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neon microcrystal

Some astronomical mysteries are found deep in the interiors of stars. Such is the case with the puzzle of Q-branch white dwarfs, a population of these evolved, dense stars that seems to have an unexpected heat source, causing them to cool more slowly than a typical white dwarf. One hypothesis proposes that Q-branch white dwarfs gain extra heat from the rapid sedimentation — sinking to the center of the star — of neutron rich neon, 22Ne. But recent molecular dynamics simulations conducted by Matt Caplan (Illinois State U.), Charles Horowitz (Indiana U.), and Andrew Cumming (McGill U., Canada) show that the tiny crystals of neon needed to speed up this sedimentation can’t exist in a stable state in the interior of a typical white dwarf — which means there must be some other mechanism at work heating Q-branch white dwarfs. The image above shows the initial state of the authors’ simulations, in which a 22Ne microcrystal (red) lies within a soup of carbon and oxygen (white) under the dense, high-pressure conditions that exist inside a white dwarf. To read more about the authors’ work, check out the article below.

Citation

“Neon Cluster Formation and Phase Separation during White Dwarf Cooling,” M. E. Caplan et al 2020 ApJL 902 L44. doi:110.3847/2041-8213/abbda0

giant impact

The panels in the image above (click the cover image for the full view, or click the video below to watch the animated version) show frames from a dramatic simulation of the collision of a planetary-mass impactor with the proto-Earth in the early solar system. In the chaos that follows the collision, the debris that doesn’t escape settles into a disk around the young Earth — a disk that will later form into our Moon. While the giant impact described here has been simulated in detail in the past, most models have yet to incorporate the effects of magnetic fields into the giant impact and the subsequent evolution of the protolunar disk. In a new study, scientists Patrick Mullen and Charles Gammie (University of Illinois at Urbana-Champaign) develop state-of-the-art simulations that include weak initial magnetic fields. The authors show how these fields are amplified by turbulence in the collision, impacting the how the disk — and, ultimately, the Moon — evolves. To read more about their results, check out the original article below.

Citation

“A Magnetized, Moon-forming Giant Impact,” P. D. Mullen and C. F. Gammie 2020 ApJL 903 L15. doi:10.3847/2041-8213/abbffd

SDO AIA active region

How can we tell how much energy is leaking out of active regions on the Sun like the one shown in this Solar Dynamics Observatory (SDO) image above? Active regions are areas with especially strong magnetic fields, and they’re often associated with solar activity. But even when the Sun is generally quiet, these regions radiate energy away — and it’s important to understand how much.

To answer this question, scientists Maria Kazachenko (University of Colorado Boulder; National Solar Observatory) and Hugh Hudson (UC Berkeley; University of Glasgow, UK) used the Extreme Ultraviolet Variability Experiment on board SDO to measure unresolved spectra of the Sun as though it were a distant star. Kazachenko and Hudson compared data from periods when the Sun had no active regions to data from when only one active region was present, allowing them to identify the radiation specifically associated with the active region.

To learn more about the authors’ work, check out the original article below. And if you want to read more about “Sun-as-a-star” studies — studies like this one that treat the Sun as an unresolved source — you can check out this other recent ApJ study, which was announced in a press release today.

SDO data of active region

This full image from the article shows the magnetic fields (top two images) and SDO AIA 211 Å maps (bottom two images) for the Sun when one active region is present (left) and no active regions are present (right). The right column shows the disk-integrated magnetic flux (top) and a line-integrated spectrum (bottom) as the active region crosses the disk. [Kazachenko & Hudson 2020]

