Instantaneous starburst of the massive clusters Westerlund 1 and NGC 3603 YC

TL;DR

This paper introduces a new method combining astrometric and Bayesian analysis to precisely determine the age spread of starburst clusters, revealing near-instantaneous star formation in Westerlund 1 and NGC 3603 YC.

Contribution

A novel two-step approach for measuring age spreads in starburst clusters using astrometry and Bayesian isochrone fitting.

Findings

Age spread in Westerlund 1 is less than 0.4 Myr.

Age spread in NGC 3603 YC is less than 0.1 Myr.

Star formation occurred almost instantaneously in both clusters.

Abstract

We present a new method to determine the age spread of resolved stellar populations in a starburst cluster. The method relies on a two-step process. In the first step, kinematic members of the cluster are identified based on multi-epoch astrometric monitoring. In the second step, a Bayesian analysis is carried out, comparing the observed photometric sequence of cluster members with sets of theoretical isochrones. When applying this methodology to optical and near-infrared high angular resolution Hubble Space Telescope (HST) and adaptive optics observations of the ~5 Myr old starburst cluster Westerlund 1 and ~2 Myr old starburst cluster NGC 3603 YC, we derive upper limits for the age spreads of 0.4 and 0.1 Myr, respectively. The results strongly suggest that star formation in these starburst clusters happened almost instantaneously.