California Institute of Technology | 2011 Jun 08
Shattered Expectations: Ultrabright Supernovae Defy ExplanationThey'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.”
Scientific American | John Matson | 2011 Jun 08
Brightest supernovae are in a class of their ownA rare, superluminous kind of stellar explosion does not fit into the usual supernova categories
From the outlook of a planet that resides next to a quiet, relatively predictable star, the circumstances that lead to dramatic stellar explosions elsewhere in the universe can sound somewhat improbable. Some such blasts, known as type Ia supernovae, occur when a small, dense star known as a white dwarf—roughly the diameter of Earth, but hundreds of thousands of times more massive—grows too large by siphoning material off a neighboring star, igniting a thermonuclear explosion. Other cataclysms, known as type II supernovae, occur when much heftier stars, some of them dozens of times as massive as the sun, implode under their own weight.
Luckily those circumstances arise infrequently enough to spare humankind the fallout of a nearby supernova. But the universe is a big place, and locally rare events such as type Ia and type II supernovae happen in relatively large numbers across the vast expanse of space. Now a sky survey has turned up a much rarer kind of supernova, one that defies the standard explanations for how such blasts work.
"They just don't look like normal supernovae," says Robert Quimby, a postdoctoral researcher at the California Institute of Technology, of the newfound phenomena. "That's the simplest way to put it." Quimby is part of the Palomar Transient Factory (PTF) project, which uses the 1.2-meter Oschin Telescope at Palomar Observatory in California to locate explosions in the universe, some of which are so distant that they occurred several billion years ago, but light from their detonation is only now reaching Earth. The PTF team described the new class of supernovae in a study published online June 8 in Nature.
Four new PTF supernovae, along with two events identified in the past several years that defied classification, all share the same unexplained traits: They are extraordinarily bright, and a spectral breakdown of their emitted light shows no trace of common supernova components such as hydrogen, iron and calcium. "If you look at thousands of supernova spectra, as I do, these immediately jump out to you as being peculiar," Quimby says. "They don't have the normal kinds of wiggles that you'd expect to see."
The extreme brightness of the new class of supernovae, some 10 times that of a typical type Ia supernova from an exploding white dwarf, rank them among the most luminous supernovae known. That luminosity enabled Quimby and his colleagues to spot a handful of the new supernovae among the 1,000-plus supernovae of all kinds that have been found by PTF, even though core-collapse supernovae appear to be 10,000 times more common.
But just what produces the brightness of the new class remains unknown. The way the supernovae fade from their peak brightness over time is inconsistent with the decay of radioactive elements, which is what powers the glow of a type Ia supernova. And in core-collapse cataclysms such as type II supernovae, heavy elements such as iron appear in the spectra, usually accompanied by hydrogen from the expanding supernova blast encountering ambient gas in the circumstellar medium.
One possible origin for the superluminous blasts is a very massive star, roughly 100 times the mass of the sun, that ejects a dense shell of hydrogen-depleted material. If it then undergoes core collapse to initiate a supernova, the supernova-driven ejecta would collide with the existing shell to glow brightly. Astronomers have found a precedent of a hydrogen-poor supernova preceded by an eruptive event, says Roger Chevalier, a professor of astronomy at the University of Virginia who did not contribute to the new research. But the scale of that eruption was far too small to explain the luminosity of the PTF group's supernovae.
Alternately, a supernova could have left behind a magnetar, a highly magnetized form of the dense stellar remnants known as neutron stars. The rapid spin of a magnetar could provide an internal power source to light up the supernova ejecta. But that scenario is wanting for observational backup as well; all known magnetars spin far too slowly to account for the glow of the superluminous blasts. "You want it to be formed with a spin rate of one to three milliseconds, and we don't have any evidence to show that magnetars form with those kinds of spin rates," Chevalier says. "So in principle at least you can produce the high luminosity in that way, but again there's a lot we don't understand."
Quimby and his colleagues are continuing to look for new events and to track fading supernovae over time to see how they evolve. They have even marshaled the Hubble Space Telescope to gather their ultraviolet spectra. "By building that whole sequence and incorporating the UV data, we can get a better handle on the physical origins of these things," Quimby says. But for the moment neither mechanism for the newfound supernova class is entirely convincing, Chevalier notes. "They both have their pluses and minuses, and I wouldn't say the community has come to an agreement about what is going on here," he says.
