Far and mid infrared observations of two ultracompact H_II regions and one compact CO clump

Abstract

Two ultracompact H II regions (IRAS 19181+1349 and 20178+4046) and one compact molecular clump (20286+4105) have been observed at far infrared wavelengths using the TIFR 1 m balloon-borne telescope and at mid infrared wavelengths using ISO. Far infrared observations have been made simultaneously in two bands with effective wavelengths of ~150 and ~210 µm, using liquid ³He cooled bolometer arrays. ISO observations have been made in seven spectral bands using the ISOCAM instrument; four of these bands cover the emission from Polycyclic Aromatic Hydrocarbon (PAH) molecules. In addition, IRAS survey data for these sources in the four IRAS bands have been processed using the HIRES routine. In the high resolution mid infrared maps as well as far infrared maps multiple embedded energy sources have been resolved. There are structural similarities between the images in the mid infrared and the large scale maps in the far infrared bands, despite very different angular resolutions of the two. Dust temperature and optical depth ( τ₁₅₀) maps have also been generated using the data from balloon-borne observations. Spectral energy distributions (SEDs) for these sources have been constructed by combining the data from all these observations. Radiation transfer calculations have been made to understand these SEDs. Parameters for the dust envelopes in these sources have been derived by fitting the observed SEDs. In particular, it has been found that the radial density distribution for three sources is different. Whereas in the case of IRAS 20178+4046 a steep distribution of the form r^-2 is favoured, for IRAS 20286+4105 it is r^-1 and for IRAS 19181+1349 it is a uniform distribution ( r⁰). Line ratios for PAH bands have generally been found to be similar to those for other compact H II regions but different to general H II regions.