The Parentage of Magnetic White Dwarfs: Implications from Their Space Motions

Transcription

1 PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC, 111:702È708, 1999 June ( The Astronomical Society of the PaciÐc. All rights resered. Printed in U.S.A. The Parentage of Magnetic White Dwarfs: Implications from Their Space Motions TARA ANSELOWITZ, RICHARD WASATONIC, KAREN MATTHEWS, EDWARD M. SION, AND GEORGE P. MCCOOK Department of Astronomy and Astrophysics, Villanoa Uniersity, Villanoa, PA 19085; emsion=ucis.ill.edu Receied 1999 February 10; accepted 1999 March 9 ABSTRACT. We hae examined the statistical properties, cooling ages, and ector components of the three-dimensional space motion U, V, W for the enlarged sample of 53 magnetic white dwarfs contained in the fourth edition of the Catalog of Spectroscopically IdentiÐed White Dwarfs (McCook & Sion). Their cooling ages range from 2 million years to 12.6 billion years. A comparison of the total kinematic samples of magnetics and DA stars oer the same luminosity range 10.0 \ M \ 16.0 reeals a elocity dispersion p() \ 39 km s~1 for 256 DA stars compared with p() \ 23 km s~1 for 26 magnetic degenerates with space motions. These results underscore the conclusion that the sample of magnetic white dwarfs appears to be predominantly descended from a young disk stellar population subcomponent characterized by relatiely small motions with respect to the Sun. This suggests both links to upper main-sequence Ap/Bp progenitors but possibly also massie near miss ÏÏ pulsar progenitors. We Ðnd preliminary eidence that the magnetic white dwarfs show a peculiar distribution in UV elocity space relatie to other spectroscopic subgroups of white dwarfs. Howeer, there is little eidence of a di erence in elocity dispersion among the hot and cool magnetic degenerates, despite their haing astly di erent cooling ages. This may be taken as indirect eidence that the magnetics represent a sample of mixed eolutionary progeny. 1. INTRODUCTION There are currently 53 magnetic white dwarfs known out of 2249 in the most recent (fourth edition) of the Catalog of Spectroscopically IdentiÐed White Dwarfs (McCook & Sion 1999). This comprises 2% of all known white dwarfs. Remarkably, while 46 magnetic white dwarfs are known to hae DA (hydrogen dominated) spectral types, seen objects hae now been identiðed as haing non-da (or helium dominated) atmospheres, including the Ðrst DB magnetics. The existence of DB magnetics may hae an important bearing on whether the DA ersus DB distinction among nonmagnetic white dwarfs is primordial or due to conectie mixing as these dying stars cool. Gien this enlarged sample of magnetic white dwarfs, are we able to shed any additional light on their origin and nature? It is especially timely to reexamine the statistics and kinematics of this astrophysically critical subgroup. This paper examines the origin question from the standpoint of the statistics and kinematics of the enlarged sample and improed data since the analysis of Sion et al. (1988). The magnetic white dwarfs are compared kinematically with a sample of DA white dwarfs lying in the same range of absolutmagnitude/t in order to minimize color and motion- dependent selection e ects. In 2, we compile the obsered fundamental stellar characteristics of the magnetic white dwarfs followed in 3by the results of our kinematic and statistical calculations. In 4 we discuss the implications and summarize our conclusions. 