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illustration of a white dwarf collecting gas from its stellar companion

With T Coronae Borealis expected to have an outburst any day now, recurrent novae are in the news. Recently, researchers reported their investigation of a recurrent nova that brightens every year.

Recurring Stellar Characters

light curves from M31N 2008-12a's eruptions from 2013 to 2022

Vertically offset light curves from M31N 2008-12a’s 2013–2022 eruptions. [Basu et al. 2024]

Recurrent novae are periodic outbursts that happen when a white dwarf — the exposed core of an evolved star with a mass of about 8 solar masses or less — snags some gas from a puffy red giant companion. Heated by the blisteringly hot surface of the white dwarf, this accreted gas ignites in a flash of nuclear fusion. This process can recur for millions of years, creating with each outburst a “guest” star that fades until the next eruption.

Known recurrent novae have outbursts anywhere from every year to every 98 years. The nova with the most recorded appearances is M31N 2008-12a, which hails from our galactic neighbor, Andromeda. Researchers have witnessed the star brighten 15 times since its discovery in 2008, and a dive into the archives dredged up three previous eruptions in 1992, 1993, and 2001. What can this collection of eruptions tell us about M31N 2008-12a’s past, present, and future?

Light curve showing the overall behavior as well as the "cusp" feature

Light curve in the i’ band, showing the overall behavior as well as the “cusp” feature. Click to enlarge. [Adapted from Basu et al. 2024]

Characterizing Outbursts

Judhajeet Basu (Indian Institute of Astrophysics and Pondicherry University) and collaborators examined optical, ultraviolet, and X-ray data to examine the behavior of M31N 2008-12a during its annual outbursts from 2017 to 2022. Their investigation showed that each outburst was roughly the same — rising rapidly to its peak in about a day, then declining sharply for 2–4 days before fading more gradually.

In some wavelength bands, the light curves show a “cusp” feature rising above the expected curve. The “cuspy” look of the light curve at certain wavelengths could be evidence for outflowing jets emerging from the poles of the star. These types of jets have been seen for other recurrent novae, like the Milky Way’s RS Ophiuchi.

From Nova to Supernova

histogram showing the frequency of days since last eruption

Demonstration of the possible increase in time between eruptions in the last few years. [Adapted from Basu et al. 2024]

Basu’s team found that while each recent outburst has looked mostly the same, the time between eruptions has gotten longer, on average, over the last seven years. The slowly increasing time between eruptions could mean one of two things: the mass of the white dwarf is decreasing over time, reducing the star’s ability to siphon gas from its companion, or the accretion rate is slowing. Calculations show that the star’s mass is increasing with time, so a decrease in the accretion rate must be responsible. This could point to anything from a change in the orbital dynamics of the system to the donor star running out of gas.

Researchers estimate that M31N 2008-12a has been experiencing nova eruptions every year for the past million years. Despite the repeated eruptions that remove mass from the white dwarf’s surface, the star is gaining more mass than it’s losing, creeping ever closer to the Chandrasekhar limit. Once the star hits this mass limit in another 20,000 years or so, it will be too massive to support itself against gravity and will undergo one final outburst as a supernova.

Citation

“Multiwavelength Observations of Multiple Eruptions of the Recurrent Nova M31N 2008-12a,” Judhajeet Basu et al 2024 ApJ 966 44. doi:10.3847/1538-4357/ad2c8e

A photograph of stars and long, horizontal, bright streaks caused by satellites.

As construction continues on the Vera Rubin Observatory, the skies above its mountaintop home grow more and more crowded following every rocket launch. Astronomers, conscious of the plans for mega-constellations of new satellites in the next few years, are rightfully worried: will these satellites and the tiny bits of debris that come with every deployment and collision affect the new telescope’s long-awaited, gigantic survey?

