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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenThu, 30 Nov 2023 19:53:51 +0000Radiative transfer in very strong magnetic fields
https://resolver.caltech.edu/CaltechETD:etd-06272007-101041
Authors: Miller, Michael Coleman
Year: 1990
DOI: 10.7907/9t01-q903
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
The study of the cooling of neutron stars has been undertaken by many researchers in the past twenty-five years, but this study has been made difficult by the inherent theoretical and observational uncertainties; most observations of their thermal X-ray flux have yielded only upper limits. More sensitive satellites such as ROSAT and AXAF may provide more positive flux information, and it is important to know how to interpret these data in terms of surface temperature. One of the most important factors in this interpretation is the effect of the surface magnetic field.
Young neutron stars are believed to have extremely strong magnetic fields, on the order of 10(12)G. These fields dominate the physics of the atmosphere. In particular, atoms in the atmospheres of neutron stars have much greater binding energies than in the zero-field case, and they are constrained to move along the field lines. We use a multiconfigurational Hartree-Fock code, modified for very strong magnetic fields, to calculate wavefunctions, energies and oscillator strengths for several atoms in representative values of the magnetic field.
We then use these simulations to construct model atmospheres for neutron stars. Because of the low mass necessary for optical depth unity in the soft X-rays (typically [...]) and because of the short time scale for gravitational separation (~ 1 - 100s), the photosphere is likely to consist of a pure element. Numerous processes could cause many elements to be important, so we investigate atmospheres consisting of pure hydrogen, helium, carbon, nitrogen and silicon in magnetic fields of 9.4 x 10(11)G, 2.35 x 10(12)G, and 4.7 x 10(12)G.
We also use the high-field energies to investigate soft X-ray lines in gamma-ray bursts. Highly ionized elements could create absorption lines in the 1-15keV range, and the identification of such lines in conjunction with cyclotron lines would determine the magnetic field and gravitational redshift on the surface of the star, which would provide clues to the equation of state on the interior. We conclude with a discussion of the prospect of identifying these lines with future satellites.https://thesis.library.caltech.edu/id/eprint/2743Radiative processes in active galactic nuclei
https://resolver.caltech.edu/CaltechETD:etd-06202007-132612
Authors: Coppi, Paolo Severo
Year: 1991
DOI: 10.7907/GDMQ-Q829
A study of processes relevant to the electron-positron pair plasmas thought to exist in Active Galactic Nuclei is undertaken. The processes considered include: Compton scattering, pair annihilation, two photon pair production, synchrotron emission, e-e bremsstrahlung, and Coulomb scattering. Approximations used in the past to treat these processes in the context of a kinetic code are examined, and improvements are presented. A two-moment scattering formalism is presented to allow for important energy dispersion effects in scattering. This improved treatment of microphysical processes is implemented in a time-dependent, kinetic code incorporating Klein-Nishina effects on both the pair and photon distributions, relativistic thermal Comptonization, and synchrotron reabsorption.
The effects of pair plasma reprocessing on the emergent radiation spectrum are examined. Time-varying and stationary spectra are computed. Good qualitative agreement with previous calculations is found, except when the differences are attributable to the improved treatment of the microphysics. These differences can be substantial, particularly in the "photon-starved" regime where the effects of Coulomb scattering by suprathermal pairs off thermal pairs significantly modify the spectra. The spectral response of the pair plasma to variations in the particle injection is found to depend sensitively on the plasma parameters. A transitional spectrum may look very different from the spectra of either the stationary initial or final states. The highest energies (gamma-rays) are found to respond most rapidly to changes and should vary more than the X-rays. Pair plasmas can produce soft X-ray excesses. This happens under conditions independently favored by current pair plasma-Compton reflection models of the hard X-ray spectrum.
https://thesis.library.caltech.edu/id/eprint/2662MHD in divergence form : a computational method for astrophysical flow
https://resolver.caltech.edu/CaltechETD:etd-08152007-144037
Authors: van Putten, Maurice H. P. M.
