Atomic diffusion and mixing in old stars VII : Chemical abundance variations in M4 (NGC 6121)

• Context. Variations in chemical abundances with evolutionary phase have been identified in previous papers of this series among stars in three globular clusters, M30 ([Fe/H] = −2.3), NGC 6397 ([Fe/H] = −2.1) and NGC 6752 ([Fe/H] = −1.6). These variations compare well with the predictions from stellar-structure models with atomic diffusion moderated by additional mixing of low and high efficiency at the respective ends of the metallicity scale.Aims. By extending these studies to higher metallicity, for a large number of elements, further empirical constraints can be provided to the nature of the additional mixing mechanism. We therefore investigate whether evolutionary abundance variations are present among stars in the globular cluster NGC 6121 (M4, [Fe/H] = −1.1).Methods. We perform a detailed chemical abundance analysis of 86 stars, ranging from the cluster turnoff point to the red giant branch just above the bump. We determine the abundances of 14 elements consistently by means of spectrum matching, using medium-resolution spectra obtained with VLT/FLAMES-GIRAFFE. Stellar parameters are obtained from UBVI broadband photometry, after correcting for differential reddening effects.Results. We observe the usual C-N-O anticorrelations and confirm the presence of a bimodal population characterised by their N content. We find systematic evolutionary variations in individual chemical abundances of weak statistical significance, but which are rather robust to uncertainties in stellar parameters and modelling assumptions, for magnesium, silicon, calcium, titanium and iron. These variations match predictions from stellar evolution models including atomic diffusion if efficient additional mixing is employed, in line with previous results. Using these models, we derive an initial lithium abundance for the cluster, 2.59 ± 0.10, which is fully compatible with those determined for M30, NGC 6397 and NGC 6752, falling slightly short of the predicted primordial BBN value.Conclusions. The observed abundance patterns of 14 elements investigated here suggest that the second generation stars in M4 were formed out of gas that was polluted by both massive (20−40 solar masses) stars and AGB stars. Element-specific abundance trends are identified in stars along the evolutionary sequence in M4. Although the significance of the individual trends is weak, they all seem to indicate that atomic diffusion is at work in M4. This is the fourth cluster in which atomic diffusion signatures are seen, giving more evidence for the assumption that atomic diffusion produces measurable surface-abundance changes in warm metal-poor stars and hence should be accounted for in stellar-evolution models and studies of Galactic chemical evolution. 

Ämnesord

NATURVETENSKAP  -- Fysik -- Astronomi, astrofysik och kosmologi (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Astronomy, Astrophysics and Cosmology (hsv//eng)

Nyckelord

stars: abundances

stars: atmospheres

stars: fundamental parameters

globular cluster and associations: M4

techniques: spectroscopic

Publikations- och innehållstyp

vet (ämneskategori)

art (ämneskategori)