Hundreds of millions of light-years away in a distant galaxy, a star orbiting a supermassive black hole is being violently ripped apart under the black hole’s immense gravitational pull. As the star is shredded, its remnants are transformed into a stream of debris that rains back down onto the black hole to form a very hot, very bright disk of material swirling around the black hole, called an accretion disc. This phenomenon—where a star is destroyed by a supermassive black hole and fuels a luminous accretion flare—is known as a tidal disruption event (TDE), and it is predicted that TDEs occur roughly once every 10,000 to 100,000 years in a given galaxy.

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TDEs are usually “once-and-done” because the extreme gravitational field of the SMBH destroys the star, meaning that the SMBH fades back into darkness following the accretion flare. In some instances, however, the high-density core of the star can survive the gravitational interaction with the SMBH, allowing it to orbit the black hole more than once. Researchers call this a repeating partial TDE.

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findings, published in Astrophysical Journal Letters, describe the capture of the star by a SMBH, the stripping of the material each time the star comes close to the black hole, and the delay between when the material is stripped and when it feeds the black hole again.

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Once bound to the SMBH, the star powering the emission from AT2018fyk has been repeatedly stripped of its outer envelope each time it passes through its point of closest approach with the black hole. The stripped outer layers of the star form the bright accretion disk, which researchers can study using X-Ray and Ultraviolet /Optical telescopes that observe light from distant galaxies.

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“Until now, the assumption has been that when we see the aftermath of a close encounter between a star and a supermassive black hole, the outcome will be fatal for the star, that is, the star is completely destroyed,” he says. “But contrary to all other TDEs we know of, when we pointed our telescopes to the same location again several years later, we found that it had re-brightened again. This led us to propose that rather than being fatal, part of the star survived the initial encounter and returned to the same location to be stripped of material once more, explaining the re-brightening phase.”

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So how could a star survive its brush with death? It all comes down to a matter of proximity and trajectory. If the star collided head-on with the black hole and passed the event horizon—the threshold where the speed needed to escape the black hole surpasses the speed of light—the star would be consumed by the black hole. If the star passed very close to the black hole and crossed the so-called “tidal radius”—where the tidal force of the hole is stronger than the gravitational force that keeps the star together—it would be destroyed. In the model they have proposed, the star’s orbit reaches a point of closest approach that is just outside of the tidal radius, but doesn’t cross it completely: some of the material at the stellar surface is stripped by the black hole, but the material at its center remains intact.

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More information: T. Wevers et al, Live to Die Another Day: The Rebrightening of AT 2018fyk as a Repeating Partial Tidal Disruption Event, The Astrophysical Journal Letters (2023). DOI: 10.3847/2041-8213/ac9f36

Source: A team of physicists devise a model that maps a star’s surprising orbit about a supermassive black hole

You may not realize it in your day-to-day life, but we are all enveloped by a giant “superbubble” that was blown into space by the explosive deaths of a dozen-odd stars. Known as the Local Bubble, this structure extends for about 1,000 light years around the solar system, and is one of countless similar bubbles in our galaxy that are produced by the fallout of supernovas. Cosmic superbubbles have remained fairly mysterious for decades, but recent astronomical advances have finally exposed key details about their evolution and structure. Just within the past few years, researchers have mapped the geometry of the Local Bubble in three dimensions and demonstrated that its surface is an active site of star birth, because it captures gas and dust as it expands into space.

Now, a team of scientists has added another layer to our evolving picture of the Local Bubble by charting the magnetic field of the structure, which is thought to play a major role in star formation. Astronomers led by Theo O’Neill, who conducted the new research during a summer research program at the Center for Astrophysics at Harvard & Smithsonian (CfA), presented “the first-ever 3D map of a magnetic field over a superbubble” on Wednesday at the American Astronomical Society’s 241st annual meeting in Seattle, Washington. The team also unveiled detailed visualizations of their new map, bringing the Local Bubble into sharper focus.

“We think that the entire interstellar medium is really full of all these bubbles that are driven by various forms of feedback from, especially, really massive stars, where they’re outputting energy in some form or another into the space between the stars,” said O’Neill, who just received an undergraduate degree in astronomy-physics and statistics from the University of Virginia, in a joint call with their mentor Alyssa Goodman, an astronomer at CfA who co-authored the new research. […] “Now that we have this map, there’s a lot of cool science that can be done both by us, but hopefully by other people as well,” O’Neill said. “Since stars are clustered, it’s not as if the Sun is super special, and is in the Local Bubble because we’re just lucky. We know that the interstellar medium is full of bubbles like this, and there’s actually a lot of them nearby our own Local Bubble.” “One cool next step will be looking at places where the Local Bubble is nearby other feedback bubbles,” they concluded. “What happens when these bubbles interact, and how does that drive start formation in general, and the overall long-term evolution of galactic structures?”