Threats to Ambitious Plans

After several decades inhabiting only the dreams and blueprints of astronomers, the Vera Rubin Observatory is finally a real, physical place. Now a building and construction site near the summit of Cerro Pachón in Chile, its concrete and steel structure already houses most of what’s needed to begin one of the most ambitious surveys of the sky ever conceived. The Legacy Survey of Space and Time, or LSST, promises to revolutionize every sub-field of astronomy from cosmology to planetary science, and scientists around the world are eagerly awaiting its kickoff.

photograph of Vera Rubin Observatory

The Vera Rubin Observatory. [RubinObs/NSF/AURA/H. Stockebrand; CC BY 4.0]

The plan is to use the largest camera ever built to photograph the entire night sky, repeatedly, for a decade. Unfortunately, though, stars and galaxies aren’t the only objects that will show up in these wide-angle images. Anything placed in orbit around Earth will blunder through the pictures as well, potentially reflecting sunlight towards the telescope as they zip along their looping trajectories. This will cause streaks and flashes in some of the images, which, without careful filtering, could either obscure or mimic the subtle signal of a fleeting astronomical event.

Tiny Pieces, Potentially Large Impact

Astronomers have known this might be a problem for a while now, and the LSST team has spent considerable time figuring out how to handle satellites and large chunks of space debris. While challenges remain and the correction techniques won’t ever be perfect, the community is prepared to handle anything large enough to be tracked by ground-based radar, or about 10 cm. But, what about smaller objects, like the bits of debris created when two satellites collide?

In a February Research Note, one astronomer voiced concerns that these tiniest pieces of space junk could overwhelm LSST’s transient detection algorithms. This prompted a team led by J. Anthony Tyson, University of California, Davis, to model more thoroughly how glints from small, nearby objects would appear in LSST images.

Closer and Faster Than The Stars

An illustration of how a nearby, moving satellite would be blurred out compared to an equally bright but faraway and stationary star. [Adapted from Tyson et al. 2024]

Thankfully, the researchers concluded that there likely isn’t much cause for alarm. While they point out that it should be possible to build filters for these events, they also point out a more important and ironic conclusion: because the objects are so close to the telescope, they’ll actually appear fainter than you might initially expect. Since the observatory is designed to concentrate light from objects that are effectively infinitely far away, objects as close as a few thousand kilometers will appear blurry and out of focus. This means a flash that otherwise would have occupied just a few pixels will be smeared out across many, and in most cases will become lost in the noise.

The authors conclude that “In general… the large population of [low Earth orbit] debris below a few centimeters in size may pose little challenge for LSST transient science.” While there are still hurdles to overcome and challenges to solve before LSST can deliver on its extraordinary promises, thankfully, dealing with tiny bits of space junk likely won’t be one of them.

Citation

“Expected Impact of Glints from Space Debris in the LSST,” J. Anthony Tyson et al 2024 ApJL 966 L38. doi:10.3847/2041-8213/ad41e6

An illustration of an exoplanet being engulfed by its home star, as 8 UMi b somehow has not been

New research may have revived the mystery of 8 Ursae Minoris b, a seemingly doomed exoplanet that shouldn’t exist.

The Planet That Shouldn’t Be: 8 UMi b

When first discovered, the exoplanet 8 Ursae Minoris b (8 UMi b; also called Halla) puzzled astronomers. The planet should have been engulfed by its host star as the star swelled into a red giant, but there was no question that the planet was there, resolutely tugging on its star as it completed each 93-day orbit.

Previously, researchers explained away this impossibility by suggesting that 8 UMi was once a lower-mass star with a close-in stellar companion. As 8 UMi began its expansion into a red giant, it swallowed its companion. The subsequent shakeup of 8 UMi’s interior changed its evolutionary path and halted its expansion, saving 8 UMi b from a fiery fate.

The key to testing this hypothesis is determining 8 UMi’s age: if the star is old — 9 billion years old or so — then the binary merger scenario is feasible. If the star is young, that would make a merger quite unlikely — and the mystery of 8 UMi b will live on.

Age Estimation

Plot of theoretical isochrones showing the position of 8 UMi

The position of 8 UMi on theoretical isochrones of various ages. This analysis yielded an age of 1.9 billion years for this star. [Adapted from Chen et al. 2024]

A team of stellar sleuths led by Huiling Chen (Peking University) set out to determine 8 UMi’s age. The team used position information and photometry data from the Gaia spacecraft as well as a high-resolution spectrum of the star from a 1.93-meter telescope at the Haute-Provence Observatory. These measurements allowed the team to determine the star’s temperature, surface gravity, and chemical composition.

