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image of the Sun releasing two coronal mass ejections

A chance alignment between Earth and a Mars-bound spacecraft has given us a rare glimpse into the movement of high-energy particles from the Sun. The data from this event can help researchers understand the radiation environment near Mars — a key factor in planning crewed missions to our neighboring planet and beyond.

Energetic Particle Parade

illustration of energetic particles being ejected by the sun

Illustration of energetic particles being ejected by the Sun. [NASA’s Goddard Space Flight Center Conceptual Image Lab]

The space between the planets in our solar system is filled with a wispy sea of charged particles that flow out from the Sun’s atmosphere. This particle population is augmented by cosmic rays — speedy protons and atomic nuclei accelerated in extreme environments across the universe — which ebb and flow against the 11-year solar activity cycle. This undulating particle background is punctuated by bursts of high-energy particles from the Sun, which can be unleashed suddenly in violent solar storms.

Spacecraft that venture out from the protection of Earth’s magnetic field must navigate this ocean of particles and weather solar storms. And if we someday wish to send astronauts to other planets, we’ll need to know how high-energy solar particles, which pose a risk to the health of astronauts and electronic systems alike, travel through the solar system.

location of Tianwen-1 relative to Earth, Mars, and other spacecraft

Location of Tianwen-1 (TW-1) relative to Solar Orbiter (SolO), Parker Solar Probe (PSP), and STEREO-A (STA), Earth, and Mars. The black arrow marks the location of the active region that launched the solar storm. [Adapted from Fu et al. 2022]

When Spacecraft Align

In a new publication, a team led by Shuai Fu (Macau University of Science and Technology), Zheyi Ding (China University of Geosciences), and Yongjie Zhang (Chinese Academy of Sciences) studied the high-energy solar particles produced in an event in November 2020, when the Sun emitted a solar flare and a massive explosion of solar plasma called a coronal mass ejection.

This event coincided with a chance alignment of multiple spacecraft along the same solar magnetic field line. This alignment meant that several spacecraft near Earth and the Tianwen-1 spacecraft en route to Mars measured the same burst of energetic particles millions of miles apart, providing a rare opportunity to study how energetic particles from the Sun travel through space along magnetic field lines.

Diffusion and Evolution

By comparing the timing of measurements from Tianwen-1 to those from three spacecraft near Earth, the team discerned that the magnetic field line that connected the spacecraft did not connect back to the origin of the particles. This means that the particles must have traveled, or diffused, across magnetic field lines to reach the spacecraft.

plots of particle flux as a function of energy at eight time steps

Comparison of proton fluence (number of particles collected per unit area) measured by spacecraft at Earth (blue) and by Tianwen-1 at 1.39 au (red). The time increases from (a) to (h). The spectra at Earth and at Tianwen-1 “break” or bend at roughly the same energy, suggesting that there is little evolution as the particles travel outward. Click to enlarge. [Fu et al. 2022]

In addition, the team found that the shape of the particle energy distribution remained the same at moderate and high energies as the particles traveled between Earth and Tianwen-1’s location at 1.39 au. This suggests that the shape of the energy distribution is determined earlier, at the time the particles are accelerated to high energies, rather than as the particles travel through space.

The November 2020 event marked the first solar energetic particle event observed by Tianwen-1, but surely not the last. The spacecraft will continue to monitor high-energy particles from its station in Mars orbit as the solar cycle revs up, collecting valuable data for understanding the radiation environment around Mars and planning future missions.

Citation

“First Report of a Solar Energetic Particle Event Observed by China’s Tianwen-1 Mission in Transit to Mars,” Shuai Fu et al 2022 ApJL 934 L15. doi:10.3847/2041-8213/ac80f5

very large telescope image of the protoplanetary disk around IM Lupi

The disk surrounding the star IM Lupi has come into clearer focus in the past few years thanks to new observations that revealed spirals, kinks, and other interesting structures. Could a hidden planet be the cause of all these features?

A Detailed Disk

radio image of the disk around IM Lupi

IM Lupi as seen at a wavelength of 1.25 mm. The white bar in the lower right-hand corner is a 10-au scale bar. [Adapted from Andrews et al. 2018]

IM Lupi is a young star with an intriguing protoplanetary disk. Observations of IM Lupi’s disk over the past few years have found that the disk doesn’t rotate evenly; there are more than a dozen “kinks” where the gas moves at a different rate than what we would expect for a smoothly rotating disk. In addition, a spiral pattern is imprinted upon the disk’s upper surface.

Previous research has suggested that these features could signal that IM Lupi’s expansive disk hides a massive planet orbiting the star at a distance of 117 au. In a new publication, a team led by Harrison Verrios (Monash University, Australia) puts that theory to the test.

