Experiment Observes Elusive Neutrino Transformation

Finding could help explain why the universe has more matter than antimatter

PASADENA, Calif.—An international team of physicists—including several from the California Institute of Technology (Caltech)—has detected and measured, for the first time, a transformation of one particular type of neutrino into another type. The finding, physicists say, may help solve some of the biggest mysteries about the universe, such as why the universe contains more matter than antimatter—a phenomenon that explains why stars, planets, and people exist at all.

The results, released online on March 8, come from the Daya Bay Reactor Neutrino Experiment, which consists of six 20-ton neutrino detectors lying beneath the mountains of southern China near Hong Kong. The paper in which the team reports its data has been submitted to the journal Physical Review Letters.

"Physicists working on five experiments around the world have been racing to measure this process," says Robert McKeown, professor of physics and leader of the Caltech team involved with the project. "Our precise measurement from the Daya Bay Experiment now provides the final clue in helping us understand neutrino transformations."

Neutrinos are fundamental, uncharged particles that zip through space at near-light speed, barely interacting with any other particles. In fact, billions of neutrinos are streaming through your body at this very second.

Neutrinos come in three types (or "flavors")—electron, muon, and tau—and can transform from one type to another via a process that is described by variables called mixing angles. There are three mixing angles, two of which have been previously measured; McKeown was part of the KamLAND experiment in Japan that helped determine the second of these mixing angles several years ago. But an accurate measurement of the third, called θ13 ("theta one three"), which describes how an electron neutrino transforms into the other flavors, has eluded physicists. Thanks to the Daya Bay Experiment, physicists have finally pinned down a precise number to describe the transformation.

Having measured all three mixing angles, physicists can now pursue the next set of ambitious experiments to study what is called CP violation, or charge-conjugation and parity violation, says McKeown. If CP violation is true, then particle reactions can occur at rates that differ from those of reactions involving the particles' antimatter counterparts.

In theory, the Big Bang should have produced equal amounts of matter and antimatter, with collisions between the two subsequently annihilating both. Had that been the case, however, there would be no stars, planets, people, or anything else made of matter. But CP violation, the thought goes, enabled the universe to have more matter than antimatter.

The Daya Bay Reactor Neutrino Experiment's six liquid-filled cylinders detect antineutrinos—the antimatter partner of the neutrino—produced by nuclear reactors in the nearby China Guangdong Nuclear Power Group. Three neutrino detectors sit about 400 meters (about a quarter of a mile) from the nuclear reactors, while the other three are located about 1700 meters (just over a mile) away.

The nuclear reactions that occur inside the energy-producing reactors produce electron antineutrinos, which can be observed by both sets of detectors. The far set of detectors measure fewer electron antineutrinos than expected because a fraction of the electron antineutrinos transform into muon and tau antineutrinos in mid-flight. The detectors cannot directly observe these muon or tau antineutrinos, but by measuring the fraction of "missing" electron antineutrinos, researchers can determine the θ13 mixing angle. In their experiments, the physicists found that the far set of detectors observed 6 percent fewer electron antineutrinos than expected, giving them the information needed to precisely calculate the value of θ13—which turned out to be 8.8 degrees.

McKeown and the Caltech group designed and built the calibration devices (three for each detector) that enabled their colleagues to understand how well the detectors would work and other crucial properties of the instruments.

The other Caltech members of the Daya Bay Collaboration are staff scientist Robert Carr, senior postdoctoral scholar in physics Dan Dwyer, Robert A. Millikan Postdoctoral Scholar in Experimental Physics Xin Qian, graduate student Hei Man (Raymond) Tsang, and postdoctoral scholar in physics Fenfang Wu. Funding for the Caltech team was provided by the National Science Foundation. The Daya Bay Collaboration involves nearly 300 researchers from 38 institutions around the world, with major contributions from—in addition to Caltech—China's Institute of High Energy Physics, Lawrence Berkeley National Laboratory, Brookhaven National Laboratory, and the University of Wisconsin, Madison.

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Vladimir Markovic Receives Clay Research Award

Caltech's Vladimir Markovic, professor of mathematics, has been chosen to receive the 2012 Clay Research Award from the Clay Mathematics Institute. 

