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Illustration of a star surrounded by a bright, extended disk of dust and gas.

How does material move through an accretion disk to the young star at its center? Surprising detections from a fortuitously angled disk have now provided new insights.

Driving Inflow

When stars are born from the collapse of a dense molecular cloud, they spend their early stages surrounded by circumstellar disks: disks of gas and dust that we understand to be accreting onto the young stars at their centers.

Taurus Molecular Cloud complex

Herschel infrared view of the Taurus Molecular Cloud complex, the home of GV Tau. [ESA/Herschel/NASA/JPL-Caltech; acknowledgement: R. Hurt (JPL-Caltech)]

How do we know that the disk matter is flowing onto the stars? Evidence for accretion comes from the high-energy light emitted when inflowing material strikes the surface of young stars, producing accretion shocks. But, though these observations provide evidence that accretion is occurring, they don’t tell us much about the mechanisms that drive these flows within the disk.

For material to move inwards within a disk, it must first lose angular momentum — but where does that momentum go? What processes remove or redistribute it? In a new study led by Joan Najita (NSF’s NOIRLab), a team of scientists presents high-resolution observations of an unusual disk — one that happens to be angled in such a way as to help us answer these questions.

Lucky Alignment

Two-panel diagram illustrating disk geometry.

Schematics representing the likely observing geometry of GV Tau N; the observer is on the right. Top: The line of sight to the disk continuum (orange) passes through the warm molecular atmosphere at larger radii (pink), producing absorption. Bottom: View of the molecular gas velocities. The combination of rotation (blue arrows) and inflow (green arrows) produces net redshifted (red arrows) absorption velocities. [Adapted from Najita et al. 2021]

Najita and collaborators used the TEXES spectrograph on the Gemini North 8-m telescope to conduct mid-infrared observations of GV Tau N, a young star surrounded by a nearly edge-on circumstellar disk. The authors’ observations revealed rare molecular absorption lines, a result of the nearly edge-on inclination of the disk.

The unique viewing angle for GV Tau N means that our sightline passes through the disk atmosphere in the inner few au of the disk — the region where planet formation is thought to occur. The molecules in this gas absorb some of the continuum light emitted by the interior disk, leaving signatures in the spectrum that provide valuable insight into the composition and motions of the gas at the surface of the inner disk.

Caught in the Act

Najita and collaborators found evidence for a variety of molecular species in the disk: acetylene (C2H2), hydrogen cyanide (HCN), water (H20), and even ammonia (NH3), which has never before been detected in an inner accretion disk. But the especially interesting result is that these molecules’ absorption lines are redshifted, lying at longer wavelengths than expected if the gas were moving in a stable circular orbit.

Spectrum showing ammonia absorption.

Spectrum showing various ammonia absorption lines from GV Tau N. [Adapted from Najita et al. 2021]

This redshift is an indication that the gas observed is flowing rapidly (about 1 au per year) inward along the disk surface — direct evidence for accretion in action. The authors show that their observations match expected mass accretion rates for active T Tauri stars: roughly a few to a few tens of Earth masses per year. The observations fit neatly with a disk accretion model in which angular momentum is redistributed within the disk, causing surface gas to flow in and accrete while the midplane of the disk spreads outward.

GV Tau N is a lucky break — its orientation allowed us to make these unique measurements. But it’s surely not alone! With more observations of systems like GV Tau N, we’ll be able to further deepen our understanding of disk accretion.

Citation

“High-resolution Mid-infrared Spectroscopy of GV Tau N: Surface Accretion and Detection of NH3 in a Young Protoplanetary Disk,” Joan R. Najita et al 2021 ApJ 908 171. doi:10.3847/1538-4357/abcfc6

Simulation showing two black holes in the process of merging. A star field makes up the background.

Theory predicts that gravitational-wave detectors should be able to observe a population of huge black holes. A new study explores what we’ll learn from these mysterious objects and when we can hope to find them.

A Preferred Size

Stellar graveyard Nov 2020

A recent version of the rapidly expanding “stellar graveyard”, a plot that shows the masses of the different components of observed compact binary mergers. Click to enlarge. [LIGO-Virgo/Northwestern U./Frank Elavsky & Aaron Geller]

So-called stellar-mass black holes — the black holes probed by gravitational-wave detectors like LIGO/Virgo — can theoretically span a broad range of sizes, from just a few solar masses to hundreds of times the mass of the Sun.

