2025/04/30 by Erba, Christiana, Ignace, Richard, Simmons, Faith +1
#FOS: Physical sciences #Solar and Stellar Astrophysics (astro-ph.SR)
paper · doi:10.48550/arxiv.2505.00183
WR 31a (Hen 3-519) is likely a post-luminous 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 fit for a strictly circular orbit to derive an orbital period of 16.7 d, 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 d. 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 evolutionary stage. However, if the loop originates because of a co-rotating interaction region (CIR), then the rotation period could be 8.5 d or 17 d. This would give an estimated equatorial rotation speed of 95 or 190 km/s. Either of these is a significant fraction of the estimated critical speed of rotational break-up at 320 km/s (for an Eddington factor of Γ=0).