On the Thermal Instability in a Contracting Cloud and Formation of a Bound Cluster

We perform linear analysis of thermal instability in a contracting large cloud filled with warm H I gas and investigate the effect of metallicity and radiation flux. When the cloud reaches critical density nf, the cloud fragments into cool, dense condensations because of thermal instability. For a lower metallicity gas cloud, the value of nf is high. Collision between condensations will produce self-gravitating clumps and stars thereafter. From the result of the calculations, we suggest that high star formation efficiency and bound cluster formation are realized in low-metallicity and/or strong-radiation environments.