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active galaxy Centaurus A

A new modeling method allows black holes and the gas that surrounds them to “talk” back and forth, painting a more realistic picture of how black holes collect material and churn out energy.

A Problem of Scale

Centaurus A

Composite image of Centaurus A, a galaxy whose appearance is dominated by the large-scale jets powered by the supermassive black hole at its center. [ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray); CC BY 4.0]

Black holes at the centers of galaxies across the universe consume gas, dust, and even stars from their surroundings. In exchange for this feast, accreting black holes emit powerful jets and radiation that disrupt and heat nearby gas. This process, known as feedback, cements the link between a black hole and its home galaxy.

Supermassive black holes, though enormous, are tiny compared to their host galaxies — the Milky Way’s central black hole’s event horizon stretches roughly 15 million miles, just a minuscule fraction of our galaxy’s half-quintillion-mile diameter. Despite this size mismatch, supermassive black holes are so powerful that they can influence entire galaxies, leaving researchers with the enormous challenge of modeling the complex processes of accretion and feedback across a wide range of spatial scales.

Building a Two-Way Radio

Typically, models of hungry black holes handle the spatial scale issue by nesting simulations spanning different scales within each other and running them in sequence, starting far from the black hole and spiraling in toward it. This strategy helps the model communicate to the black hole what’s going on around it — how much gas there is to snack on, for example — but it needs to let the black hole talk back, too. That’s where a new technique from a team led by Hyerin Cho (조혜린), Center for Astrophysics | Harvard & Smithsonian and the Black Hole Initiative, comes in.

This new technique uses general relativistic magnetohydrodynamics to model black hole accretion and feedback across seven orders of magnitude in spatial scale. The key advance is that the model spirals from large scales down to small scales — and back — hundreds of times, allowing the black hole to chat freely with its surroundings.

Focusing on Feedback

To demonstrate the new method’s capabilities, Cho and collaborators first showed that they could reproduce the standard analytical solution for a black hole accreting unmagnetized gas. Then, they moved on to a more realistic system that includes magnetic fields. Unlike the unmagnetized case, where gas swirls toward the black hole in a smooth and orderly way, the magnetized case is chaotic: random, turbulent movements as the gas is pulled toward the black hole make the accretion rate vary wildly.

simulation results showing the plasma beta and the plasma density across eight orders of magnitude of spatial scale

Maps of plasma beta (β; the ratio of thermal pressure to magnetic pressure within a plasma) and plasma density (ρ) across eight orders of magnitude in spatial scale. Click to enlarge. [Adapted from Cho et al. 2023]

Where does the turbulence come from? Cho’s team found that magnetic field lines close to the black hole are constantly rearranging, relaxing into new configurations that convert pent-up magnetic energy into kinetic energy. In other words, the reconfiguring of the magnetic field heats and accelerates the surrounding gas, prompting large-scale motions that transport energy away from the black hole — and this outward transport of energy signals that black hole feedback is actually taking place!

Importantly, Cho’s team’s results mesh with what researchers have seen for the black holes they’ve observed closely, especially the central supermassive black holes of the Milky Way and the massive elliptical galaxy Messier 87. While this two-way communication represents a huge advance in the modeling of black hole accretion and feedback, there’s more work to be done; future investigations will tackle spinning black holes surrounded by rotating gas.

Citation

“Bridging Scales in Black Hole Accretion and Feedback: Magnetized Bondi Accretion in 3D GRMHD,” Hyerin Cho et al 2023 ApJL 959 L22. doi:10.3847/2041-8213/ad1048

A star at bottom-right and a bright jet leading from it towards top-right.

Though astronomers already knew that strange things can happen in the final moments of a massive star’s life, a recent model may have just taken the top prize as the most bizarre possible ending. In their last gasps, some stars might be cut clean down the middle by a “relativistic blade.”

Raging, Raving Stars

A bright central circle with a narrow jet shooting off the screen towards upper right.

An artist’s illustration of a jet escaping from a collapsing star. [NASA, ESA and M. Kornmesser]

Massive stars are not typically inclined to go gentle into the good night. Instead, as they exhaust the last remains of their nuclear fuel, they opt to mark the ends of their short lives with complex, cataclysmic explosions. Previous work has postulated that these eruptions do not expand outwards in all directions, and that instead, the fireballs take the form of narrow jets that shoot outwards from the star’s core in opposite directions. These high-energy fountains fly outwards at relativistic speeds and burrow deep into the star’s surrounding neighborhood, sweeping up matter as they go.

Though that scenario sounds plenty dramatic, new work by Marcus DuPont and Andrew MacFadyen, New York University, suggests a potentially even more striking scene. Under the right conditions, they surmise that instead of narrow, polar jets, a blast wave could instead expand straight outward along the star’s equator, break through the surface, and cleave the dying star clean in two.

Views of two relativistic blade simulations, one for a narrow blade (top) and one for a wider blade (bottom). In both, the upper hemisphere illustrates density, while the southern illustrates pressure. [DuPont and MacFadyen 2023]

Carving a Star

DuPont and MacFadyen introduce their model of a “relativistic blade” in a playful yet highly technical publication that includes mentions of “pizza slices of uneven length” right alongside topics like “relativistic magnetohydrodynamics” and no fewer than 21 equations in the first five pages. In their telling, as a massive star begins its final collapse, its core can transform into an ultra-dense, ultra-magentized, and rapidly rotating neutron star known as a millisecond magnetar. This rotten heart would seal the star’s doomed fate: the intense winds and magnetic field surrounding the magnetar would deposit enormous quantities of energy into the area around the core that must escape to the surface. Thanks to the magenetar’s spin, that energy would be concentrated into narrow “lamina,” or blades, of outflowing matter that would propagate outwards in just a few seconds.

