Broad-band linear polarization in cool stars. I - Models and spatial effects for magnetic and scattering regions

Details Broad-band linear polarization in cool stars. I - Models and spatial effects for magnetic and scattering regions Broad-band linear polarization in cool stars. I - Models and spatial effects for magnetic and scattering regions

Jun 1991

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991apj...374..319h&link_type=abstract

Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 374, June 10, 1991, p. 319-329. Research supported by Emil Aaltonen Foundat

Statistics

Computation

12

Cool Stars, Electromagnetic Scattering, Linear Polarization, Stellar Magnetic Fields, Stellar Models, Computational Astrophysics, Spatial Distribution, Stellar Rotation, Polarization, Stars: Late-Type, Stars: Magnetic

Scientific paper

We have developed models of broad-band linear polarization (BLP) arising from magnetic regions on cool stars. The models include an improved treatment of spatial effects in which the BLP is explicitly integrated over the stellar surface. We find that for magnetic region filling factors, f larger than ≍1% of a hemisphere, direct disk-integration yields results which are often significantly different from a simple linear scaling of BLP with region area, especially for regions near the limb. In particular, the amplitude of the BLP reaches a maximum for f ≍ 24%, which is a consequence of cancellation of the polarization signal within the region itself. The line-of-sight angle at which the region exhibits maximum polarization increases with region size.
We study the effects of bipolar pairs of regions, and single regions with small-scale bipolarity. The most important effect of bipolarity is the reduction in the influence of Faraday rotation on the integrated polarization. Spatial effects become more important as the size of the bipolar spot pair is increased.
We construct similar models for Rayleigh and Thomson scattering regions in order to compare the signatures of BLP from these sources. Like magnetic BLP, scattering-induced BLP shows a maximum in polarization amplitude (at f ≍ 18%), but the line-of-sight angle of the maximum first decreases (for f ≤ 10%) and then increases with increasing region size. We also present approximate formulas for the scaling of magnetically induced and scattering-induced polarization as a function of f We discuss the importance of the differences found in the rotational phase dependence for discerning the source of the polarization. Use of the phase dependence requires detailed comparisons of polarization observed at several rotational phases, with the success of application depending on the specific geometry of the polarizing regions. The general applicability of the models depends on the accuracy in determining the instrumental and interstellar polarization (in modeling of polarization degree P) and/or the orientation of the stellar rotation axis on the plane of the sky (in modeling of Stokes parameters PQ and PU)

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