Li – O anti-correlation in NGC 6752: evidence for Li-enriched polluting gas

Shen, Zhenxing; Bonifacio, P.; Pasquini, L.; Zaggia, S.

doi:10.1051/0004-6361/201015738

Cited by 58 publications

(74 citation statements)

References 37 publications

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“…Also in NGC 6752 and 47 Tuc (Shen et al 2010;) there is one star, which, within errors, has a Li abundance compatible with the primordial abundance. In those cases, however, the star lies along a well defined Li-O correlation so that there is a continuity of Li abundances from the highest to the lowest.…”

Section: Discussionmentioning

confidence: 67%

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Lithium and sodium in the globular cluster M 4

Monaco

Villanova

Bonifacio

et al. 2012

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Context. The abundance inhomogeneities of light elements observed in globular clusters (GCs), and notably the ubiquitous Na-O anti-correlation, are generally interpreted as evidence that GCs comprise several generations of stars. There is an on-going debate as to the nature of the stars, which produce the inhomogeneous elements, and investigating the behavior of several elements is a way to shed new light on this problem. Aims. We aim at investigating the Li and Na content of the GC M 4, that is known to have a well defined Na-O anti-correlation. Methods. We obtained moderate resolution (R = 17 000-18 700) spectra for 91 main sequence (MS)/sub-giant branch stars of M 4 with the Giraffe spectrograph at the FLAMES/VLT ESO facility. Using model atmospheres analysis we measured lithium and sodium abundances. Results. We detect a weak Li-Na anti-correlation among un-evolved MS stars. One star in the sample, # 37934, shows the remarkably high lithium abundance A(Li) = 2.87, compatible with current estimates of the primordial lithium abundance. Conclusions. The shallow slope found for the Li-Na anti-correlation suggests that lithium is produced in parallel to sodium. This evidence, coupled with its sodium-rich nature, suggests that the high lithium abundance of star # 37934 may originate by pollution from a previous generations of stars. The recent detection of a Li-rich dwarf of pollution origin in the globular cluster NGC 6397 may also point in this direction. Still, no clear cut evidence is available against a possible preservation of the primordial lithium abundance for star # 37934.

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Section: Discussionmentioning

confidence: 67%

“…In the case of NGC 6752, it was already pointed out by Shen et al (2010), however, that such a scenario would imply a slope of one in the A(Li)-A(O) correlation. The fact that the observed slope is significantly different from unity implies that the material is not totally depleted in Li, requiring a production of some Li along with Na.…”

Section: Discussionmentioning

confidence: 95%

Lithium and sodium in the globular cluster M 4

Monaco

Villanova

Bonifacio

et al. 2012

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133

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“…This result is also consistent with the detection of only the strongest line of the O i triplet at 777.1 nm in the Mike spectrum. We have a very uncertain estimate of the equivalent width of about 1.9 pm, which corresponds to [O/Fe] ∼ 0.5 when taking the NLTE corrections for oxygen into account, according to the prescriptions of Shen et al (2010). The line is possibly contaminated by telluric absorption on its red wing.…”

Section: C N and Omentioning

confidence: 87%

“…In this context, Shen et al (2010) demonstrate for the more metal-rich GC NGC 6752 that the first-generation stars must have enriched some of the second-generation stars in Li. Similarly, in NGC 6397, Pasquini et al (2008) find a Li-rich but O-poor star, which also requires some pollution with Li by a prior population.…”

Section: Introductionmentioning

confidence: 93%

The Be-test in the Li-rich star #1657 of NGC 6397: evidence for Li-flash in RGB stars?

