The Palomar Transient Factory (PTF) brings together universities, observatories, and one national laboratory to hunt for supernovae and other astronomical objects. At the National Energy Research Scientific Computing Center (NERSC) Berkeley Lab processes and stores the data from PTF’s surveys, which use the Oschin Telescope at Caltech’s Palomar Observatory.
On August 25, 2010, PTF’s “autonomous machine-learning framework,” developed by Josh Bloom of Berkeley Lab’s Physics Division and Peter Nugent of the Computational Research Division (both are also with UC Berkeley’s Department of Astronomy) and their colleagues, was combing through recent data and came upon a Type IIn supernova, half a billion light years away in the constellation Hercules. The supernova was eventually labeled SN 2010mc.
Type II’s are “core collapse” supernovae, which start as precursor stars somewhere between 8 and 100 times the mass of Earth’s sun, burning much of their hydrogen down to helium, carbon, and other elements and eventually to an iron cinder. When this core reaches 1.4 solar masses, it collapses under its own weight to create a neutron star or even a black hole, releasing a tremendous amount of energy as neutrinos, magnetic fields, and shock waves – and destroying the star.
Astronomers have long suspected the story isn’t that simple, that the explosion of a Type II supernova is only the last in a series of smaller blasts that successively blow off much of the core’s enveloping matter.
Indeed, the “n” in Type IIn means that instead of the usual broad hydrogen-emission line that marks a Type II, the identifying line is narrow – probably because light from the explosion has passed through a thin sphere of hydrogen that already surrounded the star before it went supernova.
Despite many such suggestive clues, no causal proof had previously linked precursor “bumps” to an actual supernova. But soon after PTF’s Type IIn was found, Eran Ofek of Israel’s Weizmann Institute of Science led a search of previous PTF scans of the stellar neighborhood and found its likely precursor, a massive variable star that only 40 days before it went supernova had shed a huge amount of mass.
The PTF team developed a scenario and tested it against competing theoretical ideas, using evidence from several sky surveys that had also observed SN 2010mc’s precursor. They concluded that the “penultimate outburst” had blown off a hundredth of a solar mass in a shell expanding 2,000 kilometers per second, already 7 billion kilometers away from the supernova when it exploded. Earlier ejecta was detected 10 billion kilometers away, having slowed to a hundred kilometers per second.
After the supernova explosion, high-velocity ejecta passing through shells of earlier debris left a record of varying brightness and spectral features. The observations pointed to the most-likely theoretical model of what happened: turbulence-excited gravity waves drove successive episodes of mass loss, finally culminating in the collapse and explosion of the core.
The report of these results appears in Nature at http://www.nature.com/nature/journal/v494/n7435/abs/nature11877.html. For more information on the next-to-last blast from this massive star, see the NERSC press release at http://www.nersc.gov/news-publications/news/science-news/2013/a-massive-stellar-burst-before-the-supernova/.