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Annalisa Calamida (INAF-OAR)

Deep optical and Near-Infrared photometry of the globular cluster ω Cen. Annalisa Calamida (INAF-OAR). Summary Star counts and evolutionary lifetimes in ω Cen White Dwarfs in ω Cen Conclusions. G. Bono, R. Buonanno, C. E. Corsi, I. Ferraro,

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Annalisa Calamida (INAF-OAR)

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  1. Deep optical and Near-Infrared photometry of the globular cluster ω Cen Annalisa Calamida (INAF-OAR) Summary • Star counts and evolutionary lifetimes in ωCen • White Dwarfs in ω Cen • Conclusions 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)

  2. 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 • Overabundanceα-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

  3. Pancino et al. (2000) Gratton et al. (2005) • Main peak at [Fe/H] ~ -1.7 • Secondary peak at [Fe/H] ~ -1.2 • Tail up to [Fe/H] ~ -0.5 • Many different RGBs: • Metal-poor (ω1) • Metal-intermediate (ω2) • Metal-rich (ω3) U, V photometry from WFI@2-2m

  4. Blue & Red MS (Bedin et al. 2004) • Super-metal-poor population ([Fe/H] <<-2.0) -> 30% of ωCen stars!!! • Helium enhanced population (ΔY ~ 0.15) • Population of stars located behind ω Cen HST

  5. Spectra of17 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 !!

  6. WFI ACS Quest for complete star counts of HB, RG & MS stars in ω Cen B, R, Hα ACS/HST (108 frames), FOV~ 9’9’ U, B, V, I  WFI/2.2m (125 frames), FOV~ 45’45’

  7. AGB BS SGB-a EHB WDs Simultaneous reduction of all space & ground-based data: DAOPHOTII/ALLFRAME (Stetson 1994) ACS@HST 1.3 Million stars WFI-2.2m ESO/MPI 0.6 Million stars ωCen FINAL CATALOGUE: 1.7 million stars!!!

  8. Setting the “theoretical clock” • Canonical scenario (Y=0.23): • Arrival rate of stars onto the HB • r(HB) = NHB/tHB compared to • r(RG) = NRG/tRGand to • r(MS) = NMS/tMS M/M⊙= 0.80 Age = 12 Gyr Castellani et al. 2007, ApJ, 663, 1021 Excess of HB stars Discrepancy betweentheory & observations: ~30-40% for HB/RG and ~ 43% for HB/MS Pisa Evolutionary Code Cariulo et al. (2004)

  9. New theoretical clocks at fixed cluster age • He-mixed scenario: • 70% canonical + • 30% He-enriched (Red and Blue-MS) HB/RG: smaller discrepancy but still high for Y = 0.33 (15-25%) & Y = 0.42 (15-20%) HB rate ~ 24% (Y=0.42) and ~30% (Y=33) larger than MS rate ! Castellani et al. 2007, ApJ, 663, 1021

  10. 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 ωCen 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 WDs

  11. WDs in ωCen: theory DM0=13.700.10 (Del Principe et al. 2006) E(B-V)=0.110.02 (Calamida et al. 2005) • WD cooling sequences by • Althaus & Benvenuto (1998) • and atmosphere models by • Bergeron et al. (1995): • CO core + H envelope • CO core + He envelope Preliminary evidence of He core WDs in ω Cen Models by Serenelli et al. (2002) Calamida et al. 2008

  12. WDs in ωCen: star counts EHB WDs 8 ACS fields   6500 WDs !! (Individually double-checked with ROMAFOT) MS stars 18.775≤F435W≤19.025: 25133160 B=24 65125 B=24.5 118934 B=25 204745 N(WDs)/N(MS)

  13. Star counts & evolutionary lifetimes • Canonical scenario: M = 0.5Mʘ and CO core + H envelope Observations Theory • N(WDs)/N(MS) t(WDs)/t(MS) • B ≲ 24 mag0.052±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 increasing the discrepancy

