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photograph of the asteroid bennu

photographs of laboratory experiments compared to a photo of the moon's surface

Top: A Moon lander observed a glow on the lunar horizon, possibly caused by suspended particles. Middle and bottom: Lab experiments show that electrostatic lofting can lift particles several centimeters. [NASA/CU-Boulder/LASP]

Spacecraft observations have revealed particles skittering across the surfaces of airless asteroids and moons. Without wind or water, what makes these particles move?

Ejection Events

When the Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer (OSIRIS-REx) spacecraft arrived at nearby asteroid Bennu in 2018 after a two-year journey, it observed rocky particles up to 10 centimeters in size being ejected from the asteroid’s surface.

Closer to home, laboratory experiments have found that the repulsion between particles with similar electrical charges can cause them to levitate. Could the same process be responsible for the particles seen escaping from Bennu?

diagram of the model setup

Diagram depicting the simplified model surface particles (circles) and the forces at play. The red areas emit electrons, which are collected by blue areas. Under certain conditions, the electrostatic force (FES) between areas with like charge can overcome the forces of gravity (Fg) and cohesion (FCO) and allow the particle to escape. [Adapted from Hartzell et al. 2022]

Charging Up an Asteroid’s Surface

In a new publication, a team led by Christine Hartzell (University of Maryland) investigated whether this process of electrostatic lofting could cause the particle ejection events seen by OSIRIS-REx. In order for particles to be lifted in this way, they must build up enough electrical charge — from impinging solar photons as well as electrons and ions of the solar wind — to overcome the combined forces of gravity and cohesion, which work to keep the particles stuck to the asteroid’s surface.

Using a simplified model composed of particles of identical shape and size, the team found that electrostatic lofting can raise small particles off Bennu’s sunlit surface. The size of the lofted particles was strongly dependent on how cohesive the surface material is; if the cohesion is low, particles a few tenths of a millimeter in size could escape, while only micron-size particles could depart if the cohesion is high.

From Day to Night

The team also considered whether electrostatic lofting could explain the particles seen lifting off from Bennu’s night side. On the night side, the particles are not exposed to sunlight, so they mainly become charged when electrons from the solar wind curl around from the opposite side of the asteroid and strike the surface, knocking electrons free.

simulations of electric fields on bennu

Top: Electric field needed to lift particles on Bennu’s dayside for low (S=0.1; black) and high (S=0.75; blue) cohesion, compared to the expected electric field (red). Bottom: Required and present electric field strengths on the night side for low cohesion values and different densities of the solar wind. Click to enlarge. [Adapted from Hartzell et al. 2022]

The resultant electric field is about a thousand times smaller on the asteroid’s night side than it is on the day side, meaning that electrostatic lofting is unlikely to cause particles to escape from Bennu’s night side — even if the cohesion between particles is very low. The authors identified one exception: if an asteroid passes through a region with lots of high-energy electrons — such as the wake of a magnetized planet — the particles may be able to get the electrical oomph they need to leave the asteroid’s surface.

Ultimately, the team concluded that electrostatic particle lofting is unlikely to be responsible for ejecting the large particles seen by OSIRIS-REx, but it may selectively remove small particles from Bennu’s surface. However, the case isn’t fully closed: further work may show that the asteroid’s irregular shape causes strong electric fields to develop in certain areas, which could be the sites of larger particle ejections. What’s more, the planned return of a sample of Bennu’s surface in 2023 should also give scientists more information about the surface material and allow for more accurate modeling of electrostatic lofting — stay tuned!

Bonus

Curious about the fate of particles ejected from Bennu’s surface? Check out the video below to follow the trajectories of particles ejected in the four largest events observed by OSIRIS-REx. [M. Brozovic/NASA/JPL-Caltech/University of Arizona]

Citation

“An Evaluation of Electrostatic Lofting and Subsequent Particle Motion on Bennu,” Christine Hartzell et al 2022 Planet. Sci. J. 3 85. doi:10.3847/PSJ/ac5629

optical image of the northern region of the vela supernova remnant

Some supernovae unexpectedly increase in brightness in ways that our current models can’t explain. Can a new model that combines shock waves and sound waves make sense of this mystery?

