EWOCS ongoing project: Cluster dynamics

The formation and evolution of starburst clusters is still an open issue. Astronomers proposed two main hypotheses, both supported by the high central density and smooth radial density decline in starburst clusters, which is different than the typical hierarchical structure of low mass clusters (Rathborne+2015; Gutermuth+2008; Kuhn+2014, 2015a, 2015b). These cluster may have formed by a monolithic gravitational infall of highly concentrated clouds (Adams 2000; Kroupa, Aarseth & Hurley 2001; Baumgardt & Kroupa2007; Banerjee & Kroupa2013; Pfalzner & Kaczmarek 2013) or after the assemble of less dense subclusters (Allison+2009; Longmore et al. 2014).


Current studies, which are mainly focused on the massive stars, claim that the morphology, internal dynamic, and mass distribution of Westerlund 1 could be reproduced with a formation by monolithic infall, but hierarchical formation scenario is not excluded. Besides Westerlund 1 has a well defined sequence in color-magnitude diagrams, which can be well fitted by single isochrones, with no evidence for age spread >0.4 Myrs. The cluster, thus, likely formed in single and rapid star formation episode (Kudryavsteva+2012). However, this is far to be a conclusive result, since several authors suggest that the cluster is the product of more events of star formation.


Besides, Westerlund 1 massive members have a small velocity dispersion according to HST data at high astrometric precision (Cottar+2012). Westerlund 1 seems thus to be a dynamically stable cluster, that will not disperse for a long time.


The EWOCS project will provide a list of stars associated to Westerlund 1 down to about 1 Solar mass. In particular, we will be able to identify for the first time members dispersed in the cluster halo, where selection criteria not based on X-rays suffer strong contamination from unrelated stars. We will thus analyze the cluster dynamics down to low masses both in the core and in the halo. The study of the cluster dynamics will thus provide answers to the following important questions:


  • What fraction of the initial stellar population of starburst clusters is and will be lost?


  • What is the final evolution of starburst clusters?


  • What is their contribution to the field stellar population?


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