Alessandra Di Cecco

Collaborators: A. Calamida, M. Monelli, P.B. Stetson, A. R. Walker

Roma: G. Bono, R. Buonanno, C.E. Corsi, I. Ferraro, G. Iannicola, L. Pulone

Pisa: R. Becucci, S. Degl’Innocenti, P.G. Prada Moroni

Teramo: A. Pietrinferni, S. Cassisi

Absolute age of the old metal-poor GC M92

Observatory of Rome

SummarySummary

• Brief introduction to Galactic Globular Clusters (GGCs)

• Goals:

1. Age of a GGC

2. Multi-populations

3. Star counts and evolutionary lifetimes

• Conclusions

Data and reduction strategy

Analysis and results

Galactic Globular Clusters (GGCs)

History:

Johann Abraham Ihle in 1665 discovered the first GGC, M22. Today, we know more than 150 GGCs.

General Features: Most GGCs contain 105 to 106 coeval stars with the same chemical composition (Simple Stellar Population, SSP)

Metallicity range: Z= 10-2 -10-4

GCs are the oldest known components of the Galactic halo (Baade 1950)

GCs provide independent constraints:GCs provide independent constraints:

on the age of the Universe

on the evolutionary theory of stellar structures

on the formation of the Galactic spheroid

on the primordial Helium content

Globular Cluster

Isochrones are functions of Initial Helium abundance (Y),metallicity (Z)

[Fe/H]=Log(N(Fe)/N(H)) * - Log(N(Fe)/N(H))ּס

[α/Fe] ∑(O,Ne,Mg,Si,Ar,S,Ca,Ti)

[M/H]=[Fe/H]+Log(10^([α/Fe])*0.638+0.362) (Salaris et al. 1993)

Age is affected by Distance Modulus and Reddening

1.Age and 2.Multi-populations

Some GGCs cannot be explained by a SSP:

• ω Centauri has a triple split of the RGB and a double MS split (Pancino et al. 2000, Sollima et al. 2005, Bedin et al. 2004, Norris 2004).

• NGC2808 shows triple MS splitting (Piotto et al. 2007)

• Chemical AnticorrelationNGC1851 has double sub-giant branch: one is related to strong CN,ONa anticorrelations (Milone et al. 2008, Cassisi et al.2008)

NGC6388 (NaO anticorrelation, Busso et al. 2007)

M4 (NaO anticorrelation, Marino et al. 2008)

• The anticorrelation is not predicted by the canonical stellar models (AGB, fast-rotating stars, D’Ercole et al. 2008)

• It is primordial

10Gyr isocrhones (Maraston 2005)

ω Centauri

Large field-of-view

High spatial resolution to overcome the central crowding

Why M92?Why M92?• Extremely metal-poor ([Fe/H]=-2.32±0.07,Kraft & Ivans 2004)

• It is far from the Galactic Plane (E(B-V)=(0.02-0.03)±0.01, Zinn 1980, Reed et al. 1988)

• Its distance modulus was widely investigated (DM=14.65±0.09, Del Principe et al. 2005, 2006; Solima et al. 2006)

• The CMD does not present peculiar/anomalous features

• HOWEVER -- Spectroscopic measurements show evidence of strong C (~3) and N (~10) variations in evolved SGB, RGB, AGB stars. (Carbon et al. 1982)

1. Large Mosaic CCDs, i.e. MegaCam at the Canada-France-Hawaii Telescope (CFHT)

2. Advanced Camera for Surveys (ACS)

Two DatasetsTwo Datasets

M92 (NGC6341)M92 (NGC6341) RA:17 17 07 Declination:+43 08 11

M92 data: 57 images, deep and shallow exposure times

MegaCam consists of 36 CCDs (2048x4612 pxl)

Field-of-view: 1°x1°

Plate scale: 0.187’’/pxl

Filters: a set of SDSS bands (u*,g’,r’,i’,z’)

u* (353nm) g’ (486nm) r’ (626nm) i’ (762nm) z’ (835nm)

7x500s 5x250s 5x250s 5x300s 5x500s

10x30s 5x5s 5x5s 5x5s 5x15s

MegaCamMegaCam

Reduction strategy:

Elixir pre-processed data.

