The True Incidence of Magnetism Among Field White Dwarfs

We study the incidence of magnetism in white dwarfs from three large and well-observed samples of hot, cool, and nearby white dwarfs in order to test whether the fraction of magnetic degenerates is biased and whether it varies with effective temperature, cooling age, or distance. The magnetic fraction is considerably higher for the cool sample of Bergeron, Ruiz, & Leggett and the Holberg, Oswalt, & Sion sample of local white dwarfs than it is for the generally hotter white dwarfs of the Palomar-Green survey. We show that the mean mass of magnetic white dwarfs in this survey is 0.93 M_⊙ or more, so there may be a strong bias against their selection in the magnitude-limited Palomar-Green survey. We argue that this bias is not as important in the samples of cool and nearby white dwarfs. However, this bias may not account for all of the difference in the magnetic fractions of these samples. It is not clear that the magnetic white dwarfs in the cool and local samples are drawn from the same population as the hotter PG stars. In particular, two or three of the cool sample are low-mass white dwarfs in unresolved binary systems. Moreover, there is a suggestion from the local sample that the fractional incidence may increase with decreasing temperature, luminosity, and/or cooling age. Overall, the true incidence of magnetism at the level of ∼2 MG or greater is at least ∼10%, and it could be higher. Limited studies capable of detecting lower field strengths down to ∼10 kG suggest by implication that the total fraction may be substantially higher then 10%.