA Astrophysics Data System (ADS)

    Caramazza, M.; Drake, J. J.; Micela, G.; Flaccomio, E.

    2009-05-01

    We investigate the X-ray emission in the 20-40 keV band expected from the flaring low-mass stellar population in Cygnus OB2 assuming that the observed soft X-ray emission is due to a superposition of flares and that the ratio of hard X-ray to soft X-ray emission is described by a scaling found for solar flares by Isola and co-workers. We estimate a low-mass stellar hard X-ray flux in the 20-40 keV band in the range ~71031-71033 erg/s and speculate the limit of this values. Hard X-ray emission could lie at a level not much below the current observed flux upper limits for Cygnus OB2. Simbol-X, with its broad energy band (10-100 keV) and its sensitivity should be able to detect this emission and would provide insights into the hard X-ray production of flares on pre-main sequence stars.

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

    NASA Astrophysics Data System (ADS)

    Cody, Ann Marie; Hillenbrand, Lynne A.

    2014-12-01

    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 behaviorboth periodic and aperiodicon 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.

  12. A pulsation search among young brown dwarfs and very-low-mass stars

    SciTech Connect

    Cody, Ann Marie; Hillenbrand, Lynne A.

    2014-12-01

    In 2005, Palla and 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 behaviorboth periodic and aperiodicon 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.

  13. Electromagnetic pulse from supernovae. [model for old low-mass stars

    NASA Technical Reports Server (NTRS)

    Colgate, S. A.

    1975-01-01

    Upper and lower limits to the radiated electromagnetic pulse from a supernova are calculated assuming that the mass fraction of the matter expanding inside the dipole magnetic field shares energy and maintains the pressure balance in the process. A supernova model is described in which the explosion occurs in old low-mass stars containing less than 10% hydrogen in their ejecta and a remnant neutron star is produced. The analysis indicates that although the surface layer of a star of 1 g/cu thickness may be shock-accelerated to an energy factor of about 100 and may expand into the vacuum with an energy factor approaching 10,000, the equatorial magnetic field will retard this expansion so that the inner, more massive ejecta layers will effectively accelerate the presumed canonical dipole magnetic field to greater velocities than would the surface layer alone. A pulse of 10 to the 46th power ergs in a width of about 150 cm will result which will not be affected by circumstellar matter or electron self-radiation effects. It is shown that interstellar matter will attenuate the pulse, but that charge separation may reduce the attenuation and allow a larger pulse to escape.

  14. Interpreting the low-mass PMS stars in Cep OB3b

    NASA Astrophysics Data System (ADS)

    Pozzo, M.; Jeffries, R. D.; Naylor, T.

    2001-05-01

    We present the results of the UBVI CCD photometric survey of the younger subgroup (Cep OB3b) of the Cep OB3 association. We have discovered a low-mass PMS stellar association mixed with the early-type members of the subgroup. Its location in a V vs (V-I) colour-magnitude diagram is clearly separated from contaminating field stars, and encompasses the X-ray sources recently discovered in the region by Naylor & Fabian (1999). The spectroscopic follow-up of the optically selected PMS sample has confirmed the kinematic membership of several candidates, together with their PMS nature (WTTS and CTTS). The optical sample has then been cross-correlated with NIR sources found in the region by 2MASS (Second Incremental Release Point Source Catalogue), in order to look for objects with genuine IR excess, suggesting the presence of a circumstellar accretion disk.

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

    NASA Astrophysics Data System (ADS)

    Jurusik, W.; Drzazga, R. T.; Jableka, M.; Chyży, K. T.; Beck, R.; Klein, U.; Weżgowiec, M.

    2014-07-01

    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 μG, while the ordered fields reach 1-2 μG, 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.

  16. Scaled up low-mass star formation in massive star-forming cores in the G333 giant molecular cloud

    NASA Astrophysics Data System (ADS)

    Wiles, B.; Lo, N.; Redman, M. P.; Cunningham, M. R.; Jones, P. A.; Burton, M. G.; Bronfman, L.

    2016-03-01

    Three bright molecular line sources in G333 have recently been shown to exhibit signatures of infall. We describe a molecular line radiative transfer modelling process which is required to extract the infall signature from Mopra and Nanten2 data. The observed line profiles differ greatly between individual sources but are reproduced well by variations upon a common unified model where the outflow viewing angle is the most significant difference between the sources. The models and data together suggest that the observed properties of the high-mass star-forming regions such as infall, turbulence, and mass are consistent with scaled-up versions of the low-mass case with turbulent velocities that are supersonic and an order of magnitude larger than those found in low-mass star-forming regions. Using detailed radiative transfer modeling, we show that the G333 cores are essentially undergoing a scaled-up version of low mass star formation. This is an extension of earlier work in that the degree of infall and the chemical abundances are constrained by the RT modeling in a way that is not practical with a standard analysis of observational data. We also find high velocity infall and high infall mass rates, possibly suggesting accelerated collapse due to external pressure. Molecular depletion due to freeze-out onto dust grains in central regions of the cores is suggested by low molecular abundances of several species. Strong evidence for a local enhancement of 13C-bearing species towards the outflow cloud cores is discussed, consistent with the presence of shocks caused by the supersonic motions within them.

  17. The star formation history of low-mass disk galaxies: A case study of NGC 300

    NASA Astrophysics Data System (ADS)

    Kang, Xiaoyu; Zhang, Fenghui; Chang, Ruixiang; Wang, Lang; Cheng, Liantao

    2016-01-01

    Context. Since NGC 300 is a bulgeless, isolated low-mass galaxy and it has not experienced radial migration during its evolution history, it can be treated as an ideal laboratory to test the simple galactic chemical evolution model. Aims: Our main aim is to investigate the main properties of the star formation history (SFH) of NGC 300 and compare its SFH with that of M 33 to explore the common properties and differences between these two nearby low-mass systems. Methods: We construct a simple chemical evolution model for NGC 300, assuming its disk forms gradually from continuous accretion of primordial gas and including the gas-outflow process. The model allows us to build a bridge between the SFH and observed data of NGC 300, in particular, the present-day radial profiles and global observed properties (e.g., cold gas mass, star formation rate, and metallicity). By means of comparing the model predictions with the corresponding observations, we adopt the classical χ2 methodology to find out the best combination of free parameters a, b, and bout. Results: Our results show that by assuming an inside-out formation scenario and an appropriate outflow rate, our model reproduces well most of the present-day observational values. The model not only reproduces well the radial profiles, but also the global observational data for the NGC 300 disk. Our results suggest that NGC 300 may experience a rapid growth of its disk. Through comparing the best-fitting, model-predicted SFH of NGC 300 with that of M 33, we find that the mean stellar age of NGC 300 is older than that of M 33 and there is a recent lack of primordial gas infall onto the disk of NGC 300. Our results also imply that the local environment may play a key role in the secular evolution of galaxy disks.

  18. Identifying the Young Low-mass Stars within 25 pc. II. Distances, Kinematics, and Group Membership

    NASA Astrophysics Data System (ADS)

    Shkolnik, Evgenya L.; Anglada-Escud, Guillem; Liu, Michael C.; Bowler, Brendan P.; Weinberger, Alycia J.; Boss, Alan P.; Reid, I. Neill; Tamura, Motohide

    2012-10-01

    We have conducted a kinematic study of 165 young M dwarfs with ages of lsim300 Myr. Our sample is composed of stars and brown dwarfs with spectral types ranging from K7 to L0, detected by ROSAT and with photometric distances of lsim25 pc assuming that the stars are single and on the main sequence. In order to find stars kinematically linked to known young moving groups (YMGs), we measured radial velocities for the complete sample with Keck and CFHT optical spectroscopy and trigonometric parallaxes for 75 of the M dwarfs with the CAPSCam instrument on the du Pont 2.5 m Telescope. Due to their youthful overluminosity and unresolved binarity, the original photometric distances for our sample underestimated the distances by 70% on average, excluding two extremely young (lsim3 Myr) objects found to have distances beyond a few hundred parsecs. We searched for kinematic matches to 14 reported YMGs and identified 10 new members of the AB Dor YMG and 2 of the Ursa Majoris group. Additional possible candidates include six Castor, four Ursa Majoris, two AB Dor members, and one member each of the Her-Lyr and ? Pic groups. Our sample also contains 27 young low-mass stars and 4 brown dwarfs with ages lsim150 Myr that are not associated with any known YMG. We identified an additional 15 stars that are kinematic matches to one of the YMGs, but the ages from spectroscopic diagnostics and/or the positions on the sky do not match. These warn against grouping stars together based only on kinematics and that a confluence of evidence is required to claim that a group of stars originated from the same star-forming event. Based on observations collected at the W. M. Keck Observatory, the Canada-France-Hawaii Telescope, the du Pont Telescope at Las Campanas Observatory, and the Subaru Telescope. 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.

  19. Rotational Periods of Very Young Brown Dwarfs and Very Low Mass Stars in Chamaeleon I

    NASA Astrophysics Data System (ADS)

    Joergens, V.; Fernndez, M.; Carpenter, J. M.; Neuhuser, R.

    2003-09-01

    We have studied the photometric variability of very young brown dwarfs and very low mass stars (masses well below 0.2 Msolar) in the Cha I star-forming region. We have determined photometric periods in the Gunn i and R bands for the three M6.5-M7 type brown dwarf candidates Cha H? 2, Cha H? 3, and Cha H? 6 of 2.2-3.4 days. These are the longest photometric periods found for any brown dwarf so far. If interpreted as rotationally induced, they correspond to moderately fast rotational velocities, which is fully consistent with their vsini values and their relatively large radii. We have also determined periods for the two M5-M5.5 type very low mass stars B34 and CHXR 78C. In addition to the Gunn i- and R-band data, we have analyzed JHKS monitoring data of the targets, which have been taken a few weeks earlier and confirm the periods found in the optical data. Upper limits for the errors in the period determination are between 2 and 9 hr. The observed periodic variations of the brown dwarf candidates as well as of the T Tauri stars are interpreted as modulation of the flux at the rotation period by magnetically driven surface features, on the basis of a consistency with vsini values as well as R-i color variations typical for spots. Furthermore, the temperatures even for the brown dwarfs in the sample are relatively high (>2800 K) because the objects are very young. Therefore, the atmospheric gas should be sufficiently ionized for the formation of spots on one hand, and the temperatures are too high for significant dust condensation and hence variabilities due to clouds on the other hand. A comparison with rotational properties of older brown dwarfs shows that most of the acceleration of brown dwarfs takes place within the first 30 Myr or less. If magnetic braking plays a role, this suggests that the disk dissipation for brown dwarfs occurs between a few and 36 Myr. Based on observations obtained at the European Southern Observatory at La Silla in program 65.L-0629.

  20. The Low-mass Interacting Binary System OO Aql Revisited: A New Quadruple System

    NASA Astrophysics Data System (ADS)

    ?li, T.; Koak, D.; Boz, G. .; Yakut, K.

    2013-05-01

    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 1 = 1.05(2) M ?, M 2 = 0.89(2) M ?, R 1 = 1.38(2) R ?, R 2 = 1.28(2) R ?, log (L 1/L ?) = 0.258, and log (L 2/L ?) = 0.117, and the separation of the components was determined to be a = 3.333(16) R ?. 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}/{\\dot{P}})=4\\times 10^{7} yr that can be interpreted in terms of the mass transfer (dM/dt) = 5 10-8 M ? yr-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).

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

    PubMed

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

    2015-01-01

    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 α-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

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

    NASA Astrophysics Data System (ADS)

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

    2015-01-01

    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 α-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.

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

    SciTech Connect

    Icli, T.; Kocak, D.; Boz, G. C.; Yakut, K.

    2013-05-15

    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).

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

    NASA Astrophysics Data System (ADS)

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

    2015-01-01

    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.

  5. Magnetic Inhibition of Convection and the Fundamental Properties of Low-mass Stars. I. Stars with a Radiative Core

    NASA Astrophysics Data System (ADS)

    Feiden, Gregory A.; Chaboyer, Brian

    2013-12-01

    Magnetic fields are hypothesized to inflate the radii of low-mass starsdefined as less massive than 0.8 M ?in detached eclipsing binaries (DEBs). We investigate this hypothesis using the recently introduced magnetic Dartmouth stellar evolution code. In particular, we focus on stars thought to have a radiative core and convective outer envelope by studying in detail three individual DEBs: UV Psc, YY Gem, and CU Cnc. Our results suggest that the stabilization of thermal convection by a magnetic field is a plausible explanation for the observed model-radius discrepancies. However, surface magnetic field strengths required by the models are significantly stronger than those estimated from observed coronal X-ray emission. Agreement between model predicted surface magnetic field strengths and those inferred from X-ray observations can be found by assuming that the magnetic field sources its energy from convection. This approach makes the transport of heat by convection less efficient and is akin to reduced convective mixing length methods used in other studies. Predictions for the metallicity and magnetic field strengths of the aforementioned systems are reported. We also develop an expression relating a reduction in the convective mixing length to a magnetic field strength in units of the equipartition value. Our results are compared with those from previous investigations to incorporate magnetic fields to explain the low-mass DEB radius inflation. Finally, we explore how the effects of magnetic fields might affect mass determinations using asteroseismic data and the implication of magnetic fields on exoplanet studies.

  6. An ALMA survey for disks orbiting low-mass stars in the TW Hya Association

    NASA Astrophysics Data System (ADS)

    Rodriguez, David R.; van der Plas, Gerrit; Kastner, Joel H.; Schneider, Adam C.; Faherty, Jacqueline K.; Mardones, Diego; Mohanty, Subhanjoy; Principe, David

    2015-10-01

    We carried out an ALMA survey of 15 confirmed or candidate low-mass (<0.2 M?) members of the TW Hya Association (TWA) with the goal of detecting molecular gas in the form of CO emission, as well as of providing constraints on continuum emission due to cold dust. Our targets have spectral types of M4-L0 and hence represent the extreme low end of the TWA's mass function. Our ALMA survey has yielded detections of 1.3 mm continuum emission around 4 systems (TWA 30B, 32, 33, and 34), suggesting the presence of cold dust grains. All continuum sources are unresolved. TWA 34 further shows 12CO(2-1) emission whose velocity structure is indicative of Keplerian rotation. Among the sample of known ~7-10 Myr-old star/disk systems, TWA 34, which lies just ~50 pc from Earth, is the lowest mass star thus far identified as harboring cold molecular gas in an orbiting disk.

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

    NASA Astrophysics Data System (ADS)

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

    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.

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

    NASA Astrophysics Data System (ADS)

    Pitts, Mark A.

    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.

  9. THE EFFECT OF MAGNETIC ACTIVITY ON LOW-MASS STARS IN ECLIPSING BINARIES

    SciTech Connect

    Morales, Juan Carlos; Ribas, Ignasi; Jordi, Carme; Baraffe, Isabelle; Chabrier, Gilles

    2010-07-20

    In recent years, analyses of eclipsing binary systems have unveiled differences between the observed fundamental properties of low-mass stars and those predicted by stellar structure models. Particularly, radius and effective temperatures computed from models are {approx}5%-10% lower and {approx}3%-5% higher than observed, respectively. These discrepancies have been attributed to different factors, notably the high levels of magnetic activity present on these stars. In this paper, we test the effect of magnetic activity both on models and on the observational analysis of eclipsing binaries using a sample of such systems with accurate fundamental properties. Regarding stellar models, we have found that unrealistically high spot coverages need to be assumed to reproduce the observations. Tests considering metallicity effects and missing opacities on models indicate that these are not able to explain the radius discrepancies observed. With respect to the observations, we have tested the effect of several spot distributions on the light curve analysis. Our results show that spots cause systematic deviations on the stellar radii derived from light curve analysis when mainly distributed over the stellar poles. Assuming the existence of polar spots, overall agreement between models and observations is reached when {approx}35% spot coverage is considered on stellar models. Such spot coverage induces a systematic deviation in the radius determination from the light curve analysis of {approx}3% and is also compatible with the modulations observed on the light curves of these systems. Finally, we have found that the effect of activity or rotation on convective transport in partially radiative stars may also contribute to the explanation of the differences seen in some of the systems with shorter orbital periods.

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

    NASA Astrophysics Data System (ADS)

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

    2013-01-01

    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.

  11. Tidal Timelines: Evolution of Terrestrial Exoplanet Habitability Around Low Mass Stars

    NASA Astrophysics Data System (ADS)

    Mullins, K.; Barnes, R.

    2009-12-01

    The range of orbits for planetary habitability have traditionally been based on the stellar flux that allows liquid water to persist on a planetary surface. However, when considering terrestrial (rocky) planets close to a low mass star (?0.35M?), tidal effects must be considered because of the additional energy input from tidal heating. Of further interest is the time over which habitable conditions are generated by tidal interaction. Tides cause orbital evolution, during which the heat flux varies, which may cause the planet to migrate in and out of habitable zones and possibly result in sterilization. So, the heating history of a planet should be a consideration when searching for life-supporting planets. We apply heat flux limitations on habitability (based on observations within our solar system) and tidally evolve planets across a range of initial conditions of orbits and masses. Our results provide a visualization of the time a planet has spent with a favorable amount of tidal heat for habitability and/or the amount of time until the heating is no longer conducive to habitability. As a greater number of close in terrestrial planets are found, these results can provide a method for identifying those planets with the highest potential for life.

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

    SciTech Connect

    Schlieder, Joshua E.; Simon, Michal; Lepine, Sebastien; Rice, Emily; Fielding, Drummond; Tomasino, Rachael E-mail: schlieder@mpia-hd.mpg.de E-mail: erice@amnh.org E-mail: tomas1r@cmich.edu

    2012-05-15

    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.

  13. Determination of mass and orbital parameters of a low-mass star HD 213597B

    NASA Astrophysics Data System (ADS)

    Chaturvedi, Priyanka; Deshpande, Rohit; Dixit, Vaibhav; Roy, Arpita; Chakraborty, Abhijit; Mahadevan, Suvrath; Anandarao, B. G.; Hebb, Leslie; Janardhan, P.

    2014-08-01

    HD 213597 is an eclipsing binary system which was detected by the STEREO spacecraft and was speculated to host a low-mass stellar companion. We used high-resolution spectroscopy with the 10-m Hobby-Eberly Telescope and the 1.2-m telescope in Mount Abu for radial velocity (RV) measurements of this source. We performed aperture photometry for this star on the STEREO archival data and thereby confirm the transit signature. We also did follow-up ground-based photometry with a 10-inch telescope from Mt Abu. The spectroscopic RV semi-amplitude of the primary (33.39 km s-1) indicates that the secondary is an M dwarf making the system a short period F+M eclipsing binary. These RVs along with the inclination derived from our combined photometric analysis (i = 84.9), enable us to estimate the mass of the secondary as MB 0.286 M? and radius as RB 0.344 R? using an estimated mass MA 1.3 M? and radius RB 1.97 R? of the primary. Our spectral analysis returned the following parameters: Teff = 6625 121 K, [Fe/H] = -0.095 0.08 and log g = 3.72 0.22 for the primary. When log g is constrained to a value of 3.96, we derive Teff = 6753 52 K and [Fe/H] = -0.025 0.05.

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

    NASA Technical Reports Server (NTRS)

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

    2001-01-01

    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.

  15. NUMERICAL SIMULATIONS OF THERMOHALINE CONVECTION: IMPLICATIONS FOR EXTRA-MIXING IN LOW-MASS RGB STARS

    SciTech Connect

    Denissenkov, Pavel A.

    2010-11-01

    Low-mass stars are known to experience extra-mixing in their radiative zones on the red giant branch (RGB) above the bump luminosity. To determine if the salt-fingering transport of chemical composition driven by {sup 3}He burning is efficient enough to produce RGB extra-mixing, two-dimensional numerical simulations of thermohaline convection for physical conditions corresponding to the RGB case have been carried out. We have found that the effective ratio of a salt finger's length to its diameter a{sub eff} {approx}< 0.5 is more than 10 times smaller than the value needed to reproduce observations (a{sub obs} {approx}> 7). On the other hand, using the thermohaline diffusion coefficient from linear stability analysis together with a = a{sub obs} is able to describe the RGB extra-mixing at all metallicities so well that it is tempting to believe that it may represent the true mechanism. In view of these results, follow-up three-dimensional numerical simulations of thermohaline convection for the RGB case are clearly needed.

  16. Low-mass lithium-rich AGB stars in the Galactic bulge: evidence for cool bottom processing?

    NASA Astrophysics Data System (ADS)

    Uttenthaler, S.; Lebzelter, T.; Palmerini, S.; Busso, M.; Aringer, B.; Lederer, M. T.

    2007-08-01

    Context: The stellar production of the light element lithium is still a matter of debate. Aims: We report the detection of low-mass, Li-rich Asymptotic Giant Branch (AGB) stars located in the Galactic bulge. Methods: A homogeneous and well-selected sample of low mass, oxygen-rich AGB stars in the Galactic bulge has been searched for the absorption lines of Li. Using spectral synthesis techniques, we determine from high resolution UVES/VLT spectra the Li abundance in four out of 27 sample stars, and an upper limit for the remaining stars. Results: Two stars in our sample have a solar Li abundance or above; these stars seem to be a novelty, since they do not show any s-element enhancement. Two more stars have a Li abundance slightly below solar; these stars do show s-element enhancement in their spectra. Different scenarios which lead to an increased Li surface abundance in AGB stars are discussed. Conclusions: Of the different enrichment scenarios presented, Cool Bottom Processing (CBP) is the most likely one for the Li-rich objects identified here. Self-enrichment by Hot Bottom Burning (HBB) seems very unlikely as all Li-rich stars are below the HBB mass limit. Also, the ingestion of a low mass companion into the stars' envelope is unlikely because the associated additional effects are lacking. Mass transfer from a former massive binary companion is a possible scenario, if the companion produced little s-process elements. A simple theoretical estimation for the Li abundance due to CBP is presented and compared to the observed values. Based on observations at the Very Large Telescope of the European Southern Observatory, Cerro Paranal/Chile under Program 65.L-0317(A, B).

  17. Planets Around Low-mass Stars (PALMS). V. Age-dating Low-mass Companions to Members and Interlopers of Young Moving Groups

    NASA Astrophysics Data System (ADS)

    Bowler, Brendan P.; Shkolnik, Evgenya L.; Liu, Michael C.; Schlieder, Joshua E.; Mann, Andrew W.; Dupuy, Trent J.; Hinkley, Sasha; Crepp, Justin R.; Johnson, John Asher; Howard, Andrew W.; Flagg, Laura; Weinberger, Alycia J.; Aller, Kimberly M.; Allers, Katelyn N.; Best, William M. J.; Kotson, Michael C.; Montet, Benjamin T.; Herczeg, Gregory J.; Baranec, Christoph; Riddle, Reed; Law, Nicholas M.; Nielsen, Eric L.; Wahhaj, Zahed; Biller, Beth A.; Hayward, Thomas L.

    2015-06-01

    We present optical and near-infrared adaptive optics (AO) imaging and spectroscopy of 13 ultracool (>M6) companions to late-type stars (K7-M4.5), most of which have recently been identified as candidate members of nearby young moving groups (YMGs; 8-120 Myr) in the literature. Three of these are new companions identified in our AO imaging survey, and two others are confirmed to be comoving with their host stars for the first time. The inferred masses of the companions (˜10-100 MJup) are highly sensitive to the ages of the primary stars; therefore we critically examine the kinematic and spectroscopic properties of each system to distinguish bona fide YMG members from old field interlopers. The new M7 substellar companion 2MASS J02155892-0929121 C (40-60 MJup) shows clear spectroscopic signs of low gravity and, hence, youth. The primary, possibly a member of the ˜40 Myr Tuc-Hor moving group, is visually resolved into three components, making it a young low-mass quadruple system in a compact (≲100 AU) configuration. In addition, Li i λ6708 absorption in the intermediate-gravity M7.5 companion 2MASS J15594729+4403595 B provides unambiguous evidence that it is young (≲200 Myr) and resides below the hydrogen-burning limit. Three new close-separation (<1″) companions (2MASS J06475229-2523304 B, PYC J11519+0731 B, and GJ 4378 Ab) orbit stars previously reported as candidate YMG members, but instead are likely old (≳1 Gyr) tidally locked spectroscopic binaries without convincing kinematic associations with any known moving group. The high rate of false positives in the form of old active stars with YMG-like kinematics underscores the importance of radial velocity and parallax measurements to validate candidate young stars identified via proper motion and activity selection alone. Finally, we spectroscopically confirm the cool temperature and substellar nature of HD 23514 B, a recently discovered M8 benchmark brown dwarf orbiting the dustiest-known member of the Pleiades. Based on observations collected at the European Organization for Astronomical Research in the Southern Hemisphere, Chile (ESO Program 090.A-9010(A)).

  18. Magnetic inhibition of convection and the fundamental properties of low-mass stars. I. Stars with a radiative core

    SciTech Connect

    Feiden, Gregory A.; Chaboyer, Brian E-mail: brian.chaboyer@dartmouth.edu

    2013-12-20

    Magnetic fields are hypothesized to inflate the radii of low-mass starsdefined as less massive than 0.8 M {sub ?}in detached eclipsing binaries (DEBs). We investigate this hypothesis using the recently introduced magnetic Dartmouth stellar evolution code. In particular, we focus on stars thought to have a radiative core and convective outer envelope by studying in detail three individual DEBs: UV Psc, YY Gem, and CU Cnc. Our results suggest that the stabilization of thermal convection by a magnetic field is a plausible explanation for the observed model-radius discrepancies. However, surface magnetic field strengths required by the models are significantly stronger than those estimated from observed coronal X-ray emission. Agreement between model predicted surface magnetic field strengths and those inferred from X-ray observations can be found by assuming that the magnetic field sources its energy from convection. This approach makes the transport of heat by convection less efficient and is akin to reduced convective mixing length methods used in other studies. Predictions for the metallicity and magnetic field strengths of the aforementioned systems are reported. We also develop an expression relating a reduction in the convective mixing length to a magnetic field strength in units of the equipartition value. Our results are compared with those from previous investigations to incorporate magnetic fields to explain the low-mass DEB radius inflation. Finally, we explore how the effects of magnetic fields might affect mass determinations using asteroseismic data and the implication of magnetic fields on exoplanet studies.

  19. INTERACTIONS BETWEEN FORMING STARS AND DENSE GAS IN THE SMALL LOW-MASS CLUSTER CEDERBLAD 110

    SciTech Connect

    Ladd, E. F.; Wong, T.; Bourke, T. L.; Thompson, K. L.

    2011-12-20

    We present observations of dense gas and outflow activity in the Cederblad 110 region of the Chamaeleon I dark cloud complex. The region contains nine forming low-mass stars in evolutionary stages ranging from Class 0 to Class II/III crowded into a 0.2 pc region with high surface density ({Sigma}{sub YSO} {approx} 150 pc{sup -2}). The analysis of our N{sub 2}H{sup +} (J = 1{yields}0) maps indicates the presence of 13 {+-} 3 solar masses of dense (n {approx} 10{sup 5} cm{sup -3}) gas in this region, much of which is unstable against gravitational collapse. The most unstable material is located near the Class 0 source MMS-1, which is almost certainly actively accreting material from its dense core. Smaller column densities of more stable dense gas are found toward the region's Class I sources, IRS 4, 11, and 6. Little or no dense gas is colocated with the Class II and III sources in the region. The outflow from IRS 4 is interacting with the dense core associated with MMS-1. The molecular component of the outflow, measured in the (J = 1{yields}0) line of {sup 12}CO, appears to be deflected by the densest part of the core, after which it appears to plow through some of the lower column density portions of the core. The working surface between the head of the outflow lobe and the dense core material can be seen in the enhanced velocity dispersion of the dense gas. IRS 2, the Class III source that produces the optical reflection nebula that gives the Cederblad 110 region its name, may also be influencing the dense gas in the region. A dust temperature gradient across the MMS-1 dense core is consistent with warming from IRS 2, and a sharp gradient in dense gas column density may be caused by winds from this source. Taken together, our data indicate that this region has been producing several young stars in the recent past, and that sources which began forming first are interacting with the remaining dense gas in the region, thereby influencing current and future star formation activity.

  20. EVOLUTION, NUCLEOSYNTHESIS, AND YIELDS OF LOW-MASS ASYMPTOTIC GIANT BRANCH STARS AT DIFFERENT METALLICITIES

    SciTech Connect

    Cristallo, S.; Straniero, O.; Piersanti, L.; Gallino, R.; DomInguez, I.; Lederer, M. T.

    2009-05-01

    The envelope of thermally pulsing asymptotic giant branch (TP-AGB) stars undergoing periodic third dredge-up (TDU) episodes is enriched in both light and heavy elements, the ashes of a complex internal nucleosynthesis involving p, {alpha}, and n captures over hundreds of stable and unstable isotopes. In this paper, new models of low-mass AGB stars (2 M {sub sun}), with metallicity ranging between Z = 0.0138 (the solar one) and Z = 0.0001, are presented. Main features are (1) a full nuclear network (from H to Bi) coupled to the stellar evolution code, (2) a mass loss-period-luminosity relation, based on available data for long-period variables, and (3) molecular and atomic opacities for C- and/or N-enhanced mixtures, appropriate for the chemical modifications of the envelope caused by the TDU. For each model, a detailed description of the physical and chemical evolutions is presented; moreover, we present a uniform set of yields, comprehensive of all chemical species (from hydrogen to bismuth). The main nucleosynthesis site is the thin {sup 13}C pocket, which forms in the core-envelope transition region after each TDU episode. The formation of this {sup 13}C pocket is the principal by-product of the introduction of a new algorithm, which shapes the velocity profile of convective elements at the inner border of the convective envelope: both the physical grounds and the calibration of the algorithm are discussed in detail. We find that the pockets shrink (in mass) as the star climbs the AGB, so that the first pockets, the largest ones, leave the major imprint on the overall nucleosynthesis. Neutrons are released by the {sup 13}C({alpha}, n){sup 16}O reaction during the interpulse phase in radiative conditions, when temperatures within the pockets attain T {approx} 1.0 x 10{sup 8} K, with typical densities of (10{sup 6}-10{sup 7}) neutrons cm{sup -3}. Exceptions are found, as in the case of the first pocket of the metal-rich models (Z = 0.0138, Z = 0.006 and Z = 0.003), where the {sup 13}C is only partially burned during the interpulse: the surviving part is ingested in the convective zone generated by the subsequent thermal pulse (TP) and then burned at T {approx} 1.5 x 10{sup 8} K, thus producing larger neutron densities (up to 10{sup 11} neutrons cm{sup -3}). An additional neutron exposure, caused by the {sup 22}Ne({alpha}, n){sup 25}Mg during the TPs, is marginally activated at large Z, but becomes an important nucleosynthesis source at low Z, when most of the {sup 22}Ne is primary. The final surface compositions of the various models reflect the differences in the initial iron-seed content and in the physical structure of AGB stars belonging to different stellar populations. Thus, at large metallicities the nucleosynthesis of light s-elements (Sr, Y, Zr) is favored, whilst, decreasing the iron content, the overproduction of heavy s-elements (Ba, La, Ce, Nd, Sm) and lead becomes progressively more important. At low metallicities (Z = 0.0001) the main product is lead. The agreement with the observed [hs/ls] index observed in intrinsic C stars at different [Fe/H] is generally good. For the solar metallicity model, we found an interesting overproduction of some radioactive isotopes, like {sup 60}Fe, as a consequence of the anomalous first {sup 13}C pocket. Finally, light elements (C, F, Ne, and Na) are enhanced at any metallicity.

  1. Evolution, Nucleosynthesis, and Yields of Low-Mass Asymptotic Giant Branch Stars at Different Metallicities

    NASA Astrophysics Data System (ADS)

    Cristallo, S.; Straniero, O.; Gallino, R.; Piersanti, L.; Domnguez, I.; Lederer, M. T.

    2009-05-01

    The envelope of thermally pulsing asymptotic giant branch (TP-AGB) stars undergoing periodic third dredge-up (TDU) episodes is enriched in both light and heavy elements, the ashes of a complex internal nucleosynthesis involving p, ?, and n captures over hundreds of stable and unstable isotopes. In this paper, new models of low-mass AGB stars (2 M sun), with metallicity ranging between Z = 0.0138 (the solar one) and Z = 0.0001, are presented. Main features are (1) a full nuclear network (from H to Bi) coupled to the stellar evolution code, (2) a mass loss-period-luminosity relation, based on available data for long-period variables, and (3) molecular and atomic opacities for C- and/or N-enhanced mixtures, appropriate for the chemical modifications of the envelope caused by the TDU. For each model, a detailed description of the physical and chemical evolutions is presented; moreover, we present a uniform set of yields, comprehensive of all chemical species (from hydrogen to bismuth). The main nucleosynthesis site is the thin 13C pocket, which forms in the core-envelope transition region after each TDU episode. The formation of this 13C pocket is the principal by-product of the introduction of a new algorithm, which shapes the velocity profile of convective elements at the inner border of the convective envelope: both the physical grounds and the calibration of the algorithm are discussed in detail. We find that the pockets shrink (in mass) as the star climbs the AGB, so that the first pockets, the largest ones, leave the major imprint on the overall nucleosynthesis. Neutrons are released by the 13C(?, n)16O reaction during the interpulse phase in radiative conditions, when temperatures within the pockets attain T ~ 1.0 108 K, with typical densities of (106-107) neutrons cm-3. Exceptions are found, as in the case of the first pocket of the metal-rich models (Z = 0.0138, Z = 0.006 and Z = 0.003), where the 13C is only partially burned during the interpulse: the surviving part is ingested in the convective zone generated by the subsequent thermal pulse (TP) and then burned at T ~ 1.5 108 K, thus producing larger neutron densities (up to 1011 neutrons cm-3). An additional neutron exposure, caused by the 22Ne(?, n)25Mg during the TPs, is marginally activated at large Z, but becomes an important nucleosynthesis source at low Z, when most of the 22Ne is primary. The final surface compositions of the various models reflect the differences in the initial iron-seed content and in the physical structure of AGB stars belonging to different stellar populations. Thus, at large metallicities the nucleosynthesis of light s-elements (Sr, Y, Zr) is favored, whilst, decreasing the iron content, the overproduction of heavy s-elements (Ba, La, Ce, Nd, Sm) and lead becomes progressively more important. At low metallicities (Z = 0.0001) the main product is lead. The agreement with the observed [hs/ls] index observed in intrinsic C stars at different [Fe/H] is generally good. For the solar metallicity model, we found an interesting overproduction of some radioactive isotopes, like 60Fe, as a consequence of the anomalous first 13C pocket. Finally, light elements (C, F, Ne, and Na) are enhanced at any metallicity.

  2. THE LUMINOSITY AND MASS FUNCTIONS OF LOW-MASS STARS IN THE GALACTIC DISK. II. THE FIELD

    SciTech Connect

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

    2010-06-15

    We report on new measurements of the luminosity function (LF) and mass function (MF) of field low-mass dwarfs derived from Sloan Digital Sky Survey Data Release 6 photometry. The analysis incorporates {approx}15 million low-mass stars (0.1 M{sub sun}< M < 0.8 M{sub sun}), spread over 8400 deg{sup 2}. Stellar distances are estimated using new photometric parallax relations, constructed from ugriz photometry of nearby low-mass stars with trigonometric parallaxes. We use a technique that simultaneously measures Galactic structure and the stellar LF from 7 < M{sub r} < 16. We compare the LF to previous studies and convert to an MF using the mass-luminosity relations of Delfosse et al. The system MF, measured over -1.0< log M/M{sub sun} <-0.1, is well described by a lognormal distribution with M{sub o} = 0.25 M{sub sun}. We stress that our results should not be extrapolated to other mass regimes. Our work generally agrees with prior low-mass stellar MFs and places strong constraints on future theoretical star formation studies.

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

    PubMed

    Kafka, Stella

    2012-06-01

    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

  4. Direct measurements of the fundamental properties of low-mass stars and brown dwarfs

    NASA Astrophysics Data System (ADS)

    Dupuy, Trent J.

    2010-10-01

    Detailed theoretical models of stars, developed and observationally tested over the last century, now underlie most of modern astronomy. In contrast, models of lower temperature objects, brown dwarfs and gas giant planets, have only recently been developed and remained largely unconstrained by observations. Despite this lack of empirical validation, these models have become entrenched in many active areas of astronomical research, and thus rigorously testing them is imperative. Dynamical masses from visual binaries are central to this effort, but such measurements have been previously been impeded by observational limitations (ultracool binaries are faint, and their orbital separations are very small). This dissertation presents results from our program to test models using precise dynamical masses (as good as 2%) for ultracool binaries, based on infrared parallaxes, near-infrared spectroscopy, and Keck laser guide star adaptive optics astrometry for a sample of over 30 objects. In just the last 2 years, we have more than tripled the number of ultracool binaries with dynamical masses, extending these measurements to much lower temperatures in previously unexplored areas of parameter space. Our main results are summarized as follows: (1) For most field binaries, based on direct measurements of their luminosities and masses, we find that the temperatures predicted by evolutionary models are discrepant with those derived from fitting the observed spectra with model atmospheres, indicating systematic errors of [approximate]200 K in temperature (or 15%-20% in radius). We have also devised alternative model tests for these systems using their space motion and chromospheric activity to constrain their ages. (2) For the only field binary with an independent age determination from the solar-type primary in its hierarchical triple system (from age-activity-rotation relations), we find that evolutionary models systematically underpredict luminosities by a factor of [approximate]2 at a given mass, which means that model-based substellar mass determinations (e.g., for directly imaged extrasolar planets and the low-mass initial mass function) may be systematically overestimating the masses. (3) We have employed our large sample of binary orbits to carry out a novel test of the earliest evolutionary stages, by using the distribution of orbital eccentricities to distinguish between competing models of brown dwarf formation.

