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Artist's rendition of the Parker Solar Probe getting very close to the Sun

What did the Parker Solar Probe observe when it got closer to the Sun than any spacecraft has ever ventured? Astronomers peer into the solar magnetic field to find out. 

Probing the Sun’s Surface 

Obtaining high-quality observations of the Sun has been important to astronomers for centuries, in large part because the Sun is capable of unleashing space weather events that not only produce the spectacular northern lights, but can also totally knock out our power grids and GPS systems. Therefore, understanding the Sun’s atmosphere and magnetic fields, where these weather events originate, is key. 

Image of the solar corona as seen during a solar eclipse

The solar corona as seen during a solar eclipse. [NASA/Aubrey Gemignani]

The Parker Solar Probe, which launched in 2018, completed its closest encounter with the Sun last month, entering its corona (the hot, tenuous upper atmosphere). This marked the closest any spacecraft has ever gotten to the solar surface. The mission’s primary goal is to fly into the corona and sample particles and magnetic fields to get a better understanding of what drives the solar wind. A group of plasma physicists has now used these data to analyze the magnetic fields and plasma waves in the Sun’s atmosphere in unprecedented detail.  

 

Waving Hello to Results at Small and Large Scales 

Graphs showing the normal, tangential, and radial components of the magnetic field and velocity of particles within the solar corona on two different days. In general, they decrease very slightly over time

The normal, tangential, and radial components of the magnetic field and velocity of particles observed by the Parker Solar Probe on two different days. Click to enlarge. [Zhao et al. 2021]

A group led by Lingling Zhao from The University of Alabama in Huntsville, focused on properties of different types of plasma waves in the corona. Plasma waves are present everywhere in our solar system, from Jupiter’s magnetosphere to the scorching solar atmosphere. These waves transfer energy and momentum and can heat and cool plasma.

The team looked at two scales: magnetohydrodynamic (which treats the plasma as a conducting fluid) and ion-kinetic (which treats the plasma as a collection of individual particles). Using Parker Solar Probe’s observations, the authors analyzed the flow of particles at these scales and found that different types of waves occur at each scale. The presence of waves at both large and small scales supports specific models of magnetic fluctuations and instabilities within the solar system, especially close to the Sun, which could lead to a better understanding of the solar wind. The authors’ work helps to home in on how the solar wind accelerates from a gentle breeze near the solar surface to the 400-kilometer-per-second gale seen near Earth.

The Parker Solar Probe has allowed scientists to get a closer glimpse of the solar surface than ever before. Future measurements with the Parker Solar Probe and the Solar Orbiter, a satellite launched in February 2020 to study the inner heliosphere and solar wind, will give more insight into the solar wind under different conditions (such as different temperatures and speeds) and will allow us to get a better handle on what heats the solar wind. 

Bonus

Check out this video from NASA/the Johns Hopkins Applied Physics Laboratory showing the Parker Solar Probe during its closest encounter with the Sun. Streaks in the gif are coronal streamers, which are part of the magnetic field of the Sun and are usually only seen from Earth during solar eclipses. The Milky Way can be seen in the background.

Citation  

“MHD and Ion Kinetic Waves in Field-aligned Flows Observed by Parker Solar Probe,” L.-L. Zhao et al 2021 ApJ 922 188. doi:10.3847/1538-4357/ac28fb 

magnetar

Astronomers are starting to close in on the origins of fast radio bursts — powerful, fleeting flashes of radio waves seen at extragalactic distances. Highly magnetized neutron stars called magnetars might be responsible for many of these far-flung events, but how exactly do these extreme objects generate fast radio bursts?

