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By now it is well established that giant stars can explode asymmetrically, even though it seems to occur only in a limited number of cases. The latest example is SN2010jl, that appeared on the 1st November last year in the galaxy UGC 5189A, a disturbed galaxy undergoing a gravitational interaction with a nearby neighbour.
The asymmetry of this supernova, appearing visually only as a point of light, was detected by an international team of astronomers led by Ferdinando Patat (ESO, Garching), that observed the event with the CAFOS spectropolarimeter, on the Zeiss 2.2 metre telescope at Calar Alto Observatory.
This instrument allows the degree of polarization in a luminous object to be measured. Stars generally produce unpolarized light, in which the electromagnetic waves oscillate in random directions, but there are various mechanisms that can act to cause one particular direction to dominate. The dominance of one direction over another implies some kind of asymmetry in the source.
Instruments such as CAFOS allow the level of polarization to be measured, and therefore to determine whether a source is spherical or not.
Applying this complex technique to N2010jl, Patat and colleagues were able to confirm that it is not spherical.
It turns out that this supernova is a rare type IIn, the same class to which the only two previous examples of asymmetric supernovae belong; something that is certainly no coincidence.
The progenitors of these supernovae are gigantic blue stars (tens of times more massive than the Sun), that in a few million years exhaust their fuel supply and collapse in on themselves, producing tremendous explosions called "core-collapse" or "gravitational" supernovae.
During their short lives, these stars expel large quantities of material into the surrounding medium
via their very strong stellar winds, producing a kind of cocoon, which the supernova explosion then crashes into.
It appears that this cocoon is behind the polarization observed by Patat's team, and means that either the explosion was intrinsically asymmetrical (hitting a spherical cocoon) or that the cocoon was not spherical. Either way, the origin of the asymmetry remains to be explained.
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