PSF photometry was performed using DAOPHOTII, ALLSTAR, ALLFRAME programs (Stetson 1987,1994).

Local secondary standard sequence provided by Clem et al 2007.

363534333231302928

272625242322212019

181716151413121110

987654321

6.4’

14.4

’

CFHT Camera

Many background galaxies

Reflection

ACS Space DataACS Space DataField-of-view: 3’x3’

One image for each filter : F814W-F606W, F814W-F475W

We calibrated F814W i’ (I), F606W r’ (V/R) , F475W g’ (B)

The final catalogue (CFHT+ACS) consists of ~140’000 stars

Theoretical ComparisonTheoretical ComparisonThe isochrones were provided by Pisa Library (atmosphere models provided by Castelli & Kurucz 2006, diffusion):

• [a/Fe]=+0.4, [Fe/H]=–2.32 ( Kraft & Ivans 2004)

• E(B-V) and DM0 within the uncertainties: μ=DM0=14.65±0.09 E(B-V)=(0.02-0.03)±0.010

σ (g’=20)=0.02

Theoretical comparison:• Age of 11 ± 1 Gyr confirmed by further analysis performed in ACS and Johnson bands.

• Limited metallicity-[distance-reddening] degeneracy: we found a similar age using [Fe/H]=-2.01 but this Fe-abundance is only marginally in agreement with Fe measurement [Fe/H]=-2.15±0.07 by Carretta & Gratton (1997).

• No evidence of multi-population

3. Star count ratios and lifetimesPost MS evolutionary phase lifetimes ()

are directly related to the star counts (N)

N i α ,

N i /N j = i j

NGC2808

R-parameter decreases inward (Castellani et al.

2006): R=NHB/NRG

R parameter is related to the He abundance.

Iannicola et al. (2008) : the culprits are the RGB, since they decrease outwards

Omega Centauri (Castellani et al 2007)

Changing in HB luminosity function

RG/MS agrees with predicted lifetime

HB/RG is higher than predicted values ~30-40%

Star count ratios in M92Star count ratios in M92

Theoretical lifetimes Star count ratiosTheoretical lifetimes Star count ratios

(Kurucz, 0.75Mo,,[Fe/H]=-2.32) We found good agreement between theoretical We found good agreement between theoretical

HB/RGB = 0.20±0.09 0.28±0.04 lifetimes and star count ratios lifetimes and star count ratios

RGB/MS = 0.44±0.03 0.38±0.04 We found that each ratio assumes constant value moving We found that each ratio assumes constant value moving

HB/MS = 0.09±0.05 0.10±0.02 from the innermost to the outermost regions of M92. from the innermost to the outermost regions of M92.

We studied the ratios between HB, RG and MS stars along the radial distance (rg)

Selected regions for star counts in red

Conclusions

Work in progress:

We want to investigate the metallicity of M92 using an internal metallicity indicator RGB bump

We have obtained multiband photometry of GGC M92 using MegaCam and ACS and…

..we found an age of 11±1 Gyr (marginally affected by metallicity-[distance-reddening] degeneracy)

..we did not find multi-populations

..we investigated star count ratios and we found good agreement with theoretical lifetimes. Moreover, the ratios are constant as a function of

the radial distance

Relation between [M/H] and V(bump-HB)

The ‘first dredge-up’ occurs when sinking of the convective envelope reaches the thin H-burning shell.

The ‘RGB bump’ occurs when H-burning shell approaches the H-discontinuity left by over the first dredge-up.

In the luminosity range where the RGB bump takes place the RG stars spend a longer time interval.

The metallicity of a GCs is related to the V(bump)-V(HB)

RGB

BUMP

Thank you for your attention!

Degeneration1° Agreement: [Fe/H]=–2.32, DM(μ)=14.74, E(B-V) CFHT=0.035 [E(B-V)ACS=0.025]

2° Agreement: [Fe/H]=–2.01 DM(μ)=14.70, E(B-V) CFHT=0.030 [E(B-V)ACS=0.018]

State of the Art State of the Art