The LIGO/Virgo gravitational-wave detectors have discovered signals from dozens of black-hole binaries completing their final death spirals and merging. So far, these observed primary black holes have primarily fallen into a mass range below ~45 solar masses, indicating a precipitous drop in the population of binary black holes above this mass.

supernova

Artist’s impression of a supernova. Progenitor stars of a certain mass are susceptible to pair-instability supernova, preventing the formation of a black hole. [ESO/M. Kornmesser]

Avoiding an Unstable End

Why the dearth of heavier black holes? Theorists have an explanation: the pair-instability supernova mass gap. Based on our understanding of stellar evolution, black holes in a certain mass range — roughly 50–120 solar masses — shouldn’t be able to form. This mass gap arises because the progenitor stars needed to produce black holes of this size are predicted to undergo a runaway process, eventually exploding as violent supernovae that prevent remnant black holes from forming.

The formation of black holes above ~120 solar masses, however, should still be possible — so we’d expect a population of enormous far-side-of-the-mass-gap black holes to be lurking in our galaxy and beyond. In a new study, University of Chicago scientists Jose María Ezquiaga and Daniel Holz dig further into this prediction.

Hunt on the Far Side

Ezquiaga and Holz use the statistics of past black-hole binary detections and predictions of the capabilities of current and future gravitational-wave detectors to estimate what’s in store for us in terms of far-side black holes.

First, the authors show that these heavyweights would be the most massive sources detectable by LIGO/Virgo, and — if they exist — we should be able to spot up to tens of them during LIGO/Virgo’s next two observing runs (O4 and O5).

Plot of the number of detections of black holes expected as a function of mass.

The estimated maximum number of black-hole-binary mergers detected per year for various current and upcoming ground-based gravitational-wave detectors, and in 4 years for LISA. [Adapted from Ezquiaga & Holz 2021]

What’s more, far-side binaries should also lie in the observing band of LISA, the upcoming space-based gravitational-wave mission. They may dominate the population of binaries that can be observed by both LIGO/Virgo and LISA, providing valuable information about how the merger rate for black-hole binaries changes over time.

Finally, Ezquiaga and Holz show that observations of far-side binaries with LISA, LIGO/Virgo, and the Einstein Telescope (a next-generation detector) will provide an independent measure of the expansion of the universe at different redshifts: z ~ 0.4, 0.8, and 1.5, respectively. By exploiting the upper edge of the mass gap, far-side black holes can act as standard sirens and enable precision cosmology.

Soon To Be Found?

So what’s the upshot? The outlook is good for far-side black holes!

If these heavyweights exist, we should spot them within the next couple years and they’ll be able to provide us with valuable insight into a variety of science questions. If we don’t observe any within this time frame, that also provides a powerful statement about black-hole formation, demanding new theories to explain the dearth.

Citation

“Jumping the Gap: Searching for LIGO’s Biggest Black Holes,” Jose María Ezquiaga and Daniel E. Holz 2021 ApJL 909 L23. doi:10.3847/2041-8213/abe638

Photograph of a large, bright, elliptical galaxy in the midst of a broader galaxy field.

In the outer reaches of galaxies, stars don’t move quite how they should. Is this deviation due to mysterious dark matter? Or is something else at work? In a recent study, scientists turn to elliptical galaxies in search of new clues.

Weirdness in Galactic Fringes

Animated gif that shows two galaxies rotating with different behaviors.

Two rotating galaxies are shown with their rotation curves in this animated gif. Based on the distribution of visible matter, we would expect to see inner stars moving fast and outer stars moving slowly, producing a rotation curve like what’s seen on the left. Instead, we see stars moving with the same speed, producing the flat rotation curve seen on the right. [Ingo Berg]

Usually, things that orbit do so in predictable ways. In our solar system, for instance, planets orbit the Sun following predictable laws of gravitation: close-in planets speed along quickly, whereas planets farther out move more slowly.

You might think that galaxies would work the same way. Since most of a galaxy’s visible mass is concentrated at its center — just like most of our solar system’s mass (the Sun) is at its center — we would expect stars near the center of a galaxy to orbit quickly, and stars in the galaxy’s outermost fringes to orbit very slowly. Instead, we see that stars throughout galaxies move at roughly the same speeds: galaxies have flat rotation curves.

Dark matter halo

Artist’s illustration of the distribution of dark matter in the halo surrounding the visible disk of the Milky Way Galaxy. [ESO/L. Calçada]

Dark Matter? Or a Gravitational Misunderstanding?

