Variable Polarization of the Be Star Lambda Eridani: 1986--94

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Session 22 -- Stellar Spectroscopy, Atmospheres, Models, Intrinsic Variables, Theory, Part II
Display presentation, Tuesday, June 13, 1995, 9:20am - 6:30pm

[22.11] Variable Polarization of the Be Star Lambda Eridani: 1986--94

David McDavid (Limber Obs.), Marian Frueh (McDonald Obs.)

Over the past nine years of polarimetric monitoring, we have found a close correlation between the V-band linear polarization and the reported H$\alpha$ and He~I $\lambda 6678$ emission strength of the highly variable B2e star $\lambda$ Eri. Frueh recorded a large increase in polarization during the major outburst of 1987--88 (Smith, Peters, and Grady 1991, ApJ 367, 302). A smaller increase occurred during the minor episode of 1988--89, and there was almost no polarization at all during the ``quiescent'' phase of 1990--91. The 1992--93 outburst (Kambe et al.~1993, PASP 105, 1222) went undocumented because of bad weather, but in 1994 came the highest polarization for the entire period, corresponding to the strongest emission outburst (Stefl, IAUC 6098).

In a plot of the normalized Stokes parameters the data points lie nearly along a straight line through the origin, showing that the intrinsic polarization of $\lambda$ Eri has a constant position angle of about $74\deg$, with little or no interstellar component. This result is consistent with the catalog of Axon and Ellis, which gives zero polarization for six out of the ten stars within $6\deg$ of $\lambda$ Eri on the sky. The physical interpretation is that the polarization of $\lambda$ Eri is typical of most Be stars in that it can be explained by electron scattering in the dense inner region of the equatorially flattened circumstellar envelope that produces the emission lines. The exceptionally clear correlation between polarization and emission strength for this star is probably due to the relatively mild nature of its activity, which allows us to watch a low-lying envelope appear and disappear almost completely.

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