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PARASITIC VAMPIRE STARS
https://phys.org/news/2024-02-cannibalized-stars.html
https://iopscience.iop.org/article/10.3847/1538-4357/ad13ec
https://earth.com/stellar-cannibalism-scientists-find-remnants-of-devoured-stars
Stellar cannibalism: Scientists find the remnants of devoured stars
by Chrissy Sexton

“A team of astronomers has made a significant breakthrough in understanding the life cycle of close binary star systems. Using the advanced capabilities of Georgia State’s Center for High Angular Resolution Astronomy (CHARA) Array, the experts detected the dim glow of stellar remnants that have been devoured by companion stars, a phenomenon known as stellar cannibalism. The CHARA Array, which is located in Mount Wilson, California, employs six telescopes spread across its summit to function as a single colossal telescope with a diameter of 330 meters.

This unique setup allows astronomers to observe stars with very small angular separations – a task that is impossible with conventional telescopes. This technique, along with the use of sophisticated MIRC-X and MYSTIC cameras developed at the University of Michigan and Exeter University, respectively, enabled the researchers to detect the faint light of stripped stars next to their brighter, predatory companions.

The research was led by postdoctoral research associate Robert Klement. It was focused on a collection of “Be stars” – rapidly rotating stars that are thought to have unusual orbiting companions. Be stars are among the fastest rotating stars in the universe, spinning so quickly that they eject gas from their equatorial regions, forming a distinctive orbiting gas ring. The origin of these Be stars has long intrigued astronomers, with prevailing theories suggesting that they are the result of intense interactions and mass transfer between close binary stars.

“CHARA Array measurements (red ellipses) of the motion of the stripped star (dashed line) that orbits the Be star HR2142 (yellow star) every 81 days. The small black star symbols are the calculated positions of the stripped companion during the time of our observations. The orbit is circular, but appears elliptical because it is tilted with respect to the plane of the sky. Credit: Robert Klement”

Binary star systems, especially those with small separations, experience a complex evolution. As these stars age and expand, the gas from one star can start to flow to its companion, effectively feeding the latter and causing it to spin faster. This mass transfer can strip the donor star of nearly all its gas, leaving behind a hot, dense core – what Klement and his team refer to as the “stellar corpse.” Until now, the existence of these stripped stars was largely theoretical, as detection proved elusive due to their faintness and close proximity to their bright companions. However, the precision of the CHARA Array has enabled astronomers to directly observe the remnants of stellar cannibalism for the first time.

In his two-year observation program, Klement discovered faint light from stripped companions in nine out of 37 surveyed Be stars, focusing on seven targets to track the orbital motion around the Be stars. “The orbits are important because they allow us to determine the masses of stellar pairs,” explained Klement. “Our mass measurements indicate that stripped stars lost almost everything. In the case of the star HR2142, the stripped star probably went from 10 times the mass of the Sun down to about one solar mass.” The findings from this survey not only confirm the cannibalistic nature of some binary star systems, but also open new avenues for understanding more about star evolution. “This survey of Be stars – and the discovery of nine faint companion stars – truly demonstrates the power of CHARA,” said Alison Peck, a program director in the National Science Foundation’s Astronomical Sciences Division.

“Using the array’s exceptional angular resolution and high dynamic range allows us to answer questions about star formation and evolution that have never been possible to answer before.” Douglas Gies, director of the CHARA Array, noted that the research has finally uncovered a key hidden stage in the lives of close stellar pairs. “The CHARA Array survey of the Be stars has revealed directly that these stars were created through a wholesale transformation by mass transfer,” said Gies. “We are now seeing, for the first time, the result of the stellar feast that led to the stripped stars.”

“The Palomar 48 inch telescope at the Palomar Observatory in California with an image of the Milky Way in the background. The stars represent the number of supernovae discovered in each direction and the inset is an image of a galaxy after (left) and before (right) the supernova exploded. Credit: Mickael Rigault.”

SUPERNOVAE EXPLOSION DIVERSITY
https://nature.com/articles/nature10644
https://phys.org/collisions-stellar-cannibalism-diversity-white
https://scitechdaily.com/4000-supernovae-may-change-everything-we-know-about-dark-energy/
4,000 Supernovae May Change Everything We Know About Dark Energy
by Trinity College Dublin / February 14, 2025
“From star cannibalism to violent collisions, these stellar explosions, captured in unprecedented detail, are challenging our understanding of dark energy and the expanding universe. With nearly 4,000 supernovae analyzed, astrophysicists now question whether these cosmic blasts can still serve as reliable distance markers, potentially reshaping our cosmic measurements. Astrophysicists have discovered an unexpected variety in how white dwarf stars explode in deep space.

By analyzing nearly 4,000 such events recorded in detail by a next-generation astronomical survey, researchers have uncovered new insights that could improve our ability to measure cosmic distances and deepen our understanding of dark energy. For decades, the violent explosions of white dwarfs at the end of their lifecycles have been crucial in studying dark energy — the mysterious force driving the universe’s accelerating expansion.

