Kinematics of Faint White DwarfsAuthor(s): Willem J. LuytenSource: Proceedings of the National Academy of Sciences of the United States of America,Vol. 75, No. 10 (Oct., 1978), pp. 4640-4641Published by: National Academy of SciencesStable URL: http://www.jstor.org/stable/68967 .

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Proc. Natl. Acad. Sci. USA Vol. 75, No. 10, pp. 4640-4641, October 1978 Astronomy

Kinematics of faint white dwarfs (proper motion/luminosity)

WILLEM J. LUYTEN

University of Minnesota, 211 Space Science Center, Minneapolis, Minnesota 55455

Contributed by Willem J. Luyten, July 17, 1978

ABSTRACT An analysis has been made for solar motion for 128 very faint white dwarfs of color class b or a. While about 40% of these stars may be high-velocity objects, it seems defi- nitely indicated that the luminosity of all of them is considerably lower than that for the "normal" white dwarf of the same color.

Fifteen years ago, when 52 new white dwarfs had been found on the first seven pairs of plates analyzed in the Palomar Proper Motion Survey, I pointed out the possibility that degenerate stars might exist with a luminosity some four magnitudes fainter than the principal degenerate sequence (1). The recent discovery of a 19th magnitude bluish white dwarf with a large proper motion (2) gave further impetus toward a search for such low-luminosity white dwarfs.

The two catalogues of white dwarfs that have resulted from the Palomar Proper Motion Survey contain 6500 entries (3, 4), and a preliminary discussion of them was presented at the Russel Symposium in November 1977 (5). The Hertzsprung diagram given there shows more than 100 objects of the pre- liminary color classification b or a with a reduced proper motion H = m + 5 + 5 log u exceeding the value 20. Very few spectra, photoelectric magnitudes, and colors, or trigonometric paral- laxes are now available for these very faint stars, and in the absence of such more precise data, the best thing we can do is to make an analysis for solar motion from the proper motions. In this way we can obtain some preliminary indications of the kinematics as well as of the luminosities of these objects.

The list that follows gives data for 128 stars classified as b or a, and with H u> 20. Two of these stars, LP 661-3 = L 886-6 and LP 561-13 = L1126-68, were found in the Bruce Proper Motion Survey, all others in the Palomar Survey. Because more precise positions for all these stars have been given in the two white dwarf catalogues mentioned before, they are here identified only with accuracy sufficient for statistical purposes. The last four columns of Table I give, in order, the red magnitude mR, the estimated color, the total proper motion, M, and its direction 0. The least certain of these, of course, are the color estimates. These have been obtained from a comparison of the red and blue Palomar Survey plates, and we know that the blue mag- nitudes are heavily contaminated by the strong ultraviolet in the spectrum of these degenerate stars. Some stars listed here as b or a may well prove to belong to color class f or even g, and the best we can hope for is that their number will be small.

With this caveat we can now proceed to the analysis for solar motion. The apparent magnitudes of our stars range from 15.6 to 19.4, with a mean value of mR = 18.2, and all proper motions have been reduced to this magnitude before being used in the solution. Inspection of the directions of the motions suggests that some of our stars are probably high-velocity population II objects. Selecting the most likely 53 candidates for this group,

The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "ad- vertisement" in accordance with 18 U. S. C. ?1734 solely to indicate this fact.

46'

and making a separate solution for solar motion for them, we obtain A = 19?, D = +70?, q = 0. 327. The remaining 75 stars then yield A = 281?, D = -4?, q = 0. 312. These results appear to confirm that the former group are, indeed, high-velocity stars, while the latter group appear to be composed of more "normal" stars. The positions obtained for the two apices, however, differ by such extreme amounts that one might almost think the two groups of stars belong to different universes. Even assuming a solar velocity of 150 km/sec for the first group would make their mean absolute magnitude +13; the second group, whose solar velocity should obviously be much smaller, would be even fainter.

As of now, therefore, I believe it would be more realistic to use a single solar motion solution for all stars. Combining the data for the two groups, we obtain:

A = 291?, D = +31?, q = 0'225; 128 stars with mR = 18.2.

This low value of D suggests a solar velocity of around 20 km/sec, but even if we assume V = 40 km/sec, the mean par- allax comes out as f = 0'"026, leading to a mean absolute mag- nitude around +15 (red).

This is so much fainter than would have been expected of stars as blue as these that the possibility of systematic errors in the data immediately looms up. If such errors are present in the proper motions, their effect can only be negligible, because only 4 of the 128 stars have motions smaller than 0.2 annually. While it is quite possible that the red magnitudes are too faint, espe- cially for the stars south of declination -20?, these errors are not likely to be larger than 0.5 and probably average less than 0.3. All colors may have been estimated systematically too blue, but again, it does not seem likely that many of these b and a stars are, in reality, yellow degenerates, and it would seem out of the question that they are all, on the average, as yellow as color class g.

