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Why Haven't Loose Globular Clusters Collapsed yet?

Published online by Cambridge University Press:  01 September 2007

Guido De Marchi ,
Francesco Paresce  and
Luigi Pulone
Guido De Marchi
Affiliation:
ESA, Space Science Department, 2200 AG Noordwijk, Netherlands email: [email protected]
Francesco Paresce
Affiliation:
INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, 40129 Bologna, Italy email: [email protected]
Luigi Pulone
Affiliation:
INAF, Observatory of Rome, 00040 Monte Porzio Catone, Italy email: [email protected]
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Abstract

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We report on the discovery of a surprising observed correlation between the slope of the low-mass stellar global mass function (GMF) of globular clusters (GCs) and their central concentration parameter c = log(rt/rc), i.e. the logarithmic ratio of tidal and core radii. This result is based on the analysis of a sample of twenty Galactic GCs, with solid GMF measurements from deep HST or VLT data, representative of the entire population of Milky Way GCs. While all high-concentration clusters in the sample have a steep GMF, low-concentration clusters tend to have a flatter GMF implying that they have lost many stars via evaporation or tidal stripping. No GCs are found with a flat GMF and high central concentration. This finding appears counter-intuitive, since the same two-body relaxation mechanism that causes stars to evaporate and the cluster to eventually dissolve should also lead to higher central density and possibly core-collapse. Therefore, severely depleted GCs should be in a post core-collapse state, contrary to what is suggested by their low concentration. Several hypotheses can be put forth to explain the observed trend, none of which however seems completely satisfactory. It is likely that GCs with a flat GMF have a much denser and smaller core than suggested by their surface brightness profile and may well be undergoing collapse at present. It is, therefore, likely that the number of post core-collapse clusters in the Galaxy is much larger than thought so far.

Keywords

Stars: luminosity functionmass function – Galaxy: globular clusters: general
Type
Contributed Papers
Information
Proceedings of the International Astronomical Union , Volume 3 , Symposium S246: Dynamical Evolution of Dense Stellar Systems , September 2007 , pp. 161 - 165
Copyright
Copyright © International Astronomical Union 2008

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