What drives this weird behavior? The most widely accepted explanation is dark matter: the idea that there’s a lot of matter in a galaxy that isn’t concentrated in its center — we just can’t see it. This dark matter is distributed in a wide halo around the galaxy, and its gravitational tug causes outer stars to orbit faster than they would under the influence of the visible matter alone.

But dark matter isn’t the only possible explanation for galactic rotation curves: an alternative, modified Newtonian dynamics (MOND), was first introduced nearly 40 years ago. The MOND hypothesis contends that dark matter isn’t needed to explain galaxy rotation curves — because our understanding of gravity is wrong.

According to MOND, normal Newtonian gravitation applies in regions where acceleration is large — say, in the case of the Moon orbiting the Earth, planets orbiting a solar system, or in the inner regions of a galaxy. But gravity behaves slightly differently in regions where acceleration is small — like in the outer reaches of galaxies.

Critical Acceleration

How can we test MOND as a theory? One clue is whether individual galaxies stray from expected gravitational behavior at a consistent acceleration scale — some fundamental value of acceleration that effectively marks the transition from the Newtonian regime to the MOND regime. If, instead of a universal acceleration scale, we find a large amount of scatter in how galaxies deviate, that would rule out the MOND hypothesis.

Plot of the individually fitted g† for the authors' 15 elliptical galaxies.

The authors’ elliptical galaxy sample is consistent with a universal acceleration scale of g† = 1.2 x 10-10 m/s2 — which is consistent with the acceleration scale previously found for spiral galaxies. [Chae et al. 2020]

A universal acceleration scale has, in fact, been found in previous studies that explored the observed radial acceleration of spiral galaxies. Now, in a study led by Kyu-Hyun Chae (Sejong University, Republic of Korea), scientists have demonstrated that this same scale appears to extend to a sample of elliptical galaxies as well.

Chae and collaborators’ study is important because it shows that both galaxies supported by rotation (spirals) and those supported by pressure (ellipticals) behave the same way. These two independent measures underscore the universality of how galaxies’ acceleration deviates from what’s predicted from visible matter alone — and suggest that MOND isn’t out of the running just yet.

Citation

“On the Presence of a Universal Acceleration Scale in Elliptical Galaxies,” Kyu-Hyun Chae et al 2020 ApJL 903 L31. doi:10.3847/2041-8213/abc2d3

active galactic nucleus

Active galactic nuclei are exactly what they sound like — central regions of galaxies that emit enormous amounts of energy. Typically, they consist of a supermassive black hole surrounded by a hot disk of material being accreted onto the black hole. Hardly the most hospitable environment, but stars can still live in these surroundings!

Centaurus A

This composite image reveals Centaurus A, a galaxy with an active nucleus spewing fast-moving jets into its surroundings. [ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray)]

Actively Hostile Environments

It would be hard to overstate how energetic active galactic nuclei (AGN) are. Some can outshine the rest of their host galaxy at almost all detectable wavelengths! Spectra of material near the central black hole have shown that AGN environments contain a higher abundance of heavier elements than the environment of our Sun. So it’s possible that those heavier elements were produced in the accretion disk and then swept closer in towards the black hole.

But what produces heavier elements? Stars! Stars can be found near the central supermassive black holes of galaxies, like the Milky Way’s Sagittarius A*, but AGN have far more extreme environments than our placid central black hole. So what sort of stars live in AGN environments? A recent study led by Matteo Cantiello (Flatiron Institute/ Princeton University) dives into this question.

The mass and brightness of an AGN star over time. This star was modeled under specific AGN conditions. LEdd stands for Eddington luminosity, which is the maximum brightness a star can have when it has balanced its outward radiative pressure with its inward gravitational contraction. Mass loss starts roughly around when the star’s luminosity reaches the Eddington luminosity. [Cantiello et al. 2021]

What Massive Stars Make

Cantiello and collaborators were especially interested in how the evolution of stars in AGN environments differs from stellar evolution in calmer environments. To get where they are, AGN stars have to either form in accretion disks or get captured and pulled into the disks. Both models are viable and supported by looking at stellar populations around central black holes that were previously active, like Sagittarius A*.

Once in the disk, stars can rapidly accrete material and become hundreds of times more massive than the Sun. Massive stars experience more internal mixing than less massive stars, so the contents of a massive star are evenly distributed within the star’s interior. This is very different from stars like our Sun, where the outer layers of the star contain lighter elements like hydrogen and helium while inner layers are dominated by heavier elements.

