Refreshments served at 3:00 pm in the 3rd Floor Atrium
TAP Colloquium
Lucy McNeill, Kyoto University
Title: Some New Ideas in Stellar Explosions
Abstract:
White dwarf stars are the most common remnant of stars after they die. Unlike our Sun, most stars are born orbiting with a companion star. in a binary White dwarfs in binary star systems can explode as Type Ia supernova explosions, which are frequently observed in distant Galaxies (Nobel Prize in Physics 2011).
In the past few years, the Zwicky Transient Facility (ZTF) has detected dozens of nearby double white dwarf binaries which should merge in the next thousands of years. To our surprise, all of these white dwarfs are hot and large. In this talk I will summarise our recently published work (McNeill and Hirai 2025). Inspired by the success of tidal heating theory in explaining temperatures of hot-Jupiter exoplanets, we generalised the theory of tidal heating to white dwarf stars. Tidal forces inevitably heat up, and inflate white dwarfs in short period binaries (orbiting faster than ~ once per hour), consistent with the emerging observational context from ZTF. This restricts the kinds of binary star systems which make Type Ia supernovae, disfavouring the double degenerate scenario for type Ia supernovae progenitors.
Time permitting, I will also introduce some recent work related to exploding massive stars. While these rare stars only make up ~1% of the population, their terminal explosions at the collapse of their iron cores power the most energetic explosions in the Universe. For example, the core collapse of rapidly rotating massive stars are thought to produce the long gamma ray bursts which accompany Type Ic supernova explosions. I will present a new 3D simulation of a rapidly rotating Wolf-Rayet star during the final oxygen (O) shell burning phase. Rather than well-defined rigidly rotating convective regions, specific angular momentum prefers to be mixed across the whole star. This dynamical state should be realised for any massive star which undergoes a convective “shell merger”, which are thought to occur between O/Ne/C shells in at least 40% of core-collapse progenitors. I will show that pre-collapse shell mergers can provide initial conditions required for the most energetic magnetar-driven explosions ~10^52 erg, currently unattainable in 3D core-collapse explosion simulations.
Figure caption: Birds-eye view of the rotational velocity (vz) in a rapidly rotating Wolf-Rayet star's inner carbon-oxygen core region. Rotation is around the z-axis, and the 3D stellar model is sliced through the equator in the x-y plane.
Left: Rotational velocity (vz) for a 1D stellar evolution model of a rapidly rotating massive Wolf Rayet star at core collapse. The inner convective O shell is rotating rigidly (constant angular velocity)
Right: Rotational velocity for the same stellar model, simulated in 3D. For the massive star's preferred (equilibrium) rotation, inner regions are sped up compared to the 1D model. The whole CO core region is mixed in specific angular momentum.
Bio: Lucy McNeil is currently a Research Assistant Professor in the Department of Astronomy and the Hakubi Center for Advanced Research, at Kyoto University. Before that, she was a postdoctoral researcher at RIKEN's Interdisciplinary Theoretical and Mathematical Sciences program, where she is a Visiting Scientist and facilitator of the "Asymptotics in Astrophysics" working group. Previously, she held a JSPS fellowship in the Department of Physics at Kyoto University, and was also a postdoc at St. Vincent’s Institute of Medical Research in Melbourne.
She is interested in various topics in stellar structure and evolution theory, particularly tidal interactions in stellar binaries. Recently, her group utilized numerical simulations of white dwarf stars to generalize tidal heating in short period (< 1 hour orbital periods) double white dwarf binaries. Space-based gravitational wave detectors will detect ~100,000's of these short period, compact binaries in the Milky Way. However, using the current ZTF sample of ~20 nearby eclipsing white dwarf binaries we propose a generic tidal heating model to explain their surprising hot temperatures.
University press release
(Popular science article
She is also interested in angular momentum transport processes in massive stars, which set the spins of their black hole or neutron star remnants. In particular, she wants to know what are the preferred equilibrium rotation patterns in massive stars that go on to core-collapse. She is simulating hydrodynamics and nuclear burning in the very inner ~Earth sized oxygen core of massive stars. Her group can relate these 3D simulations to core-collapse supernova explosion observations, and also the observed transients associated with the final ~years before core collapse supernovae seen by ZTF. Recently, with theorists in Solar physics they developed a model for the magnetic angular momentum transport in the convective zones of massive stars during the final oxygen shell burning phase. In some cases, the rotational pattern and magnetic field strength and geometry coevolve to spin up the core region which goes on to become a neutron star or black hole.
University press release
(Popular science article