A Cosmic Conundrum

radio and X-ray image of a magnetar in a supernova remnant

X-ray and radio observations of the magnetar associated with the first fast radio burst seen in our galaxy. The magnetar is the bright blue X-ray source in the center of the supernova remnant. [Zhou et al. 2020]

The mystery of where fast radio bursts come from — some of them, anyway! — seemed to be solved when the first radio burst within the Milky Way was found to come from a magnetar — an ultra-dense stellar remnant with a magnetic field roughly 100 billion to 10 trillion times stronger than a typical refrigerator magnet. But as is so often the case in astrophysics, solving one puzzle prompted many more: how do magnetars generate these strong, brief bursts of radio waves, and do they arise from near the magnetar’s surface or from material surrounding it?

In today’s article, Andrei Beloborodov (Columbia University and Max Planck Institute for Astrophysics, Germany) explored whether a fast radio burst generated close to the surface of a magnetar could escape the confines of its magnetosphere — the region of space where charged particles bend to the will of the magnetar’s intense magnetic field.

illustration of the size of jupiter's magnetosphere

Illustration of the size of Jupiter’s magnetosphere on the night sky. Although a magnetar’s magnetosphere is physically smaller, its magnetic fields are far stronger. [NASA/ESA]

Magnetar Models

Magnetospheres have great importance within our solar system — Earth’s magnetosphere shields us from energetic particles generated by the Sun, and Jupiter’s magnetosphere would dominate the sky if our eyes were adapted to see radio wavelengths. The magnetosphere of a magnetar, though, is far stranger than these nearby examples; the magnetized neutron star at the center generates a plasma of electrons and their positively charged counterparts, positrons, that fills the magnetosphere and might prevent a radio burst generated near a magnetar’s surface from escaping into space.

Beloborodov used plasma physics equations to understand how a radio burst might interact with the charged particles and magnetic fields within a magnetar’s magnetosphere. As the radio burst travels outward, it compresses the magnetosphere, transferring its momentum to the magnetic fields and the plasma. The oscillating electrons and positrons emit gamma rays that can collide to produce even more electrons and positrons — creating a cascade of particles and gamma rays that scatter the radio wave and sap its energy. For the radio wave, there’s no escape; Beloborodov found that it’s extremely unlikely that a fast radio burst could escape the magnetosphere’s grasp if it were generated within 100,000 km of the magnetar’s surface.

More to Learn

CHIME

Photograph of the CHIME radio telescope in British Columbia. [Andre Renard/Dunlap Institute/CHIME Collaboration]

While Beloborodov’s findings rule out the possibility of fast radio bursts arising from within the inner magnetosphere, there are other ways that magnetars can power these events. Another possibility is that fast radio bursts form much farther from the magnetar where magnetospheric flares collide with the wind flowing out from the magnetar.

Luckily, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) is poised to add new observations to its existing catalog of hundreds of fast radio bursts, helping us to understand the sources of these mysterious events and the complex physics behind them.

Citation

“Can a Strong Radio Burst Escape the Magnetosphere of a Magnetar?,” Andrei M. Beloborodov 2021 ApJL 922 L7. doi:10.3847/2041-8213/ac2fa0

Planet orbiting a K dwarf star

How did the 14 Herculis planetary system get into an unusual configuration? Did the system have a run-in with another star, or was there a troublemaker within the system itself? Astronomers peek deep into the characteristics of the system to find out! 

Planets Going the Distance 

Light curve from 14 Her, showing time (JD) vs. V (magnitude)

Light curve for 14 Herculis, as measured in the ASAS-SN (All-Sky Automated Survey for Supernovae) program. [Adapted from Bardalez Gagliuffi et al. 2021]

When a star forms, leftover material from the protostellar nebula creates a disk around the star and may eventually become planets. Once these planets form, their characteristics, such as mass and inclination, can determine how the system will evolve. Some planetary systems, like our own, have relatively circular orbits, but others have strange configurations. One example of an oddball system is 14 Herculis, a middle-aged K0 dwarf star that’s orbited by two giant planets with very high eccentricities. Exactly how did the planets end up on these peculiar orbits? A team led by Daniella C. Bardalez Gagliuffi at the American Museum of Natural History examined the full orbital configuration of the system to find out. 

