A Parsec-scale Circumstellar Shell Around VX Sgr: Mass Loss and RSG/TŻO Nature
Abstract
A thin, arc-shaped shell is discovered surrounding VX Sagittarii (VX Sgr), the largest known CSM shell around an RSG at a scale r∼0.59 pc. The nebula has an extinction-corrected Hα intensity IHα∼38 R and an ionized mass ∼0.1 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 Ṁ≈4×10−5 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.
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BibTeX
@misc{shapiro2026parsecscale,
title = {{A Parsec-scale Circumstellar Shell Around VX Sgr: Mass Loss and RSG/TŻO Nature}},
author = {Julian Shapiro},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.20689},
eprint = {2610.20689},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.20689},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - A Parsec-scale Circumstellar Shell Around VX Sgr: Mass Loss and RSG/TŻO Nature
AU - Julian Shapiro
PY - 2026
DA - 2026/10/14
PB - arXiv
DO - 10.48550/arxiv.2610.20689
UR - https://arxiv.org/abs/2610.20689
AB - A thin, arc-shaped shell is discovered surrounding VX Sagittarii (VX Sgr), the largest known CSM shell around an RSG at a scale r∼0.59 pc. The nebula has an extinction-corrected Hα intensity I_Hα∼38 R and an ionized mass ∼0.1 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 Ṁ≈4×10^−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.
ER -