Two hot, low-field magnetic DA white dwarfs.

Abstract

The authors report the discovery of two hot DA white dwarf stars from the Palomar Green Survey, one of which is proved to be, and the other likely to be, a star with surface magnetic field in excess of 10⁶ gauss. PG 1658+441 has an energy distribution fitted by a 30,000 K pure hydrogen atmospheric model. Circular spectropolarimetry revealed longitudinal Zeeman components to the three lowest Balmer lines, with an indicated longitudinal field strength (B_e) at ≡0.7 megagauss. Moreover, the high-quality intensity spectra showed apparent splitting of Hβ and Hα into triplets by a mean surface field (B_s) of approximately 2.3 megagauss. PG 0136+251 is a hotter star with weaker lines; its energy distribution implies a 40,000-50,000 K hydrogen-rich atmosphere. Spectropolarimetry revealed unconvincing evidence for magnetic line splitting. However, the anomalously large widths of the Balmer line cores suggest B_s ≥ 1.3 megagauss. It is argued that two other possible broadening agents - an abnormally high surface gravity or very rapid rotation - cannot account for the shapes of its Balmer lines.