When the Planets Don’t Align 

Corner plot showing distributions for mass, inclination, eccentricity, and semi-major axis

Distributions of parameters for planet b. Click to enlarge. [Bardalez Gagliuffi et al. 2021]

14 Her was one of the first stars targeted in radial-velocity searches for exoplanets because of its proximity and brightness. Astronomers detected the closer in of its two known planets, 14 Her b, in 2003, and its outer planet, 14 Her c, four years later. Using archival radial-velocity data from the high-resolution echelle spectrometer on the Keck Telescope and proper motions inferred from the Hipparcos–Gaia Catalog of Accelerations, the team was able to calculate the orbital parameters of the two planets.

They found that planet b orbits at a semimajor axis of ~3 au and has a moderate eccentricity, and that planet c is 27 au from the star and has a highly eccentric orbit. Because the authors only had data for ~15% of 14 Her c’s orbit, they were only able to get a broad distribution of inclinations relative to 14 Her b, but the data pointed to the two orbits being misaligned by nearly 70 degrees. There are only three other known systems with giant planets that have misaligned orbits. 

Corner plot showing distributions for mass, inclination, eccentricity, and semi-major axis

Distribution of parameters for planet c. Click to enlarge. [Bardalez Gagliuffi et al. 2021]

Feeling Inclined to Search for Answers 

What caused this strange misalignment? The answer surely lies in the system’s dynamic past, but what kind of dynamic past is still up in the air. The most likely explanation is planet–planet scattering: multiple planets of similar mass formed together in circular orbits and gravitationally influenced one another, eventually leading to the ejection of one planet and affecting the orbits of the others. It’s also possible that a passing star came close enough to the system that it perturbed the orbits and launched one planet out of the system; to reach a stable configuration, the surviving planets got farther apart, and their eccentricities grew. 

Imaging the system in the mid-infrared could reveal a hidden planetary sibling that might shed further light on 14 Her’s dynamic past, so we’ll just have to wait for JWST and the Nancy Grace Roman Space Telescope to help solve this mystery! 

Citation 

“14 Her: A Likely Case of Planet–Planet Scattering,” Daniella C. Bardalez Gagliuffi et al 2021 ApJL 922 L43. doi: 10.3847/2041-8213/ac382c 

Hubble image of the Ring Nebula

When a star ends its main-sequence lifetime, loses its outer layers, and becomes a white dwarf, how massive will it be? Astronomers turn to white dwarfs’ binary companions to help answer this question.

A Frequent Finale

artist's impression of Sirius B next to Earth

Stars with masses less than roughly eight solar masses leave behind their dense cores, which are approximately the same size as Earth but substantially more massive. This artist’s impression compares the size of Sirius B, the nearest white dwarf, to Earth’s size. [ESA/NASA]

Except for the few percent of stars that will end their lives in massive cosmic explosions, nearly all the stars in our galaxy are destined to become white dwarfs. Our own star is headed that route: in five billion years or so, the Sun will exhaust its supply of core hydrogen, balloon into a red giant, and loft its outer layers into space to expose its blazing hot core. The core will shine for a further ten billion years — or more! — as a white dwarf before fading from view.

Even though the vast majority of stars become white dwarfs, many questions remain about these compact stellar remnants. One lingering uncertainty is the connection between a star’s main-sequence mass and the mass of the white dwarf it leaves behind. Is the mass of the white dwarf correlated with the star’s main-sequence mass, or do other factors, like the star’s chemical makeup, play a defining role? Pinning down the initial-to-final mass relation can help us gain a better grasp on the intricacies of how stars make the tumultuous journey from the main sequence to their final phase.

diagram of the mass-determination process

Diagram of the white dwarf mass-determination process, shown for two white dwarfs in the sample. Stellar models (solid lines) allow progenitor star lifetimes to be mapped to initial stellar masses. [Barrientos & Chanamé 2021]

Companion Chronology

Manuel Barrientos and Julio Chanamé (Pontifical Catholic University of Chile) approached this question by studying white dwarfs that are separated by thousands of astronomical units from their binary companions. These widely separated binary systems are a useful tool for studying white dwarfs; the two stars in the binary system are likely to be the same age, and the wide separation makes it unlikely that the stars have exchanged mass, which could complicate their evolutionary tracks.

