Subaru Telescope Reveals 3D Structure of Supernovae

A research group led by Dr. Masaomi Tanaka (National Astronomical Observatory of Japan), Dr. Koji Kawabata (Hiroshima University), Dr. Takashi Hattori (National Astronomical Observatory of Japan), and Dr. Keiichi Maeda (University of Tokyo, Kavli Institute for the Physics and Mathematics of the Universe) used the Faint Object Camera and Spectrograph (FOCAS) on the Subaru Telescope to conduct observations that revealed a clumpy 3D structure of supernovae (Figure 1). This finding supports a clumpy 3D scenario of supernovae explosions rather than the widely accepted bipolar explosion scenario. It advances our understanding of how supernovae explode, a process that has been a persistent mystery.

Figure 1: Schematic drawing of the 3D structure (left) and the image of SN 2009mi (right) captured with FOCAS on the Subaru Telescope. This supernova was discovered in the galaxy IC 2151 (in the direction of the constellation Lepas, about 100 million light years away) by Berto Monard in South Africa. (Credit: NAOJ)

The Mystery of Supernovae

Stars heavier than eight solar masses will end their lives with a brilliant explosion called a "supernova". A supernova ejects elements synthesized within its star that are heavier than hydrogen and helium, the main elements of the primeval Universe. The ejection of these heavier elements into interstellar space has enriched the chemical composition of the Universe.

Despite its important role in the evolution of the Universe, the process of how supernovae explosions occur has been unclear. Based on recent numerical simulations, researchers agree that supernovae would not succeed as one-dimensional, spherical events and that multi-dimensional effects are important for understanding their occurrence. Scientists have proposed two main scenarios to explain how supernovae explosions occur: (1) a bipolar explosion facilitated by rotation, and (2) a clumpy 3D explosion driven by convection. However, scientists have not known which scenario is more plausible, because they have not actually observed the shape of supernovae.


Seeing the "Shape" of Supernovae by Polarization

Although it would seem easy to see the shape of supernovae by simply taking a picture of them, observing them is really a very challenging task. Since most supernovae occur in galaxies millions or hundreds of million light years away, they only look like a point, even though they expand at a speed of 10,000 km/s.

The current research team used a special method of detection to reveal the shape of supernovae; they measured so-called "polarization", which supplies information about the direction of vibrating electromagnetic waves. They performed numerical simulations for emissions from supernovae and found clearly different polarization patterns for clumpy and bipolar explosions. An object shows various angles of polarization in a clumpy explosion while it shows a single angle of polarization in a bipolar explosion (Figure 2).

Figure 2: Schematic drawing of the polarization patterns. If a supernova has a clumpy geometry, the polarization has various angles (left), but if it has a bipolar geometry, the polarization has a single angle (right). (Credit: NAOJ)

Based on hypotheses derived from their simulations, the group used the Subaru Telescope's Faint Object Camera and Spectrograph (FOCAS) to conduct polarimetric observations of nearby supernovae; such observations measure the intensity and direction of polarization. Because the researchers did not know when the supernovae would appear, they could not assign an observing time in advance. Fortunately, Subaru Telescope has a Target of Opportunity (ToO) mode that overcomes this difficulty and enables a dynamic allocation of the observing time. Thanks to this mode, the team succeeded in conducting polarimetric observations of two so-called "stripped-envelope supernovae" (SN 2009mi and SN 2009jf), which do not have hydrogen surrounding them and are the best targets for studying explosion geometry.


Revealing a 3D Structure

The team detected the polarization from the two supernovae, which clearly indicated that the supernovae are not usually round. They also found that each supernova had various angles of polarization, a finding consistent with the scenario of a clumpy 3D explosion (Figure 3).

When the team added the two new supernovae to the ones from previous observations, they had a total of six stripped-envelope supernovae, five of which showed the signature of clumpy 3D geometry. The research showed that the clumpy 3D shape is common in supernovae. Although the bipolar explosion scenario is widely accepted, the findings of this research support the clumpy 3D scenario of supernovae explosions. Convective motion in the explosions could account for this clumpy shape. This result serves as a catalyst to further understand how supernovae explosions occur.

