MUSE spectroscopic observations of the Young Massive Cluster NGC1850

TL;DR

This study uses MUSE spectroscopy to analyze the chemical composition, dynamics, and rotation of stars in the Young Massive Cluster NGC1850, revealing bimodal oxygen abundance, rotational effects, and chemical properties consistent with the LMC's evolution.

Contribution

First detailed spectroscopic analysis of NGC1850's chemical properties and stellar rotation, linking observed features to cluster dynamics and stellar evolution.

Findings

Average metallicity of -0.31 dex for the cluster

Detection of bimodal oxygen line strength among TO stars

Correlation between stellar rotation, colour, and line broadening

Abstract

NGC1850 is the nearest Young Massive Cluster of the Local Group with a mass similar to those of Galactic globular clusters. Recent studies have revealed an extended morphology of its MSTO, which can be interpreted as a spread in either age or internal rotation. An accurate spectroscopic determination of its chemical properties is still missing. We analyse spectra obtained with MUSE in adaptive optics mode of 1167 stars in both components of this cluster (NGC1850A and NGC1850B). Thanks to this dataset, we measure an average metallicity of <[M/H]>=-0.31 +/- 0.01, a mean Ba abundance of <[Ba/Fe]>=+0.40 +/- 0.02 and a systemic radial velocity of <v_{LOS}>=251.1 +/- 0.3 km/s. The dispersion of the radial velocities suggests a dynamical mass of log(M/Ms)=4.84 +/- 0.1, while no significant systemic rotation is detected. We detect a significant bimodality in OI line strength among the TO stars of NGC1850A with ~66% of stars with [O/Fe]~-0.16 and the rest with no detectable line. The majority of O-weak stars populate preferentially the red side of the MSTO and show H lines in emission, suggesting that they are Be stars rotating close to their critical velocity. Among normal MSTO stars, red stars have on average broader line profiles than blue ones, suggesting a correlation between colour and rotational velocity. The mean metallicity of this cluster lies at the metal-rich side of the metallicity distribution of the LMC following its age-metallicity relation. The Ba and O abundances agree with those measured in the bar of this galaxy. The observed spread in OI line width among its MS stars can be interpreted as an effect of rotational mixing occurring in the envelopes of O-weak stars. The correlation between line broadening and colour suggests that the observed colour spread among turn-off stars can be due to a wide range in rotational velocity covered by these stars.