  14. He-mixed scenario:70% canonical +30% He-enriched • Y = 0.42 N(WDs)/N(MS) t(WDs)/t(MS) • B ≲ 24 mag0.052±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%

  15. Helium-core WDs (M = 0.3Mʘ): N(WDs)/N(MS) t(WDs)/t(MS) B ≲ 24 mag0.052±0.0020.07±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 ωCen 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

  16. Candidate WDs in the Near-IR Selected in separation & Sharpness < 0.5 MAD:Multi-Conjugate Adaptive Optics Demonstrator (ESO/VLT) • Wide field of View (~ 1”1”) adaptive optics correction in the J/K bands • CAMCAO: 2k2k IR camera, 0.028”/pixel • Two nights: 03/04/2007  5K (524s), 3J (524s) 04/04/2007  3K (1024s), 3J (1024s) Dimm seeing: ~ 0.6-1” Image Seeing: K: ~ 0.1" & J : ~ 0.25" K~20.5 & J~20with SNR =10

  17. Multi-Conjugate Adaptive Optics (MCAO) ISAAC@VLT: FWHM ~ 0.6” MAD@VLT FWHM ≤ 0.1”

  18. SOFI@NTT ISAAC@VLT MAD@VLT

  19. Candidate WDs in the IR First time in a globular cluster! NIR excess: 4-5 mag in K!

  20. Hα excess!!

  21. Conclusions • The discrepancy between star counts and evolutionary lifetimes suggests that a fraction of at least 10% (0.3Mʘ) of ωCen 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

  22. Many thanks to: G. Bono, R. Buonanno, S. Degl’Innocenti, P. Prada Moroni, E. Marchetti, S. D’Odorico, P. Amico, C. E. Corsi, I. Ferraro, M. Monelli

  23. 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 F625Wmag: F625W = V0.544 + I0.455

  24. Star counts & Theoretical predictions B, B-F625N & B, B-V B, B-F658N & B, U-V Arrival rate of stars onto the HB: r (HB) = NHB/tHB Arrival rate of stars onto the RGB: r(RG) = NRG/tRG Discrepancy between theory & observations ≈30-40% RG/MS Castellani et al. 2007, ApJ, 663, 1021

  25. Total rates: r(HB) ~ 39 stars/ Myr & r(MS) ~ 26 stars/Myr HB rate ~ 43% 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.

  26. r(HB)/r(RG): Smaller discrepancy but still high for: Y = 0.33 (15-25%) Y = 0.42 (15-20%) HB rate ~ 24% (Y=0.42) and ~30% (Y=33) larger than MS rate RG/MS Castellani et al. 2007, ApJ, 663, 1021

  27. 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 500pcbeyond the bulk of cluster • - Difference in distance is 10% in agreement with distance density maxima of tidal tails in Palomar5 (Freyhammer et al. 2005)

  28. (Capuzzo Dolcetta 2005) Clumps ρ= volume mass density

  29. ~ 3-4·104 binaries Binary frequency: ~ 3-4%(Mayor et al. 1996) • 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 ωCen than in NGC2808

  30. Origin of EHB stars: coalescence of two low-mass He-core WDs -> + HBs extreme mass loss episodes before the He flash: stars above the limit for He ignition -> + HBs below the limit -> + He-core WDs

  31. 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.

  32. Helium enhancement? Lee et al. (2005) Explain origin of EHB stars in NGC2808 and in ω Cen Working hypothesis: He-enriched population EHB • HB morphology: • HB becomes systematically bluer (hotter) • NGC2808 -> Helium overabundance? • (D’Antona et al. 2005) EHB

  33. 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 ωCen • AGB intermediate-mass stars ->ΔY is not enough Problems with the IMF(chemical evol. models by Romano et al. 2007)

  34. NRG/NMS vs B 18.65<BMS<19.15 The discrepancy ranges from 10% to 15% from brighter to fainter RG stars

  35. NRG/NMS vs B Marginally dependent on He content

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