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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). [ESO/O. Furtak; CC BY 4.0]

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

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

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