The GALAH Survey: Temporal Chemical Enrichment of the Galactic Disk
  We present isochrone ages and initial bulk metallicities ($\rm
[Fe/H]_{bulk}$, by accounting for diffusion) of 163,722 stars from the GALAH
Data Release 2, mainly composed of main sequence turn-off stars and subgiants
($\rm 7000 K>T_{eff}>4000 K$ and $\rm log g>3$ dex). The local age-metallicity
relationship (AMR) is nearly flat but with significant scatter at all ages; the
scatter is even higher when considering the observed surface abundances. After
correcting for selection effects, the AMR appear to have intrinsic structures
indicative of two star formation events, which we speculate are connected to
the thin and thick disks in the solar neighborhood. We also present abundance
ratio trends for 16 elements as a function of age, across different $\rm
[Fe/H]_{bulk}$ bins. In general, we find the trends in terms of [X/Fe] vs age
from our far larger sample to be compatible with studies based on small ($\sim$
100 stars) samples of solar twins but we now extend it to both sub- and
super-solar metallicities. The $\alpha$-elements show differing behaviour: the
hydrostatic $\alpha$-elements O and Mg show a steady decline with time for all
metallicities while the explosive $\alpha$-elements Si, Ca and Ti are nearly
constant during the thin disk epoch (ages $\lessapprox $ 12 Gyr). The s-process
elements Y and Ba show increasing [X/Fe] with time while the r-process element
Eu have the opposite trend, thus favouring a primary production from sources
with a short time-delay such as core-collapse supernovae over long-delay events
such as neutron star mergers.
