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Physical conditions in star forming regions around S235

Gas density and temperature in star forming regions around Sh2-235 are derived from ammonia line observations. This information is used to evaluate formation scenarios and to determine evolutionary stages of the young embedded clusters S235 East1, S235 East2, and S235 Central. We also estimate the gas mass in the embedded clusters and its ratio to the stellar mass. S235 East1 appears to be less evolved than S235 East2 and S235 Central. In S235 East1 the molecular gas mass exceeds that in the other clusters. Also, this cluster is more embedded in the parent gas cloud than the other two. Comparison with a theoretical model shows that the formation of these three clusters could have been stimulated by the expansion of the Sh2-235 HII region (hereafter S235) via a collect-and-collapse process, provided the density in the surrounding gas exceeds $3\cdot10^3$ cm$^{-3}$, or via collapse of pre-existing clumps. The expansion 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 large-scale supernova shock. Thus, triggered star formation in the studied region may come in three varieties, namely collect-and-collapse and collapse of pre-existing clumps, both initiated by expansion of the local HII regions, and triggering by an external large-scale shock. We argue that the C235 A HII region expands into a highly non-uniform medium with increasing density. It is too young to trigger star formation in its vicinity by a collect-and-collapse process. There is an age spread inside the S235 A-B region. Massive stars in the S235 A-B region are considerably younger than lower mass stars in the same area. This follows from the estimates of their ages and the ages of associated HII regions.