Observations of Globular Clusters

(of relevance for the MODEST collaboration)

Giampaolo PiottoDipartimento 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

A stro m etry P h oto m etry (?)w ith A C S

HST (!)

P h oto m etry

P re -F la m es P ro pe r M o tion

A stro m etry

W FI

V elo c ities A b un d an ces

FLAMES@ VLT

New Observational Facilities

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 20-25 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 correspondsto 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 Sequencedown to the hydrogen burning limit (HBL)

NGC6121=M4

Astrometry (1):Identify clustermembers for deep surveys

Luminosity-Radius Relation Luminosity-Radius Relation (LRR)(LRR)

The modelscannot fit themain sequence at intermediateand highmetallicities (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 functionsin differentradial bins:

Observationalconstrainton mass segregation.Set constraintson the clusterdynamical model.NGC 6121 = M4

Cluster Camera [Fe/H] NGC6397* WFPC2 -2.2NGC6121* WFPC2/ACS -1.2

NGC104 ACS -0.7NGC6791 ACS +0.4

NGC5139 ACS -1.6/-0.5

Ongoing projects

Bedin, Piotto, King, Anderson, in prep.

GO9444

GO9648

Example:47 Tucanae

CMD spanningmore than 17 magnitudes,from the RGB tip downto Mv~15,close to the HBL

ABSOLUTE MOTIONSAstrometry (2):Measure proper motions

(U,V,W)LSR = ( 53+- 3, -202+-20, 0+- 4)Km/s

, LSR = ( 54+- 3, 16+-20, 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 DISTANCESOF 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 distancesfor as many GGCs as possible,

covering a wide range of metallicitiesin 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 instrumentslike the highresolutionmultifiberspectrographFLAMES@VLT:

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

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Error budget is very important!

This is a preliminary calculation!!!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 thesisPreliminary results for 47Tuc

Model predictions

O Data

Ongoing work on proper motions: example

HST observations completedlast month

GO9899, PI: Piotto

Ongoing work on radial velocities: example

NGC2808: 2000 stars observed

(FLAMES@VLT, PI: Piotto)

In addition: NGC6121 (2600*) NGC6397 (1700*) NGC6752 (1500*)

Geometrical distance project priority list

NGC 6121 Least model dependent!NGC 2808NGC 6752NGC 6397NGC 5139NGC 104

plus 7 other clusters with at least two epoch HST observations

Why should all this beof 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 GIRAFFE/UVES+FLAMES@VLT

Cluster Giraffe UVES Ongoing ESO program(PI Gratton)

Targets:Metallicities with 0.03dexuncertainties (UVES data)

Reddening with 0.005 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 DAOPHOT 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 http://dipastro.pd.astro.it/globulars

• 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 than1Gyr (smaller forthe most metal poorclusters).

Clusters with [Fe/H]<-1.5 appears1.5-2 Gyr younger,but this second results is totallymodel dependent.

Omega Centauri:the population puzzle goes deeper

Astrometry (4):Omega Centauri.

Accurate astrometry implies accuratephotometry!

Bedin, Piotto et al. 2004, ApJL, 605, L125

The problem of the double MS and ofthe multiple SGBs and TO

While the multipleTO could be understood in termsof a metallicity (andage) spread,the doublemain sequence represents a realpuzzle.

Is it a structure inthe 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 FLAMES@VLT in order to verify this hypothesis3 HST extra orbits allocated on DDT (GO10101, PI King)

17 blue main sequence17 red main sequence33 upper SGB32 middle SGB23 lower SG

FLAMES+GIRAFFEObservatiosin June2004

First results: the double main sequence

Piotto et al., ApJL, in preparation

17x12=204 hours i.t.

RedMS:Rad. Vel.: 235+-11km/s[Fe/H]=-1.56

BlueMS: Rad. Vel.: 232+-6km/s[Fe/H]=-1.27It 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 metalrich, and possibly helium richer stars are not carbonrich 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 pollutedby 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 accurateproper 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: WFI@2.2m ESO ~12 mas/frameA post doc (Ramakant Singh Yadav)full time dedicated in Padova

NG

C 6

12

1-M

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2.2

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IN JUST ~2.8yrIN 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 theestimated total numberof 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 BSis largely independentfrom the total massand 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 mainsequence and fills the Roche lobe.The secondary gains mass and becomes a blue straggler.

Blue stragglers will form earlier inbinaries containing more massivestars, 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 ourmodel, we expect tofind predominantlyBS produced by collisions in clusterswith Mv<-8.8.These BS areexpected to bebrighter(Bailyn andPinsonneault 1995)

This prediction seems to be confirmed bythe observed luminosity function.

We have extended our investigation to open clusters…

GCs

Open clusters

NEW!!!!

The trendscontinues into the mass regimes of (relatively) old open clusters(age>0.5Gyr).

(The high noisefor open clustersis mainly due tothe small numberof red clump stars.)

Log(age)

Total Absolute Magnitude

BSS in Open Clusters

If we include the totalcluster sample, theanticorrelation with the total magnitude (mass) iseven more evident(extending the trendalready 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 extendsto Te>=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 Te=20.000K or more.

EHB are not so rare, after all!

WHY?

Horizontal Branch Extension

For each cluster wefitted a model to obtain the temperature ofits hottest stars, as an index of the HBextension.

Then we started byexploring simplepairwise correlations.

Metallicity: the first parameter

Clearly there is acorrelation betweenthe HB extension andmetallicity. The metallicity is the firstparameter, afterall.

There is also a large dispersion. Indeed, The metallicity explains only 32% of the total variance.Basically, this is the“second parameterproblem.”

New important correlations: the total absolute magnitude

The total absolutemagnitude accountsfor 19% of the totalvariance.

Note the if we remove the mostmetal rich clusters(for which the metallicity effect dominates), thecorrelation betweenthe HB extension and the total absolutemagnitude (mass) ismuch stronger.

No correlation with the central density or otherrelevant cluster

parameters

Multicomponent Analysis

PCA analysis confirms that the HB extensioncorrelates 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 thedependence of the horizontal branch in globular clusters on thecluster 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 retainedinside a globular cluster?

3) Which is the fraction of retained ejecta as a function of thecluster mass, mass function, etc.?

4) Which is the resulting chemical contamination?