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photograph of spiral and elliptical galaxies

plot of a signal from a nearby fast radio burst

Example of a signal from a nearby fast radio burst. The top panel shows the overall intensity over time, while the bottom panel shows the frequency of the burst over time. [Adapted from Bhardwaj et al. 2024]

Where do fast radio bursts come from? New research shows that the nearby host galaxies of these fleeting flashes have something in common, which may help researchers understand the origins of fast radio bursts.

Mysterious Bursts

Fast radio bursts are powerful, milliseconds-long flashes of radio waves of unknown origin. Since the first fast radio burst was discovered in 2007, astronomers have detected roughly a thousand of these mysterious signals. The source of these bursts is still up for debate, with supernovae, magnetars, colliding objects, and other energetic phenomena tapped as candidates.

To understand the origin of these bursts, it helps to know what kind of galaxies they happen in. If fast radio bursts emerge from spiral galaxies with active star formation, it could mean that the bursts are linked to “prompt” formation channels such as the deaths of short-lived massive stars. If instead bursts come from elliptical galaxies with little or no star formation, that could imply that bursts come from “delayed” channels like the slowly progressing mergers of stellar remnants. So, what kind of galaxies do fast radio bursts tend to come from?

In the Neighborhood

Mohit Bhardwaj (Carnegie Mellon University and McGill University) and collaborators turned to data from the Canadian Hydrogen Intensity Mapping Experiment (CHIME) to answer this question. Their aim was to study fast radio bursts in the local universe since a sample of nearby bursts is less likely to be affected by observational biases. To find nearby bursts, Bhardwaj’s team searched the first CHIME fast radio burst catalog for signals with low dispersion measure. On average, the farther the source, the larger the dispersion measure — essentially, because there’s more stuff in between the source and Earth to disperse the radio signal.

visible-light images of candidate host galaxies

Visible-light images of the localization regions of the four fast radio bursts in this study. The red boxes indicate the most likely host galaxy for each burst. Note that FRB 20181223C has four potential host galaxies (red and cyan boxes in the upper-left image), but only one that satisfied the source’s maximum redshift limit. Click to enlarge. [Bhardwaj et al. 2024]

The team found four cataloged bursts with dispersion measure excess (the amount left over after the contribution from the Milky Way is subtracted off) less than 100 parsecs per cubic centimeter, which corresponds to a distance of about 1.3 billion light-years. After searching the localization regions of these bursts in deep optical images from the Panoramic Survey Telescope and Rapid Response System survey, the authors found only one plausible host galaxy for each burst.

A Spiral Sample

Including the four bursts with newly identified host galaxies from this work, researchers have now localized the positions of 18 nearby fast radio bursts. What ties these burst-hosting galaxies together, if anything? As it turns out, they’re all spiral galaxies. That’s an intriguing result, but does it necessarily mean that fast radio bursts are more likely to come from spiral galaxies, or is it just easier to detect fast radio bursts from spiral galaxies?

images of the 18 fast radio burst host galaxies

The host galaxies of all 18 local universe fast radio bursts used in this study. Click to enlarge. [Bhardwaj et al. 2024]

Bhardwaj and collaborators explained that their sample selection is actually biased against bursts from spiral galaxies because these galaxies tend to have more signal-dispersing material than elliptical galaxies do, making spiral galaxies more likely to be eliminated by the dispersion measure cutoff. Given the typical ages of stars in spiral galaxies, the team suggests that the dominant formation pathway for fast radio bursts in the local universe is through core-collapse supernovae, which mark the explosive end of stars more than about eight times the mass of the Sun.

Bhardwaj’s team noted that this doesn’t mean all fast radio bursts must come from supernovae; because a small number of known bursts have arisen in unusual locations like globular clusters and non-star-forming spiral galaxies, delayed pathways like mergers of stellar remnants may be responsible for certain bursts.

Citation

“Host Galaxies for Four Nearby CHIME/FRB Sources and the Local Universe FRB Host Galaxy Population,” Mohit Bhardwaj et al 2024 ApJL 971 L51. doi:10.3847/2041-8213/ad64d1

Image of the Sharpless 2-106 star-forming region

Editor’s Note: Lexi Gault is a fourth-year graduate student at Indiana University who was recently selected as the 2024–2025 AAS Media Fellow. We’re excited to welcome Lexi to the team and look forward to featuring her writing on AAS Nova regularly!

Once thought to be a pure descendant of one the universe’s first stars, new research on star J1010+2358 uncovers a more complex history.

The First Stars and Their Violent Ends

During the universe’s debut into star formation, only hydrogen, helium, and some lithium existed. Without metals (i.e., elements heavier than helium) to efficiently cool the gas, the first generation of stars, called Population III stars, likely formed with much higher masses than any subsequent generation. Based on theoretical modeling, Pop III stars may have had masses hundreds of times the mass of the Sun; however, their true mass distribution is still under investigation.

