An investigation of the interstellar extinction – I. Towards dusty WC Wolf-Rayet stars

Abstract

The 10 μm extinction towards six WC8 or WC9 Wolf-Rayet stars is investigated. All objects show smooth dust emission suffering silicate absorption with depths well correlated with the extinction in the visible. The de-reddened spectra are well represented by emission from featureless grain components, possibly from iron or carbon grains. The extinction to the stars is found to be dominantly interstellar in origin with little extinction from the circumstellar shell. The shape of the silicate absorption is far better represented by a curve derived from the circumstellar dust shell of the M supergiant μ Cep than by that derived from the Trapezium in Orion. Combining these results with data on two reddened B supergiants in Cygnus gives a ratio of |$A_\text v/\tau_{9.7}=18.5$| using the μ Cep curve, and indicates that this ratio holds over at least two directions in the galactic plane and within the local 3 kpc. The shape of the 8–13 μm extinction in regions of star formation and molecular clouds may be different in detail to that found here.