  5. PLANETS AROUND LOW-MASS STARS (PALMS). II. A LOW-MASS COMPANION TO THE YOUNG M DWARF GJ 3629 SEPARATED BY 0.''2

    SciTech Connect

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

    2012-09-01

    We present the discovery of a 0.''2 companion to the young M dwarf GJ 3629 as part of our high-contrast adaptive optics imaging search for giant planets around low-mass stars with the Keck-II and Subaru telescopes. Two epochs of imaging confirm that the pair is comoving and reveal signs of orbital motion. The primary exhibits saturated X-ray emission which, together with its UV photometry from GALEX, points to an age younger than {approx}300 Myr. At these ages the companion lies below the hydrogen burning limit with a model-dependent mass of 46 {+-} 16 M{sub Jup} based on the system's photometric distance of 22 {+-} 3 pc. Resolved YJHK photometry of the pair indicates a spectral type of M7 {+-} 2 for GJ 3629 B. With a projected separation of 4.4 {+-} 0.6 AU and an estimated orbital period of 21 {+-} 5 yr, GJ 3629 AB is likely to yield a dynamical mass in the next several years, making it one of only a handful of brown dwarfs to have a measured mass and an age constrained from the stellar primary.

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

    NASA Astrophysics Data System (ADS)

    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

    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

  7. Star Formation in Low Mass Magnetized Cores: The Formation of Disks and Outflows

    NASA Astrophysics Data System (ADS)

    Duffin, Dennis F.

    2012-10-01

    Protostellar discs are generally thought to drive molecular outflows and jets observed in star forming regions, but there has been some debate as to how they form. The details of the driving and collimation of outflows help determine how much mass is cleared out and how much energy is fed back into the surroundings. Recently it has been argued that the magnetic brake is so strong that early protostellar disks cannot form. We have performed 3D ideal magnetohydrodynamic (MHD) simulations of collapsing Bonnor-Ebert spheres, employing sink particles within an AMR grid and using a cooling function to model radiative cooling of the gas. This allows us to follow the formation and early evolution of the accretion disc (2-8)×10^4 years further into the Class 0 phase of its evolution. We form a rotationally dominated disc with a radius of 100 AU embedded inside a transient, unstable, flattened, rotating structure extending out to 2000 AU. The inner disc becomes unstable to a warping instability due to the magnetic structure of the outflow, warping 30 deg with respect to the rotation-axis by the end of the simulation. The disc is unstable to a Parker instability and sheds magnetic loops, degrading the orientation of the mean threading field. This reduces and locally reverses the magnetic braking torque of the large scale field back upon the disc. The reduction of magnetic braking allows a nearly Keplerian disc to form and may be the key way in which low mass stellar systems produce rotationally dominated discs. We discuss the relevance of our disc misalignment concerning the formation of mis-aligned hot Jupiters. Protostellar outflows are implicated in clearing mass from collapsing cores, and limiting the final mass of newly formed stars. The details of the driving and collimation of outflows help determine how much mass is cleared out and how much energy is fed back into the surroundings. The simulations generate outflows which are precessing, kinked, contain internal shocks and extend to a scale of 0.1 pc end-to-end. Our disc-wind theory describes magneto-centrifugal driving throughout the outflow bubble. The bulk properties of the outflow agree well with observations. The outflow has two components, a larger low speed wind (vr < 1.5 km/s) dominated by a toroidal magnetic field Bφ, and an inner centrifugally driven jet dominated by Bp with speeds up to 20 km/s. The ratio of mass flux from the disk surface compared to accretion in the disk is measured to be Mout/Min ∼ 0.1 from the inner component, whereas in the outer component Mout/Min ∼1.0. The jet is misaligned and precesses as the disc warps by 30 deg with respect to the z-axis. We measure star formation efficiencies of ɛcore = 0.63 (and growing), higher than theoretical predictions of ɛcore = 0.29-0.39 and observations ɛcore = 0.33. These new results reported in this thesis, show that disks can form in strongly magnetized media, in agreement with the observations - and that outflows are not as efficient in clearing away collapsing gas as has been assumed in various theoretical models. Both of these results have important implications for disk formation, and the origin of the IMF, as described in this work.

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

    NASA Astrophysics Data System (ADS)

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

    2015-01-01

    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 β 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.

  9. PLANETS AROUND LOW-MASS STARS (PALMS). IV. THE OUTER ARCHITECTURE OF M DWARF PLANETARY SYSTEMS

    SciTech Connect

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

    2015-01-01

    We present results from a high-contrast adaptive optics imaging search for giant planets and brown dwarfs (≳1 M {sub Jup}) around 122 newly identified nearby (≲40 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 {sub ☉}) 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 {sub 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 {sub Jup}; L0{sub −1}{sup +2}; 120 ± 20 AU), GJ 3629 B (64{sub −23}{sup +30} M {sub Jup}; M7.5 ± 0.5; 6.5 ± 0.5 AU), 1RXS J034231.8+121622 B (35 ± 8 M {sub Jup}; L0 ± 1; 19.8 ± 0.9 AU), and 2MASS J15594729+4403595 B (43 ± 9 M {sub 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 {sub 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 {sub Jup} range like those orbiting HR 8799 and β Pictoris between 10-100 AU for a hot-start (cold-start) formation scenario. The frequency of brown dwarf (13-75 M {sub Jup}) companions to single M dwarfs between 10-100 AU is 2.8{sub −1.5}{sup +2.4}%. 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.

  10. Identifying the Young Low-mass Stars within 25 pc. I. Spectroscopic Observations

    NASA Astrophysics Data System (ADS)

    Shkolnik, Evgenya; Liu, Michael C.; Reid, I. Neill

    2009-07-01

    We have completed a high-resolution (R ? 60,000) optical spectroscopic survey of 185 nearby M dwarfs identified using ROSAT data to select active, young objects with fractional X-ray luminosities comparable to or greater than Pleiades members. Our targets are drawn from the NStars 20 pc census and the Moving-M sample with distances determined from parallaxes or spectrophotometric relations. We limited our sample to 25 pc from the Sun, prior to correcting for pre-main-sequence overluminosity or binarity. Nearly half of the resulting M dwarfs are not present in the Gliese catalog and have no previously published spectral types. We identified 30 spectroscopic binaries (SBs) from the sample, which have strong X-ray emission due to tidal spin-up rather than youth. This is equivalent to a 16% SB fraction, with at most a handful of undiscovered SBs. We estimate upper limits on the age of the remaining M dwarfs using spectroscopic youth indicators such as surface gravity-sensitive indices (CaH and K I). We find that for a sample of field stars with no metallicity measurements, a single CaH gravity index may not be sufficient, as higher metallicities mimic lower gravity. This is demonstrated in a subsample of metal-rich radial velocity (RV) standards, which appear to have low surface gravity as measured by the CaH index, yet show no other evidence of youth. We also use additional youth diagnostics such as lithium absorption and strong H? emission to set more stringent age limits. Eleven M dwarfs with no H? emission or absorption are likely old (>400 Myr) and were caught during an X-ray flare. We estimate that our final sample of the 144 youngest and nearest low-mass objects in the field is less than 300 Myr old, with 30% of them being younger than 150 Myr and four very young (lap10 Myr), representing a generally untapped and well-characterized resource of M dwarfs for intensive planet and disk searches. Based on observations collected at the W. M. Keck Observatory and the Canada-France-Hawaii Telescope. 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.

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

    NASA Astrophysics Data System (ADS)

    Mohanty, Subhanjoy; Jayawardhana, Ray; Basri, Gibor

    2005-06-01

    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 members of the ? Ophiuchus, Taurus, Chamaeleon I, IC 348, R Coronae Australis, Upper Scorpius, and TW Hydrae star-forming regions and young clusters, with ages from <1 to ~10 Myr. The sample contains 41 brown dwarfs (spectral types >=M6.5). We have previously presented high-resolution optical spectra for roughly half the sample; the rest are new. This is a close to complete survey of all confirmed brown dwarfs known so far in the regions examined, except in ? Oph and IC 348 (where we are limited by a combination of extinction and distance). We find that (1) classical T Tauri-like disk accretion persists in the substellar domain down to nearly the deuterium-burning limit; (2) while an H? 10% width >~200 km s-1 is our prime accretion diagnostic (following our previous work), permitted emission lines of Ca II, O I, and He I are also good accretion indicators, just as in classical T Tauri stars (we caution against a blind use of H? width alone, since inclination and rotation effects on the line are especially important at the low accretion rates in very low mass objects); (3) the Ca II ?8662 line flux is an excellent quantitative measure of the accretion rate in very low mass stars and brown dwarfs (as in higher mass classical T Tauri Stars), correlating remarkably well with the M? obtained from veiling and H? modeling; (4) the accretion rate diminishes rapidly with mass-our measurements support previous suggestions that M?~M2* (albeit with considerable scatter) and extend this correlation to the entire range of substellar masses; (5) the fraction of very low mass stellar and substellar accretors decreases substantially with age, as in higher mass stars; (6) at any given age, the fraction of very low mass stellar and substellar accretors is comparable to the accretor fraction in higher mass stars; and (7) a number of our sources with infrared excesses arising from dusty disks do not evince measurable accretion signatures, with the incidence of such a mismatch increasing with age: this implies that disks in the low-mass regime can persist beyond the main accretion phase and parallels the transition from the classical to post-T Tauri stage in more massive stars. These strong similarities at young ages, between higher mass stars on the one hand and low-mass bodies close to and below the hydrogen-burning limit on the other, are consistent with a common formation mechanism in the two mass regimes.

  12. The variation of the tidal quality factor of convective envelopes of rotating low-mass stars along their evolution

    NASA Astrophysics Data System (ADS)

    Mathis, S.

    2015-12-01

    More than 1500 exoplanets have been discovered around a large diversity of host stars (from M- to A-type stars). Tidal dissipation in their convective envelope is a key actor that shapes the orbital architecture of short-period systems and that still remains unknown. Using a simplified two-layer assumption and grids of stellar models, we compute analytically an equivalent modified tidal quality factor, which is proportional to the inverse of the frequency-averaged dissipation due to the viscous friction applied by turbulent convection on tidal waves. It leads the conversion of their kinetic energy into heat and tidal evolution of orbits and spin. During their Pre-Main-Sequence, all low-mass stars have a decrease of the equivalent modified tidal quality factor for a fixed angular velocity of their convective envelope. Next, it evolves on the Main Sequence to an asymptotic value that is minimum for 0.6M_⊙ K-type stars and that increases by several orders of magnitude with increasing stellar mass. Finally, the rotational evolution of low-mass stars strengthens tidal dissipation during the Pre-Main-Sequence.

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

    NASA Astrophysics Data System (ADS)

    Hebb, Leslie

    2006-06-01

    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.

  14. The MUSCLES Treasury Survey: Temporally- and Spectrally-Resolved Irradiance from Low-mass Exoplanet Host Stars

    NASA Astrophysics Data System (ADS)

    France, Kevin; Parke Loyd, R. O.; Youngblood, Allison; Linsky, Jeffrey; MUSCLES Treasury Survey Team

    2016-01-01

    The spectral and temporal behavior of exoplanet host stars is a critical input to models of the chemistry and evolution of planetary atmospheres. High-energy photons (X-ray to near-UV; 5 - 3200 Ang) from these stars regulate the atmospheric temperature profiles and photochemistry on orbiting planets, influencing the production of potential "biomarker" gases. It has been shown that the atmospheric signatures of potentially habitable planets around low-mass stars may be significantly different from planets orbiting Sun-like stars owing to the different UV spectral energy distribution. I will present results from a panchromatic survey (Hubble/Chandra/XMM/optical) of M and K dwarf exoplanet hosts, the MUSCLES Treasury Survey (Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems). We reconstruct the Lyman-alpha and extreme-UV (100-900 Ang) radiation lost to interstellar attenuation and create 5 Angstrom to 5 micron stellar irradiance spectra; these data will be publically available as a High-Level Science Product on MAST. We find that all low-mass exoplanet host stars exhibit significant chromospheric/transition region/coronal emission -- no "UV inactive" M dwarfs are observed. The F(far-UV)/F(near-UV) flux ratio, a driver for possible abiotic production of the suggested biomarkers O2 and O3, increases by ~3 orders of magnitude as the habitable zone moves inward from 1 to 0.1 AU, while the incident far-UV (912 - 1700 Ang) and XUV (5 - 900 Ang) radiation field strengths decrease by factors of a few across this range. Far-UV flare activity is common in 'optically inactive' M dwarfs; statistics from the entire sample indicate that large UV flares (E(300 - 1700 Ang) >= 10^31 erg) occur several times per day on typical M dwarf exoplanet hosts.

  15. A project to unveil the population of Low-Mass Star-Forming Galaxies of the Universe

    NASA Astrophysics Data System (ADS)

    Gallego, Jesus; Rodriguez-Muoz, Luca; Tresse, Laurence; Pacifici, Camilla; Charlot, Stphane; Gil de Paz, Armando; Gomez-Guijarro, Carlos; Villar, Vctor; Barro, Guillermo

    2015-08-01

    Dwarf galaxies play a key role in galaxy formation and evolution: (1) hierarchical models predict that low-mass systems merged to form massive galaxies (building block paradigm; Dekel & Silk 1986); (2) dwarf systems might have been responsible for the reionization of the Universe (Wyithe & Loeb 2006); (3) theoretical models are particularly sensitive to the density of low-mass systems at diferent redshifts (Mamon et al. 2011), being one of the key science cases for the future E-ELT (Evans et al. 2013). While the history of low-mass dark matter halos is relatively well understood, the formation history of dwarf galaxies is still poorly reproduced by the models due to the distinct evolution of baryonic and dark matter.We present constraints on the star formation histories (SFHs) of a sample of low-mass Star-Forming Galaxies (LMSFGs; 7.3 < log M?/Mo < 8.0, at 0.3 < zspec < 0.9) selected by photometric stellar mass and apparent magnitude. The SFHs were obtained through the analysis of their spectral energy distributions using a novel approach (Pacifici et al. 2012) that (1) consistently combines photometric (HST and ground-based multi-broadband) and spectroscopic (equivalent widths of emission lines from VLT and GTC spectroscopy) data, and (2) uses physically motivated SFHs with non-uniform variations of the star formation rate (SFR) as a function of time.The median SFH of our LMSFGs 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 dwarf SFGs, consistent with the cosmological downsizing trends. We find similar median SFH timescales for a slightly more massive secondary sample 8.0 < log M?/Mo < 9.1).This is a pilot study for future surveys on dwarf galaxies at high redshift.

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

    SciTech Connect

    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.; Eastman, Jason D.; Siverd, Robert J.; Scott Gaudi, B.; Niedzielski, Andrzej; Sivarani, Thirupathi; Stassun, Keivan G.; Gary, Bruce; Wolszczan, Alex; Barnes, Rory

    2010-08-01

    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.

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

    NASA Astrophysics Data System (ADS)

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

    2015-01-01

    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.

  18. Rotation of low-mass stars - A new probe of stellar evolution

    NASA Technical Reports Server (NTRS)

    Pinsonneault, M. H.; Kawaler, Steven D.; Demarque, P.

    1990-01-01

    Models of stars of various masses and rotational parameters were developed and compared with observations of stars in open clusters of various ages in order to analyze the evolution of rotating stars from the early premain sequence to an age of 1.7 x 10 to the 9th yrs. It is shown that, for stars older than 10 to the 8th yrs and less massive than 1.1 solar mass, the surface rotation rates depend most strongly on the properties of the angular momentum loss. The trends of the currently available observations suggest that the rotation periods are a good indicator of the field-star ages.

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

    NASA Technical Reports Server (NTRS)

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

    1983-01-01

    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.

  20. Extracting multipole moments of neutron stars from quasi-periodic oscillations in low mass X-ray binaries

    NASA Astrophysics Data System (ADS)

    Boshkayev, Kuantay; Rueda, Jorge; Muccino, Marco

    2015-06-01

    We consider the kilohertz quasi-periodic oscillations of low-mass X-ray binaries within the Hartle-Thorne spacetime. We show that the interpretation of the epicyclic frequencies of this spacetime with the observed kilohertz quasi-periodic oscillations, within the Relativistic Precession Model, allows us to extract the total mass M, angular momentum J, and quadrupole moment Q of the compact object in a low-mass X-ray binary. We exemplify this fact by analyzing the data of the Z-source GX 5-1. We show that the extracted multipole structure of the compact component of this source deviates from the one expected from a Kerr black hole and instead it points to a neutron star explanation.

  1. Suzaku spectra of the neutron-star low-mass X-ray binary 4U 1608-52

    NASA Astrophysics Data System (ADS)

    Lei, Yajuan; Zhang, Haotong; zhang, Yanxia

    2015-08-01

    We present the spectral analysis of the neutron-star low-mass X-ray binary 4U 1608-52 using data from four Suzaku observations in 2010 March. 4U 1608-52 is a transient atoll source, and the analyzed observations contain the “island” and “banana” states, corresponding transitional, and soft states. The spectra are fitted with the hybrid model for the soft states, which consists of two thermal components (a multicolor accretion disk and a single-temperature blackbody) plus a broken power law. The fitting results show that the continuum spectra evolve during the different states. Fe emission line is often detected in low-mass X-ray binary, however, no obviously Fe line is detected in the four observations of 4U 1608-52.

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

    SciTech Connect

    Hernandez, Jesus; Morales-Calderon, Maria; Calvet, Nuria; Hartmann, L.; Muzerolle, J.; Gutermuth, R.; Luhman, K. L.; Stauffer, J. E-mail: muzerol@stsci.ed

    2010-10-20

    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.

  3. Measurement of neutron star parameters: A review of methods for low-mass X-ray binaries

    NASA Astrophysics Data System (ADS)

    Bhattacharyya, Sudip

    2010-04-01

    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.

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

    NASA Technical Reports Server (NTRS)

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

    1995-01-01

    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.

  5. Confirmation of the Presence of Asteroid Collisional Debris in Pleiades Low Mass Stars using IRAC

    NASA Astrophysics Data System (ADS)

    Stauffer, John; Rebull, Luisa; Krick, Jessica

    2015-11-01

    We have identified four late type dwarfs in the Pleiades with unusual periodic, transit-like flux dips in their early-2015 K2 light curves. The dips are too long in duration to be due to solid bodies (lower mass stars or planets), but too short in duration to be due to star-spots on the stellar photosphere. Their most likely origin is in clouds of material orbiting the star at or near the star's corotation radius. We propose to obtain a single IRAC long-duration staring mode observation of one of these stars, in order to determine if the flux dips are also present at 4.5 microns (as they would be if the flux dips are due to dust extinction, for example). The AOR length is set to slightly longer than the measured period of the flux dips for the target star, HHJ 135.

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

    NASA Technical Reports Server (NTRS)

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

    2000-01-01

    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.

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

    NASA Astrophysics Data System (ADS)

    Matthews, Allison; Torres, Guillermo

    2015-01-01

    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.

  8. New BVI {sub C} photometry of low-mass pleiades stars: Exploring the effects of rotation on broadband colors

    SciTech Connect

    Kamai, Brittany L.; Stassun, Keivan G.; Vrba, Frederick J.; Stauffer, John R.

    2014-08-01

    We present new BVI{sub C} photometry for 350 Pleiades proper motion members with 9 < V ≲ 17. Importantly, our new catalog includes a large number of K- and early M-type stars, roughly doubling the number of low-mass stars with well-calibrated Johnson/Cousins photometry in this benchmark cluster. We combine our new photometry with existing photometry from the literature to define a purely empirical isochrone at Pleiades age (≈100 Myr) extending from V = 9 to 17. We use the empirical isochrone to identify 48 new probable binaries and 14 likely nonmembers. The photometrically identified single stars are compared against their expected positions in the color-magnitude diagram (CMD). At 100 Myr, the mid K and early M stars are predicted to lie above the zero-age main sequence (ZAMS) having not yet reached the ZAMS. We find in the B – V versus V CMD that mid K and early M dwarfs are instead displaced below (or blueward of) the ZAMS. Using the stars' previously reported rotation periods, we find a highly statistically significant correlation between rotation period and CMD displacement, in the sense that the more rapidly rotating stars have the largest displacements in the B – V CMD.

  9. A Molecular Disk Survey of Low-Mass Stars in the TW Hya Association

    NASA Astrophysics Data System (ADS)

    Rodriguez, David R.; van der Plas, Gerrit; Kastner, Joel H.; Schneider, Adam C.; Faherty, Jacqueline K.; Mardones, Diego; Mohanty, Subhanjoy; Principe, David

    2015-11-01

    We have carried out an ALMA Cycle 2 survey of 15 confirmed or candidate low-mass (<0.2M ?) members of the TW Hya Association (TWA) with the goal of detecting line emission from CO molecular gas and continuum emission from cold dust. Our targets have spectral types of M4-L0 and hence represent the extreme low end of the TWA's mass function. The survey has yielded a detection of 12CO(2-1) emission around TWA 34. This newly discovered ~10 Myr-old molecular gas disk lies just ~50pc from Earth.

  10. A Molecular Disk Survey of Low-Mass Stars in the TW Hya Association

    NASA Astrophysics Data System (ADS)

    Rodriguez, David R.; van der Plas, Gerrit; Kastner, Joel H.; Schneider, Adam C.; Faherty, Jacqueline K.; Mardones, Diego; Mohanty, Subhanjoy; Principe, David

    2016-01-01

    We have carried out an ALMA Cycle 2 survey of 15 confirmed or candidate low-mass (<0.2M ⊙) members of the TW Hya Association (TWA) with the goal of detecting line emission from CO molecular gas and continuum emission from cold dust. Our targets have spectral types of M4-L0 and hence represent the extreme low end of the TWA's mass function. The survey has yielded a detection of 12CO(2-1) emission around TWA 34. This newly discovered ~10 Myr-old molecular gas disk lies just ~50pc from Earth.

  11. Characterizing the small scale structures in the earliest stages of low-mass star formation

    NASA Astrophysics Data System (ADS)

    Vilhelm Persson, Magnus; van Dishoeck, Ewine; Tobin, John; Harsono, Daniel; Jrgensen, Jes K.

    2015-08-01

    In deeply-embedded low-mass protostars, the density and temperature distribution in the inner few hundred AUs are poorly constrained. In sources where the envelope is less massive, i.e. the Class I stage, disks with Keplerian rotation have been inferred using C18O lines. However, constraining the various disk characteristics turns out to be difficult even in this case. Continuum and molecular line observations of optically thin tracers at very high sensitivity and resolution are needed to constrain the density, temperature and kinematics. Ultimately the assumed structure affects the determination of molecular abundances.We are attempting to model high-resolution dust continuum radio-interferometric observations of a few deeply-embedded low-mass protostars with a power-law disk model embedded in a spherical envelope.We model the interferometric visibilities taken with either the Plateau de Bure Interferometer or the ALMA telescope, probing scales down to a few tens of AU in some cases. Given the assumptions, the study shows disk sizes in the deeply-embedded phase that could be slightly larger than typical found in the more evolved Class I sources. The fitting also highlights that models for the physical structure of the inner envelope, on 500-2000 AU scales, needs to be improved. With future high sensitivity observations, we could potentially also be able to constrain any vertical density and temperature structure. In this poster I will present the

  12. Small, Numerous and Close-in: The Population of Planets around Low-mass Stars.

    NASA Astrophysics Data System (ADS)

    Mulders, Gijs Dirk; Pascucci, Ilaria; apai, Daniel

    2015-08-01

    The Kepler Space Telescope has monitored stars from spectral type M to A for transiting exoplanets, covering a factor four in planet host star mass. We take advantage of this large coverage in stellar masses to establish what are the key processes in the formation and evolution of planetary systems.We derive planet occurrence rates for a range of orbital periods and planet sizes, taking into account the different observational biases that exist for stars of different mass, size, and luminosity. This uniform approach allows us to compare planet populations directly and identify scaling relations with stellar mass. We identify three trends:First, planets around lower mass stars are found closer to their hosts stars. The inner edges of the planet populations match the inner edges of the gas disks where planets halt their migration. Second, the size of the largest planets decreases with stellar mass, indicating formation in less massive disks. Third, the 3-4 times higher occurrence rate of small (1-3 earth radii) planets around M dwarfs with respect to sunlike stars indicates an increased planet migration efficiency and is inconsistent with in-situ formation models.Our findings demonstrate how exoplanet studies around stars of very different masses can pin down specific physical processes shaping the final architecture of planetary systems. We will conclude by exploring how the yield from the Kepler extended mission -- with a large number of M stars but a different detection bias -- can further our knowledge of planet formation and evolution.References:Mulders et al. 2015Mulders et al. in prep

  13. Constraining the properties of neutron star crusts with the transient low-mass X-ray binary Aql X-1

    NASA Astrophysics Data System (ADS)

    Waterhouse, A. C.; Degenaar, N.; Wijnands, R.; Brown, E. F.; Miller, J. M.; Altamirano, D.; Linares, M.

    2016-03-01

    Aql X-1 is a prolific transient neutron star low-mass X-ray binary that exhibits an accretion outburst approximately once every year. Whether the thermal X-rays detected in intervening quiescent episodes are the result of cooling of the neutron star or due to continued low-level accretion remains unclear. In this work, we use Swift data obtained after the long and bright 2011 and 2013 outbursts, as well as the short and faint 2015 outburst, to investigate the hypothesis that cooling of the accretion-heated neutron star crust dominates the quiescent thermal emission in Aql X-1. We demonstrate that the X-ray light curves and measured neutron star surface temperatures are consistent with the expectations of the crust cooling paradigm. By using a thermal evolution code, we find that ≃1.2-3.2 MeV nucleon-1 of shallow heat release describes the observational data well, depending on the assumed mass-accretion rate and temperature of the stellar core. We find no evidence for varying strengths of this shallow heating after different outbursts, but this could be due to limitations of the data. We argue that monitoring Aql X-1 for up to ≃1 yr after future outbursts can be a powerful tool to break model degeneracies and solve open questions about the magnitude, depth, and origin of shallow heating in neutron star crusts.

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

    NASA Technical Reports Server (NTRS)

    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

    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.

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

    NASA Astrophysics Data System (ADS)

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

    2014-11-01

    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.

  16. Constraining the low-mass Slope of the star formation sequence at 0.5 < z < 2.5

    SciTech Connect

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

    2014-11-10

    We constrain the slope of the star formation rate (SFR; log Ψ) to stellar mass (log M {sub *}) relation down to log (M {sub *}/M {sub ☉}) = 8.4 (log (M {sub *}/M {sub ☉}) = 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 Ψ∝log M {sub *}) than at high masses (log Ψ∝(0.3-0.6)log M {sub *}). 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 μm imaging; β-corrected UV SFRs; and Hα SFRs. The normalization of the sequence evolves differently in distinct mass regimes as well: for galaxies less massive than log (M {sub *}/M {sub ☉}) < 10 the specific SFR (Ψ/M {sub *}) is observed to be roughly self-similar with Ψ/M {sub *}∝(1 + z){sup 1.9}, whereas more massive galaxies show a stronger evolution with Ψ/M {sub *}∝(1 + z){sup 2.2-3.5} for log (M {sub *}/M {sub ☉}) = 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.

  17. The low-mass end of the fundamental relation for gravitationally lensed star-forming galaxies at 1 < z < 6

    NASA Astrophysics Data System (ADS)

    Christensen, Lise; Richard, Johan; Hjorth, Jens; Milvang-Jensen, Bo; Laursen, Peter; Limousin, Marceau; Dessauges-Zavadsky, Miroslava; Grillo, Claudio; Ebeling, Harald

    2012-12-01

    We present Very Large Telescope/X-shooter spectra of 13 galaxies in the redshift range 1 ? z ? 6, which are strongly lensed by massive galaxy clusters. Spectroscopic redshifts are measured for nine galaxies, while three sources have redshifts determined from continuum breaks in their spectra. The stellar masses of the galaxies span four orders of magnitude between 107 and 1011 M? and have luminosities at 1500 rest frame between 0.004 and 9L* after correcting for the magnification. This allows us to probe a variety of galaxy types from young, low-mass starburst galaxies to massive evolved galaxies. The lensed galaxies with stellar masses less than 1010 M? have a large scatter compared to the fundamental relation between stellar mass, star formation rates and oxygen abundances. We provide a modified fit to the fundamental relation for low-mass, low-metallicity galaxies with a weaker dependence of the metallicity on either the star formation rate or stellar mass compared to low-redshift, high-mass and high-metallicity Sloan Digital Sky Survey galaxies. Based on data from the X-shooter GTO observations collected at the European Southern Observatory VLT/Kuyuen telescope, Paranal, Chile, under programme IDs: 084.B-0351(D), 086.A-0674(A), 086.A-0674(B), 087.A-0432(A) and 087.A-0432(B). Based on HST general observer pro-grammes GO-10491, GO-11103 and GO-12166.

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

    SciTech Connect

    Biller, Beth; Benisty, Myriam; Chauvin, Gael; Olofsson, Johan; Pott, Joerg-Uwe; Mueller, Andre; Bonnefoy, Mickaeel; Henning, Thomas; Lacour, Sylvestre; Thebault, Philippe; Sicilia-Aguilar, Aurora; Tuthill, Peter; Crida, Aurelien

    2012-07-10

    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.

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

    SciTech Connect

    Rice, Emily L.; Mclean, Ian S.; Barman, T.; Prato, L.; Kirkpatrick, J. Davy

    2010-01-01

    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.

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

    NASA Astrophysics Data System (ADS)

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

    2012-09-01

    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 ~61050 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.

  1. H? photometry of low-mass stars in 47 Tucanae: chromospheric activity and exotica

    NASA Astrophysics Data System (ADS)

    Beccari, G.; De Marchi, G.; Panagia, N.; Pasquini, L.

    2014-01-01

    We have used archival Hubble Space Telescope observations obtained with the Advanced Camera for Surveys to study the H? emission properties of main-sequence stars in the globular cluster 47 Tucanae. Using a combination of multiband observations in the F606W, F814W and F658N bands, we search for stars showing H? excess emission. An accurate photometric measurement of their H? equivalent width allows us to identify objects with large H? emission, which we attribute to mass accretion rather than enhanced chromospheric activity. The spatial position of some of these stars is coincident with that of known X-ray sources, and their location in the colour-magnitude diagram allows us to classify them as active binaries or cataclysmic variables (CVs). We show that this method, commonly adopted to study accreting discs in young stellar objects, can be successfully used to identify and characterize candidate CVs.

  2. A theoretical study of acoustic glitches in low-mass main-sequence stars

    SciTech Connect

    Verma, Kuldeep; Antia, H. M.; Basu, Sarbani; Mazumdar, Anwesh E-mail: antia@tifr.res.in E-mail: anwesh@tifr.res.in

    2014-10-20

    There are regions in stars, such as ionization zones and the interface between radiative and convective regions, that cause a localized sharp variation in the sound speed. These are known as 'acoustic glitches'. Acoustic glitches leave their signatures on the oscillation frequencies of stars, and hence these signatures can be used as diagnostics of these regions. In particular, the signatures of these glitches can be used as diagnostics for the position of the second helium ionization zone and that of the base of the envelope convection zone. With the help of stellar models, we study the properties of these acoustic glitches in main-sequence stars. We find that the acoustic glitch due to the helium ionization zone does not correspond to the dip in the adiabatic index Γ{sub 1} caused by the ionization of He II, but to the peak in Γ{sub 1} between the He I and He II ionization zones. We find that it is easiest to study the acoustic glitch that is due to the helium ionization zone in stars with masses in the range 0.9-1.2 M {sub ☉}.

  3. Radiative feedback by low-mass stars in the first generation

    SciTech Connect

    Whalen, Daniel James; Hueckstaedt, Robert; Mcconkie, Thomas

    2009-01-01

    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.

  4. CONTINUED COOLING OF THE CRUST IN THE NEUTRON STAR LOW-MASS X-RAY BINARY KS 1731-260

    SciTech Connect

    Cackett, Edward M.; Miller, Jon M.; Brown, Edward F.; Cumming, Andrew; Degenaar, Nathalie; Wijnands, Rudy

    2010-10-20

    Some neutron star low-mass X-ray binaries have very long outbursts (lasting several years) which can generate a significant amount of heat in the neutron star crust. After the system has returned to quiescence, the crust then thermally relaxes. This provides a rare opportunity to study the thermal properties of neutron star crusts, putting constraints on the thermal conductivity and hence the structure and composition of the crust. KS 1731-260 is one of only four systems where this crustal cooling has been observed. Here, we present a new Chandra observation of this source approximately eight years after the end of the last outburst and four years since the last observation. We find that the source has continued to cool, with the cooling curve displaying a simple power-law decay. This suggests that the crust has not fully thermally relaxed yet and may continue to cool further. A simple power-law decay is in contrast to theoretical cooling models of the crust, which predict that the crust should now have cooled to the same temperature as the neutron star core.

  5. IUE observations of rapidly rotating low-mass stars in young clusters - The relation between chromospheric activity and rotation

    NASA Technical Reports Server (NTRS)

    Simon, Theodore

    1990-01-01

    If the rapid spindown of low-mass stars immediately following their arrival on the ZAMS results from magnetic braking by coronal winds, an equally sharp decline in their chromospheric emission may be expected. To search for evidence of this effect, the IUE spacecraft was used to observe the chromospheric Mg II emission lines of G-M dwarfs in the nearby IC 2391, Alpha Persei, Pleiades, and Hyades clusters. Similar observations were made of a group of X-ray-selected 'naked' T Tauri stars in Taurus-Auriga. The existence of a decline in activity cannot be confirmed from the resulting data. However, the strength of the chromospheric emission in the Mg II lines of the cluster stars is found to be correlated with rotation rate, being strongest for the stars with the shortest rotation periods and weakest for those with the longest periods. This provides indirect support for such an evolutionary change in activity. Chromospheric activity may thus be only an implicit function of age.

  6. Modelling the ionosphere of gas-giant exoplanets irradiated by low-mass stars

    NASA Astrophysics Data System (ADS)

    Chadney, J.; Galand, M.; Unruh, Y.; Koskinen, T.; Sanz-Forcada, J.

    2015-10-01

    The composition and structure of the upper atmosphere of Extrasolar Giant Planets (EGPs) are affected by the high-energy spectrum of the host star from soft X-rays to Extreme UltraViolet (EUV) (0.1-10 nm). This emission depends on the activity level of the star, which is primarily determined by its age [1]. In this study, we focus upon EGPs orbiting K- and M-dwarf stars of different ages. XUV spectra for these stars are constructed using a coronal model [2]. These spectra are used to drive both a thermospheric [3] and an ionospheric model, providing densities of neutral and ion species. Ionisation is included through photo-ionisation and electronimpact processes. The former is calculated by solving the Lambert-Beer law, while the latter is calculated from a supra-thermal electron transport model [4]. Planets orbiting far from the star are found to undergo Jeans escape, whereas close-orbiting planets undergo hydrodynamic escape. The critical orbital distance of transition between the two regimes is dependent on the level of stellar activity. We also find that EGP ionospheres at all orbital distances considered (0.1-1 AU) and around all stars selected (eps Eri, AD Leo, AU Mic) are dominated by the long-lived H+ ion. In addition, planets in the Jeans escape regime also have a layer in which H3 + is the major ion at the base of the ionosphere. For fast-rotating planets, densities of short-lived H3 + undergo significant diurnal variations, their peak value being determined by the stellar X-ray flux. In contrast, densities of longer-lived H+ show very little day/night variability and their value is determined by the level of stellar EUV flux. The H3 + peak in EGPs in the hydrodynamic escape regime under strong stellar illumination is pushed to altitudes below the homopause, where this ion is likely to be destroyed through reactions with heavy species (e.g., hydrocarbons, water). Infrared emissions from H3 + shall also be discussed, as well as the impact of stellar variability.

  7. From Stars to Superplanets: The Low-Mass Initial Mass Function in the Young Cluster IC 348

    NASA Astrophysics Data System (ADS)

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

    2000-10-01

    We investigate the low-mass population of the young cluster IC 348 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 narrowband imaging, that measure the strength of the 1.9 μm 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 ~0.7 to 0.015 Msolar. The mass function derived for the cluster in this interval, dN/dlogM~M0.5, is similar to that obtained for the Pleiades, but appears significantly more abundant in brown dwarfs than the mass function for companions to nearby Sunlike stars. This provides compelling observational evidence for different formation and evolutionary histories for substellar objects formed in isolation versus 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. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.

