How common is LBV S Dor variability at low metallicity?

TL;DR

This study investigates the frequency of LBV S Dor variability among B supergiants in the SMC, finding it to be extremely rare, which suggests LBVs are likely distinct from typical B supergiants in low-metallicity environments.

Contribution

The paper provides the first systematic search for S Dor variables in the SMC and concludes that LBVs are probably separate from B supergiants, with implications for stellar evolution models.

Findings

Only one S Dor-like variable found among 64 B supergiants.

No temperature variation observed in AzV 261 over a decade.

Maximum LBV phase duration in the SMC estimated at a few thousand years.

Abstract

It remains unclear whether massive star evolution is facilitated by mass loss through stellar winds only, or whether episodic mass loss during an eruptive luminous blue variable (LBV) phase is also significant. LBVs exhibit unique photometric and spectroscopic variability (termed S Dor variables). This may have tremendous implications for our understanding of the first stars, gravitational wave events, and supernovae. A key question here is whether all evolved massive stars passing through the blue supergiant phase are dormant S Dor variables transforming during a brief period, or whether LBVs are truly unique objects. By investigating the OGLE light-curves of 64 B supergiants (Bsgs) in the Small Magellanic Cloud (SMC) on a timescale of three years with a cadence of one night, the incidence of S Dor variables amongst the Bsgs population is investigated. From our sample, we find just one Bsg, AzV 261, that displays the photometric behaviour characteristic of S Doradus variables. We obtain and study a new VLT X-shooter spectrum of AzV 261 in order to investigate whether the object has changed its effective temperature over the last decade. We do not find any effective temperature variations indicating that the object is unlikely to be a LBV S Dor variable. As there is only one previous bona-fide S Dor variable known to be present in the SMC (R 40), we find the maximum duration of the LBV phase in the SMC to be at most $\sim$few 10$^3$yrs, or more likely that canonical Bsgs and S Dor LBVs are intrinsically different objects. We discuss the implications for massive star evolution in low metallicity environments, characteristic of the early Universe.