RGBs in composite stellar popula2ons

Transcription

1 RGBs in composite stellar popula2ons C. Gallart (IAC) with special thanks to A. Aparicio, S. Cassisi, S. Hidalgo & M. Monelli

2 Main uses in stellar populations studies: Measure metallicity Total SFR. SFR(t)?? RGB tip: distance This talk will discuss information from: Photometry in the optical Low resolution spectra C/M ratios, IR

3 1. Measuring metallicity distributions from the RGB: Z= Metal poor data: Saviane et al. (2000, A&A, 355, 966) analytical representation of observed RGBs Teramo Girardi Metal-rich data: fiducial lines of bulge clusters Gallart, Zoccali & Aparicio, 2005, ARAA

4 1. Measuring metallicity distributions from the RGB: Valenti et al. 2004; fiducial RGBs and several index Saviane et al (2000); analytic RGBs and M I =-3.5 index

5 1. Measuring metallicity distributions from the RGB: Intrinsic problem: RGB age metallicity degeneracy:

6 1. Measuring metallicity distributions from the RGB: Intrinsic problem: RGB age metallicity degeneracy: A real case: LMC metallici2es from CaII triplet. No correla2on between [Fe/H] and posi2on of the star on the RGB is found. X [Fe/H] [Fe/H] [Fe/H] [Fe/H] 0 0<[Fe/H] Carrera et al. (2007)

7 1. Measuring metallicity distributions from the RGB: Intrinsic problem: the RGB age metallicity degeneracy: Es2mate of the errors introduced on the metallicity deriva2on in the case that a wrong single old age is assumed. Gallart et al. 2005, ARAA Reference color: (V I) at M I = 3.0 Gallart, Zoccali & Aparicio, 2005, ARAA

8 1. Measuring metallicity distributions from the RGB: Intrinsic problem: the RGB age metallicity degeneracy: The derived metallicity range or dispersion of a composite stellar popula2on is a lower limit of the true metallicity dispersion. Use indicators of presence of intermediate age popula2on (such as AGB stars) to assess the validity of using the RGB to es2mate metallicity range.

9 1. Measuring metallicity distributions from the RGB: Breaking the RGB age metallicity degeneracy with spectra: Carrera et al. 2008: LMC metallicity

10 Thanks to the RGB age metallicity degeneracy: possibility to esbmate the age of each star from its metallicity and posibon on the RGB: Log(age)=a+b(V I)+cMv+d[M/H]+ +f(v I) 2 +g[m/h] 2 +h(v I) 3 Carrera et al. 2008

11 2. Measuring integrated star formation rates: 2x 4x Aparicio et al. (unpublished; see also Greggio 2002)

12 Measuring star formation histories? Color magnitude diagrams reaching the oldest main sequence turnoffs can provide a detailed determina2on of the SFH

13 1) Any SFH can be represented as a func2on of two independent variables: AGE and METALLICITY and given in terms of SIMPLE populabons Ψ(t,Z)= A Σ α i Ψ i 2) The distribu2ons of stars in the model and observed CMD are compared 3) By using a merit func2on, we can retrieve a quan2ta2ve es2mate of the star forma2on rate and the chemical evolu2on law.

14 Measuring star formation histories? Beyond the Local Group, in general only the RGB will be reachable. How reliable are the SFHs derived from the bright part of the CMD (RGB+bright MS)? Compare SFH obtained from the whole CMD with SFH obtained with the bright portion of the CMD alone

15 LMC: CTIO 4m LCID:

16 Distinguishing subtle details in the early SFH Monelli & LCID 2009, in prep Hidalgo & LCID 2009, in prep

17 How good are the solutions if we use only the bright part of the CMD? Cetus IC1613

18 How good are the solutions if we use only the RGB? Cetus IC1613 Girardi bright CMD BaSTI bright CMD BaSTI whole CMD

19 How good are the solutions if we use only the RGB? Cetus IC1613 Girardi bright CMD BaSTI bright CMD [M/H] Look back time (Gyr) Z 10 3 BaSTI whole CMD [M/H] Look back time (Gyr) Z 10 3

20 Two independent paths to Z(t): a comparison of results 1) SFR(t) and Z(t) from main sequence photometry alone Meschin et al. (2009) 2) Z(t) from spectra & RGB photometry Carrera et al. (2009) Log(age)=a+b(V I)+cMv+d[M/H]+ +f(v I)2+g[M/H]2+h(V I)3

21 Two independent paths to Z(t): a comparison of results 1) SFR(t) and Z(t) from main sequence photometry alone: Red line Magenta dots 2) Z(t) from spectra & RGB photometry Blue symbols Black dots

22 Conclusions: -RGB good metallicity indicator if we are sure that no age range exists (beyond very few Gyr). Other age indicators on the CMD can help, eg., presence or not of C stars. RGB width: lower limit of metallicity dispersion. -Adding CaT spectroscopy: possibility to recover Z(t) -Integrated SFR(t) can be estimated within a factor of a 2-4 from numbers counts in the upper RGB -SFH best measured from CMDs reaching the oldest main sequence turnoffs. Loose constraints can be obtained by modelling the bright part of the CMD alone -Consistent results are obtained for Z(t) obtained through modelling of the MS alone and the RGB (CMD+spectra).