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1

Infrared Detection of Very Low Mass Stars  

Microsoft Academic Search

We present in this thesis a review of very-low -mass ((TURN)0.1 M(,0)) star research, and results of two observational programs directed at the photometric detection of low mass binary companions in the infrared. Present theoretical desiderata are model atmospheres for very cool dwarf stars and determination of the minimum protostellar mass with all relevant physics included. Luminosities for these stars

Ronald George Probst

1981-01-01

2

Gravitational waves from low mass neutron stars  

SciTech Connect

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

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

2010-05-15

3

Low-mass Star Formation: Observations  

NASA Astrophysics Data System (ADS)

I briefly review recent observations of regions forming low mass stars. The discussion is cast in the form of seven questions that have been partially answered, or at least illuminated, by new data. These are the following: where do stars form in molecular clouds; what determines the IMF; how long do the steps of the process take; how efficient is star formation; do any theories explain the data; how are the star and disk built over time; and what chemical changes accompany star and planet formation. I close with a summary and list of open questions.

Evans, Neal J.

2011-04-01

4

Spectroscopic Observations of Nearby Low Mass Stars  

NASA Astrophysics Data System (ADS)

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

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

2014-01-01

5

Infrared Studies of Low Mass Star Formation  

NASA Astrophysics Data System (ADS)

My thesis consists of seven infrared studies of low mass star formation. One chapter describes an all sky survey of young stars in the IRAS Point Source Catalog (IPSC). Three chapters contain studies of the Chamaeleon I star forming region. Two chapters concern star formation in the Chamaeleon II molecular cloud. Finally, one chapter deals with star formation in B209, which is a globule in the Taurus clouds. I will summarize the results of these studies in the above mentioned four categories. Advanced statistical classification methods have been used to extract a set of 5962 young stellar object candidates from the IPSC. In the selection procedure clustering properties on the sky were used in addition to the usually explored IRAS colours. A performance analysis indicates that in low mass star forming regions 87% of these objects are indeed young stars. All IRAS catalogues (IPSC, IRAS Serendipitous Survey Catalog and IRAS Faint Source Survey) have been searched for young stars born in the Chamaeleon I molecular cloud. These studies have led to the discovery of the exciting source of Herbig-Haro objects 49 and 50. Ground-based near-infrared photometry has been obtained for a majority of Chamaeleon I members in order to construct spectral energy distributions. Bolometric luminosities have been estimated for 62 out of the 81 known members to construct a luminosity function (LF). A comparison of the LF with that of the ? Ophiuchi infrared cluster suggests that star formation has evolved further in Chamaelon I. Young stars in the Chamaeleon II star forming region have been measured in the near infrared and IRAS data has been extracted in order to construct spectral energy distributions and luminosity estimates. An additional search based on the IRAS data has been performed to find new young stellar object candidates. The spatial distribution of young stars and candidate members shows that star formation is more widespread in Chamaelon II than in the well-studied Chamaeleon I. A study of star formation in B209 revealed two new embedded sources belonging to the group of young stars born in the Taurus region. This result from a small area of the whole cloud complex suggests that our current sample of embedded objects in Taurus may be severely incomplete. One of the new objects is of sub-solar luminosity, but yet it drives a relatively strong molecular outflow. It is possible that the known sample of luminous embedded sources exciting molecular outflows has a large number of low luminosity counterparts, which are extremely difficult to discover. The embedded sources may thus have a similar kind of luminosity function as the T Tauri stars making them physically more similar to pre-main-sequence stars than to protostars. (If interested in a personal copy of my thesis, please, contact the above mentioned E-mail address.)

Prusti, Timo

1992-01-01

6

Measuring the Distance to Low-Mass Star Forming Cores  

NASA Astrophysics Data System (ADS)

The study of low-mass star formation has made great advances in the past decade with the advent of space and ground-based telescopes. However, the understanding of these regions is often hindered by a lack of knowledge of the star-forming cores' distance. We present a simple plan and preliminary results to determine the distance to areas of low-mass star formation.

Young, Chadwick; Garber, Leah; Bourke, Tyler

2010-10-01

7

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

8

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

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 luminosity function for low mass stars  

NASA Astrophysics Data System (ADS)

This paper describes a photometric survey for late M-dwarfs from COSMOS measures of R- and I-band UK Schmidt plates. The survey covers some 85 square degrees and includes about 500 M-stars within 100 pc. The luminosity function is sampled down to MR ? 17. In addition, spectra have been obtained for the six reddest stars, which have confirmed the low temperatures deduced from the photometry. Proper motions for two-thirds of the sample stars have been measured and indicate that the reddest stars are younger than the sample as a whole. The observed luminosity function has been compared with theoretical functions from the literature. There are some puzzling discrepancies for the more luminous stars, but there is unambiguous evidence for a steadily rising mass function towards the sample limit and the observations suggest that the local missing mass is in the form of low luminosity stars. The evolutionary status of the low luminosity stars in the sample is examined in the light of two hypotheses.

Hawkins, M. R. S.; Bessell, M. S.

1988-09-01

11

Mass and radius formulas for low-mass neutron stars  

NASA Astrophysics Data System (ADS)

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

Sotani, Hajime; Iida, Kei; Oyamatsu, Kazuhiro; Ohnishi, Akira

2014-05-01

12

The Formation of Low-Mass Stars and Brown Dwarfs  

NASA Astrophysics Data System (ADS)

It is estimated that ˜ 60 % of all stars (including brown dwarfs) have masses below 0. 2 M?. Currently, there is no consensus on how these objects form. I will briefly review the four main theories for the formation of low-mass objects: turbulent fragmentation, ejection of protostellar embryos, disc fragmentation, and photo-erosion of prestellar cores. I will focus on the disc fragmentation theory and discuss how it addresses critical observational constraints, i.e. the low-mass initial mass function, the brown dwarf desert, and the binary statistics of low-mass stars and brown dwarfs. I will examine whether observations may be used to distinguish between different formation mechanisms, and give a few examples of systems that strongly favour a specific formation scenario. Finally, I will argue that it is likely that all mechanisms may play a role in low-mass star and brown dwarf formation.

Stamatellos, Dimitris

13

Lithuim in Very Low-Mass Stars in the Pleiades  

Microsoft Academic Search

High-resolution, Keck Telescope echelle observations from 630 nm to 850 nm of seven Pleiads with spectral types from M5 to M6.5 reveal rather rapid rotation, with an average v sin i ~ 52 km s(-1) , and chromospheric activity in Halpha emission. The activity in these stars is not any stronger than that of other Pleiades low-mass stars, despite the

B. R. Oppenheimer; G. Basri; T. Nakajima; S. R. Kulkarni

1997-01-01

14

Lithuim in Very Low-Mass Stars in the Pleiades  

NASA Astrophysics Data System (ADS)

High-resolution, Keck Telescope echelle observations from 630 nm to 850 nm of seven Pleiads with spectral types from M5 to M6.5 reveal rather rapid rotation, with an average v sin i ~ 52 km s(-1) , and chromospheric activity in H? emission. The activity in these stars is not any stronger than that of other Pleiades low-mass stars, despite the expected high contrast of H? with their cool photospheres and their rapid rotation. This shows that the ``levelling off'' of H? equivalent widths previously noted in low-mass stars in young clusters is not related to the conventional rotation-activity connection. None of the stars previously categorized as brown dwarf candidates have lithium signatures in their spectra. They are, therefore, very low-mass stars and not brown dwarfs. However, two stars, HHJ 339 and HHJ 430, 1 and 2 magnitudes above the Pleiades zero-age main sequence, do show absorption due to Li 1 at 670.8 nm and in the subordinate feature at 812.6 nm. These two stars are also rotating very rapidly. These facts strongly suggest that these stars are rather young. Their proper motions and radial velocities agree with those measured for the Pleiades as a whole. We discuss various explanations for these stars, none of which is completely satisfactory. In one scenario they represent very late star formation in the Pleiades cluster (implying a huge range in the ages of Pleiads). This seems unpalatable given the lack of matter dense enough to form stars in the Pleiades at present. Another possibility is that these stars formed in a nearby, more recent star formation site and drifted into the Pleiades. Although the cluster recently passed through a clump of young Taurus stars, we do not see how it could ``accrete'' two of them. In our most feasible explanation, we posit that a cloud which was a member of the ``Pleiades Supercluster'' recently formed stars, which are now scattered between us and the Pleiades. HHJ 339 and HHJ 430 could be members of this group whose motion has now brought them near the older open star cluster.

Oppenheimer, B. R.; Basri, G.; Nakajima, T.; Kulkarni, S. R.

1997-01-01

15

Luminosity functions for very low mass stars and brown dwarfs  

NASA Technical Reports Server (NTRS)

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

Laughlin, Gregory; Bodenheimer, Peter

1993-01-01

16

X-ray flares in Orion low-mass stars  

NASA Astrophysics Data System (ADS)

Context: X-ray flares are common phenomena in pre-main sequence stars. Their analysis gives insights into the physics at work in young stellar coronae. The Orion Nebula Cluster offers a unique opportunity to study large samples of young low mass stars. This work is part of the Chandra Orion Ultradeep project (COUP), an ~10 day long X-ray observation of the Orion Nebula Cluster (ONC). Aims: Our main goal is to statistically characterize the flare-like variability of 165 low mass (0.1-0.3 M?) ONC members in order to test and constrain the physical scenario in which flares explain all the observed emission. Methods: We adopt a maximum likelihood piece-wise representation of the observed X-ray light curves and detect flares by taking into account both the amplitude and time derivative of the count-rate. We then derive the frequency and energy distribution of the flares. Results: The high energy tail of the energy distribution of flares is well described by a power-law with index ~2.2. We test the hypothesis that light curves are built entirely by overlapping flares with a single power law energy distribution. We constrain the parameters of this simple model for every single light curve. The analysis of synthetic light curves obtained from the model indicates a good agreement with the observed data. Comparing low mass stars with stars in the mass interval (0.9-1.2 M?), we establish that, at ~1 Myr, low mass and solar mass stars of similar X-ray luminosity have very similar flare frequencies. Conclusions: Our observational results are consistent with the following model/scenario: the light curves are entirely built by overlapping flares with a power-law intensity distribution; the intense flares are individually detected, while the weak ones merge and form a pseudo-quiescent level, which we indicate as the characteristic level.

Caramazza, M.; Flaccomio, E.; Micela, G.; Reale, F.; Wolk, S. J.; Feigelson, E. D.

2007-08-01

17

Young low mass stars in the vicinity of Sigma Scorpii  

NASA Technical Reports Server (NTRS)

The region near Sigma Scorpii, a member of the Sco-Cen OB association, is examined for signs of recent star formation. Thirteen candidate young stellar objects are identified over an 80 x 80 arcmin region centered on Sigma Sco using the Point source Catalog and a recent survey for H-alpha emission-line stars. Near-infrared photometry, improved IRAS fluxes, and optical spectra are used to determine the nature of these objects. Four definite young stars are revealed, as well as one additional such object that fell just outside of the present target region. These stars, all of spectral type K or M, are argued to have formed in the vicinity of the B1 giant star Sigma Scorpii and to represent a subsample of the low-mass members of the association.

Meyer, Michael R.; Wilking, Bruce A.; Zinnecker, Hans

1993-01-01

18

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

19

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

20

Pulsation in extremely low-mass helium stars  

NASA Astrophysics Data System (ADS)

We explore the stability of extremely low-mass stars (M < 0.25 M ?) across a wide range of composition, effective temperature, and luminosity. We identify the instability boundaries associated with radial oscillations. These are a strong function of both composition and radial order (0 <= n <= 13). The classical blue edge shifts to higher effective temperature and luminosity with decreasing hydrogen abundance. Higher-order modes are more easily excited, and small islands of instability develop. Short-period oscillations have been discovered in the low-mass pre-white dwarf component of the eclipsing binary J0247-25. If its envelope is depleted in hydrogen, J0247-25B is unstable to intermediate-order p modes. Driving is by the classical ? mechanism operating in the second helium ionization zone. The observed periods, temperature and luminosity of J0247-25B require an envelope hydrogen abundance 0.2 <= X <= 0.3.

Jeffery, C. S.; Saio, H.

2014-02-01

21

Low-mass stars: Open problems all along their evolution  

NASA Astrophysics Data System (ADS)

Although low-mass stars have a comparatively simple structure, current stellar models are far from reproducing them accurately. In light of the observational progress, in particular of asteroseismology, the deficits of the theory of stellar structure and evolution become increasingly evident, and can no longer be hidden under the carpet of observational errors. In my brief review I will discuss a number of obvious problems of the models, which are mainly - and not surprisingly so - connected with convective and other mixing processes. They begin already on the pre-main sequence and continue throughout the complete evolution. In addition to the deficits in the treatment of physical processes I will also address our difficulties in obtaining completely satisfying agreement between different numerical codes. It will be shown that this concerns already the main sequence evolution of low-mass stars without a convective core. Keeping this in mind, it is no surprise that models for stars on the Asymptotic Giant Branch differ widely.

Weiss, A.; Heners, N.

2013-03-01

22

Atmospheres From Very Low-Mass Stars to Extrasolar Planets  

NASA Astrophysics Data System (ADS)

Within the next few years, several instruments aiming at imaging extrasolar planets will see first light. In parallel, low mass planets are being searched around red dwarfs which offer more favorable conditions - both for radial velocity detection and transit studies - than solar-type stars. We review recent advancements in modeling the stellar to substellar transition. The revised solar oxygen abundances and cloud models allow to reproduce the photometric and spectroscopic properties of this transition to a degree never achieved before, but problems remain at the stellar-brown dwarf transition typical of the Teff range of characterizable exoplanets.

Allard, F.; Homeier, D.; Freytag, B.; Sharp, C. M.

2012-11-01

23

Radial Velocities of Low-mass Stars Using Telluric Lines  

NASA Astrophysics Data System (ADS)

The search for companions to the lowest mass stars offers us an exciting pathway toward the detection of Earth-like planets. Low mass-stars are intrinsically small and cool, motivating the development of observational techniques optimized for the deep red (700 to 1000 nm) and infrared wavelengths where these stars are brightest. However, Earth's atmosphere absorbs strongly in these spectral regions, and telluric features due to O2, H2O, CH4, and CO2 dominate significant portions of ground-based spectra at red wavelengths. For the specific application of radial velocity (RV) measurements of low-mass stars, wavelengths where there is significant telluric absorption present a distinct advantage. The telluric lines provide a rich set of absorption features that can readily serve as a simultaneous absorption reference, yielding RV measurements of modest precision using existing instrumentation. We describe the theoretical expectations regarding the fundamental limits of telluric lines as a "zero velocity" wavelength reference and present results from detailed simulations of absorption by Earth's atmosphere as well as the impact of atmospheric variations on RV precision. We present preliminary results from a deep red optical survey of 100 late-M stars that relies on telluric water vapor as a simultaneous reference. We demonstrate that long-term stability of 10 m/s may be achievable using a typical echelle spectrograph, theoretical models of the composition of Earth's atmosphere, and some basic assumptions about the time variability of the atmosphere. This work has been supported by the National Science Foundation through an Astronomy and Astrophysics Postdoctoral Fellowship.

Blake, Cullen; Shaw, M.

2011-09-01

24

Dust discs around low-mass main-sequence stars  

NASA Technical Reports Server (NTRS)

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

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

1988-01-01

25

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

26

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

27

Naked T Tauri stars - the low-mass pre-main sequence unveiled  

SciTech Connect

The search for low-mass premain-sequence (PMS) stars associated with X-ray sources in regions of star formation is discussed. The survey to date has revealed at least 30 low-mass PMS stars in the Tau-Aur region, and a comparable number in Oph. These stars are the naked T Tau stars, unveiled versions of the well-known classical T Tau stars. The properties of these newly discovered PMS stars and their relation to the classical T Tau stars are discussed, and it is concluded that the naked T Tau stars are the true low-mass PMS stars, and that the observable characteristics defining the classical T Tau stars are due to the interaction of an underlying, fairly normal star with a dominant circumstellar environment. The impact the naked T Tau stars are likely to have on models of the PMS evolution of low-mass stars is considered. 24 references.

Walter, F.M.

1987-01-01

28

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

NASA Technical Reports Server (NTRS)

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

Walter, Frederick M.

1987-01-01

29

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

30

Low-Mass Quark Stars or Quark White Dwarfs  

NASA Astrophysics Data System (ADS)

An equation of state is considered that, in superdense nuclear matter, results in a phase transition of the first kind from the nucleon state to the quark state with a transition parameter > 3/2 ( = Q /( N + P 0/c 2)). A calculation of the integrated parameters of superdense stars on the basis of this equation of state shows that on the stable branch of the dependence of stellar mass on central pressure (dM/dP c > 0), in the low-mass range, following the formation of a tooth-shaped break (M = 0.08 M ?, R = 200 km) due to quark formation, a new local maximum with M max = 0.082 M ? and R = 1251 km is also formed. The mass and radius of the quark core of such a star turn out to be M core = 0.005 M ? and R core = 1.7 km, respectively. Mass accretion in this model can result in two successive transitions to a neutron star with a quark core, with energy release like supernova outbursts.

Alaverdyan, G. B.; Harutyunyan, A. R.; Vartanyan, Yu. L.

2001-04-01

31

Multiplicity among Young Brown Dwarfs and Very Low Mass Stars  

NASA Astrophysics Data System (ADS)

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

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

2007-12-01

32

Structure and evolution of low-mass Population II stars  

NASA Astrophysics Data System (ADS)

The focus of the present paper is on the detailed description of the internal structures of low mass, population II stars, to clarify some issues about these stellar models and, mainly, their present reliability for observational comparisons. We then explore 1) the role of the local convective model; 2) the differences between "grey" and "non grey" models, and between models in which the photospheric boundary conditions are set at different optical depths (?ph = 3 or 100); 3) the role of the equation of state (EoS), both in the atmospheric models and in the interior. One of the major conclusions of the paper is a cautionary note about the usage of the additive volume law in EoS calculations. The dependence of the HR diagram locations and mass luminosity relations on metal and helium content are also discussed. A few comparisons with globular cluster stars show that: 1) general consistency of distance scales and morphologies in the HR diagram is found, when comparing ground based measurements in the Johnson B and V bands and observations in the HST bands; 2) a discrepancy between models and observations may exist for more metal rich clusters; 3) the plausible hypothesis that the mass function in the globular cluster NGC 6397 behaves smoothly until the lower limit of the main sequence poses constraints on the mass-luminosity relation at the lowest end of the main sequence. The evolutionary tracks are available at the WEB location http://www.mporzio.astro.it.

Montalbán, J.; D'Antona, F.; Mazzitelli, I.

2000-08-01

33

Exploring the Earliest Stage of Low-Mass Star Formation  

NASA Astrophysics Data System (ADS)

The authors present a series of studies to shed light on the earliest stage of low-mass star formation process. Their intensive survey has identified such a protostar GF 9-2 that shows the H2O maser emission, a clear signpost of protostar formation, but does not have well-developed molecular outflow. These facts indicate that the natal cloud core harboring the protostar still retains some information of the initial conditions for gravitational collapse as the core has not being dispersed by the onset of outflow. Combining single-dish radio telescope and interferometric data, they have obtained high-fidelity 3-dimensional images, yielding to analyze the density and velocity structure of the core from 0.1pc down to 0.003pc in size. Furthermore, they found clear spectroscopic evidence for the presence of infall toward the central protostar. These results have a reasonable consistency with the initial conditions given in one of the extreme paradigms for core collapse scenario - the runaway collapse solution - debated over the four decades. The authors believe that the core has been undergoing gravitational collapse for about 5,000 yrs since the protostar formation and that the gravitationally unstable state initiated the collapse 2 ×105 yrs (the free-fall time) ago.

Furuya, Ray S.

2008-12-01

34

The role of low-mass star clusters in forming the massive stars in DR 21  

NASA Astrophysics Data System (ADS)

We have studied the young low-mass pre-main sequence (PMS) stellar population associated with the massive star-forming region DR 21 by using archival X-ray Chandra observations and by complementing them with existing optical and infrared (IR) surveys. The Chandra observations have revealed for the first time a new highly extincted population of PMS low-mass stars previously missed in observations at other wavelengths. The X-ray population exhibits three main stellar density peaks, coincident with the massive star-forming regions, being the DR 21 core the main peak. The cross-correlated X-ray/IR sample exhibits a radial `Spokes-like' stellar filamentary structure that extends from the DR 21 core towards the northeast. The near-IR data reveal a centrally peaked structure for the extinction, which exhibits its maximum in the DR 21 core and gradually decreases with the distance to the N-S cloud axis and to the cluster centre. We find evidence of a global mass segregation in the full low-mass stellar cluster, and of a stellar age segregation, with the youngest stars still embedded in the N-S cloud, and more evolved stars more spatially distributed. The results are consistent with the scenario where an elongated overall potential well created by the full low-mass stellar cluster funnels gas through filaments feeding stellar formation. Besides the full gravitational well, smaller scale local potential wells created by dense stellar sub-clusters of low-mass stars are privileged in the competition for the gas of the common reservoir, allowing the formation of massive stars. We also discuss the possibility that a stellar collision in the very dense stellar cluster revealed by Chandra in the DR 21 core is the origin of the large-scale and highly energetic outflow arising from this region.

Rivilla, V. M.; Jiménez-Serra, I.; Martín-Pintado, J.; Sanz-Forcada, J.

2014-01-01

35

Very low mass stars and white dwarfs in NGC 6397  

NASA Technical Reports Server (NTRS)

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

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

1995-01-01

36

Ionization of the diffuse gas in galaxies: hot low-mass evolved stars at work  

NASA Astrophysics Data System (ADS)

We revisit the question of the ionization of the diffuse medium in late-type galaxies, by studying NGC 891, the prototype of edge-on spiral galaxies. The most important challenge for the models considered so far was the observed increase of [O III]/H?, [O II]/H? and [N II]/H? with increasing distance to the galactic plane. We propose a scenario based on the expected population of massive OB stars and hot low-mass evolved stars (HOLMES) in this galaxy to explain this observational fact. In the framework of this scenario we construct a finely meshed grid of photoionization models. For each value of the galactic altitude z we look for the models which simultaneously fit the observed values of the [O III]/H?, [O II]/H? and [N II]/H? ratios. For each value of z we find a range of solutions which depends on the value of the oxygen abundance. The models which fit the observations indicate a systematic decrease of the electron density with increasing z. They become dominated by the HOLMES with increasing z only when restricting to solar oxygen abundance models, which argues that the metallicity above the galactic plane should be close to solar. They also indicate that N/O increases with increasing z.

Flores-Fajardo, N.; Morisset, C.; Stasi?ska, G.; Binette, L.

2011-08-01

37

Radiative and Kinetic Feedback by Low-Mass Primordial Stars  

NASA Astrophysics Data System (ADS)

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

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

2010-03-01

38

Space Motions of Low-Mass Stars. III.  

NASA Astrophysics Data System (ADS)

Radial velocity observations are presented for 149 stars taken from the McCormick lists of dwarf K and M stars in a continuing program of radial velocities of faint nearby stars. The data will serve to derive a total stellar density of these kinds of stars in the solar neighborhood. These data were obtained with the spectrometer of the Vilnius University Observatory mounted on the 1.6 m Kuiper Telescope of the Steward Observatory.

Upgren, A. R.; Sperauskas, J.; Boyle, R. P.

39

Space Motions of Low-Mass Stars. II: Radial Velocities  

NASA Astrophysics Data System (ADS)

Radial velocities are presented for 53 dwarf K and M stars, eight of which are radial velocity standards. This is the second list in a program to determine space motions for all of the stars in the McCormick lists of dwarf stars. The observations reported here differ from those of the first list in that they were made using the 1.88m David Dunlap reflector. One of the stars varies in radial velocity, consistent with a spectroscopic binary with a period of about 48 days. (SECTION: Stars)

Upgren, A. R.; Harlow, J. J. B.

1996-01-01

40

A Study of the Low Mass Binary Star Ross 614  

NASA Astrophysics Data System (ADS)

We have combined photograph, MAP, interferometric, and spectroscopic data to determine the orbital characteristics and masses of the Ross 614 binary star system. Attention was first drawn to the star by Frank E. Ross (1927, AJ 37, 193) who noticed its high proper motion in a comparison of new plates with those taken at the Yerkes Observatory by E.E. Barnard. The Binary nature of the star was recognized from accelerations in the star's proper motion (D. Reuyl 1936, AJ 55, 236) and the mass of the companion was first estimated by combining measurements of McCormick and Sproul plates with a separation measured by Walter Baade at the Hale 5-m reflector (S.L. Lippincott 1955, AJ 60, 379). In her paper Lippincott notes the companion's significance as defining the lower end of the observational main sequence. Fifty six years later the star still holds that honor. With a wealth of new data spanning more than 3 additional orbits, we find her value of 0.08 solar masses to be within our error of our value.

Gatewood, G.; Han, I.; Tangren, W.

2001-12-01

41

Low Mass Star Formation in Perseus: Large Field Mapping at 1mm with Bolocam  

Microsoft Academic Search

The earliest stages of low mass star formation, enshrouded in dense cores of dust and gas, are most easily studied at long wavelengths from the far-infrared to millimeter. SIRTF will provide unparalleled sensitivity in the 4-160 micron range, but coverage at longer wavelengths is necessary to developing a more complete picture of star formation in molecular clouds. As a complement

M. L. Enoch; A. I. Sargent; S. Golwala; J. Glenn; N. J. Evans II; K. E. Young

2003-01-01

42

Search for Habitable Planets Around Low-Mass Stars Using the InfraRed Doppler Instrument  

NASA Astrophysics Data System (ADS)

We present strategies and plans for a new Doppler exoplanet survey of late-M dwarf stars to search for Earth-mass planets in the habitable zone around low-mass stars using a new near-infrared instrument for the Subaru telescope (IRD).

Omiya, M.; Sato, B.; Harakawa, H.; Kuzuhara, M.; Hirano, T.; Narita, N.; IRD Team

2013-11-01

43

Correlativity of nucleosynthesis in low-mass AGB stars.  

NASA Astrophysics Data System (ADS)

In a self-consistent treatment, an LM-AGB TP model of H-He-burning and no-branch reaction passway for s-process from Fe-Bi with correlative reaction network from C-Ne is used to reproduce the enrichment of F, C and heavy elements in the surface of MS and S stars (with Tc). The growing of the core mass and the mass loss through stellar winds are also considered. Comparisons between the computed correlation {[F/O], C/O}; {[F/O], (s/Ti)} and that of the observations are presented. It appears that fluorine and heavy elements cam be synthesized where the nucleosynthesis events occur in a fit temperature range, and then are dredged up to the surface of the star. Because F production only occurs in a narrow temperature range, the synthesis events are sensitive to temperature. The observation of this case is discussed specially.

Liu, Yongxin; Zhang, Bo; Peng, Qiuhe

1996-10-01

44

From Very Low Mass Stars to Extrasolar Planets  

NASA Astrophysics Data System (ADS)

We have modeled, using the atmosphere code Phoenix, the photosphere structures and spectral distribution of brown dwarfs and of all known Extrasolar Giant Planets (EGPs) to this day. Since EGPs are often brighter, more massive and larger than telluric planets, they will be more readily detected in the future transit surveys (COROT, KEPLER, SIMS, Eddington), and be the most directly observable using current technology (e.g. by nulling interferometry with the GENIE experiment). In this paper we summarize the spectral properties of EGPs as a function of their orbital distance, phase as viewed from the earth, mass and age, and type of primary star. We establish the most favorable observation conditions (i.e. maximum luminosity contrast of the planet to the primary star). We also explore uncertainties tied to the chemical composition of the atmosphere, and the presence of cloud layers, and study constraining cases such as HD209458b, OGLE-TR56b and Jupiter.

Allard, France; Barman, Travis S.; Paillet, Jimmy; Baraffe, I.; Chabrier, G.; Hauschildt, P. H.

2004-12-01

45

Detecting Low-Mass Stellar Companions to B Stars  

NASA Astrophysics Data System (ADS)

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 can form from the fragmentation of either a collapsing molecular core, or of the circumstellar disk surrounding a protostar. Binaries formed through fragmentation of the circumstellar disk are thought to have lower masses than those formed from a fragmenting core, and so determining the mass-ratio distribution can constraint the relative importance of the two formation mechanisms. We describe a new technique sensitive to binary companions down to a mass-ratio of roughly 0.1 using high-resolution, high signal-to-noise ratio spectra. We apply this technique to a sample of archived VLT/CRIRES observations of nearby B-stars in the CO bandhead near 2300 nm, and demonstrate the feasibility of this method to observations with optical echelle spectrographs. 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, S. E.

2013-01-01

46

New companions to nearby low-mass stars  

NASA Astrophysics Data System (ADS)

We present high-angular-resolution optical I-band imaging of 451 late K to mid-M nearby stars. These observations have been performed with Astralux and FastCam using the lucky imaging technique. We found 70 companions with separations between 0.1 and 3.5 arcsec, out of which 28 are new discoveries. We derive a total binary fraction of 20.3+ 6.9- 5.2 per cent. Follow-up observations have been carried out for 16 of these new binaries and all of them were confirmed as physically associated systems. Parallaxes are available for 70 per cent of the binary sample, providing reliable estimates of physical separations. We have measured separations and position angles as an initial stage for a systematic follow-up aimed to determine orbital parameters.

Jódar, Esther; Pérez-Garrido, Antonio; Díaz-Sánchez, Anastasio; Villó, Isidro; Rebolo, Rafael; Pérez-Prieto, Jorge A.

2013-02-01

47

Low-Mass Star Surveys with the Palomar Transient Factory  

NASA Astrophysics Data System (ADS)

The Palomar Transient Factory (PTF) is a fully-automated, wide-field survey aimed at a systematic exploration of the optical transient sky. The survey is performed using a 7.26 square degree camera installed on the 48 inch Samuel Oschin telescope at Palomar Observatory; colors and light curves for detected transients are obtained with the automated Palomar 60 inch telescope. In the standard 60 s exposures the survey reaches a depth of R=20.6 (5-sigma, median seeing). Each PTF field contains tens of thousands of stars, including several thousand M-dwarfs. As of the beginning of 2011 the PTF survey has collected more than 25 epochs on over 6000 square degrees of the sky, and more than 100 epochs on 750 square degrees. Few-millimag long-term photometric precisions are achieved on the brighter targets. Ongoing PTF stellar science programs include a search for transiting planets around 100,000 M-dwarfs, a rotation and activity study in open clusters, the monitoring for outbursts and a search for transiting planets in young stellar regions, galactic structure measurements, and a variety of other stellar variability programs.

Law, Nicholas M.; PTF Collaboration

2011-05-01

48

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

SciTech Connect

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

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

2011-10-28

49

PEGASE, an infrared interferometer to study stellar environments and low mass companions around nearby stars  

Microsoft Academic Search

PEGASE is a mission dedicated to the exploration of the environment (including habitable zone) of young and solar-type stars\\u000a (particularly those in the DARWIN catalogue) and the observation of low mass companions around nearby stars. It is a space\\u000a interferometer project composed of three free flying spacecraft, respectively featuring two 40 cm siderostats and a beam combiner\\u000a working in the visible

M. Ollivier; O. Absil; F. Allard; J.-P. Berger; P. Bordé; F. Cassaing; B. Chazelas; A. Chelli; O. Chesneau; V. Coudé du Foresto; D. Defrère; P. Duchon; P. Gabor; J. Gay; E. Herwats; S. Jacquinod; P. Kern; P. Kervella; J.-M. Le Duigou; A. Léger; B. Lopez; F. Malbet; D. Mourard; D. Pelat; G. Perrin; Y. Rabbia; D. Rouan; J.-M. Reiss; G. Rousset; F. Selsis; P. Stee; J. Surdej

2009-01-01

50

Low-Mass Star Formation and the Initial Mass Function in Young Clusters  

Microsoft Academic Search

I have used optical and near-infrared spectroscopy and imaging to measure spectral types and luminosities for young (\\/tau<10 Myr), embedded (AV=0[-]50), low-mass (0.1-1 Msolar) stars in three nearby (d<300 pc) clusters: L1495E, IC 348, and rho Ophiuchi. In conjunction with theoretical evolutionary tracks, I have derived the star formation history and initial mass function for each stellar population. A large

Kevin Lee Luhman

1998-01-01

51

Optimizing exoplanet transit searches around low-mass stars with inclination constraints  

Microsoft Academic Search

Aims. We investigate a method to increase the efficiency of a targeted exoplanet search with the transit technique by preselecting a subset of candidates from large catalogs of stars. Assuming spin-orbit alignment, this can be done by considering stars that have higher probability to be oriented nearly equator-on (inclination close to 90^{\\\\circ}). Methods. We use activity-rotation velocity relations for low-mass

E. Herrero; I. Ribas; C. Jordi; E. F. Guinan; S. G. Engle

2011-01-01

52

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

Microsoft Academic Search

The search for planets around stars with empirical methods turn out to be as one of the most remarkable astronomical topics since late 90's. New observational techniques and instrumentations which are introduced in last decade made possible the discoveries of very low-mass and planetary companions around distant stars. More than 340 exoplanets have been discovered so far (see http:\\/\\/exoplanet.eu) and

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

2010-01-01

53

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

Microsoft Academic Search

We observed thirteen very low mass (VLM; M < 0.2 M&sun;) objects in the Taurus star-forming region using near-infrared diffraction-limited imaging techniques on the W.M. Keck I 10 m telescope. Of these thirteen, five were found to be binary, with separations ranging from 0\\

Quinn M. Konopacky; A. M. Ghez; E. L. Rice

2006-01-01

54

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

NASA Technical Reports Server (NTRS)

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

2000-01-01

55

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

NASA Technical Reports Server (NTRS)

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

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

1994-01-01

56

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

NASA Technical Reports Server (NTRS)

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

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

1992-01-01

57

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

NASA Astrophysics Data System (ADS)

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

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

2014-07-01

58

Low-Mass Companions for Five Solar-Type Stars From the Magellan Planet Search Program  

NASA Astrophysics Data System (ADS)

We report low-mass companions orbiting five solar-type stars that have emerged from the Magellan precision Doppler velocity survey, with minimum (Msin i) masses ranging from 1.2 to 25 M JUP. These nearby target stars range from mildly metal-poor to metal-rich, and appear to have low chromospheric activity. The companions to the brightest two of these stars have previously been reported from the CORALIE survey. Four of these companions (HD 48265-b, HD 143361-b, HD 28185-b, and HD 111232-b) are low-mass Jupiter-like planets in eccentric intermediate- and long-period orbits. On the other hand, the companion to HD 43848 appears to be a long-period brown dwarf in a very eccentric orbit. Based on observations obtained with the Magellan Telescopes, operated by the Carnegie Institution, Harvard University, University of Michigan, University of Arizona, and the Massachusetts Institute of Technology.

