The incidence of LBV variability in the LMC

TL;DR

This study extends the search for LBV-like variability to LMC blue supergiants using two decades of OGLE photometry and Gaia DR3 data, building on prior SMC work. By applying δm-δt histograms and the von Neumann η index, the authors identify two S Dor-type variables: the known LBV HD 269216 and a new candidate [ST92] 4-13, though the latter has luminosity too low for a formal LBV classification. Spectroscopic analysis suggests [ST92] 4-13 exhibits an apparent spectral-type shift without LBV-level wind features, implying low-luminosity S Dor-like behavior that may arise from Eddington-limit physics rather than a single evolutionary stage. The inferred S Dor duty cycle is at most $\sim 10^3$ years, indicating LBVs are rare phenomena and highlighting the need for simultaneous photometric and spectroscopic confirmation in LBV classifications. Overall, the results argue for a nuanced view of LBVs, where S Dor-type variability can occur across a range of masses and metallicities, not exclusively in a canonical LBV evolutionary phase.

Abstract

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.

Figures (8)

• Figure 1: Top:Spectral types of Bsgs with OGLE data (red) and the total number of Bsgs from our sample (solid line). Bottom: The recovery fraction for Gaia multi-epoch photometry. Known LBVs are not shown.
• Figure 2: Representative example of the time series photometric analysis, where the color gradient marks the density of the points. The left panel shows the LBV candidate [ST92] 4-13 $\delta$m-$\delta$t histogram computed using OGLE-IV data, with the $\delta$m levels of 0.08 (10$\sigma$) and 0.1 mag marked. The right panel shows the same for the known LBV HD 269216, computed from Gaia multi-epoch photometry.
• Figure 3: Histogram of 1/$\eta$ values computed using OGLE and Gaia light curves, in logarithmic scale. The positions of the identified LBVs, HD 269216 and [ST92] 4-13 are marked. The best-fit Gaussian to the population except the outliers (log (1/$\eta) >$ 1) is shown by the dashed line. The dotted line shows the 3$\sigma$ cut-off.
• Figure 4: Left: $I$-band light curve of [ST92] 4-13, with the ($V-I$) color shown at the bottom. Data are from the OGLE-IV photometry. The red asterisk marks the epoch when the spectral data were taken. Right: Gaia$G$ and $(BP-RP$) magnitude and color light curves of HD 269216, shown at the top and bottom panel respectively.
• Figure 5: Kiwi-GA fit of [ST92] 4-13. See Section 4.1.2 for details.