On the nature of VX Sagitarii: Is it a TZO, a RSG or a high-mass AGB star?

TL;DR

This study analyzes VX Sagittarii's spectral and photometric data to determine its true nature, providing evidence that it is likely an extreme Asymptotic Giant Branch star rather than a red supergiant or Thorne Zytkow object.

Contribution

It offers new spectroscopic and photometric insights, including pulsation behavior and s-process element enrichment, to clarify VX Sgr's evolutionary stage and classification.

Findings

VX Sgr exhibits Mira-like variability and line doubling at maximum light.

Presence of strong Rb I lines indicates s-process enrichment.

Data supports classification as an extreme AGB star.

Abstract

Aims. We present a spectroscopic analysis of the extremely luminous red star VX Sgr based on high-resolution observations combined with AAVSO light curve data. Given the puzzling characteristics of VX Sgr, we explore three scenarios for its nature: a massive red supergiant (RSG) or hypergiant (RHG), a Thorne Zytkow object (TZO), and an extreme Asymptotic Giant Branch (AGB) star. Methods. Sampling more than one whole cycle of photometric variability, we derive stellar atmospheric parameters by using state-of-the-art PHOENIX atmospheric models. We compare them to optical and near infrared spectral types. We report on some key features due to neutral elemental atomic species such as Li I, Ca I, and Rb I. Results. We provide new insights into its luminosity, evolutionary stage as well as its pulsation period. Based on all the data, there are two strong reasons to believe that VX Sgr is some sort of extreme AGB star. Firstly, its Mira-like behaviour during active phases. VX Sgr shows light variations with amplitudes much larger than any known RSG and clearly larger than all RHGs. In addition, it displays Balmer line emission and, as shown here for the first time, line doubling of its metallic spectrum at maximum light, both characteristics typical of Miras. Secondly, unlike any known RSG or RHG, VX Sgr displays strong Rb I lines. In addition to the photospheric lines that are sometimes seen, it always shows circumstellar components whose expansion velocity is compatible with that of the OH masers in the envelope, demonstrating a continuous enrichment of the outer atmosphere with s-process elements, a behaviour that can only be explained by third dredge up during the thermal pulse phase.