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Current gravitational wave detectors are primarily sensitive to powerful sources like black hole–black hole mergers — what types of events may next-generation gravitational wave detectors reveal? A recent study explores stripped subgiant stars around supermassive black holes and how their gravitational waves may be detected in next-generation missions. 

Inspiraling Stars and Gravitational Waves

The immense gravity of supermassive black holes at the centers of galaxies drives complicated dynamics that could generate so far undetected gravitational wave signals. One predicted source of gravitational waves is a rare event known as an extreme-mass-ratio inspiral (EMRI) — a prolonged inspiraling of a stellar-mass object around a supermassive black hole, driven inward by gravitational waves. EMRIs are thought to arise when a stellar-mass object is captured into a close-in nearly circular orbit around a supermassive black hole, emitting gravitational waves due to the object’s proximity to the black hole. 

To date, most studies have explored EMRIs involving compact objects like black holes, but an EMRI with an inspiraling star is possible, though not yet well explored nor understood. In addition to generating gravitational waves, a stellar EMRI will also emit light as the supermassive black hole rips up siphoned material from the star. These events may explain recent observations of peculiar X-ray transients, but their gravitational wave counterparts remain undetected as EMRIs’ predicted signals fall outside the sensitivity range of current ground-based gravitational wave detectors. However, the Laser Interferometer Space Antenna (LISA), a space-based gravitational wave detector expected to launch in the mid-2030s, will be sensitive to the gravitational waves of EMRIs. In order to properly identify them, we must better understand what to expect.

Diagram showing the evolutionary steps of a supermassive black hole capturing a subgiant star that subsequently undergoes mass transfer and gravitational wave-driven inspiral. Click to enlarge. [Olejak et al 2025]

Modeling a Subgiant Inspiral

Seeking to better predict what the gravitational waves of stellar EMRIs may look like and what LISA may detect, Aleksandra Olejak (Max Planck Institute for Astrophysics) and collaborators used stellar evolution code to simulate the evolution of a subgiant star as it transfers mass to a supermassive black hole and gravitational waves draw in its orbit. In their models, the supermassive black hole captures the star through scattering between stars or tidal interactions with a stellar binary in which the black hole catches one star and ejects the other.

Once the star is captured, its orbit becomes circularized around the black hole, pulled in closer over time until mass transfer of the star’s outer envelope begins. During this stage, the gravitational wave signal remains too weak to be detected. However, as the star is stripped down to just a helium core, it contracts and experiences an extended gravitational wave–driven orbital decay.

Possible Detections with LISA

The strain spectral density as a function of the detected frequency of the gravitational wave. The colorful line indicates the signal from a stripped subgiant at the Milky Way’s center, and the gray lines are what would be observed at greater distances. LISA’s sensitivity to gravitational wave signals is shown with the black and red lines with detectable sources falling to the right of or above the line. Click to enlarge. [Olejak et al 2025]

With a better understanding of this process, the authors explored how such a signal compares to LISA’s sensitivity at various distances away from Earth. They found that as a star spirals inward, the gravitational wave signal grows stronger and enters LISA’s detectable frequency range. Stellar EMRIs occurring closer to Earth will result in higher signal-to-noise detections, and the gravitational waves from those occurring within the Milky Way would remain detectable for hundreds of thousands of years. The authors’ analysis revealed that, given their simulations, LISA could detect such an event out to ~3 billion light-years away. They estimate that a few systems involving similarly stripped subgiant stars could be detectable by LISA during a four-year mission, with a 1% chance of our galactic center hosting one.    

This study provides exciting potential for what is possible with LISA, and future detections of gravitational waves will shine further light onto the complicated dynamics of galactic centers and the evolution of stars around supermassive black holes.

Citation

“Supermassive Black Holes Stripping a Subgiant Star Down to Its Helium Core: A New Type of Multimessenger Source for LISA,” Aleksandra Olejak et al 2025 ApJL 987 L11. doi:10.3847/2041-8213/ade432

Illustration of an accreting supermassive black hole shrouded by dust

Astronomers have confirmed the discovery of a little red dot galaxy from when the universe was roughly half a billion years old. The galaxy, CAPERS-LRD-z9, is the most distant object to show the tell-tale broad emission lines of gas spiraling around a black hole, opening a new window onto black hole growth in the early universe.

Little Red Dots in the Spotlight

Images of six "little red dot" galaxies from JWST

Images of six “little red dot” galaxies from JWST. [NASA/ESA/CSA/I. Labbe]

Of all the discoveries JWST has enabled since its launch, none seems as enduringly mysterious as the tiny, distant galaxies nicknamed “little red dots.” These early universe objects are characterized by their small sizes, red color in JWST images, and V-shaped spectra.

But what are little red dots? Are they growing supermassive black holes busily amassing gas? Compact collections of old stars? Shredded stars spoon-feeding baby black holes? As reported today in the Astrophysical Journal Letters, new JWST data provide answers for one particularly distant dot.

JWST Takes Another Look

CAPERS-LRD-z9 was first identified as a possible high-redshift little red dot when it was observed by the Public Release IMaging for Extragalactic Research (PRIMER) survey with JWST’s Near-Infrared Camera (NIRCam). Anthony Taylor (The University of Texas at Austin) and collaborators followed up on the discovery with JWST Near-Infrared Spectrograph (NIRSpec) observations from the CANDELS-Area Prism Epoch of Reionization Survey (CAPERS). This spectrum pinned the object’s redshift at z = 9.288, corresponding to when the universe was only about half a billion years old.