Some of the most massive Pop III stars likely ended their lives in pair-instability supernovae (PISNe), a type of supernova so energetic and violent it completely rips the star apart, leaving behind no stellar remnant. Metals that formed inside these stars and within their explosions are released into the interstellar medium, creating a newly enriched cosmic soup that fuels the next generation of star formation. Finding stars born out of PISN material, and particularly descendants whose chemical signatures trace back to a single Pop III star, is no simple task, but this detection would provide powerful information about the first stars in the universe.

Measuring Chemical Abundances

With a low metallicity and unique chemical abundance pattern, the star J1010+2358 was initially suggested to have been formed from the gaseous remains of a single 260-solar-mass Pop III star. However, more recent analysis suggests that the star’s origins are more ambiguous, perhaps obtaining only 10% of its metals from such a PISN.

Plot of chemical abundance pattern

Measured chemical abundance pattern of J1010+2358 (red stars). Multiple model predictions are overlaid; the dashed gray line shows a 100% 260-solar-mass PISN contribution, indicating an ill fit to the data. Click to enlarge.
[Skúladóttir et al. 2024]

To more confidently determine J1010+2358’s ancestry, a team led by Ása Skúladóttir (University of Florence) used the Ultraviolet and Visible Echelle Spectrograph on the European Southern Observatory’s 8.2-meter Very Large Telescope to derive a more detailed abundance pattern of the star. They found the carbon and aluminum abundances to be significantly higher than predicted for pure PISN descendants. The team also remeasured a number of other elements, and the results further signal that J1010+2358 is not a descendant of a single, massive Pop III star as previously claimed.

Likely Origins of J1010+2358

How, then, did J1010+2358 obtain its interesting chemical composition? Through applying theoretical models for various progenitor combinations, Skúladóttir’s team found a best fit where the star’s metals come from a combination of a 13-solar-mass second-generation star that underwent a core-collapse supernova and a 39-solar-mass Pop III core-collapse supernova. They considered multiple scenarios with the star obtaining some of its metals from a 260-solar-mass PISN, but all plausible fits have PISN contributions too low to further characterize the mass distribution of first-generation stars.

Quality of fit results for J1010+2358 progenitors

Quality of fit results for J1010+2358 containing two progenitors, with one being a PISN of a given mass and fraction of metal contribution and the other being a 13-solar-mass second-generation star that underwent a core-collapse supernova. Values 2 and below are best quality fits and most plausible progenitor scenarios. Click to enlarge. [Skúladóttir et al. 2024]

Stars have complex histories stored within their chemical DNA, and the difficulty of identifying a star with a single first-generation predecessor underscores the importance of careful chemical abundance analyses. Although J1010+2358 is likely not a true PISN descendant, the lessons learned through this star will be critical in the quest to uncover the properties of the universe’s first stars, and upcoming high-resolution spectroscopic surveys may reveal more promising candidates in the near future.

Citation

“On the Pair-instability Supernova Origin of J1010+2358,” Ása Skúladóttir et al 2024 ApJL 968 L23. doi:10.3847/2041-8213/ad4b1a

Radar images of the near-Earth object Apophis

As you read this, an asteroid named 99942 Apophis is spiraling along a trajectory that will bring it uncomfortably close to Earth in about five years’ time. Thankfully, astronomers are very confident that this 340-meter ball of rock will miss us and instead will squeak between Earth and the Moon on 13 April 2029 before continuing on its way through the solar system. But such a close brush with disaster inspires reflections on worst-case scenarios. Even though everything we know about the laws of physics tells us that Apophis won’t collide with Earth on its current trajectory, could anything happen between now and the flyby that would change that?

Unlikely Odds

Illustration of different hypothetical trajectories Apophis could be on as it approaches Earth. An animated version can be seen here. [Wiegert 2024]

Recently, Paul Wiegert, The University of Western Ontario, took up a specific version of this question: if a separate smaller object crashed into Apophis, could that impact nudge the asteroid off its current path and onto a catastrophic collision course?

Luckily, almost definitely not. But, unsurprisingly given a situation as complicated as orbital dynamics and city-destroying impacts, there are caveats. While Wiegert estimates that the odds of Apophis stumbling into an asteroid larger than 3.6 meters across — large enough to divert Apophis into Earth — are about two in a billion, the odds of it encountering a smaller asteroid are higher. He estimates there’s about a one in a million chance that Apophis could strike an asteroid bigger than 60 centimeters, which in theory would be large enough to nudge Apophis onto an orbit that would still miss us in 2029, but would then lead to an impact later (in 2036, for example). We’d have to be doubly unlucky for this to happen, since even in this unlikely event, most small impacts leave Apophis on paths that take it farther from Earth in the future, not towards it. So, there’s certainly no need for alarm, but it will be worth monitoring Apophis in the coming years to make sure it hasn’t strayed from its safe path.

Sneaky Asteroid

There lies an unfortunate twist to this story, however: we can’t actually observe Apophis until early 2027, at best, so we have no way to know if it’s been jostled onto a dangerous trajectory until then. The asteroid has been lurking too close to the Sun for telescopes to safely see it since 2021, so our next look at it will come after a significant gap in time.

