Velocity Dispersion of Dissolving OB Associations Affected by External Pressure of Formation Environment

This paper presents a possible way to understand dissolution of OB associations (or groups). Assuming rapid escape of parental cloud gas from associations, we show that the shadow of the formation environment for associations can be partially imprinted on the velocity dispersion at their dissolution. This conclusion is not surprising as long as associations are formed in a multiphase interstellar medium, because the external pressure should suppress expansion caused by the internal motion of the parental clouds. Our model predicts a few km s as the internal velocity dispersion. Observationally, the internal velocity dispersion is ∼ 1 km s which is smaller than our prediction. This suggests that the dissipation of internal energy happens before the formation of OB associations. Subject headings: open clusters and associations: general — ISM: clouds — stars: formation — turbulence