Moving beyond mathematical musings and into numerical simulations, the pair used software developed earlier by DuPont to illustrate a few caveats of their model. For one, the blades would have to be initially more collimated than their jet counterparts, and for another, the width of the blades has a strong impact on their efficiency and breakout time. But, under a reasonable set of conditions, the simulations show that the blades would “affec[t] a clean slice through the progenitor.”

Consequences of Slashed Stars

Though everyone loves a good star-killing relativistic blade for its own sake, DuPont and MacFadyen did have practical considerations in mind when concocting such a sci-fi sounding model. It’s not currently clear where mysterious flashes of high-energy radiation known as gamma-ray bursts actually come from. Astronomers have developed several models to explain their observations of these bursts, and one of the leading ideas is the earlier-mentioned jets, which would crackle with gamma rays on their journey from their stars’ cores. These blades, too, would unleash gamma rays as they cut through their stars, and thus could also be a source of the observed flashes.

DuPont and MacFadyen suggest that how these bursts fade over time would differ between stars pierced by jets and those cut by blades, meaning that detailed observations may be able to tell the two mechanisms apart some day. They also suggest that more complex models are needed to dial what these blades would look like and how likely they are to arise. That’s great news both for serious astronomers hoping to better understand gamma-ray bursts and space-opera fans alike, who can both look forward to more work on this dramatic hypothesis.

Citation

“Stars Bisected by Relativistic Blades,” Marcus DuPont and Andrew MacFadyen 2023 ApJL 959 L23. doi:10.3847/2041-8213/ad132c

radio image of the star Betelgeuse

artist's impression of a dust cloud blocking the light from Betelgeuse

An artist’s impression of a cloud of dust partially blocking Betelgeuse from view during the Great Dimming. [ESO, ESA/Hubble, M. Kornmesser; CC BY 4.0]

Betelgeuse is a highly recognizable red supergiant star that sits at the shoulder of the constellation Orion. In 2019–2020, Betelgeuse faded to a third of its usual brightness, raising speculation that it was vying for the title of “Most Likely to Go Supernova,” but the star is likely still tens of thousands of years from its explosive end. Instead, the leading theory among astronomers is that Betelgeuse underwent a surface mass ejection, propelling some of its surface gas into space. When this surface material cooled enough to condense into dust, it blocked some of Betelgeuse’s light, causing the dramatic, months-long dimming episode.

Today, we’re introducing three recent articles that examine Betelgeuse’s intriguing behavior and uncertain past.

How Did the Great Dimming Affect Betelgeuse?

Betelgeuse's brightness and radial velocity from 2012 to 2023

Betelgeuse’s brightness (top) and radial velocity (bottom) over time. The Great Dimming, marked by the vertical purple line, appears to have brought about a new periodicity in these quantities. Click to enlarge. [MacLeod et al. 2023]

Up until the “Great Dimming,” Betelgeuse’s brightness varied somewhat regularly, with a period of about 400 days discernible from the wiggles in its light curve. Coupled with the observed movement of the star’s surface, these changes reveal that Betelgeuse is pulsating as the entire star expands and contracts together. When examining the star’s behavior post-dimming, a team led by Morgan MacLeod (Center for Astrophysics | Harvard & Smithsonian) discovered that a new strong periodicity had emerged with a cadence of 200 days.

MacLeod’s team used hydrodynamics simulations to connect the change in variability to changes in the star’s interior. The gaseous outer envelope of Betelgeuse’s atmosphere is constantly churning with large-scale convective motions, similar to the movement of boiling water, that transport heat throughout the star. MacLeod’s team modeled an unusually hot plume of gas that broke free from the star’s surface, powering the surface mass ejection that caused the dimming. When this hot gas plume rose, it could have interrupted Betelgeuse’s typical pulsations, energizing a new pulsation mode that cycles twice as quickly.

If this process is responsible for the change in Betelgeuse’s behavior, the new pulsation mode will eventually lose energy and fade away: MacLeod and collaborators predict that the star should return to its previous 400-day pulsation period within 5–10 years.

Was Betelgeuse Once Two Stars?

Most massive stars grow up with a nearby binary companion, and at some point in their lives these stars may transfer mass back and forth. A recent research article takes this concept of mass transfer to the extreme, exploring whether Betelgeuse engulfed its binary companion, fundamentally altering the course of its evolution.

plot of simulation output showing the evolution of the surface velocity and abundance of carbon, oxygen, and nitrogen

Results of stellar evolution simulations showing how the surface velocity and nitrogen, oxygen, and carbon abundances change with time. The black boxes indicate the values of these quantities when the modeled stars have similar temperatures and luminosities to present-day Betelgeuse. Click to enlarge. [Shiber et al. 2024]

Sagiv Shiber (Louisiana State University) and collaborators used three-dimensional hydrodynamics simulations to study the outcome of a merger between a 16-solar-mass star nearing the end of its hydrogen-burning lifetime and a 4-solar-mass main-sequence companion. The merger of stars with very different masses is thought to produce a red supergiant, like Betelgeuse, rather than a blue supergiant, which the merger of stars with similar masses might create. The models follow the smaller star as it’s swallowed by the larger star, spirals inward, and merges with the larger star’s core, ejecting roughly 0.6 solar mass of material in the process.

