The wide upper main sequence and main-sequence turnoff of the ∼800 Myr old star cluster NGC 1831

We present the analysis of the colour-magnitude diagram (CMD) morphology of the similar to 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 similar to 40 percent of stars being slow rotators (Omega/Omega(crit) < 0.5) and the remaining similar to 60 percent 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 kms(-1)at an age of 10 Myr, above the previously suggested threshold of 15 kms(-1)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 similar to 20 kms(-1).

Automated summary

The study analyzes the colour-magnitude diagram morphology of the star cluster NGC1831 in the Large Magellanic Cloud, suggesting that its characteristics can be fully explained in the context of rotation velocity scenario. The cluster's escape velocity is found to be above the previously suggested threshold, indicating that the threshold limit for clusters whose morphology cannot be easily explained should be at least around 20 km/s.