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Abstract:

The depth of the 9.7-microns silicate absorption feature towards several of the mid-infrared sources within 2 pc of the center of the Galaxy has been measured. The ratio of the depth of the silicate absorption to the extinction at shorter wavelengths is found to be about twice that determined for sources in the solar neighborhood. The most plausible explanation for this difference is that in the central regions of the Galaxy, there are very few carbon stars, and so the production of carbon-rich dust grains will be much reduced compared with the outer Galactic disc. That the A(V)/tau(9.7) ratio depends on the ratio of C to M-type stars lends support to those models of interstellar dust in which carbon-rich grain materials are responsible for much of the extinction in the visible and near-infrared, whilst those that produce the 9.7-micron absorption feature are formed in oxygen-rich environments.

Abstract:

Spectropolarimetric observations between 8 - 13 μm of BN, IRc 2, 3, 4 and a further region in the Orion infrared complex BNKL are presented. Apart from IRc 2, all show evidence for absorption by aligned grains and have polarization position angles directed at a region close to IRc 2, a property also shared by the self-luminous source IRc 9 at short wavelengths. The evidence strongly suggests that grains are aligned by the intrinsic angular momentum of photons from IRc 2 and it is shown that this is a quantitatively plausible mechanism for the physical conditions in this part of Orion.

Abstract:

The low-resolution spectra of six Seyfert nuclei and the quasar 3C345 are presented, together with four more starburst nuclei spectra observed in a companion study. The spectra were obtained using the UCL LH-cooled grating spectrometer with an array of 25 As:Si photoconductors in a low resolution mode covering a wavelength range of 7.7-13.3 microns. It is shown that the active galaxies are characterized by power law spectra with a tendency for the spectral index to steepen with the progression quasar-Seyfert 1-Seyfert 2 and with silicate absorption in Seyfert 2 galaxies. In contrast, the starburst galaxy spectra are dominated by emission from unidentified dust-associated features and frequently show the fine structure forbidden line of Ne II. The possible origins for the unidentified features in the starburst galaxy spectra are discussed.