Light Elements (Li and Be) in Globular Clusters

L. Pasquini , ESO

Li in Turn-Off (or close to TO)

Li in evolved stars

A new result..

Be in TO stars

Li Observations in GC: a summary

Cluster Work (Year) TO Evolved

NGC 6397 Molaro and Pasquini 94, Pasquini and Molaro 96

Castilho et al. 2000

Thevenin et al. 2001

Bonifacio et al. 2002

1, 3(2) (NTT)

7 (VLT)

12(5) (VLT)

3 (NTT)

16 (NTT)

M 92 Deliyannis et al. 95; Boesgaard et al. 98

Pilachowski et al. 2000

4, 7(3) (Keck)

61 (WIYN)

47 Tuc Pasquini and Molaro 1997 3 (NTT) 2 (NTT)

M3 Kraft et al. 1999

Pilachowski et al. 2000

1 (47) (Keck)

77 (WIYN)

NGC362 Smith et al. 1999 2 (CTIO)

M13, M15 Pilachowski et al. 2000 78,46 (WIYN)

NGC 6752 Grundhal et al. 2002 21 (VLT)

M71 Ramirez and Cohen 2002 10 (Keck)

Li in (close to) Turn-Off Stars

Early Studies: Is the Li Plateau Primordial ? At which abundance level ? Two different ‘views’ : NGC 6397 [Fe/H]=-2.0 : Li Constant, Plateau (MP94, PM96)

47 Tuc [Fe/H]=-0.7 : Li Compatible with Plateau (PM97) (same Li in 1 CN rich and 1 CN-poor star within uncertainties )

M92 [Fe/H]=-2.3?: Li variable, 1 star higher than Plateau (BDSK99) : Statistically significant? (Bonifacio 02)

BUT: Is this really the ‘right’ question (in 2004)?

Li in (close to) Turn-Off Stars

Globular Clusters are known to have abundance anomalies! Is perhaps this the critical issue?

T00, B02, find an impressive constancy and similarity to Plateau in the UVES spectra of NGC6397 stars with different [O/Fe] Ratio.

Li/H=2.34 0.056(Intrinsic scatter <0.035)

Li/H = 2.32 0.014+-0.05 for ‘Plateau’ stars.

Li in (close to) Turn-Off Stars

M92 show as well anomalous abundances:

K99 finds that M92 stars show Mg and Na anomalous abundances asIf the gas was processed through Mg-Al and Ne-Na cycling. BUT Li is “too high” for being polluted… it should have been destroyed!

Li vs. Teff for the M92 Stars (B02 analysis):

Li/H=2.36 0.19

Li in (close to) Turn-Off Stars

FACTS SEEM SIMILAR BUT CONTRADDICTORY:

Both metal poor clusters (NGC6397 and M92): Show abundance differences typical of “CNO” processing

Show Li abundances either ‘constant’, or close to, compatible with ‘Plateau’. How is this possible ?

Ventura et al.: previous AGB processing , Li AGB production and Cameron-Fowler process+ Mass LossesBUT: Fine Tuning Is required !

Li in Evolved Stars

Most works were devoted to the search of Li-rich giants. But Li-Rich giants seems to be rare in GC: Pilachowski et al. 2000, for instance, do not find any Li-Rich giant out of the 261 sampled in 4 clusters (20 mA limit).

A systematic approach was followed by Castilho et al on NGC6397 (16 stars)

and by

Grundhal et al on NGC6752 (21 stars):they both follow sistematically the stars from the SGB to the RGB.

Li in Evolved Stars

The Grundahl et al. Data on NGC6752 results are quite impressive: while a mixed situation is present for stars below the RGB bump, stars which are more luminous only present upper limits. M71 has a similar behaviour (Ramirez et al. 02)

Mixing seems secured after the ‘bump’, as expected from theory. What about the spread for fainter NGC6752 stars?

M71 data from Ramirez et al 02

Li-Na anticorrelation at the TO!

New results from the NGC6752 spectra ESO Large Programme (P.I. Gratton): 9 TO stars, high S/N ratio UVES spectra(Pasquini et al. 2005)

Qualitative agreement with AGB predictions! quantitative TBDDifferences NGC6397 ?? Same mechanism for all clusters ??

Be in TO of NGC6397

2 NGC6397 TO stars of the Thevenin et al sample. V~16 at the TO. 10 hours/star at the VLT+UVES R=40000 , S/N 8.5 and 15 respectively.

Log Be/H = -12.35 0.2

O Triplet

Be in TO of NGC6397

Oxygen is low and quite different between the stars: almost 0.6 Dex. Oxygen in NGC6397 is also found to vary in subgiants by Carretta et al. Nitrogen in the NGC6397 stars is high (N/H~ - 0.74) with respect to field stars.

Be in TO of NGC6397

Be can be used as cosmochronometer, NGC6397 formed within ~0.5 Gyr from the onset of star formation in the Galaxy.

Chemical evolution Model from Valle et al. 02

Age of 13.4 +- 0.8 fromGratton et al. 03

Be in TO of NGC6397

Unlike Li, Be cannot be produced by the Fowler-Cameron mechanism. The presence of Be points out that the gas must have been ~200 My exposed to the GCR before forming the stars we observed now. Li and Be further exclude ‘internal mixing’ as cause of chemical anomalies.

Since Be can only be destroyed but not produced during AGB phase, its presence seems incompatible with this hypothesis. AGB can still be retained only if processing occurs ‘immediately’ and the processed gas is ‘immediately’ released to the ISM. (tuning again?)

We looked at some alternative formation scenario, but we did not find a satisfactory one which explains the whole abundance patterns and does not require fine tuning.

Li and Be in Globular Clusters

Light elements, such as Li and Be, contains unique information, essential to test predictions of Globular Clusters formation and mixing theory.