Astronomical instrumentation

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.