Thermal emission from bow shocks I: 2D Hydrodynamic Models of the Bubble
  Nebula
  The Bubble Nebula (or NGC 7635) is a parsec-scale seemingly spherical
wind-blown bubble around the relatively unevolved O star BD+60$^\circ$2522. The
small dynamical age of the nebula and significant space velocity of the star
suggest that the Bubble Nebula might be a bow shock. We have run 2D
hydrodynamic simulations to model the interaction of the central star's wind
with the interstellar medium (ISM). The models cover a range of possible ISM
number densities of $n=50-200 {\rm cm}^{-3}$ and stellar velocities of
$v_{\star}=20-40$ km s$^{-1}$. Synthetic H$\alpha$ and 24 $\mu$m emission maps
predict the same apparent spherical bubble shape with quantitative properties
similar to observations. The synthetic maps also predict a maximum brightness
similar to that from the observations and agree that the maximum brightness is
at the apex of the bow shock. The best-matching simulation had
$v_{\star}\approx20$ km s$^{-1}$ into an ISM with $n\sim100 {\rm cm}^{-3}$, at
an angle of 60$^\circ$ with respect to the line of sight. Synthetic maps of
soft ($0.3-2$ keV) and hard ($2-10$ keV) X-ray emission show that the brightest
region is in the wake behind the star and not at the bow shock itself. The
unabsorbed soft X-rays have luminosity $\sim10^{32}-10^{33}$ erg s$^{-1}$. The
hard X-rays are fainter, luminosity $\sim 10^{30} - 10^{31}$ erg s$^{-1}$, and
may be too faint for current X-ray instruments to successfully observe. Our
results imply that the O star creates a bow shock as it moves through the ISM
and in turn creates an asymmetric bubble visible at optical and infrared
wavelengths, and predicted to be visible in X-rays. The Bubble Nebula does not
appear to be unique, it could be just a favourably oriented very dense bow
shock. The dense ISM surrounding BD+60$^\circ$2522 and its strong wind suggest
that it could be a good candidate for detecting non-thermal emission.
