Thermal emission from bow shocks II: 3D magnetohydrodynamic models of
  Zeta Ophiuchi
  The nearby, massive, runaway star Zeta Ophiuchi has a large bow shock
detected in optical and infrared, and, uniquely among runaway O stars, diffuse
X-ray emission is detected from the shocked stellar wind. Previous
investigation of NGC 7635 found that numerical simulations over-predict the
X-ray emission by ~10 times. Here we make the first detailed computational
investigation of the bow shock of Zeta Ophiuchi, to test whether a simple model
of the bow shock can explain the observed nebula, and to compare the detected
X-ray emission with simulated emission maps. We re-analysed archival {\it
Chandra} observations of the thermal diffuse X-ray emission from the shocked
wind region of the bow shock, finding total unabsorbed X-ray flux (0.3-2 keV
band) corresponding to a diffuse luminosity of
$L_\mathrm{X}=2.33~(0.79-3.45)\times10^{29}$ergs$^{-1}$. 3D MHD simulations
were used to model the interaction of the star's wind with a uniform ISM using
a range of stellar and ISM parameters motivated by observational constraints.
Synthetic infrared, Ha, soft X-ray, emission measure, and radio 6\,GHz emission
maps were generated from three simulations, for comparison with relevant
observations. Simulations where the space velocity of Zeta Ophiuchi has a
significant radial velocity produce infrared emission maps with opening angle
of the bow shock in better agreement with observations than for the case where
motion is fully in the plane of the sky. The simulation with the highest
pressure has the closest match, with flux level within a factor of 2 of the
observational lower limit, and emission weighted temperature of
$\log_{10}(T_\mathrm{A}/\mathrm{K})=6.4$, although the morphology of the
diffuse emission appears somewhat different. Observed X-ray emission is a
filled bubble brightest near the star whereas simulations predict brightening
towards the contact discontinuity as density increases.
