Dynamics and stellar evolution

Involved people at OAB: Bellazzini, Cacciari, Ferraro, Fusi Pecci.

Many new results are now supporting the claim that dynamical evolution of GGCs can affect their stellar populations. Schematically, both the integrated cluster colors and the properties of individual objects confirm the existence of dynamically induced variations in the evolution of many cluster members.

In this scenario, we have started a long term project which aims at using GGCs as a laboratory to study the impact of the (internal) environment and (external) dynamical effects on the evolution of the cluster stellar population. The stellar interactions can deeply affect the dynamical status of the cluster, (1) generating a wide variety of peculiar objects with respect to a normal stellar population and/or (2) producing strong signatures on the LF of canonical sequences.

1. The HST observations we obtained for this project allowed us to collect a homogeneous data-base for 8 GGCs, and 5 more are being observed during cycle 9. The analyzed data in 4 GGCs showed important results, e.g. the discovery of the spectacular population of Blue Stragglers (BSS) in M80.

   The discovery of UV objects lying in the vicinity of LLGCX sources [Ferraro et al. (2000, ApJ 537, 312)] and/or H$\alpha$-excess stars [Ferraro et al. (2000, ApJ, 542, L29)] suggests that these stars might be a new sub-class of cataclysmic variables, produced by stellar interactions in a dense core. Similar objects, with strong UV emission, have been also found in NGC288 [Bellazzini & Messineo (2000)]; for these stars a VLT-FORS2 spectroscopic follow-up has been obtained. A method for deriving the fraction of binary systems in GGCs from the secondary main sequence in CMD is also under development.

   In addition, deep ROSAT HRI X-ray observations will be used to derive a lower limit to the diffuse X-ray emission of a sample of GGCs. More than 20 hours of ROSAT observing time have been allocated to this program and preliminary results on the detection of a LLGCX in NGC288 have already been published. This part of the research is in collaboration with Rood and Sarazin (Univ. Virginia), and Paltrinieri (Univ. Roma).

2. The LF of stars below the MS TO is a powerful tool to study the modifications of the Present Mass Function (PMF) in GGCs due to internal dynamical evolution of the cluster and its interaction with the Milky Way. These arguments are very important to derive a reliable Initial Mass Function (IMF) from the observed PMF. Both the cluster internal dynamics and the tidal interaction with the Milky Way significantly contribute to modify the PMF. Our research is now concentrating on finding clear evidence for disk shocking effects on the LF of low mass stars. 24 hours of observing time with VLT-FORS1 have been allocated to this project: 3 GGCs, namely NGC6218, NGC6838, and NGC6712, have been observed.

   We have fully analyzed the NGC6712 data, and we have found the first evidence of a disrupting globular cluster. NGC6712 is a low-concentration, intermediate-high metallicity GC with some peculiar characteristics. First, it is the lowest density GC containing a low mass X-ray binary, and since LMXB's are thought to form via tidal capture in high density clusters, its presence in the core of a loose cluster is somewhat surprising. Second, it appears to be experiencing a severe interaction with the Galaxy due to its orbit which gives rise to several passages through disk and bulge.

   Our VLT observations of NGC6712 showed the effect on the cluster stellar population of a severe interaction with the disk and the bulge of the Galaxy:

   (a) We provided the first unambiguous evidence that this cluster has an inverted mass function, since the MS Luminosity Function (hence the global Mass Function) sensibly drops below the TO (Andreuzzi et al. 2001, A&A, in press). NGC6712 is the only GGCs known so far for which this effect has been detected. As suggested by its orbit, and supported by N-body simulations, this is probably due to a severe depletion of low mass stars in the MS luminosity function, probably stripped away by the tidal force of the Galaxy.

   (b) The extensive multi band (UBVR) photometry we carried out added additional support to this scenario. We discovered the presence of a UV, H$\alpha$-excess star located a few arcsec away from the optical counterpart to the LMXB; this object is an additional promising interacting binary candidate, suggesting that strong stellar interactions might have occurred at some remote stage of the cluster evolution (Ferraro et al. 2000, ApJ, 542, L29). The study of the evolved stellar population in the cluster (Paltrinieri et al. 2001, AJ, in press) revealed a surprisingly large population of BSS when compared with clusters of similar mass and central concentration. Furthermore, we discovered a luminous UV-bright object in the core, whose position in the CMD closely matches that of a star evolving in the post-AGB phase. A second object with similar characteristics has been located further out in the cluster at $\sim 4'.4$ from the center.

   The presence of a LMXB, our discovery of an additional promising interacting binary candidate and the large BSS population, provide additional support to the fact that star collisions might have occurred in the past: at that time, NGC6712 probably was a massive and concentrated cluster and collisional BSS (and other exotic objects such as interacting binaries) formed copiously via dynamical collisions. Later, these stars have migrated towards the center because of mass segregation, where we now see them. The continued action of tidal stripping and disk shocking has removed most of the cluster mass, driving it towards dissolution. What we now observe is nothing but the remnant core of a disrupting cluster and its population of peculiar objects, which are otherwise totally unexpected for its actual mass.

   This research has been carried out in collaboration with Paresce (ESO), De Marchi (STScI), Andreuzzi, Buonanno and Pulone (OAR), and Paltrinieri (Univ. Roma).