Figure 3: Observed polarization around a calcium absorption line of SN 2009jf as a function of velocity caused by expanding motion (Doppler velocity). It shows that the polarization angle changes with wavelength. (Credit: NAOJ)

Research Group

• Masaomi Tanaka (National Astronomical Observatory of Japan [NAOJ])

• Koji S. Kawabata (Hiroshima University, Japan)

• Takashi Hattori (NAOJ)

• Paolo A. Mazzali (Max Planck Institute for Astrophysics, Germany)

• Kentaro Aoki (NAOJ)

• Masanori Iye (NAOJ)

• Keiichi Maeda (University of Tokyo, Kavli Institute for the Physics and Mathematics of the Universe, Japan)

• Kenichi Nomoto (University of Tokyo, Kavli Institute for the Physics and Mathematics of the Universe, Japan)

• Elena Pian (Scuola Normale Superiore, Italy)

• Toshiyuki Sasaki (NAOJ)

• Masayuki Yamanaka (Kyoto University, Japan)

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The Most Distant and Ancient Supernovae in the Young Universe

Figure 1: The type Ia supernova in the inset above, one of 150 in the full sample, exploded some 10 billion years ago and is one of the oldest and farthest type Ia supernovae observed to date. Except for a handful of stars, all of the objects in the above image are galaxies. (Click image to enlarge.)

Figure 2: This image shows 22 out of 150 supernovae, only 10% of the Subaru Deep Field. With the exception of a few nearby Milky Way stars, each point of light in the image is a galaxy, which consists of tens of billions of stars. Every triplet of frames focuses on different aspects of one event: the galaxy before the explosion; with the supernova in progress; and isolation of light from the supernova, as shown in a digital "difference image."

Full-size image with supernovae - Full-size image without supernovae

A team of Japanese, Israeli, and U.S. astronomers used the Subaru Telescope to assemble the largest sample ever found of the most distant exploding stars called supernovae, which emitted their light about ten billion years ago, long before the Earth was formed. The researchers used this sample of ancient supernovae to determine how frequently such explosions of stars occurred in the young universe.

Supernovae have substantial importance in astrophysics. They are nature's element factories: essentially all of the elements in the periodic table that are heavier than oxygen were formed through nuclear reactions immediately preceding and during these colossal explosions. The explosions fling these elements into interstellar space, where they serve as raw materials for new generations of stars and planets. Thus, the atoms in our bodies, like the calcium atoms in our bones or the iron atoms in our blood, were created in supernovae. By tracking the frequency and types of supernova explosions back through cosmic time, astronomers can reconstruct the universe's history of element creation, from the plain mix of hydrogen and helium that existed for the first billion years or so after the Big Bang, up to the elemental richness we see today.

However, looking back in time requires looking out to great distances, which means that even these bright explosions are exceedingly faint and difficult to spot. To overcome this obstacle, the team took advantage of a combination of the Subaru Telescope's assets: the huge light-collecting power of its large 8.2 meter primary mirror; the sharpness of its images, and the wide field of view of its prime focus camera (Suprime-Cam). On four separate occasions, they pointed the telescope toward one single field called the Subaru Deep Field, which spans an area of the sky similar to that covered by the full moon and had previously been studied in great detail by Subaru scientists. By "staring" with the telescope at this single field, they let the faint light from the most distant galaxies and supernovae accumulate over several nights at a time, thus forming a very long and deep exposure of the field. Each of the four observations caught about 40 supernovae in the act of exploding among the 150,000 galaxies in the field. Altogether, the team discovered 150 explosions, including a dozen that rank among the most distant and ancient ever seen.

The team's analysis of the data showed that supernovae of the so-called "thermonuclear" type were exploding about five times more frequently in the young universe, about ten billion years ago, than they do today. Thermonuclear supernovae, often called Type-Ia supernovae, are one of the main sources of the element iron in the universe. Equally important, these explosions have served as cosmic distance markers for astronomers. Over the past decade, they have revealed that the expansion of the universe, in which all galaxies are receding from each other, is actually accelerating under the influence of mysterious dark energy. However, the nature of the thermonuclear supernovae themselves is poorly understood, and there has been fierce debate about the identity of the pre-explosion stars or stellar systems. By revealing the range of the ages of the stars that explode in this way, the team's new findings provide some important clues to solving this mystery. The results correspond closely to a scenario in which a thermonuclear supernovae is the outcome of the merger of a pair of compact stellar remnants called white dwarfs. Future observations with the next-generation Subaru imaging camera, Hyper Suprime-Cam, will permit the discovery of even larger and more distant supernova samples, and allow for further testing of this conclusion.

