M dwarf stars make up over 70% of stars by number in the Milky Way Galaxy and are known to host at least two exoplanets per star on average. Using mutually eclipsing double-lined spectroscopic binary stars (SB2 EBs), astronomers can empirically measure stellar properties of M dwarf stars including mass and radius. However, empirical measurements systematically differ from the predictions of stellar evolutionary models and show large scatter. Some M dwarf stars are outliers, with radii that are a factor of 2-to-3 larger than model predictions, assuming they were measured accurately. In this dissertation, I investigated whether the outliers, systematic offset, and the scatter seen in the mass-radius diagram are physical, using SB2 EBs with photometry from NASA's Kepler Mission and high-resolution near-infrared ground-based spectroscopy. Empirical measurements using space-based photometry and high-resolution near-infrared ground-based spectroscopy, together with Bayesian model-fitting techniques, provide significant advancements over previous measurements. For this dissertation work, a sample of Kepler EBs were carefully chosen to be detached and non-interacting. I conducted a radial velocity survey of the sample using Immersion GRating INfrared Spectrometer (IGRINS) with the Discovery Channel Telescope (DCT) and iSHELL with NASA's Infrared Telescope Facility (IRTF). Combined with high-precision Kepler data, I determined the masses and radii of the component stars in the sample. I also determined a new mass-radius relationship of M dwarf stars using the sample of Kepler EB systems. My investigation showed that the outliers in the mass-radius diagram of M dwarf stars are not physical and they are due to the quality of data and from analysis using different pipelines. I also showed that the offset and scatter in the mass-radius diagram are persistent, which are not from the measurement uncertainties. This suggests the need for an extra degree of freedom to accurately capture the discrepancies between the empirical measurements and model predictions. Lastly, I showed that reduced convective heat flow due to enhanced magnetic fields from rapid stellar rotation can account for the offset and scatter in the measurements.

The B0.2 V magnetic star τ Sco stands out from the larger population of massive magnetic OB stars due to its remarkable, superionized wind, apparently related to its peculiar magnetic field - a field which is far more complex than the mostly-dipolar fields usually observed in magnetic OB stars. τ Sco is therefore a puzzling outlier in the larger picture of stellar magnetism - a star that still defies interpretation in terms of a physically coherent model. Recently, two early B-type stars were discovered as τ Sco analogues, identified by the striking similarity of their UV spectra to that of τ Sco, which was - until now - unique among OB stars. We present the recent detection of their magnetic fields by the MiMeS collaboration, reinforcing the connection between the presence of a magnetic field and a superionized wind. We will also present ongoing observational efforts undertaken to establish the precise magnetic topology, in order to provide additional constrains for existing models attempting to reproduce the unique wind structure of τ Sco-like stars.

The magnetic (B) field in the disk of a galaxy may play an important role in the dynamics and evolution of the interstellar medium (ISM). The process by which the interstellar B-field is generated and maintained is not well understood, but the general expectation is that the generated B-field will be toroidal (parallel to the disk of the galaxy). The large-scale B-field threading the cold ISM of external galaxies can be probed via optical and near-infrared (NIR) polarimetric observations. However, scattered light can introduce false-positive B-field detections into these observations and is a source of contamination. This dissertation sets a context for observations of the Milky Way B-field by assessing the degree to which scattered light affects NIR polarimetry and reporting measurements of the B-fields in the disks of several external galaxies. The polarization properties of scattered light were investigated in order to better understand the degree to which scattered light may be a source of contamination in studies of the cold ISM B-field of external galaxies. The optical and NIR polarization of three, nearby, reflection nebulae was observed, and the wavelength dependence of the polarization percentage was measured. This wavelength dependence was found to be related to the characteristics of the scattering dust grain population with the general conclusion that the total amount of polarized, scattered light decreases with increasing wavelength. This analysis was repeated for the scattering-dominated galaxy M82 to test if similar results obtained for galaxies outside the Milky Way. Observations of this object indicate that the total amount of contamination from scattered light in this object is also less at NIR wavelengths than at optical wavelengths, and they confirm that B-field generated polarization can be detected in external galaxies. A sample of edge-on galaxies was observed for NIR polarization to measure the cold ISM B-field there. These observations indicate the likely presence of non-toroidal B-fields, localized, coherent B-field structures, and spiral disk structure detectable via NIR polarimetry. A comparison of the B-fields threading the cold and hot components of the ISM suggests that these B-fields may be dynamically distinct under certain conditions.

