The ultimate fate of a star shredded by a black hole

If a star (red trail) wanders too close to a black hole (left), it can be shredded, or spaghettied, by the intense gravity. Some of the star material swirls around the black hole, like water in a sewer, emitting many X-rays (blue). Recent studies of these so-called tidal disturbance events suggest that a significant fraction of the star’s gas is also blown outward by intense winds from the black hole, in some cases creating a cloud that obscures the disk. accretion and the high-energy events that occur within. . Credit: NASA/CXC/M. Weiss

In 2019, astronomers observed the closest example yet of a star that was shredded, or “spaghettified,” after getting too close to a massive black hole.

This tidal disturbance of a sun-like star by a black hole 1 million times more massive than itself took place 215 million light-years from Earth. Fortunately, this was the first such event bright enough for astronomers at the University of California, Berkeley to study the optical light of stellar death, specifically the polarization of light, to learn more about what that happened after the tearing of the star.

Their October 8, 2019 observations suggest that much of the star’s material was blown away at high speeds – up to 10,000 kilometers per second – and formed a spherical cloud of gas that blocked most emissions at high energy produced by the black hole engulfed the rest of the star.

Earlier, further observations of the explosion’s optical light, called AT2019qiz, revealed that much of the star’s material was being tossed outward in a powerful wind. But the new data on the polarization of light, which was essentially zero at visible or optical wavelengths when the event was brightest, tells astronomers that the cloud was likely spherically symmetrical.

“This is the first time anyone has inferred the shape of the gas cloud around a tidal spaghettied star,” said Alex Filippenko, a UC Berkeley astronomy professor and a member of the team. research.

The findings support an answer to why astronomers don’t see high-energy radiation, such as X-rays, from many of the dozens of tidal disturbance events observed to date: X-rays, which are produced by materials torn from the star. and swept into an accretion disk around the black hole before falling inward, are masked by gas blown outward by the black hole’s powerful winds.

“This observation rules out a class of solutions that have been proposed theoretically and gives us a stronger constraint on what happens to the gas around a black hole,” said UC Berkeley graduate student Kishore Patra, lead author of the study. “People have seen other evidence of wind coming out of these events, and I think this polarization study definitely reinforces that evidence, in the sense that you wouldn’t get a spherical geometry without having a sufficient amount of wind. The Interestingly, a significant fraction of the inward-rotating stellar material does not ultimately fall into the black hole – it is expelled from the black hole.”

Polarization Reveals Symmetry

Many theorists have speculated that stellar debris forms an eccentric asymmetric disk after disturbance, but an eccentric disk should show a relatively high degree of polarization, which would mean that perhaps several percent of the total light is polarized. This was not observed for this tidal disturbance event.

“One of the craziest things a supermassive black hole can do is shred a star by its enormous tidal forces,” said team member Wenbin Lu, an assistant professor of astronomy at UC. Berkeley. “These stellar tidal disturbance events are one of the few ways astronomers know of the existence of supermassive black holes at the center of galaxies and measure their properties. However, due to the extreme computational cost of numerically simulating such events, astronomers still don’t understand the complicated process after a tidal disturbance.”

A second set of observations on November 6, 29 days after the October observation, revealed that the light was very slightly polarized, about 1%, suggesting that the cloud had thinned enough to reveal the asymmetric structure of the gas. around the black hole. Both observations came from the 3-meter Shane Telescope at Lick Observatory near San Jose, California, which is equipped with the Kast Spectrograph, an instrument capable of determining the polarization of light across the entire optical spectrum. The light becomes polarized – its electric field vibrates mostly in one direction – when it scatters electrons into the gas cloud.

“The accretion disk itself is hot enough to emit most of its light in X-rays, but that light has to pass through that cloud, and there are many scatterings, absorptions, and reemissions of light before it cannot escape from this cloud,” says Patra. “With each of these processes, the light loses some of its photon energy, descending to ultraviolet and optical energies. The final scattering then determines the polarization state of the photon. So, by measuring the polarization, we can deduce the geometry of the surface where the final dispersion occurs.

Patra noted that this deathbed scenario can only apply to normal tidal disturbances – not “weird ones”, in which relativistic jets of matter are blown out of the black hole’s poles. Only further measurements of the polarization of light from these events will answer this question.

“Polarization studies are very difficult, and very few people in the world know enough about the technique to use it,” he said. “So this is uncharted territory for tidal disturbance events.”

Patra, Filippenko, Lu and UC Berkeley researcher Thomas Brink, graduate student Sergiy Vasylyev and postdoctoral fellow Yi Yang reported their observations in a paper that has been accepted for publication in the journal Royal Astronomical Society Monthly Notices.

A cloud 100 times larger than Earth’s orbit

The UC Berkeley researchers calculated that polarized light was emitted from the surface of a spherical cloud with a radius of about 100 astronomical units (au), 100 times farther from the star than Earth does. is from the sun. An optical glow of hot gas emanated from a region about 30 AU.

The 2019 spectropolarimetric observations – a technique that measures polarization at many wavelengths of light – were of AT2019qiz, a tidal disturbance event located in a spiral galaxy in the constellation Eridanus. The zero polarization of the entire spectrum in October indicates a spherically symmetric gas cloud – all polarized photons balance each other. The slight polarization of the November measurements indicates a small asymmetry. Because these tidal disturbances occur so far away, at the center of distant galaxies, they only appear as a bright spot, and polarization is one of the few indications of the shape of objects.

“These disruptive events are so far away that you can’t really resolve them, so you can’t study the geometry of the event or the structure of these explosions,” Filippenko said. “But studying polarized light actually helps us infer information about the distribution of matter in this explosion or, in this case, the shape of the gas – and possibly the accretion disk – around this hole. black.”

Death by spaghettification: scientists record the last moments of a star devoured by a black hole

More information:
Kishore C Patra et al, AT 2019qiz Tidal Disturbance Event Spectropolarimetry: A Quasi-Spherical Reprocessing Layer, Royal Astronomical Society Monthly Notices (2022). DOI: 10.1093/mnras/stac1727

Provided by University of California – Berkeley

Quote: The Ultimate Fate of a Star Shredded by a Black Hole (July 11, 2022) Retrieved July 11, 2022 from

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