Improving white dwarfs as chronometers with Gaia parallaxes and
  spectroscopic metallicities
  White dwarfs (WDs) offer unrealized potential in solving two problems in
astrophysics: stellar age accuracy and precision. WD cooling ages can be
inferred from surface temperatures and radii, which can be constrained with
precision by high quality photometry and parallaxes. Accurate and precise Gaia
parallaxes along with photometric surveys provide information to derive cooling
and total ages for vast numbers of WDs. Here we analyse 1372 WDs found in wide
binaries with MS companions, and report on the cooling and total age precision
attainable in these WD+MS systems. The total age of a WD can be further
constrained if its original metallicity is known, because the main-sequence
(MS) progenitor lifetime depends on metallicity at fixed mass, yet metallicity
is unavailable via spectroscopy of the WD. We show that incorporating
spectroscopic metallicity constraints from 38 wide binary MS companions
substantially decreases internal uncertainties in WD total ages compared to a
uniform constraint. Averaged over the 38 stars in our sample, the total
(internal) age uncertainty improves from 21.04% to 16.77% when incorporating
the spectroscopic constraint. Higher mass WDs yield better total age precision;
for 8 WDs with zero age main sequence masses $\geq$ 2.0 solar masses, the mean
uncertainty in total ages improves from 8.61% to 4.54% when incorporating
spectroscopic metallicities. We find that it is often possible to achieve 5%
total age precision for WDs with progenitor masses above 2.0 solar masses if
parallaxes with $\leq$ 1% precision and Pan-STARRS $g, r$, and $i$ photometry
with $\leq$ 0.01 mag precision are available.