Illustration of the angle between Apophis and the Sun, as seen from Earth. The yellow band denotes geometries where telescopes cannot observe the asteroid. [Wiegert 2024]

Wiegert, acknowledging both the extreme unlikeliness of a deflection but also the dire consequences of misplaced complacency, also calculates how we might tell if Apophis is on a new path once it finally reappears in the night sky. He finds that if the asteroid is even a few tenths of an arcsecond away from its predicted position, we should immediately attempt an intensive observing campaign to check its new trajectory. A deviation wouldn’t automatically spell danger, but it would indicate that something happened while we waited for Apophis to emerge, and that there’s a chance that we could be in trouble.

When it comes to something as serious as asteroid impacts, it’s good to double-check every assumption and to investigate even the most unlikely scenarios. Thankfully Earth is almost certainly safe from an asteroid impact in 2029, though up until then and beyond, astronomers will be sure to keep checking for signs of danger.

Citation

“On the Sensitivity of Apophis’s 2029 Earth Approach to Small Asteroid Impacts,” Paul Wiegert 2024 Planet. Sci. J. 5 184. doi:10.3847/PSJ/ad644d

illustration of a hot Jupiter exoplanet orbiting close to its host star

Are hot Jupiters lonely, or do they have nearby planetary companions? The answer may depend on how these close-in planets form. A recent discovery adds to the small number of known systems containing a hot Jupiter and an inner planet companion and provides clues to hot Jupiter formation.

ALMA image of the disk around the young star TW Hydrae

Planets form in protoplanetary disks, like the one pictured here around the star TW Hydrae. Interactions between young planets and the gas of the disk can cause planetary migration. [S. Andrews (Harvard-Smithsonian CfA); B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO)]

The Origins of Hot Jupiters

Hot Jupiters are among the strangest creatures in the exoplanet zoo. These worlds, which are roughly the mass of Jupiter, orbit extremely close to their host stars, achieving temperatures far hotter than the giant worlds in our solar system.

It’s not yet clear how hot Jupiters come to be. Do they form in place, vacuuming up massive amounts of gas in the blazing heat of their host stars? Do they get their start in cooler regions and then migrate to warmer pastures as they form? Or do they form far from their stars, then get kicked closer by gravitational interactions that dramatically change their orbits? To learn more, researchers are collecting all the information on hot Jupiters that they can — and they’re especially interested in the company that hot Jupiters keep.

Close Companions

So far, astronomers know of only a handful of hot Jupiters that have planetary companions whose orbits lie between the hot Jupiter and the host star. Certain theories of hot Jupiter formation, like high-eccentricity migration, predict that inner planets will be ejected or destroyed when a giant planet migrates from a distant orbit to a close-in one, becoming a hot Jupiter. To understand whether high-eccentricity migration is the main way that hot Jupiters form, as current observations suggest, researchers must understand how common these close-in companions are.

light curves for two exoplanets around TOI-1408

Normalized transit signals from the newly discovered inner planet TOI-1408 c (top) and the previously known hot Jupiter TOI-1408 b (bottom). [Adapted from Korth et al. 2024]

Judith Korth (Lund University) and collaborators used observations from the Transiting Exoplanet Survey Satellite (TESS) and ground-based telescopes to search for close-in planets around TOI-1408. TOI-1408 is known to host a hot Jupiter (TOI-1408 b) with a period of 4.42 days. The team searched TOI-1408 b’s transit signals for transit timing variations — changes in the time between transits — and found a clear signal at 2.2 days.

Searching through the TESS light curves, the team found evidence for a planet with that orbital period. They suspect that the automated TESS transit search pipeline missed the planet because of its large transit timing variations. Modeling suggests a mass of 7.6 Earth masses and a radius of 2.22 Earth radii, making TOI-1408 c a mini-Neptune planet.

Far-Out Findings

TOI-1408 b has joined the small but growing ranks of hot Jupiters with low-mass inner planet companions. This finding suggests that TOI-1408 b couldn’t have formed through high-eccentricity migration. Instead, in-situ formation or disk migration must be responsible.

The data analyzed in this work held another surprise: TOI-1408 c might not be the only new member of the TOI-1408 family! Korth’s team used radial velocity modeling to search for other planets and found clear evidence for another object in the system with a period of thousands of days. While more follow-up observations are needed to narrow in on the precise properties of this object, Korth’s team suggests it has a mass of roughly 14.6 Jupiter masses and an orbit lasting nearly 7 years.

Citation

“TOI-1408: Discovery and Photodynamical Modeling of a Small Inner Companion to a Hot Jupiter Revealed by Transit Timing Variations,” Judith Korth et al 2024 ApJL 971 L28. doi:10.3847/2041-8213/ad65fd

supernova

A kilometer below Japanese ground lies a massive cylindrical tank, its steel walls lined with more than 10,000 photomultiplier tubes that await the arrival of neutrinos. Could recent upgrades to the Super-Kamiokande neutrino detector improve our ability to spot and assess supernova explosions in real time?

