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Annalisa Calamida (INAF-OAR)Annalisa Calamida (INAF-OAR)
Deep optical and Near-Infrared Deep optical and Near-Infrared photometry of the globular cluster photometry of the globular cluster
ωω Cen Cen
G. Bono, R. Buonanno, C. E. Corsi, I. Ferraro, G. Iannicola, L. Pulone, M. Monelli, F. Caputo, V. Castellani (INAF-OAR)S. D’Odorico, E. Marchetti, P. Amico (ESO)S. Degl’Innocenti, P. Prada Moroni (Pisa University)M. Nonino (INAF-OAT)P. B. Stetson (DAO, Victoria, Canada)
Summary
Star counts and evolutionary lifetimes in ω Cen
White Dwarfs in ω Cen
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
Properties GCs ω Cen dSphs
Magnitude (MV) < -9 -10 -8/-13
Mass (M⊙) ~ 105 ~106 106-108
Metallicity spread, Δ [Fe/H] (dex) < 0.1 ~1 0.2-1.4
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)
Blue & Red MS
(Bedin et al. 2004)
Super-metal-poor population ([Fe/H] <<-2.0) -> 30% of ω Cen stars!!! Helium enhanced population (ΔY ~ 0.15)
Population of stars located behind ω Cen
HST
Spectra of 17 stars (Piotto et al. 2005):
rMS: [M/H] = -1.57 dex, bMS: [M/H] = -1.26 dex
bMS is ~ 0.3 dex more metal-rich than the rMS
CMD:Isochrone best fit
[M/H] = -1.26
Y ~ 0.35
ΔY/ΔZ > 70 !!
Quest for Quest for complete star countscomplete star counts of of HB, RG & MS stars in HB, RG & MS stars in ωω Cen Cen
B, R, Hα ACS/HST (108 frames), FOV~ 9’9’
U, B, V, I WFI/2.2m (125 frames), FOV~ 45’45’
ACS
WFI
Simultaneous reduction of all space & ground-based data:
DAOPHOTII/ALLFRAME (Stetson 1994)
ACS@HST 1.3 Million starsWFI-2.2m ESO/MPI 0.6 Million stars
ωω CenCen
FINAL CATALOGUE: 1.7 million stars!!!FINAL CATALOGUE: 1.7 million stars!!!
BS
WDs
EHB
SGB-a
AGB
Setting the “theoretical clock”M/M⊙ = 0.80Age = 12 Gyr
Castellani et al. 2007, ApJ, 663, 1021
Pisa Evolutionary CodeCariulo et al. (2004)
Canonical scenario (Y=0.23):
Arrival rate of stars onto the HB
r(HB) = NHB/tHB compared to
r(RG) = NRG/tRG and to
r(MS) = NMS/tMS
Discrepancy
between theory &
observations:
~30-40% for HB/RG
and ~ 43% for
HB/MS
Excess of HB stars
He-mixed scenario:
70% canonical +
30% He-enriched (Red and
Blue-MS)
Castellani et al. 2007, ApJ, 663, 1021
HB rate ~ 24% (Y=0.42) HB rate ~ 24% (Y=0.42) and and ~~30% (Y=33) larger 30% (Y=33) larger
than MS rate !than MS rate !
