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 Chemical inhomogeneity of GCs Li variations of TO stars in GCs –History –Our work
Abundance Anomalies in Globular clusters Homogeneous Fe abundance Homogeneous n-capture element abundances Light element abundance anomalies –C-N –Na-O –Mg-Al –etc
Chemical Anomaly of GCs: Fe Group 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 Kraft, et al., 1992: M3, M13
Chemical Anomaly of GCs: Fe Group --compared to field stars Gratton et al., 2004
Chemical Anomaly of GCs: Fe Group --compared to field stars Gratton et al., 2004
Chemical Anomaly of GCs: n-capture elements Gratton et al., 2004
The C-N & C-L anti-correlation 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 N 14 The C abundance decreases with L on the RGB (and N increases). This is known as the C-L anticorrelation This is also observed in halo field stars. Cohen, Briley, & Stetson (2002) M3, Smith 2002
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
Mg, Al… 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)
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)
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?
Li abundace in globular clusters 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 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
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.
M 92 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 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
NGC 6397 [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
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?
Our data 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
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
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.
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
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)
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
Weiss et al. (200 ０ ) for HBB production –When Al is produced, too much Na Denissenkov et al. (2001): 23 Na firstly produced then destroyed during interpulse phase - -> accurate period for both O- depletion and 23 Na production
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
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
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.
Li abundance variations and dynamics 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
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.
Primordial Li abundance Are field stars also polluted by the first generation stars?
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