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1

Gravitational waves from low mass neutron stars  

SciTech Connect

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

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

2010-05-15

2

The origin of low mass stars.  

PubMed

Recent evidence indicates that most low mass stars in the Galaxy (< 5 M [symbol: see text]) form alongside massive stars in clusters embedded in giant molecular clouds. Once their parental gas is removed, the fate of these clusters is to disperse and blend into the field population of the galactic disk. The distribution of stellar masses in the solar neighborhood, called the Initial Mass Function, is discussed in the context of the origin of low mass stars. Arguments based on the production rate of field stars are presented that point to giant molecular clouds as the primary birth sites for low mass stars. The role of observations of molecular clouds at millimeter and infrared wavelengths in confirming this picture is reviewed. Millimeter-wave observations have revealed that molecular clouds consist of low-density gas interspersed with high-density cores. Near-infrared images of these clouds indicate that stars form preferentially in these cores, with the number of young stars roughly scaling with the mass of the core. Molecular-line and near-infrared observations which characterize star formation in the nearest giant molecular cloud complex in Orion are presented. The implications for the Sun forming in a cluster environment are briefly discussed. PMID:9150571

Wilking, B A

1997-06-01

3

Constraining Star Formation: Low Mass Companions of 5 Msun Stars  

NASA Astrophysics Data System (ADS)

Creating a binary/multiple system during star formation is an effective way to manipulate angular momentum, an important ingredient in the process, leaving binary parameters as a fingerprint. We have surveyed 69 Cepheids with the Hubble Space Telescope Wide Field Camera 3 (WFC3) survey, and identified 39 possible low-mass companions to these 5 Msun supergiants, which have the surprising characteristic that there are approximately as many F-G dwarfs as K dwarfs. X-rays are the key to determining whether they are physical companions or chance alignments, and this proposal is a pilot X-ray project to identify genuine companions to test this.

Remage Evans, Nancy

2011-10-01

4

Submillimeter astronomy and low mass star formation  

NASA Technical Reports Server (NTRS)

Surveys made at wavelengths shorter than 100 microns are criticized in that they are biased against finding the coldest, most deeply obscured, and possibly youngest protostars. Large scale mapping of star forming regions in the submillimeter continuum and in lines such as CS H2CO and NH3, that are excited at high density, is identified as crucial in order to find clouds in the earliest stages of collapse into stars. The use of photometry from 1 to 1000 microns in characterizing the disks around embedded stars and T Tauri stars is outlined.

Beichman, Charles A.

1990-01-01

5

A Case Study of Low-Mass Star Formation  

E-print Network

This article synthesizes observational data from an extensive program aimed toward a comprehensive understanding of star formation in a low-mass star-forming molecular cloud. New observations and published data spanning from the centimeter wave band to the near infrared reveal the high and low density molecular gas, dust, and pre-main sequence stars in L1551.

J. Swift; Wm. J. Welch

2007-06-14

6

Very Low-Mass Stars and Brown Dwarfs  

NASA Astrophysics Data System (ADS)

Part I. Searches in Clusters, Stellar Associations and the Field: 1. Open clusters after HIPPARCOS J. S. Mermilliod; 2. Proper motions of very low mass stars and brown dwarfs in open clusters N. C. Hambly; 3. Parallaxes for brown dwarfs in clusters C. G. Tinney; 4. Very low mass stars and brown dwarfs in the Belt of Orion S. J. Wolk and F. M. Walter; 5. Photometric surveys in open clusters M. R. Zapatero Osorio; 6. The mass function of the Pleiades R. F. Jameson et al.; 7. Brown dwarfs and the low-mass initial mass function in young clusters K. L. Luhman; 8. Very low mass stars in globular clusters I. R. King and G. Piotto; 9. The DENIS very low mass star and brown dwarf results X. Delfosse and T. Forveille; 10. Preliminary results from the 2MASS core project J. Liebert et al.; Part II. Spectroscopic Properties, Fundamental Parameters and Modelling: 11. Properties of M dwarfs in clusters and the field S. L. Hawley et al.; 12. Spectroscopy of very low mass stars and brown dwarfs in young clusters E. L. Martin; 13. High resolution spectra of L type stars and brown dwarfs G. Basri et al.; 14. Modelling very low mass stars and brown dwarf atmospheres F. Allard; 15. Dust in very cool dwarfs T. Tsuji; 16. On the interpretation of the optical spectra of very cool dwarfs Ya. V. Pavlenko; 17. Absolute dimensions for M type dwarfs A. Gimenez; 18. Theory of very low mass stars and brown dwarfs I. Baraffe; Part III. Convection, Rotation and Acitivity: 19. Convection in low mass stars F. D'Antona; 20. Rotation law and magnetic field in M dwarf models G. Rudiger and M. Kuker; 21. Doppler imaging of cool dwarf stars K. G. Strassmeier; 22. X-ray Emission from cool dwarfs in clusters S. Randich; 23. X-ray variability for dM stars G. Micela and A. Marino; 24. The coronae of AD Leo and EV Lac S. Sciortino et al.; 25. Prospects of vuture X-ray missions for low mass stars and cluster stars R. Pallavicini.

Rebolo, Rafael; Rosa Zapatero-Osorio, Maria

2001-02-01

7

Production of low-mass stars. IV. Carbon stars  

SciTech Connect

Detailed stellar evolution calculations were carried out for a metal-poor case (Z = 0.001) for stars of initial masses 1.0, 1.2, 2.0, and 3.0 solar and for a metal-rich (Z = 0.02) for stars of initial masses 1.2 and 3.0 solar. The stars were evolved from the main sequence through all intermediate stages and finally through a number of helium shell flashes on the AGB. Classical third dredge-up and carbon star production are obtained in two cases, both having metallicity Z = 0.001. A value of the mixing length parameter of greater than about 1.5 appears to be a necessary condition for dredge-up in low-mass stars, and an increase in alpha leads to conditions more favorable to dredge-up. Other conditions tending to favor dredge-up are high flash strength, relatively large envelope mass, and low metallicity. 39 references.

Boothroyd, A.I.; Sackmann, I.J.

1988-05-01

8

Aging jets from low-mass stars  

NASA Astrophysics Data System (ADS)

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 Lbol approximately = 0.005 solar luminosity, and log Te approximately = 3.5, the HH 55 star is close to the main sequence and evolutionary tracks suggest an age of 3 x 107 yr.

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

1994-12-01

9

Aging jets from low-mass stars  

NASA Technical Reports Server (NTRS)

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.

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

1994-01-01

10

The evolution of very low mass stars  

NASA Technical Reports Server (NTRS)

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.

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

1986-01-01

11

Detectability of Habitable Planets around Very Low-Mass Stars  

NASA Astrophysics Data System (ADS)

We present observations of 18 very low-mass stars with Kepler light curves available in the public archive, where we performed a search for transits and characterized the photometric variability to estimate the effects of stellar activity in the detectability of habitable planets around stars at the cool end of the stellar mass distribution.

Martioli, E.; Martín, E. L.; Cabrera, J.; Solano, E.; Tata, R.

2014-10-01

12

The rotational evolution of young low mass stars  

E-print Network

Star-disk interaction is thought to drive the angular momentum evolution of young stars. In this review, I present the latest results obtained on the rotational properties of low mass and very low mass pre-main sequence stars. I discuss the evidence for extremely efficient angular momentum removal over the first few Myr of pre-main sequence evolution and describe recent results that support an accretion-driven braking mechanism. Angular momentum evolution models are presented and their implication for accretion disk lifetimes discussed.

Jerome Bouvier

2007-12-18

13

Very low mass stars, black dwarfs and planets  

Microsoft Academic Search

Hydrogen-rich stars of very low mass (M ? 0.08M?) never go through hydrogenburning thermonuclear reactions and, in a time scale much shorter than the age of the Galaxy, become completely degenerate objects or black dwarfs. The number of the very-low-mass (VLM) black dwarfs is expected to be very large and they are likely to make a significant contribution to the

Shiv S. Kumar

1994-01-01

14

Compact Stars in low-mass X-ray binaries  

E-print Network

We propose a model for compact stars in low-mass X-ray binaries(LMXBs) namely KS 1731-260, EXO 1745-248 and 4U 1608-52. Here we investigate the physical phenomena of a compact star in the LMXBs. Using our model, we have calculated central density, surface density, mass(M) and red-shift for the above mentioned compact stars, which is very much consistent with the reported data. We also obtain the possible equation of state(EOS) of the stars which is physically acceptable.

Hossein, Sk Monowar; Jafry, Md Abdul Kayum; Kalam, Mehedi

2014-01-01

15

Compact Stars in low-mass X-ray binaries  

E-print Network

We propose a model for compact stars in low-mass X-ray binaries(LMXBs) namely KS 1731-260, EXO 1745-248 and 4U 1608-52. Here we investigate the physical phenomena of a compact star in the LMXBs. Using our model, we have calculated central density, surface density, mass(M) and red-shift for the above mentioned compact stars, which is very much consistent with the reported data. We also obtain the possible equation of state(EOS) of the stars which is physically acceptable.

Sk. Monowar Hossein; Sajahan Molla; Md. Abdul Kayum Jafry; Mehedi Kalam

2014-08-11

16

Variability and Rotation in Low Mass Stars and Brown Dwarfs  

Microsoft Academic Search

The evolution of angular momentum in stars is not really understood today. In particular, very little is known about rotation\\u000a of substellar objects. Therefore, we started a systematic search for periodic variability in very low mass (VLM) stars and\\u000a Brown Dwarfs. From a deep multi-filter survey of the young open cluster IC4665, accompanied by follow-up spectroscopy with\\u000a FORS at the

Jochen Eislöffel; Alexander Scholz

2002-01-01

17

Transition Region Emission From Very Low Mass Stars  

E-print Network

We present results from our cycle 10 HST program to search for transition region emission in very low-mass, main-sequence stars in the spectral range M7-M9. Our program is aimed at 1) detecting emission; and 2) distinguishing between flaring and quiescent origin for the emission. We have obtained HST/STIS time series observations of three active, very low mass stars (VB 8, VB 10, and LHS 2065) which show persistent activity in transition region lines including Si IV, C IV, He II. Emission in transition region lines appears to be variable between exposures, but is always observed. A strong flare was also observed in one 10 minute exposure on VB 10. Our results indicate that active, very low-mass stars maintain a persistent quiescent chromosphere and transition region that is similar to those observed in active, earlier type M dwarfs, in contrast to suggestions that these low-mass, main-sequence stars exhibit only relatively strong flares and no quiescent emission.

Suzanne L. Hawley; Christopher M. Johns-Krull

2003-04-29

18

Induced formation of primordial low-mass stars  

NASA Astrophysics Data System (ADS)

We show that the explosion of the first supernovae can trigger low-mass star formation via gravitational fragmentation of the supernova-driven gas shell. If the shell mass does not exceed the host galaxy gas mass, all explosions with energies ESN?10 51 erg can lead to shell fragmentation. However, the minimum ambient density required to induce such fragmentation is much larger, n0>300 cm -3, for Type II supernovae than for pair-instability ones, which can induce star formation even at lower ambient densities. The typical mass of the unstable fragments is ˜10 4-7M?; their density is in the range 110-6×10 7 cm -3. Fragments have a metallicity strictly lower than 10 -2.6Z? and large values of the gravitational-to-pressure force ratio f?8. Based on these findings, we conclude that the second generation of stars produced by such self-propagating star formation is predominantly constituted by low-mass, long-living, extremely metal-poor (or even metal-free, if mixing is suppressed) stars. We discuss the implications of such results for Pop III star formation scenarios and for the most iron-poor halo star HE0107-5240.

Salvaterra, R.; Ferrara, A.; Schneider, R.

2004-12-01

19

Observational and Theoretical Studies of Low-Mass Star Formation  

NASA Technical Reports Server (NTRS)

Under this grant we have pursued studies of low-mass star formation with observations of candidate star-forming regions, (1) to determine the incidence of "infall asymmetry" in the spectral lines from very red young stellar objects; (2) to make detailed maps of candidate infall regions to determine the spatial extent of their infall asymmetry; (3) to compare the spatial and velocity structure of candidate infall regions with single dish and interferometer resolution; and (4) to begin a program of observations of starless dense cores to detect the presence or absence of infall motions.

Myers, Philip C.

1998-01-01

20

COUNTING LOW-MASS STARS IN INTEGRATED LIGHT  

SciTech Connect

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

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

2012-03-01

21

Angular Momentum Transport within Evolved Low-mass Stars  

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

22

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

E-print Network

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

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

2008-08-12

23

Searching for very low mass objects around nearby dMe radio stars  

NASA Astrophysics Data System (ADS)

Nearby M-dwarfs are best suited for searches of low mass companions. VLBI phase-referencing observations with sensitive telescopes are able to detect radio star flux-densities of tenths of mJy as well as to position the star on the sky with submilliarcsecond precision. We have initiated a long-term observational program, using EVN telescopes in combination with NASA DSN dishes, to revisit the kinematics of nearby, single M dwarfs. The precision of the astrometry allows us to search for possible companions with masses down to 1 Jupiter mass. In this contribution we report preliminary results of the first observation epochs, in which we could detect some of the radio stars included in our program.

Guirado, J. C.; Ros, E.; Jones, D. L.; Alef, W.; Marcaide, J. M.; Preston, R. A.

2002-06-01

24

Searching for low mass objects around nearby dMe radio stars  

E-print Network

Nearby M-dwarfs are best suited for searches of low mass companions. VLBI phase-referencing observations with sensitive telescopes are able to detect radio star flux-densities of tenths of mJy as well as to position the star on the sky with submilliarcsecond precision. We have initiated a long-term observational program, using EVN telescopes in combination with NASA DSN dishes, to revisit the kinematics of nearby, single M dwarfs. The precision of the astrometry allows us to search for possible companions with masses down to 1 Jupiter mass. In this contribution we report preliminary results of the first observation epochs, in which we could detect some of the radio stars included in our program.

J. C. Guirado; E. Ros; D. L. Jones; W. Alef; J. M. Marcaide; R. A. Preston

2002-07-01

25

The low mass stellar population of the Orion OB1 association and implications for the formation of low mass stars  

Microsoft Academic Search

The OB associations, which are fossil star formation regions, retain the\\u000aend-products of the star formation process in an open, unobscured environment.\\u000aObservations of the low mass stars in OB associations provide a far clearer\\u000apicture of the results of the star formation process than do observations of\\u000aembedded, on-going star formation regions. We review the X-ray and optical\\u000asurveys

Frederick M. Walter; Juan M. Alcala; Ralph Neuhauser; Michael Sterzik

1999-01-01

26

Gas & Stars Aging low-mass stars eject their outer layers.  

E-print Network

Recycling Gas & Stars #12;Aging low-mass stars eject their outer layers. M57:The Ring Nebula #12;Thor's Emerald Helmet Winds from high-mass stars blow bubbles of hot gas. #12;Supernova blast waves in stars are mixed back into the gas. NGC 6992: Filaments of theVeil Nebula #12;Bubbles blown by high

Barnes, Joshua Edward

27

Extreme Coronal Mass Ejections in Young Low-Mass Stars  

NASA Astrophysics Data System (ADS)

Two long-standing questions in the study of young, low-mass stars are: (1) What are the mechanisms that govern the observed order-of-magnitude decrease of stellar angular momentum during pre-main-sequence evolution, and (2) What are the physical drivers of X-ray production in these stars at up to 104 times the solar value? Application of solar flare models to the most powerful X-ray flares observed among T Tauri stars in Orion suggests that the flares are produced by magnetic loop structures with lengths of up to tens of stellar radii. We present new results demonstrating that, for the majority of these stars, the extremely large flaring structures are not anchored to or stabilized by circumstellar disks. Given the energy and size scales involved, mass losses (e.g., via stellar coronal mass ejections -- CMEs -- associated with these flares) at such large effective lever arms could shed substantial angular momentum. To begin estimating the attendant angular momentum losses of such extreme CMEs in young stars, we have assembled from the solar literature a database of ˜10,000 X-ray flares and CMEs, from which we determine for the first time the empirical relationship between solar X-ray flare energy and CME ejected mass. Finally, we demonstrate how our flare flux/CME mass relationship can be used to estimate stellar angular momentum loss via extreme CMEs in young stars.

Aarnio, A. N.; Stassun, K. G.; Matt, S. P.; Hughes, W. J.; McGregor, S. L.

2011-12-01

28

Improving PARSEC models for very low mass stars  

NASA Astrophysics Data System (ADS)

Many stellar models present difficulties in reproducing basic observational relations of very low mass stars (VLMS), including the mass-radius relation and the optical colour-magnitudes of cool dwarfs. Here, we improve PARSEC (PAdova-TRieste Stellar Evolution Code) models on these points. We implement the T- ? relations from PHOENIX BT-Settl model atmospheres as the outer boundary conditions in the PARSEC code, finding that this change alone reduces the discrepancy in the mass-radius relation from 8 to 5 per cent. We compare the models with multiband photometry of clusters Praesepe and M67, showing that the use of T- ? relations clearly improves the description of the optical colours and magnitudes. But anyway, using both Kurucz and PHOENIX model spectra, model colours are still systematically fainter and bluer than the observations. We then apply a shift to the above T- ? relations, increasing from 0 at Teff = 4730 K to ˜14 per cent at Teff = 3160 K, to reproduce the observed mass-radius relation of dwarf stars. Taking this experiment as a calibration of the T- ? relations, we can reproduce the optical and near-infrared colour-magnitude diagrams of low-mass stars in the old metal-poor globular clusters NGC 6397 and 47 Tuc, and in the intermediate-age and young solar-metallicity open clusters M67 and Praesepe. Thus, we extend PARSEC models using this calibration, providing VLMS models more suitable for the lower main-sequence stars over a wide range of metallicities and wavelengths. Both sets of models are available on PARSEC webpage.

Chen, Yang; Girardi, Léo; Bressan, Alessandro; Marigo, Paola; Barbieri, Mauro; Kong, Xu

2014-11-01

29

Low-Mass Companions to Solar-Type Stars  

E-print Network

We present preliminary results from a coronagraphic survey of young nearby Sun-like stars using the Palomar and Keck adaptive optics systems. We have targeted 251 solar analogs (F5-K5) at 20-160 pc from the Sun, spanning the 3-3000 Myr age range. The youngest (0.5" from their host stars, with sensitivity extending to planetary-mass (5-15 Mjup) objects at wider (>3") separations. Based on the discovery of a number of new low-mass (20 AU separations (probed via direct imaging) may be greater (12%) than that found from radial velocity surveys probing <4 AU separations (6%; Mazeh et al. 2003). We also report the astrometric confirmation of the first sub-stellar companion from the survey - an L4 brown dwarf at a projected distance of 44 AU from the 500 Myr-old star HD 49197. Based on this detection, we estimate that the frequency of sub-stellar companions to solar-type stars is at least 1%, and possibly of order a few per cent.

Stanimir Metchev; Lynne Hillenbrand

2004-11-25

30

Tidal Alignment of Exoplanets Around Low Mass Stars  

NASA Astrophysics Data System (ADS)

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

Penev, Kaloyan; Jackson, Brian K.

2014-06-01

31

The role of low-mass star clusters in massive star formation. The Orion Case  

NASA Astrophysics Data System (ADS)

To distinguish between the different theories proposed to explain massive star formation, it is crucial to establish the distribution, the extinction, and the density of low-mass stars in massive star-forming regions. We analyze deep X-ray observations of the Orion massive star-forming region using the Chandra Orion Ultradeep Project (COUP) catalog. We studied the stellar distribution as a function of extinction, with cells of 0.03 pc x 0.03 pc, the typical size of protostellar cores. We derived stellar density maps and calculated cluster stellar densities. We found that low-mass stars cluster toward the three massive star-forming regions: the Trapezium Cluster (TC), the Orion Hot Core (OHC), and OMC1-S. We derived low-mass stellar densities of 10^5 stars pc^-3 in the TC and OMC1-S, and of 10^6 stars pc^-3 in the OHC. The close association between the low-mass star clusters with massive star cradles supports the role of these clusters in the formation of massive stars. The X-ray observations show for the first time in the TC that low-mass stars with intermediate extinction are clustered toward the position of the most massive star, which is surrounded by a ring of non-extincted low-mass stars. This "envelope-core" structure is also supported by infrared and optical observations. Our analysis suggests that at least two basic ingredients are needed in massive star formation: the presence of dense gas and a cluster of low-mass stars. The scenario that better explains our findings assumes high fragmentation in the parental core, accretion at subcore scales that forms a low-mass stellar cluster, and subsequent competitive accretion. Finally, although coalescence does not seem a common mechanism for building up massive stars, we show that a single stellar merger may have occurred in the evolution of the OHC cluster, favored by the presence of disks, binaries, and gas accretion.

Rivilla, M. V.; Martin-Pintado, J.; Jimenez-Serra, I.; Rodriguez-Franco, A.

2013-06-01

32

Continuum Mapping of Low Mass Star Forming Cores using SCUBA  

NASA Astrophysics Data System (ADS)

Recent studies in star formation have been guided by a putative evolutionary sequence based on the spectral energy distribution (SED) of young stellar objects. We are testing this idea by tracing the changes in the distribution of matter across this sequence. We have obtained 850 and 450 micron continuum maps of 21 low mass cores with SED's ranging from Pre-protostellar to Class I (20K < Tbol < 140K) using the SCUBA bolometer array at the JCMT. By combining our maps with continuum emission at other wavelengths and using a radiative transport code, we can compare model predictions of the spatial extent of emission and the SED with observations. This method allows for a self-consistent calculation of the temperature distribution and for smoothing effects caused by a finite beam. We present observations and models of the dust density and temperature distributions as well as SEDs for our sources.

Shirley, Y. L.; Evans, N. J.; Rawlings, J. M. C.; Gregersen, E. M.

1998-12-01

33

The role of low-mass star clusters in massive star formation. The Orion case  

NASA Astrophysics Data System (ADS)

Context. Different theories have been proposed to explain the formation of massive stars: two are based on accretion processes (monolithic core accretion and competitive accretion), and another on coalescence of low- and intermediate-mass stars. To distinguish between these theories, it is crucial to establish the distribution, the extinction, and the density of young low-mass stars in massive star-forming regions. X-ray observations can penetrate the very obscured cradles of massive stars, directly sampling the distribution of the population of pre-main sequence (PMS) low-mass stars in these regions. Aims: Our aim is to analyze deep X-ray observations of the Orion massive star-forming region using the Chandra Orion Ultradeep Project (COUP) catalog, to reveal the distribution of the population and clustering of PMS low-mass stars, and to study their possible role in massive star formation. Methods: We studied the distribution of PMS low-mass stars with X-ray emission in Orion as a function of extinction with two different methods: a spatial gridding and a close-neigbors method with cells of ~0.03 × 0.03 pc2, the typical size of protostellar cores. We derived density maps of the stellar population and calculated cluster stellar densities. Results: Consistent with previous studies, we found that PMS low-mass stars cluster toward the three massive star-forming regions: the Trapezium cluster (TC), the Orion hot core (OHC), and the OMC1-S region. We derived PMS low-mass stellar densities of 105 stars pc-3 in the TC and OMC1-S, and of 106 stars pc-3 in the OHC. The close association between the low-mass star clusters with massive star cradles supports the role of these clusters in the formation of massive stars. The X-ray observations show for the first time in the TC that low-mass stars with intermediate extinction are clustered toward the position of the most massive star ?1 Ori C, which is surrounded by a ring of non-extincted PMS low-mass stars. This "envelope-core" structure is also supported by infrared and optical observations. Our analysis suggests that at least two basic ingredients are needed in massive star formation: the presence of dense gas and a cluster of low-mass stars. The scenario that better explains our findings assumes high fragmentation in the parental core, accretion at subcore scales that forms a low-mass stellar cluster, and subsequent competitive accretion. Finally, although coalescence does not seem a common mechanism for building up massive stars, we show that a single stellar merger may have occurred in the evolution of the OHC cluster, favored by the presence of disks, binaries, and gas accretion.

Rivilla, V. M.; Martín-Pintado, J.; Jiménez-Serra, I.; Rodríguez-Franco, A.

2013-06-01

34

Multiplicity Among Young Brown Dwarfs and Very Low Mass Stars  

E-print Network

Characterizing multiplicity in the very low mass (VLM) domain is a topic of much current interest and fundamental importance. Here we report on a near-IR AO imaging survey of 31 young brown dwarfs and VLM stars, 28 of which are in Chamaeleon I, using the ESO VLT. Our survey is sensitive enough to detect equal mass binaries down to separations of 0.04-0.07" (6-10 AU at 160 pc) and, typically, companions with mass ratios as low as 0.2 outside of 0.2" (~30 AU). We resolve the suspected 0.16" (~26 AU) binary Cha_Halpha 2 and present two new binaries, Hn 13 and CHXR 15, with separations of 0.13" (~20 AU) and 0.30" (~50 AU); the latter is one of the widest VLM systems discovered to date. We do not find companions around the majority of our targets, giving 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 VLMOs (72) with high angular resolution imaging to date. Its multiplicity fraction is in statistical agreement with that for VLMOs in the field. In addition 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 Myr to several Gyr. Instead, the observations to date 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.

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

2007-08-28

35

H2, CO and Dust Emission Around Low Mass Stars  

NASA Technical Reports Server (NTRS)

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.

Hollenbach, David; DeVincenzi, Donald (Technical Monitor)

2002-01-01

36

H2, CO and Dust Emission Around Low Mass Stars  

NASA Technical Reports Server (NTRS)

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.

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

2002-01-01

37

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

E-print Network

1 RV SURVEY FOR PLANETS OF BROWN DWARFS AND VERY LOW-MASS STARS IN CHA I Viki Joergens1 and Ralph, Germany ABSTRACT We have carried out a radial velocity (RV) search for planets and brown dwarf companions. It is sensitive down to Jupiter mass planets. Out of the twelve monitored very low-mass stars and brown dwarfs

Joergens, Viki

38

Low-mass stars with mass loss and low-luminosity carbon star formation  

SciTech Connect

The effects of large carbon enrichments in static stellar envelopes were investigated, using new Los Alamos opacities (including low-temperature carbon and molecular opacities) and including carbon ionizations. To search for the production of low-mass,low-luminosity carbon stars, detailed stellar evolutionary computations were carried out for a grid of low-mass stars of two different metallicities. The stars were evolved from the main sequence through all intermediate stages and through helium-shell flashes on the asymptotic giant branch. The effects of the latest nuclear reaction rates, the new Los Alamos opacities, Reimers-type wind mass loss, and detailed treatment of convection and semi-convection were investigated. Two low-luminosity carbon stars were achieved, in excellent agreement with observations. Conditions favoring dredge-up (and thus carbon-star production) include a reasonably large convective mixing length, low metallicity, relatively large envelope mass, and high flash strength. Mass loss was of major importance, tending to oppose dredge-up; the total mass-loss amounts inferred from observations suffice to prevent formation of high-mass, high-luminosity carbon stars.

Boothroyd, A.I.

1987-01-01

39

The Formation and Early Evolution of Low-Mass Stars and Brown Dwarfs  

NASA Astrophysics Data System (ADS)

The discovery of large numbers of young low-mass stars and brown dwarfs over the past decade has made it possible to investigate star formation and early evolution in a previously unexplored mass regime. In this review, we begin by describing surveys for low-mass members of nearby associations, open clusters, star-forming regions, and the methods used to characterize their stellar properties. We then use observations of these populations to test theories of star formation and evolution at low masses. For comparison to the formation models, we consider the initial mass function, stellar multiplicity, circumstellar disks, protostellar characteristics, and kinematic and spatial distributions at birth for low-mass stars and brown dwarfs. To test the evolutionary models, we focus on measurements of dynamical masses and empirical Hertzsprung-Russell diagrams for young brown dwarfs and planetary companions.

Luhman, Kevin L.

2012-09-01

40

Low Mass Star Formation in the Taurus-Auriga Clouds  

E-print Network

We review the history and structure of star formation in the Taurus-Auriga dark clouds. Our discussion includes a summary of the macroscopic cloud properties, the population of single and binary pre-main sequence stars, the properties of jets and outflows, and detailed summaries of selected individual objects. We include comprehensive tables of dark clouds, young stars, and jets in the clouds.

Scott J. Kenyon; Mercedes Gomez; Barbara A. Whitney

2008-10-07

41

An infrared search for low-mass companions of stars near the sun  

NASA Technical Reports Server (NTRS)

Using a CCD camera on the IRTF telescope on Mauna Kea, a search was conducted for low-mass companions to stars in the solar neighborhood. The K band (2.2 microns) survey includes 55 condidates closer than 12 pc, as well as eight stars in the Pleiades star cluster. Due to the saturation of the primary star image, the survey was insensitive to companions within about 2 arcsec of the primary star. The survey detected a single low-mass candidate object, a companion to the star Gliese 569, which lies near or below the hydrogen-burning mass limit and resembles extremely low-mass stars similar to VB 10 and LHS 2924.

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

1989-01-01

42

The Rôle Of Astrochemistry In Low Mass Star Formation  

Microsoft Academic Search

Molecular processes play both active and passive\\/diagnostic rles in the process of star formation. Various molecular behaviours\\u000a can be identified in star-forming regions with the result that different molecular species can be used to constain different\\u000a aspects of the infall process, such as the density structures, the kinematics and the evolutionary history of star-forming\\u000a cores. The main limitations in the

J. Rawlings

2003-01-01

43

The accretion process in neutron-star low-mass X-ray binaries  

E-print Network

There had been long-standing fundamental problems in the spectral studies of accreting neutron stars (NSs) in low-mass X-ray binaries involving the X-ray spectral decomposition, the relations between subtypes (mainly atoll ...

Lin, Dacheng

2009-01-01

44

GLOBAL STAR FORMATION REVISITED  

SciTech Connect

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

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

2009-07-20

45

A quest for activity cycles in low-mass stars  

NASA Astrophysics Data System (ADS)

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

Vida, K.; Kriskovics, L.; Oláh, K.

2013-11-01

46

Low Mass Stars and Brown Dwarfs around Sigma Orionis  

E-print Network

We present optical spectroscopy of 71 photometric candidate low-mass members of the cluster associated with Sigma Orionis. Thirty-five of these are found to pass the lithium test and hence are confirmed as true cluster members, covering a mass range of <0.055-0.3M_{sun}, assuming a mean cluster age of <5 Myr. We find evidence for an age spread on the (I, I-J) colour magnitude diagram, members appearing to lie in the range 1-7 Myr. There are, however, a significant fraction of candidates that are non-members, including some previously identified as members based on photometry alone. We see some evidence that the ratio of spectroscopically confirmed members to photometric candidates decreases with brightness and mass. This highlights the importance of spectroscopy in determining the true initial mass-function.

