Models of Infrared Emission from Axially Symmetric HII Region/Molecular Cloud Complexes

D. W. Walker

Queen Mary College

Mile End Road

London E1 4NS

U. K.

Abstract

Numerical models of dusty HII region/molecular cloud complexes that possess axial, rather than spherical, symmetry are described. The objective is to understand, by these models, the role axial geometry plays in explaining the infrared continuum emission from dust in such complexes, and to compare the model results with observations. The validity of such models may thus be checked, and their importance in understanding the process of star formation assessed.

We consider a star situated at some arbitrary point within, or near to, a static, spherical cloud of dust and hydrogen gas. Three different types of density distribution are considered, all of which are spherically symmetric about the cloud centre. The exciting star is assumed to radiate as a black-body, and values of the effective temperature and stellar radius are chosen to represent ZAMS stars ranging in spectral type from B0 to O4. The dust grains are assumed to be all of the same size and chemical composition, and the dust-to-gas ratio is constant. The wavelength form of the absorption efficiency of the dust is taken to be a power-law above a certain critical wavelength, and is unity below this wavelength.

To render the problem tractable two important assumptions are made. Firstly, scattering by the dust is ignored, and secondly, it is assumed that the dust is heated only by stellar photons, and not by the infrared emission of the dust itself. The validity of this latter assumption is investigated.

Models in which the energy source produces no ionisation are also considered. Such models are applicable to cases in which the energy source is a ZAMS star of spectral type later than B5, or a protostellar object.

In comparing the models with observation the computed surface brightness distribution is integrated over an appropriate wavelength range and convolved with a Gaussian beam in order to make comparison with broad-band observations more meaningful.

It was found that the morphology of the model HII regions varies considerably depending on the parameters, and that the infrared surface brightness distribution and the shape of the energy spectrum are sensitive to the orientation of the observer to the cloud. In certain cases the position of the peak emission was found to vary with wavelength.

By comparing model results with far-infrared observations of Cep B it was found that the O7V star HD217806 cannot be the primary heat source for Cep B. The statistical applications of the model are demonstrated by considering a simple sky model, and the expected number of detectable sources in each of the four wavelength bands of the Infrared Astronomical Satellite (IRAS) are evaluated.

D. W. Walker, Models of Infrared Emission from Axially Symmetric HII Region/Molecular Cloud Complexes, Ph.D. thesis, Queen Mary College, University of London, May 1983.