dissolving globular clusters: the fate of m 12 work in collaboration with f. paresce (inaf) and l....
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Dissolving globular clusters: the fate of M 12
Work in collaboration with F. Paresce (INAF) and L. Pulone (Obs. Rome)
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)
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)
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)
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)
Let us give it a try...
• If statistics correct, there must be clusters facing disruption now
• NGC 6712 excellent test case: Td270 Myr
(De Marchi et al. 1998)
• First VLT data stunning!
• Inverted MF, first case ever: “the making of the MW halo”
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)
(= M12)
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
(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?
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: 0.3-0.8 M
• 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 m0.1
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=1.2 105 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.
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)
-1.6 0.1 0.9
Td/Gyr (Gnedin)
2.1 14.5 0.3
Td/Gyr (Dinescu)
3.9 29.4 3.7
Td/Gyr (Baumgardt)
11.3 16.3 9.0
M/105 M 0.9 1.2 0.7
c 2.6 1.3 0.7
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
(= M12)
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?
80%
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
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
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?
c
NGC 6712 0.7 0.9
M 12 1.3 0.1
Pal 5 0.7 -0.4
NGC 6352 1.1 -0.6
NGC 6496 0.7 -0.7
NGC 288 1.0 -1.1
47 Tuc 2.0 -1.4
NGC 6397 2.5 -1.5