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JWST image of the galaxy cluster SMACS

Editor’s Note: For the remainder of 2024, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume January 3rd.

Which Came First: Supermassive Black Holes or Galaxies? Insights from JWST

Published January 2024

Main takeaway:

Joseph Silk (Sorbonne University; The Johns Hopkins University; University of Oxford) and collaborators have theorized that supermassive black holes are responsible for the high rate of star formation seen in many young galaxies. The proposed framework flips the conventional narrative of black hole and galaxy formation, suggesting that supermassive black hole growth spurred the formation of new stars rather than lagging behind the formation of stars.

Why it’s interesting:

How and when supermassive black holes formed in the early universe is one of the key unanswered questions in astronomy. The conventional theory of supermassive black hole formation suggests that galaxies formed first: gas clouds collapsed to form the first stars, which left behind stellar-mass black holes when the stars expired. A series of collisions between these stellar-mass black holes slowly built the first supermassive black holes while star formation continued busily in the background. The team’s new theory suggests that black holes and galaxies grew in tandem instead, with black hole growth playing an important role in the formation of new stars.

Solving a cosmic chicken-and-egg problem:

diagram showing the redshifts at which supermassive black holes spur and quench star formation

Diagram showing the redshifts at which supermassive black holes spur and quench star formation in their host galaxies, in the proposed framework. Click to enlarge. [Silk et al. 2024]

Observations from JWST have revealed the presence of extremely bright galaxies in the early universe, leading astronomers to wonder how these galaxies became so brilliant so quickly. Within the framework proposed by Silk’s team, the extraordinary brightness of these young galaxies is a natural consequence of the supermassive black holes at their centers; as the growing supermassive black holes accreted gas from their surroundings, they shot out powerful outflows that slammed into the surrounding gas, compressing it and triggering an explosive burst of star formation. This theorized powerful burst of star formation doesn’t last forever, though; about 1 billion years into the universe’s history, a shift in the outflowing winds of the supermassive black holes cast out the gas that fueled star formation, bringing it to a halt. Testing the predictions of this theory is likely to be difficult, though sensitive observations and intricate simulations may provide a path forward.

Citation

Joseph Silk et al 2024 ApJL 961 L39. doi:10.3847/2041-8213/ad1bf0

locations of stars in two newly discovered Milky Way substructures

Editor’s Note: For the remainder of 2024, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume January 3rd.

Shiva and Shakti: Presumed Proto-Galactic Fragments in the Inner Milky Way

Published March 2024

Main takeaway:

Illustration of the structure of the Milky Way

Illustration of the structure of the Milky Way. Click to enlarge. [Left: NASA/JPL-Caltech; right: ESA; layout: ESA/ATG medialab]

Using data from the Gaia spacecraft, Khyati Malhan and Hans-Walter Rix (Max Planck Institute for Astronomy) discovered two stellar structures within the Milky Way. The two structures, named Shiva and Shakti, each contain more than 10 billion solar masses’ worth of stars and orbit in the inner Milky Way. Though the origins of the two structures are not yet known, current data suggest that they formed more than 12 billion years ago — before the Milky Way’s spiral arms and stellar disk came to be.

Why it’s interesting:

The Gaia spacecraft makes precise measurements of the positions, distances, velocities, and chemical compositions of stars in our galaxy. Using these data, researchers have discovered many new structures and stellar populations in and around our galaxy, such as stellar streams and satellite galaxies. These structures hold important clues to the formation and evolution of our galaxy, revealing a complex history of accretion, assimilation, and migration of gas and stars.