On a Hunt for Planets

Verrios and collaborators used hydrodynamic modeling to understand how the presence of a planet would affect IM Lupi’s disk. In addition to modeling a planet-less disk as a control case, the team investigated the effects of a planet with a mass 2, 3, 5, or 7 times the mass of Jupiter orbiting the central star at a distance of 100–120 au.

Observed and modeled polarized intensity maps

Observed (left) and modeled (right) polarized intensity maps. The model shows the results for a planet with a mass of 2 Jupiter masses. Click to enlarge. [Verrios et al. 2022]

In order to compare against observations, the team generated images from their hydrodynamic models. Specifically, they modeled the emission from the disk at wavelengths of 1.25 millimeters (which traces the warm dust) and 1.6 microns (which shows the polarized light scattered off the disk). The team found that by including a planet in their simulations, they could reproduce all of the observed velocity kinks as well as the distinctive spiral pattern on the disk’s surface. The team’s models also predicted that the wake created by the planet’s motion should be visible in velocity maps, and observations match this prediction closely. Overall, the authors found that a 2–3-Jupiter-mass planet orbiting at a distance of roughly 110 au produced the best match to the observations.

Disk Disturbances Demystified

Comparison of observed and modeled velocity maps

Comparison of observed (top row) and modeled (bottom row) 1.25-mm images and velocity maps. The Δv at the bottom of each panel denotes the difference from the rest velocity of the 12CO J=2–1 transition. Click to enlarge. [Adapted from Verrios et al. 2022]

Does this study rule out the possibility that the structures in IM Lupi’s disk have a different cause, such as a gravitational instability? Protoplanetary disks are notorious for having features that mimic the signals of planets, and more work is needed to confirm the presence of a planet tucked away in IM Lupi’s disk — but the authors’ simulations suggest that a planet can explain all the intriguing features of the disk without invoking another cause.

Previously, researchers theorized that a planet could only disturb its disk in a small region immediately surrounding the planet, which would suggest that the more widespread disturbances in IM Lupi’s disk must have another cause. However, Verrios and coauthors found that widespread features popped up in their simulations, suggesting that planets can have more far-reaching effects than predicted.

Citation

“Kinematic Evidence for an Embedded Planet in the IM Lupi Disk,” Harrison J. Verrios et al 2022 ApJL 934 L11. doi:10.3847/2041-8213/ac7f44

An artist's impression of a stellar-mass black hole

At the heart of some galaxies, a disk of gas whirls around a central supermassive black hole. A recent publication explores what might happen when stars and stellar-mass black holes meet in the disks around supermassive black holes.

Getting an Assist from an Active Galactic Nucleus

plot of stellar orbital evolution

Simulated orbital evolution of four stars orbiting an active galactic nucleus. This plot shows how the orbital semimajor axis (top), eccentricity (middle), and angle between the stellar orbit and the disk rotation axis (bottom) change over time. [Yang et al. 2022]

The disks around supermassive black holes, which together power luminous active galactic nuclei across the universe, can enable interesting interactions between stars and compact objects. Stars orbiting the centers of these galaxies at all angles repeatedly pass through the disk, collecting gas and gaining mass. Over time, the friction of these repeated passages alters the stars’ orbits and brings some of the stars to new orbits that lie within the disk itself.

Once the stars are settled in the disk, they migrate inward and become susceptible to gravitational entanglements with other objects. Some of these stars will link up with stellar-mass black holes to form binary systems. From there, astronomers predict that the orbits of these binaries shrink until the black hole’s tidal forces rip the star apart, causing a small-scale version of the same phenomenon that occurs around supermassive black holes: a tidal disruption event (TDE). But how common are these theorized micro-TDEs likely to be, and what observational signatures would they have?

table of merger and disruption rates

Estimated rates of different types of binary mergers or disruptions within the disk surrounding an active galactic nucleus. Results are in units of events per cubic gigaparsec per year. Micro-TDEs refer to interactions between stars and stellar-mass black holes. [Yang et al. 2022]

Event Estimation

A research team led by Yang Yang (University of Florida) estimated the frequency of these events as well as the kind of electromagnetic signals they would emit. The team began by estimating how many stars and black holes will end up in aligned orbits within the disk of an active galactic nucleus. From there, the authors applied a random distribution of masses to the stars and black holes and calculated how quickly binary systems assembled from these populations would merge, resulting in a rate of one event per year per cubic gigaparsec (1 cubic gigaparsec = 3.5×1028 cubic light-years; about 50,000 times the volume of the Virgo Supercluster).