2. PROPERTIES AND STATISTICS OF THE MAGNETIC WHITE DWARF SAMPLE In Table 1, we present the current census of magnetic degenerates with data compiled from seeral sources. The M alues were taken from McCook & Sion (1999) with the exception of the M alue for WD 0823[253 which was adopted from Jordan (1997). Most of the other data were obtained from the compilation of Jordan (1997) unless speciðed by reference numbers to the other sources gien below. An assigned reference number (in parentheses) refers to (1) McCook & Sion (1999), (2) Schmidt & Smith (1995), (3) Jordan (1997), (4) Reimers et al. (1998), (5) Jordan et al (1999), and (6) Moran, Marsh, & Dhillon (1998). Starting with the Ðrst column, we gie the WD number, discoery name, T, dominant atmospheric chemical constituent, absolute isual magnitude, magnetic Ðeld strength (in Megagauss), rotational period, and cooling age (in units of 108 yr). It is seen that the sample encompasses a broad range of surface temperatures and hence cooling times. We determined the cooling times for each magnetic white dwarf by assuming a mean mass of 0.8 M for the entire sample and _ 702

2 PARENTAGE OF MAGNETIC WHITE DWARFS 703 TABLE 1 PROPERTIES OF MAGNETIC DEGENERATES T B field Cooling Age WD Number Name (K) Chemical Composition M (MG) P rot (t/108 yr) Reference 0000[ HE H (3) 0009] LHS H D0.09 2È20 hr 31.6 (3) 0011[ LHS H hr? 50.1 (3) 0127[ HE H (3) 0136] PG H ? (3) 0159[ MWD H (3) 0253] KPD D15000 H hr 3.16 (3) 0307[ MWD H (3) 0317[855J... REJ H (3) 0329] KUV H (3) 0503[ LHS H minutesè1 yr 79.4 (3) 0548[ G C2, CH D (3) 0553] G H hr? 57.5 (3) 0616[649J... REJ H (3) 0637] GD 77 D10000 H (3) 0728] G H... D (3) 0756] G H D (3) 0816] GD H (3) 0823[ RXS J H (5) (5) 2.8È3. (5) (5) 0912] G ? D days 22.4 (3) 0945] LB 11146b H]? (3) 1008] LHS (3) 1015] PG H hr 3.80 (3) 1017] GD H (3) 1026] LHS 2273 D6000 H D (3) 1031] PG D15000 H È hr 3.16 (3) 1036[ LP 790[ C D200 Z100 yr 25.1 (3) 1045[ HE 9000 H (3) 1127[ ESO C È30? (3) 1136[ LBQS H (3) 1211[ HE 20[25000 H... 80? 1.75 days 1.12 (3) 1220] PG H D (3) 1254] HS 10[15000 H... D (3) 1309] G H (3) 1312] PG H hr 3.16 (3) 1330] G62-46 D6050 H (3) 1349] SBS H (3) 1350[ LP 907[ H D (3) 1412] HS... H (2)... D8 (2) (2) 1440] HS H... D (3) 1533[ PG H D1 days 2.00 (3) 1639] GD H(em) D (3) 1658] PG H (3) 1743[ BPM H days 1.12 (3) 1748] G ? D200 Z100 yr 50.1??) 1814] G D7000 H \14 50 minutesè? yr 28.2 (3) 1818] G H ? (3) 1829] G H Z100 yr 30.2 (3) 1900] GW ] H Z100 yr 3.16 (3) 2201[ HE H (3) 2316] KUV H days 6.61 (3) 2329] PG (6) 11.2 (6) (3) 0003[ HE... He (4)... 20È25 (4) (4) 0026[ HE... He (4)... 10È30 (4) (4) 0041[ Feige H, He hr 1.12 (3) 0107[ HE... He (4)... 10È30 (4) (4) 0338[ HE... He (4)... 20È25 (4) (4) 0853] LB He D (3) 2010] GD He (5) È700 (5) D100 yr 2.82 (5) NOTE.ÈSee 2 for references (in parentheses).

3 704 ANSELOWITZ ET AL. then extracting the cooling times from the model grid tabulation of M. Wood (1995, priate communication). This mean mass was adopted for two reasons: (1) indiidual masses for magnetic white dwarfs are known reliably for only a small fraction of the current sample so for the purpose of this statistical study, the adoption of a mean mass is the only workable alternatie; (2) there is considerable independent empirical eidence that magnetic white dwarfs hae smaller than aerage radii and hence higher masses (Liebert 1988) than the aerage mass (0.58 M ) of _ DA stars. Moreoer, numerous published analyses of indiidual magnetics (e.g., Schmidt et al. 1992; Schmidt, Liebert, & Smith 1998) hae yielded high masses. Since the range of M is ery broad, we diided the mag- netics into two samples based upon their M : a hot ÏÏ sample in the range 10.5 \ M \ 12.5 and a cool sample ÏÏ in the range 12.5 \ M \ This subdiision enables us to (a) search for relatie di erences between two populations ÏÏ of magnetics with astly di erent mean cooling ages and (b) carry out relatie comparisons with the kinematics and statistics of other groups of white dwarfs oer a similar luminosity