  8. The young low-mass star ISO-Oph-50: extreme variability induced by a clumpy, evolving circumstellar disc

    NASA Astrophysics Data System (ADS)

    Scholz, Alexander; Mui?, Koraljka; Geers, Vincent

    2015-07-01

    ISO-Oph-50 is a young low-mass object in the 1 Myr old Ophiuchus star-forming region undergoing dramatic changes in its optical/near/mid-infrared brightness by 2-4 mag. We present new multi-band photometry and near-infrared spectra, combined with a synopsis of the existing literature data. Based on the spectroscopy, the source is confirmed as a mid-M dwarf, with evidence for ongoing accretion. The near-infrared light curves show large-scale variations, with 2-4 mag amplitude in the bands IJHK, with the object generally being bluer when faint. Near its brightest state, the object shows colour changes consistent with variable extinction of ?AV 7 mag. High-cadence monitoring at 3.6 ?m reveals quasi-periodic variations with a typical time-scale of 1-2 weeks. The best explanation for these characteristics is a low-mass star seen through circumstellar matter, whose complex variability is caused by changing inhomogeneities in the inner parts of the disc. When faint, the direct stellar emission is blocked; the near-infrared radiation is dominated by scattered light. When bright, the emission is consistent with a photosphere strongly reddened by circumstellar dust. Based on the available constraints, the inhomogeneities have to be located at or beyond 0.1 au distance from the star. If this scenario turns out to be correct, a major portion of the inner disc has to be clumpy, structured, and/or in turmoil. In its observational characteristics, this object resembles other types of young stellar objects with variability caused in the inner disc. Compared to other objects, however, ISO-Oph-50 is clearly an extreme case, given the large amplitude of the brightness and colour changes combined with the erratic behaviour. ISO-Oph-50 has been near its brightest state since 2013; further monitoring is highly encouraged.

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

    SciTech Connect

    Cristallo, S.; Dominguez, I.; Abia, C.; Piersanti, L.; Straniero, O.; Gallino, R.; Di Rico, G.; Quintini, M.; Bisterzo, S.

    2011-12-01

    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.

  10. Ionization of the diffuse gas in galaxies: Hot low-mass evolved stars at work

    NASA Astrophysics Data System (ADS)

    Flores-Fajardo, N.; Morisset, C.; Stasinska, G.; Binette, L.

    2011-10-01

    The Diffuse Ionized Medium (DIG) is visible through its faint optical line emission outside classical HII regions (Reynolds 1971) and turns out to be a major component of the interstellar medium in galaxies. OB stars in galaxies likely represent the main source of ionizing photons for the DIG. However, an additional source is needed to explain the increase of [NII]/H?, [SII]/H? with galactic height.

  11. VizieR Online Data Catalog: Planets Around Low-Mass Stars (PALMS). IV. (Bowler+, 2015)

    NASA Astrophysics Data System (ADS)

    Bowler, B. P.; Liu, M. C.; Shkolnik, E. L.; Tamura, M.

    2015-02-01

    We carried out our observations at the Keck II 10m telescope with the facility near-infrared imaging camera NIRC2 using natural guide star adaptive optics (NGS-AO) between 2010 August and 2013 August (table 3). Our NGS-AO observations at the 8.2m Subaru Telescope were obtained with the AO188 adaptive optics system coupled with the High Contrast Instrument for the Subaru Next Generation Adaptive Optics (HiCIAO) imaging instrument. (2 data files).

  12. A super-Earth transiting a nearby low-mass star.

    PubMed

    Charbonneau, David; Berta, Zachory K; Irwin, Jonathan; Burke, Christopher J; Nutzman, Philip; Buchhave, Lars A; Lovis, Christophe; Bonfils, Xavier; Latham, David W; Udry, Stphane; Murray-Clay, Ruth A; Holman, Matthew J; Falco, Emilio E; Winn, Joshua N; Queloz, Didier; Pepe, Francesco; Mayor, Michel; Delfosse, Xavier; Forveille, Thierry

    2009-12-17

    A decade ago, the detection of the first transiting extrasolar planet provided a direct constraint on its composition and opened the door to spectroscopic investigations of extrasolar planetary atmospheres. Because such characterization studies are feasible only for transiting systems that are both nearby and for which the planet-to-star radius ratio is relatively large, nearby small stars have been surveyed intensively. Doppler studies and microlensing have uncovered a population of planets with minimum masses of 1.9-10 times the Earth's mass (M[symbol:see text]), called super-Earths. The first constraint on the bulk composition of this novel class of planets was afforded by CoRoT-7b (refs 8, 9), but the distance and size of its star preclude atmospheric studies in the foreseeable future. Here we report observations of the transiting planet GJ 1214b, which has a mass of 6.55M[symbol:see text]), and a radius 2.68 times Earth's radius (R[symbol:see text]), indicating that it is intermediate in stature between Earth and the ice giants of the Solar System. We find that the planetary mass and radius are consistent with a composition of primarily water enshrouded by a hydrogen-helium envelope that is only 0.05% of the mass of the planet. The atmosphere is probably escaping hydrodynamically, indicating that it has undergone significant evolution during its history. The star is small and only 13 parsecs away, so the planetary atmosphere is amenable to study with current observatories. PMID:20016595

  13. Millimeter continuum measurements of circumstellar dust around very young low-mass stars

    NASA Technical Reports Server (NTRS)

    Terebey, S.; Chandler, C. J.; Andre, P.

    1994-01-01

    We investigate the question of disk formation during the protostar phase. We build on the results of Keene and Masson (1990) whose analysis of L1551 showed the millimeter continuum emission comes from both an unresolved circumstellar component, i.e., a disk and an extended cloud core. We model the dust continuum emission from the cloud core and show how it is important at 1.3 mm but negligible at 2.7 mm. Combining new 2.7 mm Owens Valley Interferometer data of IRAS-Dense cores with data from the literature we conclude that massive disks are also seen toward a number of other sources. However, 1.3 mm data from the IRAM 30 m telescope for a larger sample shows that massive disks are relatively rare, occurring around perhaps 5% of young embedded stars. This implies that either massive disks occur briefly during the embedded phase or that relatively few young stars form massive disks. At 1.3 mm the median flux of IRAS-Dense cores is nearly the same as T Tauri stars in the sample of Beckwith et al. (1990). We conclude that the typical disk mass during the embedded phase is nearly the same or less than the typical disk mass during the T Tauri phase.

  14. Big Fish in Small Ponds: massive stars in the low-mass clusters of M83

    SciTech Connect

    Andrews, J. E.; Calzetti, D.; McElwee, Sean; Chandar, R.; Elmegreen, B. G.; Kennicutt, R. C.; Kim, Hwihyun; Krumholz, Mark R.; Lee, J. C.; Whitmore, B.; O'Connell, R. W. E-mail: callzetti@astro.umass.edu

    2014-09-20

    We have used multi-wavelength Hubble Space Telescope WFC3 data of the starbursting spiral galaxy M83 in order to measure variations in the upper end of the stellar initial mass function (uIMF) using the production rate of ionizing photons in unresolved clusters with ages ≤ 8 Myr. As in earlier papers on M51 and NGC 4214, the uIMF in M83 is consistent with a universal IMF, and stochastic sampling of the stellar populations in the ∼<10{sup 3} M {sub ☉} clusters are responsible for any deviations in this universality. The ensemble cluster population, as well as individual clusters, also imply that the most massive star in a cluster does not depend on the cluster mass. In fact, we have found that these small clusters seem to have an over-abundance of ionizing photons when compared to an expected universal or truncated IMF. This also suggests that the presence of massive stars in these clusters does not affect the star formation in a destructive way.

  15. Low-Mass Star Formation and the Initial Mass Function in the ? Ophiuchi Cloud Core

    NASA Astrophysics Data System (ADS)

    Luhman, K. L.; Rieke, G. H.

    1999-11-01

    We have obtained moderate-resolution (R=800-1200) K-band spectra for ~100 stars within and surrounding the cloud core of ? Oph. We have measured spectral types and continuum veilings and have combined this information with results from new deep imaging. Using the latest evolutionary tracks of D'Antona & Mazzitelli to interpret the H-R diagram for ? Oph, we infer ages ranging between 0.1 and 1 Myr for the class II and III sources (i.e., those that have emerged from their natal cocoons). A few stars may be slightly older. The initial mass function (IMF) peaks at about 0.4 Msolar and slowly declines to the hydrogen-burning limit with a slope of ~-0.5 in logarithmic units (Salpeter is +1.35). Our lower limits on the numbers of substellar objects demonstrate that the IMF probably does not fall more steeply below the hydrogen-burning limit, at least down to ~0.02 Msolar. The derived IMF is consistent with previous findings that the ? Oph IMF is roughly flat from 0.05 to 1 Msolar. The exact shape of the mass function remains a function of the theoretical evolutionary tracks and, at the lowest masses, the conversion from spectral types to effective temperatures. We then make the first comparison of mass functions of stars and prestellar clumps measured in the same region. The similar behavior of the two mass functions in ? Oph supports the suggestion of Motte et al. and Testi & Sargent that the stellar mass function in young clusters is a direct product of the process of cloud fragmentation. We have also studied the very young and often still embedded class I and flat-spectrum objects. After considering the effect of extinction on the SED classifications of the sample, we find that ~17% of the ? Oph stars are class I, implying ~0.1 Myr for the lifetime of this stage. In spectra separated by 2 yr, we observe simultaneous variability in the Br? emission and K-band continuum veiling for two stars, where the hydrogen emission is brighter in the more heavily veiled data. This behavior indicates that the disk may contribute significantly to continuous K-band emission, in contrast to the proposal that the infalling envelope always dominates. Our detection of strong 2 ?m veiling (rK=1-4) in several class II and III stars, which should have disks but little envelope material, further supports this proposition. We also detect absorption features in the spectra of ~25% of class I and flat-spectrum sources, demonstrating the feasibility of studying the photospheres of extremely young protostars. Observations reported in this paper were obtained with the Multiple Mirror Telescope, operated by the Smithsonian Astrophysical Observatory and the University of Arizona.

  16. Theoretical power spectra of mixed modes in low-mass red giant stars

    NASA Astrophysics Data System (ADS)

    Grosjean, M.; Dupret, M.-A.; Belkacem, K.; Montalban, J.; Samadi, R.; Mosser, B.

    2014-12-01

    Context. CoRoT and Kepler observations of red giant stars revealed very rich spectra of non-radial solar-like oscillations. Of particular interest was the detection of mixed modes that exhibit significant amplitude, both in the core and at the surface of the stars. It opens the possibility of probing the internal structure from their innermost layers up to their surface throughout their evolution on the red giant branch, as well as on the red clump. Aims: Our objective is primarily to provide physical insight into the mechanism responsible for mixed-mode amplitudes and lifetimes. Subsequently, we aim at understanding the evolution and structure of red-giant spectra along with their evolution. The study of energetic aspects of these oscillations is also important for predicting the mode parameters in the power spectrum. Methods: Non-adiabatic computations, including a time-dependent treatment of convection, are performed and provide the lifetimes of radial and non-radial mixed modes. We then combine these mode lifetimes and inertias with a stochastic excitation model that gives us their heights in the power spectra. Results: For stars representative of CoRoT and Kepler observations, we show under which circumstances mixed modes have heights comparable to radial ones. We stress the importance of the radiative damping in determining the height of mixed modes. Finally, we derive an estimate for the height ratio between a g-type and a p-type mode. This can thus be used as a first estimate of the detectability of mixed modes. Appendices are available in electronic form at http://www.aanda.org

  17. Three-dimensional modelling of proton ingestion episodes in low-mass stars

    NASA Astrophysics Data System (ADS)

    Heap, Stuart A.; Stancliffe, Richard J.; Lattanzio, John C.; Dearborn, David S. P.

    2012-09-01

    We have modelled a dual shell flash (DSF) in a low-metallicity 1.5Msolar AGB star using the 3-dimensional hydrodynamic program "Djehuty", observing how the evolution of these events compares to 1-dimensional models, which are hypothesised to be inaccurate due to the simplifications in the treatment of convective processes. In particular, the stability of the separated convective structure following hydrogen ignition is investigated. In both models constructed, the split convective zone structure was found to be unstable, with the velocities within the inner convective zone increasing until material breaks through the gap and recombines the two regions into a large single convective region.

  18. Nonequilibrium iron oxide formation in some low-mass post-asymptotic giant branch stars

    NASA Technical Reports Server (NTRS)

    Rietmeijer, Frans J. M.

    1992-01-01

    Using experimental evidence that under highly oxidizing conditions gamma-Fe2O3 (maghemite) and Fe3O4 display refractory behavior, it is proposed that very low C/O ratios, that could be unique to evolving AGB stars, induce nonequilibrium formation of ferromagnetic iron oxide grains along with chondritic dust. The oxides are preferentially fractionated from chondritic dust in the stellar magnetic field which could account for the observed extreme iron underabundance in their photosphere. A search for the 1-2.5-micron IR absorption feature, or for diagnostic magnetite and maghemite IR absorption features, could show the validity of the model proposed.

  19. H? photometry of low mass stars in 47 Tucanae: chromospheric activity and exotica

    NASA Astrophysics Data System (ADS)

    Beccari, Giacomo; De Marchi, Guido; Panagia, Nino; Pasquini, Luca

    2013-02-01

    We have used archival Hubble Space Telescope (HST) observations obtained with the Advanced Camera for Surveys (ACS) to study the H? emission properties of main sequence (MS) stars in the globular cluster 47 Tucanae. An accurate photometric measurement of their H? equivalent width, allows us to identify objects with large H? emission. We demonstrate that this method allows us to identify and characterise candidate cataclysmic variables (CVs) with high efficiency, reconciling observations with theoretical predictions on the number of dynamically formed CVs expected in a 47 Tucanae-like globular cluster.

  20. Free inertial modes in differentially rotating convective envelopes of low-mass stars : numerical exploration

    NASA Astrophysics Data System (ADS)

    Guenel, M.; Baruteau, C.; Mathis, S.; Rieutord, M.

    2015-12-01

    Tidally-excited inertial waves in stellar convective regions are a key mechanism for tidal dissipation in stars and therefore the evolution of close-in binary or planetary systems. As a first step, we explore here the impact of latitudinal differential rotation on the properties of free inertial modes and identify the different families of modes. We show that they differ from the case of solid-body rotation. Using an analytical approach as well as numerical calculations, we conclude that critical layers --- where the Doppler-shifted frequency vanishes --- could play a very important role for tidal dissipation.

  1. Angular momentum in stars - The Kraft curve revisited

    NASA Technical Reports Server (NTRS)

    Kawaler, Steven D.

    1987-01-01

    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.

  2. Accretion onto Planetary Mass Companions of Low-mass Young Stars

    NASA Astrophysics Data System (ADS)

    Zhou, Yifan; Herczeg, Gregory J.; Kraus, Adam L.; Metchev, Stanimir; Cruz, Kelle L.

    2014-03-01

    Measurements of accretion rates onto planetary mass objects may distinguish between different planet formation mechanisms, which predict different accretion histories. In this Letter, we use Hubble Space Telescope (HST)/WFC3 UVIS optical photometry to measure accretion rates onto three accreting objects, GSC 06214-00210 b, GQ Lup b, and DH Tau b, that are at the planet/brown dwarf boundary and are companions to solar mass stars. The excess optical emission in the excess accretion continuum yields mass accretion rates of 10-9-10-11 M ? yr-1 for these three objects. Their accretion rates are an order of magnitude higher than expected from the correlation between mass and accretion rates measured from the UV excess, which is applicable if these wide planetary mass companions formed by protostellar core fragmentation. The high accretion rates and large separation from the central star demonstrate the presence of massive disks around these objects. Models for the formation and evolution of wide planetary mass companions should account for their large accretion rates. High ratios of H? luminosity over accretion luminosity for objects with low accretion rates suggest that searches for H? emission may be an efficient way to find accreting planets.

  3. ACCRETION ONTO PLANETARY MASS COMPANIONS OF LOW-MASS YOUNG STARS

    SciTech Connect

    Zhou, Yifan; Herczeg, Gregory J.; Kraus, Adam L.; Metchev, Stanimir; Cruz, Kelle L. E-mail: zhouyifan1012@gmail.com

    2014-03-01

    Measurements of accretion rates onto planetary mass objects may distinguish between different planet formation mechanisms, which predict different accretion histories. In this Letter, we use Hubble Space Telescope (HST)/WFC3 UVIS optical photometry to measure accretion rates onto three accreting objects, GSC0621400210b, GQ Lup b, and DH Tau b, that are at the planet/brown dwarf boundary and are companions to solar mass stars. The excess optical emission in the excess accretion continuum yields mass accretion rates of 10{sup 9}-10{sup 11} M {sub ?}yr{sup 1} for these three objects. Their accretion rates are an order of magnitude higher than expected from the correlation between mass and accretion rates measured from the UV excess, which is applicable if these wide planetary mass companions formed by protostellar core fragmentation. The high accretion rates and large separation from the central star demonstrate the presence of massive disks around these objects. Models for the formation and evolution of wide planetary mass companions should account for their large accretion rates. High ratios of H? luminosity over accretion luminosity for objects with low accretion rates suggest that searches for H? emission may be an efficient way to find accreting planets.

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

    NASA Astrophysics Data System (ADS)

    Veyette, Mark; Muirhead, Philip; Mann, Andrew

    2015-01-01

    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).

  5. Observational signatures of neutron stars in low-mass X-ray binaries climbing a stability peak

    NASA Astrophysics Data System (ADS)

    Kantor, E. M.; Gusakov, M. E.; Chugunov, A. I.

    2016-01-01

    In the recent papers by Gusakov et al., a new scenario describing evolution of rapidly rotating neutron stars (NSs) in low-mass X-ray binaries was proposed. The scenario accounts for a resonant interaction of normal r-modes with superfluid inertial modes at some specific internal stellar temperatures (`resonance temperatures'). This interaction results in an enhanced damping of r-mode and appearance of the `stability peaks' in the temperature - spin frequency plane, which split the r-mode instability window in the vicinity of the resonance temperatures. The scenario suggests that the hot and rapidly rotating NSs spend most of their life climbing up these peaks and, in particular, are observed there at the moment. We analyse in detail possible observational signatures of this suggestion. In particular, we show that these objects may exhibit `anti-glitches' - sudden frequency jumps on a time-scale of hours-months.

  6. Runaway Heating by R-Modes of Neutron Stars in Low-Mass X-Ray Binaries

    NASA Astrophysics Data System (ADS)

    Levin, Yuri

    1999-05-01

    Recently Andersson et al. and Bildsten have independently suggested that an r-mode instability might be responsible for stalling the neutron star spin-up in strongly accreting low-mass X-ray binaries (LMXBs). We show that if this does occur, there are two possibilities for the resulting neutron star evolution. If the r-mode damping is a decreasing function of temperature, then the star undergoes a cyclic evolution: (1) accretional spin-up triggers the instability near the observed maximum spin rate; (2) the r-modes become highly excited through gravitational radiation reaction, and in a fraction of a year (0.13 yr in a particular model that we have considered) they viscously heat the star up to T~2.5109 K; (3) r-mode gravitational radiation reaction then spins the star down in tspindown~=0.08(ffinal/130 Hz)-6 yr to a limiting rotational frequency ffinal, whose exact value depends on the not fully understood mechanisms of r-mode damping; (4) the r-mode instability shuts off; and (5) the neutron star slowly cools and is spun up by accretion for ~5106 yr, until it once again reaches the instability point, closing the cycle. The shortness of the epoch of r-mode activity makes it unlikely that r-modes are currently excited in the neutron star of any galactic LMXBs, and unlikely that advanced LIGO interferometers will see gravitational waves from extragalactic LMXBs. Nevertheless, this cyclic evolution could be responsible for keeping the rotational frequencies within the observed LMXB frequency range. If, on the other hand, the r-mode damping is temperature independent, then a steady state with constant angular velocity and Tcore~=4108 K is reached, in which r-mode viscous heating is balanced by neutrino cooling and accretional spin-up torque is balanced by gravitational radiation reaction spin-down torque. In this case (as Bildsten and Andersson et. al. have shown) the neutron stars in LMXBs could be potential sources of periodic gravitational waves, detectable by enhanced LIGO interferometers.

  7. Infrared observations of low-mass star formation in Orion - HH objects

    NASA Technical Reports Server (NTRS)

    Harvey, P. M.; Wilking, B. A.; Cohen, M.

    1982-01-01

    The results of a preliminary analysis of IR data on Herbig-Haro objects in the Orion nebula are reported. The observations were made with the high angular resolution IR photometry equipment on the NASA Kuiper Airborne Observatory and the NASA facility on Mauna Kea, HI. Data were taken in the 1-200 microns region with 40, 6, and 8 arcsec resolution. Attention was focused on NGC 1999 (HH1-3) and M78 (HH24-25) and the determination of absolute luminosities of the exciting stars. Measurements were also made of the IR energy distribution in the thermally emitting dust clouds and the point sources. Herbig-Haro objects featured compact and far IR sizes and large visual extinction, in addition to a steeply rising energy distribution up to 50-100 microns, where the luminosity emitted was concentrated.

  8. The distribution of rotational velocities for low-mass stars in the Pleiades

    NASA Technical Reports Server (NTRS)

    Stauffer, John R.; Hartmann, Lee W.

    1987-01-01

    The available spectral type and color data for late-type Pleiades members have been reanalyzed, and new reddening estimates are obtained. New photometry for a small number of stars and a compilation of H-alpha equivalent widths for Pleiades dwarfs are presented. These data are used to examine the location of the rapid rotators in color-magnitude diagrams and the correlation between chromospheric activity and rotation. It is shown that the wide range of angular momenta exhibited by Pleiades K and M dwarfs is not necessarily produced by a combination of main-sequence spin-downs and a large age spread; it can also result from a plausible spread in initial angular momenta, coupled with initial main-sequence spin-down rates that are only weakly dependent on rotation. The new reddening estimates confirm Breger's (1985) finding of large extinctions confined to a small region in the southern portion of the Merope nebula.

  9. Calibrating Core Overshooting in Low-Mass Stars with Kepler Data

    NASA Astrophysics Data System (ADS)

    Deheuvels, S.

    2015-01-01

    The extent of the chemically mixed regions associated with convective cores remains uncertain due to our poor understanding of the interface between convective and radiative zones (e.g. overshooting). This generates large uncertainties on stellar ages. So far, several studies have shown that convective cores must extend beyond the Schwarzschild boundary. However, very few constraints exist on the size of this extension and its dependency with stellar parameters. We used 3 years of high-precision photometric data from the Kepler satellite to investigate how stars whose mass lies around the limit for having a convective core (M1.2 M_?) can contribute to the longstanding question of the size of convective cores. We constrained the amount of core overshooting in 14 targets and found a tendency of overshooting to increase with stellar mass in this mass range. These results will be presented in more details in a paper in preparation.

  10. An attempt to calibrate core overshooting using the seismic properties of low-mass stars

    NASA Astrophysics Data System (ADS)

    Deheuvels, S.; Silva Aguirre, V.; Cunha, M. S.; Appourchaux, T.; Ballot, J.; Brando, I.; Lebreton, Y.; Michel, E.

    2015-09-01

    The sizes of stellar convective cores remain uncertain because of our poor understanding of the interface between convective and radiative zones. The very high precision of the seismic data provided by the CoRoT and Kepler space missions offers a great opportunity to search for the signature of convective cores in main-sequence stars. We here validate the seismic diagnostic based on the r010 ratios, which has been proposed to probe the size of convective cores, and we use it on a sample of 24 specially chosen Kepler targets. We thus constrain the extension of the core in 14 targets and find a tendency of the core extension to increase with stellar mass in this mass range. These results will be presented in more detail in a paper in preparation.

  11. The Star Formation Histories of Local Group Dwarf Galaxies. III. Characterizing Quenching in Low-mass Galaxies

    NASA Astrophysics Data System (ADS)

    Weisz, Daniel R.; Dolphin, Andrew E.; Skillman, Evan D.; Holtzman, Jon; Gilbert, Karoline M.; Dalcanton, Julianne J.; Williams, Benjamin F.

    2015-05-01

    We explore the quenching of low-mass galaxies (104 ≲ {{M}\\star } ≲ 108 {{M}⊙ }) as a function of lookback time using the star formation histories (SFHs) of 38 Local Group dwarf galaxies. The SFHs were derived by analyzing color-magnitude diagrams of resolved stellar populations in archival Hubble Space Telescope/Wide Field Planetary Camera 2 imaging. We find: (1) lower-mass galaxies quench earlier than higher-mass galaxies; (2) inside of Rvirial there is no correlation between a satellite’s current proximity to a massive host and its quenching epoch; and (3) there are hints of systematic differences in the quenching times of M31 and Milky Way (MW) satellites, although the sample size and uncertainties in the SFHs of M31 dwarfs prohibit definitive conclusions. Combined with results from the literature, we qualitatively consider the redshift evolution (z = 0-1) of the quenched galaxy fraction over ˜7 dex in stellar mass (104 ≲ {{M}\\star } ≲ 1011.5 {{M}⊙ }). The quenched fraction of all galaxies generally increases toward the present, with both the lowest and highest-mass systems exhibiting the largest quenched fractions at all redshifts. In contrast, galaxies between {{M}\\star } ˜ 108-1010 {{M}⊙ } have the lowest quenched fractions. We suggest that such intermediate-mass galaxies are the least efficient at quenching. Finally, we compare our quenching times with predictions for infall times for low-mass galaxies associated with the MW. We find that some of the lowest-mass satellites (e.g., CVn II, Leo IV) may have been quenched before infall, while higher-mass satellites (e.g., Leo I, Fornax) typically quench ˜1-4 Gyr after infall. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA constract NAS 5-26555.

  12. Motion Verified Red Stars (MoVeRS): A Catalog of Proper Motion Selected Low-mass Stars from WISE, SDSS, and 2MASS

    NASA Astrophysics Data System (ADS)

    Theissen, Christopher A.; West, Andrew A.; Dhital, Saurav

    2016-02-01

    We present a photometric catalog of 8,735,004 proper motion selected low-mass stars (KML-spectral types) within the Sloan Digital Sky Survey (SDSS) footprint, from the combined SDSS Data Release 10 (DR10), Two Micron All-Sky Survey (2MASS) point-source catalog (PSC), and Wide-field Infrared Survey Explorer (WISE) AllWISE catalog. Stars were selected using r ‑ i, i ‑ z, r ‑ z, z ‑ J, and z ‑ W1 colors, and SDSS, WISE, and 2MASS astrometry was combined to compute proper motions. The resulting 3,518,150 stars were augmented with proper motions for 5,216,854 earlier type stars from the combined SDSS and United States Naval Observatory B1.0 catalog (USNO-B). We used SDSS+USNO-B proper motions to determine the best criteria for selecting a clean sample of stars. Only stars whose proper motions were greater than their 2σ uncertainty were included. Our Motion Verified Red Stars catalog is available through SDSS CasJobs and VizieR.

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

    NASA Technical Reports Server (NTRS)

    Cui, Wei

    2000-01-01

    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.

  14. Investigating the burstiness of the star formation history of low-mass galaxies at intermediate redshifts with KECK/DEIMOS spectroscopy and CANDELS imaging

    NASA Astrophysics Data System (ADS)

    Guo, Yicheng; Koo, David C.; Faber, Sandra M.; Rafelski, Marc

    2016-01-01

    The history of gas accretion, expulsion, and recycling, and star formation of low-mass galaxies (with stellar mass below 10^9 Msun) is thought to be stochastic and bursty. We combine the deep broad-band images of CANDELS and the high-resolution optical spectroscopy from Keck/DEIMOS surveys --- TKRS, DEEP2, DEEP3, and HALO7D --- to explore the star formation histories of low-mass galaxies at intermediate redshifts (0.5?z?1.0). We study (1) the stellar mass (M)--gas-phase metallicity (Z) relation (MZR) and its scatter and (2) the ratio of star formation rates (SFRs) measured through FUV to that through H? (FUV--H? ratio). Our MZR sample is 20 times larger than those in previous studies beyond the local universe. This huge gain in sample size enables superior constraints on the MZR and its scatter in the low-mass regime. We find that the scatter of the MZR increases as mass decreases. For the FUV--H? ratio, we find that it increases with the decrease of mass and SFR. Both results can be explained by low-mass galaxies having a star formation history with more bursts than massive galaxies having. A simple model shows that the star formation occuring in starburst phases in low-mass galaxies is 5x higher than that in a constant star formation phase, while, for massive galaxies, the bursty phases of star formation is negligible. Finally, we find that our median FUV--H? ratio for low-mass galaxies is higher than that of local galaxies of the same mass, implying a redshift evolution.

  15. The Relationship Between Molecular Gas and Star Formation in Low-mass E/S0 Galaxies

    NASA Astrophysics Data System (ADS)

    Wei, Lisa H.; Vogel, Stuart N.; Kannappan, Sheila J.; Baker, Andrew J.; Stark, David V.; Laine, Seppo

    2010-12-01

    We consider the relationship between molecular gas and star formation surface densities in 19 morphologically defined E/S0s with stellar mass lsim4 × 1010 M sun, paying particular attention to those found on the blue sequence in color versus stellar mass parameter space, where spiral galaxies typically reside. While some blue-sequence E/S0s must be young major-merger remnants, many low-mass blue-sequence E/S0s appear much less disturbed and may be experiencing the milder starbursts associated with inner-disk building as spirals (re)grow. For a sample of eight E/S0s (four blue, two mid, and two red sequence) whose CARMA CO(1-0), Spitzer MIPS 24 μm, and GALEX FUV emission distributions are spatially resolved on a 750 pc scale, we find roughly linear relationships between molecular gas and star formation surface densities within all galaxies, with power-law indices N = 0.6-1.9 (median 1.2). Adding 11 more blue-sequence E/S0s whose CO(1-0) emission is not as well resolved, we find that most of our E/S0s have global 1-8 kpc aperture-averaged molecular gas surface densities overlapping the range spanned by the disks and centers of spiral galaxies. While many of our E/S0s fall on the same Schmidt-Kennicutt relation as local spirals, ~80% (predominantly on the blue sequence) are offset toward apparently higher molecular gas star formation efficiency (i.e., shorter molecular gas depletion time). Possible interpretations of the elevated efficiencies include bursty star formation similar to that in local dwarf galaxies, H2 depletion in advanced starbursts, or simply a failure of the CO(1-0) emission to trace all of the molecular gas.

  16. Magnetic inhibition of convection and the fundamental properties of low-mass stars. II. Fully convective main-sequence stars

    SciTech Connect

    Feiden, Gregory A.; Chaboyer, Brian E-mail: brian.chaboyer@dartmouth.edu

    2014-07-01

    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.

  17. The Lyman continuum escape fraction of low mass star-forming galaxies at z~1.

    NASA Astrophysics Data System (ADS)

    Rutkowski, Michael J.; Scarlata, Claudia; Haardt, Francesco; Siana, Brian D.; Rafelski, Marc; Henry, Alaina L.; Hayes, Matthew; Salvato, Mara; Pahl, Anthony; Mehta, Vihang; Beck, Melanie; Malkan, Matthew Arnold; Teplitz, Harry I.

    2016-01-01

    Star-forming galaxies (SFGs) in the high redshift universe (z>6) are believed to ionize neutral hydrogen in the intergalactic medium during the epoch of reionization. We tested this assumption by studying likely analogs of these SFGs in archival HST grism spectroscopy with GALEX UV and ground-based optical images at the redshift range in which we can directly measure the rest-frame Lyman continuum (?<912, LyC) emission. We selected ~1400 SFGs for study on the presence of strong H? emission and strongly selected against those SFGs whose GALEX FUV photometry could be contaminated by low redshift interlopers along the line of sight to produce a sample of ~600 z~1 SFGs. We made no unambiguous detection of escaping Lyman continuum radiation in individual galaxies in this sample, and stacked the individual non-detections in order to constrain the absolute Lyman continuum escape fraction, fesc<2% (3?). We sub-divided this sample and stacked SFGs to measure upper limits to fesc with respect to stellar mass,luminosity and relative orientation. For z~1 high H? equivalent width (EW>200) SFGs, we found for the first time an upper limit to fesc<9%. We discuss the implications of these limits for the ionizing emissivity of high redshift SFGs during the epoch of reionization. We conclude that reionization by SFGs is only marginally consistent with independent Planck observations of the CMB electron scattering opacity unless the LyC escape fraction of SFGs increases with redshift and an unobserved population of faint (MUV<-13 AB) SFGs contributes significantly to the UV background.

  18. The Pisa Stellar Evolution Data Base for low-mass stars

    NASA Astrophysics Data System (ADS)

    Dell'Omodarme, M.; Valle, G.; Degl'Innocenti, S.; Prada Moroni, P. G.

    2012-04-01

    Context. The last decade showed an impressive observational effort from the photometric and spectroscopic point of view for ancient stellar clusters in our Galaxy and beyond, leading to important and sometimes surprising results. Aims: The theoretical interpretation of these new observational results requires updated evolutionary models and isochrones spanning a wide range of chemical composition so that the possibility of multipopulations inside a stellar cluster is also taken also into account. Methods: With this aim we built the new "Pisa Stellar Evolution Database" of stellar models and isochrones by adopting a well-tested evolutionary code (FRANEC) implemented with updated physical and chemical inputs. In particular, our code adopts realistic atmosphere models and an updated equation of state, nuclear reaction rates and opacities calculated with recent solar elements mixture. Results: A total of 32 646 models have been computed in the range of initial masses 0.30 ÷ 1.10 M⊙ for a grid of 216 chemical compositions with the fractional metal abundance in mass, Z, ranging from 0.0001 to 0.01, and the original helium content, Y, from 0.25 to 0.42. Models were computed for both solar-scaled and α-enhanced abundances with different external convection efficiencies. Correspondingly, 9720 isochrones were computed in the age range 8 ÷ 15 Gyr, in time steps of 0.5 Gyr. The whole database is available to the scientific community on the web. Models and isochrones were compared with recent calculations available in the literature and with the color-magnitude diagram of selected Galactic globular clusters. The dependence of relevant evolutionary quantities, namely turn-off and horizontal branch luminosities, on the chemical composition and convection efficiency were analyzed in a quantitative statistical way and analytical formulations were made available for reader's convenience. These relations can be useful in several fields of stellar evolution, e.g. evolutionary properties of binary systems, synthetic models for simple stellar populations and for star counts in galaxies, and chemical evolution models of galaxies. Stellar tracks and isochrones are 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/540/A26

  19. 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

    Henderson, Calen B.; Stassun, Keivan G.

    2012-03-01

    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.

  20. Determining The Physical Properties Of Very-Low-Mass Stars And Brown Dwarfs In The Near-infrared

    NASA Astrophysics Data System (ADS)

    Rice, Emily L.

    2009-01-01

    Accurate measurements of the fundamental physical properties of very-low-mass stars and brown dwarfs are crucial for understanding cool, complex atmospheres and calibrating evolutionary models. By studying a large sample of objects bright enough for high-resolution spectroscopy, we developed methods for determining physical properties as accurately and efficiently as possible. As part of the NIRSPEC Brown Dwarf Spectroscopic Survey (BDSS; McLean et al. 2003, 2007), we are conducting a detailed comparison of observed and synthetic spectra for a sample of young M and L dwarfs and field M, L, and T dwarfs, totaling 50 objects. High-resolution near-infrared spectra from the cross-dispersed echelle spectrometer NIRSPEC on Keck II provide an unequaled combination of resolving power and wavelength coverage. Synthetic spectra were created from PHOENIX atmosphere models calculated exclusively for this project with updated line lists and solar abundances. Combined with spectral types from photometry and low-resolution spectra and surface gravity predicted by age estimates, we anticipate the precision of measurements from the high-resolution spectra to be 75 K in effective temperature, 0.2 dex in surface gravity, 1 km/s in radial velocity, and 2 km/s in projected rotational velocity. Our results also distinguish between wavelength regimes for which the models reproduce observed high-resolution spectra and regimes in which model data, such as line lists and oscillator strengths, are lacking. This work combines a rich observational data set and sophisticated atmosphere models to advance our understanding of the correspondence between spectral features of low-mass objects and the physical properties of their cool atmospheres.

  1. THE BROWN DWARF KINEMATICS PROJECT. II. DETAILS ON NINE WIDE COMMON PROPER MOTION VERY LOW MASS COMPANIONS TO NEARBY STARS ,

    SciTech Connect

    Faherty, Jacqueline K.; Shara, Michael M.; Burgasser, Adam J.; West, Andrew A.; Bochanski, John J.; Cruz, Kelle L.; Walter, Frederick M.

