2000A&A...364..633N

This paper offers an interpretation of the photometric and polarimetric variability of UXORs where the star is surrounded by an optically thick, flared circumstellar disk similar to pre-main-sequence disks. A screen of dust sporadically obscures the stellar radiation, causing a minimum of the stellar light. Using a Monte Carlo code developed by Whitney & Hartmann (1992ApJ...395..529W) we compute the polarization and colors of the observed radiation, and compare it to the available observations. The agreement is remarkably good. We find that the UXOR phenomenon occurs for systems seen in a well-defined range of inclinations, roughly between 45° and 65°-68°. About 1/2 of the optically visible Herbig Ae stars should be UXORs, i.e., have deep photometric minima accompanied by a large increase in the polarization fraction, provided that screens can form. The results are not sensitive to the disk parameters, as long as the disk intercepts about 20% of the stellar radiation. The screens causing the light minima have sizes of the order of 1-few stellar radii, optical depth τ_V∼3-5, and contain relatively small grains. We find a good fit to the observations with a MRN distribution with average radius ∼0.03µm. The lack of UXORs with low polarization in deep minima can be understood if the screens are confined in a region close to the disk plane. However, the nature and origin of the screens remain open questions.