Year: 1992
DOI: 10.7907/263j-a556
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
The equations of MHD in curved space-time are presented in divergence form for the purpose of numerical implementation. This result follows from a covariant divergence form of the single fluid theory of electro-magneto-hydrodynamics in curved space-time with general constitutive relations.
Some one- and two-dimensional shock computations are given. A pseudospectral method with weak smoothing is used in all of our computations. The pseudo-spectral method is constructed by consideration of Riemann problems in one dimension. The power of MHD in divergence form is brought about by using uniform grid-spacing and explicit time-stepping. The problems considered are shock-tube problems in transverse MHD with analytical comparison solution and a coplanar Riemann problem as discussed for nonrelativistic MHD in Brio and Wu [37]. In a limit of nonrelativistic velocities comparison is made of the results of the latter with those in [37]. In two dimensions cylindrically symmetric problems are considered for test of isotropy, independence of coordinate system and convergence (using comparison results in polar coordinates). We conclude with a computation of a shock induced vortex in jet flow with [...] 2.35, a relativistic jet computation with [...] 3.25 and, finally, computations on magnetic pressure dominated stagnation points in a 2D shock problem in nontransverse MHD.
This work is proposed for numerical study of astrophysical flows, and in particular as a "vehicle" towards the origin of jets.https://thesis.library.caltech.edu/id/eprint/3139Dynamics of neutron stars and binaries in globular clusters or, Ménages à trois: revitalizing burnt out degenerates through partner swapping
https://resolver.caltech.edu/CaltechTHESIS:08312011-085802800
Authors: Sigurdsson, Steinn
Year: 1992
DOI: 10.7907/gcf0-px44
Interaction cross-sections and collision cross-sections for a set of hard multi-mass binary-single star interactions are calculated in order to estimate three-body collision cross-sections in galactic globular clusters. The cross-sections are calculated by direct integration of binary-single star encounters, using Monte Carlo sampling to average over the three-body phase space. A number of mass-ratios physically relevant to the globular cluster environment are used. Differential energy transfer rates due to three-body interactions are calculated. Parametric
approximations for the various cross-sections calculated are found.
The results of the cross-sections are used to evaluate various formation scenarios for the pulsars PSR2127+11C (M15C) and PSR1744-24A (TER5A). In addition the contribution of the globular cluster system to the galactic birthrate of PSR1913+16 type systems is estimated.
The dynamics and interactions of a test binary population in a number of globular cluster models are calculated in a static background. The cluster method used are isotropic multi-mass King models of varying concentration and density.
The model developed is generalisable to an arbitrary cluster distribution function, including one evolving in time. Relative probabilities of different encounters are
found for binaries on arbitrary trajectories in the various cluster models. The actual interaction rates of the test population are calculated by direct integration, using Monte Carlo sampling to average over the initial binary parameters. The number of neutron stars expected to be recycled in different concentration clusters is estimated with a particular view to understanding the pulsar population observed in clusters 47Tuc and M15.
