Supernova uncertainty

Spectacular stellar explosions – supernovae – mark the end of a massive star's life, but in two separate studies reported this week in the journal Nature, a peculiar new breed of supernovae is causing controversy.

Typically, supernovae fall into two categories, resulting from either the collapse of its core or from the thermonuclear detonation of a white dwarf star. These categories can be further subdivided depending on which chemical elements are involved. For example, Type Ia supernovae arise when a white dwarf star – the burnt-out remnant of a normal star such as our Sun – accretes material from a binary companion, igniting and releasing energy as a supernova once it has reached a critical mass of 1.4 solar masses. Type Ib, Ic or Type II, on the other hand, result from the gravitational collapse of the core of a relatively young massive star boasting at least eight solar masses, that has rapidly exhausted its fuel supply. Much of the stellar material is ejected into space, leaving a dense neutron star or even a black hole in its place.

Top: NGC 1032, the host galaxy of supernova SN 2005E, before the supernova explosion. Bottom: The discovery of the supernova SN 2005E, located about 750,000 light years from the galaxy nucleus. Image: SDSS, Lick Observatory.

Recent observations of supernova event SN 2005E, which occurred within the galaxy NGC 1032, appear not to fit into either category. The first unusual observation was that a very small mass of material was ejected, just one-third the mass of the Sun. Combined with the absence of any recent star formation in the region, this observation works against the idea of core-collapse. But the star's chemical composition doesn't fit with the model of an exploding white dwarf star either – the material expelled in SN 2005E contains a higher fraction of calcium and titanium than any supernova observed so far. Instead, this signature is more typical of nuclear reactions involving helium, rather than the carbon and oxygen found in the centre of typical white dwarf stars.

Turning to computer simulations to help shed some light on the matter, one team of astronomers, led by Hagai Perets and including Paolo Mazzali and Alex Filippenko, suggest that the observations can be explained by an interacting system of two close white dwarf stars. In this scenario, the helium shell of one star is drawn onto the other. “Once the receiving star has accumulated a certain amount, the helium starts to burn explosively,” says Mazzali. “The unique processes producing certain chemical elements in these explosions could solve some of the puzzles related to chemical enrichment. This could, for example, be the main source of titanium.”

SN 2005cz taken by the Subaru telescope and marked by an arrow, lies on the edge of its elliptical host galaxy HGC4589. Image: NAOJ, Subaru telescope.

In an independent study, a team of researchers from Hiroshima University in Japan argue that SN 2005E’s progenitor star was more massive – between 10 and 12 solar masses – and that it underwent a core-collapse similar to a Type II supernova. Koji Kawabata and colleagues use their observations of a supernova with similar properties – SN 2005cz – to illustrate their point. They note that initially the supernova's brightness correlated with Type Ib supernovae, thus implying core collapse, but the rate at which the brightness decreased was more rapid, and the calcium signature in the emission much greater. This lead the team to conclude that the progenitor star had an initial mass of around 10 solar masses and that it lost its hydrogen envelope by interacting with a companion. The remnant then collapsed into a neutron star and was accompanied by ejection of no more than one solar mass of matter. But SN 2005cz occurred in an elliptical galaxy, which has ceased star formation and therefore generally do not produce core-collapse supernovae.

“It’s a confusing, muddy situation now,” says Filippenko. “But we hope that, by finding more examples of this subclass and of other unusual supernovae and observing them in greater detail, we will find new variations on the theme and get a better understanding of the physics that’s actually going on.”

To add even more confusion, last year Filippenko and colleagues reported on a supernova (SN200bj) that they suggest also explodes by ignition of helium layer on a white dwarf. “SN 2002bj is arguably similar to SN 2005E, but has some clear observational differences as well,” he says. “It was likely a white dwarf accreting helium from a companion star, though the details of the explosion seem to have been different because the spectra and light curves differ.”

A handful of calcium-rich supernovae are known, and because these supernovae, like Type Ib, show evidence for helium in their spectra shortly after they explode, and because in the later stages they show strong calcium emission lines, the UC Berkeley astronomers were the first to coin them as 'calcium-rich Type Ib supernovae.'

If these calcium-rich supernovae turn out to be the first examples of a common, new breed of supernova, they could help explain two puzzling observations: the abundance of calcium in galaxies and in life on Earth, and the concentration of positrons – the anti-matter counterpart of the electron – in the centre of galaxies. The abundance of positrons can be explained as a decay product of radioactive titanium-44, produced abundantly in this type of supernova, to scandium-44 and a positron, prior to scandium’s decay to calcium-44. Currently, the most popular explanation for this positron presence is the decay of dark matter at the core of galaxies, but could just as easily be accounted for by the third type of supernova.

“The research field of supernovae is exploding right now, if you’ll pardon the pun,” adds Filippenko. “Many supernovae with peculiar new properties have been found, pointing to a greater richness in the physical mechanisms by which nature chooses to explode stars.”