Source: We Exist Inside a Giant Space Bubble, And Scientists Have Finally Mapped It – Slashdot

(Andromeda Galaxy)

In the quest to find the outer limits of our galaxy, astronomers have discovered over 200 stars that form the Milky Way’s edge, the most distant of which is over one million light-years away—nearly halfway to the Andromeda galaxy.

The 208 stars the researchers identified are known as RR Lyrae stars, which are stars with a brightness that can change as viewed from Earth. These stars are typically old and brighten and dim at regular intervals, which is a mechanism that allows scientists to calculate how far away they are. By calculating the distance to these RR Lyrae stars, the team found that the farthest of the bunch was located about halfway between the Milky Way and the Andromeda galaxy, one of our cosmic next-door neighbors.

“This study is redefining what constitutes the outer limits of our galaxy,” said Raja GuhaThakurta in a press release. GuhaThakurta is professor and chair of astronomy and astrophysics at the University of California Santa Cruz. “Our galaxy and Andromeda are both so big, there’s hardly any space between the two galaxies.”

The Milky Way galaxy consists of a few different parts, the primary of which is a thin, spiral disk about 100,000 light-years across. Our home solar system sits on one of the arms of this disk. An inner and outer halo surround the disk, and these halos contain some of the oldest stars in our galaxy.

Previous studies have placed the edge of the outer halo at 1 million light-years from the Milky Way’s center, but based on the new work, the edge of this halo should be about 1.04 million light-years from the galactic center. Yuting Feng, a doctoral student at the university working with GuhaThakurta, led the study and is presenting the findings this week at the American Astronomical Society meeting in Seattle.

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Source: Astronomers Find the Edge of Our Galaxy

While using the Atacama Large Millimeter/submillimeter Array (ALMA) to study the masers around oddball star MWC 349A scientists discovered something unexpected: a previously unseen jet of material launching from the star’s gas disk at impossibly high speeds. What’s more, they believe the jet is caused by strong magnetic forces surrounding the star.

The discovery could help researchers to understand the nature and evolution of massive stars and how hydrogen masers are formed in space. The new observations were presented today (January 9) in a press conference at the 241st meeting of the American Astronomical Society (AAS) in Seattle, Washington.

Located roughly 3,900 light-years away from Earth in the constellation Cygnus, MWC 349A’s unique features make it a hot spot for scientific research in optical, infrared, and radio wavelengths. The massive star—roughly 30 times the mass of the sun—is one of the brightest radio sources in the sky, and one of only a handful of objects known to have hydrogen masers. These masers amplify microwave radio emissions, making it easier to study processes that are typically too small to see. It is this unique feature that allowed scientists to map MWC 349A’s disk in detail for the first time.

“A maser is like a naturally occurring laser,” said Sirina Prasad, an undergraduate research assistant at the Center for Astrophysics | Harvard & Smithsonian (CfA), and the primary author of the paper. “It’s an area in outer space that emits a really bright kind of light. We can see this light and trace it back to where it came from, bringing us one step closer to figuring out what’s really going on.”

Leveraging the resolving power of ALMA’s Band 6, developed by the U.S. National Science Foundation’s National Radio Astronomy Observatory (NRAO), the team was able to use the masers to uncover the previously unseen structures in the star’s immediate environment. Qizhou Zhang, a senior astrophysicist at CfA, and the project’s principal investigator added, “We used masers generated by hydrogen to probe the physical and dynamic structures in the gas surrounding MWC 349A and revealed a flattened gas disk with a diameter of 50 au, approximately the size of the Solar System, confirming the near-horizontal disk structure of the star. We also found a fast-moving jet component hidden within the winds flowing away from the star.”

The observed jet is ejecting material away from the star at a blistering 500 km per second. That’s akin to traveling the distance between San Diego, California, and Phoenix, Arizona, in the literal blink of an eye. According to researchers, it is probable that a jet moving this fast is being launched by a magnetic force. In the case of MWC 349A, that force could be a magnetohydrodynamic wind—a type of wind whose movement is dictated by the interplay between the star’s magnetic field and gases present in its surrounding disk.

“Our previous understanding of MWC 349A was that the star was surrounded by a rotating disk and photo-evaporating wind. Strong evidence for an additional collimated jet had not yet been seen in this system. Although we don’t yet know for certain where it comes from or how it is made, it could be that a magnetohydrodynamic wind is producing the jet, in which case the magnetic field is responsible for launching rotating material from the system,” said Prasad. “This could help us to better understand the disk-wind dynamics of MWC 349A, and the interplay between circumstellar disks, winds, and jets in other star systems.”

More information: These results will be presented during a press conference at the 241st proceedings of the American Astronomical Society on Monday, January 9th at 2:15pm Pacific Standard Time (PST).

Source: Hydrogen masers reveal new secrets of a massive star