Originally thought of as a constant and unchanging place, the Universe is full of dramas of stars emerging, dying, eating each other, colliding, etc. One of the first transient phenomena noticed were called novae (the name means "new" in Latin). Years later, supernovae were discovered. Despite their names, both novae and supernovae are events in relatively old stars, with supernovae marking the point of stellar death. Known for thousands of years, supernovae and novae remain among the most studied events in our Universe. Supernovae strongly influence the circumstellar medium, enriching it with heavy elements and shocking it, facilitating star formation. Cosmic rays are believed to be accelerated in shocks from supernovae, with small contribution possibly coming from novae. Even though the basic physics of novae is understood, many questions remain unanswered. These include the geometry of the ejecta, why some novae are luminous radio or gamma-ray sources and others are not, what is the ultimate fate of recurrent novae, etc. Supernova explosions are the primary sources of elements heavier than hydrogen and helium. The elements up to nuclear masses A around 100 can form through successive nuclear fusion in the cores of stars starting with hydrogen. Beyond iron, the fusion becomes endothermic instead of exothermic. In addition, for these nuclear masses the temperatures required to overcome the Coulomb barriers are so high that the nuclei are dissociated into alpha particles and free nucleons. Hence all elements heavier than A around 100 should have formed by some other means. These heavier nuclear species are formed by neutron capture on seed nuclei close to or heavier than iron-group nuclei. Depending on the ratio between neutron-capture timescale and beta-decay timescale, neutron-capture processes are called rapid or slow (r- and s-processes, respectively). The s-process, which occurs near the valley of stable isotopes, terminates at Bi (Z=83), because after Bi there is a gap of four elements with no stable isotopes (Po, At, Rn, Ac) until we come to stable Th. The significant abundance of Th and U in our Universe therefore implies the presence of a robust source of r-process. The astrophysical site of r-process is still under debate. Here we present a study of a candidate site for r-process, neutrino-heated winds from newly-formed strongly magnetized, rapidly rotating neutron stars ("proto-magnetars"). Even though we find such winds are incapable of synthesizing the heaviest r-process elements like U and Th, they produce substantial amounts of weak r-process (38<Z<47) elements. This may lead to a unique imprint of rotation and magnetic fields compared to such yields from otherwise analogous slowly rotating non-magnetized proto-neutron stars. Novae explosions are not as powerful as those of supernovae, but they occur much more frequently. The standard model of novae assumes a one-stage ejection of mass from the white dwarf following thermonuclear runaway. The discovery by the Fermi space telescope of gamma-rays from classical novae made the existence of shocks in novae outflows evident. The presence of shocks in novae was considered well before the discovery of gamma-ray emission; however, little previous theoretical work acknowledged the overwhelming effect of shocks on observed emission and ejecta geometry. Here we present the calculations of synchrotron radio emission from the shocks as they propagate down the density gradient and peak at the timescale of a few months. The model satisfactory fits observations and has several implications for the physics of novae.

Thermonuclear (Type Ia) supernovae are excellent distance indicators, due to their uniform peak brightness. They are also important contributors to the chemical evolution of galaxies since their explosions supply large amounts of iron peak elements to the interstellar medium. However, there is no consensus on the progenitor systems of these supernovae. As a result, different delay times from the formation of the binary system to the supernova have been proposed. Whether the observed rate of supernova Type Ia in early-type galaxies supports a progenitor channel with one or two degenerate objects has been disputed. While the predominant old population found in early-type galaxies supports longer delay times, the presence of recent star formation might indicate the opposite. In this work, we employ a double-burst model to account for the relative contribution of both populations. We show that for a DTD ∝ t^−1, convolved with star formation histories that are relevant for early-type galaxies, the supernova rate is independent of a host galaxy’s colour. Our results indicate that a DTD with no cutoff is preferred, thus favoring the double-degenerate scenario. / Graduate

Thesis (Ph. D.)--University of Texas at Austin, 1998. / Vita. Includes bibliographical references (leaves 175-187). Available also in a digital version from Dissertation Abstracts.

No description available.

No description available.