Late to the Party

time series of supernova remnant SN 1987A from 1994 to 2016

This time series of Hubble images shows the expansion of supernova remnant SN 1987A. As shock waves interacted with the ambient gas, the supernova remnant brightened dramatically. [NASA, ESA and R. Kirshner (Harvard-Smithsonian Center for Astrophysics and Gordon and Betty Moore Foundation) and P. Challis (Harvard-Smithsonian Center for Astrophysics)]

When a massive star explodes as a supernova, it briefly outshines its host galaxy before gradually fading from view. Sometimes, expanding supernova shock waves collide with nearby gas — such as circumstellar material ejected by the star before it went supernova — which causes a temporary increase in brightness. When this happens, the material being swept up is compressed, heated, and ionized, causing new emission lines to appear in the supernova’s spectrum. However, some supernovae brighten without showing new emission lines — what causes this behavior?

New work by Eric Coughlin (Syracuse University) and Jonathan Zrake (Clemson University) suggests that delayed brightening might not always be evidence for a new interaction with circumstellar material — but rather, an echo of a previous perturbation.

A Sound Solution

Coughlin and Zrake used linear perturbation theory — a way to mathematically describe the properties of a system in terms of a slowly varying background and a small perturbation in that background — to explore the scenario in which a supernova shock wave expanding into circumstellar material encounters an especially dense region of gas.

plot of the shock luminosity as a function of distance

The shock luminosity relative to the luminosity obtained when there is no density enhancement present (solid line) is plotted as a function of the shock’s position. The scaled density is shown by the dotted line. [Adapted from Coughlin & Zrake 2022]

In this scenario, Coughlin and Zrake predict that the supernova will brighten twice. As the shock wave expands into the circumstellar material, it encounters a denser region of gas and the supernova brightens for the first time and exhibits new emission lines. As the shock collects more and more material, it slows down and the brightness decreases.

So far, this is just a typical interaction between a shock wave and circumstellar material. Here’s where things change: the new model explored by Coughlin and Zrake incorporates a second wave — a slower-moving sound wave — which is launched by the initial collision between the shock wave and the denser circumstellar material. As the shock wave slows down, the sound wave catches up to it and hits it from behind. In the authors’ new framework, it’s the interaction between the initial shock wave and the secondary sound wave that causes the second brightening of the supernova — not a change in the density of the surrounding material. And since no additional material is being swept up and ionized, no new emission lines are produced.

Explaining Outliers

plot of shock luminosity as a function of time

The shock luminosity as a function of time for the density enhancement introduced in the previous figure. The increase in luminosity around 75 days agrees qualitatively with the observations of SN 2019tsf. [Adapted from Coughlin & Zrake 2022]

This model may explain the behavior of supernovae that have brightened long after their initial explosions, like SN 2019tsf, iPTF14hls, and SN2020faa. Should we expect delayed brightening to be a feature of all supernovae? Unlikely, say Coughlin and Zrake — extremely dense circumstellar material could obscure an increase in brightness, and if the density of the material decreases too quickly with distance, the sound wave won’t be able to catch up with the shock wave.

The authors note that there’s much more to explore, since real supernovae expanding into circumstellar gas are far more complex than the framework introduced in this article. Hopefully, future work will help us understand the wide variety of supernova behaviors seen so far!

Citation

“A Physical Model of Delayed Rebrightenings in Shock-interacting Supernovae without Narrow-line Emission,” Eric R. Coughlin and Jonathan Zrake 2022 ApJ 927 148. doi:10.3847/1538-4357/ac4033

Illustration of the sun setting off a coronal mass ejection headed to Earth and it hitting Earth's magnetosphere

Are there any indications as to when monster solar storms are going to happen? A team of astronomers used simulations combined with observations of our home star to show that we may have another way to tell when these massive events are coming toward us. 

An Active Atmosphere of a Seemingly Serene Star 

Space weather, particularly solar flares and coronal mass ejections (CMEs), have a direct impact on life here on Earth. Though our world is protected by the blanket of Earth’s magnetic field, if a solar storm is energetic enough, it can damage weather satellites, GPS satellites, and knock out power grids. Therefore, understanding these events is crucial to maintaining daily life in a world dependent on these technologies.  

Diagrams showing the brightening and dimming events on two separate days

Two dimming/brightening events with the difference between the base image and the observation shown by the color scale (dimmer than the original event shown in black and brighter shown in red). [Adapted from Jin et al. 2022]

Though the Sun may look like a steadily glowing sphere in the sky, it’s actually very dynamic. Sunspots grow and shrink, solar flares build and erupt, and the Sun’s outermost layer, or corona, brightens and dims. Recent studies have explored coronal dimmings and have found that the majority of them occur just after coronal mass ejections. They can therefore provide useful information about space weather events. In a new study, a team led by Meng Jin from the Lockheed Martin Solar Astrophysics Laboratory/the SETI Institute combined observations with simulations to study the relationship between coronal dimming events and coronal mass ejections. 

