Interstellar Circular Polarization of Upper Scorpius Stars

Abstract

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) ∝ (n′_l – n′_r) · (n″_l – n″_r) 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.