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New type of massive stellar death

Stars die when they have exhausted the fuel in their centres and until now it has been believed, that stars could only die in two ways – one way for the smaller and medium size stars and one way for the very massive stars.

Our sun is a middle size star. When stars that are smaller than our sun or up to 8 times more massive than the sun die, they expel the outer layers and leave behind a white dwarf in the centre

White dwarf. The Ring Nebula (M57). This is the remains of a star that died by expelling its outer layers about 6000-8000 years ago. In the centre the exposed hot core of the star - a so called white dwarf with a size similar to the earth - can still be seen. The outer layers are moving away from the white dwarf with velocities a few tenths of kilometres per second. All stars with mass less than about 8 times the mass of the sun are expected to die in a similar way. (image of planetary nebula).

Stars with a mass more than 8 times that of our sun die violently in energetic supernova explosions expelling several solar masses of chemically enriched material into the interstellar medium leaving behind either neutron stars or black holes in the centre. In this way the interstellar medium becomes more and more enriched in elements such as Oxygen and Carbon, that are essential for life.

The new discovery

In May and June of this year, two long-duration Gamma-ray bursts (GRBs) were detected by the NASA satellite Swift. GRBs are power-full bursts of gamma-rays coming from far away. There has been a tremendous progress in the study of this during the last 10 years, and it has been found that the long-duration GRBs (these have duration longer than 2 seconds) are caused by the deaths of massive stars.

A team of astrophysics from Dark Cosmology Centre (DARK) at the Niels Bohr Institute, University of Copenhagen monitored the two bursts intensively during June, July, August and September 2006. The remarkable conclusion from this monitoring was that there were no supernovae associated with these two Gamma-ray bursts.

Conclusion of this research

There are two possible conclusions: 1) that these GRBs were not caused by massive stars, or 2) that they were caused by massive stars that did not cause associated supernova explosions. Focusing on the May burst, where the team have the strongest evidence, the team has obtained deep images in very good observing conditions and spectroscopy as well. This allowed the team to localise exactly where in the host galaxy the burst occurred.

The host galaxy turns out to be a small spiral galaxy, and the burst occurred in a compact star-forming region in one of the spiral arms of the galaxy. This is strong evidence that the star(s) that made the GRB were massive, as massive stars due to their short lifetimes (few million years) are only found in star-forming regions.

Some massive stars simply collapse

The implications of this discovery are therefore this: Where as we up till know thought that massive stars died in supernova explosions expelling large materials of enriched material into the interstellar medium, it seems that this is not always the case.

The theoretical idea is that some massive stars simply collapse under the formation of a black hole (either directly as water running out of the sink, or in an indirect way where some material gets expelled, but then "falls" back and forms a black hole). Such stars would return very little chemically enriched material to the interstellar medium. It is difficult to estimate what fraction of massive stars that die in this way, but it is probably small.

Anne Dorte Bach | alfa
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