MN48: a new Galactic bona fide luminous blue variable revealed by Spitzer and SALT

TL;DR

This study confirms MN48 as a genuine Galactic luminous blue variable through spectroscopic and photometric evidence, revealing its variability and association with circumstellar nebulae, and discusses its possible origin in nearby star clusters.

Contribution

First identification of MN48 as a bona fide LBV based on spectral and brightness variability, expanding the known Galactic LBV population and their nebulae association.

Findings

MN48 exhibits typical LBV spectral changes and brightness variability.

MN48 is the 18th confirmed Galactic LBV.

Over 70% of LBVs are associated with circumstellar nebulae.

Abstract

In this paper, we report the results of spectroscopic and photometric observations of the candidate evolved massive star MN48 disclosed via detection of a mid-infrared circular shell around it with the Spitzer Space Telescope. Follow-up optical spectroscopy of MN48 with the Southern African Large Telescope (SALT) carried out in 2011--2015 revealed significant changes in the spectrum of this star, which are typical of luminous blue variables (LBVs). The LBV status of MN48 was further supported by photometric monitoring which shows that in 2009--2011 this star has brightened by approx 0.9 and 1 mag in the V and I_c bands, respectively, then faded by approx 1.1 and 1.6 mag during the next four years, and apparently started to brighten again recently. The detected changes in the spectrum and brightness of MN48 make this star the 18th known Galactic bona fide LBV and increase the percentage of LBVs associated with circumstellar nebulae to more than 70 per cent. We discuss the possible birth place of MN48 and suggest that this star might have been ejected either from a putative star cluster embedded in the HII region IRAS 16455-4531 or the young massive star cluster Westerlund 1.