Reduced Proper-motion Diagram Research Articles

DISCOVERY OF A BRIGHT, EXTREMELY LOW MASS WHITE DWARF IN A CLOSE DOUBLE DEGENERATE SYSTEM

We report the discovery of a bright (V ~ 13.7), extremely low-mass white dwarf in a close double degenerate system. We originally selected GALEX J171708.5+675712 for spectroscopic follow-up among a group of white dwarf candidates in an ultraviolet-optical reduced proper-motion diagram. The new white dwarf has a mass of 0.18 M_solar and is the primary component of a close double degenerate system (P=0.246137 d, K_1 = 288 km/s) comprising a fainter white dwarf secondary with M_2 ~ 0.9 M_solar. Light curves phased with the orbital ephemeris show evidence of relativistic beaming and weaker ellipsoidal variations. The light curves also reveal secondary eclipses (depth ~ 8 mmag) while the primary eclipses appear partially compensated by the secondary gravitational deflection and are below detection limits. Photospheric abundance measurements show a nearly solar composition of Si, Ca, and Fe (0.1-1 solar), while the normal kinematics suggest a relatively recent formation history. Close binary evolutionary scenarios suggest that extremely low mass white dwarfs form via a common-envelope phase and possible Roche-lobe overflow.

A selection of hot subluminous stars in the GALEX survey - I. Correlation with the Guide Star Catalog★

We assembled a catalogue of bright, hot subdwarf and white dwarf stars extracted from a joint ultraviolet, optical, and infrared source list. The selection is secured using colour criteria that correlate well with effective temperatures T_eff ~> 12,000 K. We built a N_UV-V versus V-J diagram for more than 60,000 bright sources using the Galaxy Evolution Explorer (GALEX) N_UV magnitude (N_UV<14), and the associated Guide Star Catalog (GSC2.3.2) photographic quick-V magnitude and the Two Micron All Sky Survey (2MASS) J and H magnitudes. This distillation process delivered a catalogue of approximately 700 sources with N_UV-V<0.5 comprising ~160 known hot subdwarf stars and another ~60 known white dwarf stars. A reduced proper-motion diagram built using the proper-motion measurements extracted from the Naval Observatory Merged Astrometric Dataset allowed us to identify an additional ~120 new hot subdwarf candidates and ~10 hot white dwarf candidates. We present a spectroscopic study of a subset of 52 subdwarfs, 48 of them analysed here for the first time, and with nine objects brighter than V ~ 12. Our sample of spectroscopically confirmed hot subdwarfs comprises ten sdO stars and 42 sdB stars suitable for pulsation and binary studies. We also present a study of 50 known white dwarfs selected in the GALEX survey and six new white dwarfs from our catalogue of subluminous candidates. Ultraviolet, optical, and infrared synthetic magnitudes employed in the selection and analysis of white dwarf stars are listed in appendix.

Exploring Halo Substructure with Giant Stars: A Diffuse Star Cloud or Tidal Debris around the Milky Way in Triangulum‐Andromeda

We report here the discovery of an apparent excess of 2MASS M giant candidates with dereddened 0.85 b > -40\degr$, and covering most of the constellations of Triangulum and Andromeda. This structure does not seem to be preferentially distributed around a clear core, but rather lies in a tenuous, clumpy cloud-like structure tens of kiloparsecs away. The reduced proper-motion diagram as well as spectroscopy of a subsample shows these excess stars to be real giants, not contaminating dwarfs. Radial velocity measurements indicate among those M giants the presence of a coherent kinematical structure with a velocity dispersion $\sigma < 17$ km s$^{-1}$. Our findings support the existence of a quite dispersed stellar structure around the Milky Way that, due to its coreless and sparse distribution, could be part of a tidal stream or a new kind of satellite galaxy.

Observations of White Dwarfs in the Solar Neighborhood

High proper motion surveys are still adding much to our understanding of the local white dwarf population and, possibly, to the Galactic halo membership of some objects. As part of our study of white dwarfs in the solar neighborhood we have observed seven white dwarfs from the revised New Luyten Two Tenths (NLTT) catalog of Salim & Gould. We found four DA white dwarfs (NLTT 529, 49985, 53468, 55932), one DZ white dwarf (NLTT 40607) that shows a close resemblance to the unusual DZ white dwarf G165-7, and two DC white dwarfs (NLTT 19138, NLTT 52404). The white dwarf candidates were chosen using the V-J reduced proper-motion diagram of Salim & Gould and an optical-infrared diagram (V-J vs. J-H). We also observed five stars from the list of local white dwarfs by Holberg, Oswalt, & Sion, that required confirmation of their spectral classification. We confirm that GD 1212 and WD 1717-345 are white dwarfs but that BPM 17113 (WD 0311-543), KUV 05097+1649, and BPM 19929 (WD 1013-559) are main-sequence F stars rather than white dwarfs. An analysis of a color-selected sample of white dwarf candidates extracted from the rNLTT catalog could contribute more than 200 new white dwarfs, most belonging to the thin disk, with a few (N < 15) kinematically selected members of an older population (thick disk or halo).

Spectroscopic Identification of White Dwarfs in the New Luyten Two-Tenths Survey

We spectroscopically identify New Luyten Two-Tenths (NLTT) stars from the reduced proper-motion diagram of Salim & Gould. Five out of 16 objects investigated are identified as white dwarfs: four hydrogen-rich DA stars (NLTT 8581, NLTT 31748, NLTT 53177, and NLTT 56805) and one helium-rich (NLTT 8435). The properties of the stars are further constrained using Two Micron All Sky Survey photometric measurements and, in the case of NLTT 31748, using International Ultraviolet Explorer spectrophotometry. The distances range from 15 to 23 pc, and therefore the sample contributes to estimates of the local density of white dwarf stars. The newly found objects are kinematically consistent with a thick or old thin disk membership. We discuss the likelihood of further identifications.

Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog

We complete construction of a catalog containing improved astrometry and new optical/infrared photometry for the vast majority of NLTT stars lying in the overlap of regions covered by POSS I and by the second incremental Two Micron All Sky Survey (2MASS) release, approximately 44% of the sky. The epoch 2000 positions are typically accurate to 130 mas, the proper motions to 5.5 mas yr-1, and the V-J colors to 0.25 mag. Relative proper motions of binary components are measured to 3 mas yr-1. The false-identification rate is ~1% for 11 V 18 and substantially less at brighter magnitudes. These improvements permit the construction of a reduced proper-motion diagram that, for the first time, allows one to classify NLTT stars into main-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We in turn use this diagram to analyze the properties of both our catalog and the NLTT catalog on which it is based. In sharp contrast to popular belief, we find that NLTT incompleteness in the plane is almost completely concentrated in MS stars, and that SDs and WDs are detected almost uniformly over the sky δ > -33°. Our catalog will therefore provide a powerful tool to probe these populations statistically, as well as to reliably identify individual SDs and WDs.

Classifying Luyten Stars Using an Optical-Infrared Reduced Proper-Motion Diagram

We present a V-J reduced proper motion (RPM) diagram for stars in the New Luyten Two-Tenths (NLTT) catalog. In sharp contrast to the RPM diagram based on the original NLTT data, this optical-infrared RPM diagram shows distinct tracks for white dwarfs, subdwarfs, and main-sequence stars. It thereby permits the identification of white-dwarf and subdwarf candidates that have a high probability of being genuine.

Absolute Proper Motions toB ∼ 22.5. IV. Faint, Low‐Velocity White Dwarfs and the White Dwarf Population Density Law

The reduced proper-motion diagram (RPMD) for a complete sample of 819 faint (B ≤ 22.5) stars with high-accuracy proper motions (σμ ~ 1 mas yr-1) in an area of 0.3 deg2 in the north Galactic pole field SA 57 is investigated. Eight stars with very large reduced proper motions are identified as faint white dwarf candidates. On the basis of larger than 6 σ measured proper motions and the lack of photometric variability over a 20 yr baseline, we discriminate these white dwarf candidates from the several times more numerous quasi-stellar objects (QSOs), which can potentially occupy a similar location in the RPMD. For comparison, less than 4 σ proper motions and photometric variability are found in all but one of 35 spectroscopically confirmed QSOs in the same field. While spectroscopic confirmation of their status as white dwarfs is a necessary, but difficult, outstanding task, we discuss the implausibility that these stars could be any kind of survey contaminant. High-quality proper motions lend confidence in our ability to separate white dwarfs from subdwarfs in the RPMD. If bona fide white dwarfs, the eight candidates found here represent a portion of the white dwarf population that hitherto has remained uninvestigated by previous surveys by virtue of the faint magnitudes and low proper motions of the stars. This faint, low-velocity sample represents an increase in the white dwarf sky surface density to B = 22.5 by an order of magnitude than that found in the previously most complete surveys to this depth. However, because the majority of the stars discovered here are at projected distances of more than a disk scale height above the Galactic midplane, their existence does not affect significantly the typical estimates of the local white dwarf density. On the other hand, as distant white dwarf candidates with low, typically thin-disk-like transverse velocities (<40 km s-1), the newly discovered stars suggest a disk white dwarf scale height larger than the values of 250-350 pc typically assumed in assessments of the local white dwarf density (and thought to characterize the Galactic old thin disk in stellar population models). Both a ⟨V/Vmax⟩ and a more complex maximum-likelihood analysis of the spatial distribution of our likely thin-disk white dwarfs yield scale heights of 400-600 pc while at the same time giving a reasonable match to the local white dwarf volume density found in other surveys (although this good match is a result of the dominance of the one relatively nearby white dwarf in the 1/Vmax density calculation). A high scale height persists even if the relatively small sample is pruned of any potential thick-disk or halo white dwarfs. While our work is not optimized toward the study of halo white dwarfs as potential MACHO objects, our results do have interesting implications for this hypothesis. We can place some direct constraints (albeit weak ones) on the contribution of halo white dwarfs to the dark matter of the Galaxy. Moreover, the elevated scale height that we measure for the thin disk could alter the interpretation of microlensing results to the extent of making white dwarfs untenable as the dominant MACHO contributor.

High proper motion stars in Kapteyn Selected Area 94

We have applied the astrometric techniques devised by Murray for analysis of 48-inch Schmidt photographic data to SuperCOSMOS scans of UK Schmidt plates centred on Kapteyn's Selected Area 94 (α=2h 53m, δ=0°). In this preliminary study, we combine astrometric data from four short-exposure V-band plates, taken in 1987 August (2 plates) and 1993 August (2 plates), with BVRI photometry from sky-limited plate material, to identify stars with proper motions exceeding 0.1 arcsec yr−1. This paper discusses the completeness of the resulting sample and presents spectroscopy of 30 stars with μ>0.2 arcsec yr−1. Based on the latter observations and the distribution in the [HV, (V−I)] reduced proper-motion diagram, we have classified stars in the complete sample as candidate white dwarfs, main-sequence dwarfs and halo subdwarfs, and derived estimates of the disc and halo luminosity functions.