Age Demographics of the Milky Way Disk and Bulge
  We use the extensive $Gaia$ Data Release 2 set of Long Period Variables to
select a sample of Oxygen-rich Miras throughout the Milky Way disk and bulge
for study. Exploiting the relation between Mira pulsation period and stellar
age/chemistry, we slice the stellar density of the Galactic disk and bulge as a
function of period. We find the morphology of both components evolves as a
function of stellar age/chemistry with the stellar disk being stubby at old
ages, becoming progressively thinner and more radially extended at younger
stellar ages, consistent with the picture of inside-out and upside-down
formation of the Milky Way's disk. We see evidence of a perturbed disk, with
large-scale stellar over-densities visible both in and away from the stellar
plane. We find the bulge is well modelled by a triaxial boxy distribution with
an axis ratio of $\sim [1:0.4:0.3]$. The oldest of the Miras ($\sim$ 9-10 Gyr)
show little bar-like morphology, whilst the younger stars appear inclined at a
viewing angle of $\sim 21^{\circ}$ to the Sun-Galactic Centre line. This
suggests that bar formation and buckling took place 8-9 Gyr ago, with the older
Miras being hot enough to avoid being trapped by the growing bar. We find the
youngest Miras to exhibit a strong peanut morphology, bearing the
characteristic X-shape of an inclined bar structure.