As to what kind of electromagnetic signals these events might produce, Yang and collaborators suggest that they would resemble the signals from TDEs around supermassive black holes — but with some key differences. The stream of accreted material that forms around a stellar-mass black hole will be hotter than the stream that forms around a supermassive black hole, producing copious X-ray emission, and micro-TDEs may reach their peak luminosity later than full-scale TDEs.

Disruption Detection

An artist's impression of a tidal disruption event.

An artist’s impression of a tidal disruption event. [NASA’s Goddard Space Flight Center]

It’s a big universe out there — where should we look for signs of micro-TDEs? Because micro-TDEs and full-scale TDEs have similar electromagnetic signals, Yang and collaborators suggest that galaxies where TDEs are rare are the first places we should look. Luckily, we already know where that might be: supermassive black holes weighing in above 100 million solar masses likely consume Sun-like stars whole rather than ripping them apart. If we observe a TDE-like signal coming from the neighborhood of such a black hole, it could be a sign of a micro-TDE instead.

At present, two candidate events have been discovered, though the authors can’t rule out the possibility that they are regular TDEs. As with any as-yet-unobserved phenomenon, more theoretical work is needed to ensure we’ll be able to identify micro-TDEs when we find them.

Citation

“Tidal Disruption on Stellar-mass Black Holes in Active Galactic Nuclei,” Y. Yang et al 2022 ApJL 933 L28. doi:10.3847/2041-8213/ac7c0b

mosaic of the large magellanic cloud from swift observatory

Across the universe, luminous galactic centers are fueled by supermassive black holes that accrete gas, dust, and stars from their surroundings. These powerful active galactic nuclei (AGN) radiate across the electromagnetic spectrum and emit jets of energetic particles, potentially shaping the evolution of the galaxies they inhabit. Collecting spectra of AGN is key to understanding the structure of the material that surrounds them and the role they may play in galaxy evolution — and a new public data release from a spectroscopic survey of AGN discovered by the Neil Gehrels Swift Observatory has expanded our ability to probe these objects.

An illustration of the Neil Gehrels Swift Observatory in front of a gamma-ray burst

An illustration of the Neil Gehrels Swift Observatory in front of a gamma-ray burst. [Spectrum and NASA E/PO, Sonoma State University, Aurore Simonnet]

Since 2005, the Neil Gehrels Swift Observatory has monitored the sky from gamma-ray to optical wavelengths, primarily in pursuit of the sources of gamma-ray bursts: extragalactic explosions potentially caused by massive stars going supernova or compact objects merging. However, Swift sees far more than just gamma-ray bursts — its Burst Alert Telescope (BAT), which scans 80% of the sky each day at X-ray and gamma-ray energies (14–195 kiloelectronvolts), has discovered hundreds of AGN in the local universe.

But detecting AGN is just the first step toward understanding the nature and importance of these objects — dedicated spectroscopic follow-up is a critical next step. Enter the BAT AGN Spectroscopic Survey (BASS): a project that aims to survey the most powerful AGN that have been detected in high-energy X-rays by Swift Observatory. In a new special issue of the Astrophysical Journal Supplement Series, the BASS team presents the latest step toward their goal of producing an immense catalog of AGN spectra.

projection of the sky with symbols indicating the locations of the active galactic nuclei being surveyed

Locations of the objects surveyed in the second BASS data release. The symbol shape and color indicates the instrument and telescope used to collect that object’s spectrum. Click to enlarge. [Koss et al. 2022]

This data release contains 1,449 optical spectra — 1,181 of which have never been released before — and 233 near-infrared spectra, all from 858 AGN in the local universe. The just published special issue presents catalogs of spectra, derived quantities, and first science results, including the following important findings:

  • The objects discovered by the Burst Alert Telescope span a wide range of properties. Namely, the black hole masses, luminosities, accretion rates, and degree to which the targets are obscured by gas and dust all vary by at least five orders of magnitude, making this survey a useful probe of a wide variety of AGN. Additionally, few of the sources observed in this survey are contained in other surveys, making the BASS project a source of unique information.
  • For the first time, the black hole mass function and the Eddington ratio distribution function — how the black hole mass and AGN luminosity vary with other factors — have been determined directly for heavily obscured objects. The resulting distribution functions show that obscured AGN are intrinsically less luminous compared to their theoretical maximum luminosities than their unobscured counterparts. This suggests that radiation plays a large role in determining the structure of the material close to an AGN.
  • plot of number of active galactic nuclei versus redshift

    Redshift distribution of AGN in the BASS second data release. [Oh et al. 2022]

    The masses of the supermassive black holes at the heart of obscured AGN tend to be underestimated when the mass is determined from measurements of hydrogen emission lines. Researchers can reduce this effect in the future studies by applying multiplicative factors when estimating masses this way.
  • New diagnostics based on mid-infrared luminosities can distinguish between obscured and unobscured AGN, yielding new candidate sources that are heavily obscured. However, separating out star-forming galaxies remains a challenge.