    2010-01-15

    We report on nine wide common proper motion systems containing late-type M, L, or T companions. We confirm six previously reported companions, and identify three new systems. The ages of these systems are determined using diagnostics for both stellar primaries and low-mass secondaries and masses for the secondaries are inferred using evolutionary models. Of our three new discoveries, the M3+T6.5 pair G 204-39 and SDSS J1758+4633 has an age constrained to 0.5-1.5 Gyr making the secondary a potentially useful brown dwarf benchmark. The G5+L4 pair G 200-28 and SDSS J1416+5006 has a projected separation of {approx}25,000 AU making it one of the widest and lowest binding energy systems known to date. The system containing NLTT 2274 and SDSS J0041+1341 is an older M4+L0 (>4.5 Gyr) pair which shows H{alpha} activity in the secondary but not the primary making it a useful tracer of age/mass/activity trends. Two of the nine systems have discrepant component ages that emerge from stellar or ultracool diagnostics indicating possible shortcomings in our understanding of the age diagnostics of stars and brown dwarfs. We find a resolved binary frequency for widely separated (>100 AU) low-mass companions (i.e., at least a triple system) which is at least twice the frequency found for the field ultracool dwarf population. The ratio of triples to binaries and quadruples to binaries is also high for this sample: 3:5 and 1:4, respectively, compared to 8 pc sample values of 1:4 and 1:26. The additional components in these wide companion systems indicates a formation mechanism that requires a third or fourth component to maintain gravitational stability or facilitate the exchange of angular momentum. The binding energies for the nine multiples discussed in this text are among the lowest known for wide low-mass systems, suggesting that weakly bound, low-to-intermediate mass (0.2 M {sub sun} < M {sub tot}< 1.0 M {sub sun}) multiples can form and survive to exist in the field (1-8 Gyr)

  2. 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

    NASA Astrophysics Data System (ADS)

    Wisniewski, John P.; Ge, Jian; Crepp, Justin R.; De Lee, Nathan; Eastman, Jason; Esposito, Massimiliano; Fleming, Scott W.; Gaudi, B. Scott; Ghezzi, Luan; Gonzalez Hernandez, Jonay I.; Lee, Brian L.; Stassun, Keivan G.; Agol, Eric; Allende Prieto, Carlos; Barnes, Rory; Bizyaev, Dmitry; Cargile, Phillip; Chang, Liang; Da Costa, Luiz N.; Porto De Mello, G. F.; Femena, Bruno; Ferreira, Leticia D.; Gary, Bruce; Hebb, Leslie; Holtzman, Jon; Liu, Jian; Ma, Bo; Mack, Claude E.; Mahadevan, Suvrath; Maia, Marcio A. G.; Nguyen, Duy Cuong; Ogando, Ricardo L. C.; Oravetz, Daniel J.; Paegert, Martin; Pan, Kaike; Pepper, Joshua; Rebolo, Rafael; Santiago, Basilio; Schneider, Donald P.; Shelden, Alaina C.; Simmons, Audrey; Tofflemire, Benjamin M.; Wan, Xiaoke; Wang, Ji; Zhao, Bo

    2012-05-01

    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 eff <~ 6000 K) primary stars. Our analysis of TYC 4110-01037-1 reveals it to be a moderately aged (lsim5 Gyr) solar-like star having a mass of 1.07 0.08 M ? and radius of 0.99 0.18 R ?. 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 ~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-1. We determine the minimum companion mass (if sin i = 1) to be 97.7 5.8 M 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 eff <~ 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.

  3. The Brown Dwarf Eclipsing Binary 2M0535-05: A Case Study for Activity Effects on Physical Properties of Low-Mass Stars and Brown Dwarfs

    NASA Astrophysics Data System (ADS)

    Stassun, K. G.

    2013-02-01

    2M0535-05 is a one-of-a-kind eclipsing binary (EB) comprising two brown dwarfs (BDs), and is an important benchmark for understanding the fundamental properties of BDs and low-mass stars. Because 2M0535-05 presents a peculiar reversal of temperatures with mass (the higher mass, magnetically active BD in the system is cooler than the lower mass companion BD), 2M0535-05 is particularly important as a case study for the effects of magnetic activity on the properties of low-mass objects. Using a large number of low-mass M-dwarfs and EBs in the field, we have developed empirical relations for determining the amount by which the temperatures and radii-and therefore the estimated masses-of low-mass stars and BDs are altered due to chromospheric activity. The relations link the amount by which an active object's temperature is suppressed, and its radius inflated, to the strength of its Hα emission. These relations are found to approximately preserve bolometric luminosity. Applying these relations to 2M0535-05 brings the activity-corrected radii and temperatures of 2M0535-05 into precise agreement with theoretical isochrones for inactive stars. The relations that we present are applicable to BDs and low-mass stars with masses below 0.8 M⊙ and for which the activity, as measured by Hα, is in the range - 4.6 < log LHα/Lbol < -3.3. We discuss implications of this work for determinations of young cluster IMFs, and discuss competing ideas for the physical mechanism by which magnetic fields alter the temperatures and radii of low-mass stars.

  4. Are the Red Halos of Galaxies Made of Low-Mass Stars? Constraints from Subdwarf Star Counts in the Milky Way Halo

    NASA Astrophysics Data System (ADS)

    Zackrisson, Erik; Flynn, Chris

    2008-11-01

    Surface photometry detections of red and exceedingly faint halos around galaxies have resurrected the old question of whether some nonnegligible fraction of the missing baryons of the universe could be hiding in the form of faint, hydrogen-burning stars. The optical/near-infrared colors of these red halos have proved very difficult to reconcile with any normal type of stellar population, but can, in principle, be explained by advocating a bottom-heavy stellar initial mass function. This implies a high stellar mass-to-light ratio and hence a substantial baryonic mass locked up in such halos. Here we explore the constraints imposed by current observations of ordinary stellar halo subdwarfs on a putative red halo of low-mass stars around the Milky Way. Assuming structural parameters similar to those of the red halo recently detected in stacked images of external disk galaxies, we find that a smooth halo component with a bottom-heavy initial mass function is completely ruled out by current star count data for the Milky Way. All viable smooth red halo models with a density slope even remotely similar to that of the stacked halo moreover contain far too little mass to have any bearing on the missing-baryon problem. However, we note that these constraints can be sidestepped if the red halo stars are locked up in star clusters, and we discuss potential observations of other nearby galaxies that may be able to put such scenarios to the test.

  5. A Widely Separated, Highly Occluded Companion to the Nearby Low-mass T Tauri Star TWA 30

    NASA Astrophysics Data System (ADS)

    Looper, Dagny L.; Bochanski, John J.; Burgasser, Adam J.; Mohanty, Subhanjoy; Mamajek, Eric E.; Faherty, Jacqueline K.; West, Andrew A.; Pitts, Mark A.

    2010-11-01

    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 (RVs), as well as a chance alignment probability of only 0.08%. Like TWA 30A, the spectrum of TWA 30B shows signatures of an actively accreting disk (H I and alkali line emission) and forbidden emission lines tracing outflowing material ([O I], [O II], [O III], [S II], and [N II]). We have also detected [C I] emission in the optical data, marking the first such detection of this line in a pre-main-sequence star. Negligible RV shifts in the emission lines relative to the stellar frame of rest (?V <~ 30 km s-1) indicate that the outflows are viewed in the plane of the sky and that the corresponding circumstellar disk is viewed edge-on. Indeed, TWA 30B appears to be heavily obscured by its disk, given that it is 5 mag fainter than TWA 30A at K band despite having a slightly earlier spectral type (M4 versus M5). The near-infrared spectrum of TWA 30B also evinces an excess that varies on day timescales, with colors that follow classical T Tauri tracks as opposed to variable reddening (as is the case for TWA 30A). Multi-epoch data show this excess to be well modeled by a black body component with temperatures ranging from 630 to 880 K and emitting areas that scale inversely with the temperature. The variable excess may arise from disk structure such as a rim or a warp at the inner disk edge located at a radial distance of ~3-5 R sun. As the second and third closest actively accreting and outflowing stars to the Sun (after TWA 3), TWA 30AB presents an ideal system for a detailed study of star and planetary formation processes at the low-mass end of the hydrogen-burning spectrum. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

  6. Sgr A* and Its Environment: Low-mass Star Formation, the Origin of X-Ray Gas and Collimated Outflow

    NASA Astrophysics Data System (ADS)

    Yusef-Zadeh, F.; Wardle, M.; Schödel, R.; Roberts, D. A.; Cotton, W.; Bushouse, H.; Arendt, R.; Royster, M.

    2016-03-01

    We present high-resolution multiwavelength radio continuum images of the region within 150″ of Sgr A*, revealing a number of new extended features and stellar sources in this region. First, we detect a continuous 2″ east–west ridge of radio emission, linking Sgr A* and a cluster of stars associated with IRS 13 N and IRS 13E. The ridge suggests that an outflow of east–west blob-like structures is emerging from Sgr A*. In particular, we find arc-like radio structures within the ridge with morphologies suggestive of photoevaporative protoplanetary disks. We use infrared Ks and L‧ fluxes to show that the emission has similar characteristics to those of a protoplanetary disk irradiated by the intense radiation field at the Galactic center. This suggests that star formation has taken place within the S-cluster 2″ from Sgr A*. We suggest that the diffuse X-ray emission associated with Sgr A* is due to an expanding hot wind produced by the mass loss from B-type main sequence stars, and/or the disks of photoevaporation of low mass young stellar objects (YSOs) at a rate of ∼10‑6 {M}ȯ yr‑1. The proposed model naturally reduces the inferred accretion rate and is an alternative to the inflow–outflow style models to explain the underluminous nature of Sgr A*. Second, on a scale of 5″ from Sgr A*, we detect new cometary radio and infrared sources at a position angle PA ∼ 50° which is similar to that of two other cometary sources X3 and X7, all of which face Sgr A*. In addition, we detect a striking tower of radio emission at a PA ∼ 50°–60° along the major axis of the Sgr A East supernova remnant shell on a scale of 150″ from Sgr A*. We suggest that the cometary sources and the tower feature are tracing interaction sites of a mildly relativistic jet from Sgr A* with the atmosphere of stars and the nonthermal Sgr A East shell at a PA ∼ 50°–60° with \\dot{M}∼ 1× {10}-7 {M}ȯ {{yr}}-1, and opening angle 10°. Lastly, we suggest that the east–west ridge of radio emission traces an outflow that is potentially associated with past flaring activity from Sgr A*. The position angle of the outflow driven by flaring activity is close to ‑90°.

  7. Common Patterns in the Evolution between the Luminous Neutron Star Low-Mass X-ray Binary Subclasses

    NASA Astrophysics Data System (ADS)

    Fridriksson, Joel K.; Homan, Jeroen; Remillard, Ronald A.

    2015-08-01

    The X-ray transient XTE J1701-462 was the first source observed to evolve through all known subclasses of low-magnetic-field neutron star low-mass X-ray binaries (NS-LMXBs), as a result of large changes in its mass accretion rate. To investigate to what extent similar evolution is seen in other NS-LMXBs we have performed a detailed study of the color-color and hardness-intensity diagrams (CDs and HIDs) of Cyg X-2, Cir X-1, and GX 13+1—three luminous X-ray binaries, containing weakly magnetized neutron stars, known to exhibit strong secular changes in their CD/HID tracks. Using the full set of Rossi X-ray Timing Explorer Proportional Counter Array data collected for the sources over the 16 year duration of the mission, we show that Cyg X-2 and Cir X-1 display CD/HID evolution with close similarities to XTE J1701-462. Although GX 13+1 shows behavior that is in some ways unique, it also exhibits similarities to XTE J1701-462, and we conclude that its overall CD/HID properties strongly indicate that it should be classified as a Z source, rather than as an atoll source. We conjecture that the secular evolution of Cyg X-2, Cir X-1, and GX 13+1—illustrated by sequences of CD/HID tracks we construct—arises from changes in the mass accretion rate. Our results strengthen previous suggestions that within single sources Cyg-like Z source behavior takes place at higher luminosities and mass accretion rates than Sco-like Z behavior, and lend support to the notion that the mass accretion rate is the primary physical parameter distinguishing the various NS-LMXB subclasses.

  8. Upper Bounds on r-mode Amplitudes from Observations of Low-mass X-Ray Binary Neutron Stars

    NASA Astrophysics Data System (ADS)

    Mahmoodifar, Simin; Strohmayer, Tod

    2013-08-01

    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 ⊙ we find dimensionless r-mode amplitudes in the range from about 1 × 10-8 to 1.5 × 10-6. For the accreting millisecond X-ray pulsar sources with known quiescent spin-down rates, these limits suggest that <~ 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 ⊙) 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.

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

    NASA Technical Reports Server (NTRS)

    Mahmoodifar, Simin; Strohmayer, Tod

    2013-01-01

    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.

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

    SciTech Connect

    Mahmoodifar, Simin; Strohmayer, Tod

    2013-08-20

    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.

  11. A COMPARISON OF BROAD IRON EMISSION LINES IN ARCHIVAL DATA OF NEUTRON STAR LOW-MASS X-RAY BINARIES

    SciTech Connect

    Cackett, Edward M.; Miller, Jon M.; Reis, Rubens C.; Fabian, Andrew C.; Barret, Didier

    2012-08-10

    Relativistic X-ray disklines have been found in multiple neutron star low-mass X-ray binaries, in close analogy with black holes across the mass scale. These lines have tremendous diagnostic power and have been used to constrain stellar radii and magnetic fields, often finding values that are consistent with independent timing techniques. Here, we compare CCD-based data from Suzaku with Fe K line profiles from archival data taken with gas-based spectrometers. In general, we find good consistency between the gas-based line profiles from EXOSAT, BeppoSAX, and RXTE and the CCD data from Suzaku, demonstrating that the broad profiles seen are intrinsic to the line and not broad due to instrumental issues. However, we do find that when fitting with a Gaussian line profile, the width of the Gaussian can depend on the continuum model in instruments with low spectral resolution, though when the different models fit equally well the line widths generally agree. We also demonstrate that three BeppoSAX observations show evidence for asymmetric lines, with a relativistic diskline model providing a significantly better fit than a Gaussian. We test this by using the posterior predictive p-value method, and bootstrapping of the spectra to show that such deviations from a Gaussian are unlikely to be observed by chance.

  12. The magnetohydrodynamical model of kilohertz quasi-periodic oscillations in neutron star low-mass X-ray binaries (II)

    SciTech Connect

    Shi, Chang-Sheng; Zhang, Shuang-Nan; Li, Xiang-Dong

    2014-08-10

    We study the kilohertz quasi-periodic oscillations (kHz QPOs) in neutron star low-mass X-ray binaries (LMXBs) with a new magnetohydrodynamics (MHD) model, in which the compressed magnetosphere is considered. The previous MHD model is reexamined and the relation between the frequencies of the kHz QPOs and the accretion rate in LMXBs is obtained. Our result agrees with the observations of six sources (4U 0614+09, 4U 1636-53, 4U 1608-52, 4U 1915-15, 4U 1728-34, and XTE 1807-294) with measured spins. In this model, the kHz QPOs originate from the MHD waves in the compressed magnetosphere. The single kHz QPOs and twin kHz QPOs are produced in two different parts of the accretion disk and the boundary is close to the corotation radius. The lower QPO frequency in a frequency-accretion rate diagram is cut off at a low accretion rate and the twin kHz QPOs encounter a top ceiling at a high accretion rate due to the restriction of the innermost stable circular orbit.

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

    SciTech Connect

    Visscher, Channon; Lodders, Katharina; Fegley, Bruce E-mail: lodders@wustl.ed

    2010-06-20

    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.

  14. Determining the Physical Properties of Very-Low-Mass Stars and Brown Dwarfs in the Near-Infrared

    NASA Astrophysics Data System (ADS)

    Rice, Emily L.; Barman, Travis S.; McLean, Ian S.; Prato, L.; Kirkpatrick, J. Davy

    2009-02-01

    Accurate measurements of the fundamental physical properties of very-low-mass stars and brown dwarfs are crucial for calibrating evolutionary models. Photometry and low-resolution spectroscopy effectively average over absorption features that sample different layers in complex cool atmospheres. By studying a large sample of objects bright enough for high-resolution spectroscopy, we can develop methods for determining physical properties as accurately and efficiently as possible. As part of the Brown Dwarf Spectroscopic Survey (BDSS [1, 2]), we are conducting a detailed comparison of observed and synthetic spectra for a sample of young M and L dwarfs and field M, L, and T dwarfs (~50 objects in total). High-resolution near-infrared spectra from NIRSPEC on Keck II provide an unequaled combination of resolving power and wavelength coverage. Synthetic spectra were created from PHOENIX atmosphere models calculated exclusively for this project with updated line lists and solar abundances. Combined with spectral types from photometric studies and low-resolution spectra and surface gravity estimates from age determination, the high-resolution spectra enable precise measurements of effective temperature and surface gravity, as well as accurate determination of radial velocity and projected rotational velocity. Our preliminary observation-model comparisons distinguish between wavelength regimes for which the models reproduce observed high-resolution spectra and regimes in which model data (line lists, oscillator strengths, etc.) are lacking.

  15. The Dynamical Evolution of Low-mass Hydrogen-burning Stars, Brown Dwarfs, and Planetary-mass Objects Formed through Disk Fragmentation

    NASA Astrophysics Data System (ADS)

    Li, Yun; Kouwenhoven, M. B. N.; Stamatellos, D.; Goodwin, S. P.

    2015-06-01

    Theory and simulations suggest that it is possible to form low-mass hydrogen-burning stars, brown dwarfs (BDs), and planetary-mass objects (PMOs) via disk fragmentation. As disk fragmentation results in the formation of several bodies at comparable distances to the host star, their orbits are generally unstable. Here, we study the dynamical evolution of these objects. We set up the initial conditions based on the outcomes of the smoothed-particle hydrodynamics simulations of Stamatellos & Whitworth, and for comparison we also study the evolution of systems resulting from lower-mass fragmenting disks. We refer to these two sets of simulations as set 1 and set 2, respectively. At 10 Myr, approximately half of the host stars have one companion left, and approximately 22% (set 1) to 9.8% (set 2) of the host stars are single. Systems with multiple secondaries in relatively stable configurations are common (about 30% and 44%, respectively). The majority of the companions are ejected within 1 Myr with velocities mostly below 5 km s-1, with some runaway escapers with velocities over 30 km s-1. Roughly 6% (set 1) and 2% (set 2) of the companions pair up into very low-mass binary systems, resulting in respective binary fractions of 3.2% and 1.2%. The majority of these pairs escape as very low-mass binaries, while others remain bound to the host star in hierarchical configurations (often with retrograde inner orbits). Physical collisions with the host star (0.43 and 0.18 events per host star for set 1 and set 2, respectively) and between companions (0.08 and 0.04 events per host star for set 1 and set 2, respectively) are relatively common and their frequency increases with increasing disk mass. Our study predicts observable properties of very low-mass binaries, low-mass hierarchical systems, the BD desert, and free-floating BDs and PMOs in and near young stellar groupings, which can be used to distinguish between different formation scenarios of very low-mass stars, BDs, and PMOs.

  16. Statistical Properties of Galactic ? Scuti Stars: Revisited

    NASA Astrophysics Data System (ADS)

    Chang, S.-W.; Protopapas, P.; Kim, D.-W.; Byun, Y.-I.

    2013-05-01

    We present statistical characteristics of 1578 ? 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 Rodrguez 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 ? 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 Rodrguez's work. All the ? 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 ? 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.

  17. A search for Wolf-Rayet stars in active star forming regions of low mass galaxies - GR8, NGC 2366, IC 2574, and NGC 1569

    NASA Astrophysics Data System (ADS)

    Drissen, Laurent; Roy, Jean-Rene; Moffat, Anthony F. J.

    1993-10-01

    We report the detection, via narrow-band 4686 A filter imagery, of possible new Wolf-Rayet stars in the most massive giant H II regions of the irregular galaxies NGC 2366 and IC 2574. One stellar knot in the post-starburst galaxy NGC 1569 also appears to contain a weak excess of light at 4686 A. A similar search yielded negative results in the very low mass galaxy GR8. The strongest 4686 A excess is located close to the secondary eastern knot in the core of NGC 2366-I (NGC 2363). If this excess is of stellar origin, about five Wolf-Rayet stars of the luminous late-type can account for the excess emission. Nebular emission wraps around this cluster in the form of a shell. The putative Wolf-Rayet stars appear to be close to the center of the large expanding H II bubble discovered by Roy et al. (1991). A possible nebular origin of the 4686 A excess is also discussed.

  18. PLANETS AROUND LOW-MASS STARS. III. A YOUNG DUSTY L DWARF COMPANION AT THE DEUTERIUM-BURNING LIMIT ,

    SciTech Connect

    Bowler, Brendan P.; Liu, Michael C.; Shkolnik, Evgenya L.; Dupuy, Trent J.

    2013-09-01

    We report the discovery of an L-type companion to the young M3.5V star 2MASS J01225093-2439505 at a projected separation of 1.''45 ( Almost-Equal-To 52 AU) as part of our adaptive optics imaging search for extrasolar giant planets around young low-mass stars. 2MASS 0122-2439 B has very red near-infrared colors similar to the HR 8799 planets and the reddest known young/dusty L dwarfs in the field. Moderate-resolution (R Almost-Equal-To 3800) 1.5-2.4 {mu}m spectroscopy reveals a near-infrared spectral type of L4-L6 and an angular H-band shape, confirming its cool temperature and young age. The kinematics of 2MASS 0122-2439 AB are marginally consistent with members of the {approx}120 Myr AB Dor young moving group based on the photometric distance to the primary (36 {+-} 4 pc) and our radial velocity measurement of 2MASS 0122-2439 A from Keck/HIRES. We adopt the AB Dor group age for the system, but the high energy emission, lack of Li I {lambda}6707 absorption, and spectral shape of 2MASS 0122-2439 B suggest a range of {approx}10-120 Myr is possible. The age and luminosity of 2MASS 0122-2439 B fall in a strip where ''hot-start'' evolutionary model mass tracks overlap as a result of deuterium burning. Several known substellar companions also fall in this region (2MASS J0103-5515 ABb, AB Pic b, {kappa} And b, G196-3 B, SDSS 2249+0044 B, LP 261-75 B, HD 203030 B, and HN Peg B), but their dual-valued mass predictions have largely been unrecognized. The implied mass of 2MASS 0122-2439 B is Almost-Equal-To 12-13 M{sub Jup} or Almost-Equal-To 22-27 M{sub Jup} if it is an AB Dor member, or possibly as low as 11 M{sub Jup} if the wider age range is adopted. Evolutionary models predict an effective temperature for 2MASS 0122-2439 B that corresponds to spectral types near the L/T transition ( Almost-Equal-To 1300-1500 K) for field objects. However, we find a mid-L near-infrared spectral type, indicating that 2MASS 0122-2439 B represents another case of photospheric dust being retained to cooler temperatures at low surface gravities, as seen in the spectra of young (8-30 Myr) planetary companions. Altogether, the low mass, low temperature, and red colors of 2MASS 0122-2439 B make it a bridge between warm planets like {beta} Pic b and cool, very dusty ones like HR 8799 bcde.

  19. Planets around Low-mass Stars. III. A Young Dusty L Dwarf Companion at the Deuterium-burning Limit

    NASA Astrophysics Data System (ADS)

    Bowler, Brendan P.; Liu, Michael C.; Shkolnik, Evgenya L.; Dupuy, Trent J.

    2013-09-01

    We report the discovery of an L-type companion to the young M3.5V star 2MASS J01225093-2439505 at a projected separation of 1.''45 (≈52 AU) as part of our adaptive optics imaging search for extrasolar giant planets around young low-mass stars. 2MASS 0122-2439 B has very red near-infrared colors similar to the HR 8799 planets and the reddest known young/dusty L dwarfs in the field. Moderate-resolution (R ≈ 3800) 1.5-2.4 μm spectroscopy reveals a near-infrared spectral type of L4-L6 and an angular H-band shape, confirming its cool temperature and young age. The kinematics of 2MASS 0122-2439 AB are marginally consistent with members of the ~120 Myr AB Dor young moving group based on the photometric distance to the primary (36 ± 4 pc) and our radial velocity measurement of 2MASS 0122-2439 A from Keck/HIRES. We adopt the AB Dor group age for the system, but the high energy emission, lack of Li I λ6707 absorption, and spectral shape of 2MASS 0122-2439 B suggest a range of ~10-120 Myr is possible. The age and luminosity of 2MASS 0122-2439 B fall in a strip where "hot-start" evolutionary model mass tracks overlap as a result of deuterium burning. Several known substellar companions also fall in this region (2MASS J0103-5515 ABb, AB Pic b, κ And b, G196-3 B, SDSS 2249+0044 B, LP 261-75 B, HD 203030 B, and HN Peg B), but their dual-valued mass predictions have largely been unrecognized. The implied mass of 2MASS 0122-2439 B is ≈12-13 M Jup or ≈22-27 M Jup if it is an AB Dor member, or possibly as low as 11 M Jup if the wider age range is adopted. Evolutionary models predict an effective temperature for 2MASS 0122-2439 B that corresponds to spectral types near the L/T transition (≈1300-1500 K) for field objects. However, we find a mid-L near-infrared spectral type, indicating that 2MASS 0122-2439 B represents another case of photospheric dust being retained to cooler temperatures at low surface gravities, as seen in the spectra of young (8-30 Myr) planetary companions. Altogether, the low mass, low temperature, and red colors of 2MASS 0122-2439 B make it a bridge between warm planets like β Pic b and cool, very dusty ones like HR 8799 bcde. 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. Based in part on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.

  20. NEUTRON STAR RADIUS MEASUREMENT WITH THE QUIESCENT LOW-MASS X-RAY BINARY U24 IN NGC 6397

    SciTech Connect

    Guillot, Sebastien; Rutledge, Robert E.; Brown, Edward F. E-mail: rutledge@physics.mcgill.ca

    2011-05-10

    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 {approx}10%: R{sub NS} = 8.9{sup +0.9}{sub -0.6} km for M{sub NS} = 1.4 M{sub sun}. Alternatively, we provide the confidence regions in mass-radius space as well as the best-fit projected radius R{sub {infinity}} = 11.9{sup +1.0}{sub -0.8} km, as seen by an observer at infinity. The best-fit effective temperature, kT{sub eff} = 80{sup +4}{sub -5} eV, is used to estimate the NS core temperature which falls in the range T{sub core} = (3.0-9.8) x 10{sup 7} K, depending on the atmosphere model considered. This makes U24 the third most precisely measured NS radius among qLMXBs, after those in {omega} Cen and M13.

  1. Low mass companions to nearby stars: Spectral classification and its relation to the stellar/substellar break

    NASA Technical Reports Server (NTRS)

    Kirkpatrick, J. Davy; Mccarthy, Donald W., Jr.

    1994-01-01

    The relationship between mass and spectral class for main-sequence stars has never been obtained for dwarfs cooler than M6; currently, the true nature of objects classified as M7, M8, M9, or later (be they stellar or substellar) is not known. In this paper, spectral types for the components in five low mass binary systems are estimated based on previously published infrared speckle measurements, red/infrared photometry, and parallax data, together with newly acquired high signal-to-noise composite spectra of the systems and revised magnitude difference relations for M dwarfs. For two of these binaries, the secondary has a smaller mass (less than 0.09 solar mass) than any object having a dynamically measured mass and a known spectral type, thus extending the spectral class/mass relation to lower masses than has previously been possible. Data from the higher mass components (0.09 solar mass less than M less than 0.40 solar mass) are consistent with earlier results; the two lowest mass objects -- though having mass errors which could place them on either side of the M dwarf/brown dwarf dividing line (Mass is about 0.08 solar mass) -- are found to have spectral types no cooler than M6.5 V. An extrapolation of the updated spectral class/mass relation to the hydrogen-burning limit suggests that objects of type M7 and later may be substellar. Direct confirmation of this awaits the discovery of a close, very late-type binary for which dynamical masses can be measured.

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

    NASA Technical Reports Server (NTRS)

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

    2012-01-01

    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.

  3. Organic Chemistry of Low-mass Star-forming Cores. I. 7 mm Spectroscopy of Chamaeleon MMS1

    NASA Astrophysics Data System (ADS)

    Cordiner, Martin A.; Charnley, Steven B.; Wirstrm, Eva S.; Smith, Robert G.

    2012-01-01

    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 106 cm-3 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 non-equilibrium carbon chemistry; C6H and HC7N column densities are 5.9+2.9 -1.3 1011 cm-2 and 3.3+8.0 -1.5 1012 cm-2, 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-]/[C6H] < 10%, consistent with previous observations in interstellar clouds and low-mass protostars. Deuterated HC3N and c-C3H2 were detected. The [DC3N]/[HC3N] ratio of approximately 4% is consistent with the value typically found in cold interstellar gas.

  4. Search for Low-mass Objects in the Globular Cluster M4. I. Detection of Variable Stars

    NASA Astrophysics Data System (ADS)

    Safonova, M.; Mkrtichian, D.; Hasan, P.; Sutaria, F.; Brosch, N.; Gorbikov, E.; Joseph, P.

    2016-02-01

    With every new discovery of an extrasolar planet, the absence of planets in globular clusters (GCs) becomes more and more conspicuous. Null detection of transiting hot Jupiters in GCs 47 Tuc, ω Cen, and NGC 6397 presents an important puzzle, raising questions about the role played by cluster metallicity and environment on formation and survival of planetary systems in densely populated stellar clusters. GCs were postulated to have many free-floating planets, for which microlensing (ML) is an established tool for detection. Dense environments, well-constrained distances and kinematics of lenses and sources, and photometry of thousands of stars simultaneously make GCs the ideal targets to search for ML. We present first results of a multisite, 69-night-long campaign to search for ML signatures of low-mass objects in the GC M4, which was chosen because of its proximity, location, and the actual existence of a planet. M4 was observed in R and I bands by two telescopes, 1 m T40 and 18-inch C18, of the Wise Observatory, Tel Aviv, Israel, from 2011 April to July. Observations on the 1 m telescope were carried out in service mode, gathering 12 to 48 20 s exposures per night for a total of 69 nights. C18 observations were done for about 4 hr a night for six nights in 2011 May. We employ a semiautomated pipeline to calibrate and reduce the images to the light curves that our group is developing for this purpose, which includes the differential photometry package DIAPL, written by Wozniak and modified by W. Pych. Several different diagnostics are employed for search of variability/transients. While no high-significance ML event was found in this observational run, we have detected more than 20 new variables and variable candidates in the M4 field, which we present here.

  5. 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

    Wisniewski, John P.; Agol, Eric; Barnes, Rory; Ge, Jian; De Lee, Nathan; Fleming, Scott W.; Lee, Brian L.; Chang, Liang; Crepp, Justin R.; Eastman, Jason; Gaudi, B. Scott; Esposito, Massimiliano; Gonzalez Hernandez, Jonay I.; Prieto, Carlos Allende; Ghezzi, Luan; Da Costa, Luiz N.; Porto De Mello, G. F.; Stassun, Keivan G.; Cargile, Phillip; Bizyaev, Dmitry; and others

    2012-05-15

    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.

  6. X-shooter spectroscopy of young stellar objects. IV. Accretion in low-mass stars and substellar objects in Lupus

    NASA Astrophysics Data System (ADS)

    Alcalá, J. M.; Natta, A.; 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.

    2014-01-01

    We present VLT/X-shooter observations of a sample of 36 accreting low-mass stellar and substellar objects (YSOs) in the Lupus star-forming region, spanning a range in mass from ~0.03 to ~1.2 M⊙, but mostly with 0.1 M⊙star. Based on observations collected at the European Souther Observatory at Paranal, under programs 084.C-0269(A), 085.C-0238(A), 086.C-0173(A), 087.C-0244(A) and 089.C-0143(A).Tables 1-4 and Appendices are available in electronic form at http://www.aanda.org

  7. The Ross Variable Stars Revisited. II

    NASA Astrophysics Data System (ADS)

    Osborn, W.; Mills, O. F.

    2012-06-01

    Better magnitudes and epochs have been determined for 190 of the 379 confirmed and suspected variable stars discovered by Ross from 1925 to 1931. Accurate positions have been determined for those objects for which unambiguous identifications had been lacking. These include a number of cases for which Ross's published coordinates have large errors.

  8. ORGANIC CHEMISTRY OF LOW-MASS STAR-FORMING CORES. I. 7 mm SPECTROSCOPY OF CHAMAELEON MMS1

    SciTech Connect

    Cordiner, Martin A.; Charnley, Steven B.; Wirstroem, Eva S.; Smith, Robert G.

    2012-01-10

    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{sup 6} cm{sup -3} 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 non-equilibrium carbon chemistry; C{sub 6}H and HC{sub 7}N column densities are 5.9{sup +2.9}{sub -1.3} Multiplication-Sign 10{sup 11} cm{sup -2} and 3.3{sup +8.0}{sub -1.5} Multiplication-Sign 10{sup 12} cm{sup -2}, 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 C{sub 4}H{sup -} and C{sub 6}H{sup -}, with anion-to-neutral ratios [C{sub 4}H{sup -}]/[C{sub 4}H] < 0.02% and [C{sub 6}H{sup -}]/[C{sub 6}H] < 10%, consistent with previous observations in interstellar clouds and low-mass protostars. Deuterated HC{sub 3}N and c-C{sub 3}H{sub 2} were detected. The [DC{sub 3}N]/[HC{sub 3}N] ratio of approximately 4% is consistent with the value typically found in cold interstellar gas.

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

    NASA Astrophysics Data System (ADS)

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

    2016-01-01

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

  10. 2M1155-79 (= T CHAMAELEONTIS B): A LOW-MASS, WIDE-SEPARATION COMPANION TO THE NEARBY, 'OLD' T TAURI STAR T CHAMAELEONTIS

    SciTech Connect

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

    2012-03-10

    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.

  11. Revisiting Forbidden Lines in T Tauri stars

    NASA Astrophysics Data System (ADS)

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

    2015-01-01

    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.

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

    NASA Astrophysics Data System (ADS)

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

    2008-02-01

    We report the discovery of a pre-main-sequence (PMS), 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 find that Par 1802 is composed of two equal-mass (0.39+/-0.03, 0.40+/-0.03 Msolar) 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 PMS stellar evolution.

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

    NASA Astrophysics Data System (ADS)

    Malo, Lison; Doyon, Ren; Feiden, Gregory A.; Albert, Loc; Lafrenire, David; Artigau, tienne; Gagn, Jonathan; Riedel, Adric

    2014-09-01

    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.

  14. BANYAN. IV. Fundamental parameters of low-mass star candidates in nearby young stellar kinematic groups—isochronal age determination using magnetic evolutionary models

    SciTech Connect

    Malo, Lison; Doyon, René; Albert, Loïc; Lafrenière, David; Artigau, Étienne; Gagné, Jonathan; Feiden, Gregory A.; Riedel, Adric E-mail: doyon@astro.umontreal.ca

    2014-09-01

    Based on high-resolution optical spectra obtained with ESPaDOnS at Canada-France-Hawaii Telescope, we determine fundamental parameters (T {sub eff}, R, L {sub 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 β 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.

  15. 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)

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

    2011-04-01

    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 Astronmico Hispano Alemn (CAHA) at Calar Alto, operated jointly by the Max-Planck Institut fr Astronomie and the Instituto de Astrofsica de Andaluca (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).

  16. The Habitable Zone Planet Finder: A Proposed High Resolution Nir Spectrograph For The Het To Discover Low Mass Exoplanets Around M Stars

    NASA Astrophysics Data System (ADS)

    Mahadevan, Suvrath; Ramsey, L.; Wolszczan, A.; Wright, J.; Endl, M.; Redman, S.

    2010-01-01

    The Habitable Zone Planet Finder (HZPF) is a proposed instrument for the 9m Hobby Eberly telescope that will be capable of discovering low mass planets around M dwarfs. HZPF will be fiber-fed, provide a spectral resolution R 50,000 and cover the wavelength range 0.9-1.65mm, the Y, J and H near infrared (NIR) bands where most of the flux is emitted by late type M stars, and where most of the radial velocity information is concentrated. Enclosed in a vacuum tank with active temperature control, fiber scrambling and mechanical agitation, HZPF is designed to achieve a radial velocity precision < 3m/s, with a desire to achieve 1m/s for the brightest targets. This instrument will enable a study of the properties of low mass planets around M dwarfs; discover planets in the habitable zones around these stars, and serve as an essential radial velocity confirmation tool for astrometric and transit detections around late M dwarfs. Radial velocity observation the NIR will also enable a search for close in planets around young active stars, complementing the search space enabled by upcoming high-contrast imaging instruments. Tests with our laboratory prototype have already demonstrated the ability to recover radial velocities in the NIR at 7-10 m/s precision from integrated sunlight. We will discuss lessons learned about calibration and NIR array performance from our tests and how they impact the design of the HZPF.

  17. BANYAN. III. Radial Velocity, Rotation, and X-Ray Emission of Low-mass Star Candidates in Nearby Young Kinematic Groups

    NASA Astrophysics Data System (ADS)

    Malo, Lison; Artigau, tienne; Doyon, Ren; Lafrenire, David; Albert, Loc; Gagn, Jonathan

    2014-06-01

    Based on high-resolution spectra obtained with PHOENIX at Gemini-South, CRIRES at VLT-UT1, and ESPaDOnS at the Canada-France-Hawaii Telescope, we present new measurements of the radial and projected rotational velocities of 219 low-mass stars. The target likely membership was initially established using the Bayesian analysis tool recently presented in Malo et al., taking into account only the position, proper motion, and photometry of the stars to assess their membership probability. In the present study, we include radial velocity as an additional input to our analysis, and in doing so we confirm the high membership probability for 130 candidates: 27 in ? Pictoris, 22 in Tucana-Horologium, 25 in Columba, 7 in Carina, 18 in Argus and 18 in AB Doradus, and 13 with an ambiguous membership. Our analysis also confirms the membership of 57 stars proposed in the literature. A subsample of 16 candidates was observed at 3 or more epochs, allowing us to discover 6 new spectroscopic binaries. The fraction of binaries in our sample is 25%, consistent with values in the literature. Of the stars in our sample, 20% show projected rotational velocities (vsin i) higher than 30 km s-1 and therefore are considered as fast rotators. A parallax and other youth indicators are still needed to fully confirm the 130 highly probable candidates identified here as new bona fide members. Finally, based on the X-ray emission of bona fide and highly probable group members, we show that for low-mass stars in the 12-120 Myr age range, the X-ray luminosity is an excellent indicator of youth and better than the more traditionally used R X parameter, the ratio of X-ray to bolometric luminosity.