Minniti, Dante; Butler, R. Paul; López-Morales, Mercedes; Shectman, Stephen A.; Adams, Fred C.; Arriagada, Pamela; Boss, Alan P.; Chambers, John E.

2009-03-01

59

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

60

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

NASA Technical Reports Server (NTRS)

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

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

1984-01-01

61

THE EFFECTS OF RADIATIVE TRANSFER ON LOW-MASS STAR FORMATION  

SciTech Connect

Forming stars emit a substantial amount of radiation into their natal environment. We use ORION, an adaptive mesh refinement (AMR) three-dimensional gravito-radiation-hydrodyanics code, to simulate low-mass star formation in a turbulent molecular cloud. We compare the distributions of stellar masses, accretion rates, and temperatures in the cases with and without radiative transfer, and we demonstrate that radiative feedback has a profound effect on accretion, multiplicity, and mass by reducing the number of stars formed and the total rate at which gas turns into stars. We also show that once the star formation reaches a steady state, protostellar radiation is by far the dominant source of energy in the simulation, exceeding viscous dissipation and compressional heating by at least an order of magnitude. Calculations that omit radiative feedback from protstars significantly underestimate the gas temperature and the strength of this effect. Although heating from protostars is mainly confined to the protostellar cores, we find that it is sufficient to suppress disk fragmentation that would otherwise result in very low-mass companions or brown dwarfs. We demonstrate that the mean protostellar accretion rate increases with the final stellar mass so that the star formation time is only a weak function of mass.

Offner, Stella S. R. [Department of Physics, University of California, Berkeley, CA 94720 (United States); Klein, Richard I. [Department of Astronomy, University of California, Berkeley, CA 94720 (United States); McKee, Christopher F. [Departments of Physics and Astronomy, University of California, Berkeley, CA 94720 (United States); Krumholz, Mark R., E-mail: soffner@berkeley.ed [Department of Astronomy, University of California, Santa Cruz, CA 95064 (United States)

2009-09-20

62

The Evolutionary Properties and Peculiar Thermal Pulses of Metal-deficient Low-Mass Stars  

Microsoft Academic Search

We investigate the evolutionary behavior of low-mass star models with very low original metal content (log Z = -10, -6, -5) and Y = 0.23. The computations have been extended from the main sequence up to the double shell burning phase. Theoretical isochrones on the H-R diagram are presented for a range of ages spanning 7-15 x 10 yr. Attention

Santi Cassisi; Vittorio Castellani; Amedeo Tornambe

1996-01-01

63

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

64

Disk-Rotation Connection: Very Low Mass Stars and Brown Dwarfs  

NASA Astrophysics Data System (ADS)

At present, how young pre-main sequence (PMS) objects effectively lose angular momentum is still not fully understood. Nevertheless, irrespective of an specific model, there is a general consensus that circumstellar accretion disks are somehow responsible for the removal of angular momentum, and therefore accreting stars should on average rotate slower than non-accreting ones. Although, many authors have investigated a rotation-disk connection in low mass pre-main sequence stars, this connection was not investigated as yet in the brown dwarf (BD) regime because of the very few samples available. Our investigation extends well down into the substellar regime, with a total sample of 732 very low mass objects, 81 of which are BDs candidates in the Orion Nebula Cluster (ONC), with measured rotational periods. We found a rotation-disk correlation in the low and very low mass stars of the sample, in which objects with NIR excess tend to rotate slower than objects without NIR excess. Interestingly, we found no correlation in the substellar regime. A tight correlation between the peak-to-peak (ptp) amplitude of the rotational modulation and the NIR excess was found however for all objects.

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

2011-12-01

65

Constraints on the low-mass IMF in young super-star clusters in starburst galaxies  

NASA Astrophysics Data System (ADS)

As evidence for variations in the initial mass function (IMF) in nearby star forming regions remains elusive we are forced to expand our search to more extreme regions of star formation. Starburst galaxies, which contain massive young clusters have in the past been reported to have IMFs different than that characterizing the field star IMF. In this thesis we use high signal-to-noise near-infrared spectra to place constraints on the shape of the IMF in extreme regions of extragalactic star formation and also try to understand the star formation history in these regions. Through high signal-to-noise near-infrared spectra it is possible to directly detect low-mass PMS stars in unresolved young super-star clusters, using absorption features that trace cool stars. Combining Starburst99 and available PMS tracks it is then possible to constrain the IMF in young super-star clusters using a combination of absorption lines each tracing different ranges of stellar masses and comparing observed spectra to models. Our technique can provide a direct test of the universality of the IMF compared to the Milky Way. We have obtained high signal-to-noise H- and K-band spectra of two young super-star clusters in the starburst galaxies NGC 4039/39 and NGC 253 in order to constrain the low-mass IMF and star formation history in the clusters. The cluster in NGC 4038/39 shows signs of youth such as thermal radio emission and strong hydrogen emission lines as well as late-type absorption lines indicative of cool stars. The strength and ratio of these absorption lines cannot be reproduced through either late-type pre-main sequence stars or red supergiants alone. We interpret the spectrum as a superposition of two star clusters of different ages over the physical region of 90 pc our spectrum represents. 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. While the superposition of clusters does not allow us to place stringent constraints on the IMF there is no evidence of a low-mass cutoff in the cluster and the IMF is consistent with a Chabrier and Kroupa IMF typical of the field. The cluster in NGC 253 shows the same signs of youth as the cluster in NGC 4038/39 and sits in front of a background population of older stars. The background population has an age of ? 12 Myr and thus contains red supergiants. After carefully subtracting this background we model the spectrum of the young cluster. We find that its IMF is consistent with a Chabrier and Kroupa IMF with a best-fit power-law slope of 1.0 in linear units. Slopes of 0.0 - 1.5 are also formally consistent with the cluster spectrum. We conclude that there is no strong evidence for an unusual IMF or a lack of low-mass stars (? 1 M? ) in either of these galaxies.

Greissl, Julia Jennifer

2010-12-01

66

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

NASA Astrophysics Data System (ADS)

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

Haurberg, Nathalie C.

67

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

68

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

NASA Astrophysics Data System (ADS)

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

Feiden, Gregory A.; Chaboyer, Brian

2012-09-01

69

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

70

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

71

HD 98800: An Opportunity to Measure True Masses for Low-Mass PMS Stars  

NASA Astrophysics Data System (ADS)

HD 98800 became interesting when IRAS found it to have a large infrared excess, indicating a substantial dust disk. But ``HD 98800'' is, in fact, a quadruple system consisting of four K and M stars, and its Hipparcos parallax has now shown that this is a pre-main sequence system. The four stars are in two visible objects, each of which is a spectroscopic binary with a period of about one year. In particular, the Ba-Bb pair is an SB2 with an estimated semi-major axis of about 20 milliarcsec. In TRANS mode, FGS1R can cleanly resolve the Ba-Bb pair and can determine the relative orbit and luminosities for the two components. POS mode observations lead to an absolute orbit and a more precise parallax than is currently available. In this program we propose to follow the HD 98800 Ba-Bb pair over the course of a full orbit during Cycle 8. The combination of FGS1R-TRANS and FGS1R-POS observations will provide gravitational masses for two low-mass PMS stars. In addition, the co nstraints of coevality and knowled ge of the astrophysical properties of the components {temperatures, luminosities, composition} make these observations a crucial test of our models of pre-main sequence evolution. These may be the first true masses determined for low-mass PMS objects, and so can provide a fundamental test of PMS evolutionary tracks.

Soderblom, David

1999-07-01

72

Discovery of a Low-Mass Companion to the Solar-Type Star TYC 2534-698-1  

Microsoft Academic Search

Brown dwarfs and low-mass stellar companions are interesting objects to study since they occupy the mass region between deuterium and hydrogen burning. We report here the serendipitous discovery of a low-mass companion in an eccentric orbit around a solar-type main-sequence star. The stellar primary, TYC 2534-698-1, is a G2V star that was monitored both spectroscopically and photometrically over the course

Stephen R. Kane; Suvrath Mahadevan; William D. Cochran; Thirupathi Sivarani; Gregory W. Henry; Michael H. Williamson

2009-01-01

73

Neutron Star Formation in Theoretical Supernovae. Low Mass Stars and White Dwarfs.  

National Technical Information Service (NTIS)

The presupernova evolution of stars that form semi-degenerate or strongly degenerate O + Ne + Mg cores is discussed. For the 10 to 13 Msub solar stars, behavior of off-center neon flashes is crucial. The 8 to 10 m/sub solar stars do not ignite neon and ev...

K. Nomoto

1986-01-01

74

The Role of Environment in the Formation of Low Mass Stars: Lessons from the Orion Molecular Clouds  

NASA Astrophysics Data System (ADS)

Low mass stars form in diverse environmental conditions, and understanding how these conditions influence the fragmentation and collapse of the molecular gas into stars is of key interest. The Orion molecular clouds are a remarkable laboratory for studying low mass star formation across the full range of environments, from crowded clusters containing massive stars, to moderate sized groups ofintermediate and low mass stars, and finally to relatively isolated low mass star formation. We present results from the Herschel Orion Protostar Survey, or HOPS, a study of over 300 protostar in the Orionclouds with the Herschel, Spitzer, Hubble and APEX telescopes. These data provide the means to identify young stars and protostars, determine the properties of the protostars, and map the column density of the dense gas in their surroundings. We examine how the properties of the protostars depend on the local environment, as traced by the surface densities of YSOs and gas, and we discuss the implications for our understanding of low mass star formation.

Megeath, S. Thomas

2014-07-01

75

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

NASA Astrophysics Data System (ADS)

We measured lithium in a sample of low-mass stars (~0.1-0.3 Msolar) of the Orion Nebula cluster. We find evidence for significant Li depletion in four high-probability members, corresponding to nuclear ages between ~15 and 30 Myr. In two cases, there is excellent agreement between the mass and age based on models of Li burning and those derived from the H-R diagram, reinforcing our early findings. For the two other stars, the nuclear age is significantly larger than the isochronal one. Several Li-depleted stars display accretion activity, veiling, and emission lines. We discuss empirical evidence in favor of the old nuclear age and the implications on the star formation history of the Orion cluster. Based on data collected at ESO-VLT, Paranal Observatory, Chile [ID 074.C-0757(A)].

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

2007-04-01

76

Neutron Star Masses and Radii from Quiescent Low-mass X-Ray Binaries  

NASA Astrophysics Data System (ADS)

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

Lattimer, James M.; Steiner, Andrew W.

2014-04-01

77

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

78

Brown dwarfs and very low mass stars in the Hyades cluster: a dynamically evolved mass function  

NASA Astrophysics Data System (ADS)

Aims: We conducted a search for brown dwarfs (BDs) and very low mass (VLM) stars in the 625 Myr-old Hyades cluster in order to derive the cluster's mass function across the stellar-substellar boundary. Methods: We performed a deep (I=23, z=22.5) photometric survey over 16 deg2 around the cluster center and followed up with K-band photometry to measure the proper motion of candidate members and with optical and near-IR spectroscopy of probable BD and VLM members. Results: We report the discovery of the first 2 BDs in the Hyades cluster. The 2 objects have a spectral type early-T and their optical and near-IR photometry as well as their proper motion are consistent with them being cluster members. According to models, their mass is 50 Jupiter masses at an age of 625 Myr. We also report the discovery of 3 new very low mass stellar members of the cluster and confirm the membership of 16 others. We combine these results with a list of previously known cluster members to build the present-day mass function (PDMF) of the Hyades cluster from 50 Jupiter masses to 3 M_?. We find the Hyades PDMF to be strongly deficient in very low mass objects and BDs compared to the IMF of younger open clusters such as the Pleiades. We interpret this deficiency as the result of dynamical evolution over the past few 100 Myr, i.e., the preferential evaporation of low mass cluster members due to weak gravitational encounters. Conclusions: We thus estimate that the Hyades cluster currently hosts about 10-15 BDs, while its initial substellar population may have amounted to up to 150-200 members. Appendices are only available in electronic form at http://www.aanda.org

Bouvier, J.; Kendall, T.; Meeus, G.; Testi, L.; Moraux, E.; Stauffer, J. R.; James, D.; Cuillandre, J.-C.; Irwin, J.; McCaughrean, M. J.; Baraffe, I.; Bertin, E.

2008-04-01

79

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

NASA Astrophysics Data System (ADS)

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

Nelson, Lorne A.

2011-05-01

80

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.

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

2012-01-01

81

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

82

Magnetised winds of low-mass stars and their impact on exoplanets  

NASA Astrophysics Data System (ADS)

The proper characterisation of stellar winds is crucial to constrain interactions between exoplanets and their surrounding environments and also essential for the study of space weather events on exoplanets. Although the great majority of exoplanets discovered so far are orbiting cool, low-mass stars with properties (mass, radius and effective temperatures) similar to solar, the stellar magnetism can be significantly different from the solar one, both in topology and intensity. Due to the current technology used in exoplanetary searches, most of the currently known exoplanets are found orbiting at extremely close distances to their host stars (< 0.1 au). The dramatic differences in stellar magnetism and orbital radius can make the interplanetary medium of exoplanetary systems remarkably distinct from the one present in the solar system. In addition, the interaction of the stellar winds with exoplanets can lead, among others, to observable signatures that are absent in our own solar system.

Vidotto, A. A.

2014-01-01

83

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

NASA Astrophysics Data System (ADS)

Using the Infrared Array Camera and the Multiband Imaging Photometer aboard the Spitzer Space Telescope, we have obtained images of the Lupus 3 star-forming cloud at 3.6, 4.5, 5.8, 8.0, and 24 ?m. We present photometry in these bands for the 41 previously known members that are in our images. In addition, we have identified 19 possible new members of the cloud based on red 3.6-8.0 ?m colors that are indicative of circumstellar disks. We have performed optical spectroscopy on six of these candidates, all of which are confirmed as young low-mass members of Lupus 3. The spectral types of these new members range from M4.75 to M8, corresponding to masses of 0.2-0.03 Msolar for ages of ~1 Myr according to theoretical evolutionary models. We also present optical spectroscopy of a candidate disk-bearing object in the vicinity of the Lupus 1 cloud, 2M 1541-3345, which Jayawardhana & Ivanov recently classified as a young brown dwarf (M~0.03 Msolar) with a spectral type of M8. In contrast to their results, we measure an earlier spectral type of M5.75+/-0.25 for this object, indicating that it is probably a low-mass star (M~0.1 Msolar). In fact, according to its gravity-sensitive absorption lines and its luminosity, 2M 1541-3345 is older than members of the Lupus clouds (?~1 Myr) and instead is probably a more evolved pre-main-sequence star that is not directly related to the current generation of star formation in Lupus.

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

2007-02-01

84

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

85

The short-period low-mass binary system CC Com revisited  

Microsoft Academic Search

In this study we determined precise orbital and physical parameters of the very short-period low-mass contact binary system CC Com. The parameters are obtained by analysis of new CCD data combined with archival spectroscopic data. The physical parameters of the cool and hot components are derived as M_{c} = 0.717(14) M&sun;, M_{h} = 0.378(8) M&sun;, R_{c} = 0.708(12) R&sun;, R_{h}

O. Köse; B. Kalomeni; V. Keskin; B. Ulas; K. Yakut

2011-01-01

86

On the kinematics of the neutron star low mass X-ray binary Cen X-4  

NASA Astrophysics Data System (ADS)

We present the first determination of the proper motion of the neutron star low mass X-ray binary Cen X-4 measured from relative astrometry of the secondary star using optical images at different epochs. We determine the Galactic space velocity components of the system and find them to be significantly different from the mean values that characterize the kinematics of stars belonging to the halo, and the thin and the thick disc of the Galaxy. The high metallicity of the secondary star of the system rules out a halo origin and indicates that the system probably originated in the Galactic disc. A statistical analysis of the galactocentric motion revealed that this binary moves in a highly eccentric (e? 0.85±0.1) orbit with an inclination of ?110° to the Galactic plane. The large Galactic space velocity components strongly support that a high natal kick as a result of a supernova explosion could have propelled the system into such an orbit from a birth place in the Galactic disc. The high Li abundance in the secondary, comparable to that of stars in star forming regions and young stellar clusters like the Pleiades, may suggest a relatively recent formation of the system. Following the orbit backwards in time, we found that the system could have been in the inner regions of the Galactic disc ~100-200 Myr ago. The neutron star might have formed at that moment. However, we cannot rule out the possibility that the system formed at a much earlier time if a Li production mechanism exists in this LMXB.

González Hernández, J. I.; Rebolo, R.; Peñarrubia, J.; Casares, J.; Israelian, G.

2005-06-01

87

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

NASA Technical Reports Server (NTRS)

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

Terebey, Susan; Surace, Jason A.

1994-01-01

88

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

89

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

SciTech Connect

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

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

2013-09-15

90

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

91

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

NASA Astrophysics Data System (ADS)

We explore the minimal conditions which enable the formation of metal-enriched solar and subsolar-mass stars. Using a one-zone semi-analytical model, we accurately follow the chemical and thermal evolution of the gas with the aim of understanding how the initial metal and dust content alters the cooling and fragmentation properties, hence the characteristic stellar mass. We find that in the absence of dust grains, gas fragmentation occurs at densities nH˜ [104-105] cm-3 when the metallicity exceeds Z˜ 10-4 Z?. The resulting fragmentation masses are ?10 M?. The inclusion of Fe and Si cooling does not affect the thermal evolution as this is dominated by molecular (mostly OH, H2O and CO) cooling even for metallicities as large as Z= 10-2 Z?. The presence of dust is the key driver for the formation of low-mass stars. We focus on three representative core-collapse supernova (SN) progenitors (a Z= 0 star with 20 M? and two Z= 10-4 Z? stars with 20 and 35 M?), and consider the effects of reverse shocks of increasing strength: these reduce the depletion factors, fdep=Mdust/(Mdust+Mmet), alter the shape of the grain size distribution function and modify the relative abundances of grain species and metal species in the gas phase. We find that the lowest metallicity at which fragmentation occurs is Z= 10-6 Z? for gas pre-enriched by the explosion of a 20 M? primordial SN (fdep? 0.22) and/or by a 35 M?, Z= 10-4 Z? SN (fdep? 0.26); it is ˜1 dex larger, when the gas is pre-enriched by a Z= 10-4 Z?, 20 M? SN (fdep? 0.04). Cloud fragmentation depends on the depletion factor and it is suppressed when the reverse shock leads to a too large destruction of dust grains. These features are all consistent with the existence of a minimum dust-to-gas ratio, ?, above which fragmentation is activated. We derive a simple analytic expression for ?, which depends on the total grain cross-section per unit mass of dust; for grain composition and properties explored in the present study, ?. When the dust-to-gas ratio of star-forming clouds exceeds this value, the fragmentation masses range between 0.01 and 1 M?, thus enabling the formation of the first low-mass stars.

Schneider, Raffaella; Omukai, Kazuyuki; Bianchi, Simone; Valiante, Rosa

2012-01-01

92

Angular Momentum Evolution of Low-Mass Stars in the Pleiades  

NASA Astrophysics Data System (ADS)

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

Krishnamurthi, Anita

93

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

94

High-resolution CO observations of young low-mass stars  

SciTech Connect

Through the use of the Owens Valley Millimeter Interferometer, CO(1-0) emission near very young low-mass stars was observed. The data consist of a snapshot visibility survey of 25 low-luminosity IRAS sources associated with dense molecular cores and full aperture synthesis maps of two cores, L1262 and L1681B. The two sources mapped show spatially compact low-velocity outflows. The momentum flux in the compact outflows is comparable to that powering larger scale low-luminosity outflows. Extrapolating from the results of the snapshot survey, at least 64 percent and perhaps all dense cores with embedded infrared sources have outflows. L1681B contains an unresolved source with velocity extent and emission asymmetries which are consistent with gravitational infall. 24 refs.

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

1989-05-01

95

A New Twist in the Evolution of Low-mass Stars  

NASA Astrophysics Data System (ADS)

We show that the evolutionary track of a low-mass red giant should make an extended zigzag on the Hertzsprung-Russel diagram just after the bump luminosity if fast internal rotation and enhanced extra mixing in the radiative zone bring the temperature gradient close to the adiabatic one. This can explain both the location and peculiar surface chemical composition of Li-rich K giants studied by Kumar et al. We also discuss a striking resemblance between the photometric and composition peculiarities of these stars and giant components of RS CVn binaries. We demonstrate that the observationally constrained values of the temperature gradient in the Li-rich K giants agree with the required rate of extra mixing only if the turbulence that is believed to be responsible for this extra mixing is highly anisotropic, with its associated transport coefficients in the horizontal direction strongly dominating over those in the vertical direction.

Denissenkov, Pavel A.

2012-07-01

96

Lyman-alpha Emitters in the HUDF: A Population of Low Mass, Star Forming Galaxies  

NASA Astrophysics Data System (ADS)

We present observations of the Ly-alpha galaxies at reshifts 4low mass objects with characteristic ages of a few 106 Myr and masses < 108 M?. These objects were identified based on the presence of strong Lyman-alpha emission in their low resolution ACS slitless spectra. We present both our morphological analysis of these sources as well as our estimates of their masses, ages, and metalicities. The latter estimates are based on a thorough SED fitting of the broad band photometry that is available for these objects and using an extensive number of models. We have combined extremely deep HST/ACS, HST/NICMOS, VLT/ISAAC, and Spitzer/IRAC observations of these objects to provide us with a continuum rest-frame wavelength coverage ranging from the UV to the optical. We observe that the color of these objects is extremely blue, and consistent with being dominated by a population of very young stars. The IRAC infrared observations, corresponding to the rest-frame optical, furthermore allow us to set upper limits on the amount of older stars present in these objects. We conclude that the Lyman-alpha emitters that we identified in the HUDF, and which should constitute a nearly complete sample between the redshifts of 4low mass objects at these redshifts.

Pirzkal, Norbert; Malhotra, S.; Rhoads, J. E.; Xu, C.

2006-12-01

97

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

98

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

99

Low Mass Star Formation in Perseus: Large Field Mapping at 1mm with Bolocam  

NASA Astrophysics Data System (ADS)

The earliest stages of low mass star formation, enshrouded in dense cores of dust and gas, are most easily studied at long wavelengths from the far-infrared to millimeter. SIRTF will provide unparalleled sensitivity in the 4-160 micron range, but coverage at longer wavelengths is necessary to developing a more complete picture of star formation in molecular clouds. As a complement to the SIRTF Legacy program "From Molecular Cores to Planet-forming Disks", we have carried out a flux-limited survey of approximately 6.5 square degrees of the Perseus molecular cloud at 1.1 mm using Bolocam, a new large format bolometer array for the Caltech Submillimeter Observatory (CSO). At millimeter wavelengths dust emission is generally optically thin and thus a good tracer of mass, while emission at infrared wavelengths is more strongly dependent on temperature and density. We will present preliminary results from this survey and discuss general characteristics of the Perseus cloud, such as the spatial distribution of dust cores, and mass statistics. Results from recent CSO/SHARCII 350 micron images of a few of the newly identified pre-protostellar and protostellar candidates will also be presented. This work is funded in part by an NSF Graduate Research Fellowship.

Enoch, M. L.; Sargent, A. I.; Golwala, S.; Glenn, J.; Evans, N. J., II; Young, K. E.

2003-12-01

100

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

NASA Technical Reports Server (NTRS)

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

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

2010-01-01

101

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.

102

Low-Mass X-Ray Binaries with Strange Quark Stars  

NASA Astrophysics Data System (ADS)

Strange quark stars (SSs) may originate from accreting neutron stars (NSs) in low-mass X-ray binaries (LMXBs). Assuming that conversion of NS matter to SSs occurs when the core density of accreting NS reaches the density of quark deconfinement, ˜5?0, where ?0˜2.7×1014 g cm-3 is nuclear saturation density, we investigate LMXBs with SSs (qLMXBs). In our simulations, about 0.1--10% of LMXBs can produce SSs, which greatly depends on the masses of nascent NSs and the fraction of transferred matter accreted by the NSs. If the conversion does not affect binary systems, LMXBs evolve into qLMXBs. We find that some observational properties (spin periods, X-ray luminosities and orbital periods) of qLMXBs are similar to those of LMXBs, and it is difficult to differentiate between them. If the conversion disturbs the binary systems, LMXBs can produce isolated SSs. These isolated SSs could be submillisecond pulsars, and their birthrate in the Galaxy is ˜5-70 Myr-1.

Zhu, Chunhua; Lü, Guoliang; Wang, Zhaojun; Liu, Jinzhong

2013-01-01

103

Class I methanol masers in low-mass star-forming regions  

NASA Astrophysics Data System (ADS)

Results of observations of Class I methanol masers in regions of low-mass star formation (MMIL) are summarized and analyzed. Four masers were detected at 44, 84, and 95 GHz towards "chemically active" bipolar outflows in the low-mass star-forming regions NGC1333 I4A, NGC 1333 I2A, HH 25, and L1157. Another maser was found at 36 GHz towards a similar outflow in NGC 2023. Thus, all the detected MMILs are associated with chemically active outflows. The brightness temperatures of the strongest 44-GHz maser spots in NGC 1333 I4A, HH 25, and L1157 exceed 2000 K, whereas the brightness temperature in NGC 1333 I2A is only 176 K, although a rotational-diagram analysis shows that this last source is also amaser. The flux densities of the newly detectedmasers are no higher than 18 Jy, and are much lower than those of strong masers in regions of high-mass star formation (MMIH). The MMIL luminosities match the maser luminosity-protostar luminosity relation established earlier for MMIHs. No MMIL variability was detected in 2004-2011. The radial velocities of the newly detected masers are close to the systemic velocities of the associated regions, except for NGC 2023, where the maser radial velocity is lower than the systemic velocity by approximately 3.5 km/s. Thus, the main MMILproperties are similar to those of MMIHs. MMILs are likely to be an extension of the MMIH population toward lower luminosities of both the masers and the associated young stellar objects. The results of VLA observations of MMILs can be explained using a turbulent-cloud model, which predicts that compact maser spots can arise in extended sources because the coherence lengths along some directions randomly appear to be longer than the mean coherence length in a turbulent velocity field. However, one must assume that the column density of methanol towardM1, the strongest maser in L1157, is appreciably higher than the mean column density of the clump B0a where the maser arises. The shape of the maser lines in L1157, forming double profiles with a red asymmetry, may indicate that the masers arise in collapsing clumps. However, although this model may be correct for L1157, it is specific to this source, since none of the other masers observed exhibited a double profile.

Kalenskii, S. V.; Kurtz, S.; Bergman, P.

2013-02-01

104

Rotational studies of very low mass stars and brown dwarfs in the Orion Nebula Cluster  

NASA Astrophysics Data System (ADS)

Rotational studies at different ages and masses are important for constraining the angular momentum evolution of young stellar objects (YSO). Of particular interest are the rotational studies of very low mass (VLM) stars and brown dwarfs (BDs), because few rotational periods are known in that mass range. We aim to extend these studies well down into the substellar regime, providing for the first time information on rotational periods for a large sample of VLM stars and BDs. This extensive rotational period study of YSOs in the 1 Myr old Orion Nebula Cluster (ONC) is based on a deep photometric monitoring campaign using the Wide Field Imager (WFI) camera on the ESO/MPG 2.2 m telescope on La Silla, Chile. Time series data with about 95 data points were taken over 19 nights. Accurate I-band photometry of 2908 stars was obtained within a magnitude range between 13 and 21 mag, i.e. three magnitudes deeper than the previous studies in the ONC ([11]). Two different power spectral analysis techniques were used to search for periodic variability. In addition, the ?2 variability test was used for the detection of irregular variables. We measured rotational periods of 487 objects with estimated masses between 0.5 Msolar and 0.015 Msolar, 124 of which are BD candidates. This is by far the most extensive and complete rotational period data set for young VLM stars and BDs. Besides the periodic variables, 808 objects show strong non-periodic (i.e. irregular) brightness variations. We studied the dependence of the period distribution on the magnitude (mass) and variability level and compared the found period distribution with that of higher-mass objects in the ONC ([11]) and with the rotational data set existing for the twice as old cluster NGC 2264 ([18]). We found that substellar objects rotate on average faster than the VLM stars, a trend which was already observed for higher mass stars. In addition, we found a clear dependence of the rotational periods on position within the field. Objects located inside the so-called Rcluster rotate on average slower, which can be explained by an age spread in the ONC, with a somewhat younger central region. The results of a comparison between the period distributions of the ONC and NGC 2264 strongly favours this hypothesis. Interesting correlations between rotational period and variability level were also found in both clusters, probably explained by different magnetic field topologies.

Rodríguez-Ledesma, Maria V.; Mundt, Reinhard; Eislöffel, Jochen; Herbst, William

2009-02-01

105

Discovery of a Low-Mass Companion to the Solar-Type Star TYC 2534-698-1  

Microsoft Academic Search

Brown dwarfs and low-mass stellar companions are interesting objects to study\\u000asince they occupy the mass region between deuterium and hydrogen burning. We\\u000areport here the serendipitous discovery of a low-mass companion in an eccentric\\u000aorbit around a solar-type main sequence star. The stellar primary, TYC\\u000a2534-698-1, is a G2V star that was monitored both spectroscopically and\\u000aphotometrically over the

Stephen R. Kane; Suvrath Mahadevan; William D. Cochran; Sivarani Thirupathi; Gregory W. Henry; Michael H. Williamson

2008-01-01

106

An Astrometric Study of the Low-Mass Binary Star Ross 614  

NASA Astrophysics Data System (ADS)

Long accepted as the quintessential low-mass star, the secondary of the nearby diminutive astrometric binary Ross 614 has attracted considerable astrophysical interest. Unfortunately, the orbital period of 16.6 yr exceeds the duration of the mission-limited studies of most space-borne or instrumental-proving observational programs. As with most such binaries, the only full-orbit studies are based on photographic materials. The last extended study of this system was based upon the plate collections of the McCormick and Sproul Observatories. The work reported here combines data from the Multichannel Astrometric Photometer, the Hipparcos Intermediate Astrometric Data, the previously unmeasured photographic plates of the Allegheny Observatory, published observations of the visual binary, and recently published radial velocities of the system. Together, these data span more than three orbits of the low-mass binary system. Limiting our analysis to the most recent observations of the binary, and five older observations that are in fair agreement with them, we find masses of 0.2228+/-0.0055 and 0.1107+/-0.0028 Msolar for the primary and secondary, respectively, with the largest source of error being the visual separations of the system. We find a parallax of 244.07+/-0.73 mas, a period of 16.595+/-0.0077 yr, and an increased estimate of the semimajor axis of 1101.2+/-8.2 mas. The latter led to a significant increase in the computed masses. All other aspects of the orbital elements and astrometry are in excellent agreement with those found in the independent study of the McCormick and Sproul plates. The importance of long-term astrometric coverage is pointed out by the fact that the orbital motion of the system only resulted in an acceleration during the compilation of the Hipparcos Catalogue. No orbital parameters or mass estimates can be discerned from these high-precision but short-term data.

