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We are investigating the optical linear polarization properties of a few Herbig AeBe (HAEBE) stars and candidates. Hillenbrand et al. (1992, ApJ, 397, 613) recently interpreted the spectral energy distributions of a large sample of these intermediate-mass, pre-main-sequence stars with accretion disk models. Polarization measurements provide an independent tool to gather geometric information on the distribution of the circumstellar gas and dust. The stars being analyzed are the confirmed HAEBE stars HD 45677, HD 53367, HD 200775 and two peculiar Be stars which are possible HAEBE objects, HD 50138 and HD 158643 (= 51 Oph).
The observed polarization is a vector sum of interstellar polarization (ISP) and the sought-after intrinsic polarization, both unknown a priori. We can recognize the presence of intrinsic polarization from the variability of the observed polarization (ISP is constant). To this end, we have combined UBV polarimetry published in the literature with spectropolarimetric observations obtained at the U. Wisconsin's Pine Bluff Observatory (PBO), which were folded with the response of UBV filters. The polarization of HD 45677, HD 53367, HD 200775, and HD 50138 shows significant variations with time, indicating intrinsic polarization. There are as yet too few observations of HD 158643 to assess its time variability. Our data indicate that the material around most of our sample stars is in a non-spherically symmetric distribution and is also not homogeneous. HD 45677 and HD 200775 exhibit linear temporal changes in the polarimetric Q-U plane, a characteristic of disk-shaped distributions of the circumstellar material.
The next step will be to determine the ISP, remove it, then investigate the intrinsic polarization. At this time, we have used the field-star method to set limits on the ISP; we also plan to use polarization observations in the H$\alpha$ emission lines to derive values of the ISP.
This work was supported by a Research Experiences for Undergraduates award funded by NSF and granted by the AAS, and by NAS5-26777.