An AWSoM Simulation of the Sun 

The team used the Alfvén Wave Solar Model (AWSoM) to reconstruct the Sun’s corona and the solar wind environment and then simulated CMEs with various characteristics, such as speed, mass, and magnetic energy. These CMEs are initiated by magnetic flux ropes erupting from the surface. Jin and collaborators also looked at observations of the dimming events from the Atmospheric Imaging Assembly and the Extreme Ultraviolet Variability Experiment, which are part of the Solar Dynamics Observatory.  

Diagrams showing brightenings and dimmings, with magnetic field lines coming from one of the diagrams

Two of the authors’ simulations of the Sun. The difference between the base image and the observation is shown by the color scale (dimmer than the original event shown in black and brighter shown in red) and field lines shown in white and green on Panel (d). [Jin et al. 2022]

Magnetospheric Modeling and Coronal Construction 

The authors’ models were able to reproduce many features observed in the coronal dimming that followed a CME eruption, which shows that dimmings can help us understand characteristics of the associated CMEs such as their energy, mass, and the magnetic configuration of the flux ropes that caused them. Using that knowledge, the team deduced that transient brightening and dimming patterns are related to plasma heating processes in the corona, while long-lasting core and remote dimmings (which are away from the source region) are caused by mass loss from CMEs. They also found that the coronal dimming patterns are significantly influenced by the interaction between the erupting flux ropes with different orientations and the solar corona as a whole. 

Diagram showing that CMEs emanate from core dimming spots and another showing core dimming and remote dimming with a topological connection with open/quasi-open field lines coming from both dimming areas

A cartoon showing the location of CMEs and magnetic field lines in relation to dimming spots. [Adapted from Jin et al. 2022]

 

This knowledge about the relationship between coronal dimmings and solar CMEs could also help us detect CMEs on stars outside of the solar system! Distinguishing between stellar flares and CMEs is difficult but important when considering habitability because CMEs are more likely to erode planetary atmospheres. With knowledge that CMEs are associated with stellar dimmings, we can get a better handle on which stars have CMEs, and therefore better judge which planets may be habitable. 

Citation 

“Coronal Mass Ejections and Dimmings: A Comparative Study Using MHD Simulations and SDO Observations,” Meng Jin et al 2022 ApJ 928 154. doi:10.3847/1538-4357/ac589

simulation of matter spiraling around a pair of black holes

When galaxies merge, the supermassive black holes at their centers spiral around each other and eventually coalesce into a single black hole. How can we track down these massive mergers?

The Hunt for Massive Mergers

illustration of an active galactic nucleus emitting a jet

Modeling the emission from supermassive black hole binaries may help us to distinguish them from active galactic nuclei powered by single black holes. [NASA/JPL-Caltech]

Since the first detection of gravitational waves from a pair of stellar-mass black holes in 2015, gravitational waves have been a powerful tool to study merging black holes. However, detecting the extremely long-wavelength gravitational waves from merging supermassive black holes — with wavelengths of up to tens of light-years! — is beyond our current capabilities. What other methods can we use to detect supermassive black holes in the midst of merging?

One possibility is to track down the electromagnetic radiation produced by the hot plasma that surrounds the black holes as they draw closer. If we can detect this radiation, we can study supermassive black holes as they merge as well as potentially identify the small fraction of active galactic nuclei that are actually powered by black hole binaries rather than by a single black hole — a population that has never been definitively detected.

plots of simulated surface brightness

Simulated surface brightness of the accretion disks around merging black holes with a total mass of one million solar masses. Results are shown for three different wavelengths (left to right: 45, 12, 0.3 nanometers) and for spinning (top row) and non-spinning (bottom row) black holes. In all simulations, a gap opens between the circumbinary disk and the mini-disks — a feature that is not present in models of accretion disks around single black holes. Click to enlarge. [Gutiérrez et al. 2022]

Verging on Merging

A team led by Eduardo Gutiérrez (Argentine Institute of Radio Astronomy and Rochester Institute of Technology) used general relativistic magnetohydrodynamics simulations to model the electromagnetic radiation generated as two supermassive black holes approach a merger.

To predict the light emitted by the system, Gutiérrez and collaborators first modeled the motion of the superheated plasma surrounding the black holes. As the black holes circle around each other, the surrounding material forms a disk that envelops both black holes as well as mini-disks that circle each black hole. A dense region called the “lump” develops on the inner edge of the larger disk, periodically feeding material to the mini-disks.

The team then simulated the winding path that photons would take through the superheated plasma and warped spacetime to reach an observer on Earth. The resultant spectrum is mainly composed of emission from the disk around the binary, the mini-disks, and the streams of material that connect the larger disk to the mini-disks.