However, massive stars are also unstable and can lose mass quickly as they teeter between expansion and collapse. Their sheer bulk also means that they will end their lives through core collapse — forming heavier and heavier elements through fusion until they run out of material to fuse and collapse onto themselves. The bottom line is that AGN stars are good at producing heavy elements and sending those elements out into the accretion disk.

A schematic showing stellar evolution in the accretion disk of an AGN. Low mass stars can be formed in or accreted by the disk, where they gain mass and eventually evolve to leave behind compact remnants near the center of the disk. [Cantiello et al. 2021]

Signs of Stellar Life and Death

So Cantiello and collaborators identified two signatures of AGN stars: high abundances of heavy elements and compact stellar remnants left behind from core collapse. There are studies showing evidence for the first signature, and interestingly, this abundance of heavy elements doesn’t seem to depend on redshift.

The second signature is a bit trickier to tease out. Before gravitational-wave observatories, our best bet would be to search for the explosions associated with core collapse in the accretion disk of an AGN. Now, we can also look for the gravitational-wave signatures of the mergers of dense objects, with an expectation of how often these mergers would occur.

Sagittarius A* is a good proving ground for the findings of this study, since our galaxy’s nucleus may approximate the aftermath of an AGN. With predictions in hand, it’s now time to observe!

Citation

“Stellar Evolution in AGN Disks,” Matteo Cantiello et al 2021 ApJ 910 94. doi:10.3847/1538-4357/abdf4f

Photograph of the rocky surface of an asteroid.

Samples from the near-Earth asteroid (162173) Ryugu recently arrived at Earth, ready for laboratory analysis. In the meantime, ground-based measurements of Ryugu’s surface are helping us to complete our picture of this nearby, rocky body.

Observations on Earth and from Earth

Illustration of a spacecraft with two sets of solar panels in the foreground, in front of a gray, rocky body.

Artist’s illustration of the Hayabusa2 spacecraft. [JAXA]

In December 2020, the Japanese spacecraft Hayabusa2 completed a daring 6-year mission, successfully landing on near-Earth asteroid Ryugu and returning a sample of this body’s material to Earth. Laboratory analysis of the sample is sure to provide valuable new insight into the structure and composition of the surface of this carbonaceous asteroid. But while we’re waiting for those results, there’s more to be learned from ground-based observations!

One way to study the surfaces of nearby objects is by making polarization measurements, which track the orientation of light waves reflected off of an object. In the case of an airless body like Ryugu, the amount of polarization measured from different viewing angles can tell us about the surface texture of the object.

In a new study led by Daisuke Kuroda (Kyoto University, Japan), scientists report the first polarization measurements of Ryugu, captured by four different observatories based in Japan and South Korea.

Plot of linear polarization vs. phase angle for 7 different objects.

Phase angle–polarization dependence of near-Earth asteroids and cometary nuclei. The solid red circles represent the polarimetric data for Ryugu, marking the highest polarization measured for such an object thus far. [Kuroda et al. 2021]

Breaking Records

Kuroda and collaborators used data gathered between September and December 2020 to measure the linear polarization of light scattered off of Ryugu as the angle between the Earth, Ryugu, and the Sun changed. The authors’ measurements covered a range of this phase angle spanning 28° to 104°.

The authors find that Ryugu exhibits the highest polarization degree ever measured for an asteroid or comet: as much as 53% of the light from Ryugu was linearly polarized at a phase angle of 100°!

This large degree of polarization is consistent with the asteroid’s low albedo. Why? Light wave orientations are scrambled by repeated reflection and refraction, resulting in lower polarization. For dark objects with poor reflectivity, like Ryugu, the reduced scattering results in higher polarization.

A Grainy Surface

Ryugu surface

Color photo of the surface of Ryugu, taken by the MASCOT lander deployed by Hayabusa2. [MASCOT/DLR/JAXA]

The authors’ next step was to use observations of space-rocks we can analyze up close and in person — in this case, meteorites found on Earth — to better understand observations of Ryugu. By comparing the polarimetric data for Ryugu to those measured in the lab for different meteorites, Kuroda and collaborators inferred that Ryugu’s surface layer is dominated by grains of submillimeter-order size.

When we complete the analysis of the actual surface material returned from Ryugu by Hayabusa2, those results will provide valuable context for the polarimetric observations presented here — and vice versa! The combination of these data will help us to learn more from future observations of our near-Earth rocky neighbors.

Citation

“Implications of High Polarization Degree for the Surface State of Ryugu,” Daisuke Kuroda et al 2021 ApJL 911 L24. doi:10.3847/2041-8213/abee25

illustration of a large red star surrounded by spherical shells of mass

Scientists have long puzzled over slow and regular variations in the brightness of many evolved giant stars. Now, clues from newly analyzed infrared observations may finally have solved this mystery.

