Interstellar extinction toward the Cas OB6 association: Where is the dust?

Margaret Murray Hanson, University of Colorado Boulder
Geoffrey C. Clayton, University of Colorado Boulder


We have completed a multiband (ultraviolet, optical, and near-infrared) study of the interstellar extinction properties of nine massive stars in IC 1805 and IC 1848, which are both part of Cas OB6 in the Perseus spiral arm. Our analysis includes determination of absolute extension over the wavelength range from 3 μm to 1250 Å. We have attempted to distinguish between foreground dust and dust local to Cas OB6. This is done by quantitatively comparing extinction laws of the least reddened sightlines (sampling mostly foreground dust) versus the most reddened sightlines (sampling a larger fraction of the dust in the Cas OB6 region). We have combined previous investigations to better understand the evolution of the interstellar medium in this active star forming region. We found no variation the extinction curve behavior between moderately reddened and heavily reddened Cas OB6 stars. None the the curves show any significant deviation from the Cardelli-Clayton-Mathis (CCM) Rv-dependent extinction. They are all consistent with that seen from diffuse dust. Most or all of the dust along the line of right may be foreground to Cas OB6. Massive star forming regions can show significant deviations from CCM behavior which have been attributed to processing of the dust grains. Any dust local to the association must exist far from the hot stars in IC 1805 and IC 1848. A previous episode of star formation may have already cleared out the region of most of the gas and dust. Evidence for this can be seen in H I and IRAS data of the region.