Estimates are also made of the binary density profile of the different concentration class clusters, and the final distribution in binary parameters. The production rate of "blue stragglers" and the interaction rate of (sub)giants and white dwarfs in the various clusters are also estimated.https://thesis.library.caltech.edu/id/eprint/6638QSO Absorption Lines and the Ionizing Field at High Redshifts
https://resolver.caltech.edu/CaltechETD:etd-09232008-081252
Authors: Zuo, Lin
Year: 1992
DOI: 10.7907/vsk7-nv30
<p>In this thesis we explore the relationship between the QSO absorption line systems and the metagalactic ionizing field at high redshifts.</p>
<p>In the first introductory chapter we describe the recent developments in the field of QSO absorption lines and the ionizing radiation background. We concentrate on compiling the new observational results and address why studies of QSO absorption line systems and ionizing field are important to our understandings of the formation and evolution of the large scale structure of the universe.</p>
<p>In the second chapter Markoff's method has been used to derive a general formalism to deal with the absorptions produced by randomly distributed discrete clouds, such as the QSO absorption line systems. Some analytical forms are obtained for the effective optical depth τ<sub>eff</sub>, the count reduction factor f<sub>c</sub>, and the optical depth probability distribution function P(τ). We demonstrate that the spectrum of ionizing background is very different from the intrinsic source spectrum. We calculate the QSO contributions to J<sub>νL</sub>(Z<sub>obs</sub>) by using simple analytical expressions. We show that because of the Lyman continuum absorption produced by QSO absorption line systems, it is very difficult to find a "clear" line of sight to conduct the Hell Gunn-Peterson test. We have also found that dust grains in the QSO damped Lyα systems produce a marginally significant obscuration for z = 3 quasars: the count reduction factor is 1⁄1.7 at z = 3. The reddening is shown to be small for a flux limited QSO sample.</p>
<p>In the following two chapters we discuss fluctuations and intensity correlation in the ionizing field. We derive the intensity probability distribution function P(J) for randomly distributed point sources. We show that absorptions by QSO absorption line systems reduce the total number of sources involved in producing the ionizing background and therefore enhance the fluctuation significantly, if QSOs are the main ionizing sources. We have calculated the intensity correlation function ξ<sub>j</sub> for randomly distributed QSOs. The QSO Lyα clouds can be used as intensity indicators to reveal the intensity correlation at high redshifts. We have measured the equivalent width correlation function ξ<sub>1⁄w</sub> for several selected QSOs and have found, in some cases, strong correlation signals at small separations. Careful examination shows that such signals are mainly generated by the lines near the QSO emission redshifts. One explanation is that the high S⁄N near QSO emission red-shifts enable us to detect very weak lines which result in the correlation signal. The other explanation is that the correlated intensities of ionizing field near QSOs have caused the observed equivalent width correlation. If this latter explanation is correct, from the affected range by QSOs we conclude that J<sub>νL</sub> is less than 10<sup>-21</sup> ergs s<sup>-1</sup>cm<sup>-2</sup>Hz<sup>-1</sup>sr<sup>-1</sup> at z ~ 3.5.</p>
<p>The last two chapters deal with the ionization structure of the QSO Lyα clouds. We solve the coupled ionization balance and radiation transfer problem (the "inverse HII region" problem) for the non-uniform spherical absorbing clouds and calculate the HI column density distribution f(N). We show that with an appropriate density gradient in the clouds we can reproduce the observed overall power law distribution and the apparent excess of absorption systems with N ≥ 2 x 10<sup>20</sup>cm<sup>-2</sup>. The calculated f(N) is not sensitive to the input ionizing spectra and is a generic feature of the density profile. In our model all QSO absorption lines have the same origin: the ionized outer envelopes produce the Lyα forest lines while the neutral cores result in damped Lyα systems. We discuss the consequences of such models and propose star-forming dwarf galaxies as primary candidates for QSO absorption line systems. Our calculations also suggest that uniform cloud models are highly unlikely. To calculate the absorptions produced by HeI and HeII in QSO Lyα clouds we also solve the ionization structure for the clouds containing both H and He. We explore the variation of HeI and HeII distributions under different input ionizing spectra and discuss how to get a self-consistent spectral shape for the ionizing background.</p>https://thesis.library.caltech.edu/id/eprint/3732The ages, speeds and offspring of pulsars
https://resolver.caltech.edu/CaltechETD:etd-09102008-134917
Authors: Hansen, Bradley Miles Stougaard
Year: 1996
DOI: 10.7907/69DE-D262
We investigate the cooling of low mass white dwarfs with helium cores. We construct a detailed numerical model using the most modern input physics, including our own calculations of low temperature hydrogen opacities. We use our models to constrain the ages of binary millisecond pulsars from the optical observations of their white dwarf companions. We use this to place limits on the initial spin periods, magnetic field decay times and accretion histories of the millisecond pulsars. Our models can also be used along with observations of spectroscopic gravities and radial velocities to place interesting constraints on the neutron star equation of state. We provide grids of temperature and luminosity as a function of age for various white dwarf masses and surface compositions to facilitate future analyses.
