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16 Psyche

Editor’s note: In these last two weeks of 2020, 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.

HST UV Observations of Asteroid (16) Psyche

Published October 2020

Main takeaway:

A study led by Tracy Becker (Southwest Research Institute) presents new ultraviolet images of the large asteroid (16) Psyche captured with the Hubble Space Telescope. These images reveal potential weathering of the asteroid’s metallic surface by the solar wind.

Why it’s interesting:

At more than 200 km in diameter, Psyche is one of the largest asteroids known in our solar system; it’s thought to be the exposed core of a failed protoplanet. One of this body’s most unusual properties is its apparent composition: its surface appears to be predominantly composed of metals like iron and nickel. The new ultraviolet observations of Psyche’s surface may help us to better understand its composition and how this asteroid holds up under the physical processes of our solar system, like bombardment by the solar wind.

Psyche

Illustration of a spacecraft orbiting the asteroid Psyche. [NASA/JPL-Caltech/Arizona State Univ./Space Systems Loral/Peter Rubin]

Why these observations are timely:

We’re going to Psyche! NASA will launch a spacecraft in 2022 that will arrive at Psyche in 2026. This mission’s goal is to spend nearly 2 years orbiting the asteroid and observing its topography, surface features, gravity, magnetism, and other characteristics to better understand the properties and origins of planetary cores. Carefully examining Psyche now will help us to get more out of this future mission.

Citation

Tracy M. Becker et al 2020 Planet. Sci. J. 1 53. doi:10.3847/PSJ/abb67e

coronal strands

Editor’s note: In these last two weeks of 2020, 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.

Is the High-Resolution Coronal Imager Resolving Coronal Strands? Results from AR 12712

Published April 2020

Main takeaway:

NASA’s High-Resolution Coronal Imager, or Hi-C mission, captured high-resolution images of the fine strands that make up the loops within the Sun’s outer atmosphere, the corona. The images, presented in a publication led by Thomas Williams (University of Central Lancaster, UK), demonstrate resolution on scales all the way down to ~200 km (~125 miles).

Why it’s interesting:

coronal strands

Top: The location of the active region explored with new Hi-C images (identified by the dotted box). Bottom: Comparison of SDO/AIA (left) and Hi-C (right) images of the coronal strands in the active region. Click for a closer look. [Williams et al. 2020]

These images show that the loops of plasma that extend above the Sun’s surface have even finer structure than previously observed. The coronal loops are constructed from collections of strands that have typical widths of ~500 km, although some are even narrower. Examining the structure of this plasma could help us to understand how the solar corona is so unexpectedly hot — the corona clocks in at more than a million degrees Fahrenheit, hundreds of times hotter than the Sun’s surface.

Why these views are unique:

Perhaps surprisingly, Hi-C is not a space telescope parked in orbit and beaming these spectacularly high resolution solar images back to Earth. Instead, this imager was launched on a sounding rocket for its third time in 2018, and it produced these remarkable observations from just 329 seconds of data taken while the rocket was high in its arc through our atmosphere. The high quality of these images — even when compared with those of established space telescopes like the Solar Dynamics Observatory — highlights the need for a permanent solar observatory with the resolving power of Hi-C.

Citation

Thomas Williams et al 2020 ApJ 892 134. doi:10.3847/1538-4357/ab6dcf

'Oumuamua

Editor’s note: In these last two weeks of 2020, 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.

Destruction of Molecular Hydrogen Ice and Implications for 1I/2017 U1 (‘Oumuamua)

Published August 2020

Main takeaway:

One more potential explanation for the strange behavior of interstellar asteroid 1I/2017 U1 (‘Oumuamua) has been dashed thanks to a study by Thiem Hoang (Korea Astronomy and Space Science Institute; Korea University of Science and Technology) and Abraham Loeb (Harvard University). Hoang and Loeb show that an ‘Oumuamua-sized body made out of molecular hydrogen ice could not have survived the interstellar journey from its birth site to our solar system.

Why it’s interesting:

outgassing 'Oumuamua

Illustration of outgassing from the surface of interstellar asteroid ‘Oumuamua. [ESA/Hubble, NASA, ESO, M. Kornmesser]

A number of mysteries were raised by the 2017 discovery of ‘Oumuamua, the first interstellar object detected visiting our solar system. Chief among these: what caused ‘Oumuamua’s unexpected non-gravitational acceleration as it sped away from the Sun and off into space? One posited theory was that this interstellar asteroid was actually a hydrogen iceberg. A composition of hydrogen ice could explain the asteroid’s unusual, elongated shape, and sublimation of this ice could drive its extra acceleration. But Hoang and Loeb’s analysis shows that an ‘Oumuamua-sized hydrogen iceberg would be destroyed within 10 million years by starlight heating. Collisional heating would also rapidly wear at such an object in its birthplace, potentially destroying it before it could escape into the interstellar medium and journey to our solar system. And even the formation of such a hydrogen iceberg seems unlikely: Hoang and Loeb show that it’s difficult to form icy grains rich in molecular hydrogen in the dense clouds that could birth an interstellar asteroid.