Using these data, Chen’s team estimated 8 UMi’s age with three different methods: stellar isochrones (theoretical relations between brightness and temperature for stars with different masses but the same age), kinematics, and chemical abundances. The three methods yielded age estimates in the range of 1.9–3.5 billion years — far younger than the nearly 9 billion years estimated for the binary merger scenario.

plots demonstrating age estimation methods using chemical abundances

Age estimates from two chemical abundance methods. These two methods yielded age estimates of 3.3 and 3.5 billion years. Click to enlarge. [Chen et al. 2024]

A Mystery Once Again

The newly calculated age for 8 UMi would make it extremely unlikely for a merger with a binary companion to be responsible for saving 8 UMi b from engulfment. How, then, does this planet exist?

While Chen and collaborators emphasize that more work is needed to solve the mystery once and for all, one of the newly derived stellar properties could provide an explanation: Chen’s team estimated 8 UMi’s mass to be 1.7 solar masses, which is about 13% larger than previous estimates. This larger mass could mean that 8 UMi is slightly more compact than expected, and it would mean that 8 UMi b’s orbital period corresponds to a slightly larger orbital distance — just large enough, perhaps, for the planet to eke out survival on the edge of its star.

Citation

“The Kinematic and Chemical Properties of the Close-in Planet Host Star 8 UMi,” Huiling Chen et al 2024 ApJL 966 L27. doi:10.3847/2041-8213/ad3bb4

Artist's impression of a supermassive black hole in a galaxy

For the first time, researchers have examined unexpectedly massive black holes during a time period called cosmic noon. These black holes may fill in the gap between over-massive black holes in the early universe and those present today.

Black Holes Then and Now

illustrations showing how a large black hole can form from the direct collapse of a massive cloud of gas

Illustration of the formation of a massive black hole seed from the collapse of a gas cloud in the early universe. Click to enlarge. [NASA/STScI/Leah Hustak]

Exactly how and when our universe’s supermassive black holes grew to their impressive size is a topic of intense debate. Using JWST, researchers discovered that some black holes in galaxies less than a billion years after the Big Bang are astonishingly large given the universe’s young age. Many of these over-massive black holes sit at the centers of low-mass galaxies, meaning that they’re also unexpectedly large given the mass of their host galaxies. These findings imply that early black holes either grew from “seeds” that were already quite massive or gained mass rapidly.

Observations of black holes in the universe today support the hypothesis that many supermassive black holes grew from massive seeds, and low-mass galaxies with over-massive black holes are common in the local universe as well as in the early universe. But what’s missing from this tale of black hole growth is what happened in between these two time periods: where’s the link between over-massive black holes in the early universe and today?

picture and spectrum of a low-mass galaxy with an active galactic nucleus

Observations of a low-mass galaxy with an active galactic nucleus. Click to enlarge. [Adapted from Mezcua et al. 2024]

Activity at Cosmic Noon

To connect these two epochs, Mar Mezcua (Institute of Space Sciences, Spain) and collaborators looked toward a period of the universe’s history known as cosmic noon. This period, when the universe was just 2–3 billion years old, is marked by high star-formation rates and fast black hole growth. To study black holes during this time period, Mezcua’s team searched for galaxies containing actively accreting supermassive black holes, also called active galactic nuclei.

Starting from a sample of more than a thousand galaxies with active galactic nuclei, the team selected 12 low-mass galaxies with high-quality data and redshifts that placed them at cosmic noon. Measurements of emission-line widths revealed that these black holes were roughly 100–1,000 times more massive than the black holes in similarly sized active galaxies in the local universe. They are also more massive than expected given the typical ratio of black hole mass to stellar mass.

Drawn from the Same Population

plot of black hole mass versus galaxy stellar mass

Black hole mass versus galaxy stellar mass for the black holes in this study (red squares), over-massive black holes in the early universe (dark purple squares), and other populations of black holes. Click to enlarge. [Mezcua et al. 2024]

When Mezcua’s team compared the over-massive black holes in their cosmic noon sample to those seen in the early universe with JWST, they found that both samples showed the same relationship between black hole mass and stellar mass. The luminosities and accretion rates were also similar. This suggests that these two groups of black holes, both of which are overly massive compared to other black holes present in their respective time periods, belong to the same population.

