On the evolution of irradiated turbulent clouds: a comparative study between modes of triggered star formation

Abstract

Gas within molecular clouds (MCs) is turbulent and unevenly distributed. Interstellar shocks such as those driven by strong fluxes of ionizing radiation (IR) profoundly affect MCs. While small dense MCs exposed to a strong flux of IR have been shown to implode due to radiation-driven shocks, a phenomenon called radiation-driven implosion, larger MCs, however, are likely to survive this flux, which, in fact, may produce new star-forming sites within these clouds. Here we examine this hypothesis using the smoothed particle hydrodynamics algorithm coupled with a ray-tracing scheme that calculates the position of the ionization front at each time-step. We present results from simulations performed for three choices of IR flux spanning the range of fluxes emitted by a typical B-type star to a cluster of OB-type stars. The extent of photoablation, of course, depends on the strength of the incident flux and a strong flux of IR severely ablates an MC. Consequently, the first star formation sites appear in the dense shocked layer along the edges of the irradiated cloud. Radiation-induced turbulence readily generates dense filamentary structure within the photoablated cloud although several new star-forming sites also appear in some of the densest regions at the junctions of these filaments. Prevalent physical conditions within an MC play a crucial role in determining the mode, i.e. filamentary as compared to isolated pockets, of star formation, the time-scale on which stars form and the distribution of stellar masses. The probability distribution functions derived for irradiated clouds in this study are intriguing due to their resemblance with those presented in a recent census of irradiated MCs. Furthermore, irrespective of the nature of turbulence, the protostellar mass functions(MFs) derived in this study follow a power-law distribution. When turbulence within the cloud is driven by a relatively strong flux of IR such as that emitted by a massive O-type star or a cluster of such stars, the MF approaches the canonical form due to Salpeter and even turns over for protostellar masses smaller than ∼0.2 M⊙.