Astronomy and Astrophysics – Astrophysics
Scientific paper
Mar 1984
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984a%26a...132..375c&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 132, no. 2, March 1984, p. 375-380.
Astronomy and Astrophysics
Astrophysics
15
Scientific paper
Narrowband polarimetric measurements of Orionis in the range 4000 Å to 8500 Å are presented. Both the degree of polarization, p(λ) and the position angle of the polarization, θp(λ), show significant changes across several TiO bands as well as a general variation with wavelength. Various ways of determining the interstellar polarization (ISP) of this star are considered. Using all available data, we conclude that it is not possible to give a reliable estimate for the ISP; it is, however, very small (< 0.03%) and for the purpose of interpreting the intrinsic effects in the p(λ) curve of α Orionis, its contribution can be ignored. Assuming that the observed polarization is purely intrinsic we show that a straightforward development of the simple hotspot model, previously proposed by us, explains the recorded behaviour. The single convection cell model is generalised by allowing two or more spots to be present simultaneously, thereby enabling not only the degree but also the position angle of polarization to be dependent on wavelength. The changes in p and θp across spectral features are due to a differential limb darkening coefficient; it is larger in the centres of the absorption bands than in the adjacent continuum and it affects the spots according to their radial distance from the centre of the stellar disk. By a suitable choice of spot parameters, the observed polarizations are mimicked by the model star. We conclude that these results warrant the establishment of a more refined model with better numerical values for the variables describing the atmosphere.
Clarke David
Schwarz Hugo E.
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