Nature News | Jon Cartwright | 2011 Jun 08
The Universe's biggest explosions can't be explained by current theories.
Some of the brightest stellar explosions in the Universe should be classified together as a new type of supernova, according to an international collaboration of researchers. The group has catalogued six explosions that cannot easily be explained by any process yet known.
When stars several times more massive than our Sun die, they explode, forming supernovae. The process varies, but the result is a massive radiation of energy that can outshine an entire galaxy. Sometimes the radiation is produced by the radioactive decay of freshly generated elements, whereas in other cases it comes from an explosive release of heat or from a collision between debris ejected from the star and material surrounding it.
Robert Quimby, an astronomer at the California Institute of Technology in Pasadena, and his colleagues are presenting a new class of supernova that is not driven by any of these processes.
In a study published online today in Nature[1], the researchers describe four previously unidentified supernovae, along with two known events that had confounded astronomers: SN 2005ap, which in 2007 was identified as the brightest supernova ever detected[2], and SCP 06F6, which made headlines in 2008 because it had a spectrum that didn't match any known types of supernova[3].
The supernovae in the new class have several distinguishing features. One is that they are very bright — about ten times more luminous than type Ia supernovae, the most commonly recorded type. Another is that their main emission is not visible light, as for most supernovae, but ultraviolet radiation.
Unrivalled power
The question is what causes their brightness. In type Ia supernovae, the lasting glow comes from the radioactive decay of isotopes such as nickel-56. But Quimby's group doesn't think that this is the case with the new supernovae, because their ultraviolet light fades away about three times too fast to match the rate of nuclear decay.
The light could come from the explosion itself, but the researchers say that the sheer brightness would require the star to give off an "unrealistic" amount of energy. The only remaining conventional explanation — that the light is generated in interactions between debris from the star and hydrogen-rich surrounding material — seems unlikely because the light that they emit shows no indication of the presence of hydrogen.
Perhaps, says Quimby's group, the exploding stars were so big — say, 100 times the mass of our Sun — that they would become very unstable, throwing off bits of material before their final explosion. That final explosion would then interact with the material previously cast off, producing a dazzling light show.
On the other hand, the early stages of the supernovae might have created spinning, highly magnetized neutron stars or 'magnetars'. The very strong magnetic field of such stars would slow down their spin, and the excess energy of their motion would be released to make the supernovae unusually bright.
The process is likely to be debated for some time. "The death of very massive stars is still quite uncertain," say Hideyuki Umeda and Ken'ichi Nomoto, astronomers at the University of Tokyo. "How mass is ejected from these stars, and how long before the explosion, is still unknown and a controversial issue."
But the new supernovae don't have to be assigned to a named class to be useful to astronomers. Their extreme brightness means they should illuminate distant parts of the Universe, perhaps literally shedding light on the formation of very faint dwarf galaxies.
- Hydrogen-poor superluminous stellar explosions - RM Quimby et al
- Nature (online 2011 Jun 08) DOI: 10.1038/nature10095
arXiv.org > astro-ph > arXiv:0910.0059 > 01 Oct 2009 (v1), 07 Jun 2011 (v2)
- Nature (online 2011 Jun 08) DOI: 10.1038/nature10095
- SN 2005ap: A Most Brilliant Explosion - RM Quimby et al
- Astrophysical Journal 668(2) L99 (2007 Oct 20) DOI: 10.1086/522862
arXiv.org > astro-ph > arXiv:0709.0302 > 2007 Sep 03
- Astrophysical Journal 668(2) L99 (2007 Oct 20) DOI: 10.1086/522862
- Discovery of an Unusual Optical Transient with the Hubble Space Telescope - K Barbary et al
- Astrophysical Journal 690(2) 1358 (2009 Jan 10) DOI: 10.1088/0004-637X/690/2/1358
arXiv.org > astro-ph > arXiv:0809.1648 > 10 Sep 2008
- Astrophysical Journal 690(2) 1358 (2009 Jan 10) DOI: 10.1088/0004-637X/690/2/1358