JWST image of SN 1987A

JWST image of SN 1987A. The optical signal of this explosion was preceded by the arrival of a burst of neutrinos, a handful of which were observed by detectors on Earth. [NASA, ESA, CSA, Mikako Matsuura (Cardiff University), Richard Arendt (NASA-GSFC, UMBC), Claes Fransson (Stockholm University), Josefin Larsson (KTH)]

Memo from a Dying Star

In 1987, 25 tiny messengers arrived at Earth following a tremendous explosion 168,000 light-years away. This signal marked the first time that these near-massless messengers — neutrinos — had ever been directly observed from a Type II supernova, the core collapse of a massive star.

The benefit of observing these particles is clear: because neutrinos so rarely interact with matter, they arrive at Earth carrying untouched information about the death of the star that produced them. And because neutrinos escape the collapsing star more readily than photons do, they arrive before the visible light from the supernova. This means that if scientists can detect and localize a supernova neutrino burst, they can notify observatories about the imminent optical signal from the supernova. Combining the neutrino and electromagnetic observations could then provide valuable insight into long-standing questions, like the mechanism of the star’s explosion.

The catch? We only expect a handful of nearby (i.e., in our galaxy) supernova explosions every century — and what’s more, observing neutrinos is no easy feat! In the nearly four decades since those 25 messengers heralded the supernova SN 1987A, we haven’t detected any other neutrinos linked to supernovae. But that doesn’t mean we haven’t been preparing.

A Salty Upgrade for Super-K

Super-Kamiokande scale model

A scale model showing the (empty) tank of the underground Super-K neutrino detector. A recent upgrade saw a gadolinium salt added to the water that fills the tank. [Adapted from Wikipedia user Motokoka; CC BY-SA 4.0]

The Super-Kamiokande (Super-K) neutrino detector has recently undergone an upgrade: scientists have dissolved a gadolinium salt in the 55,000 tons of ultrapure water that fills its underground tank. The addition of this rare-earth metal improves the detector’s ability to differentiate between neutrinos and antineutrinos, thereby enabling scientists to more accurately localize the supernova that produced an incoming burst of neutrinos.

In a recent article led by Yuri Kashiwagi (Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo), Super-K scientists have analyzed how this upgraded observatory — and a corresponding real-time alert system developed to notify optical astronomers of the explosion and tell them where to point their telescopes — will respond to a hypothetical supernova within our galaxy.

Passing the Message On

Using multiple different supernova models, Kashiwagi and collaborators simulated the neutrino burst that would be produced by a supernova exploding roughly 33,000 light-years away. Through further simulations, the team then explored how successfully Super-K would detect neutrinos from the burst, how well the supernova’s location could be identified from Super-K’s detections, and how quickly this information could be broadcast to astronomical observatories via the alerting system.

supernova localization

The blue contours on this sky map show an example of the reconstructed supernova position that Super-K neutrino observations should provide for a supernova that occurs 33,000 light-years away. In this example, the supernova’s location can be identified to within 3°. [Adapted from Kashiwagi et al. 2024]

The authors found that the simulated supernova’s location could be rapidly identified to within 3–7° on the sky, and that information could be sent to optical observatories within minutes. In many cases, this response would be sufficient for wide-field telescopes like the upcoming Vera Rubin Observatory to catch the rise of the optical signal from a supernova — and the neutrino data captured by Super-K could even help distinguish between different supernova models.

While there’s still more to learn, it seems likely that Super-K is well prepared for future nearby supernova detections. Now all that’s left is to wait for the next explosion!

Citation

“Performance of SK-Gd’s Upgraded Real-time Supernova Monitoring System,” Y. Kashiwagi et al 2024 ApJ 970 93. doi:10.3847/1538-4357/ad4d8e

JWST photograph of the Cartwheel Galaxy

Today’s Monthly Roundup is a bit of an astrophysical mishmash, highlighting some of the structures that exist in our universe. From galaxies to planets, we’ll explore where these structures come from and the tools researchers use to study them.

A Ring Galaxy

When gamma-ray telescopes like Fermi survey the sky, they see discrete sources of gamma rays as well as a diffuse background glow. Discerning exactly where these gamma rays come from is no easy task, and roughly 30% of known gamma-ray sources haven’t been identified. Many of these unassociated sources are likely active galactic nuclei (AGN) — accreting supermassive black holes that can launch powerful jets — but some may have more unusual origins.

annotated ultraviolet image of Kathryn's Wheel

An ultraviolet image (background color) of the Kathryn’s Wheel system with contours showing the Hα emission. Annotations have been added to show the various components of this system. Click to enlarge. [Adapted from Paliya & Saikia 2024]

In a recent article, Vaidehi Paliya and D. J. Saikia (Inter-University Centre for Astronomy and Astrophysics) matched the gamma-ray source 4FGL J1647.5−5724 to a galaxy called Kathryn’s Wheel. Kathryn’s Wheel is a ring galaxy, consisting of a central gas-poor galaxy surrounded by a ring of star formation. This curious system likely formed when a nearby dwarf galaxy, LEDA 3080069, shot through it like a bullet, kicking gas out of the system and triggering a shock wave that kick-started star formation in a ring around it.

