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composite image of the Sun

How does the Sun accelerate particles to relativistic energies? The answer may lie in the shocks that form between clashing plasma structures high in the Sun’s atmosphere, in its corona. The composite image above shows the Sun’s disk in extreme-ultraviolet light overlaid on a white-light image of the wispy corona captured by blocking the Sun from view. A massive eruption of plasma called a coronal mass ejection emerges from the lower-left side, while faint, streaky coronal streamers point outward in multiple directions. The white arrow indicates a streamer that has been deflected by the passage of the coronal mass ejection. Space-based observatories viewed the event from multiple perspectives, allowing a team led by Federica Frassati (National Institute for Astrophysics, Astrophysical Observatory of Turin, Italy) to reconstruct a three-dimensional view of the shock front as it expanded. The team’s results show, for the first time, that the interaction between an advancing shock front and background solar streamers can accelerate particles up to a whopping 100 megaelectronvolts. To learn more, check out the full article below.


“Acceleration of Solar Energetic Particles through CME-driven Shock and Streamer Interaction,” Federica Frassati et al 2022 ApJ 926 227. doi:10.3847/1538-4357/ac460e

simulations of the Sun's magnetic fields

simulations of the Sun's magnetic fields

Simulated magnetic fields under high solar activity (top row) and low solar activity (bottom row) conditions. The left column shows the results with no outflow, while the right column shows the resulting magnetic field when a solar wind with a velocity of 150 kilometers per second is included. Click to enlarge. [Rice & Yeates 2021]

Solar physicists have a magnetic flux problem: current models of the magnetic field threaded through the Sun’s million-degree upper atmosphere, or corona, struggle to reproduce the observed amount of open magnetic flux — the field lines that extend from the Sun’s surface into the solar system, carrying solar plasma far afield. Now, Oliver Rice and Anthony Yeates (Durham University, UK) have found a way to lessen the discrepancy between models and observations without increasing the computational cost by incorporating an outward-flowing solar wind into their model. As the simulation results above and to the right show, introducing a solar wind component draws the magnetic field lines outward, increasing the number of magnetic field lines that extend outward into the solar system and decreasing the number of loops that double back toward the Sun’s surface. While the addition of a solar wind component doesn’t fully relieve the tension between models and observations, the authors are hopeful that future developments, such as incorporating more realistic spatial variations of the solar wind, will further improve the outcome. To learn more, see the full article below.


“Global Coronal Equilibria with Solar Wind Outflow,” Oliver E. K. Rice and Anthony R. Yeates 2021 ApJ 923 57. doi:10.3847/1538-4357/ac2c71

Hubble image of Sh2-106 star forming region

The birth of a massive star is anything but calm and peaceful — a fact that is dramatically illustrated in this false-color Hubble image of the star-forming region Sh2-106 (click for the full view, or see below for the original image with scale). This spectacular nebula hides the young massive star S106 IR. Birthed from the tumultuous collapse of its parent molecular cloud, S106 IR grew via rapid accretion until a tremendous explosion rocked the region roughly 3,500 years ago, according to new research led by John Bally (University of Colorado Boulder).

Hubble image of Sh2-106 star forming region

False-color Hubble image of the Sh2-106 star-forming region. [Bally et al. 2021]

Bally and collaborators have used both new and archival observations of Sh2-106 spanning 16 years to analyze the supersonic motion of the gas sent streaming in the powerful explosion. The authors’ work provides unique insight into the fireworks possible during the final stages of a massive star’s birth. For more information (and more stunning images of Sh2-106!), check out the original article below.


“Supersonic Expansion of the Bipolar H II Region Sh2-106: A 3500 Year Old Explosion?,” John Bally et al 2022 ApJ 924 50. doi:10.3847/1538-4357/ac30de

simulation results in the x-y plane

hydrodynamic simulations at different angles

Simulations of the Be star (larger green circle) and neutron star (smaller green circle) disks. Lighter colors indicate denser material. Results are shown after 29 (top), 33 (middle), and 38 (bottom) orbital periods of the binary have passed. After the initial mass transfer, an accretion disk forms around the neutron star and elongates. Click to enlarge. [Franchini & Martin 2021]

Alessia Franchini (University of Milano-Bicocca, Italy) and Rebecca Martin (University of Nevada, Las Vegas) study binary systems made up of a neutron star and a Be star — a rapidly rotating high-mass star with hydrogen emission lines in its spectrum. Because the Be star (the letters are pronounced separately as “B e”) spins so quickly, it flings away some of its mass into a decretion disk, from which the neutron star can siphon material. Be star–neutron star binaries tend to emit bursts of X-rays once per orbital period as well as on irregular timescales, with these irregular bursts often occurring in pairs — though it’s not yet clear why. Franchini and Martin use hydrodynamic simulations, shown above and to the right, to understand the behavior of these extreme systems. Their simulations show that the first of the paired X-ray outbursts can be caused by the transfer of mass from the Be star to the neutron star, while the second outburst might be driven by changes in the eccentricity of the neutron star’s disk, though higher-resolution simulations are needed to confirm this result. To learn more, see the full article below.