We have investigated the effect of the pulsar wind on the atmospheric composition of binary companions. The spallation of atmospheric helium to hydrogen increases the cooling age of the white dwarf. We find that all white dwarf companions in binaries with orbital period < 300 days should cool as DA (hydrogen surface layer) white dwarfs, irrespective of their original hydrogen content. We investigate the effect of various wind compositions and note that, if almost all the hydrogen on the surface of a pulsar companion is the result of spallation of an ionic wind, then the D/H ratio is large.
We investigate the processes by which planets might form around a millisecond pulsar such as PSR B1257+12. We study the evolution of accretion disks of different mass, angular momentum and composition, corresponding to various proposed formation scenarios. We find that most formation scenarios require a high efficiency of conversion of metal-rich material into planets if they are to produce the observed parameters of the 1257+12 planetary system.
We have studied the distribution of pulsar proper motions in the light of the recent analysis of Lyne & Lorimer (1994). Using a simulation of the selection effects of the various surveys, and treating the censored data using survival statistics, we arrive at an estimate of the characteristic pulsar birth velocity ~ 300 km.s[superscript -1], 2/3 that of Lyne & Lorimer. We also show that the older pulsar population shows the effects of the asymmetric drift, indicating that it must be dynamically old.
https://thesis.library.caltech.edu/id/eprint/3443Order of Magnitude Physics: A Textbook with Applications to the Retinal Rod and the Density of Prime Numbers
https://resolver.caltech.edu/CaltechTHESIS:10302009-110303489
Authors: Mahajan, Sanjoy Sondhi
Year: 1998
DOI: 10.7907/8MJZ-A984
<p>I develop tools to amplify our mental senses: our intuition and reasoning abilities. The first five chapters—based on the Order of Magnitude Physics class taught at Caltech by Peter Goldreich and Sterl Phinney—form part of a textbook on dimensional analysis, approximation, and physical reasoning. The text is a resource of intuitions, problem-solving methods, and physical interpretations. By avoiding mathematical complexity, order-of-magnitude techniques increase our physical understanding, and allow us to study otherwise difficult or intractable problems. The textbook covers: (1) simple estimations, (2) dimensional analysis, (3) mechanical properties of materials, (4) thermal properties of materials, and (5) water waves.</p>
<p>As an extended example of order-of-magnitude methods, I construct an analytic model for the flash sensitivity of a retinal rod. This model extends the flash-response model of Lamb and Pugh with an approximate model for steady-state response as a function of background light I<sub>b</sub>. The combined model predicts that the flash sensitivity is proportional to I<sub>b</sub><sup>-1.3</sup>. This result roughly agrees with experimental data, which show that the flash sensitivity follows the Weber-Fechner behavior of I<sub>b</sub><sup>-1</sup> over an intensity range of 100. Because the model is simple, it shows clearly how each biochemical pathway determines the rod's response.</p>
<p>The second example is an approximate model of primality, the square-root model. Its goal is to explain features of the density of primes. In this model, which is related to the Hawkins' random sieve, divisibility and primality are probabilistic. The model implies a recurrence for the probability that a number n is prime. The asymptotic solution to the recurrence is (log n)<sup>-1</sup>, in agreement with the prime-number theorem. The next term in the solution oscillates around (log n)<sup>-1</sup> with a period that grows superexponentially. These oscillations are a model for oscillations in the density of actual primes first demonstrated by Littlewood, who showed that the number of primes ≤ n crosses its natural approximator, the logarithmic integral, infinitely often. No explicit crossing is known; the best theorem, due to to Riele, says that the first crossing happens below 7 x 10<sup>370</sup>. A consequence of the square-root model is the conjecture that the first crossing is near 10<sup>27</sup>.</p>