The results are described in a paper by Graur et al. in the October 2011 issue of the Monthly Notices of the Royal Astronomical Society. The title is "Supernovae in the Subaru Deep Field: the rate and delay-time distribution of type Ia supernovae out to redshift 2".

Team members:

O. Graur (Tel-Aviv University,Israel)
D. Poznanski (LBNL, UC Berkeley, USA; Tel-Aviv University, Israel)
D. Maoz (Tel-Aviv University,Israel)
N. Yasuda (University of Tokyo, Japan)
T. Totani (Kyoto University, Japan)
M. Fukugita (University of Tokyo, Japan)
A. V. Filippenko (UC Berkeley, USA)
R. J. Foley (Harvard/Smithsonian Center for Astrophysics, USA)
J. M. Silverman (UC Berkeley, USA)
A. Gal-Yam (Weizmann Institute of Science, Israel)
A. Horesh (Tel-Aviv University, Israel; Caltech, USA)
B. T. Jannuzi (National Optical Astronomy Observatory, USA)

Source: Subaru Telescope

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‘Zombie' Stars Key to Measuring Dark Energy

Chandra X-ray image of Tycho's supernova remnant. This Type Ia supernova was observed by Tycho Brahe in 1572, and today is just an expanding ball of gas. Astronomers used to have to wait years for a close, bright supernova to learn about them. Today big surveys are discovering supernovae by the thousands. Credit: NASA/Chandra X-ray Observatory

Supernova 1994D. The supernova is the bright point in the lower-left. It is a type Ia thermonuclear supernova like those described by Howell. The supernova is on the edge of galaxy NGC 4526, depicted in the center of the image.

Credit: NASA/Hubble Space Telescope

D. Andrew Howell
Credit: Katrina Marcinowski

(Santa Barbara, Calif.) –– "Zombie" stars that explode like bombs as they die, only to revive by sucking matter out of other stars. According to an astrophysicist at UC Santa Barbara, this isn't the plot for the latest 3D blockbuster movie. Instead, it's something that happens every day in the universe –– something that can be used to measure dark energy.

This special category of stars, known as Type Ia supernovae, help to probe the mystery of dark energy, which scientists believe is related to the expansion of the universe.

Andy Howell, adjunct professor of physics at UCSB and staff scientist at Las Cumbres Observatory Global Telescope Network (LCOGT), wrote a review article about this topic, published recently in Nature Communications. LCOGT, a privately funded global network of telescopes, works closely with UCSB.

Supernovae are stars that have been observed since 1054 A.D., when an exploding star formed the crab nebula, a supernova remnant.

More recently, the discovery of dark energy is one of the most profound findings of the last half-century, according to Howell. Invisible dark energy makes up about three-fourths of the universe. "We only discovered this about 20 years ago by using Type Ia supernovae, thermonuclear supernovae, as standard or ‘calibrated' candles," said Howell. "These stars are tools for measuring dark energy. They're all about the same brightness, so we can use them to figure out distances in the universe."

These supernovae are so bright that they shine with the approximate power of a billion suns, noted Howell.

He calls Type Ia supernovae "zombie" stars because they're dead, with a core of ash, but they come back to life by sucking matter from a companion star. Over the past 50 years, astrophysicists have discovered that Type Ia supernovae are part of binary systems –– two stars orbiting each other. The one that explodes is a white dwarf star. "That's what our sun will be at the end of its life," he said. "It will have the mass of the sun crammed into the size of the Earth."

The white dwarf stars that tend to explode as Type Ia supernovae have approximately the same mass. This was considered a fundamental limit of physics, according to Howell. However, in an article in Nature about five years ago, Howell reported his discovery of stars that go beyond this limit. These previously unknown Type Ia supernovae have more than typical mass before they explode –– a fact that confounds scientists.

Howell presented a hypothesis to understand this new class of objects. "One idea is that two white dwarfs could have merged together; the binary system could be two white dwarf stars," he said. "Then, over time, they spiral into each other and merge. When they merge, they blow up. This may be one way to explain what is going on."

