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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 

Illustration of exoplanetary systems

V1298 Tau is the youngest known planetary system containing multiple exoplanets. What can simulations tell us about how this system likely formed?

animation of the resonances of jupiter's moons

This animation (not to scale) shows the resonances exhibited by three of Jupiter’s moons. Alignments between the moons are highlighted by color changes. [Wikipedia]

Chasing Young Planetary Chains

One of the many enduring mysteries in the study of planetary systems is how they form. Some theories suggest that the movement of young planets within a protoplanetary disk causes the planets to form resonant chains — a setup in which the planets’ orbital periods are integer multiples of each other. Jupiter’s moons Ganymede, Europa, and Io are an example of this arrangement; the orbital periods of Europa and Ganymede are two and four times as long, respectively, as Io’s orbital period.

However, observations show that less than 1% of mature planetary systems are arranged in resonant chains. To understand whether planets make and then break resonant chains — or whether these chains form at all — we need to study young (<100 million years old) systems with three or more planets, only two of which are currently known.

Seeking Stability

At just 23 million years old, V1298 Tau hosts the youngest multi-planet system discovered so far. Given the orbital periods of the four known planets in the system — roughly 8, 12, 24, and 50 days — some researchers have suggested that the planets in this system are arranged in a resonant chain. To test this theory, a team of astronomers led by Roberto Tejada Arevalo (Princeton University) incorporated new estimates of the planets’ masses into a dynamical model to test the stability of the current arrangement and probe the true orbital properties of the system.

plot of posterior distributions for resonant conditions

Distributions of stable (black) and unstable (red) resonant configurations for different samples of orbital elongation or eccentricity, e, and the ratio of total planetary mass to stellar mass, μ. Click to enlarge. [Adapted from Tejada Arevalo et al. 2022]

In their analysis, Tejada Arevalo and collaborators used observations of planetary transits made by Kepler and the Transiting Exoplanet Survey Satellite (TESS) to constrain the possible orbital parameters of the system. The transits captured by Kepler and TESS give us only snapshots of the system’s behavior, and orbital parameters can oscillate over time, so the authors determined the set of orbital parameters that could lead to the transits we’ve observed. Using this set of parameters, the team then tested the stability of each orbital setup. Ultimately, the authors find that only 1% of stable orbital configurations that could generate the observations are consistent with a resonant chain, making it unlikely that V1298 Tau’s planets are arranged in such a configuration.

Finding a Solution that Resonates

What do these results imply about the possible creation — and subsequent breaking — of resonant chains in young planetary systems? The V1298 Tau system’s lack of a resonant chain arrangement implies that either the planets were never in that configuration or the breaking of the chain occurred early in the system’s formation. The dissipation of a protoplanetary disk, which tends to occur after just a few million years, may provide a natural way for resonant chains to become unstable. The team’s analysis suggests that the system’s nearly resonant configuration is consistent with a chain-breaking instability early in the system’s history.

plot of the target star locations for the Cluster Difference Imaging Photometric Survey

TESS observing footprint (gray) and target stars (blue) for the Cluster Difference Imaging Photometric Survey — a search for young exoplanets that might uncover resonant chains. Click to enlarge. [Bouma et al. 2019]

Ultimately, characterizing just one planetary system isn’t enough to draw conclusions about the formation of planetary systems as a whole. To do that, we’re going to need more detections of young planets — and, luckily, several searches are underway.

Citation

“Stability Constrained Characterization of the 23 Myr Old V1298 Tau System: Do Young Planets Form in Mean Motion Resonance Chains?,” Roberto Tejada Arevalo et al 2022 ApJL 932 L12. doi:10.3847/2041-8213/ac70e0

active galaxy Centaurus A

composite infrared and X-ray image of a molecular cloud with young stars

Clouds of cold gas, like Cepheus B shown here at infrared (red, blue, and green) and X-ray (violet) wavelengths, provide the fuel for star formation. [X-ray: NASA/CXC/PSU/K. Getman et al.; IR: NASA/JPL-Caltech/CfA/J. Wang et al.]

How does the presence of an accreting supermassive black hole affect its host galaxy’s ability to form stars? A new study examines the supply of star-forming gas in more than 10,000 galaxies to find out.

Stymied Star Formation?

Stars form from cold, dense hydrogen gas. Any process that heats this gas, disperses it, or makes it turbulent has the potential to disrupt the star-formation process. For this reason, astronomers have long suspected that the radiation and particle jets from active galactic nuclei — extremely luminous galactic centers powered by a supermassive black hole accreting matter — can inhibit star formation in the galaxies they inhabit.