Gatewood, George; Coban, Louis; Han, Inwoo

2003-03-01

107

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

NASA Astrophysics Data System (ADS)

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

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

2013-01-01

108

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

109

Discovery of a Low-mass Companion to a Metal-rich F Star with the MARVELS Pilot Project  

Microsoft Academic Search

We report the discovery of a low-mass companion orbiting the metal-rich, main sequence F star TYC 2949-00557-1 during the Multi-object APO Radial Velocity Exoplanet Large-area Survey (MARVELS) pilot project. The host star has an effective temperature T eff = 6135 ± 40 K, logg = 4.4 ± 0.1, and [Fe\\/H] = 0.32 ± 0.01, indicating a mass of M =

Scott W. Fleming; Jian Ge; Suvrath Mahadevan; Brian Lee; Jason D. Eastman; Robert J. Siverd; B. Scott Gaudi; Andrzej Niedzielski; Thirupathi Sivarani; Keivan G. Stassun; Alex Wolszczan; Rory Barnes; Bruce Gary; Duy Cuong Nguyen; Robert C. Morehead; Xiaoke Wan; Bo Zhao; Jian Liu; Pengcheng Guo; Stephen R. Kane; Julian C. van Eyken; Nathan M. De Lee; Justin R. Crepp; Alaina C. Shelden; Chris Laws; John P. Wisniewski; Donald P. Schneider; Joshua Pepper; Stephanie A. Snedden; Kaike Pan; Dmitry Bizyaev; Howard Brewington; Olena Malanushenko; Viktor Malanushenko; Daniel Oravetz; Audrey Simmons; Shannon Watters

2010-01-01

110

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

NASA Technical Reports Server (NTRS)

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

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

2001-01-01

111

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

112

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

SciTech Connect

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

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

2012-05-15

113

Spatially resolved H2 emission from a very low-mass star  

NASA Astrophysics Data System (ADS)

Context. Molecular outflows from very low-mass stars (VLMSs) and brown dwarfs have been studied very little. So far, only a few CO outflows have been observed, allowing us to map the immediate circumstellar environment. Aims: We present the first spatially resolved H2 emission around IRS54 (YLW 52), a ~0.1-0.2 M? Class I source. Methods: By means of VLT SINFONI K-band observations, we probed the H2 emission down to the first ~50 AU from the source. Results: The molecular emission shows a complex structure delineating a large outflow cavity and an asymmetric molecular jet. Thanks to the detection of several H2 transitions, we are able to estimate average values along the jet-like structure (from source position to knot D) of AV ~ 28 mag, T ~ 2000-3000 K, and H2 column density N(H2) ~ 1.7 × 1017 cm-2. This allows us to estimate a mass loss rate of ~2 × 10-10 M? yr-1 for the warm H2 component. In addition, from the total flux of the Br ? line, we infer an accretion luminosity and mass accretion rate of 0.64 L? and ~3 × 10-7M? yr-1, respectively. The outflow structure is similar to those found in low-mass Class I and CTTS. However, the Lacc/Lbol ratio is very high (~80%), and the mass accretion rate is about one order of magnitude higher when compared to objects of roughly the same mass, pointing to the young nature of the investigated source. Based on observations collected at the European Southern Observatory Paranal, Chile (ESO programme 385.C-0893(A)).Appendices are available in electronic form at http://www.aanda.orgThe reduced datacube is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/552/L2

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

2013-04-01

114

The L723 Low-Mass Star Forming Protostellar System: Resolving a Double Core  

NASA Astrophysics Data System (ADS)

We present 1.35 mm Submillimeter Array (SMA) observations around the low-mass Class 0 source IRAS 19156+1906, at the center of the LDN 723 (L723) dark cloud. We detected emission from dust as well as emission from H2CO 30,3-20,2, DCN 3-2, and CN 2-1 lines, which arise from two cores, SMA 1 and SMA 2, separated by 2farcs9 (880 AU in projected distance). SMA 2 is associated with the previously detected source VLA 2. Weak SiO 5-4 emission is detected, possibly tracing a region of interaction between the dense envelope and the outflow. We modeled the dust and H2CO emission from the two cores. The results from the modeling show that the cores have similar physical properties (density and temperature distribution) but that SMA 2 has a larger p-H2CO abundance (by a factor of 3-10) than SMA 1. The p-H2CO abundances' findings are compatible with the value of the outer part of the circumstellar envelopes associated with Class 0 sources. SMA 2 is harboring an active multiple low-mass protostellar system and powering at least one molecular outflow. In contrast, there are no known signs of outflow activity toward SMA 1. This suggests that SMA 2 is more evolved than SMA 1. The kinematics of the two sources show marginal evidence of infall and rotation motions. The mass detected by the SMA observation, which trace scales of lsim1000 AU, is only a small fraction of the mass contained in the large-scale molecular envelope, which suggests that L723 is still in a very early phase of star formation. Despite the apparent quiescent nature of the L723, fragmentation is occurring at the center of the cloud at different scales. Thus, at sime1000 AU, the cloud has fragmented in two cores: SMA 1 and SMA 2. At the same time, at least one of these cores, SMA 2, has undergone additional fragmentation at scales of sime150 AU, forming a multiple stellar system.

Girart, J. M.; Rao, R.; Estalella, R.

2009-03-01

115

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

116

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

117

The Interior Structure Constants as an Age Diagnostic for Low-mass, Pre-main-sequence Detached Eclipsing Binary Stars  

NASA Astrophysics Data System (ADS)

We propose a novel method for determining the ages of low-mass, pre-main-sequence stellar systems using the apsidal motion of low-mass detached eclipsing binaries. The apsidal motion of a binary system with an eccentric orbit provides information regarding the interior structure constants of the individual stars. These constants are related to the normalized stellar interior density distribution and can be extracted from the predictions of stellar evolution models. We demonstrate that low-mass, pre-main-sequence stars undergoing radiative core contraction display rapidly changing interior structure constants (greater than 5% per 10 Myr) that, when combined with observational determinations of the interior structure constants (with 5%-10% precision), allow for a robust age estimate. This age estimate, unlike those based on surface quantities, is largely insensitive to the surface layer where effects of magnetic activity are likely to be most pronounced. On the main sequence, where age sensitivity is minimal, the interior structure constants provide a valuable test of the physics used in stellar structure models of low-mass stars. There are currently no known systems where this technique is applicable. Nevertheless, the emphasis on time domain astronomy with current missions, such as Kepler, and future missions, such as LSST, has the potential to discover systems where the proposed method will be observationally feasible.

Feiden, Gregory A.; Dotter, Aaron

2013-03-01

118

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

SciTech Connect

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

Bochanski, John J.; Hawley, Suzanne L.; Ivezic, Zeljko [Astronomy Department, University of Washington, P.O. Box 351580, Seattle, WA 98195 (United States); Covey, Kevin R. [Department of Astronomy, Cornell University, 226 Space Sciences Building, Ithaca, NY 14853 (United States); West, Andrew A. [Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Building 37, 77 Massachusetts Avenue, Cambridge, MA 02139 (United States); Reid, I. Neill; Golimowski, David A., E-mail: jjb@mit.ed [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States)

2010-06-15

119

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

120

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

NASA Astrophysics Data System (ADS)

We have obtained 8-13 ?m 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 using a simple model previously applied to the more massive and luminous Herbig Ae/Be systems. We determined the feature strength and shape and derived the composition and typical size of the silicate dust grains. We confirm the previously reported dependency of the silicate feature strength and shape on the grain size of the amorphous silicate dust. We examine the relation between the derived dust properties and stellar and circumstellar disk parameters, such as systemic age, luminosity of H? (LH?), disk mass, and opacity power-law index ? at radio wavelengths. A possible relation is found between silicate feature strength (grain size indicator) and the LH?, which may be an indicator of accretion activity. It implies that the turbulence induced by accretion activity may be important for grain size evolution in the disk. No clear correlation between the crystallinity and the stellar/disk parameters is found. We find that on average 5%-20% in mass of the silicate dust grains is in crystalline form, irrespective of systemic age. This latter finding supports the idea that crystalline silicate is formed at an early evolutionary phase, probably at the protostellar phase, and is remaining during the later stages. Based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.

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

2006-08-01

121

Planets around Low-mass Stars (PALMS). II. A Low-mass Companion to the Young M Dwarf GJ 3629 Separated by 0.2"  

NASA Astrophysics Data System (ADS)

We present the discovery of a 0.2" companion to the young M dwarf GJ 3629 as part of our high-contrast adaptive optics imaging search for giant planets around low-mass stars with the Keck-II and Subaru telescopes. Two epochs of imaging confirm that the pair is comoving and reveal signs of orbital motion. The primary exhibits saturated X-ray emission which, together with its UV photometry from GALEX, points to an age younger than ~300 Myr. At these ages the companion lies below the hydrogen burning limit with a model-dependent mass of 46 ± 16 M Jup based on the system's photometric distance of 22 ± 3 pc. Resolved YJHK photometry of the pair indicates a spectral type of M7 ± 2 for GJ 3629 B. With a projected separation of 4.4 ± 0.6 AU and an estimated orbital period of 21 ± 5 yr, GJ 3629 AB is likely to yield a dynamical mass in the next several years, making it one of only a handful of brown dwarfs to have a measured mass and an age constrained from the stellar primary. 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.

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

2012-09-01

122

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

NASA Astrophysics Data System (ADS)

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

Duffin, Dennis F.

2012-10-01

123

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

Microsoft Academic Search

We present evolutionary models for low mass stars from 0.075 to 1 M_sun 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

Isabelle Baraffe; Gilles Chabrier; France Allard; P. H. Hauschildt

1998-01-01

124

Chemical abundances on the secondary star in the low-mass x-ray binary Cygnus X-2  

NASA Astrophysics Data System (ADS)

We present our first results in the analysis of the secondary star in the Low-Mass X-Ray binary (LMXB) Cygnus X-2, using a high resolution spectrum obtained with UES@WHT in ORM (La Palma, Tenerife). We have used a ?2 minimization procedure to create a grid of 1 500 000 LTE synthetic spectra that allow us to obtain a plausible range of values of the stellar parameters, taking into account any possible veiling from the accretion disk produced by the presence of a neutron star as the primary. We will study the chemical abundances of several elements and study their anomalies regarding Galactic trends.

Suárez-Andrés, L.; González Hernández, J. I.; Israelian, G.; Casares, J.; Rebolo, R.

2013-02-01

125

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

SciTech Connect

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

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

2010-10-20

126

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

NASA Astrophysics Data System (ADS)

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

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

1995-01-01

127

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

NASA Technical Reports Server (NTRS)

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

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

1995-01-01

128

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

NASA Astrophysics Data System (ADS)

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

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

2013-05-01

129

The scale height of low mass stars in the galactic disc - Evidence for a young population  

NASA Astrophysics Data System (ADS)

Samples of M stars have been obtained in the direction of the SGP and at galactic latitude b = -47° from R and I band UK Schmidt plates. The absolute magnitudes MR were obtained from (R-I) colour, and the space density evaluated as a function of height above the galactic plane, for a range of luminosities MR = 9 - 14. This was then used to obtain the relation between scale height and absolute magnitude. Combining the data with previously published results for more massive stars shows a steady decline in the scale height from a maximum around 300 pc for early M stars down to 80 pc for the least luminous stars in the sample. This result is interpreted as implying a small age of 5×108yr for these low luminosity stars. If these stars are indeed young, it is argued that they are probably "brown dwarfs" which have failed to ignite hydrogen and are cooling rapidly to very low luminosities.

Hawkins, M. R. S.

1988-10-01

130

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

NASA Astrophysics Data System (ADS)

We present post-processing s-process calculations for the core He-flash of a 1M? star with and Z = 10-8. Our model shows neutron densities larger than 1014cm-3, resulting in the production of s-process elements. The model reproduces the C, O, Sr, and Ba abundances of the star hyper metal-poor star HE0107-5240 within a factor of 4.

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

2012-02-01

131

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

132

New BVI C Photometry of Low-mass Pleiades Stars: Exploring the Effects of Rotation on Broadband Colors  

NASA Astrophysics Data System (ADS)

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

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

2014-08-01

133

A super-Earth transiting a nearby low-mass star  

Microsoft Academic Search

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.

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

2009-01-01

134

Modelling the spin equilibrium of neutron stars in low-mass X-ray binaries without gravitational radiation  

NASA Astrophysics Data System (ADS)

In this paper we discuss the spin equilibrium of accreting neutron stars in low-mass X-ray binaries (LMXBs). We demonstrate that, when combined with a naive spin-up torque, the observed data lead to inferred magnetic fields which are at variance with those of Galactic millisecond radio pulsars. This indicates the need for either additional spin-down torques (e.g. gravitational radiation) or an improved accretion model. We show that a simple consistent accretion model can be arrived at by accounting for radiation pressure in rapidly accreting systems (above a few per cent of the Eddington accretion rate). In our model the inner disc region is thick and significantly sub-Keplerian and the estimated equilibrium periods are such that the LMXB neutron stars have properties that accord well with the Galactic millisecond radio pulsar sample. The implications for future gravitational-wave observations are also discussed briefly.

Andersson, N.; Glampedakis, K.; Haskell, B.; Watts, A. L.

2005-08-01

135

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

136

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

NASA Astrophysics Data System (ADS)

We describe a new method to identify young, late-type stars within ~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 Å 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.; Bessell, M. S.; Zuckerman, B.; Kastner, Joel H.

2011-02-01

137

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

NASA Astrophysics Data System (ADS)

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

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

2011-06-01

138

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

139

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

NASA Technical Reports Server (NTRS)

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

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

1993-01-01

140

Detecting donor star signatures in low mass X-ray binaries  

NASA Astrophysics Data System (ADS)

Here I present an overview of the Bowen survey of low mass X-ray binaries (LMXBs). The Bowen fluorescence technique uses the fact that narrow emission lines arise on the surface of the irradiated donor of LMXBs, making possible to derive a radial velocity curve and thereby constrain the mass of the compact object. I will concentrate on the main results of the last ?4 years, and provide an example that illustrates the validity of the technique (EXO 0748-676). Furthermore I will also illustrate where the results were limited (SAX J1808.4-3658) or did not work. Finally, now that the Bowen fluorescence technique has been applied to most of the optically bright X-ray binaries I will point out several new paths to pursue in the future.

Cornelisse, R.

2013-01-01

141

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

NASA Astrophysics Data System (ADS)

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

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

2012-09-01

142

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

NASA Technical Reports Server (NTRS)

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

Simon, Theodore

1990-01-01

143

Incorporating magnetic field observations in wind models of low-mass stars  

NASA Astrophysics Data System (ADS)

Stellar winds of cool, main-sequence stars are very tenuous and difficult to observe. Despite carrying away only a small amount of the stellar mass, they are important for regulating the rotation of the star and, consequently, its activity and magnetism. As it permeates the interplanetary space, the stellar wind interacts with any exoplanet encountered on its way, until it reaches the interstellar medium (ISM). These interactions can result in complex physical processes that depend on the characteristics of the wind. To better constrain the wind characteristics, more realistic wind models that account for factors such as stellar rotation and the complex/diverse observationally-derived stellar magnetic field configurations of cool stars are required. In this paper, I present a three-dimensional model of the wind of cool stars, which adopt as boundary condition observationally-derived magnetic maps. I also discuss how these studies are relevant for, e.g., the characterisation of the interaction between stellar winds and planets/ISM, and the propagation of cosmic rays.

Vidotto, A. A.

2014-06-01

144

Lithium production by thermohaline mixing in low-mass, low-metallicity asymptotic giant branch stars  

NASA Astrophysics Data System (ADS)

We examine the effects of thermohaline mixing on the composition of the envelopes of low-metallicity asymptotic giant branch (AGB) stars. We have evolved models of 1, 1.5 and 2M? and of metallicity Z = 10-4 from the pre-main sequence to the end of the thermal pulsing asymptotic giant branch with thermohaline mixing applied throughout the simulations. We find that the small amount of 3He that remains after the first giant branch is enough to drive thermohaline mixing on the AGB and that the mixing is most efficient in the early thermal pulses, with the efficiency dropping from pulse to pulse. We note a surprising increase in the 7Li abundance, with log10?(7Li) reaching values of over 2.5 in the 1.5M? model. It is thus possible to get stars which are both C- and Li-rich at the same time. We compare our models to measurements of carbon and lithium in carbon-enhanced metal-poor stars which have not yet reached the giant branch. These models can simultaneously reproduced the observed C and Li abundances of carbon-enhanced metal-poor turn-off stars that are Li-rich.

Stancliffe, Richard J.; Angelou, George C.; Lattanzio, John C.

2010-04-01

145

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

Microsoft Academic Search

This thesis presents an extensive observational study of the Lynds dark cloud L1551 with the goal of understanding the past, present, and future star formation in the region. This is achieved by considering the connection between the diverse phenomena observed within the cloud and the cloud environment. Roughly 140 [Special characters omitted.] of total mass at ~15 K lies within

Jonathan Jay Swift

2006-01-01

146

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

147

A super-Earth transiting a nearby low-mass star  

NASA Astrophysics Data System (ADS)

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?), 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 GJ1214b, which has a mass of 6.55M? and a radius 2.68 times Earth's radius (R?), 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 13parsecs away, so the planetary atmosphere is amenable to study with current observatories.

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

2009-12-01

148

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

149

Lithium production by thermohaline mixing in low-mass, low-metallicity asymptotic giant branch stars  

Microsoft Academic Search

We examine the effects of thermohaline mixing on the composition of the envelopes of low-metallicity asymptotic giant branch (AGB) stars. We have evolved models of 1, 1.5 and 2M&sun; and of metallicity Z = 10-4 from the pre-main sequence to the end of the thermal pulsing asymptotic giant branch with thermohaline mixing applied throughout the simulations. We find that the

Richard J. Stancliffe; George C. Angelou; John C. Lattanzio

2010-01-01

150

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

NASA Astrophysics Data System (ADS)

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

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

2011-10-01

151

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

152

The Galactic Center Cloud G2—a Young Low-mass Star with a Stellar Wind  

NASA Astrophysics Data System (ADS)

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-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 × 10-8 M ? yr-1 and wind velocity 100 km s-1, the bow shock will have an emission measure (EM = n 2 vol) at a distance ~1016 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 (~3 AU for a stellar mass of 2 M ?). In this model, the emission cloud will probably survive pericenter passage, discriminating this scenario from others.

Scoville, N.; Burkert, A.

2013-05-01

153

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.

2014-04-01

154

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

155

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

NASA Astrophysics Data System (ADS)

While Population III (Pop 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 haloes. 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 temperature. In this paper, we investigate the merging of mini-haloes with masses of a few 105 M? 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 ˜60 K triggering fragmentation. Based on Jeans estimates, the expected stellar masses are about 10 M?. Our findings show that the merging scenario is a potential pathway for the formation of low-mass stars.

Bovino, S.; Latif, M. A.; Grassi, T.; Schleicher, D. R. G.

2014-07-01

156

Bounds on Compactness for Low-Mass X-Ray Binary Neutron Stars from X-Ray Burst Oscillations  

NASA Astrophysics Data System (ADS)

We have modeled X-ray burst oscillations observed with the Rossi X-Ray Timing Explorer from two low-mass X-ray binaries (LMXB): 4U 1636-53 with a frequency of 580 Hz and 4U 1728-34 at a frequency of 363 Hz. We have computed least-squares fits to the oscillations observed during the rising phase of bursts using a model that includes emission from either a single circular hot spot or a pair of circular antipodal hot spots on the surface of a neutron star. We model the spreading of the thermonuclear hot spots by assuming that the hot spot angular size grows linearly with time. We calculate the flux as a function of rotational phase from the hot spots and take into account photon deflection in the relativistic gravitational field of the neutron star, assuming the exterior spacetime is the Schwarzschild metric. We find acceptable fits with our model in a ?2 sense and use these to place constraints on the compactness of the neutron stars in these sources. For 4U 1636-53, in which detection of a 290 Hz subharmonic supports the two-spot model, we find that the compactness (i.e., mass/radius ratio) is constrained to be M/R<0.163 at 90% confidence (G=c=1). This requires a relatively stiff equation of state (EOS) for the stellar interior. For example, if the neutron star has a mass of 1.4Msolar, then its radius must be greater than 12.8 km. Fits using a single hot spot model are not as highly constraining. We discuss the implications of our findings for recent efforts to calculate the EOS of dense nucleon matter and the structure of neutron stars.

Nath, Nitya R.; Strohmayer, Tod E.; Swank, Jean H.

2002-01-01

157

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

NASA Astrophysics Data System (ADS)

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

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

1999-11-01

158

The hydrodynamical structure of circumstellar envelopes around low mass-loss rate, low outflow velocity AGB stars  

NASA Astrophysics Data System (ADS)

Recent hydrodynamical models (Winters et al. 2000b) allow the existence of pulsating, large velocity amplitude Asymptotic Giant Branch (AGB) stars with tenuous dusty circumstellar shells, which produce low mass-loss rates and simultaneously low wind velocities. The Semi-regular long-period variable L2 Pup shows a broad SiO maser feature at 86 GHz (v=1, J=2-1), indicating an outward velocity of the material close to the stellar photosphere of at least 10 km s-1 , and narrow CO (J=2-1) and (1-0) line profiles indicating an outflow velocity of the material in the circumstellar shell of only about 3 km s-1 . This can be explained in terms of our hydrodynamical models which provide large velocities in the shocked stellar atmosphere and low velocities of the circumstellar outflow. Based on observations obtained at the European Southern Observatory, La Silla, Chile.

Winters, J. M.; Le Bertre, T.; Nyman, L.-Å.; Omont, A.; Jeong, K. S.

2002-06-01

159

DISSIPATION EFFICIENCY IN TURBULENT CONVECTIVE ZONES IN LOW-MASS STARS  

SciTech Connect

We extend the analysis of Penev et al. to calculate effective viscosities for the surface convective zones of three main-sequence stars of 0.775 M{sub sun}, 0.85 M{sub sun}, and the present day Sun. In addition, we also pay careful attention to all normalization factors and assumptions in order to derive actual numerical prescriptions for the effective viscosity as a function of the period and direction of the external shear. Our results are applicable for periods that are too long to correspond to eddies that fall within the inertial subrange of Kolmogorov scaling, but no larger than the convective turnover time, when the assumptions of the calculation break down. We find moderately anisotropic viscosity, scaling linearly with the period of the external perturbation, with its components having magnitudes between three and ten times smaller than the Zahn's prescription.

Penev, K.; Sasselov, D. [Department of Astronomy, Harvard University, 60 Garden Street, Cambridge, MA 02138 (United States); Robinson, F. [Department of Geology and Geophysics, Yale University, New Haven, CT 06520-8101 (United States); Demarque, P. [Department of Astronomy, Yale University, New Haven, CT 06520-8101 (United States)

2009-10-20

160

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

161

THE FACTORY AND THE BEEHIVE. I. ROTATION PERIODS FOR LOW-MASS STARS IN PRAESEPE  

SciTech Connect

Stellar rotation periods measured from single-age populations are critical for investigating how stellar angular momentum content evolves over time, how that evolution depends on mass, and how rotation influences the stellar dynamo and the magnetically heated chromosphere and corona. We report rotation periods for 40 late-K to mid-M star members of the nearby, rich, intermediate-age ({approx}600 Myr) open cluster Praesepe. These rotation periods were derived from {approx}200 observations taken by the Palomar Transient Factory of four cluster fields from 2010 February to May. Our measurements indicate that Praesepe's mass-period relation transitions from a well-defined singular relation to a more scattered distribution of both fast and slow rotators at {approx}0.6 M{sub sun}. The location of this transition is broadly consistent with expectations based on observations of younger clusters and the assumption that stellar spin-down is the dominant mechanism influencing angular momentum evolution at 600 Myr. However, a comparison to data recently published for the Hyades, assumed to be coeval to Praesepe, indicates that the divergence from a singular mass-period relation occurs at different characteristic masses, strengthening the finding that Praesepe is the younger of the two clusters. We also use previously published relations describing the evolution of rotation periods as a function of color and mass to evolve the sample of Praesepe periods in time. Comparing the resulting predictions to periods measured in M35 and NGC 2516 ({approx}150 Myr) and for kinematically selected young and old field star populations suggests that stellar spin-down may progress more slowly than described by these relations.

Agueeros, Marcel A.; Lemonias, Jenna J. [Department of Astronomy, Columbia University, 550 West 120th Street, New York, NY 10027 (United States); Covey, Kevin R.; Batalha, Natasha [Department of Astronomy, Cornell University, 226 Space Sciences Building, Ithaca, NY 14853 (United States); Law, Nicholas M. [Dunlap Institute for Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, Ontario M5S 3H4 (Canada); Kraus, Adam [Institute for Astronomy, University of Hawaii, Honolulu, HI 96822 (United States); Bloom, Joshua S.; Cenko, S. Bradley; Poznanski, Dovi [Department of Astronomy, University of California, Berkeley, CA 94720 (United States); Kasliwal, Mansi M.; Kulkarni, Shrinivas R.; Ofek, Eran O.; Quimby, Robert M. [Cahill Center for Astrophysics, California Institute of Technology, Pasadena, CA 91125 (United States); Nugent, Peter E. [Computational Cosmology Center, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720 (United States)

2011-10-20

162

The Factory and the Beehive. I. Rotation Periods for Low-mass Stars in Praesepe  

NASA Astrophysics Data System (ADS)

Stellar rotation periods measured from single-age populations are critical for investigating how stellar angular momentum content evolves over time, how that evolution depends on mass, and how rotation influences the stellar dynamo and the magnetically heated chromosphere and corona. We report rotation periods for 40 late-K to mid-M star members of the nearby, rich, intermediate-age (~600 Myr) open cluster Praesepe. These rotation periods were derived from ~200 observations taken by the Palomar Transient Factory of four cluster fields from 2010 February to May. Our measurements indicate that Praesepe's mass-period relation transitions from a well-defined singular relation to a more scattered distribution of both fast and slow rotators at ~0.6 M sun. The location of this transition is broadly consistent with expectations based on observations of younger clusters and the assumption that stellar spin-down is the dominant mechanism influencing angular momentum evolution at 600 Myr. However, a comparison to data recently published for the Hyades, assumed to be coeval to Praesepe, indicates that the divergence from a singular mass-period relation occurs at different characteristic masses, strengthening the finding that Praesepe is the younger of the two clusters. We also use previously published relations describing the evolution of rotation periods as a function of color and mass to evolve the sample of Praesepe periods in time. Comparing the resulting predictions to periods measured in M35 and NGC 2516 (~150 Myr) and for kinematically selected young and old field star populations suggests that stellar spin-down may progress more slowly than described by these relations.

Agüeros, Marcel A.; Covey, Kevin R.; Lemonias, Jenna J.; Law, Nicholas M.; Kraus, Adam; Batalha, Natasha; Bloom, Joshua S.; Cenko, S. Bradley; Kasliwal, Mansi M.; Kulkarni, Shrinivas R.; Nugent, Peter E.; Ofek, Eran O.; Poznanski, Dovi; Quimby, Robert M.

2011-10-01

163

THE RELATIONSHIP BETWEEN MOLECULAR GAS AND STAR FORMATION IN LOW-MASS E/S0 GALAXIES  

SciTech Connect

We consider the relationship between molecular gas and star formation surface densities in 19 morphologically defined E/S0s with stellar mass {approx}<4 x 10{sup 10} M{sub sun}, paying particular attention to those found on the blue sequence in color versus stellar mass parameter space, where spiral galaxies typically reside. While some blue-sequence E/S0s must be young major-merger remnants, many low-mass blue-sequence E/S0s appear much less disturbed and may be experiencing the milder starbursts associated with inner-disk building as spirals (re)grow. For a sample of eight E/S0s (four blue, two mid, and two red sequence) whose CARMA CO(1-0), Spitzer MIPS 24 {mu}m, and GALEX FUV emission distributions are spatially resolved on a 750 pc scale, we find roughly linear relationships between molecular gas and star formation surface densities within all galaxies, with power-law indices N = 0.6-1.9 (median 1.2). Adding 11 more blue-sequence E/S0s whose CO(1-0) emission is not as well resolved, we find that most of our E/S0s have global 1-8 kpc aperture-averaged molecular gas surface densities overlapping the range spanned by the disks and centers of spiral galaxies. While many of our E/S0s fall on the same Schmidt-Kennicutt relation as local spirals, {approx}80% (predominantly on the blue sequence) are offset toward apparently higher molecular gas star formation efficiency (i.e., shorter molecular gas depletion time). Possible interpretations of the elevated efficiencies include bursty star formation similar to that in local dwarf galaxies, H{sub 2} depletion in advanced starbursts, or simply a failure of the CO(1-0) emission to trace all of the molecular gas.

Wei, Lisa H.; Vogel, Stuart N. [Department of Astronomy, University of Maryland, College Park, MD 20742-2421 (United States); Kannappan, Sheila J.; Stark, David V. [Department of Physics and Astronomy, University of North Carolina, Phillips Hall CB 3255, Chapel Hill, NC 27599-3255 (United States); Baker, Andrew J. [Department of Physics and Astronomy, Rutgers, the State University of New Jersey, 136 Frelinghuysen Road, Piscataway, NJ 08854-8019 (United States); Laine, Seppo [Spitzer Science Center, California Institute of Technology, MS 220-6, Pasadena, CA 91125 (United States)

2010-12-10

164

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

165

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

NASA Technical Reports Server (NTRS)

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

Cui, Wei

2000-01-01

166

Gas Heating, Chemistry, and Infrared Spectra in Intermediate-Aged Disks Around Low Mass Stars  

NASA Astrophysics Data System (ADS)

We model gas and dust emission from intermediate-aged ( ˜ 107 years) disks extending 1-20 AU around a solar mass central star. The models self-consistently treat thermal balance and chemistry, and calculate the vertical density and temperature structure of the gas in a disk. The models cover gas masses 10-3 - 1 M[J] and dust masses 10-7 - 10-4 M[J], so that the dust is fairly optically thin to stellar radiation. We focus on infrared emission lines from various gas species such as the rotational lines of H[2], OH, H[2]O and CO molecules and the fine structure lines of carbon, oxygen, sulfur, iron, and silicon atoms and ions, many of which are observable by the Spitzer Space Telescope. We find that the [SI] 25.23 ? m line is the strongest emission line for a wide range of disk and stellar parameters, followed by emission from [SiII] 34.8 ? m and [FeII] 26 ? m. [FeI] 24 ? m is strong when gas masses are high ( ? 0.1 M[J]). Emission from the rotational lines of H[2] is more difficult to detect, unless disk gas masses are substantial ( ? 0.1 M[J]). The models presented here will be useful in future infrared studies of the timescale for the dispersion of gas in a planet-forming disk, and testing core accretion models of giant planet formation.

Hollenbach, D.; Gorti, U.

2004-12-01

167

Statistical Properties of Galactic ? Scuti Stars: Revisited  

NASA Astrophysics Data System (ADS)

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

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

2013-05-01

168

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

169

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

170

A 3-5 ?m VLT spectroscopic survey of embedded young low mass stars II. Solid OCN-  

NASA Astrophysics Data System (ADS)

The 4.62 ?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 ?0.85% toward low-mass YSO's and ?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.

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

2005-10-01

171

The Brown Dwarf Kinematics Project. II. Details on Nine Wide Common Proper Motion Very Low Mass Companions to Nearby Stars  

NASA Astrophysics Data System (ADS)

We report on nine wide common proper motion systems containing late-type M, L, or T companions. We confirm six previously reported companions, and identify three new systems. The ages of these systems are determined using diagnostics for both stellar primaries and low-mass secondaries and masses for the secondaries are inferred using evolutionary models. Of our three new discoveries, the M3+T6.5 pair G 204-39 and SDSS J1758+4633 has an age constrained to 0.5-1.5 Gyr making the secondary a potentially useful brown dwarf benchmark. The G5+L4 pair G 200-28 and SDSS J1416+5006 has a projected separation of ~25,000 AU making it one of the widest and lowest binding energy systems known to date. The system containing NLTT 2274 and SDSS J0041+1341 is an older M4+L0 (>4.5 Gyr) pair which shows H? activity in the secondary but not the primary making it a useful tracer of age/mass/activity trends. Two of the nine systems have discrepant component ages that emerge from stellar or ultracool diagnostics indicating possible shortcomings in our understanding of the age diagnostics of stars and brown dwarfs. We find a resolved binary frequency for widely separated (>100 AU) low-mass companions (i.e., at least a triple system) which is at least twice the frequency found for the field ultracool dwarf population. The ratio of triples to binaries and quadruples to binaries is also high for this sample: 3:5 and 1:4, respectively, compared to 8 pc sample values of 1:4 and 1:26. The additional components in these wide companion systems indicates a formation mechanism that requires a third or fourth component to maintain gravitational stability or facilitate the exchange of angular momentum. The binding energies for the nine multiples discussed in this text are among the lowest known for wide low-mass systems, suggesting that weakly bound, low-to-intermediate mass (0.2 M sun < M tot< 1.0 M sun) multiples can form and survive to exist in the field (1-8 Gyr). This paper includes data gathered with the 6.5 meter Magellan Telescopes located at Las Campanas Observatory, Chile. This paper makes use of data obtained with the telescopes operated by the SMARTS consortium.

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

2010-01-01

172

The Relationship between ? max and Age t from ZAMS to RGB-Tip for Low-mass Stars  

NASA Astrophysics Data System (ADS)

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 ?max and the large frequency separation ?? are calculated using the scaling relations. For the stars, the masses of which are from 0.8 M ? to 2.8 M ?, we can obtain the ?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 ?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 ?max diagram, which is plotted by the four sets of grid models, are consistent for red giants with masses from 1.1 M ? to 2.8 M ?. For red giants, the differences of correlation coefficients between Age and ?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 ?max diagram. By general linear least-squares, we make the polynomial fitting and deduce the relationship between log(Age) and log(?max) in red giants' evolutionary state.

Tang, Y. K.; Gai, N.