He shares the prize with Brown University's Jeremy Kahn, who was previously an assistant professor of mathematics at Caltech. They were honored for their work in hyperbolic geometry.

At this year's Clay Research Conference in June at Oxford University, the institute will formally present Kahn and Markovic with the award, and both will present talks on their work.

For more information on the Clay Research Award, go to http://www.claymath.org.

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A Powerful New Astronomy Instrument Gets Ready for Hawaii

It's time to go to Hawaii—at least, if you're MOSFIRE, a new near-infrared spectrometer that's now on its way to the W. M. Keck Observatory, atop Mauna Kea. But the powerful new instrument, six feet in diameter, about a dozen feet in length, and weighing in at 4,500 pounds—10,000 if you include the mount and packing crate—isn't going there to surf. MOSFIRE, which stands for multi-object spectrometer for infrared exploration, will be the newest weapon in the Keck's arsenal to survey the cosmos, helping astronomers learn about star formation, galaxy formation, and the early universe. 

It's taken seven years to design and build MOSFIRE, which has made its home in the synchrotron high-bay facility at the southern edge of campus. Today, it's scheduled to leave Caltech for good and head to San Diego. There, it will board a boat on February 8 and cruise to Hilo, where it will be trucked up almost 14,000 feet to the peak of Mauna Kea.

MOSFIRE will take spectra—the chemical signatures of everything from stars to galaxies—at near-infrared wavelengths (0.97-2.45 microns, or millionths of a meter), which are just a bit longer than the light our eyes can see. Observing in the infrared allows researchers to probe stars and galaxies that are obscured in clouds of dust, as well as the most distant objects whose spectra have been shifted beyond optical wavelengths by the expansion of the universe. What sets MOSFIRE apart from other instruments is its superior sensitivity and ability to survey up to 46 objects at the same time. 

"There are only two or three instruments in the world that do something similar, but none are as sensitive and none are on telescopes as big as Keck's," says Chuck Steidel, the Lee A. DuBridge Professor of Astronomy at Caltech, and MOSFIRE's co-principal investigator. Steidel anticipates that MOSFIRE will be one of the Keck's workhorse instruments, used for about half of all telescope time. "It's opening up a whole new area of study," he says.

The instrument can scan the sky with a 6.1 arc minute field of view, which is about 20 percent of a full moon and nearly 100 times bigger than the Keck's current near-infrared camera. To take spectra of multiple objects, the state-of-the-art spectrometer consists of 46 pairs of sliding bars that open and close like curtains. Aligned in rows, each pair of bars blocks most of the sky, leaving a small slit between the bars that allows a sliver of light from the targeted object to leak through. Light from each slit then enters the spectrometer, which breaks down the object's light into its spectrum of wavelengths.

With its multiple-object capability, the new tool will make research in near-infrared spectroscopy many times more efficient than before. "I reckon that MOSFIRE will observe very faint targets more than a hundred times faster than has ever been possible," says Steidel, who does research on galaxy formation and observational cosmology. "All the observations that my group and I have done in near-infrared spectroscopy with Keck over the last ten years could be done in just one night with MOSFIRE."

Because everything that's even somewhat warm radiates in the infrared, all infrared instruments must be kept cold to prevent any trace of heat from the ground, the telescope, or the instrument itself from messing up the signal you're trying to detect. MOSFIRE is kept at a cool 120 kelvins (about -243 degrees Fahrenheit or -153 degrees Celsius), and will be the largest cryogenic instrument on the Keck telescopes.

Astronomers will use MOSFIRE to study the epoch of galaxy formation, as well as the so-called period of reionization, when the universe was just a half-billion to a billion years old. During this time, galaxies and quasars—objects consisting of huge black holes that spew enormous amounts of energy as they consume gas and dust—first turned on, shining brightly and ionizing the neutral gas between galaxies for the first time since the universe was only about 380,000 years old. The instrument will also be used to investigate nearby stars, young stars, how stars formed, and even brown dwarfs, which are stars not quite massive enough for nuclear fusion to ignite in their cores.

