Supernovae are involved in many of the most pressing current questions in astrophysics. In particular, Type Ia supernovae are called on to provide much of the iron in the Universe, and their bright, almost uniform explosions allow the mapping of the Universe''s expansion. In spite of this importance, the progenitor systems of Type Ia supernovae are still not unambiguously identified, and the explosion mechanism itself is poorly understood. But several trends in recent years have brought new hope of progress. On the theoretical front, progenitor systems involving a single degenerate star accreting from a companion have been shown to produce a strong, fast wind for substantial periods of time before the explosion. The effects of this wind on the system''s surroundings, and on the companion star itself, have not yet been well modeled. At the same time, evidence has been mounting that at least a few Type Ia supernovae do involve interaction with circumstellar material, while high-velocity absorption lines are now commonly seen in Type Ia spectra. Light (and infrared) echos have been seen from a few Type Ia systems, indicating in some cases dust quite close to the supernova. Finally, supernova remnants are revealing far more about their birth events than had been thought possible, primarily through X-ray spectroscopy. Evidence of ejecta is clearly seen in significant numbers of remnants with ages up to and beyond 10,000 years. In addition, some supernova remnants, such as Kepler''s, show clear evidence for prior interaction with a dense circumstellar medium.
These tantalizing clues may hold the key to the nature of not just a few anomalous Type Ia supernovae, but of the entire Type Ia phenomenon. In this project, Dr. Reynolds and collaborators will undertake a series of multi-dimensional hydrodynamic simulations designed to connect the progenitor systems, the supernovae themselves, and their remnants. The effect of a fast white-dwarf wind on the companion and on surrounding material will be studied in order to characterize the immediate pre-supernova environment. Model explosions will be set off in the midst of these environments to identify optical, infrared, and radio clues to the interaction with circumstellar material, both to explain observed systems and to interpret upper limits when emission is not seen. The supernovae will be numerically evolved to the age of young supernova remnants such as Kepler, to compare with their structure, and then followed to much larger ages, to compare with several Magellanic Cloud remnants which seemingly require strong circumstellar interaction to explain their X-ray properties.
The results of this work will have broad applicability. The question of the nature of Type Ia supernova origins is connected with some of the most significant questions in astronomy today, with implications for galactic chemical evolution as well as cosmology. This work will allow a much tighter connection to be drawn between supernova remnants, the supernovae that produce them, and the progenitors of those supernovae, to the mutual benefit of all three areas of research. Graduate students, undergraduates, and high-school students will be involved in all aspects of this work. The results of this research will also be used by Dr. Reynolds and collaborators in their continued outreach efforts to groups from amateur astronomers to elementary-school children.
Type Ia supernovae are the thermonuclear explosions of white dwarf stars, either pushed to the brink of instability from matter falling onto them from a close companion normal star, or after merging with another white dwarf star. These supernovae play a central role in many areas of astrophysics. Their uniform brightness allows them to serve as distance markers to chart the expansion of the Universe, and they produce much of the Universe's iron as well as other heavy elements. The two possible origins have considerably different consequences for astrophysics, so clarifying this issue is a high priority. A few recent Type Ia supernovae have shown evidence of interaction with surrounding gas ("circumstellar medium,'' CSM) strongly suggestive of the single-white-dwarf (SD) origin. We undertook a theoretical investigation of the interaction of the white dwarf with CSM, before, during, and long after the explosion, using hydrodynamic computer simulations. First, we found that a wind of material hypothesized to flow from the white dwarf before the explosion would have surprisingly little effect on the normal companion. Quantifying this conclusion has been difficult because of numerical effects. We also calculated possible detectable effects from a shell of CSM around a supernova, causing time-variable features in the visible-light spectrum. Much of our effort has gone into theoretical interpretation of observations of a few young supernova remnants, such as G1.9+0.3 (which we discovered, the remnant of the most recent supernova in the Milky Way), and the remnants of Kepler's supernova of 1604 AD and Tycho's of 1572 AD. We produced detailed 3D simulations of Tycho's supernova remnant, showing that its appearance in X-ray emission could be reproduced by hydrodynamic instabilities without requiring unusual CSM. Most interesting has been the investigation of Kepler's remnant. In a project led by an undergraduate, we applied advanced statistical techniques to distinguish between X-ray emission from CSM and much stronger X-rays from the remnants of the white dwarf. We then ran hydrodynamic simulations showing that the location of observed CSM was consistent with what would result from an explosion into a disk of gas, presumably from the companion. The student presented this work at a meeting of the American Astronomical Society, winning an award for undergraduate research. Other projects, several also involving undergraduates, included modeling of an unusual supernova with CSM, and describing the consequences if both pre-supernova stars have interacting stellar winds. We were also able to show that the oldest historical supernova, RCW 86 (SN 185 AD), took place in a "bubble'' evacuated by the stellar wind of the progenitor, indicating an SD origin. This result received broad publicity in Fall 2011 (see attached image). Our work has led to various papers and talks at international meetings. Beyond our own research specialties, the hydrodynamics code we used for this project is publicly available and we continue to refine it and the descriptions and documentation we provide to users anywhere. Our work has proved to be of considerable interest to the public, and has been featured on NASA public websites and in semi-popular publications such as Science News. We also frequently give talks on supernovae and their remnants in various public forums, such as Science Cafe's and presentations at the North Carolina Museum of Natural Sciences.
