Variable Polarization of WR 31a: Binary Companion or Corotating Interaction Region?

Le résumé fourni par la source

Abstract WR 31a (Hen 3-519) is likely a postluminous blue variable (LBV) star that is evolving to become a classical Wolf–Rayet star. Multicolor (UBVR) photopolarimetric observations of WR 31a were obtained over nine nights in early-2007. The linear polarization data of WR 31a trace a “loop” structure in a Stokes Q − U diagram, which is similar in all four passbands. After mean subtraction, the four loops align to form a single overall pattern. Such loops can be expected to arise from binary systems. We test the binary hypothesis with two models. The data are fitted for a strictly circular orbit to derive an orbital period of 16.7 days, requiring a high inclination perspective of i ∼ 80°. We also consider an elliptical orbit under simplifying assumptions, yielding a match for i ∼ 75° with eccentricity e ∼ 0.5 and a longer orbital period of about 70 days. The prevalence of binarity among massive stars is well known; the prospect of detecting a binary companion during the post-LBV stage of WR 31a would add to an emerging narrative of diverse interactions between massive multiple components as a function of the evolutionary stage. However, if the loop originates because of a corotating interaction region, then the rotation period could be 8.5 days or 17 days. This would give an estimated equatorial rotation speed of 95 or 190 km s−1. Either of these is a significant fraction of the estimated critical speed of rotational break-up at 320 km s−1 (for an Eddington factor of Γ = 0).

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