PLATO as it is: a legacy mission for Galactic archaeology
  Deciphering the assembly history of the Milky Way is a formidable task, which
becomes possible only if one can produce high-resolution
chrono-chemo-kinematical maps of the Galaxy. Data from large-scale astrometric
and spectroscopic surveys will soon provide us with a well-defined view of the
current chemo-kinematical structure of the Milky Way, but will only enable a
blurred view on the temporal sequence that led to the present-day Galaxy. As
demonstrated by the (ongoing) exploitation of data from the pioneering
photometric missions CoRoT, Kepler, and K2, asteroseismology provides the way
forward: solar-like oscillating giants are excellent evolutionary clocks thanks
to the availability of seismic constraints on their mass and to the tight
age-initial-mass relation they adhere to. In this paper we identify five key
outstanding questions relating to the formation and evolution of the Milky Way
that will need precise and accurate ages for large samples of stars to be
addressed, and we identify the requirements in terms of number of targets and
the precision on the stellar properties that are needed to tackle such
questions. By quantifying the asteroseismic yields expected from PLATO for
red-giant stars, we demonstrate that these requirements are within the
capabilities of the current instrument design, provided that observations are
sufficiently long to identify the evolutionary state and allow robust and
precise determination of acoustic-mode frequencies. This will allow us to
harvest data of sufficient quality to reach a 10% precision in age. This is a
fundamental pre-requisite to then reach the more ambitious goal of a similar
level of accuracy, which will only be possible if we have to hand a careful
appraisal of systematic uncertainties on age deriving from our limited
understanding of stellar physics, a goal which conveniently falls within the
main aims of PLATO's core science.