With CAPERS-LRD-z9 placed along the cosmic timeline, Taylor and coauthors turned to the question of its identity. The JWST spectra revealed a broad emission line from hydrogen gas moving at thousands of kilometers per second — evidence that CAPERS-LRD-z9 harbors an accreting supermassive black hole (an active galactic nucleus or AGN) that is spinning gas into a frenzy around it. CAPERS-LRD-z9 is the most distant object known to show this characteristic signature of a growing black hole.

JWST spectrum of CAPERS-LRD-z9

JWST NIRSpec spectrum of CAPERS-LRD-z9 with the fit from the AGN (red) plus stars (blue) model. Click to enlarge. [Taylor et al. 2025]

Next, the team examined the little red dot’s spectral energy distribution, using a model that includes an AGN and a stellar population. To reproduce the step-like discontinuity seen in CAPERS-LRD-z9’s spectrum, they used a model in which the supermassive black hole is shrouded in a shell of dusty gas. The model fit well with a black hole mass of 38 million solar masses (though the authors estimate the mass could be anywhere in the range of 4.5–316 million solar masses), an upper limit of 1 billion solar masses on the stellar mass, and a gas density of 1010 per cubic centimeter surrounding the black hole.

Seeding a Black Hole

plot of black hole mass versus redshift for detected AGN and quasars

Observed black hole masses and redshifts of several sources, including CAPERS-LRD-z9 (large red star), quasars at redshifts of z > 6 (blue squares), massive spectroscopically confirmed AGN with broad emission lines (filled symbols), and the highest-redshift AGN detected to date via X-ray or ultraviolet emission. The shaded areas show the black hole masses achievable through accretion at the Eddington rate onto a massive (red) or stellar-mass (purple) seed. Click to enlarge. [Taylor et al. 2025]

Since successfully fitting one model doesn’t immediately rule out others, Taylor’s team also applied a stars-only model to the spectrum. This model fit poorly, and combined with the presence of broad hydrogen lines from fast-moving gas, this is strong evidence that CAPERS-LRD-z9 contains an accreting supermassive black hole.

This finding raises the question of how a black hole can grow to millions of solar masses in just 500 million years. The authors showed that this is possible in two scenarios: either the black hole began as a >10,000-solar-mass “seed” that grew at the Eddington rate — the hypothetical limit at which a black hole can accrete matter — or it started out as a smaller, ~100-solar-mass seed that grew at a super-Eddington rate. The observations rule out the possibility that the black hole grew from a ~100-solar-mass seed accreting at or below the Eddington rate.

In addition to setting the record for most distant broad-line AGN known, CAPERS-LRD-z9 gives new intel on the lives of black holes in the early universe. This won’t be the last we hear about this little red dot!

Citation

“CAPERS-LRD-z9: A Gas Enshrouded Little Red Dot Hosting a Broad-Line AGN at z = 9.288,” Anthony J. Taylor et al 2025 ApJL 989 L7. doi:10.3847/2041-8213/ade789

NGC 4151

Astronomers have known about NGC 4151’s X-ray-emitting active galactic nucleus since the 1970s. Now, observations from the X-Ray Imaging and Spectroscopy Mission (XRISM) show it in a whole new light.

Getting Feedback

illustration of active galactic nucleus winds

Illustration of the winds and accretion disk of an active galactic nucleus. [NASA, and M. Weiss (Chandra X -ray Center)]

Active galactic nuclei (AGNs) are extremely luminous galactic centers powered by accretion of matter onto a supermassive black hole. These nuclei provide a venue for galaxies and their supermassive black holes to interact. This interaction goes both ways, with the galaxy providing fuel to the supermassive black hole, and the AGN injecting material, momentum, and energy back into the surrounding galaxy in a process known as feedback.

AGN feedback can dramatically reshape a galaxy’s star formation and evolution. One of the most powerful ways an accreting supermassive black hole can influence its host galaxy is through ionized winds, which can scour the surrounding galactic bulge clear of star-forming material. There is much still to learn about these winds, including how they are structured and what physical processes drive them.

New Insights from XRISM

NGC 4151 provides an excellent opportunity to learn about active galactic nucleus winds. Only about 60 million light-years away, NGC 4151 hosts an accreting 34-million-solar-mass black hole that is luminous, highly variable, and shows a complex absorption spectrum that suggests that the AGN’s intense radiation is filtering through outflowing winds.

XRISM spectrum of NGC 4151

XRISM spectrum of NGC 4151, showing the multi-component model fit. Click to enlarge. [Xiang et al. 2025]

To study the winds of NGC 4151, a team led by Xin Xiang (University of Michigan) obtained X-ray spectra of NGC 4151 from XRISM on five dates spanning a period of about 6 months.

Xiang’s team modeled the array of absorption and emission lines in the XRISM spectra to tease out the properties of NGC 4151’s winds. They found that the best match to the spectra required up to six layers of absorbing material, showing that NGC 4151’s winds are highly structured. These absorbing layers took the form of slow warm absorbers (velocities between 100 and 1,000 km/s), very fast outflows (1,000–10,000 km/s), and ultra-fast outflows (10,000–100,000 km/s; up to one-third of the speed of light). While most of these components carry away mass as fast as or faster than the black hole accretes, some appeared to lack the oomph to escape the oncoming accretion flow, suggesting that they might be “failed” winds that curl back toward the AGN.

Being Blown Away

The data showed that the winds are likely magnetocentrifugally driven, meaning that material is lifted and accelerated from the surface of the disk along magnetic field lines as the disk rotates. For some of the wind components, especially warm absorbers emerging farther from the black hole, radiation pressure may be a driver as well.