Astrophysicists are using Type Ia supernovae to build a map of the history of the universe's expansion. "What we've found is that the universe hasn't been expanding at the same rate," said Howell. "And it hasn't been slowing down as everyone thought it would be, due to gravity. Instead, it has been speeding up. There's a force that counteracts gravity and we don't know what it is. We call it dark energy."

The new findings relate to Einstein's concept of the cosmological constant. This is a term he added into his equations to make them valid. However, Einstein did it because he thought the universe was static; he didn't know the universe was expanding. When it was revealed that the universe is expanding, Einstein believed this concept was his biggest blunder. "It turns out that this cosmological constant was actually one of his greatest successes," said Howell. "This is because it's what we need now to explain the data."

He said that dark energy is probably a property of space. "Space itself has some energy associated with it," said Howell. "That's what the results seem to indicate, that dark energy is distributed everywhere in space. It looks like it's a property of the vacuum, but we're not completely sure. We're trying to figure out how sure are we of that –– and if we can improve Type Ia supernovae as standard candles we can make our measurements better."

Throughout history, people have noticed a few supernovae so bright they could be seen with the naked eye. With telescopes, astronomers have discovered supernovae farther away. "Now we have huge digital cameras on our telescopes, and really big telescopes," said Howell, "We've been able to survey large parts of the sky, regularly. We find supernovae daily." Astronomers have discovered thousands of supernovae in recent years.

During his career, Howell has used these powerful telescopes to study supernovae. Currently, besides teaching at UCSB, he is involved in LCOGT's detailed study of supernovae that is aimed at helping to understand dark energy. With this extensive network of observatories, it will be possible to study the night sky continuously.

"The next decade holds real promise of making serious progress in the understanding of nearly every aspect of supernovae Ia, from their explosion physics, to their progenitors, to their use as standard candles," writes Howell in Nature Communications. "And with this knowledge may come the key to unlocking the darkest secrets of dark energy."

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Caltech-led Astronomers Find a New Class of Stellar Explosion

The four supernovae discovered by the Palomar Transient Factory. Left: before explosion. Right: after explosion. From top to bottom, the supernovae are PTF09atu, PTF09cnd, PTF09cwl, and PTF10cwr. Credit: Caltech/Robert Quimby/Nature

The 1.2-meter Samuel Oschin Telescope at Palomar Observatory that was used to discover four supernovae of a new class. Inset: one of the newly discovered supernovae, PTF09cnd. Credit: Caltech/Scott Kardel/Robert Quimby/modified from Nature

PASADENA, Calif.—They're bright and blue—and a bit strange. They're a new type of stellar explosion that was recently discovered by a team of astronomers led by the California Institute of Technology (Caltech). Among the most luminous in the cosmos, these new kinds of supernovae could help researchers better understand star formation, distant galaxies, and what the early universe might have been like.

"We're learning about a whole new class of supernovae that wasn't known before," says Robert Quimby, a Caltech postdoctoral scholar and the lead author on a paper to be published in the June 9 issue of the journal Nature. In addition to finding four explosions of this type, the team also discovered that two previously known supernovae, whose identities had baffled astronomers, also belonged to this new class.

Quimby first made headlines in 2007 when—as a graduate student at the University of Texas, Austin—he discovered what was then the brightest supernova ever found: 100 billion times brighter than the sun and 10 times brighter than most other supernovae. Dubbed 2005ap, it was also a little odd. For one thing, its spectrum—the chemical fingerprint that tells astronomers what the supernova is made of, how far away it is, and what happened when it blew up—was unlike any seen before. It also showed no signs of hydrogen, which is commonly found in most supernovae.

At around the same time, astronomers using the Hubble Space Telescope discovered a mysterious supernova called SCP 06F6. This supernova also had an odd spectrum, though there was nothing that indicated this cosmic blast was similar to 2005ap.

