ESO Large Program 165-L0263:Distances, Ages and Metal Abundances

in Globular Cluster Dwarfs

• Raffaele Gratton

• Osservatorio Astronomico di Padova, INAF, ITALY

PI: R. Grattonco-authors: P. Bonifacio, A. Bragaglia, E. Carretta, V. Castellani,M. Centurion, A. Chieffi, R. Claudi, G. Clementini, F. D’Antona,S. Desidera, P. Francois, F. Grundhal, S. Lucatello, P. Molaro, L. Pasquini, C. Sneden, M. Spite, F. Spite, O. Straniero, M. Zoccali

VLT2 (Kueyen)+UVES12 nights in June and September 200012 nights in August and October 20016 nights in August 2002

ESO Large Program 165-L0263:

Aims

•Distances and Absolute Ages of Globular Clusters

•The O-Na anticorrelation among globular cluster TO-stars

•Lithium abundances in TO-stars and subgiants of globular clusters

Clusters selected for observations

The closest globular clusters (but M4 for which differential reddening is important)

cluster V(TO) [Fe/H]

NGC6397 16.4 -1.82NGC6752 17.2 -1.4247 Tuc 17.6 -0.70

Stars selected for observations:TO stars and early subgiants (below the RGB clump)

Field star sample:34 metal-poor stars with good parallaxesfrom the Hipparcossatellite

Green points:single stars

Red squares:binaries

ANALYSIS

Teff’s from spectra: Balmer line profiles

Analysis procedure strictly identical for field and cluster stars

Reddening free

Our spectra have R~40,000, andS/N~80-100 for stars in NGC6397,S/N~20-60 for stars in NGC 6752and 47 Tucanae..The spectral range is 3500-9000 Å.

We show the correlation between EWs measured with an authomaticprocedure on spectra of two TO stars in NGC6752 (upper panel) and NGC6397 (lower panel)

Typical errors are 3 mÅ for stars inNGC 6397, and 5 mÅ for stars inNGC 6752 and 47 Tucanae

Accurate EWs can be derived fromour spectra

Teff’s from spectra:

- Balmer line profiles

Analysis procedure strictly identical for field and cluster stars

Reddening free

Comparison between Teff’s from H and from colours(calibration by Kurucz, model without overshooting)

Zero point error27 Kr.m.s.=159 K

Reddeningzero pointerror:E(B-V)=0.008

(yielding anerror of 0.04 magin the distancesand 0.5 Gyr onthe ages)

Our Teff scale agrees verywell with that of Alonso et al. based on the IRFM

Average difference is

T(Us)-T(A)= 811 K

(r.m.s.= 83 K, 58 stars)

Eliminating nine outliers:

r.m.s.= 38 K

Results•Distances and Ages of Globular Clusters

•Impact of microscopic diffusion on models of low mass stars

•The O-Na anticorrelation among globular cluster TO-stars

•Lithium abundances in TO-stars and subgiants of globular clusters

•Comparison between abundances in GC and field stars

•Rotation of TO-stars in globular clusters

Globular Cluster Ages

• Absolute ages - lower limit to the age of the Universe - put formation of the Milky Way in a cosmological framework• Cluster distances: a step to the extragalactic

distance scale• Relative Ages - GCs as probes to reconstruct early history of Galaxy formation

Comparison between confidence range for globular clusterages and values allowed by Universe geometry

Costraints on theepoch of formationof globular clusters

True distance modulus to the LMC according various methods

Scenarios of MW formation

• Dissipational collapse (Eggen, Sandage & Lynden Bell (1962)

• Accretion (Searle & Zinn 1979)

• Numerical models suggest that both mechanism may be active in a galaxy: the relative weight may be crucial in determining galaxy morphology (spiral vs elliptical)

Field stars

For field starkinematics andchemistry can beused to show thepresence of twometal-poorpopulations:-a dissipative collapse component-an accretion component

(Gratton et al., inpreparation) time

GC relative ages

Rosenberg et al. (1999):

- metal-poor GCs all have the same age

-some age spread for metal rich GCs

End of Thick disk?