Tentatively, therefore, the conclusion seems justified that these stars are of a lower luminosity than "normal" degenerate stars of their color. It is hoped that accurate UBV (ultraviolet- blue-visual) photometry and spectra will soon become available and provide more information on this point, but a definitive answer as to the real luminosity will probably not be obtained until a substantial number of trigonometric parallaxes have been obtained for these stars, especially for those that do not appear to be high-velocity stars.

1. Luyten, W. J. (1963) Publications of the Observatory of the University of Minnesota 2, No. 17.

2. Luyten, W. J. (1978) International Astronomical Union Circular 3210.

3. Luyten, W. J. (1970) White Dwarfs (Univ. of Minnesota, Min- neapolis, MN), pp. 1-124.

4. Luyten, W. J. (1977) White Dwarfs II (Univ. of Minnesota, Min- neapolis, MN), pp. 1-103.

5. Luyten, W. J. (1978) Proper Motion Survey with the 48-Inch Schmidt Telescope, No. 51 (Univ. of Minnesota, Minneapolis, MN), pp. 1-12.

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Astronomy: Luyten

Table 1. Details for I

Right ascension Declination

(1950) (1950) LP no. hr min ? ' mR c , " , ?

824-155 0 06 -23 43 18.2 a 0.33 127 149- 25 07 +49 17 18.9 a 0.19 249 240- 56 16 +34 25 18.3 a 0.33 90 644- 86 16 - 7 18 18.7 a 0.20 84 405- 9 22 +21 21 19.0 b 0.30 83 585- 27 24 - 2 04 18.2 a 0.27 204 29- 23 41 +74 56 18.5 a 0.30 104

150- 39 43 +51 09 19.1 a 0.23 81 646- 82 102 - 406 17.7 a 0.37 100 466-257 06 +15 00 17.0 a 0.54 141

648- 81 31 - 751 18.9 b 0.21 73 588- 35 40 - 139 18.7 b 0.35 159 768-500 46 -17 26 17.6 a 1.18 188 589- 35 204 - 1 11 17.8 a 0.32 231 885- 57 22 -2909 18.6 a 0.50 96 530- 21 29 + 7 57 17.2 a 0.35 134 886- 43 46 -30 14 18.8 b 0.58 52 592- 7 307 + 042 18.9 b 0.20 108 472- 60 21 +11 38 17.9 a 0.36 127 712- 45 24 -10 54 18.0 a 0.45 143

773- 11 32 -16 05 19.0 a 0.32 80 889- 20 58 -32 06 19.0 b 0.21 117 357-186 4 09 +23 47 18.2 a 0.42 144 474-179 10 +15 15 19.0 a 0.16 142 890- 24 24 -29 03 18.0 a 0.38 193 475-247 38 + 9 19 16.3 a 0.76 184 776- 40 5 00 -15 19 18.7 a 0.26 200 85- 9 00 +57 18 18.3 b 0.30 183

416-350 04 +20 27 17.8 a 0.36 157 717- 1 06 -14 54 17.6 b 0.54 154

119- 21 09 +52 46 17.5 a 0.38 154 836- 26 19 -25 29 18.5 a 0.46 180 119- 48 26 +52 38 16.2 a 0.67 146 661- 3 6 59 - 6 23 16.4 a 0.90 185 122- 34 7 02 +51 48 18.0 a 0.45 162 485- 14 8 26 +12`03 18.0 a 0.32 269 425-348 38 +15 08 17.7 b 0.34 135 545- 40 38 + 2 58 19.0 b 0.23 175 164- 52 43 +49 37 19.1 b 0.31 159 210- 12 55 +42 41 18.2 a 0.31 232

313- 49 9 19 +29 41 18.7 a 0.24 240 126- 14 26 +56 01 17.4 a 0.36 268 668- 20 44 - 558 17.6 b 0.39 236 62-159 10 20 +64 04 17.9 a 0.43 239

212- 60 20 +39 20 17.1 a 0.46 244 848- 19 27 -26 40 18.4 a 0.41 177 610- 30 30 - 2 15 18.3 b 0.28 285 168- 8 37 +46 28 18.6 a 0.27 242 490- 56 40 +10 34 17.9 a 0.28 195 731- 5 43 -13 23 18.0 b 0.33 288

