Andrew Yee
January 26th 06, 06:06 PM
University Communications
University of Arizona
Tucson, Arizona
Contact Information:
Adam Burrows, 520-621-1795
January 24, 2006
Sounds of Star Death Near Middle C
By Lori Stiles
Scientists have made the astonishing discovery that sound might drive
supernovae 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 1.5
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 about 500
milliseconds, the inner core begins to vibrate wildly. And after 600, 700
or 800 milliseconds, this oscillation becomes so vigorous that it sends
out 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
we were 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 that they
actually generate sound waves. Typical sound frequencies are about 200 to
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."
Movies and images on related Website:
* Computer movie -- supernova explosion
http://uanews.org/20060123_12170.html
IMAGES:
[Available at http://zenith.as.arizona.edu/~burrows/briley]
PHOTO CAPTION: burrows.explosion1.tiff:
Isodensity contours for the core of an 11-solar-mass star in explosion.
The remnant neutron star is the green "dot" in the center and the outer
shells are just interior to the blast wave that has been launched. The
scale is ~2000 kilometers on a side and the time is ~800 milliseconds
after the bounce at nuclear densities of the collapsed core. The colors
reflect the entropies on the shells, where entropy is a measure of heat
content. (Taken from the work of Burrows et al. (2006), Ap.J. in press.)
PHOTO CAPTION: s25_ye_on_den_still.tga:
Isodensity contours, colored with entropy, of the supernova blast seen in
the calculations of Burrows et al. (2006, Ap.J. in press). Similar to the
figure above, this figure shows the accretion funnels still streaming onto
the inner core (purple dot). It is these accretion streams that continue
to excite the core oscillation that generates the acoustic power that
eventually explodes the star in these simulations. Green is high entropy
and brown is low entropy material.
MOVIE DESCRIPTION: s25.dual.mpeg:
Movies of the growth of the core oscillation and explosion of a
25-solar-mass star. The left panel is a movie of a color map that depicts
the density of the matter (red: high density; blue: low density) in the
inner 50 kilometers of the core starting just after bounce at nuclear
densities. The arrows are velocity vectors that follow the flow. The
oscillation of the core grows to become significant at late times. Note
the presence at times of both dipole and quadrupole character. The right
panel shows the same evolution, but with an entropy color map (red: high
entropy; blue: low entropy) in the inner 200 kilometers. The white lines
are isodensity contours. At late times the core is oscillating violently
and the object is exploding.
University of Arizona
Tucson, Arizona
Contact Information:
Adam Burrows, 520-621-1795
January 24, 2006
Sounds of Star Death Near Middle C
By Lori Stiles
Scientists have made the astonishing discovery that sound might drive
supernovae 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 1.5
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 about 500
milliseconds, the inner core begins to vibrate wildly. And after 600, 700
or 800 milliseconds, this oscillation becomes so vigorous that it sends
out 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
we were 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 that they
actually generate sound waves. Typical sound frequencies are about 200 to
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."
Movies and images on related Website:
* Computer movie -- supernova explosion
http://uanews.org/20060123_12170.html
IMAGES:
[Available at http://zenith.as.arizona.edu/~burrows/briley]
PHOTO CAPTION: burrows.explosion1.tiff:
Isodensity contours for the core of an 11-solar-mass star in explosion.
The remnant neutron star is the green "dot" in the center and the outer
shells are just interior to the blast wave that has been launched. The
scale is ~2000 kilometers on a side and the time is ~800 milliseconds
after the bounce at nuclear densities of the collapsed core. The colors
reflect the entropies on the shells, where entropy is a measure of heat
content. (Taken from the work of Burrows et al. (2006), Ap.J. in press.)
PHOTO CAPTION: s25_ye_on_den_still.tga:
Isodensity contours, colored with entropy, of the supernova blast seen in
the calculations of Burrows et al. (2006, Ap.J. in press). Similar to the
figure above, this figure shows the accretion funnels still streaming onto
the inner core (purple dot). It is these accretion streams that continue
to excite the core oscillation that generates the acoustic power that
eventually explodes the star in these simulations. Green is high entropy
and brown is low entropy material.
MOVIE DESCRIPTION: s25.dual.mpeg:
Movies of the growth of the core oscillation and explosion of a
25-solar-mass star. The left panel is a movie of a color map that depicts
the density of the matter (red: high density; blue: low density) in the
inner 50 kilometers of the core starting just after bounce at nuclear
densities. The arrows are velocity vectors that follow the flow. The
oscillation of the core grows to become significant at late times. Note
the presence at times of both dipole and quadrupole character. The right
panel shows the same evolution, but with an entropy color map (red: high
entropy; blue: low entropy) in the inner 200 kilometers. The white lines
are isodensity contours. At late times the core is oscillating violently
and the object is exploding.