SPACE
X-ray satellite XMM-newton sees ‘space clover' in a new light
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THIS MULTIWAVELENGTH IMAGE OF THE CLOVERLEAF ORC (ODD RADIO CIRCLE) COMBINES VISIBLE LIGHT OBSERVATIONS FROM THE DESI (DARK ENERGY SPECTROSCOPIC INSTRUMENT) LEGACY SURVEY IN WHITE AND YELLOW, X-RAYS FROM XMM-NEWTON IN BLUE, AND RADIO FROM ASKAP (THE AUSTRALIAN SQUARE KILOMETER ARRAY PATHFINDER) IN RED.
view moreCREDIT: X. ZHANG AND M. KLUGE (MPE), B. KORIBALSKI (CSIRO)
Astronomers have discovered enormous circular radio features of unknown origin around some galaxies. Now, new observations of one dubbed the Cloverleaf suggest it was created by clashing groups of galaxies.
Studying these structures, collectively called ORCs (odd radio circles), in a different kind of light offered scientists a chance to probe everything from supersonic shock waves to black hole behavior.
“This is the first time anyone has seen X-ray emission associated with an ORC,” said Esra Bulbul, an astrophysicist at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, who led the study. “It was the missing key to unlock the secret of the Cloverleaf’s formation.”
A paper describing the results was published in Astronomy and Astrophysics Letters on April 30.
A Serendipitous Discovery
Until 2021, no one knew ORCs existed. Thanks to improved technology, radio surveys became sensitive enough to pick up such faint signals. Over the course of a few years, astronomers discovered eight of these strange structures scattered randomly beyond our galaxy. Each is large enough to envelop an entire galaxy –– sometimes several.
“The power needed to produce such an expansive radio emission is very strong,” Bulbul said. “Some simulations can reproduce their shapes but not their intensity. No simulations explain how to create ORCs.”
When Bulbul learned ORCs hadn’t been studied in X-ray light, she and postdoctoral researcher Xiaoyuan Zhang began poring over data from eROSITA (Extended Roentgen Survey with an Imaging Telescope Array), an orbiting German/Russian X-ray telescope. They noticed some X-ray emission that seemed like it could be from the Cloverleaf, based on less than 7 minutes of observation time.
That gave them a strong enough case to assemble a larger team and secure additional telescope time with XMM-Newton, an ESA (European Space Agency) mission with NASA contributions.
“We were allotted about five-and-a-half hours, and the data came in late one evening in November,” Bulbul said. “I forwarded it to Xiaoyuan, and he came into my office the next morning and said, ‘Detection,’ and I just started cheering!”
“We really got lucky,” Zhang said. “We saw several plausible X-ray point sources close to the ORC in eROSITA observations, but not the expanded emission we saw with XMM-Newton. It turns out the eROSITA sources couldn’t have been from the Cloverleaf, but it was compelling enough to get us to take a closer look.”
Gallivanting Galaxies
The X-ray emission traces the distribution of gas within the group of galaxies like police tape around a crime scene. By seeing how that gas has been disturbed, scientists determined that galaxies embedded in the Cloverleaf are actually members of two separate groups that drew close enough together to merge. The emission’s temperature also hints at the number of galaxies involved.
When galaxies join, their higher combined mass increases their gravity. Surrounding gas begins to fall inward, which heats up the infalling gas. The greater the system’s mass, the hotter the gas becomes.
Based on the emission’s X-ray spectrum, it’s around 15 million degrees Fahrenheit, or between 8 and 9 million degrees Celsius. “That measurement let us deduce that the Cloverleaf ORC is hosted by around a dozen galaxies that have gravitated together, which agrees with what we see in deep visible light images,” Zhang said.
The team proposes the merger produced shock waves that accelerated particles to create radio emission.
“Galaxies interact and coalesce all the time,” said Kim Weaver, the NASA project scientist for XMM-Newton at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who was not involved in the study. “But the source of the accelerated particles is unclear. One fascinating idea for the powerful radio signal is that the resident supermassive black holes went through episodes of extreme activity in the past, and relic electrons from that ancient activity were reaccelerated by this merging event.”
