paper

The He abundance in NGC 1850 A and B: are we observing the early stage of formation of multiple populations in a stellar cluster?

arXiv:2312.12351 · doi:10.1093/mnras/stad3928

Abstract

We present the result of a sample of B-stars in the Large Magellanic Cloud young double stellar cluster NGC 1850 A and NGC 1850 B, observed with the integral-field spectrograph at the Very Large Telescope, the Multi Unit Spectroscopic Explorer. We compare the observed equivalent widths (EWs) of four He lines (4922 , 5015 , 6678 , and 7065 ) with the ones determined from synthetic spectra computed with different He mass fraction (Y=0.25, 0.27, 0.30 and 0.35) with the code SYNSPEC, that takes into account the non-LTE effect. From this comparison, we determined the He mass fraction of the B stars, finding a not homogeneous distribution. The stars can be divided in three groups, He-weak (Y 0.24) and the He-normal (0.24 Y 0.26) belonging to the MS of NGC 1850 A, and the He-rich stars (0.33 Y 0.38) situated in the MS associated to NGC 1850 B. We have analyzed the stellar rotation as possible responsible of the anomalous features of the He lines in the He-rich stars. We provide a simple analysis of the differences between the observed EWs and the ones obtained from the theoretical models with different rotation velocity (V = 0 and 250 Km/s). The resolution of the MUSE spectra do not allow to get a conclusive result, however our analysis support the He-enhanced hypothesis.

Accepted for publication by MNRAS, 10 pages, 8 figures

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