Potential origins of the newfound structures:

Stellar structures within our galaxy can develop in one of two ways: in situ, meaning forming out of gas and stars already present in the Milky Way, or through accretion from outside the galaxy. The orbits and chemical abundances of the stars in Shiva and Shakti tell a conflicting story about the origins of these two structures. The orbits point to an accretion origin, while the chemical abundances suggest in situ formation; this combination of features has never been seen before. Malhan and Rix suggest that these opposing conclusions can be reconciled in one of two ways: 1) the stars of the Shiva and Shakti structures belonged to the Milky Way’s diffuse, extended halo and became trapped in a resonance with the Milky Way’s central bar of stars before migrating to their current positions, or 2) the Shiva and Shakti structures are truly ancient, representing fragments of proto-galaxies that formed before the Milky Way was constructed. While each of these hypotheses has its strengths and neither is fully compatible with the data, Malhan and Rix favor the proto-galactic fragment idea. Luckily, upcoming surveys and instruments will give an even deeper look into the structure of the Milky Way, helping to clarify how our galaxy was assembled.

Citation

Khyati Malhan and Hans-Walter Rix 2024 ApJ 964 104. doi:10.3847/1538-4357/ad1885

asteroid 243 Ida

Editor’s Note: For the remainder of 2024, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume January 3rd.

Detection of Molecular H2O on Nominally Anhydrous Asteroids

Published February 2024

Main takeaway:

Using the Stratospheric Observatory for Infrared Astronomy (SOFIA) — a now-decommissioned Boeing 747 that toted an infrared telescope to 40,000 feet for more than a decade — Anicia Arredondo (Southwest Research Institute) and collaborators investigated the surface compositions of four asteroids. The spectra of two asteroids in the sample contained a feature unambiguously attributed to water, marking the first time water has been detected on the surface of an asteroid.

Why it’s interesting:

Asteroids represent material left over from the formation of the solar system. By studying asteroids, researchers hope to learn about the solar nebula from which the planets coalesced. Of particular interest is the distribution of water in the early solar system, which can tell us about how Earth got its water, as well as how planets in other star systems might develop. The four asteroids examined in this study are S-type asteroids, which are thought to form in the inner solar system, where volatile materials like water are scarce. The discovery of water molecules on these supposedly dry asteroids provides an important constraint on theories of solar system and planet formation.

spectra of four asteroids

Spectra of the four asteroids examined in this work. Two of the asteroids, (7) Iris and (20) Massalia, clearly show the 6-micron water feature, while the other two asteroids do not. Click to enlarge. [Arredondo et al. 2024]

On the definitive spectral feature:

Detecting a particular molecule in space can be difficult because the spectral features from different molecules often overlap. Many asteroids exhibit a spectral feature at 3 microns (1 micron = 10-6 meter) that arises from stretching of the chemical bond between an oxygen atom and a hydrogen atom. This bond is present in a water molecule — meaning that this feature could be due to water — but it’s also present in many other molecules. To make their definitive detection of water, Arredondo’s team searched instead for a spectral feature at 6 microns, which is solely due to water and has been detected previously on the Moon. The water molecules discovered in this study might be trapped in glass beads on the asteroids’ surfaces, adsorbed onto silicon, or bound up in minerals. The team plans to use JWST to continue their search for water on asteroid surfaces.

Citation

Anicia Arredondo et al 2024 Planet. Sci. J. 5 37. doi:10.3847/PSJ/ad18b8

faintest known Milky Way satellite Ursa Major III/UNIONS 1

Editor’s Note: For the remainder of 2024, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume January 3rd.

The Discovery of the Faintest Known Milky Way Satellite Using UNIONS

Published January 2024

Main takeaway:

A team led by Simon Smith (University of Victoria) discovered a collection of stars orbiting the Milky Way in data from the Ultraviolet Near Infrared Optical Northern Survey (UNIONS), which is carried out by three telescopes in Hawaiʻi. The newly discovered Milky Way satellite, named Ursa Major III/UNIONS 1, contains about 57 stars and has a total mass of 16 solar masses. Ursa Major III/UNIONS 1 is the faintest known Milky Way satellite.