Barrientos and Chanamé used models of stellar evolution and parameters extracted from spectroscopic and photometric observations to determine the masses of the white dwarfs, how long ago they formed, and the ages of their stellar companions. They then used evolutionary models to link the main-sequence lifetime of the white dwarf’s progenitor star — determined by subtracting the lifetime of the white dwarf from the age of the companion star — to its mass.

Far-Reaching Implications

plot of final mass versus initial mass

Initial and final masses of the stars in this work (blue symbols) and others. The two panels use different sets of stellar parameters. Click to enlarge. [Barrientos & Chanamé 2021]

The team was particularly interested in the least massive stars, for which the initial-to-final mass relation has been only loosely constrained by previous work. They found that the lowest-mass progenitor stars yielded a wide range of white-dwarf masses — some were more massive than white dwarfs that originated from progenitor stars twice as massive. However, the authors find evidence that these white dwarfs may actually be two closely situated white dwarfs, which would complicate the analysis.

The authors hope that future observations will refine the initial-to-final mass relationship further — the dispersion seen in their work isn’t predicted by models, cannot be fully explained by differences in stellar composition, and would have major implications for research that relies on precise determination of this relationship.

Citation

“Improved Constraints on the Initial-to-final Mass Relation of White Dwarfs Using Wide Binaries,” Manuel Barrientos and Julio Chanamé 2021 ApJ 923 181. doi:10.3847/1538-4357/ac2f49

extreme ultraviolet photograph of the Sun

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 Source Locations of Major Flares and CMEs in Emerging Active Regions

Published March 2021

Main takeaway:

greyscale image of the solar magnetic field

Extreme space weather events like solar flares often arise from solar active regions where magnetic field lines of opposite polarity are located close together. This map of the Sun’s magnetic field shows field lines directed out of the Sun’s surface in white and those directed inward in black. [Solar Dynamics Observatory, NASA]

A team led by Lijuan Liu (Sun Yat-sen University, University of Science and Technology of China, and Center for Excellence in Comparative Planetology, China) monitored 19 active regions on the Sun from their formation to the moment they produced a solar flare or an explosive outburst of magnetized solar plasma called a coronal mass ejection. This study allowed the team to identify and characterize the polarity inversion lines — areas where the direction of the solar magnetic field changes — from which the flares and coronal mass ejections arose.

Why it’s interesting:

A prominent focus of solar physics is to understand how extreme space weather events like solar flares and coronal mass ejections form. When directed toward Earth, these explosive events can cause geomagnetic storms: widespread disturbances of Earth’s protective magnetic field that can result in auroras, damage to power grids and spacecraft electronics, and radio communications blackouts. Part of the mitigation strategy for bad space weather is to be able to predict when it will occur, but it’s not yet clear why some active regions — places where magnetic fields bubble up through the Sun’s surface — produce flares or coronal mass ejections and others do not.

What’s the status of these active regions:

Liu and collaborators find that the complexity of the magnetic field in the active region plays a key role in whether or not it will erupt; none of the 19 active regions studied consisted of a single bipole — a region with areas of opposite magnetic polarity — but rather were made up of multiple bipoles colliding. Colliding bipoles might be the smoking gun for extreme space weather events: the more severe the collision, the more severe the resulting event. Ultimately, Liu and collaborators suggest that the characteristics of these colliding regions on the Sun’s surface could be used to forecast space weather.