Multiwavelength images of Kathryn’s Wheel show bright Hα and ultraviolet emission, both of which signal the presence of hot, massive young stars. Intense star-forming regions are known to emit gamma rays, in part because of the frequent core-collapse supernovae that shock interstellar gas and accelerate cosmic rays. A closer look at the galaxy revealed that its gamma-ray emission is stronger than expected, suggesting either a population of rapidly spinning stellar remnants called pulsars or — as a more mundane explanation — interference from a foreground Milky Way star.

In addition to triggering star formation, galaxy collisions can also activate AGN. While the current data show no signs of gamma-ray variability or relativistic jets — both of which would indicate an AGN — more observations are needed to look into the possibility further.

High-Energy Bubbles

In 2010, scientists discovered that the Milky Way has been blowing bubbles. Observations with the Fermi Gamma-ray Space Telescope revealed bubbles of gamma-ray emission extending 50 degrees above and below the plane of our galaxy. Ten years later, the eROSITA instrument on the Spectrum-Roentgen-Gamma spacecraft spotted a similar but even more extensive structure in X-rays. The origin of these structures, called the Fermi and eROSITA bubbles, is not yet known. Many researchers have suggested that a past period of AGN activity, in which the Milky Way’s central supermassive black hole accreted matter and shot out powerful jets, could be responsible.

Recently, Po-Hsun Tseng (National Taiwan University) and collaborators approached the AGN activity hypothesis from a new angle. Previous work has tested this theory under the assumption that the AGN jets emerged vertically from the plane of the galaxy, matching the orientation of the bubbles. However, this doesn’t have to be the case — the direction of an AGN’s jets is related to the spin of the black hole, which doesn’t have to be parallel to the galactic disk.

simulated gamma-ray bubbles

Simulated gamma-ray bubbles from jets emerging perpendicular to the disk (top), at a 45-degree angle to the disk (middle), and parallel to the disk (bottom). [Adapted from Tseng et al. 2024]

Using special relativistic fluid dynamics simulations, Tseng’s team examined whether angled AGN jets could produce vertical bubbles. The simulated jets emerged from the plane of the Milky Way at a 45-degree angle, remaining “on” for 120,000 years before shutting off. A key component of the team’s model is the inclusion of a thin, dense, clumpy layer of interstellar gas lying parallel to the galactic disk. When the jet collides with this layer, interactions between cosmic rays in the jet and gas within the layer produce gamma rays.

Ultimately, the team found that under certain conditions, the jets transfer their kinetic energy to the dense layer of gas without plowing through it, and the once-narrow jets instead emerge vertically from the disk as bubbles. While more modeling is needed to understand the origin of the Fermi and eROSITA bubbles, this work shows that the assumption of vertical jets need not apply.

 

A Chain of Planets

When planets form in the dusty recesses of a protoplanetary disk, their motions within the disk are thought to align the planets in a resonant chain: a setup in which the orbital periods of the planets are integer multiples of one another. For example, a three-planet system with 8-, 16-, and 32-day orbits would be in a resonant chain. If planets do link up in resonant chains when they first form, something — gravitational nudges from passing stars, for example — must break those chains, as only 1% of known planetary systems are in this configuration. It’s critical to identify the small percentage of systems with intact resonant chains since they show the initial state of a planetary system before it’s disrupted.

plot of the cumulative fraction of unstable systems over time

Cumulative fraction of simulated six-planet systems becoming unstable as a function of time. Nearly all non-resonant systems are unstable by 25 million years, while only a small fraction of six-planet resonant chains go unstable in that same stretch of time. Click to enlarge. [Lammers & Winn 2024]

Caleb Lammers and Joshua Winn (Princeton University) investigated a potential resonant chain in the HD 110067 system. If confirmed, HD 110067 would be just the third known resonant chain containing six or more planets. HD 110067 has six known transiting planets with orbital periods between 9.1 and 55 days. The planets’ orbital periods are very nearly in ratios of 3:2 and 4:3 — highly suggestive of a resonant configuration.

Lammers and Winn performed N-body simulations to assess the likelihood that the system is arranged in a resonant chain. They found that in order for HD 110067’s six planets to be dynamically stable, the planets almost certainly must be in a chain. Simulated non-resonant systems become unstable within 25 million years — just 0.3% of the current age of HD 110067’s planetary system — and even systems with as many as five planets linked in a chain are unlikely to survive in that configuration to the present day.