Unexplained Changes

Series of illustrations of a star at different stages of life, from main sequence to red giant to planetary nebula.

Artist’s illustration of the life stages of a star like the Sun, which eventually will age into a cool red giant star and then eject its outer layers to form a planetary nebula. [ESO/S. Steinhöfel]

As stars like our Sun age, they inflate to hundreds of times their main-sequence size, engulfing the orbits of their inner planets and becoming luminous red giants. In the final stages of their lives, they eventually become variable stars, exhibiting changes in their brightnesses that can be caused by intrinsic pulsations, motions of convection cells in their stellar envelopes, and even the presence of circumstellar dust.

Most of this variability of red giants is reasonably well understood, but there’s one type that has remained mysterious: so-called long secondary periods.

Common Evolution

In addition to showing ordinary variation from pulsations, long-secondary-period red giants show regular dips in their optical light curves that occur on timescales an order of magnitude longer than the pulsations — typically several months to several years.

Evolved stars with long secondary periods are surprisingly common: at least a third of luminous asymptotic giant branch stars and supergiants show these long-period variations. Yet despite their ubiquity, long secondary periods have remained unexplained for decades. Are these variations intrinsic to the aging star? Or are they caused by some external factor?

Three light curves for a sample long-secondary-period variable star.

An example set of light curves from a long-secondary-period variable star shows the primary eclipses in all three light curves. Secondary eclipses appear only in the two infrared bands (W1 and W2, the orange and red data), not in the optical band (I, the blue data). [Adapted from Soszyński et al. 2021]

Now, new research from a team of scientists led by Igor Soszyński (University of Warsaw, Poland) has used infrared observations to identify the likely culprit: dust-shrouded binary companions.

Optical vs. Infrared

Soszyński and collaborators gathered optical observations of a sample of 16,000 known long-secondary-period variable stars in the Milky Way and the nearby Magellanic Clouds. For roughly 700 of these stars, the authors then obtained corresponding light curves at two infrared wavebands from the NEOWISE-R mission.

A striking feature was immediately evident when comparing the optical and infrared observations of these variables: where the optical light curves showed a single long-period dip in brightness, the infrared light curves of about half of the stars also showed a second dip that appeared exactly out of phase with the primary dip.

Former Planet, Now Dusty Shade

What does this mean? Soszyński and collaborators argue that these second dips confirm that the long-period variability is caused by eclipses from a binary companion.

Illustration of an orbiting planet trailing a cloud of dust.

Artist’s illustration of an orbiting planet trailing a comet-like tail of dust. A dust-shrouded brown-dwarf companion is the best explanation for long-period variability seen in the light curves of many evolved stars. [Maciej Szyszko]

In the authors’ explanation, a close-in planet accumulates mass from the expanding envelope of its red-giant host, eventually growing to brown-dwarf size. As this substellar companion — shrouded in a comet-like, extended cloud of dust — passes between us and the red giant, we observe long-period eclipses in the star’s optical and infrared light. When the cloud and companion then pass behind the star, their primarily infrared emission briefly vanishes, causing the secondary eclipse observed at infrared wavelengths only.

This solution to the decades-old mystery of long secondary period variability in red giants opens a door to new discoveries. By studying the shape of the eclipses in the variable stars’ light curves, there’s plenty more we can now hope to learn about how stars like the Sun evolve alongside their planets.

Citation

“Binarity as the Origin of Long Secondary Periods in Red Giant Stars,” I. Soszyński et al 2021 ApJL 911 L22. doi:10.3847/2041-8213/abf3c9

High-contrast photograph of the stripes and bands of Jupiter’s atmosphere.

As a moon orbits around its planet, the gravitational forces of the two bodies exert mutual pulls on one another. What happens to the inside of a gas-giant planet as a consequence of the tugs of its moons? New data from the Juno orbiter at Jupiter are providing answers!

Types of Tides

We needn’t look far to see how a moon’s gravity can influence its planet: anyone who’s witnessed low or high tide on a beach has experienced the effects of our own Moon.

illustration of the force magnitude and direction at the surface of the Earth as a consequence of the Moon's gravitational pull

The Moon exerts a differential gravitational pull on the Earth that manifests itself as tidal bulges on the sides of Earth facing and opposite the Moon. [Krishnavedala]

This well-known phenomenon is an example of hydrostatic tides: the pull of the Moon’s gravitational field causes the Earth to deform, creating stationary tidal bulges on the sides of the Earth closest to and opposite from the Moon. Since the most easily deformed part of the Earth is its surface water, we see these tidal bulges predominantly in our oceans, with two high tides and two low tides evident each day.

