Bruck 88 : a young star cluster with an old age resemblance in the outskirts of the Small Magellanic Cloud

TL;DR

This study reveals that the young star cluster Bruck 88 in the Small Magellanic Cloud appears similar to old globular clusters due to a bright red giant star dominating its integrated spectrum, despite its actual youth.

Contribution

It demonstrates that a single bright red giant star can cause a young cluster to resemble an old globular cluster in integrated light, highlighting the importance of resolving individual stars.

Findings

Bruck 88 is a young cluster with an old appearance in integrated spectra.

A bright red giant star dominates the cluster's integrated light.

The cluster's true age is around 100-150 Myr, not old.

Abstract

We present spectroscopic and photometric results for the Small Magellanic Cloud (SMC) cluster Bruck 88. From the comparison of the cluster integrated spectrum with template cluster spectra we found that the Milky Way globular cluster template spectra are the ones which best resemble it. However, the extracted cluster colour magnitude diagram reveals that Bruck 88 is a young cluster (log(t) = 8.1 +- 0.1). The derived cluster age is compatible with the presence of a Bright Red Giant (BRG) star located ~ 2.6 arcsec in the sky from the cluster centre. We serendipitously observed HW 33, a star cluster located ~ 3 arcmin to the south-east from Bruck 88. We obtained for the cluster the same age than Bruck 88 and surprisingly, a BRG star located within the cluster radius also appears to be compatible with the cluster age. We estimated the MK type of the BRG star in the Bruck 88 field to be in the range G9 II/Ib - K1 III. By combining the spectrum of a star within this MK type range with a 100-150 Myr template cluster integrated spectrum, we found that a proportion 85/15 in the sense BRG/template results in a spectrum which best resembles that of Bruck 88. This result confirms that a BRG star dominates the cluster integrated spectrum, so that it causes the globular cluster appearance of its integrated light.