The Dynamics of Low-Mass Molecular Clouds in External Radiation Fields

We present the results of three-dimensional hydrodynamic calculations of the evolution of low-mass molecular clouds, performed using the numerical method of smoothed particle hydrodynamics. The clouds that we consider are subject to heating by the interstellar radiation field and by cosmic rays. They are able to cool through molecular line emission (primarily CO and its isotopes) and by emission from the fine structure lines of C⁺ and O I. We also include gas-dust thermal coupling in our models. A simplified chemical network is incorporated that models the conversion between C⁺ and CO, where the chemical balance is determined by the local flux of dissociating radiation. Calculations are performed for initially uniform density clouds, with masses in the range M = 100-400 M_⊙, sizes in the range R = 1.7-3.4 pc, with the initial number density in all cases being n = 100 cm^-3. We performed calculations for clouds with different geometrical shapes: spherical, prolate, and oblate. Additionally, we considered the effects of an anisotropic radiation field on the cloud evolution.

These are the main results:

1. Clouds that are initially Jeans stable are able to collapse because of the coupling between the dynamical and thermal evolution. This collapse results in core-halo structure where we have a cold, dense, CO core surrounded by a warmer, tenuous, C⁺ envelope.

2. A pressure gradient is set up in the clouds by the attenuation of the UV radiation field. When a cloud is anisotropically heated, this pressure gradient leads to the formation of a highly flattened cloud core when it collapses.

3. The combined thermal and dynamical evolution of the prolate and oblate clouds leads to the formation of highly elongated or flattened structures. These structures are able to fragment, typically with four to eight subcondensations forming, which have masses in the range 3-7.5 M_⊙.