Pre-main-sequence stars. III. Herbig Be/Ae stars and other selected objects

The majority of the Herbig Be/Ae stars show considerable linear polarization in contrast to other hot young stars. A dependence on evolutionary phase exists in that the unpolarized stars are on the zero-age main sequence. The amount of polarization does not seem to be correlated with circumstellar shell parameters such as reddening and infrared excesses. Also, no correlation with V sin and hence possible circumstellar disk shell orientation was found. Only little variability in apparent magnitude on the time scale of days exists, but long-term variability is common. In the Taurus region and in NGC 2264 two stars are especially unusual: RY Tau shows strong, time-variable polarization increasing strongly toward the ultraviolet; and W90, situated below the main sequence in NGC 2264, shows variability in apparent magnitude as well as amount and wavelength dependence of polarization. The new data are interpreted as indicating variations in the optical depth of the circumstellar shell. Subject headings: circumstellar shells - emission-line stars - polarization - pre-main-sequence stars - photometry