But hydrostatic tides are not the only type of tidal effect that the gravitational pull of moons can create. An additional effect is called dynamical tides, in which the gravitational influence of the moon induces oscillations in the interior of a gaseous planet. These waves could, if detected and understood, be used to probe the inside of the planet to learn about its structure and composition.

Until now, we’ve only detected the effect of hydrostatic tides in gas-giant planets — never the waves from dynamical tides. But new observations from the Juno orbiter are now shaking things up.

illustration of a three-armed spacecraft in front of Jupiter

Artist’s rendering of the Juno spacecraft. [NASA/JPL-Caltech]

A New Probe on the Scene

The Juno probe was launched in 2011, and it arrived at Jupiter and began orbiting this nearby gas giant in 2016. One of Juno’s instruments is designed to map out Jupiter’s gravitational field by making careful measurements of minute changes in Juno’s velocity as it orbits. This detailed mapping allows researchers to detect the tidal response of Jupiter to the gravitational pull of its system of moons.

Intriguingly, Juno’s measurements show a tidal response that can’t be explained exclusively by hydrostatic tides. In a new publication, California Institute of Technology scientists Benjamin Idini and David Stevenson explore whether Juno may, in fact, be seeing evidence of dynamical tides in Jupiter.

Io-Induced Waves

Photograph of Io in front of the face of Jupiter

This photograph, captured by Voyager 1 at Jupiter in 1979, shows Io and its shadow passing across Jupiter’s face. [NASA / JPL / Ian Regan]

Idini and Stevenson use perturbation theory and tidal models to calculate the predicted effect of the dynamical tides excited by Jupiter’s moons. The authors demonstrate that the deviation from hydrostatic equilibrium measured by Juno is consistent with expected dynamical tides induced by Io, the closest-in Galilean moon with the strongest gravitational influence.

The authors’ results suggest that Juno has indeed obtained the first unambiguous detection of the gravitational effect of dynamical tides in a gas-giant planet. Idini and Stevenson hope that we’ll be able to use these results — and future, even higher-precision data from Juno as it continues its extended mission — to explore Jupiter’s interior and answer long-standing questions about the inside of this gas giant.

Citation

“Dynamical Tides in Jupiter as Revealed by Juno,” Benjamin Idini and David J. Stevenson 2021 Planet. Sci. J. 2 69. doi:10.3847/PSJ/abe715

Telescopes are getting better and better at detecting the components of exoplanet atmospheres. But what can those components tell us about a planet’s climate? It turns out that water vapor may be especially useful in this regard.

Atmospheres on Tidally-Locked Planets

The Moon orbiting the Earth, with a yellow arrow showing the direction of the Moon’s rotation. The Moon’s rotational period matches its orbital period so that the same face of the Moon faces Earth at all times. Click to play. [NASA’s Scientific Visualization Studio]

As we find more and more exoplanets, we’re realizing that our solar system may be the exception to the rule! The menagerie of exoplanets we’ve discovered so far includes Jupiter-sized planets that are close to their suns, planets with two suns, and planets that take one orbit about their sun to complete one rotation on their axis — these planets are said to be tidally locked.

Just like our tidally locked Moon always shows the same face to the Earth, tidally locked planets always show the same face to their sun. So, a tidally locked planet will have a consistent dayside and nightside. This has fascinating implications for their climate, and even moreso when we consider that there could be tidally locked Earth-like planets!

A cross section of the planet’s atmosphere with a specific model pressure showing humidity with the colored contours and mass flow with the white contours. The nightside is on the left while the dayside is on the right. [Adapted from Ding & Pierrehumbert 2020]

Models of the water vapor runaway greenhouse effect — when radiation is prevented from efficiently leaving a planet — on tidally locked planets show that the nightside emits more thermal radiation than the dayside as the planet approaches the runaway greenhouse state. Since this reversal of thermal emission requires the emergence of clouds and the buildup of water vapor on the nightside of the planet, spotting it in an exoplanet’s atmosphere could be a useful indicator that the atmosphere is not dry.

To achieve nightside buildup of water vapor, the vapor must avoid being caught on the dayside in a “cold trap”, where it would be cooled, condense, and remain on the dayside. On a planet with inefficient cold trapping, the water vapor can be swept to the nightside to contribute to the thermal emission there.

