The very low-mass stellar content of the young supermassive Galactic star cluster Westerlund 1

TL;DR

This study uses deep near-infrared observations to analyze the stellar mass function of Westerlund 1, revealing a flattened mass distribution at low masses and insights into its potential evolution into a globular cluster.

Contribution

First detailed measurement of the low-mass stellar content and mass function of Westerlund 1 using HST/WFC3 data, including the identification of a flattened mass function at low masses.

Findings

Mass function flattens at low masses (~0.15 M_sun)

Peak mass similar to field and embedded clusters

Total mass estimated at 44-57 x 10^3 M_sun

Abstract

We present deep near-infrared HST/WFC3 observations of the young supermassive Galactic star cluster Westerlund 1 and an adjacent control field. The depth of the data is sufficient to derive the mass function for the cluster as a function of radius down to 0.15 M$_\odot$ in the outer parts of the cluster. We identify for the first time a flattening in the mass function (in logarithmic units) at a mass range that is consistent with that of the field and nearby embedded clusters. Through log-normal functional fits to the mass functions we find the nominal peak mass to be comparable to that of the field and nearby embedded star clusters. The width of a log-normal fit appears slightly narrow compared to the width of the field IMF, closer to the values found for globular clusters. The subsolar content within the cluster does not appear to be mass segregated in contrast to the findings for the supersolar content. The total mass of Westerlund 1 is estimated to be 44-57 $\times 10^3$ M$_\odot$ where the main uncertainty is the choice of the isochrone age and the higher mass slope. Comparing the photometric mass with the dynamically determined mass, Westerlund 1 is sufficiently massive to remain bound and could potentially evolve into a low-mass globular cluster.