1996A&A...312..243V

The extinction curves arising from the combined effect of absorption and scattering of stellar radiation in dust shells around pre-main-sequence stars have been investigated. Monte Carlo simulations of the polarized radiation transfer in an inhomogeneous spheroidal shell have been used to determine the scattered radiation. The results are presented as the normalized curves for circumstellar extinction A(λ)-A(0.3µm) in the wavelength range from 0.1µm to 1µm. The dependence of these curves on the parameters of silicate-graphite dust mixture and the shell shape, orientation and structure is examined. It is found that the circumstellar extinction of disk-like shells seen pole-on is the most influenced by scattered radiation that leads to the flattening of the extinction curves. Variations of the dust grains and dust shell parameters can produce similar effects on the extinction curves which are difficult to distinguish from each other. Then, the wavelength dependence of polarization can be used to estimate the properties of circumstellar shells. Our theoretical models are used to interpret the observations of the extinction in the ultraviolet and visual, and the visual polarization of three Herbig Ae/Be stars: AB Aur, HD 259431 and HD 200775. We found that the minimum size of grains is >0.01µm. The slope of the grain size spectrum changes in the interval from 3.2 to 5.0. In the shells around AB Aur and HD 200775, there is an excess and a depletion of silicate particles, respectively. The shell aspect ratio varies from 2.0 (AB Aur) to 5.0 (HD 200775). Apparently, the dust shell of AB Aur is seen nearly to pole-on (i≃30deg), while the other two stars approximately edge-on.