  18. BANYAN. III. Radial velocity, rotation, and X-ray emission of low-mass star candidates in nearby young kinematic groups

    SciTech Connect

    Malo, Lison; Artigau, Étienne; Doyon, René; Lafrenière, David; Albert, Loïc; Gagné, Jonathan E-mail: doyon@astro.umontreal.ca

    2014-06-10

    Based on high-resolution spectra obtained with PHOENIX at Gemini-South, CRIRES at VLT-UT1, and ESPaDOnS at the Canada-France-Hawaii Telescope, we present new measurements of the radial and projected rotational velocities of 219 low-mass stars. The target likely membership was initially established using the Bayesian analysis tool recently presented in Malo et al., taking into account only the position, proper motion, and photometry of the stars to assess their membership probability. In the present study, we include radial velocity as an additional input to our analysis, and in doing so we confirm the high membership probability for 130 candidates: 27 in β Pictoris, 22 in Tucana-Horologium, 25 in Columba, 7 in Carina, 18 in Argus and 18 in AB Doradus, and 13 with an ambiguous membership. Our analysis also confirms the membership of 57 stars proposed in the literature. A subsample of 16 candidates was observed at 3 or more epochs, allowing us to discover 6 new spectroscopic binaries. The fraction of binaries in our sample is 25%, consistent with values in the literature. Of the stars in our sample, 20% show projected rotational velocities (vsin i) higher than 30 km s{sup –1} and therefore are considered as fast rotators. A parallax and other youth indicators are still needed to fully confirm the 130 highly probable candidates identified here as new bona fide members. Finally, based on the X-ray emission of bona fide and highly probable group members, we show that for low-mass stars in the 12-120 Myr age range, the X-ray luminosity is an excellent indicator of youth and better than the more traditionally used R {sub X} parameter, the ratio of X-ray to bolometric luminosity.

  19. The Inner Edge of the Habitable Zone for Synchronously Rotating Planets around Low-mass Stars Using General Circulation Models

    NASA Astrophysics Data System (ADS)

    Kopparapu, Ravi kumar; Wolf, Eric T.; Haqq-Misra, Jacob; Yang, Jun; Kasting, James F.; Meadows, Victoria; Terrien, Ryan; Mahadevan, Suvrath

    2016-03-01

    Terrestrial planets at the inner edge of the habitable zone (HZ) of late-K and M-dwarf stars are expected to be in synchronous rotation, as a consequence of strong tidal interactions with their host stars. Previous global climate model (GCM) studies have shown that, for slowly rotating planets, strong convection at the substellar point can create optically thick water clouds, increasing the planetary albedo, and thus stabilizing the climate against a thermal runaway. However these studies did not use self-consistent orbital/rotational periods for synchronously rotating planets placed at different distances from the host star. Here we provide new estimates of the inner edge of the HZ for synchronously rotating terrestrial planets around late-K and M-dwarf stars using a 3D Earth-analog GCM with self-consistent relationships between stellar metallicity, stellar effective temperature, and the planetary orbital/rotational period. We find that both atmospheric dynamics and the efficacy of the substellar cloud deck are sensitive to the precise rotation rate of the planet. Around mid-to-late M-dwarf stars with low metallicity, planetary rotation rates at the inner edge of the HZ become faster, and the inner edge of the HZ is farther away from the host stars than in previous GCM studies. For an Earth-sized planet, the dynamical regime of the substellar clouds begins to transition as the rotation rate approaches ∼10 days. These faster rotation rates produce stronger zonal winds that encircle the planet and smear the substellar clouds around it, lowering the planetary albedo, and causing the onset of the water-vapor greenhouse climatic instability to occur at up to ∼25% lower incident stellar fluxes than found in previous GCM studies. For mid-to-late M-dwarf stars with high metallicity and for mid-K to early-M stars, we agree with previous studies.

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

    SciTech Connect

    Di Stefano, Rosanne

    2012-08-01

    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.

  1. Atmospheric Erosion Caused by Stellar Coronal Plasma Flows on Terrestrial Exoplanets within Close-In Habitable Zones of Low Mass Stars

    NASA Astrophysics Data System (ADS)

    Lammer, H.; Terada, N.; Kulikov, Yu. N.; Lichtenegger, H. I. M.; Khodachenko, M. L.; Penz, T.

    2008-04-01

    Since low mass M stars show a higher level of stellar activity compared to solar-like stars, and because of the closer orbital distance of their habitable zones compared to that of the Solar System, terrestrial exoplanets within M star habitable zones are expected to be much more strongly influenced by stellar winds and dense plasma ejected from the host star by coronal mass ejections. The efficiency of atmospheric erosion of CO_2-rich exoplanets, having the size and mass similar to that of the Earth, due to dense stellar plasma flows within close-in habitable zones of active M-type dwarf stars is investigated. Since M stars are active at the X-ray and EUV radiation (XUV) wavelengths over long time periods, we have applied a thermal balance model at various XUV flux input values for simulating the thermospheric heating by photodissociation and ionization processes, due to exothermic chemical reactions and cooling by the CO_2 IR radiation in the 15?m band. Our study shows that intense XUV radiation of active M-stars, together with the photochemical production of excited atomic oxygen results in atmospheric expansion and extended exospheres which can interact with the stellar plasma flow. Using the calculated thermospheric neutral and ion densities, we applied a 3-D magnetohydrodynamic and a test particle model for calculating the non-thermal loss rates from the extended exospheres of magnetized and non-magnetized Earth-like exoplanets. The consequences of our preliminary results for the evolution of habitable planets within active M star environments are discussed.

  2. Short-duration Lensing Events. I. Wide-orbit Planets? Free-floating Low-mass Objects? Or High-velocity Stars?

    NASA Astrophysics Data System (ADS)

    Di Stefano, Rosanne

    2012-08-01

    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.

  3. 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)

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

    2010-12-01

    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.

  4. The {sup 13}C(α,n){sup 16}O reaction as a neutron source for the s-process in AGB low-mass stars

    SciTech Connect

    Trippella, O.; Busso, M.; La Cognata, M.; Spitaleri, C.; Guardo, G. L.; Lamia, L.; Puglia, S. M.R.; Romano, S.; Spartà, R.; Kiss, G. G.; Rogachev, G. V.; Avila, M.; Koshchiy, E.; Kuchera, A.; Santiago, D.; Mukhamedzhanov, A. M.; Maiorca, E.; Palmerini, S.

    2014-05-09

    The {sup 13}C(α,n){sup 16}O reaction is considered to be the most important neutron source for producing the main component of the s-process in low mass stars. In this paper we focus our attention on two of the main open problems concerning its operation as a driver for the slow neutron captures. Recently, a new measurement of the {sup 13}C(α,n){sup 16}O reaction rate was performed via the Trojan Horse Method greatly increasing the accuracy. Contemporarily, on the modelling side, magnetic mechanisms were suggested to justify the production of the {sup 13}C pocket, thus putting the s-process in stars on safe physical ground. These inputs allow us to reproduce satisfactorily the solar distribution of elements.

  5. Collective properties of neutron-star X-ray binary populations of galaxies. II. Pre-low-mass X-ray binary properties, formation rates, and constraints

    SciTech Connect

    Bhadkamkar, H.; Ghosh, P.

    2014-04-01

    We continue our exploration of the collective properties of neutron-star X-ray binaries in the stellar fields (i.e., outside globular clusters) of normal galaxies. In Paper I of this series, we considered high-mass X-ray binaries (HMXBs). In this paper (Paper II), we consider low-mass X-ray binaries (LMXBs), whose evolutionary scenario is very different from that of HMXBs. We consider the evolution of primordial binaries up to the stage where the neutron star just formed in the supernova explosion of the primary is in a binary with its low-mass, unevolved companion, and this binary has circularized tidally, producing what we call a pre-low-mass X-ray binary (pre-LMXB). We study the constraints on the formation of such pre-LMXBs in detail (since these are low-probability events), and calculate their collective properties and formation rates. To this end, we first consider the changes in the binary parameters in the various steps involved, viz., the common-envelope phase, the supernova, and the tidal evolution. This naturally leads to a clarification of the constraints. We then describe our calculation of the evolution of the distributions of primordial binary parameters into those of pre-LMXB parameters, following the standard evolutionary scenario for individual binaries. We display the latter as both bivariate and monovariate distributions, discuss their essential properties, and indicate the influences of some essential factors on these. Finally, we calculate the formation rate of these pre-LMXBs. The results of this paper will be used in a subsequent one to compute the expected X-ray luminosity function of LMXBs.

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

    SciTech Connect

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

    2013-04-10

    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.

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

    NASA Technical Reports Server (NTRS)

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

    1986-01-01

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

  8. X-Ray Properties of Low-mass Pre-main Sequence Stars in the Orion Trapezium Cluster

    NASA Astrophysics Data System (ADS)

    Schulz, Norbert S.; Huenemoerder, David P.; Günther, Moritz; Testa, Paola; Canizares, Claude R.

    2015-09-01

    The Chandra HETG Orion Legacy Project (HOLP) is the first comprehensive set of observations of a very young massive stellar cluster that provides high-resolution X-ray spectra of very young stars over a wide mass range (0.7-2.3 {M}⊙ ). In this paper, we focus on the six brightest X-ray sources with T Tauri stellar counterparts that are well-characterized at optical and infrared wavelengths. All stars show column densities which are substantially smaller than expected from optical extinction, indicating that the sources are located on the near side of the cluster with respect to the observer as well as that these stars are embedded in more dusty environments. Stellar X-ray luminosities are well above 1031 erg s-1, in some cases exceeding 1032 erg s-1 for a substantial amount of time. The stars during these observations show no flares but are persistently bright. The spectra can be well fit with two temperature plasma components of 10 MK and 40 MK, of which the latter dominates the flux by a ratio 6:1 on average. The total emission measures range between 3-8 × 1054 cm-3 and are comparable to active coronal sources. The fits to the Ne ix He-Like K-shell lines indicate forbidden to inter-combination line ratios consistent with the low-density limit. Observed abundances compare well with active coronal sources underlying the coronal nature of these sources. The surface flux in this sample of 0.6-2.3 {M}⊙ classical T Tauri stars shows that coronal activity increases significantly between ages 0.1 and 10 Myr. The results demonstrate the power of X-ray line diagnostics to study coronal properties of T Tauri stars in young stellar clusters.

  9. Elemental abundances of low-mass stars in nearby young associations: AB Doradus, Carina Near and Ursa Major

    NASA Astrophysics Data System (ADS)

    Biazzo, K.; D'Orazi, V.; Desidera, S.; Covino, E.; Alcal, J. M.; Zusi, M.

    2012-12-01

    We present stellar parameters and abundances of 11 elements (Li, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni and Zn) of 13 F6-K2 main-sequence stars in the young groups AB Doradus, Carina Near and Ursa Major. The exoplanet-host star ? Horologii is also analysed. The three young associations have lithium abundance consistent with their age. All other elements show solar abundances. The three groups are characterized by a small scatter in all abundances, with mean [Fe/H] values of 0.10 (? = 0.03), 0.08 (? = 0.05) and 0.01 (? = 0.03) dex for AB Doradus, Carina Near and Ursa Major, respectively. The distribution of elemental abundances appears congruent with the chemical pattern of the Galactic thin disc in the solar vicinity, as found for other young groups. This means that the metallicity distribution of nearby young stars, targets of direct-imaging planet-search surveys, is different from that of old, field solar-type stars, i.e. the typical targets of radial velocity surveys. The young planet-host star ? Horologii shows a lithium abundance lower than that found for the young association members. It is found to have a slightly super-solar iron abundance ([Fe/H] = 0.16 0.09), while all [X/Fe] ratios are similar to the solar values. Its elemental abundances are close to those of the Hyades cluster derived from the literature, which seems to reinforce the idea of a possible common origin with the primordial cluster. Based on observations performed with European Southern Observatory (ESO) telescopes [programme IDs: 70.D-0081(A), 082.A-9007(A), 083.A-9011(B), 084.A-9011(B)].

  10. 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

    Noble, J. A.; Fraser, H. J.; Aikawa, Y.; Pontoppidan, K. M.; Sakon, I.

    2013-10-01

    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 μm 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.

  11. Taking into account the effects of component proximity on the spectral-line profiles of stars in low-mass X-ray binary systems

    NASA Astrophysics Data System (ADS)

    Petrov, V. S.; Antokhina, E. A.; Cherepashchuk, A. M.

    2015-05-01

    An exact calculation of CaI ?6439 absorption profiles in the spectra of optical stars in low-mass X-ray binary systems is carried out. The calculations are used to revise a formula relating the rotational broadening of lines and the component-mass ratio. In the case of modest (substantial) X-ray heating, failure to take into account the tidal-rotational deformation of the figure of the star leads to overestimation (underestimation) of the mass of the relativistic object. The radial-velocity curves of optical stars are modeled for binary systems with various parameters and X-ray heating powers k x ; corresponding tables of K corrections are presented. Refined values for the component-mass ratio q = 23 1, black-hole mass M x = 8.4 0.5, and optical-star mass M v = 0.36 0.07 for the GS 2023+338 (V404 Cyg) system are presented.

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

    NASA Astrophysics Data System (ADS)

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

    2002-12-01

    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

  13. Constraining the Low-Mass Slope of the Star Formation Sequence at 0.5 less than z less than 2.5

    NASA Technical Reports Server (NTRS)

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

    2014-01-01

    We constrain the slope of the star formation rate (SFR; log psi) to stellar mass (log stellar mass) relation down to log (stellar mass / solar mass) = 8.4 (log (stellar mass / solar mass) = 9.2) at redshift = 0.5 (redshift = 2.5) with a mass-complete sample of 39,106 starforming galaxies selected from the 3D-Hubble Space Telescope photometric catalogs, using deep photometry in the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (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 psi proportional to log stellar mass) than at high masses (log psi is proportional to (0.3-0.6) log stellar mass). 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 micrometer imaging; beta-corrected UV SFRs; and H-alpha SFRs. The normalization of the sequence evolves differently in distinct mass regimes as well: for galaxies less massive than log (stellar mass / solar mass) is equal to less than 10 the specific SFR (psi / stellar mass) is observed to be roughly self-similar with psi / stellar mass proportional to (1 + redshift) (sup 1.9), whereas more massive galaxies show a stronger evolution with psi / solar mass proportional to (1 + redshift) (sup 2.2-3.5) for log (stellar mass / solar mass) = 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.

  14. Constraining the Low-Mass Slope of the Star Formation Sequence at 0.5 Less than Z Less than 2.5

    NASA Technical Reports Server (NTRS)

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

    2014-01-01

    We constrain the slope of the star formation rate (SFR; log psi) to stellar mass relation down to log(stellar mass/solar mass) = 8.4 (log(stellar mass/solar mass) = 9.2) at z = 0.5 (z = 2.5) with a mass-complete sample of 39,106 starforming 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 psi varies as log stellar mass) than at high masses (log psi varies as (0.3-0.6) log stellar mass). 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 micrometers imaging; beta-corrected UV SFRs; and H alpha SFRs. The normalization of the sequence evolves differently in distinct mass regimes as well: for galaxies less massive than log(stellar mass/solar mass) less than 10 the specific SFR (psi/stellar mass) is observed to be roughly self-similar with psi/stellar mass varies as (1 + z)(sup)1.9, whereas more massive galaxies show a stronger evolution with psi/stellar mass varies as (1 + z)(sup2.2-3.5) for log(stellar mass/solar mass) = 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.

  15. VizieR Online Data Catalog: Spectra of low-mass stars in Upper Sco (Lodieu+, 2011)

    NASA Astrophysics Data System (ADS)

    Lodieu, N.; Dobbie, P. D.; Hambly, N. C.

    2010-11-01

    Coordinates (J2000), ZYJHK photometry from the UKIDSS Galactic Clusters Survey, and proper motions derived from the UKIDSS/2MASS cross-match (in arcsec/yr) of stars in the AAOmega field-of-view ordered by increasing Z magnitude. The last column provides a tentative estimate of the spectral type. Data obtained with the AAOmega spectrograph on the Anglo-Australian telescope in May 2007. (4 data files).

  16. The contribution of disks and envelopes to the millimeter continuum emission from very young low-mass stars

    NASA Technical Reports Server (NTRS)

    Terebey, S.; Chandler, C. J.; Andre, P.

    1993-01-01

    We investigate the question of disk formation during the protostar phase. We model the dust continuum emission from the dense cloud core using the cloud-collapse models of Terebey et al. (1984) and show that dust emission from the dense core is important when measured with large single-dish telescopes at 1.3 mm, but nearly negligible with interferometers at 2.7 mm. From published and new data, we conclude that massive disks are also seen toward a number of other sources including L1448 IRS 3, whose disk mass is estimated to be 0.5 solar mass. However, 1.3 mm data show that massive disks are relatively rare, occurring around perhaps 5 percent of young embedded stars. This implies that either massive disks occur briefly during the embedded phase or that relatively few young stars form massive disks. The median 1.3 mm flux density of IRAS-Dense cores in our sample is nearly the same as T Tauri stars in the sample of Beckwith et al. (1990). We conclude that the typical disk mass is not significantly higher during the embedded phase than during the later T Tauri phase.

  17. Molecular Opacities for Low-Mass Metal-poor AGB Stars Undergoing the Third Dredge-up

    NASA Astrophysics Data System (ADS)

    Cristallo, S.; Straniero, O.; Lederer, M. T.; Aringer, B.

    2007-09-01

    The concomitant overabundances of C, N, and s-process elements are commonly ascribed to the complex interplay of nucleosynthesis, mixing, and mass loss taking place in asymptotic giant branch (AGB) stars. At low metallicity, the enhancement of C and/or N can be up to 1000 times larger than the original iron content and significantly affects the stellar structure and its evolution. For this reason, the interpretation of the already available and still growing amount of data concerning C-rich metal-poor stars belonging to our Galaxy as well as to dwarf spheroidal galaxies would require reliable AGB stellar models for low and very low metallicities. In this paper we address the question of calculation and use of appropriate opacity coefficients, which take into account the C enhancement caused by the third dredge-up. A possible N enhancement, caused by the cool bottom process or by the engulfment of protons into the convective zone generated by a thermal pulse and the subsequent huge third dredge-up, is also considered. Based on up-to-date stellar models, we illustrate the changes induced by the use of these opacity coefficients on the physical and chemical properties expected for these stars.

  18. The Helium-Core Mass at the Helium Flash in Low-Mass Red Giant Stars: Observations and Theory

    NASA Astrophysics Data System (ADS)

    Catelan, M.; de Freitas Pacheco, J. A.; Horvath, J. E.

    1996-04-01

    The method developed by Raffelt to estimate a possible increase in the standard values of the helium-core mass at the tip of the red giant branch, Mc, from properties of the color-magnitude diagrams of Galactic globular clusters is employed. In the present study, we revise and update Raffelt's database, including also constraints from RR Lyrae pulsation, and find that a small increase, of ΔMc ≍ 0.01±0.015 Msun, cannot be ruled out with the present data and evolutionary models. Our new upper limits on ΔMc are less restrictive than those previously obtained by Raffelt, as are the corresponding constraints on novel astroparticle phenomena that may affect the evolution of low-mass red giants. Within the estimated uncertainties, however, the standard values of Mc may also be acceptable. Raffelt's method does not rule out a low envelope helium abundance in globular cluster giants, though again the standard values are compatible with the available constraints. The influence of a nonsolar ratio for the α-capture elements upon these results is also investigated. In addition, we review several aspects of the input physics employed in red giant stellar evolutionary calculations, with the purpose of evaluating possible sources of uncertainty in the value of the helium-core mass at the helium flash that is obtained from evolutionary computations, such as heat conduction by electrons in the degenerate core; Coulomb effects upon the equation of state; triple-α reaction rates and screening factors; neutrino emission rates, both standard and enhanced by a possible nonzero magnetic moment; stellar rotation; microscopic element diffusion; and energy losses by axions and weakly interacting massive particles (WIMPs).

  19. A Study of the Wide Main Sequence: The Long-Term Photometric Variability of Low Mass Stars

    NASA Astrophysics Data System (ADS)

    Pewett, Tiffany; Henry, Todd J.; Hosey, Altonio D.; Dieterich, Sergio; Jao, Wei-Chun; Winters, Jennifer G.; Riedel, Adric R.; RECONS Team

    2016-01-01

    The RECONS (REsearch Consortium On Nearby Stars, www.recons.org) team has carried out a long-term photometric variability study using the SMARTS 0.9m telescope at the Cerro Tololo Inter-American Observatory (CTIO). The program has obtained up to 15 years of observations in the V band for hundreds of M dwarf stars. This unique study has provided insight into how the ubiquitous M dwarfs change over decadal timescales, revealing their long-term magnetic cycles and how the presence or lack of such activity may affect their sizes and consequent luminosities, and thus their positions on the H-R Diagram.Using carefully vetted parallaxes and photometric colors, many measured by the RECONS team, we have created a highly accurate H-R Diagram of the nearest (within 25pc) stars using their V-K colors to represent temperatures and absolute V magnitudes as proxies for luminosities. We find that for M dwarfs, the main sequence widens significantly, by up to four magnitudes in MV, corresponding to a factor of almost 40 in optical flux. This spread implies a wide range of stellar radii for M dwarfs of the same temperature. Our study of long-term photometric variability indicates that there is a trend in cyclic activity that is highest for the most luminous red dwarfs and lowest for the rare, cool red subdwarfs. This provides valuable insight into the complex interplay of age, metallicity, and magnetic fields that molds the character of the red dwarfs.This effort has been supported by the NSF through grants AST-0908402, AST-1109445, and AST-1412026, STScI grant HST-GO-13724.001-A, and via observations made possible by the SMARTS Consortium.

  20. The Snow Line in Viscous Disks around Low-mass Stars: Implications for Water Delivery to Terrestrial Planets in the Habitable Zone

    NASA Astrophysics Data System (ADS)

    Mulders, Gijs D.; Ciesla, Fred J.; Min, Michiel; Pascucci, Ilaria

    2015-07-01

    The water-ice or snow line is one of the key properties of protoplanetary disks that determines the water content of terrestrial planets in the habitable zone. Its location is determined by the properties of the star, the mass accretion rate through the disk, and the size distribution of dust suspended in the disk. We calculate the snow-line location from recent observations of mass accretion rates and as a function of stellar mass. By taking the observed dispersion in mass accretion rates as a measure of the dispersion in initial disk mass, we find that stars of a given mass will exhibit a range of snow-line locations. At a given age and stellar mass, the observed dispersion in mass accretion rates of 0.4 dex naturally leads to a dispersion in snow-line locations of 0.2 dex. For ISM-like dust sizes, the 1? snow-line location among solar-mass stars of the same age ranges from 2 to 5 AU. For more realistic dust opacities that include larger grains, the snow line is located up to two times closer to the star. We use these locations and the outcome of N-body simulations to predict the amount of water delivered to terrestrial planets that formed in situ in the habitable zone. We find that the dispersion in snow-line locations leads to a large range in water content. For ISM-like dust sizes, a significant fraction of habitable-zone terrestrial planets around Sun-like stars remain dry, and no water is delivered to the habitable zones of low-mass M stars (\\lt 0.5 {M}? ) as in previous works. The closer-in snow line in disks with larger grains enables water delivery to the habitable zone for a significant fraction of M stars and all FGK stars. Considering their larger numbers and higher planet occurrence, M stars may host most of the water-rich terrestrial planets in the galaxy if these planets are able to hold on to their water in their subsequent evolution.

  1. The doubly eclipsing quintuple low-mass star system 1SWASP J093010.78+533859.5

    NASA Astrophysics Data System (ADS)

    Lohr, M. E.; Norton, A. J.; Gillen, E.; Busuttil, R.; Kolb, U. C.; Aigrain, S.; McQuillan, A.; Hodgkin, S. T.; Gonzlez, E.

    2015-06-01

    Our discovery of 1SWASP J093010.78+533859.5 as a probable doubly eclipsing quadruple system, containing a contact binary with P ~ 0.23 d and a detached binary with P ~ 1.31 d, was announced in 2013. Subsequently, Koo and collaborators confirmed the detached binary spectroscopically, and identified a fifth set of static spectral lines at its location, corresponding to an additional non-eclipsing component of the system. Here we present new spectroscopic and photometric observations, allowing confirmation of the contact binary and improved modelling of all four eclipsing components. The detached binary is found to contain components of masses 0.837 0.008 and 0.674 0.007M?, with radii of 0.832 0.018 and 0.669 0.018R? and effective temperatures of and K, respectively; the contact system has masses 0.86 0.02 and 0.341 0.011M?, radii of 0.79 0.04 and 0.52 0.05R?, respectively, and a common effective temperature of 4700 50 K. The fifth star is of similar temperature and spectral type to the primaries in the two binaries. Long-term photometric observations indicate the presence of a spot on one component of the detached binary, moving at an apparent rate of approximately one rotation every two years. Both binaries have consistent system velocities around -11 to -12 km s-1, which match the average radial velocity of the fifth star; consistent distance estimates for both subsystems of d = 78 3 and d = 73 4 pc are also found, and, with some further assumptions, of d = 83 9 pc for the fifth star. These findings strongly support the claim that both binaries - and very probably all five stars - are gravitationally bound in a single system. The consistent angles of inclination found for the two binaries (88.2 0.3and 86 4) may also indicate that they originally formed by fragmentation (around 9-10 Gyr ago) from a single protostellar disk, and subsequently remained in the same orbital plane. Table 1 is available in electronic form at http://www.aanda.org

  2. Orbital properties of an unusually low-mass sdB star in a close binary system with a white dwarf

    NASA Astrophysics Data System (ADS)

    Silvotti, R.; stensen, R. H.; Bloemen, S.; Telting, J. H.; Heber, U.; Oreiro, R.; Reed, M. D.; Farris, L. E.; O'Toole, S. J.; Lanteri, L.; Degroote, P.; Hu, H.; Baran, A. S.; Hermes, J. J.; Althaus, L. G.; Marsh, T. R.; Charpinet, S.; Li, J.; Morris, R. L.; Sanderfer, D. T.

    2012-08-01

    We have used 605 days of photometric data from the Kepler spacecraft to study KIC 6614501, a close binary system with an orbital period of 0.157 497 47(25) days (3.779 939 h), that consists of a low-mass subdwarf B (sdB) star and a white dwarf (WD). As seen in many other similar systems, the gravitational field of the WD produces an ellipsoidal deformation of the sdB which appears in the light curve as a modulation at two times the orbital frequency. The ellipsoidal deformation of the sdB implies that the system has a maximum inclination of 40, with i ? 20 being the most likely. The orbital radial velocity (RV) of the sdB star is high enough to produce a Doppler beaming effect with an amplitude of 432 5 ppm, clearly visible in the folded light curve. The photometric amplitude that we obtain, K1 = 85.8 km s-1, is 12 per cent less than the spectroscopic RV amplitude of 97.2 2.0 km s-1. The discrepancy is due to the photometric contamination from a close object at about 5 arcsec north-west of KIC 6614501, which is difficult to remove. The atmospheric parameters of the sdB star, Teff = 23 700 500 K and log g = 5.70 0.10, imply that it is a rare object below the extreme horizontal branch (EHB), similar to HD 188112. The comparison with different evolutionary tracks suggests a mass between 0.18 and 0.25 M?, too low to sustain core helium burning. If the mass was close to 0.18-0.19 M?, the star could be already on the final He-core WD cooling track. A higher mass, up to 0.25 M?, would be compatible with a He-core WD progenitor undergoing a cooling phase in a H-shell flash loop. A third possibility, with a mass between 0.32 and 0.40 M?, cannot be excluded and would imply that the sdB is a 'normal' (but with an unusually low mass) EHB star burning He in its core. In all these different scenarios, the system is expected to merge in less than 3.1 Gyr due to gravitational wave radiation.

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

    SciTech Connect

    Alves, Felipe O.; Girart, Josep M.; Acosta-Pulido, Jose A.; Franco, Gabriel A. P.; Lopez, Rosario E-mail: girart@ice.cat E-mail: franco@fisica.ufmg.br E-mail: falves@astro.uni-bonn.de

    2011-07-15

    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.

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

    NASA Astrophysics Data System (ADS)

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

    2012-11-01

    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.

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

    SciTech Connect

    Cackett, E. M.; Brown, E. F.; Cumming, A.; Degenaar, N.; Miller, J. M.; Fridriksson, J. K.; Wijnands, R.; Homan, J.

    2013-09-10

    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.

  6. BANYAN. V. A SYSTEMATIC ALL-SKY SURVEY FOR NEW VERY LATE-TYPE LOW-MASS STARS AND BROWN DWARFS IN NEARBY YOUNG MOVING GROUPS

    SciTech Connect

    Gagn, Jonathan; Lafrenire, David; Doyon, Ren; Malo, Lison; Artigau, tienne

    2015-01-10

    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{sup 1}. We selected highly probable candidate members of several YMGs from this sample using the Bayesian Analysis for Nearby Young AssociatioNsII tool (BANYANII). We used the most probable statistical distances inferred from BANYANII 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.

  7. 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)

    Gagn, Jonathan; Lafrenire, David; Doyon, Ren; Malo, Lison; Artigau, tienne

    2015-01-01

    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.

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

    SciTech Connect

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

    2014-12-10

    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 {sub A} = 1.0038 ± 0.0066 M {sub ☉}, M {sub B} = 0.5955 ± 0.0022 M {sub ☉}, R {sub A} = 0.980 ± 0.013 R {sub ☉}, and R {sub B} = 0.5873 ± 0.0067 R {sub ☉}. 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.

  9. 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)

    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

    We present a 3-5 μm 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 μm and 24 μm 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 μm excess in the system may be explained by the contribution from this disk. 1RXS 1609B exhibits no 3-4 μm excess, nor does its primary; however, the system as a whole has a modest 24 μm 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 μm 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.

  10. Diagnostics of models and observations in the contexts of exoplanets, brown dwarfs, and very low-mass stars.

    NASA Astrophysics Data System (ADS)

    Kopytova, Taisiya

    2016-01-01

    When studying isolated brown dwarfs and directly imaged exoplanets with insignificant orbital motion,we have to rely on theoretical models to determine basic parameters such as mass, age, effective temperature, and surface gravity.While stellar and atmospheric models are rapidly evolving, we need a powerful tool to test and calibrate them.In my thesis, I focussed on comparing interior and atmospheric models with observational data, in the effort of taking into account various systematic effects that can significantly influence the data analysis.As a first step, about 460 candidate member os the Hyades were screened for companions using diffraction limited imaging observation (both our own data and archival data). As a result I could establish the single star sequence for the Hyades comprising about 250 stars (Kopytova et al. 2015, accepted to A&A). Open clusters contain many coeval objects of the same chemical composition and age, and spanning a range of masses. We compare the obtained sequence with a set of theoretical isochrones identifying systematic offsets and revealing probable issues in the models.However, there are many cases when it is impossible to test models before comparing them with observations.As a second step, we apply atmospheric models for constraining parameters of WISE 0855-07, the coolest known Y dwarf(Kopytova et al. 2014, ApJ 797, 3). We demonstrate the limits of constraining effective temperature and the presence/absence of water clouds.As a third step, we introduce a novel method to take into account the above-mentioned systematics. We construct a "systematics vector" that allows us to reveal problematic wavelength ranges when fitting atmospheric models to observed near-infrared spectraof brown dwarfs and exoplanets (Kopytova et al., in prep.). This approach plays a crucial role when retrieving abundances for these objects, in particularly, a C/O ratio. The latter parameter is an important key to formation scenarios of brown dwarf and exoplanets. We show the way to constrain a C/O ratio while eliminating systematics effects, which significantly improves the reliability of a final result and our conclusions about formation history of certain exoplanets and brown dwarfs.

  11. CARMENES science preparation: characterisation of M dwarfs with low-resolution spectroscopy and search for low-mass wide companions to young stars

    NASA Astrophysics Data System (ADS)

    Alonso-Floriano, F. J.

    2015-11-01

    This thesis is focused on the study of low-mass objects that can be targets of exoplanet searches with near-infrared spectrographs in general and CARMENES (Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Echelle Spectrographs; see Quirrenbach et al. 2014) in particular. The CARMENES consortium comprises 11 institutions from Germany and Spain that are building a high-resolution spectrograph (R=82,000) with two channels, visible (0.55 - 1.05 um) and infrared (0.95 - 1.7 um), for the 3.5 m Calar Alto telescope. It will observe a sample of 300 M dwarfs in 600 nights of guaranteed time during at least three years, starting in January 2016. The final sample will be chosen from the 2200 M dwarfs included in the CARMENCITA input catalogue. For these stars, we have obtained and collected a large amount of data: spectral types, radial and rotational velocities, photometry in several bands, etc. Part of the e effort of the science preparation necessary for the final selection of targets for CARMENES and other near-infrared spectrographs has been collected in two publications, which are presented in this PhD thesis. In the first publication (Alonso-Floriano et al., 2015A&A...577A.128A), we obtained low-resolution spectra for 753 stars using the CAFOS spectrograph at the 2.2 m Calar Alto telescope. The main goal was to derive accurate spectral types, which are fundamental parameters for the sample selection. We used a grid of 49 standard stars, from spectral types K3V to M8V, together with a double least-square minimisation technique and 31 spectral indices previously defined by other authors. In addition, we quantified the surface gravity, metallicity and chromospheric activity of the sample, in order to detect low-gravity stars (giants and very young), metal-poor and very metal-poor stars (subdwarfs), and very active stars. In the second publication (Alonso-Floriano et al., 2015A&A...583A..85A), we searched for common proper motion companions, especially of low mass, to members of the near young beta Pictoris moving group. First, we compiled a list of 185 members and candidate members to beta Pictoris from 35 representatives studies on this moving group. Next, we used the Aladin and STILTS virtual observatory tools, as well as the PPMXL proper motion and Washington double stars catalogues. The objects that showed similar proper motions to those stars of the sample were targets of an astro-photometric follow-up. The 36 common proper motion companion eventually obtained were subjects of a study of binding energies to determine their physical ligation.

  12. RECENT STELLAR MASS ASSEMBLY OF LOW-MASS STAR-FORMING GALAXIES AT REDSHIFTS 0.3 < z < 0.9

    SciTech Connect

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

    2015-01-20

    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 {sub *}/M {sub ☉} ≤ 8.0), at 0.3 < z {sub spec} < 0.9, in the Extended-Chandra Deep Field-South field. Among them, 24 were selected with photometric stellar mass log M {sub *}/M {sub ☉} < 8.0, 0.3 < z {sub phot} < 1.0, and m {sub NB816,} {sub 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 {sub *}/M {sub ☉} ≤ 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.

  13. Spectral and timing analysis of the mHz QPOs in the neutron-star low-mass X-ray binary 4U 1636-53

    NASA Astrophysics Data System (ADS)

    Lyu, Ming; Mndez, Mariano; Zhang, Guobao; Keek, L.

    2015-11-01

    We investigate the spectral and timing properties of the millihertz quasi-periodic oscillations (mHz QPOs) in neutron-star low-mass X-ray binary 4U 1636-53 using XMM-Newton and Rossi X-ray Timing Explorer (RXTE) observations. The mHz QPOs in the XMM-Newton/RXTE observations show significant frequency variation and disappear right before type I X-ray bursts. We find no significant correlation between the mHz QPO frequency and the temperature of the neutron-star surface, which is different from theoretical predictions. For the first time we observed the full lifetime of a mHz QPO lasting 19 ks. Besides, we also measure a frequency drift time-scale 15 ks, we speculate that this is the cooling time-scale of a layer deeper than the burning depth, possibly heated by the previous burst. Moreover, the analysis of all X-ray bursts in this source shows that all type I X-ray bursts associated with the mHz QPOs are short, bright and energetic, suggesting a potential connection between mHz QPOs and He-rich X-ray bursts.

  14. On the Geometric Nature of Low-frequency Quasi-periodic Oscillations in Neutron-star Low-mass X-Ray Binaries

    NASA Astrophysics Data System (ADS)

    Homan, Jeroen; Fridriksson, Joel K.; Remillard, Ronald A.

    2015-10-01

    We report on a detailed analysis of the so-called 1 Hz quasi-periodic oscillation (QPO) in the eclipsing and dipping neutron-star low-mass X-ray binary EXO 0748-676. This type of QPO has previously been shown to have a geometric origin. Our study focuses on the evolution of the QPO as the source moves through the color-color diagram in which it traces out an atoll-source-like track. The QPO frequency increases from 0.4 Hz in the hard state to 25 Hz as the source approaches the soft state. Combining power spectra based on QPO frequency reveals additional features that strongly resemble those seen in non-dipping/eclipsing atoll sources. We show that the low-frequency QPOs in atoll sources and the 1 Hz QPO in EXO 0748-676 follow similar relations with respect to the noise components in their power spectra. We conclude that the frequencies of both types of QPOs are likely set by (the same) precession of a misaligned inner accretion disk. For high-inclination systems like EXO 0748-676 this results in modulations of the neutron-star emission due to obscuration or scattering, while for lower-inclination systems the modulations likely arise from relativistic Doppler-boosting and light-bending effects.

  15. 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)

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

    2011-08-01

    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 μm, 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.

