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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

Set of four images showing close-ups of the rocky surface of Ceres.

The images above show two regions on the surface of Ceres, the only dwarf planet in the inner solar system. The larger-scale views of the two regions (left panels) and their zoom-ins (right panels) reveal fine-debris landslides tumbling down the walls of a crater on this airless body. In a new publication led by Elizabeth Palmer (University of Southern California), a team of scientists has combined existing observations of Ceres with laboratory experiments to better understand the properties of this dwarf planet’s surface. They find that Ceres’s uppermost surface layer is much more porous than other bodies like it, providing an opportunity for it to retain small, hidden pockets of volatiles — substances that should normally evaporate from airless bodies — across its surface. The authors speculate that Ceres’s porous surface is created by activity like avalanches of fine particles, as seen above. To learn more about the authors’ results, check out the original article below.


“Exploring Ceres’s Unusual Regolith Porosity and Its Implications for Volatile Retention,” Elizabeth M. Palmer et al 2021 Planet. Sci. J. 2 182. doi:10.3847/PSJ/ac0b3e

Image of the surface of a small, rocky, rough asteroid, overlaid with a grid of ~50 colored tiles.

Recently, the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) spacecraft traveled to the near-Earth asteroid (101955) Bennu and spent two years studying the surface of this rocky body. An example view OSIRIS-REx captured using its MapCam color imager is shown as the background layer of the photo above (click for the full image!); the ~50 overlaid colored regions are tiles from a high-resolution shape model. Scientists use shape models to better interpret MapCam’s photometry of Bennu’s rough surface, since we must account for variations in the asteroid’s surface texture, color, albedo, and composition. In a new study led by Dathon Golish (Lunar and Planetary Laboratory, University of Arizona), a team of scientists has worked to improve our analysis of asteroid Bennu with photometric models. To read more about the authors’ work and results, check out the original article below.


“Regional Photometric Modeling of Asteroid (101955) Bennu,” D. R. Golish et al 2021 Planet. Sci. J. 2 124. doi:10.3847/PSJ/abfd3c

image of the solar corona in a region of quiet sun.

How does the Sun’s outermost atmosphere — the solar corona — become heated to a million degrees Kelvin? The answer may lie in what’s pictured here: the quiet Sun. This extreme ultraviolet image, taken by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) in 2019, shows a region of the Sun spanning roughly 300 x 350 arcseconds (click for the full view). The image captures the quiet Sun — the ordinary, background roiling of the corona, unblemished by large magnetic features like active regions or coronal holes. In a new study, scientists Vishal Upendran and Durgesh Tripathi (Inter University Centre for Astronomy and Astrophysics, India) combine SDO AIA images of the quiet Sun with new models to better understand how heat may be injected into the solar corona on a continuous basis — even when the Sun is quiet. They show that impulsive events — tiny, constantly occurring nanoflares — can pump enough energy into the quiet-Sun corona to explain its mysteriously large temperature. For more information, check out the original article below.


“On the Impulsive Heating of Quiet Solar Corona,” Vishal Upendran and Durgesh Tripathi 2021 ApJ 916 59. doi:10.3847/1538-4357/abf65a

A gold-wrapped spacecraft sits on the right side of the image, in a room completely covered in blue conical protrusions.

On the right side of this photograph is the integrated payload and spacecraft of Chandrayaan-2, an Indian Space Research Organisation (ISRO) lunar exploration mission. In the image, the mission’s Dual-Frequency synthetic aperture radar (DFSAR) instrument is being tested prior to launch in an anechoic chamber — a room designed to minimize reflections and echos at specific wavelengths, allowing scientists to evaluate the function of an instrument. Chandrayaan-2 launched in 2019 and is currently orbiting the Moon, mapping out the lunar surface and exploring its thin atmosphere. In a new publication, a team of scientists led by Sriram Bhiravarasu (Space Applications Centre, ISRO) provides a preliminary report on DFSAR’s performance and describes what we may be able to learn from the radar polarization measurements made by this instrument — for instance, how the rockiness of craters evolve with time on the Moon’s surface. To learn more (and to see some cool radar images of the Moon’s surface), check out the original article below.


“Chandrayaan-2 Dual-frequency Synthetic Aperture Radar (DFSAR): Performance Characterization and Initial Results,” Sriram S. Bhiravarasu et al 2021 Planet. Sci. J. 2 134. doi:10.3847/PSJ/abfdbf

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