Early phases of collapse?

Absolute Ages for GCs

• TO luminosity

• End of WD cooling sequence

• Nucleocosmochronology

Requiredistances

End of WD cooling sequenceHansen et al. (2002): 12.70.7 Gyr for M4

However, De Marchi et al. (2002) analysis of this data only indicates an age larger than 10 Gyrs

NucleocosmochronologyCayrel et al., Nature, 409, 691, 2001

Age (143 Gyr) of the extremely metal-poor star CS31082-001 from nucleocosmochronology with first identification of UII lines

Uncertainties in ages from TO

-Observational: Distances: 0.07 mag 1 Gyr

-Theoretical: Microscopic diffusion: about 1 Gyr

Distances to GCs

Currently, the most accurate method is themain sequence fitting method

In perspective, dynamical distances obtainedcombining proper motions and radial velocities(+ a dynamical model for the cluster) mayprovide distances accurate to a few percentwithin a few years from now

NGC6397King et al. 1997

Potentiality of GC dynamical distances:few per cent accuracy

Errors in dynamical distances

Anderson & King (2002) showed that astrometricaccuracy of about 1 mas can be achieved withWFPC2 on HST. Over 10 yrs, the accuracy onproper motion is equivalent to errors of about 2km/s in the transverse motion of stars within GCs

Coupled with radial velocities with accuraciesof about 1 km/s for some hundred GC stars(with e.g. FLAMES), and a model for internalmotions, this may yield distances accurate to a few per cent for most GCs

Previous Globular Cluster distances from Main-Sequence fitting to local subdwarfs

Systematic effects and total error budget associatedwith previous MS fitting distances to Globular Clusters

Effect (m-M)

Malmquist bias negligibleLutz-Kelker correction 0.02Binaries (in the field) 0.02Binaries (in clusters) 0.03

Photometric calibrations (0.01 mag) 0.04 Reddening scale (0.015 mag) 0.07 Metallicity scale (0.1 dex) 0.08

Total uncertainty (1 ) 0.12

Reddening freeTeff calibration

2 Gyr

Our results:

Line is not best fit, but the prediction of models by Chieffi & Straniero

Colour of the main sequence at MV=6

Comparing the Teff-colour relations for field and cluster stars:

Source E(B-V) NGC 6397 E(B-V) NGC6752 E(B-V)47Tuc

(b-y) 0.1780.007 0.045 0.007 0.0210.005(B-V) 0.1860.006 0.0350.007 0.0350.0091

average 0.1830.005 0.0400.005 0.0240.004

Harris 0.18 0.04 0.05Schlegel et al maps 0.187 0.056 0.032

1 Including correction in the photometry by Hesser et al. suggested by Percival et al. 2002 Astro-ph 0203157:(B-V) = 1.091 (B-V)Hesser – 0.048

Reddenings toward NGC6397, NGC6752 and 47 Tuc

NGC6397 E(B-V) 0.1830.005[Fe/H] -2.030.04

NGC6752E(B-V) 0.0400.005[Fe/H] -1.42 0.04

47 TucanaeE(B-V) 0.0240.004[Fe/H] -0.660.04

Main sequence fitting distance to NGC6397 and NGC6752

Parameter NGC6397 NGC6752 47 Tuc

[Fe/H] -2.030.04 -1.430.04 -0.66 0.04 [/Fe] 0.340.02 0.290.02 0.300.02[M/H] -1.790.04 -1.220.04 -0.45 0.04

(m-M)V 12.57 13.38 13.47 (from B-V)(m-M)V 12.62 13.15 13.57 (from b-y)(m-M)V 12.600.08 13.260.08 13.520.08 (average)(m-M)V 12.580.08 13.240.08 13.500.08 (bin. corr)

V(TO) 16.560.02 17.390.03 17.680.05 (new measure)V(HB) 13.110.10 13.840.10 14.130.10 (using Rosenberg V)MV(TO) 3.980.08 4.150.08 4.180.08MV(HB) 0.530.13 0.600.13 0.630.13

Age (Gyr) 14.21.1 14.11.1 11.51.1 (No diffusion)Age (Gyr) 13.81.1 13.71.1 11.11.1 (Diffusion).