This weak cold trap effect has mostly been modeled for planets with warm, thick atmospheres, but it is feasible for this effect to also occur on planets with thin, temperate atmospheres. A recent study done by Feng Ding (Harvard University) and Raymond Pierrehumbert (University of Oxford, UK) explores the second scenario for slowly rotating tidally locked planets.

Simulating Two Sides of a Planet

The brightness of the modeled planet as seen at a wavelength of 1,000 cm as it rotates. The different lines indicate different pressures and atmospheric conditions. The black vertical dashed line marks the superior conjunction, where the star is between the observer and the planet. Click to enlarge. [Adapted from Ding & Pierrehumbert 2020]

Ding and Pierrehumbert specifically looked at atmospheres rich in water vapor, which would allow for the necessary clouds and weak cold trap to exist. Their model planet had a period of 40 days and accounted for a variety of interactions that could occur on an Earth-like planet with an atmosphere and oceans. They were also able to vary atmospheric pressure and surface temperature and so modeled several different conditions for their planet.

It turns out that thin, temperate atmospheres with weak cold traps do show the same nightside–dayside emission difference as warm, thick atmospheres as they approach the runaway greenhouse state! Interestingly, the difference between the nightside and dayside emissions can point to the relative amount of water vapor in a planet’s atmosphere as well as the atmospheric pressure — insight we can’t gain from the planet’s transmission spectrum. Further properties of a planet’s atmosphere can be determined by observing how the brightness of the planet changes as it rotates.

The subtleties from this study can’t be picked up by our telescopes yet, but possible future missions like the Origins Space Telescope may be able to. There’s no need to rush though: there are still lots more planets to simulate!

Citation

“The Phase-curve Signature of Condensible Water-rich Atmospheres on Slowly Rotating Tidally Locked Exoplanets,” Feng Ding and Raymond T. Pierrehumbert 2020 ApJL 901 L33. doi:10.3847/2041-8213/abb941

Illustration of two black holes, each surrounded by an accretion disk, merging.

Could the biggest — literally — gravitational-wave discovery yet be something other than what it initially seemed? A new study suggests that the most massive merger of black holes detected by LIGO/Virgo may have included a surprising lightweight.

Echoes of a Surprising Merger

In May 2019, a collision of two black holes shook spacetime, registering in the LIGO and Virgo gravitational-wave detectors as the heaviest black-hole merger discovered yet. Initial analysis of GW190521 suggested that the participants in this cosmic collision were ~85 and ~66 times the mass of the Sun, and that they formed a final black hole of ~142 solar masses — an unexpectedly heavy outcome that lands in the elusive category of intermediate-mass black holes.

Stellar graveyard Nov 2020

The rapidly expanding “stellar graveyard”, a plot of the masses of the different components of observed compact binary mergers. GW190521, top center, is more massive than any other binary merger we’ve observed. [LIGO-Virgo/Northwestern U./Frank Elavsky & Aaron Geller]

But GW190521 raised eyebrows for another reason as well: the estimated masses of the two merging black holes fell between 65 and 120 solar masses, a region known as the pair-instability mass gap. This range of masses should be inherently off-limits for black holes born from collapsed stars, based on our current understanding of stellar evolution processes.

While there are many hypotheses about how mass-gap black holes could potentially form, two scientists have focused on an alternative angle: what if we were simply wrong in our estimate of GW190521’s component masses?

Checking Our Assumptions

How do we measure component masses from a gravitational-wave signal? Decades of theoretical research have produced a vast array of model signals for mergers with different parameters. By comparing the observed gravitational-wave signal to the various models, we can calculate which ones fit best. But this comparison relies on what are called priors — a set of assumptions that go into the analysis and affect the outcome.

Three plots showing the GW190521 signal at Hanford, Livingston, and Virgo

The observed gravitational-wave signal of GW190521 in each of the three detectors (black), plotted with two best-fit models: one for when the component mass ratio is between 1 and 2 (blue) and one for a mass ratio between 2 and 25 (orange). [Nitz & Capano 2021]

In a recent publication, scientists Alexander Nitz and Collin Capano (Max Planck Institute for Gravitational Physics and Leibniz University Hannover, Germany) reanalyze the gravitational-wave signal for GW190521 using a different set of priors and constraints than the original analysis completed by the LIGO collaboration.

Nitz and Capano find that their analysis admits two possible solutions for GW190521: one similar to that found by the LIGO collaboration — and another, in which the component black holes are ~16 and ~170 solar masses. This second option becomes even more heavily favored when the authors analyze the gravitational-wave signal simultaneously with an electromagnetic flare that may have been associated with the merger.