Diagram of the multiple wind components of NGC 4151

Diagram of the wind components of NGC 4151, including ultra-fast outflows (UFOs), very fast outflows (VFOs), and warm absorbers (WAs). [Xiang et al. 2025]

Altogether, the modeling paints a picture of a complex, asymmetric, time-variable, and clumpy set of winds that carry significant mass away from the AGN. Several fast-moving outflow components appear to exceed the threshold luminosity necessary to blow star-forming gas out of the galactic bulge, suggesting that star formation in NGC 4151’s galactic bulge may someday be halted entirely by these winds.

Citation

“XRISM Spectroscopy of Accretion-Driven Wind Feedback in NGC 4151,” Xin Xiang et al 2025 ApJL 988 L54. doi:10.3847/2041-8213/adee9b

Crab Nebula

With a measurement 15 years in the making, astronomers have pinned down the path of a neutron star launched by a collapsing star. This finding helps to explain how neutron stars are “kicked” into space by supernovae.

Just for Kicks

Crab Nebula pulsar

This multiwavelength image shows the pulsar — a type of neutron star — at the center of the Crab Nebula supernova remnant. [X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA-JPL-Caltech]

When massive stars reach the end of their lives, they expire in a final, fantastic cosmic display: a core-collapse supernova. These events can leave behind remnants called neutron stars, which are composed of highly compressed neutrons arranged in a sphere roughly as wide as the island of Manhattan is long.

Many neutron stars have been observed to zip through space at hundreds of kilometers per second, which suggests that these stars get a “kick” when they’re born in a supernova. However, the details of this kick are not yet clear. Are neutron stars launched into space in the opposite direction from the material ejected in the explosion, as some studies suggest? Or is the neutron star’s motion determined by the burst of neutrinos that carries off most of the supernova’s energy, as other studies hint?

Worth the Wait

To answer these questions, astronomers must compare the velocities of young neutron stars to the movement of their associated supernova remnants. In a recent research article, Tyler Holland-Ashford (NASA Goddard Space Flight Center) and collaborators measured the motion of the neutron star affiliated with the G18.9–1.1 supernova remnant, using Chandra observations made 15 years apart in September 2009 and July 2024.

Chandra images of the field containing the neutron star

Chandra images from 2009 (left) and 2024 (right; this image is the merged product of two observations). The neutron star is shown in black. Blue and red sources were used for astrometric correction. The red sources were present in both observations. Click to enlarge. [Holland-Ashford et al. 2025]

Making this measurement is trickier than it sounds, requiring more finesse than simply charting the location of the neutron star at different times. Holland-Ashford’s team used measurements from the ultra-precise star-mapping Gaia spacecraft to account for the proper motions of other sources in the images and to put the observations from the two time periods into the same frame of reference. After these adjustments, they measured the proper motion of the neutron star to be 24.7 milliarcseconds per year.

Because the distance to the supernova remnant is uncertain, this proper motion could correspond to a range of transverse velocities. For distance estimates of 6,800 and 12,400 light-years, this puts the neutron star’s transverse velocity at 264 or 474 km/s, respectively.

A Distinct Offset

supernova remnant and neutron star location

Current location of the neutron star (purple) compared to the geometric center of the supernova remnant (red) and the center of its X-ray emission (green). Even assuming the oldest reasonable age for the supernova remnant, there is a distinct offset between the neutron star’s birthplace and the center of the remnant. Click to enlarge. [Holland-Ashford et al. 2025]

What does this velocity tell us about the kick this neutron star received at birth? To probe the origin of the kick, the team first used the neutron star’s measured velocity and the likely age of the supernova remnant to trace the neutron star’s trajectory back to its birth site. This showed that the neutron star’s birthplace is offset from the center of the supernova remnant by several arcseconds, though both locations fall along the trajectory set by the neutron star’s motion. This suggests that the ejected stellar material and the newborn material were kicked in opposite directions by the supernova, most likely by a theorized “conservation-of-momentum-like” process.

Currently, Chandra is the only X-ray observatory with fine enough resolution to make the kind of measurement needed in this study. Luckily, Chandra’s observations of neutron stars stretch back to 1999, and combining this wealth of data with sensitive measurements from future high-resolution X-ray instruments should provide the long time baselines needed to trace the trajectories of more neutron stars and further explore the origins of neutron star kicks.

Citation

“Proper Motion of the Neutron Star in the Supernova Remnant G18.9–1.1,” Tyler Holland-Ashford et al 2025 ApJ 988 218. doi:10.3847/1538-4357/adeb7e

collage of multiple illustrations and images

There’s been a lot of astronomy in the news lately! Many recent news stories have featured research published in the AAS journals, so today we’re taking a look at four research articles that have recently gotten attention from the media.

New Horizons Navigates by the Stars

Starting closest to home, the first article describes a technological demonstration that took place on the edge of our solar system. In the nearly 20 years since its launch, the New Horizons spacecraft has ventured from Cape Canaveral to its current berth in the Kuiper Belt, roughly 61 au (5.7 billion miles or 9.1 billion kilometers) from Earth. In that time, New Horizons became the first spacecraft to venture close to Pluto and its moons as well as a second Kuiper Belt object called Arrokoth.

images of Proxima Centauri and Wolf 359 from New Horizons and from Earth

The difference in the position of Proxima Centauri (top row) and Wolf 359 (bottom row) as seen from New Horizons (left column) and Earth (right column) is evident by eye. [Adapted from Lauer et al. 2025]

New Horizons has also traveled a large enough distance for nearby stars to shift their positions relative to the background of more distant stars, enabling a measurement of the spacecraft’s position in space. Tod Lauer (NSF National Optical Infrared Astronomy Research Laboratory) and collaborators demonstrated this ability using New Horizons Long Range Reconnaissance Imager (LORRI) observations from April 2020, when the spacecraft was 47 au from the Sun. The stars used for the parallax measurements were Proxima Centauri and Wolf 359, two of the nearest stars to Earth. The team measured the stars’ positions relative to the positions of background stars and compared the results to observations made from Earth.

