MUSE Observations of NGC330 in the Small Magellanic Cloud: Helium Abundance of Bright Main-sequence Stars

Abstract

We present observations of the most bright main-sequence stars in the Small Magellanic Cloud stellar cluster NGC 330 obtained with the integral-field spectrograph, the Multi Unit Spectroscopic Explorer, at the Very Large Telescope. The use of this valuable instrument allows us to study both photometric and spectroscopic properties of stellar populations of this young star cluster. The photometric data provide us a precise color-magnitude diagram, which seems to support the presence of two stellar populations of ages of ≍18 Myr and ≍30 Myr assuming a metallicity of Z = 0.002. Thanks to the spectroscopic data, we derive a helium abundance of 10 main-sequence stars within the effective radius of R<SUB>eff</SUB> = 20″ of NGC 330, thus leading to an estimation of $ \langle \epsilon ({\rm{He}}) \rangle = 10.93 ± 0.05(1σ). The helium elemental abundances of stars likely belonging to the two possible stellar populations do not show differences or dichotomy within the uncertainties. Thus, our results suggest that the two stellar populations of NGC 330, if they exist, share similar original He abundances. If we consider stellar rotation velocity in our analysis, a coeval (30 Myr) stellar population, experiencing different values of rotation, cannot be excluded. In this case, the mean helium abundance $\langle \epsilon (\mathrm{He}){\rangle }_{\mathrm{rot}} obtained in our analysis is 11.00 ± 0.05 dex. We also verified that possible non-LTE (NLTE) effects cannot be identified with our analysis because of the spectral resolution and they are within our derived abundance He uncertainties. Moreover, the analysis of the He abundance as a function of the distance from the cluster center of the observed stars does not show any correlation.