MOSFIRE will also allow astronomers to do riskier—but more scientifically rewarding—research, Steidel says. Taking the spectrum of single a star or galaxy involves precious telescope time and resources. But because MOSFIRE can observe many objects at once, astronomers can afford to take extremely long exposures. Otherwise, such long exposures of single targets would be too costly.

"I'm definitely ready to do science," says Steidel, whose graduate students have planned their PhD theses around MOSFIRE. "I learned a huge amount working on the instrument, and it has been a lot of fun, but my eye has always been on what we can do with it." No one's sure what astronomers will discover, but therein lies the excitement, he says. "Sometimes serendipity is the most interesting thing that happens."

In addition to Steidel, MOSFIRE is led by co-principal investigator Ian McLean of UCLA. Caltech's Keith Matthews, who has built two previous Keck instruments, plays a leading role as chief instrument scientist. The team includes the engineering and technical staff of the Caltech Optical Observatories, the technical staff of the UCLA Infrared Lab, staff from the W. M. Keck Observatory, and optical designer Harland Epps of UC Santa Cruz. MOSFIRE was supported by the National Science Foundation and a gift from Gordon and Betty Moore.  

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Radio Stars

Caltech's newest astronomy professor searches for cosmic radio waves

Growing up in rural northwest Ireland, beyond the reach of city lights, Gregg Hallinan fell in love with the night sky. "When you didn't have bad weather, and you didn't have clouds, the skies were nothing short of spectacular," he says. "From a young age, I was obsessed with astronomy—it's all I cared for. My parents got me a telescope when I was seven or eight, and from then on, that was it."

Now, Hallinan has brought his celestial obsession to Caltech as a new assistant professor of astronomy. He explores magnetic activity around stars, planets, and those in-between objects called brown dwarfs, which are balls of gas that just aren't big enough for nuclear fusion to ignite and turn them into stars. "I consider myself incredibly lucky that I can take my passion and hobby and have it as a career," he says.

Magnetic fields play important roles throughout the universe. In our own neighborhood, the sun's magnetic field causes all the features we see on its surface, like sunspots, solar flares, and long arcs of plasma called prominences. Earth's magnetic field forms a huge bubble that shields the planet from solar wind, which contains energetic particles that can strip the Earth of its ozone layer, our protection from harmful ultraviolet radiation. On Earth and other planets, like Jupiter, magnetic fields accelerate charged particles and slam them into the magnetic poles, creating light shows we know as the northern and southern lights.

Around stars, planets, and other astronomical objects, the accelerating particles also produce radio signals that travel across space. Hallinan's most surprising and important discovery yet was one he made as a graduate student at the National University of Ireland, Galway, when he found that brown dwarfs generate regular pulses of radio emissions—a feature more commonly associated with planets like Jupiter.

After receiving his PhD in 2008, he stayed at Galway as a postdoc for two years to follow up on his thesis work. He then spent five months at the National Radio Astronomical Observatory in Socorro, New Mexico, and a year at the University of California, Berkeley, before coming to Caltech.

Studying magnetic fields and the fleeting blips of radio emission they produce has led Hallinan to become increasingly interested in those signals, which are examples of a broader type of phenomenon called radio transients. All kinds of cosmic phenomena can generate these variable radio signals, such as exploding stars, mysterious blasts called gamma ray bursts, and stars being ripped apart when venturing too close to a black hole.

One of the most exciting sources of radio transients are planets around other stars, Hallinan says, and hunting for radio-emitting planets will be a major focus of his research. Radio pulses from an exoplanet would indicate the presence of a magnetic field, and since Earth's protective magnetic field may have been crucial for allowing life to evolve, radio activity from an exoplanet could be a signature for a habitable planet. "Looking very long term, when we're characterizing habitable planets, magnetic fields could be important constraints for trying to figure out if there's life on those planets," Hallinan says. But so far, he adds, no one has detected radio emission from any exoplanet. "I'm trying to be one of the pioneers in trying to detect that radiation."

