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Predicting thermal radiation from stellar-wind bubbles using multi-dimensional simulations
Get PDFIn 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
LOFAR observations of radio burst source sizes and scattering in the solar corona
Get PDFLow frequency radio wave scattering and refraction can have a dramatic effect on the observed size and position of radio sources in the solar corona. The scattering and refraction is thought to be due to fluctuations in electron density caused by turbulence. Hence, determining the true radio source size can provide information on the turbulence in coronal plasma. However, the lack of high spatial resolution radio interferometric observations at low frequencies, such as with the LOw Frequency ARray (LOFAR), has made it difficult to determine the true radio source size and level of radio wave scattering. Here we directly fit the visibilities of a LOFAR observation of a Type IIIb radio burst with an elliptical Gaussian to determine its source size and position. This circumvents the need to image the source and then de-convolve LOFAR’s point spread function, which can introduce spurious effects to the source size and shape. For a burst at 34.76 MHz, we find full width at half maximum (FWHM) heights along the major and minor axes to be 18.8′ ± 0.1′ and 10.2′ ± 0.1′, respectively, at a plane of sky heliocentric distance of 1.75 R⊙. Our results suggest that the level of density fluctuations in the solar corona is the main cause of the scattering of radio waves, resulting in large source sizes. However, the magnitude of ε may be smaller than what has been previously derived in observations of radio wave scattering in tied-array images
The codicology of late medieval Irish legal manuscripts: A preliminary study of TCD MS H 3.18 (1337)
Get PDFOn the group generated by C, P and T: I2=T2=P2=ITP=−1, with applications to pseudo-scalar mesons
Get PDFWe study faithful representations of the discrete Lorentz symmetry operations of parity P and time reversal T, which involve complex phases when acting on fermions. If the phase of P is a rational multiple of π then P2n=1 for some positive integer n and it is shown that, when this is the case, P and T generate a discrete group, a dicyclic group (also known as a generalised quaternion group) which are generalisations of the dihedral groups familiar from crystallography. Charge conjugation C introduces another complex phase and, again assuming rational multiples of π for complex phases, TC generates a cyclic group of order 2m for some positive integer m.There is thus a doubly infinite series of possible finite groups labelled by n and m. Demanding that C commutes with P and T forces n=m=2 and the group generated by P and T is uniquely determined to be the quaternion group.
Neutral pseudo-scalar mesons can be simultaneous C and P eigenstates. T commutes with P and C when acting on fermion bi-linears so neutral pseudo-scalar mesons can also be T eigenstates. The T-parity should therefore be experimentally observable and the CPT theorem dictates that T=CP
First results from the REAL-time Transient Acquisition backend (REALTA) at the Irish LOFAR station
Get PDFModern radio interferometers such as the LOw Frequency ARray (LOFAR) are capable of producing data at hundreds of gigabits to terabits per second. This high data rate makes the analysis of radio data cumbersome and computationally expensive. While high performance computing facilities exist for large national and international facilities, that may not be the case for instruments operated by a single institution or a small consortium. Data rates for next generation radio telescopes are set to eclipse those currently in operation, hence local processing of data will become all the more important. Here, we introduce the REAL-time Transient Acquisition backend (REALTA), a computing backend at the Irish LOFAR station (I-LOFAR) which facilitates the recording of data in near real-time and post-processing. We also present first searches and scientific results of a number of radio phenomena observed by I-LOFAR and REALTA, including pulsars, fast radio bursts, rotating radio transients, the search for extraterrestrial intelligence, Jupiter, and the Sun
Variations in melt emplacement beneath the northern East African Rift from radial anisotropy
Get PDFWhere and how melt is stored in the crust and uppermost mantle is important for understanding the dynamics of magmatic plumbing systems and the evolution of rifting. We determine shear velocity and radial anisotropy in the magmatically rifting northern East African Rift to determine the locus and orientation of melt, both on and off-rift. Love and Rayleigh fundamental modes are extracted from ambient noise data from 9-26 s period and then inverted for shear velocity. VSV is 0.15 ± 0.03 km/s lower than VSH from 5-30 km depth on average. VSH>VSV across most of the study region suggests the crust is inherently horizontally layered, with the largest anisotropy in the upper 5-15 km. Effective medium theory suggests thin compositional layering of felsic and mafic intrusions can account for anisotropy up to 4%. However, to reconcile the largest observed anisotropy (6.5%), and lowest velocities, we require 2-4% partial melt oriented in sills. Along the rift, horizontally aligned radial anisotropy gets weaker north-eastwards, suggesting sills become less dominant with progressive rifting. The Erta Ale magmatic segment is the only location where VSV>VSH, suggesting the crust contains vertical micro-cracks and dykes. Overall, the results suggest during early continental breakup when the rift is narrow, sill formation is the dominant storage mechanism. As a rift widens, vertical dyke intrusion becomes dominant and is likely controlled by variations in crustal thickness and stress state
Understanding Seismic Waves Generated by Train Traffic via Modeling: Implications for Seismic Imaging and Monitoring
Get PDFSupplemental Material for article: "Understanding Seismic Waves Generated by Train Traffic via Modeling: Implications for Seismic Imaging and Monitoring
Humming trains in seismology: an opportunistic source 1 for probing the shallow crust
Get PDFUnveiling the coupling between the atmosphere and the Earth, improving our understanding of the preparation phase of earthquakes and volcanic eruptions, mitigating induced seismic hazard, discovering new natural resources all require improved imaging and monitoring of the top first kilometers of the crust. Passive seismic imaging and monitoring usually relies on blind correlations of long time series of noise. Instead, seismic interferometry applied to opportunistic sources of noise relies on an accurate understanding of noise source mechanisms, on time window and station pair selection, and on specific seismic phases extraction (surface, body). Recently, massive freight trains have been recognized as the most persistent and powerful cultural seismic sources generating tremor equivalent to magnitude 2 earthquakes and detectable up to 100 km distance. In this paper, we discuss the source mechanisms of train tremor and review some basic theory on seismic interferometry applied to opportunistic sources. We finally show two case studies of long-range body- and surface
waves retrieval in the contexts of mineral exploration in Canada and fault zone monitoring in Southern California. This approach of noise recovery to create valuable sources together with disruptive dense data acquisition technologies such as nodes or Distributed Acoustic Sensing will deeply transform our capability to explore and monitor the shallow crust with improved spatial and temporal resolution