Li Abundance of TO Li Abundance of TO stars in globular stars in globular
clustersclusters
Zhixia ShenZhixia Shen
Luca PasquiniLuca Pasquini
The Globular Cluster (GC)The Globular Cluster (GC)• The same distance, the sa
me age and [Fe/H]:GCs are good testbeds for
– stellar evolution – Nucleosynthesis in old st
ars– Galaxy chemical evolutio
n– The age of the universe
Outlines Outlines
• Chemical inhomogeneity of GCs• Li variations of TO stars in GCs
– History– Our work
Abundance Anomalies in Abundance Anomalies in Globular clustersGlobular clusters
• Homogeneous Fe abundance
• Homogeneous n-capture element abundances
• Light element abundance anomalies– C-N– Na-O– Mg-Al– etc
Chemical Anomaly of GCs: Chemical Anomaly of GCs: Fe GroupFe 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: Chemical Anomaly of GCs: Fe GroupFe Group--compared to field stars--compared to field stars
Gratton et al., 2004
Chemical Anomaly of GCs: Chemical Anomaly of GCs: Fe GroupFe Group--compared to field stars--compared to field stars
Gratton et al., 2004
Chemical Anomaly of GCs: Chemical Anomaly of GCs: n-capture elementn-capture elementss
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 N14
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 AnticorrelationO-Na Anticorrelation
Gratton et al., 2004
O-Na AnticorrelationO-Na Anticorrelation• This is readily explained by hot(ter) hydrogen burning, where t
he 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 oc
curs in Globulars - it is not seen in field halo stars
Mg, Al…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)
SummarySummary• 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)
SummarySummary1) 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 clustersLi 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 star
s, NGC 6397)– Diffusion or extra-mixing me
chanism
Li abundance of TO stars in Li abundance of TO stars in GCsGCs
• Indicator of globular cluster chemical evolution history– The low temperature fo
r 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-correl
ation; – 47 Tuc: Li-Na anti-correlation, lack of correlati
on between Li and N.
M 92M 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 92M 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 6397NGC 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…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?
NGC 6752NGC 6752
• [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
47 Tuc47 Tuc• [Fe/H] ~ -0.7• Age ~ 10 Gyr• Distance ~ 13,400 ly • m-M ~ 13.5• Li:
– Bonifacio et al. 2007
Our dataOur 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
ResultsResults
Error:Error:Li: 0.09-0.14 dexLi: 0.09-0.14 dexO: 0.17-0.26 dexO: 0.17-0.26 dex
• 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 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 The chemical component of pollution gaspollution 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: productionAGB 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 problemAGB: production problem• Quantatively, AGB can’t explain the abundance va
riation for most GCs (Fenner et al. 2004)– Too much or not enough Na while O is not depleted en
ough– 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. (200０ ) for HBB production– When Al is prod
uced, too much Na
• Denissenkov et al. (2001): 23Na firstly produced then destroyed during interpulse phase --> accurate period for both O-depletion and 23Na production
WMS: productionWMS: 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 m
ass 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 Li: pollution scenario (Prantzos & Charbonnel 2006) - AGB& Charbonnel 2006) - AGB
• If IM-AGB (4-9 solar mass)– 20-150 Myr– Before that, M* > 9Msun --> SNe-->wind of 40
0km/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
WMSWMS
• 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 Li abundance variations and dynamicsdynamics
• 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 abundacDifferent GCs, different abundace variationse 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 abundancePrimordial Li abundance
• Are field stars also polluted by the first generation stars?
Conclusions 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
The scatterThe scatter
Thank you!Thank you!
Invitation for LunchInvitation for Lunch
Time: 11:30 am today Place: The third floor of NongYuan
Everyone is welcomed!
Shen Zhixia & Wang Lan
