Features RSS

illustration of the Milky Way

Editor’s note: In these last two weeks of 2021, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

A Kiloparsec-scale Molecular Wave in the Inner Galaxy: Feather of the Milky Way?

Published November 2021

Main takeaway:

New observations from a team led by Veena Vadamattom Shaji (University of Cologne, Germany) have revealed a long, skinny cloud of molecular gas in the Milky Way. This is the first structure discovered in our galaxy analogous to the wispy gas filaments called “feathers” that emerge nearly perpendicularly from the spiral arms of other galaxies.

Why it’s interesting:

diagram of the Milky Way's spiral arms

The location of the Gangotri wave (green) on a model of the Milky Way’s spiral arms. [Veena et al. 2021]

Like trying to map a forest when you’re surrounded by trees, it’s hard to discern the structure of our galaxy when we’re tucked away inside it. Over time, our understanding of the Milky Way’s structure has coalesced into a spiral with four major arms and a central bar, plus several smaller arms and spurs. The newly discovered gas cloud, named the Gangotri wave after the glacier that feeds the Ganges River in India, contains roughly 9 million solar masses of gas, dust, and stars, and it stretches at least 6,500 light-years across. Based on the velocity of the cloud, the Gangotri wave is likely either a subbranch of the Norma arm or a filament connecting two arms.

What’s causing this feathery feature:

Although spiral-arm feathers have been explored in models, there isn’t yet a consensus on how these structures arise. Potential causes include self-gravity, shear from the Milky Way’s rotation, and instabilities like the wiggle instability. Whichever model prevails must account for the Gangotri wave’s location as well as its curious morphology: the gas filament appears to have a sinusoidal-wave-like structure in the direction perpendicular to the galactic plane, with an amplitude of 220–650 light-years. The gas, dust, and stars within the Gangotri wave all seem to follow this sinusoidal pattern, suggesting that gravitational instabilities are the likeliest cause.

Citation

V. S. Veena et al 2021 ApJL 921 L42. doi:10.3847/2041-8213/ac341f

illustration of an asteroid

Editor’s note: In these last two weeks of 2021, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

The Surface of (16) Psyche from Thermal Emission and Polarization Mapping

Published August 2021

Main takeaway:

A team led by Katherine de Kleer (California Institute of Technology) used the Atacama Large Millimeter/submillimeter Array (ALMA) to map the surface of asteroid (16) Psyche. These observations suggest that Psyche’s surface is no less than 20% metal, but the data lack the expected polarization signature of a metal-rich material.

Why it’s interesting:

Psyche has long been known to astronomers — the prefix “16” indicates that it was the sixteenth minor planet ever cataloged — but only in the past few decades has its intriguing nature been revealed; the extremely dense 220-km-wide asteroid is thought to be the core of a long-lost protoplanet. Although observations by de Kleer and collaborators indicate that Psyche’s surface is metal rich — likely in the form of iron oxides — the metal might be concentrated in dense inclusions that strongly scatter light waves. If this is a common trait among metal-rich asteroids, their emission may be less polarized than that of metal-poor asteroids, completely opposite what has been theorized.

photograph of a spacecraft

This July 2021 photograph shows the Psyche spacecraft in a testing bay at the Jet Propulsion Laboratory. [NASA/JPL-Caltech]

Why you should be psyched:

Psyche will soon be visited by a NASA mission of the same name, which is scheduled to launch in August 2022. After launch we won’t have to wait too long — in an astronomical sense, anyway — for the spacecraft’s arrival; after a quick flyby of Mars, the spacecraft will reach its destination in 2026. The mission aims to discern the origin of this strange asteroid and give us our first look at a world made of metals rather than rock or ice.

Citation

Katherine de Kleer et al 2021 Planet. Sci. J. 2 149. doi:10.3847/PSJ/ac01ec

white light and ultraviolet image of the Sun

What happens when high-energy particles from hundreds of light-years away enter our solar system? Today’s article details a clash between galactic cosmic rays and explosions of solar plasma.

