Massive Warm/Hot Galaxy Coronae: II. Isentropic Model
  We construct a new analytic phenomenological model for the extended
circumgalactic material (CGM) of $L^*$ galaxies. Our model reproduces the
OVII/OVIII absorption observations of the Milky Way (MW) and the OVI
measurements reported by the COS-Halos and eCGM surveys. The warm/hot gas is in
hydrostatic equilibrium in a MW gravitational potential, and we adopt a
barotropic equation of state, resulting in a temperature variation as a
function of radius. A pressure component with an adiabatic index of
$\gamma=4/3$ is included to approximate the effects of a magnetic field and
cosmic rays. We introduce a metallicity gradient motivated by the enrichment of
the inner CGM by the Galaxy. We then present our fiducial model for the corona,
tuned to reproduce the observed OVI-OVIII column densities, and with a total
mass of $M_{\rm gas} \approx 5.5 \times 10^{10}~{\rm M_{\odot}}$ inside $r_{\rm
cgm} \approx 280$ kpc. The gas densities in the CGM are low ($n_{\rm H} =
10^{-5} - 3 \times 10^{-4}~{\rm cm^{-3}}$) and its collisional ionization state
is modified by the metagalactic radiation field (MGRF). We show that for
OVI-bearing warm/hot gas with typical observed column densities $N_{\rm OVI}
\sim 3 \times 10^{14}~{\rm cm^{-2}}$ at large ($\gtrsim 100$ kpc) impact
parameters from the central galaxies, the ratio of the cooling to dynamical
times, $t_{\rm cool}/t_{\rm dyn}$, has a model-independent upper limit of
$\lesssim 4$. In our model, $t_{\rm cool}/t_{\rm dyn}$ at large radii is $\sim
2-3$. We present predictions for a wide range of future observations of the
warm/hot CGM, from UV/X-ray absorption and emission spectroscopy, to dispersion
measure (DM) and Sunyaev-Zeldovich CMB measurements. We provide the model
outputs in machine-readable data files, for easy comparison and analysis.
