The incidence of LBV variability in the LMC

Luminous blue variables (LBVs) exhibit unique variability features, characterized by episodic outbursts ($>$1 mag) accompanied by spectroscopic changes (S Dor variables). It is debated if all massive stars undergo an LBV-like phase during their evolution, or instead LBVs are exotic phenomena. We aim to quantify the incidence of LBV-like variability in the blue supergiant (BSgs) population of the Large Magellanic Cloud (LMC) using the OGLE survey. Here, we extend previous work in the Small Magellanic Cloud to the LMC, where we examine the light curves of 87 B Supergiants (BSgs) (out of 254 known BSgs) spanning timescales of twenty years, and 37 objects across a three year timescale for aperiodic variations resembling known S Dor variables. One blue supergiant, [ST92] 4-13 shows S Dor type photometric variations. New spectra of this object reveals a potential change in spectral type compared to the literature classification. However, based on its spectral characteristics and low luminosity and mass, we do not currently classify it as an LBV. Our study highlights the need to classify bona fide LBVs as stars undergoing both photometric and spectroscopic variations. Based on currently known stellar population of S Dor variables in the LMC, the lifetime of the S Dor phase is at most $\sim$10$^3$ yrs, in agreement with our duty cycle study based on OGLE data in the SMC. This is orders of magnitude shorter than assumed in literature. Our discovery of LBV-like variability at low luminosities may suggest that S Dor variations could arise from Eddington limit related physics over a wide range of stellar masses, rather than being linked to a unique evolutionary stage.

💡 Research Summary

This paper investigates how common Luminous Blue Variable (LBV)–type variability is among blue supergiants (BSGs) in the Large Magellanic Cloud (LMC) and uses the result to constrain the typical lifetime of the LBV phase. The authors start from the catalog of Bonanos et al. (2009), which lists 254 massive stars with reliable spectral classifications in the LMC. From this list they select objects that have multi‑epoch photometry in the OGLE‑IV/III surveys (I‑band) and in Gaia DR3 (G, BP, RP). OGLE provides roughly 1 000 measurements per star over a 20‑year baseline (2001–2020) with a typical precision of 0.01 mag, while Gaia contributes 30–80 epochs between 2014 and 2017. After cross‑matching, 87 BSGs have OGLE light curves and 49 have Gaia light curves; the overlap yields 11 stars with both data sets.

To identify irregular, aperiodic variability reminiscent of S Doradus (S Dor) outbursts, the authors employ two complementary statistical tools. First, they construct Δm–Δt density histograms for each light curve, where Δm is the absolute magnitude difference between any two observations and Δt is the time separation. Objects showing Δm > 0.1 mag (and especially > 0.5 mag) on any timescale are flagged for visual inspection. Second, they compute the von Neumann η index (1/η) as defined by Sokolovsky et al. (2017); larger 1/η values indicate stronger irregularity. The distribution of log(1/η) is fitted with a Gaussian, and any star more than three sigma above the mean is considered a candidate.

Applying these criteria, three OGLE stars stand out. Two of them (VFTS 698 and VFTS 652) are known binaries whose modest variability (< 0.5 mag) is likely driven by orbital effects, so they are discarded. The remaining object,