Exploring massive star early evolution: the case of the Herschel 36 A triple system

ABSTRACT Theoretical models show that some massive stars have not yet arrived at the zero-age main sequence (ZAMS) at the end of the accretion phase. At that time, they have lost their thick envelopes and thus could be optically visible. Although some candidates to optically observable ZAMS stars have been reported, the evolutionary status of none of them has been confirmed yet. The O-type triple system Herschel 36 A (H36A) is one of these candidates. We present the quantitative spectral analysis of the individual stellar components of H36A and investigate the evolutionary status of the system by contrasting main-sequence and pre-main-sequence models. Overall, the derived parameters suggest that the components of H36A could be pre-main-sequence stars going through the very last contraction to the ZAMS. However, the possibility of them already being on the main sequence is not yet ruled out. This study highlights the importance of considering multiple evolutionary models and shows that H36A represents a key object for understanding massive star formation and early evolution.