While galaxy group mergers are common, ORCs are very rare. And it’s still unclear how these interactions can produce such strong radio emissions.
“Mergers make up the backbone of structure formation, but there’s something special in this system that rockets the radio emission,” Bulbul said. “We can’t tell right now what it is, so we need more and deeper data from both radio and X-ray telescopes.”
The team solved the mystery of the nature of the Cloverleaf ORC, but also opened up additional questions. They plan to study the Cloverleaf in more detail to tease out answers.
“We stand to learn a lot from more thorough observations because these interactions take in all kinds of science,” Weaver says. “You’ve pretty much got everything that we deal with in the cosmos put together in this little package. It’s like a mini universe.”
For more information on ESA’s XMM-Newton mission, visit: https://science.nasa.gov/mission/xmm-newton/
JOURNAL
Astronomy and Astrophysics
ARTICLE TITLE
The galaxy group merger origin of the Cloverleaf odd radio circle system
Webb telescope probably didn’t find life on an exoplanet — yet
Claims of biosignature gas detection were premature
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ARTIST RENDERING OF THE VIEW ON A HYCEAN WORLD.
view moreCREDIT: SHANG-MIN TSAI/UCR
Recent reports of NASA’s James Webb Space Telescope finding signs of life on a distant planet understandably sparked excitement. A new study challenges this finding, but also outlines how the telescope might verify the presence of the life-produced gas.
The UC Riverside study, published in the Astrophysical Journal Letters, may be a disappointment to extraterrestrial enthusiasts but does not rule out the near-future possibility of discovery.
In 2023 there were tantalizing reports of a biosignature gas in the atmosphere of planet K2-18b, which seemed to have several conditions that would make life possible.
Many exoplanets, meaning planets orbiting other stars, are not easily comparable to Earth. Their temperatures, atmospheres, and climates make it hard to imagine Earth-type life on them.
However, K2-18b is a bit different. “This planet gets almost the same amount of solar radiation as Earth. And if atmosphere is removed as a factor, K2-18b has a temperature close to Earth’s, which is also an ideal situation in which to find life,” said UCR project scientist and paper author Shang-Min Tsai.
K2-18b’s atmosphere is mainly hydrogen, unlike our nitrogen-based atmosphere. But there was speculation that K2-18b has water oceans, like Earth. That makes K2-18b a potentially “Hycean” world, which means a combination of a hydrogen atmosphere and water oceans.
Last year, a Cambridge team revealed methane and carbon dioxide in the atmosphere of K2-18b using JWST – other elements that could point to signs of life.
“What was icing on the cake, in terms of the search for life, is that last year these researchers reported a tentative detection of dimethyl sulfide, or DMS, in the atmosphere of that planet, which is produced by ocean phytoplankton on Earth,” Tsai said. DMS is the main source of airborne sulfur on our planet and may play a role in cloud formation.
Because the telescope data were inconclusive, the UCR researchers wanted to understand whether enough DMS could accumulate to detectable levels on K2-18b, which is about 120 light years away from Earth. As with any planet that far away, obtaining physical samples of atmospheric chemicals is impossible.
“The DMS signal from the Webb telescope was not very strong and only showed up in certain ways when analyzing the data,” Tsai said. “We wanted to know if we could be sure of what seemed like a hint about DMS.”
Based on computer models that account for the physics and chemistry of DMS, as well as the hydrogen-based atmosphere, the researchers found that it is unlikely the data show the presence of DMS. “The signal strongly overlaps with methane, and we think that picking out DMS from methane is beyond this instrument’s capability,” Tsai said.
However, the researchers believe it is possible for DMS to accumulate to detectable levels. For that to happen, plankton or some other life form would have to produce 20 times more DMS than is present on Earth.
Detecting life on exoplanets is a daunting task, given their distance from Earth. To find DMS, the Webb telescope would need to use an instrument better able to detect infrared wavelengths in the atmosphere than the one used last year. Fortunately, the telescope will use such an instrument later this year, revealing definitively whether DMS exists on K2-18b.
"The best biosignatures on an exoplanet may differ significantly from those we find most abundant on Earth today. On a planet with a hydrogen-rich atmosphere, we may be more likely to find DMS made by life instead of oxygen made by plants and bacteria as on Earth,” said UCR astrobiologist Eddie Schwieterman, a senior author of the study.
