Ups!... I did it again: unveiling the hidden companion in Upsilon Sagittarii, a unique binary system at a second mass transfer stage

TL;DR

This study unveils the true nature of Upsilon Sagittarii as a binary system with a rapidly rotating B-type companion, challenging previous models and providing new insights into binary evolution and mass transfer processes.

Contribution

First composite spectral model of Upsilon Sagittarii that accurately identifies the companion and refines the primary's properties, advancing understanding of binary evolution stages.

Findings

Companion is a ~7Msun B-type star, not a helium supergiant.

The primary is less luminous and less massive than previously thought.

The system's properties support a specific binary evolution scenario.

Abstract

Upsilon Sagittarii is a hydrogen-deficient binary that has been suggested to be in its second stage of mass transfer, after the primary has expanded to become a helium supergiant following core helium exhaustion. A tentative identification of the faint companion in the ultraviolet led to mass estimates of both components that made the helium star in Upsilon Sagittarii a prototypical immediate progenitor of a type Ib/c supernova. However, no consistent model for the complex spectrum has been achieved, casting doubt on this interpretation. In the present study we provide for the first time a composite spectral model that fits the ultraviolet data, and clearly identifies the companion as a rapidly rotating slowly moving ~7Msun B-type star, unlike previously suggested. The stripped helium supergiant is less luminous than previous estimates, and, with an estimated mass smaller than 1Msun, is ruled out as a core-collapse supernova progenitor. We provide a detailed binary evolution scenario that explains the temperature and luminosity of the two components as well as the very low gravity (log g ~ 1.0 [cm/s^2]) and extreme hydrogen deficiency of the primary (atmospheric mass fraction XH ~ 0.001). The best-fitting model is a ~5Msun intermediate-mass primary with an initial orbital period of a few days, and a secondary that appears to have gained a significant amount of mass despite its high rotation. We conclude that Upsilon Sagittarii is a key system for testing binary evolution processes, especially envelope stripping and mass accretion.