A simple estimate of the effect of a thick circumstellar dust shell on photospheric line profiles

Abstract

A star embedded in an optically thick dust shell may have a line spectrum that closely resembles the spectrum of some unobscured star - e.g., VY CMa (shrouded) and alpha Her (unshrouded). This implies that the surface temperatures of the two stars are nearly equal, but since one is subject to a back-warming effect, the distributions of temperature with depth in the respective atmospheres are dissimilar. We find that effective temperatures and absorption line strengths are reduced in shrouded stars below the values that obtain in their unshrouded counterparts. For models of VY CMa that yield grain temperatures at the inner boundary of the dust shell consistent with observation, the reduction in strength of a visible line is small (not more than 15%). Thus the spectral distribution of alpha Her may be used as an approximation to that of VY CMa before it is scattered by the dust, without the necessity of first correcting for back warming.