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198Lapjs...45..457A the Astrophysical Journal Supplement The Astrophysical Journal Supplement Series, 45:457-474, 1981 March © 1981. The American Astronomical Society. All rights reserved. Printed in U.S.A. 198lApJS...45..457A THE MAGNETIC FIELDS OF WHITE DWARFS J. R. P. Angel,1 Steward Observatory, University of Arizona Ermanno F. Borra, Département de Physique, Université Laval and Observatoire Astronomique du Mont Mégantic AND J. D. Landstreet1 Department of Astronomy, University of Western Ontario Received 1980 June 20; accepted 1980 September 16 ABSTRACT In a survey program carried out over the past decade, more than 100 white dwarfs have been observed for magnetic fields by continuum circular polarization measurements. Twelve white dwarfs have been measured with greater accuracy by Zeeman measurements in absorption lines. These observations are reported in full in this paper. All known magnetic white dwarfs have surface fields greater than 5X106 gauss. No white dwarfs with weaker fields have been found by any search method, although the most accurate measurements have errors of only a few kilogauss. Field strengths in the stars showing no continuum polarization are found to be typically less than 500 kilogauss, based on an approximate treatment of atmospheric magnetic circular dichroism and radiative transfer. Our data, combined with those of Trimble and Greenstein and of Elias and Greenstein, are used to determine the probability P(B) (or an upper limit to it) of finding surface field strength B over the range 3X104-3X108 gauss. We find that BP(B), the probability per octave, is roughly constant at ~0.005 for fields in the range 3 X106-3 X 108 gauss, and does not exceed this value down to 106 gauss. Below this field strength, not enough stars have been sampled with high accuracy to set very severe limits on BP(B). While the first few magnetic white dwarfs discovered were usually cool, we do not find the currently known group to be significantly cooler than the complete sample of classified white dwarfs. All the magnetic white dwarfs that have identifiable atmospheric constituents, except LP790-29, contain some hydrogen. This hydrogen has probably not been accreted from the interstellar medium, since the fields are almost certainly strong enough to prevent accretion. Continued monitoring of all three magnetic white dwarfs with linear polarization has revealed no significant change in position angle, with an upper limit of A0<1° yr-1. This suggests that either these white dwarfs are rotating with periods of hundreds of years or more or that their magnetic and spin axes are closely aligned. Improved ephemerides are given for two magnetic white dwarfs with well-established rotation periods. The space density-to-lifetime ratio is found to be about the same for the magnetic peculiar A stars and magnetic white dwarfs, suggesting there may be an evolution from one to the other. Subject headings: stars: magnetic— stars: white dwarfs I. INTRODUCTION 1978) and Landstreet (1979). The most striking char- During the past ten years, we have observed over 100 acteristic of the magnetic white dwarfs is the very large white dwarfs for evidence of magnetic fields. For con- field strengths found. By contrast, in the large majority venience, data on the 13 single magnetic white dwarfs of white dwarfs no field can be measured even though reported to date are fisted in Table 1; five of these were some limits are 1000 times smaller than typical fields in first identified through our survey program. Reviews of Table 1. The null results are of interest because of the magnetic white dwarfs are given by Angel (1977, constraints they set on the presence of moderate strength fields in white dwarf atmospheres which might modify convection, gravitational setting, and the rate of inter- 'Visiting Astronomer, Kitt Peak National Observatory, which is operated by the Association of Universities for Research in stellar accretion. These processes may account for the Astronomy, Inc., under contract with the National Science Foun- unusual compositions of many white dwarf atmospheres dation. (Strittmatter and Wickramasinghe 1971). Furthermore, 457 © American Astronomical Society • Provided by the NASA Astrophysics Data System 458 ANGEL, BORRA, AND LANDSTREET Vol. 45 TABLE 1 Known Single Magnetic White Dwarfs Name WD Number T e (K) R (km/sec) Typical CP B e Comments 198lApJS...45..457A Composition M B o (BH)(megagauss) Typical LP B s References (%) (megagauss) L795-7 0041-10 22000 0.8 55 0.4 v l4v P = 132 min. * Feige 7 H, He v 1.8 11 no obs 18 1 G35-26 16000 1.2 264 <0.2 H, He? 0.6 0.0003 no obs 6.5 G99-37 0548-00 6200 1.0 31 0.9 3.6 0.8 54 < 0.2 3, 4 G99-47 0553+05 5600 1.2 49 0.4 3.5 H 0.6 1.3 <0.3 15 5, 6 GD90 0816+37 12000 0.7 30 < .05 0 H 1.1 480 no obs 5.5 6, 7 G195-19 0912+53 7000 1.2 87 -Iv P = 1.33 days no lines 0.6 0.07 < .2? 