Seeing Double

plot of simulated spectral energy distributions

Spectral energy distributions derived from simulations of accreting black holes. In the simulations, the mass of the single black hole is equal to the sum of the masses of the binary components. [Adapted from Gutiérrez et al. 2022]

Gutiérrez and coauthors found that the radiation from merging supermassive black holes should be detectable, and there are significant differences in the emission from merging black holes and a single black hole. Specifically, a binary system emits less energy than a single black hole, and its emission peaks at a lower frequency and decreases less sharply at frequencies above the peak. And unlike a single black hole, emission from black hole binaries should show semi-periodic behavior; because the lump that feeds material to the mini-disks has a slightly elliptical orbit, the accretion rate — and therefore the strength of the emission — increases when the lump passes closest to the mini-disks.

The authors predict that the signal from a black hole binary with a total mass of a billion solar masses would vary with periods of ~20 and ~150 days, while the emission from a million-solar-mass binary would vary on shorter timescales. Repeated X-ray observations should be able to detect this variation, determine whether the cause of the emission is one black hole or two, and give us the first-ever look at behemoth black holes moving toward a merger.

Citation

“Electromagnetic Signatures from Supermassive Binary Black Holes Approaching Merger,” Eduardo M. Gutiérrez et al 2022 ApJ 928 137. doi:10.3847/1538-4357/ac56de

representative color infrared image of the spiral galaxy messier 81

An infrared survey of nearby galaxies revealed outbursting objects that defied existing classifications. Now, new optical observations with Hubble investigate whether these unusual objects are more than meets the (infrared) eye.

A Spirited Survey

infrared image of the rho ophiuchi cloud complex

Young stellar objects in dusty nebulae, like those in the Rho Ophiuchi cloud complex shown here in an image from the Wide-field Infrared Survey Explorer (WISE), are a potential source of the SPRITEs discovered by SPIRITS. [NASA/JPL-Caltech/UCLA]

From 2014 to 2019, a team of astronomers carried out the first large-scale survey of variable objects at infrared wavelengths in nearby galaxies. The SPitzer InfraRed Intensive Transients Survey (SPIRITS) discovered a curious class of outbursting objects, dubbed eSPecially Red Intermediate-luminosity Transient Events (SPRITEs), which are more luminous than novae but less luminous than supernovae and have outbursts that last anywhere from days to years.

Unlike other types of infrared transients, which flare into view at optical wavelengths during an outburst, SPRITEs remain hidden or extremely faint at wavelengths shorter than infrared even when they’re at their brightest. What might these SPRITEs be — supernovae cloaked in clouds of dust, young stars in natal nebulae, or something else entirely?

light curves for three objects targeted

Spitzer light curves for three of the objects studied. Blue symbols indicate 4.5-micron measurements and red symbols indicate 3.6-micron measurements. The remaining symbols indicate upper limits. Click to enlarge. [Adapted from Bond et al. 2022]

Diving Deep with Hubble

In a new publication led by Howard Bond (Penn State University and Space Telescope Science Institute), a team of astronomers performed deep optical and near-infrared imaging with the Hubble Space Telescope to take a closer look at 21 SPIRITS targets, most of which were classified as SPRITEs. Their goal was to detect optical counterparts for these infrared transients and characterize their environments in order to identify the source of the outbursts.

Many of the targets were still not detected in the Hubble observations, but observations of their surroundings yielded new insights into their identities. What’s more, the team continued to monitor the targets with Spitzer, leading to entirely new classifications for many of the objects: of the 21 supposed transients, only three retained that moniker, while a dozen others were discovered to have periodic behavior, and six were found to vary irregularly. How has this affected our understanding of the SPIRITS targets and SPRITEs specifically?

From Transient to Periodic

images of the site of SPRITE 17fe

Hubble images of the site of SPRITE 17fe, which is marked with a red cross in the top image. The middle row shows a zoomed-in view of the site years before the outburst in three different wavelength bands. The bottom row shows the same field during an outburst — 0.8 years after the peak — in two near-infrared bands. [Bond et al. 2022]

Two of the three remaining SPRITEs were located in star-forming regions, suggesting that they are associated with massive stars, though one was found in a region with predominantly old stars and is likely a classical nova shrouded in dust. Thus, SPRITEs can have many causes; merging massive stars, dusty supernovae, and eruptions of bright variable stars are all possibilities.

