Sounds Near Middle 'C' Rock Stars To Death

By Lori Stiles, University Communications
Feb 10, 2006,

Scientists have made the astonishing discovery that sound might drivesuper
novae explosions. Their computer simulations say that dying stars pulse
at audible frequencies -- for instance, at about the F-note above middle C--
for a split second before they blow up.

Researchers in the 1960s began using computer models to test ideas about
what, exactly, causes stars to explode. But mathematical simulations
have so far failed to satisfactorily explain the inner workings of nature's
most spectacular blasts.

Neutrinos -- subatomic particles widely thought to power supernovae
explosions -- don't seem to be energetic enough to do the job, especially
for more massive stars. More sophisticated models that include
convective motion work a bit better, but not well enough.

Adam Burrows of The University of Arizona and colleagues at UA's
Steward Observatory, Hebrew University, and Germany's Max Planck Institute
(Potsdam) have developed computer models that simulate the full second or
more of star death, from the dynamics of core collapse through supernova
explosion. Their two-dimensional computer models allow for the fact that
supernovae outbursts are not spherical, symmetrical events.

A supernova is a massive star that has burned for 10 million to 20 million years
and developed a hot, dense 'white dwarf' star about the size
of Earth at its core. When the white dwarf reaches a critical mass
(about 15 times the mass of the sun), it collapses and creates a spherical shock
wave, all within less than half a second before the star would explode as a
supernova.However, in all the best recent simulations, the shock wave stalls. So
theorists have focused their work on what might revive the shock wave into
becoming a supernova explosion.

According to Burrow's new results, part of the problem is that other
computer models don't run long enough. His team's detailed models
involve a million steps, or about five times as many as typical models that
calculate only the first few hundred milliseconds of supernovae events.

Burrows team's simulations also characterize the natural motion of a
supernova core, something that other detailed models do not."Our
simulations show that the inner core starts to execute pulsations,"
Burrows said. "And they allow us to follow the development to
explosion for a longer time than other models do. They show that
after about500 milliseconds, the inner core begins to vibrate wildly.
And after 600,700 or 800 milliseconds, this oscillation becomes so
vigorous that it sendsout sound waves.

In these computer runs, it's these sound waves that actually cause the
star to explode, not the neutrinos."He added, "We were quite sure when
we started seeing this phenomenon that wewere seeing sound waves,
but it was so unexpected that we kept rechecking and retesting our results."

The team has used their models to make billions of calculations on computer
clusters in the UA astronomy department, at Berkeley's supercomputer
center and elsewhere, checking their analysis for the past year. They are
publishing the research in the Astrophysical Journal. Their research
is funded by the National Science Foundation, the Department of
Energy, and the Joint Institute for Nuclear Astrophysics.

The team got a clear picture of what likely happens by making
movies from their simulations.

Burrows has posted the movies on his Website at

http://zenith.as.arizona.edu/~burrows/briley

Collapsing material falls lopsidedly onto the inner core and soon
excites oscillations at specific frequencies in the simulations.
Within hundreds of milliseconds, the inner core vibrations become
so intense thatthey actually generate sound waves. Typical sound
frequencies are about 200to 400 hertz, in the audible range
bracketing middle C.

"Sound also generates pressure, which pushes the exciting streams of
infalling matter to the opposite side of the core, further driving the core
oscillations in a runaway process," Burrows said. "The sound waves
reinforce the shock wave (created by the collapsed star) until it finally explodes
aspherically."

Burrows said that others who study supernova explosions in computer
experiments will be skeptical of his team's results -- and should be.

"This is such a break from 40 years of traditional thinking that one
should be cautious trumpeting it," he said. "Nevertheless, this is
provocative and interesting. It would open up many new possibilities
and perhaps solve a long-standing problem of what triggers
supernovae explosions