A Parsec-scale Circumstellar Shell Around VX Sgr: Mass Loss and RSG/T.ZO 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 about0.59, pc. The nebula has an extinction-corrected H α intensity I Hα about38, R and an ionized mass about0.1, M. The slow 12, km,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 O - or 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 about4 times10-5, M, yr-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 Λ of the shell and SN ejecta; the interaction would likely not have a significant effect on the SN forward shock.