"The radio transient sky is virtually unexplored," he says. "We know there's stuff happening out there, but we haven't yet got the technology to systematically search for those transients." But Hallinan is working to change that, helping to lead exactly such a search for radio emission from exoplanets and other kinds of radio transients. In particular, he's enlisting the most powerful radio telescope in the world to help with the search: the Jansky Very Large Array in New Mexico. He's also working to bring a radio-transient monitoring project to Caltech's Owens Valley Radio Observatory, east of the Sierra Nevada. And with other new radio telescopes coming on line—such as the Low Frequency Array in Europe and the Long Wavelength Array in New Mexico—discoveries are on their way, Hallinan says. That especially includes exoplanets. "We're very hopeful that we'll find radio emission from other planets in the next few years."

Caltech is already the home of the Palomar Transient Factory, a project led by professor of astronomy and planetary science Shri Kulkarni that surveys the skies for flashes of light in visible wavelengths, instead of radio. "Caltech is pretty much unparalleled in the study of transient science," Hallinan says, and a radio-transient project will expand on Caltech's expertise. "The most exciting thing about radio transient work is that we don't know what's out there. You're at the cutting edge."

Aside from astronomy, he's a big fan of mixed martial arts; back in Ireland, he was a karate instructor. But these days, Hallinan is focused on the cosmos, a passion first kindled on those clear winter nights above the Irish countryside.  

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Caltech's George Helou Honored by Home Country of Lebanon

George Helou, senior research associate in physics at Caltech, has received numerous honors over the past year from his home country of Lebanon in recognition of his work in astronomy. "It is gratifying to receive these accolades from my country of origin, as an indication of the value they attach to science and education," says Helou, who is also executive director of the Infrared Processing and Analysis Center (IPAC), deputy director of the Spitzer Science Center, and director of the Herschel Science Center. For the full story on his recent honors, which include election to the Lebanese Academy of Sciences, click here

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Michael Aschbacher Wins Wolf Prize in Mathematics

Michael Aschbacher, the Shaler Arthur Hanisch Professor of Mathematics, will share the 2012 Wolf Prize in mathematics. The award recognizes his role in classifying types of mathematical objects called finite simple groups. According to the prize citation, "His impact on the theory of finite groups is extraordinary in its breadth, depth, and beauty."

"The classification of finite simple groups is one of the crowning achievements of modern mathematics," says Hirosi Ooguri, the Fred Kavli Professor of Theoretical Physics and Mathematics at Caltech. "It's wonderful that Michael is recognized as the principal architect of this work."

Aschbacher will share the prize, which includes $100,000, with Luis Caffarelli at the University of Texas, Austin, who was recognized for work on partial differential equations. They will receive the award from Israeli President Shimon Peres at a ceremony on May 13 at the Knesset in Jerusalem.

"Receiving an award such as the Wolf Prize is of course personally very satisfying," Aschbacher says. "The finite simple groups are the building blocks of finite group theory, playing a role somewhat analogous to that of prime numbers in arithmetic. As a result, the classification theorem is not only a beautiful and natural result, but it's also very useful."

Since 1978, The Wolf Prize is awarded annually in the fields of agriculture, chemistry, mathematics, medicine, physics, and the arts. Among this year's winners is singer Placido Domingo. Past winners have included notable names such as Stephen Hawking in physics, violinist Isaac Stern and architect Frank Gehry in the arts. Previous winners from Caltech include Harry Gray, Ahmed Zewail, and Rudy Marcus in chemistry; Alexander Varshavsky, and the late Seymour Benzer, Edward Lewis, and Roger Sperry in medicine. 

Aschbacher has recently garnered several awards for his work on finite simple groups. He was awarded the 2012 Leroy P. Steele Prize for Mathematical Exposition, and last year, he won the Rolf Schock Prize from the Royal Swedish Academy of Sciences. He also received the Cole Prize in Algebra and is a member of the National Academy of Sciences and the American Academy of Arts and Sciences.

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AAS Honors John Johnson for Observational Astronomical Research

John Johnson, assistant professor of astronomy at Caltech, has been named the recipient of the American Astronomical Society's 2012 Newton Lacy Pierce Prize, which is awarded for outstanding achievement in observational astronomical research based on measurements of radiation from an astronomical object.

According to the award citation, Johnson was selected "for major contributions to understanding fundamental relationships between extrasolar planets and their parent stars."