From the apparent shift in position of the two stars, the team determined the location of the spacecraft in space as well as the error in their measurement. They were able to ascertain the position of the spacecraft to within 0.44 au of its true position.

Though this measurement is far less accurate than localization with the Deep Space Network, it’s still an important step toward understanding the future prospects for autonomous spacecraft navigation via the stars. Looking ahead to future interstellar navigation systems, the team showed that measurements of a few nearby stars (Proxima Centauri and Barnard’s Star are the best bets for journeys within tens of thousands of astronomical units of the Sun) are more useful than measurements of a larger number of more distant stars. Basing measurements on a larger number of images to cut down on random scatter in the position of the star would also improve the results, as would simply using newer instruments. Ultimately, other types of navigation systems, such as those based on measurements of pulsars, are more likely to reach the precision necessary for autonomous spacecraft navigation. However, this method remains interesting, given that the straightforward nature of the imaging and analysis is already well within the capabilities of modern spacecraft systems.

First Detection of Semiheavy Water Ice Around a Low-Mass Protostar

Next up is a discovery from 457 light-years away in the Taurus molecular cloud. With a mass of 0.3–0.5 solar mass, a disk spanning 75–125 au, and a surrounding envelope of 0.9 solar mass, the isolated low-mass protostar L1527 is likely to grow into a star of similar mass to the Sun. That makes it an excellent target for investigating how planetary systems like our own acquire critical molecules like water.

Katerina Slavicinska (Leiden Observatory) and coauthors used JWST to search L1527 for semiheavy water (HDO) — a water molecule in which one of the hydrogen atoms is replaced with a deuterium atom. HDO has been detected in many locations throughout our solar system, including in Earth’s oceans, and water in our solar system tends to contain a high abundance of HDO molecules. A high deuterium abundance can be linked to formation in a cold environment, suggesting that our solar system’s water may have formed in the icy clouds out of which the Sun and the planets were born.

JWST spectrum of L1527

JWST spectrum of L1527. [Slavicinska et al. 2025]

Thanks to JWST’s high resolution and sensitivity, Slavicinska’s team was able to detect HDO ice in L1527, where previous, lower-resolution observations had only hinted at the presence of the molecule. They also measured emission from H2O ice, yielding a ratio of the abundances of the two types of water. The ice HDO/H2O ratio of L1527 is consistent with the gas HDO/H2O ratios of other isolated low-mass protostars and 4–7 times higher than the gas HDO/H2O ratios of clustered low-mass protostars.

plot of the ratio of semiheavy water to water for various solar system objects and protostars

HDO/H2O ratios for various solar system objects and protostars. Click to enlarge. [Slavicinska et al. 2025]

This difference may be due to differences in the star-forming environments of clustered protostars, or it may signal that the water in these protostars underwent gas-phase processing at some point. If environmental factors are the cause, that would suggest that L1527 should have a higher HDO/H2O ratio than objects in our solar system, since the Sun likely formed in a cluster environment.

This study demonstrates JWST’s ability to investigate the chemistry of young stars and probe the chemical evolution of protostars and planetary systems. Slavicinska and coauthors identified two important next steps to advance our understanding of the chemistry of star- and planet-forming environments: 1) measuring gas-phase and ice-phase HDO/H2O ratios in the same object to understand how gas-phase chemistry alters water around protostars and 2) measuring the HDO/H2O ratios of larger samples of isolated and clustered protostars in order to understand the impact of environmental factors.

A Massive Planet Approaches Its Doom

TOI-2109b is an ultra-hot Jupiter exoplanet that orbits its host star every 16 hours, giving it the shortest period of any known hot Jupiter. Orbiting its host star so closely, TOI-2109b is susceptible to powerful tidal forces that can lead to an exchange of angular momentum between the planet and its home star, potentially causing the planet to spiral inward and be engulfed by its star.

orbital periods, temperatures, and radii of ultra-short-period hot Jupiters

Comparison of the orbital period, temperature, and radius of TOI-2109b to other ultra-short-period Jupiters. [Alvarado-Montes et al. 2025]

Jaime Alvarado-Montes (Macquarie University) and collaborators investigated how and when TOI-2109b’s doom might come about. Critical to the discussion is the uncertain age of TOI-2109, which likely lies in the range of 1.09–2.65 billion years. If the star’s age is on the lower end of the range, TOI-2109b’s orbit would decay slowly (~4 milliseconds per year); if the star’s age is on the higher end of the range, the planet’s orbit would decay quickly (~1,100 milliseconds per year). The decay rate depends on the star’s age because planets lose their kinetic energy due to friction inside their host stars, and the efficiency of this process depends upon the interior structure of the star, which changes with age.

Alvarado-Montes and coauthors used data from the Transiting Exoplanet Survey Satellite (TESS), the CHaracterising ExOPlanet Satellite (CHEOPS), and multiple ground-based telescopes to constrain the rate of change of TOI-2109b’s orbital period. Taking into account changes in transit timing due to an outer planet candidate in the system, deviations from spherical symmetry, and other factors, the authors find a likely orbital decay rate of just 2.6 milliseconds per year. This is consistent with the rate predicted for a “young” host star, and it’s expected to shift TOI-2109b’s mid-transit time by a few seconds over a three-year period. This change is potentially detectable with high-cadence observations, helping to understand not just the fate of TOI-2109b, but of ultra-short-period planets as a whole.

