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Session 78 - Stars - Young and Old, Large and Small.
Display session, Wednesday, January 15
New observations of the O stars zeta Oph, 68 Cyg, 19 Cep, lambda Cep, and 10 Lac have been made with the AnyPol linear polarimeter on the 0.4-m telescope at Limber Observatory using the Johnson-Cousins UBVRI broadband filter system. For all five stars the wavelength dependence of the degree of polarization can be fit by a Serkowski interstellar polarization law with reasonable parameters. The polarization position angles are practically constant with wavelength and are consistent with those of neighboring stars. Comparison with earlier measurements shows no clearly defined polarization variability over time. Thus the new observations give no compelling evidence for intrinsic polarization of any of the program stars, and the simplest conclusion is that the polarization is purely interstellar for all of them.
The O stars chosen for this study are generally known from ultraviolet and optical spectroscopy to have substantial mass loss rates and variable winds, as well as occasional circumstellar emission. Their lack of intrinsic polarization in comparison with the similar Be stars may be explained by the dominance of radiation as a wind driving force due to higher luminosity, which results in lower density and less rotational flattening in the electron scattering inner envelopes.
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