A Parsec-scale Circumstellar Shell Around VX Sgr: Mass Loss and RSG/TŻO Nature

A thin, arc-shaped shell is discovered surrounding VX Sagittarii (VX Sgr), the largest known CSM shell around an RSG at a scale $r\sim 0.59\mathrm{p}\mathrm{c}$. The nebula has an extinction-corrected H$\alpha$ intensity $I_{\mathrm{H}\alpha }\sim 38\mathrm{R}$ and an ionized mass $\sim 0.1{\mathrm{M}}_{\odot }$. The slow $12{\mathrm{k}\mathrm{m}\mathrm{s}}^{-1}$ peculiar velocity of VX Sgr and the lack of far-infrared emission in the arc indicate that it is not produced by a bow shock, but is likely instead steady RSG wind or ejecta that is photoionized by a nearby $\mathrm{O}$- or $\mathrm{B}$- type star. The nature of VX Sgr is debated in the literature as a red supergiant/hypergiant (RSG/RHG), super-AGB star, or Thorne–Żytkow Object (TŻO). Using the kinematic age of the Sgr OB1 association, the super-AGB model is disfavored. A spectroscopic test of the arc is proposed to potentially distinguish an RSG from a possible TŻO. The mass-loss rate is estimated from the geometry and emission measure of the shell, finding $\dot{M}\approx 4\times {10}^{-5}{\mathrm{M}}_{\odot }{\mathrm{y}\mathrm{r}}^{-1}$. The future interaction of the supernova (SN) ejecta of VX Sgr and the surrounding circumstellar material (CSM) is briefly explored through the mass ratio $\mathrm{\Lambda }$ of the shell and SN ejecta; the interaction would likely not have a significant effect on the SN forward shock.