A Census of White Dwarfs Within 40 Parsecs of the Sun

A Census of White Dwarfs Within 40 Parsecs of the Sun

M.-M. Limoges111Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under cooperative agreement with the National Science Foundation.     P. Bergeron    Sébastien Lépine

Our aim is to compile a catalog of white dwarfs within 40 parsecs of the Sun, in which newly discovered objects would significantly increase the completeness of the current census. White dwarf candidates are identified from the SUPERBLINK proper motion database ([Lépine et al. (2005)]), which allows us to investigate stars down to a proper motion limit as low as 40 . The selection criteria and distance estimates are based on a combination of color-magnitude and reduced proper motion diagrams. Candidates with distances less than 50 parsecs are selected for spectroscopic follow-up. We present our preliminary sample of spectroscopically confirmed white dwarfs, as well as their atmospheric parameters. These parameters are obtained using the spectroscopic technique developed in [Bergeron et al. (1992)] for DA stars. DB, DQ, and DZ stars are also analyzed spectroscopically. For featureless spectra as well as those showing only , we perform a detailed photometric analysis of their energy distribution.

white dwarfs, spectroscopy, color, reduced proper motion, distance


address=Département de Physique, Université de Montréal, C.P. 6128, Succ. Centre-Ville, Montréal, Québec H3C 3J7, Canada

address=Département de Physique, Université de Montréal, C.P. 6128, Succ. Centre-Ville, Montréal, Québec H3C 3J7, Canada,

address=Department of Astrophysics, Division of Physical Sciences, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024,

1 Introduction

The current census of nearby white dwarfs is complete to within 20 parsecs from the Sun, and contains 126 white dwarfs (Holberg et al., 2008). Since it represents a sample too small for detailed statistical analysis, there is a need to extend the complete sample of white dwarfs to a larger volume. Nearby white dwarfs have been traditionally found in catalogs of stars with large proper motions. In order to improve the statistics of the local white dwarf population, we have been hunting for white dwarfs in the SUPERBLINK catalog.

The SUPERBLINK proper motion database Lépine et al. (2005) is based on a re-analysis of the POSS-I and POSS-II plates of the Digitized Sky Survey (20-45 yr baseline). Its 1.3 million stars in the Northern sky were identified with a detection level ¿ down to , and a proper motion limit as low as 0.04. Because of its low proper motion limit, the SUPERBLINK sample effectively eliminates the kinematic bias of the Luyten catalogs, while detecting all white dwarfs down to the luminosity function turn-off.

Figure 1: Optical spectra for a subset of 19 stars from our new WD sample. This sample shows 17 DA stars, and 2 featureless DC white dwarfs. The DA sample includes 4 DA+dM (PM I16171+0530, LSPM J0655+5920, PM I04586+6209 and PM I23283+3319) and two magnetic white dwarfs (PM I06019+3726 and PM I15164+2803).
Figure 2: Color-color diagram showing the WD candidates and the spectroscopically confirmed WD from our study. The solid curves show model atmospheres for WD with a pure H atmosphere for log g=7.0, 8.0 and 9.0. The dashed curve represents a model for a pure He atmosphere with log g=8.0.

2 Selection method

The first step of our analysis was to gather information about SUBERBLINK stars, as a complement to photographic , and measurements, J2000 coordinates, and proper motions available for each SUPERBLINK star (see Lépine et al. (2005)). Catalogs were queried in order to obtain GALEX far-UV () and near-UV () (Gil de Paz et al., 2009), SDSS (Adelman-McCarthy et al., 2009) or 2MASS (Skrutskie et al., 2006) photometry. Apparent magnitudes are estimated using the empirical relation . Stars are then placed in vs , vs or vs reduced proper motion diagrams, depending on the available photometry for each object. When possible, cross-correlations between the diagrams are performed. Selection criteria are applied based on each star’s position in the diagram (more details will be given in Limoges et al. 2011, in preparation). Finally, confirmed white dwarfs from McCook Sion (2006) are used to obtain color-magnitude calibrations, from which we determine an absolute magnitude and photometric distance for each candidate. Candidates with distance estimates less than 50 parsecs were selected for spectroscopic follow-up.

76 0 43 2 4 8 4 2 137
Table 1: Spectral types of our new white dwarfs sample
Catalog Name RA DEC RA DEC ST
(deg) (deg) ( yr) ( yr)
LSPM J00212640 5.447206 26.676771 0.074 0.322 DC
LSPM J00270542 6.902645 5.700896 0.266 0.273 DA
PM I003314742 8.293821 47.703468 0.060 0.052 DA
LSPM J00555948 13.992868 59.800690 0.454 0.061 DC
PM I012163440 20.407501 34.678669 0.131 0.017 DZ
LSPM J01277328 21.954573 73.479942 0.156 0.075 DA
LSPM J01452918 26.435930 29.306622 0.529 0.008 DC
LSPM J01483615 27.168560 36.258621 0.000 0.231 DA
PM I015642955 29.124598 29.926645 0.054 0.089 DA
LSPM J02061836 31.561138 18.606722 0.784 0.093 DC
Table 2: New White Dwarfs (Sample)

All spectra have been obtained at Kitt Peak with the Steward Observatory 2.3-m telescope, and the NOAO Mayall 4-m and 2.1-m telescopes. The adopted configurations allow a spectral coverage of 3200–5300 and 3800–6700, at a mean resolution of  Å FWHM. A subset of DA and DC stars from our sample of new white dwarfs is shown in Figure 1. In Figure 2, a color-color diagram displays the confirmed white dwarfs and candidates for which data were available. A summary of the spectral types for the 137 new white dwarfs is shown in Table 1. Finally, a sample of the detailed list of our new white dwarfs, including astrometry and spectral type is given in Table 2.222The complete version is available on www.astro.umontreal.ca/limoges/

3 Results

From our list of candidates, 137 white dwarfs333The DC companions in DA+DC systems are still candidates. They are not included in the total. were found. Most of these objets were already part of the literature, without spectral type or white dwarf status confirmation. The atmospheric parameters ( and ) of 114 stars were measured. DA stars were analyzed using the spectroscopic fitting technique described at length in Liebert et al. (2005) and references therein. Detailed photometric analyses of their energy distributions (Bergeron et al., 2001) were performed for cooler objects showing only the hydrogen line and featureless DC stars. Our results are summarized in Figure 3 in a mass vs diagram. Distances were then derived from the atmospheric parameters. It was found that 82 stars are closer than 50 parsecs, and 57 are within 40 parsecs. The remaining 23 cool stars will be analyzed as soon as we acquire the photometric measurements. Figures 2 and 3 demonstrate the effectiveness of our method in detecting cool white dwarfs. We are now reaching the bottom of the luminosity distribution, and on our way to obtaining a statistically complete sample of nearby white dwarfs.

Figure 3: Top panel: Spectroscopic masses of DA white dwarfs in our sample as a function of . Bottom panel: Photometric estimates of Teff; since no parallax measurements are available, is fixed at 8.0. The squares represent white dwarfs that were already known.
We would like to thank the director and staff of Steward Observatory for the use of their facilities. This work was supported in part by the NSERC Canada and by the Fund FQRNT (Québec). P.B. is a Cottrell Scholar of the Research Corporation for Science Advancement.


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