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