Why it’s interesting:

The advent of sensitive surveys has revealed much about the Milky Way’s neighborhood, including a steadily growing population of satellites. These surveys are discovering fainter and fainter satellites that blur the line between the largest star clusters and the smallest dwarf galaxies. Dwarf galaxies and star clusters have significant overlap in their masses and are distinguished by the presence or absence of dark matter: dwarf galaxies are thought to form in individual dark matter halos, while star clusters are not.

properties of Ursa Major III/UNIONS 1 compared to different types of Milky Way satellites

The brightness and size of Ursa Major III/UNIONS 1 (orange square) relative to dwarf galaxies (blue circles), globular clusters (red circles), and ambiguous satellites (black diamonds). Click to enlarge. [Smith et al. 2024]

On the nature of this faint satellite:

It’s not yet clear whether Ursa Major III/UNIONS 1 is a dwarf galaxy or a star cluster. This fact is reflected in the system’s name: newfound dwarf galaxies are named for the constellation in which they appear, while star clusters are named for the survey in which they were discovered. This satellite currently bears both kinds of names. Smith’s team found evidence that the velocities of stars in Ursa Major III/UNIONS 1 are spread fairly widely around the mean velocity of the system, suggesting that the system sits in its own dark matter halo and thus could be an astoundingly small dwarf galaxy. However, this finding is highly sensitive to the number of stars included in the analysis, highlighting the need for careful follow-up observations of this faint Milky Way satellite — and many others.

Citation

Simon E. T. Smith et al 2024 ApJ 961 92. doi:10.3847/1538-4357/ad0d9f

artist's impression of a quasar

Editor’s Note: For the remainder of 2024, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume January 3rd.

Quaia, the Gaia-unWISE Quasar Catalog: An All-Sky Spectroscopic Quasar Sample

Published March 2024

Main takeaway:

map of quasars in the Quaia catalog

The on-sky distribution of quasars in the Quaia catalog. The dearth of objects across the center of the image is due to the Milky Way’s disk. The team also developed a more refined sample of 755,850 quasars with especially reliable distance estimates. Click to enlarge. [Adapted from Storey-Fisher et al. 2024]

Beginning with 6,649,162 quasar candidates identified by the Gaia mission, Kate Storey-Fisher (New York University) and collaborators have constructed a catalog of 1,295,502 quasars spread across the sky. The catalog, named Quaia, includes redshifts for each quasar, enabling detailed studies of the large-scale structure of our universe over the course of cosmic history.

Why it’s interesting:

Quasars — short for “quasi-stellar radio sources” — are extremely luminous galactic centers powered by accreting supermassive black holes. Quasars are thought to reside in regions of dense dark matter, making them important probes of the unseen dark-matter structures that suffuse the universe. In addition to their cosmological importance, studying quasars can also clue us in to the physics of accretion, the growth of supermassive black holes, and the evolution of massive galaxies.

More details about the data:

The Gaia satellite, which began its mission in 2013, was designed to obtain precise positions, distances, and velocities for stars in the Milky Way. Fortuitously, Gaia also studiously documented its observations of objects that are far brighter and more distant than stars in our galaxy, like quasars. The initial 6.6-million-object sample of quasar candidates, however, was riddled with objects mistaken for quasars, and many of the distances to the objects — critical for cosmological studies — were wildly inaccurate. Storey-Fisher’s team incorporated data from other sources, like the Wide-field Infrared Survey Explorer and the Sloan Digital Sky Survey, to boost the purity of the sample and provide better distance estimates. You can explore the final sample for yourself at this link.

Citation

Kate Storey-Fisher et al 2024 ApJ 964 69. doi:10.3847/1538-4357/ad1328

a diagram of the inner solar system showing the orbits of the inner planets as well as the orbit of 2024 PT5

Editor’s Note: For the remainder of 2024, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume January 3rd.

A Two-Month Mini-Moon: 2024 PT5 Captured by Earth from September to November

Published September 2024

Main takeaway:

Carlos de la Fuente Marcos and Raúl de la Fuente Marcos (Complutense University of Madrid) used N-body calculations to explore the orbit of the near-Earth object 2024 PT5. They found that 2024 PT5 would accompany Earth as a “mini-moon” from 29 September 2024 to 25 November 2024. Their predictions suggested that the object, which is thought to be about 33 feet (10 meters) wide, would linger for these two months at a distance several times larger than the distance between Earth and the Moon.

