The Binary Frequency of the OB Runaway Stars.

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Scientific paper

A radial velocity survey of the bright, northern OB runaway stars has been undertaken to determine the frequency of binary stars in this high velocity group. A total of 634 high dispersion spectrograms of 36 proposed runaway stars were obtained over a two year period. Approximately half of the stars are velocity variable; these include 7 spectroscopic binaries, 1 possible (beta) Cep variable, and 10 stars with emission lines in their spectra. The latter group contains 7 of the 8 Be stars observed, and 3 luminous O stars, and we argue that their variability is caused by non-radial pulsation. Fifteen of the program stars have a peculiar radial velocity (i.e. heliocentric radial velocity corrected for solar motion and differential galactic rotation) greater than 30 km s('-1); 5 others are probable runaways on the basis of distance from the galactic plane and proper motion. Only 2 of the confirmed and probable runaways, HD 3950 and HD 198846 (Y Cyg), are binaries and both are double-lined systems. New orbital elements are presented for HD 3950 and the 5 new binary systems found among the low velocity stars. The stellar wind characteristics, interstellar features, and cluster or association membership of many runaways, and the masses derived from the two runaway binaries, suggest that only a small proportion of the group could be evolved, low mass stars. If the runaways were accelerated by a supernova explosion in a close binary system, they would probably have neutron star companions, and would eventually become massive X-ray binaries (MXRB). However, the MXRBs have moderate velocities, and the observed runaway star radial velocity limits and available X-ray data rule out collapsed companions if their orbits are similar to those of the MXRBs. The kinematical ages indicate that the runaways were ejected from young associations soon after their birth. The higher fractional abundance of runaways among more massive stars, their binary frequency, and the properties of the runaway binaries suggest that they obtained their high velocities through single-binary, binary-binary, or small n-body close encounters during the early, dynamical evolution of associations.

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