The publicly available sample of spectra developed by the BASS team provides a valuable tool for researchers wishing to study AGN in the local universe. Looking forward, the team plans to supplement their current catalog with observations of fainter sources made by the Burst Alert Telescope, expanding our understanding of these cosmic engines.

Citation

Special ApJS Issue on the BAT AGN Spectroscopic Survey Second Data Release

“BAT AGN Spectroscopic Survey XXI: The Data Release 2 Overview,” Michael J. Koss et al 2022 ApJS 261 1. doi:10.3847/1538-4365/ac6c8f

extreme-ultraviolet image and magnetic field map of a bipolar ephemeral active region

Among the smallest and most fleeting occupants of the zoo of solar phenomena are bipolar ephemeral active regions (BEARs). In today’s article, researchers studied the evolution of these regions to understand how they generate miniature solar flares — and understand what this might mean for how massive solar flares are ejected.

image of the sun's disk with its magnetic field strength indicated in greyscale

This map of the Sun’s magnetic field from the Solar Dynamics Observatory shows large bipolar active regions, where field lines pointing in opposite directions are located close together. These regions are associated with sunspots, solar flares, and coronal mass ejections. [NASA/SDO]

The Life Cycle of an Active Region

The roiling plasma of the solar surface gives rise to an amazing variety of phenomena. Many solar phenomena are linked to active regions: places where loops of the solar magnetic field emerge from beneath the solar surface. Sunspots arise from active regions, and the evolution of the plasma and magnetic fields contained within large active regions is thought to power solar flares, coronal mass ejections, and other solar outbursts.

However, a large active region might take weeks or even months from the moment it emerges to the moment it unleashes a solar flare or a coronal mass ejection. During those weeks or months, active regions will rotate out of view, hiding parts of their evolution on the far side of the Sun. Is there a way to study the full life cycle of an active region?

diagram of the magnetic field lines surrounding a BEAR

A drawing of the magnetic field orientation around a BEAR situated in a region of open solar field lines that extend out into space. The top panel shows a top-down view and the bottom panel shows a side view. [Moore et al. 2022]

A Range of Scales

To answer that question, a research team led by Ronald Moore (University of Alabama in Huntsville and NASA Marshall Space Flight Center) turned to bipolar ephemeral active regions, or BEARs, which are among the smallest active regions. BEARs contain arching magnetic field lines that rise above the solar surface, span roughly 10,000 km, and aren’t associated with sunspots. As the “ephemeral” part of the name suggests, their lives are fleeting — BEARs emerge in just half a day and disperse roughly two days later, making it possible to study their entire lives in detail.

Moore and collaborators note that the structure of the magnetic field lines above BEARs is largely the same as it is above larger active regions, and both types of active regions are associated with solar explosions: massive solar flares arise from large active regions and microflares erupt from BEARs. This suggests that all active regions, regardless of size, are governed by the same processes and evolve in the same way. By that logic, we can gain the same understanding from studying the smallest active regions as we can from the largest.

Toward a Universal Mechanism

To study the life cycles of solar BEARs and determine what makes them eject microflares, Moore and collaborators used data from the Solar Dynamics Observatory to track the magnetic field strength and extreme-ultraviolet emission from 10 BEARs as they evolved.

histogram of the number of microflares produced by each BEAR

Histogram showing the number of microflares produced by each BEAR in the team’s sample. [Moore et al. 2022]

The team found that the 10 BEARs released 43 microflares in total, with each BEAR emitting zero to 12 microflares during its lifetime. Movies of the magnetic field evolution revealed that each microflare followed an instance of flux cancellation: when the roiling motion of solar plasma brings together magnetic field lines that point in opposite directions, they “cancel” each other out and the flux that was present dissipates. Because the magnetic field structure above BEARs and other active regions is so similar, and because flux cancellation appears to be a universal process for the formation of microflares, Moore and collaborators suggest that this process drives the ejection of massive solar flares as well.

Bonus

Check out this video from the authors’ article, which shows the evolution of the extreme-ultraviolet emission and magnetic field in one of the BEARs in the sample. The left panel shows the 21.1-nanometer extreme-ultraviolet emission, the middle panel shows the magnetic field (white indicates outward-directed magnetic flux and black indicates inward-directed magnetic flux), and the right panel superimposes magnetic field strength contours on an extreme-ultraviolet image. The movie shows 16 hours of the BEAR’s evolution.

