Fragmentation of Primordial Filamentary Clouds under Far-Ultraviolet Radiation

Abstract

The collapse and fragmentation of uniform filamentary clouds under isotropic far-ultraviolet external radiation are investigated. Especially, the impact of the photodissociation of hydrogen molecules during collapse is considered. The dynamical and thermal evolutions of collapsing filamentary clouds are calculated by solving the virial equation and the energy equation while taking into account non-equilibrium chemical reactions. It is found that thermal evolution is hardly affected by external radiation if the initial density is high ($ n_0$$ \gt$ 10$ ^{2}$ cm$ ^{-3}$ ). On the other hand, if the line mass of the filamentary cloud is moderate and the initial density is low ($ n_0$$ \le$ 10$ ^{2}$ cm$ ^{-3}$ ), the thermal evolution of the filamentary cloud tends to be adiabatic, owing to the effect of external dissociation radiation. In this case, the collapse of the filamentary cloud is suppressed, and the filamentary cloud fragments into very massive clouds ($ \sim$ 10$ ^{4-5}\ M_\odot$ ) in the early stage of collapse. The analytic criterion for the filamentary clouds to fragment into such massive clouds is discussed. We also investigate the collapse and fragmentation of the filamentary clouds with an improved model. This model can partly capture the effect of run-away collapse. Also, in this model filamentary clouds with low initial density ($ n_0$$ \le$ 10$ ^{2}$ cm$ ^{-3}$ ) fragment into massive clouds ($ \sim$ 10$ ^{4}\ M_\odot$ ) owing to the effect of external radiation.