Abstract
Abundance trends in heavier elements with evolutionary phase have been shown to exist in the globular cluster NGC 6752 [Fe/H]=-1.6. These trends are a result of atomic diffusion and additional (non-convective) mixing. Studying such trends can provide us with important constraints on the extent to which diffusion modifies the internal structure and surface abundances of solar-type, metal-poor stars. Taking advantage of a larger data sample, we investigate the reality and the size of these abundance trends and address questions and potential biases associated with the various stellar populations that make up NGC6752. Based on uvby Str\"omgren photometry, we are able to separate three stellar populations in NGC 6752 along the evolutionary sequence from the base of the red giant branch down to the turnoff point. We find weak systematic abundance trends with evolutionary phase for Ca, Ti, and Fe which are best explained by stellar-structure models including atomic diffusion with efficient additional mixing. We derive a new value for the initial lithium abundance of NGC 6752 after correcting for the effect of atomic diffusion and additional mixing which falls slightly below the predicted standard BBN value. We find three stellar populations by combining photometric and spectroscopic data of 194 stars in the globular cluster NGC 6752. Abundance trends for groups of elements, differently affected by atomic diffusion and additional mixing, are identified. Although the statistical significance of the individual trends is weak, they all support the notion that atomic diffusion is operational along the evolutionary sequence of NGC 6752.
Highlights
Stellar spectroscopy of stars with spectral types F, G and K is a mature field of astrophysics
Our differential 1D-local thermodynamic equilibrium (LTE)/NLTE based spectroscopic analysis of 194 stars observed at medium-high resolution indicates weak
(∼0.1 dex) systematic abundance trends of heavy elements with evolutionary phase along the subgiant branch, in magnesium, calcium, titanium and iron. These trends are of low statistical significance taken individually, they are found to be in good agreement with those determined by independent methods in Paper I, as well as predictions from stellar structure models including atomic diffusion with efficient additional mixing
Summary
Stellar spectroscopy of stars with spectral types F, G and K is a mature field of astrophysics. At least for simple atoms like lithium and sodium, the matter-light interaction taking place in stellar atmospheres can nowadays be modelled from first principles, with due consideration of hydrodynamics and departures from local thermodynamic equilibrium (LTE). Using long-lived stars as data carriers of different phases of Galactic evolution, much has been learned about the matter cycle and the general build-up of the Galactic inventory of chemical elements with time. Our inferences are not free from uncertainties and limitations. One such inquietude is the theoretical expectation that the surface abundances of late-type stars with shallow outer convection zones are a direct function of time, due to elementseparating effects collectively referred to as atomic diffusion (Michaud et al 1984). Subject to the prevailing forces (gravity, radiation pressure), chemical elements will be pushed into/out
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