Why it’s interesting:

2024 PT5 would not be the first object to be classified as a mini-moon. Earth snags an asteroid from the swarm of near-Earth objects every 10 or 20 years; some of these objects complete one or more orbits around Earth (“temporarily captured orbiters”) while others fail to circle Earth even once (“temporarily captured flybys”). Eventually, the Sun’s persuasive gravitational pull steals these objects away from Earth. The asteroid 2006 RH120 spent almost exactly a year as a mini-moon in 2006–2007, and 2020 CD3 spent several years as Earth’s companion before departing in 2020. Some asteroids make repeated appearances, like 2022 NX1, which was last seen near Earth in 2022 and will return in 2051. So, while mini-moons are not uncommon, 2024 PT5 stands out as being one of the largest known objects in this class.

What researchers learned from 2024 PT5’s close approach:

As predicted, 2024 PT5 spent two months as a close companion to Earth, though NASA reported that the object was never fully captured by Earth’s gravity. While 2024 PT5 floated near Earth, researchers took the opportunity to study the object more closely. These observations suggested that the asteroid may have once been a part of the Moon — the actual Moon, not a miniature version — that was ejected when an object slammed into the Moon. Alternatively, the asteroid may be a fragment of a larger object that was disrupted. Though 2024 PT5 departed in late November, it’s not gone for long: its next close pass to Earth will take place in January 2025.

Citation

Carlos de la Fuente Marcos and Raúl de la Fuente Marcos 2024 Res. Notes AAS 8 224. doi:10.3847/2515-5172/ad781f

Polaris, the North Star, with the integrated flux nebula of the Milky Way

Editor’s Note: For the remainder of 2024, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume January 3rd.

The Orbit and Dynamical Mass of Polaris: Observations with the CHARA Array

Published August 2024

Main takeaway:

A research team led by Nancy Remage Evans (Smithsonian Astrophysical Observatory) used observations from the Center for High Angular Resolution Astronomy (CHARA) Array to deduce the mass of the North Star, Polaris. The team found the mass to be 5.13 ± 0.28 solar masses, which is about 50% more massive than previous estimates. The new data also show starspots on Polaris’s surface, which may explain some of the star’s properties.

Why it’s interesting:

Though a seemingly unwavering fixture in the northern night sky, Polaris is actually the nearest example of an important class of variable stars called Cepheids. The intrinsic luminosity of a Cepheid variable scales with the rate at which the star’s brightness varies. Thus, charting a Cepheid’s brightness over time provides a precise measure of the star’s luminosity. Comparing the intrinsic luminosity of the star to how bright the star appears then provides a way to measure the distance to the star. Cepheid variables are an important rung on the cosmic distance ladder, helping to measure the distances to far-off galaxies and even measure the rate of the universe’s expansion.

On amassing masses and spotting starspots:

reconstructions of Polaris's surface brightness

Two reconstructions of Polaris’s surface, suggesting the presence of dark starspots. [Evans et al. 2024]

Polaris is a member of a triple star system containing a binary pair and a third, more distant companion. The team measured Polaris’s motion along its orbit with its binary companion to calculate the star’s mass. Curiously, Polaris appears to be brighter than expected for its mass — just one of many unusual features of this star, which seems to have properties in common with Cepheids as well as with non-variable supergiant stars. This study also found evidence for the existence of starspots — cool regions of a star’s surface caused by strong magnetic fields poking up through the surface — which might help to determine the star’s rotation period.

Citation

Nancy Remage Evans et al 2024 ApJ 971 190. doi:10.3847/1538-4357/ad5e7a

multi-wavelength image of the Sun

For those of us in the Northern Hemisphere, the winter solstice is rapidly approaching — so today, let’s make up for the lack of sunshine by basking in the warmth of some solar physics research! This instance of the Monthly Roundup gives a quick overview of four recent research articles that share new findings from our home star.

Tracing Threads in a Solar Prominence

Prominences are massive arcs of cool, dense plasma that appear high above the solar surface along the edge of the Sun’s disk. (These same features, when seen on the Sun’s disk rather than silhouetted against space, are called filaments.) Occasionally, prominences erupt in massive explosions that eject solar plasma into the solar system. Other times, they gently dissipate after weeks or months.