The two groups of black holes may have different reasons for being overly massive, though: in the early universe, the presence of too-massive black holes is thought to mean that these black holes grew from massive “seed” black holes. Later, at cosmic noon, black hole feedback has had time to disrupt and heat star-forming gas, and interactions between galaxies have stripped away star-forming material. Both of these processes could cause a black hole to remain large compared to its host galaxy.

This marks the first time researchers have studied over-massive black holes during cosmic noon, and there’s much more to learn about black holes in this time period. An investigation into outflows and mergers may help researchers understand how these outsize black holes formed and grew.

Citation

“Overmassive Black Holes at Cosmic Noon: Linking the Local and the High-Redshift Universe,” Mar Mezcua et al 2024 ApJL 966 L30. doi:10.3847/2041-8213/ad3c2a

Illustration of stellar-mass black holes embedded within the accretion disk of a supermassive black hole

Researchers estimate that the accretion disks of supermassive black holes could host millions of stars. When these stars evolve into black holes, they may reshape the observational properties of the disks they call home.

Stellar Extremophiles

Countless stars across the universe have taken up residence in the vicinity of supermassive black holes, including in the dusty disks that surround black holes that are actively accreting gas, otherwise known as active galactic nuclei. Some stars are born in these black hole disks, condensing out of the dusty gas on the outskirts of the disk, where the gas is cooler and feels less of the black hole’s tidal pull. Others may be trapped there, the friction of passing through the disk eventually wearing their orbits down until the stars settle within the disk.

Whether born there or captured, many of these stars will evolve into stellar-mass black holes. Researchers estimate that in the 10–100-million-year lifetime of an active galactic nucleus, its disk may host anywhere from 100 to 100 million stellar-mass black holes. How can we tell if a supermassive black hole’s accretion disk is home to stellar-mass black holes?

Heating Up the Outskirts

plot comparing the temperature of an accretion disk with and without embedded stellar-mass black holes

Modeled temperature of an accretion disk with (blue line) and without (red line) embedded stellar-mass black holes. [Adapted from Zhou et al. 2024]

A team led by Shuying Zhou (Xiamen University) searched for the signs of stellar-mass black holes in an accretion disk by modeling an active galactic nucleus with a disk containing 1,000–100,000 black holes. The team found that because the stellar-mass black holes alter the surrounding disk by accreting some of the gas, they can potentially change the observational properties of the disk.

As stellar-mass black holes orbit within the supermassive black hole’s accretion disk, they accrete some of the gas. This accreted gas becomes extremely hot and emits X-rays that warm the gas that’s nearby. In the outskirts of the disk, where the temperature is lower, this process can heat the disk a potentially measurable amount.

comparison spectral energy distributions for models with and without black holes embedded within the accretion disk

Comparison of the model output for a static standard disk (SSD) and a disk in which stellar-mass black holes are embedded (SSD with sBHs) and a composite active galactic nucleus spectrum. Click to enlarge. [Zhou et al. 2024]

Spotting Black Holes in Black Hole Disks

Zhou’s team compared the spectral energy distributions — how energy output is distributed across different wavelengths of light — for accretion disks that host stellar-mass black holes and those that do not. For a supermassive black hole with a mass of 100 million solar masses, the presence of stellar-mass black holes in the accretion disk greatly boosts the disk’s energy output at wavelengths greater than 470 nanometers (nm) and slightly suppresses the disk’s energy output at shorter wavelengths. For more massive black holes, the energy-boosting effect happens at longer wavelengths, above about 800 nm.

In addition to altering the spectral energy distribution, the presence of stellar-mass black holes may also increase the accretion disk’s half-light radius, or the radius within which half of the disk’s light is emitted. This change is potentially measurable through microlensing of active galactic nuclei by foreground galaxies. In fact, it may have already been measured — some microlensing measurements suggest that the half-light radii of distant active galactic nuclei are 2–4 times larger than expected for a typical accretion disk.

Citation

“Stellar Black Holes Can ‘Stretch’ Supermassive Black Hole Accretion Disks,” Shuying Zhou et al 2024 ApJL 966 L9. doi:10.3847/2041-8213/ad3c3f

nebula Pa 30

In astronomy, sometimes 1 + 1 = 1. That’s the case when white dwarfs collide, creating a single massive remnant that sheds mass through powerful magnetic winds.