range, thus minimizing motiondependent selection e ects based upon color/m. 3. KINEMATICS OF MAGNETIC WHITE DWARFS For those magnetic white dwarfs with sufficient kinematic information (photometric or trigonometric parallax, proper motion, position angle), we hae computed the ector components of the space motion U, V, and W relatie to the Sun in a right-handed system following Wooley et al. (1970), where U is measured positie in the direction of the Galactic anticenter, V is measured positie in the direction of the Galactic rotation, and W is measured positie in the direction of the north Galactic pole. We hae taken the radial elocity to be zero because of the lack of indiidual radial elocities for the magnetic white dwarfs. All other information needed to compute the space motions was obtained from McCook & Sion (1999). The M alues in Table 1 were obtained from colorèabsolute magnitude correlations gien in McCook & Sion (1999). Photometric parallaxes were computed based upon multichannel spectrophotometry, Stro mgren narrowband y, b[y, u[b photometry, or UBV broadband photometry in that order of priority. If a trigonometric parallax alue greater than 0.1 was aailable, it was used in place of the photometric parallax. In Table 2 we gie the WD number, name, proper motion (arcsec yr~1), position angle (in degrees), ector elocity components U, V, W, and in the last column the total (transerse) space elocity. As stated in 1, we hae selected the DA white dwarfs (which comprise 80% of all white dwarfs) to be the spectroscopic group to use as a basis of comparison with the kinematic properties of the magnetic degenerates. Howeer, it is well known that the elocity dispersions of stellar types tend to increase with increasing total stellar age. This e ect is demonstrated in Figure 1, where we hae plotted the total motion V (in km s~1) of the magnetic sample, in each of t eight elocity bins, as a function of cooling age (in units of 109 yr). A correlation of mean total elocity with increasing cooling age is weakly indicated. By contrast, the DA stars in Sion et al. (1988) reeal a much stronger tendency toward higher elocity with increasing cooling age. Thus, kinematic di erences between the two subtypes of white dwarfs may arise if one group is substantially older than the other. To check this possibility, we hae examined the aerage T (and cooling age) of the two samples. For 256 DA stars with calculated space motions from Sion et al. (1988), the mean T \ 14,300 K. For the magnetic white dwarfs, the mean T D 15,000 K. Thus the two samples seem to be charac- terized by roughly similar aerage T and thus roughly comparable aerage cooling ages. Therefore, kinematic differences between the two samples are less likely to be simply attributed to di erences in age. In Tables 3È5 we present elocity aerages, standard deiations, and standard errors of U, V, W, and the total motion T for DA stars and magnetic stars, binned separately into the M ranges 10.00È16.00 (the total range), 10.00È (the hot ÏÏ range), and 12.50È16.00 (the cool ÏÏ range), respectiely. For the DA stars the U, V, W, and T alues are those from Sion et al. (1988). One sees that in the hot ÏÏ range in Table 4, the magnetics hae signiðcantly smaller elocity dispersions in each of the three ector components U, V, W, and in the total motion. Howeer, the sample of magnetics is ery small in this range compared with the 175 DA stars. In the cool ÏÏ range (12.50È16.00) in Table 5, the relatie numbers are more comparable. We see that the FIG. 1.ÈTotal motion (in km s~1) of magnetic white dwarfs, plotted in eight elocity bins, s. the cooling age (in units of one billion years). A weak trend toward higher total motion with increasing cooling age is seen.