A Journey Through the Solar System

cosmic rays

Artist’s impression of the shower of particles caused when a cosmic ray, a charged particle produced by an astrophysical source, hits Earth’s upper atmosphere. [J. Yang/NSF]

High-energy particles called galactic cosmic rays — mostly super-speedy protons, helium nuclei, electrons, and positrons — are thought to be accelerated in sources like distant supernovae. Some of these particles pass through our solar system, where they can cause bursts of ionization in Earth’s atmosphere and damage spacecraft electronics; they may even cause the flashes of light that sometimes cross the vision of astronauts.

Scientists have observed sudden decreases in the cosmic-ray flux when magnetized plasma explodes from the Sun’s upper atmosphere in a coronal mass ejection. These decreases in the cosmic-ray flux — dubbed Forbush decreases after the scientist who discovered them — may help us understand how galactic cosmic rays navigate the maze of magnetic fields in our solar system. Up until now, scientists have mostly used ground-based detectors to observe these interactions indirectly, but a new study has turned to an unlikely sounding spacecraft for help: the Dark Matter Particle Explorer.

plot of normalized flux over time

Example of a Forbush decrease observed by DAMPE. The 2.08–2.26-GeV cosmic-ray flux (green) decreases while the total number of particles, including energetic particles from the Sun (magenta), increases. A fit to the data is shown in the dashed purple line. The blue symbols show data from a ground-based detector. [Adapted from Alemanno et al. 2021]

Not Just for Dark Matter

The Dark Matter Particle Explorer (DAMPE) was designed to detect high-energy particles and photons that might be produced when dark matter particles decay. From its vantage point in orbit around Earth, it can detect particles with a wide range of energies, making it a great tool for studying Forbush decreases.

The DAMPE collaboration selected data from September 2017, one of the most active periods of the previous solar activity cycle. The team monitored the flux of particles over time, separating the galactic cosmic rays from the particles originating from the Sun. They identified several particle events, consisting of a jump in the number of particles from the Sun coupled with the characteristic sharp dip and slow rise of a Forbush decrease in the cosmic-ray flux.

Deciphering Forbush Decreases

multiple plots of relative count rate versus date

Forbush decreases observed by DAMPE at different energies. The solid lines show the model fits. Click to enlarge. [Alemanno et al. 2021]

The team found that cosmic rays with lower energies showed deeper decreases and took longer to recover to normal levels than cosmic rays with higher energies, though the recovery time might be constant for the highest-energy cosmic rays. The authors showed that their model, which describes the transport of charged particles across an obstacle or barrier, reproduced the data well overall, but it couldn’t account for some of the detailed behaviors seen at the low- and high-energy extremes.

The authors also highlighted the benefits of using DAMPE rather than ground-based detectors. In addition to the improved energy resolution, DAMPE was able to identify small particle events that went unnoticed by ground-based detectors. Hopefully, DAMPE will continue to help us understand how cosmic rays travel through our solar system for many years to come (in addition to detecting dark matter, of course!).

Citation

“Observations of Forbush Decreases of Cosmic-Ray Electrons and Positrons with the Dark Matter Particle Explorer,” Francesca Alemanno et al 2021 ApJL 920 L43. doi:10.3847/2041-8213/ac2de6

artist's rendition of the Imaging X-ray Polarimetry Explorer (IXPE) spacecraft with Earth in the background

It’s always exciting when a new spacecraft launches. What can we expect from the new X-ray mission from NASA and the Italian Space Agency that leapt into orbit around Earth last week?

A Brief History of X-Ray Astrophysics

a rocket prepared for launch

The eighth Orbiting Solar Observatory — the first and only mission to definitively detect the polarization of an astrophysical source — launched in 1975. [NASA]

Scientists first detected X-rays from an astrophysical source in 1962 during a short sounding rocket flight above Earth’s atmosphere. A decade later, subsequent rocket experiments hinted that astrophysical X-rays might be polarized, and that the specific orientation of the X-ray photons might reveal a wealth of information about these distant sources. It wasn’t until 1978 that these suspicions were confirmed, when the eighth Orbiting Solar Observatory mission detected polarized emission from the Crab nebula and placed upper limits on the emission from several other sources. Today, the vast landscape of polarized X-ray emission remains largely unexplored.