M. J. Kenyon; R. D. Jeffries; T. Naylor

2001-09-06

47

Interactive Winds in Low-Mass Evolved Stars  

NASA Astrophysics Data System (ADS)

Hypersonic flows occur in the late evolutionary stages of low to intermediate mass stars (M ZAMS < 8M?). We present, from an observational point of view, the story of the matter ejected and shaped by different stellar winds. From the end of the Asymptotic Giant Branch to the Planetary Nebulae phases, the temperature and density both of the star and of the gas and dust envelope show drastic variations over a few thousand of years. We present multiwavelength (X-ray to radio) signatures of the outflows, observed through spectra and images of the consecutive phases, and discuss some open questions (bipolar morphology, microstructures,...).

Acker, A.

1998-10-01

48

Numerical Results on Low Mass Star and Brown Dwarf Multiplicity  

E-print Network

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

Eduardo Delgado-Donate; Cathie Clarke

2004-12-17

49

Detection of pedestal features in dark clouds - Evidence for formation of low mass stars  

NASA Technical Reports Server (NTRS)

To assess whether B335 is unique among dark clouds or whether CO-12 pedestal features are quite common, 180 opacity class 5 and 6 Lynds clouds were surveyed. From this set of data, three additional sources were found to have pedestal features. These suggest the presence of embedded low-mass stars, though a hot differentially rotating disk cannot be excluded for B335. Estimates of the mass-loss rate required to produce stellar winds consistent with observations are comparable with mass-loss rates for T Tauri stars. Further, the pedestal feature formation rate is similar to the local low-mass star formation rate.

Frerking, M. A.; Langer, W. D.

1982-01-01

50

Modelling the Hidden Magnetic Field of Low-Mass Stars  

E-print Network

Zeeman-Doppler imaging is a spectropolarimetric technique that is used to map the large-scale surface magnetic fields of stars. These maps in turn are used to study the structure of the stars' coronae and winds. This method, however, misses any small-scale magnetic flux whose polarisation signatures cancel out. Measurements of Zeeman broadening show that a large percentage of the surface magnetic flux may be neglected in this way. In this paper we assess the impact of this 'missing flux' on the predicted coronal structure and the possible rates of spin down due to the stellar wind. To do this we create a model for the small-scale field and add this to the Zeeman-Doppler maps of the magnetic fields of a sample of 12 M dwarfs. We extrapolate this combined field and determine the structure of a hydrostatic, isothermal corona. The addition of small-scale surface field produces a carpet of low-lying magnetic loops that covers most of the surface, including the stellar equivalent of solar 'coronal holes' where the ...

Lang, P; Morin, J; Donati, J-F; Jeffers, S; Vidotto, A A; Fares, R

2014-01-01

51

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

E-print Network

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

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

2005-02-15

52

Asymmetric dark matter may alter the evolution of very low-mass stars and brown dwarfs  

Microsoft Academic Search

We study energy transport by asymmetric dark matter (ADM) in very low-mass stars and brown dwarfs. Our motivation is to explore astrophysical signatures of ADM, which may not otherwise be amenable to indirect dark matter searches. In viable models, the additional cooling of low-mass stellar cores can alter stellar properties. ADM with mass 4≲Mx\\/GeV≲10 and a spin-dependent (spin-independent) cross section

Andrew R. Zentner; Andrew P. Hearin

2011-01-01

53

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

NASA Astrophysics Data System (ADS)

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.

Tian, F.

2011-10-01

54

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

NASA Astrophysics Data System (ADS)

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.

Tian, F.

2011-12-01

55

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

NASA Astrophysics Data System (ADS)

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.

Tian, Feng

2011-09-01

56

An age-activity calibration for old low-mass stars  

NASA Astrophysics Data System (ADS)

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

Poppenhaeger, Katja

2014-09-01

57

RADIUS-DEPENDENT ANGULAR MOMENTUM EVOLUTION IN LOW-MASS STARS. I  

SciTech Connect

Angular momentum evolution in low-mass stars is determined by initial conditions during star formation, stellar structure evolution, and the behavior of stellar magnetic fields. Here we show that the empirical picture of angular momentum evolution arises naturally if rotation is related to magnetic field strength instead of to magnetic flux and formulate a corrected braking law based on this. Angular momentum evolution then becomes a strong function of stellar radius, explaining the main trends observed in open clusters and field stars at a few Gyr: the steep transition in rotation at the boundary to full convection arises primarily from the large change in radius across this boundary and does not require changes in dynamo mode or field topology. Additionally, the data suggest transient core-envelope decoupling among solar-type stars and field saturation at longer periods in very low mass stars. For solar-type stars, our model is also in good agreement with the empirical Skumanich law. Finally, in further support of the theory, we show that the predicted age at which low-mass stars spin down from the saturated to unsaturated field regimes in our model corresponds remarkably well to the observed lifetime of magnetic activity in these stars.

Reiners, Ansgar [Institut fuer Astrophysik, Georg-August-Universitaet, Friedrich-Hund-Platz 1, 37077 Goettingen (Germany); Mohanty, Subhanjoy, E-mail: Ansgar.Reiners@phys.uni-goettingen.de [Imperial College London, 1010 Blackett Laboratory, Prince Consort Road, London SW7 2AZ (United Kingdom)

2012-02-10

58

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

E-print Network

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

Anna Frebel; Jarrett L. Johnson; Volker Bromm

2007-01-12

59

Vibrational Instability of Metal-Poor Low-Mass Main-Sequence Stars  

NASA Astrophysics Data System (ADS)

We find that low-degree low-order g-modes become unstable in metal-poor low-mass stars due to the ?-mechanism of the pp-chain. Since the outer convection zone of these stars is limited only to the very outer layers, the uncertainty in the treatment of convection does not affect the result significantly. The decrease in metallicity leads to decrease in opacity and hence increase in luminosity of a star. This makes the star compact and results in decrease in the density contrast, which is favorable to the ?-mechanism instability. We find also instability for high order g-modes of metal-poor low-mass stars by the convective blocking mechanism. Since the effective temperature and the luminosity of metal-poor stars are significantly higher than those of Pop I stars, the stars showing ? Dor-type pulsation are substantially less massive than in the case of Pop I stars. We demonstrate that those modes are unstable for about 1 M ? stars in the metal-poor case.

Sonoi, Takafumi; Shibahashi, Hiromoto

60

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

NASA Astrophysics Data System (ADS)

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

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

2014-10-01

61

Rotational Evolution of Low Mass Post-T Tauri Stars at 10-30 Myr  

NASA Astrophysics Data System (ADS)

We present a complementary study to those presented by Scholz et al. (2007) of rotation and chromospheric activity for a sample of 33 young low-mass stars of the southern hemisphere with spectral types G-K belonging to the stellar associations: TWA Hya (˜10 Myr), BPMG (˜11 Myr), Upper Centaurus Lupus (UCL, ˜21 Myr), Lower Centarus Crux (LCC, ˜23 Myr), and Tucana-Horologium (THA, ˜30 Myr). Using CaII infrared triplet (IRT) chromospheric lines analysis of FEROS spectra, we detected a clear decay of the chromospheric activity with age. The rotation — activity relation shows prominent differences with those observed in main sequence stars indicating that young low-mass stars generate their magnetic fields in a different way. Theoretical predictions using a time-dependent stellar spin evolution model were also done and confirm that such kind of spin up is a consequence of the disappearance of the disks at the beginning of the post T Tauri stage.

Pinzón, G.; Rodríguez, J.

2013-04-01

62

Star formation in the lagoon nebula & low-mass stars and brown dwarfs  

NASA Astrophysics Data System (ADS)

Topic I of this thesis reports on star formation in the Lagoon Nebula. We report on deep Chandra X-Ray Observatory observations of the Lagoon Nebula (NGC 6530 and the Hourglass Nebula) totaling 233 ks. We find 1482 X-ray sources, 1130 associated with catalogued near-infrared or optical stars. These X-ray sources are mainly concentrated in the young Hourglass Nebula Cluster (HNC), the older NGC 6530 cluster, and the young M8E cluster in the southern rim. The clustering of X-ray sources near 850mum emission along the central ridge of NGC 6530, M8E, the southern ridge, and coincident with the Hourglass Nebula, provides evidence of triggered star formation. Chandra point-source density contours show a ridge of increased density between NGC 6530 and the HNC, 9 Sgr and the HNC, and class III/II contours stretching from 9 Sgr to the HNC, respectively, provide support for a proposed sequence of star formation in the Lagoon Nebula. Topic II of this thesis reports on low-mass stars and brown dwarfs (BDs). We report on Chandra X-Ray Observatory observations of the TW Hydrae BD 2MASSW J1139511-315921 (2M1139). In the combined 31 ks ACIS-S exposure, 2M1139 is detected at the 3sigma confidence level. This object is similar to another TW Hydrae BD member, CD-33 7795B (TWA 5B), previously detected in X-rays an order of magnitude more luminous than 2M1139. We find the discrepancy between their X-ray luminosities is consistent with BDs of similar spectral type in the Orion Nebula Cluster. Though rotation may play a role in the X-ray activity of ultracool dwarfs like 2M1139 and TWA 5B, the discrepancy cannot be explained by rotation alone. We discover five high proper motion spectroscopically confirmed L dwarfs by comparing WISE to 2MASS. Two of these are L dwarfs at the L/T transition within 10 pc, and three are early L dwarfs within 25 pc. Of the early L dwarfs, one is a member of the class of unusually red L dwarfs whose red spectra can not be easily attributed to youth.

Castro, Philip J.

63

Planet formation around low mass stars: the moving snow line and super-Earths  

E-print Network

We develop a semi-analytic model for planet formation during the pre-main sequence contraction phase of a low mass star. During this evolution, the stellar magnetosphere maintains a fixed ratio between the inner disk radius and the stellar radius. As the star contracts at constant effective temperature, the `snow line', which separates regions of rocky planet formation from regions of icy planet formation, moves inward. This process enables rapid formation of icy protoplanets that collide and merge into super-Earths before the star reaches the main sequence. The masses and orbits of these super-Earths are consistent with super-Earths detected in recent microlensing experiments.

Grant M. Kennedy; Scott J. Kenyon; Benjamin C. Bromley

2006-09-06

64

Clusters of Low-Mass Pre-Main Sequence Stars, Mass Functions, and Candidate Brown Dwarfs in Orion's Belt  

Microsoft Academic Search

At least 3 of the 4 O stars of the Orion OB1b sub-association, zeta , delta , and sigma Orionis, are associated with clusters of low-mass PMS stars. These clusters appear similar to the Orion nebular cluster (ONC); each consists of several hundred to a few thousand low-mass PMS stars centered on an O star system. These clusters are roughly

W. H. Sherry; F. M. Walter; S. J. Wolk

2000-01-01

65

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

E-print Network

We study energy transport by asymmetric dark matter in the interiors of very low-mass stars and brown dwarfs. Our motivation is to explore astrophysical signatures of asymmetric dark matter, which otherwise may not be amenable to conventional indirect dark matter searches. In viable models, the additional cooling of very-low mass stellar cores can alter stellar properties. Asymmetric dark matter with mass 4 < Mx/GeV < 10 and a spin-dependent (spin-independent) cross sections of sigma \\sim 10^{-37} cm^2 (sigma \\sim 10^{-40} cm^2) can increase the minimum mass of main sequence hydrogen burning, partly determining whether or not the object is a star at all. Similar dark matter candidates reduce the luminosities of low-mass stars and accelerate the cooling of brown dwarfs. Such light dark matter is of particular interest given results from the DAMA, CoGeNT, and CRESST dark matter searches. We discuss possibilities for observing dark matter effects in stars in the solar neighborhood, globular clusters, and, of particular promise, local dwarf galaxies, among other environments, as well as exploiting these effects to constrain dark matter properties.

Andrew R. Zentner; Andrew P. Hearin

2011-10-26

66

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

SciTech Connect

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

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

2012-05-01

67

S-process in extremely metal-poor, low-mass stars  

NASA Astrophysics Data System (ADS)

Context. Extremely metal-poor (EMP), low-mass stars experience an ingestion of protons into the helium-rich layer during the core He-flash, resulting in the production of neutrons through the reactions 12C(p,?)13N(?)13C(?,n)16O. This is a potential site for the production of s-process elements in EMP stars, which does not occur in more metal-rich counterparts. The signatures of s-process elements in the two most iron deficient stars observed to date, HE1327-2326 & HE0107-5240, still await for an explanation. Aims: We investigate the possibility that low-mass EMP stars could be the source of s-process elements observed in extremely iron deficient stars, either as a result of self-enrichment or in a binary scenario as the consequence of a mass transfer episode. Methods: We present evolutionary and post-processing s-process calculations of a 1 M? stellar model with metallicities of Z = 0, 10-8, and 10-7. We assess the sensitivity of nucleosynthesis results to uncertainties in the input physics of the stellar models with particular regard to the details of convective mixing during the core He-flash. Results: Our models provide the possibility of explaining the C, O, Sr, and Ba abundance for the star HE0107-5240 as the result of mass-transfer from a low-mass EMP star. The drawback of our model is that nitrogen would be overproduced and the 12C/^{13C} abundance ratio would be underproduced in comparison to the observed values if mass would be transferred before the primary star enters the asymptotic giant branch phase. Conclusions: Our results show that low-mass EMP stars cannot be ruled out as companion stars that might have polluted HE1327-2326 and HE0107-5240 and produced the observed s-process pattern. However, more detailed studies of the core He-flash and the proton ingestion episode are needed to determine the robustness of our predictions.

Cruz, M. A.; Serenelli, A.; Weiss, A.

2013-11-01

68

In infrared search for very low mass stars: The luminosity function  

Microsoft Academic Search

Infrared photometry can reveal cool companions to hot sub-main-sequence stars to very faint limtis of luminosity. We have surveyed approx.100 white dwarfs at 2.2 ..mu..m for very low mass red dwarf companions in a search complete to M\\/sub v\\/approx.21. Very few new companions were found, none with M\\/sub v\\/>15. This does not appear due to either selection effects or evolutionary

R. G. Probst

1983-01-01

69

Subaru\\/COMICS Study on Silicate Dust Processing around Young Low-Mass Stars  

Microsoft Academic Search

We have obtained 8-13 mum spectra of 30 young (1-10 Myr) low-mass pre-main-sequence stars using COMICS on the 8.2 m Subaru Telescope to examine dust evolution in protoplanetary disks. Most spectra show silicate emission features of various strengths and shapes, indicative of dust processing during the different stages of protoplanetary disk evolution. We have analyzed the observed silicate emission features

Mitsuhiko Honda; Hirokazu Kataza; Yoshiko K. Okamoto; Takuya Yamashita; Michiel Min; Takashi Miyata; Shigeyuki Sako; Takuya Fujiyoshi; Itsuki Sakon; Takashi Onaka

2006-01-01

70

An Equation of State for Low-Mass Stars and Giant Planets  

Microsoft Academic Search

We present new equations of state (EOS) for hydrogen and helium, intended for applications to low-mass stars (M < 1 Msun), brown dwarfs, and giant planets. They cover the range 2.10

D. Saumon; G. Chabrier; H. M. van Horn

1995-01-01

71

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

SciTech Connect

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

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

2013-10-20

72

Herschel Photometry of Disks around Low-mass Stars in the R CrA Cloud  

NASA Astrophysics Data System (ADS)

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 ?m. 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. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

Harvey, Paul M.; Henning, Thomas; Liu, Yao; Wolf, Sebastian

2014-11-01

73

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

NASA Astrophysics Data System (ADS)

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

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

2000-12-01

74

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

E-print Network

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

Dimitris Stamatellos; Anthony P. Whitworth

2008-09-29

75

The environmental impact of high- and low-mass stars: From formation to main sequence  

NASA Astrophysics Data System (ADS)

Throughout the entire lifetime of a star, it continuously alters the environment. Diverse processes are involved which complicates studies of stellar interaction. This thesis focuses on two different physical phenomena associated with stars: (1) the interplay of magnetic fields and collapsing clouds, and (2) the effects of radiation from massive stars. We first compare polarization measurements of 52 Galactic star-forming regions with their locations in the Galaxy. In particular, we find that there is no correlation between the average magnetic field direction of star-forming molecular clouds and the Galaxy, indicating that star formation may eventually become its own process independent of the Galaxy. Secondly, we observe the coupling of the magnetic field with the low-mass protostar L1157-mm by creating polarimetric maps at resolutions from ˜300 to 2500 AU. The inferred magnetic field lines show a well-defined hourglass morphology centered about the core -- only the second of such morphology discovered around a low-mass protostar. Next, we focus on radiation from massive stars in the Large Magellanic Cloud. We first present a survey of HII regions around massive young stellar objects (YSOs) and explore numerous relationship between parameters measured through observations of free-free and infrared emission. In particular, we find that YSO mass is a crucial consideration when exploring almost any relationship. Finally, we analyze how massive stars process dust in two classical HII regions and two superbubbles. We find that PAH emission is highest in the presence of molecular clouds with a low radiation field and that emission from very small grains are particularly high at locations where the radiation field is very strong.

Stephens, Ian William

76

Distributed Low-mass Star Formation in the IRDC G34.43+00.24  

NASA Astrophysics Data System (ADS)

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 H2 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.

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

2014-08-01

77

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

PubMed

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

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

2011-02-01

78

Complex Organic Molecules during Low-mass Star Formation: Pilot Survey Results  

NASA Astrophysics Data System (ADS)

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 CH3CN, toward two of the YSOs, and tentatively toward a third. Abundances with respect to CH3OH vary between 0.7% and 10%. This sample is combined with previous COM observations and upper limits to obtain a frequency distributions of CH3CN, HCOOCH3, CH3OCH3, and CH3CHO. We find that for all molecules more than 50% of the sample have detections or upper limits of 1%-10% with respect to CH3OH. 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. Based on observations carried out with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).

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

2014-06-01

79

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

E-print Network

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 sigma Ori and lambda Ori. Methods. We observed 98 and 49 low-mass (0.2-1.0 M_sun) stars in sigma Ori and lambda Ori respectively, using the multi-object optical spectrograph FLAMES at the VLT, with the high-resolution (R=17,000) HR15N grating (6470-6790 AA). We used radial velocities, Li and Halpha to establish cluster membership and Halpha and other optical emission lines to analyze the accretion properties of members. Results. We identified 65 and 45 members of the sigma Ori and lambda 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 sigma Ori cluster, finding values between 10^-11 and 10^-7.7 M_sun yr^-1 and in 4 stars of the lambda Ori cluster, finding values between 10^-11 and 10^-10.1 M_sun 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 sigma Ori cluster (52+-9% and 78+-16%) than in lambda Ori (28+-8% and 40+-20%) Conclusions. The different disk and accretion properties of the two clusters could be due either to the effect of the high-mass stars and the supernova explosion in the lambda Ori cluster or to different ages of the cluster populations. Further observations are required to draw a definitive conclusion.

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

2008-05-19

80

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

SciTech Connect

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

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

2010-02-10

81

REEVALUATING THE MASS-RADIUS RELATION FOR LOW-MASS, MAIN-SEQUENCE STARS  

SciTech Connect

We examine the agreement between the observed and theoretical low-mass (<0.8 M{sub Sun }) 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.

Feiden, Gregory A.; Chaboyer, Brian, E-mail: Gregory.A.Feiden.GR@Dartmouth.edu [Department of Physics and Astronomy, Dartmouth College, 6127 Wilder Laboratory, Hanover, NH 03755 (United States)

2012-09-20

82

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

NASA Technical Reports Server (NTRS)

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.

Tavani, Marco; London, Richard

1993-01-01

83

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

NASA Astrophysics Data System (ADS)

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 ? 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, ?LDB, determined from the luminosity of the LDB, LLDB, is increased by a factor of (1 - ?)-E compared to that inferred from unspotted models, where E ? 1 + dlog ?LDB/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 ? is highly uncertain with a probable range of 0.1 < ? < 0.4. For the smaller ? 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 ? 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.

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

2014-12-01

84

Searching for Clustering of Low Mass Pre-main Sequence Stars in the Orion OB1b Sub-Association  

NASA Astrophysics Data System (ADS)

We report on a project to map the spatial distribution of low mass pre-main sequence (PMS) stars around and between the stars of Orion's belt. We have UBVRI observations of ~ 3 deg(2) around and between the 3 O and B stars of Orion's belt, zeta , epsilon , and delta Orionis and sigma Orionis. We have found a well defined PMS locus in the color-magnitude diagram around each star. This allows us to select likely PMS stars photometrically. We discuss the spatial distribution of the PMS stars. We also present color-magnitude diagrams of the low mass stars in regions close to each of the O and B stars, and from selected regions between the massive stars. We will discuss the ages of PMS stars in each region.

Sherry, W. H.; Walter, F. M.; Wolk, S. J.

1999-05-01

85

Searching for Clustering of Low Mass Pre-main Sequence Stars in the Orion OB1b SubAssociation  

Microsoft Academic Search

We report on a project to map the spatial distribution of low mass pre-main sequence (PMS) stars around and between the stars of Orion's belt. We have UBVRI observations of ~ 3 deg(2) around and between the 3 O and B stars of Orion's belt, zeta , epsilon , and delta Orionis and sigma Orionis. We have found a well

W. H. Sherry; F. M. Walter; S. J. Wolk

1999-01-01

86

A Distributed Population of Low Mass Pre-Main Sequence Stars near the Taurus Molecular Clouds  

E-print Network

We present a drift scan survey covering a ~5 deg by 50 deg region toward the southern portion of the Taurus-Auriga molecular cloud. Data taken in the B,R,I filters with the Quest-2 camera on the Palomar 48-inch telescope were combined with 2MASS near-infrared photometry to select candidate young stars. Follow-up optical spectroscopy of 190 candidates led to identification of 42 new low mass pre-main sequence stars with spectral types M4-M8, of which approximately half exhibit surface gravity signatures similar to known Taurus stars while the other half exhibit surface gravity signatures similar to members of the somewhat older Upper Sco, TW Hya and Beta Pic associations. The pre-main sequence stars are spread over ~35 deg, and many are located well outside of previously explored regions. From assessment of the spatial and proper motion distributions, we argue that the new pre-main sequence stars identified far from the clouds cannot have originated from the vicinity of the 1-2 Myr-old subclusters which contain the bulk of the identified Taurus members, but instead represent a newly-identified area of recent star-formation near the clouds.

Catherine L. Slesnick; John M. Carpenter; Lynne A. Hillenbrand; Eric E. Mamajek

2006-09-01

87

Carbon star formation and neutron-rich isotope formation in low-mass asymptotic giant branch stars  

SciTech Connect

A preliminary analysis of the results of a very time-consuming program for the determination of dredge-up properties of thermally pulsing asymptotic giant branch (AGB) stars of low mass shows that: the dredge up to the surface of freshly made isotopes such as /sup 12/C does not continue throughout a star's AGB lifetime, but has a beginning and an end; only in stars of small initial metallicity and of sufficiently small mass does semicondvection play a role in the production of /sup 13/C, which can act as a source of neutrons for s-process-like isotope production; the primary reason for dredge-up properties of low-mass stars is the fact that the strength of thermal pulses reaches values that are an order of magnitude larger than in high-mass AGB stars; the minimum core mass when dredge up first begins increases with increasing metallicity and with decreasing total stellar mass; only above the critical value of mixing length over scale height does dredge up occur at all; although model stars of larger than solar metallicity can dredge up carbon, they probably do not become carbon stars, in agreement with the known lack of carbon stars in regions of high metallicity such as the galactic bulge. It is postulated that, in stars of moderate initial mass which can survive wind mass loss long enough to develop a core mass larger than those studied here, a second dredge-up phase may occur, and this may account for the abundances of Nb and Tc in stars such as R CMi and CY Cyg.

Iben, I. Jr.

1983-12-15

88

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

NASA Astrophysics Data System (ADS)

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

Warren, Steven Ray

89

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

NASA Astrophysics Data System (ADS)

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

Mace, Gregory Nathan

90

On the helium flash in low-mass Population III Red Giant stars  

E-print Network

We investigate the evolution of initially metal-free, low-mass Red Giant stars through the He core flash at the tip of the Red Giant Branch. The low entropy barrier between the helium- and hydrogen-rich layers enables a penetration of the helium flash driven convective zone into the inner tail of the extinguishing H-burning shell. As a consequence, protons are mixed into high-temperature regions triggering a H-burning runaway. The subsequent dredge-up of matter processed by He and H burning enriches the stellar surface with large amounts of helium, carbon and nitrogen. Extending previous results by Hollowell et al. (1990) and Fujimoto et al. (2000), who claimed that the H-burning runaway is an intrinsic property of extremely metal-poor low-mass stars, we found that its occurrence depends on additional parameters like the initial composition and the treatment of various physical processes. We perform some comparisons between predicted surface chemical abundances and observational measurements for extremely metal-deficient stars. As in previous investigations, our results disclose that although the described scenario provides a good qualitative agreement with observations, considerable discrepancies still remain. They may be due to a more complex evolutionary path of `real' stars, and/or some shortcomings in current evolutionary models. In addition, we analyze the evolutionary properties after the He core flash, during both the central and shell He-burning phases, allowing us to deduce some interesting differences between models whose Red Giant Branch progenitor has experienced the H-flash and canonical models. In particular, the Asymptotic Giant Branch evolution of extremely metal-deficient stars and the occurrence of thermal pulses are strongly affected by the previous RGB evolution.

H. Schlattl; S. Cassisi; M. Salaris; A. Weiss

2001-05-23

91

The Rotation of Young Low-Mass Stars and Brown Dwarfs  

NASA Astrophysics Data System (ADS)

We review the current state of our knowledge concerning the rotation and angular momentum evolution of young stellar objects and brown dwarfs from a primarily observational viewpoint. There has been a tremendous growth in the number of young, low-mass objects with measured rotation periods over the last five years, due to the application of wide field imagers on 1-2-m-class telescopes. Periods are typically accurate to 1% and available for about 1700 stars and 30 brown dwarfs in young clusters. Discussion of angular momentum evolution also requires knowledge of stellar radii, which are poorly known for pre-main-sequence stars. It is clear that rotation rates at a given age depend strongly on mass; higher-mass stars (0.4-1.2 solar mass) have longer periods than lower-mass stars and brown dwarfs. On the other hand, specific angular momentum is approximately independent of mass for low-mass pre-main-sequence stars and young brown dwarfs. A spread of about a factor of 30 is seen at any given mass and age. The evolution of rotation of solar-like stars during the first 100 m.y. is discussed. A broad, bimodal distribution exists at the earliest observable phases (~1 m.y.) for stars more massive than 0.4 solar mass. The rapid rotators (50-60% of the sample) evolve to the ZAMS with little or no angular momentum loss. The slow rotators continue to lose substantial amounts of angular momentum for up to 5 m.y., creating the even broader bimodal distribution characteristic of 30-120-m.y.-old clusters. Accretion disk signatures are more prevalent among slowly rotating PMS stars, indicating a connection between accretion and rotation. Disks appear to influence rotation for, at most, ~5 m.y., and considerably less than that for the majority of stars. This time interval is comparable to the maximum lifetime of accretion disks derived from near-infrared studies, and may be a useful upper limit to the time available for forming giant planets. If the dense clusters studied so far are an accurate guide, then the typical solar-like star may have only ~1 m.y. for this task. There is less data available for very-low-mass stars and brown dwarfs but the indication is that the same mechanisms are influencing their rotation as for the solar-like stars. However, it appears that both disk interactions and stellar winds are less efficient at braking these objects. We also review our knowledge of the various types of variability of these objects over as broad a mass range as possible with particular attention to magnetically induced cool spots and magnetically channeled variable mass accretion.

Herbst, W.; Eislöffel, J.; Mundt, R.; Scholz, A.

92

Inertial waves in differentially rotating low-mass stars and tides  

E-print Network

Star-planet tidal interactions may result in the excitation of inertial waves in the convective region of stars. Their dissipation plays a prominent role in the long-term orbital evolution of short-period planets. If the star is assumed to be rotating as a solid-body, the waves' Doppler-shifted frequency is restricted to $[-2 \\Omega, 2 \\Omega]$ ($\\Omega$ being the angular velocity of the star) and they can propagate in the entire convective region. However, turbulent convection can sustain differential rotation with an equatorial acceleration (as in the Sun) or deceleration that may modify waves propagation. We thus explore the properties of inertial modes of oscillation in a conically differentially rotating background flow whose angular velocity depends on the latitudinal coordinate only, close to what is expected in the external convective envelope of low-mass stars. We find that their frequency range is broadened by differential rotation, and that they may propagate only in a restricted part of the envelo...

Guenel, M; Mathis, S; Rieutord, M

2014-01-01

93

Chemical abundances of secondary stars in low mass X-ray binaries  

NASA Astrophysics Data System (ADS)

The high-velocity black hole in the low mass X-ray binary XTE J1118+480 could be the remnant of an massive star formed in the early stages of the evolution of the Galaxy (Mirabel et al. 2001). Alternatively, it could have been ejected from the Galactic plane as a result of a `kick' received during a supernova explosion (Gualandris et al. 2005). The chemical composition of the secondary star may provide unique clues on the origin of this black hole. Here, we present a medium-resolution optical spectra of the companion and determine the atmospheric abundances of Fe, Ca, Mg, Ni and Al. We find supersolar abundances for all these elements, rejecting the black hole was originated from the direct collapse of an ancient massive halo star. The compact object probably formed in a supernova event whose nucleosynthetic products polluted the secondary star. The observed element abundances and their ratios have been compared with a variety of supernova models for different metallicities and progenitor masses. Although we cannot definitely discard a supernova origin in the Galactic halo, the abundance pattern of the secondary star clearly suggest that black hole formed in the supernova explosion of a metal-rich massive progenitor and was violently `kicked out' from its birth place in the Galactic disc.

González Hernández, Jonay I.; Rebolo, Rafael; Israelian, Garik

2007-04-01

94

A Distributed Population of Low Mass Pre-Main Sequence Stars near the Taurus Molecular Clouds  

E-print Network

We present a drift scan survey covering a ~5 deg by 50 deg region toward the southern portion of the Taurus-Auriga molecular cloud. Data taken in the B,R,I filters with the Quest-2 camera on the Palomar 48-inch telescope were combined with 2MASS near-infrared photometry to select candidate young stars. Follow-up optical spectroscopy of 190 candidates led to identification of 42 new low mass pre-main sequence stars with spectral types M4-M8, of which approximately half exhibit surface gravity signatures similar to known Taurus stars while the other half exhibit surface gravity signatures similar to members of the somewhat older Upper Sco, TW Hya and Beta Pic associations. The pre-main sequence stars are spread over ~35 deg, and many are located well outside of previously explored regions. From assessment of the spatial and proper motion distributions, we argue that the new pre-main sequence stars identified far from the clouds cannot have originated from the vicinity of the 1-2 Myr-old subclusters which contai...

Slesnick, C L; Hillenbrand, L A; Mamajek, E E; Slesnick, Catherine L.; Carpenter, John M.; Hillenbrand, Lynne A.; Mamajek, Eric E.

2006-01-01

95

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

E-print Network

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

S. Pfalzner

2003-10-27

96

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

PubMed Central

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

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

2012-01-01

97

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

PubMed

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

Allard, F; Homeier, D; Freytag, B

2012-06-13

98

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

NASA Technical Reports Server (NTRS)

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.

