Masses and Radii of White Dwarfs

Additional model atmospheres representing the surface layers of hydrogen-rich and helium-rich white dwarfs have been calculated and are presented in tables. These model atmospheres and multicliannel photoelectric scans by Oke have been used to determine the masses of 19 and the radii of 26 stars in Oke's observing program. The median radius of the program stars, which are in general among the intrinsically brightest white dwarfs, is 0.013 Ro , and the median mass is 0.52 M0. The DA stars have effective temperatures between 6000 and 50,000 K, with N(He) < 0.008 by number. The DB stars have 12,000 < 18,000 K, and calcium abundances <0.01 of the solar value. The program stars are then used to calibrate a U - V versus T relation, and radii are determined for all white dwarfs with known distances. The median radius of this sample of stars, which contains more intrinsically faint stars than the set of program stars, is 0.0095 R0. The masses of some of these stars are found from surface gravities derived from the rn1-indices of Graham. The masses and radii found here are in satisfactory agreement with the mass-radius relation for degenerate stars and with measured gravitational redshifts. The above analyses were applied for the purpose of testing the convection-accretion hypothesis of Strittmatter and Wickramasinghe, which purports to explain the existence of the DB stars. The present results contradict their prediction that there should be a shortage of DA stars with effective temperatures between I 5,000 and I 8,000 K. I propose a modification of their hypothesis which explains the existence of DB and DC stars in the observed temperature ranges.