A Comprehensive Study of Supernovae Modeling

The evolution of massive stars, as well as their endpoints as supernovae (SNe), is important both in astrophysics and cosmology. While tremendous progress towards an understanding of SNe has been made, there are still many unanswered questions. The goal of this thesis is to study the evolution of massive stars, both before and after explosion. In the case of SNe, we synthesize supernova light curves and spectra by relaxing two assumptions made in previous investigations with the the radiative transfer code CMFGEN, and explore the effects of these two assumptions. Previous studies with CMFGEN assumed γ-rays from radioactive decay deposit all energy into heating. However, some of the energy excites and ionizes the medium. A new solver is developed to include these non-thermal excitation and ionization processes. Non-thermal excitation and ionization are crucial for forming some lines, especially Hα in the nebular phase. To investigate non-thermal effects, a comparison is made between models with, and without, the non-thermal solver. Benchmarking the solver is done by comparing the non-thermal models with observations of SN 1987A. Satisfactory agreement is achieved and possible problems are discussed. With the new solver, future studies will shed light on the mixing of material between layers of different composition in supernova explosions and put further constraints on supernova explosion models. Hubble expansion is a good approximation for most types of SNe, except Type II-P. Red supergiants are widely accepted to be the progenitors of Type II-P SNe and they have radii of hundreds to thousands of times larger than that of the Sun. Type II-P SNe “memorize” their large radii at the time of explosion for several weeks and material is still being accelerated. A time-dependent fully relativistic solver is developed to handle such cases

Our Galaxy's youngest disc

We investigate the structure of our Galaxy's young stellar disc by fitting the distribution functions (DFs) of a new family to five-dimensional Gaia data for a sample of $47\,000$ OB stars. Tests of the fitting procedure show that the young disc's DF would be strongly constrained by Gaia data if the distribution of Galactic dust were accurately known. The DF that best fits the real data accurately predicts the kinematics of stars at their observed locations, but it predicts the spatial distribution of stars poorly, almost certainly on account of errors in the best-available dust map. We argue that dust models could be greatly improved by modifying the dust model until the spatial distribution of stars predicted by a DF agreed with the data. The surface density of OB stars is predicted to peak at R\simeq5.5\mbox{kpc}, slightly outside the reported peak in the surface density of molecular gas; we suggest that the latter radius may have been under-estimated through the use of poor kinematic distances. The velocity distributions predicted by the best-fit DF for stars with measured line-of-sight velocities $v_\parallel$ reveal that the outer disc is disturbed at the level of 10 \mbox{km}~\mbox{s}^{-1} in agreement with earlier studies, and that the measured values of $v_\parallel$ have significant contributions from the orbital velocities of binaries. Hence the outer disc is colder than it is sometimes reported to be.Comment: 18 pages, 20 figures, accepted to publish on MNRA

Thermal analysis of a high-power glow discharge in flowing atmospheric air by combining Rayleigh scattering thermometry and numerical simulation

The thermal state of a glow discharge with intermediate current in flowing atmospheric air is investigated by a combination of Rayleigh scattering thermometry imaging and numerical simulation. Results from the simulation indicate that during the initial breakdown the local translational temperature can reach a huge value (e.g. 6000 K) but decreases quickly due to strong heat transfer to the surrounding cold air. In the gliding stage, the translational temperature of plasma is balanced by the input power density and the heat dissipation rate. As the gas flow rate is increased, the translational temperature in the glow plasma column diminishes. The flow affects the thermal state of plasma from two aspects. First, it promotes elongation of the plasma column to decrease the input power density. Second, the flow enhances local heat dissipation. As a result, the translational temperature is lowered due to flow. Using a two-temperature model, which considers the translational temperature, the vibrational temperature and their transitions, the non-thermal state of plasma is further analyzed. The gas flow is found to reduce the translational temperature and the vibrational-translational relaxation rate, and thus prevent thermalization of the plasma column

On the nature of supernovae Ib and Ic

Utilizing non-local thermodynamic equilibrium time-dependent radiative-transfer calculations, we investigate the impact of mixing and non-thermal processes associated with radioactive decay on Type IIb/Ib/Ic supernova (SN IIb/Ib/Ic) light curves and spectra. Starting with short-period binary models of ≾5M_⊙ helium-rich stars, originally 18 and 25M_⊙ on the main sequence, we produce 1.2B ejecta which we artificially mix to alter the chemical stratification. While the total ^(56)Ni mass influences the light-curve peak, the spatial distribution of ^(56)Ni, controlled by mixing processes, impacts both the multiband light curves and spectra. With enhanced γ-ray escape. Non-thermal electrons, crucial for the production of He_I lines, deposit a large fraction of their energy as heat, and this fraction approaches 100 per cent under fully ionized conditions. Because energy deposition is generally local well after the light-curve peak, the broad He_I line characteristics of maximum-light SN IIb/Ib spectra require mixing that places ^(56)Ni and helium nuclei to within a γ-ray mean free path. This requirement indicates that SNe IIb and Ib most likely arise from the explosions of stripped-envelope massive stars (main-sequence masses ≾25M_⊙) that have evolved through mass transfer in a binary system, rather than from more massive single Wolf-Rayet stars. In contrast, the lack of He_I lines in SNe Ic may result from a variety of causes: a genuine helium deficiency; strongly asymmetric mixing; weak mixing; or a more massive, perhaps single, progenitor characterized by a larger oxygen-rich core. Helium deficiency is not a prerequisite for SNe Ic. Our models, subject to different mixing magnitudes, can produce a variety of SN types, including IIb, IIc, Ib and Ic. As it is poorly constrained by explosion models, mixing challenges our ability to infer the progenitor and explosion properties of SNe IIb/Ib/Ic