Star formation threshold

Interesting variants on the simplest star formation laws include stochastic self-propagating star formation (Gerola Seiden 1978 Dopita 1985), self-regulating star formation (Arimoto 1989 Hensler Burkert 1990), stochastic star-formation bursts (Matteucci Tosi 1985), separate laws for the halo and disk, the latter including terms that account for cloud collisions and induced star formation from interactions between massive stars and clouds (Ferrini et al. 1992, 1994), and the existence of a threshold surface gas density for star formation (Kennicutt 1989 Chamcham, Pitts Tayler 1993). 

Figure 1. Predicted star formation rate in the halo-thick disk phase and in the thin disk phase during the evolution of the Milky Way. Notice that because of the existence of a threshold in the gas density the SFR halts in between the two major episodes of infall and oscillates in the last phases of the evolution of the disk. The models are from Chiappini et al. (2001).

The excellent imaging capabilities of the Herschel space telescope have revealed, with unprecedented detail, the structure of molecular clouds and the regions where dense cores are formed. It is now clear that dense cores form within the ubiquitous filamentary structure of interstellar clouds, as localized density and column density maxima. There is a threshold in gas colunm density for dense core formation, estimated to be at extinctions of seven magnitudes from the analysis of either deep extinction images [15] or dust sub-millimetre emission maps [16], With such a threshold, the dense cores will be commOTily well shielded from the far ultra violet (FUV) radiation, with some exceptions in massive star forming regions or in photodissociation regions (PDRs) where dense molecular gas becomes directly exposed to FUV radiation. 

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