A spectrophotometric study of some cool white dwarfs.

Abstract

Spectrophotometry from the Hale reflector and the IUE of four cool (approximately 7000 K) white dwarfs having large parallaxes is reported. Practically the entire flux is measured, together with the bolometric luminosity, temperature and radius. Stein 2051 is found to be a DC, with an astrometric binary companion of type dM4. The helium-dominated atmosphere of the DC exhibits no metallic lines in the ultraviolet. From the mass-radius relation, the radius of 0.011 earth radius gives a mass of 0.68 solar mass (light-element core) or 0.47 solar mass (iron core); astrometric solutions correspond to either 0.50 or 0.72 solar mass. Ross 627, a DA, exhibits no metals in the ultraviolet. The radius is put between 0.010 and 0.013 earth radius. The star L870-2, a DA, has the best established radius for a white dwarf (unless it itself is an unresolved binary). The radius, which is between 0.016 and 0.020 earth radius, corresponds to a maximum of 0.36 solar mass and minimum of 0.20 solar mass, for a helium core. An 8-hr exposure to investigate the near-ultraviolet spectrum of vMa2 is only partially successful. Within 500 A, the enormously strong line blanketing, probably by Mg II, Mg I and Fe II, reduces the flux by a factor of 100.