Citation

“A γ-Ray-Emitting Collisional Ring Galaxy System in Our Galactic Neighborhood,” Vaidehi S. Paliya and D. J. Saikia 2024 ApJL 967 L26. doi:10.3847/2041-8213/ad4999

“Can the Symmetric Fermi and eROSITA Bubbles Be Produced by Tilted Jets?” Po-Hsun Tseng et al 2024 ApJ 970 146. doi:10.3847/1538-4357/ad50c5

“The Six-Planet Resonant Chain of HD 110067,” Caleb Lammers and Joshua N. Winn 2024 ApJL 968 L12. doi:10.3847/2041-8213/ad50d2

TW Hydrae

A new analysis of archival data reveals shocked gas in the protoplanetary disk surrounding the young star TW Hydrae. This discovery hints at the presence of a 4-Earth-mass planet and gives researchers a rare opportunity to study the earliest stages of planet formation.

Outflows from Planetary Offspring

Baby planets form in disks surrounding young stars, but the details of this process remain unclear — especially because the planets are often blanketed with dusty gas, hiding them from view. Massive gas planets like Jupiter and Saturn are thought to form by accreting gas onto rocky cores that gradually carve out lanes in the disk.

How can we tell if accretion is happening in a protoplanetary disk? As growing planets collect gas and dust, they also launch material into their surroundings in the form of outflows. As outflowing gas pummels its surroundings, shocks form, triggering the formation of molecules like sulfur monoxide (SO). That gives researchers an in — the planet might be hidden, but emission from these shock-formed molecules can announce its position.

location of SO emission relative to continuum emission of the protoplanetary disk

Integrated intensity of SO emission (orange and green contours) overlaid on a continuum image of the disk. [Adapted from Yoshida et al. 2024]

An Archival Search

This tells us how to potentially find baby planets, but where to look? One of the best places to search for signs of planet formation is around TW Hydrae, an 8-million-year-old star less than 200 light-years away. TW Hydrae possesses the nearest known protoplanetary disk, which from our vantage point appears nearly face on, with concentric light and dark rings like a bullseye. Researchers previously found two gaps in this disk, at 26 and 42 au, that could be explained by two roughly 4-Earth-mass planets. In addition, a clump of emission at 52 au hinted at the presence of a circumplanetary disk feeding gas to a growing planet.

Tomohiro Yoshida (National Astronomical Observatory of Japan) and collaborators analyzed archival data from the Atacama Large Millimeter/submillimeter Array (ALMA) to search for signs of outflows from a baby planet in the TW Hydrae disk. The team spotted an arc of emission from SO molecules originating from a gap 42 au from the star — exactly where a planet is purported to be.

Shocking Evidence

plot of best-fitting outflow trajectories

Best-fitting outflow trajectory (orange line) from the ballistic outflow modeling. [Adapted from Yoshida et al. 2024]

What does modeling say about the origin of this emission? The authors used ballistic outflow modeling to show that the SO outflow could be explained by a growing planet with a mass of 4 Earth masses. Combining estimates of the mass-accretion and mass-loss rates, the team finds an overall rate for the growth of the planet that matches theoretical expectations for a 4-Earth-mass planet.

With evidence for outflows already in hand, Yoshida’s team plans to continue the search, conducting further observations to look for evidence of the outflow in emission from other promising molecules, like silicon monosulfide. Overall, this work solidifies another line of evidence for the presence of a planet in the 42-au gap of TW Hydrae, and we can expect future observations to illuminate this growing planetary family further!

Citation

“Outflow Driven by a Protoplanet Embedded in the TW Hya Disk,” Tomohiro C. Yoshida et al 2024 ApJL 971 L15. doi:10.3847/2041-8213/ad654c

A photograph of a featureless blue sphere.

When the next flagship planetary science mission arrives at Uranus after years of interplanetary travel, it’s going to need to know exactly where it should fly near this poorly mapped, distant world. In a recent study, astronomers attempt to strike a balance between risk and reward and present several possible trajectories.

Visiting a Lonely Planet

Setting our home planet Earth aside for obvious reasons, you might be forgiven for assuming that we’re equally familiar with the remaining seven planets in our solar system. But, even after decades of robotic exploration, this is not the case: though we’ve mapped every inch of Mars and spent years circling Saturn, humanity has only visited Uranus and Neptune once apiece. These visits, which happened more than 30 years ago and lasted only a few hours each, answered some questions about the structure and formation of these planets but left many more unresolved.

A photograph of a blue-purple sphere surrounded by concentric white rings.

An image of Uranus and its rings taken recently by JWST. [NASA, ESA, CSA, STScI]

To remedy this lopsided accrual of knowledge on each planet, in 2022 the National Academy of Sciences recommended that NASA’s next flagship planetary science mission should aim not for one of the inner rocky planets, but speed all the way on to Uranus. From orbit around this distant world, a properly designed mission should be able to tell us if the planet’s core is liquid or solid, measure the structure and speed of the winds seen previously whipping around the planet, and assess the composition of the interior.