The objects found to vary periodically did so over extremely long timescales (from 670 to more than 2,100 days) and tended to be found near star-forming regions. Comparison with known types of variable stars indicates that these slowly varying sources are likely dusty, evolved giant stars with masses in the 5–10 solar-mass range. The irregular variables detected in Hubble images might be pulsating red supergiants, while the redder variables undetected at Hubble wavelengths might be associated with mass-loss events of extremely dusty luminous blue variable stars.

There’s still much to learn about the dusty objects identified by SPIRITS. Infrared spectroscopy is likely to illuminate the nature of these sources and luckily, JWST and future observatories like the Roman Space Telescope should be more than equal to the task. For a detailed discussion of each target studied by Bond and collaborators, be sure to check out the full article cited below!

Citation

“Hubble Space Telescope Imaging of Luminous Extragalactic Infrared Transients and Variables from the Spitzer Infrared Intensive Transients Survey,” Howard E. Bond et al 2022 ApJ 928 158. doi:10.3847/1538-4357/ac5832

hubble image of spiral galaxy messier 83

Many galaxies seem to have far less visible matter than expected. In a new article, astronomers have taken the search for missing matter to the outskirts of spiral galaxies.

Mysterious Matter

hubble image of the spiral galaxy messier 51

Messier 51, the Whirlpool Galaxy, is one of the many recognizable galaxies investigated in this work. The galactic halos studied in this article likely extend hundreds of thousands of light-years into space — far larger than the galaxies’ starry disks. Messier 51’s disk is 76,000 light-years in diameter. [NASA, ESA, S. Beckwith (STScI) and the Hubble Heritage Team (STScI/AURA)]

Just 15% of all the matter in the universe is thought to be visible matter — the kind we interact with on a daily basis — with dark matter making up the remaining 85%. Though dark matter is certainly the more elusive of the two, visible matter isn’t without its mysteries — after adding up the masses of the visible components of galaxies like stars and gas clouds, most galaxies appear to have less visible matter than expected based on observations of galaxies early in the universe. Milky Way-like galaxies seem to lack about 70% of their mass while low-mass galaxies can be missing all but a few percent.

Where might this missing matter be hiding? One possibility is that much of the mass of galaxies lies in extended halos that stretch hundreds of thousands of light-years beyond the bright, starry regions that make up the main body of a galaxy. This gas is difficult to detect because of its low density and interference from intervening gas within our own galaxy. How, then, can we weigh this halo gas?

global projection of the cosmic microwave background temperature fluctuations

A map of temperature fluctuations in the cosmic microwave background from WMAP. This map shows temperature deviations of up to 200 microkelvin. [NASA / WMAP Science Team]

A Sunyaev–Zeldovich Stack

A team led by Joel Bregman (University of Michigan) searched for missing matter in galactic halos by looking for evidence of the Sunyaev–Zeldovich effect — the process through which low-energy photons from the cosmic microwave background are kicked up to higher energies through interactions with extremely hot gas. The magnitude of this effect is proportional to the mass and temperature of the gas, making it a useful probe of hot, diffuse halos that might otherwise be impossible to spot.

Bregman and collaborators used this method to investigate the halos around 12 spiral galaxies located 10–33 million light-years away — close enough to determine the spatial extent of the hot halo gas. Most of the halos were too faint to be detected individually, so the team stacked the observations from 11 of the 12 galaxies (one galaxy showed significant differences and was analyzed separately) to extract a signal and determine the average properties of the galaxies in the sample.

Luminous Matter at Large

plot of sunyaev-zeldovich signal strength and signal to noise ratio

The integrated Sunyaev–Zeldovich signal from the stack of 11 galaxies as a function of radius (left) and the signal to noise ratio (right). Click to enlarge. [Bregman et al. 2022]

Bregman and coauthors found that out to a radius of 815,000 light-years (250 kiloparsecs), each galaxy contains 98 billion solar masses of gas. Each galaxy is expected to contain about 310 billion solar masses of visible matter, so this constitutes about 30% of the galaxies’ total mass. The galaxies’ stars, star-forming gas, and cooler halo gas make up a further 30%, meaning that the remaining 40% of the visible matter in these galaxies likely resides at even larger distances.

In order to search for gas even farther out, the team hopes to stack observations from more galaxies and develop new algorithms to reduce uncertainties. Proposed cosmic microwave background detectors like the Probe of Inflation and Cosmic Origins may also aid the search for missing matter, helping us understand where present-day galaxies hide their mass.

