Dirty silicate models for hot-centred clouds

Models for spherically symmetric clouds of dirty silicate dust grains illuminated by a young hot OB star are presented and compared with the average infrared spectrum for hot-centred clouds. An excellent fit is found with a uniform density distribution across the cloud: one as steep as n(r) proportional to r-2 is ruled out. Models are given for a number of individual sources. For a class of source whose infrared spectra resemble that of the BN object in Orion (GL490, 989, 4176, 2591 and probably S140) a double-shell model is required to fit the infrared spectra: an outflowing shell of hot dust with an n(r) proportional to r-2 density distribution is surrounded by a second shell of cold dust, presumably associated with the ambient cloud of dust and molecules from which the OB star has formed. For an expansion velocity of 25 km s-1, the total mass loss rates (in gas) are estimated to be in the range 0.6-10 x 10-7 solar masses yr-1. For the remaining sources modelled (W3OH, GL961, GL4182, W75N) a uniform density distribution gives a good fit to the observed spectra. The idea that the 3.1 μ 'ice' feature could be due to organic molecules which are also responsible for the 10μ feature is untenable for the BN-type sources. (author)