Observations of Globular Clusters (of relevance for the MODEST collaboration) Giampaolo Piotto Dipartimento di Astronomia Universita’ di Padova Collaborators: Jay Anderson, Luigi R. Bedin, Santi Cassisi, Francesca De Angeli, Ivan R. King, Yazan Momany, Marco Montalto, Alejandra Recio Blanco, Sandro Villanova

Recent Instrumental Advances New instruments for both imaging and spectroscopy have strongly affected the research topics in globular cluster astronomy. We have also started to take advantage of the year baseline of images on solid state digital imagers and, overall, of more than 10 year baseline of HST imaging for for high accuracy proper motions!

High Precision Astrometry on WFPC2/ACS HST Images Just the random error remains ~0.02 pxl on the WFPC2 (~0.01 pxl on the ACS) which corresponds to 1 mas (PC) on a single imagewith N images: N : ~ 1 mas /sqrt(N) (in the PC case) (Anderson and King 2002, 2003)

(Bedin, Anderson, King, Piotto 2001, ApJL, 560, L75) Hunting the bottom of the Main Sequence down to the hydrogen burning limit (HBL) NGC6121=M4 Astrometry (1): Identify cluster members for deep surveys

Luminosity-Radius Relation (LRR) The models cannot fit the main sequence at intermediate and high metallicities (Bedin et al. 2001) NGC 6397 M4 low [M/H] intermediate [M/H] King, Anderson, Cool, Piotto, (1999)

Bedin et al. 2004, in prep Mass functions in different radial bins: Observational constraint on mass segregation. Set constraints on the cluster dynamical model. NGC 6121 = M4

Cluster Camera [Fe/H] NGC6397* WFPC NGC6121* WFPC2/ACS -1.2 NGC104 ACS -0.7 NGC6791 ACS +0.4 NGC5139 ACS -1.6/-0.5 Ongoing projects

Bedin, Piotto, King, Anderson, in prep. GO9444 GO9648 Example: 47 Tucanae CMD spanning more than 17 magnitudes, from the RGB tip down to Mv~15, close to the HBL

Present Day Mass FunctionInitial Mass Function Ongoing Projects: (in coll. with D. Heggie)HST: NGC 2808 NGC 5024VLT: NGC 6981Archive: A lot of data in both HST and VLT archive.

ABSOLUTE MOTIONS Astrometry (2): Measure proper motions

(U,V,W) LSR = ( , , 0+- 4)Km/s ,   LSR = ( , , 0+- 4)Km/s …of M4:

Once corrected  l cos b and  b for the Sun peculiar motion we can get Bedin, Piotto, King, Anderson 2003, AJ, 126, 247

Astrometry (3): GEOMETRICAL DISTANCES OF GLOBULAR CLUSTERS This is our major project, at the moment Globular cluster age measurement error is dominated by uncertainty on distance, which is at least ~10% => 0.2 mag distance modulus, which translates in a >25% error in age!!!

Direct measurements of distances are several years away (GAIA, SIM,…) and we have to rely on standard candles, whose luminosiy is still poorly known, and sometimes strongly dependent on other parameters as metallicity (e.g. RR Lyrae). We need reliable measures of distances for as many GGCs as possible, covering a wide range of metallicities in order to measure accurate ages

Our method is very simple (…in principle… ) we compare the dispersion of the internal proper motions (an angular quantity) with the dispersion of the radial velocity (a linear quantity) it is not a new idea, but now…

INTERNAL DYNAMICS (Bedin et al. 2003)

…and thanks to instruments like the high resolution multifiber spectrograph We get thousands of radial velocities per night!!!

The main source of error is the sampling error: 1/sqrt(2N). For a typical sample of 3,000 stars this implies an error of 1.3% in the distance. The distance scale obtained will not be only sound, but its uncertainty will no longer contribute to the uncertainty on the age estimates.

NGC 2808

M4 ESO-071.D-0205(A) ESO-072.D-0742(A) plus several HST GO, (last GO-10146)

Error budget is very important! This is a preliminary calculation!!! Harris 2003: 2.2 kpc… Diff= 20% closer!!! Better agreement with Peterson, Rees & Cudworth et al. d=1.72+/-0.14 kpc (Formula from Pryor & Meylan 1993)

The sources of systematic errors are: - estimates of the observational errors PLUS - mass segregation - rotation MODEL !!! - anisotropies

We fit the observed radial velocities and internal proper motions with a superposition of orbits constructed with an axisymmetric dynamical model (Schwarzschild models). The orbit library is generated using the code developped by Gebhardt et al. (2000). F. De Angeli PhD thesis Preliminary results for 47Tuc Model predictions O Data

Ongoing work on proper motions: example HST observations completed last month GO9899, PI: Piotto

