• Tue. Feb 27th, 2024

Cocoons of dying stars could be a new source of gravitational waves

Cocoons of dying stars could be a new source of gravitational waves

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As a jet escapes from a collapsed star, it burrows into a cocoon of stellar debris. Credit: Ore Gottlieb/CIERA/Northwestern University

So far, astronomers have only detected gravitational waves from binary systems—two black holes, two neutron stars, or the merger of each. Although astronomers could theoretically detect gravitational waves from a single, non-binary source, they have yet to detect these elusive signals.

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

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

“To date, LIGO has only detected gravitational waves from binary systems, but one day it will find the first non-binary source of gravitational waves,” said Northwestern’s Orr Gottlieb, who led the study. “Cocoons are one of the first places we should look for such resources.”

Gottlieb will present the research in a virtual press briefing 242nd Meeting of the American Astronomical Society. “Jetted and Turbulent Stellar Deaths: New LIGO-detectable Sources of Gravitational Waves” takes place Monday, June 5 at 12:15 p.m. EDT as part of a session on “Discoveries in Distant Galaxies.”

Gottlieb is a CIERA Fellow at the Northwestern Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). Northwestern co-authors of the study include professors Vicky Kalogera and Alexander Chekovsky, postdoctoral associates Sharan Banagiri and Jonathan Jacquemin-Ide, and graduate student Nick Kass.


Jet-cocoon evolution from birth through the black hole to breakout from the star (colormap is the logarithm of the off-axis strain amplitude, noise reflects the GW frequency). Credit: Ore Gottlieb/CIERA/Northwestern University

New source ‘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 can create powerful streams (or jets) of particles traveling at close to the speed of light. Gottlieb’s simulations modeled this process—from the star falling into the black hole to the escape of the jet.

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

“When I calculated gravitational waves from the vicinity of the black hole, I found another source that interfered with my calculations – the cocoon,” Gottlieb said. “I tried to ignore it. But I found it impossible to ignore. I realized that the cocoon was an interesting gravitational wave source.”

When jets collide with the collapsing layers of a dying star, a bubble or “cocoon” forms around the jet. Cocoons are places of turbulence where hot gases and debris mix randomly and spread out from the jet in all directions. As the energetic bubble accelerates from the jet, it disrupts space-time to create a wave of gravitational waves, Gottlieb explained.

“A jet starts from the inside of a star and then burrows its way to escape,” Gottlieb said. “It’s like when you drill a hole in a wall. The spinning drill bit hits the wall and debris flows out of the wall. The drill bit gives off that physical energy. Likewise, the jet punches through the star, which heats the material of the star. This debris forms the hot layers of a cocoon.”


360 degree view of the cocoon of a dying star (colormap is logarithmic strain amplitude). Credit: Ore Gottlieb/CIERA/Northwestern University

A call to action to look for cocoons

If the cocoons generate gravitational waves, LIGO could detect them in future runs, Gottlieb said. Researchers have typically looked for single-source gravitational waves from gamma-ray bursts or supernovae, but astronomers doubt whether LIGO can detect them.

“Jets and supernovae are very energetic explosions,” Gottlieb said. “But we can only detect gravitational waves from high-frequency, asymmetric explosions. Supernovae are spherical and symmetric, so spherical explosions don’t change the equilibrium mass distribution in the star enough to emit gravitational waves. The frequency is very small—less than the frequency band that LIGO is sensitive to.”

Instead, Gottlieb urges astronomers to turn their attention to asymmetric and highly energetic cocoons.

“Our study is a call to society to look at cocoons as a source of gravitational waves,” he said. “We know that cocoons emit electromagnetic radiation, so they can be multi-messenger events. By studying them, we can learn more about what happens in the interior of stars, the properties of jets, and their propagation in starbursts.”

The title of the study is “Jetted and turbulent stellar deaths: New LVK-detectable gravitational wave sources”.

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