feb 072013


Type II supernovae are formed when massive stars collapse, initiating giant explosions. It is thought that stars emit a burst of mass as a precursor to the supernova explosion. If this process were better understood, it could be used to predict and study supernova events in their earliest stages. New observations from a team of astronomers including Carnegie’s Mansi Kasliwal show a remarkable mass-loss event about a month before the explosion of a type IIn supernova. Their work is published on February 7 in Nature.

NASA’s Chandra X-ray Observatory reveals the complex X-ray-emitting central region of the Crab Nebula, the remnant of a Type II supernova explosion in the constellation of Taurus. This image is 9.8 light-years across. Image Credit: NASA/CXC

Several models for the supernova-creation process predict pre-explosion outbursts, but it has been difficult for scientists to directly observe this process. Observations of emission lines radiating out from type IIn supernovae are thought to represent interactions between the mass ejected during and prior to the star’s explosion.

The Palomar Transient Factory team, led by Eran Ofek of the Weizmann Institute of Science in Israel, observed an energetic outburst from a supernova called SN2010mc that radiated at least 6x1040th joules of energy and released about 2x1028th kilograms (one hundredth of a solar mass). This mass-loss was observed 40 days before the supernova exploded.

“What is surprising is the short time between the precursor eruption and the eventual supernova explosion–one month is an extremely tiny fraction of the ten million-year lifespan of a star,” Kasliwal said.

This is a qualitative sketch of a proposed model for SN2010mc. Panel A shows the supernova on the day it detonated. An inner shell (purple) represents the material ejected by the precursor star about one month earlier from the penultimate outburst. An outer shell (orange) is made up of material ejected by the precursor star prior to the penultimate burst. Panel B shows SN2010mc at day five. The supernova shock front (grey line) is moving at 10,000 kilometers per second, ionizing the inner and outer shells along the way, producing the broad and narrow hydrogen emission lines that astronomers on Earth detect. Panel C shows the object at day 20, when the supernova shock engulfs the inner shell. At this point, astronomers only detect a narrow hydrogen emission line. Image Credit: E. O. Ofek, Weizmann Institute of Science

Probability modeling showed that there was only a 0.1 percent chance that the outburst was due to random chance, indicating that the outburst and explosion are likely causally related. At the very least, such outbursts are two orders of magnitude more likely to occur in the immediate run-up to the star’s explosion than at other times in a star’s life.

By comparing their observations to three proposed models for the mechanism by which this mass is ejected the team they found that one model provided the best match. The high velocities lend credence to the idea that the mass is driven out to the envelope that form’s the star’s atmosphere by the propagation and dissipation of excited gravity waves, although more work is necessary to confirm this model.

“Our discovery of SN2010mc shows that we can mark the imminent death of a massive star. By predicting the explosion, we can catch it in the act,” Kasliwal said.

Source: Carnegie Institution for Science

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