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The White Dwarfs Within 20 Parsecs of the Sun: Kinematics and Statistics

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The White Dwarfs Within 20 Parsecs of the Sun: Kinematics and Statistics The Astronomical Journal, 138:1681–1689, 2009 December doi:10.1088/0004-6256/138/6/1681 C 2009. The American Astronomical Society. All rights reserved. Printed in the U.S.A. THE WHITE DWARFS WITHIN 20 PARSECS OF THE SUN: KINEMATICS AND STATISTICS Edward M. Sion1, J. B. Holberg2, Terry D. Oswalt3, George P. McCook1, and Richard Wasatonic1 1 Department of Astronomy & Astrophysics, Villanova University, Villanova, PA 19085, USA; [email protected], [email protected], [email protected] 2 Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 75201, USA; [email protected] 3 Department of Physics and Space Sciences, Florida Institute of Technology, Melbourne, FL 32901, USA; toswalt@fit.edu Received 2009 March 17; accepted 2009 September 15; published 2009 October 30 ABSTRACT We present the kinematical properties, distribution of spectroscopic subtypes, and stellar population subcomponents of the white dwarfs within 20 pc of the Sun. We find no convincing evidence of halo white dwarfs in the total 20 pc sample of 129 white dwarfs nor is there convincing evidence of genuine thick disk subcomponent members within 20 parsecs. Virtually, the entire 20 pc sample likely belongs to the thin disk. The total DA to non-DA ratio of the 20 pc sample is 1.6, a manifestation of deepening envelope convection which transforms DA stars with sufficiently thin H surface layers into non-DAs. The addition of five new stars to the 20 pc sample yields a revised local space density −3 −3 −3 −3 of white dwarfs of (4.9 ± 0.5)×10 pc and a corresponding mass density of (3.3 ± 0.3)×10 M pc . We find that at least 15% of the white dwarfs within 20 parsecs of the Sun (the DAZ and DZ stars) have photospheric metals that possibly originate from accretion of circumstellar material (debris disks) around them. If this interpretation is correct, this suggests the possibility that the same percentage have planets or asteroid-like bodies orbiting them. Key words: stars: kinematics – stars: statistics – techniques: photometric – techniques: spectroscopic – white dwarfs −1 1. INTRODUCTION noted that this star had a space motion of 313 km s relative to the Sun based on an assumed distance of 18 pc. Holberg et al. (2008) determined that WD1339−340 has an estimated dis- The population of local white dwarfs (WDs) is astrophysically tance of 21.2 ± 3.5 pc placing it formally outside the limits of important for a number of reasons. First, from complete samples, our 20 pc sample. Nevertheless, this star has a finite probability it offers an excellent probe of the coolest, least luminous (oldest) of being within 20 pc. component of the overall white dwarf population. Second, it Including the new stars in the local sample of 129 white samples the mix of stellar populations that evolve into the dwarfs within 20 pc, yields a revised white dwarf space density different spectroscopic subtypes in the immediate vicinity of − − of (4.9 ± 0.5) × 10 3 pc 3. The corresponding mass density is the Sun. Third, it provides a unique way of measuring the −3 −3 (3.3 ± 0.3) × 10 M pc . The completeness of the sample, local space density and mass density of