To understand astrophysical X-ray sources like pulsars, black holes, and magnetars more fully, we need more than just fleeting glimpses from sounding rockets skimming Earth’s atmosphere or data collected as part of another mission. Luckily, the Imaging X-ray Polarimetry Explorer (IXPE) — the first small satellite mission to tackle X-ray polarimetry — is poised to deliver these data during its two-year primary mission (and hopefully many years more!).

cartoon of the detector components

This schematic shows the components of the gas pixel detector. Click to enlarge. [Soffitta et al. 2021]

Gaseous Detector

Detecting X-rays from cosmic sources is challenging — capturing these high-energy photons requires creative engineering, like concentric shallow-angled mirrors that funnel X-rays toward a detector — and measuring the polarization of an X-ray source is harder still. The IXPE mission aims to do just that, using three identical detectors with a design far removed from a typical telescope.

To study the polarization of incoming X-rays, IXPE leverages the photoelectric effect — the ejection of electrons from a material exposed to light. Specifically, IXPE uses a gas pixel detector, which contains a mixture of helium and dimethyl ether in a 1–2-cm-tall gas cell. Here’s how it works: an X-ray photon passing through the instrument knocks an electron out of an atom or molecule in the gas, and this electron is launched out with enough energy to collide with other gas particles and free more electrons. Since the first electron’s trajectory depends on the direction of polarization of the incoming X-ray photon, detecting this trail of electrons allows us to work out the photon’s polarization.

New Window Into the Universe

composite X-ray, optical, and infrared image of the crab nebula

The Crab nebula — a remnant of a supernova that exploded in 1054 — was one of the first sources suspected of emitting polarized X-rays. Supernova remnants are just one of the many objects that will be studied by IXPE. [X-ray: NASA/CXC/SAO/F.Seward; Optical: NASA/ESA/ASU/J.Hester & A.Loll; Infrared: NASA/JPL-Caltech/Univ. Minn./R.Gehrz]

What might we learn from the IXPE mission? The list of topics likely to be illuminated by X-ray polarimetry reads like a wishlist for high-energy and theoretical astrophysicists: magnetic fields and emitting geometries of compact objects like neutron stars; acceleration processes in supernova remnants; scattering in accretion disks around dense stellar remnants; tests of Einstein’s general theory of relativity and calculations of black-hole spins; and measurements important to theories of quantum gravity.

With the spacecraft already in place in orbit around Earth, all we can do now is wait. Stay tuned for IXPE’s first observations in January 2022!

Citation

“The Instrument of the Imaging X-Ray Polarimetry Explorer,” Paolo Soffitta et al 2021 AJ 162 208. doi:10.3847/1538-3881/ac19b0

Optical image of the Pleiades star cluster

Like a small child sorting toy cars, astronomers are intent on organizing things into groups, from types of galaxies to types of stars. A team of astronomers set out to try to see if newly discovered star groups are actually part of previously known clusters by sorting them by their speed and location.

Clustering Together and Interacting

Stars are born when giant clouds of gas and dust become so massive that they collapse in on themselves. If a cloud is large enough, it can form multiple stars with comparable ages, and those stars can travel through space together at similar velocities. These groups of stars can stay gravitationally bound to each other as an open cluster, or they can dissociate and travel through space together as a moving group or a stellar association. Groups of stars that traverse the galaxy together can also be the result of an interaction between two groups of stars.

A team of astronomers led by Jonathan Gagné (Rio Tinto Alcan Planetarium, Canada) looked at roughly a dozen stellar groups to determine whether they’re associated with known structures such as open clusters. These results could open the door to new interpretations of how young stellar groups form and evolve.

Ageing the Systems

A graph showing color against absolute magnitude. The horizontal axis (G - G_RP) goes from 0.0 to ~1.5 and the vertical axis (absolute M_G) goes from ~14 to 0. Different colored lines indicate ages and different symbols indicate different Theia groups. All of the lines and symbols for an approximately linear trend, going from the top left of the diagram (absolute M_G ~0 and G-G_RP ~0) to the bottom right (absolute M_G ~14 and G-G_RP ~1.5)

A color–magnitude diagram showing the color of the star against its brightness. The different colors of lines show clusters with different ages. By plotting the new clusters on this map, the team could pinpoint the age of each Theia group. [Gagné et al. 2021]

The team focused on the so-called Theia groups, a large set of stellar structures recently identified within a distance of 10,000 light-years. To determine whether these new stars belong to known groups, the team needed to get a better estimate of their ages. To do this, the authors used data from the Gaia Data Release 2, which was specifically designed to find stars traveling together by measuring their position and how fast they are moving through space.

