1. Population synthesis models and the VO
RGB and AGB role in population synthesis models
Maurizio Salaris
ARI – Liverpool John Moores University
Maurizio Salaris
Astrophysics Research Institute
Liverpool John Moores University
“BaSTI for population synthesis” project in collaboration with:
Susan Percival, Santi Cassisi, Adriano Pietrinferni
BaSTI pop. synthesis project in collaboration with Susan Percival, Santi Cassisi, Adriano Pietrinferni

2. OUTLINE What/why population synthesis ? The role of RGB and AGB
Some quantitative examples of the effect of uncertainties in RGB/AGB models
Differences among population synthesis models with different RGB/AGB treatment
Empirical calibrations from integrated light: possible ?
Conclusions

3. WHAT
Stellar population synthesis models are tools for interpreting the integrated light that we observe from unresolved stellar populations (galaxies, star clusters).
A so-called ‘population synthesis model’ provides the integrated spectrum and magnitudes/colours (plus mass to light-ratios in various wavelength bands) of a stellar population with a given star formation history and IMF
BaSTI, GALAXEV, Pegase, Starburst99, Buzzoni, GALEV, Vazdekis, others

4. Population synthesis: Ingredients
Library of stellar spectra
Stellar model/isochrone library

5. isochrone is specified by (L, Teff, M, [Fe/H]) or also
Isochrones.
Each point along an
isochrone is specified by
(L, Teff, M, [Fe/H]) or also
(Teff, g, [Fe/H])
Stellar spectra.
Each spectrum is specified
by (Teff, g [Fe/H])
Integrated spectrum
Effect of dust
IMF to give number
of stars along
isochrone for a fixed
total mass of the
population
Nebular emission
Line index strengths,
integrated colours,
magnitudes
mass-to-light ratios
K-correction

6. Use of pairs of integrated colours
e.g. Hempel et al. (2003)
Age-metallicity degeneracy ↓
James, Salaris et al. (2006)

7. Use of several photometric bands
Technique described by, e.g. Anders et al. (2004)

8. Use of integrated spectra: line indices
See, e.g. Worthey et al. (1994)

9. Use of integrated spectra: spectrum fitting
GALAXEV models

10. The role of RGB and AGB stars
Solar
BaSTI

11. The role of RGB and AGB stars
From Coelho et al. (2007)

12. Some quantitative tests of the sensitivity to RGB/AGB stars
Mismatch between models and spectra for stars with Teff < 5000 K and log(g) < 3.0
3 Gyr 6 8 10 14
Red triangles  Teff K
Blue circles  log(g) dex
Asterisks  [Fe/H]±0.15 dex
Open triangles  Teff K (whole isochrone)
[Fe/H]= − −

13. Number of RGB stars above the HB level reduced by a factor 2
6 Gyr 8 10 14
[Fe/H]= −1.3 −1.0 −0.7 −
[α/Fe]=+0.4

14. Mismatch between models and spectra of RGB stars
Blue circles  Teff K
Mismatches in g and [Fe/H] have negligible effect on these colours
14 Gyr 10 8 6 3
Blue circles  N/2
‘Wrong’ RGB star counts above the ZAHB
[Fe/H]=−0.7 −

15. Varying the RGB mass loss
η Reimers increased from 0.2 (reference) to 0.4 for the 14 Gyr old sequence ↓

16. Differences between different sets of models
BaSTI
Maraston (2005)
bol V K

17. Different AGB contribution  different ages

18. Statistical fluctuations of integrated magnitudes !!
Can we calibrate AGB/RGB modelling on the integrated colours of star clusters ?
Statistical fluctuations of integrated magnitudes !!
See also analytical work by Cervino & Luridiana

20. Fluctuation of  luminosity ratios

21. Co-add clusters with similar properties
BaSTI (black)
Mar05 (red)
Padua 08 (blue)
Co-added data from Gonzalez et al. (2004). Individual cluster data from Kyeong et al. (2003)

22. Different result from Mucciarelli et al. 2009
Maraston 2005 models

23. SBF and AGB calibration
Definition of fluctuation luminosity
BaSTI
Data from Gonzalez et al. (2004)

24. Calibration on resolved star clusters INTEGRATED SPECTRA
Index fluctuations in star clusters

25. CONCLUSIONS
Inconsistent Teff and g scales between isochrones and spectra have a relatively small effect on the main Lick-type spectral indices, and a larger effect on colour-colour diagrams
Star counts along the RGB and AGB have potentially a stronger effect on the ages derived from integrated colours and spectral indices
Mass loss along the RGB has a very strong effect on ages derived from integrated colours and spectral indices.
The overall modelling of the AGB phase has a strong influence on ages and metallicities derived from colour-colour diagrams.
Empirical calibrations of the AGB contribution to the integrated fluxes (and spectral indices) are problematic and at present lead to some contradicting results.