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The incidence of LBV variability in the LMC

This study analyzes two decades of OGLE survey data for 87 blue supergiants in the Large Magellanic Cloud to quantify the incidence of LBV-like variability, finding only one candidate that lacks full LBV classification and concluding that the S Dor phase is extremely short-lived ( $\sim$ 10 $^3$ years) and may occur across a wide range of stellar masses rather than being a unique evolutionary stage.

Original authors: V. M. Kalari, J. S. Vink, C. Furey, R. Salinas, A. Udalski, M. Pawlak

Published 2026-04-29

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CC BY 4.0

Original authors: V. M. Kalari, J. S. Vink, C. Furey, R. Salinas, A. Udalski, M. Pawlak

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: Hunting for "Sneezing" Stars

Imagine the universe is a crowded neighborhood of stars. Most stars are like steady, reliable neighbors who keep the same brightness and temperature for millions of years. But then, there is a rare, dramatic type of star called a Luminous Blue Variable (LBV).

Think of an LBV as a star that has a massive, unpredictable "sneeze" or "tantrum." For a short time, it might suddenly get much brighter, change its color, and blow off a huge cloud of gas, before settling back down. Astronomers have long debated: Is this "tantrum" something every massive star goes through when it gets older, or is it a rare, exotic personality trait that only a few special stars have?

This paper is like a neighborhood watch report. The authors went out and checked the "light curves" (the history of how bright the stars are) of 87 massive blue stars in the Large Magellanic Cloud (a small galaxy next to our own) to see how many of them are throwing these tantrums.

The Detective Work: Looking Back in Time

To catch these stars in the act, the astronomers needed a long history book.

The Tool: They used data from the OGLE survey, which has been taking pictures of the sky almost every night for 20 years. It's like having a security camera that never sleeps.
The Target: They looked at 87 "Blue Supergiants" (huge, bright, blue stars).
The Comparison: They also checked a shorter 3-year dataset from the Gaia satellite to see if they could spot quick changes.

The Findings: One Real Sneeze, One False Alarm

After analyzing the data, here is what they found:

The Known Star (HD 269216): They confirmed that one star they already knew about is indeed having a tantrum. It's changing brightness and color just like a classic LBV.
The New Candidate ([ST92] 4-13): They found a new star that was getting brighter and dimmer in a way that looked exactly like an LBV tantrum. It changed brightness by nearly a full magnitude (a lot!) over 20 years.
- The Twist: When they took a closer look at this new star's "fingerprint" (its spectrum), they realized something was off. It wasn't bright or massive enough to be a "true" LBV. It was more like a "pretend" LBV. It had the behavior (the light changes) but not the identity (the massive size and specific gas clouds) of a real one.
The Rest of the Crowd: The other 85 stars? They were mostly quiet. They didn't show these dramatic, decade-long tantrums.

The Big Conclusion: It's Not a "Phase," It's a "Trait"

The most important takeaway from this paper is about the timing.

The Old Idea: Some scientists thought that maybe all massive stars go through this LBV phase, but it just lasts a long time (like 100,000 years), so we just haven't seen it happen to everyone yet.
The New Reality: This paper shows that the "tantrum" phase is actually incredibly short—maybe only 1,000 years (or even less).

The Analogy:
Imagine you are looking at a room full of 1,000 people. You want to know if everyone eventually gets a case of the hiccups.

If the hiccups lasted 100 years, you'd expect to see many people hiccuping right now.
But if the hiccups only last 1 minute, you might look at the whole room for 20 years and only see one person hiccuping.

That is what this paper found. The "LBV phase" is so short that even with 20 years of watching, we only see it happen to a tiny, tiny fraction of stars. This suggests that LBVs are not a standard "middle age" for all massive stars. Instead, they are rare, special, and perhaps a bit exotic.

A New Theory: The "Eddington Limit"

The paper also suggests a reason why these stars might act this way. It's not necessarily because they are reaching a specific age in their life story. Instead, it might be a physics problem.

Think of a star as a balloon. If you blow too much air into it (too much energy pushing out), the balloon gets unstable and wobbles. The authors suggest that when a star gets too close to its "Eddington limit" (the point where its own light pushes against its own gravity), it starts to wobble and change brightness. This could happen to stars of different sizes, not just the biggest, most massive ones.

Summary

What they did: Watched 87 massive blue stars for 20 years.
What they found: Only one star was definitely an LBV, and one other acted like one but wasn't massive enough to be one.
What it means: The "LBV tantrum" is a very rare and very short event. It is likely not something every massive star goes through. To be a "real" LBV, a star needs to show both the light changes and the specific massive star characteristics. If it only has the light changes, it might just be a regular star having a temporary glitch.

1. Problem Statement and Context

Luminous Blue Variables (LBVs) are massive stars characterized by episodic outbursts (often $>1$ mag) and spectroscopic changes (S Doradus variables). A central debate in stellar astrophysics concerns the nature of LBVs:

The Standard View (Conti Scenario): LBVs are a brief, transitional evolutionary phase between core hydrogen-burning O-stars and the Wolf-Rayet (WR) phase, lasting $\sim 10^4$ years.
Alternative Views: LBVs could be direct supernova progenitors with longer lifetimes ( $\sim 10^5$ years), or the phenomenon could be a physical instability (Eddington limit related) occurring across a wide range of masses and evolutionary stages, rather than a unique phase.

Previous studies (e.g., Kalari et al. 2018 in the Small Magellanic Cloud) suggested LBV-like variability might be rare, but were limited by short observational baselines (3 years), potentially missing the decadal timescales typical of S Dor variability. This paper aims to quantify the incidence of LBV-like variability in the Large Magellanic Cloud (LMC) using a significantly extended temporal baseline to determine if S Dor variability is a common trait of Blue Supergiants (BSgs) or a rare, specific evolutionary state.

