Multiple populations of Hbeta emission line stars in the Large Magellanic Cloud cluster NGC1971

TL;DR

This study suggests that Hbeta emission from Be stars causes the extended Main Sequence turnoff in the young LMC cluster NGC1971, providing a new perspective on the eMSTO phenomenon.

Contribution

It is the first observational evidence linking Hbeta emissions in Be stars to the eMSTO feature in young star clusters.

Findings

Blue and red stars in the eMSTO show no abundance inhomogeneities.

Hbeta emissions are likely responsible for the eMSTO appearance.

Photometric passbands centered on Hbeta can reveal Be star populations.

Abstract

We revisited the young Large Magellanic Cloud star cluster NGC1971 with the aim of providing additional clues to our understanding of its observed extended Main Sequence turnoff (eMSTO), a feature common seen in young stars clusters, which was recently argued to be caused by a real age spread similar to the cluster age (~160 Myr). We combined accurate Washington and Stromgren photometry of high membership probability stars to explore the nature of such an eMSTO. From different ad hoc defined pseudo colors we found that bluer and redder stars distributed throughout the eMSTO do not show any inhomogeneities of light and heavy-element abundances. These 'blue' and 'red' stars split into two clearly different groups only when the Washington $M$ magnitudes are employed, which delimites the number of spectral features responsible for the appearance of the eMSTO. We speculate that Be stars populate the eMSTO of NGC1971 because: i) Hbeta contributes to the M passband; ii) Hbeta emissions are common features of Be stars and; iii) Washington M and T1 magnitudes show a tight correlation; the latter measuring the observed contribution of Halpha emission line in Be stars, which in turn correlates with Hbeta emissions. As far as we are aware, this is the first observational result pointing to Hbeta emissions as the origin of eMSTOs observed in young star clusters. The presence outcome will certainly open new possibilities of studying eMSTO from photometric systems with passbands centered at features commonly seen in Be stars.