Hot degenerates in the montreal-cambridge-tololo survey. II. Two new hybrid white dwarfs, MCT 0128-3846 and MCT 0453-2933 and the nature of the dab stars

Abstract

We report the discovery, in the course of a survey for high-latitude, ultraviolet-excess objects in the southern hemisphere, of two new hybrid DAB stars. Both objects exhibit a weak He I λ4471 line superposed onto the usual Balmer line spectrum. Analyses of the Balmer series with both homogeneous and stratified models show these stars to have spectroscopic effective temperatures around 27,000 K. However, low­ dispersion ultraviolet observations with the lU E reveal energy distributions considerably flatter than those of normal DA stars near that effective temperature. These energy distributions are inconsistent as well as those of stratified models calculated at the temperature and hydrogen-layer mass determined from the optical spectrum. We demonstrate instead that consistent fits to the optical and ultraviolet spectrum, as well as to the energy distribution, can be achieved if these objects are unresolved, composite systems consisting of a DA white dwarf together with a DB or DBA star. The DAB spectral class thus appears inhomogeneous, and may contain both composite systems and stratified stars.