The advent of high-resolution solar imaging has revealed new features in solar prominences, including threads that are 3,500–28,000 kilometers long and just 210 kilometers wide. These threads are thought to form when magnetic flux tubes become filled with cool and dense plasma. Recently, Yuxiang Song (Chinese Academy of Sciences; University of Science and Technology of China) and collaborators used the New Vacuum Solar Telescope to examine the behavior of 35 threads in a solar prominence.

displacement and intensity oscillations in a thread in a solar prominence

Example of a displacement oscillation (top row) and an intensity oscillation (bottom row) in a solar prominence thread. Click to enlarge. [Song et al. 2024]

They found that 29 of the 35 threads exhibited displacement oscillations, meaning that the threads wiggled around in a sinusoidal way. The average period of these spatial oscillations was 26 minutes. Eight of the 35 threads showed intensity oscillations; their brightness varied with a typical period of 7.7 minutes. Seven of the threads showed both displacement and intensity oscillations. These values are consistent with observations of threads in other solar prominences.

What causes the displacement and intensity oscillations in the threads? The thread oscillations appear to originate with oscillations deeper in the Sun’s atmosphere, which propagate outward and excite waves in the solar magnetic field. The waves, in turn, generate the oscillations in the threads. However, the authors raised the possibility that the two types of oscillations examined in this work have different causes. Future work is needed to disentangle the question of solar threads.

Studying the Sun from the Moon

Gone are the days when astronomers could observe the Sun only from Earth’s surface. Now, a spacecraft fleet scrutinizes the Sun from almost every angle. Among the spacecraft with an eye on the Sun is Chang’E-2: a lunar orbiter that carries an instrument called the Solar X-ray Monitor

A team led by Man-Hei Ng and Chi-Long Tang (Macau University of Science and Technology and China National Space Administration) used data from the Solar X-ray Monitor to study conditions in solar flares emerging from two active regions on the Sun. Solar flares are brief, brilliant flashes of light that are powered by the rearrangement of solar magnetic fields. Ng and Tang’s team aimed to determine which elements and ions were present in these solar flares. These data give clues as to how matter and energy are transported from the lower, denser regions of the Sun’s atmosphere to the superheated solar corona and beyond.

plot of elemental abundances relative to the photospheric abundances

Examples of elements experiencing the inverse first ionization potential effect (iron; top) and the first ionization potential effect (calcium; bottom). The plots show the ratio of the measured elemental abundance with respect to the abundance of that element within the solar photosphere. Click to enlarge. [Adapted from Ng et al. 2024]

Previous research has shown that elements that require relatively little energy (<10 electronvolts) to ionize and elements that require a relatively large amount of energy to ionize behave differently. Generally, easily ionized elements are more prevalent in flaring regions than at the denser, cooler solar surface. This effect is called the first ionization potential effect. This behavior sometimes flips in regions where the solar magnetic field is especially complicated; the reversed behavior is called the inverse first ionization potential effect.

In this study, Ng and Tang’s team observed the inverse first ionization potential effect in iron for the first time, and they confirmed the existence of this effect for silicon. They also noted another curious effect for the first time: heavy elements like iron took longer to return to their pre-flare levels than lighter elements did, showing that inertia plays an important role. Intriguingly, one of the solar flares showed yet another rare effect: a post-flare change in the elemental abundances from values typical of the solar corona to those more similar to deeper layers of the Sun’s atmosphere, suggesting that this plasma was sourced from below, in the chromosphere or photosphere.