These supernovae also play a key role in creating essential elements like titanium, iron, and nickel, forged under the extreme heat and pressure of their detonation. This research marks a significant breakthrough in our understanding of stellar explosions, supported by a newly released dataset and 21 related studies published today (February 14) in a special issue of Astronomy & Astrophysics. This unique dataset of nearly 4,000 nearby supernovae is many times larger than previous similar samples and has allowed crucial breakthroughs in understanding how these white dwarfs explode.

The sample was obtained by Zwicky Transient Facility (ZTF), a Caltech-led astronomical sky survey, with key involvement of researchers at Trinity College Dublin, led by Prof. Kate Maguire in the School of Physics. “Thanks to ZTF’s unique ability to scan the sky rapidly and deeply, it has been possible to discover new explosions of stars up to one million times fainter than the dimmest stars visible to the naked eye,” highlights Prof. Kate Maguire. One of the key results, led by the group at Trinity, is the discovery that there are multiple exotic ways that white dwarfs can explode, including in collisions of two stars in luminous stellar spectacles, as well as the cannibalism of stars by their companions in double star systems.

This is only possible with this sample due to the ability to discover very faint blips combined with large sample sizes. And the surprising diversity may have implications for the use of these supernovae to measure distances in the Universe since the constraints on the properties of dark energy crucially demand that these explosions can be standardized. “The diversity of ways that white dwarf stars can blow up is much greater than previously expected, resulting in explosions that range from being so faint they are barely visible to others that are bright enough to see for many months to years afterward,” says Prof. Maguire.”

“The Milky Way and its halo of stars, many of which have been cannabalised via collisions with other galaxies (Image: ESA/Gaia/DPAC, T Donlon et al. 2024; Background Milky Way and Magellanic Clouds: Stefan Payne-Wardenaar)”

GALACTIC CANNIBALISM
https://space.com/gaia-reveals-spiral-arms-milky-way
https://academic.oup.com/mnras/article/531/1/1422/7675507
https://space.com/gaia-space-telescope-milky-way-cannibalism
The Milky Way’s last major act of galactic cannibalism was surprisingly recent
by Robert Lea / June 10, 2024

“New findings from the Gaia space telescope indicate the Milky Way may have cannibalized a small galaxy not too long ago, cosmically speaking. In fact, the last major collision between our galaxy and another seems to have occurred billions of years later than previously suspected. The Milky Way has been long understood to have grown via a series of violent collisions, which see smaller galaxies ripped apart by the immense gravitational influence of our solar system’s spiral home. These collisions distribute stars from the devoured galaxy across the halo that surrounds the main disk of the Milky Way and its distinctive spiral arms.

These bouts of galactic cannibalism also send “wrinkles” rippling through the Milky Way that affect different “families” of stars, with different origins, in different ways. With its ability to precisely pinpoint the position and motion of over 100,000 stars local to the solar system within the full catalog of stellar bodies in monitors, Gaia aims to retell the history of the Milky Way by counting its wrinkles. “We get wrinklier as we age, but our work reveals that the opposite is true for the Milky Way. It’s a sort of cosmic Benjamin Button, getting less wrinkly over time,” Thomas Donlon, study team leader of the Rensselaer Polytechnic Institute and University of Alabama scientist, said in a statement. “By looking at how these wrinkles dissipate over time, we can trace when the Milky Way experienced its last big crash – and it turns out this happened billions of years later than we thought.” These galactic wrinkles were only discovered by Gaia in 2018; this marks the first time they have been extensively researched to reveal the timing of the collision that created them.

The halo of our galaxy is populated with stars that have strange orbits, with many of these believed to be the “leftovers” of galaxies the Milky Way once devoured. Many of those stars are believed to be the wreckage of the so-called “last major merger,” referring to the last time the Milky Way experienced a significant collision with another galaxy. Scientists think this final major collision may have involved a massive dwarf galaxy, and the event is known as the Gaia-Sausage-Enceladus (GSE) merger. It is thought to have infused the Milky Way with stars on orbits that bring them close to the Galactic Center. The GSE event is thought to have happened between eight and 11 billion years ago when the Milky Way was in its infancy.

Since 2020, Thomas and his team have been comparing the Milky Way’s wrinkles to simulations of how galactic collisions and mergers could have created them. However, the Gaia observations of these strangely orbiting stars — released as part of the space telescope’s Data Release 3 in 2022 — indicate these odd stellar bodies could have been deposited by a different merger event. “We can see how the shapes and number of wrinkles change over time using these simulated mergers. This lets us pinpoint the exact time when the simulation best matches what we see in real Gaia data of the Milky Way today — a method we used in this new study, too,” explained Donlon. “By doing this, we found that the wrinkles were likely caused by a dwarf galaxy colliding with the Milky Way around 2.7 billion years ago. We named this event the Virgo Radial Merger.”

