On the origin of UV-dim stars: a population of rapidly rotating shell stars?

TL;DR

This study investigates the origin of UV-dim stars in star clusters, proposing they are rapidly rotating shell stars observed nearly edge-on, with their prevalence decreasing in older clusters due to stellar spin-down.

Contribution

It introduces the hypothesis that UV-dim stars are rapidly rotating shell stars and analyzes their occurrence across clusters of different ages to support this idea.

Findings

UV-dim stars are more common in younger clusters.

The fraction of UV-dim stars decreases in clusters older than 2 Gyr.

Older clusters lack the extended main sequence turn-off, indicating less rapid rotation.

Abstract

The importance of stellar rotation in setting the observed properties of young star clusters has become clearer over the past decade, with rotation being identified as the main cause of the observed extended main sequence turn-off (eMSTO) phenomenon and split main-sequences. Additionally, young star clusters are observed to host large fractions of rapidly rotating Be stars, many of which are seen nearly equator-on through decretion disks that cause self-extinction (the so called "shell stars"). Recently, a new phenomenon has been reported in the $\sim1.5$ Gyr star cluster NGC 1783, where a fraction of the main sequence turn-off stars appears abnormally dim in the UV. We investigate the origin of these "UV-dim" stars by comparing the UV colour-magnitude diagrams of NGC 1850 ($\sim100$ Myr), NGC 1783 ($\sim1.5$ Gyr), NGC 1978 ($\sim2$ Gyr) and NGC 2121 ($\sim2.5$ Gyr), massive star clusters in the Large Magellanic Cloud. While the younger clusters show a non-negligible fraction of UV-dim stars, we find a significant drop of such stars in the two older clusters. This is remarkable as clusters older than $\sim$2 Gyr do not have an eMSTO, thus a large populations of rapidly rotating stars, because their main sequence turn-off stars are low enough in mass to slow down due to magnetic braking. We conclude that the UV-dim stars are likely rapidly rotating stars with decretion disks seen nearly equator-on (i.e., are shell stars) and discuss future observations that can confirm or refute our hypothesis.