When Stellar Remnants Collide

Nearly all of the stars in the Milky Way, including the Sun, are fated to become white dwarfs. These Earth-sized objects are the super-hot crystallized cores of stars that have lost their outer layers after ballooning into red giants. When two white dwarfs collide, the collision can trigger a supernova, create a neutron star — an object even denser than a white dwarf — or merge the two white dwarfs into one.

In a recent research article, Yici Zhong (University of Tokyo) and collaborators modeled the properties of post-merger white dwarfs, focusing on their fast-moving magnetized winds. The results may be applicable to an unusual class of supernovae that are faint, fade quickly, and fail to fully explode — leaving behind a white dwarf remnant.

Magnetic Outflows

plot of radial wind speed versus latitude

Radial velocity of the white dwarf’s wind as a function of latitude. The wind is fastest near the star’s equator. [Adapted from Zhong et al. 2024]

When two white dwarfs merge into one, the resulting white dwarf is expected to rotate rapidly and be highly magnetized. This combination of speedy spinning and strong magnetic field launches a thick and powerful wind from the star’s surface and carries mass away from the star. To learn more about the magnetic winds of white dwarfs, Zhong’s team carried out numerical magnetohydrodynamics simulations of a post-merger white dwarf.

Zhong’s team found that the wind doesn’t emerge from the entire surface of the star equally and is instead fastest and most luminous near the star’s equator. Nor does the wind blow steadily: some of the gas launched from the star’s surface gets trapped in the magnetic field, and a periodic rearranging of the magnetic field ejects bubbles of this trapped gas.

The Winds of WD J005311

These findings may give researchers new ways to study a recently discovered white dwarf called WD J005311, which appears to be the remnant of a collision of white dwarfs that triggered a supernova. Unlike most supernovae caused by colliding white dwarfs, the explosion didn’t destroy the stars completely, and the surviving star’s 16,000-kilometer-per-second winds pummel the supernova remnant from within.

Modeled torque, luminosity, and mass-loss rate of the wind over time

Modeled torque, luminosity, and mass-loss rate of the wind over time. Click to enlarge. [Zhong et al. 2024]

Zhong’s team used their model to estimate the star’s properties, which have been challenging to measure directly. They estimated the star’s mass at 1.1–1.3 solar masses and its magnetic field at 20–50 million Gauss (about 20–50 million times stronger than Earth’s).

The team’s model also predicts that the star’s wind blows outward 20–40% faster at its equator than its poles, and this asymmetry could be detected through optical spectroscopy. The uneven speed means that the wind exerts more pressure on the surrounding nebula along its equator, possibly creating an uneven shock that could be seen in future X-ray observations.

Citation

“The Optically Thick Rotating Magnetic Wind from a Massive White Dwarf Merger Product. II. Axisymmetric Magnetohydrodynamic Simulations,” Yici Zhong et al 2024 ApJ 963 26. doi:10.3847/1538-4357/ad1f5c

illustration of a tidal disruption event

Is the gamma-ray burst GRB 191019A a typical long-duration gamma-ray burst from a dying massive star, an anomalously long burst from colliding objects, or something else entirely?

Powerfully Mysterious

illustration of a neutron star merger

An illustration of a neutron star merger, which is one way to create a gamma-ray burst. [ESA 2002/Medialab]

Gamma-ray bursts are among the most mysterious phenomena in the universe. These powerful flashes of gamma rays have conventionally been divided into two categories according to their length: “long” bursts are those that last more than about two seconds, while “short” bursts are those that are shorter than two seconds.

With time and accumulating data, these length-based classifications have become associated with different sources: long gamma-ray bursts seem to arise from core-collapse supernovae, when the curtain closes on stars substantially more massive than the Sun, and short gamma-ray bursts seem to happen when two extraordinarily dense objects like neutron stars collide. However, as the tally of gamma-ray bursts has grown, so has the list of events that fail to fall neatly into these two categories.

artist's impression of GRB 191019A

This illustration depicts GRB 191019A as resulting from the collision of two stars within the dense environment of a galactic nucleus. Today’s article suggests that the event took place far closer to the galaxy’s supermassive black hole. [International Gemini Observatory/NOIRLab/NSF/AURA/M. Garlick/M. Zamani; CC BY 4.0]

An Alternative Hypothesis

In October 2019, researchers discovered the gamma-ray burst GRB 191019A, which lasted just over a minute and appeared at first to be a standard long burst arising from a supernova. Later, this interpretation was called into question: no associated supernova emission was spotted, the burst was slightly fainter than expected for its type, and its host galaxy wasn’t as vigorously star forming as the galactic hosts of long gamma-ray bursts tend to be. Together, these findings suggested that GRB 191019A may be part of an emerging class of gamma-ray bursts that arise from colliding objects despite lasting longer than two seconds.