Citation

“Hot Extended Galaxy Halos around Local L* Galaxies from Sunyaev–Zeldovich Measurements,” Joel N. Bregman et al 2022 ApJ 928 14. doi:10.3847/1538-4357/ac51de

illustration of a black hole

It sounds like something out of science fiction: a miniature black hole barrels through the densely packed remnant of a dead star, consuming it from the inside. Strange though it may seem, this scenario might hold the key to discerning between stars composed of different kinds of exotic matter.

Strange Stars, Indeed

illustration of a neutron star on a map of manhattan

Comparison of the size of a neutron star to the size of Manhattan. A strange star of the same mass would be slightly smaller. [NASA/Goddard Space Flight Center]

When a massive star expires, its outer layers explode away from its collapsed core. This core is so dense that electrons and protons are squished together, forming a star composed of neutrons. However, a neutron star’s evolution might not stop there: it’s possible that the extreme conditions inside the star could convert the neutrons into elementary particles called strange quarks. The resulting strange star would be only slightly smaller than the original neutron star, making it difficult to distinguish between them observationally. Since we can’t peer into their interiors, how can we tell if a compact stellar remnant is a neutron star or a strange star?

In new article, Ze-Cheng Zou (邹泽城) and Yong-Feng Huang (黄永锋) from Nanjing University, China, propose an intriguing way to detect strange stars: by watching them be consumed by a tiny black hole. Specifically, Zou and Huang posit that we can differentiate between the gravitational waves emitted as a neutron star or strange star encounters a planet-mass primordial black hole — one that formed spontaneously from a dense spot in the universe less that one second after the Big Bang.

plot of black hole mass over time

Demonstration of the how the mass of the black hole changes as it accretes matter from the neutron star or strange star. [Zou & Huang 2022]

Insights from Inspiraling

Zou and Huang modeled the outcome of a clash between a neutron star or strange star and a planet-mass primordial black hole and predicted the gravitational waves that would be emitted as a result. The authors consider black holes from slightly more massive than Mercury to as massive as Jupiter — with radii of 0.3 to 300 centimeters — and strange stars and neutron stars of 1.4 solar masses.

As the black hole tunnels into the star, its progress gradually slows as a result of accreting stellar material and emitting gravitational waves. Though the modeled neutron star and strange star have the same mass and similar radii (12.6 and 11.0 km, respectively), they are theorized to have different internal structures, which alters the rate at which the black hole spirals inward. This distinction is imprinted on the emitted gravitational-wave signal — but can we tell the difference?

Revealed by Gravitational Waves

Amplitude of gravitational-wave signals as a function of frequency and black hole mass (left) and distance (right). Sensitivities of existing and future detectors are shown for comparison. Click to enlarge. [Adapted from Zou & Huang 2022]

Though the two scenarios produce gravitational waves with similar amplitudes, the shapes of the signals are different — and detecting them is not outside the reach of our current gravitational-wave observatories. The Laser Interferometer Gravitational-Wave Observatory (LIGO) should be able to detect these events up to 32,000 light-years away, depending on the black hole’s mass, while future detectors should be sensitive to events 10,000 times more distant.

Observing these curious collisions might tell us more than just the composition of the dense stellar remnants — it also might tell us something about the composition of dark matter. If we observe primordial black holes interacting with neutron stars — or strange stars — it could help constrain the abundance of primordial black holes and determine what fraction of the total mass of dark matter they comprise. With strange stars, exciting results are clearly the norm!

Citation

“Gravitational-wave Emission from a Primordial Black Hole Inspiraling inside a Compact Star: A Novel Probe for Dense Matter Equation of State,” Ze-Cheng Zou and Yong-Feng Huang 2022 ApJL 928 L13. doi:10.3847/2041-8213/ac5ea6

illustration of a binary star system

How do binaries in circular orbits come to be, especially for pairs of solar-type stars? Using stellar models, a researcher has brought us closer to understanding how the loss of energy in a system eventually leads it to circularize. 

Good Things Come in Pairs 

When you look out into the night sky, you may just see a bunch of singular points of light, but it is estimated that up to 85% of all stars are in multi-star systems — binaries, triples, or even quadruple systems. Around ~50% of stars like our Sun are actually in pairs. Binaries have been observed in all different shapes and sizes, from very elliptical orbits where the orbital period is on the order of multiple years to very close circular binaries that have orbital periods of days. Circular orbits could be caused by a number of physical mechanisms that gradually reduce a binary’s eccentricity over time, but we still don’t fully understand which mechanisms are at work or how. Adrian Barker (University of Leeds) has looked at circular solar-type binary systems to try to help solve the mystery. 