Using two techniques not previously applied to Cassiopeia A (Cas A), we measure the reddening toward its expansion center. An estimate of AV from the near-IR [Fe II] lines is hampered by uncertain atomic data, though the spatial variation in their flux ratio allows relative measurement of the extinction in regions without previous optical estimates. We use a second technique based on the broad-band IR shape of the synchrotron emission, and find Aᵥ = 6.2 ± 0.6 for a knot 13" from the expansion center. Assuming a plausible lower limit on the apparent magnitude of the SN in outburst, the ⁵⁶Ni yield was 0.058 < M(Ni) < 0.16M⊙. With the ⁴⁴Ti mass from published gamma-ray observations, this implies a ⁴⁴Ca/ ⁵⁶Fe ratio consistent with the solar abundance. Recently published Spitzer Space Telescope IRS observations detect dust and line emission from cold gas interior to Cas A’s reverse shock. Using simple physical arguments and new hydrodynamic, non-equilibrium photoionization calculations, we infer the physical conditions in this material. We find that the mid-IR bright clumps are photoionized by the SNR shocks, over-dense relative to the expected average in the interior of the remnant, and have abundances consistent with incomplete oxygen burning. The lack of detectable iron lines indicates that any Si-burning material still interior to the reverse shock must be far more tenuous than the clumps of O-burning ashes. Finally, we present calculations from a new multi-dimensional hydrodynamics and non-equilibrium ionization and cooling code designed to model the emission from SNR shocks. Two-dimensional simulations of a shock-cloud interaction in a pure-oxygen plasma, with flow parameters relevant to Cas A, show a wider range of temperatures and ionization states than is typical in single-zone or 1D calculations, indicating that fluid and cooling instabilities play a role in producing the observed spectra of radiative shocks in metal-rich gas.

Core-collapse supernovae (CCSN) are among the most energetic explosions in the universe, liberating ~1053 erg of gravitational binding energy of the stellar core. Most of this energy ( ~99%) is emitted in neutrinos and only 1% is released as electromagnetic radiation in the visible spectrum. Energy radiated in the form of gravitational waves (GWs) is about five orders smaller. Nevertheless, this energy corresponds to a very strong GW signal and, because of this CCSN are considered as one of the prime sources of gravitational waves for interferometric detectors. Gravitational waves can give us access to the electromagnetically hidden compact inner core of supernovae. They will provide valuable information about the angular momentum distribution and the baryonic equation of state, both of which are uncertain. Furthermore, they might even help to constrain theoretically predicted SN mechanisms. Detection of GW signals and analysis of the observations will require realistic signal predi ctions from the non-parameterized relativistic numerical simulations of CCSN. This dissertation presents the gravitational wave signature of core-collapse v supernovae. Previous studies have considered either parametric models or nonexploding models of CCSN. This work presents complete waveforms, through the explosion phase, based on first-principles models for the first time. We performed 2D simulations of CCSN using the CHIMERA code for 12, 15, and 25M non-rotating progenitors. CHIMERA incorporates most of the criteria for realistic core-collapse modeling, such as multi-frequency neutrino transport coupled with relativistic hydrodynamics, eective GR potential, nuclear reaction network, and an industry-standard equation of state. / Based on the results of our simulations, I produced the most realistic gravitational waveforms including all postbounce phases of core-collapse supernovae: the prompt convection, the stationary accretion shock instability, and the corresponding explosion. Additionally, the tracer particles applied in the analysis of the GW signal reveal the origin of low-frequency component in the prompt part of gravitational waveform. Analysis of detectability of the GW signature from a Galactic event shows that the signal is within the band-pass of current and future GW observatories such as AdvLIGO, advanced Virgo, and LCGT. / by Konstantin Yakunin. / Thesis (Ph.D.)--Florida Atlantic University, 2011. / Includes bibliography. / Electronic reproduction. Boca Raton, Fla., 200?. Mode of access: World Wide Web.