Ongoing work on radial velocities: example NGC2808: 2000 stars observed PI: Piotto) In addition: NGC6121 (2600*) NGC6397 (1700*) NGC6752 (1500*)

Geometrical distance project priority list NGC 6121 Least model dependent! NGC 2808 NGC 6752 NGC 6397 NGC 5139 NGC 104 plus 7 other clusters with at least two epoch HST observations

Why should all this be of interest for MODEST? From the various proper motion projects we get: 1)Accurate proper motions AND radial velocities for up to a few thousand stars, from the cluster center to many core radii from the center; 2)Mass functions, in a few cases down to 0.1 solar masses; 3)Mass segregation; 4)For a selected number of clusters, accurate distances and ages; 5)In some cases, absolute motion

Accurate Reddening and Metallicity measurement with Cluster Giraffe UVES Ongoing ESO program (PI Gratton) Targets: Metallicities with 0.03dex uncertainties (UVES data) Reddening with mag. uncertainty (GIRAFFE data Coupled with the geometric distance project we should be able to measure GC ages with a few 100 Myr uncertainties

74 GC cores observed with the WFPC2 in the F439W and F555W band [all clusters with (m-M) B <18]; Data reduced with D AOPHOT and ALLFRAME; Data calibrated to both HST Flight and standard Johnson B, V systems following Dolphin (2000); Completeness available for all the CMD branches ( 7100 experiments with more than 5 million artificial stars) Photometric data and completeness are available at The database has proven to be a mine of information Piotto et al. (2002), A&A, 391, 945

Relative Ages of Galactic Globular Clusters Within each single bin, GCs are coeval, with an age dispersion less than 1Gyr (smaller for the most metal poor clusters). Clusters with [Fe/H]<-1.5 appears Gyr younger, but this second results is totally model dependent.

Omega Centauri:the population puzzle goes deeper Astrometry (4): Omega Centauri. Accurate astrometry implies accurate photometry!

Bedin, Piotto et al. 2004, ApJL, 605, L125 The problem of the double MS and of the multiple SGBs and TO

While the multiple TO could be understood in terms of a metallicity (and age) spread, the double main sequence represents a real puzzle.

Is it a structure in the background? Bedin, Piotto, Anderson et al. 2004, ApJL, 605, L125 Leon, Meylan, & Combes 2000

Bedin et al. (2004) have proposed an alternative explanation for the Omega Centauri double main sequence: It represents a population of super-helium rich stars (Y>0.30), which might be originated by material polluted by intermediate mass (1.5-3 solar masses) AGB star ejecta. This would be consistent with: 1) The increase of s-process elements with metallicity found by Smith et al. (2000) 2) The anomalously hot horizontal branch 3) The lack of correlation between period shift and metallicity among RRLyr stars (Gratton et al. 1986) ESO DDT project (PI Piotto) approved for 15hr at in order to verify this hypothesis 3 HST extra orbits allocated on DDT (GO10101, PI King)

17 blue main sequence 17 red main sequence 33 upper SGB 32 middle SGB 23 lower SG FLAMES +GIRAFFE Observatios in June2004

First results: the double main sequence Piotto et al., ApJL, in preparation 17x12=204 hours i.t. RedMS: Rad. Vel.: km/s [Fe/H]=-1.56 BlueMS: Rad. Vel.: km/s [Fe/H]=-1.27 It is more metal rich!

Other chemical elements: Red Main Sequence: [C/Fe]=0.0 [N/Fe]~1.0 [Ba/Fe]=0.4 Blue Main Sequence [C/Fe]=0.0 [N/Fe]~1.0 [Ba/Fe]=0.7 The blue main sequence stars are richer in Ba (s-process element), but NOT carbon rich. This is the second important result. The fact that there is no significant radial velocity difference and no significant difference in proper motion make the background object explanation even more unlike. The only other possibility is indeed a strong He overabundance

An overabundance of helium (Y~0.40) indeed can reproduce the observed blue main sequence. The fact that the more metal rich, and possibly helium richer stars are not carbon rich seems to exclude that the cloud has been contaminated by AGB ejecta. According to Thielemann et al. (1996) SNe from 8-12 solar mass stars should produce a huge amount of helium. Material polluted by these SNe could in principle originate stars with the chemical content of the blueMS stars in Omega Centauri.

Future Plans: Observations: 1) Reduce the new ACS/HST images (foreseen for June 2005) to follow the two MSs in Omega Cen down to the hydrogen burning limit; Use the first epoch of the same field for accurate proper motions of the stars in the two MSs 2) With improved ACS photometry search for main sequence splits and/or broadening in other globular clusters. Theory (of interest for MODEST!) 1)Investigate the fraction of material ejected by SNe from 8-12 solar mass stars that can be retained within the cluster (see also proposal at the end of the talk).