Citation

Lijuan Liu et al 2021 ApJ 909 142. doi:10.3847/1538-4357/abde37

photograph of Saturn's moon Enceladus

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 Science Case for a Return to Enceladus

Published July 2021

Main takeaway:

A team led by Morgan Cable (Jet Propulsion Laboratory) proposes that Saturn’s icy moon Enceladus should be the target of a future spacecraft mission. The moon’s water plumes are unique in the solar system and provide unparalleled access to the salty, organic-rich ocean beneath the crust — an environment brimming with astrobiological promise.

Why it’s interesting:

photograph of enceladus with plumes

A backlit view of Enceladus’s south polar plumes. [NASA/JPL-Caltech/Space Science Institute]

Enceladus is only 504 kilometers (313 miles) in diameter — you could drive around it in roughly a day at a leisurely 65 kilometers (40 miles) per hour — but this tiny moon is one of the most promising places to search for life beyond Earth. Its icy crust is dotted with small craters and its southern hemisphere is crossed by several long fissures. As the Cassini spacecraft flew by Enceladus in 2005, it spotted cryovolcanoes erupting through the fissures, spraying water into space. The presence of salt in the plumes indicated that they emerged from an ocean deep enough to reach the moon’s rocky core. Analysis of gravity measurements and observations of Enceladus’s slight wobble confirmed the presence of a global ocean, which is likely to be a persistent rather than transient feature.

What kind of mission would be best:

Given the existing evidence for an organic-rich water ocean, Cable and coauthors say we already know that Enceladus is habitable, so the logical next step is to send a dedicated life-finding mission. It may be possible to probe Enceladus’s oceans in a minimally invasive way by sending a spacecraft to fly repeatedly through its plumes and analyze the droplets. A lander or rover could undertake more sensitive experiments, searching for life-signaling organic molecules like amino acids and lipids. A mission to Enceladus would require roughly 11 years to travel from Earth to its home in orbit around the moon — so what are we waiting for?

Citation

Morgan L. Cable et al 2021 Planet. Sci. J. 2 132. doi:10.3847/PSJ/abfb7a

photograph of the Moon's surface

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.

Lunar Gravitational-wave Antenna

Published March 2021

Main takeaway:

A team led by Jan Harms (Gran Sasso Science Institute, Italy) revisited the idea of using the Moon to detect gravitational waves. The proposed detector would use an array of highly sensitive seismometers to measure the subtle vibrations of the Moon caused by gravitational waves passing through it.

Why it’s interesting:

The observatory proposed by Harms and collaborators hearkens back to theories from the 1960s, which introduced the idea that a gravitational wave passing through an elastic body would cause it to vibrate, and that we could use these minuscule oscillations to study gravitational waves. Though scientists have considered measuring the vibrations that pass through Earth to detect gravitational waves, the Moon is likely a better option; the Moon has moonquakes and meteorite impacts (which would have to be accounted for in the data analysis), but it’s seismically quieter than Earth. Notably, there was an attempt to deploy a gravitational-wave detector on the Moon in 1972, but a failure of a temperature regulator made the data unusable.

Why the team is over the Moon for this concept:

schematic of seismometer locations

A potential layout of lunar seismometers for the first phase of the Lunar Gravitational-wave Antenna. [Harms et al. 2021]

Because the proposed Lunar Gravitational-wave Antenna would rotate along with the Moon every 27.3 days, its detectors would move through space in a way unlike any existing gravitational-wave detector, which may aid in estimating the parameters of the gravitational-wave signal. If the array is extended to cover a large area of the Moon, it might become sensitive enough to study the orientation of incoming gravitational waves, which would help to distinguish between candidate sources for gravitational-wave signals. Overall, the proposed Lunar Gravitational-wave Antenna would likely complement existing and planned gravitational-wave observatories while greatly improving our ability to triangulate and analyze signals. Although the sensitive hardware necessary for such an undertaking requires further development, the authors posit that the detector could be achieved within the next decade.

Citation

Jan Harms et al 2021 ApJ 910 1. doi:10.3847/1538-4357/abe5a7

infrared image of Messier 87

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.