  16. Calibrating convective-core overshooting with eclipsing binary systems. The case of low-mass main-sequence stars

    NASA Astrophysics Data System (ADS)

    Valle, G.; Dell'Omodarme, M.; Prada Moroni, P. G.; Degl'Innocenti, S.

    2016-03-01

    Context. Double-lined eclipsing binaries have often been adopted in literature to calibrate the extension of the convective-core overshooting beyond the border defined by the Schwarzschild criterion. Aims: In a robust statistical way, we quantify the magnitude of the uncertainty that affects the calibration of the overshooting efficiency parameter β that is owing to the uncertainty on the observational data. We also quantify the biases on the β determination that is caused by the lack of constraints on the initial helium content and on the efficiencies of the superadiabatic convection and microscopic diffusion. Methods: We adopted a modified grid-based SCEPtER pipeline to recover the β parameter from synthetic stellar data. Our grid spans the mass range [1.1; 1.6] M⊙ and evolutionary stages from the zero-age main sequence (MS) to the central hydrogen depletion. The β estimates were obtained by generalising the maximum likelihood technique described in our previous works. As observational constraint, we adopted the effective temperatures, [Fe/H], masses, and radii of the two stars. Results: By means of Monte Carlo simulations, adopting a reference scenario of mild overshooting β = 0.2 for the synthetic data, and taking typical observational errors into account, we found both large statistical uncertainties and biases on the estimated values of β. For the first 80% of the MS evolution, β is biased by about -0.04, with the 1σ error practically unconstrained in the whole explored range [0.0; 0.4]. In the last 5% of the evolution the bias vanishes and the 1σ error is about 0.05. The 1σ errors are similar when adopting different reference values of β. Interestingly, for synthetic data computed without convective-core overshooting, the estimated β is biased by about 0.12 in the first 80% of the MS evolution, and by 0.05 afterwards. Assuming an uncertainty of ±1 in the helium-to-metal enrichment ratio ΔY/ ΔZ, we found a large systematic uncertainty in the recovered β value, reaching 0.2 at the 60% of the MS evolution. Taking into account both the helium abundance indetermination and 1σ statistical uncertainty, we found that in the terminal part of the MS evolution the error on the estimated β values ranges from -0.05 to + 0.10, while β is basically unconstrained throughout the explored range at earlier evolutionary stages. We quantified the impact of a uniform variation of ±0.24 in the mixing-length parameter αml around the solar-calibrated value. The largest bias occurs in the last 5% of the evolution with an error on the estimated median β from -0.03 to + 0.07. In this last part, the 1σ uncertainty that addresses statistical and systematic error sources ranges from -0.09 to + 0.15. Finally, we quantified the impact of a complete neglect of diffusion in the stellar evolution computations. In this case, the 1σ uncertainty that addresses statistical and systematic error sources ranges from -0.08 to + 0.08 in the terminal 5% of the MS, while β is practically unconstrained in the first 80% of the MS. Conclusions: The calibration of the convective core overshooting with double-lined eclipsing binaries - in the explored mass range and with both components still in their MS phase - appears to be poorly reliable, at least until further stellar observables, such as asteroseismic ones, and more accurate models are available.

  17. 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)

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

    2014-12-01

    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

  18. Evolution, Nucleosynthesis, and Yields of AGB Stars at Different Metallicities. III. Intermediate-mass Models, Revised Low-mass Models, and the ph-FRUITY Interface

    NASA Astrophysics Data System (ADS)

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

    2015-08-01

    We present a new set of models for intermediate-mass asymptotic giant branch (AGB) stars (4.0, 5.0, and 6.0 M⊙) at different metallicities (-2.15 ≤ [Fe/H] ≤ +0.15). This set integrates the existing models for low-mass AGB stars (1.3 ≤ M/M⊙ ≤ 3.0) already included in the FRUITY database. We describe the physical and chemical evolution of the computed models from the main sequence up to the end of the AGB phase. Due to less efficient third dredge up episodes, models with large core masses show modest surface enhancements. This effect is due to the fact that the interpulse phases are short and, therefore, thermal pulses (TPs) are weak. Moreover, the high temperature at the base of the convective envelope prevents it from deeply penetrating the underlying radiative layers. Depending on the initial stellar mass, the heavy element nucleosynthesis is dominated by different neutron sources. In particular, the s-process distributions of the more massive models are dominated by the 22Ne(α,n)25Mg reaction, which is efficiently activated during TPs. At low metallicities, our models undergo hot bottom burning and hot third dredge up. We compare our theoretical final core masses to available white dwarf observations. Moreover, we quantify the influence intermediate-mass models have on the carbon star luminosity function. Finally, we present the upgrade of the FRUITY web interface, which now also includes the physical quantities of the TP-AGB phase for all of the models included in the database (ph-FRUITY).

  19. Angular momentum redistribution by mixed modes in evolved low-mass stars. II. Spin-down of the core of red giants induced by mixed modes

    NASA Astrophysics Data System (ADS)

    Belkacem, K.; Marques, J. P.; Goupil, M. J.; Mosser, B.; Sonoi, T.; Ouazzani, R. M.; Dupret, M. A.; Mathis, S.; Grosjean, M.

    2015-07-01

    The detection of mixed modes in subgiants and red giants by the CoRoT and Kepler space-borne missions allows us to investigate the internal structure of evolved low-mass stars, from the end of the main sequence to the central helium-burning phase. In particular, the measurement of the mean core rotation rate as a function of the evolution places stringent constraints on the physical mechanisms responsible for the angular momentum redistribution in stars. It showed that the current stellar evolution codes including the modelling of rotation fail to reproduce the observations. An additional physical process that efficiently extracts angular momentum from the core is thus necessary. Our aim is to assess the ability of mixed modes to do this. To this end, we developed a formalism that provides a modelling of the wave fluxes in both the mean angular momentum and the mean energy equations in a companion paper. In this article, mode amplitudes are modelled based on recent asteroseismic observations, and a quantitative estimate of the angular momentum transfer is obtained. This is performed for a benchmark model of 1.3 M⊙ at three evolutionary stages, representative of the evolved pulsating stars observed by CoRoT and Kepler. We show that mixed modes extract angular momentum from the innermost regions of subgiants and red giants. However, this transport of angular momentum from the core is unlikely to counterbalance the effect of the core contraction in subgiants and early red giants. In contrast, for more evolved red giants, mixed modes are found efficient enough to balance and exceed the effect of the core contraction, in particular in the hydrogen-burning shell. Our results thus indicate that mixed modes are a promising candidate to explain the observed spin-down of the core of evolved red giants, but that an other mechanism is to be invoked for subgiants and early red giants.

  20. LONG-DURATION X-RAY FLASH AND X-RAY-RICH GAMMA-RAY BURSTS FROM LOW-MASS POPULATION III STARS

    SciTech Connect

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

    2012-11-10

    Recent numerical simulations suggest that Population III (Pop III) stars were born with masses not larger than {approx}100 M {sub Sun} and typically {approx}40 M {sub Sun }. 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 {approx}10{sup 5} s in the observer frame and a peak luminosity of {approx}5 Multiplication-Sign 10{sup 50} erg s{sup -1}. Assuming that the E {sub p}-L {sub p} (or E {sub p}-E {sub {gamma},iso}) correlation holds for Pop III GRBs, we find that the spectrum peak energy falls at approximately a few keV (or {approx}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 {sub p}-E {sub {gamma},iso} correlation holds, we have the possibility to detect Pop III GRBs at z {approx} 9 as long-duration X-ray-rich GRBs by EXIST. Conversely, if the E {sub p}-L {sub p} correlation holds, we have the possibility to detect Pop III GRBs up to z {approx} 19 as long-duration X-ray flashes by Lobster.

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

    SciTech Connect

    Tang, Y. K.; Gai, N. E-mail: ning.gai@hotmail.com

    2013-07-10

    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.

  2. 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)

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

    2012-11-01

    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

  3. Evolution of thermally pulsing asymptotic giant branch stars. IV. Constraining mass loss and lifetimes of low mass, low metallicity AGB stars

    SciTech Connect

    Rosenfield, Philip; Dalcanton, Julianne J.; Weisz, Daniel; Williams, Benjamin F.; Marigo, Paola; Girardi, Léo; Gullieuszik, Marco; Bressan, Alessandro; Dolphin, Andrew; Aringer, Bernhard

    2014-07-20

    The evolution and lifetimes of thermally pulsating asymptotic giant branch (TP-AGB) stars suffer from significant uncertainties. In this work, we analyze the numbers and luminosity functions of TP-AGB stars in six quiescent, low metallicity ([Fe/H] ≲ –0.86) galaxies taken from the ACS Nearby Galaxy Survey Treasury sample, using Hubble Space Telescope (HST) photometry in both optical and near-infrared filters. The galaxies contain over 1000 TP-AGB stars (at least 60 per field). We compare the observed TP-AGB luminosity functions and relative numbers of TP-AGB and red giant branch (RGB) stars, N{sub TP-AGB}/N{sub RGB}, to models generated from different suites of TP-AGB evolutionary tracks after adopting star formation histories derived from the HST deep optical observations. We test various mass-loss prescriptions that differ in their treatments of mass loss before the onset of dust-driven winds (pre-dust). These comparisons confirm that pre-dust mass loss is important, since models that neglect pre-dust mass loss fail to explain the observed N{sub TP-AGB}/N{sub RGB} ratio or the luminosity functions. In contrast, models with more efficient pre-dust mass loss produce results consistent with observations. We find that for [Fe/H] ≲ –0.86, lower mass TP-AGB stars (M ≲ 1 M{sub ☉}) must have lifetimes of ∼0.5 Myr and higher masses (M ≲ 3 M{sub ☉}) must have lifetimes ≲ 1.2 Myr. In addition, assuming our best-fitting mass-loss prescription, we show that the third dredge-up has no significant effect on TP-AGB lifetimes in this mass and metallicity range.

  4. Eclipsing Binaries as Astrophysical Laboratories: CM Draconis - Accurate Absolute Physical Properties of Low Mass Stars and an Independent Estimate of the Primordial Helium Abundance

    NASA Astrophysics Data System (ADS)

    McCook, G. P.; Guinan, E. F.; Saumon, D.; Kang, Y. W.

    1997-05-01

    CM Draconis (Gl 630.1; Vmax = +12.93) is an important eclipsing binary consisting of two dM4.5e stars with an orbital period of 1.2684 days. This binary is a high velocity star (s= 164 km/s) and the brighter member of a common proper motion pair with a cool faint white dwarf companion (LP 101-16). CM Dra and its white dwarf companion were once considered by Zwicky to belong to a class of "pygmy stars", but they turned out to be ordinary old, cool white dwarfs or faint red dwarfs. Lacy (ApJ 218,444L) determined the first orbital and physical properties of CM Dra from the analysis of his light and radial velocity curves. In addition to providing directly measured masses, radii, and luminosities for low mass stars, CM Dra was also recognized by Lacy and later by Paczynski and Sienkiewicz (ApJ 286,332) as an important laboratory for cosmology, as a possible old Pop II object where it may be possible to determine the primordial helium abundance. Recently, Metcalfe et al.(ApJ 456,356) obtained accurate RV measures for CM Dra and recomputed refined elements along with its helium abundance. Starting in 1995, we have been carrying out intensive RI photoelectric photometry of CM Dra to obtain well defined, accurate light curves so that its fundamental properties can be improved, and at the same time, to search for evidence of planets around the binary from planetary transit eclipses. During 1996 and 1997 well defined light curves were secured and these were combined with the RV measures of Metcalfe et al. (1996) to determine the orbital and physical parameters of the system, including a refined orbital period. A recent version of the Wilson-Devinney program was used to analyze the data. New radii, masses, mean densities, Teff, and luminosities were found as well as a re-determination of the helium abundance (Y). The results of the recent analyses of the light and RV curves will be presented and modelling results discussed. This research is supported by NSF grants AST-9315365 and AST-9528506 which we gratefully acknowledge.

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

    SciTech Connect

    Bowler, Brendan P.; Liu, Michael C.; Cieza, Lucas A.; Kraus, Adam L.; Shkolnik, Evgenya L.; Dupuy, Trent J.; Tamura, Motohide

    2012-07-10

    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.

  6. USING HIGH-RESOLUTION OPTICAL SPECTRA TO MEASURE INTRINSIC PROPERTIES OF LOW-MASS STARS: NEW PROPERTIES FOR KOI-314 AND GJ 3470

    SciTech Connect

    Pineda, J. Sebastian; Bottom, Michael; Johnson, John A.

    2013-04-10

    We construct high signal-to-noise 'template' spectra by co-adding hundreds of spectra of nearby dwarfs spanning K7 to M4, taken with Keck/HIRES as part of the California Planet Search. We identify several spectral regions in the visible (370-800 nm) that are sensitive to the stellar luminosity and metallicity. We use these regions to develop a spectral calibration method to measure the mass, metallicity, and distance of low-mass stars, without the requirement of geometric parallaxes. Testing our method on a sample of nearby M dwarfs, we show that we can reproduce stellar masses to about 8%-10%, metallicity to {approx}0.15 dex, and distance to 11%. We were able to make use of HIRES spectra obtained as part of the radial velocity monitoring of the star KOI-314 to derive a new mass estimate of 0.57 {+-} 0.05 M{sub Sun }, a radius of 0.54 {+-} 0.05 R{sub Sun }, a metallicity, [Fe/H], of -0.28 {+-} 0.10, and a distance of 66.5 {+-} 7.3 pc. Using HARPS archival data and combining our spectral method with constraints from transit observations, we are also able to derive the stellar properties of GJ 3470, a transiting planet hosting M dwarf. We estimate a mass of 0.53 {+-} 0.05 M{sub Sun }, a radius of 0.50 {+-} 0.05 R{sub Sun }, a metallicity, [Fe/H], of 0.12 {+-} 0.12, and a distance of 29.9{+-}{sub 3.4}{sup 3.7} pc.

  7. XTE J1701-462 AND ITS IMPLICATIONS FOR THE NATURE OF SUBCLASSES IN LOW-MAGNETIC-FIELD NEUTRON STAR LOW-MASS X-RAY BINARIES

    SciTech Connect

    Homan, Jeroen; Fridriksson, Joel K.; Remillard, Ronald A.; Lewin, Walter H. G.; Van der Klis, Michiel; Wijnands, Rudy; Altamirano, Diego; Mendez, Mariano; Lin Dacheng; Casella, Piergiorgio; Belloni, Tomaso M.

    2010-08-10

    We report on an analysis of Rossi X-Ray Timing Explorer data of the transient neutron star low-mass X-ray binary (NS-LMXB) XTE J1701-462, obtained during its 2006-2007 outburst. The X-ray properties of the source changed between those of various types of NS-LMXB subclasses. At high luminosities, the source switched between two types of Z source behavior and at low luminosities we observed a transition from Z source to atoll source behavior. These transitions between subclasses primarily manifest themselves as changes in the shapes of the tracks in X-ray color-color (CD) and hardness-intensity diagrams (HID), but they are accompanied by changes in the kHz quasi-periodic oscillations, broadband variability, burst behavior, and/or X-ray spectra. We find that for most of the outburst the low-energy X-ray flux is a good parameter to track the gradual evolution of the tracks in CD and HID, allowing us to resolve the evolution of the source in greater detail than before and relate the observed properties to other NS-LMXBs. We further find that during the transition from Z to atoll, characteristic behavior known as the atoll upper banana can equivalently be described as the final stage of a weakening Z source flaring branch, thereby blurring the line between the two subclasses. Our findings strongly suggest that the wide variety in behavior observed in NS-LXMBs with different luminosities can be linked through changes in a single variable parameter, namely the mass accretion rate, without the need for additional differences in the neutron star parameters or viewing angle. We briefly discuss the implications of our findings for the spectral changes observed in NS-LMXBs and suggest that, contrary to what is often assumed, the position along the color-color tracks of Z sources is not determined by the instantaneous mass accretion rate.

  8. Testing Model Atmospheres for Young Very-low-mass Stars and Brown Dwarfs in the Infrared: Evidence for Significantly Underestimated Dust Opacities

    NASA Astrophysics Data System (ADS)

    Tottle, Jonathan; Mohanty, Subhanjoy

    2015-05-01

    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 ?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 ({{T}eff}) several hundred Kelvin lower than predicted by the standard pre-main sequence (PMS) spectral type-{{T}eff} conversion scale (based on theoretical evolutionary models). It is only in the mid-M types that the two temperature estimates agree. (2) The {{T}eff} discrepancy in the early M types (corresponding to stellar masses ? 0.4 {{M}? } at ages of a few Myr) probably arises from remaining uncertainties in the treatment of atmospheric convection within the atmospheric models, whereas in the late M types it is likely due to an underestimation of dust opacity. (3) The empirical and model-atmosphere J-band bolometric corrections are both roughly flat, and similar to each other, over the M-type {{T}eff} range. Thus the model atmospheres yield reasonably accurate bolometric luminosities ({{L}bol}), but lead to underestimations of mass and age relative to evolutionary expectations (especially in the late M types) due to lower {{T}eff}. We demonstrate this for a large sample of young Cha I and Taurus sources. (4) The trends in the atmospheric model J-Ks colors, and their deviations from the data, are similar at PMS and main sequence ages, suggesting that the model dust opacity errors we postulate here for young ages also apply at field ages.

  9. Observation vs. theory: testing the synthetic IR colours of young very low mass stars/brown dwarfs using the evolutionary tracks

    NASA Astrophysics Data System (ADS)

    Tottle, Jonathan; Mohanty, Subhanjoy

    2013-07-01

    Our ability to accurately derive stellar properties from spectral energy distributions (SEDs) depends on how well they can be fit with atmospheric models. The AMES-Dusty synthetic spectra (Allard et al., 2001), which incorporate dust grains suspended in the stellar atmosphere, are commonly used to fit SEDs of very low mass stars (VLMS) and brown dwarfs (BDs). Recently, the same group has produced an updated model named BT-Settl (Allard et al., 2012) that allow these grains to gradually settle out of the atmosphere at cooler temperatures. Using these models it is now possible to produce the NIR colours across the main sequence from spectral types M to T. However, one significant area in which these Dusty and Settl models have not been thoroughly tested is in PMS VLMS/BDs. We use empirical IR colours of PMS M-dwarfs to show that both of these models show significant discrepancies with observations. We find that the synthetic spectra imply a temperature up to 500K cooler than expected for these objects from the theoretical evolutionary tracks for their estimated ages. We postulate that the problem lies mainly with the spectra; and if so, we conjecture that an incorrect H2O opacity may be to blame, aided by additional dust effects.

  10. Discovery and Observations of ASASSN-13db, an EX Lupi-type Accretion Event on a Low-mass T Tauri Star

    NASA Astrophysics Data System (ADS)

    Holoien, Thomas W.-S.; Prieto, Jose L.; Stanek, K. Z.; Kochanek, C. S.; Shappee, B. J.; Zhu, Z.; Sicilia-Aguilar, A.; Grupe, D.; Croxall, K.; Adams, J. J.; Simon, J. D.; Morrell, N.; McGraw, S. M.; Wagner, R. M.; Basu, U.; Beacom, J. F.; Bersier, D.; Brimacombe, J.; Jencson, J.; Pojmanski, G.; Starrfield, S. G.; Szczygieł, D. M.; Woodward, C. E.

    2014-04-01

    We discuss ASASSN-13db, an EX Lupi-type ("EXor") accretion event on the young stellar object (YSO) SDSS J051011.01-032826.2 (hereafter SDSSJ0510) discovered by the All-Sky Automated Survey for SuperNovae (ASAS-SN). Using archival photometric data of SDSSJ0510 we construct a pre-outburst spectral energy distribution and find that it is consistent with a low-mass class II YSO near the Orion star forming region (d ~ 420 pc). We present follow-up photometric and spectroscopic observations of the source after the ΔV ~ -5.4 mag outburst that began in 2013 September and ended in early 2014. These data indicate an increase in temperature and luminosity consistent with an accretion rate of ~10-7 M ⊙ yr-1, three or more orders of magnitude greater than in quiescence. Spectroscopic observations show a forest of narrow emission lines dominated by neutral metallic lines from Fe I and some low-ionization lines. The properties of ASASSN-13db are similar to those of the EXor prototype EX Lupi during its strongest observed outburst in late 2008.

  11. DISCOVERY AND OBSERVATIONS OF ASASSN-13db, AN EX LUPI-TYPE ACCRETION EVENT ON A LOW-MASS T TAURI STAR

    SciTech Connect

    Holoien, Thomas W.-S.; Stanek, K. Z.; Kochanek, C. S.; Shappee, B. J.; Croxall, K.; Wagner, R. M.; Basu, U.; Beacom, J. F.; Jencson, J.; Prieto, Jose L.; Zhu, Z.; Sicilia-Aguilar, A.; Grupe, D.; Adams, J. J.; Simon, J. D.; Morrell, N.; McGraw, S. M.; Bersier, D.; Brimacombe, J.; Pojmanski, G.; and others

    2014-04-20

    We discuss ASASSN-13db, an EX Lupi-type ({sup E}Xor{sup )} accretion event on the young stellar object (YSO) SDSS J051011.01–032826.2 (hereafter SDSSJ0510) discovered by the All-Sky Automated Survey for SuperNovae (ASAS-SN). Using archival photometric data of SDSSJ0510 we construct a pre-outburst spectral energy distribution and find that it is consistent with a low-mass class II YSO near the Orion star forming region (d ∼ 420 pc). We present follow-up photometric and spectroscopic observations of the source after the ΔV ∼ –5.4 mag outburst that began in 2013 September and ended in early 2014. These data indicate an increase in temperature and luminosity consistent with an accretion rate of ∼10{sup –7} M {sub ☉} yr{sup –1}, three or more orders of magnitude greater than in quiescence. Spectroscopic observations show a forest of narrow emission lines dominated by neutral metallic lines from Fe I and some low-ionization lines. The properties of ASASSN-13db are similar to those of the EXor prototype EX Lupi during its strongest observed outburst in late 2008.

  12. Evolution of the habitable zone of low-mass stars. Detailed stellar models and analytical relationships for different masses and chemical compositions

    NASA Astrophysics Data System (ADS)

    Valle, G.; Dell'Omodarme, M.; Prada Moroni, P. G.; Degl'Innocenti, S.

    2014-07-01

    Context. The habitability of an exoplanet is assessed by determining the times at which its orbit lies in the circumstellar habitable zone (HZ). This zone evolves with time following the stellar luminosity variation, which means that the time spent in the HZ depends on the evolution of the host star. Aims: We study the temporal evolution of the HZ of low-mass stars - only due to stellar evolution - and evaluate the related uncertainties. These uncertainties are then compared with those due to the adoption of different climate models. Methods: We computed stellar evolutionary tracks from the pre-main sequence phase to the helium flash at the red-giant branch tip for stars with masses in the range [0.70-1.10] M?, metallicity Z in the range [0.005-0.04], and various initial helium contents. By adopting a reference scenario for the HZ computations, we evaluated several characteristics of the HZ, such as the distance from the host star at which the habitability is longest, the duration of this habitability, the width of the zone for which the habitability lasts one half of the maximum, and the boundaries of the continuously habitable zone (CHZ) for which the habitability lasts at least 4 Gyr. We developed analytical models, accurate to the percent level or lower, which allowed to obtain these characteristics in dependence on the mass and the chemical composition of the host star. Results: The metallicity of the host star plays a relevant role in determining the HZ. The importance of the initial helium content is evaluated here for the first time; it accounts for a variation of the CHZ boundaries as large as 30% and 10% in the inner and outer border. The computed analytical models allow the first systematic study of the variability of the CHZ boundaries that is caused by the uncertainty in the estimated values of mass and metallicity of the host star. An uncertainty range of about 30% in the inner boundary and 15% in the outer one were found. We also verified that these uncertainties are larger than that due to relying on recently revised climatic models, which leads to a CHZ boundary shift within 5% with respect to those of our reference scenario. We made an on-line tool available that provides both HZ characteristics and interpolated stellar tracks. On-line habitable zone calculator and track interpolator are available at http://astro.df.unipi.it/stellar-models/HZ/. The C code is also 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/567/A133

  13. Winds from Low Mass Protostars

    NASA Astrophysics Data System (ADS)

    Shu, Frank H.; Lizano, Susana; Adams, Fred C.; Ruden, Steven P.

    In its last stages, star formation in molecular clouds includes the onset of a stellar wind that helps to clear away the surrounding placenta of gas and dust, thereby making the young stellar object optically visible. The authors discuss new observational evidence that the emerging wind is largely neutral and atomic in low-mass protostars. They then suggest a simple theoretical mechanism for the generation of such powerful neutral winds.

  14. Daily multiwavelength Swift monitoring of the neutron star low-mass X-ray binary Cen X-4: evidence for accretion and reprocessing during quiescence

    NASA Astrophysics Data System (ADS)

    Bernardini, F.; Cackett, E. M.; Brown, E. F.; D'Angelo, C.; Degenaar, N.; Miller, J. M.; Reynolds, M.; Wijnands, R.

    2013-12-01

    We conducted the first long-term (60 d), multiwavelength (optical, ultraviolet, UV, and X-ray) simultaneous monitoring of Cen X-4 with daily Swift observations from 2012 June to August, with the goal of understanding variability in the low-mass X-ray binary Cen X-4 during quiescence. We found Cen X-4 to be highly variable in all energy bands on time-scales from days to months, with the strongest quiescent variability a factor of 22 drop in the X-ray count rate in only 4 d. The X-ray, UV and optical (V band) emission are correlated on time-scales down to less than 110 s. The shape of the correlation is a power law with index ? about 0.2-0.6. The X-ray spectrum is well fitted by a hydrogen neutron star (NS) atmosphere (kT = 59-80 eV) and a power law (with spectral index ? = 1.4-2.0), with the spectral shape remaining constant as the flux varies. Both components vary in tandem, with each responsible for about 50 per cent of the total X-ray flux, implying that they are physically linked. We conclude that the X-rays are likely generated by matter accreting down to the NS surface. Moreover, based on the short time-scale of the correlation, we also unambiguously demonstrate that the UV emission cannot be due to either thermal emission from the stream impact point, or a standard optically thick, geometrically thin disc. The spectral energy distribution shows a small UV emitting region, too hot to arise from the accretion disc, that we identified as a hotspot on the companion star. Therefore, the UV emission is most likely produced by reprocessing from the companion star, indeed the vertical size of the disc is small and can only reprocess a marginal fraction of the X-ray emission. We also found the accretion disc in quiescence to likely be UV faint, with a minimal contribution to the whole UV flux.

  15. Coronal mass ejection (CME) activity of low mass M stars as an important factor for the habitability of terrestrial exoplanets. I. CME impact on expected magnetospheres of Earth-like exoplanets in close-in habitable zones.

    PubMed

    Khodachenko, Maxim L; Ribas, Ignasi; Lammer, Helmut; Griessmeier, Jean-Mathias; Leitner, Martin; Selsis, Franck; Eiroa, Carlos; Hanslmeier, Arnold; Biernat, Helfried K; Farrugia, Charles J; Rucker, Helmut O

    2007-02-01

    Low mass M- and K-type stars are much more numerous in the solar neighborhood than solar-like G-type stars. Therefore, some of them may appear as interesting candidates for the target star lists of terrestrial exoplanet (i.e., planets with mass, radius, and internal parameters identical to Earth) search programs like Darwin (ESA) or the Terrestrial Planet Finder Coronagraph/Inferometer (NASA). The higher level of stellar activity of low mass M stars, as compared to solar-like G stars, as well as the closer orbital distances of their habitable zones (HZs), means that terrestrial-type exoplanets within HZs of these stars are more influenced by stellar activity than one would expect for a planet in an HZ of a solar-like star. Here we examine the influences of stellar coronal mass ejection (CME) activity on planetary environments and the role CMEs may play in the definition of habitability criterion for the terrestrial type exoplanets near M stars. We pay attention to the fact that exoplanets within HZs that are in close proximity to low mass M stars may become tidally locked, which, in turn, can result in relatively weak intrinsic planetary magnetic moments. Taking into account existing observational data and models that involve the Sun and related hypothetical parameters of extrasolar CMEs (density, velocity, size, and occurrence rate), we show that Earth-like exoplanets within close-in HZs should experience a continuous CME exposure over long periods of time. This fact, together with small magnetic moments of tidally locked exoplanets, may result in little or no magnetospheric protection of planetary atmospheres from a dense flow of CME plasma. Magnetospheric standoff distances of weakly magnetized Earth-like exoplanets at orbital distances

  16. Dust Masses of Disks around 8 Brown Dwarfs and Very Low-mass Stars in Upper Sco OB1 and Ophiuchus

    NASA Astrophysics Data System (ADS)

    van der Plas, G.; Ménard, F.; Ward-Duong, K.; Bulger, J.; Harvey, P. M.; Pinte, C.; Patience, J.; Hales, A.; Casassus, S.

    2016-03-01

    We present the results of ALMA band 7 observations of dust and CO gas in the disks around 7 objects with spectral types ranging between M5.5 and M7.5 in Upper Scorpius OB1, and one M3 star in Ophiuchus. We detect unresolved continuum emission in all but one source, and the 12CO J = 3 ‑ 2 line in two sources. We constrain the dust and gas content of these systems using a grid of models calculated with the radiative transfer code MCFOST, and find disk dust masses between 0.1 and 1 M⊕, suggesting that the stellar mass/disk mass correlation can be extrapolated for brown dwarfs (BDs) with masses as low as 0.05 M⊙. The one disk in Upper Sco in which we detect CO emission, 2MASS J15555600, is also the disk with the warmest inner disk, as traced by its H–[4.5] photometric color. Using our radiative transfer grid, we extend the correlation between stellar luminosity and mass-averaged disk dust temperature, originally derived for stellar mass objects, to the BD regime to < {T}{dust}> ≈ 22{({L}*/{L}ȯ )}0.16 {{K}}, applicable to spectral types of M5 and later. This is slightly shallower than the relation for earlier spectral type objects and yields warmer low-mass disks. The two prescriptions cross at 0.27 L⊙, corresponding to masses between 0.1 and 0.2 M⊙ depending on age.

  17. Planets around Low-mass Stars (PALMS). I. A Substellar Companion to the Young M Dwarf 1RXS J235133.3+312720

    NASA Astrophysics Data System (ADS)

    Bowler, Brendan P.; Liu, Michael C.; Shkolnik, Evgenya L.; Dupuy, Trent J.; Cieza, Lucas A.; Kraus, Adam L.; Tamura, Motohide

    2012-07-01

    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 2farcs4 (~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+2 -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 ~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 ~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 ~200 Myr M9 brown dwarf LP 944-20. Assuming an age of 50-150 Myr, evolutionary models imply a mass of 32 ± 6 M 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. 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.

  18. Observing and modeling the dynamic atmosphere of the low mass-loss C-star R Sculptoris at high angular resolution

    NASA Astrophysics Data System (ADS)

    Sacuto, S.; Aringer, B.; Hron, J.; Nowotny, W.; Paladini, C.; Verhoelst, T.; Hfner, S.

    2011-01-01

    Context. We study the circumstellar environment of the carbon-rich star R Sculptoris using the near- and mid-infrared high spatial resolution observations from the ESO-VLTI focal instruments VINCI and MIDI, respectively. Aims: These observations aim at increasing our knowledge of the dynamic processes in play within the very close circumstellar environment where the mass loss of AGB stars is initiated. Methods: We first compare the spectro-interferometric measurements of the star at different epochs to detect the dynamic signatures of the circumstellar structures at different spatial and spectral scales. We then interpret these data using a self-consistent dynamic model atmosphere to discuss the dynamic picture deduced from the observations. Since the hydrodynamic computation needs stellar parameters as input, a considerable effort is first applied to determining these parameters. Results: Interferometric observations do not show any significant variability effect at the 16 m baseline between phases 0.17 and 0.23 in the K band, and for both the 15 m baseline between phases 0.66 and 0.97 and the 31 m baseline between phases 0.90 and 0.97 in the N band. We find fairly good agreement between the dynamic model and the spectrophotometric data from 0.4 to 25 ?m. The model agrees well with the time-dependent flux data at 8.5 ?m, whereas it is too faint at 11.3 and 12.5 ?m. The VINCI visibility measurements are reproduced well, meaning that the extension of the model is suitable in the K-band. In the mid-infrared, the model has the proper extension to reveal molecular structures of C2H2 and HCN located above the stellar photosphere. However, the windless model used is not able to reproduce the more extended and dense dusty environment. Conclusions: Among the different explanations for the discrepancy between the model and the measurements, the strong nonequilibrium process of dust formation is one of the most probable. The transition from windless atmospheres to models with considerable mass-loss rates occurs in a very narrow range of stellar parameters, especially for the effective temperature, the C/O ratio, and the pulsation amplitude. A denser sampling of such critical regions of the parameter space with additional models might lead to a better representation of the extended structures of low mass-loss carbon stars like R Sculptoris. The complete dynamic coupling of gas and dust and the approximation of grain opacities with the small-particle limit in the dynamic calculation could also contribute to the difference between the model and the data. Based on observations made with the Very Large Telescope Interferometer at Paranal Observatory under programs 60.A-9220, 074.D-0601, 077.D-0294 (French Guaranteed Time Observation), 078.D-0112 (Belgian Guaranteed Time Observation), and 078.D-0122 (French Guaranteed Time Observation).

  19. Determining the Metallicity of Low-Mass Stars and Brown Dwarfs: Tools for Probing Fundamental Stellar Astrophysics, Tracing Chemical Evolution of the Milky Way and Identifying the Hosts of Extrasolar Planets

    NASA Astrophysics Data System (ADS)

    West, A. A.; Bochanski, J. J.; Bowler, B. P.; Dotter, A.; Johnson, J. A.; Lépine, S.; Rojas-Ayala, B.; Schweitzer, A.

    2011-12-01

    We present a brief overview of a splinter session on determining the metallicity of low-mass dwarfs that was organized as part of the Cool Stars 16 conference. We review contemporary spectroscopic and photometric techniques for estimating metallicity in low-mass dwarfs and discuss the importance of measuring accurate metallicities for studies of Galactic and chemical evolution using subdwarfs, creating metallicity benchmarks for brown dwarfs, and searching for extrasolar planets that are orbiting around low-mass dwarfs. In addition, we present the current understanding of the effects of metallicity on stellar evolution and atmosphere models and discuss some of the limitations that are important to consider when comparing theoretical models to data.

  20. Abundance Anomaly of the 13C Isotopic Species of c-C3H2 in the Low-mass Star Formation Region L1527

    NASA Astrophysics Data System (ADS)

    Yoshida, Kento; Sakai, Nami; Tokudome, Tomoya; Lpez-Sepulcre, Ana; Watanabe, Yoshimasa; Takano, Shuro; Lefloch, Bertrand; Ceccarelli, Cecilia; Bachiller, Rafael; Caux, Emmanuel; Vastel, Charlotte; Yamamoto, Satoshi

    2015-07-01

    The rotational spectral lines of c-C3H2 and two kinds of the 13C isotopic species, c-{}13{{CCCH}}2 ({C}2v symmetry) and c-{{CC}}13{{CH}}2 (Cs symmetry), have been observed in the 1-3 mm band toward the low-mass star-forming region L1527. We have detected 7, 3, and 6 lines of c-C3H2, c-{}13{{CCCH}}2, and c-{{CC}}13{{CH}}2, respectively, with the Nobeyama 45 m telescope and 34, 6, and 13 lines, respectively, with the IRAM 30 m telescope, where seven, two, and two transitions, respectively, are observed with both telescopes. With these data, we have evaluated the column densities of the normal and 13C isotopic species. The [c-C3H2]/[c-{}13{{CCCH}}2] ratio is determined to be 310 80, while the [c-C3H2]/[c-{{CC}}13{{CH}}2] ratio is determined to be 61 11. The [c-C3H2]/[c-{}13{{CCCH}}2] and [c-C3H2]/[c-{{CC}}13{{CH}}2] ratios expected from the elemental 12C/13C ratio are 60-70 and 30-35, respectively, where the latter takes into account the statistical factor of 2 for the two equivalent carbon atoms in c-C3H2. Hence, this observation further confirms the dilution of the 13C species in carbon-chain molecules and their related molecules, which are thought to originate from the dilution of 13C+ in the gas-phase C+ due to the isotope exchange reaction: {}13{{{C}}}++{CO}\\to {}13{CO}+{{{C}}}+. Moreover, the abundances of the two 13C isotopic species are different from each other. The ratio of c-{}13{{CCCH}}2 species relative to c-{{CC}}13{{CH}}2 is determined to be 0.20 0.05. If 13C were randomly substituted for the three carbon atoms, the [c-{}13{{CCCH}}2]/[c-{{CC}}13{{CH}}2] ratio would be 0.5. Hence, the observed ratio indicates that c-{{CC}}13{{CH}}2 exists more favorably. Possible origins of the different abundances are discussed. Based on observations carried out with the IRAM 30 m Telescope and the NRO 45 m Telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain). NRO is a branch of the National Astronomical Observatory of Japan, National Institutes of Natural Sciences, Japan.

