Garrod, R.T.; (2005) The chemistry of transient dense cores in interstellar clouds. Doctoral thesis , University of London.

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Abstract

I model the chemical effects on dark clouds of their being composed of small (<0.1 pc), transient (order of 1 Myr), dense cores (which are only observed with high-resolution interferometry) within which most of the clouds' mass resides, surrounded by a diffuse background gas. I investigate the chemical evolution of the cores, and the resultant chemical enhancement of the background gas by cyclical processing through core formation and dissipation. I approximate the MHD mechanism of Falleh Hartquist (2002), by which large transient density inhomogeneities may be produced by slow-mode waves in a cold plasma, into a multi-point 1-D chemical code. Molecular freeze-out onto dust grains and subsequent re-injection into the gas phase are switched on or off according to the attainment of a critical visual extinction at individual points. I explore a grid of parameter space in five physical and chemical variables. Among a number of conclusions, I find firstly that multi-point chemical codes are highly necessary for modelling dark cloud regions, due to the large chemical variation over time and space. I find that the core chemistry is young at all times, in keeping with observations, and that the abundance and spatial extent of several important species is significantly enhanced by the cycling process. Further to this, I construct (non-interacting) assemblies of such cores, producing convolved maps mimicking observational constraints, and present evidence that such collections of transient dense cores may reproduce the morphologies observed in both low- and high-resolution molecular line studies of dark clouds. I further modify the convolution method in a number of ways, using this mapping procedure to test hypotheses derived from the chemical analysis. Hence I obtain observational diagnostics for the determination of physical and chemical conditions in dark clouds.

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