Expanding shells in low and high density environments

Abstract

The gravitational instability of expanding shells evolving in a homogeneous and static medium is discussed. In the low density environment (n = 1 cm−3), the fragmentation starts in shells with diameters of a few 100 pc and fragment masses are in the range of 5 × 103 − 106 M ⊙. In the high density environment (n = 10 5 − 107 cm−3), shells fragment at diameters of ∼ 1 pc producing clumps of stellar masses. The mass spectrum in both environments is approximated by a power law dN/dm ∼ m−2.3. This is close to the slope of the stellar IMF. To reproduce the observed mass spectrum of clouds (the spectral index close to ∼ −2.0) we have to assume, that the cloud formation time is independent of the cloud size, similarly to the Jeans unstable medium.