  1. Characterization of Low-mass, Wide-separation Substellar Companions to Stars in Upper Scorpius: Near-infrared Photometry and Spectroscopy

    NASA Astrophysics Data System (ADS)

    Lachapelle, François-René; Lafrenière, David; Gagné, Jonathan; Jayawardhana, Ray; Janson, Markus; Helling, Christiane; Witte, Soeren

    2015-03-01

    We present new 0.9-2.45 μm spectroscopy (R˜ 1000 ), and Y, J, H, Ks, {{L}\\prime } photometry, obtained at Gemini North, of three low-mass brown dwarf companions on wide orbits around young stars of the Upper Scorpius OB association: HIP 78530 B, [PGZ 2001] J161031.9-191305 B, and GSC 06214-00210 B. We use these data to assess the companions’ spectral type, temperature, surface gravity, and mass, as well as the ability of the BT-SETTL and Drift-Phoenix atmosphere models to reproduce the spectral features of young substellar objects. For completeness, we also analyze the archival spectroscopy and photometry of the Upper Scorpius planetary mass companion 1RXS J160929.1-210524 b. Based on a comparison with model spectra we find that the companions, in the above order, have effective temperatures of 2700 ± 100, 2500 ± 200, 2300 ± 100, and 1700 ± 100 K. These temperatures are consistent with our inferred spectral types, respectively M7 β, M9 γ, M9 γ, and L4 γ, obtained from spectral indices and comparisons with templates. From bolometric luminosities estimated from atmosphere model spectra adjusted to our photometry, and using evolution models at 5-10 Myr, we estimate masses of 21-25, 28-70, 14-17, and 7-12 MJup, respectively. [PGZ 2001] J161031.9-191305 B appears significantly overluminous for its inferred temperature, which explains its higher mass estimate. Synthetic spectra based on the BT-Settl and Drift-Phoenix atmosphere models generally offer a good fit to our observed spectra, although our analysis has highlighted a few problems. For example, the best fits in the individual near-infrared bands occur at different model temperatures. Also, temperature estimates based on a comparison of the broadband magnitudes and colors of the companions to synthetic magnitudes from the models are systematically lower than the temperature estimates based on a comparison with synthetic spectra.

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

    SciTech Connect

    Guillot, Sebastien; Rutledge, Robert E.; Servillat, Mathieu; Webb, Natalie A. E-mail: rutledge@physics.mcgill.ca

    2013-07-20

    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.

  3. STATISTICAL PROPERTIES OF GALACTIC {delta} SCUTI STARS: REVISITED

    SciTech Connect

    Chang, S.-W.; Kim, D.-W.; Byun, Y.-I.; Protopapas, P. E-mail: kim@mpia-hd.mpg.de

    2013-05-15

    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.

  4. 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)

    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

    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 fr 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

  5. REVISITING THE FIRST GALAXIES: THE EPOCH OF POPULATION III STARS

    SciTech Connect

    Muratov, Alexander L.; Gnedin, Oleg Y.; Zemp, Marcel; Gnedin, Nickolay Y.

    2013-08-10

    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.

  6. Revisiting quark stars under the influence of strong magnetic fields

    NASA Astrophysics Data System (ADS)

    Menezes, Debora

    2013-03-01

    Quark matter at finite temperature and subject to strong magnetic fields is possibly present in the early stages of heavy ion collisions and in the interior of protoneutron stars. We use the mean field approximation to investigate this type of quark matter described by the Nambu-Jona-Lasinio model. The energy per baryon of magnetized quark matter becomes more bound than nuclear matter made of iron nuclei, for magnetic fields around 1019 G. When the su(3) NJL model is applied to stellar matter, the maximum mass configurations are always above 1.45 solar masses and may be as high as 1.9 solar masses for a central magnetic field of 1018 G. These numbers are within the masses of observed neutron stars but exclude the recently measured star with 1.97 solar mass.

  7. The origin of fluorine: abundances in AGB carbon stars revisited

    NASA Astrophysics Data System (ADS)

    Abia, C.; Cunha, K.; Cristallo, S.; de Laverny, P.

    2015-09-01

    Context. Revised spectroscopic parameters for the HF molecule and a new CN line list in the 2.3 ?m region have recently become available, facilitating a revision of the F content in asymptotic giant branch (AGB) stars. Aims: AGB carbon stars are the only observationally confirmed sources of fluorine. Currently, there is no consensus on the relevance of AGB stars in its Galactic chemical evolution. The aim of this article is to better constrain the contribution of these stars with a more accurate estimate of their fluorine abundances. Methods: Using new spectroscopic tools and local thermodynamical equilibrium spectral synthesis, we redetermine fluorine abundances from several HF lines in the K-band in a sample of Galactic and extragalactic AGB carbon stars of spectral types N, J, and SC, spanning a wide range of metallicities. Results: On average, the new derived fluorine abundances are systematically lower by 0.33 dex with respect to previous determinations. This may derive from a combination of the lower excitation energies of the HF lines and the larger macroturbulence parameters used here as well as from the new adopted CN line list. Yet, theoretical nucleosynthesis models in AGB stars agree with the new fluorine determinations at solar metallicities. At low metallicities, an agreement between theory and observations can be found by handling the radiative/convective interface at the base of the convective envelope in a different way. Conclusions: New fluorine spectroscopic measurements agree with theoretical models at low and at solar metallicity. Despite this, complementary sources are needed to explain its observed abundance in the solar neighbourhood.

  8. Revisiting the Flowers-Ruderman instability of magnetic stars

    NASA Astrophysics Data System (ADS)

    Marchant, Pablo; Reisenegger, Andreas; Akgn, Taner

    2011-08-01

    In 1977, Flowers & Ruderman described a perturbation that destabilizes a purely dipolar magnetic field in a fluid star. They considered the effect of cutting the star in half along a plane containing the symmetry axis and rotating each half by 90 in opposite directions, which would cause the energy of the magnetic field in the exterior of the star to be greatly reduced, just as it happens with a pair of aligned magnets. We formally solve for the energy of the external magnetic field and check that it decreases monotonically along the entire rotation. We also describe the instability using perturbation theory, and show that it happens due to the work done by the interaction of the magnetic field with surface currents. Finally, we consider the stabilizing effect of adding a toroidal field by studying the potential energy perturbation when the rotation is not done along a sharp cut, but with a continuous displacement field that switches the direction of rotation across a region of small but finite width. Using these results, we estimate the relative strengths of the toroidal and poloidal fields needed to make the star stable to this displacement and show that the energy of the toroidal field required for stabilization is much smaller than the energy of the poloidal field. We also show that, contrary to a common argument, the Flowers-Ruderman instability cannot be applied many times in a row to reduce the external magnetic energy indefinitely.

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

    SciTech Connect

    Muratov, Alexander L.; Gnedin, Oleg Y.; Zemp, Marcel; Gnedin, Nickolay Y.

    2013-08-01

    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.

  10. The Wolf-Rayet star HD 211853 revisited

    NASA Astrophysics Data System (ADS)

    Panov, K. P.; Seggewiss, W.

    1990-01-01

    New photometric data of the Wolf-Rayet (WR) quadruple system HD 211853 (GP Cep) are presented. A thorough discussion of all data available, photometric as well as spectroscopic, leads to the conclusion that the system consists of two pairs of stars (periods 6.6884 and 3.46877 days) both hosting a WR component. In all photometric data sets, ranging from 1962 to 1988, two periods of light variability are present: a 3.46877 d period consistent with the shorter spectroscopic period as proposed by Massey (1981) and a 2.34607 d period which has not been seen before. The origin of the corresponding light variations is discussed.

  11. The Giant Star Ca II Ionization Problem: Mass Loss Revisited

    NASA Astrophysics Data System (ADS)

    Harper, G. M.; Brown, A.; Ayres, T.; Sim, S. A.

    2004-01-01

    Pioneering studies of winds from non-coronal evolved late-type stars were plagued by uncertainties in the Ca ionization balance that resulted in very large uncertainties in derived mass-loss rates. Here we re-examine the Ca II ionization balance in these stellar winds using FUSE spectra which reveal for the first time the flux from the photoionizing radiation field shortward of 1045 Angstroms. We present a FUSE 912-1185 Angstrom spectroscopic survey of evolved late-K and M stars; including the M giants alpha Cet (M1.5 III) gamma Cru (M3.5 III) beta Gru (M4.5 III) and R Dor (M8e III). Trends are presented of the high excitation emission of C III fluorescent Fe II pumped by H Lyman alpha and continuum fluxes with spectral-type. Using FUSE spectra of alpha Tau (K5 III) supplemented with partial redistribution calculations of H Lyman alpha and beta together with UV and radio data we present a detailed study of alpha Tau's wind ionization balance and derive new limits on its mass-loss rate

  12. Star cluster evolution in the Magellanic Clouds revisited

    NASA Astrophysics Data System (ADS)

    de Grijs, Richard; Goodwin, Simon P.

    2009-03-01

    The evolution of star clusters in the Magellanic Clouds has been the subject of significant recent controversy, particularly regarding the importance and length of the earliest, largely mass-independent disruption phase (referred to as “infant mortality”). Here, we take a fresh approach to the problem, using a large, independent, and homogeneous data set of UBVR imaging observations, from which we obtain the cluster age and mass distributions in both the Large and Small Magellanic Clouds (LMC, SMC) in a self-consistent manner. We conclude that the (optically selected) SMC star cluster population has undergone at most ~30% (1σ) infant mortality between the age range from about 3-10 Myr, to that of approximately 40-160 Myr. We rule out a 90% cluster mortality rate per decade of age (for the full age range up to 109 yr) at a > 6σ level. Using a simple approach, we derive a “characteristic” cluster disruption time-scale for the cluster population in the LMC that implies that we are observing the initial cluster mass function (CMF). Preliminary results suggest that the LMC cluster population may be affected by <10% infant mortality.

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

    SciTech Connect

    Guever, Tolga; Oezel, Feryal

    2013-03-01

    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 }.

  14. EFFECTS OF ROTATIONALLY INDUCED MIXING IN COMPACT BINARY SYSTEMS WITH LOW-MASS SECONDARIES AND IN SINGLE SOLAR-TYPE STARS

    SciTech Connect

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

    2012-08-20

    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{sub Sun} 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.

  15. 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)

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

    2015-01-01

    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.

  16. MULTI-WAVELENGTH CHARACTERIZATION OF STELLAR FLARES ON LOW-MASS STARS USING SDSS AND 2MASS TIME-DOMAIN SURVEYS

    SciTech Connect

    Davenport, James R. A.; Becker, Andrew C.; Kowalski, Adam F.; Hawley, Suzanne L.; Schmidt, Sarah J.; Hilton, Eric J.; Sesar, Branimir; Cutri, Roc

    2012-03-20

    We present the first rates of flares from M dwarf stars in both red optical and near-infrared (NIR) filters. We have studied {approx}50,000 M dwarfs from the Sloan Digital Sky Survey (SDSS) Stripe 82 area and 1321 M dwarfs from the Two Micron All Sky Survey (2MASS) Calibration Scan Point Source Working Database that overlap SDSS imaging fields. We assign photometric spectral types from M0 to M6 using (r - i) and (i - z) colors for every star in our sample. Stripe 82 stars each have 50-100 epochs of data, while 2MASS Calibration stars have {approx}1900 epochs. From these data we estimate the observed rates and theoretical detection thresholds for flares in eight photometric bands as a function of spectral type. Optical flare rates are found to be in agreement with previous studies, while the frequency per hour of NIR flare detections is found to be more than two orders of magnitude lower. An excess of small-amplitude flux increases in all bands exhibits a power-law distribution, which we interpret as the result of flares below our detection thresholds. In order to investigate the recovery efficiency for flares in each filter, we extend a two-component flare model into the NIR. Quiescent M0-M6 spectral templates were used with the model to predict the photometric response of flares from u to K{sub s} . We determine that red optical filters are sensitive to flares with u-band amplitudes {approx}>2 mag, and NIR filters to flares with {Delta}u {approx}> 4.5 mag. Our model predicts that M0 stars have the best color contrast for J-band detections, but M4-M6 stars should yield the highest rate of NIR flares with amplitudes of {Delta}J {>=} 0.01 mag. Characterizing flare rates and photometric variations at longer wavelengths is important for predicting the signatures of M dwarf variability in next-generation surveys, and we discuss their impact on surveys such as the Large Synoptic Survey Telescope.

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

    NASA Astrophysics Data System (ADS)

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

    2014-10-01

    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 γ 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.

  18. A study of the motion of the star Gliese 623 with a low-mass dark companion on the basis of observations at Pulkovo

    NASA Astrophysics Data System (ADS)

    Shakht, N. A.

    Photographic observations of the nearby star Gliese 623 (AC 48 1595/1589) with an optically invisible companion of extremely low stellar mass have been made during the years 1979-1995 by means of 26-inch refractor at Pulkovo. The relative proper motion and relative parallax have been obtained on the basis of 89 plates (580 individual positions). The residuals with a mean error of 0.011 arcsec have been calculated. The motion of the photocentre evoked by the companion has been estimated by means of these residuals. The following dynamic elements are chosen for the photocentric orbit: P = 3.76 years, e = 0.51, To = 1984.3. The most stable geometrical element is the great semi-axis of the photocentre which equals 0.052 0.007 arcsec (m.e.). The values of i, and have shown some change for different intervals of observations and are due to systems of reference stars. The lower limit of the mass of the companion is estimated as 0.09 0.03 solar masses. The positions of the main star on the each plate are given.

  19. Discovery of the near-infrared counterpart to the luminous neutron-star low-mass X-ray binary GX 3+1

    SciTech Connect

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

    2014-10-01

    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 K{sub s} = 15.8 ± 0.1 mag star as the counterpart is based on the presence of a Br γ 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 (L{sub X} ≈ 10{sup 36}-10{sup 37} erg s{sup –1}) than GX 3+1, where optically thin synchrotron emission from a jet probably dominates the NIR flux.

  20. THIRTY NEW LOW-MASS SPECTROSCOPIC BINARIES

    SciTech Connect

    Shkolnik, Evgenya L.; Hebb, Leslie; Cameron, Andrew C.; Liu, Michael C.; Neill Reid, I. E-mail: Andrew.Cameron@st-and.ac.u E-mail: mliu@ifa.hawaii.ed

    2010-06-20

    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.

  1. CoRoT 101186644: A transiting low-mass dense M-dwarf on an eccentric 20.7-day period orbit around a late F-star. Discovered in the CoRoT lightcurves

    NASA Astrophysics Data System (ADS)

    Tal-Or, L.; Mazeh, T.; Alonso, R.; Bouchy, F.; Cabrera, J.; Deeg, H. J.; Deleuil, M.; Faigler, S.; Fridlund, M.; Hébrard, G.; Moutou, C.; Santerne, A.; Tingley, B.

    2013-05-01

    We present the study of the CoRoT transiting planet candidate 101186644, also named LRc01_E1_4780. Analysis of the CoRoT lightcurve and the HARPS spectroscopic follow-up observations of this faint (mV = 16) candidate revealed an eclipsing binary composed of a late F-type primary (Teff = 6090 ± 200 K) and a low-mass, dense late M-dwarf secondary on an eccentric (e = 0.4) orbit with a period of ~20.7 days. The M-dwarf has a mass of 0.096 ± 0.011 M⊙, and a radius of 0.104-0.006+0.026 R⊙, which possibly makes it the smallest and densest late M-dwarf reported so far. Unlike the claim that theoretical models predict radii that are 5-15% smaller than measured for low-mass stars, this one seems to have a radius that is consistent and might even be below the radius predicted by theoretical models. Based on observations made with the 1-m telescope at the Wise Observatory, Israel, the Swiss 1.2-m Leonhard Euler telescope at La Silla Observatory, Chile, the IAC-80 telescope at the Observatory del Teide, Canarias, Spain, and the 3.6-m telescope at La Silla Observatory (ESO), Chile (program 184.C-0639).

  2. Black Hole-Neutron Star Mergers with a Hot Nuclear Equation of State: Outflow and Neutrino-cooled Disk for a Low-mass, High-spin Case

    NASA Astrophysics Data System (ADS)

    Deaton, M. Brett; Duez, Matthew D.; Foucart, Francois; O'Connor, Evan; Ott, Christian D.; Kidder, Lawrence E.; Muhlberger, Curran D.; Scheel, Mark A.; Szilagyi, Bela

    2013-10-01

    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 ? neutron star, 5.6 M ? black hole), high-spin (black hole J/M 2 = 0.9) system with the K 0 = 220 MeV Lattimer-Swesty EOS. We find that about 0.08 M ? of nuclear matter is ejected from the system, while another 0.3 M ? 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 Ye 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 ? ~ 1054 erg s-1), but the neutrino luminosity decreases by an order of magnitude over 50 ms of post-merger evolution.

  3. CHARACTERIZING THE COOL KEPLER OBJECTS OF INTERESTS. NEW EFFECTIVE TEMPERATURES, METALLICITIES, MASSES, AND RADII OF LOW-MASS KEPLER PLANET-CANDIDATE HOST STARS

    SciTech Connect

    Muirhead, Philip S.; Hamren, Katherine; Schlawin, Everett; Lloyd, James P.; Rojas-Ayala, Barbara; Covey, Kevin R.

    2012-05-10

    We report stellar parameters for late-K and M-type planet-candidate host stars announced by the Kepler Mission. We obtained medium-resolution, K-band spectra of 84 cool (T{sub eff} {approx}< 4400 K) Kepler Objects of Interest (KOIs) from Borucki et al. We identified one object as a giant (KOI 977); for the remaining dwarfs, we measured effective temperatures (T{sub eff}) and metallicities [M/H] using the K-band spectral indices of Rojas-Ayala et al. We determine the masses (M{sub *}) and radii (R{sub *}) of the cool KOIs by interpolation onto the Dartmouth evolutionary isochrones. The resultant stellar radii are significantly less than the values reported in the Kepler Input Catalog and, by construction, correlate better with T{sub eff}. Applying the published KOI transit parameters to our stellar radius measurements, we report new physical radii for the planet candidates. Recalculating the equilibrium temperatures of the planet-candidates assuming Earth's albedo and re-radiation fraction, we find that three of the planet-candidates are terrestrial sized with orbital semimajor axes that lie within the habitable zones of their host stars (KOI 463.01, KOI 812.03, and KOI 854.01). The stellar parameters presented in this Letter serve as a resource for prioritization of future follow-up efforts to validate and characterize the cool KOI planet candidates.

  4. 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

    Milone, A. P.; Aparicio, A.; Monelli, M. E-mail: aparicio@iac.es; and others

    2012-08-01

    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.

  5. Extinct 93Zr in Single Presolar SiC Grains from Low Mass Asymptotic Giant Branch Stars and Condensation from Zr-depleted Gas

    NASA Astrophysics Data System (ADS)

    Kashiv, Y.; Davis, A. M.; Gallino, R.; Cai, Z.; Lai, B.; Sutton, S. R.; Clayton, R. N.

    2010-04-01

    Synchrotron X-ray fluorescence was used in this study for the first time to measure trace element abundances in single presolar grains. The abundances of Zr and Nb were determined in SiC grains of the KJF size-separate. These grains are most likely from C-rich asymptotic giant branch stars (mainstream grains). Comparison of the data with s-process calculations suggests that the relatively short-lived isotope 93Zr (t 1/2 = 1.5 × 106 yr) condensed into the grains. The Nb/Zr ratios of the majority of the grains are higher than the s-process and CI chondrite ratios. This is probably due to grains condensing from stellar gas that was depleted in Zr, potentially because of earlier condensation of ZrC, but not depleted in Nb. However, grain contamination with solar system Nb is possible as well. Upper limits on the initial 93Zr/Zr ratios in the grains agree with the ratios observed in late-type S stars.

  6. 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

    NASA Astrophysics Data System (ADS)

    Milone, A. P.; Marino, A. F.; Cassisi, S.; Piotto, G.; Bedin, L. R.; Anderson, J.; Allard, F.; Aparicio, A.; Bellini, A.; Buonanno, R.; Monelli, M.; Pietrinferni, A.

    2012-08-01

    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 ~65% and ~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. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.

  7. Water in low-mass star-forming regions with Herschel. The link between water gas and ice in protostellar envelopes

    NASA Astrophysics Data System (ADS)

    Schmalzl, M.; Visser, R.; Walsh, C.; Albertsson, T.; van Dishoeck, E. F.; Kristensen, L. E.; Mottram, J. C.

    2014-12-01

    Aims: Our aim is to determine the critical parameters in water chemistry and the contribution of water to the oxygen budget by observing and modelling water gas and ice for a sample of eleven low-mass protostars, for which both forms of water have been observed. Methods: A simplified chemistry network, which is benchmarked against more sophisticated chemical networks, is developed that includes the necessary ingredients to determine the water vapour and ice abundance profiles in the cold, outer envelope in which the temperature increases towards the protostar. Comparing the results from this chemical network to observations of water emission lines and previously published water ice column densities, allows us to probe the influence of various agents (e.g., far-ultraviolet (FUV) field, initial abundances, timescales, and kinematics). Results: The observed water ice abundances with respect to hydrogen nuclei in our sample are 30-80 ppm, and therefore contain only 10-30% of the volatile oxygen budget of 320 ppm. The keys to reproduce this result are a low initial water ice abundance after the pre-collapse phase together with the fact that atomic oxygen cannot freeze-out and form water ice in regions with Tdust ? 15 K. This requires short prestellar core lifetimes ?0.1 Myr. The water vapour profile is shaped through the interplay of FUV photodesorption, photodissociation, and freeze-out. The water vapour line profiles are an invaluable tracer for the FUV photon flux and envelope kinematics. Conclusions: The finding that only a fraction of the oxygen budget is locked in water ice can be explained either by a short pre-collapse time of ?0.1 Myr at densities of nH ~ 104 cm-3, or by some other process that resets the initial water ice abundance for the post-collapse phase. A key for the understanding of the water ice abundance is the binding energy of atomic oxygen on ice. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

  8. The Habitable Zone Planet Finder Project: A Proposed High Resolution NIR Spectrograph for the Hobby Eberly Telescope (HET) to Discover Low Mass Exoplanets around M Stars

    NASA Astrophysics Data System (ADS)

    Mahadevan, S.; Ramsey, L.; Redman, S.; Zonak, S.; Wright, J.; Wolszczan, A.; Endl, M.; Zhao, B.

    2010-10-01

    Radial velocity precision in the NIR is now approaching the level necessary to detect exoplanets around mid-late M stars that are very faint in the optical and emit most of their flux in the NIR. The Penn State Pathfinder prototype instrument has already demonstrated 7-10 ms-1 precision on sunlight, and similar precision has been reported at the Pathways conference using CRIRES and an ammonia gas-cell. We discuss the science goals that motivate a stable cross-dispersed, high-resolution NIR spectrograph on a large telescope, as well as the path leading from the Pathfinder prototype to one such possible instrument—the fiber-fed Habitable Zone Planet Finder (HZPF) on the Hobby Eberly Telescope (HET). We also discuss wavelength calibration issues specific to the NIR, and our ongoing exploration with Pathfinder to mitigate these issues.

  9. A network of filaments detected by Herschel in the Serpens core. A laboratory to test simulations of low-mass star formation

    NASA Astrophysics Data System (ADS)

    Roccatagliata, V.; Dale, J. E.; Ratzka, T.; Testi, L.; Burkert, A.; Koepferl, C.; Sicilia-Aguilar, A.; Eiroa, C.; Gaczkowski, B.

    2015-12-01

    Context. Filaments represent a key structure during the early stages of the star formation process. Simulations show that filamentary structures commonly formed before and during the formation of cores. Aims: The Serpens core is an ideal laboratory for testing the state of the art of simulations of turbulent giant molecular clouds. Methods: We used Herschel observations of the Serpens core to compute temperature and column density maps of the region. We selected the early stages of a recent simulation of star-formation, before stellar feedback was initiated, with similar total mass and physical size as the Serpens core. We also derived temperature and column density maps from the simulations. The observed distribution of column densities of the filaments was analyzed, first including and then masking the cores. The same analysis was performed on the simulations as well. Results: A radial network of filaments was detected in the Serpens core. The analyzed simulation shows a striking morphological resemblance to the observed structures. The column density distribution of simulated filaments without cores shows only a log-normal distribution, while the observed filaments show a power-law tail. The power-law tail becomes evident in the simulation if the focus is only the column density distribution of the cores. In contrast, the observed cores show a flat distribution. Conclusions: Even though the simulated and observed filaments are subjectively similar-looking, we find that they behave in very different ways. The simulated filaments are turbulence-dominated regions; the observed filaments are instead self-gravitating structures that will probably fragment into cores. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendices are available in electronic form at http://www.aanda.org

  10. 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

    Deaton, M. Brett; Duez, Matthew D.; Foucart, Francois; O'Connor, Evan; Ott, Christian D.; Scheel, Mark A.; Szilagyi, Bela; Kidder, Lawrence E.; Muhlberger, Curran D. E-mail: m.duez@wsu.edu

    2013-10-10

    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 ν} ∼ 10{sup 54} erg s{sup –1}), but the neutrino luminosity decreases by an order of magnitude over 50 ms of post-merger evolution.

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

    NASA Astrophysics Data System (ADS)

    Mottram, J. C.; Kristensen, L. E.; van Dishoeck, E. F.; Bruderer, S.; San Jos-Garca, I.; 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.

    2014-12-01

    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 for the conditions expected in various parts of the protostar, particularly the outflow. Aims: We constrain and quantify the physical conditions probed by water in the outflow-jet system for Class 0 and I sources. Methods: We present velocity-resolved Herschel HIFI spectra of multiple water-transitions observed towards 29 nearby Class 0/I protostars as part of the WISH guaranteed time key programme. The lines are decomposed into different Gaussian components, with each component 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 use non-LTE radex models to constrain the excitation conditions present in the two outflow-related components. Results: Water emission at the source position 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 H2 densities of order 105 - 108 cm-3 and H2O column densities of order 1016 - 1018 cm-2. H2O emission originates in compact emitting regions: for the spot shocks these correspond to point sources with radii of order 10-200 AU, while for the cavity shocks these come from a thin layer along the outflow cavity wall with thickness of order 1-30 AU. Conclusions: Water emission at the source position traces two distinct kinematic components in the outflow; J shocks at the base of the outflow or in the jet, and C shocks in a thin layer in the cavity wall. The similarity of the physical conditions is in contrast to off-source determinations which show similar densities but lower column densities and larger filling factors. We propose that this is due to the differences in shock properties and geometry between these positions. Class I sources have similar excitation conditions to Class 0 sources, but generally smaller line-widths and emitting region sizes. We suggest that it is the velocity of the wind driving the outflow, rather than the decrease in envelope density or mass, that is the cause of the decrease in H2O intensity between Class 0 and I sources. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendices are available in electronic form at http://www.aanda.orgReduced spectra are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/572/A21

  12. Discovery of low mass objects in Taurus

    NASA Technical Reports Server (NTRS)

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

    1989-01-01

    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.

  13. The CO/SiO radiative instability in cool star atmospheres revisited

    NASA Technical Reports Server (NTRS)

    Cuntz, M.; Muchmore, D. O.

    1994-01-01

    We revisit the formation of radiative instabilities in cool star atmospheres and compare our results with those given by Muchmore, Nuth, & Stencel. We have considered the combined influence of CO and SiO molecules and have computed models for a grid of effective temperatures and geometrical dilution factors for the stellar radiation. Our results are based on the analysis of the energy balance of gas elements with prescribed thermodynamic properties. Our results show that radiative instabilities are most likely primarily caused by CO, whereas SiO is expected to play only a minor role, except when the CO density is reduced compared to Local Thermodynamic Equilibrium (LTE) values or the CO band can be assumed to be optically thick. The onset of radiative instabilities is expected to be strongly modified when dynamic phenomena such as stochastic shocks are present. Our results provide strong evidence that dust formation can most likely occur via a radiative instability alone. Therefore, we present a revised version of the Muchmore et al. dust formation paradigm, which also considers hydrodynamic cooling. The new paradigm is particularly relevant in cases where dust is formed relatively close to the stellar photosphere.

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

    NASA Technical Reports Server (NTRS)

    Harper, Graham M.

    2002-01-01

    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.

  15. Contribution of Low Mass Galaxies to Reionization

    NASA Astrophysics Data System (ADS)

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

    2015-01-01

    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.

  16. The ACS LCID project. X. the star formation history of IC 1613: Revisiting the over-cooling problem

    SciTech Connect

    Skillman, Evan D.; Hidalgo, Sebastian L.; Monelli, Matteo; Gallart, Carme; Aparicio, Antonio E-mail: shidalgo@iac.es E-mail: carme@iac.es [Instituto de Astrofsica de Canarias, Va Lctea s and others

    2014-05-01

    We present an analysis of the star formation history (SFH) of a field near the half-light radius in the Local Group dwarf irregular galaxy IC 1613 based on deep Hubble Space Telescope Advanced Camera for Surveys imaging. Our observations reach the oldest main sequence turn-off, allowing a time resolution at the oldest ages of ?1 Gyr. Our analysis shows that the SFH of the observed field in IC 1613 is consistent with being constant over the entire lifetime of the galaxy. These observations rule out an early dominant episode of star formation in IC 1613. We compare the SFH of IC 1613 with expectations from cosmological models. Since most of the mass is in place at early times for low-mass halos, a naive expectation is that most of the star formation should have taken place at early times. Models in which star formation follows mass accretion result in too many stars formed early and gas mass fractions that are too low today (the 'over-cooling problem'). The depth of the present photometry of IC 1613 shows that, at a resolution of ?1 Gyr, the star formation rate is consistent with being constant, at even the earliest times, which is difficult to achieve in models where star formation follows mass assembly.

  17. 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

    NASA Astrophysics Data System (ADS)

    Fleming, Scott W.; Ge, Jian; Barnes, Rory; Beatty, Thomas G.; Crepp, Justin R.; De Lee, Nathan; Esposito, Massimiliano; Femenia, Bruno; Ferreira, Leticia; Gary, Bruce; Gaudi, B. Scott; Ghezzi, Luan; Gonzlez Hernndez, Jonay I.; Hebb, Leslie; Jiang, Peng; Lee, Brian; Nelson, Ben; Porto de Mello, Gustavo F.; Shappee, Benjamin J.; Stassun, Keivan; Thompson, Todd A.; Tofflemire, Benjamin M.; Wisniewski, John P.; Wood-Vasey, W. Michael; Agol, Eric; Allende Prieto, Carlos; Bizyaev, Dmitry; Brewington, Howard; Cargile, Phillip A.; Coban, Louis; Costello, Korena S.; da Costa, Luis N.; Good, Melanie L.; Hua, Nelson; Kane, Stephen R.; Lander, Gary R.; Liu, Jian; Ma, Bo; Mahadevan, Suvrath; Maia, Marcio A. G.; Malanushenko, Elena; Malanushenko, Viktor; Muna, Demitri; Nguyen, Duy Cuong; Oravetz, Daniel; Paegert, Martin; Pan, Kaike; Pepper, Joshua; Rebolo, Rafael; Roebuck, Eric J.; Santiago, Basilio X.; Schneider, Donald P.; Shelden, Alaina; Simmons, Audrey; Sivarani, Thirupathi; Snedden, Stephanie; Vincent, Chelsea L. M.; Wan, Xiaoke; Wang, Ji; Weaver, Benjamin A.; Weaver, Gwendolyn M.; Zhao, Bo

    2012-09-01

    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 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 1 = 1.21 0.08 M ? and R 1 = 1.09+0.15 - 0.13 R ?. 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 ?, 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 ~0.3-0.4 M ?. 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 {2, 3} <~ 1.0 M ?, provided they are not stellar remnants. If the tertiary is not a stellar remnant, then it likely has a mass of ~0.5-0.6 M ?, 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.

  18. 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

    Fleming, Scott W.; Ge Jian; De Lee, Nathan; Jiang Peng; Lee, Brian; Nelson, Ben; Barnes, Rory; Beatty, Thomas G.; Gaudi, B. Scott; Shappee, Benjamin J.; Crepp, Justin R.; Esposito, Massimiliano; Femenia, Bruno; Gonzalez Hernandez, Jonay I.; Ferreira, Leticia; Porto de Mello, Gustavo F.; Gary, Bruce; Hebb, Leslie; Stassun, Keivan; Ghezzi, Luan; and others

    2012-09-15

    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.

  19. Searching for Low-mass Companions of Cepheids, Part II

    NASA Astrophysics Data System (ADS)

    Remage Evans, Nancy; Tingle, E.; Bond, H. E.; Schaefer, G. H.; Mason, B.; Karovska, M.; Wolk, S.; Pillitteri, I.; DePasquale, J.; Guinan, E.; Engle, S.

    2012-01-01

    The formation of a binary/multiple system is an effective way to manipulate angular momentum during the star-formation process. The properties of binary systems (separations and mass ratios) are thus the ``fingerprints" of the process. Low mass companions are the most difficult to identify particularly for massive stars. We are conducting a snapshot survey of the nearest Cepheids (5 Msun stars) using the Hubble Space Telescope Wide Field Camera 3 (WFC3) to discover possible resolved low mass companions. The color-magnitude combination is the first approach to identifying probable physical companions. The distributions of mass and separation for these stars will be discussed. Financial suppoet was provided by Hubble grant GO-12215.01-A and the Chandra X-ray Center NASA contract NAS8-03060.

  20. Deuteration in low-mass protostars

    NASA Astrophysics Data System (ADS)

    Parise, B.

    2004-09-01

    Despite the low deuterium abundance in the Universe (D/H ~ 1.5e-5), high abundances of deuterated molecules are detected in star forming regions, with a fractionation (i.e. the ratio of deuterated over main isotopomer) higher than the cosmic abundance of deuterium by several orders of magnitude. Particularly, warm dense gas in hot cores around low-mass protostars is enriched in deuterated species, with even high observed abundances of doubly-deuterated species such as D2CO. These deuterated molecules provide valuable tools to probe the physical conditions occurring during star formation. Deuteration is thought to be driven by the small energy differences between a deuterated species and the normal isotope. Because the temperatures indicated by the fractionation are much lower than the present gas temperatures in hot cores, the observed deuterations are thought to reflect a previous cold phase. Likely these molecules formed during the preceding prestellar core phase -- either in the gas phase or on the grain surface -- and were stored in an ice mantle which evaporated once the YSO heated its environment above the ice sublimation temperature. We study in this thesis the physical and chemical processes leading to the high molecular deuteration observed in low-mass protostellar environments. We present observations of deuterated molecules (namely methanol, formaldehyde and water) both in the gas and in the icy mantles of dust grains in the envelope surrounding such objects. Millimeter observations unveiled a high deuteration of methanol in the gas of the envelope. In particular, triply-deuterated methanol was detected with a fractionation CD3OH/CH3OH ~ 1% in IRAS16293-2422. The observed fractionations are consistent with the scenario of formation of methanol on dust grain surfaces. Deuterated methanol and formaldehyde were then searched for and detected on a sample of low-mass Class 0 protostars, suggesting that this high deuteration is common in this class of objects. Analysis of the gas-phase water emission in the IRAS16293-2422 envelope leads paradoxically to a fractionation one order of magnitude lower, in agreement with the upper limit on water deuteration in ices, derived by near-infrared observations towards slightly more evolved objects. The last chapter of the thesis presents a grain chemistry model that studies in details water fractionation.

  1. Low-mass companions to Bright Giants

    NASA Astrophysics Data System (ADS)

    Niedzielski, A.; Wolszczan, A.; Nowak, G.; Adamw, M.; Deka, B.; Grecka, M.; Kowalik, K.

    2014-04-01

    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?ski 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.

  2. Statistical Characteristics of Low-Mass Companions (Extrasolar Planets)

    NASA Astrophysics Data System (ADS)

    Heacox, W. D.

    1998-12-01

    Statistical characteristics for recently discovered low-mass companions (``extrasolar planets") to solar-like stars, considered as a group, have been compared to those of stellar companions to solar-like stars in an unbiased survey. Nonparametric statistical models have been used to derive distributions of characteristics not individually observable in all systems, and to correct for observational selection of low-mass companions. The semi-major axis distributions of the two populations are of the same smooth, monotonically decreasing, scale-free form, strongly suggestive of dissipative orbital evolution by transfer of energy and angular momentum to dispersed material in young systems. The mass distribution of low-mass companions, corrected for observational selection due to limited radial velocity precision, is probably a power law with a relatively large negative index; this may be an extension of the stellar companion mass distribution if the latter's roll-over near 0.2 solar masses is an observational selection effect, as seems possible. There is no statistically significant evidence for the existence of two distinct populations (e.g., planets and brown dwarfs) of low-mass companions, and no statistically significant difference between the orbital characteristics of low-mass and stellar companions to solar-like primaries.

  3. NA62 Low Mass Spectrometer

    NASA Astrophysics Data System (ADS)

    Palladino, V.

    2014-06-01

    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.

  4. Feedback in low-mass galaxies in the early Universe.

    PubMed

    Erb, Dawn K

    2015-07-01

    The formation, evolution and death of massive stars release large quantities of energy and momentum into the gas surrounding the sites of star formation. This process, generically termed 'feedback', inhibits further star formation either by removing gas from the galaxy, or by heating it to temperatures that are too high to form new stars. Observations reveal feedback in the form of galactic-scale outflows of gas in galaxies with high rates of star formation, especially in the early Universe. Feedback in faint, low-mass galaxies probably facilitated the escape of ionizing radiation from galaxies when the Universe was about 500 million years old, so that the hydrogen between galaxies changed from neutral to ionized-the last major phase transition in the Universe. PMID:26156371

  5. Young, Low-Mass Spectroscopic Binaries in Nearby Moving Groups

    NASA Astrophysics Data System (ADS)

    Flagg, Laura; Shkolnik, Evgenya L.; Weinberger, Alycia J.; Bowler, Brendan P.; Kraus, Adam L.; Liu, Michael C.