2013-07-01

173

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

SciTech Connect

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

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

2012-01-15

174

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

NASA Astrophysics Data System (ADS)

Context. While it is currently unclear from a theoretical standpoint which forces and processes dominate the formation of high-mass stars, and hence determine the mode in which they form, much of the recent observational evidence suggests that massive stars are born in a similar manner to their low-mass counterparts. Aims: This paper aims to investigate the hypothesis that the embedded luminous star AFGL 2591-VLA 3 (2.3 × 105 L? at 3.33 kpc) is forming according to a scaled-up version of a low-mass star formation scenario. Methods: We present multi-configuration Very Large Array (VLA) 3.6 cm and 7 mm, as well as Combined Array for Research in Millimeter Astronomy C18O and 3 mm continuum observations to investigate the morphology and kinematics of the ionized gas, dust, and molecular gas around AFGL 2591. We also compare our results to ancillary Gemini North near-IR images, and model the near-IR to sub-mm spectral energy distribution (SED) and Two Micron All Sky Survey (2MASS) image profiles of AFGL 2591 using a Monte-Carlo dust continuum radiative transfer code. Results: The observed 3.6 cm images uncover for the first time that the central powering source AFGL 2591-VLA 3 has a compact core plus collimated jet morphology, extending 4000 AU eastward from the central source with an opening angle of <10° at this radius. However, at 7 mm VLA 3 does not show a jet morphology, but instead compact (<500 AU) emission, some of which (<0.57 mJy of 2.9 mJy) is estimated to be from dust emission. The spectral index of AFGL 2591-VLA 3 between 3.6 cm and 7 mm was found to be between 0.4 and 0.5, similar to that of an ionized wind. If the 3.6 cm emission is modelled as an ionized jet, the jet has almost enough momentum to drive the larger-scale flow. However, assuming a shock efficiency of 10%, the momentum rate of the jet is not sufficient to ionize itself via only shocks, and thus a significant portion of the emission is instead likely created in a photoionized wind. The C18O emission uncovers dense entrained material in the outflow(s) from these young stars. The main features of the SED and 2MASS images of AFGL 2591-VLA 3 are also reproduced by our model dust geometry of a rotationally flattened envelope with and without a disk. Conclusions: The above results are consistent with a picture of massive star formation similar to that seen for low-mass protostars. However, within its envelope, AFGL 2591-VLA 3 contains at least four other young stars, constituting a small cluster. Therefore it appears that AFGL 2591-VLA 3 may be able to source its accreting material from a shared gas reservoir while still exhibiting the phenomena expected during the formation of low-mass stars. Figures 11 and 12 are available in electronic form at http://www.aanda.orgFITS files 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/551/A43

Johnston, K. G.; Shepherd, D. S.; Robitaille, T. P.; Wood, K.

2013-03-01

175

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

176

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

SciTech Connect

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

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

2013-08-20

177

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

SciTech Connect

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

Cackett, Edward M. [Department of Physics and Astronomy, Wayne State University, 666 W. Hancock St, Detroit, MI 48201 (United States); Miller, Jon M. [Department of Astronomy, University of Michigan, 500 Church St, Ann Arbor, MI 48109-1042 (United States); Reis, Rubens C.; Fabian, Andrew C. [Institute of Astronomy, University of Cambridge, Madingley Rd, Cambridge, CB3 0HA (United Kingdom); Barret, Didier, E-mail: ecackett@wayne.edu [Institut de Recherche en Astrophysique et Planetologie, 9, Avenue du Colonel Roche, BP 44346, F-31028 Toulouse Cedex 4 (France)

2012-08-10

178

Dust emission from star-forming regions. III - The Rho Ophiuchus cloud - Where are the low-mass protostars?  

NASA Astrophysics Data System (ADS)

The dust emission of the densest part of the Rho Oph cloud as indicated by CO-18 and DCO(+) line emission has been surveyed, and three structureless cores of densities of about 4 x 10 exp 5 to 10 exp 6/cu cm and masses of 15, 3, and 1 solar mass have been found along with another core of about 15 solar masses, which contains four high-density (about 10 exp 8/cu cm) condensations with masses ranging from 0.3 to 3 solar masses. These could be the low-mass counterparts of the more massive isothermal protostars detected previously in the cloud core associated with NGC 2024. The following picture of low-to-medium mass star formation emerges: structureless static cores with densities of 4 x 10 exp 5 to 10 exp 6/cu cm, which could be stabilized against collapse by magnetic fields, form the first evolutionary stage. When collapse takes place, about one-third of the core mass goes into isothermal condensations, which contract on free-fall time scales of 4 x 10 exp 4 yr and become outflow sources prior to the formation of a central stellar core. The short lifetime explains why isothermal protostars are so rare.

Mezger, P. G.; Sievers, A.; Zylka, R.; Haslam, C. G. T.; Kreysa, E.; Lemke, R.

1992-11-01

179

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

SciTech Connect

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

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

2010-06-20

180

Revisiting binary stars in population synthesis models  

NASA Astrophysics Data System (ADS)

We report results of a population synthesis model that follows the evolution of single and binary stars. In this model, we include the two He white dwarfs merger channel, suggested by Han et al., for the formation of extreme horizontal branch (EHB) stars. The physical parameters of the resulting EHB stars are derived from the Bag of Stellar Tracks and Isochrones data base by Pietrinferni et al., and are thus realistic and observationally supported. The predictions of this model are in good agreement with traditional population synthesis models, except when the spectrum of the stellar population is dominated by binary stars or their products, e.g., EHB stars in the ultraviolet (UV) of early-type galaxies (ETGs). We reproduce successfully the observed colour-magnitude diagram and spectral energy distribution of the metal-rich open cluster NGC 6791. The stellar population in this cluster may be archetypal of the stellar population in ETGs that show the UV excess phenomenon. Our models should be appropriate to study the UV upturn in ETGs.

Hernández-Pérez, Fabiola; Bruzual, Gustavo

2013-05-01

181

Fluorine in Asymptotic Giant Branch Carbon Stars Revisited  

NASA Astrophysics Data System (ADS)

A re-analysis of the fluorine abundance in three Galactic asymptotic giant branch (AGB) carbon stars (TX Psc, AQ Sgr, and R Scl) has been performed from the molecular HF (1-0) R9 line at 2.3358 ?m. High resolution (R ~ 50,000) and high signal-to-noise spectra obtained with the CRIRES spectrograph and the VLT telescope or from the NOAO archive (for TX Psc) have been used. Our abundance analysis uses the latest generation of MARCS model atmospheres for cool carbon-rich stars. Using spectral synthesis in local thermodynamic equilibrium, we derive for these stars fluorine abundances that are systematically lower by ~0.8 dex in average with respect to the sole previous estimates by Jorissen et al. The possible reasons of this discrepancy are explored. We conclude that the difference may rely on the blending with C-bearing molecules (CN and C2) that were not properly taken into account in the former study. The new F abundances are in better agreement with the prediction of full network stellar models of low-mass AGB stars. These models also reproduce the s-process elements distribution in the sampled stars. This result, if confirmed in a larger sample of AGB stars, might alleviate the current difficulty to explain the largest [F/O] ratios found by Jorissen et al. In particular, it may not be necessary to search for alternative nuclear chains affecting the production of F in AGB stars.

Abia, C.; Recio-Blanco, A.; de Laverny, P.; Cristallo, S.; Domínguez, I.; Straniero, O.

2009-04-01

182

The SLoWPoKES catalog of low-mass ultra-wide binaries: A cool stars resource for testing fundamental properties and for constraining binary formation theory  

NASA Astrophysics Data System (ADS)

We present results from the Sloan Low-mass Wide Pairs of Kinematically Equivalent Stars (SLoWPoKES) catalog of ultra-wide (103-105.5 AU), low-mass (K5-M7) common proper motion binaries. We constructed a Galactic model, based on empirical stellar number density and 3D velocity distributions, to select bona fide pairs with probability of chance alignment < 5 %, making SLoWPoKES an efficient sample for followup observations. Our initial catalog contains 1342 disk dwarf, subdwarf, and white dwarf-red dwarf systems and is the largest collection of low-mass, wide binaries ever assembled. The diversity - in mass, metallicity, age, and evolutionary states - of SLoWPoKES pairs makes it a valuable resource of coeval laboratories to examine and constrain the physical properties of low-mass stars. SLoWPoKES pairs show signatures of two (or more) formation modes in the distribution of the physical separation and higher-order multiplicity. Neither dynamical dissipation of primordial triples/quadruples or dynamical capture of ejected stars can explain the observed populations by itself. We use follow-up spectroscopic observations to recalibrate the metallicity-sensitive ?{TiO/CaH} index by assuming that both members of the binary system have the same composition. Our new formulation is a significantly better tracer of absolute metallicity, particularly for the early-type M dwarfs. The catalogs are publicly available on a custom data visualization portal.

Dhital, S.; West, A. A.; Stassun, K. G.; Law, N. M.

2013-02-01

183

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

184

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

185

On the Bolometric Quiescent Luminosity and Luminosity Swing of Black Hole Candidate and Neutron Star Low-Mass X-Ray Transients  

Microsoft Academic Search

Low-mass X-ray transients hosting black hole candidates display on average a factor of ~100 larger swing in the minimum (quiescent) to maximum (outburst) X-ray luminosity than neutron star systems do, despite the fact that the swing in the mass inflow rate is likely in the same range. Advection-dominated accretion flows, ADAFs, were proposed to interpret such a difference, because the

Sergio Campana; Luigi Stella

2000-01-01

186

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

187

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

188

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

NASA Astrophysics Data System (ADS)

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

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

2010-01-01

189

Destruction of wide binary stars in low-mass elliptical galaxies: implications for initial mass function estimates  

NASA Astrophysics Data System (ADS)

We discuss the effects of destruction of wide binaries in the nuclei of the lower mass giant elliptical galaxies. We show that the numbers of barium stars and extrinsic S stars should be dramatically reduced in these galaxies compared to what is seen in the largest elliptical galaxies. Given that the extrinsic S stars show strong Wing-Ford band and Na I D absorption, we argue that the recent claims of different initial mass functions from the most massive elliptical galaxies versus lower mass ellipticals may be the result of extrinsic S stars, rather than bottom-heavy initial mass function.

Maccarone, Thomas J.

2014-07-01

190

Destruction of wide binary stars in low-mass elliptical galaxies: implications for initial mass function estimates  

NASA Astrophysics Data System (ADS)

We discuss the effects of destruction of wide binaries in the nuclei of the lower mass giant elliptical galaxies. We show that the numbers of barium stars and extrinsic S stars should be dramatically reduced in these galaxies compared to what is seen in the largest elliptical galaxies. Given that the extrinsic S stars show strong Wing-Ford band and Na I D absorption, we argue that the recent claims of different initial mass functions from the most massive elliptical galaxies versus lower mass ellipticals may be the result of extrinsic S stars, rather than bottom-heavy initial mass function.

Maccarone, Thomas J.

2014-04-01

191

The Mass and Radius of the Neutron Star in the Bulge Low-mass X-Ray Binary KS 1731-260  

NASA Astrophysics Data System (ADS)

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

Özel, Feryal; Gould, Andrew; Güver, Tolga

2012-03-01

192

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

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

193

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

194

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

NASA Astrophysics Data System (ADS)

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

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

2005-06-01

195

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

196

PP 13S, a young, low-mass FU Orionis-type pre-main sequence star  

NASA Astrophysics Data System (ADS)

We present near-IR imaging and spectroscopy, together with sub-mm/mm photometry and {(12) CO} molecular line maps of the cometary nebula PP 13S. Previous models have been unable to resolve whether PP 13S is a young, pre-main sequence star or an old, evolved object. Our new observations prove conclusively that PP 13S is a young stellar object with a luminosity of ~ 30Lsun and an accretion disk with an inclination of ~ 40°. It has a {(12) CO} J=2-1 outflow, strong and broadened CO overtone band absorption, and vibrationally excited H_2 emission. We conclude that PP 13S has all the characteristics of an FU Orionis-type star, and although no outburst has been observed, we suggest that PP 13S should be added to the list of FUor pre-main sequence stars. Our study also includes PP13N, which we resolve into a young double star system.

Sandell, Goeran; Aspin, Colin

1998-05-01

197

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

NASA Astrophysics Data System (ADS)

Use of the highly sensitive Hokupa'a/Gemini curvature wave front sensor has allowed direct adaptive optics (AO) guiding on very low mass (VLM) stars with SpT = M8.0-L0.5. A survey of 39 such objects detected nine VLM binaries (seven of which were discovered for the first time to be binaries). Most of these systems are tight (separation <5 AU) and have similar masses (?Ks<0.8 mag; 0.852.4 mag and consist of a VLM star orbited by a much cooler L7-L8 brown dwarf companion. On the basis of this flux-limited (Ks<12 mag) survey of 39 M8.0-L0.5 stars (mainly from the 2MASS sample of Gizis et al.), we find a sensitivity-corrected binary fraction in the range 15%+/-7% for M8.0-L0.5 stars with separations greater than 2.6 AU. This is less than the 32%+/-9% measured for more massive M0-M4 dwarfs over the same separation range. It appears M8.0-L0.5 binaries (as well as L and T dwarf binaries) have a much smaller semimajor axis distribution peak (~4 AU) compared to more massive M and G dwarfs, which have a broad peak at larger ~30 AU separations. We also find no VLM binary systems (defined here as systems with Mtot<0.185Msolar) with separations greater than 15 AU. We briefly explore possible reasons why VLM binaries are slightly less common, nearly equal in mass, and much more tightly bound compared to more massive binaries. We investigate the hypothesis that the lack of wide (a>20 AU) VLM/brown dwarf binaries may be explained if the binary components were given a significant differential velocity kick. Such a velocity kick is predicted by current ``ejection'' theories, where brown dwarfs are formed because they are ejected from their embryonic minicluster and therefore starved of accretion material. We find that a kick from a close triple or quadruple encounter (imparting a differential kick of ~3 km s-1 between the members of an escaping binary) could reproduce the observed cutoff in the semimajor axis distribution at ~20 AU. However, the estimated binarity (<~5%) produced by such ejection scenarios is below the 15%+/-7% observed. Similarly, VLM binaries could be the final hardened binaries produced when a minicluster decays. However, the models of Sterzik & Durisen and Durisen, Sterzik, & Pickett also could not produce a VLM binary fraction of 15% and a G star binary fraction of 57%. The observed VLM binary frequency could possibly be produced by cloud core fragmentation. However, our estimate of a fragmentation-produced VLM binary semimajor axis distribution contains a significant fraction of ``wide'' VLM binaries with a>20 AU in contrast to observation. In summary, more detailed theoretical work will be needed to explain these interesting results that show VLM binaries to be a significantly different population from more massive M & G dwarf binaries.

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

2003-04-01

198

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

199

Collective Properties of Neutron-star X-Ray Binary Populations of Galaxies. II. Pre-low-mass X-Ray Binary Properties, Formation Rates, and Constraints  

NASA Astrophysics Data System (ADS)

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

Bhadkamkar, H.; Ghosh, P.

2014-04-01

200

Emission line diagnostics for accretion and outflows in young very low-mass stars and brown dwarfs  

NASA Astrophysics Data System (ADS)

We discuss accretion and outflow properties of three very low-mass young stellar objects based on broad-band mid-resolution X-Shooter/VLT spectra. Our targets (FU Tau A, 2M1207-39, and Par-Lup3-4) have spectral types between M5 and M8, ages between 1Myr and ~ 10Myr, and are known to be accreting from previous studies. The final objective of our project is the determination of mass outflow to accretion rate for objects near or within the substellar regime as a probe for the T Tauri phase of brown dwarfs and the investigation of variability in the accretion and outflow processes.

Stelzer, B.; Alcalá, J. M.; Whelan, E.; Scholz, A.

2014-01-01

201

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

202

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

203

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

NASA Astrophysics Data System (ADS)

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 revisit their results, analysing 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 cannot be omitted when interpreting multiplicity data. A new analysis of the Corona Australis region is performed within the standard model. It is found that this region is likely as unevolved as Taurus and an initial density of ?190 M? pc-3 is required to produce the presently observed binary population, which is close to its present-day density.

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

2014-07-01

204

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

205

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

206

Timing Spectroscopy of Quasi-Periodic Oscillations in the Low-Mass X-ray Neutron Star Binaries  

NASA Astrophysics Data System (ADS)

Precise simultaneous measurements of the frequencies of the two kiloHertz quasi-periodic oscillations (referred in the literature as upper and lower kHz QPOs) cast doubts on the validity of the simple beat-frequency interpretation and some of the modifications introduced to explain the results of the varying frequency difference. A new model explains the variation of the frequency difference suggesting that the upper kHz QPO, namely, ?h is an upper hybrid frequency of the Keplerian oscillator under the influence of the Coriolis force and the lower kHz QPO is the Keplerian frequency ?K. Such an oscillator has two branches characterized by high frequency ?h (around 1 kHz) and by low frequency ?L (around 50 Hz). The frequency ?L depends strongly on the angle, ? between the normal to the neutron star disk and ? - the angular velocity of the magnetosphere surrounding the neutron star. In the lower part of the QPO spectrum (around 10 Hz), this model identifies the frequency of radial viscous oscillations ?v (previously called ``extra noise component") and the break frequency ?b which is associated with the diffusive process in the transition region (the innermost part of the disk). According to this model, all frequencies (namely ?h, ?L, ?b and ?v) have specific dependences on ?K. This talk focuses on the verification of the predicted relations. For three sources, namely Sco X-1, 4U 1728-34 and 4U 0614+09 we present a comprehensive classification of QPO within the framework of this model. We also demonstrate that for four sources the data confirm the constancy of ? angle (an analytic combination of the observable frequencies) for a given source in the wide range of kHz QPO frequencies. For the source 4U 0614+09 it is 15.5o +/- 1o, for 4U 1728-34 it is 8.3 +/- 1.0o, which is slightly larger than ?=5.5 +/- 0.5o previously found for Sco X-1 and ?=3.9 +/- 0.2o for 4U 1702-42.

Titarchuk, L.; Osherovich, V.

207

New insights into the earliest phases of low-mass star formation with the Herschel Space Observatory  

NASA Astrophysics Data System (ADS)

The Herschel Space Observatory has been revolutionizing our understanding of the the earliest phases of star formation. In this contribution, we describe early results from the Gould Belt Survey, a Herschel Key Project to map 15 nearby molecular clouds in continuum emission from 70 ?m to 500 ?m. In particular, I describe how the sensitive and wide maps of the Aquila Rift have strongly confirmed the similarity between the shapes of the stellar Initial Mass Function and the prestellar core mass function (CMF). Also, the Herschel map sensitivity to larger scale emission has revealed that prestellar cores form almost exclusively within dense filaments that exceed a critical mass per unit length defined by temperature (and gravity). Finally, filaments in three clouds, IC 5146, Polaris and Aquila, are found to have similar widths of ˜0.1 pc, approximately the scale where the turbulent velocity equals the sound speed of 10 K gas. This common width suggests filaments themselves are formed through collisional shocks of turbulent flows and evolve in quasi-virial balance through mass accretion.

Di Francesco, J.

208

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

209

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

210

SPECTRAL ENERGY DISTRIBUTIONS OF YOUNG STARS IN IC 348: THE ROLE OF DISKS IN ANGULAR MOMENTUM EVOLUTION OF YOUNG, LOW-MASS STARS  

SciTech Connect

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 {mu}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.; Stassun, Keivan G. [Department of Physics and Astronomy, Vanderbilt University, 6301 Stevenson Center, Nashville, TN 37235 (United States); Covey, Kevin R. [Department of Astronomy, Cornell University, 226 Space Sciences Building, Ithaca, NY 14853 (United States)

2011-08-15

211

Structure and evolution of low-mass W Ursae Majoris type systems - III. The effects of the spins of the stars  

NASA Astrophysics Data System (ADS)

In a previous paper, using Eggleton's stellar evolution code, we have discussed the structure and evolution of low-mass W Ursae Majoris (W UMa) type contact binaries with angular momentum loss owing to gravitational radiation or magnetic braking. We find that gravitational radiation is almost insignificant for cyclic evolution of low-mass W UMa type systems, and it is possible for angular momentum to be lost from W UMa systems in a magnetic stellar wind. The weaker magnetic activity shown by observations in W UMa systems is likely caused by the lower mass of the convective envelopes in these systems than in similar but non-contact binaries. The spin angular momentum cannot be neglected at any time for W UMa type systems, especially for those with extreme mass ratios. The spin angular momenta of both components are included in this paper and they are found to have a significant influence on the cyclic evolution of W UMa systems. We investigate the influence of the energy transfer on the common convective envelopes of both components in detail. We find that the mass of the convective envelope of the primary in contact evolution is slightly more than that in poor thermal contact evolution, and that the mass of the convective envelope of the secondary in contact evolution is much less than that in poor thermal contact evolution. Meanwhile, the rate of angular momentum loss of W UMa type systems is much lower than that of poor thermal contact systems. This is indeed caused by the lower masses of the convective envelopes of the components in W UMa type systems. Although the models with angular momentum loss for W UMa systems exhibit cyclic evolution, they seem to show that a W UMa system cannot continue this type of cyclic evolution indefinitely, and it might coalesce into a fast-rotating star after about 1200 cycles of evolution (about 7.0 × 109 yr).

Li, Lifang; Han, Zhanwen; Zhang, Fenghui

2005-06-01

212

Herschel/HIFI observations of high-J CO lines in the NGC 1333 low-mass star-forming region  

NASA Astrophysics Data System (ADS)

Herschel/HIFI observations of high-J lines (up to Ju = 10) of 12CO, 13CO and C18O are presented toward three deeply embedded low-mass protostars, NGC 1333 IRAS 2A, IRAS 4A, and IRAS 4B, obtained as part of the Water In Star-forming regions with Herschel (WISH) key program. The spectrally-resolved HIFI data are complemented by ground-based observations of lower-J CO and isotopologue lines. The 12CO 10-9 profiles are dominated by broad (FWHM 25-30 km s-1) emission. Radiative transfer models are used to constrain the temperature of this shocked gas to 100-200 K. Several CO and 13CO line profiles also reveal a medium-broad component (FWHM5-10 km s-1), seen prominently in H2O lines. Column densities for both components are presented, providing a reference for determining abundances of other molecules in the same gas. The narrow C18O 9-8 lines probe the warmer part of the quiescent envelope. Their intensities require a jump in the CO abundance at an evaporation temperature around 25 K, thus providing new direct evidence for a CO ice evaporation zone around low-mass protostars. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendices and acknowledgements (pages 5 to 7) are only available in electronic form at http://www.aanda.org

Y?ld?z, U. A.; van Dishoeck, E. F.; Kristensen, L. E.; Visser, R.; Jørgensen, J. K.; Herczeg, G. J.; van Kempen, T. A.; Hogerheijde, M. R.; Doty, S. D.; Benz, A. O.; Bruderer, S.; Wampfler, S. F.; Deul, E.; Bachiller, R.; Baudry, A.; Benedettini, M.; Bergin, E.; Bjerkeli, P.; Blake, G. A.; Bontemps, S.; Braine, J.; Caselli, P.; Cernicharo, J.; Codella, C.; Daniel, F.; di Giorgio, A. M.; Dominik, C.; Encrenaz, P.; Fich, M.; Fuente, A.; Giannini, T.; Goicoechea, J. R.; de Graauw, Th.; Helmich, F.; Herpin, F.; Jacq, T.; Johnstone, D.; Larsson, B.; Lis, D.; Liseau, R.; Liu, F.-C.; Marseille, M.; McCoey, C.; Melnick, G.; Neufeld, D.; Nisini, B.; Olberg, M.; Parise, B.; 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.; Dieleman, P.; Jellema, W.; Ossenkopf, V.; Schieder, R.; Stutzki, J.

2010-10-01

213

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

214

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

215

Evolution of Thermally Pulsing Asymptotic Giant Branch Stars. IV. Constraining Mass loss and Lifetimes of Low Mass, Low Metallicity AGB Stars  

NASA Astrophysics Data System (ADS)

The evolution and lifetimes of thermally pulsating asymptotic giant branch (TP-AGB) stars suffer from significant uncertainties. In this work, we analyze the numbers and luminosity functions of TP-AGB stars in six quiescent, low metallicity ([Fe/H] <~ –0.86) galaxies taken from the ACS Nearby Galaxy Survey Treasury sample, using Hubble Space Telescope (HST) photometry in both optical and near-infrared filters. The galaxies contain over 1000 TP-AGB stars (at least 60 per field). We compare the observed TP-AGB luminosity functions and relative numbers of TP-AGB and red giant branch (RGB) stars, N TP-AGB/N RGB, to models generated from different suites of TP-AGB evolutionary tracks after adopting star formation histories derived from the HST deep optical observations. We test various mass-loss prescriptions that differ in their treatments of mass loss before the onset of dust-driven winds (pre-dust). These comparisons confirm that pre-dust mass loss is important, since models that neglect pre-dust mass loss fail to explain the observed N TP-AGB/N RGB ratio or the luminosity functions. In contrast, models with more efficient pre-dust mass loss produce results consistent with observations. We find that for [Fe/H] <~ –0.86, lower mass TP-AGB stars (M <~ 1 M ?) must have lifetimes of ~0.5 Myr and higher masses (M <~ 3 M ?) must have lifetimes <~ 1.2 Myr. In addition, assuming our best-fitting mass-loss prescription, we show that the third dredge-up has no significant effect on TP-AGB lifetimes in this mass and metallicity range. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

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

2014-07-01

216

XTE J1701-462 AND ITS IMPLICATIONS FOR THE NATURE OF SUBCLASSES IN LOW-MAGNETIC-FIELD NEUTRON STAR LOW-MASS X-RAY BINARIES  

SciTech Connect

We report on an analysis of Rossi X-Ray Timing Explorer data of the transient neutron star low-mass X-ray binary (NS-LMXB) XTE J1701-462, obtained during its 2006-2007 outburst. The X-ray properties of the source changed between those of various types of NS-LMXB subclasses. At high luminosities, the source switched between two types of Z source behavior and at low luminosities we observed a transition from Z source to atoll source behavior. These transitions between subclasses primarily manifest themselves as changes in the shapes of the tracks in X-ray color-color (CD) and hardness-intensity diagrams (HID), but they are accompanied by changes in the kHz quasi-periodic oscillations, broadband variability, burst behavior, and/or X-ray spectra. We find that for most of the outburst the low-energy X-ray flux is a good parameter to track the gradual evolution of the tracks in CD and HID, allowing us to resolve the evolution of the source in greater detail than before and relate the observed properties to other NS-LMXBs. We further find that during the transition from Z to atoll, characteristic behavior known as the atoll upper banana can equivalently be described as the final stage of a weakening Z source flaring branch, thereby blurring the line between the two subclasses. Our findings strongly suggest that the wide variety in behavior observed in NS-LXMBs with different luminosities can be linked through changes in a single variable parameter, namely the mass accretion rate, without the need for additional differences in the neutron star parameters or viewing angle. We briefly discuss the implications of our findings for the spectral changes observed in NS-LMXBs and suggest that, contrary to what is often assumed, the position along the color-color tracks of Z sources is not determined by the instantaneous mass accretion rate.

Homan, Jeroen; Fridriksson, Joel K.; Remillard, Ronald A.; Lewin, Walter H. G. [MIT Kavli Institute for Astrophysics and Space Research, 70 Vassar Street, Cambridge, MA 02139 (United States); Van der Klis, Michiel; Wijnands, Rudy; Altamirano, Diego [Astronomical Institute 'Anton Pannekoek', University of Amsterdam, Science Park 904, 1098 XH, Amsterdam (Netherlands); Mendez, Mariano [Kapteyn Astronomical Institute, Groningen University, 9700 AV, Groningen (Netherlands); Lin Dacheng [Centre d'Etude Spatiale des Rayonnements, UMR 5187, 9 Av. du Colonel Roche, BP 44346, 31028 Toulouse Cedex 4 (France); Casella, Piergiorgio [School of Physics and Astronomy, University of Southampton, Southampton, Hampshire, SO17 1BJ (United Kingdom); Belloni, Tomaso M., E-mail: jeroen@space.mit.ed [INAF-Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate (Italy)

2010-08-10

217

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

218

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

219

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

220

Variable Stars of the DRACO DWARF Spheroidal Glaxay: Revisited.  

National Technical Information Service (NTIS)

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 (RRL) stars, 9 anomalous Cephe...

H. A. Smith H. C. Harris K. Kinemuchi L. A. Snyder N. A. Silbermann

2008-01-01

221

Coronal mass ejection (CME) activity of low mass M stars as an important factor for the habitability of terrestrial exoplanets. I. CME impact on expected magnetospheres of Earth-like exoplanets in close-in habitable zones.  

PubMed

Low mass M- and K-type stars are much more numerous in the solar neighborhood than solar-like G-type stars. Therefore, some of them may appear as interesting candidates for the target star lists of terrestrial exoplanet (i.e., planets with mass, radius, and internal parameters identical to Earth) search programs like Darwin (ESA) or the Terrestrial Planet Finder Coronagraph/Inferometer (NASA). The higher level of stellar activity of low mass M stars, as compared to solar-like G stars, as well as the closer orbital distances of their habitable zones (HZs), means that terrestrial-type exoplanets within HZs of these stars are more influenced by stellar activity than one would expect for a planet in an HZ of a solar-like star. Here we examine the influences of stellar coronal mass ejection (CME) activity on planetary environments and the role CMEs may play in the definition of habitability criterion for the terrestrial type exoplanets near M stars. We pay attention to the fact that exoplanets within HZs that are in close proximity to low mass M stars may become tidally locked, which, in turn, can result in relatively weak intrinsic planetary magnetic moments. Taking into account existing observational data and models that involve the Sun and related hypothetical parameters of extrasolar CMEs (density, velocity, size, and occurrence rate), we show that Earth-like exoplanets within close-in HZs should experience a continuous CME exposure over long periods of time. This fact, together with small magnetic moments of tidally locked exoplanets, may result in little or no magnetospheric protection of planetary atmospheres from a dense flow of CME plasma. Magnetospheric standoff distances of weakly magnetized Earth-like exoplanets at orbital distances

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

2007-02-01

222

THE FREQUENCY OF LOW-MASS EXOPLANETS  

SciTech Connect

We report first results from the Anglo-Australian Telescope Rocky Planet Search-an intensive, high-precision Doppler planet search targeting low-mass exoplanets in contiguous 48 night observing blocks. On this run, we targeted 24 bright, nearby and intrinsically stable Sun-like stars selected from the Anglo-Australian Planet Search's main sample. These observations have already detected one low-mass planet reported elsewhere (HD 16417b), and here we reconfirm the detection of HD 4308b. Further, we have Monte Carlo simulated data from this run on a star-by-star basis to produce robust detection constraints. These simulations demonstrate clear differences in the exoplanet detectability functions from star to star due to differences in sampling, data quality and intrinsic stellar stability. They reinforce the importance of star-by-star simulation when interpreting the data from Doppler planet searches. These simulations indicate that for some of our target stars we are sensitive to close-orbiting planets as small as a few Earth masses. The two low-mass planets present in our 24-star sample indicate that the exoplanet minimum mass function at low masses is likely to be a flat {alpha} {approx} -1 (for dN/dM {proportional_to} M {sup {alpha}}) and that between 15% {+-} 10% (at {alpha} = -0.3) and 48% {+-} 34% (at {alpha} = -1.3) of stars host planets with orbital periods of less than 16 days and minimum masses greater than 3 M {sub +}.

O'Toole, S. J. [Anglo-Australian Observatory, P.O. Box 296, Epping 1710 (Australia); Jones, H. R. A. [Centre for Astrophysics Research, University of Hertfordshire, Hatfield, AL 10 9AB (United Kingdom); Tinney, C. G.; Bailey, J.; Wittenmyer, R. A. [Department of Astrophysics, School of Physics, University of NSW, 2052 (Australia); Butler, R. P. [Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington DC 20015-1305 (United States); Marcy, G. W. [Department of Astronomy, University of California, Berkeley, CA 94720 (United States); Carter, B. [Faculty of Sciences, University of Southern Queensland, Toowoomba, Queensland 4350 (Australia)], E-mail: otoole@aao.gov.au

2009-08-20

223

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

224

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

225

Planets around Low-mass Stars (PALMS). I. A Substellar Companion to the Young M Dwarf 1RXS J235133.3+312720  

NASA Astrophysics Data System (ADS)

We report the discovery of a brown dwarf companion to the young M dwarf 1RXS J235133.3+312720 as part of a high contrast imaging search for planets around nearby young low-mass stars with Keck-II/NIRC2 and Subaru/HiCIAO. The 2farcs4 (~120 AU) pair is confirmed to be comoving from two epochs of high-resolution imaging. Follow-up low- and moderate-resolution near-infrared spectroscopy of 1RXS J2351+3127 B with IRTF/SpeX and Keck-II/OSIRIS reveals a spectral type of L0+2 -1. The M2 primary star 1RXS J2351+3127 A exhibits X-ray and UV activity levels comparable to young moving group members with ages of ~10-100 Myr. UVW kinematics based the measured radial velocity of the primary and the system's photometric distance (50 ± 10 pc) indicate it is likely a member of the ~50-150 Myr AB Dor moving group. The near-infrared spectrum of 1RXS J2351+3127 B does not exhibit obvious signs of youth, but its H-band morphology shows subtle hints of intermediate surface gravity. The spectrum is also an excellent match to the ~200 Myr M9 brown dwarf LP 944-20. Assuming an age of 50-150 Myr, evolutionary models imply a mass of 32 ± 6 M Jup for the companion, making 1RXS J2351+3127 B the second lowest-mass member of the AB Dor moving group after the L4 companion CD-35 2722 B and one of the few benchmark brown dwarfs known at young ages. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

Bowler, Brendan P.; Liu, Michael C.; Shkolnik, Evgenya L.; Dupuy, Trent J.; Cieza, Lucas A.; Kraus, Adam L.; Tamura, Motohide

2012-07-01

226

Revisiting The First Galaxies: The Epoch of Population III Stars  

NASA Astrophysics Data System (ADS)

We study the formation of the first galaxies using new hydrodynamic cosmological simulations with the ART code. Our simulations feature a recently developed model for dust-based formation of molecular gas. Here, we develop and implement a new recipe for the formation of metal-free Pop III stars. We reach a spatial resolution of 2 pc at z=10 and resolve star-forming galaxies with the masses above 10^6 solar masses. We find the epoch during which Pop III stars dominate the energy and metal budget of the universe to be short-lived. While these stars seed their host galaxies with metals, they cannot drive significant outflows to enrich the IGM in our simulations. Feedback from pair instability supernovae causes Pop III star formation to self-terminate within their host galaxies, but is not strong enough to suppress star formation in external galaxies. Within any individual galaxy, Pop II stars overtake Pop III stars within ~50-150 Myr. A threshold of M = 3 * 10^6 solar masses separates galaxies that lose a significant fraction of their baryons due to Pop III feedback from those that do not. Understanding the nature of the transition between Pop III and Pop II star formation is of key importance for studying the dawn of galaxy formation.

