Faint Supernovae Remain Unexplained

The joy — and the frustration — of astronomy is that so much remains that we don’t know. Supernovae, for example, are phenomena that astronomers like to think they understand. But Mansi Kasliwal (Caltech) and her colleagues have observed something strange: a supernova called SN2007ax, which they claim is the faintest and reddest Type Ia supernova yet observed. Using optical, ultraviolet, and near-infrared observations, they conclude that supernovae like SN2007ax prove that we still do not fully grasp the scope of physical processes involved in exploding stars.

Kepler's Supernova remnant
SN1604, or Kepler's Supernova remnant, has long sparked debate amongst astronomers as to whether it actually was a Type Ia supernova. This image combines data from three space-based telescopes: the Chandra X-ray Observatory, the Spitzer Space Telescope and the Hubble Space Telescope.
Usually when people hear “supernova” they think of the cataclysmic death throws of a massive star. But that’s not the only kind of supernova out there. Type Ia supernovae occur when a white dwarf (the core of a dead star that wasn’t massive enough to explode) sucks up enough material from a more massive companion to ignite runaway nuclear fusion in the white dwarf’s interior, destroying the dwarf.

Astronomers use Type Ia supernovae as “standard candles.” Generally, these supernovae brighten and fade at the same rate and reach peak brightness at similar luminosities, so astronomers measure the difference between their absolute and apparent brightnesses to determine their distances. Type Ia studies led to the discovery that the universe’s expansion rate is accelerating.

Not all Type Ia are standard, though. There’s a subclass of unusually faint supernovae that Kasliwal’s team claims “has been purposefully overlooked” by astronomers. These subluminous Type Ia supernovae present a mystery. They are redder, fade more quickly, and have different spectra than their dependable siblings. SN2007ax, for example, was 10 times fainter at its peak luminosity than a normal Type Ia.

Using observations from several instruments, including the two Keck telescopes on Mauna Kea, Kasliwal’s team also found that SN2007ax’s ejecta are expanding at a medium velocity and have a smaller amount of radioactive nickel than normal Type Ia events. The values are similar to those for other known subluminous supernovae.

“These subluminous supernovae are not good standard candles,” says Kasliwal. She adds, however, that since they’re so much fainter than normal Type Ia explosions, astronomers find fewer of them far away from us. As a small percentage of the total observations, these subluminous supernovae should not seriously threaten the results that use Type Ia as reference points — such as the universe’s accelerating expansion.

Binary white dwarf
In this NASA illustration, a close pair of orbiting white dwarf stars throw off spiral waves of gravitational radiation.
The most exciting thing about these events is what they imply for the explosion process, Kasliwal explains. SN2007ax may be “just the tip of the iceberg.” Although it probably marks a white dwarf’s death, slightly fainter Type Ia supernovae could arise from a completely different physical process — perhaps even a violent thermonuclear flash produced by helium accreting onto a rapidly revolving binary system of white dwarfs.

Neither this model nor other exotic theories fit all the observed characteristics for these rebellious supernovae, though. The point is that astronomers just don’t know what’s going on in every supernova. Until they do, our understanding of these enigmatic explosions will remain incomplete.

The team’s results will appear in the Astrophysical Journal Letters.

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