A massive outburst may give a month’s advance notice of when certain giant stars will go supernova. That’s not great for evacuation plans, but perfect for observers who want to catch a supernova in action.Three years ago a giant star gave us a signal of its impending destruction just 40 days before it happened. In a fit of frenzy, the star sent gas hurtling outward at 2,000 kilometers per second (4.5 million miles per hour), more than twice the speed of the fastest solar wind. Six weeks later the entire star exploded as a Type IIn supernova, leaving behind a tiny, dense stellar corpse.
Eran Ofek (Weizmann Institute of Science, Israel) and his colleagues chronicle the star’s last days in today’s Nature. Using the light curve and spectrum of Supernova 2010mc, Ofek and his colleagues measured the mass of the pre-supernova blowoff; it amounted to 1% of the mass of the Sun. That’s extraordinary by any stellar standard. Such an event so soon before the ultimate explosion would be very improbable if the two events were unrelated — 40 days is very short compared to such a massive star’s 10-million-year lifespan. The authors estimate that there’s just a 0.1% chance that the two events lined up randomly.
Instead, Ofek and his colleagues suggest that just before the star went supernova its core was already unstable, fusing heavy elements at a frantic rate to forestall the inevitable collapse. The rapid fusion set gas sloshing around the star’s interior like waves on the ocean. (Such waves are called gravity waves, which incidentally have nothing to do with the more famous, Einstein-inspired gravitational waves of spacetime. Just like with ocean waves, gravity acts as the restoring force.) The gravity waves turned into shock waves as they traveled to the star’s surface, carrying an enormous amount of energy and throwing off the star’s outermost layer.
Forty days later, the unavoidable core collapse tore the star apart. As the debris raced outward, it slammed smack into the slower gas from the pre-supernova outburst, leaving tell-tale signatures of the collision in SN 2010mc’s light curve and spectrum.
This model might be common to all exploding stars of a certain type called Type IIn. Another paper recently accepted for publication in the Monthly Notices of the Royal Astronomical Society examines supernova 2009ip, which may have undergone a similar pre-supernova outburst in under two months before the star went supernova.
“The light curves of SN 2010mc and SN 2009ip (during its 2012 activity) are remarkably similar,” says Jon Mauerhan (University of Arizona), who was not involved in the Nature article. “That suggests the same physical mechanism could be at work in both cases.”
There’s a key difference: 2009ip started out as a supernova impostor. The star, known as a luminous blue variable, emitted outbursts in 2009, 2010, and then in 2012. Because Mauerhan and his colleagues were already watching the star, tracking its brightness and measuring its spectra, they were able to watch as the star finally went supernova for real about one month after the 2012 outburst. While the 2009 and 2010 outbursts were likely due to a different process, the 2012 pre-supernova flare-up may have been similar in nature to 2010mc.
“There is still so much mystery surrounding pre-supernova outbursts from massive stars,” Mauerhan explains. “With the development of efficient telescope surveys, such as the Large Synoptic Survey Telescope (LSST), more and more examples of pre-supernova activity from massive stars will be revealed and will improve our understanding of the physics involved.” The LSST is due to begin watching the sky in 2021.
E. Ofek et al. “An Outburst from a Massive Star 40 Days Before a Supernova Explosion.” Nature, 2013 February 7.
J. Mauerhan et al. “The Unprecedented 2012 Outburst of SN 2009ip: A Luminous Blue Variable Becomes a True Supernova.” Monthly Notices of the Royal Astronomical Society, accepted.