The quest for the initial stellar mass function in starburst clusters
The initial stellar mass function describes the relative numbers of low-mass and high-mass stars in a stellar population. The population can be a defined volume of stars in the field of the Galaxy, an old globular cluster, or a young star cluster. While the MF is measured in these diverse environments, only young, and massive star clusters provide a prestine view of the initial mass distribution at the time when the stars were formed. Starburst clusters have the additional advantage to cover the full mass range known in the present-day universe, from the hydrogen burning limit to solar mass stars up to stars with more than 100 solar masses. Despite their very young age of only a few million years (in stellar terms), it is challenging to measure the initial stellar mass function even in starburst clusters. The rapid evolution of starburst clusters hampers the MF derivation in two ways: 1) High-mass stars burn their fuel extremely quickly, change their brightness rapidly and vanish in less than 10 Myr, rendering the determination of their masses even at young ages difficult. 2) The cluster itself evolved on short dynamical timescales of a few Myr, when the high-mass stars sink into the core and the low-mass members are distributed in a halo around the dense core. A detailed understanding of the cluster evolution and the entire region possibly containing cluster stars is required to measure the prestine, initial mass distribution.
The present-day mass function of the Arches, as an example, reveals a core with
a flat mass function caused by an exceptionally high density of massive stars. The stellar mass distribution observed in less extreme star-forming regions such as the Orion region has a significantly steeper slope. The canonical value used throughout the universe when individual cluster stars cannot be resolved is Gamma = -1.3 (Salpeter 1955). While this value seems to match moderate star-forming regions extremely well, the majority of stars likely formed in strong starburst events. These starbursts as observed in distant galaxies bring forth hundreds of starburst clusters with masses at least one order of magnitude more massive than the Milky Way starburst clusters. The question that arises is whether our interpretation of the light from distant, unresolved starburst clusters is biased by the assumption of a universal mass function, and whether or not such an assumption is justified even in the closest examples of resolved starburst clusters.
The cluster membership problem
Ideally, the entire cluster population in a starburst has to be measured to derive its initial
mass function. What is observed, however, is the present-day brightness distribution of stars in the cluster field. As all starburst clusters in the Milky Way are observed towards the center of the Galaxy or along spiral arms, contamination by field stars biases the measured cluster MF. In near-by star-forming regions, cluster membership and hence the full spatial distribution of cluster stars is established from sharing the cluster proper motion. Due to the large distances of several kiloparsecs and the high number density of objects along the line of sight, this method could not be emploid for starburst clusters until now. With the high spatial resolution and astrometric precision of adaptive optics observations at 8-10 m class telescopes (Keck, VLT), it has become feasible to conduct cluster membership studies in Milky Way starburst clusters. These observational campaigns are designed to yield the total extent of each cluster population, the spatial distribution of cluster members, especially in a potential low-mass stellar halo around each cluster, and hence theunbiased initial stellar MF as prestinely as possible.
The colour-magnitude diagram of the Arches cluster illustrates the influence of field contamination on the stellar light and mass distribution. Below 20 Msun, clusters members
(black dots) cannot be distinguished from field stars (red triangles) based on their colours alone. Proper motion membership information was used to separate field stars from the cluster population in this diagram. These proper motion studies will lead to revised, and much more accurate, stellar mass functions in all known Milky Way starburst clusters.