Discovery of a Mysterious Long-Period Transient

Finally, Fengqiu Adam Dong (National Radio Astronomy Observatory; Green Bank Observatory) and coauthors recently described their discovery of an unusual long-period transient radio signal. Long-period radio transients exhibit signals that repeat with periods ranging from 10 seconds to multiple hours. While the exact origin of these signals remains unknown, researchers suspect that magnetic white dwarfs and neutron stars are the cause, with the two sources perhaps representing different classes under the long-period radio transient umbrella.

The signal was detected by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) radio telescope, which was searching for bursts of radio emission from pulsars within our galaxy. The newly discovered radio signal, which comes from a source named CHIME J1634+44, has a primary period of 841 seconds and a secondary period of 4,206 seconds, making it decidedly un-pulsar-like. Dong’s team performed follow-up observations of CHIME J1634+44 with the Very Large Array, the Neil Gehrels Swift Observatory, and the Green Bank Telescope. The Very Large Array and the Green Bank Telescope each detected two bursts from the source, bringing the total number of bursts reported in this study to 89.

images of CHIME J1634+44

Continuum images of CHIME J1634+44 from the Very Large Array. The two panels show the autocorrelation function of the left-hand (left) and right-hand (right) circularly polarized components of the signal. CHIME J1634+44 is only detected in one of these two images, which suggests a 100% circularly polarized signal. [Dong et al. 2025]

CHIME J1634+44 is unusual in a number of ways. Its emission is almost entirely circularly polarized, meaning that the plane in which the electric and magnetic waves of the radio signal oscillates rotates in a circle as the signal travels through space. The time between pulses is also decreasing, slowly but steadily. If the pulsation period coincides with an object’s spin period, this means that the object is spinning faster as time goes on; this could occur if the source of the bursts is accreting material from a companion. If instead the pulsation period is linked to the orbital period of an object in a binary system, emission of gravitational waves could be causing the orbit to shrink.

What kind of object could produce this signal? Dong and coauthors considered binary systems containing either a white dwarf or a neutron star. Neither scenario perfectly fits the available evidence, but the team concluded that a binary system containing a neutron star is the likeliest source of the long-period, highly polarized, steadily accelerating pulses of CHIME J1634+44. The strongest evidence in favor of this scenario is that neutron stars are known to exhibit strongly polarized pulses of similar luminosity to those from CHIME J1634+44. Dong’s team expects that CHIME J1634+44 will remain an important test of theories of long-period radio transients — and as new information emerges about its identity, CHIME J1634+44 might just find its way into the news again!

Citation

“A Demonstration of Interstellar Navigation Using New Horizons,” Tod R. Lauer et al 2025 AJ 170 22. doi:10.3847/1538-3881/addabe

“HDO Ice Detected Toward an Isolated Low-Mass Protostar with JWST,” Katerina Slavicinska et al 2025 ApJL 986 L19. doi:10.3847/2041-8213/addb45

“Orbital Decay of the Ultra-Hot Jupiter TOI-2109b: Tidal Constraints and Transit-Timing Analysis,” Jaime A. Alvarado-Montes et al 2025 ApJ 988 66. doi:10.3847/1538-4357/ade057

“CHIME/Fast Radio Burst Discovery of an Unusual Circularly Polarized Long-Period Radio Transient with an Accelerating Spin Period,” Fengqiu Adam Dong et al 2025 ApJL 988 L29. doi:10.3847/2041-8213/adeaab

Through detailed simulations of gas and dust, a recent study revealed that the behavior of dust within protoplanetary disks is a bit more complex than previously assumed.

Dust Traps in Protoplanetary Disks

As a planet forms within a protoplanetary disk — dust and gas orbiting a new star — tidal interactions between the budding body and the dusty material surrounding it can create pressure bumps where dust builds up. These dust traps appear as rings in observations of protoplanetary disks.

Dust traps are thought to play a critical role in the disk’s evolution and the early stages of planet formation. Dust traps may prevent solid material from migrating inward, starving the inner disk and impeding planet growth interior to the trap. These reservoirs may also serve as a chemical barrier, keeping volatile materials like water from moving to the inner regions of a disk.

While a perfect dust trap completely isolates material from the rest of the disk, recent observations and 2D simulations have shown that dust traps may be a bit more permeable — leaking smaller sized grains, mixing material, and changing the disk’s appearance. However, these results only account for two dimensions of the complex three-dimensional environment in which dust traps reside. Thus, 3D hydrodynamical simulations are necessary to provide more realistic details of dust dynamics within planet-hosting protoplanetary disks. 

Dusty Simulations

Dust-gas density ratios for 3d models at 1500 orbits

Z-axis averaged dust–gas density ratios (top) and dust–gas surface density ratios for the 3D simulations after 1,500 orbits. For the simulations with higher diffusion and lower planet mass, there is clear leaking of dust beyond the dust trap ring (edges marked with dotted red lines). Click to enlarge. [Huang et al 2025]

In a recent study, Pinghui Huang (Chinese Academy of Sciences; University of Victoria) and collaborators performed multiple 2D and 3D numerical simulations of gas and dust within a protoplanetary disk with a forming planet. The simulations varied the mass of the planet and the level of turbulent diffusion — how well material and energy flow and mix within the gas. These variations allowed the authors to explore how dust traps behave within different types of systems. 

