Discovery of the first symbiotic star in NGC6822

TL;DR

This paper reports the discovery of the first symbiotic star in NGC6822, an important step in understanding extragalactic symbiotic systems and their stellar components.

Contribution

It presents the identification and characterization of the first symbiotic star in NGC6822, including its spectral features and stellar classification, expanding knowledge of extragalactic symbiotic stars.

Findings

The star is an S-type symbiotic with a red giant and a hot companion.

The red giant is an AGB carbon star, pulsating with a 142-day period.

Most extragalactic symbiotic stars likely contain AGB stars.

Abstract

We report the discovery of the first symbiotic star (V=21.6, K_S=15.8 mag) in the Local Group dwarf irregular galaxy NGC6822. This star was identified during a spectral survey of Ha emission-line objects using the Southern African Large Telescope (SALT) during its performance-verification phase. The observed strong emission lines of HI and HeII suggest a high electron density and T* < 130 000 K for the hot companion. The infrared colours allow us to classify this object as an S-type symbiotic star, comprising a red giant losing mass to a compact companion. The red giant is an AGB carbon star, and a semi-regular variable, pulsating in the first overtone with a period of 142 days. Its bolometric magnitude is M_bol=-4.4 mag. We review what is known about the luminosities of extragalactic symbiotic stars, showing that most, possibly all, contain AGB stars. We suggest that a much larger fraction of Galactic symbiotic stars may contain AGB stars than was previously realised.