Main parameters for NGC6397, NGC6752 and 47 Tuc

Comparison with previous data:Main Sequence Fitting Method

[Fe/H] E(B-V) (m-M)V

NGC 6397Reid 1998 -1.82 0.19 12.830.15Us -2.030.04 0.1830.005 12.580.08 NGC6752Reid 1998 -1.42 0.04 13.280.15Carretta 2000 -1.43 0.0350.005 13.340.04Us -1.430.04 0.0400.005 13.240.08 47 TucanaeReid 1998 -0.70 0.04 13.680.15Carretta 2000 -0.67 0.0550.007 13.570.09Percival 2002 -0.67 0.0550.007 13.370.11Us -0.660.04 0.0240.004 13.500.08

Comparison with other data:White Dwarf cooling sequence

Distances from white dwarf cooling sequence are independenton metallicity, but have a dependence on reddening similar tothat from Main Sequence Fitting

NGC 6752Renzini et al. 1996 E(B-V)=0.04 0.02 13.170.030.10Us 0.0400.005 13.240.020.08

47 TucanaeZoccali et al. 2001 E(B-V)=0.0550.02 13.270.030.10Us 0.0240.004 13.500.020.08

The age difference between 47 Tuc and the two otherclusters is real?

A similar age difference is given by the horizontal method

The horizontalage parameteris from Rosenberg et al.

Calibration of Relative ages from the horizontal method

14 Gyr

12 Gyr

8 Gyr

10 Gyr

Relative ages from Rosenberg et al. 1999

End of Thick disk?

Early phases of collapse?

Effect (m-M)

Malmquist bias negligibleLutz-Kelker correction 0.02Binaries (in the field) 0.02Binaries (in clusters) 0.03

Reddening scale (0.008 mag) 0.04 Metallicity scale (0.04 dex) 0.03

Total uncertainty (1 ) 0.07

Reddening freeTeff calibration

Systematic effects and total error budget associatedwith the MS fitting distances to Globular Clusters

1 Gyr

Model Error Budget

Error Source Distribution Limits (Gyr)

Convection Flat -0.4,0.4

Code Flat -0.4,0.4

Diffusion Gaussian 0.4 (@-0.4)

Solar Mv Flat -0.3,0.3

Absolute Age Error Budget

• Distance modulus 0.07 mag 1.0 Gyr• Model uncertainties (Carretta et al. 2000):

0.6 Gyr• Chaboyer et al. (2001): ages are likely 4% smaller

due to diffusion• Best age estimate: 13.7 0.8 0.6 Gyr• This corresponds to a redshift of z4 and

very likely >1

Epoch of formation of GCs

z>3 for the oldest GCsz>1.3 for the youngest GCs

Limit on M

M <0.57 at 95%

HB and RR Lyrae magnitudes

Mv(HB) = (0.220.05)([Fe/H]+1.5)+(0.560.07)

This distance scale is 0.12 mag shorter than that previously obtained from the Main Sequence FittingMethod (Carretta et al. 2000)

It is 0.03 mag shorter than the best distance scaleproposed by Carretta et al. (2000)

Clementini et al. 2003

Dependence of the RR Lyrae magnitude on metallicity for variables in the bar of the LMC (photometric data from the Danish 1.5 m telescope and spectroscopic data from FORS1 at VLT UT1)

Distance estimates for the LMC

Comparison between various distance estimates for the LMC

Old

New

Microscopic diffusion is a basic physical mechanism, thatshould be included in stellar models

It is needed to adequately reproduce the run of the sound speedwithin the solar interior as derived from helioseismogical data