If this is the case, galaxies with active galactic nuclei should have smaller reservoirs of cold, star-forming hydrogen gas than those without, either because the gas has been heated up or blown away. However, searches for a decline in cold gas have so far come up short. Can a new analysis find trends that have gone unnoticed in previous studies?

Grouping Galaxies

A team led by Hong Guo (Shanghai Astronomical Observatory, China) analyzed a massive sample of galaxies observed by the Sloan Digital Sky Survey and Arecibo Observatory to determine what effect — if any — the presence of an active galactic nucleus has on the host galaxy’s supply of cold, star-forming gas. Guo and collaborators grouped the >10,000 galaxies in their sample according to their masses, star-formation rates, and luminosity of their active galactic nuclei.

plots showing how galaxies in the sample were categorized

Separation of the sample into galaxies with and without active galactic nuclei. The classification is shown according to the Eddington parameter (ratio of luminosity and black hole mass) on the left and according to the luminosity on the right. Click to enlarge. [Guo et al. 2022]

Using a stacking technique to determine the average mass of cold hydrogen gas contained in the galaxies in each bin, the authors compared the supply of star-forming gas between galaxies with active galactic nuclei and those without. The authors find that cold gas is depleted in active galactic nuclei-hosting galaxies with masses up to 10 billion solar masses. For galaxies in this mass range, the depletion of cold gas is greatest in galaxies with high star-formation rates and highly luminous active galactic nuclei. However, this trend is weak or absent in higher-mass galaxies — those from 10 to 100 billion solar masses.

A High-Mass Hypothesis

plots of neutral hydrogen mass as a function of star-formation rate, galaxy mass, and whether or not the galaxy has an active galactic nucleus

Active galactic nucleus (AGN) hosting galaxies with masses up to 10 billion solar masses have less star-forming gas available than galaxies without AGN (left column). This trend is not robust among more massive galaxies (center and right columns). Click to enlarge. [Guo et al. 2022]

Why did Guo and collaborators find that the presence of an active galactic nucleus correlates with a lower supply of star-forming gas in certain galaxies, while previous works found no such trend in any galaxies? The authors speculate that binning their galaxy sample by mass and star-formation rate allowed them to uncover the trend; since the mass of star-forming gas also depends on a galaxy’s star-formation rate, lumping galaxies of all star-formation rates together might mask any correlation.

As to why only the galaxies in the lowest mass bin showed this behavior, Guo and coauthors suggest that high-mass galaxies may simply be too large for their entire reservoir of cold gas to be heated or disrupted while the active galactic nucleus is “on.” In this case, an active galactic nucleus might have a large impact on the inner regions of the galaxy, but that impact may not extend far enough out to be observed. Future high-resolution surveys of cold gas should illuminate this issue further, especially in low-mass galaxies where active galactic nuclei have an outsize impact.

Citation

“Cold Gas Reservoirs of Low- and High-mass Central Galaxies Differ in Response to Active Galactic Nucleus Feedback,” Hong Guo et al 2022 ApJL 933 L12. doi:10.3847/2041-8213/ac794f

Artist's depiction of two black holes nearing a merger.

The detection of gravitational waves paved a new avenue for the study of binary black holes across cosmic time. What can we learn about the evolution of black hole binaries from the systems we’ve detected with gravitational waves?

Examining Gravitational-Wave Events

plot of the masses of the compact objects discovered with gravitational waves

The rapidly expanding “stellar graveyard,” a plot that shows the masses of compact objects observed via gravitational waves and other means. Click to enlarge. [Visualization: LIGO-Virgo-KAGRA / Aaron Geller / Northwestern]

With our detections of merging black hole binaries ever increasing, we can start to answer fundamental questions about how these fascinating systems form and evolve. And since black holes are an endpoint of stellar evolution, studying how the population of black holes may have changed over time can also provide insights into stellar evolution.

In a new publication, a team led by Sylvia Biscoveanu (Massachusetts Institute of Technology) took advantage of the wealth of black hole data to search for trends in the spins of black holes in binary systems. Using a collection of 69 binary black hole events in the third Gravitational-Wave Transient Catalog, the team aimed to determine if the spins of the black holes are correlated with their masses or with the redshift at which the binary lies.

plot of black hole spin distribution for several redshift values

Each shaded region indicates the 90% credible region for the black hole effective spin distribution at a different redshift (z). Click to enlarge. [Biscoveanu et al. 2022]

Calculations from a Curated Catalog

Biscoveanu and collaborators found that the average spin of binary black holes varied little with redshift (a proxy for cosmic time) or mass, but the distribution of spins broadened at higher redshifts. In other words, black holes in binary pairs had the same average spin 10 billion years ago as they do today, but there were more black holes spinning more rapidly  — both with higher positive and higher negative spin — in the past than there are now.

