Volume 496, Number 2, March III 2009
|Page(s)||503 - 511|
|Published online||14 January 2009|
Scattering polarization due to light source anisotropy
II. Envelope of arbitrary shape
Department of Physics & Astronomy, East Tennessee State University, Johnson City, TN, USA e-mail: email@example.com
2 Kelvin Bldg, Department of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ, Scotland, UK
3 Now at PO Box 87946, Riyadh 11652, Saudi Arabia
Accepted: 12 December 2008
Aims. We consider the polarization arising from scattering in an envelope illuminated by a central anisotropic source. This work extends the theory introduced in a previous paper (Al-Malki et al. 1999) in which scattering polarization from a spherically symmetric envelope illuminated by an anisotropic point source was considered. Here we generalize to account for the more realistic expectation of a non-spherical envelope shape.
Methods. Spherical harmonics are used to describe both the light source anisotropy and the envelope density distribution functions of the scattering particles. This framework demonstrates how the net resultant polarization arises from a superposition of three basic “shape” functions: the distribution of source illumination, the distribution of envelope scatterers, and the phase function for dipole scattering.
Results. Specific expressions for the Stokes parameters and scattered flux are derived for the case of an ellipsoidal light source inside an ellipsoidal envelope, with principal axes that are generally not aligned. Two illustrative examples are considered: (a) axisymmetric mass loss from a rapidly rotating star, such as may apply to some Luminous Blue Variables, and (b) a Roche-lobe filling star in a binary system with a circumstellar envelope.
Conclusions. As a general conclusion, the combination of source anisotropy with distorted scattering envelopes leads to more complex polarimetric behavior such that the source characteristics should be carefully considered when interpreting polarimetric data.
Key words: polarization / stars: circumstellar matter / stars: binaries: close / stars: rotation / stars: winds, outflows
© ESO, 2009
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