Microscopic diffusion

Kraft, Sneden and coworkers:The O-Na anticorrelation for giants in globular clusters

Diffusion causes sedimentation of heavy elements, mainly He

Timescale for sedimentation is given by:

K Mcz /(M Tcz3/2)

where K is a constant, Mcz is the mass and Tcz the temperatureat the base of the convective envelope, and M the star mass

Effects of microscopic diffusion

Due to the low mass of the convective envelope,in low mass (M~0.8 M0), metal-poor ([Fe/H]-2) stars near theTO, also O and Fe are expected to be depleted significantly

The net effects of sedimentation are:- ages are reduced by about 10%- Li abundances may be significantly reduced with respect to the original value

Observations of TO and subgiants in NGC6397 (M~0.8 M0,[Fe/H]=-2.0) allow to costrain sedimentation effects

Star S/N [Fe/H] [O/Fe] TO-stars

1543 91 -2.02 0.16 1622 82 -2.02 0.11 1905 92 -2.06 0.11201432 97 -2.00 0.08202765 59 -2.02 0.21 <> -2.020.01

Subgiants 669 91 -2.01 0.26 793 105 -2.04 <0.26206810 85 -2.10 0.48

<> -2.050.03

Abundances in stars of NG6397

Model [Fe/H] TO-subgiants

Castellani et al. 2001 -0.25 for [Fe/H]= -2.0Salasnich et al. 2000 -0.29 for [Fe/H]= -1.3 -0.78 for [Fe/H]= -2.3Chieffi & Straniero 1997 -0.38 for [Fe/H]= -2.3Chaboyer et al. 2001 <-0.28 for [Fe/H]= -2.0

NGC6397 +0.030.04 for [Fe/H]= -2.0

Conclusion: Models predict much larger sedimentation due to microscopicdiffusion than actually observed. There should be somemechanism that prevents sedimentation

Prediction of models with microscopic diffusion (0.8 Mo)

- Diffusion computations assume full ionization and neglect the effects of radiation pressure: they may then overestimate sedimentation

- Chaboyer et al. (2001) propose that mixing occurs at the base of the outer convective envelope. They found that our observations could be explained by an (ad hoc) mixing region of 0.005 Mo near the stellar surface, where diffusion is inhibited. When diffusion is included according to this recipe, ages are reduced by 4% with respect to model without diffusion

Discussion

Richard et al. 2002

Models withdiffusion andradiation pressurepredicts largeoverabundancesof Fe at the endof the MS. Theseoverabundancesdisagree withobservations

These largeoverabundancesmay be eliminatedif some turbulence atthe base of theouter convectiveenvelope is introduced

Richard et al. 2002

Richard et al. 2002

When turbulenceis included in themodels, only amoderate depletionof Li (<0.2 dex) is predicted for starson the Spite plateau

Li in NGC 6397Bonifacio et al. 2002

A&A, 390, 91

Li doublet inTO-stars of NGC6397Line strength is the same in all stars

Average Li abundance:log n(Li)=2.34r.m.s=0.056 dexMaximum intrinsic scatter0.035 dexThis is to be fulfilled by stellar models whichpredict Li depletion.

Li abundances infield and (Na-poor) cluster stars.

They occupy thesame location

Dilution factor is about 15 for both field (Gratton et al. 2000) andcluster stars, in agreement with theoretical predictions

Spite’splateau

Lithium abundancesand primordial nucleosynthesis

(figure from Suzuki et al. 2000)

If our Li abundance in NGC6397 (log n(Li)=2.34) is primordial Li then the baryonic density is:bh2=0.0160.004orbh2=0.0050.002WMAP:bh2=0.02240.0009

WMAP

Variations among MS starsin 47 Tuc (Briley et al. 1994)

Variations in the strength of CH and CN bands

Noticed since early seventies (Osborn 1971) from DDO photometry and spectroscopy

Bimodal distribution along the RGB (Norris & Smith 1980s)

NGC6752

Presence of elements processed through the complete CNO-cycle.At these temperatures 22Ne+p 23Na(Denissenkov & Denissenkova1990; Langer & Hoffman 1995;Cavallo et al. 1996).