The team analyzed synthetic black hole data and applied new models to existing data to rule out the possibility that the broadening is 1) caused by the increase in uncertainty in the spin measurements of high-redshift binary systems, 2) a reflection of an underlying correlation between other factors, such as black hole mass and redshift, or 3) a consequence of applying the wrong model to the data. These analyses suggested that the trend is real. In fact, a distribution that widens as redshift increases should be easier to rule out than other trends, since rapidly spinning black holes are overall easier to detect than those that spin slowly, so failing to detect them at high redshift would rule out a broadening distribution.

three plots of black hole spin distributions for different redshift values

Black hole spin distributions recovered using three different models. These plots demonstrate that the observed trend is not the result of the authors’ initial model choice. [Biscoveanu et al. 2022]

Taking a Broad View of Black Hole Spin

A broadening of the spin distribution with redshift could have many physical causes. It could indicate that there are several formation pathways for black hole binaries, and each pathway has a different redshift dependence and different spin distribution. Another possibility is that black hole binaries might form via only one pathway but then evolve in such a way that the spins change over time.

However, while both hypotheses can lead to black holes with high positive spin at high redshift, they can’t yet explain the increase in black holes with high negative spin, which are necessary to create the broad spin distribution we observe. One possibility is that these systems with negative spin — in which the spin of an individual black hole is off kilter with respect to the angular momentum of the binary system — could form if black holes in the early universe got a larger gravitational “kick” when they’re born than they do today. There’s still much to investigate, and as our catalog of binary black hole systems grows, our answers are likely to evolve further!

Citation

“The Binary Black Hole Spin Distribution Likely Broadens with Redshift,” Sylvia Biscoveanu et al 2022 ApJL 932 L19. doi:10.3847/2041-8213/ac71a8

Star surrounded by a planet-forming disk

In mystery thriller books, the authors always lead you to suspect that the culprit is someone outside the group: the gardener or the locksmith, perhaps. Sometimes, however, the answer is right in front of you, and the perpetrator is in the inner circle. A group of astronomers has recently reached the same conclusion: that an ongoing orbital alignment mystery seen in some stellar systems isn’t caused by disruptions from the outside, but rather comes from within the stellar systems themselves. 

A cartoon of a protoplanetary disk: the star is at the middle surrounded a small gap and then by the inner disk, there's another gap, and then the outer disk is present. The angular momentum vectors all point up but at slightly different angles.

A schematic of the geometry of a misaligned system, showing the angular momentum direction of the star, the inner disk, and the outer disk. [Epstein-Martin et al. 2022]

A Mystery Arises 

When a stellar system forms, everything is thought to be aligned: the star forms, it ignites nuclear fusion, and all of the leftover gas and dust orbit in a single plane and in the same direction that the star spins. This theoretical picture initially seemed to fit the planetary systems we had found…. that is, until recent space missions started uncovering thousands of new planets and began to tilt this theory on its head. All of a sudden, astronomers were discovering stars whose spin axes were misaligned with the orbits of their planets. But how does this situation arise? Shouldn’t the angular momentum of the system extend to the stellar spin axes, and everything should be aligned? According to recent exoplanet discoveries, apparently not!  

These questions remain a hot topic in the field of planet formation. One proposed explanation is that an external companion star could exert a torque on a star-forming region and misalign everything. In counterpoint, a team of astronomers led by Marguerite Epstein-Martin (California Institute of Technology and Columbia University) posit that the troublemaker was instead internal: forces within the disk itself.

A plot of stellar age [years]on the x-axis (going from ~10^5 to 10^7) against angular momentum (in AU^2 solar masses/yr) on the y-axis (going from ~10^-3 to ~10^1). The star is the bottom curve then the inner disk (A_in = 5 - 10 AU, a shaded region), and the outermost disk (A_out = 40 - 90 AU, a shaded region). All slowly slopes downward to the right.