In a recent article, a team led by Robert Eyles-Ferris (University of Leicester) explored another possibility: that GRB 191019A isn’t a gamma-ray burst at all. Instead, Eyles-Ferris’s team proposed that rather than a supernova or a cosmic collision, what caused the event was a star that wandered too close to a supermassive black hole and was summarily torn apart — a tidal disruption event.

Examining a Rare Phenomenon

Some aspects of GRB 191019A, such as its location near the center of a galaxy, naturally align with this alternative hypothesis. Other factors, such as the event’s length, seem at odds with this explanation — tidal disruption events tend to play out over the course of months rather than minutes. Eyles-Ferris and collaborators hypothesized that GRB 191019A isn’t just any tidal disruption event but an ultra-deep one, in which the doomed star is so stretched out by the black hole’s tidal forces that the star wraps all the way around the black hole and collides with itself, launching a relativistic jet in the process.

Using a mathematical model, Eyles-Ferris and coauthors showed that GRB 191019A’s luminosity and time scale are consistent with what’s expected for an ultra-deep tidal disruption event. If this hypothesis is correct, GRB 191019A is the first example of an ultra-deep tidal disruption event and just the fifth known jetted tidal disruption event. As rare as these events are likely to be, other instances will crop up somewhere in the vastness of the universe, and Eyles-Ferris’s team has a way to pick them out: a flash of low-energy X-rays that erupts the moment the star collides with itself.

Citation

“Ultradeep Cover: An Exotic and Jetted Tidal Disruption Event Candidate Disguised as a Gamma-ray Burst,” R. A. J. Eyles-Ferris et al 2024 ApJL 965 L20. doi:10.3847/2041-8213/ad3922

A photograph of a bright white dot trailed by a fainter white stream.

A bold NASA experiment demonstrated that when an asteroid runs into something really hard, all of the ejected material reflects extra light back to Earth and makes the bruised asteroid appear slightly brighter for a short time. Recently, astronomers have built on that finding to estimate the chances of observing a similar flash out in the wilds of the main belt when two asteroids bump into each other by chance.

A DART of Inspiration

Back in September of 2022, NASA did its best to avenge the dinosaurs by slamming a small spacecraft into the moon of an asteroid. Aside from their vindictive motives, the agency had another, slightly more important reason for such an aggressive act. Astronomers and planetary defense experts wanted to watch how the asteroid would respond to a high-speed collision, and knowing exactly when one would occur allowed them to be ready with their telescopes. The Double Asteroid Redirection Test (DART) mission, as it was called, was a fabulous success, and the scientists involved got plenty of data along with the satisfaction of carrying out revenge 65 million years in the making.

A close-up view of the aftermath of the DART impact captured by a nearby spacecraft. [ASI/NASA/APL]

Moments after the spacecraft plowed into its target, the presumably surprised asteroid seemed to grow rapidly brighter. This temporary flash was caused in part by ejecta rapidly fleeing the crash site and reflecting additional sunlight back to Earth. As the material dispersed and the asteroid continued on its slightly altered course, it faded back to its original brightness.

Now, many months after the initial excitement, a team of astronomers led by Eran Ofek, Weizmann Institute of Science, have hit upon a realization: if asteroids flash during a collision, could we detect the small blips caused by an impact between two natural asteroids, instead of an asteroid and a human-made spacecraft?

Estimating Collision Rates

To answer their question, Ofek and collaborators needed to combine the answers to two smaller questions. First, would these natural collisions be bright enough to observe from Earth? Second, how often would these collisions actually take place? Both questions are non-trivial to solve, though following the success of DART, the first was slightly easier to tackle.