After allowing the merger product to evolve until its temperature and luminosity were similar to Betelgeuse’s, Shiber’s team calculated the surface composition and rotation rate of the resulting star, since these quantities are potentially measurable. Post-merger stars appear to have more nitrogen at their surface than stars that have evolved normally, and they may rotate more rapidly than typical stars, as well. Observations suggest that Betelgeuse is both rapidly rotating and rich in nitrogen, supporting the hypothesis that it was once two stars.

Is Betelgeuse Actually Rotating Rapidly?

If Betelgeuse is a mashup of two stars, simulations suggest that it should be rotating rapidly. Several years ago, researchers studied Betelgeuse with the Atacama Large Millimeter/submillimeter Array (ALMA) and interpreted the observations to mean that the star was rotating at a rate of 5 kilometers per second — twice the pace expected for a single star. A team led by Jing-Ze Ma (马竟泽) from the Max Planck Institute for Astrophysics investigated this critical piece of evidence by testing whether large-scale convection — the roiling motion that causes material to rise and sink within a star, helping to transport heat from the star’s interior to its surface — could be mistaken for rapid stellar rotation.

intensity and radial velocity for a simulated star, simulated ALMA observations, and actual ALMA observations

The intensity (top row) and radial velocity (bottom row) for a simulated star, simulated ALMA observations, and actual ALMA observations. Click to enlarge. [Ma et al. 2024]

The team performed three-dimensional simulations of red supergiant stars that are not rotating, and processed the output to mimic what ALMA would see if looking at these stars. Ultimately, Ma and coauthors found that ALMA data can be misleading, with a 90% chance of large-scale convective motions being misinterpreted as stellar rotation at a rate of 2 kilometers per second or faster.

As always, researchers need more data to investigate this issue further. One or more ALMA observations could clarify whether Betelgeuse is truly rotating rapidly, supporting the hypothesis that it’s the result of a stellar merger, or if the star’s massive convective motions have been misleading.

Citation

“Left Ringing: Betelgeuse Illuminates the Connection Between Convective Outbursts, Mode Switching, and Mass Ejection in Red Supergiants,” Morgan MacLeod et al 2023 ApJ 956 27. doi:10.3847/1538-4357/aced4b

“Betelgeuse as a Merger of a Massive Star with a Companion,” Sagiv Shiber et al 2024 ApJ 962 168. doi:10.3847/1538-4357/ad0e0a

“Is Betelgeuse Really Rotating? Synthetic ALMA Observations of Large-Scale Convection in 3D Simulations of Red Supergiants,” Jing-Ze Ma et al 2024 ApJL 962 L36. doi:10.3847/2041-8213/ad24fd

cartoon showing different types of exoplanets

Exoplanet K2-18b made headlines when researchers reported that JWST observations of the planet were consistent with a habitable ocean world. Now, another team has published a different interpretation of the data, suggesting that the purported water world is instead a gas-rich planet with no habitable surface.

Everybody Wants to Rule the Find a Habitable World

An artist's impression of K2-18b as an ocean world

An artist’s impression of K2-18b as an ocean world. [NASA, ESA, CSA, Joseph Olmsted (STScI)]

The small, cool star K2-18 is home to two planets, one of which has garnered plenty of attention in the decade since its discovery. Recently, JWST data of K2-18b, an 8.6-Earth-mass planet, revealed the presence of atmospheric carbon dioxide and methane. Some researchers have interpreted these data, coupled with the non-detection of ammonia, water, and carbon monoxide, to mean that K2-18b is a Hycean world: a rocky planet covered in oceans.

To make matters more interesting, the same research team reported weak evidence for dimethyl sulfide, a compound that on Earth forms almost exclusively due to life. This led many onlookers to the eyebrow-raising conclusion that K2-18b is not just habitable but inhabited.

These intriguing interpretations, however, are far from settled. Is K2-18b truly a habitable ocean world, or could alternative explanations fit the JWST data equally well?

simulation of the atmosphere of K2-18b as a gas-rich planet without a habitable surface

Example simulation output for K2-18b as a gas-rich planet without a habitable surface. Click to enlarge. [Wogan et al. 2024]

Water World or Gas Planet?

A team led by Nicholas Wogan (NASA Ames Research Center and University of Washington) tackled this question by applying two sets of models to the JWST data. The first set describes rocky planets with surface oceans, with and without life, and the second set describes gaseous planets without a surface and without life. The models predict the planet’s photochemistry — chemical reactions in the atmosphere driven by photons from the host star — and climate.

Wogan’s team found that K2-18b is unlikely to be a lifeless water world, since this type of planet wouldn’t contain enough methane in its atmosphere to produce the signal seen in the JWST observations. Intriguingly, a water world with microbial life is more promising: acetotrophic methanotrophs — a tongue-twisting name for simple methane-producing organisms — may be able to produce the supply of methane seen in the planet’s atmosphere.

