When a star is shredded

“One of the craziest things a supermassive black hole can do is to shred a star by its enormous tidal forces.” 

 Wenbin Lu, UC Berkeley assistant professor of astronomy. 
If a star (red trail) wanders too close to a black hole (left), it can be shredded, or spaghettified, by the intense gravity. Some of the star’s matter swirls around the black hole, like water down a drain, emitting copious X-rays (blue). Recent studies of these so-called tidal disruption 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 accretion disk and the high-energy events happening within. Image credit: NASA/CXC/M. Weiss

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

That tidal disruption of a sun-like star by a black hole 1 million times more massive than itself took place 215 million light years from Earth. Luckily, this was the first such event bright enough that astronomers from the University of California, Berkeley, could study the optical light from the stellar death, specifically the light’s polarization, to learn more about what happened after the star was torn apart.

Their observations on Oct. 8, 2019, suggest that a lot of the star’s material was blown away at high speed — up to 10,000 kilometers per second — and formed a spherical cloud of gas that blocked most of the high-energy emissions produced as the black hole gobbled up the remainder of the star.

Earlier, other observations of optical light from the blast, called AT2019qiz, revealed that much of the star’s matter was launched outward in a powerful wind. But the new data on the light’s polarization, which was essentially zero at visible or optical wavelengths when the event was at its brightest, tells astronomers that the cloud was likely spherically symmetric.

Alex Filippenko, UC Berkeley professor of astronomy said:

“This is the first time anyone has deduced the shape of the gas cloud around a tidally spaghetiffied star.”

“These stellar tidal disruption events are one of very few ways astronomers know the existence of supermassive black holes at the centers of galaxies and measure their properties. However, due to the extreme computational cost in numerically simulating such events, astronomers still do not understand the complicated processes after a tidal disruption, ” added Wenbin Lu, UC Berkeley assistant professor of astronomy. 

The team: Patra, Filippenko, Lu and UC Berkeley researcher Thomas Brink, graduate student Sergiy Vasylyev and postdoctoral fellow Yi Yang reported their observations in a paper , Spectropolarimetry of the tidal disruption event AT 2019qiz: a quasispherical reprocessing layer, that has been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society.

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