The effect of physical processes and the chemical feedback on the evolution of molecular clouds

How the prospect of star formation in molecular clouds is affected by different physical and chemical processes is still not clearly understood, although various theoretical models have considered some of the possible processes in the investigation of stellar evolution. Nevertheless, the roles of many very basic physical and chemical processes in the evolution of molecular clouds have not been fully evaluated. The main objective of the work presented in this thesis is to carry out the first quantitative investigation on the importance of chemical cooling, cosmic ray heating, gas-dust interaction and photo-electric heating by FUV radiation on cloud evolution.

First, the effect of micro-physical and chemical cooling on the evolution of molecular clouds are investigated by numerical simulations based on Smoothed Particle Hydrodynamics (SPH) involving various physical or chemical feedback mechanisms. Based on the classical concept of Jeans Mass, which is used to predict the potential for gravitational collapse of a molecular cloud at hydrostatic equilibrium, a newly defined quantity called Modified Jeans Mass (MJM) is used to describe the minimum mass needed for a cloud to collapse under a certain set of initial physical conditions. Comparison of MJMs can reveal the extent of the roles played by the micro-physical and chemical cooling processes in the evolution of molecular clouds.

A set of intensive numerical simulations using different combinations of the basic micro-physical processes and chemical feedback was designed. Various models of simulation which include different physical or chemical feedback mechanisms result in a range of MJMs, to isolate these effects from those influenced by externally imposed FUV radiation these models were run with zero FUV input. Comparison of the MJMs as well as following the evolution of the physical properties of molecular clouds enables a quantified description of the importance of the individual physical and chemical feedback processes, as well as combinations of these, on the prospect of star formation.

Secondly, the model which represented the conditions observed in nature, that is with all the microphysical and chemical processes activated, was subjected to FUV radiation and the effects of the resulting photo-electric heating on the cloud's evolution was tracked. This set of simulations studied the variation of MJMs resulting from molecular clouds with a range of initial densities being subjected to a range of different intensities of FUV radiation. It is found that a power law fitting can be applied to describe the behaviour of MJM vs initial density, i.e., M_MJM = a, n_i^b with a and b being different for different simulation models.

The detailed description for the evolution of the physical properties is presented for a representative molecular cloud in each set of simulations, and the general evolutionary features of all of the molecular clouds in the same set of simulation are summarised. The analysis on the calculated mass left in the condensed core formed when a cloud collapses, essentially the `seed' for potential star formation, is also performed. It can be concluded that: a) the different microphysical and chemical cooling processes do indeed contribute to the dynamic evolution of a molecular cloud toward star formation, although chemical cooling is the most important of these; b) the effect of FUV radiation on the evolution of molecular clouds depends strongly on the initial density and on the intensity of the FUV. For low density clouds the loss of material because of photo-evaporation is the major effect reducing the chances of star formation, in high density clouds shock propagation is the predominant effect promoting the formation of protostar seeds.