An Uneven Pair?

What does this outcome tell us? The masses in Nitz and Capano’s second solution both lie outside of the pair-instability mass gap, neatly resolving the paradox previously created by this merger.

If the authors’ interpretation is correct, then GW190521 would represent the first detected intermediate-mass-ratio inspiral — a type of merger in which one component is substantially larger than the other. This signal then provides an exciting milestone and an opportunity to learn more about the different types of dramatic collisions that occur in our galaxy.

Citation

“GW190521 May Be an Intermediate-mass Ratio Inspiral,” Alexander H. Nitz and Collin D. Capano 2021 ApJL 907 L9. doi:10.3847/2041-8213/abccc5

illustration of flow lines with kinks in them streaming off of the sun

Parker Solar Probe

Artist’s illustration of the Parker Solar Probe. A special ApJS issue features around 50 articles detailing early results from this mission. [NASA/Johns Hopkins APL/Steve Gribben]

On its first journey dipping into the Sun’s outermost atmosphere — the extended corona — the Parker Solar Probe detected something unexpected: strange kinks in the streams of plasma flowing off of the Sun. Scientists have since developed a detailed explanation for the unusual behavior the probe found.

A Kink in the Plan

Launched in 2018, the Parker Solar Probe (PSP) is on a mission to orbit the Sun, dipping ever deeper into our star’s outer corona to make measurements of the magnetic field and plasma conditions there. The probe is currently approaching its eighth perihelion pass, where it will graze an altitude just 15 solar radii or so above the Sun’s surface! But the PSP was already making surprising detections on its very first perihelion pass in 2018.

switchbacks

Animation of magnetic switchbacks propagating outward through the corona. [NASA’s Goddard Space Flight Center/Conceptual Image Lab/Adriana Manrique Gutierrez]

The solar wind — a flow of charged particles that streams off of the Sun — travels radially outward into the solar system on large scales. But close to the Sun, the PSP found, the wind’s flow structure is more complex. PSP flew through what are termed switchbacks: S-shaped hitches in the flow lines that propagate outward, representing sudden changes in magnetic field orientation and plasma velocity.

The discovery of this phenomenon put theorists to work. What causes switchbacks, and what might they tell us about the production of the solar wind and plasma processes around the Sun? In a study led by Gary Zank (University of Alabama in Huntsville), a team of scientists present a model that may explain these strange phenomena.

Searching for (Re)connection

three-panel diagram illustrating the stages of interchange reconnection and switchback production

Diagram of the authors’ model for production of switchbacks by interchange reconnection: a) a closed coronal loop and open magnetic field lines approach each other; b) the first field line reconnects, producing an open field line with an S-shaped structure; c) the switchback is launched, propagating both upward and downward, as a new reconnection event begins on the next field line. [Adapted from Zank et al. 2020]

In Zank and collaborators’ model, switchbacks originate high in the solar corona. There, coronal loops — loops of magnetic field anchored in the Sun’s photosphere and arcing up into the corona — extend out to large distances (~6 times the radius of the Sun!). When these loops move near open magnetic field lines in the solar wind, a process called interchange reconnection can occur, in which the magnetic field lines of the coronal loop suddenly break and then reattach themselves to the lines of the open field, forming a lower-energy configuration and releasing a burst of plasma.

The authors derive the mathematical theory describing how a switchback produced from this reconnection would propagate through the inhomogeneous solar corona. The reconnection event launches magnetic field deflections both out into the solar system and down toward the solar surface. The authors show that the most extreme of these deflections take the form of the S-shaped structures that PSP observed as the most dramatic switchbacks.

Matching Theory to Observation

Zank and collaborators demonstrate that their model is both qualitatively and quantitatively consistent with PSP’s observations of switchbacks in its first perihelion pass. Not only does the theory well reproduce a single switchback, but it also shows how interchange reconnection events can occur in quick succession for loops made of multiple magnetic field lines, resulting in clustering of switchbacks consistent with PSP’s measurements.

We’re still in early stages of understanding these strange phenomena, but more help is on the way! PSP continues to amass a vast trove of data as it repeatedly swings through the Sun’s atmosphere — which we can hope will soon bring the mystery of switchbacks to a close.

Citation

“The Origin of Switchbacks in the Solar Corona: Linear Theory,” G. P. Zank et al 2020 ApJ 903 1. doi:10.3847/1538-4357/abb828

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