Simon, Theodore

2005-01-01

99

Effects of non-standard neutrino emission on the evolution of low-mass stars  

NASA Astrophysics Data System (ADS)

Using the {Pools et al. (1995)} version of the STARS code with updated numerical tables for neutrino plasmon decay ({Kantor et al. 2007}), along with the reinterpretation of the Reimers mass-loss prescription by {Schröder et al. (2005)}, we analyze the consequences of enhanced neutrino emission on the internal structure and late evolution of the degenerated cores in low-mass stars, the non-standard increase in tip-RGB luminosity and the impact on the calibration of the Reimers mass-loss mechanism and the changes driven in post-RGB phases. With synthetic spectra generated with the PHOENIX code {Baron & Hauschildt et al. (1997)}, we also study the dependence of the non-standard increase in brightness on the selected NIR photometric band. By comparing our stellar evolutionary models with the synthetic spectra and the photometric data base of ?-Cen by {Sollima et al. (2004)}, we find the limit value ?_{?}? 2.2× 10^{-12}?_{B}.

Arceo-Díaz, S.; Schröder, K.-P.; Jack, D.; Zuber, K.

2014-10-01

100

The Atomic and Molecular Content of Disks around Very Low-mass Stars and Brown Dwarfs  

NASA Astrophysics Data System (ADS)

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+, H2, CO2, and tentative detections of H2O 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 H2 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 C2H2/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/H2O 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 H2O 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.

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

2013-12-01

101

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

E-print Network

We present here a detailed pulsational study applied to low-mass He-core white dwarfs, based on full evolutionary models representative of these objects. 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. We found substantial differences in the seismic properties of white dwarfs with $M_* \\gtrsim 0.20 M_{\\odot}$ and the extremely low-mass (ELM) white dwarfs ($M_* \\lesssim 0.20 M_{\\o...

Córsico, A H; Althaus, L G; Hermes, J J

2012-01-01

102

Clusters of Low-Mass Pre-Main Sequence Stars, Mass Functions, and Candidate Brown Dwarfs in Orion's Belt  

NASA Astrophysics Data System (ADS)

At least 3 of the 4 O stars of the Orion OB1b sub-association, ? , ? , and ? Orionis, are associated with clusters of low-mass PMS stars. These clusters appear similar to the Orion nebular cluster (ONC); each consists of several hundred to a few thousand low-mass PMS stars centered on an O star system. These clusters are roughly 4 pc in radius. These PMS clusters in seem to be evolved analogs of the ONC. Like the ONC they should be excellent places to search for very low-mass stars and brown dwarfs because such low mass objects are at their brightest when they are very young. Recent observational work has shown that the ? Orionis cluster is a rich hunting ground for young brown dwarfs. We have used our moderately deep VRI photomery combined with 2MASS JHK photometry to derive mass functions for the ? and ? Orionis clusters and for PMS stars around ? Orionis by fitting our data to the tracks of Baraffe et al. (1998). Our mass function reaches the base of the main sequence where we find several candidate brown dwarf in the ? Orionis and ? Orionis clusters, and around ? Orionis. We expect that these regions, like the ? Orionis cluster, will also be fertile hunting grounds for very-low mass stars and brown dwarfs.

Sherry, W. H.; Walter, F. M.; Wolk, S. J.

2000-12-01

103

Third Dredge-up in Low Mass Stars: Solving the LMC Carbon Star Mystery  

E-print Network

A long standing problem with asymptotic giant branch (AGB) star models has been their inability to produce the low-luminosity carbon stars in the Large and Small Magellanic Clouds. Dredge-up must begin earlier and extend deeper. We find this for the first time in our models of LMC metallicity. Such features are not found in our models of SMC metallicity. The fully implicit and simultaneous stellar evolution code STARS has been used to calculate the evolution of AGB stars with metallicities of Z=0.008 and Z=0.004, corresponding to the observed metallicities of the Large and Small Magellanic Clouds, respecitively. Third dredge-up occurs in stars of 1Msol and above and carbon stars were found for models between 1Msol and 3Msol. We use the detailed models as input physics for a population synthesis code and generate carbon star luminosity functions. We now find that we are able to reproduce the carbon star luminosity function of the LMC without any manipulation of our models. The SMC carbon star luminosity function still cannot be produced from our detailed models unless the minimum core mass for third dredge-up is reduced by 0.06Msol.

Richard J. Stancliffe; Robert G. Izzard; Christopher A. Tout

2004-10-08

104

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)

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.

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

105

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

NASA Astrophysics Data System (ADS)

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.

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

2014-07-01

106

Constraining the properties of spots on Pleiades very low mass stars  

E-print Network

We present results of a multi-filter monitoring campaign for very low mass (VLM) stars in the Pleiades. Simultaneous to our I-band time series (Scholz & Eisloeffel 2004), which delivered photometric periods for nine VLM stars, we obtained light curves in the J- and H-band. One VLM star with M~0.15 Ms (BPL129) shows a period in all three wavelength bands. The amplitudes in I, J, and H are 0.035, 0.035, and 0.032 mag. These values are compared to simulations, in which we compute the photometric amplitude as a function of spot temperature and filling factor. The best agreement between observations and models is found for cool spots with a temperature contrast of 18-31% and a very low surface filling factor of 4-5%. We suggest that compared to more massive stars VLM objects may have either very few spots or a rather symmetric spot distribution. This difference might be explained with a change from a shell to a distributed dynamo in the VLM regime.

Alexander Scholz; Jochen Eisloeffel; Dirk Froebrich

2005-05-27

107

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

SciTech Connect

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.

Jonker, P. G. [SRON, Netherlands Institute for Space Research, 3584 CA, Utrecht (Netherlands); Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, Massachusetts (United States); Astronomical Institute, Utrecht University, 3508 TA, Utrecht (Netherlands)

2008-02-27

108

c2d Spitzer IRS spectra of embedded low-mass young stars: gas-phase emission lines  

Microsoft Academic Search

A survey of mid-IR gas-phase emission lines of H2, H2O and various atoms\\u000atoward a sample of 43 embedded low-mass young stars in nearby star-forming\\u000aregions is presented. The sources are selected from the Spitzer \\

Fred Lahuis; Ewine F. van Dishoeck; Jes K. Jørgensen; Geoffrey A. Blake; Neal J. Evans II

2010-01-01

109

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

SciTech Connect

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

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

2012-08-10

110

The low-mass interacting binary system OO Aql revisited: a new quadruple system  

E-print Network

In this study we present photometric and spectroscopic variation analysis and orbital period study of a low-mass interacting system OO Aql. Simultaneous solution of the light and radial velocity curves provide us a determination of new set of stellar physical parameters for the primary and the secondary companion as M$_{1}$ = 1.05(2) M$_{\\odot}$, M$_{2}$ = 0.89(2) M$_{\\odot}$, R$_{1}$ = 1.38(2) R$_{\\odot}$, R$_{2}$ = 1.28(2) R$_{\\odot}$, $\\log{(L_1/L_{\\odot})} = 0.258$ and $\\log{(L_2/L_{\\odot})} = 0.117$ and the separation of the components were determined a = 3.333(16) R$_{\\odot}$. Newly obtained parameters yield the distance of the system as 136(8) pc. Analyses of the mid-eclipse times indicate a period increase of $\\frac{P}{\\dot{P}}=4\\times 10^{7}$ yr that can be interpreted in terms of the mass transfer $\\frac{dM}{dt}=5\\times 10^{-8}$ M$_{\\odot}$/yr 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...

?çli, Tu?çe; Boz, G Çisem; Yakut, Kadri

2013-01-01

111

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

NASA Astrophysics Data System (ADS)

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

Cody, Ann Marie; Hillenbrand, Lynne A.

2014-12-01

112

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

E-print Network

We investigate how magnetic properties of Magellanic-type and perturbed objects are related to star-forming activity, galactic type, and mass. We present radio and magnetic properties of 5 Magellanic-type and 2 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 5 Magellanic-type galaxies and 5 dwarf ones. The radio emission of low-mass galaxies at 4.85/8.35 GHz is closely connected with their optical discs. The strengths of total magnetic field are within 5-9 \\mu G, while the ordered fields reach 1-2 \\mu G. The magnetic field strengths are well correlated with the surface density of SFR and manifest a power-law relation with an exponent of 0.25 extending a similar relation found for dwarf galaxies. The production of magnetic energy per supernova event is similar for all the various galaxies. It constitutes about 3% of the individual SN energy release. We show that the total magnetic field energy in galaxies is almo...

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

2014-01-01

113

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

SciTech Connect

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

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

2011-05-01

114

The Search for Low-mass Companions of B Stars in the Carina Nebula Cluster Trumpler 16  

NASA Astrophysics Data System (ADS)

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

Evans, Nancy Remage; DeGioia-Eastwood, Kathleen; Gagné, Marc; Townsley, Leisa; Broos, Patrick; Wolk, Scott; Nazé, Yaël; Corcoran, Michael; Oskinova, Lida; Moffat, Anthony F. J.; Wang, Junfeng; Walborn, Nolan R.

2011-05-01

115

Ages of evolved low mass stars: Central stars of planetary nebulae and white dwarfs  

NASA Astrophysics Data System (ADS)

We have developed several methods to estimate the ages of central stars of planetary nebulae (CSPN), which are based either on observed nebular properties or on data from the stars themselves. Our goal is to derive the age distribution of these stars and compare the results with empirical distributions for CSPN and white dwarfs. We have initially developed three methods based on nebular abundances, using (i) an age-metallicity relation which is also a function of the galactocentric distance; (ii) an age-metallicity relation obtained for the galactic disk, and (iii) the central star masses derived from the observed nitrogen abundances. In this work we present two new, more accurate methods, which are based on kinematic properties: (I) in this method, the expected rotation velocities of the nebulae around the galactic centre at their galactocentric distances are compared with the predicted values for the galactic rotation curve, and the differences are attributed to the different ages of the evolved stars; (II) we determine directly the U, V, W, velocity components of the stars, as well as the velocity dispersions, and use the dispersion-age relation by the Geneva-Copenhagen survey. These methods were applied to two large samples of galactic CSPN. We conclude that most CSPN in the galactic disk have ages under 5 Gyr, and that the age distribution is peaked around 1 to 3 Gyr.

Maciel, W. J.; Rodrigues, T. S.; Costa, R. D. D.

2013-03-01

116

Isotropic star in low-mass X-ray binaries and X-ray pulsars  

E-print Network

We present a model for compact stars in the low mass X-ray binaries(LMXBs) and X-ray pulsars using a metric given by John J. Matese and Patrick G. Whitman \\citep{Matese and Whitman1980}. Here the field equations are reduced to a system of two algebraic equations considering the isotropic pressure. Compact star candidates 4U 1820-30(radius=10km) in LMXBs, and Her X-1(radius=7.7km), SAX J 1808.4-3658(SS1)(radius=7.07km) and SAX J 1808.4-3658(SS2)(radius=6.35km) in X-ray pulsars satisfy all the energy conditions, TOV-equation and stability condition. From our model, we have derived mass($M$), central density($\\rho_{0}$), suface density($\\rho_{b}$), central pressure($p_{0}$), surface pressure($p_{b}$) and surface red-shift($Z_{s}$) of the above mentioned stars, which are very much consistant with the observed/reported datas\\citep{N. K. Glendenning1997,Gondek2000}. We have also observe the adiabatic index($\\gamma$>4/3) of the above steller objects.

Kalam, Mehedi; Molla, Sajahan

2014-01-01

117

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

E-print Network

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 and rendering the planet sterile to life. However, for planets orbiting low-mass stars, 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 ($T_{eff}=2670 K$) 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 ...

Van Laerhoven, Christa; Greenberg, Richard

2014-01-01

118

Rotation and variability of very low mass stars and brown dwarfs near Epsilon Ori  

E-print Network

We explore the rotation and activity of very low mass (VLM) objects by means of a photometric variability study. Our targets in the vicinity of Epsilon Ori belong to the OriOB1b population in the Orion star-forming complex. In this region we selected 143 VLM stars and brown dwarfs (BDs), whose photometry in RIJHK is consistent with membership of the young population. The variability of these objects was investigated using a densely sampled I-band time series covering four consecutive nights with altogether 129 data points per object. Our targets show three types of variability: Thirty objects, including nine BDs, show significant photometric periods, ranging from 4h up to 100h, which we interpret as the rotation periods. Five objects, including two BDs, exhibit variability with high amplitudes up to 1 mag which is at least partly irregular. This behaviour is most likely caused by ongoing accretion and confirms that VLM objects undergo a T Tauri phase similar to solar-mass stars. Finally, one VLM star shows a strong flare event of 0.3 mag amplitude. The rotation periods show dependence on mass, i.e. the average period decreases with decreasing object mass, consistent with previously found mass-period relationships in younger and older clusters. The period distribution of BDs extends down to the breakup period, where centrifugal and gravitational forces are balanced. Combining our BD periods with literature data, we found that the lower period limit for substellar objects lies between 2h and 4h, more or less independent of age. Contrary to stars, these fast rotating BDs seem to evolve at constant rotation period from ages of 3 Myr to 1 Gyr, in spite of the contraction process. Thus, they should experience strong rotational braking.

Alexander Scholz; Jochen Eisloeffel

2004-10-05

119

THE FORMATION OF LOW-MASS BINARY STAR SYSTEMS VIA TURBULENT FRAGMENTATION  

SciTech Connect

We characterize the infall rate onto protostellar systems forming in self-gravitating radiation-hydrodynamics simulations. Using two dimensionless parameters to determine the disks' susceptibility to gravitational fragmentation, we infer limits on protostellar system multiplicity and the mechanism of binary formation. We show that these parameters give robust predictions even in the case of marginally resolved protostellar disks. We find that protostellar systems with radiation feedback predominately form binaries via turbulent fragmentation, not disk instability, and predict that turbulent fragmentation is the dominant channel for binary formation for low-mass stars. We clearly demonstrate that systems forming in simulations including radiative feedback have fundamentally different parameters than those in purely hydrodynamics simulations.

Offner, Stella S. R. [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States); Kratter, Kaitlin M.; Matzner, Christopher D. [Department of Astronomy and Astrophysics, 50 St. George Street, University of Toronto, Toronto, Ontario M5R 3H4 (Canada); Krumholz, Mark R. [Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064 (United States); Klein, Richard I., E-mail: soffner@cfa.harvard.ed [Department of Astronomy, University of California, Berkeley, CA 94720 (United States)

2010-12-20

120

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

NASA Astrophysics Data System (ADS)

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

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

2014-07-01

121

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

SciTech Connect

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

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

2013-05-15

122

>From binaries to asymmetric outflows: The influence of low-mass companions around AGB stars  

NASA Astrophysics Data System (ADS)

The study of intermediate mass, evolved stars is undergoing renewed interest due to recent observational and theoretical results suggesting that binarity i fundamental for shaping post-Asymptotic Giant Branch and Planetary Nebula outflows. Despite extensive research, the physical mechanism responsible for transitioning from a spherical Asymptotic Giant Branch (AGB) star to an asymmetric post-AGB object is poorly understood. In an effort to understand how binaries may produce asymmetries, this thesis presents several theoretical studies which explore the effect of low-mass companions on evolved star outflows. This thesis consists of four separate projects: (1) Close companions may become engulfed by the evolved star and in-spiral during a common envelope phase. Common envelope evolution can lead to three different consequences: (i.) equatorial ejection of material (ii.) spin-up of the envelope resulting in an explosive dynamo-driven jet and (iii.) tidal shredding of the companion into an accretion disk which ejects a poloidal wind. (2) In addition, we study a dynamical, large-scale a-O interface dynamo operating in an AGB star in both an isolated setting and a setting in which a low-mass companion is embedded inside the envelope. The back reaction of the fields on the shear is included and differential rotation and rotation deplete via turbulent dissipation and Poynting flux. For the isolated star, the shear must be resupplied in order to sufficiently sustain the dynamo. Furthermore, we investigate the energy requirements that convection must satisfy to accomplish this by analogy to the Sun. For the common envelope case, a robust dynamo results, unbinding the envelope under a range of conditions. (3)Wide binaries can interact with the wind of the evolved primary. The gravitational influence of the secondary focuses material in the equatorial plane. The companion induces spiral shocks which may anneal amorphous grains into crystalline dust. This work presents a physical mechanism to produce crystalline dust in AGB star binaries. (4) We present a spectral modeling technique which constrains the geometry of evolved star nebulae. We apply our technique to HD 179821 which exhibits a double peaked spectral energy distribution (SED) with a sharp rise from ~ 8 - 20 mm. Such features have been associated with dust shells or inwardly truncated circumstellar disks. In order to compare SEDs from both systems, we employ a spherically symmetric radiative transfer code and compare it to a radiative, inwardly truncated disc code. As a case study, we model the broad-band SED of HD 179821 using both codes. Shortward of 40 mm, we find that both models produce equivalent fits to the data. However, longward of 40 mm, the radial density distribution and corresponding broad range of disc temperatures produce excess emission above our spherically symmetric solutions and the observations. For HD 179821, our best fit consists of a T eff = 7000 K central source characterized by t V ~ 1.95 and surrounded by a radiatively driven, spherically symmetric dust shell. The extinction of the central source reddens the broad- band colours so that they resemble a T eff = 5750 K photosphere. We believe that HD 179821 contains a hotter central star than previously thought. Our results provide an initial step towards a technique to distinguish geometric differences from spectral modeling.

Nordhaus, Jason T.

123

Patterns of X-ray, Chromospheric, and Radio Emission in Low-mass Stars: Fast and Slow Magnetic Reconnection  

Microsoft Academic Search

Magnetic reconnection events in the atmospheres of low-mass dwarf stars can be classified as either slow or fast, depending on whether ohmic diffusion or Hall currents dominate in the reconnection process. We suggest that the separation of reconnection into slow and fast categories can help to explain some systematics of low-mass dwarfs as regards their emissions in X-rays, Halpha, and

D. J. Mullan

2010-01-01

124

Sloan Low-mass Wide Pairs of Kinematically Equivalent Stars (SLoWPoKES): A Catalog of Very Wide, Low-mass Pairs  

NASA Astrophysics Data System (ADS)

We present the Sloan Low-mass Wide Pairs of Kinematically Equivalent Stars (SLoWPoKES), a catalog of 1342 very-wide (projected separation gsim500 AU), low-mass (at least one mid-K to mid-M dwarf component) common proper motion pairs identified from astrometry, photometry, and proper motions in the Sloan Digital Sky Survey. A Monte Carlo-based Galactic model is constructed to assess the probability of chance alignment for each pair; only pairs with a probability of chance alignment <=0.05 are included in the catalog. The overall fidelity of the catalog is expected to be 98.35%. The selection algorithm is purposely exclusive to ensure that the resulting catalog is efficient for follow-up studies of low-mass pairs. The SLoWPoKES catalog is the largest sample of wide, low-mass pairs to date and is intended as an ongoing community resource for detailed study of bona fide systems. Here, we summarize the general characteristics of the SLoWPoKES sample and present preliminary results describing the properties of wide, low-mass pairs. While the majority of the identified pairs are disk dwarfs, there are 70 halo subdwarf (SD) pairs and 21 white dwarf-disk dwarf pairs, as well as four triples. Most SLoWPoKES pairs violate the previously defined empirical limits for maximum angular separation or binding energies. However, they are well within the theoretical limits and should prove very useful in putting firm constraints on the maximum size of binary systems and on different formation scenarios. We find a lower limit to the wide binary frequency (WBF) for the mid-K to mid-M spectral types that constitute our sample to be 1.1%. This frequency decreases as a function of Galactic height, indicating a time evolution of the WBF. In addition, the semi-major axes of the SLoWPoKES systems exhibit a distinctly bimodal distribution, with a break at separations around 0.1 pc that is also manifested in the system binding energy. Compared with theoretical predictions for the disruption of binary systems with time, we conclude that the SLoWPoKES sample comprises two populations of wide binaries: an "old" population of tightly bound systems, and a "young" population of weakly bound systems that will not survive more than a few Gyr. The SLoWPoKES catalog and future ancillary data are publicly available on the Internet for utilization by the astronomy community.

Dhital, Saurav; West, Andrew A.; Stassun, Keivan G.; Bochanski, John J.

2010-06-01

125

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

NASA Astrophysics Data System (ADS)

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

Bowler, Brendan Peter

126

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

NASA Astrophysics Data System (ADS)

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

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

2013-07-01

127

New light on dark stars. Red dwarfs, low-mass stars, brown dwarfs  

Microsoft Academic Search

This book presents a comprehensive discussion of both the astrophysical structure of individual M dwarf and brown dwarf star, and their collective statistical properties as a Galactic stellar population. The first section of the book discusses M dwarfs and brown dwarfs as individual objects - their observational properties, formation, internal structure and atmospheres. The second section deals with M dwarfs

I. N. Reid; S. L. Hawley

2000-01-01

128

No disks around low-mass stars and brown dwarfs in the young sigma Orionis cluster?  

E-print Network

We report on the analysis of 2MASS near-infrared data of a sample of low-mass stars and brown dwarfs in the sigma Orionis cluster. Youth and cluster membership have been spectroscopically confirmed using the Li I spectral line. We find little evidence in the JHKs colour-colour diagram for near-infrared excess emission for these cluster members. By comparison with model expectations, at most 2 out of 34 stars show (H-K) colour consistent with a near-infrared excess. This scarcity of near-infrared signatures of circumstellar disks in the lower-mass and substellar regimes of this cluster contrasts with findings in younger clusters, hinting at an age dependence of the disk frequency. Taking into account the apparent cluster age, our result supports the idea of a relatively fast (few Myr) disk dissipation and extends this conclusion to the substellar regime. We also find some evidence that, in this cluster, the disk frequency as measured by the Ks-band excess may be mass dependent.

J. M. Oliveira; R. D. Jeffries; M. J. Kenyon; S. A. Thompson; Tim Naylor

2001-12-13

129

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

E-print Network

We simulate the evolution of a 10^9 Msun dark matter halo in a cosmological setting with an adaptive-mesh refinement code as an analogue to local low luminosity dwarf irregular and dwarf spheroidal galaxies. The primary goal of our study is to investigate the roles of reionization and supernova feedback in determining the star formation histories of low mass dwarf galaxies. We include a wide range of physical effects, including metal cooling, molecular hydrogen formation and cooling, photoionization and photodissociation from a metagalactic background, a simple prescription for self-shielding, star formation, and a simple model for supernova driven energetic feedback. We carry out simulations excluding each major effect in turn. We find that reionization is primarily responsible for expelling most of the gas in our simulations, but that supernova feedback is required to disperse the dense, cold gas in the core of the halo. Moreover, we show that the timing of reionization can produce an order of magnitude dif...

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

2012-01-01

130

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

NASA Astrophysics Data System (ADS)

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

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

131

VLTI/AMBER detection of a K=9.5 very low mass star.  

NASA Astrophysics Data System (ADS)

The precise determination of both the dynamical mass and infrared photometry of the close companion to AB Dor A, AB Dor C (0.090 M_?), has provided an important benchmark for calibration of theoretical evolutionary models of low-mass young stars. However, comparison of the observed magnitudes of AB Dor C with these models suggests that they could overpredict the flux of this object, a trend also found in other young systems. One of the ambiguities remaining in AB Dor C is the possible binary nature of this star; in fact, should AB Dor C be close binary (˜10 mas separation), it could reconcile observations and models. We will report on VLTI/AMBER observations of AB Dor C addressed to discriminate between both scenarios: AB Dor C as a single object or a binary brown dwarf. We used a non-standard ``off-axis'' fringe tracking that allowed the detection of AB Dor C. This is, to our knowledge, one of the weakest object detected by an infrared interferometer (K_s=9.5). Based on observations made with ESO Telescopes at the Paranal Observatory under programme ID 090.C-0559(A).

Guirado, J. C.; Marcaide, J. M.; Berger, J.-P.; Mérand, A.; Martí-Vidal, I.

132

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

NASA Astrophysics Data System (ADS)

Magnetic fields are hypothesized to inflate the radii of low-mass stars—defined 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.

Feiden, Gregory A.; Chaboyer, Brian

2013-12-01

133

A 3-5$?$m VLT spectroscopic survey of embedded young low mass stars II; OCN$^-$  

E-print Network

The 4.62$\\mu$m (2164.5 cm$^{-1}$) `XCN' band has been detected in the $M$-band spectra of 34 deeply embedded young stellar objects (YSO's), observed with high signal-to-noise and high spectral resolution with the VLT-ISAAC spectrometer, providing the first opportunity to study the solid OCN$^-$ abundance toward a large number of low-mass YSO's. It is shown unequivocally that at least two components, centred at 2165.7 cm$^{-1}$ (FWHM = 26 cm$^{-1}$) and 2175.4 cm$^{-1}$ (FWHM = 15 cm$^{-1}$), underlie the XCN band. Only the 2165.7-component can be ascribed to OCN$^-$, embedded in a strongly hydrogen-bonding, and possibly thermally annealed, ice environment based on laboratory OCN$^-$ spectra. In order to correct for the contribution of the 2175.4-component to the XCN band, a phenomenological decomposition into the 2165.7- and the 2175.4-components is used to fit the full band profile and derive the OCN$^-$ abundance for each line-of-sight. The same analysis is performed for 5 high-mass YSO's taken from the ISO-SWS data archive. Inferred OCN$^-$ abundances are $\\leq$ 0.85 % toward low-mass YSO's and $\\leq$ 1 % toward high-mass YSO's, except for W33 A. Abundances are found to vary by at least a factor of 10--20 and large source-to-source abundance variations are observed within the same star-forming cloud complex on scales down to 400 AU, indicating that the OCN$^-$ formation mechanism is sensitive to local conditions. The inferred abundances allow quantitatively for photochemical formation of OCN$^-$, but the large abundance variations are not easily explained in this scenario unless local radiation sources or special geometries are invoked. Surface chemistry should therefore be considered as an alternative formation mechanism.

F. A. van Broekhuizen; K. M. Pontoppidan; H. J. Fraser; E. F. van Dishoeck

2005-08-25

134

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

NASA Astrophysics Data System (ADS)

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

Pitts, Mark A.

135

Interactions Between Forming Stars and Dense Gas in a Small Low Mass Cluster  

NASA Astrophysics Data System (ADS)

We present observations of dense gas and outflow activity in the Cederblad 110 region of the Chamaleleon I Dark Cloud. The region contains eight 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 (?_YSO 150 pc^-2). The analysis of our N2H+ (J=1-0) maps indicates the presence of 15 solar masses of dense (n 10^5 cm^-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. Smaller column densities of more stable dense gas are found toward the region's Class I sources. Little or no dense gas is colocated with the Class II and III sources in the region. The outflow from the Class I source IRS 4 is interacting with the dense core associated with MMS 1. The molecular component of the outflow appears to be deflected by the densest part of the core, after which it plows through some of the lower column density portions of the core. The working surface at the head of the outflow lobe can be seen in the enhanced velocity dispersion of the dense gas. The Class III source IRS 2 may also be influencing the dense gas in the region. A dust temperature gradient across the core is consistent with warming from the 3.4 Lo source, 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.

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

2011-05-01

136

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

SciTech Connect

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.

Morales, Juan Carlos; Ribas, Ignasi; Jordi, Carme [Institut d'Estudis Espacials de Catalunya (IEEC), Edif. Nexus, C/ Gran Capita, 2-4, 08034 Barcelona (Spain); Gallardo, Jose [Departamento de AstronomIa, Universidad de Chile, Casilla 36-D, Santiago (Chile); Baraffe, Isabelle; Chabrier, Gilles, E-mail: morales@ieec.uab.e [Ecole Normale Superieure de Lyon, CRAL (UMR CNRS 5574), Universite de Lyon (France)

2010-07-20

137

The Effect of Feedback and Reionization on Star Formation in Low-mass Dwarf Galaxy Halos  

NASA Astrophysics Data System (ADS)

I will present a set of high resolution simulations of a 109 M? dark matter halo in a cosmological setting done with an adaptive-mesh refinement code as a mass analogue to local low-luminosity dwarf spheroidal galaxies. The primary goal of our simulations is to investigate the roles of reionization and supernova feedback in determining the star formation histories of low mass dwarf galaxies. We include a wide range of physical effects, including metal cooling, molecular hydrogen formation and cooling, photoionization and photodissociation from a metagalactic (but not local) background, a simple prescription for self-shielding, star formation, and a simple model for supernova driven energetic feedback. We find that reionization is primarily responsible for expelling most of the gas in our simulations, but that supernova feedback is required to disperse the dense, cold gas in the core of the halo. Moreover, we show that the timing of reionization can produce an order of magnitude difference in the final stellar mass of the system. For our full physics run with reionization at z=9, we find a stellar mass of about 105 M? at z=0, and a mass-to-light ratio within the half-light radius of approximately 130 M?/L?, consistent with observed low-luminosity dwarfs. However, the resulting median stellar metallicity is 0.06 Z?, considerably larger than observed systems. In addition, we find star formation is truncated between redshifts 4 and 7, at odds with the observed late time star formation in isolated dwarf systems but in agreement with Milky Way ultrafaint dwarf spheroidals. We investigate the efficacy of energetic feedback in our simple thermal-energy driven feedback scheme, and suggest that it may still suffer from excessive radiative losses, despite reaching stellar particle masses of about 100 M?, and a comoving spatial resolution of 11 pc. This has led us to pursue improvements in our supernova feedback model to include kinetic as well as thermal energy in the proportions predicted by Sedov-Taylor models on the scale of a few parsecs, which is approximately the resolution of our simulations at high redshift when the star formation rate peaks.

Simpson, Christine M.; Bryan, G.; Johnston, K. V.; Smith, B. D.; Mac Low, M.; Sharma, S.; Tumlinson, J.

