California Institute of Technology
A joke Io Kleiser tells about her research is “if it doesn’t explode, I’m not interested.” The line conveys her fascination with supernovae, the cosmic light shows of detonating stars.
The universe’s unfathomable distance and time scales first attracted Kleiser, a doctoral candidate at the California Institute of Technology, to astrophysics. She turned to supernovae because of their variety and immediacy in a field that measures time in billions of years. When astronomers spot an exploding star, “the entire community lights up. Everyone is pointing their telescopes at the same place. There’s this urgency.” Meanwhile, “on the theoretical and modeling side you have people saying, ‘here’s the data that just came in. We need to come up with a model that explains it.’”
Kleiser, a Department of Energy National Nuclear Security Administration Stewardship Science Graduate Fellowship (DOE NNSA SSGF) recipient, is one of those people. Her simulations explore supernovae as labs for high-energy and exotic physics and calculate how a star’s development affects the supernova that ends its life. Kleiser especially examines how mass the star loses, perhaps to a companion star, may affect the resulting explosion. Such mass losses typically sap lighter elements like hydrogen as the star expands.
In her research with Sterl Phinney at Caltech and Dan Kasen at the University of California, Berkeley, Kleiser models stars that have lost all their outer hydrogen to see if some of them lead to supernovae that fade within weeks of the explosion. “That’s the main thrust of my thesis: trying to figure out what scenarios can give rise to supernovae that look like this because they don’t behave the way we would normally expect.”
To capture these exotic astronomical events, Kleiser uses two computer codes: MESA, which evolves the stars up to the ends of their lives, and SEDONA, which calculates radiation transport through matter flying off the supernova and predicts what astronomers should see from the explosions. Kleiser herself wrote a hydrodynamics code that makes stars modeled with MESA explode and connects it to SEDONA. “That was probably my favorite part of this whole project,” she adds.
New, intense telescope surveys are discovering more of the fading supernovae Kleiser models, making her predictions relevant to current work. With multiple model runs, she and her colleagues have been able to predict how luminous and long-lasting a supernova should be under a particular set of circumstances, such as explosion energy and star radius. The results could help observers, theorists and modelers connect examples of this new supernovae class to physical conditions that may have produced them. “That’s really exciting, and new lines of evidence have supported the claims we have made about these (phenomena).” Some of her calculations have run on supercomputers at the DOE National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory.
Kleiser’s 2015 Lawrence Livermore National Laboratory (LLNL) practicum dealt with earthly detonations: inertial confinement fusion (ICF) experiments at the National Ignition Facility. NIF focuses powerful laser beams on the interior of a gold capsule called a hohlraum, generating an X-ray bath that implodes a BB-sized capsule containing hydrogen isotopes. If all goes well, the compressed hydrogen nuclei fuse, producing energy.
For some low-compression tests, researchers add a dose of xenon gas to the hydrogen, using it something like a tracer to help calculate temperatures and pressures inside the imploding capsule. But the xenon also can affect nuclear reaction readings. Working with LLNL physicist (and DOE NNSA SSGF alumna) Laura Berzak Hopkins, Kleiser used the lab’s HYDRA radiation hydrodynamics code to simulate capsule implosions with varying xenon levels to measure how the gas affects early compression.
The models showed something researchers had suspected: neutron production (a fusion marker) peaks twice, first from the shock of the X-ray pulse and then from compression of the hydrogen fuel. Kleiser connected characteristics of these two peaks to xenon content and the density of gas in the capsule. The double peak probably would be present without xenon, but Kleiser suspects it affects the relative heights of the spikes and the time between them.
Kleiser expects to graduate in spring 2018. She’s ruled out working in academia but finds the remaining options a bit overwhelming. “There are so many different things I could do that are exciting. It’s a little trickier than the last transition” to graduate school.
Image caption: This rendering, done by one of Io Kleiser’s colleagues, shows a simulation of the entropy of a differentially rotating and highly magnetized supernova progenitor in three dimensions. Red colors indicate high entropy (hot) material; blue represents low entropy (cold) material. Strongly magnetized material is continuously launched from the surface of the proto-neutron star in the center but is severely distorted such that, instead of a clean jet observed in the ultra-strong magnetic field case, two giant polar lobes are formed. Credit: Philipp Moesta, TAPIR, California Institute of Technology.