The extinction of HD 200775 by dust in NGC 7023.

Abstract

The extinction properties of the Be shell star HD 200775 caused by dust in the bright reflection nebula NGC 7023 are discussed on the basis of direct observations of HD 200775. Reddening curves obtained between 5500 and 6800 A by a Reticon linear array are found to differ from any simple curve, lacking diffuse interstellar features except at 6284 and possibly 5780 A and exhibiting a very broad band structure. IUE spectra between wavelengths of 1250 and 3200 A reveal a UV reddening curve in which the 2175-A absorption is weak and the 1540-A feature is absent, although a rapid rise typical of the average extinction curve is present at wavenumbers greater than 6/micron. Comparison of the extinction curve of HD 200775 with that of the shell star Theta 1 Orionis reveals the form of the diffuse absorption bands at 2175 and 1540 A. A three-component dust model consisting of fine grains accounting for UV absorption, large grains for the optical and a volatile mantle responsible for the diffuse absorption features and broadband structure is suggested to account for the observations.