white dwarfs which however, remains at 80% since the addition of the companion are currently of critical interest because: (1) they represent a of GJ86 (at 10.8 pc) contributes to the number of white dwarfs history of star formation and stellar evolution in the Galactic within 13 pc upon which the stellar density is based. plane; (2) the luminosity function of these stars can be used to In this work, we use the enlarged local sample to examine the place a lower limit on the age of the Galactic disk (Liebert 1988; kinematical properties, distribution of spectroscopic subtypes, Oswalt et al. 1996); (3) cool white dwarfs have been suggested and stellar population subcomponents of the white dwarfs as the origin of the MACHO lensing objects seen in lensing sampled in this volume of space around the Sun. surveys (Oppenheimer et al. 2001; Kawaler 1996); and (4) they are important to understanding the overall mass density of the 2. DISTRIBUTION OF SPECTROSCOPIC SUBTYPES IN Galactic plane (Bahcall 1984). THE LOCAL WHITE DWARF POPULATION Recently, Holberg et al. (2008, hereafter, LS08) completed a detailed survey of the local population of white dwarfs ly- Table 1 presents the sample of white dwarfs within 20 pc ing within 20 pc of the Sun which they estimated to be 80% of the Sun. The basic observational data from which space complete. Their sample contained 124 individual degenerate motions have been computed are given in Table 1, which stars, including both members of four unresolved double de- contains by column: (1) the WD number; (2) the coordinates generate binaries, one of which was a suspected new dou- (R.A. and decl. are in decimal degrees); (3) DIST (distance ble degenerate binary (WD0423+120). Since the publication in parsecs); (4) PM (proper motion in arcsec yr−1); (5) P.A. of LS08, we have added seven additional white dwarfs to the (position angle in degrees); and (6) the method of distance 20 pc sample, bringing the present total to 131 degenerate stars. determination, denoted by p for trigonometric parallax, s for Two of these additional stars are close Sirius-like companions spectrophotometric distances, and a for a weighted average to nearby K stars (Holberg 2009), GJ86 (WD0208−510), and of the parallax and photometric distances according to their HD27442 (WD0415−594), discovered during exoplanet inves- respective uncertainties. Distance estimates are taken from tigations (see Mugrauer & Neuhauser¨ 2005 and Chauvin et al. LS08 which are based on both trigonometric parallaxes and 2007, respectively). Of the remaining five single white dwarfs, spectrophotometric distances. Whenever possible, preference three stars, WD0011−721, WD0708−670, and WD1116−470, was given to trigonometric parallax distances. In LS08, the are from Subasavage et al. (2008), and one star, WD1315−781, photometric distances were computed based upon spectroscopic is from Subasavage et al. (2009). We have excluded one inter- and photometric measurements using the techniques described esting high-velocity star, WD1339−340. Lepine et al. (2005) in Holberg et al. (2008). 