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The VLT Measures the Shape of a Type Ia Supernova

August 05, 2003

First Polarimetric Detection of Explosion Asymmetry has Cosmological Implications [1]

An international team of astronomers [2] has performed new and very detailed observations of a supernova in a distant galaxy with the ESO Very Large Telescope (VLT) at the Paranal Observatory (Chile). They show for the first time that a particular type of supernova, caused by the explosion of a "white dwarf", a dense star with a mass around that of the Sun, is asymmetric during the initial phases of expansion.

The significance of this observation is much larger than may seem at a first glance. This particular kind of supernova, designated "Type Ia", plays a very important role in the current attempts to map the Universe. It has for long been assumed that Type Ia supernovae all have the *same intrinsic brightness*, earning them a nickname as "standard candles".

If so, differences in the observed brightness between individual supernovae of this type simply reflect their different distances. This, and the fact that the peak brightness of these supernovae rivals that of their parent galaxy, has allowed *to measure distances of even very remote galaxies*. Some apparent discrepancies that were recently found have led to the discovery of *cosmic acceleration*.

However, this first clearcut observation of explosion asymmetry in a Type Ia supernova means that the exact brightness of such an object will depend on the angle from which it is seen. Since this angle is unknown for any particular supernova, this obviously introduces an amount of uncertainty into this kind of basic distance measurements in the Universe which must be taken into account in the future.

Fortunately, the VLT data also show that if you wait a little - which in observational terms makes it possible *to look deeper into the expanding fireball* - then it becomes more spherical. Distance determinations of supernovae that are performed at this later stage will therefore be more accurate.

[1]: This is a coordinated ESO/Lawrence Berkeley National Laboratory/Univ. of Texas Press Release. The LBNL press release is available on http://www.lbl.gov/Science-Articles/Archive/Phys-supernovae-shape-up.html.

[2]: The team consists of Lifan Wang, Dietrich Baade, Peter Hoeflich, Alexei Khokhlov, J. Craig Wheeler, Daniel Kasen, Peter E. Nugent, Saul Perlmutter, Claes Fransson, and Peter Lundqvist.

European Southern Observatory (ESO)