Not So Fast…

simulated spectra of K2-18b as an ocean world with and without life and as a gas-rich planet

JWST transmission spectra (black and gray points with error bars) and modeled spectra for K2-18b as a lifeless ocean world (top left), an ocean world with life (bottom left), and a lifeless gas-rich planet (bottom right). Click to enlarge. [Wogan et al. 2024]

As exciting as this sounds, Wogan and collaborators found that the uninhabitable gas-rich exoplanet model fits the JWST data equally well, and this model may pose fewer problems. Not only does the ocean-world model require life to explain its atmospheric composition, it’s also hard to reconcile the necessary cool surface temperature with the high likelihood of the planet experiencing a runaway greenhouse effect.

This isn’t the last word on K2-18b — there are features in the planet’s spectrum that aren’t well fit by a lively ocean world or a lifeless gas-rich planet, and both models have their challenges. Future data from JWST might dredge up a detection of ammonia, which would point to a gaseous planet, or dimethyl sulfide, which would tilt the scales considerably toward an inhabited water world. In the meantime, the hunt for habitable planets goes on.

Citation

“JWST Observations of K2-18b Can Be Explained by a Gas-Rich Mini-Neptune with No Habitable Surface,” Nicholas F. Wogan et al 2024 ApJL 963 L7. doi:10.3847/2041-8213/ad2616

Hubble Space Telescope image of a dwarf starburst galaxy

Researchers have spotted an unusual pair of interacting dwarf galaxies: one with many bright young stars, the other with none. New research examines how two seemingly similar galaxies can have such different outcomes, shedding light on why quiescent dwarf galaxies are so rare.

A Clash of Small Galaxies

photograph of the dwarf galaxies UGS 5205 and PGC 027864

A visible-light and infrared view of UGC 5205 (left) and PGC 027864 (right). [Adapted from Kado-Fong et al. 2024]

Dwarf galaxies contain anywhere from a thousand to a billion stars, making them far smaller than galaxies like the Milky Way. Despite their small size, dwarf galaxies are the sites of abundant star formation: aside from the unlucky galaxies that have had their star-forming gas stripped away by a larger galaxy, 99% of dwarf galaxies in the field are actively forming new stars. Researchers suspect that this statistic has something to do with collisions, which quench star formation in massive galaxies but appear to trigger it in dwarf galaxies.

Enter UGC 5205 and PGC 027864, a pair of dwarf galaxies that have responded in remarkably different ways to their interaction. PGC 027864 is a typical dwarf starburst galaxy ablaze with new stars, but UGC 5205 is eerily quiet. Previous observations show that the two nearly equal-mass galaxies each contain abundant star-forming gas, making UGC 5205’s lack of new stars a real mystery. Now, new data have solved the mystery of the missing star formation.

optical and radio observations of quiescent dwarf galaxy UGC 5205 and its starbursting companion, PGC 027864

Contours from the new Very Large Array data showing the location of neutral hydrogen gas (green) overlaid on a grayscale optical image. [Kado-Fong et al. 2024]

A Close Look at Cold Gas

Erin Kado-Fong (Yale University) and collaborators collected new observations of the interacting galaxy pair with the radio dishes of the Very Large Array. These new high-resolution observations show that while UGC 5205 contains an adequate amount of star-forming fuel, most of it has been expelled from the main body of the galaxy into three tidal structures. PGC 027864’s gas, on the other hand, looks mostly normal, with just a small tidal tail. While the reason for the different behavior isn’t clear — perhaps PGC 027864’s gas was more concentrated — this shows that UGC 5205’s star-formation shutdown is due to a lack of neutral hydrogen gas.

How long ago did this rare quiescent dwarf galaxy’s star formation shut down? Optical and ultraviolet images of the galaxy pair show that though UGC 5205 shines brightly in the ultraviolet, indicating abundant star formation on the order of 100 million years ago, it’s invisible at optical wavelengths that trace star formation in the past 10 million years. By modeling the light emitted by different populations of stars, Kado-Fong’s team found that UGC 5205 likely underwent a quenching event 100–300 million years ago.

Quiescent for Now

plot of the star formation rate and mass of this study's quiescent dwarf galaxy and starbursting dwarf galaxy compared to galaxies on the star-forming main sequence

The star formation rates and masses of the two galaxies in this study compared to low-mass galaxies on the star-forming main sequence. The two galaxies have similar rates measured from ultraviolet (UV) light, which traces star formation on the order of 100 million years ago, but dramatically different rates measured from Hα, which traces star formation in the last 10 million years. [Kado-Fong et al. 2024]

This work shows that interactions between dwarf galaxies can cause quiescence as well as star formation. But if starbursting dwarf galaxies are created through collisions, and it’s possible to make quiescent dwarf galaxies this way, why are quiescent dwarfs so rare? Kado-Fong and collaborators explained that this type of quenching is probably only temporary: UGC 5205’s expelled star-forming gas is likely still gravitationally bound to the galaxy, and it will eventually fall back toward the galaxy and trigger star formation. The team estimates that UGC 5205 will spend no more than 560 million years in a quenched state before returning to a normal star-forming state.

Citation

“Dwarf–Dwarf Interactions Can Both Trigger and Quench Star Formation,” Erin Kado-Fong et al 2024 ApJ 963 37. doi:10.3847/1538-4357/ad18cb

Illustration of an accretion disk swirling around a supermassive black hole

Astronomers have discovered a handful of supermassive black holes that are associated with bursts of X-rays that recur every few hours or days. New work expands on a theory that explains these X-ray outbursts and explores how we can spot them at other wavelengths.

