Fragmentation of Primordial Filamentary Clouds under Far-Ultraviolet Radiation

Abstract

Collapse and fragmentation of uniform filamentary clouds under isotropic far-ultraviolet external radiation are investigated. Especially, impact of photodissociation of hydrogen molecules during collapse is investigated. Dynamical and thermal evolution of collapsing filamentary clouds are calculated by solving virial equation and energy equation with taking into accounts non-equilibrium chemical reactions. It is found that thermal evolution is hardly affected by the external radiation if the initial density is high ($n_0 > 10^2 \mathrm{cm^{-3}}$). On the other hand, if line mass of the filamentary cloud is moderate and initial density is low ($n_0 \le 10^2 \mathrm{cm^{-3}}$), thermal evolution of the filamentary cloud tends to be adiabatic owing to the effect of the external dissociation radiation. In this case, 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. Analytic criterion for the filamentary clouds to fragment into such massive clouds is discussed. We also investigate 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, the filamentary clouds with low initial density ($n_0 \le 10^2 \mathrm{cm^{-3}}$) fragment into massive clouds ($\sim 10^4 M_\odot$) owing to the effect of the external radiation.