“Eccentric Neutron Star Disk Driven Type II Outburst Pairs in Be/X-ray Binaries,” Alessia Franchini and Rebecca G. Martin 2021 ApJL 923 L18. doi:10.3847/2041-8213/ac4029

simulations of gas density and temperature in star-forming clouds with different radii.

When dusty molecular clouds collapse to form stars, competing forces vie for control. Gravity pulls ever inward while escaping photons, hot ionized gas, and stellar winds work to heat and disperse the cloud and bring star formation to a halt. In a new article, a team led by Lachlan Lancaster (Princeton University) used hydrodynamic modeling to explore the interplay between stellar winds and star formation. In the image above (click for a larger view!), simulations show how gas density (top row) and temperature (bottom row) vary in star-forming clouds of different sizes. The team’s modeling shows that the extent to which stellar winds disrupt star formation depends on the density of the cloud, with hot stellar winds cooling rapidly and driving out less material in dense clouds. Lancaster and collaborators discovered another role stellar winds play: in dense clusters, up to 1% of a young star’s mass can come from wind particles from stellar neighbors, with potential implications for chemical abundances in cluster stars. To learn more, see the full article below.


“Star Formation Regulation and Self-pollution by Stellar Wind Feedback,” Lachlan Lancaster et al 2021 ApJL 922 L3. doi:10.3847/2041-8213/ac3333

a solar filament at optical and ultraviolet wavelengths, as well as the filament's Doppler shift

five views of a solar filament

Top row: 656.3-nm emission (left) and magnetic-field strength (right) of a solar filament seen in 2019. Bottom row: detailed 656.3-nm emission (left), 30.4-nm emission (center), and velocity (right), zoomed in on the filament. The filament is indicated by the yellow arrows in the left and center panels. The arrows in the right panel indicate the twisted structure. [Guo et al. 2021]

What causes solar plasma to erupt into interplanetary space? Astronomers use highly detailed images of the solar surface to address this question, which is key for predicting the onset of solar eruptions that might impact Earth. A team led by Yilin Guo (National Astronomical Observatories, Chinese Academy of Sciences) analyzed images and data from the New Vacuum Solar Telescope, the Solar Dynamics Observatory, and the Interface Region Imaging Spectrograph to study a solar filament — an enormous arc of plasma suspended above the Sun’s surface. Solar filaments can linger for days or months before breaking free, but the cause of the eruption isn’t always clear. In Guo and collaborators’ observations of a solar filament that appeared in 2019, there is evidence that magnetic reconnection rearranged the magnetic field structure near the footprint of the filament, forcing the tightly twisted rope of plasma to move. Observations like these can help scientists understand the causes of solar eruptions and bring us one step closer to predicting them. To learn more, see the full article and animation below.


Check out this video from the authors’ article, which shows how the filament evolves over time through a variety of perspectives. Clockwise from top left: H-alpha (optical), velocity map, magnetic field strength, 17.1 nm (extreme ultraviolet), 30.4 nm (extreme ultraviolet), and 170 nm (far ultraviolet).


“Dynamical Evolution of an Active-region Filament Driven by Magnetic Reconnection,” Yilin Guo et al 2021 ApJ 920 77. doi:10.3847/1538-4357/ac1ac6

a coronal hole stretching diagonally across an extreme ultraviolet image of the Sun

four panels with the results of different detection methods

Top left: A 19.3-nanometer image of a coronal hole. Top right: Largest and smallest boundaries of the hole derived by any mapping method. Bottom left: Results of individual mapping methods. Bottom right: Darker areas represent greater agreement between the mapping methods. [Linker et al. 2021]

Coronal holes — named for their dark appearance in X-ray and ultraviolet images of the Sun — are regions where the hot, tenuous solar atmosphere is relatively cool and dense. These areas are thought to be a source of “open” magnetic field lines, a magnetic field configuration that allows solar plasma to escape into the solar system, where it can collide with Earth and other planets. However, astronomers are still working to find effective ways to map these regions, which is key for measuring the amount of open magnetic flux they contain. A team led by Jon Linker (Predictive Science Inc.) tested six mapping methods on images of a large coronal hole taken in 2010, seen in the full image to the right. Linker and collaborators found that these methods tended to produce underestimates of the amount of open magnetic flux compared to measurements made by spacecraft sampling the Sun’s magnetic field in situ, which may partially explain the long-standing deficit in open magnetic flux measured from coronal hole images. Fully understanding the Sun’s magnetic flux may require a mission to image the Sun’s polar regions, which are difficult to photograph from the plane of the solar system. To learn more, check out the full article below.


