2020A&A...644A..98P

We revisited the young Large Magellanic Cloud star cluster NGC 1971 with the aim of providing additional clues to our understanding of its observed extended main-sequence turnoff (eMSTO), a feature commonly seen in young star clusters that has recently been argued to be caused by a real age spread similar to the cluster age (∼160 Myr). We combined accurate Washington and Stromgren photometry of stars with high membership probability to explore the nature of this 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 are split into two clearly different groups only when the Washington M magnitudes are employed, which delimites the number of spectral features that cause the appearance of the eMSTO. We speculate that Be stars populate the eMSTO of NGC 1971 because (i) Hβ contributes to the M passband, (ii) Hβ emissions are common features of Be stars, and (iii) the Washington M and T₁ magnitudes are tightly correlated; the latter measuring the observed contribution of Hα emission line in Be stars, which is in turn correlated with Hβ emissions. This is the first observational result to our knowledge that indicates that Hβ emissions are the origin of eMSTOs observed in young star clusters. Our results certainly open new possibilities of studying eMSTO from photometric systems with passbands centered at features commonly seen in Be stars.