Citation

“Active Region Irradiance during Quiescent Periods: New Insights from Sun-as-a-star Spectra,” Maria D. Kazachenko and Hugh S. Hudson 2020 ApJ 901 64. doi:10.3847/1538-4357/abada6

plasmoid ejection

How could a magnetar — a powerfully magnetized neutron star — produce the brief flashes of radio emission and X-rays we’ve recently spotted from SGR 1935+2154, the first equivalent of a fast radio burst in our own galaxy? Magnetars have crusts that can suddenly crack and shear, shaking the magnetic fields of the star in what’s known as a magnetar quake. Using numerical simulations, a team of scientists led by Yajie Yuan (CCA, Flatiron Institute) has shown that waves from a magnetar quake can propagate to the star’s magnetosphere, converting into blobs of magnetized plasma. These plasmoids accelerate outward from the star, driving blast waves into the surrounding magnetar wind that generate simultaneous X-ray and radio bursts.

plasmoid ejection zoom-in

A zoom-in of ejecta propagating through the magnetosphere surrounding a magnetized neutron star. [Adapted from Yuan et al. 2020]

The simulation frame above spans roughly 15 x 108 cm by 30 x 108 cm. It provides a large-scale view of the magnetar and its magnetosphere 50 milliseconds into the authors’ simulation, as the ejected plasmoid blobs reach the outer regions of the magnetosphere. The image to the right is from just 10 milliseconds into the simulation, showing a detailed view of the ejecta early on. Both images are adapted from the original figures; to see the originals and to learn more about the authors’ results, check out the original article below.

Citation

“Plasmoid Ejection by Alfvén Waves and the Fast Radio Bursts from SGR 1935+2154,” Yajie Yuan et al 2020 ApJL 900 L21. doi:10.3847/2041-8213/abafa8

twin jets in close binaries

The stills above (click for the full view!) represent different time stages in the formation of a close stellar binary from a collapsing cloud of gas. In a recent study, two researchers from Kyushu University in Japan, Yu Saiki and Masahiro Machida, conduct numerical simulations to track the complicated process of a binary’s formation and evolution over ~400 years. In the above frames, the top left panel shows the fragmentation of the gas cloud into two cores roughly 9,000 years after the cloud initially begins to collapse. The succeeding panels show how the separation between these two protostars shrinks over the next several hundred years and disks of gas form around each star and around the binary pair. Saiki and Machida’s simulations also show the high-velocity jets driven from each protostar in the process (see the video below), and how the twin jets tangle on large scales as the stars orbit one another. The characteristics revealed in these simulations neatly reproduce our observations of protobinary systems. For more information, check out the original article linked below the video.

Citation

“Twin Jets and Close Binary Formation,” Yu Saiki and Masahiro N. Machida 2020 ApJL 897 L22. doi:10.3847/2041-8213/ab9d86

dusty ring evolution

These stills from a 2D hydrodynamic simulation show how a ring of dust and gas surrounding a newly born star might behave as it evolves. The frames illustrate the dust-to-gas ratio after 260 (left), 600 (center), and 1,740 (right) orbits of the dusty ring around the star. These simulations were conducted as part of a study led by Pinghui Huang (Chinese Academy of Sciences and Tsinghua University, China; Los Alamos National Laboratory; Rice University). The results demonstrate how such a ring can become unstable at its edges, forming small vortices that develop into many clumps of dust. Each of these clumps contains at least 10% of Earth’s mass, potentially forming the seeds from which baby planets can grow in the environment around the young star. For more information on the authors’ results, check out the original article below.

Citation

“Meso-scale Instability Triggered by Dust Feedback in Dusty Rings: Origin and Observational Implications,” Pinghui Huang et al 2020 ApJ 893 89. doi:10.3847/1538-4357/ab8199

eclipse composite

This stunning image of the Sun and its corona (click for a closer look) is composited from hundreds of individual frames captured by Nicolas Lefaudeux at the Cerro Tololo Inter-American Observatory in Chile. The occasion: a team effort to image a total solar eclipse in July 2019.

In a recent Research Note led by Christian Lockwood (Williams College), you can read about how the team gathered images (like those composited above) using three different observatories in Chile during the eclipse. Lockwood and collaborators then combined these ground-based images — which had high resolution and a wide field of view — with close-in observations of the solar disk made by space-based satellites.