This dissertation deals with the following topics related to the magnetic activity of neutron stars and black holes: (I) Magnetic field evolution of neutron stars: We develop a numerical code which models the internal magnetic field evolution of neutron stars in axisymmetry. Our code includes the Hall drift and Ohmic effects in the crust, and the drift of superconducting flux tubes and superfluid vortices inside the liquid core. We enforce the correct hydromagnetic equilibrium in the core. We also model the elastic deformation of the crust and its feedback on the magnetic field evolution. We find that (i) The Hall attractor found by Gourgouliatos and Cumming in the crust also exists for B-fields which penetrate the core. (ii) If the flux tube drift is fast in the core, the pulsar magnetic fields are depleted on the Ohmic timescale (~150 Myr for hot neutron stars, or ~1.8 Gyr for cold neutron stars such as recycled pulsars, depending on impurity levels). (iii) The outward motion of superfluid vortices during the rapid spin-down of a young highly magnetized pulsar, can partially expel magnetic flux from the core when 𝐵 ≲ 10¹³ G. (II) Neutron star quakes and glitches: We develop a theoretical model to explain the remarkable null pulse coincident with the 2016 glitch in Vela rotation. We propose that a crustal quake associated with the glitch strongly disturbed the Vela magnetosphere and thus interrupted its radio emission. We develop the first numerical code which models the global dynamics of a neutron star quake. Our code resolves the elasto-dynamics of the entire crust and follows the evolution of Alfven waves excited in the magnetosphere. We find that Alfven waves launched by the quake become de-phased in the magnetosphere, and generate strong electric currents, capable of igniting electric discharge. Most likely, the discharge floods the magnetosphere with electron-positron plasma, quenching the pulsar radio emission. The observed ~0.2 s duration of the disturbance indicates that the crust is magnetically coupled to the superconducting core of the neutron star. (III) Pulsar magnetospheres and radio emission: We present an extreme high resolution kinetic plasma simulation of a pulsar magnetosphere using the Pigeon code. The simulation shows from first-principles how and where radio emission can be produced in pulsar magnetospheres. We observe the self-consistent formation of electric gaps which periodically ignite electron-positron discharge. The gaps form above the polar-cap, and in the bulk return-current. Discharge of the gaps excites electromagnetic modes which share several features with the radio emission of real pulsars. We also observe the excitation of plasma waves and charge bunches by streaming instabilities in the outer magnetosphere. (IV) Black hole magnetospheres and no-hair theorem: We explore the evolution of highly magnetized magnetospheres on Kerr black holes by performing general relativistic kinetic plasma simulations with the GRZeltron code, and general relativistic resistive magnetohydrodynamics simulations with the BHAC code. We show that a dipole magnetic field on the event horizon opens into a split-monopole and reconnects in a plasmoid-unstable current-sheet. The plasmoids are ejected from the magnetosphere, or swallowed by the black hole. The no-hair theorem is satisfied, in the sense that all components of the stress-energy tensor decay exponentially in time. We measure the decay time of magnetic flux on the event horizon for plasmoid-dominated reconnection in collisionless and collisional plasma.

The evolution of galaxies rely on a wide variety of physics, and numerical simulations are one of the main tools used to study them. In this thesis we develop a framework for what models can be used to realistically simulate galaxies and study their evolution. We begin with setting specific requirements on the numerical resolution of galaxies, and then test the effects of different stellar feedback models on isolated disk galaxies. We then investigate the addition of magnetic fields into the simulations, and what role they play in determining the contents, behaviour, and star formation, within the interstellar medium of galaxies. / Thesis / Master of Science (MSc) / We develop a framework used to realistically simulate the evolution of galaxies. Specifically we investigate the addition of supernova and magnetic field models, and provide solutions to eliminate the dependence of those models on numerical resolution.