“The map of the Milky Way is changing thanks to recent releases
of data from the European Gaia mission. (Image: NASA JPL)”

“For the wrinkles of stars to be as clear as they appear in Gaia data, they must have joined us less than three billion years ago — at least five billion years later than was previously thought,” team member Heidi Jo Newberg, also of Rensselaer Polytechnic Institute, said. “New wrinkles of stars form each time the stars swing back and forth through the center of the Milky Way. If they’d joined us eight billion years ago, there would be so many wrinkles right next to each other that we would no longer see them as separate features.”

The recent examination of Gaia’s observations calls into question whether a massive ancient merger in the early history of the Milky Way is really needed to explain the strange orbits of some stars in the galactic. It also casts doubt on all the stars previously associated with the GSE merger. “This result — that a large portion of the Milky Way only joined us within the last few billion years — is a big change from what astronomers thought up until now,” Donlon said. “Many popular models and ideas about how the Milky Way grows would expect a recent head-on collision with a dwarf galaxy of this mass to be very rare.” The team also thinks that the Virgo Radial Merger brought to our galaxy a family of other small dwarf galaxies and star clusters, all of which would also have been devoured by the Milky Way at around the same time.

Future investigation and data from Gaia could show if any objects previously associated with the GSE event are actually connected to the more recent Virgo Radial Merger. This new research is the latest in a treasure trove of results emerging from Gaia data that are rewriting the history of the Milky Way. Such cosmic revisionism has been made possible thanks to Gaia’s unique ability to explore a vast number of stars over Earth, allowing the space telescope to compile an unrivaled map of the positions, distances and motions of around 1.5 billion stars thus far. “The Milky Way’s history is constantly being rewritten, in no small part, thanks to new data from Gaia,” Donlon concluded. “Our picture of the Milky Way’s past has changed dramatically from even a decade ago, and I think our understanding of these mergers will continue to change rapidly.”

“The core of globular cluster 47 Tucanae is home to many blue stragglers, rejuvenated stars that glow with the blue light of young stars. A ground-based telescope image (on the left) shows the entire crowded core of 47 Tucanae, located 15,000 light-years away in the constellation Tucana. Peering into the heart of the globular cluster’s bright core, the Hubble Space Telescope’s Wide Field and Planetary Camera 2 separated the dense clump of stars into many individual stars (image on right). Some of these stars shine with the light of old stars; others with the blue light of blue stragglers. The yellow circles in the Hubble telescope image highlight several of the cluster’s blue stragglers.”

ROGUE STARS & BLUE STRAGGLERS
https://hubblesite.org/contents/news-releases/1997/news-1997-35.html
https://universetoday.com/rare-oppurtunity-to-watch-blue-straggler-forming
https://astronomy.com/stellar-cannibalism-key-to-formation-of-overweight-stars
‘Stellar cannibalism’ is key to formation of overweight stars / January 14, 2009

“Researchers have discovered that the mysterious overweight stars known as blue stragglers are the result of ‘stellar cannibalism’ where plasma is gradually pulled from one star to another to form a massive, unusually hot star that appears younger than it is. The process takes place in binary stars – star systems consisting of two stars orbiting around their common center of mass. This helps to resolve a long-standing mystery in stellar evolution.

Blue stragglers are found throughout the universe in globular clusters of about 100,000 stars, tightly bound by gravity. According to conventional theories, the massive blue stragglers found in these clusters should have died long ago, because all stars in a cluster are born at the same time and should therefore be at a similar phase. These massive rogue stars, however, appear to be much younger than the other stars and are found in virtually every observed cluster.

Dr. Christian Knigge from Southampton University in United Kingdom, who led the study, said, “The origin of blue stragglers has been a long-standing mystery. The only thing that was clear is that at least two stars must be involved in the creation of every single blue straggler, because isolated stars this massive simply should not exist in these clusters.”

Professor Alison Sills from the McMaster University in Hamilton, Ontario, Canada, said, “We’ve known of these stellar anomalies for 55 years. Over time, two main theories have emerged: that blue stragglers were created through collisions with other stars; or that one star in a binary system was ‘reborn’ by pulling matter off its companion.” The researchers looked at blue stragglers in 56 globular clusters. They found that the total number of blue stragglers in a given cluster did not correlate with the predicted collision rate – dispelling the theory that blue stragglers are created through collisions with other stars.

They did, however, discover a connection between the total mass contained in the core of the globular cluster and the number of blue stragglers observed within it. Since more massive cores also contain more binary stars, they were able to infer a relationship between blue stragglers and binaries in globular clusters. They also showed that this conclusion is supported by preliminary observations that directly measured the abundance of binary stars in cluster cores.

All of this points to ‘stellar cannibalism’ as the primary mechanism for blue straggler formation. Knigge said, “This is the strongest and most direct evidence to date that most blue stragglers, even those found in the cluster cores, are the offspring of two binary stars. In our future work, we will want to determine whether the binary parents of blue stragglers evolve mostly in isolation, or whether dynamical encounters with other stars in the clusters are required somewhere along the line in order to explain our results.”

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