Shri Kulkarni, Caltech's John D. and Catherine T. MacArthur Professor of Astronomy and Planetary Science and a coauthor on the paper, recruited Quimby to become a founding member of the Palomar Transient Factory (PTF). The PTF is a project that scans the skies for flashes of light that weren't there before—flashes that signal objects called transients, many of which are supernovae. As part of the PTF, Quimby and his colleagues used the 1.2-meter Samuel Oschin Telescope at Palomar Observatory to discover four new supernovae. After taking spectra with the 10-meter Keck telescopes in Hawaii, the 5.1-meter telescope at Palomar, and the 4.2-meter William Herschel Telescope in the Canary Islands, the astronomers discovered that all four objects had an unusual spectral signature.

Quimby then realized that if you slightly shifted the spectrum of 2005ap—the supernova he had found a couple of years earlier—it looked a lot like these four new objects. The team then plotted all the spectra together. "Boom—it was a perfect match," he recalls.

The astronomers soon determined that shifting the spectrum of SCP 06F6 similarly aligned it with the others. In the end, it turned out that all six supernovae are of the same type, and that they all have spectra that are very blue—with the brightest wavelengths shining in the ultraviolet.

According to Quimby, the two mysterious supernovae—2005ap and SCP 06F6—had looked different from one another because 2005ap was 3 billion light-years away while SCP 06F6 was 8 billion light-years away. More distant supernovae have a stronger cosmological redshift, a phenomenon in which the expanding universe stretches the wavelength of the emitted light, shifting supernovae spectra toward the red end.

The four new discoveries, which had features similar to 2005ap and SCP 06F6, were at an intermediate distance, providing the missing link that connected the two previously unexplained supernovae. "That's what was most striking about this—that this was all one unified class," says Mansi Kasliwal, a Caltech graduate student and coauthor on the Nature paper.

Even though astronomers now know these supernovae are related, no one knows much else. "We have a whole new class of objects that can't be explained by any of the models we've seen before," Quimby says. What we do know about them is that they are bright and hot—10,000 to 20,000 Kelvin; that they are expanding rapidly at 10,000 kilometers per second; that they lack hydrogen; and that they take about 50 days to fade away—much longer than most supernovae, whose luminosity is often powered by radioactive decay. So there must be some other mechanism that's making them so bright.

One possible model that would create an explosion with these properties involves a pulsating star about 90 to 130 times the mass of the sun. The pulsations blow off hydrogen-free shells, and when the star exhausts its fuel and explodes as a supernova, the blast heats up those shells to the observed temperatures and luminosities.

A second model requires a star that explodes as a supernova but leaves behind what's called a magnetar, a rapidly spinning dense object with a strong magnetic field. The rotating magnetic field slows the magnetar down as it interacts with the sea of charged particles that fills space, releasing energy. The energy heats the material that was previously blown off during the supernova explosion and can naturally explain the brightness of these events.

The newly discovered supernovae live in dim, small collections of a few billion stars called dwarf galaxies. (Our own Milky Way has 200–400 billion stars.) The supernovae, which are almost a hundred times brighter than their host galaxies, illuminate their environments like distant street lamps lighting up dark roads. They work as a kind of backlight, enabling astronomers to measure the spectrum of the interstellar gas that fills the dwarf galaxies in which the supernovae reside, and revealing each galaxy's composition. Once an observed supernova fades a couple of months later, astronomers can directly study the dwarf galaxy—which would have remained undetected if it weren't for the supernova.

These supernovae could also reveal what ancient stars might have been like, since they most likely originate from stars around a hundred times more massive than the sun—stars that would have been very similar to the first stars in the universe.

“It is really amazing how rich the night sky continues to be," Kulkarni says. "In addition to supernovae, the Palomar Transient Factory is making great advances in stellar astronomy as well.”

In addition to Quimby, Kasliwal, and Kulkarni, 24 other authors—11 of whom are from Caltech—contributed to the work described in the Nature paper, "A new class of hydrogen-poor super-luminous stellar explosions." This research was supported by the National Science Foundation, the United States-Israel Binational Science Foundations, the Israeli Science Foundation, the Department of Energy, the Gordon & Betty Moore foundation, Gary and Cynthia Bengier, the Richard and Rhoda Goldman Fund, and the Royal Society. The Palomar Transient Factory is a collaboration between Caltech, Columbia University, Las Cumbres Observatory Global Telescope, Lawrence Berkeley Laboratory, UC Berkeley, University of Oxford, and the Weizmann Institute of Science (Israel).

Written by Marcus Woo

Deborah Williams-Hedges
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