2004MNRAS.348L..13B

We present low resolution 10-µm spectra of the newly identified diffuse interstellar medium (ISM) sightlines towards StRS 217 and StRS 371 and a new spectrum of Cyg OB2 no. 12 obtained at the UK Infrared Telescope (UKIRT) with the Michelle imager/spectrometer. As previously, the µ Cephei emissivity better represents the profile of diffuse medium silicates than does the broader Trapezium emissivity which is characteristic of dust in molecular clouds. These spectra together with seven spectra published by Roche & Aitken comprise the majority of the silicates data set for the diffuse ISM. The observed sample includes sources in the first galactic quadrant at heliocentric distances of about 1.0-4.2 kpc. We find the silicate optical depth, c_sil =A_V/(19.2±0.6) and the ratio of c_silto heliocentric distance, d, is in the range 0.1 ≲c_sil/d ≲ 0.7 kpc^–1; for StRS 217 the ratio could be as high as c_sil/d ≲ 1.2 kpc^–1. However, this ratio is dominated by uncertainties in the distance estimates and does not indicate the degree of clumping in the diffuse interstellar medium.