Citation

“Bipolar Ephemeral Active Regions, Magnetic Flux Cancellation, and Solar Magnetic Explosions,” Ronald L. Moore et al 2022 ApJ 933 12. doi:10.3847/1538-4357/ac6181

mosaic of four hubble images showing the evolution of the scars left by a comet impact on Jupiter

Gravitational nudges can dislodge comets from the icy outer regions of a planetary system and send them on a collision course with the system’s planets. What kind of planets are likely to ensnare these inbound comets, and which are likely to wave them away?

Comets on the Move

telescope image of ʻOumuamua

The interstellar object ʻOumuamua is the dot at the center of this five-minute exposure taken with the William Herschel Telescope. Due to the object’s high speed, the other sources in the frame appear as streaks of light. [Alan Fitzsimmons (ARC, Queen’s University Belfast), Isaac Newton Group]

When the interstellar object ʻOumuamua sped through the solar system, its arrival confirmed what many astronomers had long suspected: space is teeming with debris that has been kicked out of planetary systems by gravitational interactions.

But the same gravitational interactions that can launch comets into interstellar space can instead send them careening into the inner regions of the planetary systems where they were born. When that happens, the comets can settle into new orbits, be consumed by their host star, or, as a new publication explores, be accreted by the planets in the system. What determines whether a planet will collect comets that wander close to it, and how might the accretion of comets affect our interpretation of exoplanet spectra?

plot of the ratio of accreted to scattered planets as a function of planetary and stellar parameters

The ratio of accreted comets to scattered comets as a function of the Safronov number, which increases with the mass and orbital distance of the planet and decreases with the mass of the host star and the radius of the planet. This plot shows the results for a Jupiter-size planet orbiting the Sun at varying orbital distances. Jupiter, WASP-77 Ab (a hot Jupiter), and HR 8799 b (a directly imaged planet) are marked on the plot. Click to enlarge. [Seligman et al. 2022]

Accreted or Scattered?

A team led by Darryl Seligman (University of Chicago) developed a set of equations that predict which planets are most likely to accrete inbound comets. The equations describe the likelihood of a planet accreting a comet into its atmosphere or scattering a comet into a new orbit (or out of the system entirely) as a function of the properties of the planet — its mass and orbital distance — and those of the comet — mainly its eccentricity, which is a measure of how circular or elongated its orbit is.

The team used their metric to determine which of the previously detected exoplanets are likely to have added cometary material to their atmospheres. Seligman and collaborators found that, in general, planets categorized as warm Jupiters, super-Earths, and sub-Neptunes are more likely to ensnare passing comets than colder, more massive planets.

Composition Imposition

plot of comet accretion efficiency for the planets to be observed by JWST in the next year

The ratio of accreted to scattered comets for many of the exoplanets to be observed by JWST. [Seligman et al. 2022]

What does it mean for our understanding of distant planetary systems if exoplanets accrete a large amount of cometary material? Potentially, quite a lot! Planetary composition is thought to relate to where in a protoplanetary disk a planet formed. However, if planets accumulate cometary material — which bears the chemical signature of having formed far out in the disk — estimates of a planet’s birthplace based on its atmospheric composition might be inaccurate.

Seligman and collaborators note several reasons that their estimates are likely an upper limit on the amount of cometary material that planets accumulate. For example, if comets in other planetary systems tend to disintegrate or lose their volatile compounds quickly, the likelihood of exoplanet–comet encounters — and the effect they have on an exoplanet’s atmospheric composition — could drop.

This issue is a timely one, since based on the team’s metric, nearly all of the exoplanets that JWST will observe within the next year have the potential to have accreted cometary material.

Citation

“Inferring Late-stage Enrichment of Exoplanet Atmospheres from Observed Interstellar Comets,” Darryl Z. Seligman et al 2022 ApJL 933 L7. doi:10.3847/2041-8213/ac786e

spitzer space telescope infrared image of L1157

The dark, dusty clouds surrounding young, hot protostars are the sites of molecule formation. What can new radio observations tell us about the potential for molecule formation in the shocked surroundings of a nearby protostar system?

Making Molecules

A visible-light image of the interstellar dark cloud Lynds 1157. Infrared or radio observations are needed to reveal the young stars hidden by the dust. [NASA/JPL-Caltech/AURA]

Over the past century, astronomers have discovered more than a hundred kinds of molecules in space. Exactly how these molecules form and survive in the cold, tenuous gas of the interstellar medium is an active area of research. One of several ways that molecules are thought to form is in the wake of a shock wave, which condenses and warms the interstellar medium, helping lone atoms link up in the vastness of space.

