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

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Presentation on theme: "Dissolving globular clusters: the fate of M 12 Work in collaboration with F. Paresce (INAF) and L. Pulone (Obs. Rome)"— Presentation transcript:

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

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

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: M 

10 Multimass model of cluster in equilibrium (Michie-King) Must reproduce surface brightness profile and velocity dispersion Must reproduce radial MF variations From local to global Underlying GMF very flat: dN/dm  m 0.1

11 Dynamical state gives no clue on history Simple mass segregation model fits data with flat global MF. But relaxation time short ( t rh ~ 0.7 Gyr ). M= 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. NGC 6397 M 12 NGC 6712  (MF index) T d /Gyr (Gnedin) T d /Gyr (Dinescu) T d /Gyr (Baumgardt) M/10 5 M  c

13 Space motion parameters weakest link Previous models based on incorrect orbit of M 12, giving R p ~ 3 kpc (Dauphole et al. 1996, Scholz et al. 1996) Orbit revision based on Hipparcos reference system: irregular orbit, R p ~ 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 (= M12)

14 Large fraction of original mass lost If IMF typical of GCs, mass lost is 80% or 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? 80%

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(r t /r c ) 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? c  NGC M Pal NGC NGC NGC Tuc NGC


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