The extended Main Sequence Turn Off cluster NGC1856: rotational evolution in a coeval stellar ensemble

TL;DR

This study investigates the extended main sequence turnoff in the young Magellanic Cloud cluster NGC 1856, attributing it to a combination of rapid and slow stellar rotation within a coeval population.

Contribution

It demonstrates that the split in the main sequence can be explained by two stellar populations with the same age but different rotation rates, highlighting the role of rotation in cluster features.

Findings

Approximately 2/3 of stars are rapid rotators

The slow rotator population may be linked to binary interactions

No red clump progeny observed for slow rotators

Abstract

Multiple or extended turnoffs in young clusters in the Magellanic Clouds have recently received large attention. A number of studies have shown that they may be interpreted as the result of a significant age spread (several 10^8yr in clusters aged 1--2 Gyr), while others attribute them to a spread in stellar rotation. We focus on the cluster NGC 1856, showing a splitting in the upper part of the main sequence, well visible in the color m_{F336W}-m_{F555W}$, and a very wide turnoff region. Using population synthesis available from the Geneva stellar models, we show that the cluster data can be interpreted as superposition of two main populations having the same age (~350Myr), composed for 2/3 of very rapidly rotating stars, defining the upper turnoff region and the redder main sequence, and for 1/3 of slowly/non-rotating stars. Since rapid rotation is a common property of the B-A type stars, the main question raised by this model concerns the origin of the slowly/non-rotating component. Binary synchronization is a possible process behind the slowly/non-rotating population; in this case, many slowly/non-rotating stars should still be part of binary systems with orbital periods in the range from 4 to 500 days. Such periods imply that Roche lobe overflow occurs, during the evolution of the primary off the main sequence, so most primaries may not be able to ignite core helium burning, consistently why the lack of a red clump progeny of the slowly rotating population.