Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006aas...209.2616d&link_type=abstract
2007 AAS/AAPT Joint Meeting, American Astronomical Society Meeting 209, #26.16; Bulletin of the American Astronomical Society, V
Astronomy and Astrophysics
Astronomy
Scientific paper
We report on the discovery of a surprising correlation between the shape of the stellar global mass function (GMF) of globular clusters (GCs) and their central concentration parameter c=log(rt/rc). This result is based on the analysis of a sample of twenty Galactic GCs with solid GMF measurements from deep HST or VLT data. Clusters of low central concentration tend to have a flatter GMF than high-concentration objects and must therefore have lost a conspicuous fraction of their original stars via evaporation or tidal stripping. Conversely, all high concentration clusters in the sample have a steep GMF, which we infer does not deviate substantially from the IMF. This correlation is counter-intuitive, since the same two-body relaxation mechanism that drives a cluster towards higher central density and possibly core collapse should also eventually cause its dissolution via evaporation. Therefore, more concentrated clusters should have lost a larger fraction of their stars and have a shallower GMF than low concentration clusters, contrary to what is observed. Even if the leading cause of cluster dissolution is not evaporation but tidal stripping (via disc or bulge shocking), these mechanisms should in any case accelerate the evolution of a cluster towards higher central concentration. It is possible that the GCs that have lost a significant fraction of their original mass and have already undergone core collapse have also recovered a normal radial density profile. If true, this implies that we may have so far seriously underestimated the number of post core-collapse clusters and that an unknown number of them may be lurking in the Milky Way. The relevance of the observed structure and central concentration of clusters as indicators of their dynamical state would then be undermined as would our understanding of the past and future evolution of the Galactic GC system.
de Marchi Guido
Paresce Francesco
Pulone Luigi
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