The “sudden death” of stars

By | May 15, 2023

Do the stars die? The answer is that they die and suddenly when they get close to black holes. This is confirmed by an international group of astronomers, in which Greek researchers from Crete also participate.

The Universe is a violent place, where even the life of a star can be cut short. This happens when you are in a “bad” neighborhood, that is, the neighborhood of a massive black hole. These black holes are millions, to billions, of the mass of the Sun and are located at the centers of galaxies.

As the star approaches the black hole, it begins to experience its increasing gravity, until that gravity becomes stronger than the forces holding the star together. Then the star will be destroyed and its gas will be swallowed, in part, by the black hole.

These sudden star deaths are called tidal disruption events (TDEs). After the star dies, its gas will form an accretion disk on its way to the black hole. This results in a flare that is usually seen in the optical and ultraviolet spectrum, as well as X-rays, but sometimes even in the radio part of the spectrum and γ-rays.

Until recently, only a few TDEs were known. Not because they are rare, but because there have not been many experiments capable of detecting them. In recent years that has changed, giving scientists the tools they need to begin to understand how matter behaves at very close distances from a black hole.

But this has led to more questions than answers. Observations from optical telescope experiments revealed that a large number of these TDEs do not produce X-rays. This contradicts what scientists previously believed about TDEs. The dominant model calls for the rapid formation of an accretion disk, especially bright in X-rays, immediately after the star is destroyed.

The polarization of light may be the key to solving this mystery. An international team of astronomers, including members of the Institute of Astrophysics (IA) at the Institute of Technology and Research (ITE) and the University of Crete, published a study in the journal Science arguing that for non-X-ray TDEs, The formation of the accretion disk is not rapid.

Instead, tidal shock waves are formed as gas from the star flows around the black hole. These shock waves are bright in optical and ultraviolet light and form what our telescopes observe as TDE. The incremental disk is created later.

“The polarization of light can provide unique information about processes in these astrophysical systems,” says Yannis Liodakis, lead author of the study, an astronomer at the Finnish Center for Astronomy with ESO (FINCA) and a graduate of the University of Crete. “The polarized light that we measured from TDE could only be explained if we were looking at these tidal shock waves.”

The team received word in late 2020 from the Gaia satellite of a new TDE in a nearby galaxy called AT2020mot. AT2020mot was then observed at a wide range of wavelengths, from radio to X-ray, with several different telescopes.

Particularly important were the observations of polarization in the optical spectrum made at the Skinakas observatory using the unique RoboPol polarimeter, which made this discovery possible.

“The RoboPol polarimeter at the Skinakas observatory has really been the cornerstone of our studies trying to understand supermassive black holes,” said Nikos Mandarakas, an AI and University of Crete doctoral student who led the observations and data analysis with RoboPol.

The scientists found that the optical light from AT2020mot was highly polarized and its polarization changed over time. Despite many attempts, none of the radio or X-ray telescopes have been able to detect TDE before, during, or even months after its peak.

Combining all this information and after comparing the observations with theoretical models, the team of astronomers realized that the data was more consistent with the scenario in which stellar gas collides with itself while orbiting the black hole and forms shock waves in the center and eccentricity of the trajectory

The shock waves amplify and order the magnetic field in stellar gas leading to highly polarized light. The level of optical polarization was very high and the fact that it changed over time made it very difficult to
explained by most models.

Scientists will continue to observe the polarized light coming from TDEs and may find out more about what happens after a star dies soon.

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