Astronomers Watch A Massive Star End Its Life
January 11, 2013

Supernova In Action – Gives Clues To How Eta Carinae May End Its Life

April Flowers for -- Your Universe Online

Astronomers announced this week that a massive star they have been watching repeatedly mimic a supernova since 2009 has finally exploded for real.

The team, led by the University of Arizona, presented their findings to the American Astronomical Society meeting and they will be published in the Monthly Notices of the Royal Astronomical Society. This new study provides critical information on the final death throes of massive stars in the years leading up to their explosion.

The new supernova, SN 2009ip, is a luminous extragalactic transient first detected in 2009 in the spiral galaxy NGC 7259. NGC 7259 lies 67 million light years away in the constellation Piscis Austrinus. The outburst of SN 2009ip was actually recognized to be a non-terminal event - one in which the culprit star survived - soon after discovery in 2009.

The research team searched the existing data archives to find that the stellar progenitor of this outburst had been serendipitously detected in multiple images of host galaxy taken throughout the decade prior to discovery. This data revealed that the star was blue and hot, emitted about 1 million times as much energy as the Sun, contained 50-80 Suns worth of mass, and varied wildly in brightness. Stars such as this one are known as luminous blue variables (LBV's). LBV's are stars that have nearly exhausted their hydrogen fuel, and, for reasons which are poorly understood, undergo brief recurrent episodes of explosive mass loss.

Eta Carinae, from our own galaxy, is the most famous example of a LBV. Eta Carinae underwent a luminous outburst in 1843, which was witnessed by astronomer John Herschel. Observations of the location of this historic event, including Hubble Space Telescope images, have revealed an expanding nebula containing over 10 Suns worth of material hurling into surrounding space at a velocity of approximately 600 km/s.

The outbursts of LBV stars are so luminous that they can be seen at extragalactic distances. They are sometimes referred to as "supernova imposters" because their radiation characteristics mimic true supernovae. SN 2009ip was one of these imposters with an initial outburst that lasted for 1-2 weeks, and ejected an envelope of hydrogen-rich mass at a speed of roughly 600 km/s, as determined by measurements of the object's spectrum. SN2009ip exploded again one year later and again faded quite rapidly. A third event happened after nearly 2 years of quiescence on July 24 2012. This time, it also began to exhibit evidence for very fast material in its spectrum. Instead exhibiting expansion velocities of 600 km/s, like the impostor eruptions in 2009 and 2010, the bulk velocity of the hydrogen-rich material during the 2012 event had reached values closer to 10,000 km/s. these spectral velocities were measured using the Steward Observatory's Bok Telescope.

"The detection of material moving this fast was our first indication that SN 2009ip had finally exploded for real", says Dr. Jon Mauerhan of the University of Arizona, "but we were a bit puzzled by the fact that the brightness of SN 2009ip had not yet reached typical supernova luminosities".

Because the prior impostor eruptions had launched matter into space at much slower velocities than the supernova explosion, the team predicted that a violent collision would likely occur once the fast supernova material caught up with the outer, slower material emitted during the prior impostor events in 2009 and 2010. That's exactly what happened just a few days later.  On September 22 the impact occurred, generating a factor of 100 increase in brightness at visual wavelengths, which was observed using Lick Observatory's Nickel telescope.

"At this point, it is difficult to deny that a true supernova has occurred", says Mauerhan.

SN 2009ip brightened continually for two weeks. After peaking out, the supernova began a gradual decline in brightness, and has been fading ever since. The team predicts that it is entirely possible that the SN will encounter more shells of impostor debris as it expands, and may produce additional temporary increases in brightness. "We have every reason to keep watching SN 2009ip. The fact that SN 2009ip exploded while in the LBV phase is extremely interesting, as the situation is at odds with standard stellar evolutionary theory", says Mauerhan.

Stars with initial masses greater than about 30 Suns will shed most or all of their hydrogen envelopes via strong winds and outbursts, and become helium-rich Wolf-Rayet stars, before exploding as supernovae according to standard theory predictions. SN 2009ip has confirmed a long-held theory that stars can actually blow up in the LBV phase, before becoming Wolf-Rayet stars. This is important in the context of Eta Carinae in our own galaxy. These findings suggest that stars like Eta Carinae could potentially blow up at any time.

"When Eta Carinae finally does blow up, it's going to be quite a display", says Mauerhan.

When Eta Carinae does explode, the fast explosion will ram into the 10 solar masses of material ejected by the star during its earlier outbursts. This will result in  very luminous explosions that could likely be a visible from Earth during the daylight.

SN 2009ip has left the team with more questions than answers. For example, the physical mechanism responsible for the star's pre-supernova outbursts is an open question with several possibilities on the table. Since the progenitor star of SN 2009ip was very massive, it is a potential candidate for a physical process called "the pair-instability mechanism", occurring deep in the hot core of the massive star. The collision of high-energy gamma rays with atomic nuclei can create electron-positron pairs. These reduce the thermal pressure inside the star, followed by gravitational contraction of the core, which drives the temperatures and pressures up even further, accelerating the rate of nuclear reactions. The final, sudden nuclear release could potentially lead to giant outbursts. A massive star can endure several such pair-instability outbursts before finally exploding as a supernova, and this is a possibility for SN 2009ip, according to current theories.

On the other hand, the pre-supernova activity of SN 2009ip could be related to the latest stages of nuclear burning. When the hydrogen fuel in their cores is gone, massive stars begin to fuse helium, followed by successive phases of carbon, neon, oxygen, and silicon burning, which eventually results in the formation of an iron core. A star cannot support itself against gravity via the fusion of iron, after this point, so the star collapses and produces a supernova. The burning phases all occur within several years of iron core creation. Brief energetic outbursts of mass experienced by the star in the years before supernova are caused by the ignition of the burning phases.

Telling the difference between an actual supernova and an imposter explosion is very difficult from an observational standpoint, but SN 2009ip could offer potential clues as it fades. Radioactive materials created in the explosion of an iron core cooled slowly, decaying over the course of a year and generating light with a characteristic fading time.

"We will certainly be watching SN 2009ip closely to elucidate the nature of this unprecedented explosion", says Mauerhan.