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Scientists from around the world discuss supernovae in Garching

An international conference on ``The Physics of Supernovae'' took place on the Garching science campus on July 29 -31, 2002. More than 100 participants presented their research around these powerful cosmic explosions in talks and posters.

Supernova 1987A

Supernova 1987A, a core-collapse supernova, exploded in 1987 in the Large Magellanic Cloud. The image was taken by the Hubble Space Telescope (HST) in 1999. (NASA/STScI)

Supernova 1994D

Supernova 1994D, a typical thermonuclear supernova, as seen by HST. (High-z search team/NASA/STScI)

Supernova 2002ap

Supernova 2002ap, a possible so-called hypernova which may be linked to the production of gamma ray bursts. (Mnt. Hopkins/CfA)

Supernovae are astronomical events that almost outshine their host galaxy for a few weeks. They are explained by explosions of stars that either collapse under their own gravity at the end of their lives and expel part of their mass in a huge shock wave (core-collapse supernovae, spectroscopically classified as types II and Ib,c), or that ignite nuclear fusion at high densities following a long phase of cooling and completely burn in a matter of seconds (thermonuclear, or type Ia, supernovae).

The conference began with presentations describing the possible progenitor stars of both classes. Progenitors of core-collapse supernovae are stars with masses more than ten times that of our sun, whose evolution may depend sensitively on their original chemical composition. This allows predictions of the changing properties and rates of these events as the universe grows older. Thermonuclear supernovae, on the other hand, are believed to follow the extended transfer of mass onto a white dwarf star in a binary system. Here, too, the initial metalicity of the system can affect the mass transfer rate, but the effect is difficult to compute in detail. Owing to the importance of type Ia supernovae as cosmological standard candles, understanding the dependence of this effect on cosmic age has gained further relevance.

On the first afternoon, the current state of theoretical modeling was discussed. This line of work, forming one of the main research efforts at the MPA, requires enormous computational resources on high-performance computers in order to recreate the final few seconds of an exploding star. In the case of core-collapse supernovae, an accurate description of the interaction of neutrinos born in the core with the stellar material further outside is of utmost importance. Much progress was made in this field during the last year but it is still unclear whether the current generation of models predicts successful explosions. The modeling of thermonuclear supernovae has seen some progress, as well; three-dimensional simulations of exploding white dwarfs are currently being carried out at the MPA.

The second day focussed primarily on the observations. Supernovae are observed in all available spectral bands ranging from infrared to ultraviolet, and with increasingly tight temporal coverage. The talks emphasized the similarities and correlations within the classes as well as the sometimes surprising differences of important details. The challenges of the upcoming supernova-surveys at large cosmological distances were also mentioned.

One of the topics of the third day was the impact of supernovae on the chemical evolution of galaxies. Here, statistical models can be compared with the observed spectra of extremely metal-poor stars where traces of the earliest supernovae can be found, and with meteoritic abundances from the time the solar system was formed. The end of the conference saw a discussion of the perhaps most impressive cosmic explosions, the so-called gamma-ray bursts. One of the most promising models is preliminarily termed ``hypernova'' and looks in some respects like a peculiar type Ib,c supernova. It includes a jet, streaming out from nearby a newly formed black hole with a velocity close to the speed of light, piercing through the shell of a collapsing star and transforming its kinetic energy into gamma rays later on. It was described what these events would look like to observers with different inclinations relative to the jet axis. There are indications that such phenomena have already been observed.

Jens Niemeyer

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