annalisa calamida 5 maggio 2009 metallicity distribution of red-giants in ω cen
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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