Annalisa CalamidaAnnalisa Calamida 5 Maggio 2009

Metallicity distribution of red-giants in Metallicity distribution of red-giants in ωω Cen 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. Degl’Innocenti, P. Prada Moroni (Universita’ di Pisa)

Summary

Why Omega Centauri?

Omega Centauri red giants metallicity distributions

Conclusions

Why ω Cen?• Most luminous (MV~ -10) and most massive (M ~ 5·106M⊙) galactic globular cluster

• Retrograde orbit (zmax= 1 Kpc; Ra = 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

0.2-1.4~1< 0.1Metallicity spread, Δ [Fe/H] (dex)

106-108~106~ 105Mass (M⊙)

-8/-13-10< -9Magnitude (MV)

dSphsω CenGCsProperties

Many different RGBs:

– Metal-poor (ω1)

– Metal-intermediate (ω2)

– Metal-rich (ω3)

U, V photometry from WFI@2-2m

– 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: questionsRGB-a (ω3): [Fe/H] ~ - 0.8

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

2305470y

1804670b

1904110v

3003500u

Half Width

λpeak (Å)BandStrömgren 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

mm11 versus CI planes versus CI planes

M92: [Fe/H] ~ –2.2

M13: [Fe/H] ~ –1.65

NGC1851: [Fe/H] ~ –1.3

NGC104 : [Fe/H] ~ –0.7

0

00/CI

CImHFe

Validation with GCsValidation with GCs

-0.73 0.10

-1.27 0.10

-1.40 0.12

-1.54 0.12

-1.91 0.14

Literature

M71NGC362NGC288NGC6752NGC6397

-1.85 0.17

-2.060.17

-1.93 0.24

-0.69 0.17

-1.28 0.20

-1.26 0.10

-1.56 0.15

Semi-empirical

-0.67 0.27

-1.34 0.26

-1.28 0.11

-1.76 0.18

Theoretical

-0.65 0.22

-1.26 0.26

-1.30 0.16

-1.55 0.21

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:

<Δ[Fe/H]> = 0.050.02 dex

σ ([Fe/H]) = 0.14 dex

Diamonds: labelled as CN-strong

stars by Norris

CN abundance

peculiarities

ω Cen RGs metallicity distributionsEmpirical 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 thanu-y

Hilker & Richtler

(2000)

• We provided new empirical, theoretical and semi-empirical

metallicity calibrations of the Strömgren m1 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 IIConclusions II

)/(

)/(log]/[ 10 XZ

XZHFe

Z = abundance by mass of metals X = abundance by mass of hydrogen(Z/X)⊙ =0.0245 for the sun