Discovery of Weak CN Stars in the Large Magellanic Cloud and Comparison to Stellar Models

Abstract The recent discovery that red supergiants in M31 and M33 contain a weak CN spectral absorption feature at ​​​​​​∼8000 Å prompted us to analyze a large CTIO 4 m Hydra spectral database of stars in the Large Magellanic Cloud (LMC). We find 751 confirmed weak CN stars in the LMC, present an overview of their spectral and photometric properties, and compare them with the weak CN stars in M31 and M33. We use a method for automatically identifying weak CN stars based on a spectral template and find that weak CN stars are almost exclusively short-lived massive stars in the core He-burning evolutionary phase. We derive a mean main-sequence age of 40 Myr for these stars through a Bayesian approach to isochrone fitting that is consistent across all three galaxies. We then examine possible evolutionary processes and atmospheric conditions that could lead to weak CN stars using a suite of low-resolution model spectra. We find a preferred range of values for T eff and log g in O-rich stars that would result in the CN absorption and also speculate that this feature could arise due to the N enhancement expected in fast-rotating stars, though more targeted modeling is required.