Photo Credit: Emily BartlettOn Monday, the astronomy community grew as University of Texas graduate student Sean Couch earned his PhD by successfully defending his doctoral thesis. Dr. Couch finished his PhD in just four years, which is a good year faster than the average. As you might guess, speed isn't the important factor, it is the quality of the dissertation and the student. And in both of these cases, Sean is also wildly successful.
I got to know Sean when I taught a course in observing techniques for graduate students three summers ago. Although he is a theorist, Sean dared to take the course, and he managed not to break any telescopes. He even successfully took some images of red supergiant stars and found that they were varying in brightness for reasons that we didn't understand at the time (I've since found that this behavior was known, though not well understood). I later learned that this project, which he developed, was heavily assisted by some of the other graduate students -- Sean's original idea was to look at the star Betelgeuse, which is so bright that it would have burned out our camera, and which is not visible during the summer when we had use of the telescopes. So perhaps it is best that Sean stuck with theory.
Sean's dissertation involves computer simulations of supernovae, or exploding stars. Modeling supernovae on a computer is very difficult. The stars that explode are very large, sometimes as big as Jupiter's orbit, or nearly 200 million miles across. But the initial explosion takes place in the central hundred miles or so of the star, and the nuclear reactions that power the supernova take place on an atomic scale. Even the powerful Texas Ranger supercomputer that Sean used for some of his work cannot precisely simulate all the parts of an exploding star, so the models have to be simplified. As time goes on and computers become more and more capable of performing fast calculations, the models are made more complex.
Sean's work involves adding jets to supernova explosions. By "jets" I mean focused beams of material that come out at the north and south poles of the star. Many astronomers have already studied what happens if you put a jet of material moving at nearly the speed of light into an exploding star: you get a gamma-ray burst. But gamma-ray bursts are rare, and probably only occur in a tiny fraction of exploding stars.
Jets. however, are very common in astronomical objects, and only very rarely are these jets moving near the speed of light. Most often these jets are just squirting material out into space at moderate speeds, maybe many miles a second, but far slower than the speed of light. Jets tend to occur where material is falling inward large distances due to conservation of angular momentum (think a figure skater starting a spin and pulling her arms in to spin faster and faster); as the rate of spinning gets faster and faster, some material is flung outward at high speed. When a supernova starts, the interior of the star collapses and the material making up the star falls inward due to gravity, setting up these exact conditions!
Sean found that, when adding more typical jets to supernova models, the outcome of his models better match what we actually see in supernova explosions than in models without jets. Some of these observations include supernova nebulae that are not perfectly round, and that elements that we think are made deep in the explosion (and so should be found closest to the explosion site) are often seen furthest away from the explosion's center. Jets, which originate deep in the star and are decidedly not round, fit these and other observations.
The one thing which Sean has not been able to do yet is to prove what kind of jet occurs in tthe typical supernova, meaning what physical process actually starts the jet going. Magnetic fields getting twisted up seems like the best bet, but there are other ways of getting material to flow outward. As Sean's advisor, Professor Craig Wheeler, elegantly put it to me last night, "It could be that a Klingon Battle Cruiser that starts [the jet]. But I'd like to think it is a magnetohydrodynamic jet." But it's always good to have a problem to work on.
Next month, Sean and his wife Theresa will be moving to Chicago, where Sean will begin a prestigious postdoctoral fellowship at the University of Chicago, home to a large and vibrant group of supernova theorists. Congratulations, Sean, on successfully completing your PhD, and best wishes in the future!