Muratov, Alexander; Gnedin, O. Y.; Gnedin, N. Y.; Zemp, M. K.

2013-01-01

227

The core helium flash revisited III. From Pop I to Pop III stars  

Microsoft Academic Search

Degenerate ignition of helium in low-mass stars at the end of the red giant\\u000abranch phase leads to dynamic convection in their helium cores. One-dimensional\\u000a(1D) stellar modeling of this intrinsically multi-dimensional dynamic event is\\u000alikely to be inadequate. Previous hydrodynamic simulations imply that the\\u000asingle convection zone in the helium core of metal-rich Pop I stars grows\\u000aduring the

Miroslav Mocak; Simon W. Campbell; Ewald Mueller; Konstantinos Kifonidis

2010-01-01

228

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

SciTech Connect

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

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

2013-07-20

229

HIGH-RESOLUTION OBSERVATIONS OF DUST CONTINUUM EMISSION AT 340 GHz FROM THE LOW-MASS T TAURI STAR FN TAURI  

SciTech Connect

FN Tau is a rare example of a very low-mass T Tauri star that exhibits a spatially resolved nebulosity in near-infrared scattering light. To directly derive the parameters of a circumstellar disk around FN Tau, observations of dust continuum emission at 340 GHz are carried out with the Submillimeter Array (SMA). A point-like dust continuum emission was detected with a synthesized beam of {approx}0.''7 in FWHM. From the analysis of the visibility plot, the radius of the emission is estimated to be <=0.''29, corresponding to 41 AU. This is much smaller than the radius of the nebulosity, 1.''85 for its brighter part at 1.6 {mu}m. The 340 GHz continuum emission observed with the SMA and the photometric data at lambda <= 70 {mu}m are explained by a power-law disk model whose outer radius and mass are 41 AU and (0.24-5.9) x 10{sup -3} M{sub sun}, respectively, if the exponent of dust mass opacity (beta) is assumed to be 0-2. The disk model cannot fully reproduce the flux density at 230 GHz obtained with the IRAM 30 m telescope, suggesting that there is another extended 'halo' component that is missed in the SMA observations. By requiring the halo not to be detected with the SMA, the lower limit to the size of the halo is evaluated to be between 174 AU and 574 AU, depending on the assumed beta value. This size is comparable to the near-infrared nebulosity, implying that the halo unseen with the SMA corresponds to the origin of the near-infrared nebulosity. The halo can contain mass comparable to or at most 8 times greater than that of the inner power-law disk, but its surface density should be lower than that at the outer edge of the power-law disk by more than 1 order of magnitude. The physical nature of the halo is unclear, but it may be the periphery of a flared circumstellar disk that is not described well in terms of a power-law disk model, or a remnant of a protostellar envelope having flattened structure.

Momose, Munetake [College of Science, Ibaraki University, Bunkyo 2-1-1, Mito, Ibaraki 310-8512 (Japan); Ohashi, Nagayoshi [Academia Sinica Institute of Astronomy and Astrophysics, P.O. Box 23-141, Taipei 10617, Taiwan (China); Kudo, Tomoyuki; Tamura, Motohide [National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588 (Japan); Kitamura, Yoshimi, E-mail: momose@mx.ibaraki.ac.j [Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510 (Japan)

2010-03-20

230

On the stability of the thermal Comptonization index in neutron star low-mass X-ray binaries in their different spectral states  

NASA Astrophysics Data System (ADS)

Context. Most of the spectra of neutron star low-mass X-ray binaries (NS LMXBs), whether they are persistent or transient, are characterized by the presence of a strong thermal Comptonization bump, which is thought to originate in the transition layer (TL) between the accretion disk and the NS surface. The observable quantities that characterize this component, which is dominating the emission below 30 keV, are the spectral index ? and the rollover energy, both related to the electron temperature and optical depth of the plasma. Aims: Starting from observational results on a sample of NS LMXBs in different spectral states, we formulate the problem of X-ray spectral formation in the TL of these sources. We predict a stability of the thermal Comptonization spectral index in different spectral states if the energy release in the TL is much higher than the intercepted flux coming from the accretion disk. Methods: We use an equation for the energy balance and the radiative transfer diffusion equation for a slab geometry in the TL to derive a formula for the thermal Comptonization index ?. We show that in this approximation the TL electron temperature kTe and optical depth ?0 can be written as a function of the energy flux from the disk intercepted by the corona (TL) and that in the corona itself, Qdisk/Qcor. Because the spectral index ? depends on kTe and ?0, this in turn leads to a relation ? = f(Qdisk/Qcor), with ? ~ 1 when Qdisk/Qcor ? 1. Results: We show that the observed spectral index ? for the sample of sources here considered lies in a belt around 1 ± 0.2 apart for the case of GX 354-0. Comparing our theoretical predictions with observations, we claim that this result, which is consistent with the condition Qdisk/Qcor ? 1, can give us constraints on the accretion geometry of these systems, an issue that seems difficult to be solved with only the spectral analysis method.

Farinelli, R.; Titarchuk, L.

2011-01-01

231

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

232

Revisiting NGC 3109: A Systematic Blue Massive Stars Analysis  

NASA Astrophysics Data System (ADS)

In the last years thoughtful quantitative analyses of extragalactic blue massive stars have shown not only that these studies are doable even at large distances (e.g. ˜2 Mpc away), but also essential for a better understanding of host galaxies and stellar evolution, in environments with different metallicities. Carrying out a systematic analysis is mandatory. We present in this work the tools and the FASTWIND stellar grids designed for overcoming this issue, measuring stellar parameters and chemical abundances. We have applied these techniques to the complete sample of blue massive stars observed in NGC 3109, a low metallicity irregular galaxy at 1.3 Mpc, by the ARAUCARIA project and presented by Evans et al. (2007). We report the first systematic quantitative analysis in this galaxy, together with the stellar parameters and the evolution stages of these objects. The chemical composition obtained will shed new light about the chemical composition and distribution along NGC 3109.

Castro, Norberto; Urbaneja, Miguel A.; Evans, Chris; Garcia, Miriam; Herrero, Artemio; Bresolin, Fabio

2013-06-01

233

Sulphur and zinc abundances in Galactic halo stars revisited  

Microsoft Academic Search

Aims:Based on a new set of sulphur abundances in very metal-poor stars and an improved analysis of previous data, we aim at resolving current discrepancies on the trend of S\\/Fe vs. Fe\\/H and thereby gain better insight into the nucleosynthesis of sulphur. The trends of Zn\\/Fe and S\\/Zn will also be studied. Methods: High resolution VLT\\/UVES spectra of 40 main-sequence

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

2007-01-01

234

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

Microsoft Academic Search

The Habitable Zone Planet Finder (HZPF) is a proposed instrument for the 9m Hobby Eberly telescope that will be capable of discovering low mass planets around M dwarfs. HZPF will be fiber-fed, provide a spectral resolution R 50,000 and cover the wavelength range 0.9-1.65mm, the Y, J and H near infrared (NIR) bands where most of the flux is emitted

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

2010-01-01

235

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

SciTech Connect

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

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

2013-03-01

236

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

237

The core helium flash revisited. III. From Population I to Population III stars  

NASA Astrophysics Data System (ADS)

Context. Degenerate ignition of helium in low-mass stars at the end of the red giant branch phase leads to dynamic convection in their helium cores. One-dimensional (1D) stellar modeling of this intrinsically multi-dimensional dynamic event is likely to be inadequate. Previous hydrodynamic simulations imply that the single convection zone in the helium core of metal-rich Pop I stars grows during the flash on a dynamic timescale. This may lead to hydrogen injection into the core and to a double convection zone structure as known from one-dimensional core helium flash simulations of low-mass Pop III stars. Aims: We perform hydrodynamic simulations of the core helium flash in two and three dimensions to better constrain the nature of these events. To this end we study the hydrodynamics of convection within the helium cores of a 1.25 M_? metal-rich Pop I star (Z = 0.02), and, for the first time, a 0.85 M_? metal-free Pop III star (Z = 0) near the peak of the flash. These models possess single and double convection zones, respectively. Methods: We use 1D stellar models of the core helium flash computed with state-of-the-art stellar evolution codes as initial models for our multidimensional hydrodynamic study, and simulate the evolution of these models with the Riemann solver based hydrodynamics code Herakles, which integrates the Euler equations coupled with source terms corresponding to gravity and nuclear burning. Results: The hydrodynamic simulation of the Pop I model involving a single convection zone covers 27 h of stellar evolution, while the hydrodynamic simulations of a double convection zone, in the Pop III model, span 1.8 h of stellar life. We find differences between the predictions of mixing length theory and our hydrodynamic simulations. The simulation of the single convection zone in the Pop I model shows a strong growth of the size of the convection zone due to turbulent entrainment. We therefore predict that for the Pop I model a hydrogen injection phase (i.e., hydrogen injection into the helium core) will commence after about 23 days, which should eventually lead to a double convection zone structure known from 1D stellar modeling of low-mass Pop III stars. Our two and three-dimensional hydrodynamic simulations of the double (Pop III) convection zone model show that the velocity field in the convection zones is different from that given by stellar evolutionary calculations. The simulations suggest that the double convection zone decays quickly, the flow eventually being dominated by internal gravity waves. The decay could be an artefact caused by the mapping of the initial stellar model to the numerical grid of our hydrodynamics code.

Mocák, M.; Campbell, S. W.; Müller, E.; Kifonidis, K.

2010-09-01

238

Hipparcos preliminary astrometric masses for the two close-in companions to HD 131664 and HD 43848. A brown dwarf and a low mass star  

Microsoft Academic Search

[abridged] We attempt to improve on the characterization of the properties\\u000a(orbital elements, masses) of two Doppler-detected sub-stellar companions to\\u000athe nearby G dwarfs HD 131664 and HD 43848. We carry out orbital fits to the\\u000aHipparcos IAD for the two stars, taking advantage of the knowledge of the\\u000aspectroscopic orbits, and solving for the two orbital elements that can

A. Sozzetti; S. Desidera

2009-01-01

239

A POSSIBLE SIGNATURE OF LENSE-THIRRING PRECESSION IN DIPPING AND ECLIPSING NEUTRON-STAR LOW-MASS X-RAY BINARIES  

SciTech Connect

Relativistic Lense-Thirring precession of a tilted inner accretion disk around a compact object has been proposed as a mechanism for low-frequency ({approx}0.01-70 Hz) quasi-periodic oscillations (QPOs) in the light curves of X-ray binaries. A substantial misalignment angle ({approx}15 Degree-Sign -20 Degree-Sign ) between the inner-disk rotation axis and the compact-object spin axis is required for the effects of this precession to produce observable modulations in the X-ray light curve. A consequence of this misalignment is that in high-inclination X-ray binaries the precessing inner disk will quasi-periodically intercept our line of sight to the compact object. In the case of neutron-star systems, this should have a significant observational effect, since a large fraction of the accretion energy is released on or near the neutron-star surface. In this Letter, I suggest that this specific effect of Lense-Thirring precession may already have been observed as {approx}1 Hz QPOs in several dipping/eclipsing neutron-star X-ray binaries.

Homan, Jeroen, E-mail: jeroen@space.mit.edu [Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 70 Vassar Street, Cambridge, MA 02139 (United States)

2012-12-01

240

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

241

THIRTY NEW LOW-MASS SPECTROSCOPIC BINARIES  

SciTech Connect

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

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

2010-06-20

242

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

243

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

244

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

245

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

NASA Technical Reports Server (NTRS)

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

Harper, Graham M.

2002-01-01

246

Chemistry in low-mass protostellar and protoplanetary regions  

Microsoft Academic Search

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

Ewine F. van Dishoeck

2006-01-01

247

Iron-line and continuum variations in the XMM-Newton and Suzaku spectra of the neutron-star low-mass X-ray binary 4U 1636-53  

NASA Astrophysics Data System (ADS)

We used six simultaneous XMM-Newton and Rossi X-ray Timing Explorer plus five Suzaku observations to study the continuum spectrum and the iron emission line in the neutron-star low-mass X-ray binary 4U 1636-53. We modelled the spectra with two thermal components (representing the accretion disc and boundary layer), a Comptonized component (representing a hot corona), and either a Gaussian or a relativistic line component to model an iron emission line at ˜6.5 keV. For the relativistic line component, we used either the DISKLINE, LAOR or KYRLINE model, the latter for three different values of the spin parameter. The fitting results for the continuum are consistent with the standard truncated disc scenario. We also find that the flux and equivalent width of the iron line first increase and then decrease as the flux of the Comptonized component increases. This could be explained either by changes in the ionization state of the accretion disc where the line is produced by reflection, or by light bending of the emission from the Comptonized component if the height at which this component is produced changes with mass accretion rate.

Lyu, Ming; Méndez, Mariano; Sanna, Andrea; Homan, Jeroen; Belloni, Tomaso; Hiemstra, Beike

2014-05-01

248

First multi-color photometric study and preliminary elements for the low-mass ratio, possible progenitors of merging stars, W UMa systems TYC 3836-0854-1 and TYC 4157-0683-1  

NASA Astrophysics Data System (ADS)

Here we present the first CCD multi-color B, V and Ic light curves of the eclipsing binary stars TYC 3836-0854-1 and TYC 4157-0683-1, the data were obtained in four nights in the year 2010 and three nights in the year 2012 for the first one and in four nights in the year 2010 for the second one. Based on our data the short orbital periods of the systems are confirmed and revised to P = 0.4155590 days for TYC 3836-0854-1 and P = 0.3960676 days for TYC 4157-0683-1. Our observations of TYC 3836-0854-1 show symmetric light curves in all passbands with brightness in both maxima at the same level, while the light curve of TYC 4157-0683-1 appear to exhibit the typical O’Connell effect, with Maximum I brighter than Maximum II. By analyzing simultaneously the complete light curves with the 2003 version of the Wilson-Devinney code (2005 revision), photometric solutions were determined. Both the systems shows a small difference between the components temperatures of ?T = 14 K for TYC 3836-0854-1 and ?T = 149 K for TYC 4157-0683-1. The orbital inclination is i = 78°.6 and i = 79°.7 respectively. The systems are found to be a high fill-out, extreme mass ratio overcontact binary with a mass ratio of q = 0.206 and a fill-out factor of f = 59.2% for TYC 3836-0854-1 and q = 0.150 and a fill-out factor of f = 76.3% for TYC 4157-0683-1, suggesting that both the systems are in the late stage of overcontact evolution. It is known that deep (f>50%), low-mass ratio (q<0.25) overcontact binary stars (DLMR) are a very important resource for understanding the phenomena of Blue Straggler/FK Com-type stars that is an unsolved problem in stellar astrophysics. One of the possible explanations for their formation is from the coalescence of W UMa-type overcontact binary systems. The absolute dimensions of both the systems are estimated from the logTeff - logL diagram and their dynamical evolution is inferred.

Acerbi, F.; Barani, C.; Martignoni, M.

2014-08-01

249

Formation of Low-Mass X-Ray Binaries  

Microsoft Academic Search

Low-mass X-ray binaries (LMXBs) are bright X-ray sources that consist of a compact object (neutron star or black hole) accreting from a low-mass companion that fills its Roche lobe and tidally loses mass. A study of the origin and properties of such systems formed in the Galactic disk is presented, involving the statistical modeling of the evolution of a primordial

Vassiliki Kalogera

1997-01-01

250

THE BINARY FRACTION OF LOW-MASS WHITE DWARFS  

SciTech Connect

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

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

2011-04-01

251

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

252

Spitzer IRS Observations of Low-Mass Seyfert Galaxies  

NASA Astrophysics Data System (ADS)

The Sloan Digital Sky Survey has made it possible to identify the first samples of active galaxies with estimated black hole masses below ~ 106 M?. We have obtained Spitzer IRS low-resolution spectra, covering 5-38 ?m, of a sample of 41 Seyfert galaxies with low-mass black holes. Our sample includes SDSS-selected objects from the low-mass Seyfert 1 sample of Greene & Ho (2004) and the low-mass Seyfert 2 sample of Barth et al. (2008), as well as NGC 4395 and POX 52. The goals of this work are to examine the dust emission properties of these objects and investigate the relationship between type 1 and type 2 AGNs at low luminosities and low masses, to search for evidence of star formation, and to use emission-line diagnostics to constrain physical conditions within the narrow-line regions. Here we present preliminary results from this project.

Thornton, Carol E.; Barth, Aaron J.; Ho, Luis C.; Greene, Jenny E.

2010-05-01

253

Accurate Low-mass Stellar Models of KOI-126  

NASA Astrophysics Data System (ADS)

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

Feiden, Gregory A.; Chaboyer, Brian; Dotter, Aaron

2011-10-01

254

New Low-Mass Members of Nearby Young Moving Groups  

NASA Astrophysics Data System (ADS)

Young, co-evolving stars, free of the obscuring effects of their formation region, exist near the Sun in loose associations with each constituent having a common motion through the Galaxy. These associations are collectively known as the nearby young moving groups (NYMGs). Most known members of the NYMGs are spectral type F, G, and K (1.5 - 0.7 MSun), with few of spectral type M (<0.7 MSun). Since low-mass M stars are the dominant stellar component of the universe (comprising ˜75% of all stars), the census of known members in the NYMGs is probably incomplete. The under-sampling of low-mass members is a consequence of their low-luminosity, wide sky distribution, and a lack of reliable youth indicators in the low-mass regime. NYMG members are important because they provide: 1) Well characterized samples of nearby young stars for study of their physical and kinematic properties and 2) Prime targets for direct exoplanet imaging. I have used a general technique to identify low-mass candidates of the NYMGs using proper motion and photometry. Candidates are then screened for secondary evidence of group membership, such as indicators of youth and consistent radial velocity, to identify likely new group members. Here I present results from a survey of candidates of the beta Pictoris (˜10 Myr) and AB Doradus (˜70 Myr) NYMGs. The selection technique and follow up observations have identified more than 50 low-mass likely members of these groups with spectral types later than K2. Among the likely new members are many visual binaries and objects that are potentially benchmark young brown dwarfs. The expanded member sample has also allowed for preliminary studies of the astrophysics of young late-type stars, including spectroscopic gravity indicators, and provided new targets for the direct imaging of exoplanets.

Schlieder, Joshua Edward

255

The Eta-Earth Survey for Low-Mass Exoplanets  

NASA Astrophysics Data System (ADS)

The NASA-UC Eta-Earth Survey by the California Planet Search (CPS) group is a systematic search for low-mass planets ( 3-30 Earth masses) orbiting the nearest 230 GKM stars suitable for high-precision Doppler observations at Keck Observatory. These 1 m/s measurements of a well-defined sample of nearby stars will provide one of the first estimates of the fraction of stars with Earth-like planets. The talk will describe recently announced super-Earths and Neptune-mass planets from the Eta-Earth Survey, including the 9 Earth mass planet HD 7924b and others. These low-mass planets are extraordinarily valuable in their own right, as targets for transit and other follow-up studies, and as windows into the mechanisms of planet formation. Low mass planets show several emerging trends in their orbital parameters and in the characteristics of their host stars. For example, super-Earth and Neptune-mass planets preferentially orbit stars with lower mass (Mstar < 0.8 solar masses), in contrast to the trend seen in higher mass planets. The survey observations are nearly complete, allowing us to place initial constraints on the population of super-Earth and Neptune-mass planets, and, using planet-formation theory, to extrapolate to the fraction of Earth-mass planets and 1 year orbits.

Howard, Andrew; Marcy, G.; Fischer, D.; Johnson, J.; Wright, J.; Valenti, J.; Anderson, J.; Piskunov, N.; Isaacson, H.; Brewer, J.; Clubb, K.; Lin, D.; Ida, S.

2010-01-01

256

The Pulsating Low-Mass He-Core White Dwarfs  

NASA Astrophysics Data System (ADS)

Recent years have witnessed the discovery of many low-mass (? 0.45 M?) white dwarf (WD) stars — expected to harbor He cores— in the field of the Milky Way and in several galactic globular and open clusters. Recently, three pulsating objects of this kind have been discovered: SDSS J184037.78+642312.3, J111215.82+111745.0, and J151826.68+065813.2. Motivated by these very exciting findings, and in view of the valuable asteroseismological potential of these objects, we present here the main outcomes of a detailed theoretical study on the seismic properties of low-mass He-core WDs based on fully evolutionary models representative of them. This study is aimed at providing a theoretical basis from which to interpret present and future observations of variable low-mass WDs.

Córsico, A. H.; Althaus, L. G.; Romero, A. D.

2013-12-01

257

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

NASA Astrophysics Data System (ADS)

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

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

2004-04-01

258

The role of general relativity in the evolution of low-mass X-ray binaries  

Microsoft Academic Search

We study the evolution of low-mass X-ray binaries hosting a neutron star and of millisecond binary radio pulsars using numerical simulations that take into account the detailed evolution of the companion star, of the binary system, and of the neutron star. According to general relativity, when energy is released during accretion or due to magnetodipole radiation during the pulsar phase,

Giuseppe Lavagetto; Luciano Burderi; F. D'Antona; T. di Salvo; Rosario Iaria; N. R. Robba

2005-01-01

259

The kinematics of very low mass dwarfs: Splinter session summary  

NASA Astrophysics Data System (ADS)

Kinematic investigations are being increasingly deployed in studies of the lowest mass stars and brown dwarfs to investigate their origins, characterize their atmospheres, and examine the evolution of their physical parameters. This article summarizes the contributions made at the ``Kinematics of Very Low Mass Dwarfs'' splinter session. Results discussed include analysis of kinematic distributions of M, L, and T dwarfs; theoretical tools for interpreting these distributions; identifications of very low mass halo dwarfs and wide companions to nearby stars; radial velocity variability among young and very cool brown dwarfs; and the search and identification of M dwarfs in young moving groups. A summary of discussion points at the conclusion of the splinter sesseion is also presented.

Burgasser, A. J.; Faherty, J. K.; Schmidt, S.; West, A. A.; Zapatero Osorio, M. R.; Pineda, J. S.; Burningham, B.; Nicholls, C.; Sanderson, R.; Shkolnik, E.; Rodriguez, D.; Riedel, A.; Joergens, V.

2013-02-01

260

REVISITING THE COSMIC STAR FORMATION HISTORY: CAUTION ON THE UNCERTAINTIES IN DUST CORRECTION AND STAR FORMATION RATE CONVERSION  

SciTech Connect

The cosmic star formation rate density (CSFRD) has been observationally investigated out to redshift z {approx_equal} 10. However, most of the theoretical models for galaxy formation underpredict the CSFRD at z {approx}> 1. Since the theoretical models reproduce the observed luminosity functions (LFs), luminosity densities (LDs), and stellar mass density at each redshift, this inconsistency does not simply imply that theoretical models should incorporate some missing unknown physical processes in galaxy formation. Here, we examine the cause of this inconsistency at UV wavelengths by using a mock catalog of galaxies generated by a semi-analytic model of galaxy formation. We find that this inconsistency is due to two observational uncertainties: the dust obscuration correction and the conversion from UV luminosity to star formation rate (SFR). The methods for correction of obscuration and SFR conversion used in observational studies result in the overestimation of the CSFRD by {approx}0.1-0.3 dex and {approx}0.1-0.2 dex, respectively, compared to the results obtained directly from our mock catalog. We present new empirical calibrations for dust attenuation and conversion from observed UV LFs and LDs into the CSFRD.

Kobayashi, Masakazu A. R. [Astronomy Data Center, National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588 (Japan)] [Astronomy Data Center, National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588 (Japan); Inoue, Yoshiyuki [Kavli Institute for Particle Astrophysics and Cosmology, Department of Physics and SLAC National Accelerator Laboratory, Stanford University, Stanford, CA 94305 (United States)] [Kavli Institute for Particle Astrophysics and Cosmology, Department of Physics and SLAC National Accelerator Laboratory, Stanford University, Stanford, CA 94305 (United States); Inoue, Akio K., E-mail: kobayashi@cosmos.phys.sci.ehime-u.ac.jp [College of General Education, Osaka Sangyo University, 3-1-1 Nakagaito Daito, Osaka 574-8530 (Japan)

2013-01-20

261

Triggered star formation in bright-rimmed clouds: the Eagle nebula revisited  

Microsoft Academic Search

A three-dimensional smoothed particle hydrodynamics model has been extended to study the radiation-driven implosion effect of massive stars on the dynamical evolution of surrounding molecular clouds. The new elements in the upgraded code are the inclusion of Lyman continuum in the incident radiation flux and the treatment of hydrogen ionization process; the introduction of ionization heating and recombination cooling effects;

J. Miao; Glenn J. White; R. P. Nelson; M. A. Thompson; L. K. Morgan

2006-01-01

262

FUV spectra of evolved late-K and M stars: mass loss revisited  

NASA Astrophysics Data System (ADS)

The FUV radiation field of evolved late-K and M stars, which is dominated by H Lyman emission lines, controls the ionization balance of key atomic and ionic species, the dissociation equilibrium of molecular H_2 and CO, and provides crucial information on the excitation of hydrogen. FUSE observations of the FUV flux below 1044Å{ } will allow us to constrain the Ca II/Ca III balance and remove a key uncertainty in previous studies on stellar mass-loss rates. The FUV photoionization flux was previously unknown, but FUSE spectra can unlock the wealth of information from early optical studies of Ca II. We propose to construct detailed radiative transfer models from the H Lyman ? and ? line profiles. These models will provide constraints on the hydrogen ionization, the dominant source of electrons in chromospheres, and the formation of the Lyman continuum which is not directly observable. FUSE will also allow us to observe wind induced line profile asymmetries and to constrain with great sensitivity the volume of ˜ 3× 105 K plasma which traditional ``non-coronal'' stars are now thought to possess. We propose deep exposures of the brightest K bright giant (? TrA K3 II), the brightest K supergiant (? Vel K4 Ib), and two of the brightest M giants (? Cru M3 III, ? Gru M5 III). These stars have been studied extensively in the UV and optical, and we have recently obtained the first radio detections for all these targets. We will incorporate the knowledge gathered from the FUV into our unified chromospheric/wind models for these stars. These spectra will provide a valuable complement to the spectral-types and evolutionary phases of the GTO targets. FUV spectra of the M stars will provide crucial input physics for circumstellar photochemistry.

Harper, Graham

263

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

264

Composition of low-mass exoplanets  

NASA Astrophysics Data System (ADS)

The research field of exoplanets is rapidly advancing with more than 450 objects detected. Furthermore, last year there were two milestone discoveries: CoRoT-7b and GJ 1214b. These are the first two low-mass exoplanets with measured masses and radii. They both have fairly similar masses but very different radii. The first step for characterizing these objects is to infer their composition. Through internal structure and evolution models, atmospheric escape and knowledge of hot-Jupiters I will present the restuls on their composition and origin. While CoRoT-7b's density is similar to Earth's and thus can be inferred to be rocky, we show that an equally good fit to the data is achieved with some amount of H2O vapor. If CoRoT-7b is terrestrial, it is depleted in iron relative to Earth. On the other hand, GJ 1214b has a substantial amount of volatiles given its relatively large size. However, owing to the intrinsic compositional degeneracy we can only place upper limits to the different compositional end-members (i.e. amount of H2O, silicate mantles, iron cores). A basic question concerning the composition of low-mass planets is unresolved: in which cases is the ratio of refractory elements similar to that of the star (i.e. Fe/Si ratio)? While this is a reasonable assumption for Earth, it is debated for Mars and not the case for Mercury. It is understood that during the last stages of formation, giant impacts and atmospheric erosion can have a large impact on the final composition of a forming planet. With more super-Earth data and structure models, we will begin to answer this question and in this way learn more about the formation of the solar system planets. However, for this we need to better understand giant impacts and atmospheric erosion on super-Earths. As a first step in this direction, I will present results on atmospheric erosion of highly-irradiated planets (like CoRoT-7b) by considering a hydrodynamically escaping atmosphere to place upper limits to the density of terrestrial exoplanets).

Valencia, Diana; Guillot, Tristan

2010-05-01

265

Tidal evolution of close binary stars. I - Revisiting the theory of the equilibrium tide  

NASA Technical Reports Server (NTRS)

The theory of the equilibrium tide in stars that possess a convective envelope is reexamined critically, taking recent developments into account and treating thermal convection in the most consistent way within the mixing-length approach. The weak points are identified and discussed, in particular, the reduction of the turbulent viscosity when the tidal period becomes shorter than the convective turnover time. An improved version is derived for the secular equations governing the dynamical evolution of close binaries of such type.

Zahn, J.-P.

1989-01-01

266

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

267

The r-process in Proto-neutron-star Wind Revisited  

NASA Astrophysics Data System (ADS)

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 ? and 2.4 M ? 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 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 ? 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 ? 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 ~ 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 ?.

Wanajo, Shinya

2013-06-01

268

Spitzer IRS Observations of Low-Mass Seyfert Galaxies  

NASA Astrophysics Data System (ADS)

The Sloan Digital Sky Survey has made it possible to identify the first samples of active galaxies with estimated black hole masses below 106 solar masses. We have obtained Spitzer IRS low-resolution spectra, covering 5-30 microns, of a sample of 41 Seyfert galaxies with low-mass black holes. Our sample includes SDSS-selected objects from the low-mass Seyfert 1 sample of Greene & Ho (2004) and the low-mass Seyfert 2 sample of Barth et al. (2008), as well as NGC 4395 and POX 52. The goals of this work are to examine the dust emission properties of these objects and investigate the relationship between Type 1 and Type 2 AGNs at low luminosities and low masses, to search for evidence of star formation, and to use emission-line diagnostics to constrain physical conditions within the narrow-line regions. We will present preliminary results from this project, including measurements of continuum shapes and dust temperatures, narrow-line region diagnostics, and PAH features, derived using the IDL code PAHFIT (Smith et al. 2007).

Thornton, Carol E.; Barth, A. J.; Greene, J. E.; Ho, L. C.

2009-05-01

269

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

270

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

271

Very Low Mass Stellar and Substellar Companions to Solar-like Stars from MARVELS. V. A Low Eccentricity Brown Dwarf from the Driest Part of the Desert, MARVELS-6b  

NASA Astrophysics Data System (ADS)

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

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

2013-06-01

272

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

273

Binary Star Orbits. III. Revisiting the Remarkable Case of Tweedledum and Tweedledee  

NASA Astrophysics Data System (ADS)

Two of the most challenging objects for optical interferometry in the middle of the last century were the close components (FIN 332) of the wide visual binary STF2375 (= WDS 18455+0530 = HIP 92027 = ADS 11640). Each component of the wide pair was found to have subcomponents of approximately the same magnitude, position angle, and separation and, hence, were designated by the tongue-in-cheek monikers "Tweedledum and Tweedledee" by the great visual interferometrist William S. Finsen in 1953. They were later included in a list of "Double Stars that Vex the Observer" by W.H. van den Bos in 1958. While speckle interferometry has reaped a rich harvest investigating the close inteferometric binaries of Finsen, the "Tweedles" have continued to both fascinate and exasperate due to both the great similarity of the close pairs and the inherent 180° ambiguity associated with interferometry. Detailed analysis of all published observations of the system has revealed several errors which are here corrected, allowing for determination of these orbital elements which resolve the quadrant ambiguity. A unique software filter was developed which allowed subarrays from archival ICCD speckle data from 1982 to be re-reduced. Those data, combined with new and unpublished observations obtained in 2001-2009 from NOAO 4 m telescopes, the Mount Wilson 100 inch telescope and the Naval Observatory Flagstaff Station 61 inch telescope as well as high-quality unresolved measures all allow for the correct orbits to be determined. Co-planarity of the multiple system is also investigated.

Mason, Brian D.; Hartkopf, William I.; McAlister, Harold A.

2010-07-01

274

Prospects for the detection of planets around very low-mass primaries  

NASA Astrophysics Data System (ADS)

Progress in the understanding of very low-mass stars and brown dwarfs has developed at a fast pace in the last decade. Several pieces of evidence indicate that these very low-mass objects form in a manner similar to stars, and hence it appears natural to that they could host planetary systems. We are carrying out studies of the binary properties of very low-mass primaries, some new results will be presented. We have also embarked on a project to design, build and exploit a high-precision near-infrared echelle spectrograph for planet dection around very low-mass primaries. This instrument, called NAHUAL, is being designed for installation at the 10.4 m Gran Telescopio Canarias in La Palma, Canary Islands. The current status of the project will be discussed, as well as opportunities for the latino-american scientific community to participate.

Martín, E.; Urania Cabral, C. V.