2013-01-01

138

c2d Spitzer IRS spectra of embedded low-mass young stars: gas-phase emission lines  

Microsoft Academic Search

Context. A survey of mid-infrared gas-phase emission lines of H2, H2O and various atoms toward a sample of 43 embedded low-mass young stars in nearby star-forming regions is presented. The sources are selected from the Spitzer ``Cores to Disks'' (c2d) legacy program. Aims: The environment of embedded protostars is complex both in its physical structure (envelopes, outflows, jets, protostellar disks)

F. Lahuis; E. F. van Dishoeck; J. K. Jørgensen; G. A. Blake; N. J. Evans

2010-01-01

139

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

E-print Network

ATMOSPHERIC CHEMISTRY IN GIANT PLANETS, BROWN DWARFS, AND LOW-MASS DWARF STARS. II. SULFUR to model sulfur and phosphorus chemistry in giant planets, brown dwarfs, and extrasolar giant planets (EGPs atmospheres of giant planets and T dwarfs. In hotter objects, several P-bearing gases (e.g., P2, PH3, PH 2, PH

Fegley Jr., Bruce

140

High spatial resolution mid-infrared observations of the low-mass young star TW Hya  

E-print Network

We want to improve knowledge of the structure of the inner few AU of the circumstellar disk around the nearby T Tauri star TW Hya. Earlier studies have suggested the existence of a large inner hole, possibly caused by interactions with a growing protoplanet. We used interferometric observations in the N-band obtained with the MIDI instrument on the Very Large Telescope Interferometer, together with 10 micron spectra recorded by the infrared satellite Spitzer. The fact that we were able to determine N-band correlated fluxes and visibilities for this comparatively faint source shows that MIR interferometry can be applied to a large number of low-mass young stellar objects. The MIR spectra obtained with Spitzer reveal emission lines from HI (6-5), HI (7-6), and [Ne II] and show that over 90% of the dust we see in this wavelength regime is amorphous. According to the correlated flux measured with MIDI, most of the crystalline material is in the inner, unresolved part of the disk, about 1 AU in radius. The visibilities exclude the existence of a very large (3-4 AU radius) inner hole in the circumstellar disk of TW Hya, which was required in earlier models. We propose instead a geometry of the inner disk where an inner hole still exists, but at a much reduced radius, with the transition from zero to full disk height between 0.5 and 0.8 AU, and with an optically thin distribution of dust inside. Such a model can comply with SED and MIR visibilities, as well as with visibility and extended emission observed in the NIR at 2 micron. If a massive planet was the reason for this inner hole, as has been speculated, its orbit would have to be closer to the star than 0.3 AU. Alternatively, we may be witnessing the end of the accretion phase and an early phase of an inward-out dispersal of the circumstellar disk.

Th. Ratzka; Ch. Leinert; Th. Henning; J. Bouwman; C. P. Dullemond; W. Jaffe

2007-07-02

141

In infrared search for very low mass stars: The luminosity function  

SciTech Connect

Infrared photometry can reveal cool companions to hot sub-main-sequence stars to very faint limtis of luminosity. We have surveyed approx.100 white dwarfs at 2.2 ..mu..m for very low mass red dwarf companions in a search complete to M/sub v/approx.21. Very few new companions were found, none with M/sub v/>15. This does not appear due to either selection effects or evolutionary effects on precursor systems. The cumulative body of evidence suggests termination of the hydrogen-burning main sequence at M/sub v/roughly-equal17.5, M/sub bol/roughly-equal12.8. Our survey and other results indicate the luminosity function declines for >13, with a resulting decline in the mass function. The mass density in objects of M<0.2 M unaccounted for in the Luyten luminosity function is < or approx. =0.005 M pc/sup -3/ in the solar neighborhood. It is suggested that a massive Galactic halo is not composed of substellar-mass objects; however, if it is, such a population could be identified with the Space Telescope without confusion with similar dish population objects.

Probst, R.G.

1983-11-01

142

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

NASA Astrophysics Data System (ADS)

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.

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

2014-08-01

143

UV excess measures of accretion onto young very low-mass stars and brown dwarfs  

E-print Network

Low-resolution spectra from 3000-9000 AA of young low-mass stars and brown dwarfs were obtained with LRIS on Keck I. The excess UV and optical emission arising in the Balmer and Paschen continua yields mass accretion rates ranging from 2e-12 to 1e-8 Mo/yr. These results are compared with {\\it HST}/STIS spectra of roughly solar-mass accretors with accretion rates that range from 2e-10 to 5e-8 Mo/yr. The weak photospheric emission from M-dwarfs at <4000 A leads to a higher contrast between the accretion and photospheric emission relative to higher-mass counterparts. The mass accretion rates measured here are systematically 4-7 times larger than those from H-alpha emission line profiles, with a difference that is consistent with but unlikely to be explained by the uncertainty in both methods. The accretion luminosity correlates well with many line luminosities, including high Balmer and many He I lines. Correlations of the accretion rate with H-alpha 10% width and line fluxes show a large amount of scatter. Our results and previous accretion rate measurements suggest that accretion rate is proportional to M^(1.87+/-0.26) for accretors in the Taurus Molecular Cloud.

Gregory J. Herczeg; Lynne A. Hillenbrand

2008-01-23

144

New Very Low Mass Binaries in the Taurus Star-Forming Region  

E-print Network

We surveyed thirteen very low mass (VLM; M 4-sigma). Using the theoretical models of Baraffe et al. (1998), we find that all five new companions, as well as one of the primaries, are likely brown dwarfs. The discovery of these systems therefore increases the total number of known, young VLM binaries by ~50%. These new systems, along with other young VLM binaries from the literature, have properties that differ significantly from older field VLM binaries in that the young systems have wider separations and lower mass ratios, supporting the idea that VLM binaries undergo significant dynamical evolution ~5 - 10 Myr after their formation. The range of separations of these binaries, four of which are over 30 AU, argues against the ejection scenario of brown dwarf formation. While several of the young, VLM binaries discovered in this study have lower binding energies than the previously suggested minimum for VLM binaries, the apparent minimum is still significantly higher than that found among higher mass binaries. We suggest that this discrepancy may be due to the small mass of a VLM binary relative to the average perturbing star, leading to more substantial changes in their binding energy over time.

Q. M. Konopacky; A. M. Ghez; E. L. Rice; G. Duchene

2007-03-21

145

High spatial resolution mid-infrared observations of the low-mass young star TW Hya  

E-print Network

We want to improve knowledge of the structure of the inner few AU of the circumstellar disk around the nearby T Tauri star TW Hya. Earlier studies have suggested the existence of a large inner hole, possibly caused by interactions with a growing protoplanet. We used interferometric observations in the N-band obtained with the MIDI instrument on the Very Large Telescope Interferometer, together with 10 micron spectra recorded by the infrared satellite Spitzer. The fact that we were able to determine N-band correlated fluxes and visibilities for this comparatively faint source shows that MIR interferometry can be applied to a large number of low-mass young stellar objects. The MIR spectra obtained with Spitzer reveal emission lines from HI (6-5), HI (7-6), and [Ne II] and show that over 90% of the dust we see in this wavelength regime is amorphous. According to the correlated flux measured with MIDI, most of the crystalline material is in the inner, unresolved part of the disk, about 1 AU in radius. The visibil...

Ratzka, T; Henning, T; Bouwman, J; Dullemond, C P; Jaffe, W

2007-01-01

146

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

NASA Astrophysics Data System (ADS)

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

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

2013-06-01

147

The Schweizer-Middleditch star revisited  

E-print Network

We have re-observed and re-analysed the optical spectrum of the Schweizer-Middleditch star, a hot subdwarf which lies along almost the same line-of-sight as the centre of the historic SN1006 supernova remnant (SNR). Although this object is itself unlikely to be the remnant of the star which exploded in 1006AD, Wellstein et al. (1999) have demonstrated that it could be the remnant of the donor star in a pre-supernova Type Ia interacting binary, if it possesses an unusually low mass. We show that, if it had a mass of 0.1-0.2 Msun, the SM star would lie at the same distance (~800pc) as the SNR as estimated by Willingale et al. (1995). However, most distance estimates to SN1006 are much larger than this, and there are other convincing arguments to suggest that the SM star lies behind this SNR. Assuming the canonical subdwarf mass of 0.5 Msun, we constrain the distance of the SM star as 1050 pc

M. R. Burleigh; U. Heber; D. O'Donoghue; M. A. Barstow

2000-02-10

148

Interactions between Forming Stars and Dense Gas in the Small Low-mass Cluster Cederblad 110  

NASA Astrophysics Data System (ADS)

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 (?YSO ~ 150 pc-2). The analysis of our N2H+ (J = 1?0) maps indicates the presence of 13 ± 3 solar masses of dense (n ~ 105 cm-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?0) line of 12CO, 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.

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

2011-12-01

149

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

SciTech Connect

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.

Ladd, E. F. [Department of Physics and Astronomy, Bucknell University, Lewisburg, PA 17837 (United States); Wong, T. [Department of Astronomy, University of Illinois, Urbana, IL 61801 (United States); Bourke, T. L. [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States); Thompson, K. L., E-mail: ladd@bucknell.edu [Department of Physics and Astronomy, University of Kentucky, Lexington, KY 40506 (United States)

2011-12-20

150

Water in low-mass star-forming regions with Herschel . HIFI spectroscopy of NGC 1333  

NASA Astrophysics Data System (ADS)

“Water In Star-forming regions with Herschel” (WISH) is a key programme dedicated to studying the role of water and related species during the star-formation process and constraining the physical and chemical properties of young stellar objects. The Heterodyne Instrument for the Far-Infrared (HIFI) on the Herschel Space Observatory observed three deeply embedded protostars in the low-mass star-forming region NGC 1333 in several H_216O, H_218O, and CO transitions. Line profiles are resolved for five H_216O transitions in each source, revealing them to be surprisingly complex. The line profiles are decomposed into broad (>20 km s-1), medium-broad (~5-10 km s-1), and narrow (<5 km s-1) components. The H_218O emission is only detected in broad 110-101 lines (>20 km s-1), indicating that its physical origin is the same as for the broad H_216O component. In one of the sources, IRAS4A, an inverse P Cygni profile is observed, a clear sign of infall in the envelope. From the line profiles alone, it is clear that the bulk of emission arises from shocks, both on small (?1000 AU) and large scales along the outflow cavity walls (~10 000 AU). The H2O line profiles are compared to CO line profiles to constrain the H2O abundance as a function of velocity within these shocked regions. The H2O/CO abundance ratios are measured to be in the range of ~0.1-1, corresponding to H2O abundances of ~10-5-10-4 with respect to H2. Approximately 5-10% of the gas is hot enough for all oxygen to be driven into water in warm post-shock gas, mostly at high velocities. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Tables 2 and 3 (page 6) are only available in electronic form at http://www.aanda.org

Kristensen, L. E.; Visser, R.; van Dishoeck, E. F.; Y?ld?z, U. A.; Doty, S. D.; Herczeg, G. J.; Liu, F.-C.; Parise, B.; Jørgensen, J. K.; van Kempen, T. A.; Brinch, C.; Wampfler, S. F.; Bruderer, S.; Benz, A. O.; Hogerheijde, M. R.; Deul, E.; Bachiller, R.; Baudry, A.; Benedettini, M.; Bergin, E. A.; Bjerkeli, P.; Blake, G. A.; Bontemps, S.; Braine, J.; Caselli, P.; Cernicharo, J.; Codella, C.; Daniel, F.; de Graauw, Th.; di Giorgio, A. M.; Dominik, C.; Encrenaz, P.; Fich, M.; Fuente, A.; Giannini, T.; Goicoechea, J. R.; Helmich, F.; Herpin, F.; Jacq, T.; Johnstone, D.; Kaufman, M. J.; Larsson, B.; Lis, D.; Liseau, R.; Marseille, M.; McCoey, C.; Melnick, G.; Neufeld, D.; Nisini, B.; Olberg, M.; Pearson, J. C.; Plume, R.; Risacher, C.; Santiago-García, J.; Saraceno, P.; Shipman, R.; Tafalla, M.; Tielens, A. G. G. M.; van der Tak, F.; Wyrowski, F.; Beintema, D.; de Jonge, A.; Dieleman, P.; Ossenkopf, V.; Roelfsema, P.; Stutzki, J.; Whyborn, N.

2010-10-01

151

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

E-print Network

We constrain the slope of the star formation rate ($\\log\\Psi$) to stellar mass ($\\log\\mathrm{M_{\\star}}$) relation down to $\\log(\\mathrm{M_{\\star}/M_{\\odot}})=8.4$ ($\\log(\\mathrm{M_{\\star}/M_{\\odot}})=9.2$) 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 ($\\log\\mathrm{\\Psi}\\propto\\log\\mathrm{M_{\\star}}$) than at high masses ($\\log\\mathrm{\\Psi}\\propto(0.3-0.6)\\log\\mathrm{M_{\\star}}$). 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$\\mu$m 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(\\mathrm{M_{\\s...

Whitaker, Katherine E; 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

152

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

SciTech Connect

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.

Cristallo, S.; Straniero, O.; Piersanti, L. [INAF-Osservatorio Astronomico di Collurania, 64100 Teramo (Italy); Gallino, R. [Dipartimento di Fisica Generale, Universita di Torino, 10125 Torino (Italy); DomInguez, I. [Departamento de Fisica Teorica y del Cosmos, Universidad de Granada, 18071 Granada (Spain); Lederer, M. T. [Institut fuer Astronomie, Tuerkenschanzstrasse 17, A-1180 Wien (Austria)

2009-05-01

153

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

NASA Astrophysics Data System (ADS)

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.

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

2009-05-01

154

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

PubMed

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

Kafka, Stella

2012-06-01

155

Discovery of a Wide Substellar Companion to a Nearby Low-Mass Star  

E-print Network

We report the discovery of a wide (135+/-25 AU), unusually blue L5 companion 2MASS J17114559+4028578 to the nearby M4.5 dwarf G 203-50 as a result of a targeted search for common proper motion pairs in the Sloan Digital Sky Survey and the Two Micron All Sky Survey. Adaptive Optics imaging with Subaru indicates that neither component is a nearly equal mass binary with separation > 0.18", and places limits on the existence of additional faint companions. An examination of TiO and CaH features in the primary's spectrum is consistent with solar metallicity and provides no evidence that G 203-50 is metal poor. We estimate an age for the primary of 1-5 Gyr based on activity. Assuming coevality of the companion, its age, gravity and metallicity can be constrained from properties of the primary, making it a suitable benchmark object for the calibration of evolutionary models and for determining the atmospheric properties of peculiar blue L dwarfs. The low total mass (M_tot=0.21+/-0.03 M_sun), intermediate mass ratio (q=0.45+/-0.14), and wide separation of this system demonstrate that the star formation process is capable of forming wide, weakly bound binary systems with low mass and BD components. Based on the sensitivity of our search we find that no more than 2.2% of early-to-mid M dwarfs (9.0 0.06 M_sun.

Jacqueline Radigan; David Lafrenière; Ray Jayawardhana; René Doyon

2008-08-11

156

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

NASA Astrophysics Data System (ADS)

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

Rodriguez, David R.

2011-01-01

157

UM 625 REVISITED: MULTIWAVELENGTH STUDY OF A SEYFERT 1 GALAXY WITH A LOW-MASS BLACK HOLE  

SciTech Connect

UM 625, previously identified as a narrow-line active galactic nucleus (AGN), actually exhibits broad H{alpha} and H{beta} lines whose width and luminosity indicate a low black hole (BH) mass of 1.6 Multiplication-Sign 10{sup 6} M{sub Sun }. We present a detailed multiwavelength study of the nuclear and host galaxy properties of UM 625. Analysis of Chandra and XMM-Newton observations suggests that this system contains a heavily absorbed and intrinsically X-ray weak ({alpha}{sub ox} = -1.72) nucleus. Although not strong enough to qualify as radio loud, UM 625 does belong to a minority of low-mass AGNs detected in the radio. The broadband spectral energy distribution constrains the bolometric luminosity to L{sub bol} Almost-Equal-To (0.5-3) Multiplication-Sign 10{sup 43} erg s{sup -1} and L{sub bol}/L{sub Edd} Almost-Equal-To 0.02-0.15. A comprehensive analysis of Sloan Digital Sky Survey and Hubble Space Telescope images shows that UM 625 is a nearly face-on S0 galaxy with a prominent, relatively blue pseudobulge (Sersic index n = 1.60) that accounts for {approx}60% of the total light in the R band. The extended disk is featureless, but the central {approx}150-400 pc contains a conspicuous semi-ring of bright, blue star-forming knots, whose integrated ultraviolet luminosity suggests a star formation rate of {approx}0.3 M{sub Sun} yr{sup -1}. The mass of the central BH roughly agrees with the value predicted from its bulge velocity dispersion but is significantly lower than that expected from its bulge luminosity.

Jiang Ning; Dong Xiaobo; Yang Huan; Wang Junxian [Key laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Chinese Academy of Sciences, Hefei, Anhui 230026 (China); Ho, Luis C., E-mail: jnac@mail.ustc.edu.cn, E-mail: xbdong@ustc.edu.cn, E-mail: lho@obs.carnegiescience.edu [Observatories of the Carnegie Institution for Science, 813 Santa Barbara Street, Pasadena, CA 91101 (United States)

2013-06-10

158

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

E-print Network

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

Viki Joergens; Ralph Neuhäuser

2003-06-23

159

A Wide-Field Survey for Low Mass Star Formation around the Galactic Massive Young Cluster NGC 3603  

NASA Astrophysics Data System (ADS)

NGC 3603 is one of the most massive (~10,000 solar masses) and compact (5~pc) young star clusters known in the Milky Way. It is therefore a candidate young globular cluster, with properties similar to massive young clusters found in other galaxies such as the LMC, M51 and the Antenna. Due to its proxmity and low extinction, NGC 3603 is one of the few massive clusters where it is possible to detect the low mass stars that dominate the total mass of the cluster. NGC 3603 lies in the center of a 40 pc diameter, 100,000 solar mass molecular cloud complex; little is known about the stars forming in this complex. We propose the first systematic survey of this complex using deep 3.6 and 4.5 micron Spitzer imaging of a 59 by 57 pc region centered on NGC 3603. By combining this data with scheduled VLT JHK imaging of the same field, we can detect and identify low to intermediate mass stars with disks and protostars in the cloud complex. Our goal is to probe the relationship between spatially extended OB associations and compact clusters of OB stars. Our method is to compare the spatial distribution of intermediate to low mass stars in the extended NGC 3603 complex to that found in OB associations like Orion. Specifically, is NGC 3603 part of a large, extended complex containing both clustered and distributed star formation, and consequently similar to nearby associations? Or did NGC 3603 result from a distinct, compact mode of star formation? These observations will give us unique insight into the process of massive star cluster formation in other galaxies and how this process may differ from star formation near the Sun.

Megeath, Tom; Chandar, Rupali; Nuernberger, Dieter; Zinnecker, Hans

2009-04-01

160

On the detection of (habitable) super-Earths around low-mass stars using Kepler and transit timing variation method  

NASA Astrophysics Data System (ADS)

We present the results of an extensive study of the detectability of Earth-sized planets and super-Earths in the habitable zones of cool and low-mass stars using transit timing variation method. We have considered a system consisting of a star, a transiting giant planet, and a terrestrial-class perturber, and calculated TTVs for different values of the parameters of the system. To identify ranges of the parameters for which these variations would be detectable by Kepler, we considered the analysis presented by Ford et al. (Transit timing observations from Kepler: I. Statistical analysis of the first four months. ArXiv:1102.0544, 2011) and assumed that a peak-to-peak variation of 20 s would be within the range of the photometric sensitivity of this telescope. We carried out simulations for resonant and non-resonant orbits, and identified ranges of the semimajor axes and eccentricities of the transiting and perturbing bodies for which an Earth-sized planet or a super-Earth in the habitable zone of a low-mass star would produce such TTVs. Results of our simulations indicate that in general, outer perturbers near first- and second-order resonances show a higher prospect for detection. Inner perturbers are potentially detectable only when near 1:2 and 1:3 mean-motion resonances. For a typical M star with a Jupiter-mass transiting planet, for instance, an Earth-mass perturber in the habitable zone can produce detectable TTVs when the orbit of the transiting planet is between 15 and 80 days. We present the details of our simulations and discuss the implication of the results for the detection of terrestrial planets around different low-mass stars.

Haghighipour, Nader; Kirste, Sabrina

2011-10-01

161

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

E-print Network

Intermediate resolution (R=7000) spectroscopy is presented for 76 photometrically selected very low mass (0.04low mass stars and brown dwarfs in small separation (a<1au) binary systems is larger than in field M-dwarfs, or the distribution of separations is much less skewed towards large separations. This conclusion hinges critically on the correct identification of the small number of binary candidates, but is significant even when only Li-rich candidate members are considered. Broadened H alpha emission, indicative of circum(sub)stellar accretion discs is found in 5 or 6 of the candidate cluster members, 3 of which probably have substellar masses. The fraction of accretors (10+/-5 per cent) is similar to that found in stars of higher mass in the cluster using H alpha emission as a diagnostic, but much lower than found for very low mass stars and brown dwarfs of younger clusters. The timescale for accretion rates to drop to less than 10^{-11} M_sun/yr is hence less than the age of the Sigma Ori cluster (3 to 7 Myr) for most low-mass objects (abridged).

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

2004-09-30

162

Chemical changes during star formation: high- versus low-mass YSOs  

E-print Network

Recent observational studies of high- and low-mass YSOs at (sub)millimeter and infrared wavelengths are reviewed, and chemical diagnostics of the different physical components are summarized. Methods for determining the temperature, density and abundance profiles in the envelopes are outlined, and are illustrated for one high-mass and one low-mass YSO. The combination of (sub)millimeter and infrared data gives a nearly complete chemical inventory of the gas and solid state material. In both high- and low-mass YSOs, the chemical characteristics are dominated by freeze-out in the cold outer part of the envelope and evaporation of ices in the warm inner part. Abundance jumps of factors of ~100 in selected molecules are found in the warm gas for both types of objects. Potential differences include (i) the complex hot core chemistry, which has been observed so far only for high-mass YSOs; (ii) the high level of deuterium fractionation seen only in low-mass YSOs; (iii) the effects of internal or external UV and X-rays; (iv) the relative importance of shocks versus thermal heating of the envelope; and (v) the importance of geometrical effects.

Ewine F. van Dishoeck

2003-01-25

163

Deuterium chemistry of dense gas in the vicinity of low-mass and massive star forming regions  

E-print Network

The standard interstellar ratio of deuterium to hydrogen (D/H) atoms is $\\sim 1.5 \\times 10^{-5}$. However, the deuterium fractionation is in fact found to be enhanced, to different degrees, in cold, dark cores, hot cores around massive star forming regions, lukewarm cores, and warm cores ({\\it hereafter}, hot corinos) around low-mass star forming regions. In this paper, we investigate the overall differences in the deuterium chemistry between hot cores and hot corinos. We have modelled the chemistry of dense gas around low-mass and massive star forming regions using a gas-grain chemical model. We investigate the influence of varying the core density, the depletion efficiency of gaseous species on to dust grains, the collapse mode and the final mass of the protostar on the chemical evolution of star forming regions. We find that the deuterium chemistry is, in general, most sensitive to variations of the depletion efficiency on to grain surfaces, in agreement with observations. In addition, the results showed ...

Awad, Zainab; Bayet, Estelle; Caselli, Paola

2014-01-01

164

How does a low-mass cut-off in the stellar IMF affect the evolution of young star clusters?  

NASA Astrophysics Data System (ADS)

We investigate how different stellar initial mass functions (IMFs) can affect the mass-loss and survival of star clusters. We find that IMFs with radically different low-mass cut-offs (between 0.1 and 2 M?) do not change cluster destruction time-scales as much as might be expected. Unsurprisingly, we find that clusters with more high-mass stars lose relatively more mass through stellar evolution, but the response to this mass-loss is to expand and hence significantly slow their dynamical evolution. We also argue that it is very difficult, if not impossible, to have clusters with different IMFs that are initially `the same', since the mass, radius and relaxation times depend on each other and on the IMF in a complex way. We conclude that changing the IMF to be biased towards more massive stars does speed up mass-loss and dissolution, but that it is not as dramatic as might be thought.

Kouwenhoven, M. B. N.; Goodwin, S. P.; de Grijs, R.; Rose, M.; Kim, Sungsoo S.

2014-12-01

165

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

E-print Network

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

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

2014-01-01

166

Evolutionary models for solar metallicity low-mass stars: mass-magnitude relationships and color-magnitude diagrams  

E-print Network

We present evolutionary models for low mass stars from 0.075 to 1 $\\msol$ for solar-type metallicities [M/H]= 0 and -0.5. The calculations include the most recent interior physics and the latest generation of non-grey atmosphere models. We provide mass-age-color-magnitude relationships for both metallicities. The mass-M$_V$ and mass-M$_K$ relations are in excellent agreement with the empirical relations derived observationally. The theoretical color-magnitude diagrams are compared with the sequences of globular clusters (47 Tucanae) and open clusters (NGC2420 and NGC2477) observed with the Hubble Space Telescope. Comparison is also made with field star sequences in $M_V$-$(V-I)$, $M_K$-$(I-K)$ and $M_K$-$(J-K)$ diagrams. These comparisons show that the most recent improvements performed in low-mass star atmosphere models yield now reliable stellar models in the near-infrared. These models can be used for metallicity, mass, temperature and luminosity calibrations. Uncertainties still remain, however, in the optical spectral region below $T_{eff} \\sim 3700K$, where predicted (V-I) colors are too blue by 0.5 mag for a given magnitude. The possible origins for such a discrepancy, most likely a missing source of opacity in the optical and the onset of grain formation are examined in detail.

Isabelle Baraffe; Gilles Chabrier; France Allard; Peter Hauschildt

1998-05-02

167

The effect of starspots on the ages of low-mass stars determined from the lithium depletion boundary  

E-print Network

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 starspots blocking a fraction $\\beta$ of their photospheric flux. Starspots slow the descent along Hayashi tracks, leading to lower core temperatures and less Li destruction at a given mass and age. The age, $\\tau_{\\rm LDB}$, determined from the luminosity of the LDB, $L_{\\rm LDB}$, is increased by a factor $(1-\\beta)^{-E}$ compared to that inferred from unspotted models, where $E \\simeq 1 + d\\log \\tau_{\\rm LDB}/d \\log L_{\\rm LDB}$ and has a value $\\sim 0.5$ at ages $< 80$ Myr, decreasing to $\\sim 0.3$ for older stars. Spotted stars have virtually the same r...

Jackson, R J

2014-01-01

168

Angular momentum evolution of low-mass pre-main sequence stars via extreme coronal mass ejections  

NASA Astrophysics Data System (ADS)

The angular momentum evolution of cool stars during the pre-main sequence phase of stellar evolution remains a major outstanding problem. Multiple processes are likely involved in the transfer of mass and angular momentum within and out of the star+disk system. The role of coronal mass ejections (CMEs), energetic events which shed mass and magnetic flux in the Sun, has yet to be fully explored in the context of pre-main sequence stars. It is well established that young, solar-type stars exhibit X-ray activity levels up to four orders of magnitude higher than the present-day Sun, suggesting that CMEs associated with these extreme X-ray flares could be an important process for expelling mass and angular momentum. We present a novel approach to modeling the CMEs of low-mass pre-main sequence stars that uses a solar-calibrated CME model and observed X-ray flare rates for young stars. We derive mass loss rates via stellar CMEs and calculate their attendant angular momentum losses during the pre-main sequence phase. While we find the mass loss rates to be modest, ˜10 % of steady-state stellar wind values from the literature, the angular momentum losses can be substantial, potentially counteracting the effects of initial stellar spin-up due to contraction in tens of Myr.

Aarnio, A. N.; Matt, S. P.; Stassun, K. G.

2013-02-01

169

High-resolution spectroscopy of ROSAT low-mass pre-main sequence stars in Orion  

NASA Astrophysics Data System (ADS)

High-resolution spectroscopic observations of the lithium-rich stars found on the basis of the ROSAT All-Sky Survey (RASS) in the general direction of the Orion star forming region are presented. Different properties are derived from the spectra and analyzed: i) the equivalent widths of the lithium 6708 Ä absorption line have been measured and a revision of spectral types has been performed; ii) radial velocities (RV) and projected rotational velocities, vsin i, have been derived by application of cross-correlation techniques. A relatively large number of spectroscopic binaries and of suspected spectroscopic binary and multiple systems are found among the stars in the sample. Based on the strength of the lithium line with respect to young open cluster ZAMS stars of the same spectral type, the pre-main sequence nature is confirmed for more than 70% of the stars in the sample. The interrelation of the derived observational properties, such as kinematics, lithium abundance, age and projected rotational velocity of the stars in the sample are analyzed also in connection with the spatial location of the objects. In particular, the comparison between the kinematics of the stars and that of the gas, provided by the CO and CS molecular emission observations, reveals different degrees of clustering of the stars with respect to the cloud material and different kinematical groups can be distinguished. The sample of RASS lithium-rich stars found in the general direction of Orion appears to be a mixture of true Orion stars and, possibly, stars belonging to the Gould Belt. Based on observations carried out at the Calar Alto observatory, the Multiple Mirror Telescope (a joint facility of the Smithsonian Institution and the University of Arizona) and the European Southern Observatory, La Silla, Chile under proposal number 60.C-0170

Alcalá, J. M.; Covino, E.; Torres, G.; Sterzik, M. F.; Pfeiffer, M. J.; Neuhäuser, R.

2000-01-01

170

Report on the ESO Workshop "Dynamics of Low-mass Stellar Systems: From Star Clusters to Dwarf Galaxies"  

NASA Astrophysics Data System (ADS)

The dynamics of low-mass stellar systems is not only an interesting subject in its own right, but is also intimately linked to global theories of structure formation, the physics of gravity, and the shape of the stellar initial mass function. Given the wealth of new information gathered very recently in this field, the time was ripe to hold a dedicated meeting on this topic. The workshop brought together a mix of about 100 astronomers who work on both the observation and theory of the dynamics of dwarf galaxies and star clusters, and a total of around 60 oral presentations and about 25 posters were presented.

Mieske, S.; Gieles, M.

2011-06-01

171

STAR FORMATION HISTORY OF A YOUNG SUPER-STAR CLUSTER IN NGC 4038/39: DIRECT DETECTION OF LOW-MASS PRE-MAIN SEQUENCE STARS  

SciTech Connect

We present an analysis of the near-infrared spectrum of a young massive star cluster in the overlap region of the interacting galaxies NGC 4038/39 using population synthesis models. Our goal is to model the cluster population as well as provide rough constraints on its initial mass function (IMF). The cluster shows signs of youth, such as thermal radio emission and strong hydrogen emission lines in the near-infrared. Late-type absorption lines are also present which are indicative of late-type stars in the cluster. The strength and ratio of these absorption lines cannot be reproduced through either late-type pre-main sequence (PMS) stars or red supergiants alone. Thus, we interpret the spectrum as a superposition of two star clusters of different ages, which is feasible since the 1'' spectrum encompasses a physical region of {approx}90 pc and radii of super-star clusters (SSCs) are generally measured to be a few parsecs. One cluster is young (<= 3 Myr) and is responsible for part of the late-type absorption features, which are due to PMS stars in the cluster, and the hydrogen emission lines. The second cluster is older (6 Myr-18 Myr) and is needed to reproduce the overall depth of the late-type absorption features in the spectrum. Both are required to accurately reproduce the near-infrared spectrum of the object. Thus, we have directly detected PMS objects in an unresolved SSC for the first time using a combination of population synthesis models and PMS tracks. This analysis serves as a testbed of our technique to constrain the low-mass IMF in young SSCs as well as an exploration of the star formation history of young UC H II regions.