By using reference stars from five groups of different known ages, Gagné and collaborators fit each Theia group on a color-magnitude diagram, which plots brightness against color. Because the known clusters follow certain trends, placing the new groups on the diagram allowed the authors to find which cluster, if any, the Theia groups were associated with. From this, the authors could estimate how old the groups are.

Sorting into Categories

Top plot: velocities of the Theia groups. Most are clustered around the same place but Theia 369 extends vertically in both directions from the group, and Theia 301 extends to the right (higher positive velocity). Bottom: locations of all of the clusters; the Sun is right in the middle of the AB Doradus moving group, the Theia 369 surrounds the Pleiades, Theia 301 extends like a tail beyond the AB Doradus moving group and Theia 234 beyond that, and Theia 368 is below the AB Doradus moving group.

An example of how the team plotted their data for the Pleiades system. Left: the spatial extent of various groups compared to the Pleiades, showing Theia 369 is likely to be associated with it. Right: the velocities of the various clusters. This shows that Theia 369 contains members of the Pleiades and Theia 301 may constitute its trailing tail. [Adapted from Gagné et al. 2021]

When presented with a group of toy cars, a child might sort them by model and color. In the case of Gagné and collaborators’ stellar clusters, once their ages were determined, the team sorted them by how fast they are moving and their sky location. The authors plotted the velocities and spatial distributions of both the Theia groups and known structures, and they found that many of the moving groups and Theia structures are extensions of larger associations of stars or open clusters. This means that these groups are more extended than previously thought, and some might consist of tails that are co-moving with the cores of these clusters.

Some groups may have similar velocities but aren’t physically close to each other, such as Theia 301, which may be extensions of current clusters. For example, Theia 301 and AB Doradus may be trailing tidal tails of the Pleiades. Additional data from Gaia and other missions will provide better constraints on velocities and ages so these groups can properly be sorted where they belong!

Citation

“A Number of Nearby Moving Groups May Be Fragments of Dissolving Open Clusters,” Jonathan Gagné et al 2021 ApJL 915 L29. doi:10.3847/2041-8213/ac0e9a

a glowing sphere is surrounded by magnetic field lines. a bright burst of energy extends from one side of the sphere

On Christmas day in 2010, the Swift Observatory detected gamma rays from a mysterious source for a whopping 28 minutes. More than a decade later, astronomers are still theorizing about the cause of this unusual gamma-ray burst.

a glowing central supernova is surrounded by a cloud of gas and dust ejected by a red giant, and two jets of material emerge from the north and south poles

Digital rendering of the aftermath of a collision between a neutron star and a red giant, which is one possible explanation for GRB 101225A. [NASA/Goddard Space Flight Center]

An Intriguing Event

GRB 101225A, dubbed the “Christmas burst” because of its holiday debut, has inspired plenty of theories due to its unusually long-duration gamma-ray emission — and its unexpectedly brief, flaring X-ray emission. The proposed causes range from long-distance (a neutron star billions of light-years away being engulfed by a red-giant companion) to local (a neutron star 10,000 light-years away snacking on a comet). Further hypotheses include a collapsing ultra-low-metallicity blue supergiant star or even a never-before-seen class of astrophysical phenomena.

The authors of today’s article put forward a new hypothesis: what if GRB 101225A’s unusual emission comes from something we’ve seen before — but viewed from a different angle?

optical and X-ray flux as a function of time for three potential off-axis gamma-ray bursts

X-ray and optical afterglow (AG) emission from GRB 101225A (black and orange symbols, respectively) as well as two other events potentially involving off-axis jets (blue and pink symbols). The solid lines show the results of the jet model, while the dashed lines are empirical fits to the data. The x-axis indicates the time since the Burst Alert Telescope (BAT) observations began. Click to enlarge. [Zou et al. 2021]

Modeling Multiwavelength Emission

The motivation for this theory, developed by Le Zou (Guangxi University, China) and collaborators, came from observations of a more recent gamma-ray burst: GRB 170817A. The gravitational-wave signal that preceded this gamma-ray burst indicated that it resulted from two neutron stars colliding, potentially leaving behind a massive newborn magnetar — a neutron star with an extremely strong magnetic field — with a relativistic jet. Further analysis of the event’s multiwavelength emission suggested that the jet was viewed at an angle, potentially as much as a few tens of degrees off center.