High-Resolution Imaging of a Solar Current Sheet

Many of the explosive events that take place in the Sun’s atmosphere owe their existence to magnetic reconnection: the process of rearranging magnetic fields in a way that releases pent-up magnetic energy, providing a way to accelerate particles to high speeds. Pankaj Kumar (American University; NASA Goddard Space Flight Center) and coauthors recently reported on high-resolution solar images that reveal this process in great detail.

extreme-ultraviolet images of the Sun, showing the location of a solar current sheet

Top: A broad view of the Sun, including the active region studied in this work, indicated in white. Bottom: A zoomed-in view of the active region, showing the location of the current sheet (CS). [Adapted from Kumar et al. 2024]

The images, which come from the Solar Dynamics Observatory spacecraft and the 1.6-meter Goode Solar Telescope, have an incredible resolution of just 50 kilometers. They show a current sheet — a surface that separates regions of oppositely directed magnetic fields, such as those found in active regions on the Sun. (Active regions are areas on the Sun’s surface where the magnetic field is particularly strong and complex; these regions are the sites of solar activity like solar flares.) While short-lived current sheets have been seen before, this study marks the first time a current sheet has been directly imaged over such a long duration; the current sheet in this study persisted for about 20 hours and covered an area of roughly 2 arcseconds by 6 arcseconds.

Kumar’s team observed plasma flowing out from and in toward the current sheet at semi-regular intervals. These quasi-periodic flows provide evidence of particle acceleration via magnetic reconnection and show that reconnection helps to heat the active region plasma. Overall, the observations presented in this work provide evidence for the magnetic breakout model, a widely used model of solar eruptive events like flares and coronal mass ejections. The breakout model describes how magnetic field lines enclosing the dense plasma of a solar filament/prominence reconfigure and release the filament into space.

In addition to providing support for the breakout model, the new observations can serve as a jumping-off point for modeling and laboratory studies of magnetic reconnection. The results of this study even have implications for stars other than the Sun, helping to explain the quasi-periodic pulsations seen at X-ray wavelengths on other stars.

The Sun as a Particle Accelerator

Solar flares and coronal mass ejections — immense eruptions of solar plasma and magnetic fields — are the sites of particle acceleration and X-ray production. Understanding how much of the energy produced in the events goes toward accelerating electrons to breakneck speeds is critical to understanding the Sun’s role as a particle accelerator.

high-energy image of the Sun from the Solar Dynamics Observatory

A 17.1-nanometer image of the Sun from the Solar Dynamics Observatory. The annotations indicate the locations of the acceleration region, the hard (i.e., high-energy) X-ray sources, and the radio emission at various frequencies. Click to enlarge. [Adapted from James & Reid 2024]

Working toward this goal, Alexander James and Hamish Reid (University College London) analyzed data from a solar radio burst that took place in 2013. Solar radio bursts are brief periods of intense radio emission that are caused by fast-moving electrons and often accompany coronal mass ejections or solar flares. This particular burst was associated with a solar flare and a coronal mass ejection.

Using multi-wavelength observations of the flare, coronal mass ejection, and solar radio burst, James and Reid estimated the speeds of the electron beams they observed escaping from the acceleration site. This analysis showed extremely high speeds for electrons in the beams, ranging from 44% to 59% of the speed of light. The team then used simulations to learn about the properties of the region in which these beams were accelerated, finding that the acceleration occurred in a region that stretched some 15,000–100,000 kilometers in the direction tangent to the Sun’s surface, but just 1,000 kilometers vertically.

This work marks the first time researchers have estimated the properties of escaping electron beams from remote-sensing observations. A comparison of the results from remote-sensing data and those from the Solar Orbiter spacecraft, which ventures close to the Sun every 6 months, will provide further insight into the particle-accelerating abilities of our home star.

Citation

“Thread Displacement and Intensity Oscillations in a Quiescent Prominence,” Yuxiang Song et al 2024 ApJ 975 280. doi:10.3847/1538-4357/ad813c

“Unveiling Mass Transfer in Solar Flares: Insights from Elemental Abundance Evolutions Observed by Chang’E-2 Solar X-Ray Monitor,” Man-Hei Ng et al 2024 ApJ 972 123. doi:10.3847/1538-4357/ad5da3

“Direct Imaging of a Prolonged Plasma/Current Sheet and Quasiperiodic Magnetic Reconnection on the Sun,” Pankaj Kumar et al 2024 ApJ 973 74. doi:10.3847/1538-4357/ad63a2

“Estimating the Total Energy Content in Escaping Accelerated Solar Electron Beams,” Alexander W. James and Hamish A. S. Reid 2024 ApJ 976 128. doi:10.3847/1538-4357/ad7b38

MWC 758 protoplanetary disk

Did vortices sculpt the crescent-shaped clumps of dust around the young star MWC 758? Using data from the Atacama Large Millimeter/submillimeter Array (ALMA), researchers have mapped the motions of the dust clumps and weighed in on the vortex hypothesis.