Polarimetric Properties of Event Horizon Telescope Targets from ALMA

Published March 2021

Main takeaway:

A team led by Ciriaco Goddi (Radboud University and Leiden Observatory–Allegro, the Netherlands) used the Atacama Large Millimeter/submillimeter Array (ALMA) to measure the polarization — how orderly or randomly the electric and magnetic fields of light waves are oriented — of the two supermassive black holes targeted by the Event Horizon Telescope as well as that of 12 active galactic nuclei. The team’s observations will help us calibrate, analyze, and interpret future very long-baseline interferometry (VLBI) data from the Event Horizon Telescope and the Global mm-VLBI Array.

Why it’s interesting:

infographic of the Event Horizon Telescope and Global mm-VLBI Array facilities locations

This infographic details the locations of the participating telescopes of the Event Horizon Telescope (cyan) and the Global mm-VLBI Array (yellow). Click to enlarge. [ESO/O. Furtak]

The Event Horizon Telescope collaboration combined data from telescopes across the globe to study supermassive black holes in unprecedented detail, resulting in an image of the black hole at the center of one of the most massive nearby galaxies, Messier 87, in 2019. Now, astronomers have used ALMA — one of the world’s largest arrays of radio telescopes — to study the polarization of the galaxies imaged by the Event Horizon Telescope, as well as a dozen galaxies that host active galactic nuclei. Polarization data allows astronomers to measure the strength and orientation of magnetic fields in distant sources, which likely play a key role in the accretion of material onto black holes as well as the acceleration of relativistic jets in active galactic nuclei.

What the survey showed:

Goddi and collaborators find that the radio emission from active galactic nuclei is strongly polarized, with the highest degree of polarization seen in blazars — active galactic nuclei emitting relativistic jets that are pointed directly at us. In the case of Event Horizon Telescope target Messier 87, the direction of polarization is highly variable on the timescale of a few days, which the team explains with a new model that combines an event-horizon-scale variable source with a larger static source. With more observatories being added to the Event Horizon Telescope, we should soon be able to study the magnetic field near Messier 87’s central black hole in even more detail!

Citation

Ciriaco Goddi et al 2021 ApJL 910 L14. doi:10.3847/2041-8213/abee6a

photograph of radio telescopes

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 JPL Planetary and Lunar Ephemerides DE440 and DE441

Published February 2021

Main takeaway:

A team led by Ryan Park (Jet Propulsion Laboratory) has updated the mathematical model used to calculate the positions of the Sun and planets. The new releases, DE440 and DE441, are best used for calculations covering the years 1550–2650 and −13200–17191, respectively.

Why it’s interesting:

transit of venus across the Sun

Transits of solar system planets (Venus is shown in this composite extreme ultraviolet image) are one of the many events that can be predicted using JPL’s database. [SDO/NASA]

NASA’s Jet Propulsion Laboratory (JPL) has tracked the locations and trajectories of major solar system bodies for decades and publishes these ephemerides for the community to use. The latest updates incorporate seven years of new data of the planets from ground-based observations as well as measurements of spacecraft positions from the Deep Space Network radio telescopes. These planetary positions are used in myriad ways — from modeling the evolution of the solar system to pinpointing the moment Jupiter will transit in front of the Sun as seen from Saturn.

Why these updates are important:

Tiny errors in a planet’s position, propagated forward or backward in time, can lead to big inaccuracies. This is especially an issue for the outer planets, which move slowly and have been visited by fewer spacecraft, giving us less data — for example, between 2003 and 2016, there were no Jupiter-orbiting spacecraft. When you’re trying to launch a $10 billion spacecraft and need to know the precise position of the Moon for it to go smoothly (just a completely hypothetical example…), you’re going to want the best data available!

Citation

Ryan S. Park et al 2021 AJ 161 105. doi:10.3847/1538-3881/abd414

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

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