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Li Abundance of TO stars in globular clusters

Li Abundance of TO stars in globular clusters. Zhixia Shen Luca Pasquini. The Globular Cluster (GC). The same distance, the same age and [Fe/H]:GCs are good testbeds for stellar evolution Nucleosynthesis in old stars Galaxy chemical evolution The age of the universe. Outlines.

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Li Abundance of TO stars in globular clusters

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  1. Li Abundance of TO stars in globular clusters Zhixia Shen Luca Pasquini

  2. The Globular Cluster (GC) • The same distance, the same age and [Fe/H]:GCs are good testbeds for • stellar evolution • Nucleosynthesis in old stars • Galaxy chemical evolution • The age of the universe

  3. Outlines • Chemical inhomogeneity of GCs • Li variations of TO stars in GCs • History • Our work

  4. Abundance Anomalies in Globular clusters • Homogeneous Fe abundance • Homogeneous n-capture element abundances • Light element abundance anomalies • C-N • Na-O • Mg-Al • etc

  5. Most globular clusters (GCs) have a very uniform distribution of Fe group elements - all the stars have the same [Fe/H]. Several years ago people believed that this indicated that the cluster was well-mixed when the stars formed Now, no the 3rd dredge-up Chemical Anomaly of GCs: Fe Group Kraft, et al., 1992: M3, M13

  6. Chemical Anomaly of GCs: Fe Group--compared to field stars Gratton et al., 2004

  7. Chemical Anomaly of GCs: Fe Group--compared to field stars Gratton et al., 2004

  8. Chemical Anomaly of GCs: n-capture elements Gratton et al., 2004

  9. Large spread in Carbon and Nitrogen in many GCs: The first negative correlation (anticorrelation) : C is low when N is high. The anticorrelation is explicable in terms of the CN cycle, where C is burnt to N14 The C abundance decreases with L on the RGB (and N increases). This isknown as the C-L anticorrelation This is also observed in halo field stars. M3, Smith 2002 The C-N & C-L anti-correlation Cohen, Briley, & Stetson (2002)

  10. O-Na Anticorrelation Gratton et al., 2004

  11. O-Na Anticorrelation • This is readily explained by hot(ter) hydrogen burning, where the ON and NeNa chains are operating - the ON reduces O, while the NeNa increases Na (T ~ 30 million K) • Where this occurs is still debatable. • The amazing thing about this abundance trend is that it only occurs in Globulars - it is not seen in field halo stars

  12. Mg-Al anticorrelation in (some) GCs. This can also be explained through high-temperature (T~ 65 million K) proton capture nucleosynthesis, via the MgAl chain (Mg depleted, Al enhanced). It does not occur in field stars... The light elements also show various correlations among themselves---> (Kraft, et al, 1997. Giants) Mg, Al…

  13. Summary • All these anticorellations point to hydrogen burning -- the CN, ON, MgAl, NeNa cycles/chains -- at various temperatures. • CN, ON, NeNa: T~20 MK-40 MK(?) • MgAl: T~40 MK-65 MK(?) • Previously, the most popular site* for this is at the base of the convective envelope in AGB stars - Hot Bottom Burning • And now, maybe winds from massive stars (WMS)

  14. Summary 1) Heavy Elements are uniform throughout cluster • No the 3rd dredge-up 2) C and N (only) have been shown (conclusively) to vary with evolution/luminosity. • Most likely ongoing deep mixing on RGB, but not very deep mixing. 3) Light elements (C – Al) show spreads to varying degrees, and are linked through the (anti)correlations. Spreads are seen in non-evolved stars also. • Inhomogeneous light element pollution; could be • pre-formation: AGB? WMS? • intrinsic stellar pollution (i.e. deep mixing), Non-evolved star? • accretion (Bondi-Hoyle?, binaries?, planets?). Fe? Mass of accretion material (O depletion to 1/10, 9:1 accretion mass?)? Subgaints?

  15. Among the light elements Li has a special role. Li is produced in Big Bang nucleosynthesis,enriched during the galaxy evolution,and destroyed in the stellar interior WMAP: A(Li)=2.64 Li-plaue: 2.1-2.3 (halo stars, NGC 6397) Diffusion or extra-mixing mechanism Li abundace in globular clusters

  16. Li abundance of TO stars in GCs • Indicator of globular cluster chemical evolution history • The low temperature for Li depletion (2.5 MK) • CNO circle: ~30 MK • TO stars: unevolved

  17. History • M 92: can’t be trusted • NGC 6397: Li abundance is an constant • NGC 6752: Li-O correlation;Li-Na/N anti-correlation; • 47 Tuc: Li-Na anti-correlation, lack of correlation between Li and N.

  18. One of the most metal-poor: [Fe/H] = -2.2 One of the oldest: 16Gyr (according to Grundahl et al 2000) m-M=14.6 Distance = 27,000 ly M 92

  19. Boesgaard et al. 1998 V ~ 18 Keck I 1.5-6.5 hr R ~ 45,000 S/N: 20-40 Reanalysis of Bonifacio et al. (2002): a variation of only 0.18 dex M 92

  20. [Fe/H] ~ -2.0 Age ~ 13-14 Gyr Distance ~ 7,200 ly One of the closest m-M ~ 12.5 Li: Bonifacio et al. 2002 NGC 6397

  21. Something interesting… • For a long time, people believed that whereas NGC6752 shows much variation, NGC6397 does not (Gratton et al 2001) • [O/Fe] = 0.21 • [Na/Fe] = 0.20 • Star-to-star  0.14 dex • Can be explained by obs error and variance in atmospheric parameters • Carretta et al. (2004): Na, O variations in NGC 6397 • Li? • Lack of Li-N correlation?

