15-20 March 2020
Europe/Berlin timezone

Thermal emission from bow shocks: Multi-dimensional Hydrodynamic and Magneto-Hydrodynamic Models of the Bubble Nebula

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The Bubble Nebula (or NGC 7635) is a parsec-scale seemingly spherical windblown 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$^{−1}$. 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 Hα and 24 µm emission maps were produced to compare with observations and synthetic maps of soft (0.3−2 keV) and hard (2−10 keV) X-ray emission were produced. The predicted unabsorbed soft X-rays have a luminosity of ∼ 10$^{32}$ − 10$^{33}$ erg s$^{−1}$. The hard X-rays are predicted to be much fainter: ∼ 10$^{30}$ − 10$^{31}$ erg s−$^{−1}$, and may be too faint for current X-ray instruments to successfully observe. We are following up this 2D study with 3D magneto-hydrodynamic simulations of the Bubble Nebula. Synthetic Hα, 24 µm, and X-ray emission maps have again been produced to compare with observational data. 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. This is the first description of the Bubble Nebula that quantitatively agrees with available observational data and makes a clear prediction for X-ray observations. Our results can test the feedback efficiency from stellar winds to the interstellar medium.

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