Given the complexities of searching far-flung planets for signs of life, some wonder about the researchers continued motivations.
“Why do we keep exploring the cosmos for signs of life? Imagine you’re camping in Joshua Tree at night, and you hear something. Your instinct is to shine a light to see what’s out there. That’s what we’re doing too, in a way,” Tsai said.
JOURNAL
The Astrophysical Journal Letters
ARTICLE TITLE
Biogenic sulfur gases as biosignatures on temperate sub-Neptune waterworlds
‘Baby asteroid’ just a toddler in space years, researchers say
ITHACA, N.Y. – An asteroid discovered last November is in fact a solar system toddler – just 2-3 million years old, a Cornell University-led research team estimates using novel statistical calculations.
The team derived the age of Selam, a “moonlet” circling the small asteroid Dinkinesh in the main asteroid belt between Mars and Jupiter, based only on dynamics, or how the pair moves in space. Their calculation agrees with one by NASA’s Lucy mission based on an analysis of surface craters, the more traditional method for dating asteroids.
The new method complements that work and has some advantages: It doesn’t require an expensive spacecraft to capture close-up images; could be more accurate in cases where asteroid surfaces have undergone recent changes; and can be applied to the secondary bodies in dozens of other known binary systems, which account for 15% of near-Earth asteroids, the researchers said.
“Finding the ages of asteroids is important to understanding them, and this one is remarkably young when compared to the age of the solar system, meaning it formed somewhat recently,” said Colby Merrill, a doctoral student in the field of aerospace engineering. “Obtaining the age of this one body can help us to understand the population as a whole.”
Merrill is the first author of “Age of (152830) Dinkinesh-Selam Constrained by Secular Tidal-BYORP Theory,” published in Astronomy & Astrophysics.
Merrill, a dynamics expert who was part of NASA’s Double Asteroid Redirection Test (DART) mission, was watching closely when the Lucy spacecraft flew by Dinkinesh on Nov. 1, 2023, and unexpectedly found Selam. The latter turned out to be “an extraordinarily unique and complex body,” Merrill said – a so-called “contact binary” consisting of two lobes that are essentially rubble piles stuck together, and the first of its kind seen orbiting another asteroid.
Binary asteroids are dynamically complex and fascinating objects that are engaged in a sort of tug of war, the researchers said. Gravity acting on the objects causes them to physically bulge and results in tides, which slowly reduce the system’s energy. Meanwhile, the sun’s radiation also alters the binary system’s energy with an effect termed the Binary Yarkovsky-O’Keefe-Radzievskii-Paddack (BYORP) effect. Eventually, the system will reach an equilibrium where tides and BYORP are equally strong – a stalemate in the tug of war.
Assuming those forces were in equilibrium and plugging in asteroid data shared publicly by the Lucy mission, the researchers calculated how long it would have taken for Selam to reach its current state after forming from surface material ejected by a rapidly spinning Dinkinesh. Along the way, the team said it improved upon preexisting equations that assumed both bodies were equally dense and ignored the secondary body’s mass. Running roughly 1 million calculations with varying parameters, the results produced a median age for Selam of 3 million years old, with 2 million being the most likely result.
Researchers hope to apply their new aging method to other binary systems where dynamics have been well characterized, even without close flybys.
“Used in tandem with crater counting, this method could help better constrain a system’s age,” said Alexia Kubas, a doctoral student in the field of astronomy and space sciences and paper co-author. “If we use two methods and they agree with each other, we can be more confident that we’re getting a meaningful age that describes the current state of the system.”
For additional information, see this Cornell Chronicle story.
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JOURNAL
Astronomy and Astrophysics
DOI
Hungry, hungry white dwarfs: solving the puzzle of stellar metal pollution
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PLANETESIMAL ORBITS AROUND A WHITE DWARF. INITIALLY, EVERY PLANETESIMAL HAS A CIRCULAR, PROGRADE ORBIT. THE KICK FORMS AN ECCENTRIC DEBRIS DISK WHICH WITH PROGRADE (BLUE) AND RETROGRADE ORBITS (ORANGE).
view moreCREDIT: CREDIT: STEVEN BURROWS/MADIGAN GROUP/JILA
Dead stars known as white dwarfs, have a mass like the Sun while being similar in size to Earth. They are common in our galaxy, as 97% of stars are white dwarfs. As stars reach the end of their lives, their cores collapse into the dense ball of a white dwarf, making our galaxy seem like an ethereal graveyard.