Investigating Quasi-periodic Eruptions

For the past few decades, researchers have seen X-ray flares coming from the centers of certain galaxies. These flares, dubbed quasi-periodic eruptions, last about an hour, and individual flares are separated by a few hours to a few days. The brightness changes from flare to flare, and the flares don’t recur at precise intervals, giving the events their name. The cause of quasi-periodic eruptions isn’t yet known, but leading models involve a star being partially torn apart by a supermassive black hole or an object interacting with gas accreting onto a black hole.

diagram showing the star–disk–black hole model proposed by the authors

A simple diagram showing the proposed model. A star orbits a supermassive black hole on an orbit that is slightly elliptical (exaggerated here for clarity) and inclined relative to the accretion disk (yellow). The X’s mark the places where the star passes through the disk. The time between crossings at A and B is shorter than the time between crossings at B and A. [AAS Nova/Kerry Hensley]

Previously, Itai Linial (Institute for Advanced Study and Columbia University) and Brian Metzger (Columbia University and Flatiron Insitute) proposed a model that explained a curious feature seen in two quasi-periodic eruption sources: a repeating pattern in which pairs of flares are separated by alternating long and short intervals. In this model, a star near a supermassive black hole collides repeatedly with an accretion disk. The disk might form when the star loses gas to the black hole’s gravitational pull, or it might come from an unrelated star that’s being ripped apart. Each time the star plunges through the disk, shocks heat a spray of gas that emits an X-ray flare. If the star’s orbit is elliptical and inclined relative to the accretion disk, this model naturally explains why the flares aren’t precisely periodic: flares are more closely spaced when the star travels a shorter distance between disk crossings, and they’re farther apart when the star travels farther between crossings.

An Ultraviolet Possibility

Now, Linial and Metzger have explored the implications of this model further, delving into the properties of quasi-periodic eruptions to understand if these events can only be observed at X-ray wavelengths, or if it’s possible to see them at other wavelengths as well. Because there are several planned and proposed ultraviolet survey missions, the team focused on the possibility of observing quasi-periodic eruptions in the ultraviolet. By outlining the equations that describe a star punching through a thin disk of gas around a supermassive black hole, the authors predicted how the brightness and timing of the flares depend on the size and mass of the star, how quickly the black hole is accreting gas from the disk, and other factors.

plot summarizing the conditions under which quasi-periodic eruptions emerge in the X-ray versus the ultraviolet

Luminosity of quasi-periodic eruptions as a function of accretion rate. This plot demonstrates how a system might transition from producing X-ray flares to ultraviolet flares, or vice versa, as the accretion rate changes. Click to enlarge. [Linial & Metzger 2024]

To be able to spot a flare, it must outshine the hot, glowing gas of the disk, which may be possible in two cases. In the first, stars on short orbits around relatively massive black holes — in the few-million-solar-mass range — produce bright X-ray flares. In the second, stars on long orbits around less-massive black holes — in the few-hundred-thousand-solar-mass range — produce bright ultraviolet flares. It may even be possible for X-ray flares to reinvent themselves as ultraviolet flares after fading from view at X-ray wavelengths, or vice versa, as the system evolves: if the accretion disk formed out of a disrupted star, the accretion rate will eventually drop, shifting the peak of the flare emission from X-rays to ultraviolet over time. It’ll be fascinating to see these predictions tested when future ultraviolet surveys get underway!

Citation

“Ultraviolet Quasiperiodic Eruptions from Star–Disk Collisions in Galactic Nuclei,” Itai Linial and Brian D. Metzger 2024 ApJL 963 L1. doi:10.3847/2041-8213/ad2464

Accurate models of supernova spectra are typically pretty slow, but neural networks are both fast and capable of mimicking many other codes. Recently, a team used the latter facts to overcome the former challenge and created a set of neural networks capable of rapidly modeling real supernova data.

Complex Explosions

As one might expect when dealing with one of the most violent, rapid, and energetic processes in the universe, one must take great care when modeling a supernova. In the seconds, hours, and days after the initial explosion, many different processes unfold on many different scales: unstable elements birthed from the raw power of the eruption decay into more durable forms; heavy elements are accelerated to mind-boggling speeds; nearby material begins to glow as the temperatures approach values beyond comprehension. Each of these processes and others affect the spectra that we measure here on Earth, and each of them must be closely tracked in order to correctly explain our observations.

The immense complexity of the problem places large demands on the codes that simulate the aftermath of a supernova. Despite many different simplifications and approximations, the inescapable fact remains that each of these simulations take a lot of time to run. For example, one code named TARDIS requires about one CPU hour to transform inputs such as total luminosity, time after explosion, and the structure of the ejecta into an output model spectrum. For astronomers who want to compare their real data to potentially millions of models to find the best-fitting values for those input parameters, that runtime won’t cut it.

A recently adopted unspoken code among the astronomy community states that if one scientist says “complex model” and “slow runtime” three times fast, another will appear with a machine learning solution to the problem. A team led by Xingzhuo Chen, Texas A&M University, just published a neural-network-accelerated supernova inference framework that answers this latest call.

Neural Networks and Nickel

A loss vs. epoch plot, showing exponentially decreasing trends for several curves.