2009-05-01

275

Fundamental Properties of a New Low-Mass Pre-Main-Sequence Ecliping Binary System  

NASA Astrophysics Data System (ADS)

Studies of eclipsing binary stars are key to accurate, empirical measurements of the masses, radii, and temperatures of stars, which are necessary for understanding stellar evolution and for deriving well- calibrated theoretical models. However, only six pre-main-sequence eclipsing binaries with masses below 1 solar mass have so far been published. We have discovered TYC 7310-503-1 to be a low-mass, pre-main- sequence eclipsing binary in the Upper-Centaurus-Lupus star-forming region with component star masses of ~ 0.8 and ~ 0.5 M_?; this is one of the lowest mass young stellar eclipsing binaries yet found, and the first that is associated with a star-forming region other than Orion. Here we propose precise time-series photometry and spectroscopy of TYC 7310-503-1 which will permit the determination of accurate stellar masses and radii for this very young, very low-mass benchmark system.

Stassun, Keivan G.; Hebb, Leslie; Stempels, Eric; Collier-Cameron, Andrew

2009-02-01

276

Composition of low-mass exoplanets  

Microsoft Academic Search

The research field of exoplanets is rapidly advancing with more than 450 objects detected. Furthermore, last year there were two milestone discoveries: CoRoT-7b and GJ 1214b. These are the first two low-mass exoplanets with measured masses and radii. They both have fairly similar masses but very different radii. The first step for characterizing these objects is to infer their composition.

Diana Valencia; Tristan Guillot

2010-01-01

277

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

278

Resolving the Luminosity Problem in Low-mass Star Formation  

NASA Astrophysics Data System (ADS)

We determine the observational signatures of protostellar cores by coupling two-dimensional radiative transfer calculations with numerical hydrodynamical simulations that predict accretion rates that both decline with time and feature short-term variability and episodic bursts caused by disk gravitational instability and fragmentation. We calculate the radiative transfer of the collapsing cores throughout the full duration of the collapse, using as inputs the core, disk, protostellar masses, radii, and mass accretion rates predicted by the hydrodynamical simulations. From the resulting spectral energy distributions, we calculate standard observational signatures (L bol, T bol, L bol/L smm) to directly compare to observations. We show that the accretion process predicted by these models reproduces the full spread of observed protostars in both L bol-T bol and L bol-M core space, including very low luminosity objects, provides a reasonable match to the observed protostellar luminosity distribution, and resolves the long-standing luminosity problem. These models predict an embedded phase duration shorter than recent observationally determined estimates (0.12 Myr versus 0.44 Myr), and a fraction of total time spent in Stage 0 of 23%, consistent with the range of values determined by observations. On average, the models spend 1.3% of their total time in accretion bursts, during which 5.3% of the final stellar mass accretes, with maximum values being 11.8% and 35.5% for the total time and accreted stellar mass, respectively. Time-averaged models that filter out the accretion variability and bursts do not provide as good of a match to the observed luminosity problem, suggesting that the bursts are required.

Dunham, Michael M.; Vorobyov, Eduard I.

2012-03-01

279

ROSAT Observations of Low Mass Disk Galaxies: No Evidence of Baryonic Blow Out  

NASA Technical Reports Server (NTRS)

To test the hypothesis that galctic winds associated with star formation in low mass disk galaxies can be an effective means of relocating cold disk gas to a warm tenuous halo, we have obtained long exposure ROSAT PSPC observations of three such galaxies.

Bothun, Gregory D.; Eriksen, James

1994-01-01

280

Very Low Mass Objects in the Coronet Cluster: The Realm of the Transition Disks  

Microsoft Academic Search

We present optical and IR spectra of a set of low-mass stars and brown dwarfs in the Coronet cluster (aged ~1 Myr), obtained with the multifiber spectrograph FLAMES on the VLT and with the IRS instrument on Spitzer. Most of the objects had been selected via their X-ray emission in a deep Chandra survey. The optical spectra reveal spectral types

Aurora Sicilia-Aguilar; Thomas Henning; Attila Juhász; Jeroen Bouwman; Gordon Garmire; Audrey Garmire

2008-01-01

281

Identification of Very Low Mass Brown Dwarfs in IC348  

NASA Astrophysics Data System (ADS)

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

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

2010-08-01

282

Deep, Low Mass Ratio Overcontact Binaries. II. IK Persei  

NASA Astrophysics Data System (ADS)

BV photometric light curves of the eclipsing binary IK Per were obtained during three nights in 2002 December. The photometric elements were computed using the Wilson-Devinney code. The results reveal that IK Per is an A-type overcontact binary system with a low mass ratio of q=0.17 and a large degree of overcontact of 60%. The asymmetry of the light curves (i.e., the O'Connell effect) is explained by spot models. The observed long-term orbital period decrease [dP/dt=-(2.5+/-0.09)×10-7 days yr-1] is probably influenced by the presence of a third body in the system. The low mass ratio, high degree of overcontact, and secular orbital decrease all indicate that the situation of IK Per resembles those of FG Hya, GR Vir, and AW UMa. Because of the decrease of their orbital periods, the shrinking of the inner and outer critical Roche lobes will lead the common envelopes to overlap more, and finally the systems will evolve into single rapid-rotation stars.

Zhu, L.-Y.; Qian, S.-B.; Soonthornthum, B.; Yang, Y.-G.

2005-06-01

283

Chemistry in low-mass protostellar and protoplanetary regions.  

PubMed

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

van Dishoeck, Ewine F

2006-08-15

284

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.

van Dishoeck, Ewine F.

2006-01-01

285

Spitzer IRS Observations of Low-Mass Seyfert Galaxies  

NASA Astrophysics Data System (ADS)

We present results from Spitzer IRS observations of a sample of 41 Seyfert galaxies with estimated black hole masses below 106 solar masses, including objects from the SDSS-selected samples of Seyfert 1 galaxies from Greene & Ho (2004) and Seyfert 2 galaxies from Barth et al. (2008), as well as NGC 4395 and POX 52. We use the IDL code PAHFIT (Smith et al. 2007) to derive measurements of continuum shapes and narrow emission line and PAH luminosities from the low-resolution spectra in order to examine the dust emission properties of these objects and investigate the relationship between Type 1 and Type 2 AGNs at low luminosities and low masses, to search for evidence of star formation, and to use emission-line diagnostics to constrain physical conditions within the narrow-line regions.

Thornton, Carol E.; Barth, A. J.; Ho, L. C.; Greene, J. E.

2010-01-01

286

THE ENIGMATIC YOUNG, LOW-MASS VARIABLE TWA 30  

SciTech Connect

TWA 30 is a remarkable young (7 {+-} 3 Myr), low-mass (0.12 {+-} 0.04 M{sub sun}), late-type star (M5 {+-} 1) residing 42 {+-} 2 pc away from the Sun in the TW Hydrae Association (TWA). It shows strong outflow spectral signatures such as [S II], [O I], [O II], [O III], and Mg I], while exhibiting weak H{alpha} emission (-6.8 {+-} 1.2 A). Emission lines of [S II] and [O I] are common to T Tauri stars still residing in their natal molecular clouds, while [O III] and Mg I] emission lines are incredibly rare in this same population; in the case of TWA 30, these latter lines may arise from new outflow material colliding into older outflow fronts. The weak H{alpha} emission and small radial velocity shifts of line emission relative to the stellar frame of rest (generally {approx_lt}10 km s{sup -1}) suggest that the disk is viewed close to edge-on and that the stellar axis may be inclined to the disk, similar to the AA Tau system, based on its temporal changes in emission/absorption line strengths/profiles and variable reddening (A{sub V} = 1.5-9.0). The strong Li absorption (0.61 {+-} 0.13 A) and common kinematics with members of the TWA confirm its age and membership to the association. Given the properties of this system such as its proximity, low mass, remarkable outflow signatures, variability, and edge-on configuration, this system is a unique case study at a critical time in disk evolution and planet-building processes.

Looper, Dagny L.; Rayner, John; Pitts, Mark A. [Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822 (United States); Mohanty, Subhanjoy [Imperial College London, 1010 Blackett Laboratory, Prince Consort Road, London SW7 2AZ (United Kingdom); 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); Mamajek, Eric E. [Department of Physics and Astronomy, University of Rochester, P.O. Box 270171, 500 Wilson Boulevard, Rochester, NY 14627-0171 (United States); Herczeg, Gregory J. [Max-Planck-Institut fuer extraterrestriche Physik, Giessenbachstrase, 85748 Garching (Germany); West, Andrew A. [Department of Astronomy, Boston University, 725 Commonwealth Avenue, Boston, MA 02215 (United States); Faherty, Jacqueline K. [Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794-3800 (United States); Kirkpatrick, J. Davy, E-mail: dagny@ifa.hawaii.ed [Infrared Processing and Analysis Center, MS 100-22, California Institute of Technology, Pasadena, CA 91125 (United States)

2010-05-01

287

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

288

Mapping the Local Halo: Statistical Parallax Analysis of SDSS Low-mass Subdwarfs  

NASA Astrophysics Data System (ADS)

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, ?. We also developed a photometric metallicity parameter, ?(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.; Savcheva, Antonia; West, Andrew A.; Hawley, Suzanne L.

2013-02-01

289

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

290

Low-mass Visual Companions to Nearby G-dwarfs  

NASA Astrophysics Data System (ADS)

A complete census of wide visual companions to nearby G-dwarf stars can be achieved by selecting candidates from the Two Micron All Sky Survey (2MASS) Point-Source Catalog and checking their status by second-epoch imaging. Such data are obtained for 124 candidates with separations up to 20'', 47 of which are shown to be new physical low-mass stellar companions. A list of visual binaries with G-dwarf primaries is produced by combining newly found companions with historical data. Maximum likelihood analysis leads to a companion frequency of 0.13 ± 0.015 per decade of separation. The mass ratio is distributed almost uniformly, with a power-law index between -0.4 and 0. The remaining uncertainty in the index is related to modeling of the companion detection threshold in 2MASS. These findings are confirmed by an alternative analysis of wider companions in 2MASS, removing the contamination by background stars statistically. Extension of this work will lead to a complete detection of visual companions—a necessary step toward reaching unbiased multiplicity statistics over the full range of orbital periods and, eventually, understanding the origin of multiple systems.

Tokovinin, Andrei

2011-02-01

291

The Low-Mass Content of the Hyades stellar cluster  

NASA Astrophysics Data System (ADS)

The Hyades cluster is an ideal target to study the dynamical evolution of a star cluster over the entire mass range due to its intermediate age and proximity to the Sun. We extend the Hyades mass function towards lower masses into the brown dwarf regime and use the full three-dimensional spatial information to characterize the dynamical evolution of the cluster. In addition, once binaries are identified, the new members will allow us to characterize the evolution of ultra-cool dwarfs at an age of 650 Myr. We perform a kinematic and photometric selection using the PPMXL and Pan-STARRS1 sky surveys, to search for cluster members up to 30 pc from the cluster centre. We determine our detection efficiency and field star contamination rate to derive the cluster luminosity and mass function. A minimum spanning tree algorithm is used to quantify the mass segregation. As likeliest candidates, we discover 43 new Hyades member candidates with mass estimates below 0.43 Msun, and double the number of low-mass member candidates. The cluster is significantly mass segregated. The extension of the mass function towards lower masses provides an even clearer signature than estimated before.

Goldman, Bertrand; Roeser, Siegfried; Schilbach, Elena; Henning, Thomas; Olczak, Christoph; Magnier, Eugene A.

2013-07-01

292

LOW-MASS VISUAL COMPANIONS TO NEARBY G-DWARFS  

SciTech Connect

A complete census of wide visual companions to nearby G-dwarf stars can be achieved by selecting candidates from the Two Micron All Sky Survey (2MASS) Point-Source Catalog and checking their status by second-epoch imaging. Such data are obtained for 124 candidates with separations up to 20'', 47 of which are shown to be new physical low-mass stellar companions. A list of visual binaries with G-dwarf primaries is produced by combining newly found companions with historical data. Maximum likelihood analysis leads to a companion frequency of 0.13 {+-} 0.015 per decade of separation. The mass ratio is distributed almost uniformly, with a power-law index between -0.4 and 0. The remaining uncertainty in the index is related to modeling of the companion detection threshold in 2MASS. These findings are confirmed by an alternative analysis of wider companions in 2MASS, removing the contamination by background stars statistically. Extension of this work will lead to a complete detection of visual companions-a necessary step toward reaching unbiased multiplicity statistics over the full range of orbital periods and, eventually, understanding the origin of multiple systems.

Tokovinin, Andrei, E-mail: atokovinin@ctio.noao.edu [Cerro Tololo Inter-American Observatory, Casilla 603, La Serena (Chile)

2011-02-15

293

Molecular Outflows in the Substellar Domain: Millimeter Observations of Young Very Low Mass Objects in Taurus and ? Ophiuchi  

NASA Astrophysics Data System (ADS)

We report here our search for molecular outflows from young very low mass stars and brown dwarfs in Taurus and ? Ophiuchi. Using the Submillimeter Array and the Combined Array for Research in Millimeter-wave Astronomy, we have observed four targets at 1.3 mm wavelength (230 GHz) to search for CO J = 2 ? 1 outflows. A young very low mass star MHO 5 (in Taurus) with an estimated mass of 90 M J , which is just above the hydrogen-burning limit, shows two gas lobes that are likely outflows. While the CO map of MHO 5 does not show a clear structure of outflow, possibly due to environment gas, its position-velocity diagram indicates two distinct blue- and redshifted components. We therefore conclude that they are components of a bipolar molecular outflow from MHO 5. We estimate an outflow mass of 7.0 × 10-5 M sun and a mass-loss rate of 9.0 × 10-10 M sun. These values are over two orders of magnitude smaller than the typical ones for T Tauri stars and somewhat weaker than those we have observed in the young brown dwarf ISO-Oph 102 of 60 M J in ? Ophiuchi. This makes MHO 5 the first young very low mass star showing a bipolar molecular outflow in Taurus. The detection boosts the scenario that very low mass objects form like low-mass stars but in a version scaled down by a factor of over 100.

Phan-Bao, Ngoc; Lee, Chin-Fei; Ho, Paul T. P.; Tang, Ya-Wen

2011-07-01

294

CFFF low mass flow DCW generator operation  

NASA Astrophysics Data System (ADS)

A summary of testing of the low mass flow diagonal conducting sidewall MHD generator in the CFFF is given. These summaries include details of the powered generator tests conducted during the 1985 LMF4 test series. A presentation of experimental generator electrical data collected during these tests is included. The quality of these data is discussed and a review of representative data presentations is made as a means of identifying phenomena associated with coal-fired MHD generators. Unique characteristics of coal slag effects upon electrical performance are seen in the voltage profiles and power characteristics for the generator. Fundamental theoretical analyses of the generator are used to qualify the levels of generator performance that were demonstrated during testing. These analyses are directed at isolating possible sources that have caused performance deficiencies and anomalies seen in the test data.

Lineberry, J. T.; Galanga, F. L.; Frazier, J. W.

1986-01-01

295

CCD time-series photometry of the globular cluster NGC 5053: RR Lyrae, Blue Stragglers and SX Phoenicis stars revisited  

NASA Astrophysics Data System (ADS)

We report the results of CCD V, r and I time-series photometry of the globular cluster NGC 5053. New times of maximum light are given for the eight known RR Lyrae stars in the field of our images, and their periods are revised. Their V light curves were Fourier decomposed to estimate their physical parameters. A discussion on the accuracy of the Fourier-based iron abundances, temperatures, masses and radii is given. New periods are found for the five known SX Phe stars, and a critical discussion of their secular period changes is offered. The mean iron abundance for the RR Lyrae stars is found to be [Fe/H] ~ -1.97 +/- 0.16 and lower values are not supported by the present analysis. The absolute magnitude calibrations of the RR Lyrae stars yield an average true distance modulus of 16.12 +/- 0.04 or a distance of 16.7 +/- 0.3 kpc. Comparison of the observational colour magnitude diagram (CMD) with theoretical isochrones indicates an age of 12.5 +/- 2.0 Gyr for the cluster. A careful identification of all reported blue stragglers (BS) and their V, I magnitudes leads to the conclusion that BS12, BS22, BS23 and BS24 are not BS. On the other hand, three new BS are reported. Variability was found in seven BS, very likely of the SX Phe type in five of them, and in one red giant star. The new SX Phe stars follow established Period-Luminosity relationships and indicate a distance in agreement with the distance from the RR Lyrae stars. Based on observations collected at the Indian Astrophysical Observatory, Hanle, India. E-mail: armando@astroscu.unam.mx (AAF); giridhar@iiap.res.in (SG); dan.bramich@hotmail.co.uk (DMB)

Arellano Ferro, A.; Giridhar, Sunetra; Bramich, D. M.

2010-02-01

296

Lense-Thirring Precession and Quasi-periodic Oscillations in Low-Mass X-Ray Binaries  

Microsoft Academic Search

Relativistic dragging of inertial frames around fast-rotating collapsed stars is substantial and can give rise to observable effects. We consider kHz quasi-periodic oscillation (QPO) sources, low-mass X-ray binaries (LMXRBs) containing an accreting neutron star. Within beat frequency models, both the Keplerian frequency of the innermost region of the accretion disk ( ~0.3-1.2 kHz) and the neutron star spin frequency (

Luigi Stella; Mario Vietri

1998-01-01

297

HIGH-PRECISION DYNAMICAL MASSES OF VERY LOW MASS BINARIES  

SciTech Connect

We present the results of a three year monitoring program of a sample of very low mass (VLM) field binaries using both astrometric and spectroscopic data obtained in conjunction with the laser guide star adaptive optics system on the W. M. Keck II 10 m telescope. Among the 24 systems studied, 15 have undergone sufficient orbital motion, allowing us to derive their relative orbital parameters and hence their total system mass. These measurements more than double the number of mass measurements for VLM objects, and include the most precise mass measurement to date (<2%). Among the 11 systems with both astrometric and spectroscopic measurements, six have sufficient radial velocity variations to allow us to obtain individual component masses. This is the first derivation of the component masses for five of these systems. Altogether, the orbital solutions of these low mass systems show a correlation between eccentricity and orbital period, consistent with their higher mass counterparts. In our primary analysis, we find that there are systematic discrepancies between our dynamical mass measurements and the predictions of theoretical evolutionary models (TUCSON and LYON) with both models either underpredicting or overpredicting the most precisely determined dynamical masses. These discrepancies are a function of spectral type, with late-M through mid-L systems tending to have their masses underpredicted, while one T-type system has its mass overpredicted. These discrepancies imply that either the temperatures predicted by evolutionary and atmosphere models are inconsistent for an object of a given mass, or the mass-radius relationship or cooling timescales predicted by the evolutionary models are incorrect. If these spectral-type trends are correct and hold into the planetary mass regime, the implication is that the masses of directly imaged extrasolar planets are overpredicted by the evolutionary models.

Konopacky, Q. M.; Ghez, A. M.; McLean, I. S. [University of California, Los Angeles, Division of Astronomy and Astrophysics, Los Angeles, CA 90095-1562 (United States); Barman, T. S. [Lowell Observatory, 1400 W. Mars Hill Road, Flagstaff, AZ 86001 (United States); Rice, E. L. [American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024-5192 (United States); Bailey, J. I. [Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48105 (United States); White, R. J. [Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30303 (United States); Duchene, G., E-mail: konopacky1@llnl.go, E-mail: ghez@astro.ucla.ed, E-mail: mclean@astro.ucla.ed, E-mail: barman@lowell.ed, E-mail: baileyji@umich.ed, E-mail: white@chara.gsu.ed, E-mail: gduchene@berkeley.ed [Astronomy Department, University of California, Berkeley, 601 Campbell Hall, Berkeley, CA 94720-3411 (United States)

2010-03-10

298

ROTATIONAL VELOCITIES OF INDIVIDUAL COMPONENTS IN VERY LOW MASS BINARIES  

SciTech Connect

We present rotational velocities for individual components of 11 very low mass (VLM) binaries with spectral types between M7 and L7.5. These results are based on observations taken with the near-infrared spectrograph, NIRSPEC, and the Keck II laser guide star adaptive optics system. We find that the observed sources tend to be rapid rotators (v sin i > 10 km s{sup -1}), consistent with previous seeing-limited measurements of VLM objects. The two sources with the largest v sin i, LP 349-25B and HD 130948C, are rotating at {approx}30% of their break-up speed, and are among the most rapidly rotating VLM objects known. Furthermore, five binary systems, all with orbital semimajor axes {approx}<3.5 AU, have component v sin i values that differ by greater than 3{sigma}. To bring the binary components with discrepant rotational velocities into agreement would require the rotational axes to be inclined with respect to each other, and that at least one component is inclined with respect to the orbital plane. Alternatively, each component could be rotating at a different rate, even though they have similar spectral types. Both differing rotational velocities and inclinations have implications for binary star formation and evolution. We also investigate possible dynamical evolution in the triple system HD 130948A-BC. The close binary brown dwarfs B and C have significantly different v sin i values. We demonstrate that components B and C could have been torqued into misalignment by the primary star, A, via orbital precession. Such a scenario can also be applied to another triple system in our sample, GJ 569A-Bab. Interactions such as these may play an important role in the dynamical evolution of VLM binaries. Finally, we note that two of the binaries with large differences in component v sin i, LP 349-25AB and 2MASS 0746+20AB, are also known radio sources.

Konopacky, Q. M.; Macintosh, B. A. [Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550 (United States); Ghez, A. M. [UCLA Division of Astronomy and Astrophysics, Los Angeles, CA 90095-1562 (United States); Fabrycky, D. C. [Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064 (United States); White, R. J. [Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30303 (United States); Barman, T. S. [Lowell Observatory, 1400 W. Mars Hill Rd., Flagstaff, AZ 86001 (United States); Rice, E. L. [American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024-5192 (United States); Hallinan, G. [Department of Astrophysics, California Institute of Technology, MC 249-17, Pasadena, CA 91125 (United States); Duchene, G., E-mail: macintosh1@llnl.gov, E-mail: konopacky@di.utoronto.ca, E-mail: ghez@astro.ucla.edu, E-mail: fabrycky@ucolick.org, E-mail: white@chara.gsu.edu, E-mail: barman@lowell.edu, E-mail: erice@amnh.org, E-mail: gh@astro.caltech.edu, E-mail: gduchene@berkeley.edu [Astronomy Department, University of California, Berkeley, Hearst Field Annex B-20, CA 94720-3411 (United States)

2012-05-01

299

Probing Binary Formation Theories with the Largest Catalog of Ultra-wide, Low-mass Binaries  

NASA Astrophysics Data System (ADS)

We present results from the Sloan Low-mass Wide Pairs of Kinematically Equivalent Stars (SLoWPoKES) catalogs of ultra-wide (10^3-5.5 AU), low-mass (K5-M7) visual binaries, comprising of over 100,000 pairs. We constructed a Galactic model, based on empirical stellar number density and 3D velocity distributions, to select bona fide pairs with probability of chance alignment ?5%, making SLoWPoKES an efficient sample for followup observations. The diversity - in mass, metallicity, age, and evolutionary states - of SLoWPoKES pairs makes it a valuable resource of coeval laboratories to examine and constrain the physical properties of low-mass stars. SLoWPoKES appears to contain two populations of wide binaries, with a break at projected physical separation of 0.1 pc, suggesting that they were formed via different mechanisms or have significantly different dynamical history. Followup high-resolution imaging has revealed that the multiplicity in "individual" stars in the ultra-wide binaries (higher-order multiplicity) is significantly higher than in tighter binaries or low-mass field stars. This is consistent with the premise that ultra-wide systems are the result of dynamical widening via transfer of angular momentum from the outer orbit to the inner orbit, followed by dissipation via interactions with Galactic tide and giant molecular clouds. Indeed, we find that the higher-order multiplicity decreases with Galactic height, evidence that they are destroyed over time. This bimodality, however, is also consistent with recent theoretical predictions, which show that the ultra-wide binaries are not formed primordially but during dissipation of star clusters. Our data do not rule out either scenario but indicate neither mechanism can form all of the observed wide binaries. We conclude that multiple processes, not all of which are primordial, are likely responsible for the observed distribution of stellar binaries.

Dhital, Saurav; West, Andrew A.; Stassun, Keivan G.; Law, Nicholas M.; Massey, Angela P.

2013-07-01

300

Formation of Low-Mass X-Ray Binaries  

NASA Astrophysics Data System (ADS)

Low-mass X-ray binaries (LMXBs) are bright X-ray sources that consist of a compact object (neutron star or black hole) accreting from a low-mass companion that fills its Roche lobe and tidally loses mass. A study of the origin and properties of such systems formed in the Galactic disk is presented, involving the statistical modeling of the evolution of a primordial ensemble of binaries through evolutionary stages until the onset of the X-ray phase. For the completion of this study, understanding of the effects of supernova explosions on orbital dynamics, as well as knowledge of the binary characteristics of nascent LMXBs are required. Observational evidence exists in support of the idea that kicks are imparted to neutron stars at their birth. An analytical method is developed for studying the effect of supernova explosions on the orbital dynamics of a binary population with initially circular orbits. Expressions for the distribution of systems over post-SN orbital separations, eccentricities, and center-of-mass velocities are derived, and their dependence on the kick and binary characteristics is studied. This analysis is a necessary tool for population studies of binaries that experience supernova explosions (see Kalogera 1996). The binary properties of LMXBs at the onset of the X-ray phase are studied, and the donor masses and orbital separations are found to be constrained by the requirement that mass transfer be stable and the age of the systems be shorter than the age of the Galaxy. It is also shown that super-Eddington mass transfer allows relatively massive donors to remain in hydrostatic and thermal equilibrium, and that observed long-period LMXBs with evolved donors as well as ultra-short-period LMXBs with hydrogen deficient donors must have survived a phase of super-Eddington mass transfer (see Kalogera & Webbink 1996). Formation of LMXBs has been suggested to occur when primordial binaries with extreme mass ratios evolve through a common envelope phase and the exposed helium core subsequently explodes as a supernova. The complete set of structural and evolutionary constraints on the properties of LMXB progenitors is identified, and it is shown that (i) their orbital separations are restricted to a narrow range, and (ii) short-period LMXBs are formed only if kicks are imparted to neutron stars at birth. Population synthesis calculations are performed with the use of a semi-analytical method which offers major advantages in terms of statistical accuracy and computational efficiency. The results of an extensive parameter study indicate that the predicted birth rates essentially reflect the choice of the -- mostly unknown -- characteristics of the primordial binary population, while the properties of the nascent LMXBs are primarily determined by stability and age constraints and the efficiency of angular momentum losses. The random natal kicks imparted to neutron stars weaken the dependence of the results on pre-SN evolution and hamper the distinction between formation paths that involve a common-envelope phase (see Kalogera & Webbink 1997). In light of the importance of neutron stars kicks in LMXB formation, a new evolutionary path is proposed, the direct-supernova mechanism, which does not invoke common-envelope evolution. Instead, the required small post-SN binary orbits are achieved because of a kick velocity of appropriate magnitude and direction relative to the pre-SN orbital velocity. The efficiency of this new mechanism strongly depends on the average kick magnitude, and can account for one third of the LMXB population for kicks of ~ 100kms(-1) . More importantly, the direct-supernova path provides the only natural way for the formation of binary millisecond pulsars in very-long-period orbits (see Kalogera 1997). (SECTION: Dissertation Summary)

Kalogera, V.

1997-12-01

301

Are Low-Mass Galaxy Clusters Overconcentrated?  

NASA Astrophysics Data System (ADS)

The Sloan Bright Arcs Survey (SBAS) at Fermilab has discovered and confirmed 19 strong-lensing systems in the Sloan Digital Sky Survey. We used the WIYN telescope to take follow-up data on 10 of these systems, studying both the properties of the galaxy clusters and the properties of the strong gravitational lenses. We have found that the majority of our systems are lower-mass clusters, those with mass ? 1014 M?. Using this data we have found evidence to support other groups' findings of an overconcentration problem among galaxy clusters, the idea that galaxy clusters are more concentrated than ?CDM would predict. It has recently been suggested that the overconcentration problem is most significant among low-mass clusters. We present our results for the relation between Einstein radius of the strong lenses and cluster mass (M200). We show that the Einstein radii of the clusters are typically larger than would be expected based on current models, indicating that the clusters are overconcentrated.

Wiesner, Matthew P.; Lin, H.; Allam, S.; Annis, J.; Buckley-Geer, E.; Diehl, H.; Kubik, D.; Kubo, J.; Tucker, D.

2012-01-01

302

The Orbital Light Curve of the Low-Mass X-ray Binary V1408 Aquilae  

NASA Astrophysics Data System (ADS)

We have obtained high-speed optical photometry of the low mass X-ray binary V1408 Aquilae (= 4U 1957+115), totaling 126 hours of data obtained on 29 nights. The orbital light curve of the system has a large-amplitude and roughly-sinusoidal modulation. We fit the data with synthetic light curves calculated from a model that includes: (1) a black hole primary surrounded by (2) an axisymmetric, optically-thick, physically-thin, viscous, steady-state accretion disk, that is fed from (3) a low-mass, cool secondary star. The secondary star is strongly heated by radiation from the disk. The varying aspect of the secondary's heated face dominates the orbital light curve and is the source of the quasi-sinusoidal modulation. We present the results of an extensive exploration of the model's parameter space.

Gomez, Sebastian; Mason, Paul A.; Robinson, Edward L.

2014-06-01

303

Identification and Follow-Up Observations of Low-Mass Eclipsing Binaries from Kepler  

Microsoft Academic Search

An outstanding problem in Astronomy for the past 15+ years has been that the radii of low-mass, (M < 1.0 M&sun;), main-sequence stars in eclipsing binary systems are consistently about 15% larger than predicted by theoretical models. The main cause is hypothesized to be rapid rotation due to binary spin-up, as all but one of the currently known systems have

Jeffrey Coughlin; M. Lopez-Morales; R. I. Marzoa; T. Harrison; N. Ule; D. Hoffman

2011-01-01

304

Enhanced Extra Mixing in Low-Mass Red Giants: Lithium Production and Thermal Stability  

Microsoft Academic Search

We show that canonical extra mixing with a diffusion coefficient Dmix~109 cm2 s-1, which is thought to start working in the majority of low-mass stars when they reach the bump luminosities on the red giant branch (RGB), cannot lead to an Li flash or a thermal instability, as has been proposed. The abundance levels of 7Li measured in the most

Pavel A. Denissenkov; Falk Herwig

2004-01-01

305

Deuterated water in low-mass protostars  

NASA Astrophysics Data System (ADS)

In addition to its dominant role in the cooling of warm gas and in the oxygen chemistry, water is a primordial species in the emergence of life, and comets may have brought a large fraction to Earth to form the oceans. Observations of deuterated water are an important complement for studies of H2O to understand how water forms and how it has evolved from cold prestellar cores to protoplanetary disks and consequently oceans for the Earth's specific, but probably not isolated, case. Several deuterated water transitions were observed with the Herschel/HIFI (Heterodyne Instrument for Far Infrared) instrument towards three low-mass protostars: IRAS 16293-2422, NGC1333 IRAS4A and NGC1333 IRAS4B. In the first source, both HDO and D2O lines are detected, thanks to the unbiased spectral survey carried out by the CHESS key program (Vastel et al. 2010, Coutens et al. 2013a). In the framework of a collaboration between the CHESS, WISH and HEXOS programs, two HDO key lines were observed towards the two other protostars. In addition, complementary observations were carried out with several ground-based single-dish telescopes (IRAM-30m, JCMT, APEX). We used the non-LTE RATRAN spherical model (Hogerheijde & van der Tak 2000) to determine the HDO abundance distribution throughout the protostellar envelope. An abundance jump at 100 K is required to reproduce the line profiles. Indeed, water molecules trapped in the icy grain mantles thermally desorb in the hot corinos, the inner warm regions of the protostellar envelopes. We also obtain that it is necessary to add a water-rich external absorbing layer to reproduce the absorbing components of the HDO and D2O fundamental transitions in all sources (Coutens et al. 2012, 2013a,b). The results derived for the different sources will be then presented and discussed.

Coutens, Audrey; Vastel, Charlotte; Chess Collaboration; Wish Collaboration; Hexos Collaboration

2013-07-01

306

Ursa Major: A Missing Low-Mass CDM Halo?  

NASA Astrophysics Data System (ADS)

The recently discovered Ursa Major dwarf spheroidal (dSph) galaxy candidate is about 5-8 times less luminous than the faintest previously known dSphs, And IX, Draco, and Ursa Minor. In this Letter, we present velocity measurements of seven color-magnitude-selected Ursa Major candidate stars. Two of them are apparent nonmembers based on metallicity and velocity, and the remaining five stars yield a systemic heliocentric velocity of v¯=-52.45+/-4.27 km s-1 and a central line-of-sight velocity dispersion of 1/2=9.3+11.7-1.2 km s-1, with 95% confidence that 1/2>6.5 km s-1. Assuming that UMa is in dynamical equilibrium, it is clearly dark matter-dominated and cannot be a purely stellar system like a globular cluster. It has an inferred central mass-to-light ratio of M/L~500 Msolar/Lsolar and, based on our studies of other dSphs, may possess a much larger total mass-to-light ratio. UMa is unexpectedly massive for its low luminosity-indeed, UMa appears to be the most dark matter-dominated galaxy yet discovered. The presence of so much dark matter in UMa immediately suggests that it may be a member of the missing population of low-mass galaxies predicted by the cold dark matter (CDM) paradigm. Given the weak correlation between dSph mass and luminosity, it is entirely likely that a population of dark dwarfs surrounds our Galaxy.

