Very low-mass stellar content of the young supermassive Galactic star cluster Westerlund 1^⋆

¹ Gemini Observatory, Casilla 603, La Serena, Chile
e-mail: manderse@gemini.edu
² Space Telescope Science Institute, 3700 San Martin Dr Baltimore, MD 21218, USA
³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁴ Argelander Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
⁵ Research & Scientific Support Department, ESA ESTEC, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands
⁶ Institute for Astronomy ETH, Physics Department, HIT J 22.4, 8093 Zurich, Switzerland
⁷ SOFIA-USRA, NASA Ames Research Center, MS N211-3, Moffett Field, CA 94035, USA

Received: 16 October 2013
Accepted: 10 February 2016

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_⊙ 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 initial mass function, 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 × 10³M_⊙ 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 lowmass globular cluster.