Berkeley 51, a young open cluster with four yellow supergiants

TL;DR

Berkeley 51 is a young, obscured open cluster at about 5.5 kpc with four yellow supergiants, characterized by a large B-type star population and an age of approximately 60 million years, based on combined photometric and spectroscopic analysis.

Contribution

This study provides a comprehensive analysis of Berkeley 51 using optical and infrared data, revealing its young age, stellar composition, and the presence of yellow supergiants, which was previously uncertain.

Findings

Cluster age estimated at ~60 million years.

Presence of four yellow supergiants near the instability strip.

Cluster distance approximately 5.5 kpc.

Abstract

The heavily obscured open cluster Berkeley~51 shows characteristics typical of young massive clusters, even though the few previous studies have suggested older ages. We combine optical ($UBV$) and 2MASS photometry of the cluster field with multi-object and long-slit optical spectroscopy for a large sample of stars. We apply classical photometric analysis techniques to determine the reddening to the cluster, and then derive cluster parameters via isochrone fitting. We find a large population of B-type stars, with a main sequence turn-off at B3$\,$V, as well as a large number of supergiants with spectral types ranging from F to M. We use intermediate resolution spectra of the evolved cool stars to derive their stellar parameters and find an essentially solar iron abundance. Under the plausible assumption that our photometry reaches stars still close to the ZAMS, the cluster is located at $d\approx5.5\:$kpc and has an age of $\sim60\:$Ma, though a slightly younger and more distant cluster cannot be ruled out. Despite the apparent good fit of isochrones, evolved stars seem to reside in positions of the CMD far away from the locations where stellar tracks predict Helium burning to occur. Of particular interest is the presence of four yellow supergiants, two on the ascending branch and two others close to or inside the instability strip.