Lines that all start out together but diverge at different ages, some going vertical and some staying horizontal

The age of a star plotted against the quality factor, an inverse measure of dissipation efficiency. This shows that inertial wave dissipation is very efficient during the pre-main-sequence phase when the star is young but becomes less efficient later on in the stars’ lifetime. The different colored lines represent the different star masses in solar masses. [Barker 2022]

Making Waves in the Field of Binary Evolution 

One possible way that the orbits of binary stars could become circular over time is through tidal dissipation: the loss of energy in a binary system due to gravitational distortions of the member stars. Barker tested this theory by modeling solar-type and low-mass stars in binary systems with the Modules for Experiments in Stellar Astrophysics (MESA) stellar evolution code. For the modeled binary systems, Barker studied the convection zones of the stars and worked out the tidal dissipation due to inertial waves — oscillations that take place within the star — in the systems. He also explored relations such as how stellar radius evolves with age and how the circularization period (the maximum orbital period in which binary orbits are perfectly circular) evolves with time.  

 

Multiple lines showing the different masses starting out as horizontal lines and all going vertical at different ages.

Age of the system plotted against the maximum binary period that circularizes, showing how energy dissipation from the inertial waves is capable of circularizing orbits out to 10-day periods before a few million years. The different colored lines represent the different star masses. [Barker 2022]

Coming Full Circle 

Previous theoretical work on this subject found that inertial wave dissipation in binary systems could help circularize binary orbits, but it didn’t seem strong enough to account for full circularization of solar-type main-sequence star binaries. While other studies searched for other mechanisms to explain it, this study concludes that tidal dissipation due to inertial waves is sufficient to explain the circularization of binary orbits and the synchronization of the spins of the stars. Barker also found that the maximum period that circularizes due to this mechanism increases with binary age, consistent with what observations of main-sequence binary systems have shown. 

While this study focused on a highly simplified model and involved proof-of-concept calculations, the author hopes that other studies will look at tidal dissipation in more detail with more sophisticated calculations while also focusing on the dynamical evolution of stellar populations.  

Citation 

“Tidal Dissipation Due to Inertial Waves Can Explain the Circularization Periods of Solar-type Binaries,” Adrian J. Barker 2022 ApJL 927 L36. doi:10.3847/2041-8213/ac5b63 

artist's impression of an earth-like planet orbiting a star

What determines whether a planet is dotted with continents or has oceans as far as the eye can see? Astronomers turn to models to understand how bulk planetary properties affect surface oceans on rocky, water-rich exoplanets.

Land Ho?

If a planet’s surface is perfectly smooth, water will wash across it to form a global ocean of constant depth. If instead the surface is roughened with mountains and valleys, the planet can host more water before its surface is fully covered. But what factors control whether a planet’s surface is flat and featureless or textured with topography, and thus how much water it can hold before becoming an ocean world?

cartoon of dynamic topography

A schematic showing the generation of dynamic topography [Adapted from Guimond et al. 2022]

A new article led by Claire Marie Guimond (University of Cambridge, UK) explores how the distribution of dry land and oceans on exoplanetary surfaces might be impacted by dynamic topography. Dynamic topography arises when a planet’s core is hotter than its surface, causing the material in between — the mantle — to undergo a slow churning on million-year timescales. This convection applies pressure to the planet’s crust, inducing upwellings and depressions up to a kilometer in size. Unlike plate tectonics, which is largely responsible for the diversity of topography on Earth but doesn’t occur on all planets, dynamic topography is expected to be a nearly universal feature of rocky planets.

plots of topography in meters as a function of planet age, mass, core mass fraction, and uranium and thorium abundances relative to solar

Plots of the change in a planet’s root-mean-square topography as a function of four potentially observable quantities. The quantities held fixed in each set of simulations are listed in the upper right corner of each panel. Click to enlarge. [Adapted from Guimond et al. 2022]

Modeling Mantle Convection

Guimond and collaborators first used complex two-dimensional numerical models to understand how a planet’s dynamic topography scales with factors like the thickness and composition of the mantle. These trends formed the backbone of less computationally expensive models, with which the team explored how a planet’s mass, age, radioactive element abundance, and core mass impact the size of its surface features.

The team found that the typical height of surface features increases with increasing planetary age and core mass and decreases with increasing planetary mass and radioactive element abundance. Planetary mass and radioactive element abundance had the strongest effect, suggesting that these factors are key to predicting a planet’s topography. The modeled surface features tended to be in the hundred-meter range, though the most massive planets had subtle hills and valleys of only a few tens of meters.