Before such a mission can begin its investigation in ~2050, however, scientists and engineers need to painstakingly plan out exactly where the spacecraft needs to go and what it needs to carry if it’s going to answer the questions we create it to resolve. A recent study led by Marzia Parisi, Jet Propulsion Laboratory, adds to this effort by considering which orbital trajectory would yield the most useful scientific measurements.

Planning Ahead

Scientists can measure things like the density structure of a planet by tracking how a probe’s speed changes as it travels along its orbit. The closer the spacecraft can get to the planet, the better, since that’s where the subtle effects will be most pronounced.

Top, a diagram of a sphere surrounded by different colored curving lines. Bottom, a 2D projection of those curving lines.

Top: Uranus and three potential types of trajectories. The safest, in purple, was ultimately disfavored over the more risky, but more rewarding, yellow and green trajectories. Bottom: a 2D map projection of the same tracks. [Parisi et al. 2024]

But, mission design is always a game of tradeoffs, and here is no exception. The closer a spacecraft gets to the planet, the greater its risk for accidentally dipping into the upper atmosphere and careening inwards. Complicating matters further, Uranus also has a series of rings that are relatively poorly mapped. No one wants the mission to end after decades of work in a collision with a bit of ice, so engineers have another motivator to keep the spacecraft in a wide orbit.

It would be ideal if mission designers could avoid the dangerous inner region altogether and always remain outside the rings. As Parisi and collaborators demonstrate, however, if the probe stays in this safe region throughout the planned 90-day mission, it won’t be able to conclusively differentiate between solid and liquid core models. If they instead accept the risk and “plunge” between the rings and the surface once per orbit, they’d be able to achieve the mission objectives with only eight laps around the planet.

While we’re still years away from deals with contractors to actually build the mission, studies like this are essential to design the future flagship. The final trajectory decisions won’t be made for a while, but if we ever see photos taken by a robot streaking between Uranus’s cloud tops and rings, we’ll be able to trace its journey to that moment to articles like this.

Citation

“Uranus Orbiter and Probe: A Radio Science Investigation to Determine the Planet’s Gravity Field, Depth of the Winds, and Tidal Deformations,” Marzia Parisi et al 2024 Planet Sci. J. 5 116. doi:10.3847/PSJ/ad4034

infrared Hubble Ultra-Deep Field

Fifteen years ago, the Hubble Space Telescope gazed intently at the infrared glow of galaxies in a tiny fraction of the sky. New research shows how this patch of space has changed since then.

Ultra-Deep and Ultra-Famous

visible-light Hubble Ultra-Deep Field

The Hubble Ultra-Deep Field at visible wavelengths. [NASA, ESA, S. Beckwith (STScI), and the HUDF Team]

The Hubble Ultra-Deep Field is perhaps one of the most recognizable images of our universe. Assembled from observations made in 2003–2004, the visible-light Ultra-Deep Field showcases 10,000 galaxies that stretch back to less than a billion years after the Big Bang.

In 2008–2009 and 2012, Hubble revisited the region, this time piecing together infrared portraits that revealed even more distant galaxies that were absent from the visible-light view. The most distant galaxies in the infrared images appear as they were when the universe was just 2–5% of its current age. What can we learn from comparing these archival images to new images of the same patch of sky?

Paying Another Visit

With Hubble happily still operational (knock on wood!), Matthew Hayes (Stockholm University) and collaborators turned the telescope’s infrared camera toward the Ultra-Deep Field once again in 2023, aiming to find differences between the new images and the original infrared deep-field images from 2008–2009 and 2012.

Artist's impression of an active galactic nucleus surrounded by a dusty accretion disk

Illustration of an active galactic nucleus. [NASA/SOFIA/Lynette Cook]

In particular, the team hoped to find evidence for faint active galactic nuclei: accreting supermassive black holes at the centers of galaxies. As an active galactic nucleus gulps down varying amounts of gas from its surroundings, its brightness changes like a flickering flame — and these brightness changes are potentially detectable in the set of Ultra-Deep Field images. By cataloging active galactic nuclei in the early universe, researchers hope to pin down how the number of supermassive black holes has changed with cosmic time. This information can help determine how black holes form and evolve.

Spotted: Black Holes

Hayes’s team used two methods to search for sources with varying brightness — potential active galactic nuclei — in the new and archival Hubble images:

  1. Subtracting one image from another to identify objects that appear in only one image
  2. Comparing the brightness of the centers of galaxies between images
photometric variability of active galactic nuclei in the Hubble ultra-deep field

Demonstration of the photometric variability of the two active galactic nuclei. The source at z = 2.0 is shown in the top row, and the source at z = 3.2 is shown in the bottom row. Click to enlarge. [Hayes et al. 2024]

In total, they spotted 71 objects whose brightness varied significantly over the time period. Of the eight objects of interest presented in this work, two are active galactic nuclei at redshifts of z = 2.0 and 3.2 (about 2 to 3 billion years after the Big Bang). Three other objects are likely active galactic nuclei at redshifts beyond z = 6 (less than a billion years after the Big Bang) that couldn’t be definitively cataloged. A further three objects appear to be supernovae, one of which is perched on the edge of a disk galaxy and two of which have no apparent host galaxy.

