A Supernova Surprise
Strange explosions in space keep Mansi Kasliwal (MS ’07) awake at night. As a graduate student at Caltech, Kasliwal has been pointing a half dozen telescopes at the sky to catch these cosmic blasts—fleeting flashes of light that last from a few days to a few months. She’s a member of the Palomar Transient Factory (PTF), a project whose automated telescopes on Mount Palomar continuously scan the heavens, looking for bright spots that weren’t there just a day or two before.
Many of these fleeting flashes belong to a class of explosion called a Type II supernova, something that forms when a dying star spends the last of its fuel and collapses. Another sort of explosion, known as a Type Ia supernova, happens in a binary system consisting of a bloated red-giant star and its white-dwarf partner. About the same size as Earth, a white dwarf is a low-mass star at its last life stage. As the red giant, which has health issues of its own, sloughs off its outer layers, some of the matter accumulates on the white dwarf’s surface. Under the right conditions, a nuclear explosion can ignite. A third class, the classical nova, is a calmer version of a Type Ia supernova that’s 1,000 times fainter.
Astronomers classify supernovae by a dozen or so characteristics: their maximum brightness, the time it takes for them to dim, and the wiggles and bumps found in their spectra—which are like supernovae fingerprints, revealing the chemical composition of the explosion. Members of the PTF team measured spectra of their finds during follow-up observations using the
Palomar Observatory, the MDM Observatory on Kitt Peak in Arizona, the Gemini telescopes in Hawaii and Chile, and the W. M. Keck Observatory, also in Hawaii.
In just about a year’s worth of observing time, the PTF has found over 1,000 so-called transients. Most are likely to be novae and supernovae—when confirmed, they will account for nearly a fifth of the total number discovered in the past century—and 97 percent of them fit the profiles of one of the three usual suspects. Among the remaining 3 percent are half a dozen explosions that are complete mysteries. A few last for several months, like Type II supernovae do, but they are about 100 times dimmer. Others are as brief as regular novae, but as bright as supernovae. They’re nothing like any cosmic explosions we’ve seen before, so what are they? “There are lots of stories, lots of ideas that people have thought of,” Kasliwal says. But none of these theories can explain everything about each object—except for one.
In February 2010, an amateur astronomer named Douglas Rich discovered a supernova that was quickly confirmed by another amateur, Paul Burke. Soon after, the PTF made the same discovery independently. The object, dubbed PTF10bhp, was particularly short-lived, shining for only about five days before fading away—rivaling only one other supernova as the fastest ever known. It turns out that all of the characteristics of this supernova—such as its peak luminosity, decay time, and spectrum— perfectly match those of a theoretical model devised by astrophysicist Lars Bildsten at UC Santa Barbara.
In this scenario, two white dwarfs zip around each other in a tight orbit, taking less than an hour for each revolution. One white dwarf is composed of helium, while the other is made of carbon and oxygen. Helium from the lower-mass star spills over to the higher-mass one. When all the mass has transferred over, the system can become unstable, and runaway reactions can lead to a thermonuclear explosion. The explosion is called a Type .Ia supernova—pronounced “point one A”—because it’s about one-tenth as bright, lasts one-tenth as long, and has about one-tenth the amount of nickel as a Type Ia supernova.
Bildsten’s model makes very specific predictions about the supernova’s peak luminosity, how long the supernova takes to brighten and decay, and the presence of calcium and titanium lines as seen in its spectrum—the latter a weird feature not observed in ordinary supernovae. One by one, Kasliwal was able to check each prediction off the list, as PTF10bhp satisfied every requirement from the model. More work will be needed to confirm that PTF10bhp is indeed a Type .Ia, but such a perfect fit between observation and theory is a reason to rejoice. “It’s very interesting because there have been very few theoretical models for these things,” she says. “It’s certainly very rare, and certainly exciting.”
Kasliwal has discovered some other oddballs, as well. For example, she’s found two supernovae about 130,000 light-years away from their host galaxies—farther than the diameter of the Milky Way. These supernovae appear to be massive stars, and their shorter lifetimes are insufficient for them to have made the long voyages from their host galaxies. They must have formed near their current locations, out in the galactic boonies surrounded by nary a wisp of gas or dust—perhaps similar to the conditions in which the first stars in the universe were born. Studying these supernovae, then, is a way to understand the evolution of the very first stars.
“I really did not think before I came to Caltech that I could actually be a part of a project where you start with some crazy brainstorm and see it happen,” Kasliwal says. “Mother Nature’s revealing these rare opportunities to teach you about new physics you could never have dreamed of. There’s so much territory that’s completely unexplored. Nature always catches you by surprise—it’s fantastic.”
Kasliwal will finish her PhD in June. Having been awarded a prestigious Hubble Fellowship and a Carnegie-Princeton Fellowship, she will begin her postdoctoral work this fall at the Carnegie Institution for Science, just up the street from Caltech. —MW