  22. [Fe/H] ~－1.43 Age ~ 13 Gyr Distance ~13,000 ly Log (M/M0) = 5.1 (DaCosta’s thesis, 1977) m-M ~ 13.13 Li: Pasquini et al. 2005 NGC 6752

  23. [Fe/H] ~ -0.7 Age ~ 10 Gyr Distance ~ 13,400 ly m-M ~ 13.5 Li: Bonifacio et al. 2007 47 Tuc

  24. TO stars: V = 17.0-17.3; (B-V)=0.4-0.51 With the same temperature and mass, at the same stage VLT-FLAMES/GIRAFFE, medusa mode For Li 6708Å, R~17,000, S/N ~ 80-100 For O 7771-7775Å, R~18,400, S/N ~ 40-50 Our data

  25. Results Error:Li: 0.09-0.14 dexO: 0.17-0.26 dex

  26. Li variation: 1.7-2.5, 0.8 dex • The upper bundary is consistent with the prediction of WMAP • Not all stars have Li • Li-O correlation: • Possibility > 99.9% (ASURV) • Can’t be made by TO star themselves • For CNO circle, Te > 30 MK • In the center of TO: 20 MK • Li depletion: 2.5 MK • Large dispersion in Li-O correlation

  27. Explanation • The Li/O-rich stars, which are also Na poor, have a composition close to the "pristine" one, while the Li/O-poor and Na-rich stars are progressively contaminated. • The contamination gas is from • the Hot bottom burning (HBB) of an AGB star or • Wind of massive stars.

  28. The chemical component of pollution gas • If we assume a primordial Li abundance of 2.64, given the observed lower boundary of 1.8, more than 80% of the gas should be polluted for such stars. • If primordial [O/Fe] = 0.4, [O/Fe] of the most Li-poor stars are -0.3, then the pollution gas should have O/H~6.6 • Pasquini et al. (2005) for pollution gas: • A(Li) ~2.0, Na/H > 5.4, O/H<7.0, N/H~7.4

  29. AGB or WMS: production • The results of Pasquini et al. (2005) for NGC 6752 is qualitatively consistent with the AGB model of Venture et al. (2002) • The lack of N in 47 Tuc: WMS is more possible (Bonifacio et al. 2007) • For metal-poor AGB stars, the reaction from O to N is quite efficient (Denissenkov et al. 1997 etc)

  30. AGB: production problem • Quantatively, AGB can’t explain the abundance variation for most GCs (Fenner et al. 2004) • Too much or not enough Na while O is not depleted enough • When Mg needs to be burnt, it is produced • C+N+O can’t be constant as observed • AGB models depends on two uncertain factors: • Mass loss rate • Efficiency of convective transport

  31. Weiss et al. (200０) for HBB production • When Al is produced, too much Na • Denissenkov et al. (2001): 23Na firstly produced then destroyed during interpulse phase --> accurate period for both O-depletion and 23Na production

  32. WMS: production • Decressin et al. (2007): • Fast rotate models of metal-poor ([Fe/H]=-1.5) massive stars from 20-120 solar mass • Surface chemical composition changes with mass loss • Based on Li abundances: • 30% primordial gas is added to the winds • The model could reproduce C,N,O and Li variation • But failed in Mg

  33. Li: pollution scenario (Prantzos & Charbonnel 2006) - AGB • If IM-AGB (4-9 solar mass) • 20-150 Myr • Before that, M* > 9Msun --> SNe-->wind of 400km/s --> no Li-rich primordial gas left • Li-production? Hard to get A(Li)=2.5 • After that, 2-4Msun stars eject almost the same amount of material as IM-AGB • Maybe no HBB, but the third dredge-up --> C and s-process elements variation

  34. WMS • In 20 Myr, massive stars evolve and slowly release gas through winds. The gas is mixed with primordial material. • The shock wave of SNe induce the formation of the new stars • After 20 Myr, wind ejecta from low mass stars (<10 Msun) won’t form stars because of no trigger.

  35. AGB: the ejecta will concentrate to the center of the GC In 47 Tuc, most CN-rich stars near the center However, in NGC 6752: Red: A(Li) < 2.0 Green: 2.0 < A(Li) < 2.3 Black: A(Li) > 2.3 Li abundance variations and dynamics

  36. Different GCs, different abundace variations • Bekki et al. (2007): GCs come from dwarf galaxies in dark halo at early age. The pollution gas is from outside IM-AGB field stars • The difference of GCs • Can’t produce the abundance variation pattern • Supported by Gnedin & Prieto (2006): all GCs 10 kpc away from the Galaxy center are from satellite galaxies.

  37. Primordial Li abundance • Are field stars also polluted by the first generation stars?

  38. Conclusions • Li variation is exist in GCs • Li abundance is correlated with Na and O • A mixing of contamination gas and primordial gas is needed • The contamination gas may comes from WMS • Next work: • The large scatter in Li-O correlation • New data of 47 Tuc

  39. The scatter

  40. Thank you! Invitation for Lunch Time: 11:30 am today Place: The third floor of NongYuan Everyone is welcomed! Shen Zhixia & Wang Lan

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