The Physical Parameters of Four WC-type Wolf-Rayet Stars in the Large Magellanic Cloud: Evidence of Evolution

Abstract

We present a spectral analysis of four LMC WC-type Wolf-Rayet (WR) stars (BAT99-8, BAT99-9, BAT99-11, and BAT99-52) to shed light on two evolutionary questions surrounding massive stars. The first is: are WO-type WR stars more oxygen enriched than the WC-type stars, indicating further chemical evolution, or are the strong high-excitation oxygen lines in the WO-type stars an indication of higher temperatures. This study will act as a baseline for answering the question of where WO-type stars fall in WR evolution. Each star's spectrum, extending from 1100~\AA\ to 25000~\AA, was modeled using \cmfgen\ to determine the star's physical properties such as luminosity, mass-loss rate, and chemical abundances. The oxygen abundance is a key evolutionary diagnostic, and with higher resolution data and an improved stellar atmosphere code, we found the oxygen abundance to be up to a factor of 5 lower than previous studies. The second evolutionary question revolves around the formation of WR stars: Do they evolve by themselves or is a close companion star necessary for their formation? Using our derived physical parameters, we compared our results to the Geneva single-star evolutionary models and the BPASS binary evolutionary models. We found that both the Geneva solar metallicity models and BPASS LMC metallicity models are in agreement with the four WC-type stars, while the Geneva LMC metallicity models are not. Therefore, these four WC4 stars could have been formed either via binary or single-star evolution.