Despite their prevalence, the chemical makeup of these stellar remnants has been a conundrum for astronomers for years. The presence of heavy metal elements—like silicon, magnesium, and calcium—on the surface of many of these compact objects is a perplexing discovery that defies our expectations of stellar behavior.
“We know that if these heavy metals are present on the surface of the white dwarf, the white dwarf is dense enough that these heavy metals should very quickly sink toward the core,” explains JILA graduate student Tatsuya Akiba. “So, you shouldn't see any metals on the surface of a white dwarf unless the white dwarf is actively eating something.”
While white dwarfs can consume various nearby objects, such as comets or asteroids (known as planetesimals), the intricacies of this process have yet to be fully explored. However, this behavior could hold the key to unraveling the mystery of a white dwarf's metal composition, potentially leading to exciting revelations about white dwarf dynamics.
In results reported in a new paper in The Astrophysical Journal Letters, Akiba, along with JILA Fellow and University of Colorado Boulder Astrophysical and Planetary Sciences professor Ann-Marie Madigan and undergraduate student Selah McIntyre, believe they have found a reason why these stellar zombies eat their nearby planetesimals. Using computer simulations, the researchers simulated the white dwarf receiving a “natal kick” during its formation (which has been observed) caused by asymmetric mass loss, altering its motion and the dynamics of any surrounding material.
In 80% of their test runs, the researchers observed that, from the kick, the orbits of comets and asteroids within a range of 30 to 240 AU of the white dwarf (corresponding to the Sun–Neptune distance and beyond) became elongated and aligned. Furthermore, around 40% of subsequently eaten planetesimals come from counter-rotating (retrograde) orbits.
The researchers also extended their simulations to examine the white dwarf's dynamics after 100 million years. They found that the white dwarf’s nearby planetesimals still had elongated orbits and moved as one coherent unit, a result never seen before.
“This is something I think is unique about our theory: we can explain why the accretion events are so long-lasting,” states Madigan. “While other mechanisms may explain an original accretion event, our simulations with the kick show why it still happens hundreds of millions of years later.”
These results explain why the heavy metals are found on the surface of a white dwarf, as that white dwarf continuously consumes smaller objects in its path.
It’s All About Gravity
As Madigan’s research group at JILA focuses on gravitational dynamics, looking at the gravity surrounding white dwarfs seemed like a natural focus of study.
“Simulations help us understand the dynamics of different astrophysical objects,” Akiba says. “So, in this simulation, we throw a bunch of asteroids and comets around the white dwarf, which is significantly bigger, and see how the simulation evolves and which of these asteroids and comets the white dwarf eats.”
The researchers hope to take their simulations to greater scales in future projects, looking at how white dwarfs interact with larger planets.
As Akiba elaborates, “Other studies have suggested that asteroids and comets, the small bodies, might not be the only source of metal pollution on the white dwarf’s surface. So, the white dwarfs might eat something bigger, like a planet.”
Discovering More about Solar System Formation
These new findings further reveal more about the formation of white dwarfs, which is important in understanding how solar systems change over millions of years. They also help shed light on the origins and future evolution of our solar system, revealing more about the chemistry involved.
“The vast majority of planets in the universe will end up orbiting a white dwarf,” Madigan says. “It could be that 50% of these systems get eaten by their star, including our own solar system. Now, we have a mechanism to explain why this would happen.”
“Planetesimals can give us insight into other solar systems and planetary compositions beyond where we live in our solar region” McIntyre adds. “White dwarfs aren't just a lens into the past. They're also kind of a lens into the future.”
JOURNAL
The Astrophysical Journal Letters
METHOD OF RESEARCH
Computational simulation/modeling
ARTICLE TITLE
Tidal Disruption of Planetesimals from an Eccentric Debris Disk Following a White Dwarf Natal Kick
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