Greissl, Julia; Meyer, Michael R. [Steward Observatory, University of Arizona, Tucson, AZ 85721 (United States); Christopher, Micol H.; Scoville, Nick Z., E-mail: jgreissl@as.arizona.ed [California Institute of Technology, Pasadena, CA 91125 (United States)

2010-02-20

172

Neutron star cooling in transiently accreting low mass binaries: a new tool for probing nuclear matter  

E-print Network

We explore, using an exact cooling code, the thermal evolution of a neutron star undergoing episodes of intense accretion, alternated by long periods of quiescence (e.g. Soft X-Ray Transients). We find that the soft component of the quiescent luminosity of Aql X-1, 4U 1608-522 and of SAX J1808.4-3658 can be understood as thermal emission from a cooling neutron star with negligible neutrino emission. In the case of Cen X-4 strong neutrino emission from the inner core is necessary to explain the observation: this may indicate that the neutron star of Cen X-4 is heavier than 1.4 Msun. This study opens the possibility of using the quiescent emission of Soft X-Ray Transients as a tool for probing the core superfluidity in relation to the mass of the neutron star.

Andrea Possenti; Monica Colpi; Dany Page; Ulrich Geppert

2001-02-02

173

Using rotation, magnetic activity and lithium to estimate the ages of low mass stars  

E-print Network

The rotation rate, level of magnetic activity and surface lithium abundance are age-dependent quantities in stars of about a solar mass and below. The physical reasons for the evolution of these phenomena are qualitatively understood, but accurate quantitative models remain dependent on empirical calibration using the Sun and stars of known age, chiefly in clusters. In this work I review the status of these ``empirical age indicators'', outlining the astrophysics of their time dependence, describing the measurements, assessing the precision (and accuracy) of age estimates when applied to individual stars, and identifying their principle limitations in terms of the mass and age ranges over which they are useful. Finally, I discuss the ``lithium depletion boundary'' technique which, in contrast to the empirical methods, appears to provide robust, almost model-independent ages that are both precise and accurate, but which is only applicable to coeval groups of stars.

Jeffries, R D

2014-01-01

174

Using rotation, magnetic activity and lithium to estimate the ages of low mass stars  

NASA Astrophysics Data System (ADS)

The rotation rate, level of magnetic activity and surface lithium abundance are age-dependent quantities in stars of about a solar mass and below. The physical reasons for the evolution of these phenomena are qualitatively understood, but accurate quantitative models remain dependent on empirical calibration using the Sun and stars of known age, chiefly in clusters. In this work I review the status of these "empirical age indicators", outlining the astrophysics of their time dependence, describing the measurements, assessing the precision (and accuracy) of age estimates when applied to individual stars, and identifying their principle limitations in terms of the mass and age ranges over which they are useful. Finally, I discuss the "lithium depletion boundary" technique which, in contrast to the empirical methods, appears to provide robust, almost model-independent ages that are both precise and accurate, but which is only applicable to coeval groups of stars.

Jeffries, R. D.

2014-11-01

175

A New Method to Identify Nearby, Young, Low-mass Stars  

E-print Network

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

Rodriguez, David R; Zuckerman, B; Kastner, Joel H

2010-01-01

176

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

SciTech Connect

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

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

2011-02-01

177

X-ray spectroscopy of neutron star low-mass X-ray binaries  

E-print Network

In this thesis, I present work spanning a variety of topics relating to neutron star lowmass X-ray binaries (LMXBs) and utilize spectral information from X-ray observations to further our understanding of these sources. ...

Krauss, Miriam Ilana

2007-01-01

178

Herbig-Haro Objects - Tracers of the Formation of Low-mass Stars and Sub-stellar Objects  

E-print Network

Herbig-Haro objects (HHOs) are caused by outflows from young objects. Since the outflow relies on mass accretion from a circumstellar disk, it indicates ongoing growth. Recent results of infrared observations yielded evidence for disks around brown dwarfs. This suggests that at least a certain fraction of brown dwarfs forms like stars. Thus, young sub-stellar objects might cause HHOs as well. We present selected results of a general survey for HHOs based on DSS-II plates and CCD images taken with the Tautenburg Schmidt telescope. Numerous young objects could be identified due to their association with newly detected HHOs. In some cases the luminosity is consistent with very low-mass stars or close to sub-stellar values. This holds for L1415-IRS and a few infrared sources embedded in other dark clouds (e.g., GF9, BHR111). The question on the minimum mass for outflow activity is addressed.

Bringfried Stecklum; Helmut Meusinger; Dirk Froebrich

2007-07-05

179

High-resolution spectroscopy of ROSAT low-mass pre-main sequence stars in Orion?  

Microsoft Academic Search

High-resolution spectroscopic observations of the lithium-rich stars found on the basis of the ROSAT All-Sky Survey (RASS) in the general direction of the Orion star form- ing region are presented. Different properties are derived from the spectra and analyzed: i) the equivalent widths of the lithium 6708 ? A absorption line have been measured and a revision of spectral types

J. M. Alcal; E. Covino; G. Torres; M. F. Sterzik; M. J. Pfeiffer

180

High-resolution spectroscopy of ROSAT low-mass pre-main sequence stars in Orion  

Microsoft Academic Search

High-resolution spectroscopic observations of the lithium-rich stars found on the basis of the ROSAT All-Sky Survey (RASS) in the general direction of the Orion star forming region are presented. Different properties are derived from the spectra and analyzed: i) the equivalent widths of the lithium 6708 Ä absorption line have been measured and a revision of spectral types has been

J. M. Alcalá; E. Covino; G. Torres; M. F. Sterzik; M. J. Pfeiffer; R. Neuhäuser

2000-01-01

181

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

NASA Astrophysics Data System (ADS)

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

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

2014-11-01

182

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

SciTech Connect

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

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

2012-07-10

183

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

SciTech Connect

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

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

2010-01-01

184

Constraining Mass-Loss & Lifetimes of Low Mass, Low Metallicity AGB Stars  

E-print Network

The evolution and lifetimes of thermally pulsating asymptotic giant branch (TP-AGB) stars suffer from significant uncertainties. We present a detailed framework for constraining model luminosity functions of TP-AGB stars using resolved stellar populations. We show an example of this method that compares various TP-AGB mass-loss prescriptions that differ in their treatments of mass loss before the onset of dust-driven winds (pre-dust). We find that models with more efficient pre-dust driven mass loss produce results consistent with observations, as opposed to more canonical mass-loss models. Efficient pre-dust driven mass-loss predicts for [Fe/H] < -1.2, lower mass TP-AGB stars (M < 1 Msun) must have lifetimes less than about 1.2 Myr.

Rosenfield, Philip; Girardi, Leo; Dalcanton, Julianne J; Bressan, Alessandro; Gullieuszik, Marco; Weisz, Daniel; Williams, Benjamin F; Dolphin, Andrew; Aringer, Bernhard

2014-01-01

185

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

SciTech Connect

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.

Cackett, Edward M.; Miller, Jon M. [Department of Astronomy, University of Michigan, 500 Church St., Ann Arbor, MI 48109-1042 (United States); Brown, Edward F. [Department of Physics and Astronomy, National Superconducting Cyclotron Laboratory, and Joint Institute for Nuclear Astrophysics, Michigan State University, East Lansing, MI 48824 (United States); Cumming, Andrew [Department of Physics, McGill University, 3600 rue University, Montreal, QC, H3A 2T8 (Canada); Degenaar, Nathalie; Wijnands, Rudy, E-mail: ecackett@ast.cam.ac.u [Astronomical Institute 'Anton Pannekoek', University of Amsterdam, Kruislaan 403, 1098 SJ, Amsterdam (Netherlands)

2010-10-20

186

Black hole-like hysteresis and accretion states in neutron star low-mass X-ray binaries  

NASA Astrophysics Data System (ADS)

We have systematically studied a large sample of the neutron star low-mass X-ray binaries (LMXBs) monitored by the Rossi X-ray Timing Explorer (50 sources; 10000+ observations). We find that the hysteresis patterns between Compton-dominated and thermal-dominated states, typically observed in black hole LMXBs, are also common in neutron star systems. These patterns, which also sample intermediate states, are found when looking at the evolution of both X-ray colour and fast variability of 10 systems accreting below ˜30 per cent of the Eddington luminosity (LEdd). We show that hysteresis does not require large changes in luminosity and it is the natural form that state transitions take at these luminosities. At higher accretion rates, neutron stars do not show hysteresis, and they remain in a thermal-dominated, low-variability state, characterized by flaring behaviour and fast colour changes. Only at luminosities close to LEdd, are high variability levels seen again, in correspondence to an increase in the fractional contribution of the Comptonization component. We compare this behaviour with that observed in LMXBs harbouring black holes, showing that the spectral, timing and multiwavelength properties of a given source can be determined by its location in the fast variability-luminosity diagram, which, therefore, provides a common framework for neutron star and black hole accretion states.

Muñoz-Darias, T.; Fender, R. P.; Motta, S. E.; Belloni, T. M.

2014-10-01

187

Formation of the first low-mass stars from cosmological initial conditions  

NASA Astrophysics Data System (ADS)

We simulate the formation of a metal-poor (10-2 Z?) stellar cluster in one of the first galaxies to form in the early Universe, specifically a high-redshift atomic cooling halo (z ˜ 14). This is the first calculation that resolves the formation of individual metal-enriched stars in simulations starting from realistic cosmological initial conditions. We follow the evolution of a single dense clump among several in the parent halo. The clump forms a cluster of ˜40 stars and sub-stellar objects within 7000 yr and could continue forming stars ˜5 times longer. Protostellar dust heating has a negligible effect on the star formation efficiency, at least during the early evolutionary stages, but it moderately suppresses gaseous fragmentation and brown dwarf formation. We observe fragmentation in thin gaseous filaments and sustained accretion in larger, rotating structures as well as ejections by binary interactions. The stellar initial mass function above 0.1 M?, evaluated after ˜104 yr of fragmentation and accretion, seems in agreement with the recent measurement in ultrafaint dwarf spheroidal Galactic satellites of Geha et al. L116

Safranek-Shrader, Chalence; Milosavljevi?, Miloš; Bromm, Volker

2014-05-01

188

The Mass-Function of Low Mass Halo Stars: Limits on Baryonic Halo Dark Matter  

E-print Network

We derive mass functions (MF) for halo red dwarfs (the faintest hydrogen burning stars) and then extrapolate to place limits on the total mass of halo brown dwarfs (stars not quite massive enough to burn hydrogen). The mass functions are obtained from the luminosity function of a sample of 114 local halo stars in the USNO parallax survey (Dahn \\etal 1995). We use stellar models of Alexander \\etal (1996) and make varying assumptions about metallicity and about possible unresolved binaries in the sample. We find that the MF for halo red dwarfs cannot rise more quickly than $1/m^2$ as one approaches the hydrogen burning limit. Using recent results from star formation theory, we extrapolate the MF into the brown-dwarf regime. We see that likely extrapolations imply that the total mass of brown dwarfs in the halo is less than $\\sim 3\\%$ of the local mass density of the halo ($\\sim 0.3\\%$ for the more realistic models we consider). Our limits apply to brown dwarfs in the halo that come from the same stellar population as the red dwarfs.

David S. Graff; Katherine Freese

1996-02-10

189

A Theoretical Study of Acoustic Glitches in Low-mass Main-sequence Stars  

NASA Astrophysics Data System (ADS)

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 ?1 caused by the ionization of He II, but to the peak in ?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 ?.

Verma, Kuldeep; Antia, H. M.; Basu, Sarbani; Mazumdar, Anwesh

2014-10-01

190

Radiative feedback by low-mass stars in the first generation  

SciTech Connect

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

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

2009-01-01

191

Rotational velocities of low-mass stars in the Pleiades and Hyades  

E-print Network

We have obtained high-resolution spectra of 89 M dwarf members of the Pleiades and Hyades and have derived radial velocities, H-alpha equivalent widths, and spectroscopic rotational velocities for these stars. Typical masses of the newly-observed Pleiades and Hyades stars are ~ 0.4 M_{\\sun} and ~ 0.2 M_{\\sun}, respectively. We combine our new observations with previously published data to explore the rotational evolution of young stars with M activity is identical in both clusters, implying that the lower activity in the Hyades is a result of the lower rotation rates. We show that a simple scaling of the Pleiades rotational distribution for M \\leq 0.4 M_{\\sun}, corrected for the effects of structural evolution, matches that of the Hyades if the average angular momentum loss from the Pleiades to the Hyades age is factor of \\approx 6. This suggests that the distribution of initial angular momenta and disk-locking lifetimes for the lowest mass stars was similar in both clusters. We argue that this result provides further evidence for a saturation of the angular momentum loss rate at high rotational velocities.

Donald M. Terndrup; John R. Stauffer; Marc H. Pinsonneault; Alison Sills; Yongquan Yuan; Burton F. Jones; Debra Fischer; Anita Krishnamurthi

1999-11-30

192

High spatial resolution mid-infrared observations of the low-mass young star TW Hydrae  

Microsoft Academic Search

Aims:We want to improve knowledge of the structure of the inner few AU of the circumstellar disk around the nearby T Tauri star TW Hya. Earlier studies have suggested the existence of a large inner hole, possibly caused by interactions with a growing protoplanet. Methods: We used interferometric observations in the N-band obtained with the MIDI instrument on the Very

Th. Ratzka; Ch. Leinert; Th. Henning; J. Bouwman; C. P. Dullemond; W. Jaffe

2007-01-01

193

Planetary protection in the extreme environments of low-mass stars  

NASA Astrophysics Data System (ADS)

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

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

2014-08-01

194

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

SciTech Connect

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

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

2011-12-01

195

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

PubMed

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

Charbonneau, David; Berta, Zachory K; Irwin, Jonathan; Burke, Christopher J; Nutzman, Philip; Buchhave, Lars A; Lovis, Christophe; Bonfils, Xavier; Latham, David W; Udry, Stéphane; 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

196

High spatial resolution mid-infrared observations of the low-mass young star TW Hya  

Microsoft Academic Search

We want to improve knowledge of the structure of the inner few AU of the\\u000acircumstellar disk around the nearby T Tauri star TW Hya. Earlier studies have\\u000asuggested the existence of a large inner hole, possibly caused by interactions\\u000awith a growing protoplanet. We used interferometric observations in the N-band\\u000aobtained with the MIDI instrument on the Very Large

Ch. Leinert; J. Bouwman; C. P. Dullemond; W. Jaffe; Sterrewacht Leiden

2007-01-01

197

A study of circumstellar disk properties in low-mass stars and brown dwarfs  

Microsoft Academic Search

We present Spitzer Space Telescope IRAC and MIPS observations for a sample of eight M dwarfs: six dMe, one dM, and one sdMe star. All of our targets are found to have Spectral Energy Distributions (SEDs) which are fitted within the error bars by a purely photospheric spectrum out to 24m m . The estimated ages for all are >10

Basmah Riaz

2008-01-01

198

Big Fish in Small Ponds: Massive Stars in the Low Mass Clusters of M83  

E-print Network

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 $\\leq$ 8 Myr. As in earlier papers on M51 and NGC 4214, the upper end of the stellar IMF in M83 is consistent with an universal IMF, and stochastic sampling of the stellar populations in the $\\lessapprox$ 10$^{3}$ Msun 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.

Andrews, J E; Chandar, R; Elmegreen, B G; Kennicutt, R C; Kim, Hwihyun; Krumholz, Mark R; Lee, J C; McElwee, Sean; O'Connell, R W; Whitmore, B

2014-01-01

199

Molecular opacities for low-mass metal-poor AGB stars undergoing the Third Dredge Up  

E-print Network

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 stars. At low metallicity, the enhancement of C and/or N may 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. Basing on up-to-date stellar models, we illustrate the changes induced by the use of these opacity on the physical and chemical properties expected for these stars.

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

2007-06-14

200

THE GALACTIC CENTER CLOUD G2-A YOUNG LOW-MASS STAR WITH A STELLAR WIND  

SciTech Connect

We explore the possibility that the G2 gas cloud falling in toward SgrA* is the mass-loss envelope of a young T Tauri star. As the star plunges to smaller radius at 1000-6000 km s{sup -1}, a strong bow shock forms where the stellar wind is impacted by the hot X-ray emitting gas in the vicinity of SgrA*. For a stellar mass-loss rate of 4 Multiplication-Sign 10{sup -8} M{sub Sun} yr{sup -1} and wind velocity 100 km s{sup -1}, the bow shock will have an emission measure (EM = n {sup 2} vol) at a distance {approx}10{sup 16} cm, similar to that inferred from the IR emission lines. The ionization of the dense bow shock gas is potentially provided by collisional ionization at the shock front and cooling radiation (X-ray and UV) from the post shock gas. The former would predict a constant line flux as a function of distance from SgrA*, while the latter will have increasing emission at lesser distances. In this model, the star and its mass-loss wind should survive pericenter passage since the wind is likely launched at 0.2 AU and this is much less than the Roche radius at pericenter ({approx}3 AU for a stellar mass of 2 M{sub Sun }). In this model, the emission cloud will probably survive pericenter passage, discriminating this scenario from others.

Scoville, N. [California Institute of Technology, MC 249-17, 1200 East California Boulevard, Pasadena, CA 91125 (United States); Burkert, A. [University Observatory Munich, Scheinerstrasse 1, D-81679 Munich (Germany)

2013-05-10

201

Big Fish in Small Ponds: Massive Stars in the Low-mass Clusters of M83  

NASA Astrophysics Data System (ADS)

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 lap103 M ? 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.

Andrews, J. E.; Calzetti, D.; Chandar, R.; Elmegreen, B. G.; Kennicutt, R. C.; Kim, Hwihyun; Krumholz, Mark R.; Lee, J. C.; McElwee, Sean; O'Connell, R. W.; Whitmore, B.

2014-09-01

202

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

NASA Astrophysics Data System (ADS)

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

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

2014-11-01

203

Low Mass Stars and Substellar Objects in the NGC 1333 Molecular Cloud  

E-print Network

We present the results of near-infrared imaging and low-resolution near- infrared spectroscopy of low mass objects in the NGC 1333 molecular cloud. A JHK survey of an 11.4' x 11.7' area of the northern cluster was conducted to a sensitivity of K < 16 mag. Using near-infrared magnitudes and colors from this and previously published surveys, twenty-five brown dwarf candidates were selected toward the high extinction cloud core. Spectra in the K band were obtained and comparisons of the depths of water vapor absorption bands in our candidate objects with a grid of dwarf,subgiant, and giant standards were made to derive spectral types. These data were then used to derive effective temperatures and stellar luminosities which, when combined with theoretical tracks and isochrones for pre-main sequence objects, resulted in estimates for their masses and ages. The models suggest a median age for the sample of < 1 Myr with substellar masses for at least 9 of the candidates including the x-ray flare source ASR 24. Surface gravities have been estimated for the brown dwarf candidates and, for a given spectral type,found to resemble more closely dwarfs than giants. Using the near-infrared imaging data and age estimates from the spectroscopic sample, an extinction-limited sample in the northern cluster was defined. Consistent with recent studies of other young clusters, this sample exhibits an accretion disk frequency of 0.75 +-0.20 and a mass spectrum slope across the hydrogen-burning limit of alpha < 1.6 where dN/dM ~ M^-(alpha).

Bruce A. Wilking; Michael R. Meyer; Thomas P. Greene; Ayman Mikhail; Glenn Carlson

2003-12-13

204

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

205

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

NASA Astrophysics Data System (ADS)

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.

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

2012-09-01

206

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

E-print Network

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

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

2004-04-19

207

The circumstellar environment of TMR-1 - A young, low-mass star in the Taurus molecular ring  

SciTech Connect

Near-infrared camera and Owens Valley millimeter interferometer data are presented that reveal the circumstellar environment of TMR-1 on scales of roughly 1000 AU. TMR-1 is a deeply embedded, young, low-mass star located in a rotating ring of material in the TMC-1/Heiles Cloud 2 region of the Taurus molecular cloud. The stellar wind cavity, as outlined by the extended infrared continuum emission at 2.2 microns, overlaps with the redshifted outflow detected in the (C-12)O data. High-density circumstellar material is seen in (C-13)O in emission and at 2.2 microns in absorption. The morphology suggests the high-density gas may either be part of a 1000 AU disk or may represent infalling material from the molecular cloud core. TMR-1 appears to be in an early wind-clearing phase of protostar formation. 25 refs.

Terebey, S.; Beichman, C.A.; Gautier, T.N.; Hester, J.J. (JPL, Pasadena, CA (USA) Palomar Observatory, Pasadena, CA (USA))

1990-10-01

208

The circumstellar environment of TMR-1 - A young, low-mass star in the Taurus molecular ring  

NASA Technical Reports Server (NTRS)

Near-infrared camera and Owens Valley millimeter interferometer data are presented that reveal the circumstellar environment of TMR-1 on scales of roughly 1000 AU. TMR-1 is a deeply embedded, young, low-mass star located in a rotating ring of material in the TMC-1/Heiles Cloud 2 region of the Taurus molecular cloud. The stellar wind cavity, as outlined by the extended infrared continuum emission at 2.2 microns, overlaps with the redshifted outflow detected in the (C-12)O data. High-density circumstellar material is seen in (C-13)O in emission and at 2.2 microns in absorption. The morphology suggests the high-density gas may either be part of a 1000 AU disk or may represent infalling material from the molecular cloud core. TMR-1 appears to be in an early wind-clearing phase of protostar formation.

Terebey, S.; Beichman, C. A.; Gautier, T. N.; Hester, J. J.

1990-01-01

209

A UKIDSS-based search for low-mass stars and small stellar clumps in off-cloud parts of young star-forming regions *  

NASA Astrophysics Data System (ADS)

The form and universality of the mass function of young and nearby star-forming regions is still under debate. Its relation to the stellar density, its mass peak and the dependency on most recent models shows significant differencies for the various regions and remains unclear up to date. We aim to get a more complete census of two of such regions. We investigate yet unexplored areas of Orion and Taurus-Auriga, observed by the UKIDSS survey. In the latter, we search for low-mass stars via photometric and proper motion criteria and signs for variability. In Orion, we search for small stellar clumps via nearest-neighbor methods. Highlights in Taurus would be the finding of the missing low-mass stars and the detection of a young cluster T dwarf. In Orion, we discovered small stellar associations of its OB1b and OB1c populations. Combined with what is known in literature, we will provide by this investigations a general picture of the results of the star-forming processes in large areas of Taurus and Orion and probe the most recent models. Based on data of the UKIRT (operated by the Joint Astronomy Centre on behalf of the Science and Technology Facilities Council of the U.K.) Infrared Deep Sky Survey (UKIDSS).Supported by the Marie Curie Research Training Network `CONSTELLATION' under grant no. MRTN-CT-2006-035890.

Perger, M.; Lodieu, N.; Martín, E. L.; Barrado Y Navascués, D.

2011-07-01

210

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

E-print Network

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 inner-most layers up to their surface along their evolution on the red giant branch as well as on the red-clump. Our objective is primarily to provide physical insight into the physical mechanism responsible for mixed-modes amplitudes and lifetimes. Subsequently, we aim at understanding the evolution and structure of red giants spectra along with their evolution. The study of energetic aspects of these oscillations is also of great importance to predict the mode parameters in the power spectrum. 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 a...

Grosjean, M; Belkacem, K; Montalban, J; Samadi, R; Mosser, B

2014-01-01

211

Accretion onto Planetary Mass Companions of Low-mass Young Stars  

NASA Astrophysics Data System (ADS)

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.

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

2014-03-01

212

A study of circumstellar disk properties in low-mass stars and brown dwarfs  

NASA Astrophysics Data System (ADS)

We present Spitzer Space Telescope IRAC and MIPS observations for a sample of eight M dwarfs: six dMe, one dM, and one sdMe star. All of our targets are found to have Spectral Energy Distributions (SEDs) which are fitted within the error bars by a purely photospheric spectrum out to 24m m . The estimated ages for all are >10 Myr, suggesting that enough disk dissipation has occurred within the inner several AU of the star. Scaling from Houdebine's model of the AU Mic chromosphere, we have computed the free-free infrared excesses for a range of densities. Our Spitzer 24m m data shows that the chromospheres in two of our targets are less dense than in AU Mic by a factor of 10 or more. Our models also indicate that the chromospheric contribution to the observed AU Mic emission at submillimeter wavelengths is only about 2%. We present Spitzer IRAC, MIPS and IRS observations for three sub-stellar members of the TW Hydrae Association (TWA): 2MASSW J1207334-393254 (2M1207), SSSPM J1102-3431 (SSSPM 1102), and 2MASS J1139511-315921 (2M1139). The near- to mid-infrared SEDs indicate the presence of flat optically thick disks around 2M1207 and SSSPM 1102, and a transition disk around 2M1139. 2M1207 shows absorption in the 10 m m silicate feature, with a peak near 11.3 m m due to crystalline forsterite. The absorption can be attributed to a close to edge-on disk. No silicate absorption/emission is observed towards SSSPM 1102. We have performed detailed modeling of these two brown dwarf disks. The best-fits have been obtained using a flat disk of mass 10 -4 [Special characters omitted.] , M of 10 -10 [Special characters omitted.] /yr, and an inclination angle of 75=B0 for 2M1207, whereas a disk mass of 10 -5 [Special characters omitted.] , M of 10 -11 [Special characters omitted.] /yr, and an inclination angle of 63° provides a good fit to SSSPM 1102. Modeling of the 10 m m silicate feature requires the presence of large (>50 m m ) grains in the disk midplane, which indicates substantial grain growth and dust settling in both brown dwarf disks. Our analysis of the stellar and sub-stellar members of the TWA indicates that while only 6 out of 25 stellar members show excess emission at these mid- infrared wavelengths, all of the TWA brown dwarfs that have been observed so far with Spitzer show signs of disks around them, resulting in a disk fraction of at least 60%. A comparison with younger clusters indicates that by the age of the TWA (~10 Myr), the disk fraction for brown dwarfs has not decreased, whereas it drops by a factor of ~2 for the higher mass stars. This suggests longer disk decay time scales for brown dwarfs compared to higher mass stars.

Riaz, Basmah

213

Temperatures and Radii of Low-Mass Dwarf Stars Estimated from Near Infrared Spectra  

NASA Astrophysics Data System (ADS)

We present estimates of the temperatures and radii of M dwarfs targeted by the MEarth transiting planet survey. The fundamental properties of M dwarfs are difficult to constrain by direct measurement, and we instead use empirical relationships that are based on the strengths of near infrared spectral features. We establish our relationships for radius and temperature using cool dwarfs with interferometric measurements. Our calibrations use the equivalent widths of H-band spectral features as tracers of these parameters and have an accuracy of 0.03 solar radii and 60 K for late K to mid M dwarfs. We validate our method by comparing our inferred stellar parameters to absolute magnitudes, which we calculate using 2MASS magnitudes and parallaxes from Dittmann et al. (2014). We also identify candidate over-luminous objects within our sample, which may be binaries or young stars.

Newton, Elisabeth R.; Charbonneau, David; Irwin, Jonathan

2014-06-01

214

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

NASA Astrophysics Data System (ADS)

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

Feiden, Gregory A.; Chaboyer, Brian

2014-07-01

215

Brown Dwarf and Very Low Mass Stars Search in Orion OB1a and OB1b asociations  

NASA Astrophysics Data System (ADS)

The initial results of the first large-scale optical-infrared search for brown dwarfs in the Orion star forming region are presented. Optical observations were performed with the Jürgen Stock Schmidt telescope and the QUEST-1 camera at the National Astronomical Observatory of Venezuela, covering an area of 180 deg^2 of which ? 32deg^2, that include a portion of OB1a and most of OB1b sub-associations, are studied in this work. R and I band observations were processed with a coadding technique, increasing the limit magnitude to I = 20.5 and R = 21.5 with completitude to I = 19.0 and R = 20.0. Infrared photometry in J, H and K bands was obtained from the Two Micron All Sky Survey data base. Spectroscopic observations of a first sample of candidates was performed with HECTOSPEC and allowed the confirmation of 15 and 9 bonna fide very low mass stars and brown dwarfs respectively.

Downes, J. J.; Briceño, C.; Hernández, J.

2006-06-01

216

c2d Spitzer IRS spectra of embedded low-mass young stars: gas-phase emission lines  

E-print Network

A survey of mid-IR gas-phase emission lines of H2, H2O and various atoms toward a sample of 43 embedded low-mass young stars in nearby star-forming regions is presented. The sources are selected from the Spitzer "Cores to Disks" (c2d) legacy program. The environment of embedded protostars is complex both in its physical structure (envelopes, outflows, jets, protostellar disks) and the physical processes (accretion, irradiation by UV and/or X-rays, excitation through slow and fast shocks) which take place. A key point is to spatially resolve the emission in the Spitzer-IRS spectra. An optimal extraction method is used to separate both spatially unresolved (compact, up to a few 100 AU) and spatially resolved (extended, 1000 AU or more) emission from the IRS spectra. The results are compared with the c2d disk sample and literature PDR and shock models to address the physical nature of the sources. Both compact and extended emission features are observed. Warm (Tex few 100 K) H2, observed through the pure rotatio...

Lahuis, Fred; Jørgensen, Jes K; Blake, Geoffrey A; Evans, Neal J

2010-01-01

217

The Pisa Stellar Evolution Data Base for low-mass stars  

NASA Astrophysics Data System (ADS)

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 ?-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

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

2012-04-01

218

Effect of episodic accretion on the structure and the lithium depletion of low-mass stars and planet-hosting stars  

NASA Astrophysics Data System (ADS)

Following up our recent analysis devoted to the impact of non steady accretion on the location of young low-mass stars or brown dwarfs in the Herzsprung-Russell diagram, we perform a detailed analysis devoted to the effect of burst accretion on the internal structure of low-mass and solar type stars. We find that episodic accretion can produce objects with significantly higher central temperatures than the ones of the non accreting counterparts of same mass and age. As a consequence, lithium depletion can be severely enhanced in these objects. This provides a natural explanation for the unexpected level of lithium depletion observed in young objects for the inferred age of their parent cluster. These results confirm the limited reliability of lithium abundance as a criterion for assessing or rejecting cluster membership. They also show that lithium is not a reliable age indicator, because its fate strongly depends on the past accretion history of the star. Under the assumption that giant planets primarily form in massive disks prone to gravitational instability and thus to accretion burst episodes, the same analysis also explains the higher Li depletion observed in planet hosting stars. At last, we show that, depending on the burst rate and intensity, accretion outbursts can produce solar mass stars with lower convective envelope masses, at ages less than a few tens of Myr, than predicted by standard (non or slowly accreting) pre-main sequence models. This result has interesting, although speculative, implications for the recently discovered depletion of refractory elements in the Sun.

Baraffe, I.; Chabrier, G.

2010-10-01

219

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  

NASA Astrophysics Data System (ADS)

We have conducted a long-term, wide-field, high-cadence photometric monitoring survey of ~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 LX /L bol ? -3.3). However, we find a significant positive correlation between LX /L bol and corotation radius, suggesting that the observed X-ray luminosities are regulated by centrifugal stripping of the stellar coronae. The period-mass relationship in NGC 6530 is broadly similar to that of the Orion Nebula Cluster (ONC), but the slope of the relationship among the slowest rotators differs from that in the ONC and other young clusters. We show that the slope of the period-mass relationship for the slowest rotators can be used as a proxy for the age of a young cluster, and we argue that NGC 6530 may be slightly younger than the ONC, making it a particularly important touchstone for models of angular momentum evolution in young, low-mass stars.

