Dissolving globular clusters: the fate of M 12

1. Dissolving globular clusters: the fate of M 12 Work in collaboration with F. Paresce (INAF) and L. Pulone (Obs. Rome)

2. Globular clusters as cosmology probes • Product of star formation at high redshift (z>5) • Comfortably located nearby, stars can be studied individually • Oldest objects around whose age can be determined reliably (concordance model)

3. The price of studying the past in the present • Stars above 0.8 Mhave evolved (WD only trace) • Stars interact dynamically, mass segregation • Gravothermal (core) collapse • Mass distribution changes with time and place, but can be predicted for isolated clusters (De Marchi et al. 2000)

4. Globular clusters ‘feel’ the Galaxy • Evaporation (relaxation) • Disc shocking (compression) • Bulge stripping • (Tidal tails) • Stars in periphery are lost preferentially, but they are also lower mass (segregation) • Integration over orbit and time modifies MF, possibly erasing original IMF properties (Vesperini & Heggie 1997)

5. Modelling interaction possible, but difficult • Space motion parameters often uncertain, unknown • Galactic potential not well defined (models) • Present clusters just small fraction of original population • Model predictions meaningful in a statistical sense, give likely evolution of GC system (Gnedin & Ostriker 1997; Aguilar, Hut & Ostriker 1988)

6. First VLT data stunning! • Inverted MF, first case ever: “the making of the MW halo” (De Marchi et al. 1998) Let us give it a try... • If statistics correct, there must be clusters facing disruption now • NGC 6712 excellent test case:Td270 Myr

7. NGC 6712 result gives confidence in models • Proper motion and radial velocity studies describe cluster orbit in detail (Dauphole et al. 1996; Odenkirchen et al. 1997) • More complex models attempt description of individual clusters’ history (Dinescu et al. 1999; Baumgardt & Makino 2003) • Present MF could be rolled back to IMF: appealing! (= M15)

8. (= M12) • However, MF of M 12 very flat, most low mass stars missing. Were they lost? Or were they never there? (De Marchi et al. 2006) Sanity check to test reliability • M 12 (NGC 6218) perfect case: similar to NGC 6712 (mass, [Fe/H]) but very different history: Td=15 Gyr vs Td=270 Myr • Should have steep MF, no signs of stripping or tidal tails

9. From luminosity to mass • Deep cluster photometry from core to half-mass radius • Accurate completeness study as a function of radius • Luminosity function varies with radius -> segregation • Conversion from magnitude to mass trustworthy: 0.3-0.8M

10. Underlying GMF very flat: dN/dm m0.1 From local to global • Multimass model of cluster in equilibrium (Michie-King) • Must reproduce surface brightness profile and velocity dispersion • Must reproduce radial MF variations

11. Dynamical state gives no clue on history • Simple mass segregation model fits data with flat global MF. But relaxation time short (trh~0.7 Gyr). • M=1.2105 M, c=1.3, M/L=1.7 typical of loose clusters. Disrupted? Recently? • No tidal tail (Lehmann & Scholz 1997). Not needed if disruption process very old. • Data alone cannot tell whether tidal disruption or flat IMF.

12. Tidal stripping most likely the culprit • Young star forming regions show steep IMF, not flat. • Cluster well away from centre of Galaxy show (similar) steep global MF: cannot be born flat • Different models are inconsistent. Error most likely in Galactic potential: disc shocking gets rid of stars very fast. • Models of tidal interaction presently not adequate to describe clusters’ fate.

13. (= M12) Space motion parameters weakest link • Previous models based on incorrect orbit of M 12, giving Rp~ 3 kpc (Dauphole et al. 1996, Scholz et al. 1996) • Orbit revision based on Hipparcos reference system: irregular orbit, Rp ~ 600 pc (Odenkirchen et al. 1997) • Predicted Td drastically reduced: 16.3 Gyr -> 4.5 Gyr (Baumgardt 2005) • Revised models in agreement with presently flat GMF

14. 80% Large fraction of original mass lost • If IMF typical of GCs, mass lost is 80% or 5.105 M • Over 1 million stars lost to the Milky Way halo • When and how? Not recently (~1 Gyr) or no equipartition would have been reached • Very old process or very slow?

15. The way forward • Need serious mapping of clusters space motion parameters, but most importantly of Galaxy structure to constrain models • Gaia will provide 3D structure of thin and thick disc; cluster distances and proper motions (orbits) • Gaia will set most stringent constraints, but knowledge of MF still needed down to < 0.5 Mto map internal dynamics

16. In the meanwhile... • GMF remains best diagnostic tool to test past interaction of GCs with MW • Space motion parameters and tidal tails are instantaneous quantities, GMF shows global effect integrated over time • GMF measurement conceptually simple and comfortably doable for many GCs with the VLT • Relatively deep (V ~ R ~ 26) photometry and good radial coverage

17. Central concentration may be the key... • Interesting trend between MF slope and King central concentration parameter: c = log(rt/rc) • Clusters with c < 1.2 usually have shallow MF, data scarce • Proposed VLT survey of sample of nearby low c clusters • Core collapse and evaporation governed by the same process: two body relaxation! • How many collapsed clusters have gone unnoticed?