Johnson's primary research focus is on the detection and characterization of exoplanets. Recently, he led a team of astronomers that discovered the three smallest confirmed planets ever detected outside our solar system.

"I'm proud because this reflects well on my entire team here at Caltech, as well as my collaborators at other institutions," said Johnson. "We put in a lot of hard work and we strive to produce the best, highest impact, and most trustworthy exoplanet science out there. This award is a nice validation that we're on the right track."

As the recipient of the Pierce Prize, Johnson will receive a cash award and has been invited to give the plenary talk at the AAS meeting in Long Beach next year.

"I can remember sitting in a plenary talk as a grad student thinking about how nice it would be to one day do research worthy of such an audience," says Johnson. "It looks like I've arrived."

 

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Astronomers Release Unprecedented Data Set on Celestial Objects that Brighten and Dim

PASADENA, Calif.—Astronomers from the California Institute of Technology (Caltech) and the University of Arizona have released the largest data set ever collected that documents the brightening and dimming of stars and other celestial objects—two hundred million in total.

The night sky is filled with objects like asteroids that dash across the sky and others—like exploding stars and variable stars-that flash, dim, and brighten. Studying such phenomena can help astronomers better understand the evolution of stars, massive black holes in the centers of galaxies, and the structure of the Milky Way. These types of objects were also essential for the recent discovery of dark energy-the mysterious energy that dominates the expansion of the universe—which earned last year's Nobel Prize.

Using the Catalina Real-Time Transient Survey (CRTS), a project led by Caltech, the astronomers systematically scanned the heavens for these dynamic objects, producing an unprecedented data set that will allow scientists worldwide to pursue new research.

"Exploring variable objects and transient phenomena like stellar explosions is one of the most vibrant and growing research areas in astrophysics," says S. George Djorgovski, professor of astronomy at Caltech and principal investigator on the CRTS. "In many cases, this yields unique information needed to understand these objects."

The new data set is based on observations taken with the 0.7-meter telescope on Mt. Bigelow in Arizona. The observations were part of the Catalina Sky Survey (CSS), a search for Near-Earth Objects (NEOs)—asteroids that may pose a threat to Earth-conducted by astronomers at the University of Arizona. By repeatedly taking pictures of large swaths of the sky and comparing these images to previous ones, the CRTS is able to monitor the brightness of about half a billion objects, allowing it to search for those that dramatically brighten or dim. In this way, the CRTS team identified tens of thousands of variables, maximizing the science that can be gleaned from the original data.

The new data set contains the so-called brightness histories of a total of two hundred million stars and other objects, incorporating over 20 billion independent measurements. "This set of objects is an order of magnitude larger than the largest previously available data sets of their kind," says Andrew Drake, a staff scientist at Caltech and lead author on a poster to be presented at the meeting of the American Astronomical Society in Austin on January 12. "It will enable many interesting studies by the entire astronomical community."

One of the unique features of the survey, Drake says, is that it emphasizes an open-data philosophy. "We discover transient events and publish them electronically in real time, so that anyone can follow them and make additional discoveries," he explains.

"It is a good example of scientific-data sharing and reuse," Djorgovski says. "We hope to set an example of how data-intensive science should be done in the 21st century."

The data set include over a thousand exploding stars called supernovae, including many unusual and novel types, as well as hundreds of so-called cataclysmic variables, which are pairs of stars in which one spills matter onto another, called a white dwarf; tens of thousands of other variable stars; and dwarf novae, which are binary stars that dramatically change in brightness.

"We take hundreds of images every night from each of our telescopes as we search for hazardous asteroids," adds Edward Beshore, principal investigator of the University of Arizona's asteroid-hunting CSS. "As far back as 2005, we were asking if this data could be useful to the community of astronomers. We are delighted that we could forge this partnership. In my estimation, it has been a great success and is a superb example of finding ways to get greater value from taxpayers' investments in basic science."

The team says they soon plan to release additional data taken with a 1.5-meter telescope on Mt. Lemmon in Arizona and a 0.5-meter telescope in Siding Spring in Australia.