Shock waves can be produced by outflows from newly forming stars called protostars, which are still wrapped in dense clouds of gas and dust. Luckily, infrared and radio observations allow us to draw back this dusty curtain and peer into the birthplaces of young stars and watch as they collect gas and shoot out jets of material. In a new publication, a team led by Siyi Feng (冯思轶) from Xiamen University presents new radio data that probes the surroundings of a young protostellar system at the heart of the dark cloud Lynds 1157 — one of the best places to study how shocks impact interstellar chemistry.

maps of the Lynds 1157 jet in ammonia emission

Example maps of an outflowing jet from Lynds 1157 in two emission lines of ammonia. The shocks are located at the places labeled B0, B1, and B2, while smaller structures are labeled with additional letters. The protobinary is labeled “mm.” [Adapted from Feng et al. 2022]

Peering at Protostars

Previous observations of Lynds 1157 have shown that the region hosts organic molecules like methanol and cyanoacetylene — a clear sign of ongoing interstellar chemistry. What makes the region especially interesting is the series of shocks that have formed along a jet that flows outward from the central source, which is likely a protobinary system. Observations show that the outermost shock is 1,000 years old, while the inner shocks are younger, allowing us to study how the temperature and density of the gas changed over time as the shocks passed through.

Using the Karl G. Jansky Very Large Array, Feng and collaborators observed emission lines of ammonia (NH3) to make high-resolution maps of Lynds 1157 and measure how the temperature and density of the gas vary throughout the cloud.

Studying Shocks

maps of temperature, density, and the ratio of ortho to para ammonia

Maps of the mean temperature (left), density (center), and ratio of ammonia molecules in an excited state to those in an unexcited state (right). Click to enlarge. [Adapted from Feng et al. 2022]

The ammonia emission lines trace the jet as it moves outward from the central protobinary, and the observations show that the gas is warmest close to the protobinary, cooler farther out along the jet, and densest at the locations of the shocks. And at the locations of the shocks, the team found evidence for ammonia molecules in an excited state, a clear indication that the gas has been heated by the shocks.

The team’s observations show that the passage of shocks heated and compressed the gas, and that as the shocks moved outward, the gas cooled. This illustrates that shocks can provide the warm, dense environment needed for molecules to form. The measurements made in this work should enable detailed chemical modeling, allowing for an even better understanding of how shocks have transformed the gas around these young protostars and paved the way for molecule formation.

Citation

“A Detailed Temperature Map of the Archetypal Protostellar Shocks in L1157,” S. Feng et al 2022 ApJL 933 L35. doi:10.3847/2041-8213/ac75d7

transmission spectrum of exoplanet WASP-96 b taken by JWST

This week, we got to see the spectacular first spectrum of an exoplanet’s atmosphere taken by JWST. Have you ever wondered how researchers use models to determine the properties of distant planets’ atmospheres from their spectra?

Myriad Models

Though models come in many forms, most fall into two categories: computational and analytical.

A computational model of an exoplanet’s atmosphere incorporates all our knowledge of atmospheric physics to predict what happens when photons from the planet’s host star navigate the maze of atoms and molecules in an exoplanet’s atmosphere on their way to our telescopes. Computational models generate synthetic spectra, which researchers can then compare to a planet’s actual spectrum to constrain the properties of the planet’s atmosphere, like its temperature, density, and composition.

In an analytical model, all that physics gets boiled down to a set of comparatively simple mathematical expressions that describe how the input parameters (e.g., the properties of a planet and its atmosphere) relate to the model output (i.e., a spectrum). By developing these mathematical expressions for how an exoplanet’s spectrum varies with different properties of its atmosphere, researchers can get a better sense of which physical properties affect a planet’s spectrum and why.

Analytical Avenues

flowchart illustrating different types of spectral modeling techniques

A flowchart showing analytical and numerical (computational) modeling methods. The method used in this work is represented by the yellow shapes on the left-hand side. Click to enlarge. [Matchev et al. 2022]

In today’s article, a team of researchers at the University of Florida led by Konstantin Matchev demonstrated how symbolic regression can be used to develop an analytical model of exoplanet spectra. Symbolic regression is a way of sifting through mathematical functions to determine a set of simple and accurate functions that describe how inputs and outputs are related.

For the specific case of exoplanet spectra, the authors first simplified their model by lowering the number of input variables. For example, instead of the planet’s radius and the host star’s radius being two separate variables, the authors use the ratio of the two radii as a single dimensionless variable.

