New and Odds on Globular Cluster Stellar Populations: an Observational Point of View

(The Snapshot Database)

G.Piotto, I. King, S. Djorgovskiand

G. Bono, S. Cassisi, M. Catelan, F. De Angeli,

G. Meylan, A. Recio Blanco, A. Renzini, M. Rich,

I. Saviane, M. Zoccali.

•(69+3) GC cores observed with the WFPC2 in the F439W and F555W band [all clusters with (m-M)B<18];

•Data reduced with DAOPHOT and ALLFRAME;

•Data calibrated to both HST Flight and standard Johnson B, V systems following Dolphin (2000);

•Completeness available for all the CMDs branches (2300 experiments with more than 1.5 million artificial stars)

The Snapshot database: an example (1)

The Snapshot database: an example (2)

All the CMDs are available on the Web

http://menhir.pd.astro.it

Data files on specific clusters available upon request to piotto@pd.astro.it

The data files will be on the web after summer

What did we already learn from the snapshot database?

• Study of specific objects:

• Properties of the blue HB extended tails (Piotto et al 1999)• The RGB Bump:

• The Helium Content (R Parameter) (Zoccali et al. 2000, ApJL, 538, 289)• Evolutionary times (Zoccali and Piotto 2000, A&A, 358, 943)

Multimodal HB in NGC 2808 (Sosin et al. 1997, ApJL, 480, L35) ;Metal rich clusters with blue HB tails: NGC 6388 and NGC 6441 (Rich et al. 1997, ApJL, 484, L25) ;The anomalous HB in NGC 6273 (Piotto et al. 1999, AJ, 118, 1727) ;The blue straggler population in NGC 6362 and NGC 6934 (Piotto et al. 1998 , AJ, 117, 264) .

The Delta V(HB-Bump) problem (Zoccali et al. 1999, ApJL, 518,L49); Star counts across the bump (Bono et al. ApJL, 546, L49)

•All the GCs (in our sample) with an extended blue HB tail show at least one gap along it.

• Some of these gaps may be occurring at a particular value of the stellar mass (about 0.53 solar masses) , common to a number of different clusters.

•A good general agreement has been found between the observed and theoretical LFs at any metallicity [M/H]<-0.7

What are we investigating?

1. Galactic Globular Cluster Relative Ages;

2. Calibration of the Mixing Lenght Parameter;

3. The Blue Straggler Population;

4. Color and Population Gradients.

Relative Ages of Galactic Globular Clusters(see A. Rosenberg talk for details)

• Most of the clusters are coeval (all the GCs with [Fe/H]<-1.1);

• A fraction of the intermediate and high metallicity clusters are significantly younger (by 15-20%).

Possible interpretation:

1) GC formation must have started at the same zero age throughout the halo (at least out to 25Kpc);

2) A fraction (40%) of the clusters with [Fe/H]>-1.1 formed at later times (15-20% of the halo age).

Calibrating the mixing length parameter

Definition: WHB is the distance in color between the HB turning down (at B-V=0) and the RGB at 0.5 magnitudes above the HB turning down.

• The trend of WHB vs. [Fe/H] is well reproduced by the models;

• Uncertainties on the transformations and on the alpha-elements enhancement do not allow to set the absolute value of the mixing length;

•WHB shows that the mixing length does not depend on metallicity in the interval –2.1<[Fe/H]<-0.5.

Blue Stragglers (1)

•Blue stragglers (BS) are present in all of our 64 CMDs;•3000 BS have been extracted from 54 GCs;•The location in the CMD depends on metallicity;•The brightest BD have always a mass less than 1.6 solar masses,•In all cases, BS are significantly more concentrated than other cluster stars.

Blue Stragglers (2)

• The relative number of BS correlates with the total absolute visual magnitude (i.e. total mass): the less massive the cluster the higher the relative number of BS

Blue Stragglers (3)

• There is some possible correlation of the relative fraction of BS with the central density r of the cluster and the concentration c, but only for logr<3.5 and c<1.5

• There is a possible dependence on the half mass relaxation time, but only for relaxation times shorter than 1Gyr.

Blue Stragglers (3)

• There is some possible correlation of the relative fraction of BS with the central density r of the cluster and the concentration c, but only for logr<3.5 and c<1.5

• There is a possible dependence on the half mass relaxation time, but only for relaxation times shorter than 1Gyr.

Blue Stragglers (4)

• The LFs of BS depend on cluster concentration;

• Comparison with (old) models by Baylin & Pinsonneault (1995) show some evidence that the LFs of the BS in more concentrated clusters could be of collisional origin, while BS in less dense clusters could come from merge of primordial binaries;

•New evolutionary models are urgently needed to interpret this large amount of new BS data.

Blue Stragglers (5)

•The empirical results of our investigation seem to show that in less massive clusters the primordial binaries which might result in BS have an higher probability to survive (in the cluster cores). Why?

•In higher density cluster cores the BS formation is probably dominated by collisions, but why there is no correlation with the cluster density and concentration parameter?

• More dynamical models are needed to interpret these new results on the relative number of BS.

Color and Population Gradients

•We confirm the presence of color gradients in M15 and M30. We found evidences of gradients in NGC 6342 and 47 Tuc;• No other significant color gradients have been identified;•As for the population gradients, only the BS are significantly more concetrated than any other cluster stars.

Future Projects

•We plan to upgrade the database reducing all the UV WFPC2 images overlapping the snapshot fields (ongoing HST programs and HST archive).

•We are obtaining second epoch WFPC2 images of the nearest snapshot clusters for proper motion determination (new data for 9 clusters + archive).

•Extension of the snapshot database with a coverage of the entire cluster fields, out to the tidal radius (32 clusters already observed with groundbased wide field imaging facilities; completion of the database by year 2002).

• A new census of the HB morphologies, and their dependence on the cluster parameters.