Exploding star could be new type

The image on the left shows NGC 1032, the host galaxy of the supernova, before the supernova explosion. The discovery of the supernova SN 2005E is shown on the right.

The image on the left shows NGC 1032, the host galaxy of the supernova, before the supernova explosion. The discovery of the supernova SN 2005E is shown on the right.

  • Possible new class of supernovae
  • Could explain abundance of calcium in our Galaxy

In the past decade, robotic telescopes have turned astronomers’ attention to scores of strange exploding stars, one-offs that may or may not point to new and unusual physics.

But supernova (SN) 2005E, discovered five years ago by the University of California, Berkeley’s Katzman Automatic Imaging Telescope (KAIT), is one of eight known “calcium-rich supernovae” that seem to stand out as horses of a different colour.

“With the sheer numbers of supernovae we’re detecting, we’re discovering weird ones that may represent different physical mechanisms compared with the two well-known types, or may just be variations on the standard themes,” said Alex Filippenko, KAIT director and UC Berkeley professor of astronomy.

“But SN 2005E was a different kind of ‘bang.’ It and the other calcium-rich supernovae may be a true suborder, not just one of a kind.”

And then there were three

Filippenko is co-author of a paper in the journal Nature describing SN 2005E and arguing that it is distinct from the two main classes of supernovae.

The Type Ia supernovae are thought to be old, white dwarf stars that accumulate matter from a companion until they undergo a thermonuclear explosion that blows them apart entirely.

The Katzman Automatic Imaging Telescope

The supernova was spotted by the Katzman Automatic Imaging Telescope

Type Ib/c or Type II supernovae are thought to be hot, massive and short-lived stars that explode and leave behind black holes or neutron stars.

In the case of SN 2005E, the team of astronomers thinks the original star was a low-mass white dwarf stealing helium from a companion star until the temperature and pressure ignited a thermonuclear explosion – a massive fusion bomb – that blew off at least the outer layers of the star and perhaps blew the entire star to smithereens.

The researchers calculate that about half of the mass thrown out was calcium, which means that a couple of such supernova every 100 years would be enough to produce the high abundance of calcium detected in galaxies like our own Milky Way, and the calcium present in all life on Earth.

More supernova confusion

Interestingly, a team of researchers from Hiroshima University in Japan argue in the same issue of Nature that SN 2005E’s original, or progenitor, star was massive – between 8 and 12 solar masses – and that it underwent a core-collapse similar to a Type II supernova.

“It’s a confusing, muddy situation now,” said 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 make things even muddier, Filippenko and former UC Berkeley post-doctoral fellow Dovi Poznanski, currently at Lawrence Berkeley National Laboratory and also co-author on the Nature paper, reported last November another supernova, SN 2002bj, that they believe explodes by a similar mechanism: ignition of a helium layer on a white dwarf.

“SN 2002bj is arguably similar to SN 2005E, but has some clear observational differences as well,” Filippenko said. “It was likely a white dwarf [stealing] helium from a companion star, though the details of the explosion seem to have been different because the spectra and light curves differ.”

Adapted from information issued by University of California – Berkeley / Sloan Digital Sky Survey / Lick Observatory.

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