Hubble Space Telescope survey of Magellanic Cloud star clusters. UV-dim stars in young clusters

TL;DR

This study uses Hubble Space Telescope data to identify UV-dim stars in young Magellanic Cloud clusters, revealing that slow rotators are common and providing insights into stellar rotation effects on cluster populations.

Contribution

It is the first comprehensive investigation of UV-dim stars in clusters younger than 200 Myr, linking UV dimness to slow stellar rotation.

Findings

UV-dim stars are common in young clusters.

Most UV-dim stars are slow rotators on the blue main sequence.

NGC1850 and BHRT5b are not gravitationally bound.

Abstract

Young and intermediate-age star clusters of both Magellanic Clouds exhibit complex color-magnitude diagrams. In addition to the extended main-sequence turn-offs (eMSTOs), commonly observed in star clusters younger than ~2 Gyr, the clusters younger than ~800 Myr exhibit split main sequences (MSs). These comprise a blue MS, composed of stars with low-rotation rates, and a red MS, which hosts fast-rotating stars. While it is widely accepted that stellar populations with different rotation rates are responsible for the eMSTOs and split MSs, their formation and evolution are still debated. A recent investigation of the ~1.7 Gyr old cluster NGC1783 detected a group of eMSTO stars extremely dim in UV bands. Here, we use multi-band Hubble Space Telescope photometry to investigate five star clusters younger than ~200 Myr, including NGC1805, NGC1818, NGC1850, and NGC2164 in the Large Magellanic Cloud, and the Small-Magellanic Cloud cluster NGC330. We discover a group of bright MS stars in each cluster that are significantly dim in the F225W and F275W bands, similar to what is observed in NGC1783. Our result suggests that UV-dim stars are common in young clusters. The evidence that most of them populate the blue MS indicates that they are slow rotators. As a byproduct, we show that the star clusters NGC1850 and BHRT5b exhibit different proper motions, thus corroborating the evidence that they are not gravitationally bound.