NEXT STEP FOR ASTROMETRY: GROUND-BASED ASTROMETRY Example: ESO ~12 mas/frame A post doc (Ramakant Singh Yadav) full time dedicated in Padova

NGC M4 IN JUST ~2.8yr

Blue Stragglers from the snapshot catalog Blue stragglers (BS) are present in all of our 74 CMDs; Almost 3000 BSs have been extracted from 62 GCs; The location of BSs in the CMD depends on metallicity; The brightest BSs have always a mass less than 1.6 solar masses; In all GCs, BSs are significantly more concentrated than other cluster stars.

Piotto, De Angeli et al. (2004, ApJL, 605, L125) Ns represents the density of stars in a cluster. (i.e. the observed number of stars has been divided by the fraction of the cluster light sampled by our WPFC2 images, and then divided by the total cluster light). There is a significant correlation between the BSS frequency and the total cluster luminosity (mass) and a very mild anticorrelation with the central collision rate.

Here, we plot the estimated total number of stars, obtained from the observed counts, divided by the fraction of the cluster light sampled by our images Note that: 1)The total number of HB stars scales linearly with Mv, or the total mass, as expected. 2) The number of BS is largely independent from the total mass and the collision rate.

Evolutionary pathway to produce Blue Stragglers in GCs Davies, Piotto, De Angeli 2004, MNRAS, 349, 129 A more massive main sequence star exchanges into a binary containing two main sequence stars. The primary evolves off the main sequence and fills the Roche lobe. The secondary gains mass and becomes a blue straggler. Blue stragglers will form earlier in binaries containing more massive stars, i.e. in high collision rate clusters. Given the finite lifetime of a blue straggler, the blue straggler population (from primordial binaries) in the most crowded clusters today could be lower than in very sparse clusters.

Production of Blue Stragglers in GCs Davies, Piotto, De Angeli 2004

Blue Straggler Luminosity Function On the basis of our model, we expect to find predominantly BS produced by collisions in clusters with Mv<-8.8. These BS are expected to be brighter (Bailyn and Pinsonneault 1995) This prediction seems to be confirmed by the observed luminosity function.

We have extended our investigation to open clusters… GCs Open clusters NEW!!!! The trends continues into the mass regimes of (relatively) old open clusters (age>0.5Gyr). (The high noise for open clusters is mainly due to the small number of red clump stars.)

Log(age) Total Absolute Magnitude BSS in Open Clusters If we include the total cluster sample, the anticorrelation with the total magnitude (mass) is even more evident (extending the trend already observed for GCs). Apparently, there is also a correletion with the cluster age, with older clusters having more BSS

Extended horizontal branches 22 out of the 74 clusters of the snapshot database show a blue tail which extends to T e >=20.000K, entering into the EHB regime. A number of these have been identified as EHB clusters within the snapshot project. In practice, 25-30% of the clusters of our sample have a blue tail extended to T e =20.000K or more. EHB are not so rare, after all! WHY?

Horizontal Branch Extension For each cluster we fitted a model to obtain the temperature of its hottest stars, as an index of the HB extension. Then we started by exploring simple pairwise correlations.

Metallicity: the first parameter Clearly there is a correlation between the HB extension and metallicity. The metallicity is the first parameter, afterall. There is also a large dispersion. Indeed, The metallicity explains only 32% of the total variance. Basically, this is the “second parameter problem.”

New important correlations: the total absolute magnitude The total absolute magnitude accounts for 19% of the total variance. Note the if we remove the most metal rich clusters (for which the metallicity effect dominates), the correlation between the HB extension and the total absolute magnitude (mass) is much stronger.

No correlation with the central density or other relevant cluster parameters

Multicomponent Analysis PCA analysis confirms that the HB extension correlates with [Fe/H] and Mv (i.e. total mass)

Why the dependence on the total cluster mass? A possible explanation could be related to what we have found in Omega Centauri: self pollution! IF a significant fraction of the material lost by intermediate mass AGB stars and/or SNe can be retained by the cluster and contaminate the medium from which less massive stars are still forming,we would end with low mass stars richer in helium. Stars richer in helium would become bluer HB stars. In this scenario: more massive clusters would be able to retain material from the AGB/SNe ejecta than less massive ones, and therefore would end with more extended HBs as observed! D’Antona et al. (2002)

A proposal for MODEST collaboration The new results in Omega Centauri and on the extension of the dependence of the horizontal branch in globular clusters on the cluster total mass rise a number of questions. 1)Can the ejecta from SNe generated by 8-12 solar mass stars be retained inside a globular cluster? 2) Can the ejecta from intermediate mass AGB stars be retained inside a globular cluster? 3) Which is the fraction of retained ejecta as a function of the cluster mass, mass function, etc.? 4) Which is the resulting chemical contamination?