Hayes’s team used these results to place a lower limit on the number density of black holes during the epoch of reionization, when radiation from the first stars and galaxies transformed the universe from opaque to transparent. They found that the number density of black holes in this time period is similar to the present day value, providing a critical test of black hole formation models. These are just the first results from this campaign, so you can look forward to more ultra-deep-field findings in the future!

Citation

“Glimmers in the Cosmic Dawn: A Census of the Youngest Supermassive Black Holes by Photometric Variability,” Matthew J. Hayes et al 2024 ApJL 971 L16. doi:10.3847/2041-8213/ad63a7

optical image of Cygnus X-1

Astronomers recently tracked a famous X-ray binary system through a change in its accretion state. What does this transition tell us about how black holes accrete gas?

Accretion Questions

illustration of an X-ray binary system

An artist’s impression of an X-ray binary, in which a compact object accretes material from a companion star and emits X-rays during intermittent outbursts [ESO/L. Calçada; CC BY 4.0]

X-ray binaries contain a star and a compact object — either a black hole or a neutron star. As the compact object ensnares gas from its stellar companion, a number of X-ray-bright features can emerge: the gas collects in a super-hot accretion disk and in a tenuous structure called the corona, and transient outflowing jets can appear.

Astronomers have discovered hundreds of X-ray binaries in the Milky Way, but there are still many open questions about the accretion process: What’s the origin of the corona, and how is it structured? Does it sit high above the compact object, or does it hover just above the surface of the disk? What’s the connection between the disk, the corona, and the jets?

One way to potentially answer these questions is to track an X-ray binary as it undergoes a state transition, shifting from producing more low-energy X-rays (“soft state”) to more high-energy X-rays (“hard state”). State transitions are thought to occur when a binary changes how it’s accreting gas, so observing a binary across state transitions can reveal whether the binary’s geometry changes in different accretion modes. Luckily, one of the best-studied X-ray binaries in our galaxy recently gave researchers a chance to study a state transition with a powerful observatory.

From Hard to Soft

Cygnus X-1 is an X-ray binary containing a 41-solar-mass supergiant star and a 21-solar-mass black hole. Over decades of monitoring, scientists have witnessed Cygnus X-1 repeatedly transition between soft and hard states. Phase switches happen randomly, and a phase can last weeks or years.

illustration of the favored coronal geometry

Side view of the favored coronal geometry; a wedge-shaped corona (blue) lies parallel to the accretion disk (yellow). The arrows show the direction of the black hole’s spin. [AAS Nova/Kerry Hensley]

Since the launch of the new Imaging X-ray Polarimetry Explorer (IXPE) spacecraft in 2021, Cygnus X-1 has held steady in a hard state. When researchers examined Cygnus X-1’s hard-state behavior with IXPE, they found that the X-ray emission was unexpectedly strongly polarized. In other words, the orientation of the X-rays as they traveled through space was more orderly than expected. Based on these observations, a “lamppost” model — in which the corona is situated above the black hole’s poles — is now disfavored. Instead, researchers favor a model in which the corona lies parallel to the accretion disk.

Soft-State Insights

polarization degree and angle for Cygnus X-1 hard and soft states

Left: Polarization degree and polarization angle for the hard (blue) and soft (red) states. Right: Polarization degree and angle as a function of energy for the soft state. Click to enlarge. [Steiner et al. 2024]

In April 2023, Cygnus X-1 transitioned out of its long-lived hard state, giving researchers their first opportunity to study the system’s soft-state behavior with IXPE. James Steiner (Center for Astrophysics ∣ Harvard & Smithsonian) and collaborators analyzed five epochs of IXPE data spread over two months. They found that while Cygnus X-1’s X-ray emission in the soft state is less polarized — 2% polarization compared to 4% in the hard state — the two states were otherwise similar; the polarization angle is parallel to the outflowing jet, and the degree of polarization increases with the temperature of the gas.

Using a fully relativistic spectral model, Steiner’s team found that the corona likely lies parallel to the accretion disk, just as in the hard state. While there are many similarities between the hard and soft states of this system, the team suggested that the majority of the polarized light in each state comes from a different source.

In the hard state, X-ray photons become polarized when they scatter off the corona. In the soft state, a substantial fraction of the X-ray photons from the accretion disk are bent back toward the disk by the black hole’s immense gravity, and they become polarized when they are reflected off the surface of the disk. In other words, X-rays from the accretion disk undergo gravitational lensing — showing that the same process that bends the light from distant galaxies is at work in a system billions of times less massive!

Citation

“An IXPE-led X-Ray Spectropolarimetric Campaign on the Soft State of Cygnus X-1: X-Ray Polarimetric Evidence for Strong Gravitational Lensing,” James F. Steiner et al 2024 ApJL 969 L30. doi:10.3847/2041-8213/ad58e4

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