The angular momentum ranges for the three components in the authors’ modeled system of a star and its surrounding disk. The yellow line shows a solar-mass star, the red region shows the inner disk, and the purple region shows the range of values for an outer disk. This figure clearly shows the hierarchy of angular momentum in the system. [Epstein-Martin et al. 2022]

An Unexpected Suspect 

Protoplanetary disks are typically modeled as rigid objects. However, recent observations show ~85% of the disks that can be resolved contain gaps with misalignments between the inner and outer disks, which result from the formation of massive planets or the presence of a stellar binary companion. The team theorized that in these cases of an inner and outer disk misalignment, the outer disk can play the role of a stellar companion and influence the dynamics of the inner disk–star orientation. From there, the team used equations to model these systems and found that there’s a hierarchy to the angular momentum within the system: the outer disk has the largest angular momentum and will apply a torque on the inner disk, which will itself exert a torque on the star. This is analogous to the dynamics between a star, an unbroken disk, and an outside companion that caused the misalignment in previous theories.  

Identification of the Culprit 

By using a series of complex equations that represent the dynamics in these misaligned systems, the team determined that, given the timescale of the contraction of the star and the lifetime of the disk, there would be just enough time for the disruptions to take place that cause the star’s misalignment with its eventual planets’ orbits. Though this does fit observations, the team notes that they employed several simplifications and assumptions. Overall, this study provides a new avenue for the formation of disks misaligned from the spins of their host star. So next time, don’t get distracted by the outside characters, because the culprit could come from within! 

Citation 

“Generating Stellar Obliquity in Systems with Broken Protoplanetary Disks,” Marguerite Epstein-Martin et al 2022 ApJ 931 42. doi: 10.3847/1538-4357/ac5b79 

Collage of binary star systems

Many stars travel through space with a binary companion, and large-scale surveys allow us to study enormous numbers of these stellar pairs. What do these surveys tell us about the characteristics of binary stars in the Milky Way?

hubble image of interacting galaxies with tidal tails

Tidal forces are perhaps best known for generating tidal tails and streams in interacting galaxies, but a galaxy’s tidal pull can have subtle effects on binary star systems as well. [NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M. Clampin (STScI), G. Hartig (STScI), the ACS Science Team and ESA]

Stars Awash in the Galactic Tide

The orbital parameters of a binary star system — namely, the distance between the stars and how eccentric (non-circular) their orbits are — can encode information about the formation and evolution of the binary system as well as the evolution of the stars themselves. The orbits of binary stars are susceptible to outside influence, too; gravitational nudges from passing stars, nearby gas clouds, and the overall tidal pull of the galaxy can change a binary system’s orbits over time.

When we examine the orbits of binary systems in the Milky Way with observations from the sky-mapping Gaia spacecraft, we find unexpected trends in the orbital parameters of binary systems near the Sun. Namely, among binary systems separated by large distances (>1,000 au), there are more systems with highly eccentric orbits than expected. What’s the origin of this trend?

Nature vs. Nurture

plot of initial and final eccentricity distribution functions from the model

The final distribution of eccentricities (black lines) and best-fitting power laws (green lines) acquired from various initial distributions (red lines). These results show that a superthermal eccentricity distribution, as is seen in binary systems near the Sun, can only arise from a distribution that is initially superthermal. [Adapted from Hamilton 2022]

As Chris Hamilton (Institute for Advanced Study) explains in a recent research article, understanding the current orbits of binary stars in the Milky Way requires separating the effects of nature (the eccentricities that the binary systems are born with) and nurture (the outside gravitational effects of passing stars and the background galactic pull).

Hamilton approached this problem by modeling the effects of the Milky Way’s overall gravitational pull on populations of synthetic binary stars in the outer regions of the galaxy. In order to test the effects of nature as well as nurture, Hamilton modeled populations with different initial eccentricity distributions: uniform (all eccentricities are equally common), thermal (the binaries have reached statistical equilibrium through gravitational interactions; represented by the gray lines in the figure to the right), subthermal (fewer eccentric binaries than the thermal case), and superthermal (more eccentric binaries than the thermal case, as we see near the Sun).

Maybe They’re Born with It, Maybe It’s the Tidal Influence of the Milky Way

The model results show that the tidal pull of the Milky Way tends not to change the eccentricity distribution of a population of binary stars. Put another way, this means that the high number of wide, eccentric binary systems in the solar neighborhood can’t have been caused by the Milky Way’s gravitational influence — another factor, such as the combined effects of individual gravitational nudges from passing stars and gas clouds, must have caused this trend, or binary systems in the solar neighborhood must be born with a similar distribution of eccentricities.

As is so often the case, there’s plenty more work to be done to understand this issue fully. In particular, modeling the effects of gravitational tugs from passing stars and applying new techniques to study the time evolution of binary systems will be critical to reaching a conclusion.

Citation

“On the Phase-mixed Eccentricity and Inclination Distributions of Wide Binaries in the Galaxy,” Chris Hamilton 2022 ApJL 929 L29. doi:10.3847/2041-8213/ac6600

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