What this means for dark matter:

The implications of this hydrogen challenge extend beyond ‘Oumuamua. Primordial snowballs — theorized bodies made of molecular hydrogen ice in the early universe — have been proposed as one potential component of dark matter. But Hoang and Loeb’s work shows that such snowballs are unlikely to have been able to form, and even if they had, they would be unable to survive to present day. This suggests we must look elsewhere — both for an explanation for dark matter and for an explanation for ‘Oumuamua’s mysteries.

Citation

Thiem Hoang and Abraham Loeb 2020 ApJL 899 L23. doi:10.3847/2041-8213/abab0c

K2-18b

Editor’s note: In these last two weeks of 2020, 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 Interior and Atmosphere of the Habitable-zone Exoplanet K2-18b

Published February 2020

Main takeaway:

A team of scientists led by Nikku Madhusudhan (University of Cambridge, UK) investigated the atmospheric and interior conditions of K2-18b, an exoplanet located ~124 light-years away that is roughly 2.6 Earth radii in size and 8.6 Earth masses in weight. They found that this planet could have the right conditions to support liquid water beneath its atmosphere on its surface.

Why it’s interesting:

super-Earth

Artist’s impression of a super-Earth exoplanet orbiting its host. [Jack Madden/Cornell University]

Most studies of potentially habitable worlds have focused on Earth-like, rocky planets. K2-18b represents a different category: a super-Earth that’s between Earth and Neptune in its properties. Madhusudhan and collaborators’ analysis demonstrates that the potential for habitable conditions aren’t restricted to Earth-like planets — larger, less dense bodies might also present the right conditions to support life.

What kind of world K2-18b might be:

Madhusudhan and collaborators use observations of K2-18b’s atmosphere and its bulk properties to constrain this planet’s structure. The resulting possible scenarios for K2-18b range from  a rocky world surrounded by an enormous hydrogen and helium atmospheric envelope, to a water world with a very thin atmosphere. Future observations with observatories like the upcoming James Webb Space Telescope should help us to learn more about this potentially habitable world.

Citation

Nikku Madhusudhan et al 2020 ApJL 891 L7. doi:10.3847/2041-8213/ab7229

Sgr Astar

Editor’s note: In these last two weeks of 2020, 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.

Time Variations in the Flux Density of Sgr A* at 230 GHz Detected with ALMA

Published April 2020

Main takeaway:

Recent observations show that the supermassive black hole at the center of the Milky Way, Sagittarius A*, flickers on a regular timescale of 30 minutes, with slower variations on hour-long timescales. The observations of this twinkling are presented in a publication led by Yuhei Iwata (Keio University).

Why it’s interesting:

The twinkles around this 4-million-solar-mass black hole, detected at millimeter wavelengths within 700 minutes of ALMA data across 10 days, may be caused by radio hot spots in the inner disk of material falling onto Sgr A*. Because this material is moving at incredibly high speeds, emitted photons from hot spots get a boost that make them shine brightly enough for us to detect. If this picture is correct, we can use our observations of these flickers to learn more about the strange physics that occurs close in around black holes.

Why this matters for imaging:

M87 EHT image

The first detailed image of a black hole, M87*, taken with the Event Horizon Telescope. [Adapted from EHT collaboration et al 2019]

Wondering why the Event Horizon Telescope released a picture of the black hole at the center of the galaxy M87, yet they haven’t announced images of the much closer supermassive black hole within our own galaxy? It’s largely due to the difference in scales: since M87* is much larger than Sgr A* (more than a thousand times more massive!), the motions around M87* occur on substantially longer timescales than the motions around Sgr A*. The steady nature of the black hole in M87 makes it much easier to photograph than the twinkling, ever-changing Sgr A* — as the timescales for flicking measured here by Iwata and collaborators demonstrate. Because Sgr A* is so tricky to capture, we’ll have to be patient as we wait for images of it!

Citation

Yuhei Iwata et al 2020 ApJL 892 L30. doi:10.3847/2041-8213/ab800d

alignment planes

Editor’s note: In these last two weeks of 2020, 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.

