CIRCUMSTELLAR EXTINCTION OF PRE-MAIN-SEQUENCE STARS

The extinction curves arising from the combined effect of absorption a nd scattering of stellar radiation in dust shells around pre-main-sequ ence stars have been investigated. Monte Carlo simulations of the pola rized radiation transfer in an inhomogeneous spheroidal shell have bee n used to determine the scattered radiation. The results are presented as the normalized curves for circumstellar extinction A(lambda) - A(0 .3 mu m) in the wavelength range from 0.1 mu m to 1 mu m. The dependen ce of these curves on the parameters of silicate-graphite dust mixture and the shell shape, orientation and structure is examined. It is fou nd that the circumstellar extinction of disk-like shells seen pole-on is the most influenced by scattered radiation that leads to the flatte ning 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 waveleng th dependence of polarization can be used to estimate the properties o f circumstellar shells. Our theoretical models are used to interpret t he observations of the extinction in the ultraviolet and visual, and t he visual polarization of three Herbig Ae/Be stars: AB Aur, HD 259431 and HD 200775. We found that the minimum size of grains is greater tha n or similar to 0.01 mu m. The slope of the grain size spectrum change s 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, resp ectively. The shell aspect ratio varies from 2.0 (AB Aur) to 5.0 (HD 2 00775). Apparently, the dust shell of AB Aur is seen nearly to pole-on (i approximate to 30 degrees), while the other two stars approximatel y edge-on.