An investigation of the interstellar extinction

The depth of the 9.7-μm silicate absorption feature towards several of the mid-infrared sources within 2 pc of the centre 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 neighbourhood. 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. (author)