731- 8 44 -10 35 18.9 a 0.27 161 612- 1 1107 - 2 57 18.1 a 0.35 297 552- 14 09 + 3 54 18.3 b 0.38 255 432- 40 18 +15 58 18.9 a 0.21 195 906- 24 21 -32 07 18.5 a 0.24 282 793- 51 31 -1652 18.7 b 0.39 268 375- 11 37 +23 03 18.0 a 0.34 150 613- 32 40 - 152 19.0 a 0.18 144 673- 38 43 - 3 29 18.2 a 0.27 123 494- 46 1206' +13 12 18.8 a 0.19 268

216- 71 10 +3851 19.0 a 0.26 256 614- 59 12 - 217 17.8 a 0.56 138 734- 44 13 -1239 19.1 a 0.15 280 39- 76 15 +6906 16.8 a 0.47 272

Proc. Natl. Acad. Sci. USA 75 (1978) 4641

28 faint white dwarfs

Right ascension Declination

(1950) (1950) LP no. hr min m

? mR c , " ,

554- 74 12 17 + 3 39 18.9 a 0.20 343 171- 40 37 +45 42 16.0 a 0.89 230 616- 31 52 - 2 20 19.0 a 0.23 228 172- 65 13 14 +47 57 18.6 a 0.21 148 737- 22 19 -10 43 18.4 b 0.30 261 618- 6 31 + 032 18.3 a 0.41 297 219- 52 38 +40 48 18.8 a 0.19 154 738- 10 43 -15 14 17.4 a 0.36 268 799- 57 59 -1917 18.7 b 0.28 262 739- 19 14 01 -14 58 18.0 a 0.40 203

500- 28 24 +10 03 18.1 b 0.34 165 134- 22 28 +50 38 18.3 b 0.31 326 22- 49 36 +76 57 19.4 a 0.13 131

914- 20 44 -30 23 18.1 a 0.32 235 561- 13 48 + 7 46 15.6 a 0.92 243 562- 23 1514 + 6 58 18.8 a 0.22 142 327- 62 25 +32 37 18.5 a 0.24 86 42-107 30 +73 45 19.2 a 0.21 118

743- 24 39 -13 47 19.0 b 0.20 314 683- 32 42 - 5 12 18.1 a 0.33 153

274- 28 54 +38 19 18.1 a 0.31 277 136- 39 16 00 +53 55 18.0 a 0.38 173 684- 16 02 - 7 18 17.8 a 0.49 213 744- 37 14 -11 43 18.7 a 0.24 251 504- 52 17 +11 13 18.2 a 0.30 347 805- 21 34 -15 54 18.8 a 0.23 238

9-175 43 +80 45 18.0 a 0.50 336 686- 41 1704 - 833 17.7 a 0.43 217 447- 20 12 +19 39 18.3 a 0.29 251 179- 47 18 +48 58 18.6 a 0.22 343

332- 29 28 +29 44 18.5 a 0.33 337 508- 1 29 +12 01 18.8 b 0.26 240 139- 34 58 +54 16 18.5 a 0.30 358 182- 44 1836 +5046 19.1 b 0.25 217 230- 20 59 +42 56 18.7 a 0.26 201 870- 49 20 13 -23 10 18.8 a 0.20 135 872- 14 46 -21 44 18.4 a 0.38 184 456- 37 2104 +1935 18.1 a 0.36 92 873- 8 08 -21 41 16.7 a 0.55 172 517- 28 17 +11 08 18.2 a 0.37 94

517- 51 24 +15 29 18.9 a 0.19 198 697- 37 25 - 8 35 18.7 a 0.30 83 107- 22 26 +58 42 18.8 a 0.19 37 458- 1 29 +20 12 19.1 b 0.22 104

11-114 47 +83 14 18.8 a 0.20 351 930- 54 48 -27 56 18.9 a 0.18 122 342- 33 51 +33 15 18.5 b 0.32 40 639- 20 22 04 - 1 18 18.7 a 0.25 237 759- 56 11 -1010 18.5 a 0.24 85 287- 35 12 +37 13 16.7 a 0.56 220

520- 54 39 +13 17 17.1 a 0.42 171 461- 17 50 +20 55 17.7 a 0.43 54 345- 23 56 +29 34 18.0 a 0.41 232 521- 63 23 00 +13 48 18.0 b 0.31 79 762- 21 05 -13 31 18.5 b 0.43 24 462- 22 16 +20 03 18.1 b 0.35 64 642- 52 19 + 046 19.0 a 0.27 207 643- 20 35 + 016 17.8 a 0.34 63 703- 66 43 - 600 19.0 a 0.25 117 291- 16 44 +3811 19.1 a 0.20 130

703- 6 50 -310 17.5 b 0.66 66 583- 71 52 + 532 18.7 b 0.24 152 880-356 52 -32 38 17.0 a 0.53 95 149- 9 55 +50 40 18.4 a 0.35 77

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