An investigation of the interstellar extinction - II. Towards the mid-infrared sources in the Galactic centre

The depth of the 9.7-μm 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 AV/τ9.7 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-μm absorption feature are formed in oxygen-rich environments.