The distribution of brighness and distance to potential asteroid collisions. [Ofek et al. 2024]

By scaling the actual light curve collected after the DART mission came to its abrupt conclusion, the team could establish what the artificial impact would have looked like had the spacecraft and its target been teleported to the main belt of asteroids. The second question about the frequency of these collisions was more challenging to answer. Unfortunately, astronomers have nearly no idea how common asteroids the size of DART are in the solar system, since asteroids that small don’t reflect enough light to be observable from Earth. Luckily, the larger asteroids that they can observe seem to follow a power-law distribution in size, so by extrapolating that empirical relation to smaller objects, they can get a sense of how many unseen asteroids swarm in the dark.

Potential for Discoveries

Combining their insights, the researchers established that about 7,000 asteroid collisions could be detectable every year, plus or minus about an order of magnitude. That’s thrilling, but also puzzling: if multiple asteroids slam into each other and flash every night, why haven’t we seen them? The team offered two solutions: first, we actually have detected this phenomenon, and collisions make up at least a subset of the three or so “active asteroids” that are spotted each year. Second, these flashes are just too short to be caught by standard surveys, which would have to observe the same spot in the sky at least twice in one hour to resolve the brightening. Happily, that means with the right survey design, we could potentially recover many more of these collisions. Perhaps in the years to come astronomers will observe asteroid collisions even when they aren’t the cause of them.

Citation

“Asteroid Collisions: Expected Visibility and Rate,” Eran O. Ofek et al 2024 AJ 167 190. doi:10.3847/1538-3881/ad2c03

the cratered surface of the dwarf planet Ceres

Many of the craters on the dwarf planet Ceres are home to deposits of water ice. New research examines the ice-trapping properties these cold, shadowed craters and finds that the ice must be surprisingly young.

Dwarf Planet Discoveries

aerial view of a crater wall on Ceres

The Dawn spacecraft captured this image of a crater wall on Ceres from an altitude of about 26 miles (43 kilometers). [NASA/JPL-Caltech/UCLA/MPS/DLR/IDA]

Ceres, the only dwarf planet in the inner solar system, is a rocky world roughly a quarter of the diameter of the Moon. Ceres is of great interest to researchers because it’s likely one of the few surviving protoplanets in the solar system, the others having been ejected from the solar system, destroyed through collisions, or incorporated into planets.

From 2015 to 2018, NASA’s Dawn mission examined Ceres from orbit, reaching an altitude as low as 22 miles (35 kilometers) above the dwarf planet’s surface. Dawn peered into the craters at Ceres’s poles, many of which never see direct sunlight during Ceres’s 4.6-Earth-year orbit around the Sun, and discovered ice on some of the shadowed crater floors. Now, researchers have examined the locations and properties of these ice deposits to estimate how long they’ve been present.

map of permanently shadowed regions at Ceres's north pole

The slope of the terrain surrounding Ceres’s north pole (grayscale) with the locations of permanently shadowed regions colored by the maximum obliquity at which they remain in shadow. Click to enlarge. [Schorghofer et al. 2024]

Crater Considerations

Norbert Schorghofer (Planetary Science Institute) and collaborators used a detailed model of Ceres’s shape based on Dawn data to predict which of Ceres’s craters remain shadowed as its axial tilt, or obliquity, changes. Influenced by tidal forces from the Sun and Jupiter, Ceres’s obliquity varies from 2 degrees to 20 degrees over the course of 24,000 years. The team found that when Ceres reaches its maximum obliquity, there are no crater floors that remain shadowed throughout a Ceres year.

To understand what this means for the history of ice deposits on Ceres, Schorghofer’s team estimated how long it would take for ice on Ceres disappear through sublimation, transitioning directly from solid to gas. They found that when exposed to direct sunlight, ice deposits on Ceres sublimate rapidly, disappearing at a rate of more than a centimeter per year. If sublimation rates were small, that could mean that ice on Ceres could survive for billions of years. Instead, this result means that Ceres’s ice can’t survive long when exposed to sunlight and must be young.