8, 9 PG1015+01 10000 : 1.5 v P = 99 min. no ident,v no obs 10 LP790-29 8600 8 C2 no obs 11 BPM25114 20000 0.9 44 1.5 v? 8-10(v?) P = 2.84 days? H 0.9 18 no obs 25 6, 12 G240-72 1748+70 6000 0.7 59 -1.5 Broad depression in spectrum no ident 1.1 16 2.5 9, 13 G227-35 1829+54 7000 0.9 43 -3 no ident 0.9 20 <.2 14 Grw+70°8247 1900+70 12000 0.9 49 -4 50? Unidentified weak lines no ident 0.9 10 4 >100? 4, 15 GD229 2010+31 22000 1.5 33 -1.5 Unidentified strong lines no ident 0.4 1.4 3 9, 16 Notes to Table 1 Table 1 lists the common name(s), WD number in the catalogue of McCook and Sion (1977), effective temperature Te based on available photometry or spectrophotometry, and chemical composition based on atomic or molecular species identified in the spectrum. R is the radius in units of /?o/100> derived from Te and a distance based on measured or photometric parallax. M is the mass in solar units from the mass-radius relationship for carbon cores. The variable otot is the total space motion, obtained by combining the observed transverse motion - and an assumed radial velocity of 27 km s *. Z?0(BH) is the minimum field to prevent accretion assuming Bondi-Hoyle type accretion (see § VI). Typical CP and typical LP are typical values of continuum circular and linear polarization (%) observed in blue hght; “no obs” indicates that no observations are available. Be and Bs are effective longitudinal and mean surface fields determined from modeling as described in the references given in the final column. References.—(1) Liebert et al: 1977; (2) Greenstein 1978; (3) Angel and Landstreet (1974); (4) Angel 1978; (5) Liebert, Angel, and Landstreet 1975; (6) Wickramasinghe and Martin 1979; (7) Angel et al. 1974a; (8) Angel, Illing, and Landstreet 1972; (9) Landstreet and Angel 1980; (10) Green et al. 1980; (11) Liebert et al. 1978; (12) Kemp 19776; (13) Angel et al. 19746; (14) Angel, Hintzen, and Landstreet 1975; (15) Landstreet and Angel 1975; (16) Landstreet and Angel 1974. our results show that detectable magnetic fields are The most extensively used search technique, because much less common in white dwarfs than in neutron of its value in detecting very strong fields, has been the stars, a fact which has to be considered in the wider measurement of continuum circular polarization. This context of the origin and evolution of stellar magnetism. effect of a magnetic field was first pointed out by Kemp Two techniques are generally used to search for mag- (1970). A discussion of the expected strength of polari- netic fields in white dwarfs. The more sensitive, which zation in white dwarf atmospheres is given below (§ II). can be applied to stars with strong atomic lines, is Essentially the circular polarization depends linearly on to attempt to detect the Zeeman effect. Searches for the mean longitudinal field over the disk of the star (the linear and quadratic Zeeman effects in DA stars have effective field) as been made by Angel and Landstreet (1970 a), Preston (1970), Trimble and Greenstein (1972), and Elias and Greenstein (1974). These give typical upper limits of 105 gauss using the linear Zeeman effect, and ~ 106 gauss using the quadratic effect. where Be is the mean field projected on the line of sight, © American Astronomical Society • Provided by the NASA Astrophysics Data System No. 3, 1981 MAGNETIC FIELDS OF WHITE DWARFS 459 V and I are the circular and total intensity Stokes communicating many of their new classifications in parameters, and y depends on the composition and advance of publication). Three objects are old planetary m temperature of the atmosphere and the wavelength of nuclei selected for radii estimated to be <0.01Ro fr° observation. The value of y may be calculated ap- the list given by Abell (1966). We included these in the 198lApJS...45..457A proximately. It may also be determined empirically for observing program because of the suggestion by stars with fields measured from the Zeeman effect. It is Fontaine, Thomas, and Van Horn (1973) that these found that y~5X108 gauss for \«0.5 fim, within a white dwarfs may be expected to be magnetic. factor of 2 or so dependent on composition and temper- Data were obtained using the 2.1 and 2.7 m telescopes ature. of McDonald Observatory, the 2.1 and 1.2 m tele- Two previous surveys by us, giving data to ~0.1% scopes of Kitt Peak National Observatory, the 2.3 m accuracy for 29 white dwarfs, have been published (Angel telescope of Steward Observatory, the 1.2 m telescope of and Landstreet 1970Z?; Landstreet and Angel 1971).
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