Complicated Disks

How planets form is one of the most pressing questions in astronomy. Observations increasingly show that protoplanetary disks, the sites of planet formation, are complex objects. These disks feature rings, gaps, spirals, and vortices, any of which might signal the presence of baby planets.

Among the intriguing features seen in protoplanetary disks are crescents: asymmetric regions where the density of dust is enhanced. These regions are readily visible in observations by ALMA and have been found in several protoplanetary disks.

Researchers suspect that crescents are caused by swirling regions called vortices. Like debris caught in an eddy in a stream, dust could theoretically become trapped in a vortex, forming the clumps seen in images — and potentially creating a perfect dusty ecosystem for planets to form.

The Causes of Crescents

Recently, a team led by I-Hsuan Genevieve Kuo (Academia Sinica Institute of Astronomy and Astrophysics, Taiwan; University of Arizona) investigated the causes of crescents in the protoplanetary disk around the star MWC 758, also called HD 36112. MWC 758 is a 1.5–2-solar-mass star that is about 3.5 million years old and less than 500 light-years away. Its disk sports several intriguing features, including two crescents and a spiral. Previous research has attributed these features to the presence of one or more planets.

continuum images of MWC 758

Images of the MWC 758 disk taken at a wavelength of 1.3 mm in 2017 (left) and 2021 (right). Click to enlarge. [Kuo et al. 2024]

Vortex theory predicts that vortices in a protoplanetary disk will revolve around the central star at the Keplerian velocity (i.e., following the predictions of Kepler’s laws of motion). To test this theory, Kuo’s team used ALMA data from 2017 and 2021 to measure the motion of the two crescents. They found that the crescents moved in the direction of the disk’s rotation, with the inner crescent moving 50% slower than expected for the vortex hypothesis and the outer crescent moving 33% faster than expected.

Spiral vs. Vortex

observed and expected azimuthal velocity of dust clumps

Observed azimuthal (i.e., around the disk) velocity of the dust clumps (green and dark blue triangles) and expected velocity for Keplerian rotation (aqua circles). Click to enlarge. [Kuo et al. 2024]

Does this finding necessarily rule out the vortex hypothesis? Kuo and coauthors first investigated and eliminated the possibility that imperfections in the disk, like warps or eccentricity, were the cause of the mismatch with theory. They then noted that the motion of the crescents matches the Keplerian velocity at a radius of about 0.46 arcsecond. If a planet were present at that radius, there’s a chance it could throw the system off-kilter and produce the observed behavior — but MWC 758’s putative planets are located well inside and outside this radius.

Instead, the non-Keplerian rotation of the dust clumps might be caused by the interaction of vortices and spirals. In this case, the vortices themselves, which are invisible to us, are moving in the way predicted by theory, but the spiral knocks the (visible) dust off course.

Luckily, this prediction is testable. If vortices and spirals are vying for control of the dust crescents in MWC 758’s disk, their power struggle would be less effective on large dust grains than on small dust grains. In other words, large dust grains should adhere more closely to the expected Keplerian velocity than small dust grains do. Future high-resolution observations at different wavelengths, which probe grains of different sizes, may provide an answer.

Citation

“ALMA Observations of Proper Motions of the Dust Clumps in the Protoplanetary Disk MWC 758,” I-Hsuan Genevieve Kuo et al 2024 ApJL 975 L33. doi:10.3847/2041-8213/ad86c1

Are there places in the galaxy that are better suited for habitability? A new study suggests that the Sun’s trek through the Milky Way may contribute additional complexity to the search for life in the galaxy. 