Anisotropy of Long-period Comets Explained by Their Formation Process

Published August 2020

Main takeaway:

New modeling by Arika Higuchi (University of Occupational and Environmental Health, Japan) suggests that there is more than one plane of alignment for our solar system. In addition to the ecliptic plane, which is tilted at ~60° relative to the Milky Way’s plane and within which the planets and many minor bodies reside, there exists a second alignment plane — the “empty ecliptic” — tilted at ~60° in the other direction relative to the galactic plane.

Why it’s interesting:

Comet NEAT

Photograph of a passing comet, as seen from Earth. [NASA/NOAO/NSF/T. Rector/Z. Levay/L.Frattare]

Long-period comets — comets that take tens of thousands of years to orbit — often have orbits that are not aligned with the ecliptic plane. Their orbits can’t be explained by simple scattering off of other solar system bodies, because even their aphelia — the points in their orbits farthest from the Sun — don’t lie near the ecliptic. The second alignment plane proposed by Higuchi could explain the orbits of these observed comets and solve the puzzle of how our solar system evolved into its current structure.

What’s causing this second plane:

The orbits of solar system bodies are initially determined by the angular momentum of the young solar nebula from which they formed, and the orbits are later shaped by the local gravitational forces of the Sun and larger solar system bodies as objects interact. But there’s an additional factor at play: the small but still significant gravitational influence of the Milky Way galaxy in which our solar system resides. Higuchi shows that when we take into account all of these forces, two fundamental alignment planes arise in the solar system: the well-known ecliptic, where most bodies likely form, and the empty ecliptic, which some comets scatter into over time.

Citation

Arika Higuchi 2020 AJ 160 134. doi:10.3847/1538-3881/aba94d

luminous matter distribution

Editor’s note: In these last two weeks of 2020, 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.

Implications of Symmetry and Pressure in Friedmann Cosmology. III. Point Sources of Dark Energy that Tend toward Uniformity

Published September 2020

Main takeaway:

A study led by Kevin Croker (University of Hawai’i at Mānoa) explores the possibility that the first generation of massive stars in the universe — Population III stars — collapsed at the end of their lives not into black holes, but instead into GEneric Objects of Dark Energy (GEODEs). These point sources of dark energy, distributed between galaxies, would explain the universe’s accelerating expansion.

Why it’s interesting:

geodes

Some theories propose that objects we’ve previously thought were black holes, like the central object in the galaxy M87 recently imaged by the Event Horizon Telescope, might instead be GEODEs. The inset diagram shows the structure of a GEODE. [EHT collaboration; NASA/CXC/Villanova University]

Scientists have long attempted to explain the universe’s accelerating expansion and the nature of dark energy. A great challenge is finding a solution that’s consistent with all observations — especially those that inform our understanding of the formation of structure in the early universe. Croker and collaborators demonstrate that, because rapidly spinning GEODEs repel each other, they tend to move away from galaxies and into the empty spaces between clusters of matter. This means that the formation of GEODEs at redshifts of z = 8 to 20 doesn’t disrupt the observed structure formation within the universe — so the GEODE scenario is neatly consistent with our observations across a broad range of scales.

Why the existence of GEODEs is tough to prove:

GEODEs are exotic theorized objects that consist of a spinning layer surrounding a core of dark energy — but, to us, they should largely appear to mimic black holes! Just like black holes, they are massive but don’t produce light, so they’re ordinarily invisible. The collision of two GEODEs is expected to produce a gravitational-wave signal that looks just like that from two black holes merging. For this reason, the discovery that GEODEs and black holes move differently through space is especially intriguing — this might provide us with a way to distinguish them.

Citation

K. S. Croker et al 2020 ApJ 900 57. doi:10.3847/1538-4357/abad2f

NGC-1559

A galaxy’s stars hold clues to its history. So, if we look at the stellar populations of several galaxies along with the properties of the galaxies themselves, we may be able to tease out trends in galaxy evolution.

The number of new stars formed versus the number of existing stars for different types of galaxies. Star-forming galaxies hang above the “main sequence”, with “starburst” galaxies lying at the extreme. [V. Buat]

Galaxies and the Stars that Make Them

To begin with, what stellar properties can you use to study galaxy evolution? Age is a key one. A stellar population with more old stars suggests that the galaxy formed most of its stars a long time ago, while a stellar population dominated by young stars points to more recent star formation.

The abundances of different elements are also insightful. Astronomers often consider the abundance of elements that aren’t hydrogen and helium relative to hydrogen (metallicity) and the abundance of alpha elements, like carbon and oxygen, relative to iron. The former property can tell us how many generations of stars the galaxy has hosted, and the latter can tell us how long the galaxy was forming stars.