By putting these overlapping observations together, the team could paint a full picture of the Sun’s tenuous, extended outer atmosphere during solar minimum. To learn more about the project, check out the article below.

Citation

“Compositing Eclipse Images from the Ground and from Space,” Christian A. Lockwood et al 2020 Res. Notes AAS 4 133. doi:10.3847/2515-5172/abacb5

basalt oxidization

Is Venus still volcanically active today? A new study led by Kyra Cutler (USRA’s Lunar and Planetary Institute; University of Birmingham, UK) investigated this question in an unusual way: by examining how rocks age in a laboratory. The photo above shows a sample of alkali basalt before and after it was exposed to 7 weeks of oxidization in a furnace to reproduce conditions similar to those on Venus’s surface. The mineralogical changes of the rock can be easily seen here — particularly the formation of hematite, visible as small white specks — and it’s even more dramatically evident in the reflectance spectra captured to mimic remote observations of Venus. Cutler and collaborators’ experiments indicate that if the basalt on the surface of Venus contains olivine or glass, some lava flows we’ve observed can only be a few years old. And even in the unlikely event that the basalt is fully crystalline instead, it’s still at most decades to hundreds of years old. These results strongly indicate that Venus is currently volcanically active. To learn more about the authors’ work, check out the article below.

Citation

“Experimental Investigation of Oxidation of Pyroxene and Basalt: Implications for Spectroscopic Analyses of the Surface of Venus and the Ages of Lava Flows,” K. S. Cutler et al 2020 Planet. Sci. J. 1 21. doi:10.3847/PSJ/ab8faf

hollow ice sphere

You’re looking at a frozen, hollow shell of ice roughly 20 cm in diameter and 3 cm thick. In a new laboratory study, scientists Kathryn Harriss and Mark Burchell (University of Kent, UK) have studied what happens when a shell like this is shot with a small, high-speed projectile, causing the ice shell to explode into pieces. You can watch a slow motion video of their experiment below!

This process simulates the possible high-speed collisions and catastrophic disruptions of icy bodies — like the frozen moons of Saturn and Jupiter — in the early solar system. By exploring how a hollow ice sphere responds to impact, Harriss and Burchell hope to better understand the relative roles of a body’s core and its surface layers in determining what happens during a catastrophic disruption. Which is more important in a collision: an icy object’s crust or its core? Check out the original article, linked below, for more information on what the authors learned.

Citation

“Catastrophic Disruption of Hollow Ice Spheres,” Kathryn H. Harriss and Mark J. Burchell 2020 Planet. Sci. J. 1 19. doi:10.3847/PSJ/ab8f34

atmospheric ion loss

Could TOI-700 d, an Earth-sized planet in the habitable zone of an early-type M-dwarf star, have held on to its atmosphere over long timescales? This question is crucial to understanding whether this recent discovery from the Transiting Exoplanet Survey Satellite (TESS) is likely to have a habitable surface. In a recent study led by Chuanfei Dong (Princeton University), a team of scientists conducted a series of state-of-the-art simulations to model the atmospheric escape from TOI-700 d as the planet is bombarded by its host’s stellar wind. The plots above show the O+ ion density and magnetic field lines in various cases from the authors’ simulations (see the original image for scales and additional detail). Though TOI-700 d’s atmospheric ion escape rates could be a few orders of magnitude higher than the rates typical of the terrestrial planets in our own solar system, the authors show that TOI-700 d could still retain a substantial atmosphere for more than a billion years — so it may well be worth exploring this planet further in the future! For more information, check out the full article below.

Citation

“Atmospheric Escape From TOI-700 d: Venus versus Earth Analogs,” Chuanfei Dong et al 2020 ApJL 896 L24. doi:10.3847/2041-8213/ab982f

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