https://thesis.library.caltech.edu/id/eprint/5338Dynamics of Spinning Compact Binaries in General Relativity
https://resolver.caltech.edu/CaltechETD:etd-05222003-161626
Authors: Hartl, Michael David
Year: 2003
DOI: 10.7907/KZ5M-MR27
<p>This thesis investigates the dynamics of binary systems composed of spinning compact objects (such as white dwarfs, neutron stars, and black holes) in the context of general relativity. In particular, we use the method of Lyapunov exponents to determine whether such systems are chaotic. Compact binaries are promising sources of gravitational radiation for both ground- and space-based gravitational-wave detectors, and radiation from chaotic orbits would be difficult to detect and analyze. For chaotic orbits, the number of waveform templates needed to match a given gravitational-wave signal would grow exponentially with increasing detection sensitivity, rendering the preferred matched filter detection method computationally impractical. It is therefore urgent to understand whether the binary dynamics can be chaotic, and, if so, how prevalent this chaos is.</p>
<p>We first consider the dynamics of a spinning compact object orbiting a much more massive rotating black hole, as modeled by the Papapetrou equations in Kerr spacetime. We find that many initial conditions lead to positive Lyapunov exponents, indicating chaotic dynamics. The Lyapunov exponents come in positive/negative pairs, a characteristic of Hamiltonian dynamical systems. Despite the formal existence of chaotic solutions, we find that chaos occurs only for physically unrealistic values of the small body's spin. As a result, chaos will not affect theoretical templates in the extreme mass-ratio limit for which the Papapetrou equations are valid. Chaos will therefore not affect the ability of space-based gravitational-wave detectors (such as LISA, the Laser Interferometer Space Antenna) to perform precision tests of general relativity using extreme mass-ratio inspirals.</p>
<p>We next consider the dynamics of spinning black-hole binaries, as modeled by the post-Newtonian (PN) equations, which are valid for orbital velocities much smaller than the speed of light. We study thoroughly the special case of quasi-circular orbits with comparable mass ratios, which are particularly relevant from the perspective of gravitational wave generation for LIGO (the Laser Interferometer Gravitational-wave Observatory) and other ground-based interferometers. In this case, unlike the extreme mass-ratio case, we find chaotic solutions for physically realistic values of the spin. On the other hand, our survey shows that chaos occurs in a negligible fraction of possible configurations, and only for such small radii that the PN approximation is likely to be invalid. As a result, at least in the case of comparable mass black-hole binaries, theoretical templates will not be significantly affected by chaos.</p>
<p>In a final, self-contained chapter, we discuss various methods for the calculation of Lyapunov exponents in systems of ordinary differential equations. We introduce several new techniques applicable to constrained dynamical systems, developed in the course of studying the dynamics of spinning compact binaries.</p>
<p>Considering the Papapetrou and post-Newtonian systems together, our most important general conclusion is that we find no chaos in any relativistic binary system for orbits that clearly satisfy the approximations required for the equations of motion to be physically valid.</p>https://thesis.library.caltech.edu/id/eprint/1940Black Hole Mergers and Their Electromagnetic Counterparts
https://resolver.caltech.edu/CaltechTHESIS:05312011-152117816
Authors: Bode, Jason Nathaniel
Year: 2011
DOI: 10.7907/YVVG-R137
