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

H. M. Tabernero, R. Dorda+2 more|arXiv (Cornell University)|Nov 18, 2020

Stellar, planetary, and galactic studiesPhysics and Astronomy103 references32 citations

TL;DR

This study re-evaluates VX Sagittarii as a super-AGB star through high-resolution spectroscopy and AAVSO photometry, finding it exhibits Mira-like pulsations, extreme luminosity (Mbol ≈ −8.6 ± 0.6 mag), and strong Rb i lines with blueshifted circumstellar components—evidence of ongoing s-process enrichment. The results challenge its classification as a red supergiant or Thorne–Żytkow object, positioning VX Sgr as the most luminous known AGB star and a strong candidate for a massive super-AGB star with initial mass >10 M⊙.

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.

Motivation & Objective

• To resolve the long-standing ambiguity in VX Sagittarii’s nature, which has been classified as a red supergiant (RSG), red hypergiant (RHG), or Thorne–Żytkow object (TZO).
• To determine whether VX Sgr's extreme luminosity, large-amplitude variability, and unusual spectral features align with AGB, RSG, or TZO evolutionary models.
• To assess the role of pulsation, convection, and s-process nucleosynthesis in shaping its atmospheric and circumstellar properties.

Proposed method

• High-resolution spectroscopy using PHOENIX atmospheric models to derive stellar parameters (Teff, log g, [Fe/H], vsini) across 11 epochs.
• Photometric analysis of AAVSO light curves to measure variability periods and amplitudes, including a 757-day main period and a long-term 28,279-day (≈77 yr) signal.
• Spectral feature analysis focusing on Li i, Ca i, and Rb i lines to detect photospheric and circumstellar components.
• Line-doubling detection in metallic lines during maximum light to confirm pulsational Mira-like behavior.
• Comparison of circumstellar expansion velocities with OH maser velocities to infer s-process element supply.
• Cross-verification of spectral types and stellar parameters to assess consistency across observational phases.

Experimental results

Research questions

• RQ1 Is VX Sagittarii best explained as a massive red supergiant, a Thorne–Żytkow object, or an extreme asymptotic giant branch (AGB) star?
• RQ2 What drives the extreme photometric and spectroscopic variability observed in VX Sgr, particularly the 660-day pulsation and the 77-year modulation?
• RQ3 Why does VX Sgr display strong Rb i lines with persistent blueshifted circumstellar components, and what does this imply about s-process nucleosynthesis?
• RQ4 How does the observed luminosity (Mbol ≈ −8.6 ± 0.6 mag) compare to known AGB stars, and does it exceed the luminosity limit for canonical AGB evolution?
• RQ5 Can the presence of Balmer line emission and line doubling during maximum light be attributed to pulsational Mira-like behavior rather than binary interaction or TZO physics?

Key findings

• VX Sagittarii exhibits Mira-like pulsations with a dominant period of 757 days and a long-term modulation at 28,279 days (≈77 years), the latter previously unreported and potentially linked to thermal pulses in a massive AGB star.
• The star’s bolometric magnitude is Mbol ≈ −8.6 ± 0.6 mag, making it the most luminous known AGB star, exceeding the typical luminosity limit of −8 mag for canonical AGB stars.
• Line doubling in metallic absorption lines during maximum light confirms Mira-like pulsational behavior, while Balmer line emission further supports this classification.
• The persistent presence of blueshifted Rb i circumstellar components with expansion velocities matching OH masers provides direct evidence of continuous s-process element enrichment via third dredge-up.
• The absence of Li i and Ca i features rules out a Thorne–Żytkow object origin, while the high luminosity and mass estimates (10–12 M⊙) support its classification as a super-AGB star.
• The observed spectral type variability (from M4 to M10) and extreme photometric amplitude (up to 6 mag in V-band) are inconsistent with RSGs but consistent with AGB stars in extreme pulsational states.