New class of stellar explosion very bright and somewhat hard to explain

Blue, brilliant and bizarre. That’s how astronomers describe six supernovas that form a brand new class. Like other supernovas, these oddballs are stellar explosions that produce spectacular light shows as their glowing debris roils the gas around them. But unlike known supernovas, these newly discovered objects emit much of their light at ultraviolet wavelengths and bear no trace of hydrogen, an element in abundance in most other stellar blasts.

SUPERSTRANGE This composite image combining infrared, visible-light and ultraviolet observations shows the superluminous supernova PTF09cnd (blue dot), which belongs to a new class of supernova. Quimby et al, Nature

The very existence of these explosions defies ready explanation, says astronomer Robert Quimby of Caltech. He and his colleagues describe the findings June 8 online in Nature.

The new class consists of four supernovas the team recently discovered and two others that have baffled astronomers in the few years since they were found. None of the six are extremely remote, but members of this new class are so bright that they should be visible from more than 12 billion light-years away and may serve as new probes of distant galaxies that would otherwise be too faint to be observed.

In contrast to all other known supernovas, the luminosity of this group of six can’t be explained by any of three standard mechanisms: the radioactive decay of newly forged elements expelled in the explosions, the collision of debris from the explosion with the surrounding hydrogen-rich gas, or the heat deposited by a shock wave traveling through a star just prior to its explosion.

Richard McCray of the University of Colorado at Boulder says he is convinced that Quimby’s team has identified a new class that is unlike any other extremely luminous supernova (SN: 7/18/09, p. 9).

Quimby and his collaborators have two ideas for how these luminous explosions may have formed. In one scenario, the core of the exploding star immediately forms a rapidly rotating, highly magnetized cinder called a magnetar. The strong magnetic field forces the cinder to slow down, converting the rotational energy into a power source that boosts the brightness of the supernova.

The second possible explanation is that just years before one of these stars explodes, it expels a huge bubble of hydrogen-poor material. When the explosion erupts, the expanding debris rams into the previously cast-off material, boosting the brightness.

Stars with helium envelopes that long ago ejected their outer shell of hydrogen are already known, says McCray. If such a star ejected its helium envelope before exploding, “that would do the trick” to explain the brightness, he notes.

Ultraviolet observations could identify a host of elements in the explosions and might indicate which of the two proposals is correct, Quimby says. The detection of a large amount of unburned carbon and oxygen, for example, would favor the model in which debris from the explosion had run into a bubble of previously ejected, unburned material.

With large sky surveys — such as the Palomar Transient Factory that Quimby’s team used — finding thousands of supernovas, it’s no surprise that astronomers have discovered some brilliant oddballs, notes theorist Stan Woosley of the University of California, Santa Cruz.

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