SCYON Abstract

Received on January 27 2007

Why haven't loose globular clusters collapsed yet?

AuthorsGuido De Marchi (1), Francesco Paresce (2) and Luigi Pulone (3)
Affiliation
(1) ESA, Space Science Department, Keplerlaan 1, 2200 AG Noordwijk, Netherlands
(2) INAF, Viale del Parco Mellini 84, 00186 Rome, Italy
(3) INAF, Osservatorio di Roma, Via di Frascati 33, 00040 Monte Porzio Catone, Italy
Accepted byAstrophysical Journal
Contactgdemarchi@rssd.esa.int
URLhttp://arxiv.org/abs/astro-ph/0701613
Links

Abstract

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. All the high-concentration clusters in the sample have a steep GMF, most likely reflecting their initial mass function. Conversely, 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, more concentrated clusters should have lost proportionately more stars and have a shallower GMF than low concentration clusters, contrary to what is observed. It is possible that severely depleted GCs have also undergone core collapse and have already recovered a normal radial density profile. It is, however, more 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. In either case, we may have so far seriously underestimated the number of post core-collapse clusters and many may be lurking in the Milky Way.