Abstract
The Bubble Nebula (or NGC 7635) is a parsec-scale seemingly spherical wind-blown bubble around the relatively unevolved O star BD+60°2522. The young dynamical age of the nebula and significant space velocity of the star suggest that the Bubble Nebula might be a bow shock. We ran 2D hydrodynamic simulations to model the interaction of the wind of the central star with the interstellar medium (ISM). The models cover a range of possible ISM number densities of n = 50−200 cm−3 and stellar velocities of v* = 20−40 km s−1. Synthetic Hα and 24 μm 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* ≈ 20 km s−1 into an ISM with n ∼ 100 cm−3, at an angle of 60° 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 a luminosity of ∼1032−1033 erg s−1. The hard X-rays are fainter: ∼1030−1031 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 simply be a favourably oriented, very dense bow shock. The dense ISM surrounding BD+60°2522 and its strong wind suggest that it could be a good candidate for detecting non-thermal emission.
Highlights
Most stars in the universe have winds in the form of gas ejected from their upper atmosphere
We ran 2D hydrodynamic simulations to model the interaction of the wind of the central star with the interstellar medium (ISM)
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
Summary
Most stars in the universe have winds in the form of gas ejected from their upper atmosphere. As a star moves through the ISM the interaction between its stellar wind and the surrounding medium can produce instabilities that mix material between the adiabatically shocked wind and the photoionized gas in the wake of the bow shock This can create a mixing region with plasma temperatures of ∼106 K (Toalá et al 2016), a strongly cooling boundary layer that efficiently removes thermal pressure support from the bubble. Moore et al 2002a), and/or might be caused by motion of BD+60◦2522 in the north direction The latter possibility is supported by the Gaia DR2 data indicating that BD+60◦2522 is moving towards the brightest (northern) rim of the Bubble Nebula with a transverse peculiar velocity of 28 ± 3 km s−1 (see Fig. 1 and Appendix A), which is typical of runaway stars. Recombination cooling of H, with rate from Hummer (1994)
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