Astronomers in the UK and Sweden say that they have found important clues in a mystery surrounding a type of supernova that could be the main source of calcium in the universe. The team looked at "calcium-rich supernovae", which are relatively weak and short-lived stellar explosions that appear to occur inexplicably far from the centres of galaxies. Its study suggests that such supernovae probably involve a white dwarf and neutron star that are ejected from a galaxy in an initial supernova and then merge sometime later to create the second explosion.

Supernovae are exploding stars that can outshine entire galaxies before fading after a few weeks or months. They occur either when a star no longer produces enough energy to prevent gravitational collapse, or when a star suddenly acquires large amounts of matter from a disintegrating neighbour.

Vast amounts of calcium

These explosions are an important source of elements in the universe that are heavier than oxygen. In 2003 astronomers discovered a new type of supernova that produces vast amounts of calcium. Observations suggest that about half of the material produced by these explosions is calcium and this could explain the large quantities of the element seen in galaxies like the Milky Way – and here on Earth where calcium is essential for life.

A puzzling aspect of the 12 known calcium-rich supernovae is that they appear to occur at large distances – tens of thousands of light-years – from any possible host galaxies. This is unlike other types of supernova, which tend to occur in regions that are populated by lots of other stars.

Now, Joseph Lyman and colleagues at the University of Warwick, Lund Observatory and the University of Leicester have used the Very Large Telescope in Chile and the Hubble Space Telescope to study the regions surrounding these supernovae and have confirmed that they exist far from the nearest possible host galaxy.

How did they get there?

"We present observations, going just about as faint as you can go, to show there is in fact nothing at the location of these transients – so the question becomes, how did they get there?" he ponders. Furthermore, the observations revealed no remnant stars in the vicinity of the supernovae, which means that they are unlikely to be formed by the explosion of a very massive star that has been ejected from a galaxy.

What the team did notice, however, is that the light given off by the supernovae is similar to that seen during short-duration gamma-ray bursts (SGRBs). These also occur in isolated regions of space and are thought to involve the collision of two neutron stars or a neutron star with a black hole. While SGRBs are much dimmer than calcium-rich supernova – and do not appear to produce much calcium – this coincidence suggested to Lyman and colleagues that a neutron star might be at the heart of the mystery.

Fleeing companions

Neutron stars are themselves produced in supernovae and often emerge with high velocities that can take them far from their galaxies of origin. Furthermore, neutron stars can sometimes exist in binary systems. Putting all of this together, Lyman and colleagues believe that calcium-rich supernovae occur when a high-velocity neutron star is accompanied by a white-dwarf companion as it speeds away from its galaxy of origin. Material being sucked into the neutron star from the white dwarf will eventually trigger a supernova, which the astronomers calculate would provide the right mechanism for producing both lots of calcium and the amounts of light seen from calcium-rich supernovae.

While this latest work can explain many of the mysterious properties of calcium-rich supernovae, it is by no means the last word on the subject. Indeed, the team's calculations suggest that the merger of a neutron star and a white dwarf should produce a burst of high-energy gamma rays. As a result, the team recommends that the gamma-ray output of any new calcium-rich supernovae be monitored.

The observations will be published in Monthly Notices of the Royal Astronomical Society and are available in a preprint on the arXiv server.