Predicting thermal radiation from stellar-wind bubbles using multi-dimensional simulations

Green, Samuel (2021) Predicting thermal radiation from stellar-wind bubbles using multi-dimensional simulations. Doctoral thesis, DIAS, UCD.

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In this thesis, I present our project to investigate thermal emission from stellar wind bubbles. Hot massive stars have strong winds, large wind-blown bubbles, and drive interstellar turbulence. The effectiveness of stellar winds driving turbulence is still very uncertain and we need to study bow shocks to constrain some of the physical processes involved. After the introduction in Chapter 1 I present a description of computational methods used and algorithm development undertaken as part of this thesis. In Chapter 3 I present 2D hydrodynamic simulations of the stellar wind bubble NGC 7635 (Bubble Nebula) to model the interaction of the wind of the central star with the Interstellar Medium (ISM). The main result from the synthetic optical and infrared emission maps is that we find the same morphological spherical bubble shape with similar quantitative aspects supporting our hypothesis that the wind bubble is a bow shock. Soft (0.3 − 2 keV) X-ray emission-map predictions of what an X-ray satellite could observe were also produced. These emission maps show that the majority of X-ray emission occurs in the wake behind the star and not with the bow shock itself. The soft X-ray luminosity of the nebula is predicted in the region of ∼1e32-1e33 erg/s.
Chapter 4 presents the results from the new method implemented for postprocessing 3D nested-grid PION simulations with the TORUS Monte Carlo radiative transfer code. This upgrade allows us to now compare results from 3D nested-grid simulations to observational data, allowing us to produce more realistic models of stellar-wind bubbles.
Chapter 5 follows up our 2D study of NGC7635 with a detailed 3D study of the bow shock of Zet Ophiuchi. 3D magneto-hydrodynamic (MHD) simulations were run to model the interaction of the star's wind with the ISM. We chose stellar and ISM parameters appropriate for comparison with Zeta Ophiuchi. We set up a simulation to obtain a bow shock with approximately the correct size compared with observations consistent with the position and width of the infrared arc seen in Spitzer and WISE data. The maximum brightness values for our 24um and H-alpha synthetic data are also comparable with the corresponding observational data. In contrast with the results of Chapter 3, emission maps show that the majority of X-ray emission occurs at the apex of the bow shock at the contact discontinuity. Calculated thermal X-ray emission from the simulated wind bubble does not show a comparable luminosity (∼1e28 erg/s) to measurements from Chandra diffuse X-ray observations (2e29 erg/s).
Finally, Chapter 6 revisits our 2D hydrodynamic study of NGC7635 with a detailed 3D MHD study. Simulations were run to model the interaction of the central star's wind with the ISM. Initial simulations with a uniform ISM density failed to agree with observational data, but a calculation where the star is moving from a low-density region up a density gradient into dense gas best matches the the morphological features of the Spitzer and HST observational data. The maximum brightness of the synthetic 24um emission-map matches the Spitzer image with 3600 MJy/ster, however, the H-alpha emission is ∼4 times fainter than the observations. We produced soft (0.3 − 2 keV) X-ray emission-map predictions to be compared with XMM-Newton data. For the density gradient simulation the majority of the X-ray emission is only coming from the apex at the contact discontinuity with a soft X-ray flux of ∼8e−15 erg/cm2/s, luminosity of ∼1e31 erg/s. This luminosity is consistent with the observational upper limits of 9e30 erg/s.
Our aim for this project was to use simulations of wind bubbles, compared with X-ray observations, to constrain the properties of the wind-ISM interface. This would then lead on to constraining the energetics of the wind-driven feedback from massive stars. The initial 2D simulations had strong mixing in the wake behind the star, producing bright soft X-ray emission. However, this result was not confirmed in 3D MHD simulations, and could be an artefact of the constrained geometry of the 2D axisymmetric calculations. Uncertainties in observational data (e.g. radial velocity for Zeta Ophiuchi) and sensitivity of X-ray luminosity to simulation setup (e.g. density gradient for the Bubble Nebula) complicate our efforts to draw strong conclusions from this work, but progress in this field has been made.

Item Type: Thesis (Doctoral)
Divisions: School of Cosmics Physics > Astronomy and Astrophysics
Date Deposited: 11 Mar 2024 13:12
Last Modified: 12 Mar 2024 02:06

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