Thermal emission from bow shocks I: 2D hydrodynamic models of the Bubble Nebula

Green, Samuel and Mackey, Jonathan and Haworth, Thomas J. and Gvaramadze, Vasilii V. and Duffy, Peter (2019) Thermal emission from bow shocks I: 2D hydrodynamic models of the Bubble Nebula. Astronomy & Astrophysics, 625. A4. ISSN 0004-6361

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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: ∼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 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.

Item Type: Article
Divisions: School of Cosmics Physics > Astronomy and Astrophysics
Date Deposited: 07 May 2019 10:19
Last Modified: 17 Dec 2022 10:13

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