Henderson, Calen B.; Stassun, Keivan G.

2012-03-01

220

Are the red halos of galaxies made of low-mass stars? Constraints from subdwarf star counts in the Milky Way halo  

E-print Network

Surface photometry detections of red and exceedingly faint halos around galaxies have resurrected the old question of whether some non-negligible fraction of the missing baryons of the Universe could be hiding in the form of faint, hydrogen-burning stars. The optical/near-infrared colours 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 discuss potential observations of other nearby galaxies that may be able to put such scenarios to the test.

E. Zackrisson; C. Flynn

2008-09-17

221

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

SciTech Connect

We report the discovery of TWA 30B, a wide ({approx}3400 AU), co-moving M dwarf companion to the nearby ({approx}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 ({Delta}V {approx}< 30 km s{sup -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 {approx}3-5 R{sub 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.

Looper, Dagny L.; Pitts, Mark A. [Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822 (United States); Bochanski, John J.; Burgasser, Adam J. [MIT Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Avenue, Building 37-664B, Cambridge, MA 02139 (United States); Mohanty, Subhanjoy [Imperial College London, 1010 Blackett Laboratory, Prince Consort Road, London, SW7 2AZ (United Kingdom); Mamajek, Eric E. [Department of Physics and Astronomy, University of Rochester, P.O. Box 270171, 500 Wilson Boulevard, Rochester, NY 14627-0171 (United States); Faherty, Jacqueline K. [Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794-3800 (United States); West, Andrew A., E-mail: dagny@ifa.hawaii.ed [Department of Astronomy, Boston University, 725 Commonwealth Avenue, Boston, MA 02215 (United States)

2010-11-15

222

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

NASA Technical Reports Server (NTRS)

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

Mahmoodifar, Simin; Strohmayer, Tod

2013-01-01

223

The Magnetohydrodynamical Model of Kilohertz Quasi-periodic Oscillations in Neutron Star Low-mass X-Ray Binaries (II)  

NASA Astrophysics Data System (ADS)

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.

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

2014-08-01

224

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

SciTech Connect

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

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

2012-09-10

225

c2d Spitzer IRS spectra of embedded low-mass young stars: gas-phase emission lines  

NASA Astrophysics Data System (ADS)

Context. A survey of mid-infrared gas-phase emission lines of H2, H2O and various atoms toward a sample of 43 embedded low-mass young stars in nearby star-forming regions is presented. The sources are selected from the Spitzer “Cores to Disks” (c2d) legacy program. Aims: The environment of embedded protostars is complex both in its physical structure (envelopes, outflows, jets, protostellar disks) and the physical processes (accretion, irradiation by UV and/or X-rays, excitation through slow and fast shocks) which take place. The mid-IR spectral range hosts a suite of diagnostic lines which can distinguish them. A key point is to spatially resolve the emission in the Spitzer-IRS spectra to separate extended PDR and shock emission from compact source emission associated with the circumstellar disk and jets. Methods: An optimal extraction method is used to separate both spatially unresolved (compact, up to a few hundred AU) and spatially resolved (extended, thousand AU or more) emission from the IRS spectra. The results are compared with the c2d disk sample and literature PDR and shock models to address the physical nature of the sources. Results: Both compact and extended emission features are observed. Warm (T_ex few hundred K) H2, observed through the pure rotational H2 S(0), S(1) and S(2) lines, and [S i] 25 ?m emission is observed primarily in the extended component. [S i] is observed uniquely toward truly embedded sources and not toward disks. On the other hand hot (T_ex ? 700 K) H2, observed primarily through the S(4) line, and [Ne ii] emission is seen mostly in the spatially unresolved component. [Fe ii] and [Si ii] lines are observed in both spatial components. Hot H2O emission is found in the spatially unresolved component of some sources. Conclusions: The observed emission on ?1000 AU scales is characteristic of PDR emission and likely originates in the outflow cavities in the remnant envelope created by the stellar wind and jets from the embedded young stars. Weak shocks along the outflow wall can also contribute. The compact emission is likely of mixed origin, comprised of optically thick circumstellar disk and/or jet/outflow emission from the protostellar object.

Lahuis, F.; van Dishoeck, E. F.; Jørgensen, J. K.; Blake, G. A.; Evans, N. J.

2010-09-01

226

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

NASA Astrophysics Data System (ADS)

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 ?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 ~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 ?6707 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, ? 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 ? 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.

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

2013-09-01

227

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

SciTech Connect

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

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

2012-05-15

228

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

NASA Technical Reports Server (NTRS)

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

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

2012-01-01

229

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

NASA Astrophysics Data System (ADS)

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

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

230

Centaurus Star-Forming Field Revisited  

NASA Astrophysics Data System (ADS)

We analyze the structure of the star-forming field in Centaurus based on intermediate-band uvby? photometry of a large sample of O-B9 -stars. The derived precise homogeneous photometric distances and color excesses allow us to reveal spatially coherent groups and layers and to revise the membership and distance of the Cen OB1 association. In particular, we are seeking a correlation between the distribution of the massive OB-stars and that of ionized and neutral interstellar material that would allow a better understanding of the interactions among various ISM components in the Galactic stars-forming fields. For the purpose we combine the photometric findings with several multi-wavelength surveys (Wisconsin H-Alpha Mapper Northern Sky Survey, Southern H-Alpha Sky Survey Atlas, MSX Galactic Plane Survey, WISE All-Sky Data Release, CO survey of the Milky Way, and Southern Galactic Plane Survey). This allows us to map the OB-star distribution together with the super-shells of neutral and ionized material located toward Centaurus. Acknowledgments. This work was supported by NSF grant AST-0708950.

Kaltcheva, Nadia; Golev, V.; Moran, K.

2013-01-01

231

RCW 103 -- Revisiting a cooling neutron star  

E-print Network

Recent observations of the compact source embedded within the supernova remnant RCW 103 rekindle interest in the origin of this object's emission. We contrast several models in which neutron-star cooling powers RCW 103. Specifically, either the presence of an accreted envelope or a sufficiently intense magnetic field can account for the X-ray emission from this object.

Jeremy S. Heyl; Lars Hernquist

1998-05-07

232

Statistical Analysis of the Relationship Between Rotation, Disks, and X-rays Among Low-Mass Pre-Main-Sequence Stars  

NASA Astrophysics Data System (ADS)

Two fundamental outstanding questions in the study of young, low-mass stars are (1) the evolution of angular momentum, and (2) the origin of X-ray emission. Indeed, low-mass stars are observed to decrease their angular momentum content by nearly an order of magnitude during the first ˜100 Myr. These stars are also observed to produce X-rays at levels up to 3-4 orders of magnitude higher than the present-day Sun. However, the physical processes governing angular momentum loss and X-ray production during the pre-main-sequence (PMS) phase remain poorly understood. Circumstellar disks are thought to regulate stellar rotation via magnetic star-disk interaction. Stellar magnetic fields, possibly driven by rotation, may also be implicated in the production of X-rays, which may in turn affect disk ionization and thus the efficiency of star-disk coupling. Finally, all of these stellar properties - rotation, disks, X-rays - are dependent in varying degrees on stellar mass. Using a sample of 400 PMS stars with known rotation periods, disk tracers from Spitzer, and X-ray luminosities from Chandra, we perform a multivariate statistical analysis on the relationships among these manifold mutually correlated variables. Our aim is to discern the fundamental relationship between disks and stellar rotation, and between rotation and X-ray emission. Here we report preliminary results.

Stassun, Keivan; Ardila, D.; Matt, S.; Feigelson, E.

2006-12-01

233

BANYAN. IV. Fundamental parameters of low-mass star candidates in nearby young stellar kinematic groups - Isochronal Age determination using Magnetic evolutionary models  

E-print Network

Based on high resolution optical spectra obtained with ESPaDOnS at CFHT, we determine fundamental parameters (\\Teff, R, \\Lbol, \\logg\\ and metallicity) for 59 candidate members of nearby young kinematic groups. The candidates were identified through the BANYAN Bayesian inference method of \\citet{2013malo}, 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 to field stars with the goal to constrain 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 $\\beta$Pictoris moving grou...

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

2014-01-01

234

The HST/ACS Coma Cluster Survey - X. Nuclear star clusters in low-mass early-type galaxies: scaling relations  

NASA Astrophysics Data System (ADS)

We present scaling relations between structural properties of nuclear star clusters and their host galaxies for a sample of early-type dwarf galaxies observed as part of the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) Coma Cluster Survey. We have analysed the light profiles of 200 early-type dwarf galaxies in the magnitude range 16.0 < mF814W < 22.6 mag, corresponding to -19.0 < MF814W < -12.4 mag. Nuclear star clusters are detected in 80 per cent of the galaxies, thus doubling the sample of HST-observed early-type dwarf galaxies with nuclear star clusters. We confirm that the nuclear star cluster detection fraction decreases strongly towards faint magnitudes. The luminosities of nuclear star clusters do not scale linearly with host galaxy luminosity. A linear fit yields L_nuc ˜ L_gal^{0.57± 0.05}. The nuclear star cluster-host galaxy luminosity scaling relation for low-mass early-type dwarf galaxies is consistent with formation by globular cluster (GC) accretion. We find that at similar luminosities, galaxies with higher Sérsic indices have slightly more luminous nuclear star clusters. Rounder galaxies have on average more luminous clusters. Some of the nuclear star clusters are resolved, despite the distance of Coma. We argue that the relation between nuclear star cluster mass and size is consistent with both formation by GC accretion and in situ formation. Our data are consistent with GC inspiralling being the dominant mechanism at low masses, although the observed trend with Sérsic index suggests that in situ star formation is an important second-order effect.

den Brok, Mark; Peletier, Reynier F.; Seth, Anil; Balcells, Marc; Dominguez, Lilian; Graham, Alister W.; Carter, David; Erwin, Peter; Ferguson, Henry C.; Goudfrooij, Paul; Guzmán, Rafael; Hoyos, Carlos; Jogee, Shardha; Lucey, John; Phillipps, Steven; Puzia, Thomas; Valentijn, Edwin; Kleijn, Gijs Verdoes; Weinzirl, Tim

2014-12-01

235

The standard model of low-mass star formation applied to massive stars: a multi-wavelength picture of AFGL 2591  

E-print Network

This paper aims to investigate the hypothesis that the embedded luminous star AFGL2591-VLA3 (2.3E5Lsun at 3.33kpc) is forming according to a scaled-up version of a low-mass star formation scenario. We present multi-configuration VLA 3.6cm and 7mm, as well as CARMA C18O and 3mm continuum observations to investigate the morphology and kinematics of the ionized gas, dust, and molecular gas around AFGL2591. We also compare our results to ancillary near-IR images, and model the SED and 2MASS image profiles of AFGL2591 using a dust continuum radiative transfer code. The observed 3.6cm images uncover for the first time that the central powering source AFGL2591-VLA3 has a compact core plus collimated jet morphology, extending 4000AU eastward from the central source with an opening angle of <10deg at this radius. However, at 7mm VLA3 does not show a jet morphology, but instead compact (<500AU) emission, some of which (<0.57 mJy of 2.9mJy) is estimated to be from dust. We determine that the momentum rate of th...

Johnston, K G; Robitaille, T P; Wood, K

2012-01-01

236

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

SciTech Connect

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.

Cordiner, Martin A.; Charnley, Steven B.; Wirstroem, Eva S. [Astrochemistry Laboratory and Goddard Center for Astrobiology, NASA Goddard Space Flight Center, Greenbelt, MD 20770 (United States); Smith, Robert G., E-mail: martin.cordiner@nasa.gov [School of Physical, Environmental, and Mathematical Sciences, University of New South Wales, Australian Defence Force Academy, Canberra, ACT 2600 (Australia)

2012-01-10

237

Low-mass, helium-enriched PG1159 stars: a possible evolutionary origin and the implications for their pulsational stability properties  

E-print Network

We examine a recently-proposed evolutionary scenario that could explain the existence of the low-mass, helium-enriched PG1159 stars. We assess the overstability of pulsation g-modes of stellar models as evolution proceeds along the PG1159 domain. Stellar models are extracted from the full evolution of a 1 Mo model star that experiences its first thermal pulse as a late thermal pulse (LTP) after leaving the AGB. The evolutionary stages corresponding to the born-again episode and the subsequent helium sub-flashes are taken into account in detail. Under reasonable mass-loss rate assumptions, the evolutionary scenario reproduces the high helium abundances observed in some PG1159 stars. We find that, despite the high helium abundance in the driving layers, there exists a narrow region in the log g-log Teff diagram for which the helium-enriched PG1159 sequence exhibits unstable pulsation modes with periods in the range 500 to 1600 s. In particular, the nonpulsating helium-enriched PG1159 star, MCT 0130-1937, is located outside the theoretical instability domain. Our results suggest that MCT 0130-1937 is a real non-pulsating star and that the lack of pulsations should not be attributed to unfavorable geometry. Our study hints at a consistent picture between the evolutionary scenario that could explain the existence of helium-enriched PG1159 stars and the nonvariable nature of MCT 0130-1937. We also present theoretical support for the unusually high helium abundance observed in the nonpulsating PG1159 star HS 1517+7403. We suggest that HS 1517+7403 could be a transition object linking the low-mass helium-rich O(He) stars with the helium-enriched PG1159 stars via the evolutionary connection K1-27-->HS 1517+7403-->MCT 0130-1937.

L. G. Althaus; A. H. Córsico; M. M. Miller Bertolami

2007-03-13

238

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

NASA Astrophysics Data System (ADS)

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

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

2012-03-01

239

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

SciTech Connect

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

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

2012-03-10

240

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

NASA Astrophysics Data System (ADS)

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

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

2011-04-01

241

Star/Galaxy Separation Revisited : Into the Zone of Avoidance  

E-print Network

The problem of automated separation of stars and galaxies on photographic plates is revisited with two goals in mind : First, to separate galaxies from everything else (as opposed to most previous work, in which galaxies were lumped together with all other non-stellar images). And second, to search optically for galaxies at low Galactic latitudes (an area that has been largely avoided in the past). This paper demonstrates how an artificial neural network can be trained to achieve both goals on Schmidt plates of the Digitised Sky Survey. Here I present the method while its application to large numbers of plates is deferred to a later paper. Analysis is also provided of the way in which the network operates and the results are used to counter claims that it is a complicated and incomprehensible tool.

A. Naim

1997-01-29

242

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

E-print Network

The early-K star T Cha, a member of the relatively nearby (D ~ 100 pc) epsilon Cha Association, is a relatively "old" (age ~7 Myr) T Tauri star that is still sporadically accreting from an orbiting disk whose inner regions are evidently now being cleared by a close, substellar companion. We report the identification, via analysis of proper motions, serendipitous X-ray imaging spectroscopy, and followup optical spectroscopy, of a new member of the epsilon Cha Association that is very likely a low-mass companion to T Cha at a projected separation of ~38 kAU. The combined X-ray and optical spectroscopy data indicate that the companion, T Cha B (= 2M1155-79), is a weak-lined T Tauri star (wTTS) of spectral type M3 and age ~<10 Myr. The serendipitous X-ray (XMM-Newton) observation of T Cha B, which targeted T Cha, also yields serendipitous detections of two background wTTS in the Chamaeleon cloud complex, including one newly discovered, low-mass member of the Cha cloud pre-MS population. T Cha becomes the third...

Kastner, Joel H; Montez, R; Murphy, S J; Bessell, M S; Sacco, G G

2012-01-01

243

The production of low mass carbon stars - Carbon-rich dredge up or oxygen-rich mass loss?  

NASA Technical Reports Server (NTRS)

Conventional theory explains the origin of carbon stars as due to dredge up of carbon enriched material from the stellar core during helium flash events late in the life of solar mass AGB stars. This relatively efficient process, however, seems to produce a larger C/O ratio than observed (Lambert et al., 1987). A secondary effect which could contribute to the appearance of carbon stars, is the selective removal of oxygen from the atmosphere by radiative force expulsion of oxygen-rich dust grains. Calculations for this scenario are presented, which evaluate the degree of momentum coupling between the grains and gas under the thermodynamical conditions of AGB star atmospheres.

Stencel, R. E.; Pesce, J. E.; Macgregor, K. M.

1989-01-01

244

Revisiting the universality of (multiple) star formation in present-day star formation regions  

E-print Network

Populations of multiple stars inside clustered regions are known to change through dynamical interactions. The efficiency of binary disruption is thought to be determined by stellar density. King and collaborators recently investigated the multiplicity properties in young star forming regions and in the Galactic field. They concluded that stellar density-dependent modification of a universal initial binary population (the standard or null hypothesis model) cannot explain the observations. We re-visit their results, analyzing the data within the framework of different model assumptions, namely non-universality without dynamical modification and universality with dynamics. We illustrate that the standard model does account for all known populations if regions were significantly denser in the past. Some of the effects of using present-day cluster properties as proxies for their past values are emphasized and that the degeneracy between age and density of a star forming region can not be omitted when interpreting...

Marks, Michael; Giersz, Mirek; Pfalzner, Susanne; Pflamm-Altenburg, Jan; Oh, Seungkyung

2014-01-01

245

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

SciTech Connect

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

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

2012-08-01

246

The 13C(?,n)16O reaction as a neutron source for the s-process in AGB low-mass stars  

NASA Astrophysics Data System (ADS)

The 13C(?,n)16O 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 13C(?,n)16O 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 13C pocket, thus putting the s-process in stars on safe physical ground. These inputs allow us to reproduce satisfactorily the solar distribution of elements.

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

2014-05-01

247

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

SciTech Connect

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

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

2013-04-10

248

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

249

NLTE in a Hot Hydrogen Star: Auer & Mihalas Revisited  

NASA Astrophysics Data System (ADS)

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

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

2003-01-01

250

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

NASA Technical Reports Server (NTRS)

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

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

1986-01-01

251

The high latitude low mass star forming region Cometary Globule 12: two compact cores and a C18O hot spot  

NASA Astrophysics Data System (ADS)

With a galactic latitude of 21^0 and at the distance of 630 pc CG 12 lies more than 200 pc above the plane. CG 12 has an associated low mass stellar cluster and a reflection nebula, NGC 5367. A strong IRAS point source and a highly collimated outflow indicate that star formation is still going on. The head of the globule has been observed using NIR imaging (NTT sofi), mm continuum (SEST Simba) and sub mm (APEX) and mm (SEST) spectroscopy. The molecular material is distributed in a 10' North-South elongated lane with two compact maxima separated by 3'. Strong C^18O (3-2), (2-1) and (1-0) emission is detected in both maxima and both have an associated compact 1.2 mm continuum source. The Northern core, CG 12 N, is cold and is possibly still pre-stellar. A stellar source with a NIR reflection nebulosity is observed near CG 12 N. The observed C^18O line ratios are similar to those observed in Class 0 sources. A remarkable C^18O (3-2) hot spot is detected in the direction of the Southern core, CG 12 S. It lies at the edge of a dense cloud core detected both in high density tracers (CS (3-2), H^13CO^+ (1-0) and DCO^+(2-1)) and in the 1.2 mm continuum. The hot spot also lies on the axis of a highly collimated bipolar molecular outflow with a driving source most probably embedded in the dense core. This is the first detection of such a compact, warm object in a low mass star forming region. NIR imaging reveals a bright cone-like feature with a faint counter cone in the centre of CG 12 S. The total mass (> 100 M[sun]) and the linear size of the CG 12 head (~3 pc) are similar to those of other nearby low mass star forming regions. Even though the most evolved stars in CG 12 lie already on ZAMS the cloud contains also proto-stellar sources and a pre-stellar core.

Haikala, L. K.; Juvela, M.; Harju, J.; Lehtinen, K.; Mattila, K.; Olberg, M.; Dumke, M.

252

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

NASA Astrophysics Data System (ADS)

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

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

2014-10-01

253

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

SciTech Connect

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

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

2013-10-01

254

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

E-print Network

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

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

2008-09-18

255

The Chamaeleon II low-mass star-forming region: radial velocities, elemental abundances, and accretion properties ?  

NASA Astrophysics Data System (ADS)

Context. Knowledge of radial velocities, elemental abundances, and accretion properties of members of star-forming regions is important for our understanding of stellar and planetary formation. While infrared observations reveal the evolutionary status of the disk, optical spectroscopy is fundamental to acquire information on the properties of the central star and on the accretion characteristics. Aims: Existing 2MASS archive data and the Spitzer c2d survey of the Chamaeleon II dark cloud have provided disk properties of a large number of young stars. We complement these data with optical spectroscopy with the aim of providing physical stellar parameters and accretion properties. Methods: We use FLAMES/UVES and FLAMES/GIRAFFE spectroscopic observations of 40 members of the Chamaeleon II star-forming region to measure radial velocities through cross-correlation technique, lithium abundances by means of curves of growth, and for a suitable star elemental abundances of Fe, Al, Si, Ca, Ti, and Ni using the code MOOG. From the equivalent widths of the H?, H?, and the He i ?5876, ?6678, ?7065 Å emission lines, we estimate the mass accretion rates, ?acc, for all the objects. Results: We derive a radial velocity distribution for the Chamaeleon II stars, which is peaked at ?Vrad? = 11.4 ± 2.0 km s-1. We find dependencies of ?acc ? M?1.3 and of ?acc ? Age-0.82 in the ~0.1-1.0 M? mass regime, as well as a mean mass accretion rate for Chamaeleon II of ?acc ~ 7-5+26 × 10-10 M? yr-1. We also establish a relationship between the He i ?7065 Å line emission and the accretion luminosity. Conclusions: The radial velocity distributions of stars and gas in Chamaeleon II are consistent. The spread in ?acc at a given stellar mass is about one order of magnitude and can not be ascribed entirely to short timescale variability. Analyzing the relation between ?acc and the colors in Spitzer c2d and 2MASS bands, we find indications that the inner disk changes from optically thick to optically thin at ?acc ~ 10-10 M? yr-1. Finally, the disk fraction is consistent with the age of Chamaeleon II. Based on FLAMES (GIRAFFE+UVES) observations collected at the Very Large Telescope (VLT; Paranal, Chile). Program 076.C-0385(A).Tables 5-7, and Appendices A and B are available in electronic form at http://www.aanda.org

Biazzo, K.; Alcalá, J. M.; Covino, E.; Frasca, A.; Getman, F.; Spezzi, L.

2012-11-01

256

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

NASA Astrophysics Data System (ADS)

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)].

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

2012-12-01

257

The helium-core mass at the helium flash in low-mass red giant stars observations and theory  

E-print Network

The method developed by Raffelt (1990a,b,c) 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, by \\Delta\\Mc \\approx 0.01\\pm 0.015 \\Msun, cannot be ruled out with the present data and evolutionary models. Our new upper limits on \\Delta\\Mc are less restrictive than those previously obtained by Raffelt, as are the corresponding constraints on novel astroparticle phenomena which 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 ...

Catelan, M

1995-01-01

258

Very Low Mass Stars and Brown Dwarf Candidates in Orion OB1a and OB1b  

NASA Astrophysics Data System (ADS)

VRI images within the belt of Orion and the Ori OB1a association reveal a pre-main sequence locus extending to below our completeness limit of about V=21. We report here on followup JHK imaging and optical and near--IR spectroscopy of the faintest and reddest of the PMS candidates. We find that they are unreddened mid-to-late M ``stars'' which fall on a few million year isochrone. Masses are largely substellar, reaching as low as about 0.02 M? (20 Jovian masses). The space density of the substellar objects is high. % we will speculate on why the density is %so large in this fossil star forming region.

Walter, Frederick M.; Sherry, William H.; Wolk, Scott J.

2003-06-01

259

Relativistic Iron Line Emission from the Neutron Star Low-mass X-ray Binary 4U 1636-536  

E-print Network

We present an analysis of XMM-Newton and RXTE data from three observations of the neutron star LMXB 4U 1636-536. The X-ray spectra show clear evidence of a broad, asymmetric iron emission line extending over the energy range 4-9 keV. The line profile is consistent with relativistically broadened Fe K-alpha emission from the inner accretion disk. The Fe K-alpha line in 4U 1636-536 is considerably broader than the asymmetric iron lines recently found in other neutron star LMXBs, which indicates a high disk inclination. We find evidence that the broad iron line feature is a combination of several K-alpha lines from iron in different ionization states.

Dirk Pandel; Philip Kaaret; Stephane Corbel

2008-08-15

260

Very Low Mass Stars and Brown Dwarf Candidates in Orion OB1a and OB1b  

Microsoft Academic Search

VRI images within the belt of Orion and the Ori OB1a association reveal a pre-main sequence locus extending to below our completeness limit of about V=21. We report here on followup JHK imaging and optical and near--IR spectroscopy of the faintest and reddest of the PMS candidates. We find that they are unreddened mid-to-late M ``stars'' which fall on a

Frederick M. Walter; William H. Sherry; Scott J. Wolk

2003-01-01

261

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

NASA Astrophysics Data System (ADS)

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).

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

2010-11-01

262

Evolution of the habitable zone of low-mass stars. Detailed stellar models and analytical relationships for different masses and chemical compositions  

E-print Network

We study the temporal evolution of the habitable zone (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. 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] Msun, metallicity Z in the range [0.005 - 0.04], and various initial helium contents. 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 composit...

Valle, G; Moroni, P G Prada; Degl'Innocenti, S

2014-01-01

263

H{alpha} ACTIVITY OF OLD M DWARFS: STELLAR CYCLES AND MEAN ACTIVITY LEVELS FOR 93 LOW-MASS STARS IN THE SOLAR NEIGHBORHOOD  

SciTech Connect

Through the McDonald Observatory M Dwarf Planet Search, we have acquired nearly 3000 high-resolution spectra of 93 late-type (K5-M5) stars over more than a decade using the High Resolution Spectrograph on the Hobby-Eberly Telescope. This sample provides a unique opportunity to investigate the occurrence of long-term stellar activity cycles for low-mass stars. In this paper, we examine the stellar activity of our targets as reflected in the H{alpha} feature. We have identified periodic signals for six stars, with periods ranging from days to more than 10 years, and find long-term trends for seven others. Stellar cycles with P {>=} 1 year are present for at least 5% of our targets. Additionally, we present an analysis of the time-averaged activity levels of our sample, and search for correlations with other stellar properties. In particular, we find that more massive, earlier type (M0-M2) stars tend to be more active than later type dwarfs. Furthermore, high-metallicity stars tend to be more active at a given stellar mass. We also evaluate H{alpha} variability as a tracer of activity-induced radial velocity (RV) variation. For the M dwarf GJ 1170, H{alpha} variation reveals stellar activity patterns matching those seen in the RVs, mimicking the signal of a giant planet, and we find evidence that the previously identified stellar activity cycle of GJ 581 may be responsible for the recently retracted planet f in that system. In general, though, we find that H{alpha} is not frequently correlated with RV at the precision (typically 6-7 m s{sup -1}) of our measurements.

Robertson, Paul; Endl, Michael; Cochran, William D.; Dodson-Robinson, Sarah E., E-mail: paul@astro.as.utexas.edu [Department of Astronomy and McDonald Observatory, University of Texas at Austin, Austin, TX 78712 (United States)

2013-02-10

264

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

SciTech Connect

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

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

2013-09-10

265

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

E-print Network

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

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

2014-01-01

266

Developing an Evolutionary Sequence for the Earliest Stages of Low-mass Star Formation: Results of the ARO-GBT Starless Core Survey  

NASA Astrophysics Data System (ADS)

Dense starless cores are the earliest observable phase of low-mass star formation. Despite a considerable observational effort over the past decade, a coherent evolutionary sequence has yet to be developed for the incipient stages of star formation. Recent observational evidence suggests that cores with the same central density may be evolving at very different rates, even within the same molecular cloud. The chemical structure of the core is an essential tool for characterizing the physical state and rate of evolution of the core. Chemical processes such as the abundance ratio of early-time versus late-time molecular species, the amount of freeze-out of gas phase species, and the degree of deuteration may be used to elucidate the evolutionary state. We present the results from a chemical mapping survey of 25 nearby (D < 400 pc) low-mass starless cores for which radiative transfer models of the dust continuum emission, mapped by SCUBA at submillimeter wavelengths, have constrained the physical structure (n(r), T(r), I_ISRF) of the cores. The cores were observed with the Arizona Radio Observatory's 10-m and 12-m telescopes and the Green Bank 100-m telescope in 10 transitions: NH3 (1,1) and (2,2), o-NH2D 1_11-1_01, CCS 1_2 - 2_1, C3S 4-3, HC5N 9-8, HC7N 21-20, C18O 2-1, C17O 2-1, and p-H2CO 1_01-0_00. The chemical, physical, and kinematical states of these cores are compared to develop an evolutionary sequence.

Shirley, Yancy L.; Myers, P. C.; Walker, C. K.; Hedden, A. S.

2007-12-01

267

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

E-print Network

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

Lynne A. Hillenbrand

2008-12-06

268

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  

Microsoft Academic Search

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

Maxim L. Khodachenko; Ignasi Ribas; Helmut Lammer; Jean-Mathias Grießmeier; Martin Leitner; Franck Selsis; Carlos Eiroa; Arnold Hanslmeier; Helfried K. Biernat; Charles J. Farrugia; Helmut O. Rucker

2007-01-01

269

Spectroscopic confirmation of very low-mass stars and brown dwarf candidates in nearby, young moving groups  

NASA Astrophysics Data System (ADS)

Extending the current census of young nearby associations toward lower masses is essential to study the IMF at a few Myr down to substellar masses and understand the evolution and atmospheres of young brown dwarfs. Based on the proper motions and photometric properties of a large sample of red stars derived from a correlation of 2MASS with WISE, we identified 190 highly probable late-type (> M5) members of the Beta Pictoris and TW Hydrae moving groups and Tucana-Horologium association, as well as 6 other young associations. As the youth (i.e. low surface gravity) of such objects should be apparent from the shape of their near-infrared spectrum as well as some atomic lines (e.g. KI doublets), we propose to use OSIRIS to obtain near-infrared spectra and confirm the membership of 75 candidates only visible in the southern hemisphere. This program will unveil a significant population of young brown dwarfs, down to the planetary mass regime, in close-by young moving groups, providing high-quality template spectra as benchmarks for evolutionary models.