“Coronal Hole Detection and Open Magnetic Flux,” Jon A. Linker et al 2021 ApJ 918 21. doi:10.3847/1538-4357/ac090a

Grayscale image of Pluto's nightside and diagrams showing Pluto's orientation

Grayscale image of Pluto's nightside and diagrams showing Pluto's orientation

Two versions of the Charon-lit image of Pluto’s southern hemisphere recovered. with different image-processing techniques (top right and bottom left). The top left and bottom right panels show the viewing orientation. [Lauer et al. 2021]

After the New Horizons spacecraft completed its flyby of Pluto in 2015, it turned to snap pictures of the dwarf planet’s wintry southern hemisphere. Although the Sun hasn’t shined directly on latitudes south of 39°S on Pluto in decades, this region does have another source of illumination: sunlight reflected from Pluto’s closest moon, Charon. In a new publication, a team led by Tod Lauer (National Optical Infrared Astronomy Research Laboratory) carefully recovered this Charon-lit image of Pluto, seen in the left panel of the image above. How bright is Charon light? Lauer and coauthors report that it’s just shy of the flux from a first-quarter Moon. In order to retrieve the faint image of Pluto’s southern regions, the team had to remove scattered sunlight that is three orders of magnitude brighter than the light from Charon. With the scattered light removed, Lauer and collaborators could discern a bright area in Pluto’s southern hemisphere that they suggest is an expanse of nitrogen or methane ice.


“The Dark Side of Pluto,” Tod R. Lauer et al 2021 Planet. Sci. J. 2 214. doi:10.3847/PSJ/ac2743

Simulation of the density of the Milky Way's dark matter halo

New simulations show how the outer regions of the Milky Way are shaped by the motion of its largest satellite galaxy, the Large Magellanic Cloud (LMC). Using 100 million simulated dark matter particles, a team led by Nicolás Garavito-Camargo (Steward Observatory, University of Arizona) shows that the LMC creates a wake in the Milky Way’s dark matter halo, like a ship traveling across the ocean, as it swings past our galaxy. The friction between the LMC and the Milky Way’s halo will cause the smaller galaxy to lose energy and eventually merge with the Milky Way. The gravitational influence of the LMC introduces an asymmetry into the roughly spherical dark matter halo, which is shown in the above image and in the video below. By pairing simulations like this one with the wealth of data from upcoming all-sky surveys, astronomers should be able to account for this asymmetry and probe the underlying shape of the Milky Way’s dark matter halo to learn more about how our galaxy formed on cosmological timescales. To read more about their results, check out the article below.


Check out this video, which shows how the structures seen in the authors’ simulations relate to the positions of the Milky Way and the LMC. [NASA/JPL-Caltech/NSF/R. Hurt/N. Garavito-Camargo & G. Besla]


“Quantifying the Impact of the Large Magellanic Cloud on the Structure of the Milky Way’s Dark Matter Halo Using Basis Function Expansions,” Nicolás Garavito-Camargo et al 2021 ApJ 919 109. doi:10.3847/1538-4357/ac0b44

close-up image of the interior of an asteroid fragment

two panels showing the interior of two different asteroid fragments

The interior structure of two carbonaceous chondrites: meteorite Los Vientos 123 (top) and meteorite El Médano 463 (bottom). Click to enlarge. [Pinto et al. 2021]

Have you ever wanted to look inside an asteroid? Today’s image (see the full version to the right) shows the interior structure of a type of meteorite called a carbonaceous chondrite — a remnant of a rocky, carbon-containing asteroid thought to have formed far from the Sun early in solar system history. A team led by Gabriel Pinto (Université de Lorraine, France) determined the composition of several carbonaceous chondrites by bombarding them with high-energy electrons and measuring the electrons that bounced back (top panel; meteorite Los Vientos 123) or the X-rays that were emitted (bottom panel; meteorite El Médano 463). The shading colors in the bottom panel indicate the chemical element, while the outline colors in both panels indicate the component type — for example, iron-oxide-rich components that formed in a high-temperature environment are outlined in blue. The authors found that the sizes and types of components in these meteorites support the theory that planetesimals formed via gravitational collapse of clumps of small particles. For more information, check out the article below!


“Constraints on Planetesimal Accretion Inferred from Particle-size Distribution in CO Chondrites,” Gabriel A. Pinto et al 2021 ApJL 917 L25. doi:10.3847/2041-8213/ac17f2

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