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The Age of the R127 & R128 Clusters: Implications for the LBV

Mojgan Aghakhanloo, Jeremiah W. Murphy, Nathan Smith, Joseph Guzman

Abstract

We infer the age of the R127 and R128 clusters in the Large Magellanic Cloud (LMC) using Strömgren photometry from the literature and the age-dating algorithm, \textit{Stellar Ages}. Analysis using single-star evolutionary models shows a substantial discrepancy between the relative numbers of bright blue stars and lower-mass stars as compared to expectations from a Salpeter mass function, and yields a younger age for the brightest blue stars than for the rest of the cluster. This inconsistency reflects an emerging trend among young clusters in the Local Group. In general, the resolution may be binary evolution or very rapid rotation, although in the specific case of R127 and R128 clusters, unknown incompleteness in the data may also affect the relative numbers of low- and high-mass stars. The discrepancy grows toward fainter magnitudes, suggesting that the dataset is likely incomplete. However, when the five brightest stars are excluded, the observed and expected counts become consistent, demonstrating that the brightest stars are peculiar. These findings have direct implications for the luminous blue variable (LBV) R127, which is the only confirmed LBV in the LMC located within a young stellar cluster. LBVs have traditionally been considered products of single-star evolution, although there is growing recognition that binary interactions may play a critical role in their evolution. A more complete dataset, particularly deeper imaging with the Hubble Space Telescope, is needed to confirm whether the apparent absence of coeval stars arises solely from observational incompleteness or the broader trend of inconsistency in young cluster modeling.

The Age of the R127 & R128 Clusters: Implications for the LBV

Abstract

We infer the age of the R127 and R128 clusters in the Large Magellanic Cloud (LMC) using Strömgren photometry from the literature and the age-dating algorithm, \textit{Stellar Ages}. Analysis using single-star evolutionary models shows a substantial discrepancy between the relative numbers of bright blue stars and lower-mass stars as compared to expectations from a Salpeter mass function, and yields a younger age for the brightest blue stars than for the rest of the cluster. This inconsistency reflects an emerging trend among young clusters in the Local Group. In general, the resolution may be binary evolution or very rapid rotation, although in the specific case of R127 and R128 clusters, unknown incompleteness in the data may also affect the relative numbers of low- and high-mass stars. The discrepancy grows toward fainter magnitudes, suggesting that the dataset is likely incomplete. However, when the five brightest stars are excluded, the observed and expected counts become consistent, demonstrating that the brightest stars are peculiar. These findings have direct implications for the luminous blue variable (LBV) R127, which is the only confirmed LBV in the LMC located within a young stellar cluster. LBVs have traditionally been considered products of single-star evolution, although there is growing recognition that binary interactions may play a critical role in their evolution. A more complete dataset, particularly deeper imaging with the Hubble Space Telescope, is needed to confirm whether the apparent absence of coeval stars arises solely from observational incompleteness or the broader trend of inconsistency in young cluster modeling.
Paper Structure (11 sections, 15 figures)

This paper contains 11 sections, 15 figures.

Table of Contents

  1. Introduction
  2. Observations
  3. Age-dating algorithm (Stellar Ages)
  4. Evolutionary models
  5. Sibling Probability
  6. Results
  7. Most Likely Age
  8. The five brightest stars
  9. Population-level inconsistencies with single-star evolutionary models
  10. Summary
  11. Most Likely Ages using Different Models

Figures (15)

  • Figure 1: Strömgren photometry for 147 stars associated with the R127 & R128 clusters and their surrounding field stars. The data are obtained from Figure 8 of H03 and represent the most complete dataset available for this region. R127 is the brightest star in the region.
  • Figure 2: Gaia DR3 data within 1 of R127. 349 out of 887 stars lack magnitude information. The plot shows only stars brighter than magnitude 18.5, comprising 169 sources. A gap around the 16th magnitude suggests that the Gaia DR3 data are incomplete along the main sequence in this region and therefore unsuitable for age-dating the environment of R127.
  • Figure 3: Single-star evolutionary isochrones for LMC-like metallicity ([Fe/H] = $-0.40$) at different ages. The left panel represents MIST models, the middle panel shows MIST models with a $\Omega_{ZAMS}/\Omega_{crit}=0.4$, and the right panel shows PARSEC isochrones without rotation. The isochrones are very similar during the main-sequence phase, which is why we obtain consistent results when using different sets of isochrones (see Appendix \ref{['sec:appendix']}).
  • Figure 4: Normalized weights as a function of log age marginalized over metallicity and rotation. The central points represent the median age, while the width of the violin indicates the distribution of possible weights. Although there are minor peaks, no strong peak emerges. This underscores the value of a method that provides age constraints on individual stars, especially in cases where the population lacks a clear age signature.
  • Figure 5: The most likely ages inferred for individual stars. The two brightest stars, R127 and R128, have estimated ages of log${10}$(t/yr)$\sim$ 6.8-7.0, while the other bright cluster members are around log${10}$(t/yr)$\sim$ 6.4 old.
  • ...and 10 more figures