The simulations showed that the embedded planet will perturb the gas and dust, producing density shocks that create gaps and, subsequently, pressure bumps where dust traps coalesce. From their analysis, the authors found that dust traps become leakier at higher levels of diffusion and when the embedded planet is lower in mass. Essentially, if the gas flows and mixes more efficiently, the perturbations of the planet are erased more quickly, and if the planet is sufficiently small, its ability to disrupt the disk is much weaker. Dust remains coupled to the gas, flowing through these weak traps without becoming stuck. Additionally, the 3D simulations show higher amounts of leakage compared to the 2D simulations, which the authors attributed to the asymmetric and complex vertical geometry of the disk.

Trapping over time.

Flux-trapping ratio (left) and mass-trapping ratio (right) as a function of time for the 2D (top) and 3D (bottom) simulations. The higher-mass planet in Model A causes more flux and mass-trapping than the lower-mass planets and more turbulent systems. Additionally, the 3D simulations show significantly lower flux and mass-trapping than the 2D simulations. Click to enlarge. [Huang et al 2025]

Implications and Comparison to Observations

What then are the consequences of leaky dust traps? In planet formation theory, dust traps determine the mass at which a planet creates a sufficient pressure bump that isolates small pebbles and dust exterior to its orbit. For perfect dust traps, this isolation of material from the planet and inner disk creates a clear chemical distinction between the inner and outer disk. However, as shown by the 3D simulations, dust traps are imperfect, allowing small particles to filter through; the authors suggest this may mean that the growing planet slows but does not stop the migration of solid materials in a disk.

Recent observations of protoplanetary disks reveal the presence of larger volatiles within the inner disk. Specifically, the disk PDS 70 shows water emission in its inner disk despite having two confirmed giant planets orbiting in the outer disk. Without leaky dust traps, volatiles like water would be trapped in the pressure bumps created by these planets. However, as the authors have shown, the complex reality of dust dynamics within protoplanetary disks allows heavier elements to leak through, enriching the inner disk. Further observations and detailed 3D simulations will allow astronomers to understand the extent of leaky dust traps and reveal the realistic conditions driving early planet formation.

Citation

“Leaky Dust Traps in Planet-Embedded Protoplanetary Disks,” Pinghui Huang et al 2025 ApJ 988 94. doi:10.3847/1538-4357/addd1f

Betelgeuse and its companion star

Astronomers may have directly imaged a companion star to the famous red supergiant Betelgeuse at last. Though the tentative detection only loosely constrains the star’s physical properties, it appears to be a 1.6-solar-mass pre-main-sequence star. The discoverers suggest the name Siwarha, Arabic for “Her Bracelet,” given that the star circles Betelgeuse, “Hand of the Giant.”

Betelgeuse in the Spotlight

constellation Orion

A photograph of the constellation Orion. Betelgeuse is the bright yellow star located left of center. [E. Slawik/NOIRLab/NSF/AURA/M. Zamani; CC BY 4.0]

Situated at the shoulder of the constellation Orion, the red supergiant Betelgeuse is one of the most recognizable stars in the night sky. It’s also the subject of countless scientific studies and amateur observations that stretch back for centuries.

Betelgeuse is a variable star. On top of its well-constrained 400-day pulsation period, the star has exhibited two behaviors that have drawn interest in recent years: a deep, prolonged dimming episode in 2019–2022 that is thought to be due to an immense ejection of mass from the star’s surface, and a 6-year variation in the star’s photometry, astrometry, and radial velocity.

Multiple studies have attributed the 6-year variability to one or more companion stars. In general, these studies predict that the companion star circles Betelgeuse on a tight, 6-year, nearly edge-on orbit. As reported in a new article published in the Astrophysical Journal Letters, observations have likely proved these predictions correct.

Speckle Detection

Steve Howell (NASA Ames Research Center) and collaborators searched for Betelgeuse’s predicted companion star using the 8.1-meter Gemini North telescope. The team used the ‘Alopeke instrument in speckle imaging mode, which involves taking thousands of milliseconds-long snapshots to avoid smearing due to atmospheric fluctuations.

Betelgeuse and its companion star in observations

Observations of Betelgeuse from 2020 (left) and 2024 (right). An arrow points to the companion. Note that the image processing introduces a 180º ambiguity in the location of the companion, which is resolved by analyzing phase information. [Howell et al. 2025]

The team imaged Betelgeuse in 2020 and 2024. The 2020 observations coincided with when the companion star was predicted to lie behind Betelgeuse from our vantage point, and no companion appears in these images. The 2024 observations were taken just a few days after the companion’s predicted greatest angular separation from the star — and show evidence for a star just beside Betelgeuse.

More to Learn

Howell and coauthors performed additional analyses to rule out interference from hot pixels, cosmic rays, or atmospheric diffraction. They also considered that the star could lie in the background or foreground, but they found both possibilities unlikely. After further calculations, the team estimated that the companion is six magnitudes fainter than Betelgeuse and separated by just 52 milliarcseconds at an angle of 115º. These quantities agree well with predictions for the companion’s position.

location of Betelgeuse in 2020 and 2024

The on-sky proper motion of Betelgeuse from 2020 to 20204. The companion is shown as the small black circle. This shows that if the companion star were actually a background star, it would have been plainly visible in the 2020 observations. [Howell et al. 2025]

Given the tentative detection — at the level of 1.5σ — it’s difficult to pin down the properties of the companion star. The team placed the star’s mass between 1.4 and 2.0 solar masses, with the likeliest value being 1.6 solar masses. If the companion has the same age as Betelgeuse, this star would be on the cusp of joining the main sequence. However, it may never reach that stage of life; Betelgeuse’s demise in a core-collapse supernova is imminent, in the astronomical sense, and the companion may spiral in to merge with Betelgeuse even before then. (It currently orbits at just 4 au — closer than the distance between the Sun and Jupiter.)