In addition to Djorgovski, Drake, and Beshore, the team includes staff scientist Ashish Mahabal, computational scientist Matthew Graham, postdoctoral scholar Ciro Donalek, and research scientist Roy Williams from Caltech. Researchers from other institutions include Steve Larson, Andrea Boattini, Alex Gibbs, Al Grauer, Rik Hill, and Richard Kowalski from the University of Arizona; Mauricio Catelan from Universidad Catholica in Chile; Eric Christensen from the Gemini Observatory in Hawaii; and Jose Prieto from Princeton University. The Caltech research is supported by the National Science Foundation. The work done at the University of Arizona is supported by NASA.

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Aschbacher Receives Steele Prize

Michael Aschbacher, the Shaler Arthur Hanisch Professor of Mathematics, has been awarded the 2012 Leroy P. Steele Prize for Mathematical Exposition by the American Mathematical Society (AMS). Aschbacher, along with coauthors Richard Lyons of Rutgers University, Steve Smith of the University of Illinois at Chicago, and Ronald Solomon of Ohio State University, were recognized for a paper on the classification of certain types of groups, which are fundamental mathematical objects.

"AMS prizes are big honors, and we are proud that Michael has gotten this prize," says Barry Simon, the IBM Professor of Mathematics and Theoretical Physics." Over the last few decades, Aschbacher has played a leading role in the classification of so-called finite simple groups-an achievement that, Simon says, is one of the major mathematical accomplishments of the last 50 years, earning Aschbacher the Rolf Schock Prize from the Royal Swedish Academy of Sciences last year. "The mathematical proof involved in that work is so complicated that even experts in the specialty haven't absorbed it all," Simon says. "What Aschbacher and his coauthors got the Steele Prize for is an exposition for professional mathematicians that's one part of a wider program. Making this material accessible to mathematicians in different fields of mathematics is an important accomplishment."

In addition to the Schock Prize, Aschbacher has also received the Cole Prize in Algebra from AMS, and he is a member of the National Academy of Sciences and the American Academy of Arts and Sciences. The Steele Prize was awarded for the paper titled "The classification of finite simple groups: groups of characteristic 2 type," published in Mathematical Surveys and Monographs, Vol. 172.

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The "Supernova of a Generation" Shows Its Stuff

Astronomers determine how the brightest and closest stellar explosion in 25 years blew up

PASADENA, Calif.—It was the brightest and closest stellar explosion seen from Earth in 25 years, dazzling professional and backyard astronomers alike. Now, thanks to this rare discovery—which some have called the "supernova of a generation"—astronomers have the most detailed picture yet of how this kind of explosion happens. Known as a Type Ia supernova, this type of blast is an essential tool that allows scientists to measure the expansion of the universe and understand the very nature of the cosmos.

"What caused these explosions has divided the astronomical community deeply," says Shri Kulkarni, the John D. and Catherine T. MacArthur Professor of Astronomy and Planetary Sciences. But this new supernova—dubbed SN2011fe—can help astronomers solve this longstanding mystery. "SN2011fe is like the Rosetta Stone of Type Ia supernovae," says Kulkarni, who is also the principal investigator on the Palomar Transient Factory (PTF). Led by the California Institute of Technology (Caltech), the PTF is designed to survey the skies for transient flashes of light that last for a few days or months, such as those emitted by exploding stars.

On August 24, the PTF team discovered the supernova in one of the arms of the Pinwheel Galaxy (also called M101), 21 million light years away. They caught the supernova just 11 hours after it exploded.

"Never before have we seen a stellar thermonuclear explosion so soon after it happened," says Lars Bildsten, professor of theoretical astrophysics at the Kavli Institute for Theoretical Physics at UC Santa Barbara, and member of the PTF team, which described its supernova findings in the December 15 issue of the journal Nature.

The PTF team uses an automated system to search for supernovae, and because they were able to point their telescopes at SN2011fe so quickly after its detonation, the astronomers were able to put together a blow-by-blow analysis of the explosion, determining that the supernova involves a dense, Earth-sized object called a white dwarf and, most likely, a main-sequence star (a star in the main stage of its life).

