The little-studied cluster Berkeley 90. III. Cluster parameters

TL;DR

This study precisely characterizes the open cluster Berkeley 90, revealing its distance, age, mass, and stellar population, providing insights into the environment of extremely massive stars and their formation.

Contribution

First detailed parameter determination of Berkeley 90 using multi-wavelength data, enhancing understanding of massive star environments and cluster properties.

Findings

Distance of 3.5 kpc with uncertainties

Cluster age up to 3 million years

Mass around 1000 to 1500 solar masses

Abstract

The open cluster Berkeley 90 is the home to one of the most massive binary systems in the Galaxy, LS III +46$^{\circ}$11, formed by two identical, very massive stars (O3.5 If* + O3.5 If*), and a second early-O system (LS III +46$^{\circ}$12 with an O4.5 IV((f)) component at least). Stars with spectral types earlier than O4 are very scarce in the Milky Way, with no more than 20 examples. The formation of such massive stars is still an open question today, and thus the study of the environments where the most massive stars are found can shed some light on this topic. To this aim, we determine the properties and characterize the population of Berkeley 90 using optical, near-infrared and WISE photometry and optical spectroscopy. This is the first determination of these parameters with accuracy. We find a distance of $3.5^{+0.5}_{-0.5}$ kpc and a maximum age of 3 Ma. The cluster mass is around $1000$ $M_{\odot}$ (perhaps reaching $1500$ $M_{\odot}$ if the surrounding population is added), and we do not detect candidate runaway stars in the area. There is a second population of young stars to the Southeast of the cluster that may have formed at the same time or slightly later, with some evidence for low-activity ongoing star formation.