The day the supernova exploded (a) it was surrounded by a shell of
matter ejected a month earlier (purple) with a radius of 7,000,000,000
kilometers, moving 2,000 kilometers a second. An outer shell (orange)
had been ejected earlier and was moving slower. By day 5 (b) the shock
front (black circle) was moving 10,000 kilometers a second, and by day
20 (c) had engulfed the inner shell, exposing the debris of the exploded
core. (Sketch adapted from Ofek et al, Palomar Transient Factory). Large Image
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 will appear in the February 7, 2013 issue of Nature at http://www.nature.com/nature/index.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/.
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