Coolest Subdwarf Research Articles

Accurate mass and radius determinations of a cool subdwarf in an eclipsing binary.

Cool subdwarfs are metal-poor low-mass stars that formed during the early stages of the evolution of our Galaxy. Because they are relatively rare in the vicinity of the Sun, we know of few cool subdwarfs in the solar neighbourhood, and none with both the mass and the radius accurately determined. This hampers our understanding of stars at the low-mass end of the main-sequence. Here we report the discovery of SDSS J235524.29+044855.7 as an eclipsing binary containing a cool subdwarf star, with a white dwarf companion. From the light-curve and the radial-velocity curve of the binary we determine the mass and the radius of the cool subdwarf and we derive its effective temperature and luminosity by analysing its spectral energy distribution. Our results validate the theoretical mass-radius-effective temperature-luminosity relations for low-mass low-metallicity stars.

MULTIPLICITY OF THE GALACTIC SENIOR CITIZENS: A HIGH-RESOLUTION SEARCH FOR COOL SUBDWARF COMPANIONS

Cool subdwarfs are the oldest members of the low mass stellar population. Mostly present in the galactic halo, subdwarfs are characterized by their low metallicity. Measuring their binary fraction and comparing it to solar metallicity stars could give key insights into the star formation process early in the history of the Milky Way. However, because of their low luminosity and relative rarity in the solar neighborhood, binarity surveys of cool subdwarfs have suffered from small sample sizes and incompleteness. Previous surveys have suggested that the binary fraction of red subdwarfs is much lower than for their main sequence cousins. Using the highly efficient RoboAO system, we present the largest yet high-resolution survey of subdwarfs, sensitive to angular separations, down to 0.15 arcsec, and contrast ratios, up to 6 magnitude difference, invisible in past surveys. Of 344 target cool subdwarfs, 40 are in multiple systems, 16 newly discovered, for a binary fraction of 11.6 percent and 1.8 percent error. We also discovered 6 triple star systems for a triplet fraction of 1.7 percent and 0.7 percent error. Comparisons to similar surveys of solar metallicity dwarf stars gives a 3 sigma disparity in luminosity between companion stars, with subdwarfs displaying a shortage of low contrast companions.

COOL SUBDWARF INVESTIGATIONS. II. MULTIPLICITY

Cool subdwarfs of types K and M are the fainter counterparts of cool main sequence dwarfs that dominate the Galactic population. In this paper we present the results of an optical speckle survey of 62 confirmed cool subdwarf systems within 60 pc. We have resolved two new companions and confirmed two previously known companions with separations 0\farcs13 to 3\farcs29. After including previously known wide companions and all known spectroscopic binaries, we determine the multiplicity rate of cool subdwarfs to be 26$\pm$6%, which is somewhat lower than comparable main sequence stars, which have a multiplicity rate of 37$\pm$5%. We find that only 3% of the cool subdwarfs surveyed have companions within 10 AU, 3% have companions between 10 and 100 AU, and 14% have companions beyond 100 AU. The other 6% of cool subdwarfs are spectroscopic binaries. This is very different from K/M dwarfs that have most companions (13%) at separations closer than 10 AU. However, because a search for close binaries among a large sample of nearby cool subdwarfs remains elusive, it is not yet settled whether or not the multiplicity rates are significantly different. Nonetheless, several different observational results and theories pointing to a possible dearth of subdwarf multiples are discussed.

Mapping the Local Galactic Halo. I. Optical Photometry of Cool Subdwarf Candidates

Optical (BVRI) photometric measurements of a sample of 564 candidate cool subdwarfs in the nearby halo are presented. The stars generally span the color range 0.4 < (B - V) < 1.8; hence, the sample is composed of early F- through early M-type stars on the subdwarf sequence. The sample is selected from the revised NLTT catalog of Gould and Salim and Salim and Gould via a reduced proper motion diagram. The photometry is precise and accurate; in particular, for stars with 9 < V < 13.5 the photometry is accurate to 0.013, 0.015, 0.012, and 0.013 mag in V, B - V, V - R, and V - I, respectively. For stars with 13.5 < V < 16 the photometry is accurate to 0.022, 0.018, 0.013, and 0.018 mag in the same bands.

A Search for Cool Subdwarfs: Stellar Parameters for 134 Candidates11Based in part on observations obtained with the Hobby‐Eberly Telescope, which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Stanford University, Ludwig‐Maximillians‐Universität München, and Georg‐August‐Universität Göttingen.

ABSTRACTThe results of a search for cool subdwarfs are presented. Kinematic (U, V, and W) and stellar parameters (Teff, log g, [Fe/H], and ξt) are derived for 134 candidate subdwarfs based on high‐resolution spectra. The observed stars span 4200 K < Teff < 6400 K and −2.70 < [Fe/H] < 0.25 including only eight giants (log g < 4.0). Of the sample, 100 stars have MgH bands present in their spectra. The targets were selected by their large reduced proper motion, by the offset from the solar‐metallicity main sequence, or from the literature. We confirm the claims made by Ryan that the NLTT catalog is a rich source of subdwarfs and verify the success of the reduced proper‐motion constraint in identifying metal‐poor stars.

Spectroscopy of New High Proper Motion Stars in the Northern Sky. I. New Nearby Stars, New High-Velocity Stars, and an Enhanced Classification Scheme for M Dwarfs

We define an enhanced spectral classification scheme for M dwarf stars, and use it to derive spectral classification of 104 northern stars with proper motions larger than 0.5/yr which we discovered in a survey of high proper motion stars at low galactic latitudes. The final tally is as follows: 54 M dwarfs, 25 sdK and sdM subdwarfs, 14 esdK and esdM extreme subdwarfs, and 11 DA and DC white dwarfs. Among the most interesting cases, we find one star to be the coolest subdwarf ever reported (LSR2036+5059, with spectral type sdM7.5), a new M9.0 dwarf only about 6pc distant (LSR1835+3259), and a new M6.5 dwarf only 7pc from the Sun (LSR2124+4003). Spectroscopic distances suggests that 27 of the M dwarfs, 3 of the white dwarfs, and one of the subdwarfs (LSR2036+5059) are within 25pc of the Sun, making them excellent candidates for inclusion in the solar neighborhood census. Estimated sky-projected velocities suggest that most of our subdwarfs and extreme subdwarfs have halo kinematics. We find that several white dwarfs and non metal-poor M dwarfs also have kinematics consistent with the halo, and we briefly discuss their possible origin.

PROBING THE LHS CATALOG. I. NEW NEARBY STARS AND THE COOLEST SUBDWARF

ABSTRACT We present moderate resolution spectroscopy of 112 cool dwarf stars to supplement the observations we have already presented in the Palomar/MSU Nearby-Star Spectroscopic Survey. The sample consists of 72 suspected nearby stars added to the The Preliminary Third Catalog of Nearby Stars since 1991 as well as 40 faint red stars selected from the LHS catalog. LHS 1826 is more metal-poor and cooler than the coolest previously known extreme subdwarf, LHS 1742a. LHS 2195 is a very late M dwarf of type M 8 V, probably at a distance of ten parsecs. LHS 1937 is an M 7 V star at ~ 20 parsecs. Three other previously unobserved LHS stars have estimated distances that place them within 25 parsecs.