Physical conditions in star-forming regions around s235

Abstract

Gas density and temperature in star-forming regions around Sh2-235 are derived from ammonialine observations. This information is used to evaluate formation scenarios and to determineevolutionary stages of the young embedded clusters S235 East 1, S235 East 2 and S235 Central.We also estimate the gas mass in the embedded clusters and its ratio to the stellar mass.S235 East 1 appears to be less evolved than S235 East 2 and S235 Central. In S235 East 1 themolecular gas mass exceeds that in the other clusters. Also, this cluster is more embeddedin the parent gas cloud than the other two. Comparison with a theoretical model shows thatthe formation of these three clusters could have been stimulated by the expansion of theSh2-235 HII region (hereafter S235) via a collect-and-collapse process, provided the densityin the surrounding gas exceeds 3 × 103 cm-3, or via collapse of pre-existing clumps. Theexpansion of S235 cannot be responsible for star formation in the southern S235 A-B region.However, formation of the massive stars in this region might have been triggered by a largescalesupernova shock. Thus, triggered star formation in the studied region may come in threevarieties, namely collect-and-collapse and collapse of pre-existing clumps, both initiated byexpansion of the localHII regions, and triggered by an external large-scale shock.We argue thatthe S235A HII region expands into a highly non-uniform medium with increasing density. It istoo young to trigger star formation in its vicinity by a collect-and-collapse process. There is anage spread inside the S235 A-B region. Massive stars in the S235 A-B region are considerablyyounger than lower mass stars in the same area. This follows from the estimates of their agesand the ages of associated HII regions. © 2013 The Authors