Over the past ten years it has become increasingly clear that most, if not all, galaxies have super-massive black holes lurking in their cores. The implications for this are large as they not only have significant effects on the host galaxies, far beyond what would have been naively expected, but would provide several significant gravitational wave sources to the Laser Interferometer Space Antenna (LISA). This thesis is primarily concerned with these gravitational wave sources and the possible electromagnetic counterparts. In particular, when two galaxies merge, it leads to the ultimate merger of their individual SMBHs. If gas is present near the time of merger a circumbinary disk forms around the binary. By assuming the disk is pressureless, and looking at the limits of this approximation, in Chapter 2 we develop an analytic theory of the reaction of such a gaseous disk to the gravitational wave mass loss and recoil kicks which occur during a SMBH merger. However, to understand the effects of finite pressure, in Chapter 3 we develop a one-dimensional hydrodynamic code. The efficiency of the code and the power of the analytic solution allow us to explain the entirety of possible reactions. These results are also favorably compared with far more complicated 3D relativistic magneto-hydrodynamics simulations. LISA will not see only the mergers of two SMBHs, it would also see the inspirals of stellar-mass objects into a SMBH. In Chapter 4 we discuss a new channel of formation of these extreme mass ratio inspirals (EMRIs). This new channel of EMRI formation is rich physically and, in particular, almost always requiring either the Kozai mechanism or an as-of-yet unnoticed phenomenon which we dub the reverse Kozai mechanism. We find that this channel of EMRI formation produces modest numbers of EMRIs when compared to the primary channel of EMRI formation, which, under optimistic detection scenarios for the most recent LISA design, results in the plausible detection of several. Finally, an unrelated project that considers solving the self-similar Type-II strong-shock problem in slightly asymmetric media is given in Chapter 5. We show that the results can even be applied to explosions along weak discontinuities in the density. https://thesis.library.caltech.edu/id/eprint/6482The Dynamics of White Dwarfs, Black Holes and Stellar Cusps
https://resolver.caltech.edu/CaltechTHESIS:08152012-212758368
Authors: Wegg, Christopher
Year: 2013
DOI: 10.7907/YCAT-ES15
<p>This thesis contains topics related mostly to the dynamics of white dwarfs (chapter 2), the dynamics of stars around binary super massive black holes (chapters 4, 5 and 6) and dynamics in the singular isothermal sphere (chapter 7).</p>
<p>In chapter 2 the kinematics of young (< 3x10<sup>8</sup>yr) galactic white dwarfs are investigated. A relationship between the mass and kinematics of white dwarfs is demonstrated, whereby high- mass white dwarfs have low velocity dispersion. This is the result of less scattering during the shorter lifetime of their more massive precursors. The kinematics of the highest-mass white dwarfs (> 0.95 Msun) are also investigated, and it is shown that they are consistent with the majority being formed via single-star evolution from massive progenitor stars.</p>
<p>In chapter 3 it is shown that the coolest, oldest white dwarfs can be identified photometrically from their unique colors, and five new ultracool white dwarfs are spectroscopically confirmed.</p>
<p>In chapter 4 it is shown that close binary supermassive black holes (SMBHs) should produce a burst of tidal disruptions of up to 0.1 yr<sup>−1</sup> as they form. The quiescent rate is ~10<sup>−5</sup> yr<sup>−1</sup> per galaxy, and it is therefore shown that binary SMBHs can potentially be identified via multiple tidal disruptions from the same system.
</p>
<p>In chapter 5 we perform more extensive simulations of the dynamics of stars around binary SMBHs to better quantify and understand the stellar dynamics. By incorporating general relativistic corrections, we also investigate the processes undergone by compact remnants orbiting the binary SMBHs, analyzing both objects that plunge directly into the SMBHs, and those that undergo extreme mass ratio inspirals (EMRIs). The potential used to mimic general relativistic precession in these simulations is novel, and more accurate for the type of nearly parabolic orbits considered in this work: It is described in chapter 6.
</p>
<p>In chapter 7 an analytic solution to the manner in which stars diffuse in the background of a singular isothermal sphere is developed. It is shown a self-similar solution should exist, and this solution is found.
