Astrophysics
Shing Chi Leung Research
Supernovae are explosions of stars. The fascinating explosions are one of the most energetic and brightest events in the universe. They reveal to us the interplay of many facets of physics, the gravity, the hydrodynamics, the microphysics of photons, atoms, nuclei and fundamental particles, all in the same system. They involve the most extreme environment in terms of density and temperature, which is hardly reproducible in terrestrial laboratories.
Supernovae are also important players in the universe. Without supernovae, there will be no elements heavier than lithium – the explosion produces and releases these “metals” (in astrophysics elements heavier than Li are metals) to the surrounding. The explosions are the major energy sources for exciting gas flow in the galactic scale. It shapes how next star generations form. Some of their subclasses (the Type Ia supernovae) are universal that can be used as the standard candle – which leads to the discovery of the accelerating expansion of the universe, the evidence of dark energy. Thus, understanding supernova not only opens the door to understand the physics in extreme environments, but also builds the foundation in many other astrophysics fields.
We are in a great epoch of supernova research because the computational power allows us to create realistic explosion simulations. There are high power telescopes to resolve structure or detailed chemical composition. These telescopes and surveys are built for searching these transient objects, the James Webb Space Telescope (JWST), Zwicky Transient Factory (ZTF). A few more exciting telescopes are planned later this decade: The Large Synoptic Space Telescope (LSST), X-Ray Imaging and Spectrographic Mission (XRISM). These newly operating telescope will push the theoretical modeling to the next level. Among all, the JWST and LSST will be the main driving force for finding very early supernovae and a substantial amount of new supernovae (100000+ per year) to help us model supernovae quantitatively. They will unveil the many open questions related to the explosion mechanisms and their consequence.
My research focuses on building the numerical models to explain the observed features in real supernovae, its brightness (luminosity), the chemical signature (or spectra) and their pre-explosion activities. Very often when I encountered unusual objects, they are opportunities for us to re-examine if our understanding in supernova explosion mechanism is comprehensive. In most cases, we need to explore inside the standard parameter space to locate the best models. Occasionally we need to extend the parameter space or re-consider the prescription of current physical models and see how we can improve them to make them more realistic. In some cases, we might even need less standard approach (e.g., effects of dark matter) to help us reconcile certain unusual events.
To do all my research, I developed a pipeline in connecting several state-of-the-art simulations codes which can evolve a star from its birth until its death as a supernova, they contain the stellar evolution code MESA, my hydrodynamics explosion code, radiative transfer code SNEC and the nucleosynthesis code TORCH. Each code is dedicated to a particular phase or physics involved in a supernova explosion. The pipeline allows me to follow the star from its birth (the Zero-age Main-sequence star), over all the stellar evolutionary phases, until the onset of explosion. By plugging in relevant physics, I have modeled the following classes of supernovae (links towards detailed explanation in my research page):
- Type Ia supernova and its variants
- Electron capture supernova
- Collapsar
- Pulsational pair-instability supernova
- Nova
Each class of supernovae is very interesting given the significantly different physics involved, even though they all appear to be fascinating cosmic fireworks. The difference in the involved physics gives rise to the many special facets of supernovae (rich in certain chemical elements, asymmetry of the explosion, very rapid or very slowly evolving luminosity).
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If you are interested in understanding the Universe using computer simulations, joining the research group to extend the functionality of the code, or even just to build some interesting data science projects, let’s chat! Currently the code and data analysis use Fortran and Python to do the job.