Kleyna, Jan T.; Wilkinson, Mark I.; Evans, N. Wyn; Gilmore, Gerard

2005-09-01

307

Magnetic activity and orbital periods of five low-mass eclipsing binaries  

NASA Astrophysics Data System (ADS)

We report 15 new VRI light curves of five low-mass eclipsing binaries (NSVS 02502726, NSVS 07453183, NSVS 11868841, NSVS 06550671 and NSVS 10653195) that were observed between 2010 and 2012. We analysed our new data together with three published spectroscopic observations and seven published light curves using a modified version of the Wilson-Devinney program. Orbital solutions of the five low-mass eclipsing binaries were revised and new star-spot parameters were obtained. We found that spot locations on the five low-mass eclipsing binaries changed over several years. However, the star-spots for NSVS 07453183 and NSVS 06550671 were stable for several months. More interestingly, for NSVS 02502726, the spots within a star-spot longitude region of 180°-360° indicated a magnetic activity cycle of 5.9(±0.2) yr. Moreover, we detected the first flare-like event on NSVS 07453183 at phase 0.39. The observations of the chromospheric activity indicators (H? and H? lines) revealed that NSVS 10653195 and NSVS 06550671 were active. For NSVS 02502726, we found a weak continuous secular decrease at a smaller rate of dp/dt = -2.1(0.8) × 10-7 d yr-1 than the previous result. For NSVS 07453183, the O-C times appeared to increase at cycle 6000, and this was followed by a decrease at cycle 6500.

Zhang, Li-Yun; Pi, Qing-feng; Yang, Yuan-Gui

2014-08-01

308

The Very Low Mass Component of the Gliese 105 System  

NASA Astrophysics Data System (ADS)

Multiple-epoch, multicolor images of the astrometric binary Gliese 105A and its very low mass companion Gliese 105C have been obtained using the Hubble Space Telescope's Wide Field Planetary Camera 2 (WFPC2) and near-infrared camera and multiobject spectrometer (NICMOS). The optical and near-infrared colors of Gl 105C strongly suggest a spectral type of M7 V for that star. Relative astrometric measurements spanning 3 yr reveal the first evidence of Gl 105C's orbital motion. Previous long-term astrometric studies at Sproul and McCormick Observatories have shown that the period of Gl 105A's perturbation is ~60 yr. To satisfy both the observed orbital motion and Gl 105A's astrometric period, Gl 105C's orbit must have an eccentricity of ~0.75 and a semimajor axis of ~15 AU. Measurements of Gl 105A's radial velocity over 12 yr show a linear trend with a slope of 11.3 m s-1 yr-1, which is consistent with these orbital constraints and a nearly face-on orbit. As no other faint companions to Gl 105A have been detected, we conclude that Gl 105C is probably the source of the 60 yr astrometric perturbation.

Golimowski, David A.; Henry, Todd J.; Krist, John E.; Schroeder, Daniel J.; Marcy, Geoffrey W.; Fischer, Debra A.; Butler, R. Paul

2000-10-01

309

WISE Detection of Low-Mass X-ray Binaries  

NASA Astrophysics Data System (ADS)

I will report on the results from our search for the Wide-field Infrared Survey Explorer detection of the Galactic low-mass X-ray binaries. Among 187 catalogued binaries, we find 13 counterparts and two candidate counterparts. For the 13 counterparts, two (4 0614+091 and G 339-4) have already been confirmed by previous studies to have a jet and one (GR 1915+105) to have a candidate circumbinary disk, from which the detected infrared emission arose. Having collected the broad-band optical and near-infrared data in literature and constructed flux density spectra for the other 10 binaries, we identify that three (A0620-00, XTE J1118+480, and GX 1+4) are candidate circumbinary disk systems, four (Cen X-4, 4U 1700+24, 3A 1954+319, and Cyg X-2) had thermal emission from their companion stars, and three (Sco X-1, Her X-1, and Swift J1753.5-0127) are peculiar systems with the origin of their infrared emission rather uncertain. Discussion of the results and WISE counterparts' brightness distribution will be provided, which suggests that more than half of the LMXBs would have a jet, a circumbinary disk, or the both.

Wang, Zhongxiang

2014-08-01

310

Formation of Millisecond Pulsars from Intermediate- and Low-Mass X-Ray Binaries  

NASA Astrophysics Data System (ADS)

We present a systematic study of the evolution of intermediate- and low-mass X-ray binaries consisting of an accreting neutron star of mass 1.0-1.8 M ? and a donor star of mass 1.0-6.0 M ?. In our calculations we take into account physical processes such as unstable disk accretion, radio ejection, bump-induced detachment, and outflow from the L 2 point. Comparing the calculated results with the observations of binary radio pulsars, we report the following results. (1) The allowed parameter space for forming binary pulsars in the initial orbital period-donor mass plane increases with increasing neutron star mass. This may help explain why some millisecond pulsars with orbital periods longer than ~60 days seem to have less massive white dwarfs than expected. Alternatively, some of these wide binary pulsars may be formed through mass transfer driven by planet/brown-dwarf-involved common envelope evolution. (2) Some of the pulsars in compact binaries might have evolved from intermediate-mass X-ray binaries with anomalous magnetic braking. (3) The equilibrium spin periods of neutron stars in low-mass X-ray binaries are in general shorter than the observed spin periods of binary pulsars by more than one order of magnitude, suggesting that either the simple equilibrium spin model does not apply or there are other mechanisms/processes spinning down the neutron stars.

Shao, Yong; Li, Xiang-Dong

2012-09-01

311

Computational Star Formation (IAU S270)  

NASA Astrophysics Data System (ADS)

1. Historical introduction; 2. Individual star formation: observations; 3. Low-mass star formation: observations; 4. Individual star formation: theory; 5. Formation of clusters: observations; 6. Formation of clusters: theory; 7. Numerical methods: MHD; 8. Numerical methods: radiative dynamics; 9. Local star formation processes; 10. Star formation feedback; 11. Star formation on galactic scales; 12. Special purpose hardware; 13. Computational methods; 14. Radiation diagnostics of star formation; 15. Large scale star formation; 16. Cosmological star formation; 17. Computational star formation: Summary; Index.

Alves, João.; Elmegreen, Bruce G.; Girart, Josep M.; Trimble, Virginia

2011-05-01

312

Stars  

NASA Astrophysics Data System (ADS)

We will begin our study with a more or less superficial inspection of the "forest" of stars that we see in the skies. The first thing we notice is that, as sources of light, they are much weaker than the Sun. Second, their apparent colors vary; from a bluish-white in most of them to a reddish-yellow, which is rarer. There is also a third aspect, though it is not very obvious to the naked eye: most of the stars group themselves in small families of two, three or more members. A good example is the Alpha Centauri, the closest star to us, which, in fact, is a triple system of stars. Another is the group of 7 stars that make up the Pleiades, which will be discussed later on. In fact, almost half of the stars are double systems with only two members, called binary stars. Most of these double stars, though together, are separated by several astronomical units (one astronomical unit, AU, is the distance from Earth to the sun: see Chapter 1), and revolve around each other over periods of several years. And yet the revolutions of some binary stars, separated by much smaller distances, occur in only a few hours! These stars are so close to each other that they can share enveloping material. Often this exchange occurs in a somewhat violent manner. Local explosions may occur, expelling matter away from the system. In other binary systems, where one of the components is a very compact, dense star, companion material flows more calmly, making up a light disk around the compact star.

Capelato, Hugo Vicente

1999-01-01

313

Heavy water stratification in a low-mass protostar  

NASA Astrophysics Data System (ADS)

Context. Despite the low elemental deuterium abundance in the Galaxy, enhanced molecular deuterium fractionation has been found in the environments of low-mass star-forming regions and, in particular, the Class 0 protostar IRAS 16293-2422. Aims: The key program Chemical HErschel Surveys of Star forming regions (CHESS) aims at studying the molecular complexity of the interstellar medium. The high sensitivity and spectral resolution of the Herschel/HIFI (Heterodyne Instrument for Far-Infrared) instrument provide a unique opportunity to observe the fundamental 11,1-00,0 transition of ortho-D2O at 607 GHz and the higher energy 21,2-10,1 transition of para-D2O at 898 GHz, both of which are inaccessible from the ground. Methods: The ortho-D2O transition at 607 GHz was previously detected. We present in this paper the first tentative detection for the para-D2O transition at 898 GHz. The spherical Monte Carlo radiative transfer code RATRAN was used to reproduce the observed line profiles of D2O with the same method that was used to reproduce the HDO and H218O line profiles in IRAS 16293-2422. Results: As for HDO, the absorption component seen on the D2O lines can only be reproduced by adding an external absorbing layer, possibly created by the photodesorption of the ices at the edges of the molecular cloud. The D2O column density is found to be about 2.5 × 1012 cm-2 in this added layer, leading to a D2O/H2O ratio of about 0.5%. At a 3? uncertainty, upper limits of 0.03% and 0.2% are obtained for this ratio in the hot corino and the colder envelope of IRAS 16293-2422, respectively. Conclusions: The deuterium fractionation derived in our study suggests that the ices present in IRAS 16293-2422 formed on warm dust grains (~15-20 K) in dense (~104-5 × 104 cm-3) translucent clouds. These results allow us to address the earliest phases of star formation and the conditions in which ices form. Based on Herschel/HIFI observations. Herschel is an ESA space observatory with scientific instruments provided by European-led principal Investigator consortia and with important participation from NASA.

Coutens, A.; Vastel, C.; Cazaux, S.; Bottinelli, S.; Caux, E.; Ceccarelli, C.; Demyk, K.; Taquet, V.; Wakelam, V.

2013-05-01

314

THE SURVEY OF H I IN EXTREMELY LOW-MASS DWARFS (SHIELD)  

SciTech Connect

We present first results from the Survey of H I in Extremely Low-mass Dwarfs (SHIELD), a multi-configuration Expanded Very Large Array (EVLA) study of the neutral gas contents and dynamics of galaxies with H I masses in the 10{sup 6}-10{sup 7} M{sub sun} range detected by the Arecibo Legacy Fast ALFA (ALFALFA) survey. We describe the survey motivation and concept demonstration using Very Large Array imaging of six low-mass galaxies detected in early ALFALFA data products. We then describe the primary scientific goals of SHIELD and present preliminary EVLA and WIYN 3.5 m imaging of the 12 SHIELD galaxies. With only a few exceptions, the neutral gas distributions of these extremely low-mass galaxies are centrally concentrated. In only one system have we detected H I column densities higher than 10{sup 21} cm{sup -2}. Despite this, the stellar populations of all of these systems are dominated by blue stars. Further, we find ongoing star formation as traced by H{alpha} emission in 10 of the 11 galaxies with H{alpha} imaging obtained to date. Taken together these results suggest that extremely low-mass galaxies are forming stars in conditions different from those found in more massive systems. While detailed dynamical analysis requires the completion of data acquisition, the most well-resolved system is amenable to meaningful position-velocity analysis. For AGC 749237, we find well-ordered rotation of 30 km s{sup -1} at {approx}40'' distance from the dynamical center. At the adopted distance of 3.2 Mpc, this implies the presence of a {approx}>1 x 10{sup 8} M{sub sun} dark matter halo and a baryon fraction {approx}<0.1.

Cannon, John M.; Engstrom, Eric; Allan, John; Erny, Grace; Fliss, Palmer; Smith, AnnaLeigh [Department of Physics and Astronomy, Macalester College, 1600 Grand Avenue, Saint Paul, MN 55105 (United States)

2011-09-20

315

IR light curves of four new low-mass eclipsing binaries  

NASA Astrophysics Data System (ADS)

In the last decade a significant amount of observational effort has been made to determine the physical parameters of low mass stars (M < 1 Msol). The most recent observations appear to show a discrepancy between the mass-radius relation predicted by the models and that obtained from the observational data. This discrepancy appear to be explained by assuming the presence of strong magnetic fields in the stars which produce larger radii than models predict. The best source of precise mass and radius measurements are double-lined, detached, eclipsing binaries (DDEBs). By measuring the radial velocities and light curves of these systems, we can derive their stellar masses and radii with accuracies of about 1-2%. However, the number of known low-mass DDEBs is small and for many of the known systems the measured masses and radii have large error bars. In the year 2005 we began an observational campaign to measure the visual and near-IR light-curves for a sample of DDEB candidates with low-mass components and observable from the Northern Hemisphere (delta > -9 deg). The objects of this sample were identified in large scale photometric surveys (NSVS, ASAS, SWASP). For the photometric observations we are using the Carlos Sanchez (JK bands) and the IAC80 (VRI bands) telescopes, both at Observatorio del Teide, Canary Islands, Spain. The IR-band light-curves are less affected by the presence of photospheric spots, which are a common feature in these stars with convective atmospheres and magnetic activity, and can, in principle, provide more precise radius measurements. In this poster we present the complete JK-bands light curves and the models of four of the low-mass DDEB observed in our program: NSVS10441882, NSVS07453183, NSVS10653195, and NSVS02502726.

Iglesias Marzoa, R.; Lopez-Morales, M.; Arevalo Morales, J.; Torres, G.; Lazaro Hernando, C.; Tamazian, V. S.; Szentgyorgyi, A.; Carnerero Martin, M. I.; Moreno Otero, M. A.; de La Fuente Guillen, D.

2011-11-01

316

Detection and Mass Determination of Low- mass Companions to Nearby M Dwarfs - Continuation  

NASA Astrophysics Data System (ADS)

This is a continuation proposal. We know of 188 M dwarfs within 10 parsecs, yet there are very few mass determinations for these faintest and most common stellar constituents of our Galaxy. Additional low-mass binaries are badly needed to determine better the mass-luminosity relation at the lower end of the main sequence. Furthermore, no unambiguous brown dwarfs have been identified, though theory would predict that they exist in great numbers. Nearby M dwarfs provide an ideal hunting ground for low-mass secondaries, whether they are stellar or substellar. Companions can be detected in short- period orbits that can be mapped with a small investment of telescope time, allowing each object's mass to be determined. The identification and follow-up of brown dwarfs and very low mass stars provides insight into such diverse problems as the {purported} galactic missing mass and the formation of stellar and planetary systems. Using HST Fine Guidance Sensor 3, we will acquire POS and TRANS mode observations of one M dwarf primaries with suspected, very low-mass secondaries. TRANS mode observations {0sec point010 resolution} will either confirm these companions or allow more stringent upper mass estimates to be made for unresolved companions. POS mode observations for the targets will provide more precise determinations of astrometric perturbations and will, in fact, be capable of detecting companions down to 0.01 Msun, the regime of high-mass planets.

Benedict, George

1996-07-01

317

Building the low-mass end of the red sequence with local post-starburst galaxies  

NASA Astrophysics Data System (ADS)

We present a study of local post-starburst galaxies (PSG) using the photometric and spectroscopic observations from the Sloan Digital Sky Survey (SDSS) and the results from the Galaxy Zoo project. We find that 74% of the local PSG are of indeterminate morphology. These local PSG also occupy a well-defined space within the color-stellar mass diagram, most notably in the low-mass end of the green valley below the transition mass described by Kauffmann et al. to be the mass division between low-mass star-forming galaxies and high-mass passively-evolving bulge-dominated galaxies. Our analysis suggests that it is likely that a local PSG will quickly transform into low-mass early-type galaxies as the stellar morphologies of the green PSG largely resemble that of the early-type galaxies within the same mass range. We propose that the current populations of PSG represent one population of galaxies which are rapidly transitioning from the blue cloud to the red sequence and subsequently, contributing towards the build-up of the low-mass end of the red sequence. This finding is consistent with the idea of downsizing where the build-up of smaller galaxies occurs at later epochs.

Wong, O. Ivy; Schawinski, K.; Kaviraj, S.; Masters, K.; Nichol, R.; Lintott, C.; Keel, W.; Darg, D.; Bamford, S.; Galaxy Zoo Team

2011-05-01

318

Life Cycles of Stars  

NSDL National Science Digital Library

This Powerpoint presentation inroduces younger students to the life cycles of stars. Topics include stellar nurseries, types of stars, supernovae, the fates of stars of either high or low mass, and the creation of heavier elements by continued fusion of successively heavier elements.

319

Low-Mass Eclipsing Binaries from Kepler: Reaching the Natural Rotation Rates of M and K Dwarfs  

NASA Astrophysics Data System (ADS)

An outstanding problem in stellar astrophysics is that the radii of low-mass, main-sequence stars in eclipsing binary systems are consistently 10-15% larger than predicted by stellar models. This inflation is hypothesized to be primarily due to enhanced magnetic activity as a result of their binarity, and thus artificially enhanced rotation rates. Thus, such an effect should diminish with increasing period, but only a small number of low-mass eclipsing binary systems are known in general, fewer are well-studied with precise light and radial-velocity curves, and barely any of these are at long periods. In addition to exploring the physics of low-mass stars, research into this area helps to better characterize the radii of extrasolar planets around low-mass stars, whose values are typically dependent on those assumed for the host star. We have previously presented results from our search for new low-mass eclipsing binary systems via our Kepler Guest Observer programs and a search through the publicly available data. We identified over 100+ low-mass eclipsing binaries suitable for ground-based follow-up, with 30 of them having periods greater than 10 days, and found preliminary evidence for a trend of decreasing stellar radii with increasing orbital period. In this presentation we present results of our ongoing effort to obtain ground-based multi-color light and radial velocity curves of these systems via the Kitt Peak National Observatory 4-meter, the Apache Point Observatory 3.5-meter, and New Mexico State University 1-meter telescopes. We also present preliminary modeling of these data combined with that from the Kepler mission, and examine what future work is needed to make progress in this area. The presenter acknowledges funding from a National Science Foundation Graduate Research Fellowship.

Coughlin, Jeffrey; Harrison, T.; Lopez-Morales, M.; Ule, N.

2012-01-01

320

Galaxy Zoo: building the low-mass end of the red sequence with local post-starburst galaxies  

NASA Astrophysics Data System (ADS)

We present a study of local post-starburst galaxies (PSGs) using the photometric and spectroscopic observations from the Sloan Digital Sky Survey and the results from the Galaxy Zoo project. We find that the majority of our local PSG population have neither early- nor late-type morphologies but occupy a well-defined space within the colour-stellar mass diagram, most notably, the low-mass end of the 'green valley' below the transition mass thought to be the mass division between low-mass star-forming galaxies and high-mass passively evolving bulge-dominated galaxies. Our analysis suggests that it is likely that local PSGs will quickly transform into 'red', low-mass early-type galaxies as the stellar morphologies of the 'green' PSGs largely resemble that of the early-type galaxies within the same mass range. We propose that the current population of PSGs represents a population of galaxies which is rapidly transitioning between the star-forming and the passively evolving phases. Subsequently, these PSGs will contribute towards the build-up of the low-mass end of the 'red sequence' once the current population of young stars fade and stars are no longer being formed. These results are consistent with the idea of 'downsizing' where the build-up of smaller galaxies occurs at later epochs. This publication has been made possible by the participation of more than 250 000 volunteers in the Galaxy Zoo project. Their contributions are individually acknowledged at .

Wong, O. I.; Schawinski, K.; Kaviraj, S.; Masters, K. L.; Nichol, R. C.; Lintott, C.; Keel, W. C.; Darg, D.; Bamford, S. P.; Andreescu, D.; Murray, P.; Raddick, M. J.; Szalay, A.; Thomas, D.; Vandenberg, J.

2012-02-01

321

Prized results from HARPS. Low-mass/habitable/transiting planets orbiting M dwarfs  

NASA Astrophysics Data System (ADS)

Searching for planets around stars with different masses probes the outcome of planetary formation for different initial conditions. The low-mass M dwarfs are also the most frequent stars in our Galaxy and potentially therefore, the most frequent planet hosts. This has motivated our search for planets around M dwarfs with HARPS. That observing program has now run for almost a decade and detected most of the known low-mass planets orbiting M dwarfs (m sin i < 20 M?), including the least massive (GJ581e, msini = 1.9 M?) and the first potentially habitable planets (GJ581c&d GJ667Cc, GJ163c). This proceeding shortly reviews the detections made with HARPS, reports on the occurrence of planets around M dwarfs and how they mesh up with planet formation theory. It also highlights our sensitivity to low-mass habitable planets, the first direct measure of ??, and the recent detection of a transiting planet the size of Uranus.

Bonfils, Xavier; Bouchy, François; Delfosse, X.; Forveille, T.; Gillon, M.; Lovis, C.; Mayor, M.; Neves, V.; Pepe, F.; Perrier, C.; Queloz, D.; Santos, N.; Ségransan, D.; Udry, S.

2013-04-01

322

Extremely Low Mass: The Circumstellar Envelope of a Potential Proto-Brown Dwarf  

NASA Technical Reports Server (NTRS)

What is the environment for planet formation around extremely low mass stars? Is the environment around brown dwarfs and extremely low mass stars conducive and sufficiently massive for planet production? The determining conditions may be set very early in the process of the host object's formation. IRAS 16253-2429, the source of the Wasp-Waist Nebula seen in Spitzer IRAC images, is an isolated, very low luminosity ("VeLLO") Class 0 protostar in the nearby rho Ophiuchi cloud. We present VLA ammonia mapping observations of the dense gas envelope feeding the central core accreting system. We find a flattened envelope perpendicular to the outflow axis, and gas cavities that appear to cradle the outflow lobes as though carved out by the flow and associated (apparently precessing) jet, indicating environmental disruption. Based on the NH3 (1,1) and (2,2) emission distribution, we derive the mass, velocity fields and temperature distribution for the envelope. We discuss the combined evidence for this source to be one of the youngest and lowest mass sources in formation yet known, and discuss the ramifications for planet formation potential in this extremely low mass system.

Wiseman, Jennifer

2011-01-01

323

Discovery of Pulsations in He-core, Extremely Low Mass White Dwarfs  

NASA Astrophysics Data System (ADS)

After an extensive search, we announce the discovery of the first three extremely low mass (ELM, ? 0.25 M?), putatively He-core pulsating white dwarfs (WDs). The objects are by far the coolest and the lowest-mass pulsating WD known. The first to be published, SDSS J1840+6423 (Hermes et al. 2012), has Teff = 9140±170 K and log g = 6.22±0.06, which corresponds to a mass of ˜ 0.17 M?. The second and third pulsating ELM WDs have similarly low masses. SDSS J1112+1117 has Teff = 9400±490 K and a log g = 5.99±0.12. SDSS J1518+0658 has Teff = 9810±320 K and a log g = 6.66±0.06. These low-mass pulsating WDs greatly extend the DAV (or ZZ Ceti) instability strip, and begin to bridge the gap in surface gravity between WDs and main-sequence stars. Consistent with the expectation that these ELM WDs are the product of binary evolution, all three of these stars have an unseen binary companion, with 4.2-14.6 hr orbital periods, in each case most likely another WD.

Hermes, J. J.; Montgomery, M. H.; Winget, D. E.; Brown, W. R.; Kilic, M.; Kenyon, S. J.; Harrold, S. T.; Bell, K.; Pelletier, J.; Rostopchina, A.

2013-01-01

324

SDSS J184037.78+642312.3: THE FIRST PULSATING EXTREMELY LOW MASS WHITE DWARF  

SciTech Connect

We report the discovery of the first pulsating extremely low mass (ELM) white dwarf (WD), SDSS J184037.78+642312.3 (hereafter J1840). This DA (hydrogen-atmosphere) WD is by far the coolest and the lowest-mass pulsating WD, with T{sub eff} = 9100 {+-} 170 K and log g = 6.22 {+-} 0.06, which corresponds to a mass of {approx}0.17 M{sub Sun }. This low-mass pulsating WD greatly extends the DAV (or ZZ Ceti) instability strip, effectively bridging the log g gap between WDs and main-sequence stars. We detect high-amplitude variability in J1840 on timescales exceeding 4000 s, with a non-sinusoidal pulse shape. Our observations also suggest that the variability is multi-periodic. The star is in a 4.6 hr binary with another compact object, most likely another WD. Future, more extensive time-series photometry of this ELM WD offers the first opportunity to probe the interior of a low-mass, presumably He-core WD using the tools of asteroseismology.

Hermes, J. J.; Montgomery, M. H.; Winget, D. E. [Department of Astronomy, University of Texas at Austin, Austin, TX 78712 (United States); Brown, Warren R.; Kenyon, Scott J. [Smithsonian Astrophysical Observatory, 60 Garden St, Cambridge, MA 02138 (United States); Kilic, Mukremin, E-mail: jjhermes@astro.as.utexas.edu [Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks St., Norman, OK 73019 (United States)

2012-05-10

325

Dissociative shocks in the inner 100 AU of low-mass protostars  

NASA Astrophysics Data System (ADS)

Even for low-mass protostars (Lbol < 100 Lsun) star formation is a violent process. The inner dense envelope is illuminated by X-rays and UV radiation from the accreting protostar, while the same inner envelope is exposed to the protostellar jet and wind, both causing shocks in the dense gas. Thus, the chemical and physical conditions along the outflow cavities are significantly different from the conditions in the bulk of the cold envelope. The hot gas (T > 500 K) remains largely uncharacterized in spite of the fact that it is observed toward nearly every low-mass protostar with Herschel-PACS. Recent observations obtained with Herschel-HIFI as part of the "Water in star-forming regions with Herschel" program (WISH) will be presented. The velocity-resolved line profiles of water and related hydrides (OH+, OH, CH+) point to a physical origin of the hot gas to be in dissociative shocks located in the inner few 100 AU of the protostar. Complementary SubMillimeter Array (SMA) data shed further light on both the spatial location and excitation conditions in these shocks. Finally, ALMA Science Verification data show how and where gas is put in motion on small scales (~ 100 AU). All of these observations pave the way for future studies of shocks and entrainment in low-mass protostars.

Kristensen, L. E.

2013-07-01

326

Investigating the Processes Driving Low-Mass Galaxy Evolution with Gas Metallicities of Starburst Galaxies  

NASA Astrophysics Data System (ADS)

There appears to be a ``fundamental" relationship that links the stellar masses, star-formation rates (SFRs), and gas metallicities of local galaxies. It has been used to constrain the major processes in galaxy evolution. However, it is unclear whether (1) this observed relation holds at earlier cosmic time, and (2) if it applies to low-mass galaxies and/or those with relatively higher specific SFRs (sSFRs). We request follow-up Hectospec spectroscopy %and DEIMOS spectroscopy to obtain gas metallicity measurements in key unexplored domains of galaxy parameter space. We will target Ntarget low-mass high equivalent width (EW) emission-line galaxies at zrange in the Subaru Deep Field (SDF). This sample is a factor of almost 4 larger than the existing data for galaxies with similar redshifts, SFRs and stellar masses. The SDF is ideal for such a survey because of its unique multi-wavelength imaging data that allow us to (1) identify a much higher surface density of high-EW star-forming galaxies over a wide redshift range than in any other survey, and (2) determine stellar masses and SFRs for individual galaxies. With the largest spectroscopic sample of low mass and/or high sSFR galaxies, we will determine the relationships between metallicity, stellar mass, and SFRs for dwarf galaxies. We will examine if the same galaxy evolution processes in massive galaxies also hold for lower mass galaxies over the past six billion years.

Ly, Chun; Malkan, Matthew; Nagao, Tohru; Hayashi, Masao; Kashikawa, Nobunari; Shimasaku, Kazuhiro; Motohara, Kentaro

2013-02-01

327

Origin of water around deeply embedded low-mass protostars  

NASA Astrophysics Data System (ADS)

Although water is such an important molecule for our life here on earth, its way from the surface of dust grains to planets is not well understood. Studies of water in star forming regions show that it is the most abundant molecule in the surface ices on dust grains (e.g., Gibb et al. 2004), an important coolant (e.g., Watson et al. 2007), and thus, highly influential for the oxygen based chemistry (e.g., van Dishoeck & Blake 1998). The water is locked up as ice on grains in cold and shielded regions, but close to the forming protostar, where the temperature reaches 90 ˜100 K, water ice evaporates from the grains, raising its abundance in the gas-phase. Exactly which mechanism that regulates when and where this happens is still debated. In the poster we will present high-angular resolution (˜0.7''), ground based interferometric observations of an isotopologue of water toward the four low-mass protostars NGC 1333 IRAS 2A, IRAS 4A (NW & SE), and IRAS 4B. Watson et al. (2007) detected mid-IR water emission towards IRAS 4B with the Spitzer Space Telescope - but not the other Class 0 sources. Here the maps show compact emission stemming from the inner 100 AU in all of the sources, except IRAS 4A-SE. Although properties (mass, luminosity, age) are similar for the sources, we detect large differences in relative abundances between water and complex organic molecules in the spatially resolved emission. One source, IRAS 2A, show water emission in the outflow close to the protostar.

Persson, M. V.; Jorgensen, J. K.; van Dishoeck, E. F.

2011-05-01

328

A New Method to Search for Quiescent Low-Mass X-ray Binary  

NASA Astrophysics Data System (ADS)

We explore a new method in searching for quiescent Low-Mass X-ray Binary (qLMXB). To date, almost all the accretion-powered BH-LMXBs, which stay in their quiescent state most of the time, were only found during their X-ray outburst. Our method explores a new way to find accretion binaries in their quiescent states. We search objects with spectral types earlier than K and M_V more than 2-? brighter than that expected for a main-sequence star, then look for stars in the above sample with log(F_X/F_R) (0.5-2. keV) more than 2-? greater than that seen in typical subgiant stars. Most likely there is an accretion disk responsible for the extra X-ray emission. We show one example target of this study, with its X-ray and optical data.

Zhao, Ping; Grindlay, J. E.; Hong, J.; Servillat, M.; Van Den Berg, M.

2013-01-01

329

Revealing the physics of R modes in low-mass x-ray binaries.  

PubMed

We consider the astrophysical constraints on the gravitational-wave-driven r-mode instability in accreting neutron stars in low-mass x-ray binaries. We use recent results on superfluid and superconducting properties to infer the core temperature in these neutron stars and show the diversity of the observed population. Simple theoretical models indicate that many of these systems reside inside the r-mode instability region. However, this is in clear disagreement with expectations, especially for the systems containing the most rapidly rotating neutron stars. The inconsistency highlights the need to reevaluate our understanding of the many areas of physics relevant to the r-mode instability. We summarize the current status of our understanding, and we discuss directions for future research which could resolve this dilemma. PMID:21981488

Ho, Wynn C G; Andersson, Nils; Haskell, Brynmor

2011-09-01

330

Revealing the Physics of r Modes in Low-Mass X-Ray Binaries  

SciTech Connect

We consider the astrophysical constraints on the gravitational-wave-driven r-mode instability in accreting neutron stars in low-mass x-ray binaries. We use recent results on superfluid and superconducting properties to infer the core temperature in these neutron stars and show the diversity of the observed population. Simple theoretical models indicate that many of these systems reside inside the r-mode instability region. However, this is in clear disagreement with expectations, especially for the systems containing the most rapidly rotating neutron stars. The inconsistency highlights the need to reevaluate our understanding of the many areas of physics relevant to the r-mode instability. We summarize the current status of our understanding, and we discuss directions for future research which could resolve this dilemma.

Ho, Wynn C. G.; Andersson, Nils; Haskell, Brynmor [School of Mathematics, University of Southampton, Southampton, SO17 1BJ (United Kingdom); Astronomical Institute ''Anton Pannekoek'', University of Amsterdam, 1098XH Amsterdam (Netherlands)

2011-09-02

331

Modeling the luminosity function of galactic low-mass X-ray binaries  

NASA Astrophysics Data System (ADS)

The evolution of the family of binaries with a low-mass star and a compact neutron star companion (low-mass X-ray binaries (LMXBs) with neutron stars) ismodeled by the method of population synthesis. Continuous Roche-lobe filling by the optical star in LMXBs is assumed to be maintained by the removal of orbital angular momentum from the binary by a magnetic stellar wind from the optical star and the radiation of gravitational waves by the binary. The developed model of LMXB evolution has the following significant distinctions: (1) allowance for the effect of the rotational evolution of a magnetized compact remnant on themass transfer scenario in the binary, (2) amore accurate allowance for the response of the donor star to mass loss at the Roche-lobe filling stage. The results of theoretical calculations are shown to be in good agreement with the observed orbital period-X-ray luminosity diagrams for persistent Galactic LMXBs and their X-ray luminosity function. This suggests that the main elements of binary evolution, on the whole, are correctly reflected in the developed code. It is shown that most of the Galactic bulge LMXBs at luminosities L x > 1037 erg s-1 should have a post-main-sequence Roche-lobe-filling secondary component (low-mass giants). Almost all of the models considered predict a deficit of LMXBs at X-ray luminosities near ˜1036.5 erg s-1 due to the transition of the binary from the regime of angular momentum removal by a magnetic stellar wind to the regime of gravitational waves (analogous to the widely known period gap in cataclysmic variables, accreting white dwarfs). At low luminosities, the shape of the model luminosity function for LMXBs is affected significantly by their transient behavior-the accretion rate onto the compact companion is not always equal to the mass transfer rate due to instabilities in the accretion disk around the compact object. The best agreement with observed binaries is achieved in the models suggesting that heavy neutron stars with masses 1.4-1.9 M ? can be born.

Kuranov, A. G.; Postnov, K. A.; Revnivtsev, M. G.

