This image shows the inner regions of a collapsing, rapidly spinning massive star. The colors indicate entropy, which roughly corresponds to heat: Red regions are very hot, while blue regions are cold. The black arrows indicate the direction of the flow of stellar material. The two white curves with black outlines indicate the neutrino (top) and gravitational-wave (bottom) signals. This frame shows a simulation about 10.5 milliseconds after the stellar core has become a dense proto-neutron star. |
Each century, about two massive stars in our own galaxy explode, producing
magnificent supernovae. These stellar explosions send fundamental, uncharged
particles called neutrinos streaming our way and generate ripples called
gravitational waves in the fabric of space-time. Scientists are waiting for the
neutrinos and gravitational waves from about 1,000 supernovae that have already
exploded at distant locations in the Milky Way to reach us. Here on Earth,
large, sensitive neutrino and gravitational-wave detectors have the ability to
detect these respective signals, which will provide information about what
happens in the core of collapsing massive stars just before they explode.
If we are to understand that data, however, scientists will
need to know in advance how to interpret the information the detectors collect.
To that end, researchers at the California Institute of Technology (Caltech)
have found through computer simulation what they believe will be an
unmistakable signature of a feature of such an event: if the interior of the dying
star is spinning rapidly just before it explodes, the emitted neutrino and
gravitational-wave signals will oscillate together at the same frequency.
“We saw this correlation in the results from our
simulations and were completely surprised,” says Christian Ott, an
assistant professor of theoretical astrophysics at Caltech and the lead author
on a paper describing the correlation, which appears in Physical Review D.
“In the gravitational-wave signal alone, you get this oscillation even at
slow rotation. But if the star is very rapidly spinning, you see the oscillation
in the neutrinos and in the gravitational waves, which very clearly proves that
the star was spinning quickly—that’s your smoking-gun evidence.”
Scientists do not yet know all the details that lead a
massive star—one that is at least 10 times as massive as the Sun—to become a
supernova. What they do know (which was first hypothesized by Caltech
astronomer Fritz Zwicky and his colleague Walter Baade in 1934) is that when
such a star runs out of fuel, it can no longer support itself against gravity’s
pull, and the star begins to collapse in upon itself, forming what is called a
proto-neutron star. They also now know that another force, called the strong nuclear
force, takes over and leads to the formation of a shock wave that begins to
tear the stellar core apart. But this shock wave is not energetic enough to
completely explode the star; it stalls part way through its destructive work.
There needs to be some mechanism—what scientists refer to
as the “supernova mechanism”—that completes the explosion. But what
could revive the shock? Current theory suggests several possibilities.
Neutrinos could do the trick if they were absorbed just below the shock,
re-energizing it. The proto-neutron star could also rotate rapidly enough, like
a dynamo, to produce a magnetic field that could force the star’s material into
an energetic outflow, called a jet, through its poles, thereby reviving the
shock and leading to explosion. It could also be a combination of these or
other effects. The new correlation Ott’s team has identified provides a way of
determining whether the core’s spin rate played a role in creating any detected
supernova.
It would be difficult to glean such information from
observations using a telescope, for example, because those provide only
information from the surface of the star, not its interior. Neutrinos and
gravitational waves, on the other hand, are emitted from inside the stellar
core and barely interact with other particles as they zip through space at the
speed of light. That means they carry unaltered information about the core with
them.
The ability neutrinos have to pass through matter,
interacting only ever so weakly, also makes them notoriously difficult to
detect. Nonetheless, neutrinos have been detected: Twenty neutrinos from
Supernova 1987a in the Large Magellanic Cloud were detected in February 1987.
If a supernova went off in the Milky Way, it is estimated that current neutrino
detectors would be able to pick up about 10,000 neutrinos. In addition,
scientists and engineers now have detectors—such as the Laser Interferometer
Gravitational-Wave Observatory, or LIGO, a collaborative project supported by
the National Science Foundation and managed by Caltech and Massachusetts
Institute of Technology (MIT)—in place to detect and measure gravitational
waves for the first time.
Ott’s team happened across the correlation between the
neutrino signal and the gravitational-wave signal when looking at data from a
recent simulation. Previous simulations focusing on the gravitational-wave
signal had not included the effect of neutrinos after the formation of a
proto-neutron star. This time around, they wanted to look into that effect.
“To our big surprise, it wasn’t that the
gravitational-wave signal changed significantly,” Ott says. “The big
new discovery was that the neutrino signal has these oscillations that are
correlated with the gravitational-wave signal.” The correlation was seen
when the proto-neutron star reached high rotational velocities—spinning about
400 times per second.
Future simulation studies will look in a more fine-grained
way at the range of rotation rates over which the correlated oscillations
between the neutrino signal and the gravitational-wave signal occur. Hannah
Klion, a Caltech undergraduate student who recently completed her freshman
year, will conduct that research this summer as a Summer Undergraduate Research
Fellowship (SURF) student in Ott’s group. When the next nearby supernova
occurs, the results could help scientists elucidate what happens in the moments
right before a collapsed stellar core explodes.