<p> A review of the conducting paper analogue for plotting two-dimensional electric and magnetic fields is given. An improved capacitively-coupled conducting paper analogue is described that will map either single or multiple, uniformly-distributed-source Poissonian fields. A DEW map construction technique is detailed that uses a thin tape dielectric and silver-painted source electrodes. Equipotentials on the conducting paper surface correspond to lines of constant magnetic vector potential or flux lines. Differential voltages are analogous to flux density. The equipotential distribution is plotted using a null technique with a unique point on the map surface held at virtual ground potential. The time-varying equations governing the capacitively- coupled analogue are derived. As a result of these equations, an analogue for the skin effect phenomenon in conductors has been demonstrated.</p> / Thesis / Master of Engineering (ME)

This project aims to obtain and interpret the measurements of the mean longitudinal magnetic field of the chemically peculiar star V1148 Ori. To achieve this aim 12 spectropolarimetric observations obtained by the CFHT using the spectropolarimeter ESPaDOnS were used. The method used to extract the magnetic field signatures from the spectra is called least-squares deconvolution. This method yields line-averaged profiles with a high signal-to-noise ratio. These mean line profiles are necessary to compute the mean longitudinal field. Results of the mean longitudinal field measurements were plotted as a function of the rotational phase, and to this, a sinusoidal function describing a dipolar field was fitted. The dipolar field parameters were computed for two different stellar radii. Inconsistent values for the stellar radii were obtained from the literature, and therefore we calculated two values for each of the parameters. For the surface polar field strength, we found BR1 = 17.38±0.30 kG and BR2 = 12.81±0.22 kG. The calculations involving one of the stellar radii gave results more consistent with previous findings. However, the discrepancy in parameter values could not be accounted for by the small uncertainties. So no definite conclusions can be drawn about the dipolar field parameters. Our fit aligns well with our longitudinal field measurements, no clear indication of any significant deviation from our model assumption, which suggests that the mean longitudinal field is consistent with a large-scale dipolar-like structure.

Magnetic fields play a crucial roll in the stellar activity and evolutionof stars. Despite much research there is much that we do notunderstand. Among Ap stars, empirical evidence has suggested a minimumthreshold for the dipolar magnetic field strength of Bp ≈ 300G.This thesis studies the magnetic field of the Ap star EP UMa usingthe oblique rotator model, which is modeling the star’s magnetic fieldas a dipole. The magnetic field was calculated through the StokesV- and I-spectrum emitted by the star. In total 16 observations ofthe Stokes V and I spectrum were used, collected from the spectropolarimeterNARVAL. These spectra were then analysed using the leastsquares deconvolution method, creating average Stokes V and I profiles,through which the magnetic field were calculated. The result ofthis study indicates a magnetic field of EP UMa with polar strengthof 74G ≤ Bp ≤ 196G, which is well below the suggested minimumthreshold.

This paper focuses on the polarized profiles of resonance scattering lines that form in magnetized disks. Optically thin lines from Keplerian planar disks are considered. Model line profiles are calculated for simple field topologies of axial fields (i.e., vertical to the disk plane) and toroidal fields (i.e., purely azimuthal). A scheme for discerning field strengths and geometries in disks is developed based on Stokes Q − U diagrams for the run of polarization across line profiles that are Doppler-broadened by the disk rotation. A discussion of the Hanle effect for magnetized disks in which the magnetorotational instability (MRI) is operating is also presented. Given that the MRI has a tendency to mix the vector field orientation, it may be difficult to detect the disk fields with the longitudinal Zeeman effect, since the amplitude of the circularly polarized signal scales with the net magnetic flux in the direction of the observer. The Hanle effect does not suffer from this impediment, and so a multi-line analysis could be used to constrain field strengths in disks dominated by the MRI.

京都大学 / 新制・課程博士 / 博士(工学) / 甲第24584号 / 工博第5090号 / 新制||工||1975(附属図書館) / 京都大学大学院工学研究科電気工学専攻 / (主査)教授 篠原 真毅, 教授 大村 善治, 教授 小嶋 浩嗣, 教授 引原 隆士 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM