Gravitational redshift and mass-radius relation in white dwarfs

This paper presents gravitational redshifts obtained from observations of H-alpha in nine DA white dwarfs, which are members of wide binaries or common proper motion pairs. Within the error bars, the data follow the theoretical zero-temperature relation for the pure carbon models of Hamada and Salpeter (1961). Due to the remaining relatively large errors in radii and the intrinsically narrow mass distribution of white dwarfs around a mean solar mass of 0.58, the results do not provide an empirical proof for the shape of the relation. Assuming the validity of this relation, it is possible to derive very accurate masses. Although the sample is small, it confirms results of other studies using larger, but less well-observed, samples. The shifts obtained for CD-38 deg 10980 are different from those obtained from the sharp ultraviolet features and indicate that they probably do not originate in the photosphere. Line shifts were also determined for two DB stars. The lines, however, are blueshifted due to the dominating influence of pressure broadening. The present state of the theory of Stark shifts in helium lines precludes an interpretation in terms of gravitational redshifts.