2014-01-01

332

A survey of [HDCO]\\/[H2CO] and [DCN]\\/[HCN] ratios towards low-mass protostellar cores  

Microsoft Academic Search

We present observations of [HDCO]\\/[H2CO] and [DCN]\\/[HCN] ratios towards a selection of low-mass protostellar cores in three different star forming regions. The best fit to the observed [HDCO]\\/[H2CO] ratios is ~ 0.05-0.07, similar to the values observed towards the dark clouds, TMC-1 and L134N. [DCN]\\/[HCN] ratios are ~ 0.04, higher than those seen in TMC-1, around the low-mass protostar IRAS

H. Roberts; G. A. Fuller; T. J. Millar; J. Hatchell; J. V. Buckle

2002-01-01

333

THE FREQUENCY OF LOW-MASS EXOPLANETS. III. TOWARD {eta}{sub +} AT SHORT PERIODS  

SciTech Connect

Determining the occurrence rate of 'super-Earth' planets (m sin i < 10 M{sub +}) is a critically important step on the path toward determining the frequency of Earth-like planets ({eta}{sub +}), and hence the uniqueness of our solar system. Current radial-velocity surveys, achieving precisions of 1 m s{sup -1}, are now able to detect super-Earths and provide meaningful estimates of their occurrence rate. We present an analysis of 67 solar-type stars from the Anglo-Australian Planet Search specifically targeted for very high precision observations. When corrected for incompleteness, we find that the planet occurrence rate increases sharply with decreasing planetary mass. Our results are consistent with those from other surveys: in periods shorter than 50 days, we find that 3.0% of stars host a giant (msin i > 100 M{sub +}) planet, and that 17.4% of stars host a planet with msin i < 10 M{sub +}. The preponderance of low-mass planets in short-period orbits is in conflict with formation simulations in which the majority of super-Earths reside at larger orbital distances. This work gives a hint as to the size of {eta}{sub +}, but to make meaningful predictions on the frequency of terrestrial planets in longer, potentially habitable orbits, low-mass terrestrial planet searches at periods of 100-200 days must be made an urgent priority for ground-based Doppler planet searches in the years ahead.

Wittenmyer, Robert A.; Tinney, C. G.; Bailey, J.; Horner, J. [Department of Astrophysics, School of Physics, University of NSW, NSW 2052 (Australia); Butler, R. P. [Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington DC 20015-1305 (United States); O'Toole, Simon J. [Australian Astronomical Observatory, P.O. Box 296, Epping, NSW 1710 (Australia); Jones, H. R. A. [Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield, Herts, AL10 9AB (United Kingdom); Carter, B. D., E-mail: rob@phys.unsw.edu.au [Faculty of Sciences, University of Southern Queensland, Toowoomba, Queensland 4350 (Australia)

2011-09-01

334

The magnetohydrodynamics model of twin kilohertz quasi-periodic oscillations in low-mass X-ray binaries  

NASA Astrophysics Data System (ADS)

We suggest an explanation for the twin kilohertz quasi-periodic oscillations (kHz QPOs) in low-mass X-ray binaries (LMXBs) based on magnetohydrodynamics (MHD) oscillation modes in neutron star magnetospheres. Including the effect of the neutron star spin, we derive several MHD wave modes by solving the dispersion equations, and propose that the coupling of the two resonant MHD modes may lead to the twin kHz QPOs. This model naturally relates the upper, lower kHz QPO frequencies with the spin frequencies of the neutron stars, and can well account for the measured data of six LMXBs.

Shi, Changsheng; Li, Xiang-Dong

2009-01-01

335

M dwarfs in the Local Milky Way: The Field Low-Mass Stellar Luminosity and Mass Functions  

SciTech Connect

Modern sky surveys, such as the Sloan Digital Sky Survey (SDSS) and the Two-Micron All Sky Survey, have revolutionized how Astronomy is done. With millions of photometric and spectroscopic observations, global observational properties can be studied with unprecedented statistical significance. Low-mass stars dominate the local Milky Way, with tens of millions observed by SDSS within a few kpc. Thus, they make ideal tracers of the Galactic potential, and the thin and thick disks. In this thesis dissertation, I present my efforts to characterize the local low-mass stellar population, using a collection of observations from the Sloan Digital Sky Survey (SDSS). First, low-mass stellar template spectra were constructed from the co-addition of thousands of SDSS spectroscopic observations. These template spectra were used to quantify the observable changes introduced by chromospheric activity and metallicity. Furthermore, the average ugriz colors were measured as a function of spectral type. Next, the local kinematic structure of the Milky Way was quantified, using a special set of SDSS spectroscopic observations. Combining proper motions and radial velocities (measured using the spectral templates), along with distances, the full UVW space motions of over 7000 low-mass stars along one line of sight were computed. These stars were also separated kinematically to investigate other observational differences between the thin and thick disks. Finally, this dissertation details a project designed to measure the luminosity and mass functions of low-mass stars. Using a new technique optimized for large surveys, the field luminosity function (LF) and local stellar density profile are measured simultaneously. The sample size used to estimate the LF is nearly three orders of magnitude larger than any previous study, offering a definitive measurement of this quantity. The observed LF is transformed into a mass function (MF) and compared to previous studies.

Bochanski, John J., Jr.; /Washington U., Seattle, Astron. Dept.

2006-06-01

336

The growth of galactic bulges through mergers in ? CDM haloes revisited - I. Present-day properties  

NASA Astrophysics Data System (ADS)

We use the combined data sets of the Millennium I and II cosmological simulations to revisit the impact of mergers in the growth of bulges in central galaxies in the ? cold dark matter (?CDM) scenario. We seed galaxies within the growing CDM haloes using semi-empirical relations to assign stellar and gaseous masses, and an analytic treatment to estimate the transfer of stellar mass to the bulge of the remnant after a galaxy merger. We find that this model roughly reproduces the observed correlation between the bulge-to-total mass (B/T) ratio and stellar mass (M*) in present-day central galaxies as well as their observed demographics, although low-mass B/T < 0.1 (bulgeless) galaxies might be scarce relative to the observed abundance. In our merger-driven scenario, bulges have a composite stellar population made of (i) stars acquired from infalling satellites, (ii) stars transferred from the primary disc due to merger-induced perturbations and (iii) newly formed stars in starbursts triggered by mergers. We find that the first two are the main channels of mass assembly, with the first one being dominant for massive galaxies, creating large bulges with different stellar populations than those of the inner discs, while the second is dominant for intermediate/low-mass galaxies and creates small bulges with similar stellar populations to the inner discs. We associate the dominion of the first (second) channel to classical (pseudo) bulges, and compare the predicted fractions to observations. We emphasize that our treatment does not include other mechanisms of bulge growth such as intrinsic secular processes in the disc or misaligned gas accretion. Interestingly, we find that the evolution of the stellar and gaseous contents of the satellite as it spirals towards the central galaxy is a key ingredient in setting the morphology of the remnant galaxy, and that a good match to the observed bulge demographics occurs when this evolution proceeds closely to that of the central galaxy.

Zavala, Jesus; Avila-Reese, Vladimir; Firmani, Claudio; Boylan-Kolchin, Michael

2012-12-01

337

MUSTANG 3.3mm Observations of Low-mass Cores  

NASA Astrophysics Data System (ADS)

We present observations of low-mass star-forming cores (L1527, L1448C, B335, L483) observed during 2009 with MUSTANG, the 3.3mm bolometer camera on the 100-m Green Bank Telescope. The 3.3mm emission may be used to constrain the fraction of emission coming from the envelope and disk. We present our initial model comparisons to observations of the dust emission surrounding the protostars. Comparison is made to interferometric observations of L1527. These observations demonstrate the feasibility of active surface observations with aperture efficiencies above 20% at 3mm with the GBT.

Shirley, Yancy L.; Mason, B. S.; Mangum, J. G.; Dicker, S. S.; Korngut, P. M.; Devlin, M. J.

2010-01-01

338

Magnetic Field Alignment With The Inner Envelope Of Low-mass Cores  

NASA Astrophysics Data System (ADS)

We present 350 ?m SHARP/CSO polarization results on seven low-mass cores with class 0 protostars. These cores are part of a larger program to test magnetically regulated star formation models. One key prediction of these models is that the magnetic field symmetry axis in a core is aligned with the symmetry axis of the flattened YSO inner envelope (aka pseudodisk). The inclination angle of a YSO can alter the projected degree of alignment. After taking the inclination angle into account, we find evidence for an alignment between the magnetic field and pseudodisk symmetry axes. This work is funded by NSF grant AST-0909030.

Chapman, Nicholas L.; Novak, G.; Davidson, J.; Matthews, T.; Matthews, B. C.; Goldsmith, P.; Volgenau, N. H.; Vaillancourt, J. E.; Looney, L.; Kwon, W.; Houde, M.; Peng, R.; Li, Z.

2013-01-01

339

Long-term variability of low-mass X-ray binaries  

NASA Astrophysics Data System (ADS)

We consider modulations of mass captured by the compact object from the companion star's stellar wind in Low Mass X-ray Binaries with late type giants. Based on 3D simulations with two different hydrodynamic codes used Lagrangian and Eulerian approaches - the SPH code GADGET and the Eulerian code PLUTO, we conclude that a hydrodynamical interaction of the wind matter within a binary system even without eccentricity results in variability of the mass accretion rate with characteristic time-scales close to the orbital period. Observational appearances of this wind might be similar to that of an accretion disc corona/wind.

Filippova, E.; Revnivtsev, M.; Parkin, E. R.

2014-01-01

340

Radial velocity variations in EX Lup: hints for a low-mass close companion  

NASA Astrophysics Data System (ADS)

EXors are low-mass pre-main sequence objects producing repetitive optical outbursts attributed to highly enhanced accretion from the circumstellar disk onto the star. One type of outburst theories requires a close stellar or sub-stellar companion that perturbs the inner part of the disk and triggers the onset of the enhanced accretion. Here, we look for a possible companion to EX Lup, the prototype of the EXor class, using radial velocity (RV) observations. The RVs show large periodic variations that can be explained by the presence of a close companion in the brown dwarf mass range. Chromospheric activity or starspots are less likely to explain the observed RV curve.

Kóspál, Ágnes; Mohler-Fischer, Maren; Sicilia-Aguilar, Aurora; Ábrahám, Péter; Curé, Michel; Henning, Thomas; Kiss, Csaba; Launhardt, Ralf; Moór, Attila; Müller, André

2013-07-01

341

THE METALLICITY EVOLUTION OF LOW-MASS GALAXIES: NEW CONSTRAINTS AT INTERMEDIATE REDSHIFT  

SciTech Connect

We present abundance measurements from 26 emission-line-selected galaxies at z {approx} 0.6-0.7. By reaching stellar masses as low as 10{sup 8} M{sub Sun }, these observations provide the first measurement of the intermediate-redshift mass-metallicity (MZ) relation below 10{sup 9} M{sub Sun }. For the portion of our sample above M > 10{sup 9} M{sub Sun} (8/26 galaxies), we find good agreement with previous measurements of the intermediate-redshift MZ relation. Compared to the local relation, we measure an evolution that corresponds to a 0.12 dex decrease in oxygen abundances at intermediate redshifts. This result confirms the trend that metallicity evolution becomes more significant toward lower stellar masses, in keeping with a downsizing scenario where low-mass galaxies evolve onto the local MZ relation at later cosmic times. We show that these galaxies follow the local fundamental metallicity relation, where objects with higher specific (mass-normalized) star formation rates (SFRs) have lower metallicities. Furthermore, we show that the galaxies in our sample lie on an extrapolation of the SFR-M{sub *} relation (the star-forming main sequence). Leveraging the MZ relation and star-forming main sequence (and combining our data with higher-mass measurements from the literature), we test models that assume an equilibrium between mass inflow, outflow, and star formation. We find that outflows are required to describe the data. By comparing different outflow prescriptions, we show that momentum, driven winds can describe the MZ relation; however, this model underpredicts the amount of star formation in low-mass galaxies. This disagreement may indicate that preventive feedback from gas heating has been overestimated, or it may signify a more fundamental deviation from the equilibrium assumption.

Henry, Alaina; Martin, Crystal L.; Finlator, Kristian [Department of Physics, University of California, Santa Barbara, CA 93106 (United States); Dressler, Alan, E-mail: alaina.henry@nasa.gov [Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101 (United States)

2013-06-01

342

Lakatos Revisited.  

ERIC Educational Resources Information Center

Revisits and reviews Imre Lakatos' ideas on "Falsification and the Methodology of Scientific Research Programmes." Suggests that Lakatos' framework offers an insightful way of looking at the relationship between theory and research that is relevant not only for evaluating research programs in theoretical physics, but in the social sciences as…

Court, Deborah

1999-01-01

343

The low-mass population of the ? Ophiuchi molecular cloud  

NASA Astrophysics Data System (ADS)

Context. Star formation theories are currently divergent regarding the fundamental physical processes that dominate the substellar regime. Observations of nearby young open clusters allow the brown dwarf (BD) population to be characterised down to the planetary mass regime, which ultimately must be accommodated by a successful theory. Aims: We hope to uncover the low-mass population of the ? Ophiuchi molecular cloud and investigate the properties of the newly found brown dwarfs. Methods: We used near-IR deep images (reaching completeness limits of approximately 20.5 mag in J and 18.9 mag in H and Ks) taken with the Wide Field IR Camera (WIRCam) at the Canada France Hawaii Telescope (CFHT) to identify candidate members of ? Oph in the substellar regime. A spectroscopic follow-up of a small sample of the candidates allows us to assess their spectral type and subsequently their temperature and membership. Results: We select 110 candidate members of the ? Ophiuchi molecular cloud, from which 80 have not previously been associated with the cloud. We observed a small sample of these and spectroscopically confirm six new brown dwarfs with spectral types ranging from M6.5 to M8.25. Based on observations obtained with WIRCam, a joint project of CFHT, Taiwan, Korea, Canada, France, at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institute National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii. Based on observations made at the ESO La Silla and Paranal Observatory under program 083.C-0092. Based in part on data collected at Subaru Telescope, and obtained from the SMOKA, which is operated by the Astronomy Data Center, National Astronomical Observatory of Japan. Research supported by the Marie Curie Research Training Network CONSTELLATION under grant No. MRTN-CT- 2006-035890.Table 4 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/515/A75

Alves de Oliveira, C.; Moraux, E.; Bouvier, J.; Bouy, H.; Marmo, C.; Albert, L.

2010-06-01

344

Bayesian search for low-mass planets around nearby M dwarfs - estimates for occurrence rate based on global detectability statistics  

NASA Astrophysics Data System (ADS)

Due to their higher planet-star mass ratios, M dwarfs are the easiest targets for detection of low-mass planets orbiting nearby stars using Doppler spectroscopy. Furthermore, because of their low masses and luminosities, Doppler measurements enable the detection of low-mass planets in their habitable zones that correspond to closer orbits than for solar-type stars. We re-analyse literature Ultraviolet and Visual Echelle Spectrograph (UVES) radial velocities of 41 nearby M dwarfs in a combination with new velocities obtained from publicly available spectra from the HARPS-ESO spectrograph of these stars in an attempt to constrain any low-amplitude Keplerian signals. We apply Bayesian signal detection criteria, together with posterior sampling techniques, in combination with noise models that take into account correlations in the data and obtain estimates for the number of planet candidates in the sample. More generally, we use the estimated detection probability function to calculate the occurrence rate of low-mass planets around nearby M dwarfs. We report eight new planet candidates in the sample (orbiting GJ 27.1, GJ 160.2, GJ 180, GJ 229, GJ 422, and GJ 682), including two new multiplanet systems, and confirm two previously known candidates in the GJ 433 system based on detections of Keplerian signals in the combined UVES and High Accuracy Radial velocity Planet Searcher (HARPS) radial velocity data that cannot be explained by periodic and/or quasi-periodic phenomena related to stellar activities. Finally, we use the estimated detection probability function to calculate the occurrence rate of low-mass planets around nearby M dwarfs. According to our results, M dwarfs are hosts to an abundance of low-mass planets and the occurrence rate of planets less massive than 10 M? is of the order of one planet per star, possibly even greater. Our results also indicate that planets with masses between 3 and 10 M? are common in the stellar habitable zones of M dwarfs with an estimated occurrence rate of 0.21^{+0.03}_{-0.05} planets per star.

Tuomi, Mikko; Jones, Hugh R. A.; Barnes, John R.; Anglada-Escudé, Guillem; Jenkins, James S.

2014-06-01

345

Discovery of a Low Mass Bipolar Molecular Outflow from L1014-IRS with the Submillimeter Array  

NASA Astrophysics Data System (ADS)

With the discovery of the faint embedded source L1014-IRS, the Spitzer Space Telescope has opened a new window for the study of very low mass objects and the formation of brown dwarfs. Using the Submillimeter Array we report the discovery of a compact low mass bipolar molecular outflow from L1014-IRS and confirm its association with the L1014 dense core at 200 pc. Consequently, L1014-IRS is the lowest luminosity (L ˜ 0.09 L?) and perhaps the lowest mass source known to be driving a bipolar molecular outflow, which is one of the smallest known in size ( ˜500 AU), mass (< 10-4 M?), and energetics (e.g., with a force < 10-7 M? km s-1 yr-1). These results suggest that L1014-IRS is either a very young protostar yet to accrete a significant fraction of its final mass, or will remain substellar, which would suggest that brown dwarfs can form in a manner broadly similiar to low mass stars. T.L.B. acknowledges support from the Submillimeter Array Fellowship Program. The work of A.C. was supported by a Smithsonian Predoctoral Fellowship at the Smithsonian Astrophysical Observatory. P.C.M. acknowledges support from NASA Origins of Solar Systems Program Grant NAG 5-13050.

Bourke, T. L.; Crapsi, A.; Myers, P. C.; Evans, N. J., II; Wilner, D. J.; Huard, T. L.; Jorgensen, J. K.; Young, C. H.

2005-12-01

346

A fundamental problem in our understanding of low-mass galaxy evolution  

NASA Astrophysics Data System (ADS)

Recent studies have found a dramatic difference between the observed number density evolution of low-mass galaxies and that predicted by semi-analytic models. Whilst models accurately reproduce the z = 0 number density, they require that the evolution occurs rapidly at early times, which is incompatible with the strong late evolution found in observational results. We report here the same discrepancy in two state-of-the-art cosmological hydrodynamical simulations, which is evidence that the problem is fundamental. We search for the underlying cause of this problem using two complementary methods. First, we consider a narrow range in stellar mass of log (Mstar/(h-2 M?)) = 9-9.5 and look for evidence of a different history of today's low-mass galaxies in models and observations. We find that the exclusion of satellite galaxies from the analysis brings the median ages and star formation rates of galaxies into reasonable agreement. However, the models yield too few young, strongly star-forming galaxies. Secondly, we construct a toy model to link the observed evolution of specific star formation rates with the evolution of the galaxy stellar mass function. We infer from this model that a key problem in both semi-analytic and hydrodynamical models is the presence of a positive instead of a negative correlation between specific star formation rate and stellar mass. A similar positive correlation is found between the specific dark matter halo accretion rate and the halo mass, indicating that model galaxies are growing in a way that follows the growth of their host haloes too closely. It therefore appears necessary to find a mechanism that decouples the growth of low-mass galaxies, which occurs primarily at late times, from the growth of their host haloes, which occurs primarily at early times. We argue that the current form of star formation-driven feedback implemented in most galaxy formation models is unlikely to achieve this goal, owing to its fundamental dependence on host halo mass and time.

Weinmann, Simone M.; Pasquali, Anna; Oppenheimer, Benjamin D.; Finlator, Kristian; Mendel, J. Trevor; Crain, Robert A.; Macciò, Andrea V.

2012-11-01

347

Low-Mass Stirling Convertor Assembly Progress Update  

NASA Astrophysics Data System (ADS)

Infinia is developing the next generation of space-ready Stirling Convertor Assemblies. Infinia has previously proposed a Low-Mass Stirling Convertor Assembly (SCA) design employing a flux-concentrating, moving-iron linear alternator. This paper describes further development of that proposed machine, including additional improvements and advancements. One significant change is a new, lighter-weight moving-magnet alternator design. Infinia has shown progress in the low mass design's development and testing: namely in the area of flexure spring rates, welding techniques on the flange, and a change to the alternator configuration. Progress has been made with the flat-top heater head design and its capabilities, as well. The changes described in this paper will significantly reduce the mass and increase the power density of the low-mass design.

Qiu, Songgang; Augenblick, John E.; Redinger, Darin L.

2006-01-01

348

Low-mass X-ray binary populations in galaxy outskirts: Globular clusters and supernova kicks  

NASA Astrophysics Data System (ADS)

For the first time, we have systematically explored the population of discrete X-ray sources in the outskirts of early-type galaxies. Based on a broad sample of 20 galaxies observed with Chandra we detected overdensity of X-ray sources in their outskirts. The overdensity appears as halos of resolved sources around the galaxies. These halos are broader than stellar light, extending out to at least ~10re (re is the effective radius). These halos are composed of sources fainter than ~5 × 1038 erg/s, whereas the more luminous sources appear to follow the distribution of the stellar light, suggesting that the excess source population consists of neutron star binaries. Dividing the galaxy sample into four groups according to their stellar mass and specific frequency of globular clusters (GCs), we find that the extended halos are present in all groups except for the low-mass galaxies with low GC content. We propose that the extended halos may be comprised of two independent components, low-mass X-ray binaries (LMXBs) located in GCs, which are known to have a wider distribution than the stellar light, and neutron star LMXBs kicked out of the main body of the parent galaxy by supernova explosions. The available deep optical and X-ray data of NGC 4365 support this conclusion. For this galaxy we identified 60.1 ± 10.8 excess sources in the (4-10)re region of which ~40% are located in GCs, whereas ~60% are field LMXBs. We interpret the latter as kicked neutron star LMXBs. We discuss the implications of these results for the natal kick distributions of black holes and neutron stars.

Zhang, Z.; Gilfanov, M.; Bogdán, Á.

2013-08-01

349

A new class of pulsating white dwarf of extremely low mass: the fourth and fifth members  

NASA Astrophysics Data System (ADS)

We report the discovery of two new pulsating extremely low-mass (ELM) white dwarfs (WDs), SDSS J161431.28+191219.4 (hereafter J1614) and SDSS J222859.93+362359.6 (hereafter J2228). Both WDs have masses <0.25 M? and thus likely harbour helium cores. Spectral fits indicate these are the two coolest pulsating WDs ever found. J1614 has Teff = 8880 ± 170 K and log g = 6.66 ± 0.14, which corresponds to a ˜0.19 M? WD. J2228 is considerably cooler, with a Teff = 7870 ± 120 K and log g = 6.03 ± 0.08, which corresponds to an ˜0.16 M? WD, making it the coolest and lowest mass pulsating WD known. There are multiple ELM WDs with effective temperatures between the warmest and coolest known ELM pulsators that do not pulsate to observable amplitudes, which questions the purity of the instability strip for low-mass WDs. In contrast to the CO-core ZZ Ceti stars, which are believed to represent a stage in the evolution of all such WDs, ELM WDs may not all evolve as a simple cooling sequence through an instability strip. Both stars exhibit long-period variability (1184-6235 s) consistent with non-radial g-mode pulsations. Although ELM WDs are preferentially found in close binary systems, both J1614 and J2228 do not exhibit significant radial-velocity variability, and are perhaps in low-inclination systems or have low-mass companions. These are the fourth and fifth pulsating ELM WDs known, all of which have hydrogen-dominated atmospheres, establishing these objects as a new class of pulsating WD.

Hermes, J. J.; Montgomery, M. H.; Gianninas, A.; Winget, D. E.; Brown, Warren R.; Harrold, Samuel T.; Bell, Keaton J.; Kenyon, Scott J.; Kilic, Mukremin; Castanheira, Barbara G.

2013-12-01

350

ROSAT observations of quiescent low mass disk galaxies: No evidence of baryonic blow out  

NASA Technical Reports Server (NTRS)

To test the hypothesis that galactic winds associated with star formation in low mass disk galaxies can be an effective means of relocating cold disk gas to a warm tenuous halo, we have obtained long exposure ROSAT Position Sensitive Proportional Counter (PSPC) observations of three such galaxies. The sensitivity of the PSPC to the presence of an extended, approximately 0.15 KEV halo of 10(exp 9) solar mass of gas, is quite high for the exposure times we used. We failed to detect this halo in all three cases and the observed x-ray luminosity of the galaxy is two orders of magnitude less than the hypothetical case in which the mass of gas that has been expelled by previous generations of star formation is equal to the stellar mass of the galaxy itself. This limit is much less than the actual mass of cold gas in these galaxies. Thus, we were unable to verify directly the presence of significant galactic winds in these three galaxies either because they are not operative, because their halos are not sufficiently massive to aid in the retention of this gas, or because the amount of injected gas is just a small percentage of the cold disk gas. If the latter reason is emblematic of low mass galaxies then we would not expect the detection of halos. We also report here the serendipitous detection of Abell 1560, a distance class 7 cluster of unknown redshift.

Bothun, Gregory D.; Eriksen, James; Schombert, James M.

1994-01-01

351

Probing the Low Mass X-ray Binary-Globular Cluster Link in Early Type Galaxies  

NASA Astrophysics Data System (ADS)

High resolution Chandra observations reveal large populations of low mass X-ray binaries around distant galaxies. As in the Milky Way, globular clusters appear to be especially fertile environments for low mass X-ray binary (LMXB) formation, with a probability of hosting a LMXB that is at least two orders of magnitude larger than for field stars. We have studied the X-ray properties of LMXBs and the optical properties of globular clusters (that host roughly 40% of the LMXBs) around a sample of 4 early type galaxies. This study allows us to isolate and understand the preferred physical characteristics of globular clusters that lead to the formation of LMXBs via dynamical processes in globular clusters, probe the ancestry of the field LMXB population and correlations with the star formation history of the galaxy, and the importance of globular clusters to the X-ray emission from early type galaxies. One of the most striking results of our analysis is that LMXBs are approximately 3 times more likely to form in red, metal-rich globular clusters than in metal-poor ones, an observation for which there is currently no convincing explanation.

Kundu, A.; Maccarone, T. J.; Zepf, S. E.

2002-12-01

352

Evidence of early disk-locking among low-mass members of the Orion Nebula Cluster  

NASA Astrophysics Data System (ADS)

Context: We present new high-resolution spectroscopic observations for 91 pre-main sequence stars in the Orion Nebular Cluster (ONC) with masses in the range 0.10-0.25~M_? carried out with the multi-fiber spectrograph flames attached to the UT2 at the Paranal Observatory. Aims: Our aim is to better understand the disk-locking scenario in very low-mass stars. Methods: We have derived radial velocities, projected rotational velocities, and full width at 10% of the H? emission peak. Using published measurements of infrared excess (?(I_C-K)), as disk tracer and equivalent width of the nead-infrared Ca II line ?8542, mid-infrared difference [3.6]-[8.0] ?m derived by Spitzer data, and 10% H? width as diagnostic of the level of accretion, we looked for any correlation between projected angular rotational velocity divided by the radius (v sin i/R) and presence of disk and accretion. Results: For 4 low-mass stars, the cross-correlation function is clearly double-lined, indicating that the stars are SB2 systems. The distribution of rotation periods derived from our v sin i measurements is unimodal with a peak of a few days, in agreement with previous results for M<0.25~M_?. The photometric periods were combined with our v sin i to derive the equatorial velocity and the distribution of rotational axes. Our < sin i> is lower than the one expected for a random distribution, as previously found. We find no evidence of a population of fast rotators close to the break-up velocity. A clear correlation between v sin i/R and ?(I_C-K) has been found. While a spread in the rotation rates is seen for stars with no circumstellar disk (?(I_C-K)<0.3), stars with a circumstellar disk (?(I_C-K)>0.3) show an abrupt drop in their rotation rates by a factor of ~5. On the other hand, only a partial correlation between v sin i and accretion is observed when other indicators are used. The X-ray coronal activity level (log L_X/L_bol) shows no dependence on v sin i/R, suggesting that all stars are in a saturated regime limit. The critical velocity is probably below our v sin i detection limit of 9 km s-1. Conclusions: The ONC low-mass stars in our sample, close to the hydrogen burning limit, at present do not seem to be locked, but the clear correlation we find between rotation and infrared color excess suggests that they were locked once. In addition, the percentage of accretors seems to scale inversely to the stellar mass. Based on the flames Science Verification proposal 60.A-9145(A) and the flames proposal 76.C-0524(A). Table [see full textsee full textsee full textsee full text] is only available in electronic form at http://www.aanda.org

Biazzo, K.; Melo, C. H. F.; Pasquini, L.; Randich, S.; Bouvier, J.; Delfosse, X.

2009-12-01

353

Neutron stars  

NASA Astrophysics Data System (ADS)

Neutron stars are laboratories for dense matter and gravitational physics. Observations of neutron stars from sources such as radio pulsars, low-mass X-ray binaries, X-ray bursts and thermally-emitting neutron stars are setting bounds to neutron star masses, radii, rotation rates, temperatures and ages. Mass measurements constrain the equation of state at the highest densities and set firm bounds to the highest possible density of cold matter. Radii constrain the equation of state in the vicinity of the nuclear saturation density and yield information about the density dependence of the nuclear symmetry energy. Laboratory measurements and theoretical studies of pure neutron matter are in remarkable agreement with observational bounds.

Lattimer, James M.

2014-05-01

354

A Quadrupole mass spectrometer for resolution of low mass isotopes  

Microsoft Academic Search

The qualitative and quantitative identification of low mass isotopes in the mass range 1–6 u poses certain difficulties when\\u000a attempting to achieve the required resolution with an instrument suitable for deployment within a process environment. Certain\\u000a adjacent species present in the process sample (HT and D2) require a resolution greater than 930 to achieve an accurate measurement. We demonstrate here

Jeyan Sreekumar; Thomas J. Hogan; Stephen Taylor; Phillip Turner; Christopher Knott

2010-01-01

355

Low Mass Muscle Actuators (LoMMAs) Using Electroactive Polymers  

NASA Technical Reports Server (NTRS)

NASA is using actuation devices for many space applications and there is an increasing need to cut their cost as well as reduce their size, mass, and power consumption. Existing transducing actuators, such as piezoceramics, are inducing limited displacement levels. Potentially, electroactive polymers (so called EAP) can be formed as inexpensive, low-mass, low-power, miniature muscle actuators that are superior to the widely used actuators.

Bar-Cohen, Y.; Xue, T.; Joffe, B.; Lih, S. S.; Willis, P.; Simpson, J.; Smith, J.; Clair, T.; Shahinpoor, M.

1997-01-01

356

Formation of Low-Mass X-Ray Binaries. I. Constraints on Hydrogen-rich Donors at the Onset of the X-Ray Phase  

Microsoft Academic Search

We identify and quantify the set of constraints that neutron star-normal star binaries must satisfy in order to become observable low-mass X-ray binaries (LMXBs). These constraints are related to (i) the thermal and hydrostatic equilibrium of the donors, (ii) the degree to which the mass transfer process is conservative, and (iii) the age of the systems. They divide the parameter

Vassiliki Kalogera; Ronald F. Webbink

1996-01-01

357

ORBITAL MIGRATION OF LOW-MASS PLANETS IN EVOLUTIONARY RADIATIVE MODELS: AVOIDING CATASTROPHIC INFALL  

SciTech Connect

Outward migration of low-mass planets has recently been shown to be a possibility in non-barotropic disks. We examine the consequences of this result in evolutionary models of protoplanetary disks. Planet migration occurs toward equilibrium radii with zero torque. These radii themselves migrate inwards because of viscous accretion and photoevaporation. We show that as the surface density and temperature fall the planet orbital migration and disk depletion timescales eventually become comparable, with the precise timing depending on the mass of the planet. When this occurs, the planet decouples from the equilibrium radius. At this time, however, the gas surface density is already too low to drive substantial further migration. A higher mass planet, of 10 M {sub +}, can open a gap during the late evolution of the disk, and stops migrating. Low-mass planets, with 1 or 0.1 M {sub +}, released beyond 1 AU in our models avoid migrating into the star. Our results provide support for the reduced migration rates adopted in recent planet population synthesis models.

Lyra, Wladimir; Mac Low, Mordecai-Mark [Department of Astrophysics, American Museum of Natural History, 79th Street at Central Park West, New York, NY 10024 (United States); Paardekooper, Sijme-Jan, E-mail: wlyra@amnh.or, E-mail: mordecai@amnh.or [Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Wilberforce Road, Cambridge CB3 OWA (United Kingdom)

2010-06-01

358

A CCD-based search for very low mass members of the Pleiades cluster  

NASA Technical Reports Server (NTRS)

We have obtained deep charge coupled device (CCD)V and I images of a number of fields near the center of the Pleiades open cluster. We have also obtained imaging data for Praesepe, a very similar cluster in terms of distance and richness but nearly 10 times older than the Pleiades. Because brown dwarfs are predicted to become much fainter and cooler between Pleiades and Praesepe ages, this provides a powerful differential technique for placing constraints on the brown dwarf population in open clusters. Combined with our previously reported observations, we now have data for about 0.4 sq deg in the Pleiades, corresponding roughly to 5% of the area of that cluster. We have searched the new CCD frames for additional Pleiades brown dwarf candidates. Two possible candidates are present, the faintest of which has V approximately equal to 22.5, (V-I)(sub K) approximately equal to 4.6. Because we do not have proper motion data and the colors of these objects are not redder than the reddest known field stars, it is possible that some or all of our candidates are somewhat higher mass field stars rather than Pleiades-age brown dwarfs. Even if all six of the proposed brown dwarf candidates in our 0.4 sq deg field are Pleiades members, the relatively small number found suggests that low mass