1681 1682 SION ET AL. Vol. 138 Table 1 Observational Data Used in Space Motions WD Type R.A. Decl. DIST PM P.A. Method 0000−345 DCP9 000.677 −34.225 12.65 0.899 217.337 a 0008+423 DA6.8 002.842 +42.668 17.94 0.237 193.4s 0009+501 DAH7.7 002.413 +50.428 11.03 0.718 216.0p 0011−134 DCH8.4 003.554 −13.177 19.49 0.911 217.7p 0011−721 DA8.0 003.457 −71.831 17.80 0.326 141.300 s 0038−226 DQ9.3 010.354 −22.347 09.88 0.567 229.004 p 0046+051 DZ8.1 012.291 +05.388 04.32 2.978 155.538 p 0108+277 DAZ9.6 017.686 +27.970 13.79 0.227 219.321 s 0115+159 DQ6 019.500 +16.172 15.41 0.648 181.805 p 0121−429 DAH7.9 021.016 −42.773 17.67 0.540 155.143 p 0135−052 DA6.9 024.497 −04.995 12.35 0.681 120.838 p 0141−675 DA7.8 025.750 −67.282 09.70 1.048 198.279 s 0148+467 DA3.8 028.012 +47.001 16.06 0.124 000.568 a 0148+641 DA5.6 027.966 +64.431 17.13 0.285 123.857 s 0208+396 DAZ7.0 032.836 +39.922 16.13 1.145 115.746 a 0208−510 DA10 032.500 −50.133 10.8 2.192 72.666 p 0213+427 DA9.4 034.281 +42.977 19.67 1.047 125.065 a 0230−144 DC9.5 038.157 −14.197 15.38 0.687 177.114 a 0233−242 DC9.3 038.840 −24.013 15.67 0.622 189.015 s 0245+541 DAZ9.7 042.151 +54.383 10.35 0.573 227.827 p 0310−688 DA3.1 047.628 −68.600 10.15 0.111 158.097 p 0322−019 DAZ9.7 051.296 −01.820 16.81 0.909 164.625 p 0326−273 DA5.4 052.203 −27.317 19.73 0.850 071.629 s 0341+182 DQ7.7 056.145 +18.436 19.01 1.199 150.771 p 0344+014 DQ9.9 056.778 +01.646 19.90 0.473 150.400 s 0357+081 DC9.2 060.111 +08.235 17.46 0.535 222.273 a 0413−077 DAP3.1 063.839 −07.656 05.04 4.088 213.216 p 0415−594 DA3.8 064.122 −59.302 18.23 0.174 195.838 p 0423+120 DA8.2 066.473 +12.196 17.36 0.244 335.866 p 0426+588 DC7.1 067.802 +58.978 05.53 2.426 147.602 p 0433+270 DA9.3 069.187 +27.164 17.85 0.276 124.196 p 0435−088 DQ8.0 069.447 −08.819 09.51 1.574 171.103 p 0457−004 DA4.7 074.930 −00.377 17.67 0.293 142.872 s 0548−001 DQP8.3 087.831 −00.172 11.07 0.251 025.810 p 0552−041 DZ11.8 088.789 −04.168 06.45 2.376 166.966 p 0553+053 DAP8.9 089.106 +05.536 07.99 1.027 204.993 p 0642−166 DA2 101.288 −16.713 02.63 1.339 204.057 p 0644+025 DA8 101.789 +02.517 17.83 0.423 272.571 a 0644+375 DA2.5 101.842 +37.526 15.41 0.962 193.561 p 0655−390 DA8.0 104.274 −39.159 17.20 0.340 242.600 s 0657+320 DC10.1 105.215 +31.962 15.19 0.691 149.362 p 0659−063 DA7.8 105.478 −06.463 12.13 0.898 184.980 a 0708−670 DC 107.217 −67.108 17.50 0.246 246.300 s 0727+482.1 DA10.0 112.678 +48.199 11.01 1.286 190.069 a 0727+482.2 DA10.0 112.697 +48.173 11.20 1.286 190.069 a 0728+642 DAP11.2 113.378 +64.157 13.40 0.266 171.352 s 0736+053 DQZ6.5 114.827 +05.227 03.50 1.259 214.574 p 0738−172 DAZ6.7 115.086 −17.413 09.28 1.267 115.137 p 0743−336 DC10.6 116.396 −34.176 15.20 1.736 352.670 p 0747+073.1 DC12.1 117.563 +07.193 18.25 1.804 173.414 a 0747+073.2 DC11.9 117.563 +07.193 18.25 1.804 173.414 a 0749+426 DC11.7 118.305 +42.500 19.74 0.420 165.845 s 0751−252 DA10.0 118.485 −25.400 18.17 0.426 300.200 p 0752−676 DC10.3 118.284 −67.792 07.05 2.149 135.866 a 0806−661 DQ4.2 121.723 −66.304 19.17 0.398 132.700 p 0821−669 DA9.8 125.361 −67.055 10.65 0.758 327.600 p 0839−327 DA5.3 130.385 −32.943 08.07 1.600 322.056 a 0840−136 DZ10.3 130.701 −13.786 19.30 0.272 263.000 s 0912+536 DCP7 138.983 +53.423 10.31 1.563 223.997 p 0955+247 DA5.8 149.451 +24.548 18.83 0.420 219.848 s 1009−184 DZ7.8 153.007 −18.725 18.00 0.519 268.200 p 1019+637 DA7.3 155.787 +63.461 13.93 0.379 053.160 s 1033+714 DC9 159.260 +71.182 20.00 1.917 256.008 s 1036−204 DQP10.2 159.731 −20.682 14.29 0.628 333.300 p 1043−188 DQ8.1 161.412 −19.114 17.57 1.978 251.636 a 1055−072 DA6.8 164.396 −07.523 11.96 0.827 276.328 a No.
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