Supernova Archaeology: Uncovering the origin of Type Ia supernovae
Type Ia supernovae result from the thermonuclear explosion of a white dwarf after it has accumulated sufficient mass from some as-yet-uncertain companion star. Presently, there are two competing models for how this occurs. In the merger scenario, a pair of white dwarfs merge to trigger an explosion. Alternatively, in the standard accretion model, a white dwarf accretes matter stably from a red giant or main sequence star. In the latter scenario, the white dwarf should be a strong source of X-ray and extreme-UV emission for 0.1 to 1 million years prior to explosion. Our team has demonstrated that such powerful ionizing sources would have a considerable impact on their surrounding environment, both on an individual and galactic scale. The absence of any such detection has provided critical insight into which progenitor scenarios are possible, marking a significant step forward in solving this longstanding mystery. (Woods & Gilfanov, 2013, 2014, 2016; Johansson, Woods, et al., 2014, 2016; Woods, Ghavamian, et al., 2017)
Titans of the Early Universe: The most massive stars that ever lived
With the upcoming launch of the James Webb Space Telescope, astronomers are preparing to peer deeper than ever before into the early life of the Universe and the first stars. Among the most unusual denizens of the high-redshift Cosmos are the truly supermassive stars, reaching as much as 100,000 times the mass of our Sun. The conditions necessary for the formation of these behemoths were only possible early in the primordial Universe. Pristine gas clouds, irradiated by adjacent newborn galaxies, could collapse on much faster timescales than feasible in modern star formation, producing the immense accretion rates necessary for such stars to grow to large masses before collapsing. These primordial titans are thought to be the likely progenitors of the most massive high-redshift quasars now observed. However, there remains very little known about how supermassive stars live and die. We are now carrying out a concerted effort to determine the final masses and fates of these astonishing objects as a function of the conditions under which they form. (Woods, Heger, et al., 2017; Haemmerlé, Woods, et al., in press).
Dancing with the Stars: The lives of interacting binaries
Most of the stars in our Galaxy are gravitationally bound to another star in a system known as a “binary”. In many of these binaries, the stars are close enough to each other that they will interact at some point during their evolution. One of the most dramatic such events is the “common envelope” phase, which occurs when a red giant begins losing mass in an unstable manner to some binary companion. This leads to a runaway where the core of the giant and the companion star are engulfed in a common envelope of matter, which is expelled only after the two have spiraled-in to a much closer orbit. These astounding transformations are essential to the formation of a vast menagerie of exotic and extremely energetic objects in the Galaxy, including accreting compact objects like white dwarfs, neutron stars, and black holes. Some of my early work centred around improving our understanding of when and how a common envelope phase begins, and how compact binaries form (Woods & Ivanova, 2011; Woods, Ivanova, et al., 2012). Together with my former student, Dr. Hai-liang Chen, we have thoroughly investigated how populations of accreting white dwarf binaries form, contribute to the UV background in some galaxies, and produce novae and supernovae (Chen, Woods, et al., 2014, 2015, 2016).