The Binary Origins of Hot Subdwarfs: New Radial Velocities

Multiple radial velocity observations have been obtained for a large sample of local field subdwarf B (sdB) stars over a period of two years. SdB stars appear to fall into three distinct groups based on their kinematic and spectroscopic properties. The sdB's in Group I have no detectable spectral lines from a cool companion, and show only small or insignificant velocity variations. They represent about 35% of all sdB stars, which defines the upper limit for the total fraction of non-binary sdB's. Group II sdB's are single-lined spectroscopic binaries, comprising about 45% of the sample. Their spectra resemble those in the first group, but they have significant or large velocity variations and probable orbital periods on the order of a day. The third group contains the remaining 20% of sdB's, those showing additional spectral lines from a cool (FGK) main sequence or subgiant companion. All Group III sdB's have slowly varying or nearly constant velocities, indicating periods of many months to several years. Group II sdB's are obvious post-common envelope systems, in strong contrast to the wide binaries of Group III. Current data are insufficient to rule out the possibility that some sdB's in Group I might be analogs of Group III binaries, with undetectably faint companions. The clear division into three groups with such disparate properties suggests very different evolutionary histories even though the current physical states are essentially indistinguishable.