</p>https://thesis.library.caltech.edu/id/eprint/7185The Atmospheric Dynamics of Pulsar Companions
https://resolver.caltech.edu/CaltechTHESIS:06112015-211414622
Authors: Jermyn, Adam Sean
Year: 2015
DOI: 10.7907/Z90Z716M
<p>Pulsars emit radiation over an extremely wide frequency range, from radio through gamma. Recently, systems in which this radiation significantly alters the atmospheres of low-mass pulsar companions have been discovered. These systems, ranging from ones with highly anisotropic heating to those with transient X-ray emissions, represent an exciting opportunity to investigate pulsars through the changes they induce in their companions. In this work, we present both analytic and numerical work investigating these phenomena, with a particular focus on atmospheric heat transport, transient phenomena, and the possibility of deep heating via gamma rays. We find that certain classes of binary systems may explain decadal-timescale X-ray transient phenomena, as well as the formation of so-called redback companion systems. We also posit an explanation for the formation of high-eccentricity millisecond pulsars with white dwarf companions. In addition, we examine the temperature anisotropy induced by the Pulsar in its companion, and demonstrate that this may be used to infer properties of both the companion and the Pulsar wind. Finally, we explore the possibility of spontaneously generated banded winds in rapidly rotating convecting objects.</p>https://thesis.library.caltech.edu/id/eprint/9019Astrophysical Applications of Quantum Mechanics
https://resolver.caltech.edu/CaltechTHESIS:06082018-000259902
Authors: Murchikova, Elena M.
Year: 2018
DOI: 10.7907/PEBS-ZJ88
From an outside point of view, astrophysics and quantum mechanics as subclasses of the physical sciences could not be further from each other. Yet these two sides of nature are deeply intertwined. The influence of quantum mechanics on astrophysics and astrophysics on quantum mechanics has been profound: spectral lines as diagnostics, radiative transport, the interiors of celestial bodies, neutrino oscillations, constraints on neutrino mass and graviton mass. In this work, I discuss several applications of quantum mechanics in astrophysics: (1) I examine the use of submm recombination lines of H, He and He<sup>+</sup> to probe the extreme ultraviolet luminosity of starbursts and Active Galactic Nuclei. (2) I use the hydrogen recombination line H30α to study the accretion zone of the Milky Way's Galactic Center black hole. I discuss detection of an accretion disk of radius <0.008 pc, consisting of ~10<sup>4</sup> K gas the disk properties, and its importance in the context of accretion on the black hole. (3) I carry out an extensive study and comparison of M1 closure schemes for neutrino radiation transport, using the protoneutron star interior as a background. (4) I study first-order hydrodynamics of a chiral fluid on a vortex background, and in an external magnetic field, as the precursor for a study of neutron star interiors. I show that there are two previously undiscovered modes describing heat waves propagating along the vortex and magnetic field.
https://thesis.library.caltech.edu/id/eprint/11044Modeling Rapidly Fading Supernovae as Nickel-Free Core-Collapse Explosions of Extended Helium Stars
https://resolver.caltech.edu/CaltechTHESIS:09032018-174351545
Authors: Kleiser, Io K.
Year: 2019
DOI: 10.7907/5YEJ-EX71
<p>Supernovae are the engines of the universe, pulling material out of the furnaces of stars and spewing it out into their galaxies. As some of the most powerful explosions since the Big Bang, they influence not only the chemical but also mechanical evolution of the galaxies they inhabit. They induce star formation and produce the building blocks of planets, organisms, and ultimately, civilizations. Understanding the connections between the supernovae we observe and the stars that would have produced them is a critical piece of understanding this process.</p>
<p>Unfortunately, we rarely have the ability to observe the progenitor stars of supernovae directly; it is usually difficult to predict when a given star will explode, and most are in galaxies too distant to allow observation of individual stars. Instead, we typically must leverage our understanding of the explosions themselves to reveal the nature of the stars that produced them. Using analytical and numerical calculations, it is possible to predict the supernovae from certain types of stars and work backwards.</p>
<p>In this thesis, we present a new model for previously elusive rapidly fading supernovae, which we believe are due to the core-collapse explosions of massive stars inside extended hydrogen-free envelopes or previously ejected mass shells. This model requires not only pre-explosion stellar radii of unprecedented size for hydrogen-free stars but also a lack of radioactive nickel, which is usually present in supernovae. We show our process from simple toy models to self-consistent explosions of stellar models and compare our results to existing rapidly fading supernovae. Understanding these unusual transients will shed light on the many possible ways stars behave shortly before death and also may be critical for understanding the population of core-collapse supernovae as a whole.</p>https://thesis.library.caltech.edu/id/eprint/11170