Zou and coauthors explored the possibility that GRB 101225A was also caused by a newly formed magnetar with a jet, and its unusual gamma-ray and X-ray signatures arise as a result of our off-axis view.

To test this theory, the team modeled the emission from a relativistic jet colliding with the surrounding interstellar medium and compared their results to the observations. The authors find that the optical data are consistent with the afterglow produced by a narrow jet viewed from just a few degrees off center.

A More Detailed View

X-ray flux over time. the observations and data both show semi-periodic behavior, but do not match exactly except at a few timesteps

Observed (black) and modeled (red) X-ray flux from GRB 101225A. [Adapted from Zou et al. 2021]

The authors suggest that their model also explains the flaring behavior seen in GRB 101225A’s X-ray emission hours after the onset of the event. If GRB 101225A is caused by a magnetar, we should see semi-periodic oscillations in the X-ray flux caused by the precession of the magnetar’s rotation. The team’s X-ray model roughly reproduces the observed flares with periods of 250 and 488 seconds, suggesting that the flaring behavior could be due to the precession of a magnetar.

The team’s results are still tentative, and more data could help tease out the cause of this unusual event. Much like the observations of gravitational waves from GRB 170817A enhanced our understanding of that event, so will gravitational-wave observations of future GRB 101225A-like events. The team estimated that current gravitational-wave observatories aren’t sensitive enough to detect a GRB 101225A-like event, but future observatories should be. As we learn more about this varied class of events, we should be able to refine our understanding and put our many theories to the test.

Citation

“GRB 101225A as Orphan Dipole Radiation of a Newborn Magnetar with Precession Rotation in an Off-axis Gamma-ray Burst,” Le Zou et al 2021 ApJL 921 L1. doi:10.3847/2041-8213/ac2ee4

dark dust lanes spiral around the glowing central bulge of M106

How do galaxies form stars for billions of years without running out of gas? Theory suggests that galaxies siphon material from the tenuous intergalactic medium or nearby galaxies, but it’s challenging to detect the cold gas trails that would bolster this theory. A new article details observations that may explain how a Milky Way-like galaxy gets its gas.

Sustaining Star Formation

FAST radio telescope

The 500-m FAST radio dish, built into a natural basin in southwest China. [Xinhua/Ou Dongqu]

Across the universe, galaxies are making stars long after their star-forming days should have ended, and it’s not always clear why. When a galaxy in a dense cluster environment consumes another galaxy, the influx of gas can spur a new rush of star formation — but are collisions the only way for a galaxy to gain star-forming material?

Another possibility — and one that applies to more isolated galaxies as well as their cluster counterparts — is that galaxies can steal gas from small satellite galaxies, even from a great distance, which would create long trails of cold, low-density hydrogen gas. A team of astronomers led by Ming Zhu (National Astronomical Observatories, Chinese Academy of Sciences) searched for streams of neutral hydrogen gas using the Five-hundred-meter Aperture Spherical radio Telescope (FAST), the world’s largest single-dish radio telescope.

hydrogen density map with density contours connecting M106 to NGC 4288. several other satellite galaxies are present, including NGC 4248 and S5

FAST hydrogen density map of the M106 environment with density contours (blue lines). The green lines are density contours from the Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) Survey. [Adapted from Zhu et al. 2021]

A Northern Sky Survey

Zhu and collaborators scanned the northern sky on a hunt for the tell-tale gas trails that can signal a galaxy accreting material. In a new publication, the team reported on their discovery of a trail of hydrogen gas connected to spiral galaxy Messier 106. Messier 106, also known as M106, has a similar luminosity and structure to the Milky Way, but somewhat more massive satellite galaxies.

