Capture of field stars by molecular clouds

Abstract

Field stars can be captured by massive interstellar clouds. Stars encounter interstellar clouds with relative velocities that have a distribution. Close encounters in which stars enter a cloud with relative velocities less than the escape velocity results in gravitational capture of the stars by the cloud. Using typical values for the parameters characterizing the molecular clouds and the field stars of different types, the numbers of the captured stars are estimated, which are found to be significantly large to have observable consequences. When star formation in the cloud gives rise to the formation of a star cluster, it would contain two populations of stars: the young newly born stars and the older field stars that were captured by the cloud during its long lifetime prior to star formation. It is suggested that the very low mass stars on the main sequence of the Pleiades cluster and evolved stars occupying anomalous positions on the color-magnitude diagrams of some other young clusters are captured field stars.