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

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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 Å 5015 Å 6678 Å and 7065 Å) 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 (Vsin i = 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.
Funding Information
  • NASA
  • ESA
  • Space Telescope Science Institute

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