Model of the expansion of H II region RCW 82

Abstract

This paper aims to resolve the problem of formation of young objects observed in the RCW 82 H {\small II} region. In the framework of a classical trigger model the estimated time of fragmentation is larger than the estimated age of the H {\small II} region. Thus the young objects could not have formed during the dynamical evolution of the H {\small II} region. We propose a new model that helps resolve this problem. This model suggests that the H {\small II} region RCW 82 is embedded in a cloud of limited size that is denser than the surrounding interstellar medium. According to this model, when the ionization--shock front leaves the cloud it causes the formation of an accelerating dense gas shell. In the accelerated shell, the effects of the Rayleigh--Taylor (R-T) instability dominate and the characteristic time of the growth of perturbations with the observed magnitude of about 3 pc is 0.14 Myr, which is less than the estimated age of the H {\small II} region. The total time $t_{\sum}$, which is the sum of the expansion time of the H {\small II} region to the edge of the cloud, the time of the R-T instability growth, and the free fall time, is estimated as $0.44<t_{\sum} < 0.78$ Myr. We conclude that the young objects in the H {\small II} region RCW 82 could be formed as a result of the R-T instability with subsequent fragmentation into large-scale condensations.