Scientists have long suspected that Type Ia supernovae involve a binary system of two stars in orbit around each other, with one of those stars being a white dwarf. The white dwarf, which is made out of carbon and oxygen, explodes when matter from its companion star spills over onto its surface. But no one is sure what kind of star the companion is. Scientists have suggested that it's another white dwarf, a main-sequence star, a helium star, or a star in a late life stage that's puffed up into a red giant.

Still, because the explosion always involves a white dwarf, its overall brightness and behavior is relatively predictable, making it a useful tool for measuring distances. Since all Type Ia supernovae produce about the same amount of light, those that appear dimmer must be farther away. In this way, by measuring the brightness of supernovae, astronomers can use them as cosmic meter sticks to determine the size of the universe—and how fast it's expanding. In fact, the work that earned the 2011 Nobel Prize in physics—the discovery that the expansion of the universe is speeding up—was based on observations using Type Ia supernovae.

"This discovery is exciting because the supernova's infancy and proximity allows us to directly see what the progenitor system is," explains Mansi Kasliwal, an astronomer at the Carnegie Institution for Science who is a recent Caltech doctoral graduate and a coauthor on the paper. "We have expected for a while that a Type Ia supernova involves a carbon-oxygen white dwarf, but now we have direct evidence."

In the case of SN2011fe, the researchers were also able to deduce, by process of elimination, that the companion star is most likely a main-sequence star. How do they know?

If the companion was a red giant, the explosion of the white dwarf would send a shock wave through the red giant, heating it. This scenario would have generated several tens of times more light than the astronomers observed. Additionally, it happens that the Hubble Space Telescope took images of the location where SN2011fe lived before it blew up. When the researchers looked at the data, they found no evidence of red giants or helium stars.

If the companion was another white dwarf, the interactions between the companion and the explosion would produce light in the optical and ultraviolet wavelengths. Since none of this sort of radiation was seen coming from SN2011fe, it is less likely that the companion was a white dwarf.

These results—which they describe in a companion paper in the same issue of Nature—along with X-ray and radio observations that also fail to see any evidence for red giants or helium stars, rule those out as the companion. Caltech postdoc Assaf Horesh is the lead author on the paper describing the X-ray and radio data, which will be published in The Astrophysical Journal
 
The astronomers have also observed, in unprecedented detail, the material that's blown off during the explosion. In particular, the team detected oxygen hurtling out from the supernova at speeds of over 20,000 kilometers per second—the first time anyone has seen high-speed oxygen coming from a Type Ia supernova, according to the researchers. "These observations probe the thin, outermost layers of the explosion," Bildsten says. "These are the parts that are moving the fastest, for which we have never been able to see this mix of atomic elements."

Not only was the supernova detected quickly, but the data processing—performed by researchers led by Peter Nugent, staff scientist at Lawrence Berkeley National Laboratory—was also done within hours. The machine-learning algorithms developed by Joshua Bloom, an associate professor at UC Berkeley, also helped make the fast find possible. And because the astronomers caught the blast so soon after it ignited, and because it's so close, the researchers say SN2011fe will become one of the best-studied supernovae ever.

"The rapid discovery and classification of SN2011fe—all on the same night—is a testament to the great teamwork between all the researchers from over a half a dozen institutions," Kulkarni says. "The future looks very bright. Soon we should be finding supernovae at an even younger age and thereby better understand how these explosions happen."

Nugent is the lead author on the Nature paper, which is titled, "Supernova 2011fe from an exploding carbon-oxygen white dwarf star." The lead author on the companion paper, "Exclusion of a luminous red giant as a companion star to the progenitor of supernova SN 2011fe," is Weidong Li of UC Berkeley. The Astrophysical Journal paper is titled, "Early radio and X-ray observations of the youngest nearby type Ia supernova PTF11kly (SN 2011fe)."

The Palomar Transient Factory (PTF) uses the 48-inch Oschin Schmidt telescope and the 60-inch telescope of the Palomar Observatory of Caltech for its observations and is a collaboration between Caltech, Columbia University, Las Cumbres Observatory Global Telescope, Lawrence Berkeley National Laboratory, Oxford University, UC Berkeley, and the Weizmann Institute of Science.

NOTE: Weidong Li died on December 12, just before the publication of these papers.

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