Dying stars' cocoons could be new source of gravitational waves

Date:
June 5, 2023
Source:
Northwestern University
Summary:
Although astrophysicists theoretically should be able to detect
gravitational waves from a single, non-binary source, they have yet
to uncover these elusive signals. Now researchers suggest looking
at a new, unexpected and entirely unexplored place: The turbulent,
energetic cocoons of debris that surround dying massive stars.


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FULL STORY ========================================================================== Although astrophysicists theoretically should be able to detect
gravitational waves from a single, non-binary source, they have yet
to uncover these elusive signals. Now researchers suggest looking at a
new, unexpected and entirely unexplored place: The turbulent, energetic
cocoons of debris that surround dying massive stars.

Now Northwestern University researchers suggest looking at a new,
unexpected and entirely unexplored place: The turbulent, energetic
cocoons of debris that surround dying massive stars.

For the first time ever, the researchers have used state-of-the-art
simulations to show that these cocoons can emit gravitational waves. And, unlike gamma-ray burst jets, cocoons' gravitational waves should be
within the frequency band that the Laser Interferometer Gravitational-Wave Observatory (LIGO) can detect.

"As of today, LIGO has only detected gravitational waves from binary
systems, but one day it will detect the first non-binary source of gravitational waves," said Northwestern's Ore Gottlieb, who led the
study. "Cocoons are one of the first places we should look to for this
type of source." Gottlieb will present this research during a virtual
press briefing at the 242nd meeting of the American Astronomical
Society. "Jetted and turbulent stellar deaths: New LIGO-detectable
sources of gravitational waves" will take place on Monday, June 5, as a
part of a session on "Discoveries in Distant Galaxies." Gottlieb is a
CIERA Fellow at Northwestern's Center for Interdisciplinary Exploration
and Research in Astrophysics (CIERA). Northwestern co-authors of the
study include professors Vicky Kalogera and Alexander Tchekovskoy,
postdoctoral associates Sharan Banagiri and Jonatan Jacquemin-Ide and
graduate student Nick Kaaz.

New source was 'impossible to ignore' To conduct the study, Gottlieb
and his collaborators used new state-of-the-art simulations to model the collapse of a massive star. When massive stars collapse into black holes,
they may create powerful outflows (or jets) of particles traveling close
to the speed of light. Gottlieb's simulations modeled this process --
from the time the star collapses into a black hole until the jet escapes.

Initially, he wanted to see whether or not the accretion disk that
forms around a black hole could emit detectable gravitational waves. But something unexpected kept emerging from his data.

"When I calculated the gravitational waves from the vicinity of the black
hole, I found another source disrupting my calculations -- the cocoon," Gottlieb said. "I tried to ignore it. But I found it was impossible
to ignore. Then I realized the cocoon was an interesting gravitational
wave source." As jets collide into collapsing layers of the dying star, a bubble, or a "cocoon," forms around the jet. Cocoons are turbulent places, where hot gases and debris mix randomly and expand in all directions from
the jet. As the energetic bubble accelerates from the jet, it perturbs space-time to create a ripple of gravitational waves, Gottlieb explained.

"A jet starts deep inside of a star and then drills its way out to
escape," Gottlieb said. "It's like when you drill a hole into a wall. The spinning drill bit hits the wall and debris spills out of the wall. The
drill bit gives that material energy. Similarly, the jet punches through
the star, causing the star's material to heat up and spill out. This
debris forms the hot layers of a cocoon." Call to action to look at
cocoons If cocoons do generate gravitational waves, then LIGO should be
able to detect them in its upcoming runs, Gottlieb said. Researchers
have typically searched for single-source gravitational waves from
gamma-ray bursts or supernovae, but astrophysicists doubt that LIGO
could detect those.

"Both jets and supernovae are very energetic explosions," Gottlieb
said. "But we can only detect gravitational waves from higher frequency, asymmetrical explosions. Supernovae are rather spherical and symmetrical,
so spherical explosions do not change the balanced mass distribution
in the star to emit gravitational waves. Gamma-ray bursts last dozens
of seconds, so the frequency is very small -- lower than the frequency
band that LIGO is sensitive to." Instead, Gottlieb asks astrophysicists
to redirect their attention to cocoons, which are both asymmetrical and
highly energetic.

"Our study is a call to action to the community to look at cocoons as a
source of gravitational waves," he said. "We also know cocoons to emit electromagnetic radiation, so they could be multi-messenger events. By
studying them, we could learn more about what happens in the innermost
part of stars, the properties of jets and their prevalence in stellar explosions." The study, "Jetted and turbulent stellar deaths: New LVK-detectable gravitational wave sources," was supported by the National Science Foundation, NASA and the Fermi Cycle 14 Guest Investigator program.These advanced simulations were made possible by the Department of Energy's Oak Ridge National Laboratory supercomputer Summit and National
Energy Research Scientific Computing Center's supercomputer Perlmutter
through the ASCR Leadership Computing Challenge computational time award.

* RELATED_TOPICS
o Space_&_Time
# Black_Holes # Stars # Astrophysics # Solar_Flare #
Cosmic_Rays # Galaxies # Astronomy # Sun
* RELATED_TERMS
o Gravitational_wave o Jupiter o General_relativity o
Teleportation o Galaxy o Dark_matter o Red_supergiant_star
o Stellar_nucleosynthesis

========================================================================== Story Source: Materials provided by Northwestern_University. Original
written by Amanda Morris. Note: Content may be edited for style and
length.


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Link to news story: https://www.sciencedaily.com/releases/2023/06/230605181202.htm

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