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Current Research Highlight :: April 2004

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Looking into the heart of a supernova

Based on the presently most detailed and most elaborate computer simulations of supernova explosions of massive stars researcher at the Max-Planck-Institut für Astrophysik in Garching by Munich have predicted the gravitational wave signal produced by these events.

Fig. 1: Radial scales in a blue giant star of about 20 solar masses. The star has radius of about 30 million kilometers, while the core is about ten thousand times smaller (radius of a few thousand kilometers) at the onset of its collapse. The neutron star formed by the collapse is still about 100 times smaller in size. At a scale of 1:1000000 the neutron would be the size of a marble, and if located at the Marienplatz in downtown Munich the surface of the star would have a radius of 30km passing through the city of Freising.

Fig. 4: A gravitational wave signal from the famous Supernova 1987A, which exploded in the Large Magellanic Cloud at a distance of about 150,000 lightyears, would have been detectable with the LIGO II interferometer experiment (see Gravitational wave bursts from core collapse supernovae). The picture shows a Hubble Space Telescope image (Space Telescope Science Institute, Hubble Heritage Team, AURA/STScI/NASA).

Supernovae are dramatic explosions of red or blue giant stars which can be detected millions of light years away because for several weeks they shine as bright as a whole galaxy consisting of hundreds of billions of stars. This amazing optical outburst commences when the explosion wave, generated in the optically obscured stellar center, eventually reaches the surface layers of the star. As giant stars have very large radii (30 to 500 million km) in spite of the large speed of the explosion wave ( 10000km/sec), the spectacular optical outburst begins only hours after the actual onset of the catastrophe, which occurs in the very center of the star. There the burnt out stellar core containing a mass comparable to that of our sun collapses in a fraction of a second to a neutron star thereby liberating the energy which causes the supernova explosion (Fig.1).

The only means to get direct and immediate information about the supernova "engine" is from observations of neutrinos emitted by the forming neutron star, and through gravitational waves which are emitted when the collapse does not proceed perfectly symmetrically. Contrary to electromagnetic waves, which are oscillations of the electromagnetic field in spacetime, gravitational waves are oscillations of the fabric of spacetime itself. According to Albert Einstein, who first predicted their existence, gravitational waves are produced whenever matter (or equivalently energy) is accelerated aspherically. However, measurable signals are only produced by astrophysical sources involving very strong gravitational fields and velocities close to the speed of light, both of which are encountered, for example, in aspherical supernovae, or during the merger process of two neutron stars resulting in the formation of a black hole.

The researchers at the Max-Planck-Institut für Astrophysik could show that for a supernova exploding in our Galaxy or its neigbourhood, the gravitational wave signal should be detectable by gravitational wave detectors presently in operation or under construction . Contrary to what was common wisdom before, they find that the dominant contribution to the gravitational wave signal is not necessarily produced when the collapse of a rotating stellar core is stopped at neutron star densities (the so-called bounce signal), but instead by violent turbulent mass motions (Fig.2) which take place inside the forming neutron star and in its immediate neigbourhood independent whether the star is rotating or not (Fig.3). Because the mass motions stir up the whole center of the star the neutrinos produced during the event are emitted asymmetrically, and hence also cause a strong gravitational wave signal. According to the Max-Planck researchers a measurement of the gravitational wave signal of a galactic supernova together with a measurement of its neutrino signal would provide important insights about the working of the "heart" of a supernova.

Ewald Müller

Literature:
Müller, E., Rampp, M., Buras, R., Janka, H.-T. und Shoemaker, D.H., Astrophysical Journal 603 (2004) 221-230

Fig. 2: Four snapshots illustrating the violent mass motions in a rotating supernova model. The side length of the plots is 600 km, and the numbers in the top left corners give the times since maximum compression (bounce). The red-orange regions are rising bubbles of hot matter. The location of the explosion wave is visible as the sharp deformed green-blue discontinuity, and the oblate blue ellipse at the center indicates the forming neutron star which is flattened by centrifugal forces.

Fig. 3: Gravitational wave signal due to violent mass flow (solid line) and asymmetric neutrino emission (thin line) predicted for a rotating supernova explosion model. The insert shows an enlargement of the bounce signal previously thought to be always the dominant contribution to the gravitation wave signal of a supernova explosion.

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