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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Several years ago people believed that this indicated that the cluster was well-mixed when the stars formed
Now, no the 3rd dredge-upChemical Anomaly of GCs: Fe Group
Kraft, et al., 1992: M3, M13
Gratton et al., 2004
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 2002The C-N & C-L anti-correlation
Cohen, Briley, & Stetson (2002)
Gratton et al., 2004
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…
1) Heavy Elements are uniform throughout cluster
2) C and N (only) have been shown (conclusively) to vary with evolution/luminosity.
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.
Li-plaue: 2.1-2.3 (halo stars, NGC 6397)
Diffusion or extra-mixing mechanismLi abundace in globular clusters
Error:Li: 0.09-0.14 dexO: 0.17-0.26 dex
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) > 2.3Li abundance variations and dynamics
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