A search for binarity using Far-Ultraviolet Spectroscopic Explorer observations of DAO white dwarfs

We report on a search for evidence of binarity in Far-Ultraviolet Spectroscopic Explorer (FUSE) observations of DAO white dwarfs. Spectra recorded by FUSE are built up from a number of separate exposures. Observation of changes in the position of photospheric heavy element absorption lines between exposures, with respect to the stationary interstellar medium lines, would reveal radial velocity changes - evidence of the presence of a binary system. This technique is successful in picking out all the white dwarfs already known to be binaries, which comprise five out of the sample of 16, but significant radial velocity shifts were found for only one additional star, Ton 320. This object is also known to have an infrared excess. DAOs can be separated broadly into low- or normal-mass objects. Low-mass white dwarfs can be formed as a result of binary evolution, but it has been suggested that the lower mass DAOs evolve as single stars from the extended horizontal branch, and we find no evidence of binarity for eight out of the 12 white dwarfs with relatively low mass. The existence of higher mass DAOs can also be explained if they are within binary systems, but of the four higher mass stars in the sample studied, PG 1210+533 and LB 2 do not exhibit significant radial velocity shifts, although there were only two exposures for the former object, and the latter has an infrared excess.