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

This paper presents a possible way to understand dissolution of OBassociations (or groups). Assuming rapid escape of parental cloud gas fromassociations, we show that the shadow of the formation environment forassociations can be partially imprinted on the velocity dispersion at theirdissolution. This conclusion is not surprising as long as associations areformed in a multiphase interstellar medium, because the external pressureshould suppress expansion caused by the internal motion of the parental clouds.Our model predicts a few km s$^{-1}$ as the internal velocity dispersion.Observationally, the internal velocity dispersion is $\sim 1$ km s$^{-1}$ whichis smaller than our prediction. This suggests that the dissipation of internalenergy happens before the formation of OB associations.Comment: 6 pages. AJ accepte