4 PARENTAGE OF MAGNETIC WHITE DWARFS 705 TABLE 2 MOTIONS OF MAGNETIC DEGENERATES WD Number Name k h M U V W T 0000[ HE ] LHS [ [ [ LHS [ [ [ [ HE ] PG [ MWD ] KPD [ MWD [855J... REJ ] KUV [ LHS [ [ [ [ G ] G [ [ [649J... REJ ] GD [ ] G ] G [ ] GD [1.610 [ [ [ RXS J ] G [ [ ] LB 11146b ] LHS ] PG ] GD [ [ ] LHS [ [ ] PG [ LP 790[ [ HE [ ESO [2.153 [ [ LBQS [ HE ] PG ] HS ] G [ [ ] PG ] G [ [ [ ] SBS [ LP 907[ [ [0.858 [ ] HS ] HS [ PG ] GD [3.724 [ ] PG [ BPM [4.789 [1.514 [ ] G [ ] G [ [ [ ] G ] G [ ] GW ] [ HE ] KUV ] PG [ HE [ HE [ Feige [ [4.755 [ [ HE [ HE ] LB [ [ ] GD

5 706 ANSELOWITZ ET AL. TABLE 3 M RANGE 10.00È16.00 Sample N Velocity Aerage SD SE U DA stars ^ Magnetics [ ^ V DA stars [ ^ Magnetics [ ^ W DA stars [ ^ Magnetics [ ^ T DA stars ^ Magnetics ^ TABLE 5 M RANGE 12.50È16.00 Sample N Velocity Aerage SD SE U DA stars ^ Magnetics [ ^ V DA stars [ ^ Magnetics [ ^ W DA stars [ ^ Magnetics [ ^ T DA stars ^ Magnetics ^ elocity dispersion of total motion for the magnetics is 38% smaller than the DA stars in the same M range. Finally, a comparison of the total samples of magnetics and DA stars oer the full range 10.0 \ M \ 16.0 in Table 3 reeals a p() of39kms~1 for 256 DA stars compared with p() \ 23 km s~1 for 26 magnetics. These results, based upon a kinematic sample of magnetics double the size of that analyzed in Sion et al. (1988), strongly support the conclusion that the sample of magnetic white dwarfs appear to be predominantly descended from a young disk stellar population subcomponent characterized by relatiely small motions with respect to the Sun. This enlarged sample strengthens links to upper main-sequence Ap/Bp progenitors but possibly also massie near miss ÏÏ pulsar progenitors. The three helium-dominated magnetics for which we hae space motions reeal low elocities as well. Finally, there is a paucity of genuine old disk and halo space motions in the magnetic sample. Figure 2 displays the total sample of DA stars and Figure 3 the total sample of magnetics in the U ersus V elocity plane. The Ðlled and open circles in each Ðgure denote the cool ÏÏ and hot ÏÏ samples, respectiely. The marked di erence in dispersion between the DA stars and the magnetics is eident. Curiously, the distribution of the magnetics in UV elocity space reeals a possibly real asymmetry compared with the DA sample. The U component is measured positie in the direction of the Galactic anticenter, and the magnetics, albeit a relatiely small sample, appear to show preferential motion toward positie U. In contrast, the hot TABLE 4 M RANGE 10.00È12.50 Sample N Velocity Aerage SD SE U DA stars ^ Magnetics ^ V DA stars [ ^ Magnetics... 7 [ ^ W DA stars [ ^ Magnetics ^ T DA stars ^ Magnetics ^ FIG. 2.ÈTotal sample of DA stars in the U s. V elocity plane. The Ðlled and open circles denote the cool ÏÏ and hot ÏÏ samples, respectiely, as deðned in the text.

6 PARENTAGE OF MAGNETIC WHITE DWARFS 707 FIG. 3.ÈTotal sample of magnetics in the U s. V elocity plane. The Ðlled and open circles denote the cool ÏÏ and hot ÏÏ samples, respectiely, as deðned in the text. Note that the distribution of the magnetics in UV elocity space reeals a possibly real asymmetry compared with the DA sample in the preious Ðgure. and cool DA samples reeal relatie symmetry in the positie and negatie U directions. We hae examined other spectroscopic subgroups (the DBs, DQs, and DZs) and again Ðnd symmetry between positie and negatie U. A preliminary conclusion is that the magnetic white dwarfs may show a peculiar distribution in elocity space relatie to other spectroscopic subgroups of white dwarfs. 4. DISCUSSION AND CONCLUSIONS The range of cooling ages of the sample is absolutely ast, extending from as short as 2 million years (RE J0317[855) to as long as 12.6 billion years (G243-4). Note, howeer, that these cooling ages are only ery approximate as a result of our assumption of a mean mass for all magnetics. For example, detailed cooling ages deried by Bergeron, Ruiz, & Leggett (1997) use log g or trigonometric parallaxes to determine the radius/mass. Hence, they use the correct mass in the determination of cooling ages. None of their stars has a cooling age greater than 10 Gyr. Neertheless, for the purposes of our statistical study these coarser cooling