Kinematics and binaries in young stellar aggregates

Abstract

Internal kinematics, spectroscopic binaries and galactic motion are investigated for the trapezium system BD+00.1617 (which lies at the heart of the young open cluster Bochum 2) by means on 73 high resolution Echelle+CCD spectra secured over the period 1994-98. Two of the three O-type member stars are found to be binaries on close and highly eccentric orbits of 6.8 and 11.0 day period. The spectra of the two binaries show large variations in the half-intensity and equivalent widths of the HeI absorption lines, which are intrinsic to the primaries and are not correlated to the orbital phase. Astrometric and radial velocities exclude that one of the component is leaving as a runaway star, but upper limits are still compatible with the trapezium evaporating at very low relative velocities. The projected rotational velocities of the three constituent O--type stars are low. This conforms to expectations from the high frequency of binaries. The observed radial velocity of Bochum 2 agrees with the Hron (1987) expression for the Galaxy rotation inside the latter's quoted errors.