He abundance in NGC 1850 A and B: Are we observing the early stage of the formation of multiple populations in a stellar cluster?
Date Issued
2024
Author(s)
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 $\mathring{\mathrm A}$, 5015 $\mathring{\mathrm A}$, 6678
$\mathring{\mathrm A}$, and 7065 $\mathring{\mathrm A}$) 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 $\lt$ 0.24) and the He-normal (0.24 $\leqslant$ Y
$\leqslant$ 0.26) belonging to the MS of NGC 1850 A, and the He-rich stars
(0.33 $\leqslant$ Y $\leqslant$ 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$\sin{i}$ = 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.
Volume
528
Issue
1
Start page
909
Issn Identifier
0035-8711
Rights
open.access
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