At higher temperatures, also26Mg+p 27Al

From Langer et al. 1993

A first mixing episode occurs at the base of the RGB, due to theinward penetration of the outer convective envelope in regions wheresome H-burning (through uncomplete CN-cycle) occurred duringthe latest phases of MS evolution (first dredge-up: Iben 1964).

First dredge up causes only minor effect in metal-poor stars

At the same phases, dilution (by a factor of ~20) of the surface Liabundance occurs

Mixing episodes along the RGB evolution of small mass stars

The maximum inward penetration of the outer convective envelopeat the base of the RGB creates a discontinuity in molecular weight(-barrier) that prevents further mixing, until is canceled by theoutward expansion of the H-burning shell (RGB bump) (Sweigart& Mengel 1979; Charbonnel 1994).

Further deep mixing (due e.g. to meridional circulations activated by core rotation) is possible only after the RGB bump

Role of the molecular weight barrier

Molecular weight-barrier along the RGB(from Charbonnel et al. 1998)

Field stars conform this theoretical paradigma (Gratton et al. 2000)

However abundances ofO and Na are not affected:

mixing is not deep enough to reach regionswhere complete CNO cycle occurs

There is a systematic difference between field and cluster stars

Important: this might be correlated with the 2nd parameter effect- Systematic different core-rotation core and total mass at He-flash- Mixing of He

It may also affect HB magnitudes (and then distance scales)

Possible hints for a correlationbetween the 2nd parameter andthe Na-O anticorrelation may besuggested by these graphs byCarretta et al. (1996)

What is going on in cluster stars?

There are mainly two scenarios:

- Deep mixing episodes: may only occur along the RGB, after the bump (temperature is not large enough in TO-stars)

- Pollution: should be present independent of the evolutionary phase (the material comes from now extincted TP AGB stars, undergoing hot bottom burning). Pollution might occur: . on protostars (Cottrell & Da Costa) . on already formed stars (D’Antona, Gratton & Chieffi)

Not distinguishable from observations of bright giants

Observations of stars fainter than the bump

Mass lost by TP-AGB stars

If we represent the mass function as:

f(m)k m-(1+)

and 1, then the mass lost by TP-AGB stars is comparable to the mass in stars with mass m<1 Mo

(those presently seen in GC).

This mass is lost at low velocity, and could perhapsbe kept within the cluster.These stars might have hot bottom burning, attemperatures where complete CNO cycle occurs

The O-Na anticorrelation among globular cluster stars

There are mainly two scenarios:

- Deep mixing episodes: may only occur along the RGB (temperature is not large enough in TO-stars)

- Pollution: should be present independent of the evolutionary phase (the material comes from now extincted TP AGB stars, undergoing hot bottom burning). Pollution might occur: . on protostars (Cottrell & Da Costa) . on already formed stars (D’Antona, Gratton & Chieffi)

Our observations of TO-stars in NGC6752 (a cluster which exhibits a clear O-Na among giants) allows to make a definitive test on the deep mixing scenarios

Na doublet at 8183-94 Åin TO-stars of NGC6752(these stars have virtuallyidentical atmospheric parameters)

There is a clear star-to-starvariation in Na abundances

OI triplet at 7771-75 Åin TO-stars of NGC6752.

These stars have virtuallyidentical atmosphericparameters.

There is a clear star-to-starvariation in O-abundances,anticorrelated withvariations in Na abundances

The O-Na anticorrelationamong stars in NGC6752.Filled squares: TO starsEmpty squares: subgiants.

The observed anticorrelationis very similar to thatobserved in giants

The Mg-Al anticorrelationamong stars in NGC6752.Upper panel: TO starsLower panel: subgiants.

