The K2 Galactic Archaeology Program Data Release 2: Asteroseismic
  results from campaigns 4, 6, & 7
  Studies of Galactic structure and evolution have benefitted enormously from
Gaia kinematic information, though additional, intrinsic stellar parameters
like age are required to best constrain Galactic models. Asteroseismology is
the most precise method of providing such information for field star
populations $\textit{en masse}$, but existing samples for the most part have
been limited to a few narrow fields of view by the CoRoT and Kepler missions.
In an effort to provide well-characterized stellar parameters across a wide
range in Galactic position, we present the second data release of red giant
asteroseismic parameters for the K2 Galactic Archaeology Program (GAP). We
provide $\nu_{\mathrm{max}}$ and $\Delta \nu$ based on six independent pipeline
analyses; first-ascent red giant branch (RGB) and red clump (RC) evolutionary
state classifications from machine learning; and ready-to-use radius & mass
coefficients, $\kappa_R$ & $\kappa_M$, which, when appropriately multiplied by
a solar-scaled effective temperature factor, yield physical stellar radii and
masses. In total, we report 4395 radius and mass coefficients, with typical
uncertainties of $3.3\% \mathrm{\ (stat.)} \pm 1\% \mathrm{\ (syst.)}$ for
$\kappa_R$ and $7.7\% \mathrm{\ (stat.)} \pm 2\% \mathrm{\ (syst.)}$ for
$\kappa_M$ among RGB stars, and $5.0\% \mathrm{\ (stat.)} \pm 1\% \mathrm{\
(syst.)}$ for $\kappa_R$ and $10.5\% \mathrm{\ (stat.)} \pm 2\% \mathrm{\
(syst.)}$ for $\kappa_M$ among RC stars. We verify that the sample is nearly
complete -- except for a dearth of stars with $\nu_{\mathrm{max}} \lesssim
10-20\mu$Hz -- by comparing to Galactic models and visual inspection. Our
asteroseismic radii agree with radii derived from Gaia Data Release 2
parallaxes to within $2.2 \pm 0.3\%$ for RGB stars and $2.0 \pm 0.6\%$ for RC
stars.