Then, the team used machine learning to extract analytic expressions that relate their input variables to a set of synthetic hot-Jupiter spectra generated by a separate analytic model. By working with the output from another analytic model, the authors were able to gauge the success of their technique; if successful, their model should extract the exact expressions that the target model is based on. Ultimately, the authors found that their symbolic regression method was able to discard unimportant variables, generate the correct expressions, and determine which input variables have the largest impact on the output spectra.

simple schematic representation of degenerate variables

An illustration of the degeneracies between pressure (P0), gas absorption cross section per unit mass (κ), gravity (g), temperature (T), mean molecular mass (m), planet radius (R0), and stellar radius (Rs). Two degenerate variables are connected by a line and three variables by a triangle. [Matchev et al. 2022]

Weighing the Options

Why use this technique when powerful computational models are already available? Computational models can eat up hours of computing time, and the results don’t always give physical intuition into the system being modeled. And while computational and analytical models both have an issue with degeneracy — a scenario in which multiple combinations of inputs give the same output — analytical models can help us pinpoint which variables are degenerate and develop a physical understanding of why. For example, the authors’ analytical model tells us that there is a degeneracy between the planet’s temperature and the strength of its gravitational pull. Physically, this arises because a hotter planet would have a puffier atmosphere, but so would a planet with a weaker gravitational pull.

Though analytical models may not be appropriate for all situations, they’re an important tool for studying exoplanet atmospheres — and new exoplanet spectra from JWST should provide an excellent challenge for all models!

Citation

“Analytical Modeling of Exoplanet Transit Spectroscopy with Dimensional Analysis and Symbolic Regression,” Konstantin T. Matchev et al 2022 ApJ 930 33. doi:10.3847/1538-4357/ac610c

polarized-light photograph of the solar corona during a solar eclipse

Researchers have created a way to measure the performance of models of the Sun’s tenuous upper atmosphere, or corona. What does this new framework tell us about some of the most common coronal models?

An illustration of the regions of the Sun's atmosphere and interior.

An illustration of the regions of the Sun’s atmosphere and interior. Click to enlarge. [NASA/Goddard]

Seeking Answers about the Solar Atmosphere

The Sun’s superheated corona plays an important role in generating space weather, launching the solar wind, and accelerating energetic particles from the Sun. Without being able to sample the solar corona directly — even the Sun-skimming Parker Solar Probe won’t venture into the densest part of the corona during its planned closest approach in 2025 — we have to rely on our ability to model complex plasma physics in order to interpret our observations made from afar.

Astronomers have created a wide variety of models to probe the behavior of the solar corona, but while these models have been compared against data, rarely have they been compared to each other in a systematic way. Now, a team led by Samuel Badman (University of California, Berkeley) has developed a new way to assess the output of multiple models of the Sun’s corona.

Coronal Comparisons

As Badman and collaborators note, models are often assessed according to their ability to reproduce a single feature, like the structure of wispy coronal streamers seen during a solar eclipse or the strength of the solar wind magnetic field at Earth’s orbit. However, this makes it difficult to compare models to each other, and it might even obscure poor performance on other important metrics.

plot demonstrating how the magnetic field structure metric is devised

Development of the magnetic field structure metric. The modeled (top) and measured (middle) magnetic field directions are shown. The bottom panel marks where those quantities agree. Click to enlarge. [Badman et al. 2022]

To remedy this issue, Badman and coauthors developed a framework to compare several coronal models to data as well as to each other. Specifically, the authors compared outputs from three models — ranging from relatively simple to highly complex — to three types of data:

  1. Extreme-ultraviolet images of the Sun’s disk that reveal the locations of coronal holes (i.e., where the Sun’s magnetic field lines extend out into the solar system rather than looping back to the Sun’s surface)
  2. Visible-light images of coronal streamers captured by blocking the light from the Sun’s disk
  3. Magnetic field measurements made by spacecraft between Earth and the Sun

 

plot of model performance on test 2

Comparison of model performance on the white-light neutral line (WL NL) metric, which measures the models’ ability to recreate the structure of the corona. The more complex Wang–Sheeley–Arge (WSA) and Magnetohydrodynamic Algorithm outside a Sphere (MAS) models perform better on this metric than the simpler potential field source surface (PFSS) models. [Adapted from Badman et al. 2022]

Optimizing Output

By comparing model predictions to these types of data, the authors were able to quantify how well the models reproduced the characteristics of the solar corona close to the Sun as well as conditions in the solar wind at Earth’s orbit. The authors’ analysis revealed that none of the three models studied performed well on all three of the tests.

For example, tuning the least complex of the three models to get the best match to the structure of coronal streamers worsened its performance on the other two tests. The other, more complex models made better predictions of the positions of coronal holes, the shapes of coronal streamers, or both, but these models still struggled to match the magnetic field measurements made by spacecraft farther out from the Sun.

Overall, the team’s results show that their framework is a valuable tool for making comparisons between models. Going forward, the authors hope to create an open-source tool to make this framework more easily accessible to researchers looking to assess the performance of their own models.

