New type of massive stellar death

So far we have thought that the signature of the death of a massive star was an energetic explosion called a ‘supernova’. New observations show that this is not always the case. On the contrary, a team led by Danish researchers have now discovered that some massive stars die by collapsing into a black hole returning very little material into the interstellar medium. The new discovery is published in the prestigious scientific magazine Nature.

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 medium-sized 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.

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 of 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 tremendous progress in the study of GRBs during the last 10 years, and it has been found that the long-duration GRBs (with durations longer than 2 sec) are caused by the deaths of massive stars.

A team of astrophysicists from the DARK Cosmology Centre at the Niels Bohr Institute, Copenhagen University monitored two bursts (GRB060505 and GRB060614) 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 has the strongest evidence, the team has obtained deep images in very good observing conditions and spectroscopy as well. This allowed the team to localize exactly where in its 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 now 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 into 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.

For the following images, please see: http://www.astro.ku.dk/dark/inter/index.php?action=get_child&id=62&t=Outreac h_text&page=pressreleases&submenu=pressreleases

Fig.1: 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 kilometers 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).

Fig 2: Supernova. The Crab nebula (M1). This is the remains of a star that exploded as a supernova in the year 1054 (it was seen among others by Chinese Astronomers). In the centre of the nebula there are the remains of the core of the exploding star – a neutron star. This is an extremely compact object with roughly the same mass as the sun, but a radius of only about 10 kilometers. The outer layers of the star are now moving away from the neutron star with velocity of order 10000 kilometers per second. Stars with mass more than about 8 times the mass of the sun were until now believed to die in a similar way leaving behind either a neutron star or a black hole.

Fig 3: Galaxy. The galaxy in which the gamma-ray burst was observed May 5 2006. The galaxy is a spiral galaxy at a distance of 1300 million light-years. The yellow arrow shows where the star collapsed and made a gamma-ray burst, namely in a star-forming region in one of the spiral arms of the galaxy.

Fig. 4: Gamma-ray bursts are formed when very massive stars that are very rapidly rotating exhaust the fuel in the centre and collapse. In the centre a compact object is formed either a neutron star or a black hole and somehow a jet is launched along the rotation action of the star. Erupting through the star surface, the jet of stellar material sweeps through space at nearly the speed of light, colliding with intervening gas, producing additional emission of photons. These emissions are believed responsible for the “afterglow” of progressively less energetic photons ranging from X-rays over visible light to radio waves. It was with the afterglow light that it was possible to localize the May 5 gamma-ray burst to the star-forming region in the spiral arm of a galaxy 1300 million light-years from earth.

For further information contact:

Johan Fynbo, Astrophysicist, Ph.D.
DARK Cosmology Centre, Niels Bohr Institute, Copenhagen University
Phone: +45 3532-5983, e-mail: fynbo@astro.ku.dk