Type Ia supernovae are important, but mysterious cosmological tools. Their standard brightnesses have enabled cosmologists to measure extreme distances and to discover dark energy. However, the nature of their progenitor mechanisms remains elusive, with many competing models offering only partial clues to their origins. Here, type Ia supernova delay times are explored using analytical models. Combined with a new observation technique, this model places new constraints on the characteristic time delay between the formation of stars and the first type Ia supernovae. This derived delay time (500 million years) implies low-mass companions for single degenerate progenitor scenarios.In the latter portions of this dissertation, two progenitor mechanisms are simulated in detail; white dwarf collisions and mergers. From the first of these simulations, it is evident that white dwarf collisions offer a viable and unique pathway to producing type Ia supernovae.Many of the combinations of masses simulated produce sufficient quantities of 56 Ni (up to 0.51 solar masses) to masquerade as normal type Ia supernovae. Other combinations of masses produce 56 Ni yields that span the entire range of supernova brightnesses, from the very dim and underluminous, with 0.14 solar masses, to the over-bright and superluminous, with up to 1.71 solar masses. The 56 Ni yield in the collision simulations depends non-linearly on total system mass, mass ratio, and impact parameter.Using the same numerical tools as in the collisions examination, white dwarf mergers are studied in detail. Nearly all of the simulations produce merger remnants consisting of a cold, degenerate core surrounded by a hot accretion disk. The properties of these disks have strong implications for various viscosity treatments that have attempted to pin down the accretion times. Some mass combinations produce super-Chandrasekhar cores on shorter time scales than viscosity driven accretion. A handful of simulations also exhibit helium detonations on the surface of the primary that bear a resemblance to helium novae.Finally, some of the preliminary groundwork that has been laid for constructing a new numerical tool is discussed. This new tool advances the merger simulations further than any research group has done before, and has the potential to answer some of the lingering questions that the merger study has uncovered. The results of thermal diffusion tests using this tool have a remarkable correspondence to analytical predictions. master's degree in physics, to now completing my PhD dissertation. I was Evan's first graduate student, but you would think he'd been advising students for years, such was his ability to lead me to self-discovery and to intellectual fulfillment. I consider Evan a great friend of mine, and I believe this is among the most important factors that have aided me in my pursuits.

Type Ia supernovae (SNe Ia) play an important role in astrophysics, especially in the study of cosmic evolution. Several progenitor models for SNe Ia have been proposed in the past. In this paper we carry out a detailed study of the He star donor channel, in which a carbon–oxygen white dwarf (CO WD) accretes material from a He main‐sequence star or a He subgiant to increase its mass to the Chandrasekhar mass. Employing Eggleton's stellar evolution code with an optically thick wind assumption, and adopting the prescription of Kato & Hachisu for the mass accumulation efficiency of the He‐shell flashes on to the WDs, we performed binary evolution calculations for about 2600 close WD binary systems. According to these calculations, we mapped out the initial parameters for SNe Ia in the orbital period–secondary mass (log Pi−Mi2) plane for various WD masses from this channel. The study shows that the He star donor channel is noteworthy for producing SNe Ia (∼1.2 × 10−3 yr−1 in our Galaxy), and that the progenitors from this channel may appear as supersoft X‐ray sources. Importantly, this channel can explain SNe Ia with short delay times (≲108 yr), which is consistent with the recent observational implications of young populations of SN Ia progenitors.

Context. The identity of the progenitor systems of Type Ia supernovae (SNe Ia) is still uncertain. In the single-degenerate scenario, the interaction between the supernova blast wave and the outer layers of a main sequence companion star strips off hydrogen-rich material which is then mixed into the ejecta. Strong contamination of the supernova ejecta with stripped material could lead to a conflict with observations of SNe Ia. This constrains the single-degenerate progenitor model. Aims. In this work, our previous simulations based on simplified progenitor donor stars have been updated by adopting more realistic progenitor-system models that result from fully detailed, state-of-the-art binary evolution calculations. Methods. We use Eggleton's stellar evolution code including the optically thick accretion wind model and taking into account the possibility of the effects of accretion disk instabilities to obtain realistic models of companion stars for different progenitor systems. The impact of the supernova blast wave on these companion stars is followed in three-dimensional hydrodynamic simulations employing the smoothed particle hydrodynamics code GADGET3. Results. For a suite of main sequence companions, we find that the mass of the material stripped from the companions range from 0.11 M to 0.18 M . The kick velocity delivered by the impact is between 51 km s −1 and 105 km s −1 . We find that the stripped mass and kick velocity depend on the ratio of the orbital separation to the radius of a companion, a f /R. They can be fitted in good approximation by a power law for a given companion model. However, we do not find a single power law relation holding for different companion models. This implies that the structure of the companion star is also important for the amount of stripped material. Conclusions. With more realistic companion star models than those employed in previous studies, our simulations show that the hydrogen masses stripped from companions are inconsistent with the best observational limits ( 0.01 M ) derived from SN Ia nebular spectra. However, a rigorous forward modeling from the results of impact simulations with radiation transfer is required to reliably predict observable signatures of the stripped hydrogen and to conclusively assess the viability of the considered SN Ia progenitor scenario.