Cepheids and rotation in NGC 1866 (Costa et al.)

Vol. 631
In section 5. Galactic structure, stellar clusters and populations

Multiple stellar populations in NGC 1866. New clues from Cepheids and colour-magnitude diagram

by G. Costa, L. Girardi, A. Bressan, Y. Chen, P. Goudfrooij, P. Marigo, T.S. Rodrigues, and A. Lanza 2019, A&A, 631, A128 alt

GC 1866 of the Magellanic Clouds is a young cluster with a turn-off mass of 4−5 Msol. The cluster not only contains a large number of evolved stars but also an extremely high number of Cepheid variables. Recent Hubble Space Telescope observations show the presence of an extended main-sequence turn-off (eMSTO) and a split main sequence, similar to what is observed in a few other clusters. Fast rotation is believed to contribute to the eMSTOs in clusters up to ages as long as 2 Gyr, and the cluster may also arguably harbor multiple stellar populations. The authors adopt the new PARSEC stellar evolutionary tracks with overshooting and rotation to obtain the ages and initial rotation velocities of five well-studied Cepheids of the cluster. Four Cepheids are found to belong to a young, initially slowly rotating, population. The fifth one, however, is significantly older with a high initial rotation velocity. The observed multiple main sequences and turn-offs indicate that the younger population is mainly made of slowly rotating stars, as is the case of the four younger Cepheids. However, the older population is mainly made of initially fast rotating stars, as is the case of the fifth Cepheid. The study reinforces that some young clusters harbor multiple populations. And more importantly, the study may indicate that the older first population may inherit the angular momentum from the parent cloud, while stars of the second younger population do not.