ages can still be instructie. Are the cooling ages all consistent with origins from young disk stars (i.e., the Bp and Ap stars)? The estimated age of the disk is only 10 billion years. Despite the long total stellar ages of white dwarfs, which tend to smooth out kinematic di erences among spectroscopic subgroups, the magnetic degenerates are kinematically distinct from other subgroups. Oerall, the space motions of the enlarged sample proide een stronger support that the magnetic white dwarfs are predominantly associated with formation from young massie progenitors, i.e., the Ap and Bp stars or more massie objects close to the lower mass limit for neutron star formation. This explanation is reinforced by the fact that the surface magnetic Ñuxes of magnetic white dwarfs and Ap stars are similar, i.e., about BR2\1024È 5 ] 1026 R2 G cm2 (Chanmugam 1994). Moreoer, the 9 surface magnetic Ñuxes of neutron stars are also similar (1023È1025) to the aboe range. Note howeer that, in iew of the rotation range of magnetics (see Table 1), a link to the Ap stars based upon angular momentum conseration cannot be made (Liebert 1995; Angel, Landstreet, & Borra 1981). On the other hand, as seen in Table 1, some magnetics hae rotation periods in the range of orbital periods/ rotation rates of magnetic CVs suggesting the possibility that some magnetics may be extinct magnetic CVs with the binary companion and mass transfer a ects no longer obserationally detectable. If the magnetic white dwarfs were descended exclusiely from Ap/Bp star progenitors, then one would expect a difference in elocity dispersion between the hotter magnetic degenerates with shorter cooling ages and the cooler magnetic degenerates with much longer cooling ages. This follows from the elocity inñation of a gien star in Galactic orbit due to increasing tidal encounters with increasing age. We Ðnd no such eidence of a di erence in dispersion among the hot and cool magnetic degenerates. This may be taken as indirect eidence that the magnetics represent a sample of mixed eolutionary progeny. Finally, do the magnetic white dwarfs hae increasing Ðeld strength with decreasing T (increasing cooling age)? Is there a higher frequency of magnetic white dwarfs among cool degenerates than among hot degenerates. These are questions beyond the scope of this paper, but Liebert & Sion (1979) did Ðnd preliminary eidence, based upon a much smaller sample, to suggest that there is a greater frequency with decreasing T. In this connection, Fabrika & Valyain (1998) presented new results supporting this possibility. Perhaps the fossil Ðelds are partially buried at white dwarf formation and gradually di use outward as the star cools. We leae these questions for followup inestigations. We thank the referee for seeral useful comments. This work was supported by NSF grant AST to Villanoa Uniersity. Two of us (T. A., K. M.) were supported by summer undergraduate research assistantships from the NASA Delaware Space Grant Colleges Consortium.

7 708 ANSELOWITZ ET AL. Angel, R. J. P., Landstreet, J., & Borra, G. 1981, ApJS, 45, 457 Bergeron, P., Ruiz, M.-T., & Leggett, S. 1997, ApJS, 108, 339 Chanmugam, G. 1992, ARA&A, 30, 143 Fabrika, S. N., & Valyain, G. G. 1999, in ASP Conf. Ser. 169, The 11th European Workshop on White Dwarfs, ed. J.-E. Solheim & E. Meistas (San Francisco: ASP), in press Jordan, S. 1997, in White Dwarfs: Proceedings of the 10th European Workshop on White Dwarfs, ed. J. Isern, M. Hernanz, & E. Garcia-Berro (Dordrecht: Kluwer), 399 Jordan, S., et al. 1999, in ASP Conf. Ser. 169, The 11th European Workshop on White Dwarfs, ed. J.-E. Solheim & E. Meistas (San Francisco: ASP), in press Liebert, J. 1988, PASP, 100, 1302 REFERENCES Liebert, J. 1995, in Magnetic Cataclysmic Variables, ed. P. Stockman (Berlin: Springer), 59 Liebert, J., & Sion, E. M. 1979, Astrophys. Lett., 20, 53 McCook, G. P., & Sion, E. M. 1999, ApJS, 121, 1 Moran, G., Marsh, T., & Dhillon, V. 1998, MNRAS, 299, 218 Reimers, D., et al. 1998, A&A, preprint Schmidt, G., Liebert, J., & Smith, P. 1998, AJ, 116, 451 Schmidt, G., & Smith, P. 1995, ApJ, 448, 305 Schmidt, G., et al. 1992, ApJ, 398, L57 Sion, E. M., Fritz, M., McMullin, J. P., & Lallo, M. D. 1988, AJ, 96, 251 Wooley, R., et al. 1970, Ann. R. Obs., 5