Mystery surrounds star’s explosive death

An unusual outburst from a supposed supernova has astronomers arguing whether the star has actually exploded, or if it is still awaiting its violent death.

Eta Carinae's shell
Eta Carina brightened considerably in the 19th century, expelling material which can be seen today surrounding the star. Credit: ESA/NASA

Somewhere in the Universe, a massive star is entering its death throes. Lately, it has been burning heavier and heavier elements in its core, but that has now become too much for it to handle. Its core suddenly collapses, causing shockwaves to ripple outwards into the surrounding layers of the star. The shockwaves march relentlessly to the surface, where they proceed to drive off the outer layers of the star.

The release of energy is phenomenal, and the ensuing brightness temporarily outshines that of the galaxy where the star resides. This is a core collapse supernova, and astronomers identify them both by their luminosity, and by the elements that are flung into space by the explosion.

In 2009, astronomers discovered what they described as an “apparent” supernova, and named it SN 2009ip. The sudden increase in brightness turned out to be nothing more than a supernova impostor; a massive star that mimics a supernova explosion with its sporadic violent outbursts. Evidence for this came from trawling through old images from the Hubble Space Telescope, which identified the star prior to its outburst in 2009.

The star is a trickster known as a luminous blue variable; a massive, post main-sequence star that is prone to expelling chunks of its mass at a time. These intense outbursts are blazingly bright and can liberate more stellar material than the total mass lost from the stellar winds during the entire main-sequence lifetime of the star. One famous luminous blue variable is Eta Carina, which became one of the brightest stars in the sky during the 19th century Great Eruption. Images today show the material that was expelled during that eruption engulfing the star.

Supernova impostors are usually caught out as the spectrum from an outburst should be different than an actual supernova explosion, but SN 2009ip decided to throw a spanner in the works. Another outburst from the luminous blue variable was seen in 2010, and a third began in July 2012. Some researchers believe that this outburst morphed into a real supernova explosion, and proved fatal for the restless star. Others believe the trickster is still alive and that it has yet to blow itself apart. So which is it?

A terminal outburst…
The initial increase in brightness seen in July was fainter than what is normal for a supernova, although it is not unheard of for a supernova to have a low initial luminosity and to take its time climbing to higher luminosities. However, this dimness could also be explained as an ordinary outburst from the luminous blue variable just prior to the real supernova explosion.

Jon Mauerhan of the University of Arizona, and his team, used spectra to detect super speedy material high-tailing it away from SN 2009ip in September 2012. Spectra reveal the chemical behaviour of elements with an array of absorption or emission lines, and these lines can be shifted from their normal position if a gas is moving. “Overall, this makes the line fatter in width,” explains Mauerhan. “The faster the material is moving, the fatter the lines will be. So the line thickness provides a direct estimate of velocity.”

The broad emission lines of the 2012 outburst showed the bulk of the material was moving away at around 8000 kilometres per second – speeds which are indicative of a supernova explosion. Shortly after the detection of the broad lines, SN 2009ip increased dramatically in luminosity and its spectrum changed into one resembling other supernovae. The rapid brightening was likely caused by the material ejected during the supernova crashing into material that had been ejected during its outbursts.

…or a supernova impostor?
On the other hand, Andrea Pastorello of the INAF Astronomical Observatory of Padova in Italy, along with colleagues, detected high speed material in 2011, a year before the supposed fatal outburst. Some of the material was moving at speeds of up to 13,000 kilometres per second – speeds which are normally associated with a supernova. The existence of high speed material didn’t culminate with a supernova in 2011, so why should it mean that a supernova occurred in 2012? Pastorello thinks that the energy released from colliding shells of material mimicked a supernova, and that star is still there and has yet to explode as a real supernova.

Mauerhan is sceptical about this solution, saying that the high speeds seen in 2011 were only observed in absorption lines, and thus only a small amount of material was moving at this speed while the bulk of the material moved at a slower speed. In contrast, the broad emission lines seen in 2012 indicate that almost all of the material was moving at high speed. “There is a fundamental difference between the nature of the high-velocity material in 2011 versus 2012. This situation is *much different* than what we saw in 2011, and the physical processes are not the same. The behaviour in 2012 is entirely unprecedented.”

Yet another theory suggests that the supposed supernova was a “mergerburst,” where two massive stars collided. The scientists behind this theory think that the previous outbursts were due to close fly-bys between the two stars and they predict that there will be no more outbursts now that the stars have merged.

Only time will tell
We will have to wait and see who turns out to be right, but SN 2009ip might not reveal its secrets for some time. A similar event occurred with SN 1961V, with scientists arguing over the true nature of the event. Half a century later and the elusive object is still enshrouded in material from various outbursts, making it impossible to conclude whether it has erupted in a supernova or not.

Pastorello tells Skymania News that once the circumstellar material stops hiding SN 2009ip, we should be able to solve the mystery with space based telescopes. “My forecast is that – when the interaction will attenuate – we will have good chance to see in a few years traces of the survived central star (if any), or if there will be evidence of elements produced in the supernova nucleosynthesis or, alternatively, possible future rebrightenings of the transient, which would be a clear indication that the star had survived the recent explosions.”

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