Li abundance of to stars in globular clusters
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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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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


  • 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

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

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

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)

O-Na Anticorrelation

Gratton et al., 2004

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 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…


  • 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)


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?

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

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.

One of the most metal-poor:

[Fe/H] = -2.2

One of the oldest:


(according to Grundahl et al 2000)


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

M 92

[Fe/H] ~ -2.0

Age ~ 13-14 Gyr

Distance ~ 7,200 ly

One of the closest

m-M ~ 12.5


Bonifacio et al. 2002

NGC 6397

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?

[Fe/H] ~-1.43

Age ~ 13 Gyr

Distance ~13,000 ly

Log (M/M0) = 5.1 (DaCosta’s thesis, 1977)

m-M ~ 13.13


Pasquini et al. 2005

NGC 6752

[Fe/H] ~ -0.7

Age ~ 10 Gyr

Distance ~ 13,400 ly

m-M ~ 13.5


Bonifacio et al. 2007

47 Tuc

TO stars:

V = 17.0-17.3; (B-V)=0.4-0.51

With the same temperature and mass, at the same stage


For Li 6708Å, R~17,000, S/N ~ 80-100

For O 7771-7775Å, R~18,400, S/N ~ 40-50

Our data


Error:Li: 0.09-0.14 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


  • 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. (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

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


  • 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.

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

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?


  • 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 scatter

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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