New type of exploding star may be a dud
Courtesy of the University of Chicago and World Science staff
A type of oddly dim, exploding star is probably a sort of dud – one that could nonetheless throw light on the nature of the mysterious ‘dark energy’ pervading space, astronomers say.
The seemingly failed outbursts are said to come from variants of exploding stars called type Ia supernovae, which are found by the thousands.
Most type 1a supernovae look similar to one another. For this reason, they are used as cosmic distance indicators. If you saw many lamps at many different distances, but knew they were all of same type of lamp, you could use their differing brightnesses to figure out the distance to all of them just by knowing the distance to one. To astronomers, type Ia supernovae serve as such lamps, or ‘standard candles.’
However, these blasts reveal more than distances: data on their brightness has also persuaded astronomers that the expansion of the universe is accelerating. Scientists have dubbed the unknown cause behind this acceleration ‘dark energy.’
About 20 type Ia supernovae look peculiar, though.
‘A little odd’
They are ‘a little bit odd,’ said George Jordan, a computational scientist at the University of Chicago. Comparing odd type Ia supernovae to normal ones may let astrophysicists more precisely define the nature of dark energy, he added. Jordan and colleagues have concluded that the peculiar type Ia supernovae are probably ‘white dwarf’ stars that failed to detonate.
‘They ignite an ordinary flame and they burn, but that isn’t followed by a triggering of a detonation wave that goes through the star,’ Jordan said. These findings were based on simulations on Intrepid, the Blue Gene/P supercomputer at Argonne National Laboratory. Details are to appear in the journal Astrophysical Journal Letters.
Type Ia supernovae are blasts that happen to white dwarfs – stars that have shrunk to Earth size after having burned most or all of their nuclear fuel. Most or all white dwarfs occur in binary systems, those that consist of two stars orbiting one another.
The peculiar type Ia supernovae are anywhere from 10 to 100 times fainter than normal ones, which are brighter and therefore more easily detected. Astrophysicists have estimated that they may account for some 15% of all type Ia supernovae.
A comparatively recent discovery
The first of the dim supernovae was discovered in 2002, noted Robert Fisher, a physicist at the University of Massachusetts Dartmouth and co-author of the paper. Called SN 2002cx, it is considered the most peculiar type Ia supernova known. The dimmest of the lot, however, was discovered in 2008. ‘If the brightness of a standard supernova could be thought of as a single 60-watt light bulb, the brightness of this 2008 supernova would be equivalent to a small fraction of a single candle or a few dozen fireflies,’ Fisher noted.
Scientists have been simulating type Ia supernova explosions for years based on a scenario called ‘gravitationally confined detonation.’ A white dwarf starts to burn near its centre. The ignition point burns outward, floating toward the surface like a bubble. After it breaks the surface, a cascade of hot ash flows around the star and collides with itself on the opposite end, triggering a detonation.
‘We took the normal GCD scenario and asked what would happen if we pushed this to the limits and see what happens when it breaks,’ Jordan said. In the failed detonation scenario, the white dwarf experiences more ignition points that are closer to the core, which fuels more burning than in the detonation scenario.
No big bang
‘The extra burning causes the star to expand more, preventing it from achieving temperatures and pressures high enough to trigger detonation,’ explained co-author Daniel van Rossum of The University of Chicago’s Flash Centre.
Therefore, instead of blowing apart, the white dwarf remains mostly intact, though some pieces burn away. This failed detonation scenario looks quite similar to the peculiar type Ia explosions, the researchers said. The simulations, they added, resulted in phenomena that astronomers now can look for or have already found in their telescopic observations. These phenomena could include white dwarfs that display unusual compositions, asymmetric surface characteristics and a kick that sends the stars flying off at speeds of hundreds of kilometres per second.
‘No one had ever suggested that white dwarfs could be kicked at such velocities,’ said Hagai Perets of the Technion-Israel Institute of Technology, a collaborator in the work.
Normal type Ia supernovae look fairly uniform, but the asymmetric characteristics of their peculiar cousins means the latter would often look much different from one another, depending on their viewing angle from Earth. The asymmetric explosion also produces the kick, the researchers said, which may be powerful enough to free the white dwarf from a stellar companion’s gravity or even from its home galaxy.
‘I had never heard of such strange white dwarfs,’ Perets said. However, when he conducted a literature search, he added, he found reports of white dwarfs with properties that this irregular composition could explain.
This image, based on supercomputer simulations, shows the asymmetric surface characteristic of a white dwarf that fails to blow up. (Image: Brad Gallagher, George Jordan/Flash)
Source: World Science, http://www.world-science.net