The training progress of the neural networks as a function of iterations through the training data. [Chen et al. 2024]

Chen and collaborators set out to model real Type Ia supernovae using neural networks capable of rapidly converting spectra into underlying physical parameters. The first step required creating and training the networks. To do this, the team fed TARDIS 108,389 different input values and compiled the resulting spectra into a training library. They then let several neural networks loose in this library and tasked them with learning how to convert the spectra into their original inputs, or essentially how to undo all of TARDIS’s hard work. After a few tens of iterations through the library, the networks could faithfully mimic TARDIS in reverse, and the team was left with a several networks capable of constraining different physical parameters when shown a spectrum.

The inferred nickel abundance over time for several real supernovae. The expected decay rate is shown in the solid line. [Chen et al. 2024]

The researchers then handed over about 1,000 spectra of about 100 distinct supernovae to their trained networks and analyzed the resulting predictions. One quantity they focused on was the nickel content of the ejecta and how it evolved over time. They found that for some supernovae, the radioactive nickel depleted at exactly the predicted half-life of the unstable isotopes. Others, however, either had nickel stick around for longer than expected or fade away faster than the familiar rate. Exactly why this might be is unclear, and the mismatched cases could point to issues with the approximations made by TARDIS itself, or with assumed spherical symmetry/smoothness of their ejecta model.

Even still, the authors state that their work “represent[s] a significant improvement in the quality of the spectral modeling of [supernovae] compared to similar models in the literature.” By using this technique of harnessing neural networks to speed up complex physical models, astronomers can look forward to many more breakthroughs that wouldn’t be possible without an AI-assisted boost.

Citation

“Artificial Intelligence Assisted Inversion (AIAI): Quantifying the Spectral Features of 56Ni of Type Ia Supernovae,” Xingzhuo Chen et al. 2024 ApJ 962 125. doi:10.3847/1538-4357/ad0a33

illustration of a planet orbiting an M dwarf and losing its atmosphere

Which planets are most likely to lose their atmospheres? New research finds a surprising relationship between planet size and atmospheric escape and suggests that the smallest planets aren’t losing their atmospheres the fastest.

The Role of Atmospheric Loss

illustration of Mars losing ions from its atmosphere during a solar storm

An illustration of Mars losing ions from its atmosphere during a solar storm. [NASA/GSFC]

Don’t panic, but Earth is slowly losing its atmosphere — very slowly, at a rate so small that the Sun will balloon into a red giant and engulf our planet long before its atmosphere is whisked away. Atmospheric escape is an unavoidable reality for any planet encased in gas, but around our calm, middle-aged star, atmospheric loss is minimal.

Other planets in distant star systems are not as lucky: pummeled by fierce radiation and howling stellar winds, many exoplanets will lose most or all of their atmospheres. A planet’s ability to hold on to its atmosphere shapes its habitability, the possibility of harboring long-lived oceans, and other critical factors. With exoplanet habitability a persistently hot topic in astronomy, it’s critical to know which planets are most likely to have atmospheres.

What’s Radius Got to Do with It?

To approach this question, a team led by Laura Chin (Boston University) investigated how a rocky exoplanet’s size affects its ability to hang on to its atmosphere. Using fluid dynamics models, Chin and collaborators simulated the atmospheric loss due to stellar radiation and stellar winds. High-energy radiation ionizes atmospheric particles, allowing them to be picked up and carried away by the magnetized stellar wind, and imparts enough energy to some particles to allow them to escape the planet’s gravitational pull.

model output showing the escape of oxygen ions

Model output showing the escape of oxygen ions from planets of varying sizes. Click to enlarge. [Chin et al. 2024]

The team modeled Venus-like exoplanets with radii ranging from half of Venus’s radius (RV; roughly the size of Mars) to 2.25 RV (roughly the limit beyond which exoplanets are gaseous rather than rocky). Similar to Venus, the simulated planets do not have global magnetic fields, and their atmospheres are mainly carbon dioxide. Because the smallest stars in our galaxy — M dwarfs — are both the most numerous stars and are likely to host small, rocky planets, the team used stellar properties similar to those of M dwarfs: powerful, dense stellar winds and searing ultraviolet light.

A Surprising Finding

plot of atmospheric escape rate as a function of planetary radius

Escape rate as a function of planetary radius for three types of ions (colored lines) and all atmospheric particles (black line). [Chin et al. 2024]

How quickly a planet without a global magnetic field loses its atmosphere through interactions with stellar winds is a combination of the planet’s gravitational pull, which determines how snugly its atmosphere clings to the planet, and the planet’s size, which determines the area that gets hit by the wind. Intriguingly, the combination of these factors creates a sweet spot for atmospheric escape: rocky planets with radii around 0.75 RV have the highest rates of atmospheric escape.

The increase in the escape rate for planets with radii from 0.5 to 0.75 RV appears to be driven by the increase in the planet’s size; a larger planet’s atmosphere absorbs more starlight (i.e., energy), creating more ions that can be stolen away by a magnetized stellar wind and boosting the escape rate. Above 0.75 RV, the decrease in the escape rate is driven by the increase in the planet’s gravitational pull, which makes it harder for atoms to depart the atmosphere.

This result contrasts with expectations that the atmospheric escape rate is largest for the smallest planets. Future surveys of rocky planets around M-dwarf stars may be able to search for a connection between planet size and atmosphere thickness, testing this finding.

