Thermal emission from bow shocks I: 2D Hydrodynamic Models of the Bubble Nebula

Abstract

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 $n=50-200{\mathrm{cm}}^{-3}$ 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 $v_{\star }\approx 20$ km s${}^{-1}$ into an ISM with $n\sim 100{\mathrm{cm}}^{-3}$, 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 $\sim 10^{32}-10^{33}$ 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.

Accepted for publication in Astronomy & Astrophysics

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• Simulations of Magnetised Stellar-Wind Bubbles