High-precision abundances of elements in Kepler LEGACY stars

Verification of trends with stellar age^⋆,⋆⋆

Stellar Astrophysics Centre, Department of Physics and AstronomyAarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
e-mail: pen@phys.au.dk

Received: 28 August 2017
Accepted: 29 September 2017

Abstract

Context. A previous study of solar twin stars has revealed the existence of correlations between some abundance ratios and stellar age providing new knowledge about nucleosynthesis and Galactic chemical evolution.

Aims. High-precision abundances of elements are determined for stars with asteroseismic ages in order to test the solar twin relations.

Methods. HARPS-N spectra with signal-to-noise ratios S/N ≳ 250 and MARCS model atmospheres were used to derive abundances of C, O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zn, and Y in ten stars from the Kepler LEGACY sample (including the binary pair 16 Cyg A and B) selected to have metallicities in the range − 0.15 < [ Fe / H ] < + 0.15 and ages between 1 and 7 Gyr. Stellar gravities were obtained from seismic data and effective temperatures were determined by comparing non-LTE iron abundances derived from Fe i and Fe ii lines. Available non-LTE corrections were also applied when deriving abundances of the other elements.

Results. The abundances of the Kepler stars support the [X/Fe]-age relations previously found for solar twins. [Mg/Fe], [Al/Fe], and [Zn/Fe] decrease by ~ 0.1 dex over the lifetime of the Galactic thin disk due to delayed contribution of iron from Type Ia supernovae relative to prompt production of Mg, Al, and Zn in Type II supernovae. [Y/Mg] and [Y/Al], on the other hand, increase by ~ 0.3 dex, which can be explained by an increasing contribution of s-process elements from low-mass AGB stars as time goes on. The trends of [C/Fe] and [O/Fe] are more complicated due to variations of the ratio between refractory and volatile elements among stars of similar age. Two stars with about the same age as the Sun show very different trends of [X/H] as a function of elemental condensation temperature T_c and for 16 Cyg, the two components have an abundance difference, which increases with T_c. These anomalies may be connected to planet-star interactions.