In the past decade, robotic telescopes have turned astronomers' attention to strange exploding stars that may point to new and unusual physics. An international team of astronomers has uncovered a supernova whose origin cannot be explained by any previously known mechanism and which promises exciting new insights into stellar explosions.
SN2005E was first spotted on January 13, 2005 in the nearby galaxy NGC1032. Since then, scientists have carried out various observations of it using different telescopes including the Keck, the world's largest, at Mauna Kea, Hawaii. Analysis of the collected data, theoretical modeling and interpretation led to the conclusion that SN2005E wasn't a typical supernova.
Supernovae result from the collapse of very massive stars or by thermonuclear detonation on the surface of white dwarf stars composed mainly of carbon and oxygen.
"But this one, although it appears to be from a white dwarf system, is devoid of carbon and oxygen. Instead it's rich in helium. It's surprisingly different," said Dae-Sik Moon of the University of Toronto's Department of Astronomy & Astrophysics.
The supernova explosion is the most energetic and brilliant event that happens in the universe," says Moon. "It is rich with information, not only about how stars die, but to understanding the origin of life and the expansion of the universe." Most heavy elements are believed to be created in stars and spread through supernova explosions. Also, scientists use the brightness of supernovae to make estimates of the acceleration of the universe.
But supernova (SN) 2005E, discovered five years ago by the University of California, Berkeley's Katzman Automatic Imaging Telescope (KAIT), is one of eight known "calcium-rich supernovae" that seem to stand out.
"With the sheer numbers of supernovae we're detecting, we're discovering weird ones that may represent different physical mechanisms compared with the two well-known types, or may just be variations on the standard themes," said Alex Filippenko, KAIT director and UC Berkeley professor of astronomy. "But SN 2005E was a different kind of 'bang.' It and the other calcium-rich supernovae may be a true suborder, not just one of a kind."
SN 2005E is distinct from the two main classes of supernovae: the Type Ia supernovae, thought to be old, white dwarf stars that accrete matter from a companion until they undergo a thermonuclear explosion that blows them apart entirely; and Type Ib/c or Type II supernovae, thought to be hot, massive and short-lived stars that explode and leave behind black holes or neutron stars.
The team of astronomers, led by Hagai Perets, now at the Harvard-Smithsonian Center for Astrophysics, and Avishay Gal-Yam of the Weizmann Institute of Science in Rehovot, Israel, presented evidence that the original star was a low-mass white dwarf stealing helium from a binary companion until the temperature and pressure ignited a thermonuclear explosion - a massive fusion bomb - that blew off at least the outer layers of the star and perhaps obliterated the entire star.
The researchers calculate that about half of the mass thrown out was calcium, which means that a couple of such supernova every 100 years would be enough to produce the high abundance of calcium observed in galaxies like our own Milky Way, and the calcium present in all life on Earth.
It was SN 2005E, which went off about 110 million years ago in the spiral galaxy NGC 1032 in the constellation Cetus, that initially drew the attention of Perets, Gal-Yam and their colleagues.
Using data provided by Filippenko and Li, as well as by the W. M. Keck Observatory in Hawaii, the Palomar Observatory in Los Angeles and the Liverpool Observatory in the United Kingdom (U.K.), they created a detailed picture of the explosion. The small amount of mass ejected in the explosion, estimated at 30 percent the mass of our sun, and the fact that the galaxy in which the explosion occurred was old with few hot, giant stars, led them to the conclusion that a low-mass white dwarf was involved.
In addition, the newly discovered supernova threw off unusually high levels of the elements calcium and radioactive titanium, which are the products of a nuclear reaction involving helium rather than the carbon and oxygen involved in Type Ia supernovae.
"We know that SN 2005E came from the explosion of an old, low-mass star because of its specific location in the outskirts of a galaxy devoid of recent star formation," said Filippenko. "And the presence of so much calcium in the ejected gases tells us that helium must have exploded in a nuclear runaway."
Interestingly, a team of researchers from Hiroshima University in Japan argue in the same issue of Nature that SN 2005E's original, or progenitor, star was massive - between 8 and 12 solar masses - and that it underwent a core-collapse similar to a Type II supernova.
"It's a confusing, muddy situation now," said Filippenko. "But we hope that, by finding more examples of this subclass and of other unusual supernovae and observing them in greater detail, we will find new variations on the theme and get a better understanding of the physics that's actually going on."
Filippenko and UC Berkeley research astronomer Weidong Li first reported an unusual calcium-rich supernova in 2003, and since then, KAIT has discovered several more, including SN 2005E on Jan. 13, 2005. Because these supernovae, like Type Ib, show evidence for helium in their spectra shortly after they explode, and because in the later stages they show strong calcium emission lines, the UC Berkeley astronomers were the first to refer to them as "calcium-rich Type Ib supernovae."
The paper's authors note that, if these eight calcium-rich superonovae are the first examples of a common, new type of supernova, they could explain two puzzling observations: the abundance of calcium in galaxies and in life on Earth, and the concentration of positrons - the anti-matter counterpart of the electron - in the center of galaxies. The latter could be the result of the decay of radioactive titanium-44, produced abundantly in this type of supernova, to scandium-44 and a positron, prior to scandium's decay to calcium-44. The most popular explanation for this positron presence is the decay of putative dark matter at the core of galaxies.
"Dark matter may or may not exist," says Gal-Yam, "but these positrons are perhaps just as easily accounted for by the third type of supernova."
Filippenko and Li hope that KAIT and other robotic telescopes scanning distant galaxies every night in search of new supernovae will turn up more examples of calcium-rich or even stranger supernovae.
"The research field of supernovae is exploding right now, if you'll pardon the pun," joked Filippenko. "Many supernovae with peculiar new properties have been found, pointing to a greater richness in the physical mechanisms by which nature chooses to explode stars."
Casey Kazan via University of California - Berkeley and University of Toronto