Twelve hours of data — the deepest radio observations of this system yet — revealed a filamentary trail of gas stretching at least 424,000 light-years between M106 and a likely satellite galaxy, NGC 4288. This gas filament could be a sought-after sign that M106 is accreting gas from a satellite over a great distance, but it could also be evidence of a tidal tail caused by a galaxy pulling gas away from M106 as it swings by. Which scenario is more likely?

Tidal Tail or Accretion Stream?

ultraviolet images show two lines of star clusters off the southern edge of one of M106's spiral arms

Left: Neutral hydrogen density contours overlaid on a near-ultraviolet image of M106. Right: Zoomed-in image of the area contained in the black rectangle with far-ultraviolet data in blue and near-ultraviolet data in yellow. Click to enlarge. [Zhu et al. 2021]

Zhu and collaborators point out a few key factors that support the accretion stream hypothesis: the gas filament doesn’t appear to contain any stars, it doesn’t sweep outward along a spiral arm, and it lacks the characteristic density gradient of a tidal tail. One of the strongest pieces of evidence that M106 is actively accreting gas is the presence of young star clusters where the filament attaches to the galaxy — evidence that star formation has already been sparked by infalling gas.

Follow-up observations will be needed to confirm the accretion-stream hypothesis, including verification of NGC 4288’s satellite galaxy status. One thing is certain: the team’s results highlight the importance of using deep radio observations to study interactions between galaxies, since optical images gave little to no sign that M106 and NGC 4288 were engaged in a gaseous tug-of-war.

Bonus

Check out this video from the authors’ article, which shows a rotating three-dimensional view of their FAST data cube. East and north indicate the right ascension and declination of the target, respectively, while the z direction shows the velocity of the gas.

Citation

“FAST Discovery of a Long HI Accretion Stream toward M106,” Ming Zhu et al 2021 ApJL 922 L21. doi:10.3847/2041-8213/ac350a

eight images of Venus passing in front of the Sun, which has sunspots scattered across its surface

Stars throughout the galaxy eject ultra-powerful stellar flares, but we enjoy relatively mild solar weather. Studying when and where stellar flares arise may provide the key to figuring out if the Sun is unusually calm compared to other stars like it — or if extreme solar flares are in our future.

Stars Near and Far

a large, dark sunspot surrounded by a pattern of solar granules

An extreme close-up of a sunspot taken by the Daniel K. Inouye Solar Telescope in Hawai’i. [NSO/AURA/NSF]

We’ve observed many stars emitting superflares — outbursts of light across the electromagnetic spectrum that are roughly 10,000 times more energetic than a typical solar flare. It’s unknown why many Sun-like stars have superflares while our own star does not, but solar flares and stellar superflares are both thought to be associated with relatively cool and dim regions where a star’s magnetic field lines poke through its surface — known as sunspots or starspots.

How can we study starspots on a star hundreds of light-years away? This is where exoplanets can play an important role: as an exoplanet passes in front of its parent star, it can overlap with a starspot. Because starspots are cooler than the surrounding stellar surface, the star’s light curve will show a small bump when the planet obscures it. By fitting models to the observed variations in the light curve, we can estimate the properties of the occulted starspots — and test whether larger starspots are responsible for more energetic superflares.

synthetic star disk with the transit latitudes of the three planets marked, plus an example light curve of a transit of one of the planets

Top panel: Synthesized disk image with Kepler-411’s three planets marked. Bottom panel: Light curve for a sample planetary transit with the best-fitting model in red. The vertical dotted line marks the location of the starspot in the transit light curve. Click to enlarge. [Araújo & Valio 2021a]

Kepler Eyes a Stellar System

Alexandre Araújo and Adriana Valio (Mackenzie Presbyterian University, Brazil) applied this technique to Kepler-411, a slightly smaller and cooler star than the Sun, to understand the connection between its abundant starspots and frequent superflares. Kepler-411 has three confirmed transiting planets — one super-Earth and two mini-Neptunes — which allows for spot detection at the latitudes at which the planets transit: −11°, −21°, and −49°.

