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abstract: 'We present preliminary calculations of electron scattering polarizations from models of structured cool star envelopes. We note that net polarizations from unresolved sources can result from non-spherical scattering envelopes and/or anisotropic illumination from a photosphere that has brightness variations. The resultant polarizations are quite small (hundreths of a percent); however, Rayleigh scattering from molecular opacity and/or dust scattering from the more extended envelope under similar considerations may produce higher polarizations.'
author:
- 'R. Ignace$^1$, G. D. Henson$^1$, J. Carson$^2$'
title: Polarization from the Structured Envelopes of Cool Evolved Stars
---

The variable nature and complex envelopes of cool evolved stars offer numerous mechanisms for the creation of polarized light, which in turn probes structure in the envelopes and flows of these star. Observations of variable polarization lead to constraints on physical models for the envelope dynamics. We consider an electron scattering chromosphere with a radial density of scatterers $n \propto \exp[(r-R)/H)]
\times \sin^2[k(r-R)]$ illuminated by a photosphere at $r=R$. Figure \[ignace:fig1\] shows a $Q-U$ diagram for a chromospheric envelope with $\tau=1$, scale height $H=0.01R$, and lateral structure given by a spherical harmonic of $l=3, m=2$. Different curves are for different viewing inclinations, and the points are for different azimuthal perspectives. The large circle is for a pole-on view, with progressive curves increasing the viewing angle by $15^\circ$ with the vertical line representing the equatorial view.

Expanding on Al-Malki (1992) and Al-Malki et al. (1999), we consider the converse case of a spherically symmetric (but radially structured) chromosphere that is anisotropically illuminated. Results are shown in Figure \[ignace:fig2\]: left for a pole-bright star, and right for an equator-bright star. Note that these are axisymmetric.

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This project was partially funded by a partnership between the NSF (AST-0552798), the REU, and the DoD ASSURE programs.

Al-Malki, M. B., 1992, Ph. D. Thesis, University of Glasgow Al-Malki, M.B., Simmons, J., Ignace, R., Brown, J.C., Clarke, D., 1999, A&A, 347, 919