Visando aprimorar as medidas de distâncias cosmológicas feitas com a observação de supernovas de tipo Ia (SNIas), nós realizamos dois estudos: primeiro, verificamos as relações entre as propriedades das SNIas e de seus ambientes; segundo, avaliamos os dados de supernovas que poderiam ser obtidos com um mapeamento (survey) fotométrico do céu em filtros de banda estreita. Na primeira parte deste trabalho, nós utilizamos dados do Sloan Digital Sky Survey para confirmar as relações publicadas na literatura entre as propriedades das SNIas e a taxa de formação estelar de suas galáxias hospedeiras: SNIas em galáxias passivas tendem a ser mais breves, mais luminosas após correções, e sua cor influencia menos a sua magnitude quando comparadas a SNIas em galáxias ativas. Nós então analisamos SNIas em aglomerados ricos de galáxias e concluímos que elas são ainda mais breves do que a SNIa média em galáxias passivas. Essa característica está relacionada com a idade mais alta das galáxias passivas em ambientes densos e com a preferência dessas galáxias em hospedar SNIas bastante breves. As demais características das SNIas foram consideradas compatíveis com SNIas de campo dentro da precisão alcançada. Esse tipo de estudo deve impactar a determinação de distâncias uma vez que, em diversos casos, SNIas em redshifts diferentes habitam ambientes diferentes. O segundo tema desta tese trata de estudar as características de um novo método fotométrico aplicado às supernovas: o imageamento em filtros de banda estreita. Utilizando simulações de fotometria e tomando como base o projeto Javalambre Physics of the accelerating universe Astrophysical Survey (J-PAS), mostramos que um mapeamento realista em filtros de banda estreita é capaz de produzir amostras massivas de SNIas em redshifts intermediários (0 < z < 0;5) com baixa contaminação (3% ou menos), com um redshift fotométrico altamente preciso (sz =0;005) e com boa precisão na parametrização de suas curvas de luz. Além disso, filtros de banda estreita são capazes de resolver quase todas as características espectrais das supernovas. Junto com a rica informação que esse mapeamento pode prover sobre as galáxias observadas, esse tipo de projeto deve trazer contribuições significativas para o melhor entendimento das supernovas e das relações com seus ambientes. / Aiming at improvements in cosmological distance measurements performed with type Ia supernovae (SNe Ia) observations, we carried out two studies: first, we analyzed the relationships between the properties of SNe Ia and those of their environments; second, we assessed what kind of SNe data could be attained by a photometric survey using narrowband filters. In the first part of this work we used data from the Sloan Digital Sky Survey to confirm previously reported relations between SNe Ia properties and their hosts star formation rate: SNe Ia in passive galaxies tend to be briefer, more luminous after corrections and their colors do not impact their magnitudes as much when compared to SNe Ia in active galaxies. We then analyzed the properties of SNe Ia inside rich galaxy clusters and concluded that they are even briefer than the average SNe Ia in passive galaxies. This characteristic is related to the older age of passive galaxies in dense regions and their preference for hosting very brief SNe Ia. Other properties of these SNe Ia were considered compatible under the precision attained. This kind of study may influence distance determinations since, in many cases, SNe Ia at different redshifts populate different environments. On the second part of this work we studied the characteristics of a new photometric method applied to supernovae: the narrowband filter imaging of the sky. Using photometry simulations and taking the Javalambre Physics of the accelerating universe Astrophysical Survey (J-PAS) project as our fiducial survey, we showed that a realistic narrowband survey is capable of producing massive SNe Ia samples at intermediate redshifts (0 < z < 0;5) with low contamination rate (3% or less), a highly precise photometric redshift (sz = 0;005) and a good precision on their light curve parametrization. Moreover, narrowband filters are able to resolve almost every supernova spectral feature. Together with the rich galaxy information this kind of survey can provide, it should present significant contributions to the better understanding of supernovae and their relationships with their environments.

Astrophysical compact objects --- white dwarfs (WDs), neutron stars (NSs), and stellar mass black holes (BHs) --- mark the endpoints of normal stellar evolution. Their birth is often associated with dramatic explosions known as core-collapse supernovae (SNe). Such SNe are archetypal ``transients'' --- astronomical events which produce detectable emission for only a limited period of time (measurable over human timescales). This dissertation investigates the astrophysical implications of the formation and destruction of compact objects with particular focus on the transient phenomena that may be produced in such events. Part I is devoted to the ``death'' of compact objects by their coalescence with a binary companion. Such compact object binaries are driven towards merger by the extraction of orbital energy in the form of gravitational-waves (GW), and are thus prime targets for current and future GW detectors. In the first two chapters of Part I we consider the merger of a WD with a NS companion, beginning with Chapter 2, in which we explore the nuclearly-reactive accretion flow produced in the aftermath of such mergers and the possible `SN-like' transient it may give rise to. We continue in Chapter 3 by proposing that the late-time evolution of this post-merger accretion disk may result in terrestrial planet formation, broadly consistent with the mysterious ``pulsar planets'' observed orbiting PSR B1257+12. We shift our attention in the next couple chapters of this first part of the dissertation to binary NS mergers. In Chapter 4 we address the question of disk formation in the aftermath of the collapse of a rigidly-rotating supramassive NS, which is directly applicable to various models of gamma-ray bursts (GRBs). In Chapter 5 we utilize both GW and electromagnetic signatures of the first observed NS merger GW170817 to place new constraints on the NS equation of state. Finally, in Part II of this dissertation, we explore the connection between transient phenomena ranging from long- and ultra-long- GRBs, to energetic super-luminous SNe (SLSNe) and fast radio bursts (FRB), and relate these to the ``birth'' of a rapidly rotating highly-magnetized NS, a millisecond ``magnetar''. In Chapter 6 we show that both jetted and thermal transients (namely a GRB and a SLSN) can be powered simultaneously by such magnetars, and explore the various observational implications of this connection. We end with Chapter 7 in which we study the photo-ionization of the medium surrounding a newly born magnetar, discussing the observational signatures related to the escape of this ionizing radiation. We additionally address the propagation of radio waves and the dispersion measure induced by such photo-ionization and apply these to show that FRBs are broadly consistent with having young magnetars as their progenitors.

Thesis (Ph. D.)--University of Oregon, 2006. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 113-116). Also available for download via the World Wide Web; free to University of Oregon users.