The wide upper main sequence and main sequence turnoff of the ~ 800 Myr old star cluster NGC1831

Abstract: We present the analysis of the colour-magnitude diagram (CMD) morphology of the ~ 800 Myr old star cluster NGC1831 in the Large Magellanic Cloud, exploiting deep, high-resolution photometry obtained using the Wide Field Camera 3 onboard the Hubble Space Telescope. We perform a simultaneous analysis of the wide upper main sequence and main sequence turn-off observed in the cluster, to verify whether these features are due to an extended star formation or a range of stellar rotation rates, or a combination of these two effects. Comparing the observed CMD with Monte Carlo simulations of synthetic stellar populations, we derive that the morphology of NGC1831 can be fully explained in the context of the rotation velocity scenario, under the assumption of a bimodal distribution for the rotating stars, with ~40% of stars being slow-rotators ($\Omega$ / $\Omega_{crit}$ < 0.5) and the remaining ~ 60% being fast rotators ($\Omega$ / $\Omega_{crit}$ > 0.9). We derive the dynamical properties of the cluster, calculating the present cluster mass and escape velocity, and predicting their past evolution starting at an age of 10 Myr. We find that NGC1831 has an escape velocity $v_{esc}$ = 18.4 km/s, at an age of 10 Myr, above the previously suggested threshold of 15 km/s, below which the cluster cannot retain the material needed to create second-generation stars. These results, combined with those obtained from the CMD morphology analysis, indicate that for the clusters whose morphology cannot be easily explained only in the context of the rotation velocity scenario, the threshold limit should be at least ~ 20 km/s.