Pasquini¹,

Koch²,

Smiljanić³

et al. 2014

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Context. The Li-rich turn-off star recently discovered in the old, metal-poor globular cluster NGC 6397 could represent the smoking gun for some fundamental, but very rare episode of Li enrichment in globular clusters and in the early Galaxy. Aims. We aim to understand the nature of the Li enrichment by performing a spectroscopic analysis of the star, in particular of its beryllium (Be) abundance, and by investigating its binary nature.Methods. We used the VLT/UVES spectrograph to observe the near UV region where the Be ii resonance doublet and the NH bands are located. We also re-analyzed an archival Magellan/MIKE spectra for C and O abundance determination.Results. We could not detect the Be ii lines and derive an upper limit of log(Be/H) < −12.2 that is consistent with the Be observed in other stars of the cluster. We could detect a weak G-band, which implies a mild carbon enhancement [C/Fe]+0.4 ± 0.2. We could not detect the UV NH band, showing that this star is less N-enhanced than other stars of the cluster, and we derive an upper limit [N/Fe] < 0.0. For oxygen we could not convincingly detect any of the near UV OH lines, which implies that oxygen cannot be strongly enhanced in this star. This is consistent with the detection of the strongest line of the O i triplet at 777 nm, which is contaminated by telluric absorptions but is consistent with [O/Fe] ∼ 0.5. Combining the UVES and Mike data, we could not detect any variation in the radial velocity greater than 0.95 kms −1 over 8 years.Conclusions. The chemical composition of the star strongly resembles that of "first generation" NGC 6397 stars, with the huge Li as the only deviating abundance. Not detecting Be rules out two possible explanations of the Li overabundance: capture of a substellar body and spallation caused by a nearby type II SNe. Discrepancies are also found with respect to other accretion scenarios, except for contamination by the ejecta of a star that has undergone the RGB Li-flash. This is at present the most likely possibility for explaining the extraordinary Li enrichment of this star.

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“…Unsurprisingly, a large amount of work has been done in this direction in the past few decades (for a review see, e.g., Gratton et al 2004;Carretta at al. 2010), which has resulted, for example, in the discoveries of abundance anticorrelations for Na-O (Kraft 1994;Gratton et al 2001;Carretta et al 2009a), Mg-Al (see, e.g., Carretta et al 2009b), Li-Na (Pasquini et al 2005;Bonifacio et al 2007), and correlation for Li-O (Pasquini et al 2005;Shen et al 2010).…”

Section: Introductionmentioning

confidence: 99%

Barium abundance in red giants of NGC 6752

Dobrovolskas¹,

Kučinskas²,

Andrievsky³

et al. 2012

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Aims. We study the effects related to departures from non-local thermodynamic equilibrium (NLTE) and homogeneity in the atmospheres of red giant stars, to assess their influence on the formation of Ba II lines. We estimate the impact of these effects on the barium abundance determinations for 20 red giants in Galactic globular cluster NGC 6752. Methods. One-dimensional (1D) local thermodynamic equilibrium (LTE) and 1D NLTE barium abundances were derived using classical 1D ATLAS9 stellar model atmospheres. The three-dimensional (3D) LTE abundances were obtained for 8 red giants on the lower RGB, by adjusting their 1D LTE abundances using 3D-1D abundance corrections, i.e., the differences between the abundances obtained from the same spectral line using the 3D hydrodynamical and classical 1D stellar model atmospheres. The 3D-1D abundance corrections were obtained in a strictly differential way using the 3D hydrodynamical and classical 1D codes CO 5 BOLD and LHD. Both codes utilized identical stellar atmospheric parameters, opacities, and equation of state. Results. The mean 1D barium-to-iron abundance ratios derived for 20 giants are [Ba/Fe] 1D LTE = 0.24 ± 0.05(stat.) ± 0.08(sys.) and [Ba/Fe] 1D NLTE = 0.05 ± 0.06(stat.) ± 0.08(sys.). The 3D-1D abundance correction obtained for 8 giants is small (∼+0.05 dex), thus leads to only minor adjustment when applied to the mean 1D NLTE barium-to-iron abundance ratio for the 20 giants, [Ba/Fe] 3D+NLTE = 0.10 ± 0.06(stat.) ± 0.10(sys.). The intrinsic abundance spread between the individual cluster stars is small and can be explained in terms of uncertainties in the abundance determinations. Conclusions. Deviations from LTE play an important role in the formation of barium lines in the atmospheres of red giants studied here. The role of 3D hydrodynamical effects should not be dismissed either, even if the obtained 3D-1D abundance corrections are small. This result is a consequence of subtle fine-tuning of individual contributions from horizontal temperature fluctuations and differences between the average temperature profiles in the 3D and 1D model atmospheres: owing to the comparable size and opposite sign, their contributions nearly cancel each other. This fine-tuning is characteristic of the particular set of atmospheric parameters and the element investigated, hence should not necessarily be a general property of spectral line formation in the atmospheres of red giant stars.

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Li – O anti-correlation in NGC 6752: evidence for Li-enriched polluting gas

Cited by 58 publications

References 37 publications

Lithium and sodium in the globular cluster M 4

Lithium and sodium in the globular cluster M 4

The Be-test in the Li-rich star #1657 of NGC 6397: evidence for Li-flash in RGB stars?

Barium abundance in red giants of NGC 6752

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