Gagne, Jonathan; Doyon, Rene; Lafreniere, David; Malo, Lison; Artigau, Etienne

2013-02-01

270

The influence of turbulence during magnetized core collapse and its consequences on low-mass star formation  

NASA Astrophysics Data System (ADS)

Context. Theoretical and numerical studies of star formation have shown that a magnetic field can greatly influence both disk formation and its fragmentation, with even relatively low magnetic field strengths being able to prevent these processes. However, very few studies have investigated the combined effects of magnetic field and turbulence. Aims: We study the collapse of turbulent, magnetized prestellar cores, focusing on the effects of magnetic diffusion, and misalignment between rotation axis and magnetic field, on the formation of disks, fragmentation, and the generation of outflows. Methods: We performed three-dimensional, adaptive-mesh, numerical simulations of magnetically super-critical collapsing dense cores of 5 M? using the magneto-hydrodynamic code Ramses. A turbulent velocity field is imposed as initial conditions, characterized by a Kolmogorov power spectrum. Different levels of turbulence (a laminar case, as well as subsonic and supersonic cases) and magnetization (from weak to strong magnetization) are investigated, as are three realizations for the turbulent velocity field. Results: The turbulent velocity field imposed as initial conditions contains a non-zero angular momentum, which is responsible for a misalignment of the rotation axis with respect to the initial magnetic field, and an effective turbulent diffusivity in the vicinity of the core. Both effects are responsible for a significant decrease in the magnetic braking, and they facilitate the formation of early massive disks. These disks can fragment even with ? ~ 5 at late times, in contrast to simulations of 1 M? cores, where fragmentation is prevented for these values of ?. Slow asymmetric outflows are always launched, and they carry a mass comparable to that of the adiabatic first core. Conclusions: Because of turbulence-induced misalignment and magnetic diffusivity, massive disk formation is possible; nevertheless, their mass and size are much more reduced than for disks formed in unmagnetized collapsing cores. We find that for ? ? 5 fragmentation can occur.

Joos, M.; Hennebelle, P.; Ciardi, A.; Fromang, S.

2013-06-01

271

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

E-print Network

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 Msun and a radius R = 0....

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

2014-01-01

272

Radial and rotational velocities of young brown dwarfs and very low-mass stars in the Upper Scorpius OB association and the ? Ophiuchi cloud core  

NASA Astrophysics Data System (ADS)

We present the results of a radial velocity (RV) survey of 14 brown dwarfs (BDs) and very low-mass (VLM) stars in the Upper Scorpius OB association (UScoOB) and three BD candidates in the ? Ophiuchi dark cloud core. We obtained high-resolution echelle spectra at the Very Large Telescope using the Ultraviolet and Visual Echelle Spectrograph (UVES) at two different epochs for each object, and measured the shifts in their RVs to identify candidates for binary/multiple systems in the sample. The average time separation of the RV measurements is 21.6 d, and our survey is sensitive to the binaries with separation <0.1 au. We found that four out of 17 objects (or 24+16-13 per cent by fraction) show a significant RV change in 4-33 d time-scale, and are considered as binary/multiple `candidates'. We found no double-lined spectroscopic binaries in our sample, based on the shape of cross-correlation curves. The RV dispersion of the objects in UScoOB is found to be very similar to that of the BD and VLM stars in Chamaeleon I (Cha I). We also found the distribution of the mean rotational velocities (v sin i) of the UScoOB objects is similar to that of the Cha I, but the dispersion of v sin i is much larger than that of the Cha I objects.

Kurosawa, Ryuichi; Harries, Tim J.; Littlefair, S. P.

2006-11-01

273

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

NASA Astrophysics Data System (ADS)

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

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

2011-08-01

274

LONG-DURATION X-RAY FLASH AND X-RAY-RICH GAMMA-RAY BURSTS FROM LOW-MASS POPULATION III STARS  

SciTech Connect

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.

Nakauchi, Daisuke; Kashiyama, Kazumi; Nakamura, Takashi [Department of Physics, Kyoto University, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502 (Japan); Suwa, Yudai [Yukawa Institute for Theoretical Physics, Kyoto University, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502 (Japan); Sakamoto, Takanori [Center for Research and Exploration in Space Science and Technology (CRESST), NASA Goddard Space Flight Center, Greenbelt, MD 20771 (United States)

2012-11-10

275

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

SciTech Connect

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

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

2013-07-10

276

The multiplicity of exoplanet host stars - New low-mass stellar companions of the exoplanet host stars HD125612 and HD212301  

E-print Network

Aims: We present new results from our ongoing multiplicity study of exoplanet host stars, carried out with SofI/NTT. We provide the most recent list of confirmed binary and triple star systems that harbor exoplanets. Methods: We use direct imaging to identify wide stellar and substellar companions as co-moving objects to the observed exoplanet host stars, whose masses and spectral types are determined with follow-up photometry and spectroscopy. Results: We found two new co-moving companions of the exoplanet host stars HD125612 and HD212301. HD125612B is a wide M4 dwarf (0.18 Msun) companion of the exoplanet host star HD125612, located about 1.5 arcmin (~4750 AU of projected separation) south-east of its primary. In contrast, HD212301B is a close M3 dwarf (0.35 Msun), which is found about 4.4 arcsec (~230 AU of projected separation) north-west of its primary. Conclusions: The binaries HD125612AB and HD212301AB are new members in the continuously growing list of exoplanet host star systems of which 43 are presently known. Hence, the multiplicity rate of exoplanet host stars is about 17%. Based on observations obtained on La Silla in ESO programs 079.C-0099(A), 080.C-0312(A)

M. Mugrauer; R. Neuhaeuser

2008-12-13

277

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

E-print Network

We present Chandra-HETGS observations of the Herbig Ae star HD 104237 and the associated young stars comprising lower mass stars, in the 0.15-1.75\\msol mass range, in their pre-main sequence phase. The brightest X-ray source in the association is the central system harboring the Herbig Ae primary, and a K3 companion. Its X-ray variability indicates modulation possibly on time scales of the rotation period of the Herbig Ae star, and this would imply that the primary significantly contributes to the overall emission. The spectrum of the Herbig Ae+K3 system shows a soft component significantly more pronounced than in other K-type young stars. This soft emission is reminiscent of the unusually soft spectra observed for the single Herbig Ae stars HD 163296 and AB Aur, and therefore we tentatively attribute it to the Herbig Ae of the binary system. The HETGS spectrum shows strong emission lines corresponding to a wide range of plasma temperatures. The He-like triplet of MgXI and NeIX suggest the presence of plasma at densities of about $10^{12}$ cm$^{-3}$, possibly indicating accretion related X-ray production mechanism. The analysis of the zero-order spectra of the other sources indicates X-ray emission characteristics typical of pre-main sequence stars of similar spectral type, with the exception of the T Tauri HD104237-D, whose extremely soft emission is very similar to the emission of the classical T Tauri star TW Hya, and suggests X-ray production by shocked accreting plasma.

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

2008-07-01

278

X-ray Emission from Young Stellar Objects in the \\epsilon Chamaeleontis Group: the Herbig Ae Star HD 104237 and Associated Low-Mass Stars  

E-print Network

We present Chandra-HETGS observations of the Herbig Ae star HD 104237 and the associated young stars comprising lower mass stars, in the 0.15-1.75\\msol mass range, in their pre-main sequence phase. The brightest X-ray source in the association is the central system harboring the Herbig Ae primary, and a K3 companion. Its X-ray variability indicates modulation possibly on time scales of the rotation period of the Herbig Ae star, and this would imply that the primary significantly contributes to the overall emission. The spectrum of the Herbig Ae+K3 system shows a soft component significantly more pronounced than in other K-type young stars. This soft emission is reminiscent of the unusually soft spectra observed for the single Herbig Ae stars HD 163296 and AB Aur, and therefore we tentatively attribute it to the Herbig Ae of the binary system. The HETGS spectrum shows strong emission lines corresponding to a wide range of plasma temperatures. The He-like triplet of MgXI and NeIX suggest the presence of plasma ...

Testa, Paola; Schulz, Norbert S; Ishibashi, Kazunori

2008-01-01

279

Evolution of Thermally Pulsing Asymptotic Giant Branch Stars IV. Constraining Mass-Loss & Lifetimes of Low Mass, Low Metallicity AGB Stars  

E-print Network

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] $\\lesssim -0.86$) galaxies taken from the ANGST sample, using 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 RGB stars, to models generated from different suites of TP-AGB evolutionary tracks after adopting star formation histories (SFH) 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 TP-AGB/RGB ratio or the luminosity functions. In ...

Rosenfield, Philip; Girardi, Leo; Dalcanton, Julianne J; Bressan, Alessandro; Gullieuszik, Marco; Weisz, Daniel; Williams, Benjamin F; Dolphin, Andrew; Aringer, Bernhard

2014-01-01

280

Constraints on First-Stars Models From Observations of Local Low-Mass Dwarf Galaxies and Galactic Metal-Poor Halo Stars  

NASA Astrophysics Data System (ADS)

The first metal-free stars in the universe had hard ionizing photon spectra and unique element yields from their supernovae, leaving signatures in the reionization of the intergalactic medium and in the metal enrichment of gas in the early universe. Here, we examine the metal abundances in a variety of systems in the local universe, from very metal-poor Galactic halo stars to ultra-faint dwarf spheroidal galaxies, and compare them with the latest theoretical models of massive stars with and without rotation. We confirm the similar abundance patterns found in the ultra-faint dwarfs and metal-poor halo stars by recent studies, and find new trends of interest in a variety of individual elements spanning metallicity values of [Fe/H] from about -2 to -5. We also compare our results with the abundances found in the very metal-deficient nearby dwarf irregular galaxy Leo P, which was recently discovered in the Arecibo ALFALFA survey. We comment on the similarities and differences between abundance trends in gas-rich dwarf galaxy systems like Leo P versus gas-poor ones like the ultra-faint dwarf spheroidals, and on the possibility of such systems hosting populations of the first stars. This work has been supported by NSF grant AST-1211005 and by Research Corporation through the Cottrell College Science Award.

Yung, Long Yan; Venkatesan, A.

2014-01-01

281

RADIAL VELOCITY SURVEY FOR PLANETS AND BROWN DWARF COMPANIONS TO VERY YOUNG BROWN DWARFS AND VERY LOW-MASS STARS IN CHA I WITH UVES AT THE VLT. V. Joergens, Leiden Observatory /  

E-print Network

RADIAL VELOCITY SURVEY FOR PLANETS AND BROWN DWARF COMPANIONS TO VERY YOUNG BROWN DWARFS AND VERY and the existence of planets around brown dwarfs are key parameters for planet and brown dwarf formation theories young brown dwarfs and very low-mass stars are suitable targets for searches for RV planets. Three

Joergens, Viki

282

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

SciTech Connect

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

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

2012-07-10

283

USING HIGH-RESOLUTION OPTICAL SPECTRA TO MEASURE INTRINSIC PROPERTIES OF LOW-MASS STARS: NEW PROPERTIES FOR KOI-314 AND GJ 3470  

SciTech Connect

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.

Pineda, J. Sebastian; Bottom, Michael; Johnson, John A., E-mail: jspineda@astro.caltech.edu [Department of Astronomy, California Institute of Technology, 1200 East California Avenue, Pasadena, CA 91125 (United States)

2013-04-10

284

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

E-print Network

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

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

2014-01-01

285

Discovery and Observations of ASASSN-13db, an EX Lupi-type Accretion Event on a Low-mass T Tauri Star  

NASA Astrophysics Data System (ADS)

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 ?V ~ -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.

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.; Szczygie?, D. M.; Woodward, C. E.

2014-04-01

286

The effect of accretion on the measurement of neutron star mass and radius in the low-mass X-ray binary 4U 1608-52  

NASA Astrophysics Data System (ADS)

Spectral measurements of thermonuclear (type I) X-ray bursts from low-mass X-ray binaries have been used to measure neutron star (NS) masses and radii. A number of systematic issues affect such measurements and have raised concerns as to the robustness of the methods. We present analysis of the X-ray emission from bursts observed from 4U 1608-52 at various persistent fluxes. We find a strong dependence of the burst properties on the flux and spectral hardness of the persistent emission before burst. Bursts occurring during the low accretion rate (hard) state exhibit evolution of the blackbody normalization consistent with the theoretical predictions of NS atmosphere models. However, bursts occurring during the high accretion rate (soft) state show roughly constant normalization, which is inconsistent with the NS atmosphere models and therefore these bursts cannot be easily used to determine NS parameters. We analyse the hard-state burst to put the lower limit on the NS radius R in 4U 1608-52 of 12 km (for masses 1.0-2.4 M?). We constrain R to be between 13 and 16 km for masses 1.2-1.6 M?. The best agreement with the theoretical NS mass-radius relations is achieved for source distances in the range 3.1-3.7 kpc. We expect that the radius limit will be 10 per cent lower if spectral models including rapid rotation are used instead.

Poutanen, Juri; Nättilä, Joonas; Kajava, Jari J. E.; Latvala, Outi-Marja; Galloway, Duncan K.; Kuulkers, Erik; Suleimanov, Valery F.

2014-08-01

287

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

E-print Network

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

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

2014-01-01

288

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)

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.

Tottle, Jonathan; Mohanty, Subhanjoy

2013-07-01

289

Winds from Low Mass Protostars  

NASA Astrophysics Data System (ADS)

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.

Shu, Frank H.; Lizano, Susana; Adams, Fred C.; Ruden, Steven P.

290

HIGH- AND LOW-MASS STAR-FORMING REGIONS FROM HIERARCHICAL GRAVITATIONAL FRAGMENTATION. HIGH LOCAL STAR FORMATION RATES WITH LOW GLOBAL EFFICIENCIES  

SciTech Connect

We investigate the properties of 'star-forming regions' in a previously published numerical simulation of molecular cloud formation out of compressive motions in the warm neutral atomic interstellar medium, neglecting magnetic fields and stellar feedback. We study the properties (density, total gas + stars mass, stellar mass, velocity dispersion, and star formation rate (SFR)) of the cloud hosting the first local, isolated 'star formation' event and compare them with those of the cloud formed by the central, global collapse event. In this simulation, the velocity dispersions at all scales are caused primarily by infall motions rather than by random turbulence. We suggest that the small-scale isolated collapses may be representative of low- to intermediate-mass star-forming regions, with gas masses (M{sub gas}) of hundreds of solar masses, velocity dispersions sigma{sub v} approx 0.7 km s{sup -1}, and SFRs approx3 x 10{sup -5} M{sub sun} yr{sup -1}, while the large-scale, massive ones may be representative of massive star-forming regions, with M{sub gas} of thousands of solar masses, sigma{sub v}approx a few km s{sup -1}, and SFRs approx3 x 10{sup -4} M{sub sun} yr{sup -1}. We also compare the statistical distributions of the physical properties of the dense cores appearing in the central region of massive collapse with those from a recent survey of the massive star-forming region in the Cygnus X molecular cloud, finding that the observed and simulated distributions are in general very similar. However, we find that the star formation efficiency per free-fall time (SFE{sub ff}) of the high mass region, similar to that of OMC-1, is low, approx0.04. In the simulated cloud, this is not a consequence of a 'slow' SFR in a nearly hydrostatic cloud supported by turbulence, but rather of the region accreting mass at a high rate. Thus, we find that measuring a low SFE{sub ff} may be incorrectly interpreted as implying a lifetime much longer than the core's local free-fall time, and an SFR much slower than that given by the free-fall rate, if the accretion is not accounted for. We suggest that rather than requiring a low value of the SFE{sub ff} everywhere in the Galaxy, attaining a globally low specific SFR requires star formation to be a spatially intermittent process, so that most of the mass in a giant molecular cloud (GMC) is not participating in the SF process at any given time. Locally, the specific SFR of a star-forming region can be much larger than the global GMC's average.

Vazquez-Semadeni, Enrique; Gomez, Gilberto C.; Ballesteros-Paredes, Javier [Centro de RadioastronomIa y AstrofIsica, Universidad Nacional Autonoma de Mexico, Apdo. Postal 3-72, Morelia, Michoacan 58089 (Mexico); Jappsen, A.-Katharina [School of Physics and Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA (United Kingdom); Klessen, Ralf S. [Zentrum fuer Astronomie der Universitaet Heidelberg, Institut fuer Theoretische Astrophysik, 69120 Heidelberg (Germany)

2009-12-20

291

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)

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.

Bernardini, F.; Cackett, E. M.; Brown, E. F.; D'Angelo, C.; Degenaar, N.; Miller, J. M.; Reynolds, M.; Wijnands, R.

2013-12-01

292

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

NASA Astrophysics Data System (ADS)

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

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

2013-07-01

293

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

NASA Astrophysics Data System (ADS)

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

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

2014-10-01

294

Two New Bursting Neutron Star Low-mass X-Ray Binaries: Swift J185003.2-005627 and Swift J1922.7-1716  

NASA Astrophysics Data System (ADS)

We discuss the origin of two triggers of Swift's Burst Alert Telescope (BAT) that occurred in 2011. The triggers were identified with Swift J185003.2-005627, a previously unknown X-ray source, and the known but unclassified X-ray transient Swift J1922.7-1716. We investigate the BAT data and follow-up observations obtained with Swift's X-ray and ultraviolet/optical telescopes to demonstrate that both triggers are consistent with thermonuclear X-ray bursts. This implies that both sources are neutron star low-mass X-ray binaries. The total duration of ~= 7 minutes and estimated energy output of ~= (3-7) × 1039 erg fall in between that of normal and intermediately long X-ray bursts. From the observed peaks of the X-ray bursts, we estimate a distance of <~ 3.7 kpc for Swift J185003.2-005627 and <~ 4.8 kpc for Swift J1922.7-1716. We characterize the outburst and quiescent X-ray properties of the two sources. They have comparable average outburst luminosities of ~= 1035-36 erg s-1, and a quiescent luminosity equal to or lower than ~= 2 × 1032 erg s-1 (0.5-10 keV). Swift J185003.2-005627 returned to quiescence ~= 20 d after its BAT trigger, while Swift J1922.7-1716 appears to exhibit long accretion outbursts that last several months to years. We identify a unique counterpart for Swift J1922.7-1716 in the ultraviolet/optical data. Finally, we serendipitously detect a flare lasting ~= 500 s from an uncataloged X-ray/optical object that we tentatively classify as a flaring M-dwarf.

Degenaar, N.; Linares, M.; Altamirano, D.; Wijnands, R.

2012-11-01

295

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)

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.

West, A. A.; Bochanski, J. J.; Bowler, B. P.; Dotter, A.; Johnson, J. A.; Lépine, S.; Rojas-Ayala, B.; Schweitzer, A.

2011-12-01

296

STATISTICAL PROPERTIES OF GALACTIC {delta} SCUTI STARS: REVISITED  

SciTech Connect

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

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

2013-05-15

297

REVISITING THE FIRST GALAXIES: THE EPOCH OF POPULATION III STARS  

SciTech Connect

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

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

2013-08-10

298

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

NASA Astrophysics Data System (ADS)

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

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

2013-03-01

299

The Variable Stars of the Draco Dwarf Spheroidal Galaxy - Revisited  

E-print Network

We present a CCD survey of variable stars in the Draco dwarf spheroidal galaxy. This survey, which has the largest areal coverage since the original variable star survey by Baade & Swope, includes photometry for 270 RR Lyrae stars, 9 anomalous Cepheids, 2 eclipsing binaries, and 12 slow, irregular red variables, as well as 30 background QSOs. Twenty-six probable double-mode RR Lyrae stars were identified. Observed parameters, including mean V and I magnitudes, V amplitudes, and periods, have been derived. Photometric metallicities of the ab-type RR Lyrae stars were calculated according to the method of Jurcsik & Kovacs, yielding a mean metallicity of = -2.19 +/- 0.03. The well known Oosterhoff intermediate nature of the RR Lyrae stars in Draco is reconfirmed, although the double-mode RR Lyrae stars with one exception have properties similar to those found in Oosterhoff type II globular clusters. The period-luminosity relation of the anomalous Cepheids is rediscussed with the addition of the new Draco anomalous Cepheids.

K. Kinemuchi; H. C. Harris; Horace A. Smith; N. A. Silbermann; L. A. Snyder; A. P. LaCluyze; C. L. Clark

2008-08-19

300

Report on the ESO/MPE/MPA/ExcellenceCluster/LMU Joint Astronomy Workshop The Formation and Early Evolution of Very Low-mass Stars and Brown Dwarfs  

NASA Astrophysics Data System (ADS)

The topics discussed at the workshop ranged from the structure and fragmentation of molecular clouds to the formation of individual very low-mass objects, their multiplicity, physical structure, mass distribution and early evolution. Each topic was introduced by two reviews on the status of our theoretical and observational understanding of the field, followed by presentations of new results and discussions. In this report we provide a brief summary of some of the areas discussed at the workshop.

Petr-Gotzens, M.; Testi, L.

2011-12-01

301

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

SciTech Connect

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

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

2013-08-01

302

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

NASA Astrophysics Data System (ADS)

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

Lyu, Ming; Méndez, Mariano; Altamirano, Diego

2014-12-01

303

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

E-print Network

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

Lyu, Ming; Altamirano, Diego

2014-01-01

304

The mass and the radius of the neutron star in the transient low mass X-ray binary SAX J1748.9-2021  

E-print Network

of Engineering and Natural Sciences, Sabanci University, Orhanli, Tuzla, Istanbul 34956, Turkey 2 University, the cooling tails of X-ray bursts can be analyzed to measure the angular size of the neutron star, which can 2006). In particular, a combination of the apparent angular size obtained during the cooling tails of X

Yanikoglu, Berrin

305

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

NASA Astrophysics Data System (ADS)

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

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

2014-10-01

306

EFFECTS OF ROTATIONALLY INDUCED MIXING IN COMPACT BINARY SYSTEMS WITH LOW-MASS SECONDARIES AND IN SINGLE SOLAR-TYPE STARS  

SciTech Connect

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.

Chatzopoulos, E.; Robinson, Edward L.; Wheeler, J. Craig, E-mail: manolis@astro.as.utexas.edu [Department of Astronomy, University of Texas at Austin, Austin, TX (United States)

2012-08-20

307

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

NASA Astrophysics Data System (ADS)

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

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

2012-08-01

308

Water in Low-Mass Star-Forming Regions with Herschel: The Link Between Water Gas and Ice in Protostellar Envelopes  

E-print Network

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., FUV field, initial abundances, timescales, and kinematics). Results: The observed water ice abundances with respect to hydrogen nuclei in our sample are 30-80ppm, and therefore contain only 10-30% of the volatile oxygen budget of 320ppm. The keys to reprodu...

Schmalzl, M; Walsh, C; Albertsson, T; van Dishoeck, E F; Kristensen, L E; Mottram, J C

2014-01-01

309

MULTI-WAVELENGTH CHARACTERIZATION OF STELLAR FLARES ON LOW-MASS STARS USING SDSS AND 2MASS TIME-DOMAIN SURVEYS  

SciTech Connect

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.

Davenport, James R. A.; Becker, Andrew C.; Kowalski, Adam F.; Hawley, Suzanne L.; Schmidt, Sarah J.; Hilton, Eric J. [Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195 (United States); Sesar, Branimir [Division of Physics, Mathematics and Astronomy, Caltech, Pasadena, CA 91125 (United States); Cutri, Roc, E-mail: jrad@astro.washington.edu [Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125 (United States)

2012-03-20

310

Discovery of the near-infrared counterpart to the luminous neutron-star low-mass X-ray binary GX 3+1  

E-print Network

Using the High Resolution Camera onboard 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_s=15.8+-0.1 mag star as the counterpart is based on the presence of a Br-gamma emission line in a NIR spectrum taken with the FIRE 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 ~ 1e36 to 1e37 erg/s) than G...

Berg, Maureen van den; Fridriksson, Joel K; Linares, Manuel

2014-01-01

311

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

NASA Astrophysics Data System (ADS)

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

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

2014-10-01

312

Evolution of black hole low-mass binaries  

Microsoft Academic Search

Several evolutionary sequences with low mass secondaries as donors and black holes as accretors are calculated. Adopting a simple estimate for the efficiency of magnetic braking we have determined the bifurcation period Pbif, separating converging from diverging binary systems, to be ~ 1 day. It is shown that in converging binary systems, similar to neutron star binaries with low-mass secondaries,

E. Ergma; A. Fedorova

1998-01-01

313

Water in star-forming regions with Herschel (WISH). III. Far-infrared cooling lines in low-mass young stellar objects  

NASA Astrophysics Data System (ADS)

Context. Understanding the physical phenomena involved in the earlierst stages of protostellar evolution requires knowledge of the heating and cooling processes that occur in the surroundings of a young stellar object. Spatially resolved information from its constituent gas and dust provides the necessary constraints to distinguish between different theories of accretion energy dissipation into the envelope. Aims. Our aims are to quantify the far-infrared line emission from low-mass protostars and the contribution of different atomic and molecular species to the gas cooling budget, to determine the spatial extent of the emission, and to investigate the underlying excitation conditions. Analysis of the line cooling will help us characterize the evolution of the relevant physical processes as the protostar ages. Methods. Far-infrared Herschel-PACS spectra of 18 low-mass protostars of various luminosities and evolutionary stages are studied in the context of the WISH key program. For most targets, the spectra include many wavelength intervals selected to cover specific CO, H2O, OH, and atomic lines. For four targets the spectra span the entire 55-200 ?m region. The PACS field-of-view covers ~47" with the resolution of 9.4". Results. Most of the protostars in our sample show strong atomic and molecular far-infrared emission. Water is detected in 17 out of 18 objects (except TMC1A), including 5 Class I sources. The high-excitation H2O 818-707 63.3 ?m line (Eu/kB = 1071 K) is detected in 7 sources. CO transitions from J = 14-13 up to J = 49 - 48 are found and show two distinct temperature components on Boltzmann diagrams with rotational temperatures of ~350 K and ~700 K. H2O has typical excitation temperatures of ~150 K. Emission from both Class 0 and I sources is usually spatially extended along the outflow direction but with a pattern that depends on the species and the transition. In the extended sources, emission is stronger off source and extended on &?10,000 AU scales; in the compact sample, more than half of the flux originates within 1000 AU of the protostar. The H2O line fluxes correlate strongly with those of the high-J CO lines, both for the full array and for the central position, as well as with the bolometric luminosity and envelope mass. They correlate less strongly with OH fluxes and not with [O I] fluxes. In contrast, [O I] and OH often peak together at the central position. Conclusions. The PACS data probe at least two physical components. The H2O and CO emission very likely arises in non-dissociative (irradiated) shocks along the outflow walls with a range of pre-shock densities. Some OH is also associated with this component, most likely resulting from H2O photodissociation. UV-heated gas contributes only a minor fraction to the CO emission observed by PACS, based on the strong correlation between the shock-dominated CO 24-23 line and the CO 14-13 line. [O I] and some of the OH emission probe dissociative shocks in the inner envelope. The total far-infrared cooling is dominated by H2O and CO, with the fraction contributed by [O I] increasing for Class I sources. Consistent with previous studies, the ratio of total far-infrared line emission over bolometric luminosity decreases with the evolutionary state. Appendices A-J are available in electronic form at http://www.aanda.org

Karska, A.; Herczeg, G. J.; van Dishoeck, E. F.; Wampfler, S. F.; Kristensen, L. E.; Goicoechea, J. R.; Visser, R.; Nisini, B.; San José-García, I.; Bruderer, S.; ?niady, P.; Doty, S.; Fedele, D.; Y?ld?z, U. A.; Benz, A. O.; Bergin, E.; Caselli, P.; Herpin, F.; Hogerheijde, M. R.; Johnstone, D.; Jørgensen, J. K.; Liseau, R.; Tafalla, M.; van der Tak, F.; Wyrowski, F.

2013-04-01

314

Characterizing the Cool Kepler Objects of Interests. New Effective Temperatures, Metallicities, Masses, and Radii of Low-mass Kepler Planet-candidate Host Stars  

NASA Astrophysics Data System (ADS)

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 eff <~ 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 eff) and metallicities [M/H] using the K-band spectral indices of Rojas-Ayala et al. We determine the masses (M sstarf) and radii (R sstarf) 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 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.

Muirhead, Philip S.; Hamren, Katherine; Schlawin, Everett; Rojas-Ayala, Bárbara; Covey, Kevin R.; Lloyd, James P.

2012-05-01

315

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

SciTech Connect

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

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

2012-08-01

316

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

SciTech Connect

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

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

2013-10-10

317

Black Hole-Neutron Star Mergers with a Hot Nuclear Equation of State: Outflow and Neutrino-Cooled Disk for a Low-Mass, High-Spin Case  

E-print Network

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 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_{\\odot} neutron star, 5.6 M_{\\odot} black hole), high spin (black hole J/M^2=0.9) system with the K_0=220 MeV Lattimer-Swesty equation of state. We find that about 0.08 M_{\\odot} of nuclear matter is ejected from the system, while another 0.3 M_{\\odot} forms a hot, compact accretion disk. The primary effects of the escaping neutrinos are (i) to make the disk much denser and more compact, (ii) to cause the average electron fraction Y_e of the disk to rise to about 0.2 and then gradually decrease again, and (iii) to gradually cool the disk. The disk is initially hot (T~6 MeV) and luminous in neutrinos (L_{\

M. Brett Deaton; Matthew D. Duez; Francois Foucart; Evan O'Connor; Christian D. Ott; Lawrence E. Kidder; Curran D. Muhlberger; Mark A. Scheel; Bela Szilagyi

2013-04-11

318

CHARACTERIZING THE COOL KEPLER OBJECTS OF INTERESTS. NEW EFFECTIVE TEMPERATURES, METALLICITIES, MASSES, AND RADII OF LOW-MASS KEPLER PLANET-CANDIDATE HOST STARS  

SciTech Connect

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.

Muirhead, Philip S. [Current address: Department of Astronomy, California Institute of Technology, 1200 East California Boulevard, MC 249-17, Pasadena, CA 91125 (United States); Hamren, Katherine [Current address: Department of Astronomy and Astrophysics, University of California Santa Cruz, 1156 High Street, Santa Cruz, CA 95064 (United States); Schlawin, Everett; Lloyd, James P. [Department of Astronomy, Cornell University, 122 Sciences Drive, Ithaca, NY 14853 (United States); Rojas-Ayala, Barbara [Current address: Department of Astrophysics, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024 (United States); Covey, Kevin R., E-mail: philm@astro.caltech.edu [Hubble Fellow. (United States)

2012-05-10

319

Water in low-mass star-forming regions with Herschel. The link between water gas and ice in protostellar envelopes  

NASA Astrophysics Data System (ADS)

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.

Schmalzl, M.; Visser, R.; Walsh, C.; Albertsson, T.; van Dishoeck, E. F.; Kristensen, L. E.; Mottram, J. C.