Young Ice

area of shadowed regions as a function of latitude and obliquity

The area of shadowed regions as a function of latitude and obliquity. When Ceres’s obliquity reaches 20 degrees, there are no places that remain shadowed all year. [Adapted from Schorghofer et al. 2024]

Currently, ice deposits exist in regions that remain shadowed until Ceres’s axial tilt reaches 10 degrees. The last time Ceres’s tilt reached that value was just 6,000 years ago, which means that the ice can be no more than 6,000 years old. That’s remarkably young for a 4-billion-year-old protoplanet! Where is this ice coming from?

Ceres contains a lot of ice hidden under just a few centimeters of rocky surface material, so anything that disturbs the surface, such as landslides or impacts, could expose the ice. The authors find that the most likely source of Ceres’s surface ice is an impact that created a temporary atmosphere that condensed into ice in its craters.

Finally, the authors looked into the possibility of other types of ice, such as carbon dioxide ice, lining crater floors on Ceres. Despite being incredibly cold, Ceres’s craters are too warm to trap gases other than water vapor as ice.

Citation

“History of Ceres’s Cold Traps Based on Refined Shape Models,” Norbert Schorghofer et al 2024 Planet. Sci. J. 5 99. doi:10.3847/PSJ/ad3639

photograph of the supernova SN 2022jox and its host galaxy

If caught just a few days later, SN 2022jox would’ve looked like just another ordinary core-collapse supernova, but early observations set it apart, revealing the gas expelled in the star’s final years.

The Extraordinary Made (More) Ordinary

illustration of a red supergiant star surrounded by thick circumstellar material

An illustration of a red supergiant star surrounded by thick circumstellar material. [NAOJ]

It was once considered extraordinary to be able to observe a supernova just hours after the first light from the explosion reached Earth. With the advent of new surveys that scan the sky and hunt tirelessly for such flashes, it’s no longer exceptionally rare to catch supernovae early on — but it’s still extraordinarily useful.

These once-elusive early observations give a critical look at what the star was up to before its collapse. Observations suggest that in the final years of their lives, many massive stars lose mass through winds or eruptions. In the first few hours or days after a supernova explosion, the expanding ejecta collides with the material previously lost by the star, creating a burst of short-lived emission lines called a flash spectrum. Once the flash spectrum fades, this crucial information about the star’s immediate surroundings is lost.

spectra of SN 2022jox

Spectra of SN 2022jox taken 0.8–9.9 days after explosion. Click to enlarge. [Andrews et al. 2024]

Early Observations

In a recent research article, Jennifer Andrews (NSF’s NOIRLab and Gemini Observatory) and collaborators analyzed the flash spectrum and light curve of the supernova SN 2022jox. The event was caught soon after exploding by the Distance Less Than 40 Mpc Survey, which aims to find supernovae in nearby galaxies (within about 130 million light-years) within one day of the explosion reaching Earth.

At just 0.8, 1.3, and 1.5 days after the explosion, spectra of SN 2022jox showed narrow emission lines from hydrogen, helium, carbon, and nitrogen. Using radiative transfer modeling, Andrews’s team found that the gas surrounding the supernova was likely lost by its progenitor star at the rate of a thousandth to a hundredth of a solar mass per year — a high value for a quiescent red supergiant star, but typical for other supernovae with flash features in their spectra.

Critical First Days

Multi-band light curves of SN 2022jox

Multi-band light curves of SN 2022jox. The right panel shows a zoom-in of the first week of data. Click to enlarge. [Andrews et al. 2024]

Just a few days later, SN 2022jox’s narrow spectral lines flattened out, and the supernova’s evolution in the days that followed was typical for a core-collapse supernova: its peak brightness, the time it took to reach peak brightness, and the way its color changed over time were all normal. This shows the importance of catching supernovae as soon as possible after explosion — without catching the flash spectrum in those critical first days, SN 2022jox would have looked like any other normal core-collapse supernova.

Months after SN 2022jox was detected, the authors saw evidence for the expanding supernova interacting with circumstellar gas once again. Taken together, the very early and very late observations of this supernova suggest that it may be common for circumstellar material to be present around “ordinary” supernovae, and observations over a wide time frame are needed to detect these cases.

Citation

“SN 2022jox: An Extraordinarily Ordinary Type II SN with Flash Spectroscopy,” Jennifer E. Andrews et al 2024 ApJ 965 85. doi:10.3847/1538-4357/ad2a49

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