Atomic Diffusion and Mixing in Old Stars. I. Very Large Telescope FLAMES-UVES Observations of Stars in NGC 6397

By A. Korn, F. Grundahl, O. Richard, Paul Barklem, L. Mashonkina, R. Collet, B. Gustafsson, N. P. D. O. Astronomy, S. Physics, Uppsala University and 14 more. 2007

163 citations recorded by the source corpus.

We present a homogeneous photometric and spectroscopic analysis of 18 stars along the evolutionary sequence of the metal-poor globular cluster NGC 6397 ([Fe/H] ≈ -2), from the main-sequence turnoff point to red giants below the bump. The spectroscopic stellar parameters, in particular stellar parameter differences between groups of stars, are in good agreement with broadband and Strömgren photometry calibrated on the infrared flux method. The spectroscopic abundance analysis reveals, for the first time, systematic trends of iron abundance with evolutionary stage. Iron is found to be 30% less abundant in the turnoff point stars than in the red giants. An abundance difference in lithium is seen between the turnoff point and warm subgiant stars. The impact of potential systematic errors on these abundance trends (stellar parameters, the hydrostatic and LTE approximations) is quantitatively evaluated and found not to alter our conclusions significantly. Trends for various elements (Li, Mg, Ca, Ti, and Fe) are compared with stellar structure models including the effects of atomic diffusion and radiative acceleration. Such models are found to describe the observed element-specific trends well, if extra (turbulent) mixing just below the convection zone is introduced. It is concluded that atomic diffusion and turbulent mixing are largely responsible for the subprimordial stellar lithium abundances of warm halo stars. Other consequences of atomic diffusion in old metal-poor stars are also discussed.

Subfields: Astronomy and Astrophysics

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Departures from LTE for neutral Li in late-type stars (2009, background): “Lithium depletion through atomic diffusion has been suggested as a solution to the discrepancy between the Spite plateau abundance and the predicted value of the primordial lithium abundance (see e.g. Korn et al. 2007; Lind et al. 2009).”
Gaia FGK benchmark stars: Effective temperatures and surface gravities (2015, background): “HD 84937 has a metallicity similar to the globular cluster NGC 6397, for which Korn et al. (2007) estimated the ef- fects of atomic diffusion.”
THE ABUNDANCES OF NEUTRON-CAPTURE SPECIES IN THE VERY METAL-POOR GLOBULAR CLUSTER M15: A UNIFORM ANALYSIS OF RED GIANT BRANCH AND RED HORIZONTAL BRANCH STARS (2011, background): “Similarly, Korn et al. (2007)(53) surveyed turn off (TO) and RGB stars of NGC 6397 and found a metallicity offset of about 0.15 dex (with the TO stars reporting consistently lower values of [Fe/H]).”
Ages and fundamental properties of Kepler exoplanet host stars from asteroseismology (2015, background): “…is clear for the Sun (e.g., Guzik & Cox 1993; Christensen-Dalsgaard, Proffitt & Thompson 1993) and has been growing in recent years for star clusters (see e.g., Korn et al. 2007; Gruyters et al. 2013; Önehag, Gustafsson & Korn 2014) revealing the importance of this process in low-mass stars.”
First stars XII. Abundances in extremely metal-poor turnoff stars,and comparison with the giants (2009, background): “Another possibility is that the abundances in EMP turnoff stars are seriously affected by diffusion (see e.g. Korn et al. 2007; Lind et al. 2008).”
THE MOST METAL-POOR STARS. II. CHEMICAL ABUNDANCES OF 190 METAL-POOR STARS INCLUDING 10 NEW STARS WITH [Fe/H] ⩽ −3.5,, (2012, background): “At present, the observational tests have been limited to the moderately metal-poor globular clusters NGC 6397 (Korn et al. 2007; Lind et al. 2008, 2009b) and NGC 6752 (Korn 2010).”
Multiple populations in globular clusters (2012, unknown): “…NGC 2808 (Carretta et al. 2006); NGC 6218 (Johnson et al. 2006, Carretta et al. 2007b); NGC 6752 (Carretta et al. 2007a, Yong et al. 2008); NGC 6397 (Korn et al. 2007; Carretta et al. 2009a,b; Lind et al. 2011); NGC 6388 (Carretta et al. 2007c); NGC 6441 (Gratton et al. 2006, 2007); NGC 6712…”
Near-Field Cosmology with Extremely Metal-Poor Stars (2015, background): “If, for example, one uses the result of Korn et al. (2007) for the globular cluster NGC 6397 ([Fe/H] = −1.9) as a guide, the Li depletion factor between the MSTO and Teff = 5100K on the RGB is ∼ 1.3 dex, which leads to a main sequence abundance for SM 0313−6708 of A(Li)MS ∼ 2.0, not unlike the…”
Signatures of intrinsic Li depletion and Li-Na anti-correlation in the metal-poor globular cluster NGC 6397 , (2009, methodology): “…metal-poor globular clusters and its Li content has been documented down to the magnitude of the cluster TO point in several studies (Pasquini & Molaro 1996; Castilho et al. 2000; Thévenin et al. 2001; Bonifacio et al. 2002; Korn et al. 2006, 2007), however, with rather poor number statistics.”