Separation of the intrinsic and interstellar linear polarization components of southern hemisphere early-type emission-line stars

Polarization measurements in the Hα, Hβ emission features of southern Be-shell stars and blue supergiants are presented. Using these observations, together with all previous published broad-band data, the intrinsic and interstellar components are separated by an intercomparison of three methods. To standardize this approach each method is briefly reviewed with discussion of possible difficulties and misuse. The largest intrinsic polarizations were found for p Car, ∊ Cap, π Aqr and κ Aps (⁠|pB_ 1.0| per cent), with intermediate polarizations of ∼ 0.5 per cent for μ Ara, η Cen, δ Cen and weak polarization (⁠|pB_ 0.15| per cent) for μ Cen, α Col and ω Car. The star p Car exhibited reductions in polarization across the Hα and Hβ lines commensurate with a simple depolarization mechanism based on unpolarized emission-line flux and hence consistent with the trend of observations of previously measured Be stars. In contrast with this situation, the Hα lines in α Ara and δ Cen show the expected polarization reduction but not to the level predicted by the simple unpolarized emission-line flux model. These stars also exhibit polarimetric evidence of intrinsic reddening. The shell star η Cen was found to have a currently active envelope and we discovered an increase in polarization at the narrow core of Hβ and polarization structure in Hα. These effects we attribute to shell-line absorption. Clearly, these new measurements indicate more complex phenomena associated with Balmer-line polarimetry than first envisaged. No line profile polarization effects were found for the supergiants observed although variable polarization was certainly confirmed for the binary μ Sgr. Preliminary line profile polarization results for Wolf–Rayet and Oef stars are mentioned.