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