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Interstellar Circular Polarization of Upper Scorpius Stars

Published online by Cambridge University Press:  14 August 2015

James C. Kemp*
Affiliation:
Institute for Astronomy, University of Hawaii, Honolulu, H.I. 96822, U.S.A.

Abstract

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Circular polarization of interstellar or perhaps circumstellar origin has been clearly established in two reddened stars in the Upper Scorpius region, o-Sco and σ-Sco A; and detected preliminarily in a third, HD 154445. In the Corning 5–60 filter band λλ 3800–4600, measured values of 104q were: o-Sco, –(2.9 ± 0.2); σ-Sco A, –(1.7 ± 0.2); and HD 154445, +(2.8 ± 0.8). Complete wavelength dependences q(λ) for o-Sco and σ-Sco A are under study, but so far are known to have the following similarity. Each shows a broad maximum (of negative q) centered in the range λλ 3800–4300; and the values are smaller by a factor ~ 4, but still negative, in the range λλ 5000–6000. The effect here is most likely due to a twisting grain alignment along the line of sight. In that case a simple model predicts q(λ) ∝ (nlnr) · (nlnr) θ0z02, where n = n′ +i n″ is a formal, complex refractive index (in van de Hulst's forward-scattering approximation), l and r referring to longitudinal and transverse grain axes, and θ0 is the total twist angle over the interaction length z0. An alternate mechanism would be multiple scattering (at large angles, not along the line of sight) within asymmetrical, well-localized clouds around the individual stars. The similar sign and magnitudes of q for o-Sco and σ-Sco A, which are 1.5° apart in the sky, argue for the line-of-sight mechanism – for example through a large cloud in front of the entire local region. Comparative studies of many stars in Upper Scorpius will be of obvious interest.

Type
Part IV Interstellar Polarization
Copyright
Copyright © Reidel 1973 

References

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