Luminosity function of GCs 2. Tidal radius and Tidal Halo of GCs 3

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Luminosity function of GCs 2. Tidal radius and Tidal Halo of GCs 3 Powered By Docstoc
					 Observatory of Rome

Absolute age of the old metal-poor GC M92

                           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
•    Brief introduction to Galactic Globular Clusters (GGCs)

•    Goals:

    1.   Age of a GGC                              Data and reduction
    2.   Multi-populations
                                                   Analysis and results
         (previous talk by P. Ventura)
    3.   Star counts and evolutionary lifetimes

•    Conclusions
Galactic Globular Clusters (GGCs)
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)
                                                           Globular Cluster
Metallicity range: Z= 10-2 -10-4

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

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
1.Age          and 2.Multi-populations
Isochrones are functions of Initial Helium abundance (Y),
metallicity (Z)
[Fe/H]=Log(N(Fe)/N(H)) * - Log(N(Fe)/N(H))‫סּ‬
                                                                               10Gyr isocrhones
[α/Fe] ∑(O,Ne,Mg,Si,Ar,S,Ca,Ti)                                               (Maraston 2005)

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

Age is affected by Distance Modulus and Reddening

Some GGCs cannot be explained as a SSP:                           ω Centauri

•   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)

•   NGC1851 and NGC6388 have double sub-giant
    branch: CN-strong/weak (Milone et al. 2008, Cassisi et

•   The anticorrelation (CNONa) is not predicted by
    the canonical stellar models (AGB, fast-rotating
    stars) (Gratton et al. 2006, D’Ercole et al. 2008)
               M92 (NGC6341)
               RA:17 17 07        Declination:+43 08 11

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)

    Two Datasets
    Large field-of-view                            1. Large Mosaic CCDs, i.e. MegaCam at the
                                                      Canada-France-Hawaii Telescope (CFHT)
    High spatial resolution to
    overcome the central crowding                  2. Advanced Camera for Surveys (ACS)
                                   MegaCam                       CFHT Camera
                                                                  1   2    3    4    5    6    7    8     9

MegaCam consists of 36 CCDs (2048x4612 pxl)
                                                                 10   11   12   13   14   15   16   17   18
Field-of-view: 1°x1°
Plate scale: 0.187’’/pxl                                         19   20   21   22   23   24   25   26   27

Filters: a set of SDSS bands (u*,g’,r’,i’,z’)                    28   29   30   31   32   33   34   35   36

M92 data: 57 images, deep and shallow exposure times
u* (353nm)   g’ (486nm)   r’ (626nm)   i’ (762nm)   z’ (835nm)

7x500s       5x250s       5x250s       5x300s       5x500s

10x30s       5x5s         5x5s         5x5s         5x15s

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.
Many         Reflection
     ACS Space Data
Field-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 Comparison
  The 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 lifetimes
Post MS evolutionary phase lifetimes (t)
are directly related to the star counts (N)

                      N iα t ,

                  N i /N j = t i /t j

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%

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
                         Star count ratios in M92
   We studied the ratios between HB, RG and
   MS stars along the radial distance (rg)

      Selected regions for star counts in red

Theoretical lifetimes            Star count ratios
(Kurucz, 0.75Mo,,[Fe/H]=-2.32)                       We found good agreement between theoretical
HB/RGB =           0.20±0.09         0.28±0.04       lifetimes and star count ratios
RGB/MS =           0.44±0.03         0.38±0.04       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.
  We have obtained multiband photometry of GGC M92 using MegaCam and ACS

  ..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

Work in progress:

        We plan to investigate the metallicity of M92 using an internal
        metallicity indicator  RGB bump
    Relation between [M/H] and V(bump-HB)
                                 The ‘first dredge-up’ occurs when sinking of the
                      RGB        convective envelope reaches the thin H-burning
                      BUMP       shell.

                                 The ‘RGB bump’ occurs when H-burning shell
                                 approaches the H-discontinuity left by over the first
                                 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)
Thank you for
your attention!
1° 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]
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