Annalisa Calamida 5 Maggio 2009 Metallicity distribution of red-giants in ω Cen

This work has been made in collaboration with: G. Bono, R. Buonanno (Universita' di Roma Tor Vergata) C. E. Corsi, G. Iannicola, L. Pulone, I. Ferraro, M. Monelli + others (INAF-OAR) P. B. Stetson (DAO, Victoria, Canada) L. M. Freyhammer (, UK) F. Grundahl (Department of Physics and Astronomy, Denmark) S. Cassisi, A. Pietrinferni, M. Del Principe, A. Piersimoni (INAF-OAC) S. DeglInnocenti, P. Prada Moroni (Universita di Pisa)

Summary Why Omega Centauri? Omega Centauri red giants metallicity distributions Conclusions

Why ω Cen? Most luminous (M V ~ -10) and most massive (M ~ 5·10 6 M ) galactic globular cluster Retrograde orbit (z max = 1 Kpc; R a = 6 Kpc) Metallicity dispersion : -2.2 < [Fe/H] < -0.5 Overabundance α-elements (O, Mg, Si, Ca) Overabundance s -elements (Ba, Mo, La, Zi) Evidence of primordial chemical self-enrichment in short time scales (1-3 Gyr) Relic of a dwarf galaxy accreted on the Milky Way Merging of two stellar systems ~1~1< 0.1Metallicity spread, Δ [Fe/H] (dex) ~10 6 ~ 10 5 Mass (M ) -8/-13-10< -9Magnitude (M V ) dSphsω CenGCsProperties

Many different RGBs: – Metal-poor (ω1) – Metal-intermediate (ω2) – Metal-rich (ω3) U, V photometry from – Main peak at [Fe/H] ~ -1.7 – Secondary peak at [Fe/H] ~ -1.2 – Tail up to [Fe/H] ~ -0.5 Gratton et al. (2005) Pancino et al. (2000)

Literature: questions RGB-a (ω3): [Fe/H] ~ SGB-a: best-fit with old (~ 17 Gyr), low-metallicity isochrones SGB-a Split MS at V ~ 20mag, blue MS (Bedin et al. 2004) Best-fit with [Fe/H] ~ -1.0, d = 1.6 Kpc Hypothesis: Super-metal-poor population ([Fe/H] <<-2.0) Helium enhanced population (ΔY ~ 0.15) Population of stars located behind ω Cen (Ferraro et al. 2004)

Strömgren indices m1 = (v-b)-(b-y) measures the blanketing strenght in the region of λ = 4100Å It used to estimates metallicity but it is affected by reddening y b v u Half Width λ peak (Å)Band Strömgren 1963 (b-y) is a temperature indicator and is not sensitive to metallicity v filter includes many iron absorption lines and the CN band at λ = 4215Å u filter relates to the Balmer discontinuity (λ = 3647Å )

Danish Dataset Images acquisition (L.M. Freyhammer) Danish Telescope, 1.54m ESO (La Silla): Our Strömgren dataset: u, v, b, y, (110), seeing ~ 1 Hilker dataset: 1993/1995: v, b, y (210), seeing ~ 1.2 Grundahl dataset: 1999: u, v, b, y (27) + HD standards, seeing ~ 1 Grundahl frames Our frames Hilker frames

m 1 versus CI planes m 1 versus CI planes M92: [Fe/H] ~ –2.2 M13: [Fe/H] ~ –1.65 NGC1851: [Fe/H] ~ –1.3 NGC104 : [Fe/H] ~ –0.7

Validation with GCs Literature M71NGC362NGC288NGC6752NGC Semi-empirical Theoretical Empirical Testing GCs: –2.0 [Fe/H] –0.6 N6752, N288, N362: E(B-V) 0.03 N6397: E(B-V) ~ 0.18 & M71: E(B-V) ~ 0.31 [Fe/H] are in the Zinn & West metallicity scale

ω Cen RGs metallicities Sample of 40 ROA stars with high- dispersion spectroscopy by Norris & Da Costa (1995): 30 RGs in common Good agreement for all the relations: = dex σ ([Fe/H]) = 0.14 dex Diamonds: labelled as CN-strong stars by Norris CN abundance peculiarities

ω Cen RGs metallicity distributions Empirical relations Sharp cut-off al low metallicities ([Fe/H] < -2.2 dex) and a meta-rich tail up to [Fe/H] ~ 0 dex CN-degenaracy at [Fe/H] phot ~ -1 dex ?-> it seems that v-y is more sensitive to CN abundance variations than u-y Hilker & Richtler (2000)

We provided new empirical, theoretical and semi-empirical metallicity calibrations of the Strömgren m 1 index The new calibrations gives reliable metallicity estimates (σ 0.25 dex) for GCs and field red giants Preliminary evidence that at least ~1/3 of ω Cen RG stars show CN-abundance peculiarities Metallicity distribution of ω Cen RGs: evidence of a sharp cut-off at low metallicities (very few metal-poor stars), two main metallicity peaks at [Fe/H] ~ -1.8,-1.3 dex and a metal-rich shoulder at [Fe/H]~ -0.4 dex Conclusions II

We can define the reddening-free indices (Stetson 1991) : [c] = c · (b-y) [m] = m · (b-y) Using Cardelli et al. (1989) extinctions curves we derived values for the Danish Stroemgren filters: E (u-y)/E(B-V) = 1.84 (1.825*) E(c 1 )/E(b-y) = 0.2 E (v-y)/ E(B-V) = (1.236) E(m 1 )/E(b-y) = E(b-y)/E(B-V) = 0.70 (0.74) * Crawford values (1976)

Z = abundance by mass of metals X = abundance by mass of hydrogen (Z/X) = for the sun