2. Methodology

The study employed a multi-faceted approach combining photometric time-series analysis with spectroscopic follow-up:

Sample Selection:
- The authors utilized the Bonanos et al. (2009) catalog of massive stars in the LMC.
- They identified 254 known Blue Supergiants (BSgs) and LBVs.
- Photometric Coverage: They cross-matched these stars with OGLE-IV (2001–2020) and Gaia DR3 (2014–2017) data.
- Final Sample: 87 BSgs with OGLE data (spanning $\sim$ 20 years) and 37 BSgs with Gaia data (spanning $\sim$ 3 years) were analyzed.
- Note: Known LBVs were excluded from the statistical incidence calculation to avoid bias, though they were analyzed for comparison.
Variability Analysis Techniques:
- $\delta m - \delta t$ Histograms: To detect aperiodic variability, the authors computed the distribution of magnitude differences ( $\Delta m$ ) versus time differences ( $\Delta t$ ) for all pairs of observations. This identifies the incidence of variability at specific timescales.
- von Neumann $\eta$ Index: A statistical test for randomness in time-series data. A high $1/\eta$ value indicates irregular, non-smooth variability.
- Spectral Analysis: For candidates showing photometric variability, the authors obtained new high-resolution spectra using the GHOST spectrograph at Gemini South (for the primary candidate) and compared them with archival data and Gaia XP spectra.
- Stellar Parameter Estimation: Used the Kiwi-GA genetic algorithm coupled with the Fastwind non-LTE atmosphere code to derive effective temperature ( $T_{\text{eff}}$ ), surface gravity ( $\log g$ ), mass loss rates, and luminosity.

3. Key Results

A. Detection of Variability

OGLE Sample (20-year baseline): Only one object, [ST92] 4-13, exhibited significant aperiodic variability resembling S Dor variables (magnitude changes of $\sim 0.7-0.8$ mag over years). Two other candidates (VFTS 698 and VFTS 652) showed variability but were identified as binaries and discarded.
Gaia Sample (3-year baseline): Only HD 269216, a known LBV, showed significant S Dor-like variability. Other stars showed minor fluctuations ( $<0.5$ mag) likely due to noise or sparse sampling.

B. Characterization of [ST92] 4-13

Photometry: The star showed a magnitude decrease of 0.7 mag (2009–2012) followed by an increase, consistent with S Dor behavior.
Spectroscopy:
- Literature classified it as B1 Ia.
- New GHOST spectra (2024) suggest a spectral type change to B2.5 (a $\sim 5,000$ K drop in $T_{\text{eff}}$ ).
- The spectrum shows Balmer emission lines and P Cygni profiles in He I, but lacks the classic P Cygni profiles in lower Balmer lines typically seen in LBVs during quiescence.
Physical Parameters:
- $T_{\text{eff}} \approx 17,000$ K.
- $\log g \approx 1.9$ .
- Luminosity: $\log(L/L_\odot) \approx 4.6$ . This is nearly an order of magnitude lower than typical LBVs (which are usually $>5.0$ ).
Classification: Despite S Dor-like photometry, the low luminosity and lack of specific LBV spectral signatures (e.g., forbidden lines, strong mass loss features) lead the authors to not classify it as a bona fide LBV.

C. Characterization of HD 269216

Confirmed as a known LBV.
Gaia data confirms a brightening trend during the 2014–2016 outburst, consistent with a transition from late-B to A-F spectral types.

D. Duty Cycle Estimation

Based on the detection of only 1 candidate (HD 269216) and 1 ambiguous object ([ST92] 4-13) out of $\sim 250$ BSgs, the authors estimate the duty cycle of the S Dor phase to be $\sim 10^3$ years.
This is significantly shorter than the often-assumed $10^4$ years and aligns with the SMC results from Kalari et al. (2018).

4. Key Contributions and Significance

Refined Definition of LBVs: The paper argues that photometric variability alone is insufficient to classify a star as an LBV. A "bona fide" LBV requires the combination of S Dor photometric variability and specific spectroscopic changes (e.g., P Cygni profiles, spectral type shifts) along with high luminosity. Objects with only photometric variability should be classified as "LBV candidates" (LBVc).
Short S Dor Lifetime: The study provides strong observational evidence that the S Dor phase is extremely short-lived ( $\sim 10^3$ years), suggesting it is a rare, transitory event rather than a long-lived evolutionary stage for all massive stars.
Physical Mechanism vs. Evolutionary Phase: The discovery of S Dor-like variability in a low-luminosity star ([ST92] 4-13) suggests that the underlying physics (potentially related to the Eddington limit and wind-envelope interactions) may operate across a wide range of stellar masses and evolutionary stages, not just in the specific evolutionary window of high-mass LBVs.
Binary Influence: The study highlights that binary interactions (mass transfer) could mimic S Dor light curves in lower-mass systems, complicating the identification of true LBVs in photometric surveys like LSST.

5. Conclusion

The authors conclude that LBVs are likely rare, unique evolutionary objects rather than a common phase for all Blue Supergiants. The incidence of true LBV variability is low, and the S Dor phase is fleeting. Future surveys must prioritize spectroscopic confirmation alongside photometry to avoid misclassifying binary systems or low-mass variables as LBVs. The findings support models where LBV-like behavior is driven by physical instabilities near the Eddington limit, which can occur in various stellar contexts, but the "classic" LBV remains a distinct, high-luminosity phenomenon.