High-precision abundances of elements in solar-type stars. Evidence of
  two distinct sequences in abundance-age relations
  HARPS spectra with signal-to-noise ratios S/N > 600 at 6000 A were analysed
with MARCS model atmospheres to obtain 1D LTE abundances of C, O, Na, Mg, Al,
Si, Ca, Ti, Cr, Fe, Ni, Sr, and Y for 72 nearby solar-type stars with
metallicities in the range -0.3 < [Fe/H] < +0.3 and ASTEC stellar models were
used to determine stellar ages from effective temperatures, luminosities
obtained via Gaia DR2 parallaxes, and heavy element abundances. The resulting
age-metallicity distribution appears to consist of two distinct populations: a
sequence of old stars with a steep rise of [Fe/H] to about +0.3 dex at an age
of ~7 Gyr and a younger sequence with [Fe/H] increasing from -0.3 dex to about
+0.2 dex over the last six Gyr. Furthermore, the trends of several abundance
ratios, [O/Fe], [Na/Fe], [Ca/Fe], and [Ni/Fe], as a function of stellar age
split into two corresponding sequences. The [Y/Mg]-age relation, on the other
hand, shows no offset between the two age sequences and has no significant
dependence on [Fe/H], but the components of a visual binary star, zeta
Reticuli, have a large and puzzling deviation. The split of the age-metallicity
distribution into two sequences may be interpreted as evidence of two episodes
of accretion of gas onto the Galactic disk with a quenching of star formation
in between. Some of the [X/Fe]-age relations support this scenario but other
relations are not so easy to explain, which calls for a deeper study of
systematic errors in the derived abundances as a function of [Fe/H], in
particular 3D non-LTE effects.
