HII Bubbles and Shocks in Molecular Clouds

Abstract

Supersonic gas velocities and large density gradients are observed in compact HII regions. Possible causes of such flows are examined analytically. It is shown that density gradients in either the HII region or the exterior molecular cloud can naturally give supersonic gas speeds. Cases discussed include the swelling of uniform HII bubbles into clouds with decreasing densities, isothermal shock propagation within the ionized region, and homologous expansion of non-uniform HII bubbles. Supersonic flows are readily generated under reasonable conditions. Thus the observed supersonic motions and density gradients in compact HII regions may be intimately related.