Citation

“Role of Planetary Radius on Atmospheric Escape of Rocky Exoplanets,” Laura Chin et al 2024 ApJL 963 L20. doi:10.3847/2041-8213/ad27d8

star-forming region NGC 6357

The star J1010+2358 may have descended from just one of the first stars, which would make it a powerful probe of the elusive first generation of stars. However, new research finds that its properties are consistent with a range of stellar ancestries.

Seeking the First Generation of Stars

The first stars in the universe collapsed into being in clouds of pristine gas containing just hydrogen, helium, and a tiny amount of lithium. This simple set of chemical ingredients likely allowed the first generation of stars to attain enormous masses, although the exact distribution of their masses is unknown. These early stars created new elements in their cores and scattered them throughout the universe in expansive clouds of metal-enriched gas.

Plot of measured and predicted chemical abundances for J1010+2358

Measured chemical abundances for J1010+2358 (black circles) and model predictions for several different stellar ancestries. The prediction for a single 260-solar-mass ancestor is shown in red. Click to enlarge. [Adapted from Koutsouridou et al. 2024]

While massive stars from this first generation are long gone from the Milky Way, their descendants may still roam the galaxy. Finding these descendants — especially those that can trace their material back to a single member of the first generation — would provide a powerful way to study the first stars in the universe. Recently, researchers claimed to have found such a star, called J1010+2358.

The star’s overall lack of metals (elements heavier than helium, in astronomer parlance) and curious chemical abundance pattern suggest that it was made from gas left behind by a 260-solar-mass star; J1010+2358 is especially lacking in elements with odd atomic numbers, such as sodium, compared to even-numbered elements. Now, a team led by Ioanna Koutsouridou (University of Florence) has investigated whether J1010+2358 is truly the descendant of a single, massive member of the first generation of stars.

A Study in Stellar Genealogy

Koutsouridou’s team examined whether J1010+2358 contains material passed down from only a single 260-solar-mass star, or if it contains material from several stars. Using chemical abundance modeling, the team found that J1010+2358 must have descended from a 260-solar-mass star — but it could have other stellar parents as well. In fact, without being able to measure several critical chemical elements in J1010+2358’s spectrum, it’s only possible to say that the reported stellar ancestor contributed at least 10% of J1010+2358’s metals.

plot of allowed and excluded initial mass functions

Allowed and excluded initial mass functions based on the non-detection of single-ancestor stars in the Stellar Abundances for Galactic Archaeology database (red contours) and the properties of J1010+2358. The green and blue contours show the constraints placed by J1010+2358 if it inherited 70% or 90%, respectively, of its metals from a single star. [Koutsouridou et al. 2024]

While J1010+2358 may have more than one stellar parent, its properties can still help researchers probe the generation of stars that came before it. Using models of how the Milky Way’s chemical enrichment evolved over time, Koutsouridou and collaborators used the non-detection of stars enriched by just one first-star ancestor to constrain the masses of the first stars. The strength of the constraint depends on how much of J1010+2358’s material came from its 260-solar-mass ancestor; only if more than 70% of the star’s metals came from a single ancestor can its chemical abundance pattern constrain the possible mass distribution of the first stars.

The hunt for descendants of the first stars goes on: high-resolution surveys continue to dredge up stars with just one first-generation stellar ancestor, and future observations may fill in the missing elemental abundance measurements in J1010+2358’s spectrum and clarify its family tree.

Citation

“True Pair-instability Supernova Descendant: Implications for the First Stars’ Mass Distribution,” Ioanna Koutsouridou et al 2024 ApJL 962 L26. doi:10.3847/2041-8213/ad2466

multiwavelength image of the Crab Nebula

When a massive star goes supernova, the explosion can leave behind a pulsar: the core of a dead star containing 1–2 times the mass of the Sun in a sphere just 20 kilometers across. Pulsars are almost entirely composed of neutrons and spin extremely quickly — the fastest recorded pulsar spins 716 times every second, meaning that a point on its surface moves at roughly a quarter of the speed of light. Pulsars emit beams of radio waves from their poles, and an observer on Earth sees pulses of radio emission in time with the star’s rotation. The word pulsar comes from pulsating radio source.

Observing pulsars helps us understand the evolution of massive stars, provides a way to study the physics of ultra-dense materials, and gives us a means to search for the background gravitational hum of supermassive black holes in colliding galaxies. Today, we’ll take a look at three recent research articles that explore fundamental questions in pulsar science.

How Do We Find Pulsars?

Jocelyn Bell Burnell discovered the first pulsar by chance in 1967, when the characteristic pulses popped up in radio observations taken with a new telescope. Today, researchers design surveys tuned to the particular properties of pulsars to make them stand out from other signals in the sky. Namely, radio surveys can search for sources with steep spectra — in other words, signals that are far brighter at low frequencies than at high frequencies — or strongly polarized light.

Ziteng Wang (Curtin University) and collaborators used the Australian Square Kilometre Array Pathfinder (ASKAP), a 36-dish radio interferometer, to search for circularly polarized signals from pulsars. In addition to known stars and pulsars, the observations pinpointed a strongly circularly polarized source with no known counterpart at other wavelengths. The team followed up on this promising discovery with the 64-meter Murriyang radio telescope at Parkes Observatory and found a pulsar with a rotation period of 78.72 milliseconds. The pulsar, cataloged as PSR J1032−5804, has an estimated age of 34,600 years, making it relatively young and possibly still associated with a visible supernova remnant. The team found a compact region of emission surrounding the pulsar, but they couldn’t rule out the possibility that the material belongs to unrelated nebulae.