Galactic Habitable Zones

Illustration of the solar system habitable zone

Illustration of a star system’s habitable zone. Too close to the star, shown in red, the planet will be too hot to support life, and farther away, the planet is too cold. The zone in the middle, shown in green, is the distance from the star that is just the right temperature for a planet to host liquid water, a crucial ingredient of life. Click to enlarge. [NASA]

Earth sits in a comfortable seat, not too hot and not too cold, just the right distance from the Sun to maintain an environment suitable for life. This region, known as the habitable zone, is frequently investigated when astronomers search for life in other star systems. While a planet’s proximity to its host star heavily influences its properties, the star system’s overall location in the galaxy can also impact the chances for life. 

In crowded areas and active star-forming regions, energetic events like supernovae can subject their neighboring star systems to harmful radiation. In the galaxy’s outskirts, there has not been enough chemical enrichment to provide the materials necessary for life. This implies that our solar system resides in a galactic habitable zone, and this region may host other life-harboring systems.

But in the search for habitable planetary systems, it is not sufficient to only consider a star’s current position in the galaxy. As evidenced by its chemical makeup, the Sun likely formed closer to the galactic center and migrated outwards to its current orbital radius, experiencing a wide variety of conditions as it moved through the galaxy. While the Sun’s current orbit is suitable for life, could its migration history have influenced the solar system’s habitability? 

Probability density of stars' final locations after travel through the Milky Way in tested galaxy models

Simulation results showing how different models of galactic central bars and spiral arms transport stars over time. The top panel shows that for a steady model, stars are not significantly transported away from their birth location. The bottom panel shows that for models with dynamic changes in both the central bar and the spiral arms, this causes sufficient migration to explain the Sun’s migration pathway from an inner radius to its current location. Click to enlarge. [Baba et al 2024]

Migration Through the Milky Way

To understand the drivers and conditions of the Sun’s migration, Junichi Baba (Kagoshima University; National Astronomical Observatory of Japan) and collaborators perform simulations of various models of the galaxy. The authors find that dynamic changes in the Milky Way’s central bar and its spiral arms are sufficient to transport the Sun from its origin to its current position in the galaxy.

Though the Milky Way’s bar and arms drive the Sun’s migration through the galaxy, the specific path the Sun takes depends on whether or not it becomes trapped in corotation resonance with the central bar — orbiting the galactic center at the same speed as the bar’s rotation. The authors test a trapped and untrapped migration path and find that these scenarios produce significantly different migration pathways. In the trapped scenario, the Sun’s orbital radius oscillates more drastically, while in the untrapped case the Sun more gradually shifts to its current position. These pathways subsequently expose the solar system to different environments within the galaxy.

Sustaining Life

Histories of surrounding environmental changes along the sun's potential orbits

Histories of environmental changes surrounding the Sun along its potential orbits. The orange line shows the path of a trapped orbit, where the Sun experiences more dramatic environmental changes than an untrapped orbit (shown in green). Click to enlarge. [Baba et al 2024]

How does the Sun’s path through the galaxy impact the habitability of the solar system? The authors investigate the galactic environments the Sun would encounter on both migration pathways, including environmental factors like star formation, gamma-ray bursts, and interactions with comets and other stars. In the trapped case, the solar system encounters significantly more of each environmental component. On one hand, exposure to star formation and highly energetic gamma-ray bursts would expose any life forms to significant and deadly radiation. On the other hand, more exposure to comets and metal-enriched star-forming regions provides more opportunities for life-forming molecules to be deposited into the solar system. 

This study highlights the importance of considering how star systems have migrated through the galaxy over their histories. Their current galactic environment is not the only influence on their ability to harbor life. Thus, as the search for life continues in exoplanet systems, tracing the orbital paths these systems have trudged is crucial for understanding their habitability.

Citation

“Solar System Migration Points to a Renewed Concept: Galactic Habitable Orbits,” Junichi Baba et al 2024 ApJL 976 L29. doi:10.3847/2041-8213/ad9260

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