As for galaxies themselves, the overall mass and size of a galaxy can be linked to the stars it holds. Another useful quantity is velocity dispersion, or how stars move within the galaxy. Recently, more subtle properties like the surface distribution of mass across a galaxy and its gravitational potential have been used as probes of galaxy evolution. In a new study, a group of researchers led by Tania Barone (The Australian National University/The University of Sydney) explored how these structural properties could be related to stellar properties.

Starting from the upper-left plot and moving clockwise: log of age versus gravitational potential, log of age versus surface mass density, metallicity versus surface mass density, and metallicity versus gravitational potential; best-fit relations are shown by the solid line. Galaxies plotted in yellow have a smaller radius than galaxies plotted in blue. The inset plots show the fit residuals, the difference between the measured and best-fit age/metallicity. If the slope of the residual distribution (given by the numbers in the inset) is small, it’s likely that the relation actually exists. Click to enlarge. [Adapted from Barone et al. 2020]

Tying Things Together

Barone and collaborators focused on star-forming galaxies in this study. While all galaxies could be said to be star-forming, a galaxy must be forming stars at a higher than average rate to be labeled a star-forming galaxy. This distinction is important since combining results from different types of galaxies could obscure any existing trends.

Age and elemental abundances can be calculated under different assumptions, one based on the brightness of the galaxy and the other based on the galaxy’s mass. However, Barone and collaborators found that under both of these assumptions, age appears to correlate with surface mass density and metallicity correlates with gravitational potential. Interestingly, both these relations were also seen in a similar study of early-type galaxies that formed when the universe was younger.

So what do these correlations say about galaxy evolution? The age–mass-density relation suggests that star formation is dictated by the availability and density of star-forming gas. The metallicity–gravitational-potential relation suggests that galaxies with lower potentials have a harder time keeping material from being whisked away by astronomical winds.

One way to explore these relations further — aside from observations — is using cosmological simulations. For now, this study is a great example of how we can assemble the clues we have to solve the puzzle of galaxy evolution!

Citation

“Gravitational Potential and Surface Density Drive Stellar Populations. II. Star-forming Galaxies,” Tania M. Barone et al 2020 ApJ 898 62. doi:10.3847/1538-4357/ab9951

coronal hole

coronal hole schematic

Coronal holes form where magnetic field lines open into space (B) instead of looping back to the solar surface (A). [Sebman81]

Want to be able to predict the flurries of high-energy particles that slam into Earth from the Sun, triggering dramatic geomagnetic storms? Then you’ll first need to track the holes in the Sun’s outer atmosphere. In a new study, scientists explore whether we can train computers to identify these patterns.

Tracking the Dark

The Sun’s outer atmosphere — its corona — is far from uniform. Both across the Sun’s face and over time, the corona varies in its density and corresponding appearance, looking brighter in some regions and forming large, dark coronal holes in others.

Coronal holes represent regions of low-density plasma containing magnetic field lines that open out into space rather than looping back down to the Sun’s surface. These areas of open field lines provide an excellent escape point for the solar wind, the high-energy particles that flow off of the Sun.

For this reason, identifying the locations, sizes, and timescales for formation and evolution of coronal holes is critical for predicting solar wind behavior and forecasting space weather. In addition, tracking coronal holes over solar cycles can shed light on how magnetic fields drive solar activity and may even help us to better predict upcoming solar activity.

A Full-Surface View

synoptic map process

The process of making a synoptic map from daily solar disk images. Click to enlarge. [NSO]

So how do we identify and track coronal holes? It’s surprisingly difficult to map out holes in the Sun — especially large ones like these! Coronal holes often don’t fit within a single image of the Sun’s disk, as viewed from Earth; instead, we have to wait for the Sun to rotate through its month-long period and take successive images as it turns.

To get a comprehensive view of the entire solar surface, we transform these individual disk images into a shared coordinate system, and then we stack and combine the daily images within each rotation period, creating what’s known as a synoptic map for each rotation.

If we can then identify the coronal holes in a uniform way within each of these synoptic maps, we’ll have a useful data set for studying how coronal holes vary across the Sun’s surface and over time! In a new publication, a team of scientists led by Egor Illarionov (Moscow State University, Russia) has found a way to do this that uses computers to do the heavy lifting.

synoptic maps and CH boundaries

Overlaid synoptic maps and algorithm-identified coronal hole boundaries (green outlines) for three different solar rotation periods: two during solar minima (a and c) and one during a solar maximum (b). [Illarionov et al. 2020]

Training the Machine

Illarionov and collaborators use a training set of daily solar disk images to teach a convolutional neural network — a type of machine learning algorithm that’s especially good at analyzing images — how to identify coronal holes. The strength of a convolutional neural network is that it works equally well analyzing any image shape; the authors can therefore use this same algorithm, once trained, to accurately identify coronal holes in the synoptic solar maps.