PSR J1032−5804 is notable because its pulses are highly scattered by interstellar gas and dust. Highly scattered pulsar signals are hard to detect because scattering broadens and weakens the signal, especially at lower frequencies where pulsars should be at their brightest. Wang’s team has shown that searching at relatively high frequencies — the team’s observations were made at 3 gigahertz — is a viable way to detect scattered pulsars.

The field surrounding PSR J1032−5804 shown at, from left to right, radio, infrared, and visible wavelengths, as well as a composite of all three. [Wang et al. 2024]

How Do Pulsars Make Their Pulses?

Pulsars may be most famous for their characteristic pulses of radio emission, but the origin of those pulses is still under debate. To understand what powers these radio beacons, researchers use detailed simulations that track the behavior of individual particles to understand how they behave under the exotic conditions present at the surface of a pulsar. To date, localized simulations have been able to produce radio waves from a pulsar’s poles, and global simulations have discerned the source of pulsars’ gamma-ray pulses (10% or so of pulsars produce gamma-ray pulses in addition to radio pulses), but radio pulses have not yet been seen in global simulations.

Ashley Bransgrove (Columbia University and Princeton University) and collaborators carried out high-resolution global simulations of a pulsar’s magnetosphere: the region immediately surrounding a pulsar where its strong magnetic field dominates the motion of charged particles. The simulations show how the rapid rotation of the pulsar lofts charged particles from its surface and accelerates them, filling the magnetosphere with gamma rays and a dense sea of electrons and their positively charged counterparts, positrons. Near the pulsar’s poles and farther out in its magnetosphere, gaps form where the electric current is mismatched, and pairs of electrons and positrons are generated in these gaps. When the gaps discharge — think of a spark, or lightning — they excite waves in the plasma and, subsequently, electromagnetic waves. The emitted radiation is similar in frequency and luminosity to observed pulsars, suggesting that electric discharge may generate the radio waves that pulsars are known for.

The team notes that it’s too soon to apply their simulations to observations of individual pulsars, and more work is needed to understand the role of gamma-ray emission, explore the details of electron–positron pair production, and extend the work to pulsars whose spin axes and magnetic axes are misaligned.

plot of simulation output showing magnetic field lines and plasma density

Simulation output showing the magnetic field lines (green curves) and plasma density (background color) in a pulsar’s magnetosphere. [Bransgrove et al. 2023]

How Do Pulsars Interact with Their Surroundings?

map showing the location of the Boomerang within the supernova remnant

The location of the Boomerang within the supernova remnant surrounding the pulsar PSR J2229+6114. [Pope et al. 2024]

When pulsars are young, they’re swaddled in the gas and dust of their surrounding supernova remnants. This leads young pulsars to create a pulsar wind nebula: a glowing cloud of gas energized by winds of relativistic charged particles streaming off the pulsar. A recent article authored by the Nuclear Spectroscopic Telescope Array (NuSTAR) and Very Energetic Radiation Imaging Telescope Array System (VERITAS) collaborations presents multiwavelength observations of the Boomerang, a 10,000-year-old pulsar wind nebula well known for its complex structure.

The teams combined archival data from radio telescopes and the Chandra X-ray Observatory with newly collected data from NuSTAR, VERITAS, and the Fermi Gamma-ray Space Telescope to probe the nebula’s multiwavelength behavior. These observations revealed that the nebula appears far larger at radio wavelengths than at X-ray wavelengths, a common feature of pulsar wind nebulae due to the difference sources of emission: the nebula’s size at radio wavelengths is set by outflowing particles, while its size at X-ray wavelengths comes from the rate at which electrons lose energy as they spiral around magnetic field lines and emit X-rays. The nebula’s size even varies across X-ray wavelengths, appearing smaller at shorter wavelengths.

Judging from how the nebula’s size changes with wavelength, its overall energy output, and its X-ray emission over the past two decades, the authors provide a new estimate on its distance and magnetic field strength, finding it to be more distant and with a far weaker magnetic field than previously thought. By modeling how the nebula’s energy output may have evolved over time, the team also found that the Boomerang is, well, boomeranging! Roughly 1,000 years ago, a backwards-moving supernova shock wave crashed into the expanding nebula, crushing the nebula and temporarily reversing its expansion. Today, the nebula is re-expanding in the wake of the shock wave, showcasing how pulsars dynamically interact with their surroundings.

Citation

“Discovery of a Young, Highly Scattered Pulsar PSR J1032-5804 with the Australian Square Kilometre Array Pathfinder,” Ziteng Wang et al 2024 ApJ 961 175. doi:10.3847/1538-4357/ad0fe8

“Radio Emission and Electric Gaps in Pulsar Magnetospheres,” Ashley Bransgrove et al 2023 ApJL 958 L9. doi:10.3847/2041-8213/ad0556

“A Multiwavelength Investigation of PSR J2229+6114 and Its Pulsar Wind Nebula in the Radio, X-ray, and Gamma-ray Bands,” I. Pope et al 2024 ApJ 960 75. doi:10.3847/1538-4357/ad0120

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