A Radial Velocity Survey of the Cyg OB2 Association

We conducted a radial velocity survey of the Cyg OB2 association over a 6 yr (1999-2005) time interval to search for massive close binaries. During this time we obtained 1139 spectra on 146 OB stars to measure mean systemic radial velocities and radial velocity variations. We spectroscopically identify 73 new OB stars for the first time, the majority of which are likely to be association members. Spectroscopic evidence is also presented for a B3 Iae classification and temperature class variation (B3-B8) on the order of 1 yr for Cyg OB2 No. 12. Calculations of the initial mass function with the current spectroscopic sample yield Γ = -2.2 ± 0.1. Of the 120 stars with the most reliable data, 36 are probable and 9 are possible single-lined spectroscopic binaries. We also identify three new and eight candidate double-lined spectroscopic binaries. These data imply a lower limit on the massive binary fraction of 30%-42%. The calculated velocity dispersion for Cyg OB2 is 2.44 ± 0.07 km s-1, which is typical of open clusters. No runaway OB stars were found.