  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.