HB/RG: smaller discrepancy but still high for Y = 0.33 (15-25%) & Y = 0.42 (15-20%)
New theoretical clocksat fixed cluster age
Current findings indicate that a mix of stellar populations made
with 70% canonical He (Y=0.23) and 30% He-enhanced (Y=0.33
or Y=0.42) only partially account for the observed excess of HB stars in ω Cen
Working hypothesis:
Hot He-flashers
Huge mass-loss (binarity) before He-core flash, Castellani & Castellani 1993, D’Cruz et al. (1996), Sweigart (1997), Castellani et al. (2006) )
He-core WDsHe-core WDs
DM0=13.700.10 (Del Principe et al. 2006)
E(B-V)=0.110.02 (Calamida et al. 2005)
WDs in WDs in ωω Cen: theoryCen: theory
WD cooling sequences by
Althaus & Benvenuto (1998)
and atmosphere models by
Bergeron et al. (1995):
1. CO core + H envelope
2. CO core + He envelope
Preliminary evidence of He core WDs in ω
Cen
Models by Serenelli et al. (2002)Calamida et al. 2008
8 ACS fields 6500 WDs !! (Individually double-
checked with ROMAFOT)
EHB
WDs
WDs in WDs in ωω Cen: star countsCen: star counts
65125
118934
204745
MS stars
18.775≤F435W≤19.025:
25133160B=24
B=24.5
B=25
N(WDs)/N(MS)
Star counts & evolutionary lifetimes
N(WDs)/N(MS) t(WDs)/t(MS)
B ≲ 24 mag 0.052±0.002 0.021±0.003 (~2.5)
B 24.5 mag≲ 0.095±0.002 0.048±0.007 (~2)
B 25 mag≲ 0.163±0.004 0.12 ±0.02 (~1.5)
Similar discrepancy for CO core + He envelope
An increase in the WDs mass increases the discrepancy
Completeness problems go in the direction of Completeness problems go in the direction of
increasing the discrepancyincreasing the discrepancy
Canonical scenario: M = 0.5Mʘ and CO core + H envelope Observations Theory
He-mixed scenario: 70% canonical + 30% He-enriched
Y = 0.42 N(WDs)/N(MS) t(WDs)/t(MS)
B 24 mag≲ 0.052±0.002 0.024±0.003
B 24.5 mag≲ 0.095±0.002 0.057±0.008
B 25 mag≲ 0.163±0.004 0.11±0.02
Smaller discrepancy but still high!!!
A decrease in cluster age, t =10 Gyr, does not remove
the discrepancy
A change in the distance modulus of 0.2mag affects the
lifetime ratios for MWD=0.5M⊙ of ~18%
Helium-core WDs (M = 0.3MHelium-core WDs (M = 0.3Mʘʘ): ):
N(WDs)/N(MS) t(WDs)/t(MS)
B ≲ 24 mag 0.052±0.002 0.07±0.01
B 24.5 mag≲ 0.095±0.002 0.18±0.03
B 25 mag≲ 0.163±0.004 0.33±0.05
A fraction ≥10% of ω Cen WDs could
be He-core WDs (0.3Mʘ)
This fraction decreases if we
account for a smaller WD mass
(lower limit 0.17-0.2M⊙ )
He-core WDs have already been identified in
NGC6397 (Taylor et al. 2001), 47 Tuc
(Edmonds et al. 2001), NGC6791 (Kalirai et
al. 2007)….
X-ray/Hα excess, variability, suggesting
their binarity
MADMAD: Multi-Conjugate Adaptive Optics Demonstrator (ESO/VLT)
Wide field of View (~ 1”1”) adaptive optics correction in the J/K bands
CAMCAO: 2k2k IR camera, 0.028”/pixel
Two nights:
03/04/2007 5K (524s), 3J (524s)
04/04/2007 3K (1024s), 3J (1024s)
Dimm seeing: ~ 0.6-1”
Image Seeing:
K: ~ 0.1" & J : ~ 0.25"K~20.5 & J~20 with SNR =10
Candidate WDs in the Near-Candidate WDs in the Near-IRIR
Selected in separation &Sharpness < 0.5
ISAAC@VLT:FWHM ~ 0.6”
MAD@VLT FWHM ≤ 0.1”
Multi-Conjugate Adaptive Optics (MCAO)
SOFI@NTT ISAAC@VLT
MAD@VLT
Candidate WDs in the IRCandidate WDs in the IR
First time in a globular cluster!
NIR excess: 4-5 mag in K!
HHαα excess!! excess!!
