Lithium and sodium in the globular cluster M 4

Monaco, L.; Villanova, S.; Bonifacio, P.; Caffau, E.; Geisler, D.; Marconi, G.; Momany, Y.; Ludwig, H.‐G.

doi:10.1051/0004-6361/201117709

Cited by 78 publications

(140 citation statements)

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“…The detection of Li-rich stars in M30 adds to the ever-growing list of globular clusters that host Li-rich stars. Monaco et al (2012) reported the detection of a Li-rich dwarf in M4 with a Li abundance compatible with the predicted primordial lithium abundance based on standard BBN (Coc et al 2013). In addition to M4, there are at least three other globular clusters that seem to harbour stars with Li abundances similar to the primordial Li abundance, NGC 6752 (Shen et al 2010) and 47 Tuc (D'Orazi et al 2010), or well above the primordial Li abundance, NGC 6397 (Koch et al 2011).…”

Section: A Li-rich Subgiantmentioning

confidence: 76%

Atomic diffusion and mixing in old stars

Gruyters

Lind

Richard

et al. 2016

A&A

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Context. The prediction of the Planck-constrained primordial lithium abundance in the Universe is in discordance with the observed Li abundances in warm Population II dwarf and subgiant stars. Among the physically best motivated ideas, it has been suggested that this discrepancy can be alleviated if the stars observed today had undergone photospheric depletion of lithium. Aims. The cause of this depletion is investigated by accurately tracing the behaviour of the lithium abundances as a function of effective temperature. Globular clusters are ideal laboratories for such an abundance analysis as the relative stellar parameters of their stars can be precisely determined. Methods. We performed a homogeneous chemical abundance analysis of 144 stars in the metal-poor globular cluster M30, ranging from the cluster turnoff point to the tip of the red giant branch. Non-local thermal equilibrium (NLTE) abundances for Li, Ca, and Fe were derived where possible by fitting spectra obtained with VLT/FLAMES-GIRAFFE using the quantitative-spectroscopy package SME. Stellar parameters were derived by matching isochrones to the observed V vs. V − I colour-magnitude diagram. Independent effective temperatures were obtained from automated profile fitting of the Balmer lines and by applying colour-T eff calibrations to the broadband photometry. Results. Li abundances of the turnoff and early subgiant stars form a thin plateau that is broken off abruptly in the middle of the SGB as a result of the onset of Li dilution caused by the first dredge-up. Abundance trends with effective temperature for Fe and Ca are observed and compared to predictions from stellar structure models including atomic diffusion and ad hoc additional mixing below the surface convection zone. The comparison shows that the stars in M30 are affected by atomic diffusion and additional mixing, but we were unable to determine the efficiency of the additional mixing precisely. This is the fourth globular cluster (after NGC 6397, NGC 6752, and M4) in which atomic diffusion signatures are detected. After applying a conservative correction (T6.0 model) for atomic diffusion, we find an initial Li abundance of A(Li) = 2.48 ± 0.10 for the globular cluster M30. We also detected a Li-rich SGB star with a Li abundance of A(Li) = 2.39. The finding makes Li-rich mass transfer a likely scenario for this star and rules out models in which its Li enhancement is created during the RGB bump phase.

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Section: A Li-rich Subgiantmentioning

confidence: 76%

Atomic diffusion and mixing in old stars

Gruyters

Lind

Richard

et al. 2016

A&A

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“…Its logarithmic ( 7 Li) non-local thermodynamic equilibrium (NLTE) surface abundance is ≈4.2, i.e., an order of magnitude above the maximum main-sequence Li abundance, however, the currently accepted primordial Li abundance (2.72 dex; Cyburt et al 2008), is at odds with the WMAP satellite measurements, implying a primordial Li production three to five times higher than observed (e.g., Sbordone et al 2010). The hypothesis of Li pollution from a previous generation of stars has also found further evidence from recent detections of Li-rich dwarfs in globular clusters (Koch et al 2010;Monaco et al 2012). Therefore, a detailed study of the surface lithium of a key star may also shed some light on these discrepancies.…”

Section: Introductionmentioning

confidence: 89%

Evidence for enhanced mixing on the super-meteoritic Li-rich red giant HD 233517

Strassmeier

Carroll

Weber

et al. 2015

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Context. HD 233517 is among the most Li-rich stars in the sky. It is a rapidly rotating, single K giant thought to be on its first ascent on the red giant branch. The star has also the highest known infrared excess among any of the known first-ascent giants. Aims. We revisit the physical parameters of the system and aim to map its surface temperature distribution. Methods. New time-series photometry and high-resolution spectroscopy were obtained with our robotic facilities STELLA and Amadeus Automatic Photoelectric Telescope (APT) in 2007-2011. Inverse line-profile modelling is performed on a total of 167 échelle spectra and six Doppler images are presented.Results. Light and radial-velocity variations suggest a stellar rotation period of 47.6 ± 0.3 d. The atmospheric parameters agree with previous studies and verify a super-meteoritic log 7 Li abundance of 4.29 ± 0.10 with undetected 6 Li, while the metals are generally deficient by -0.4 dex with respect to the Sun. We determine a lower than normal isotopic carbon ratio of 12 C/ 13 C = 9 +4 −2 . Our Doppler images indicate warm and cool spots with an average temperature contrast of just ±65 K with respect to the effective temperature.Doppler maps from Li i 670.78 reveal practically identical surface morphology, with a higher average contrast of ±160 K and errors that are five times larger. Reconstructions with simultaneously 1617 and 3007 spectral lines showed both a signal degradation with respect to our 56-line final image. An error analysis indicates an average temperature error per surface pixel of just ±4 K. Conclusions. HD 233517 appears to be an old (≈10-Gyr) single 0.95-M giant currently undergoing mild mass loss in the form of a wind. The cool and warm photospheric features are interpreted to be merely locations of suppressed and enhanced convection, respectively, probably intermingled by a yet undetected weak magnetic field. The low carbon-isotope ratio is indicative of extra mixing rather than of an engulfing event. We tentatively conclude that HD 233517 operates an enhanced non-axisymmetric mixing process that leads to an inhomogeneous super-granulation pattern on the surface in form of large cool and warm features.

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“…While it is fully representative of the remainder of the GC giant and TO stars in terms of metallicity, α-element abundance ratios, and stellar parameters, it has a Li abundance about 100 times higher than any other NGC 6397 stars. With an exceptional abundance of log( 7 Li/H)+12 = 4.2 (assuming non-LTE), it is one of the stars with the highest Li ever observed (Koch et al , 2012, and only very few other dwarf or giant stars show comparable levels of enhancement (Reyniers & Van Winckel 2001;Deliyannis et al 2002;Monaco et al 2012;Adamów et al 2012). What produced such a high Li abundance in a TO star of a GC?…”

Section: Introductionmentioning

confidence: 99%

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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Lithium and sodium in the globular cluster M 4

Cited by 78 publications

References 50 publications

Atomic diffusion and mixing in old stars

Atomic diffusion and mixing in old stars

Evidence for enhanced mixing on the super-meteoritic Li-rich red giant HD 233517

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

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