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Dying stars may be a source of gravitational waves

Until now, astrophysicists have only managed to detect gravitational waves from binary systems, such as the mergers of either two black holes, two neutron stars, or one of each. However, a team of scientists led by Northwestern University has recently found a single, non-binary source that could also emit such waves: the turbulent, energetic cocoons of debris surrounding dying massive stars.

When massive stars collapse into black holes, they often create powerful outflows, or jets, of particles travelling close to the speed of light. As these jets collide into collapsing layers of the dying star, a bubble, or cocoon forms around the jets, consisting of random mixtures of hot gases and debris expanding in all directions. 

By using state-of-the-art simulations to model the collapses of massive stars, the experts showed that such cocoons could also be a source of gravitational waves. Moreover, unlike gamma-ray burst jets, cocoons’ gravitational waves should be within the frequency band the Laser Interferometer Gravitational-Wave Observatory (LIGO) could reliably 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 study lead author Ore Gottlieb, a research fellow in Astrophysics at Northwestern. “Cocoons are one of the first places we should look to for this type of source.”

The team makes an unexpected discovery

Initially, Gottlieb and his colleagues wanted to clarify whether the accretion disk which forms around a black hole could emit detectable gravitational waves. However, something unexpected emerged from the simulation.

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

Cocoons are highly turbulent places and, as they accelerate from the jet, they perturb the space-time continuum and create a ripple of gravitational waves. “A jet starts deep inside of a star and then drills its way out to escape. 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,” Gottlieb said.

Implications of the study

If cocoons generate gravitational waves, LIGO should be able to detect them much better than it could detect waves emerging from gamma ray bursts or supernovae. While jets, supernovae, and gamma ray bursts are all highly energetic explosions, LIGO can only detect gravitational waves emerging from higher frequency, asymmetrical explosions. 

Since supernovae are rather spherical and symmetrical and gamma ray bursts have a very small frequency, detecting gravitational waves arising from them is much more challenging. Instead, the asymmetrical and highly energetic cocoons could be much more accessible sources for reliably identifying such waves.

“Our study is a call to action to the community to look at cocoons as a source of gravitational waves. 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,” Gottlieb concluded.

The study was presented on Monday, June 5, 2023, during a virtual press briefing at the 242nd meeting of the American Astronomical Society.

More about massive stars

Massive stars are stars that have a mass significantly greater than the Sun’s. Although there isn’t a strict definition, astronomers usually refer to stars with more than about 8 solar masses as “massive stars”.

These stars are exceptional in many ways. They have shorter lifetimes, more intense radiation, and more dramatic deaths than lower-mass stars.

Shorter lifetimes

Stars shine by fusing hydrogen into helium in their cores, which releases energy. The rate of this fusion reaction increases dramatically with the star’s mass. So, while the Sun will spend about 10 billion years in the main sequence stage, a 10 solar mass star will exhaust its core hydrogen in just 20 million years.

Intense radiation

The energy output from a star (its luminosity) also increases dramatically with mass. The most massive stars can have luminosities a million times greater than that of the Sun. This energy is often emitted at ultraviolet or even X-ray wavelengths, producing a “stellar wind” that can blow away substantial amounts of the star’s own mass over its lifetime.

Dramatic deaths

When the core hydrogen in a massive star is exhausted, it can begin to fuse heavier elements, all the way up to iron. But fusing elements heavier than iron consumes energy rather than releasing it. Once the core contains a significant amount of iron, the star collapses under its own gravity, leading to a supernova explosion. The core of the star is left behind as a neutron star or black hole.

Massive stars also play a critical role in the evolution of galaxies. Their powerful stellar winds and supernova explosions can trigger the formation of new stars. The supernovae also spread heavy elements throughout the galaxy, contributing to the material that forms new stars and planets.

Despite their importance, massive stars are relatively rare. For example, in a region where there might be a thousand Sun-like stars, there might only be one or two stars with 10 times the mass of the Sun. But because of their high luminosity, these massive stars are often the most visible ones in their host galaxies.

Image Credit: Ore Gottlieb/CIERA/Northwestern University

By Andrei Ionescu, Staff Writer

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