Citation

“Constraining Global Coronal Models with Multiple Independent Observables,” Samuel T. Badman et al 2022 ApJ 932 135. doi:10.3847/1538-4357/ac6610

Zoom-in of the surface of a magnetar with a bright burst erupting from it with magnetic field lines surrounding it

In the wild world of fast radio bursts, we may finally be converging on an explanation of what causes these outbursts. Is the answer magnetic reconnection, a phenomenon that occurs everywhere from the Sun to Earth’s magnetosphere?

An illustration of reconnection near the Earth; Earth in the middle with its magnetic field lines on either side of the planet extending outward, with the Sun pushing on one side

An illustration of magnetic reconnection in Earth’s magnetosphere. The solar wind puts pressure on Earth’s magnetic field lines, causing them to reconnect with the solar wind magnetic field. They’re then peeled back by the motion of the solar wind to the night side of the planet, where they reconnect. [NASA]

Making a [Re]Connection 

Fast radio bursts are one of the hottest topics in astronomy at the moment. These millisecond bursts of radio emission first erupted onto the scene in 2007 and, since then, they’ve continued to puzzle astronomers with their many mysteries. The leading picture of how fast radio bursts are produced suggests they’re caused by flares of electromagnetic radiation from magnetars. Magnetars are neutron stars with such high magnetic fields (some of the strongest in the universe!) that if a magnetar was situated between Earth and the Moon, it would wipe out all of our credit cards and hard drives. A team led by Jens Mahlmann (Princeton University) has taken this idea a step further, positing that fast radio bursts could be caused by magnetic reconnection in the plasma flowing outward from a magnetar.

In the top right corner, a magnetar (represented by a sphere). Next to it, there are three lines showing the low-frequency pulse, which then leads to a wave showing the current sheet in the magnetar wind in the bottom right corner. A zoom-in of the current sheet is shown in the bottom left (which is essentially colored lines showing the intensity of the current).

A diagram of the magnetar wind and the current sheet that ultimately causes reconnection. [Adapted from Mahlmann et al 2022]

Pressure That’ll Tip, Tip, Tip ’til [Magnetic Field Lines] Just Go Pop 

Magnetic reconnection occurs when stressed magnetic field lines snap and come back together, releasing energy as they do so. Think of a rubber band: when you put pressure on a rubber band by stretching it, its elastic potential energy is converted into kinetic energy when the rubber band is released. The same is true with magnetic reconnection: pressure is put on field lines until they snap into a new configuration, transferring magnetic energy into kinetic energy. This energy of “snapping” throws electrons outward in two jets, flinging them into space. This phenomenon is seen in many systems throughout the universe, including in solar flares and in our own magnetosphere, causing the northern lights. The energy released in reconnection can be tremendous — the energy contained in a gallon of a magnetar’s magnetic field is equal to the energy stored in 1018 gallons of gasoline — and it could provide enough energy to power the fast radio bursts that we’ve observed throughout the universe. 

Four panels showing a zoom-in of the current sheet over time as the flare triggers magnetic reconnection.

An example of simulations of the reconnection in the current sheets caused by magnetar flares. From panels (a) to (d), the current sheet gets more and more compressed, with magnetic reconnection beginning in (b). Note that time increases from panel (a) to panel (d) and the length scale increases as the pulse moves outward. The colors represent the current density. To see an animation of this figure, [click here]! [Adapted from Mahlmann et al 2022]

A Current Theory 

To test the theory that magnetic reconnection is the source of fast radio bursts, Mahlmann and collaborators simulated the area around a magnetar that has a wind of plasma propagating from its surface (aptly named the “magnetar wind”). The authors investigated the outcome of a magnetar flare traveling through the magnetar wind and colliding with a current sheet — a region in which an electric current flows between magnetic field lines that point in opposite directions. They found that the magnetar flare would trigger a magnetic pulse, causing a compression of the magnetic field lines in the current sheet, which would then snap and reconnect. A fraction of the energy released would escape the wind as radio emission. The authors calculated that, given a strong enough magnetic pulse in the magnetosphere, the resulting radio burst would be bright enough to see outside our own galaxy. 

This theory, which builds upon one of the leading explanations of how fast radio bursts are produced, might be the key to understanding these bursts. The simulations in this study are conducted in two dimensions, but the team hopes that future studies will explore the 3D realm of the intricate plasma physics that governs behavior near magnetars. 

Citation  

“Electromagnetic Fireworks: Fast Radio Bursts from Rapid Reconnection in the Compressed Magnetar Wind,” J. F. Mahlmann et al 2022 ApJL 932 L20. doi:10.3847/2041-8213/ac7156 

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