The discrepancy between star counts and evolutionary
lifetimes suggests that a fraction of at least 10% (0.3Mʘ)
of ω Cen WDs are He-core WDs, thus supporting results
based on the evolved component (HB) hot He-flashers (Calamida et al. 2008)
We identified for the first time in a globular cluster 9 WD
candidates with NIR excess (6 of them show also Hα
excess)
We identified ~30 HB stars with Hα excess, ~50 with NIR
excess and 13 with both of them
ConclusionsConclusions
Many thanks to:
G. Bono, R. Buonanno, S. Degl’Innocenti, G. Bono, R. Buonanno, S. Degl’Innocenti,
P. Prada Moroni, E. Marchetti, P. Prada Moroni, E. Marchetti,
S. D’Odorico, P. Amico,S. D’Odorico, P. Amico,
C. E. Corsi, I. Ferraro, M. MonelliC. E. Corsi, I. Ferraro, M. Monelli
To refer the Johnson B mag to the F435W mag :B = F435W + 0.03(0026) - 0.0015( 0.001)F435W
And V and I mag to the F625W mag:
F625W = V0.544 + I0.455
Star counts & Theoretical predictions
Discrepancy
between theory & observations
≈ 30-40%
Arrival rate of stars onto the HB:
r (HB) = NHB/tHB
Arrival rate of stars onto the RGB:
r(RG) = NRG/tRG
B, B-F658N & B, U-V
B, B-F625N & B, B-V
RG/MSCastellani et al. 2007, ApJ, 663, 1021
Total rates:
r(HB) ~ 39 stars/ Myr & r(MS) ~ 26 stars/Myr
HB rate HB rate ~ ~ 43% larger than MS rate43% larger than MS rate
Excess of HB stars
Discrepancy marginally affected by the assumed metal
abundance and field star contamination.
Hot HB stars are systematically hotter than field stars and the selected MS stars cover a very narrow magnitude range.
r(HB)/r(RG):
Smaller discrepancy but
still high for:
Y = 0.33 (15-25%)
Y = 0.42 (15-20%)
HB rate HB rate ~ 24%~ 24% (Y=0.42) and (Y=0.42) and ~~30%30% (Y=33) larger than MS (Y=33) larger than MS raterate
RG/MSCastellani et al. 2007, ApJ, 663, 1021
Best fit with 2 isochrones:
Δμ = 0.2 mag
ΔE(B-V) = 0.03mag
-1.1 < [Fe/H] < -0.8
Coeval to bulk of stars
- ω3-branch might be a chunk of
stars located 500pc beyond the
bulk of cluster
- Difference in distance is 10% in
agreement with distance density
maxima of tidal tails in Palomar5
(Freyhammer et al. 2005)
(Capuzzo Dolcetta 2005)
ρ = volume mass density
Clumps
Binary frequency: ~ 3-4% (Mayor et al. 1996) ~ 3-4·104 binaries
• Central density, log ρC 3.12 LV/Mº
• Concentration,c (log rt /rc ) 1.24 (Trager et al. 1995)
• Half-mass relaxation time, trh 2·1011 Gyrs > cluster age
Presence of primordial binaries among the giants with periods
200 P 4000 days
Cluster AgeS Experiment (6/2004): ~ 30 eclipsing binaries &
~ 30 contact binaries (mostly short period, P < 1day)
Collision rate (prob. that a star centrally located exp. a collision in 1 year) is one order of magnitude smaller in ω Cen than in NGC2808
Origin of EHB stars:
1) coalescence of two low-mass He-core WDs -> + HBs
2) extreme mass loss episodes before the He flash:
a. stars above the limit for He ignition -> + HBs
b. below the limit -> + He-core WDs
The ‘separation index’ quantifies the degree of
crowding (Stetson et al. 2003)
The current sep value (sep > 3) corresponds to stars that
have required a correction of less than 6% for light
contributed by known neighbours.
Explain origin of EHB stars in
NGC2808 and in ω Cen
Working hypothesis:He-enriched population
EHB
EHB
HB morphology:
HB becomes systematically bluer (hotter)
NGC2808 -> Helium overabundance? (D’Antona et al. 2005)
Helium enhancement?Lee et al. (2005)
bMS: Metal-intermediate population with ΔY ~ 0.10-0.15
Requires: ΔY/ ΔZ > 70 (canonical value ~ 3)
Y produced from:
SNe with M > 20M⊙
Winds of low Z rotating massive stars
Gas ejected from field stellar pop. that sorrounded ω Cen
AGB intermediate-mass stars ->ΔY is not enough
Problems with the IMF (chemical evol. models by
Romano et al. 2007)
The discrepancy ranges from 10% to 15% from brighter to fainter RG stars
NRG/NMS vs B
18.65<BMS<19.15
NRG/NMS vs B
Marginally dependent on He content