A still from a conventional supernova simulation depicts the rebounding shockwave (visible as the front between inwards-falling outer gases and outwards-blowing stellar materials). Lighter colors indicate regions of greater density. (Image courtesy of Tony Li)
A cataclysmic explosion that lights up an entire galaxy is hard enough to fit in your mind's eye, much less a computer.
As a summer research student in the SULI program at Stanford Linear Accelerator Center, Tony Li has learned that, despite 40 years of development, simulations of stars which are nearing the requisite conditions for a supernova have failed to undergo the critical transformation. A supernova is the intensely brilliant explosion of a dying star's matter bursting through its own shock front. Working with astrophysicist Shizuka Akiyama, Li hopes to make progress in this direction with a model that, unlike previous ones, operates three-dimensionally and grapples with the effects of a very strong magnetic field on core collapse.
Extremely massive stars at the end of their lifetime undergo gravitational implosion when they've run out of fuel to burn. Deprived of the outward pressure formerly supplied by the fusion process, the star's iron core begins to disassociate into subatomic particles in an effort to compress as much as possible. At some point, the star succumbs to a gravitational free-fall that involves the entire star, converting virtually all of its potential energy to outrushing neutrinos. Collapsing outer materials and plasma collide with the star's core, transferring much energy to reactions inside the core, and the rest to a rebounding shockwave that, though dampened, begins to push against the star's gaseous envelope.
Recent consensus has emerged that this is the true precipice of a supernova. Thermal pressure produced by the neutrinos is supposed to cook the stellar bubble until it explodes. But in current simulations of large stars, no explosion occurs, and the shockwave merely sits inside the shell.
Because scientists can't create their own experiments with stars in a laboratory, astrophysicists rely heavily on computer models to study the respective contributions of parameters in a given physical system.
Equations representing the collected body of physical and astronomical knowledge form a code that, when fed with initial conditions, runs the system and produces profiles for components like temperature, density, and velocity of outwards-blowing stellar matter.
The computer program has limits, however. As Li says, "The catch with our code is that we don't have every single detail of physics in it." Supernova science is highly complex, pooling cosmic-scale physics of gravity with the particle level and calling for equations of state that no one understands for the extreme density and temperature conditions involved.
Only recently has computing power advanced to include 3-dimensional grids, a capacity crucial for simulating the inherently multidimensional effects of magnetic fields. Previously, scientists used 2-D models and were obliged to make assumptions about the symmetry of the core's rotation.
Now that computing power has reached this stage, attention in the field is expanding beyond the conventional theory of neutrino heating and looking to other sources that could provide the missing energy.
Akiyama believes that a magnetic field effect within the neutron star could be factor that revives the retarding shockwave. Magnetic tension caused by differential rotation causes magnetic fluid inside the iron core to fluctuate chaotically, creating flashes of spectacular field strength where field lines are briefly closer. The leap in magnetic force in these areas would produce an added kick to the rebounding stellar plasma, because plasma has a magnetic component. Because the instability serves to dramatically amplify existing magnetic field strength, its addition to the theory means that a huge amount of energy could be created from initially weak fields.
Currently writing and modifying code for a 3-D simulation, Li aims to fulfill one step in the larger picture of Akiyama's research. A battery of tests will narrow the models down to those most susceptible to the magnetic field effect. The future will see extensions of this program, and a better understanding of how a very powerful magnetic field contributes to not just supernovae, but even more energetic products of core collapse, such as gamma ray bursts and magnetars.