The team closed with a call to the community to turn their instruments toward Betelgeuse on 26 November 2027, when the companion will once again be at its greatest angular separation. Our exploration of this long-sought-after star has just begun!

Citation

“Probable Direct Imaging Discovery of the Stellar Companion to Betelgeuse,” Steve B. Howell et al 2025 ApJL 988 L47. doi:10.3847/2041-8213/adeaaf

Milky Way center

New research shows that a past collision between the Milky Way’s central black hole and a smaller black hole could explain the dynamics of the S-stars, a group of stars that orbit precariously close to our galaxy’s supermassive black hole.

Journey to the Center of the Milky Way

The center of the Milky Way harbors a supermassive black hole (Sagittarius A* or Sgr A*) with a mass of 4 million Suns. The nearest neighbors of this behemoth are a compact disk of massive young stars and a collection of stars called the S-stars, which inhabit the innermost 48 light-days of our galaxy.

While the stars in the disk are orderly, arranged on orbits with moderate eccentricities and low inclinations, the S-stars orbit Sgr A* every which way, careening around the black hole with a wide range of eccentricities and inclinations. So far, the cause of this unusual distribution of stars is unknown.

When Black Holes Collide

Past research has struggled to explain the S-stars’ orbits. A successful theory of S-star origins must account for the stars’ eccentric orbits (e = 0.61, on average) and high orbital inclinations (i = 79º, on average), and it must produce these characteristics within the 15-million-year lifetime of the stars.

diagram showing the proposed formation pathway for the Milky Way's S-stars

Schematic showing the proposed formation pathway for the Milky Way’s S-star population. Click to enlarge. [Akiba et al. 2025]

In a recent research article, a team led by Tatsuya Akiba (University of Colorado Boulder) offered a new hypothesis to explain these traits: Sgr A* absorbed a smaller black hole in the not-so-distant past, and the aftermath of the merger created the distribution of stars seen today.

The premise is not far-fetched: at the ripe old age of nearly 14 billion years, the Milky Way has likely gulped down neighboring dwarf galaxies and globular clusters multiple times. When a black hole embedded within one of these meals of stars and gas merges with the Milky Way’s central black hole, the collision causes Sgr A* to recoil — potentially rearranging the stars in its vicinity in the process.

Radical Reorganization

Akiba and collaborators used N-body simulations to explore the outcomes of such a collision. In their simulations, Sgr A* is situated within an axisymmetric disk of stars or gas. A smaller black hole falls toward Sgr A*, spirals inward, and merges with the larger black hole. The asymmetric emission of gravitational waves causes Sgr A* to recoil, warping the surrounding collection of stars and gas into an eccentric disk. For the eccentricity of the disk to match what is seen in the disk of stars surrounding Sgr A* today, the incoming black hole must have a mass of roughly 200,000 solar masses.

simulation results

Simulation snapshots taken at the simulation onset (left column) and after 2 million years (right column). Orbits with inclination greater than 30 degrees are shown in magenta. Note the difference in scale between the rows. [Akiba et al. 2025]

After the merger, the stars orbiting farther out torqued the inclinations and eccentricities of the innermost stars up to high values through what’s called the eccentric Kozai–Lidov mechanism. After 2 million years of simulation time, this interaction produced a stellar distribution similar to what is seen today: a disk of moderate eccentricity surrounding “S-stars” with eccentric and highly inclined orbits.

While the properties of the modeled S-stars don’t exactly match the properties of the real deal — the simulated stars have, on average, lower inclinations and eccentricities — the authors noted that this work represents a first foray into their hypothesis. Modeling that explores a broader range of parameter space is needed to fully understand the origins of the stars at the center of the Milky Way.

Citation

“On the Formation of S Stars from a Recent Massive Black Hole Merger in the Galactic Center,” Tatsuya Akiba et al 2025 ApJL 987 L27. doi:10.3847/2041-8213/addc5d

Lagoon and Trifid Nebulae

Editor’s Note: Shortly after the publication of this article, we became aware that the disk described in this work was previously discovered by Wei-Hao Wang, as reported in the Astronomer’s Telegram in July 2024.

On 23 June 2025, the public got its first look at images from the NSF–DOE Vera C. Rubin Observatory — and the discoveries are already starting to roll in.

Trifid and Lagoon

Trifid Nebula closeup

A closeup of the Trifid Nebula from one of the Rubin Observatory first-look images. [Adapted from NSF–DOE Vera C. Rubin Observatory; CC BY 4.0]

One of the first-look images from Rubin features two photogenic star-forming regions in the Milky Way: the Trifid Nebula (Messier 20) and the Lagoon Nebula (Messier 8). The image, which was constructed from 678 exposures totaling 7.2 hours of observations, demonstrates Rubin’s ability to quickly cover large swaths of sky.

The Trifid and Lagoon nebulae are both HII regions, making them ideal places to search for circumstellar and protoplanetary disks, as well as proplyds — protoplanetary disks that are in the process of being evaporated by the intense radiation from nearby massive stars. The glowing gas of an HII region provides the background illumination needed to pick out the silhouettes of dark and dusty disks.

disk candidate

The Trifid Nebula disk candidate identified in this work. [Adapted from Zamani & Rector 2025]

Disk Detected

Published today in the Research Notes of the AAS, Mahdi Zamani (Zamani Scientific Visualizations & Imaging) and Travis Rector (University of Alaska Anchorage) reported the results of their search for proplyds in the Rubin image of the Trifid and Lagoon nebulae. The team detected one candidate circumstellar or protoplanetary disk on the edge of the Trifid Nebula, surrounded by tenuous, filamentary clouds of gas and dust.