2014-12-01

320

Chemistry of Low Mass Substellar Objects  

E-print Network

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

Katharina Lodders; Bruce Fegley, Jr

2006-01-17

321

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

E-print Network

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

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

2014-01-01

322

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)

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

Mottram, J. C.; Kristensen, L. E.; van Dishoeck, E. F.; Bruderer, S.; San José-García, 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

323

WISHes coming true: low-mass protostars as chemical fountains  

Microsoft Academic Search

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

L. E. Kristensen; E. van Dishoeck; U. Yildiz; R. Visser; G. Herczeg; J. Jorgensen; T. van Kempen; M. Hogerheijde

2011-01-01

324

The Young, Low-Mass Population of Orion's Belt  

NASA Astrophysics Data System (ADS)

I have completed a BVRI survey of 5 deg2 of the belt of Orion. I found that the pre-main sequence (PMS) population dominates a distinct locus of the V versus V-I color-magnitude diagram (CMD). I have statistically identified the low-mass PMS population by fitting the distribution of field and PMS stars on the V versus V-I CMD. This allowed me to count the number of PMS stars as a function of V magnitude and V-I color. While photometry alone can not select individual PMS stars with certainty, most of the stars which I identified as likely PMS stars have an 80% or greater likelihood of being PMS stars. The PMS stars of the belt consist of 2 populations. The low-mass PMS stars around ? and ? Orionis are consistent with membership in the Orion OB1b sub-association. The PMS population is best fit by a 3 Myr isochrone. The width of the PMS locus is inconsistent with an age spread greater than 2 Myrs. The low-mass stars near ? Orionis are primarily from the 10 Myr old Orion OB1a sub-association. The highest density of PMS stars is found in the ? Orionis cluster. In the region around ? Ori I identified ~350 PMS stars in the mass range 1.4>M>0.2Msun. The spatial distribution of low-mass PMS stars around ? Orionis is consistent with a King model. The radius of the cluster is about 30' or ~4pc (d=450pc). I have used the initial mass function of Kroupa (2002) to estimate the total mass, number of stars, and the number of brown dwarfs in the cluster. The total mass is ~320Msun. There are about 750 stars and 400 brown dwarfs (0.075>M>0.01Msun) in the cluster. Given the total mass of the cluster, it is probably unbound.

Sherry, W. H.

2002-12-01

325

Stochastic Gravitational Wave Background from Neutron Star r-mode Instability Revisited  

E-print Network

We revisit the possibility and detectability of a stochastic gravitational wave background (SGWB) produced by a cosmological population of newborn neutron stars (NSs) with r-mode instabilities. We show that the resultant SGWB is insensitive to the choice of CSFR models, but depends strongly on the evolving behavior of CSFR at low redshifts. Our results show that the dimensionless energy density $\\Omega_{\\rm{GW}}$ could have a peak amplitude of $\\simeq (1-3.5) \\times10^{-8}$ in the frequency range $(200-1000)$~Hz. However, such a high mode amplitude is unrealistic as it is known that the maximum value is much smaller and at most $10^{-2}$. A realistic estimate of $\\Omega_{\\rm{GW}}$ should be at least 4 orders of magnitude lower ($\\sim 10^{-12}$), which leads to a pessimistic outlook for the detection of r-mode background. We consider different pairs of terrestrial interferometers (IFOs) and compare two approaches to combine multiple IFOs in order to evaluate the detectability of this GW background. Constraints...

Zhu, Xing-Jiang; Zhu, Zong-Hong

2011-01-01

326

The ACS LCID project. X. The Star Formation History of IC 1613: Revisiting the Over-Cooling Problem  

E-print Network

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 which are too low today (the "over-cooling problem"). The depth of the present photometry of IC 1613 shows that...

Skillman, Evan D; Weisz, Daniel R; Monelli, Matteo; Gallart, Carme; Aparicio, Antonio; Bernard, Edouard J; Boylan-Kolchin, Michael; Cassisi, Santi; Cole, Andrew A; Dolphin, Andrew E; Ferguson, Henry C; Mayer, Lucio; Navarro, Julio F; Stetson, Peter B; Tolstoy, Eline

2014-01-01

327

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)

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.

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; González Hernández, 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

328

Why Low-Mass Black-Hole Binaries Are Transient  

E-print Network

We consider transient behavior in low-mass X-ray binaries. In short-period neutron-star systems (orbital period less than ~ 1d) irradiation of the accretion disk by the central source suppresses this except at very low mass transfer rates. Formation constraints however imply that a significant fraction of these neutron star systems have nuclear-evolved main-sequence secondaries and thus mass transfer rates low enough to be transient. But most short-period low-mass black-hole systems will form with unevolved main-sequence companions and have much higher mass transfer rates. The fact that essentially all of them are nevertheless transient shows that irradiation is weaker, as a direct consequence of the fundamental black-hole property - the lack of a hard stellar surface.

Andrew King; Ulrich Kolb; Ewa Szuszkiewicz

1997-05-06

329

The ACS LCID Project. X. The Star Formation History of IC 1613: Revisiting the Over-cooling Problem  

NASA Astrophysics Data System (ADS)

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. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program 10505.

Skillman, Evan D.; Hidalgo, Sebastian L.; Weisz, Daniel R.; Monelli, Matteo; Gallart, Carme; Aparicio, Antonio; Bernard, Edouard J.; Boylan-Kolchin, Michael; Cassisi, Santi; Cole, Andrew A.; Dolphin, Andrew E.; Ferguson, Henry C.; Mayer, Lucio; Navarro, Julio F.; Stetson, Peter B.; Tolstoy, Eline

2014-05-01

330

Very Low Mass Objects in Orion OB1a and b Frederick M. Walter  

E-print Network

are observing Orion OB1, the largest nearby OB association and fossil star formation region (FSFR). In FSFRs Orionis. Keywords: Stars: formation; Stars: low mass, brown dwarfs PACS: 95.75.Fg, 95.85.Jq, 95.85.Kr, 97 star forming regions, evolutionary models are unreliable, because the predicted observables

Walter, Frederick M.

331

Low-mass companions to Bright Giants  

NASA Astrophysics Data System (ADS)

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

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

2014-04-01

332

The Young Low-Mass Population of Orion's A Dissertation Presented  

E-print Network

The Young Low-Mass Population of Orion's Belt A Dissertation Presented by William Henry Sherry OB1b sub-association, a #24;3 Myr old fossil star forming re- gion located in the belt of Orion. I School. Graduate School ii #12; Abstract of the Dissertation The Young Low-Mass Population of Orion

Walter, Frederick M.

333

Testing the correlation between low mass planets and debris disks  

NASA Astrophysics Data System (ADS)

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

Kalas, Paul

2014-10-01

334

The Young, Low-Mass Population of Orion's Belt  

Microsoft Academic Search

I have completed a BVRI survey of 5 deg2 of the belt of Orion. I found that the pre-main sequence (PMS) population dominates a distinct locus of the V versus V-I color-magnitude diagram (CMD). I have statistically identified the low-mass PMS population by fitting the distribution of field and PMS stars on the V versus V-I CMD. This allowed me

W. H. Sherry

2002-01-01

335

Possibility of s-wave pion condensates in neutron stars revisited  

E-print Network

We examine possibilities of pion condensation with zero momentum (s-wave condensation) in neutron stars by using the pion-nucleus optical potential U and the relativistic mean field (RMF) models. We use low-density phenomenological optical potentials parameterized to fit deeply bound pionic atoms or pion-nucleus elastic scatterings. Proton fraction (Y_p) and electron chemical potential (mu_e) in neutron star matter are evaluated in RMF models. We find that the s-wave pion condensation hardly takes place in neutron stars and especially has no chance if hyperons appear in neutron star matter and/or b_1 parameter in U has density dependence.

A. Ohnishi; D. Jido; T. Sekihara; K. Tsubakihara

2008-10-20

336

Rotation and variability of young very low mass objects  

E-print Network

Variability studies are an important tool to investigate key properties of stars and brown dwarfs. From photometric monitoring we are able to obtain information about rotation and magnetic activity, which are expected to change in the mass range below 0.3 solar masses, since these fully convective objects cannot host a solar-type dynamo. On the other hand, spectroscopic variability information can be used to obtain a detailed view on the accretion process in very young objects. In this paper, we report about our observational efforts to analyse the variability and rotational evolution of young brown dwarfs and very low mass stars.

Alexander Scholz; Ray Jayawardhana; Jochen Eisloeffel; Dirk Froebrich

2005-08-23

337

DETECTION OF LOW-MASS-RATIO STELLAR BINARY SYSTEMS  

SciTech Connect

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.

Gullikson, Kevin; Dodson-Robinson, Sarah [Astronomy Department, University of Texas, 1 University Station C1400, Austin, TX 78712 (United States)

2013-01-01

338

Coronal Mass Ejection (CME) Activity of Low Mass M Stars as An Important Factor for The Habitability of Terrestrial Exoplanets. II. CME-Induced Ion Pick Up of Earth-like Exoplanets in Close-In Habitable Zones  

Microsoft Academic Search

Atmospheric erosion of CO2 -rich Earth-size exoplanets due to coronal mass ejection (CME)-induced ion pick up within close-in habitable zones of active M-type dwarf stars is investigated. Since M stars are active at the X-ray and extreme ultraviolet radiation (XUV) wave-lengths over long periods of time, we have applied a thermal balance model at various XUV flux input values for

Helmut Lammer; Herbert I. M. Lichtenegger; Yuri N. Kulikov; Jean-Mathias Grießmeier; N. Terada; Nikolai V. Erkaev; Helfried K. Biernat; Maxim L. Khodachenko; Ignasi Ribas; Thomas Penz; Franck Selsis

2007-01-01

339

The low-mass dispersed population around the Lupus clouds  

NASA Astrophysics Data System (ADS)

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 H? 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).

Comerón, F.; Spezzi, L.; López Martí, B.; Merín, B.

2013-06-01

340

The young low-mass population of Orion's belt  

NASA Astrophysics Data System (ADS)

I have conducted a BVRI survey covering ˜2.7 deg2 of the Orion OB1b sub-association, a ˜3 Myr old fossil star forming region located in the belt of Orion. I have developed a procedure to use single epoch photometry to estimate the number of PMS stars and field stars in the PMS locus as a function of color. I detected ˜180 likely members of the ? Orionis cluster. The cluster has a radial profile which is consistent with a King model that has a King radius of r0 ?1 pc. The central density of the cluster is ˜30 stars pc-3 in the mass range 0.2 ? M ? 1.0 M? . The ? Orionis cluster is the richest region of Orion OB1b and the only region that where a clustered population has been detected. The PMS locus for the ? Ori region is quite narrow for stars near 0.8 M? but becomes wider at lower masses. At least part of this width must be due to a combination of the intrinsic variability of the PMS stars, binary stars, and observational errors. Part of this width may be caused by the stars in the cluster having formed over a period of several Myrs. It is impossible to interpret the width of the PMS locus in terms of an age-spread without knowing the expected width of a single age (isochronal) population. I constructed several simulated populations of PMS stars that were either coeval or had a range of ages. I compared my models with the observed width of the PMS locus and concluded that the age-spread among members of the ? Ori cluster is less than one million years. I found a significant distributed population of low-mass PMS stars in the western belt with about 240 low-mass PMS stars per square degree. Roughly half the stars are members of the 11 Myr (D ˜ 330 pc) Orion OB1a sub- association which overlaps Orion OB1b. I found no evidence that these stars are arranged in clusters around ? Ori and ? Ori.

Sherry, William Henry

2003-12-01

341

The rotation of very low mass objects  

NASA Astrophysics Data System (ADS)

This dissertation contains an investigation of the rotation of very low mass objects, i.e. Brown Dwarfs and stars with masses <0.4 MS. Today, it is well-established that there are large populations of such VLM objects in open clusters and in the field, but our knowledge about their physical properties and evolution is still very limited. Contrary to their solar-mass siblings, VLM objects are fully convective throughout their evolution. Thus, they are not able to form a large-scale magnetic field like for example the sun. The magnetic field, in turn, is crucial for the regulation of rotation: Magnetic interaction between star and circumstellar disk ("disk-locking") and angular momentum losses through stellar winds have dominant influence on the rotational evolution. Thus, we can expect major differences in the rotational behaviour of VLM objects and solar-mass stars. The best method to investigate stellar rotation is to measure rotation periods. If a star exhibits surface features which are asymmetrically distributed, its brightness may be modulated with the rotation period. Thus, this dissertation is based on the analysis of photometric time series. Open clusters are an ideal environment for such a project, since they enable one to follow many objects at the same time. Additionally, they allow one to investigate the age and mass dependence of rotation, because distance and age of the clusters are known in good approximation. For this thesis, five open clusters were observed, which span an age range from 3 to 750 Myr. In three of them (SigmaOri, EpsilonOri, IC4665), VLM objects were identified by means of colour magnitude diagrams. The candidate lists for these three regions comprise at least 100 objects, for which photometry in at least three wavelength bands is available. About a fifth to a third of these candidates could be contaminating field stars in the fore- or background of the clusters. For the remaining two clusters (Pleiades and Praesepe), objects from the literature were selected as targets for the variability study. Masses for all these candidates were estimated by comparing the photometry with stellar evolutionary tracks. For each of the clusters, at least one photometric monitoring campaign was carried out; three of them were observed twice. Subsequently, the magnitudes of the VLM objects were measured relative to non-variable stars in the same fields. The difference image analysis procedure was used to improve the precision for two time series. That way, a photometric precision between 5 and 20 mmag was reached for the brightest stars. A comparison of several period search techniques showed that periodogram analysis delivers by far the best results for the available time series data. Beside the Scargle and CLEAN periodogram, the period search includes several independent and robust control procedures, to assure the reliability of the results. Additionally, a test to identify even non-periodic variability was implemented. For 87 candidates, a photometric rotation period was determined, 80 of these objects have masses <0.4 MS. Thus, this work increases the number of known VLM rotation periods in the age range between 3 and 750 Myr by a factor of 14. Altogether, about 30-50% of the candidates are variable. In the two youngest clusters, several objects show variability with very high amplitudes between 0.2 and 1.1 mag. Their lightcurves contain in the most cases a periodic component, but additionally irregular brightness variations. For two VLM stars, a flare event was detected. The origin of the periodic variability is surface features co-rotating with the objects. In most cases, these surface features are cool magnetically induced spots. From the lightcurves, it can be concluded that the spot properties change on timescales of at most two or three weeks. The amplitudes of the lightcurves are in the VLM regime by a factor of 2.4 smaller than for solar-mass stars, indicating a change of the spot properties with mass. The best explanation for this phenomenon is a more symmetric spot distributi

Scholz, Alexander

2004-10-01

342

Metallic Fingers and Metallicity Excess in Exoplanets' Host Stars: The Accretion Hypothesis Revisited  

Microsoft Academic Search

While the metallicity excess observed in the central stars of planetary systems is confirmed by all recent observations, the reason for this excess is still a subject of debate: is it primordial, or the result of accretion, or both? The basic argument against an accretion origin is related to the mass of the outer convective zones, which varies by more

Sylvie Vauclair

2004-01-01

343

Population models of low-mass binaries  

NASA Astrophysics Data System (ADS)

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

Kolb, Ulrich

2014-09-01

344

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

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.

Ma Bo; Ge Jian; De Lee, Nathan; Fleming, Scott W.; Lee, Brian L.; Wang Ji [Department of Astronomy, University of Florida, 211 Bryant Space Science Center, Gainesville, FL 32611-2055 (United States); Barnes, Rory; Agol, Eric [Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195-1580 (United States); Crepp, Justin R. [Department of Physics, University of Notre Dame, 225 Nieuwland Science Hall, Notre Dame, IN 46556 (United States); Dutra-Ferreira, Leticia; Porto de Mello, G. F. [Observatorio do Valongo, Universidade Federal do Rio de Janeiro, Ladeira do Pedro Antonio, 43, CEP: 20080-090, Rio de Janeiro, RJ (Brazil); Esposito, Massimiliano; Femenia, Bruno; Gonzalez Hernandez, Jonay I. [Instituto de Astrofisica de Canarias, C/Via Lctea S/N, E-38200 La Laguna (Spain); Gaudi, B. Scott [Department of Astronomy, Ohio State University, 140 West 18th Avenue, Columbus, OH 43210 (United States); Ghezzi, Luan [Laboratorio Interinstitucional de e-Astronomia (LIneA), Rio de Janeiro, RJ 20921-400 (Brazil); Hebb, Leslie; Stassun, Keivan G. [Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235 (United States); Wisniewski, John P. [Homer L Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W Brooks St, Norman, OK 73019 (United States); Bizyaev, Dmitry, E-mail: boma@astro.ufl.edu [Apache Point Observatory, P.O. Box 59, Sunspot, NM 88349-0059 (United States); and others

2013-01-01

345

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

SciTech Connect

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

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

2013-06-20

346

New Low-Mass Members of Nearby Young Moving Groups  

NASA Astrophysics Data System (ADS)

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

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

2012-08-01

347

The thermonuclear production of 19F by Wolf-Rayet stars revisited  

NASA Astrophysics Data System (ADS)

New models of rotating and non-rotating stars are computed for initial masses between 25 and 120 M_? and for metallicities Z = 0.004, 0.008, 0.020, and 0.040 with the aim of reexamining the wind contribution of Wolf-Rayet (WR) stars to the 19F enrichment of the interstellar medium. Models with an initial rotation velocity ?i = 300 km s-1 are found to globally eject less 19F than the non-rotating models. We compare our new predictions with those of Meynet & Arnould (2000, A&A, 355, 176), and demonstrate that the 19F yields are very sensitive to the still uncertain 19F (? ,p) 22Ne rate and to the adopted mass loss rates. Using the recommended mass loss rate values that take into account the clumping of the WR wind and the NACRE reaction rates, when available, we obtain WR 19F yields that are significantly lower than predicted by Meynet & Arnould (2000, A&A, 355, 176) and that would make WR stars non-important contributors to the galactic 19F budget. In view, however, of the large nuclear and mass loss rate uncertainties, we consider that the question of the WR contribution to the galactic 19F remains quite open.

Palacios, A.; Arnould, M.; Meynet, G.

2005-11-01

348

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

SciTech Connect

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

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

2013-06-15

349

Nitrogen line spectroscopy of O-stars. I. Nitrogen III emission line formation revisited  

NASA Astrophysics Data System (ADS)

Context. Evolutionary models of massive stars predict a surface enrichment of nitrogen, due to rotational mixing. Recent studies within the VLT-FLAMES survey of massive stars have challenged (part of) these predictions. Such systematic determinations of nitrogen abundances, however, have been mostly performed only for cooler (B-type) objects. For the most massive and hottest stars, corresponding results are scarce. Aims: This is the first paper in a series dealing with optical nitrogen spectroscopy of O-type stars, aiming at the analysis of nitrogen abundances for stellar samples of significant size, to place further constraints on the early evolution of massive stars. Here we concentrate on the formation of the optical N iii lines at ??4634-4640-4642 that are fundamental for the definition of the different morphological "f"-classes. Methods: We implement a new nitrogen model atom into the NLTE atmosphere/spectrum synthesis code fastwind, and compare the resulting optical Niii spectra with other predictions, mostly from the seminal work by Mihalas & Hummer (1973, ApJ, 179, 827, "MH"), and from the alternative code cmfgen. Results: Using similar model atmospheres as MH (not blanketed and wind-free), we are able to reproduce their results, in particular the optical triplet emission lines. According to MH, these should be strongly related to dielectronic recombination and the drain by certain two-electron transitions. However, using realistic, fully line-blanketed atmospheres at solar abundances, the key role of the dielectronic recombinations controlling these emission features is superseded - for O-star conditions - by the strength of the stellar wind and metallicity. Thus, in the case of wind-free (weak wind) models, the resulting lower ionizing EUV-fluxes severely suppress the emission. As the mass loss rate is increased, pumping through the N iii resonance line(s) in the presence of a near-photospheric velocity field (i.e., the Swings-mechanism) results in a net optical triplet line emission. A comparison with results from cmfgen is mostly satisfactory, except for the range 30 000 K ? Teff ? 35 000 K, where cmfgen triggers the triplet emission at lower Teff than fastwind. This effect could be traced down to line overlap effects between the N iii and O iii resonance lines that cannot be simulated by fastwind so far, due to the lack of a detailed O iii model atom. Conclusions: Since the efficiency of dielectronic recombination and "two electron drain" strongly depends on the degree of line-blanketing/-blocking, we predict the emission to become stronger in a metal-poor environment, though lower wind-strengths and nitrogen abundances might counteract this effect. Weak winded stars (if existent in the decisive parameter range) should display less triplet emission than their counterparts with "normal" winds. Appendices A-C are available in electronic form at http://www.aanda.org

Rivero González, J. G.; Puls, J.; Najarro, F.

2011-12-01

350

Coronal mass ejection (CME) activity of low mass M stars as an important factor for the habitability of terrestrial exoplanets. II. CME-induced ion pick up of Earth-like exoplanets in close-in habitable zones.  

PubMed

Atmospheric erosion of CO2-rich Earth-size exoplanets due to coronal mass ejection (CME)-induced ion pick up within close-in habitable zones of active M-type dwarf stars is investigated. Since M stars are active at the X-ray and extreme ultraviolet radiation (XUV) wave-lengths over long periods of time, 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 CO2 infrared radiation in the 15 microm band. Our study shows that intense XUV radiation of active M stars results in atmospheric expansion and extended exospheres. Using thermospheric neutral and ion densities calculated for various XUV fluxes, we applied a numerical test particle model for simulation of atmospheric ion pick up loss from an extended exosphere arising from its interaction with expected minimum and maximum CME plasma flows. Our results indicate that the Earth-like exoplanets that have no, or weak, magnetic moments may lose tens to hundreds of bars of atmospheric pressure, or even their whole atmospheres due to the CME-induced O ion pick up at orbital distances

Lammer, Helmut; Lichtenegger, Herbert I M; Kulikov, Yuri N; Griessmeier, Jean-Mathias; Terada, N; Erkaev, Nikolai V; Biernat, Helfried K; Khodachenko, Maxim L; Ribas, Ignasi; Penz, Thomas; Selsis, Franck

2007-02-01

351

Observational dynamics of low-mass stellar systems  

E-print Network

The last fifteen years have seen the discovery of new types of low-mass stellar systems that bridge the gap between the once well-separated regimes of galaxies and of star clusters. Whether such objects are considered galaxies depends also on the definition of the term "galaxy", and several possible criteria are based on their internal dynamics (e.g. the common concept that galaxies contain dark matter). Moreover, studying the internal dynamics of low-mass stellar systems may also help understand their origin and evolutionary history. The focus of this paper is on two classes of stellar systems at the interface between star clusters and dwarf galaxies: ultra-compact dwarf galaxies (UCDs) and diffuse Galactic globular clusters (GCs). A review of our current knowledge on the properties of UCDs is provided and dynamical considerations applying to diffuse GCs are introduced. In the following, recent observational results on the internal dynamics of individual UCDs and diffuse Galactic globular clusters are presen...

Frank, Matthias J

2014-01-01

352

Methanol abundance in low mass protostars  

E-print Network

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

S. Maret

2004-11-12

353

Asteroid 21 Lutetia: Low Mass, High Density  

E-print Network

in 1852, is one of the larger main-belt asteroids. In 2004, it became the flyby target as- teroid for the Rosetta spacecraft mission. An important characteristic of an asteroid is its bulk density, derived fromAsteroid 21 Lutetia: Low Mass, High Density M. Pätzold,1 * T. P. Andert,2 S. W. Asmar,3 J. D

Weiss, Benjamin P.

354

Low mass rolling element for bearings  

NASA Technical Reports Server (NTRS)

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

Parker, R. J. (inventor)

1973-01-01

355

Effect of low mass dark matter particles on the Sun  

SciTech Connect

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.

Taoso, Marco [IFIC-CSIC-Universitat de Valencia, Ed. Instituts, Apt. 22085, 46071 Valencia (Spain); Iocco, Fabio [Institut d'Astrophysique de Paris, UMR 7095-CNRS, Universite Pierre et Marie Curie, 98 bis Boulevard Arago 75014, Paris (France); Meynet, Georges; Eggenberger, Patrick [Geneva Observatory, University of Geneva, Maillettes 51, 1290 Sauverny (Switzerland); Bertone, Gianfranco [Institut d'Astrophysique de Paris, UMR 7095-CNRS, Universite Pierre et Marie Curie, 98 bis Boulevard Arago 75014, Paris (France); Institute fuer Theoretische Physik, Universitaet Zuerich, Winterthurerstrasse 190, CH-8057 Zuerich (Switzerland)

2010-10-15

356

A Unified Model of Low Mass X-ray Binaries  

NASA Astrophysics Data System (ADS)

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.

Balucinska-Church, M.; Church, M.

2014-07-01

357

WISHes coming true: low-mass protostars as chemical fountains  

NASA Astrophysics Data System (ADS)

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.

Kristensen, L. E.; van Dishoeck, E.; Yildiz, U.; Visser, R.; Herczeg, G.; Jorgensen, J.; van Kempen, T.; Hogerheijde, M.; WISH Team

2011-05-01

358

Calorimetry of low mass Pu239 items  

SciTech Connect

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

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

2010-01-01

359

An Observational Perspective of Low-Mass Dense Cores I: Internal Physical and Chemical Properties  

E-print Network

Low-mass dense cores represent the state of molecular gas associated with the earliest phases of low-mass star formation. Such cores are called "protostellar" or "starless," depending on whether they do or do not contain compact sources of luminosity. In this chapter, the first half of the review of low-mass dense cores, we describe the numerous inferences made about the nature of starless cores as a result of recent observations, since these reveal the initial conditions of star formation. We focus on the identification of isolated starless cores and their internal physical and chemical properties, including morphologies, densities, temperatures, kinematics, and molecular abundances. These objects display a wide range of properties since they are each at different points on evolutionary paths from ambient molecular cloud material to cold, contracting, and centrally concentrated configurations with significant molecular depletions and, in rare cases, enhancements.

J. Di Francesco; N. J. Evans II; P. Caselli; P. C. Myers; Y. Shirley; A. Aikawa; M. Tafalla

2006-02-17

360

The discovery of a low mass, pre-main-sequence stellar association around gamma Velorum  

E-print Network

We report the serendipitous discovery of a population of low mass, pre-main sequence stars (PMS) in the direction of the Wolf-Rayet/O-star binary system gamma^{2} Vel and the Vela OB2 association. We argue that gamma^{2} Vel and the low mass stars are truly associated, are approximately coeval and that both are at distances between 360-490 pc, disagreeing at the 2 sigma level with the recent Hipparcos parallax of gamma^{2} Vel, but consistent with older distance estimates. Our results clearly have implications for the physical parameters of the gamma^{2} Vel system, but also offer an exciting opportunity to investigate the influence of high mass stars on the mass function and circumstellar disc lifetimes of their lower mass PMS siblings.

M. Pozzo; R. D. Jeffries; T. Naylor; E. J. Totten; S. Harmer; M. Kenyon

2000-02-17

361

Chemistry in low-mass protostellar and protoplanetary regions  

PubMed Central

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

van Dishoeck, Ewine F.

2006-01-01

362

The SDSS\\/2MASS Low Mass Stellar Luminosity Function: The Calibration Region  

Microsoft Academic Search

We present an initial measurement of the luminosity and mass functions of low mass stars as constructed from a catalog of matched Sloan Digital Sky Survey (SDSS) and 2 Micron All Sky Survey (2MASS) point sources. We have assembled a photometric catalog of 25,000+ matched SDSS and 2MASS point sources, spanning over 30 square degrees on the sky. We have

K. R. Covey; S. L. Hawley; J. J. Bochanski

2005-01-01

363

The Circumstellar Environment of Low Mass Star Forming Regions  

NASA Technical Reports Server (NTRS)

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.

Butner, Harold M.

1999-01-01

364

The Circumstellar Environment of Low Mass Star Forming Regions  

NASA Technical Reports Server (NTRS)

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.

Butner, Harold M.

1997-01-01

365

Shock Chemistry in Low-Mass Star-Forming Regions  

NASA Astrophysics Data System (ADS)

We have conducted spectral line surveys in the 3 mm band with the 45 m telescope of the Nobeyama Radio Observatory toward the L1157 B1 and L1448 B1/R1 shocked regions. L1157 B1 is a famous shocked region formed by the interaction between the outflow from the protostar IRAS 20386+6751 and the ambient gas, while L1448 B1/R1 are formed from internal bow shocks within the extremely energetic jet from the protostar L1448 mm. As a result, we found a grate difference of the chemical composition between the two shocked regions.

Yamaguchi, T.; Takano, S.; Sakai, N.; Watanabe, Y.; Yamamoto, S.; NRO Line Survey Team Members

2013-10-01

366

MAPPING THE LOCAL HALO: STATISTICAL PARALLAX ANALYSIS OF SDSS LOW-MASS SUBDWARFS  

SciTech Connect

We present a statistical parallax study of nearly 2000 M subdwarfs with photometry and spectroscopy from the Sloan Digital Sky Survey (SDSS). Statistical parallax analysis yields the mean absolute magnitudes, mean velocities, and velocity ellipsoids for homogenous samples of stars. We selected homogeneous groups of subdwarfs based on their photometric colors and spectral appearance. We examined the color-magnitude relations of low-mass subdwarfs and quantified their dependence on the newly refined metallicity parameter, {zeta}. We also developed a photometric metallicity parameter, {delta}{sub (g-r)}, based on the g - r and r - z colors of low-mass stars and used it to select stars with similar metallicities. The kinematics of low-mass subdwarfs as a function of color and metallicity were also examined and compared to main-sequence M dwarfs. We find that the SDSS subdwarfs share similar kinematics to the inner halo and thick disk. The color-magnitude relations derived in this analysis will be a powerful tool for identifying and characterizing low-mass metal-poor subdwarfs in future surveys such as Gaia and LSST, making them important and plentiful tracers of the stellar halo.

Bochanski, John J. [Haverford College, 370 Lancaster Ave., Haverford, PA 19041 (United States); Savcheva, Antonia; West, Andrew A. [Department of Astronomy, Boston University, 725 Commonwealth Avenue, Boston, MA 02215 (United States); Hawley, Suzanne L., E-mail: jbochans@haverford.edu [Astronomy Department, University of Washington, Box 351580, Seattle, WA 98195 (United States)

2013-02-01

367

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

SciTech Connect

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

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

2010-01-01

368

Low Mass Members in Nearby Young Moving Groups Revealed  

NASA Astrophysics Data System (ADS)

We are now ready to expand our program that identifies highly probable low-mass members of the nearby young moving groups (NYMGs) to stars of mass ~ 0.1 Msun. This is important 1) To provide high priority targets for exoplanet searches by direct imaging, 2) To complete the census of the membership in the NYMGs, and 3) To provide a well-characterized sample of nearby young stars for detailed study of their physical properties and multiplicity (the median distances of the (beta) Pic and AB Dor groups are ~ 35 pc with ages ~ 12 and 50 Myr respectively). Our proven technique starts with a proper motion selection algorithm, proceeds to vet the sample for indicators of youth, and requires as its last step the measurement of candidate member radial velocities (RVs). So far, we have obtained all RV measurements with the high resolution IR spectrometer at the NASA-IRTF and have reached the limits of its applicability. To identify probable new members in the south, and also those of the lowest mass, we need the sensitivity of PHOENIX at Gemini-S and NIRSPEC at Keck-II.

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

2010-08-01

369

Modeling water emission from low-mass protostellar envelopes  

E-print Network

Within low-mass star formation, water vapor plays a key role in the chemistry and energy balance of the circumstellar material. The Herschel Space Observatory will open up the possibility to observe water lines originating from a wide range of excitation energies.Our aim is to simulate the emission of rotational water lines from envelopes characteristic of embedded low-mass protostars. A large number of parameters that influence the water line emission are explored: luminosity, density,density slope and water abundances.Both dust and water emission are modelled using full radiative transfer in spherical symmetry. The temperature profile is calculated for a given density profile. The H2O level populations and emission profiles are in turn computed