Weighty Water Worlds and Low-Mass Land Planets

plot of basin capacity, planet mass, and surface water fraction

Basin capacity as a function of planet mass. If a given surface water mass fraction exceeds the basin capacity, the planet will have a global ocean (labeled “water planets”). If the water mass fraction is less than the basin capacity, the planet will have dry land (labeled “land planets”). [Adapted from Guimond et al. 2022]

How much water can a planet hold before all its surface features are submerged beneath a global ocean? This quantity depends on the maximum height of the planet’s surface features, since a planet with an Everest-sized mountain will require more water to be fully covered than one with small rolling hills. Intriguingly, the authors found that the most massive planets can support the least water before turning into water worlds since they have the shallowest surface features.

Aside from dynamic topography, planets are likely to have many other surface processes at play, like impacts from asteroids and comets, volcanism, and plate tectonics, all of which might boost the amount of high-and-dry land available on a planet. The authors hope that further modeling as well as observations of exoplanets and Earth itself can increase our ability to predict the surface conditions of distant planets.

Citation

“Blue Marble, Stagnant Lid: Could Dynamic Topography Avert a Waterworld?,” Claire Marie Guimond et al 2022 Planet. Sci. J. 3 66. doi:10.3847/PSJ/ac562e

photograph of the solar corona during a total solar eclipse

How does the temperature of the solar atmosphere leap from 6000K to more than a million kelvin over a distance equal to just 0.3% of the Sun’s radius? Today’s article explores the coronal heating conundrum with a model that has earned its “awesome” moniker.

Heating Up the Solar Atmosphere

illustration of the Sun's layers

An illustration of the different layers of the Sun. The Sun’s atmosphere gets progressively warmer with increasing distance from the core, from the photosphere at 6000K, to the chromosphere at 10000K, to the corona at 1 million kelvin. [NASA]

Understanding how the Sun’s rarefied upper atmosphere, or corona, reaches its scorching million-kelvin temperature is one of the most important problems in solar physics. The corona is the site of explosive solar activity like solar flares and coronal mass ejections, as well as the launching-off point for the diffuse solar wind that suffuses the solar system.

Solar physicists have identified a wide array of processes that could potentially heat the corona. Undulating plasma waves that carry energy outward from the dense solar surface to the tenuous corona are one potential source of heating, but it’s unclear when or where this process might dominate over other heating methods. In a new publication led by Tong Shi (University of Michigan), a team of scientists has explored coronal heating by plasma waves with a high-resolution solar model.

plot of radial magnetic field strength for the Sun

The team elected to model a solar active region that appeared on July 13, 2018. The active region can be identified by the paired dark red and blue spots at a longitude of approximately 300. The color scale shows the strength and direction of the magnetic field in the radial direction in Gauss. Click to enlarge. [Shi et al. 2022]

Wave Simulations

The Alfvén Wave Solar Model (AWSoM) simulates heat transport by Alfvén waves — a type of plasma wave in which oscillating ions and magnetic fields pass energy back and forth. AWSoM has been used to model a variety of solar phenomena, including predicting the conditions the Parker Solar Probe would experience on its first close approach to the Sun.

Now, Shi and collaborators have applied AWSoM to a solar active region for the first time. AWSoM has performed well in its previous applications, but reproducing the conditions in and around an active region — an area of the Sun’s surface with particularly strong magnetic fields — is a critical test. Since AWSoM simulates the entire Sun rather than just a small area, attempting to capture the fine details of a complex active region promised to push the boundaries of AWSoM’s computational abilities.

From Global to Local

example images of the solar surface based on model output

Extreme-ultraviolet and X-ray images of the Sun from the Solar Dynamics Observatory (left column) and corresponding images based on model output (center and right columns). [Shi et al. 2022]

Shi and collaborators used AWSoM to model plasma densities and temperatures and produce outputs like spectra and ultraviolet images of the Sun’s disk that could be compared to spacecraft observations. Overall, the team’s results show that the Alfvén wave heating processes incorporated in their model are able to heat the solar corona to roughly the temperature we observe, even in the intricate magnetic loops that arc over the active region.

The authors estimate that by incorporating more levels of refinement in their model grid, they achieved their highest resolution ever, even approaching the levels seen in ultra-detailed images from spacecraft like the Solar Dynamics Observatory and Hinode. However, even AWSoM’s unparalleled resolution may not be sufficient to understand the physics of the transition region — a narrow range of altitudes between the solar surface and the corona over which the temperature abruptly increases. With further development, AWSoM should allow us to refine our understanding of coronal heating even more.

Citation

“AWSoM Magnetohydrodynamic Simulation of a Solar Active Region with Realistic Spectral Synthesis,” Tong Shi et al 2022 ApJ 928 34. doi:10.3847/1538-4357/ac52ab

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