During 590 days of Kepler observations, Kepler-411’s three planets collectively made 176 trips across its disk, passing over 198 starspots in total. Araújo and Valio estimated that the starspots averaged 34,000 km in diameter — about the width of Earth, Venus, and Mars placed side by side. The team also spotted 65 superflares in the Kepler light curves, the most energetic of which was about 200 times stronger than the strongest solar flare ever observed.

A Potential Correlation

starspot areas and superflare energies as a function of time. there is a spike in both quantites around day 1400

Starspot areas (top panel) and superflare energies (bottom panel) are shown in blue asterisks, with the 21-day running average of each quantity overplotted in red. [Araújo & Valio 2021]

Do larger starspots produce stronger stellar flares? Perhaps: the authors found a correlation between starspot area and flare energy, but it depended on how the measurements were averaged. The correlation emerged when the data were averaged over periods of 16–35 days, with the strongest correlation at 21 days — almost exactly two stellar rotation periods — but the correlation disappeared or reversed when averaged over other time periods. The authors suspect that this time period may be related to the typical lifetime of Kepler-411’s starspots.

Further work is needed to understand the connection between starspots and superflares, since this technique only allows us to measure starspots at a few latitudes, while flares can arise from any active region on the star. If we’re really lucky, we’ll spy an exoplanet passing in front of a starspot during a flare, which will allow us to pinpoint the flare’s exact location.

Citation

“Kepler-411 Star Activity: Connection between Starspots and Superflares,” Alexandre Araújo and Adriana Valio 2021 ApJL 922 L23. doi:10.3847/2041-8213/ac3767

cooled black lava is crossed by bright cracks showing hotter lava below. gas billows from multiple bright areas.

Say you’ve measured an exoplanet’s mass and radius. How do you know if this distant planet is a waterlogged ocean world or a desiccated desert? Astronomers turn to increasingly sophisticated models of planetary interiors to answer this question.

A Case of Planetary Magmatism

a cutaway model of Earth's interior, with the layers labeled. from the outside to the inside: crust, upper mantle, mantle, outer core, inner core

A diagram of Earth’s interior. Earth’s mantle is mostly solid, but it can melt into magma and be extruded onto the surface as lava through volcanoes. [NASA (Adapted from Goddard Media Studios)]

To figure out how much water an exoplanet harbors, astronomers use models to convert observed properties like the planet’s mass and radius into estimates of its composition, including how much water it contains. It’s equally important to understand where an exoplanet keeps its water: in its atmosphere, on its surface, or sequestered deep in its interior. A new article explores the possibility that water-rich planets might hide their water deep in their interiors, dissolved in an ocean of magma.

If an exoplanet orbits close to its parent star or has a thick atmosphere that traps heat, its mantle — the layer between the core and the surface — might be magma rather than solid rock. When water dissolves into this molten material, it seeps into the planet’s interior and reduces the supply of surface water. In today’s article, Caroline Dorn (University of Zurich, Switzerland) and Tim Lichtenberg (University of Oxford, UK) explore the effects of global magma oceans on the observed and inferred properties of exoplanets.

Modeling Molten Mantles

Dorn and Lichtenberg tested the effects of different planetary interiors by modeling exoplanets with iron cores, silicate-rich mantles, and an outer layer of pure water in the form of ice, liquid, or steam. The team solved fluid mechanics equations to determine the equilibrium states of planets with three different interior structures:

cartoon of the three planetary interior models

Schematic of the three models used in this work. [Dorn & Lichtenberg 2021]

  • A: mantle is dry, rocky, and solid throughout
  • B: mantle contains melted regions that do not sequester water (dry melt–solid interior)
  • C: mantle contains melted regions that do sequester water (wet melt–solid interior).

The authors found that a planet’s interior structure has a discernible effect on its size. For planets of the same mass, those with wet melt–solid interiors can be as much as 16% smaller than those with dry melt–solid interiors or rocky interiors. This can be explained on the molecular level: water molecules nestled snugly between silicate molecules in the mantle increase a planet’s radius less than freewheeling water molecules in the atmosphere or on the surface.

