Department of Physics, Engineering Physics & Astronomy

Department of Physics, Engineering Physics & Astronomy
Department of Physics, Engineering Physics & Astronomy

A Clumpy Model for Photo Dissociation Regions: Effects of Metallicity

S. Jeyakumar
Universität Koeln, Germany

Tuesday, August 27, 2002
10:00 AM @ Stirling 201

Abstract:

Most of the mass of the Inter-Stellar Medium (ISM) is in the form of cold neutral molecular clouds. These clouds are the sites of star formation. The UV radiation from hot stars heat the nearby molecular gas establishing a region called Photon Dominated Regions (PDRs), where the physical and the chemical processes are dominated by the UV field. The UV radiation not only heats the molecular gas but also regulates the star formation process itself.

Moreover in low metallicity systems, such as LMC, SMC and dwarf galaxies, a reduced number of dust grains and heavy elements per hydrogen atom, plays a significant role on the chemistry and heating of the gas in PDRs. The UV radiation penetrates much deeper into the medium dissociating CO and producing a large layer of CI and CII. Observations show that the ratio of [CII]-158 micron line emission to the CO rotational line emission is much higher for dwarf galaxies than the Milky Way. Thus making CII as the tracer of molecular hydrogen in these systems rather than CO. We study the effects of reduced metallicity on the temperature and chemical structure of PDRs.

Additionally, the models of PDRs were traditionally treated in a semi-infinite plane parallel geometry where the medium is heated by the UV field from one direction. However, many observational evidences suggest that molecular clouds are clumpy. The clump spectrum analysis of the molecular clouds show a power law spectrum of mass. We present here a clumpy (PDR) model which takes into account the observed clump mass spectrum. We use this model to study the dependence of the important cooling lines of CII, CI and CO on metallicity. We also compare our results with the results obtained using a semi-analytical approach by Bolatto et al., (1999).