Using this approach, Illarionov and collaborators construct a coronal hole catalog spanning 2010–2020. They conduct preliminary analysis of the evolution of coronal holes between solar minimum and solar maximum, and they compare these maps to maps of magnetic flux across the same times.

Looking to dive in, yourself? The authors make their code and the resulting coronal hole maps publicly available. With this dataset on hand, we can hope to soon learn more from these holes in the Sun.

Citation

“Machine-learning Approach to Identification of Coronal Holes in Solar Disk Images and Synoptic Maps,” Egor Illarionov et al 2020 ApJ 903 115. doi:10.3847/1538-4357/abb94d

gas-giant transit

What’s going on around the hot Jupiter exoplanet HAT-P-41b? This planet’s atmosphere is harboring a mystery, recently revealed by observations that span infrared through ultraviolet (UV) light.

Expanding the Spectrum

transmission spectroscopy

As a star’s light filters through a planet’s atmosphere on its way to Earth, the atmosphere absorbs certain wavelengths depending on its composition. [European Southern Observatory]

At the latest count, we’ve discovered nearly 10,000 confirmed and candidate exoplanets. One way to study these distant worlds is using transmission spectroscopy: we can explore a host star’s spectrum as its planet passes in front of it. The light that filters through the planet’s atmosphere is imprinted with spectral signatures that we can then analyze to learn about the physics and chemistry at work in the planet’s atmosphere.

Ideally, we’d gather spectroscopic observations of planet transits across a broad range of wavelengths, because each region of the electromagnetic spectrum provides an additional constraint for atmospheric models. But so far we’ve probed fewer than 20 exoplanets in the UV, a regime that can reveal critical details of atmospheric physics.

In a new study, a team led by Nikole Lewis (Cornell University) adds one more planet to this collection, the hot Jupiter HAT-P-41b — but what these scientists found was unexpected.

HAT-P-41b's transmission spectrum

HAT-P-41b’s transmission spectrum from 0.2 to 5.0 μm (red/blue/gray data points) plotted along with several atmospheric models (green, teal, and purple lines). [Lewis et al. 2020]

Unexpected Absorption

Lewis and collaborators conducted one of the most comprehensive explorations of an exoplanet atmosphere yet, combining high-precision Hubble and Spitzer observations to construct a transmission spectrum spanning UV through infrared wavelengths. They then use multiple different approaches to conduct detailed atmospheric modeling for HAT-P-41b from their observations.

The result? The authors show that HAT-P-41b can’t be neatly described by the models we usually fit to hot Jupiters. To fit the full range of observations, there must be a molecular species that readily absorbs near-UV light present in unexpectedly large quantities in this hot Jupiter’s atmosphere. Lewis and collaborators argue that the most likely candidate is H- (the hydrogen anion, or a hydrogen atom with an extra, easily-ejected electron), although other candidates include CrH, AlO, and VO.

A New Chemistry

What does this mean? Based on the temperature measured for HAT-P-41b (a toasty ~1,700 K), the chemistry we typically assume for hot Jupiter atmospheres falls several orders of magnitude short of producing enough H- to explain observations. Instead, the authors posit that there must be some new chemistry, not yet taken into account, that produces the excess H- abundance — possibly driven by the intense ultraviolet radiation from this planet’s F-type host star.

JWST

An artist’s illustration of the upcoming James Webb Space Telescope. [NASA/JWST]

This striking discovery has broad implications. If correct, it likely means that other hot Jupiters also contain large amounts of H- (or an equivalent near-UV absorber) in their atmospheres. In turn, this would imply that our understanding of hot Jupiter atmospheres needs to be substantially reworked.

If nothing else, the Lewis and collaborators’ results demonstrate the importance of broad-spectrum observations of planetary atmospheres, as well as the benefit of using multiple reduction and interpretation techniques to analyze the observations. This careful approach will become even more valuable with the upcoming launch of the James Webb Space Telescope and the new era of exoplanet atmosphere exploration it will usher in!

Citation

“Into the UV: The Atmosphere of the Hot Jupiter HAT-P-41b Revealed,” N. K. Lewis et al 2020 ApJL 902 L19. doi:10.3847/2041-8213/abb77f

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