How Much Water?

inferred water mass fraction as a function of planetary radius, mass, and interior model

Inferred water mass fraction for a range of planet masses for each of the authors’ three models for the planets’ interiors. [Dorn & Lichtenberg 2021]

Dorn and Lichtenberg also tested how our estimates of a planet’s water content depend on the choice of interior model. They found that the inferred amount of water can vary by up to an order of magnitude based on the choice of model, with wet melt–solid planets resulting in more water-rich estimates than dry melt–solid planets or rocky planets. Their results underscore the important difference between a planet’s bulk water content and its surface water content, a distinction that future models and observations may help to illuminate.

Additionally, these results raise interesting questions about a planet’s supply of water over long timescales; with water safely contained beneath the planet’s surface rather than settled on its surface or suspended in its atmosphere, it may be less likely to be lost to space.

Citation

“Hidden Water in Magma Ocean Exoplanets,” Caroline Dorn and Tim Lichtenberg 2021 ApJL 922 L4. doi:10.3847/2041-8213/ac33af

A volcanic eruption appears on the limb of Jupiter's moon Io

What’s going on under the surface of the most volcanically active body in the solar system, and what’s driving its massive plumes? Observations of hot spots on Jupiter’s moon Io reveal more detail than ever before and may help us get closer to solving the mysteries of this volcanic world. 

Tidal Heating 

Volcanoes and geological activity are not unique to Earth but are common in the solar system. One of the most volcanically active bodies in the solar system is Jupiter’s moon Io. Io gets stretched and squeezed as it circles the gas giant. Just as Earth’s oceans react to the pull of the Moon, Io’s crust reacts to the pull of Jupiter in a process called tidal heating. The surface bulges up and down by as much as 100 meters during its rotation, and this tidal heating forms the dynamic surface of Io. 

Hot spots on Io with Europa slightly covering it

An infrared image of the beginning of the occultation of Io by Europa. The dark area on the left is Europa and the hotspot regions are the pateras, which are broad, bowl-shaped volcanic craters. [de Kleer et al. 2021]

Fine-Detailing the Surface 

In order to study the volcanic processes of Io in detail, very high spatial resolution is needed. Until now, astronomers have only been able to distinguish one volcano from the other. A team led by Katherine de Kleer at the California Institute of Technology looked at four volcanoes on Io’s surface using a new method and was able to map the moon’s volcanic emission in more detail than ever before.  

Occultation Localization 

The team observed Io with the Large Binocular Telescope Interferometer, located in southeastern Arizona, during an occultation by Jupiter’s other moon, Europa. As Europa passed in front of Io, de Kleer’s team monitored the volcanic moon’s brightness to pinpoint the moment Europa obscured each hot spot. A step-like pattern in the light curve corresponds to the disappearances and reappearances of the hot spots, which can pinpoint the locations of emission.

Motion of Europa's shadow across Io

The motion of Europa (dark shadow) across Io’s face during the occultation observed by de Kleer et al., with the volcanoes of interest labeled on Io’s surface. [de Kleer et al. 2021]

The authors observed the light curves of four regions — Loki Patera, Pillan Paterna, Kurdalagon Patera, and areas of emission on Ulgen Patera and N Lerna Region — and were able to map where the emission comes from for all four. They also took into account previous observations of Pillian and Kurdalagon Patera eruptions, which helped them understand the time evolution of these sources. By combining previous observations with new occultation data, they were able to better localize the emission regions, and even discovered that Kurdalagon Patera has emission arising from two separate areas.  

Graphs of the time vs. intensity of the Kurdalagon Patera hotspot

The light curve of Kurdalagon Patera during the occultation with the best fit models. The evidence for the two emitting areas is shown in the beginning (∆t = 122-126) and end (∆t = 131-133) of the graph. [de Kleer et al. 2021]

Localizations of the hot spots will help astronomers better understand where Io’s emission is coming from, and help them understand the exact nature of the volcanism. These observations mark the highest resolution study of these volcanoes — resolution that can only usually be obtained by Io flybys. High quality ground-based observations like those used in this study are rare but offer unique opportunities to study Io’s surface, helping us get closer to solving the mysteries of this dynamic moon. 

Citation 

“Resolving Io’s Volcanoes from a Mutual Event Observation at the Large Binocular Telescope,” K. de Kleer et al 2021 PSJ 2 227. doi:10.3847/PSJ/ac28fe 

1 … 40 41 42 43 44 … 111