IUE and Voyager observations of very hot O-type subdwarfs

摘要

Twelve newly-discovered subdwarf O stars were observed with IUE and complementary observations of two of these were obtained with Voyager 1. It was found that these stars all have effective temperatures of at least 60,000 K and that they occupy a region in the H-R diagram between the hottest white dwarfs and the central stars of planetary nebulae, i.e., that they are the hottest known O-type subdwarfs. Evidence that the abundances of H, He, C, N, and possibly Fe vary widely from star to star is presented. It is believed that these stars represent the state of evolution immediately preceding the white dwarf state, and that the abundance differences are due to diffusion and convective mixing in surface layers.