New Scientist | Astrophile | Victoria Jaggard | 2012 Oct 05
Object: The supernova impostor SN 2009ip
Hideout: Galaxy NGC 7259, about 66 million light-years away
For years, SN 2009ip had been a fraud. The star's first dazzling outburst turned heads in 2009, leading astronomers to label it a supernova. But the brightness faded in a matter of days, revealing it to be an impostor. The massive star had shed some weight, but it wasn't experiencing its death throes.
It tried twice more, flaring but ultimately fading in such a nice, predictable way that people didn't pay it much heed. Now, though, it seems the star that cried wolf has finally met with an explosive end.
Spectral signatures from the most recent brightening show the characteristics of a type IIn supernova. These are produced when the core of a very massive star collapses, and the implosion sends out a blast wave that rips it apart. Although the light initially fades, the blast then travels through a dense shell of material shed by previous outbursts, heating things up and causing the supernova to shine even brighter.
This is just what happened to SN 2009ip. The star had a relatively dim flare-up on 24 July that faded, but by 26 September it had brightened considerably, with material shooting away from the blast zone at 13,000 kilometres per second – a sure-fire sign of a supernova.
Impossible explosion?
"No one could deny it at this point," says Jon Mauerhan of the Steward Observatory in Tucson, Arizona, who leads the team that made these observations. "The spectra show high velocities and luminosities that you can't account for otherwise."
SN 2009ip is only the second supernova impostor seen to blow up for real, and the first to do so while it still has a hydrogen shell, something that astronomers weren't sure was possible.
After a massive star uses up the hydrogen fuel in its core, it begins to fuse heavier elements, accruing them in onion-like layers within an outer shell of hydrogen.
At this stage some stars become luminous blue variables, so called because they go through episodic changes in brightness, including brilliant outbursts that look a lot like supernovae.
"We think when a star starts burning oxygen, it has a couple of years left at the most and could be very unstable," says Mauerhan. "This can cause the star to go through temper tantrums" – the outbursts of an impostor. After a while, these stars lose their hydrogen shell completely.
Blue surprise
Until now, most models for stellar evolution said that a luminous blue variable had to lose its hydrogen shell completely before it could explode. The only other time we have witnessed an impostor's death was SN 2006jc, which had already lost its hydrogen when it went through core collapse.
But the spectra from SN 2009ip show strong lines of hydrogen. That has captivated Mauerhan: "This is the object of the year."
Further studies of SN 2009ip and its aftermath will help tease out the physics of these exotic supernovae, says Armin Rest, an astronomer at the Space Telescope Science Institute in Maryland, who was not part of the study team.
Backyard blast
Rest is especially excited that astronomers have found pictures of the star before its first observed outburst in archived Hubble data. "This is the first time that we have such an eruption followed by a supernova for which there is even an image of the progenitor, in addition to a good set of observations during the eruptions," says Rest. "It is truly an amazing and exciting event."
The discovery may offer clues to the fate of Eta Carinae, a luminous blue variable in our cosmic backyard – about 7500 light-years away. This super-giant was seen erupting in 1843, and since then astronomers have been speculating about what will happen during its grand finale.
Of course, predicting its demise just got a bit harder, because SN 2009ip shows that supernova impostors can bite the dust at any time. But when it happens, we'll know. "The previous outburst was visible to the naked eye, so when it actually blows up, we're going to see it," says Mauerhan.
The Unprecedented Third Outburst of SN 2009ip: A Luminous Blue Variable Becomes a Supernova - Jon C. Mauerhan et al
- arXiv.org > astro-ph > arXiv:1209.6320 > 27 Sep 2012
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