The Rubin data don’t show evidence for ionized gas surrounding the disk, so it’s not yet clear if the object should be classified as a proplyd. The disk has a projected distance of 7.5 light-years from HD 164492A, the O-type star whose intense radiation is responsible for energizing the surrounding nebula, and may also be capable of ionizing the disk studied here.

Disk Detected

infrared image of the stars and protostars in the orion nebula

The Orion molecular cloud complex is among the most active star-forming regions in the Milky Way, home to hundreds of protostars and thousands of pre-main-sequence stars. [ESO/H. Drass et al.; CC BY 4.0]

This disk candidate is remarkable for both its size and location. With an estimated diameter of 4,000 au, it’s larger than most known circumstellar disks. Finding a disk outside the nearby Orion Nebula is also comparatively rare, since searches in more distant star-forming regions require finer resolution than the nearby Orion Nebula. (Orion is about 1,300 light-years away, and the Trifid Nebula is about 4,000 light-years away.)

Researchers have searched the Trifid Nebula previously for signs of protoplanetary disks, but while those searches found evidence for disks around hot stars near the center of the cluster, this particular disk candidate went undetected. Previous searches likely didn’t cover a large enough area of the nebula, or perhaps lacked the resolution needed to differentiate between the dusty disk and the nearby filamentary gas clouds. With Rubin’s broad field of view and precise resolution, the observatory was well-positioned to spot this disk candidate.

Citation

“The Potential Discovery of a Circumstellar Disk in M20 from Rubin First Look,” Mahdi Zamani and T. A. Rector 2025 Res. Notes AAS 9 172. doi:10.3847/2515-5172/ade982

Infinity Galaxy

Researchers have discovered a rare ring-galaxy duo that appears to harbor a supermassive black hole formed through direct collapse — a process similar to what may have jump-started the growth of the first supermassive black holes in the universe.

Searching for Oddballs

To find something remarkable, sometimes you have to go looking for it. As described in an article published today in the Astrophysical Journal Letters, a research team led by Pieter van Dokkum (Yale University; Dragonfly Focused Research Organization) recently struck gold in their search for unusual objects.

Van Dokkum and coauthors searched for interesting objects in publicly available images from COSMOS-Web, a JWST program dedicated to understanding how galaxies have evolved over the course of cosmic history. Having already published their discovery of a complete Einstein ring in the COSMOS-Web field, the team is now revealing a second finding: a galaxy featuring two bright, compact nuclei, two starry rings, and an unexpected inhabitant right in the center.

four views of the Infinity Galaxy

Four views of the Infinity Galaxy from Hubble and JWST. [van Dokkum et al. 2025]

Gathering Data

To learn more about this strange object, named the Infinity Galaxy for its resemblance to the infinity symbol, the team gathered data from the Hubble Space Telescope, the Keck I telescope, the Chandra X-ray Observatory, and the Very Large Array. The resulting multiwavelength portrait allowed the team to weigh the two nuclei, showing them to be massive — containing 80 billion and 180 billion solar masses of stars — and extremely compact. The new data also revealed that the cloud of gas between the nuclei contains a 1-million-solar-mass black hole.

The nuclei, the rings, and the gas between them appear to be the result of two disk galaxies that shot through one another, forming a pair of collisional ring galaxies. To achieve the infinity-symbol shape, the galaxies must have met one another face on, forming two parallel nucleus–ring systems that we see from an angle of about 40º. As the galaxies collided, some of their gas would have been torn away, left tangled together in the void between the two nuclei.

To the Infinity Galaxy and Beyond

radio and X-ray observations of the Infinity Galaxy

Radio (left) and X-ray (right) observations of the Infinity Galaxy, demonstrating that the black hole candidate is a strong radio and X-ray source. Click to enlarge. [Adapted from van Dokkum et al. 2025]

That explains the nuclei, the rings, and the gas between them — but where did the black hole come from? It’s possible that the black hole’s position between the two nuclei is simply a coincidence, either due to a chance alignment with an unrelated galaxy that hosts the black hole, or because the black hole happened to end up there after being ejected from one of the galaxies involved in the collision, or even from another galaxy that merged with the Infinity Galaxy.

The authors favor a different explanation, in which the location of the black hole is no coincidence. In this scenario, the colliding galaxies crushed a gas cloud between them so forcefully that the condensed gas collapsed directly into a black hole. The authors estimated that the collision happened 50 million years ago and created a 300,000-solar-mass black hole that subsequently grew to its current mass of 1 million solar masses. Direct collapse has been proposed to be a source of the seeds of supermassive black holes in the early universe, though there are some differences between the proposed process in the early universe and what may be happening in the Infinity Galaxy.

Van Dokkum and collaborators concluded their article by saying that future observations could clarify whether the black hole is truly associated with the Infinity Galaxy — and as reported in a press release today, preliminary analysis of follow-up observations with JWST show just that. With the black hole now definitively placed within the Infinity Galaxy, the evidence for direct collapse is strengthened, though more work is needed to probe this possibility.

Citation

“The ∞ Galaxy: A Candidate Direct-Collapse Supermassive Black Hole Between Two Massive, Ringed Nuclei,” Pieter van Dokkum et al 2025 ApJL 988 L6. doi:10.3847/2041-8213/addcfe

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