The True Incidence of Magnetism Among Field White Dwarfs

We study the incidence of magnetism in white dwarfs from three large andwell-observed samples of hot, cool, and nearby white dwarfs in order to testwhether the fraction of magnetic degenerates is biased, and whether it varieswith effective temperature, cooling age, or distance. The magnetic fraction isconsiderably higher for the cool sample of Bergeron, Ruiz, and Leggett, and theHolberg, Oswalt, and Sion sample of local white dwarfs that it is for thegenerally-hotter white dwarfs of the Palomar Green Survey. We show that themean mass of magnetic white dwarfs in this survey is 0.93 solar masses or more,so there may be a strong bias against their selection in the magnitude-limitedPalomar Green Survey. We argue that this bias is not as important in thesamples of cool and nearby white dwarfs. However, this bias may not account forall of the difference in the magnetic fractions of these samples. It is not clear that the magnetic white dwarfs in the cool and local samplesare drawn from the same population as the hotter PG stars. In particular, twoor threee of the cool sample are low-mass white dwarfs in unresolved binarysystems. Moreover, there is a suggestion from the local sample that thefractional incidence may increase with decreasing temperature, luminosity,and/or cooling age. Overall, the true incidence of magnetism at the level of 2megagauss or greater is at least 10%, and could be higher. Limited studiescapable of detecting lower field strengths down to 10 kilogauss suggest byimplication that the total fraction may be substantially higher than 10%.Comment: 16 pages, 2 figures, Astronomical Journal in press -- Jan 2003 issu