    2016-01-01

    Young spectroscopic binaries (SB) allow us to determine dynamical masses of young stars, which are needed to constrain evolutionary models and processes. During a search for new young moving group (YMG) members with ages between 10 and 300 Myr, we collected optical spectra of many nearby low-mass stars. These data allow us to measure key youth indicators and kinematics. The spectra may also reveal double-lined SBs. Here we present several new, young, low-mass SBs. We calculated mass ratios and systemic velocities for these systems. Additionally, one of these SBs is not a kinematic match to any known YMG, yet has an age of less than 20 Myr. It may be the first system of a yet-to-be identified YMG.

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

    NASA Astrophysics Data System (ADS)

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

    2004-04-01

    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.

  7. DETECTION OF LOW-MASS-RATIO STELLAR BINARY SYSTEMS

    SciTech Connect

    Gullikson, Kevin; Dodson-Robinson, Sarah

    2013-01-01

    O- and B-type stars are often found in binary systems, but the low binary mass-ratio regime is relatively unexplored due to observational difficulties. Binary systems with low mass ratios may have formed through fragmentation of the circumstellar disk rather than molecular cloud core fragmentation. We describe a new technique sensitive to G- and K-type companions to early B stars, a mass ratio of roughly 0.1, using high-resolution, high signal-to-noise spectra. We apply this technique to a sample of archived VLT/CRIRES observations of nearby B stars in the CO bandhead near 2300 nm. While there are no unambiguous binary detections in our sample, we identify HIP 92855 and HIP 26713 as binary candidates warranting follow-up observations. We use our non-detections to determine upper limits to the frequency of FGK stars orbiting early B-type primaries.

  8. The low-mass dispersed population around the Lupus clouds

    NASA Astrophysics Data System (ADS)

    Comerón, F.; Spezzi, L.; López Martí, B.; Merín, B.

    2013-06-01

    Context. Many star forming regions are known to have associated dispersed populations, whose members are located away from known current star forming sites. Their origin is unclear, and any identification of the members through relatively short-lived signatures of youth can miss them. Aims: We aim at confirming membership of a sample of cool stars identified in a previous work in the Lupus 1, 3, and 4 clouds as candidate members. Most of them do not display near- or mid-infrared excess or any other easily recognizable signatures of youth. Methods: We use low-resolution spectroscopy in the red part of the spectrum, including the Hα region, to accurately determine spectral types and probe surface gravity-sensitive features that provide reliable criteria for distinguishing cool giant stars, young stellar objects, and evolved dwarf stars. Results: Most of the candidate members of a possible dispersed population around Lupus 1 are found to be background K or early M giants. However, about half of the observed members of Lupus 3 are confirmed as young objects, including both low-mass stars and brown dwarfs. The distributed population is compared to the sample of lightly obscured members projected closer to the densest parts of the Lupus 3 star forming cloud, and the estimated ages of the members of both sets are found to be consistent with a single distribution. However, we find statistical indications (although at a low significance level) of a decrease in the frequency of infrared excesses in the distributed population. Some nongiant members are also identified with gravity-sensitive features typical of more evolved stars, and we argue that these may belong to an older population associated with the Gould Belt, similar to what is observed in the direction of other nearby star forming regions. We also confirm two additional, very low-mass members of Lupus 4. Conclusions: Although some of its members have already been known previous to this work, our results emphasize the richness of the low-mass distributed population around Lupus 3 and the existence of much less numerous dispersed populations around Lupus 1 and Lupus 4. The apparent spatial segregation as a function of the abundance of circumstellar material favors dynamical ejection from the main star forming cloud as the mechanism that gives rise to the dispersed population. Based on observations collected with the Very Large Telescope (VLT) at the European Southern Observatory, Paranal, Chile, under observing program 086.C-0546(A).

  9. Meteor Beliefs Project: Shakespeare revisited and the Elizabethan stage's `blazing star'

    NASA Astrophysics Data System (ADS)

    Gheorghe, Andrei Dorian; McBeath, Alastair

    2007-06-01

    Some fresh Shakespearean citations of meteors, further to those given previously in the Project, are presented, along with a discussion of the Elizabethan stage's use of the `blazing star', with especial reference to the great comet of 1577.

  10. The rapid dispersal of low-mass virialized clusters

    NASA Astrophysics Data System (ADS)

    Moeckel, Nickolas; Holland, Christopher; Clarke, Cathie J.; Bonnell, Ian A.

    2012-09-01

    Infant mortality brought about by the expulsion of a star cluster's natal gas is widely invoked to explain cluster statistics at different ages. While a well-studied problem, most recent studies of gas expulsion's effect on a cluster have focused on massive clusters, with stellar counts of the order of 104. Here we argue that the evolutionary time-scales associated with the compact low-mass clusters typical of the median cluster in the solar neighbourhood are short enough that significant dynamical evolution can take place over the ages usually associated with gas expulsion. To test this, we perform N-body simulations of the dynamics of a very young star-forming region, with initial conditions drawn from a large-scale hydrodynamic simulation of gravitational collapse and fragmentation. The subclusters we analyse, with populations of a few hundred stars, have high local star formation efficiencies and are roughly virialized even after the gas is removed. Over 10 Myr, they expand to a similar degree as would be expected from gas expulsion if they were initially gas rich, but the expansion is purely due to the internal stellar dynamics of the young clusters. The expansion is such that the stellar densities at 2 Myr match those of young stellar objects in the solar neighbourhood. We argue that at the low-mass end of the cluster mass spectrum, a deficit of clusters at tens of Myr does not necessarily imply gas expulsion as a disruption mechanism.

  11. s-Processing in AGB Stars Revisited. II. Enhanced 13C Production through MHD-induced Mixing

    NASA Astrophysics Data System (ADS)

    Trippella, O.; Busso, M.; Palmerini, S.; Maiorca, E.; Nucci, M. C.

    2016-02-01

    Slow neutron captures are responsible for the production of about 50% of elements heavier than iron, mainly occurring during the asymptotic giant branch phase of low-mass stars (1 ≲ M/M⊙ ≲ 3), where the main neutron source is the 13C(α, n)16O reaction. This last reaction is activated from locally produced 13C, formed by partial mixing of hydrogen into the He-rich layers. We present here the first attempt to describe a physical mechanism for the formation of the 13C reservoir, studying the mass circulation induced by magnetic buoyancy without adding new free parameters to those already involved in stellar modeling. Our approach represents the application to the stellar layers relevant for s-processing of recent exact analytical 2D and 3D models for magneto-hydrodynamic processes at the base of convective envelopes in evolved stars in order to promote downflows of envelope material for mass conservation during the occurrence of a dredge-up phenomenon. We find that the proton penetration is characterized by small concentrations, but is extended over a large fractional mass of the He-layers, thus producing 13C reservoirs of several 10‑3 M⊙. The ensuing 13C-enriched zone has an almost flat profile, while only a limited production of 14N occurs. In order to verify the effects of our new findings we show how the abundances of the main s-component nuclei can be accounted for in solar proportions and how our large 13C-reservoir allows us to solve a few so far unexplained features in the abundance distribution of post-AGB objects.

  12. Five New Low-Mass Eclipsing Binary Systems

    NASA Astrophysics Data System (ADS)

    Coughlin, Jeffrey L.; Lpez-Morales, M.; Shaw, J. S.

    2006-12-01

    We present the discovery of five new low-mass eclipsing binaries with masses between 0.54 and 0.95 M?, their photometric light curves, and preliminary models. This is part of a continuing campaign to increase the available data on these interesting systems. Once radial-velocity curves are completed, physical parameters will be determined with an error of less than 2-3%, thus allowing for a rigorous examination of stellar models in the lower-main sequence. Our initial analysis seems to support the current findings that low-mass stars have greater radii than models predict, most likely due to the presence of strong magnetic fields. This work is funded by a partnership between the National Science Foundation (NSF AST-0552798) Research Experiences for Undergraduates (REU) and the Department of Defense (DoD) ASSURE (Awards to Stimulate and Support Undergraduate Research Experiences) programs.

  13. Very-low-mass Stellar and Substellar Companions to Solar-like Stars from Marvels. III. A Short-period Brown Dwarf Candidate around an Active G0IV Subgiant

    NASA Astrophysics Data System (ADS)

    Ma, Bo; Ge, Jian; Barnes, Rory; Crepp, Justin R.; De Lee, Nathan; Dutra-Ferreira, Leticia; Esposito, Massimiliano; Femenia, Bruno; Fleming, Scott W.; Gaudi, B. Scott; Ghezzi, Luan; Hebb, Leslie; Gonzalez Hernandez, Jonay I.; Lee, Brian L.; Porto de Mello, G. F.; Stassun, Keivan G.; Wang, Ji; Wisniewski, John P.; Agol, Eric; Bizyaev, Dmitry; Cargile, Phillip; Chang, Liang; Nicolaci da Costa, Luiz; Eastman, Jason D.; Gary, Bruce; Jiang, Peng; Kane, Stephen R.; Li, Rui; Liu, Jian; Mahadevan, Suvrath; Maia, Marcio A. G.; Muna, Demitri; Nguyen, Duy Cuong; Ogando, Ricardo L. C.; Oravetz, Daniel; Pepper, Joshua; Paegert, Martin; Allende Prieto, Carlos; Rebolo, Rafael; Santiago, Basilio X.; Schneider, Donald P.; Shelden, Alaina; Simmons, Audrey; Sivarani, Thirupathi; van Eyken, J. C.; Wan, Xiaoke; Weaver, Benjamin A.; Zhao, Bo

    2013-01-01

    We present an eccentric, short-period brown dwarf candidate orbiting the active, slightly evolved subgiant star TYC 2087-00255-1, which has effective temperature T eff = 5903 42 K, surface gravity log (g) = 4.07 0.16 (cgs), and metallicity [Fe/H] = -0.23 0.07. This candidate was discovered using data from the first two years of the Multi-object APO Radial Velocity Exoplanets Large-area Survey, which is part of the third phase of Sloan Digital Sky Survey. From our 38 radial velocity measurements spread over a two-year time baseline, we derive a Keplerian orbital fit with semi-amplitude K = 3.571 0.041 km s-1, period P = 9.0090 0.0004 days, and eccentricity e = 0.226 0.011. Adopting a mass of 1.16 0.11 M ? for the subgiant host star, we infer that the companion has a minimum mass of 40.0 2.5 M Jup. Assuming an edge-on orbit, the semimajor axis is 0.090 0.003 AU. The host star is photometrically variable at the ~1% level with a period of ~13.16 0.01 days, indicating that the host star spin and companion orbit are not synchronized. Through adaptive optics imaging we also found a point source 643 10 mas away from TYC 2087-00255-1, which would have a mass of 0.13 M ? if it is physically associated with TYC 2087-00255-1 and has the same age. Future proper motion observation should be able to resolve if this tertiary object is physically associated with TYC 2087-00255-1 and make TYC 2087-00255-1 a triple body system. Core Ca II H and K line emission indicate that the host is chromospherically active, at a level that is consistent with the inferred spin period and measured v rotsin i, but unusual for a subgiant of this T eff. This activity could be explained by ongoing tidal spin-up of the host star by the companion.

  14. VERY-LOW-MASS STELLAR AND SUBSTELLAR COMPANIONS TO SOLAR-LIKE STARS FROM MARVELS. III. A SHORT-PERIOD BROWN DWARF CANDIDATE AROUND AN ACTIVE G0IV SUBGIANT

    SciTech Connect

    Ma Bo; Ge Jian; De Lee, Nathan; Fleming, Scott W.; Lee, Brian L.; Wang Ji; Barnes, Rory; Agol, Eric; Crepp, Justin R.; Dutra-Ferreira, Leticia; Porto de Mello, G. F.; Esposito, Massimiliano; Femenia, Bruno; Gonzalez Hernandez, Jonay I.; Ghezzi, Luan; Hebb, Leslie; Stassun, Keivan G.; Wisniewski, John P.; Bizyaev, Dmitry; and others

    2013-01-01

    We present an eccentric, short-period brown dwarf candidate orbiting the active, slightly evolved subgiant star TYC 2087-00255-1, which has effective temperature T{sub eff} = 5903 {+-} 42 K, surface gravity log (g) = 4.07 {+-} 0.16 (cgs), and metallicity [Fe/H] = -0.23 {+-} 0.07. This candidate was discovered using data from the first two years of the Multi-object APO Radial Velocity Exoplanets Large-area Survey, which is part of the third phase of Sloan Digital Sky Survey. From our 38 radial velocity measurements spread over a two-year time baseline, we derive a Keplerian orbital fit with semi-amplitude K = 3.571 {+-} 0.041 km s{sup -1}, period P = 9.0090 {+-} 0.0004 days, and eccentricity e = 0.226 {+-} 0.011. Adopting a mass of 1.16 {+-} 0.11 M{sub Sun} for the subgiant host star, we infer that the companion has a minimum mass of 40.0 {+-} 2.5 M{sub Jup}. Assuming an edge-on orbit, the semimajor axis is 0.090 {+-} 0.003 AU. The host star is photometrically variable at the {approx}1% level with a period of {approx}13.16 {+-} 0.01 days, indicating that the host star spin and companion orbit are not synchronized. Through adaptive optics imaging we also found a point source 643 {+-} 10 mas away from TYC 2087-00255-1, which would have a mass of 0.13 M{sub Sun} if it is physically associated with TYC 2087-00255-1 and has the same age. Future proper motion observation should be able to resolve if this tertiary object is physically associated with TYC 2087-00255-1 and make TYC 2087-00255-1 a triple body system. Core Ca II H and K line emission indicate that the host is chromospherically active, at a level that is consistent with the inferred spin period and measured v{sub rot}sin i, but unusual for a subgiant of this T{sub eff}. This activity could be explained by ongoing tidal spin-up of the host star by the companion.

  15. Formation of elongated galaxies with low masses at high redshift

    NASA Astrophysics Data System (ADS)

    Ceverino, Daniel; Primack, Joel; Dekel, Avishai

    2015-10-01

    We report the identification of elongated (triaxial or prolate) galaxies in cosmological simulations at z ≃ 2. These are preferentially low-mass galaxies (M* ≤ 109.5 M⊙), residing in dark matter (DM) haloes with strongly elongated inner parts, a common feature of high-redshift DM haloes in the Λ cold dark matter cosmology. Feedback slows formation of stars at the centres of these haloes, so that a dominant and prolate DM distribution gives rise to galaxies elongated along the DM major axis. As galaxies grow in stellar mass, stars dominate the total mass within the galaxy half-mass radius, making stars and DM rounder and more oblate. A large population of elongated galaxies produces a very asymmetric distribution of projected axis ratios, as observed in high-z galaxy surveys. This indicates that the majority of the galaxies at high redshifts are not discs or spheroids but rather galaxies with elongated morphologies.

  16. Revisiting the variable star population in NGC 6229 and the structure of the horizontal branch

    NASA Astrophysics Data System (ADS)

    Arellano Ferro, A.; Mancera Pia, P. E.; Bramich, D. M.; Giridhar, Sunetra; Ahumada, J. A.; Kains, N.; Kuppuswamy, K.

    2015-09-01

    We report an analysis of new V and I CCD time series photometry of the distant globular cluster NGC 6229. The principal aims were to explore the field of the cluster in search of new variables, and to Fourier decompose the RR Lyrae light curves in pursuit of physical parameters. We found 25 new variables: 10 RRab, 5 RRc, 6 SR, 1 CW, 1 SX Phe, and 2 that we were unable to classify. Secular period changes were detected and measured in some favourable cases. The classifications of some of the known variables were rectified. The Fourier decomposition of RRab and RRc light curves was used to independently estimate the mean cluster value of [Fe/H] and distance. From the RRab stars we found [Fe/H]UVES = -1.31 0.01(statistical) 0.12(systematic) ([Fe/H]ZW = -1.42) and a distance of 30.0 1.5 kpc, and from the RRc stars we found [Fe/H]UVES = -1.29 0.12 and a distance of 30.7 1.1 kpc, respectively. Absolute magnitudes, radii and masses are also reported for individual RR Lyrae stars. Also discussed are the independent estimates of the cluster distance from the tip of the red giant branch, 34.9 2.4 kpc and from the period-luminosity relation of SX Phe stars, 28.9 2.2 kpc. The distribution of RR Lyrae stars in the horizontal branch shows a clear empirical border between stable fundamental and first overtone pulsators which has been noted in several other clusters; we interpret it as the red edge of the first overtone instability strip.

  17. 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

    NASA Astrophysics Data System (ADS)

    De Lee, Nathan; Ge, Jian; Crepp, Justin R.; Eastman, Jason; Esposito, Massimiliano; Femenía, Bruno; Fleming, Scott W.; Gaudi, B. Scott; Ghezzi, Luan; González Hernández, Jonay I.; Lee, Brian L.; Stassun, Keivan G.; Wisniewski, John P.; Wood-Vasey, W. Michael; Agol, Eric; Allende Prieto, Carlos; Barnes, Rory; Bizyaev, Dmitry; Cargile, Phillip; Chang, Liang; Da Costa, Luiz N.; Porto De Mello, G. F.; Ferreira, Leticia D.; Gary, Bruce; Hebb, Leslie; Holtzman, Jon; Liu, Jian; Ma, Bo; Mack, Claude E., III; Mahadevan, Suvrath; Maia, Marcio A. G.; Nguyen, Duy Cuong; Oravetz, Audrey; Oravetz, Daniel J.; Paegert, Martin; Pan, Kaike; Pepper, Joshua; Malanushenko, Elena; Malanushenko, Viktor; Rebolo, Rafael; Santiago, Basilio X.; Schneider, Donald P.; Shelden Bradley, Alaina C.; Wan, Xiaoke; Wang, Ji; Zhao, Bo

    2013-06-01

    We describe the discovery of a likely brown dwarf (BD) companion with a minimum mass of 31.7 ± 2.0 M 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 ⊙, and thus it is a high-confidence BD companion. It has a moderately long orbital period of 47.8929^{+0.0063}_{-0.0062} days with a low eccentricity of 0.1442^{+0.0078}_{-0.0073}, and a semi-amplitude of 1644^{+12}_{-13} m s-1. Moderate resolution spectroscopy of the host star has determined the following parameters: T 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 * = 1.11 ± 0.11 M ⊙ and R * = 1.06 ± 0.23 R ⊙ with an age consistent with less than ~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.

  18. 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

    De Lee, Nathan; Stassun, Keivan G.; Cargile, Phillip; Ge, Jian; Fleming, Scott W.; Lee, Brian L.; Chang Liang; Crepp, Justin R.; Eastman, Jason; Gaudi, B. Scott; Esposito, Massimiliano; Femenia, Bruno; Gonzalez Hernandez, Jonay I.; Allende Prieto, Carlos; Ghezzi, Luan; Wisniewski, John P.; Wood-Vasey, W. Michael; Agol, Eric; Barnes, Rory; Bizyaev, Dmitry; and others

    2013-06-15

    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.

  19. Tidal evolution of close binary stars. I - Revisiting the theory of the equilibrium tide

    NASA Technical Reports Server (NTRS)

    Zahn, J.-P.

    1989-01-01

    The theory of the equilibrium tide in stars that possess a convective envelope is reexamined critically, taking recent developments into account and treating thermal convection in the most consistent way within the mixing-length approach. The weak points are identified and discussed, in particular, the reduction of the turbulent viscosity when the tidal period becomes shorter than the convective turnover time. An improved version is derived for the secular equations governing the dynamical evolution of close binaries of such type.

  20. New Low-Mass Members of Nearby Young Moving Groups

    NASA Astrophysics Data System (ADS)

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

    2012-08-01

    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.

  1. Low-Mass Companions in Two Large Spectroscopic Binary Surveys

    NASA Astrophysics Data System (ADS)

    Latham, D. W.; Stefanik, R. P.

    Two large radial-velocity surveys for spectroscopic binaries using the CfA Digital Speedometers are now nearing completion. The first is a survey of nearly 1500 stars in the Carney-Latham proper-motion sample, while the second is a survey of more than 1500 nearby G dwarfs. Altogether more than 50,000 velocity observations have been accumulated over a period of 15 years. We will report on the frequency and orbital characteristics of the binaries found in these surveys, with emphasis on the low-mass companions near the bottom of the main sequence.

  2. Leo P: An Unquenched Very Low-mass Galaxy

    NASA Astrophysics Data System (ADS)

    McQuinn, Kristen B. W.; Skillman, Evan D.; Dolphin, Andrew; Cannon, John M.; Salzer, John J.; Rhode, Katherine L.; Adams, Elizabeth A. K.; Berg, Danielle; Giovanelli, Riccardo; Girardi, Léo; Haynes, Martha P.

    2015-10-01

    Leo P is a low-luminosity dwarf galaxy discovered through the blind H i Arecibo Legacy Fast ALFA survey. The H i and follow-up optical observations have shown that Leo P is a gas-rich dwarf galaxy with active star formation, an underlying older population, and an extremely low oxygen abundance. We have obtained optical imaging with the Hubble Space Telescope to two magnitudes below the red clump in order to study the evolution of Leo P. We refine the distance measurement to Leo P to be 1.62 ± 0.15 Mpc, based on the luminosity of the horizontal branch stars and 10 newly identified RR Lyrae candidates. This places the galaxy at the edge of the Local Group, ˜0.4 Mpc from Sextans B, the nearest galaxy in the NGC 3109 association of dwarf galaxies of which Leo P is clearly a member. The star responsible for ionizing the H ii region is most likely an O7V or O8V spectral type, with a stellar mass ≳25 M⊙. The presence of this star provides observational evidence that massive stars at the upper end of the initial mass function are capable of being formed at star formation rates as low as ˜10-5 M⊙ yr-1. The best-fitting star formation history (SFH) derived from the resolved stellar populations of Leo P using the latest PARSEC models shows a relatively constant star formation rate over the lifetime of the galaxy. The modeled luminosity characteristics of Leo P at early times are consistent with low-luminosity dSph Milky Way satellites, suggesting that Leo P is what a low-mass dSph would look like if it evolved in isolation and retained its gas. Despite the very low mass of Leo P, the imprint of reionization on its SFH is subtle at best, and consistent with being totally negligible. The isolation of Leo P, and the total quenching of star formation of Milky Way satellites of similar mass, implies that the local environment dominates the quenching of the Milky Way satellites. Based on observations made with the NASA/ESA Hubble Space Telescope (HST), obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

  3. The Low-mass Astrometric Binary LSR 1610–0040

    NASA Astrophysics Data System (ADS)

    Koren, Seth C.; Blake, Cullen H.; Dahn, Conard C.; Harris, Hugh C.

    2016-03-01

    Even though it was discovered more than a decade ago, LSR 1610‑0040 remains an enigma. This object has a peculiar spectrum that exhibits some features typically found in L subdwarfs, and others common in the spectra of more massive M dwarf stars. It is also a binary system with a known astrometric orbital solution. Given the available data, it remains a challenge to reconcile the observed properties of the combined light of LSR 1610‑0040AB with current theoretical models of low-mass stars and brown dwarfs. We present the results of a joint fit to both astrometric and radial velocity measurements of this unresolved, low-mass binary. We find that the photocentric orbit has a period P=633.0+/- 1.7 days, somewhat longer than previous results, eccentricity of e=0.42+/- 0.03, and we estimate that the semimajor axis of the orbit of the primary is {a}1≈ 0.32 {{AU}}, consistent with previous results. While a complete characterization of the system is limited by our small number of radial velocity measurements, we establish a likely primary mass range of 0.09–0.10 {M}ȯ from photometric and color–magnitude data. For a primary mass in this range, the secondary is constrained to be 0.06–0.075 {M}ȯ , making a negligible contribution to the total I-band luminosity. This effectively rules out the possibility of the secondary being a compact object such as an old, low-mass white dwarf. Based on our analysis, we predict a likely angular separation at apoapsis comparable to the resolution limits of current high-resolution imaging systems. Measuring the angular separation of the A and B components would finally enable a full, unambiguous solution for the masses of the components of this system.

  4. Low mass rolling element for bearings

    NASA Technical Reports Server (NTRS)

    Parker, R. J. (inventor)

    1973-01-01

    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.

  5. The active W UMa type binary star V781 Tau revisited

    NASA Astrophysics Data System (ADS)

    Li, K.; Gao, D.-Y.; Hu, S.-M.; Guo, D.-F.; Jiang, Y.-G.; Chen, X.

    2016-02-01

    In this paper, new determined BVRcIc light curves and radial velocities of V781 Tau are presented. By analyzing the light curves and radial velocities simultaneously, we found that V781 Tau is a W-subtype medium contact binary star with a mass ratio of q=2.207±0.005 and a contact degree of f=21.6(±1.0) %. The difference between the two light maxima was explained by a dark spot on the less massive primary component. The change of the orbital period of V781 Tau was also investigated. A secular decrease at a rate of -6.01(±2.28)× 10^{-8} d/yr and a cyclic modulation with a period of 44.8 ± 5.7 yr and an amplitude of 0.0064±0.0011 day were discovered. The continuous period decrease may be caused by angular momentum loss due to a magnetic stellar wind. The Applegate mechanism failed to explain the cyclic modulation. It is highly possible that the cyclic oscillation is the result of the light travel time effect by a third companion.

  6. Timing analysis of the isolated neutron star RX J0720.4-3125 revisited

    NASA Astrophysics Data System (ADS)

    Cropper, Mark; Haberl, Frank; Zane, Silvia; Zavlin, Vyacheslav E.

    2004-07-01

    We present a reanalysis of the X-ray data for RX J0720.4-3125 presented in our previous paper, Zane et al., using more data recently available from XMM-Newton and Chandra. This analysis also corrects the ROSAT data used in that paper to the barycentric dynamical time (TDB) system, incorporates the revised XMM-Newton barycentric correction available since then, and corrects the definition of the instantaneous period in the maximum likelihood periodogram search. However, we are now unable to find a single coherent period that is consistent with all ROSAT, Chandra and XMM-Newton data sets. From an analysis of the separate data sets, we have derived limits on the period change of at 99 per cent confidence level. This is stronger than the value presented in Zane et al., but sufficiently similar that their scientific conclusions remain unchanged. We examine the implications in more detail, and find that RX J0720.4-3125 can have been born as a magnetar provided that it has a young age of ~104 yr. A more conservative interpretation is that the field strength has remained relatively unchanged at just over 1013 G, over the ~106-yr lifetime of the star.

  7. The Active Contact Binary Ty UMa Revisited: is It a Quadruple Star?

    NASA Astrophysics Data System (ADS)

    Li, K.; Hu, S.-M.; Guo, D.-F.; Jiang, Y.-G.; Gao, Y.-G.; Chen, X.; Odell, Andrew P.

    2015-04-01

    TY UMa is an F-type eclipsing binary star. Four-color light curves and radial velocities of this system were presented and simultaneously analyzed using the W-D code. It is found that TY UMa is a W-subtype shallow contact binary system (f=13.4%) with a mass ratio of q = 2.523. In order to explain the asymmetric light curve of this binary, a dark spot on the less massive component was employed. Our newly determined 31 times of minimum light, including those collected from the literature, have been used to analyze orbital period changes of TY UMa. The complicated period variation could be sorted into a secular period increase at a rate of dp/dt\\=\\+5.18(+/- 0.21) {{10}-7} days yr-1, a 51.7 yr periodic modulation (A3 = 0.0182 days), and a very small amplitude cyclic oscillation with a period of 10.0 yr (A4 = 0.0015 days). The long-term increase of the period can be explained by mass transfer from the less massive component to the more massive one. The Applegate mechanism may impossibly explain the two cyclic components in the period. The two cyclic variations are very likely caused by the light travel time effect of third and fourth components, suggesting that TY UMa is a quadruple system.

  8. BINARY STAR ORBITS. III. REVISITING THE REMARKABLE CASE OF TWEEDLEDUM AND TWEEDLEDEE

    SciTech Connect

    Mason, Brian D.; Hartkopf, William I.; McAlister, Harold A. E-mail: wih@usno.navy.mi

    2010-07-15

    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{sup 0} 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.

  9. A Unified Model of Low Mass X-ray Binaries

    NASA Astrophysics Data System (ADS)

    Balucinska-Church, M.; Church, M.

    2014-07-01

    We present a unified physical model of Low Mass X-ray Binaries explaining the basic Atoll and Z-track types of source. In all LMXB with luminosity above 1-2.10^{37} erg/s, we have a new fundamental result that the temperature of the Comptonizing ADC corona equals that of the neutron star, i.e. there is thermal equilibrium. This equilibrium explains the properties of the basic Banana State of Atoll sources. Below this luminosity, equilibrium breaks down, T_ADC rising towards 100 keV by an unknown heating mechanism, explaining the Island State. Above 5.10^{37} erg/s flaring begins in the GX-Atolls which we show is unstable nuclear burning. Above 1.10^{38} erg/s, LMXB are seen as Z-track sources. Flaring in these and the GX-Atolls occurs when the mass accretion rate to the neutron star falls to the critical value for unstable nuclear burning on the star. Below 2.10^{37} erg/s, a different unstable burning: X-ray bursting, takes over. We show that the Normal Branch of the Z-track consists simply of increasing mass accretion rate, as is the Banana State in Atolls. In the Horizontal Branch, a measured, strongly increasing radiation pressure of the neutron star disrupts the inner disk launching the relativistic jets seen on this branch.

  10. Effect of low mass dark matter particles on the Sun

    SciTech Connect

    Taoso, Marco; Iocco, Fabio; Meynet, Georges; Eggenberger, Patrick; Bertone, Gianfranco

    2010-10-15

    We study the effect of dark matter (DM) particles in the Sun, focusing, in particular, on the possible reduction of the solar neutrinos flux due to the energy carried away by DM particles from the innermost regions of the Sun, and to the consequent reduction of the temperature of the solar core. We find that in the very low-mass range between 4 and 10 GeV, recently advocated to explain the findings of the DAMA and CoGent experiments, the effects on neutrino fluxes are detectable only for DM models with a very small, or vanishing, self-annihilation cross section, such as the so-called asymmetric DM models, and we study the combination of DM masses and spin-dependent cross sections which can be excluded with current solar neutrino data. Finally, we revisit the recent claim that DM models with large self-interacting cross sections can lead to a modification of the position of the convective zone, alleviating or solving the solar composition problem. We show that when the 'geometric' upper limit on the capture rate is correctly taken into account, the effects of DM are reduced by orders of magnitude, and the position of the convective zone remains unchanged.

  11. WISHes coming true: low-mass protostars as chemical fountains

    NASA Astrophysics Data System (ADS)

    Kristensen, L. E.; van Dishoeck, E.; Yildiz, U.; Visser, R.; Herczeg, G.; Jorgensen, J.; van Kempen, T.; Hogerheijde, M.; WISH Team

    2011-05-01

    Water is a key molecule for tracing the chemical and physical processes taking place during the formation of low-mass stars in the Galaxy. Water is a unique molecule for tracing all physical components of a star-forming object: cold quiescent gas in the outer envelope, warm gas in the inner turbulent envelope, the molecular jet, shocks along the outflow cavity walls and UV-heated cavity walls. The "Water in star-forming regions with Herschel" (WISH; van Dishoeck et al. 2011) program uses a combination of high spectral and spatial resolution observations of water and related molecules to study each component and its associated chemistry. The chemistry is directly reflected in the physical processes listed above: molecule formation on grain mantles, non-thermal and thermal desorption from grain surfaces to the gas phase, molecule survival, destruction and reformation under extreme shock conditions and photo-dissociation and -chemistry. Recent results from WISH will be presented here. These include a detailed comparison of the properties of water across the entire sample of some 30 low-mass young stellar objects observed, and reveal how the chemistry and physics couple over a wide range of temperatures and densities. A comparison will also be made to other chemical tracers, such as CH3OH, a well-known grain-surface product. Interpreting these data requires a combination of state-of-the-art chemical and physical models. New advances in the 2D modeling, interpretation and understanding of the densest parts of the interstellar medium surrounding protostars will be highlighted.

  12. Calorimetry of low mass Pu239 items

    SciTech Connect

    Cremers, Teresa L; Sampson, Thomas E

    2010-01-01

    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.

  13. Evidence for the Rapid Formation of Low-mass Early-type Galaxies in Dense Environments

    NASA Astrophysics Data System (ADS)

    Liu, Yiqing; Peng, Eric W.; Blakeslee, John; Côté, Patrick; Ferrarese, Laura; Jordán, Andrés; Puzia, Thomas H.; Toloba, Elisa; Zhang, Hong-Xin

    2016-02-01

    We explore the environmental dependence of star formation timescales in low-mass galaxies using the [α/Fe] abundance ratio as an evolutionary clock. We present integrated [α/Fe] measurements for 11 low-mass ({M}\\star ∼ {10}9 {M}ȯ ) early-type galaxies (ETGs) with a large range of cluster-centric distance in the Virgo Cluster. We find a gradient in [α/Fe], where the galaxies closest to the cluster center (the cD galaxy, M87) have the highest values. This trend is driven by galaxies within a projected radius of 0.4 Mpc (0.26 times the virial radius of Virgo A), all of which have super-solar [α/Fe]. Galaxies in this mass range exhibit a large scatter in the [α/Fe]–σ diagram, and do not obviously lie on an extension of the relation defined by massive ETGs. In addition, we find a correlation between [α/Fe] and globular cluster specific frequency (SN), suggesting that low-mass ETGs that formed their stars over a short period of time were also efficient at forming massive star clusters. The innermost low-mass ETGs in our sample have [α/Fe] values comparable to that of M87, implying that environment is the controlling factor for star formation timescales in dense regions. These low-mass galaxies could be the surviving counterparts of the objects that have already been accreted into the halo of M87, and may be the link between present-day low-mass galaxies and the old, metal-poor, high-[α/Fe], high-SN stellar populations seen in the outer halos of massive ETGs.

  14. The Circumstellar Environment of Low Mass Star Forming Regions

    NASA Technical Reports Server (NTRS)

    Butner, Harold M.

    1999-01-01

    The final technical report of the NASA grant project is presented. The goals of the grant were to: (1) analyze the data from the Far-Infrared (FIR) Camera on board the Kuiper Airborne Observatory (KAO); (2) acquire additional data at other wavelengths for models and (4) to develop source models for the Young stellar objects (YSOs)under study. The complete Spectral Energy Distribution (SED) from 10 microns out to 1.3 mm for all sources being studied have been obtained. The FIR imaging data was processed to reveal the maximum angular resolution possible, which allows us to model the disk. To model the disk we have the high resolution millimeter interferometry data. In summary the results to date are: (1) the vast majority of embedded YSOs in Taurus are compact at 100 microns. The models mos consistent with our data and other observations are either dominated by disk emissions, or envelopes that have relatively steep density gradients; (2) the submillimeter/millimeter photometer suggests that models are very successful. Disk emission plays an important role and must be considered when predicting the overall emission. (3) in the two cases, where we seem to have extended emission, we have to investigate other possible source models than a Shu collapse.

  15. The Circumstellar Environment of Low Mass Star Forming Regions

    NASA Technical Reports Server (NTRS)

    Butner, Harold M.

    1997-01-01

    We have obtained the complete SED from 10 microns out to 1.3 mm for all of our sources. We have the FIR imaging data, processed to reveal the maximum angular resolution possible, which allows us to model the disk. To model the disk, we have high resolution millimeter interferometry data.

  16. Rotational velocities of newly discovered, low-mass members of the Alpha Persei cluster

    NASA Technical Reports Server (NTRS)

    Stauffer, John R.; Hartmann, Lee W.; Jones, Burton F.

    1989-01-01

    About 30 new, low-mass members of the young open cluster Alpha Persei are identified via a proper-motion study and subsequent photometric and spectroscopic observations. Membership in the cluster is confirmed for a number of the fainter proper-motion candidates from Heckman, Dieckvoss, and Kox (1956). Coordinates, finding charts, BVRI photometry, and rotational velocities are provided for most of the stars. At least two of the stars show peculiar H-alpha emission profiles, with weak but very broad emission wings, and relatively narrow absorption reversals. The rotational velocity distribution for low-mass stars in the Alpha Per cluster are compared with recently derived rotational velocity distributions for T Tauri stars, placing strong constraints on the mechanisms for angular momentum loss during pre-main-sequence evolution.

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

    SciTech Connect

    Hwang, K.-H.; Han, C.; Ryu, Y.-H.; Udalski, A.; Kubiak, M.; Szymanski, M. K.; Pietrzynski, G.; Soszynski, I.; Szewczyk, O.; Ulaczyk, K.; Wyrzykowski, L.; Bond, I. A.; Beaulieu, J.-P.; Dominik, M.; Horne, K.; Gould, A.; Gaudi, B. S.; Abe, F.; Botzler, C. S.; Hearnshaw, J. B.

    2010-11-01

    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.

  18. Chemistry in low-mass protostellar and protoplanetary regions.

    PubMed

    van Dishoeck, Ewine F

    2006-08-15

    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

  19. Chemistry in low-mass protostellar and protoplanetary regions

    PubMed Central

    van Dishoeck, Ewine F.

    2006-01-01

    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

  20. Brief search for low-mass objects

    NASA Astrophysics Data System (ADS)

    Krishna Kumar, C.

    1985-03-01

    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.