Na rich stars are Al-richand Mg-poor.

This is most clear amongsubgiants.

C-N anticorrelation in subgiants of NGC6752

CN-band at 3883 Å G-band

Stars are ordered according to increasing Na abundance

[N/Fe]=1.0

[N/Fe]=1.1

[N/Fe]=1.3

[N/Fe]=0.0

[N/Fe]=1.2

[N/Fe]=1.3

[N/Fe]=1.2

[N/Fe]=1.45

[N/Fe]=1.5

[C/Fe]=-0.05

[C/Fe]=-0.40

[C/Fe]=-0.15

[C/Fe]=-0.35

[C/Fe]=-0.35

[C/Fe]=-0.65

[C/Fe]=-0.60

[C/Fe]=-0.25

[C/Fe]=-0.35

C and N abundances in NGC6752 subgiants

[(C+N)/Fe]=0

All O transformed into N

C and N abundances in subgiants of NGC6397

[N/Fe]=1.4

[N/Fe]=1.3

[N/Fe]=1.5

[C/Fe]=+0.05

[C/Fe]=-0.10

[C/Fe]=0.0

Very high N abundance ![O/Fe]=+0.210.05 but[(C+N+O)/Fe]=+0.580.10

Conclusions:• The O-Na anticorrelation is present among TO-stars and

subgiants in NGC6752. For the same stars, also a Mg-Al anticorrelation is observed

• This clearly rules out deep mixing as explanation for the O-Na anticorrelation

• The sum of C+N abundances is not constant: a substantial fraction of O is transformed into N in some NGC6752 stars

• N is overabundant by a large factor in subgiants of NGC6397: while O is almost solar, the sum of C+N+O is overabundant as in halo field stars

Comparison between abundances in GC and field stars

Abundances for a sample of 140 field stars with good parallaxes

Aims

• Comparison between abundances in field and cluster stars

• Comparison between abundances in thick disk and halo stars

The field star sample

• 140 subdwarfs and subgiants (this selection allows to reduce concern related to reddening and to derive homogenous abundances and kinematics)

/<0.2 from Hipparcos

• MV>2.5

Data sources

• Parallaxes and proper motion from Hipparcos

• Radial velocities from Simbad

• Equivalent widths from high quality spectra: UVES, McDonald, SARG, Nissen et al., Fullbright, Prochaska et al.

• B-V and b-y from Simbad catalogue

• Information about binarity from Simbad and Carney et al. surveys

Abundance analysis• Kurucz model atmospheres without

overshooting

• Temperatures from colours (zero point given by the H profile fitting)

• gravities from parallaxes, bolometric corrections from Kurucz, and evolutionary masses (assuming an age of 12 Gyr)

• microturbulent velocities eliminating trends of abundances with expected equivalent widths

• Non-LTE effects for O and Na

Thick disk vs Halo

• Halo stars:

• eccentricity > 0.5

• OR Maximum height above plane > 2Kpc

• OR [Fe/H]<-2

• Thick disk stars: not halo stars with:

• eccentricity > 0.2

• OR Maximum height above plane > 0.8 Kpc

• OR [Fe/H]<-0.8

RESULTS

Papers- Na-O anticorrelation and tests of microscopic diffusion: Gratton et al. A&A, 369, 87- Li in NGC 6397: Bonifacio et al. 2002 A&A, 390, 91- Distances and Ages of Globular Clusters: Gratton et al. 2003submitted to A&A- Rotation in TO-stars: Lucatello et al. 2003, A&A, in press- Abundances in field stars – data: Gratton et al. 2003, A&A, in press- Abundances in field stars – discussion: Gratton et al. 2003, A&A, in press- Abundances in 47 Tuc: Carretta et al., 2003, in preparation- Abundances in M30 and M55: Carretta et al. 2003, in preparation- n-rich elements in NGC6752: James et al. 2003, in preparation- Li in 47 Tuc: Pasquini et al. 2003, in preparation