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artist's impression of colliding black holes

When researchers scour the detections of merging black holes made by gravitational wave observatories, they use models and statistics to make careful inferences about the population of black holes in our universe. In a recent article, researchers explored whether an emerging trend in gravitational wave data is real or an artifact of previous analysis methods.

A New Window on the Universe

Illustration of the first black hole merger detected by LIGO

Illustration of the first black hole merger detected by LIGO. [Aurore Simmonet (Sonoma State University)]

The detection of gravitational waves from merging black holes in 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO) gave scientists a new way to investigate black holes. By analyzing the spacetime ripples from colliding black holes, researchers hope to understand how the black holes formed (through the collapse of massive stars or the successive mergers of existing black holes?) and how they came to exist in binary systems (by first belonging to a stellar binary system or by forming solo and linking up with another black hole later?).

One potential result that has emerged from several analyses of gravitational wave signals is that the effective spins and the ratios of the masses of merging black hole binaries appear to be anticorrelated. But as with all results that are extracted delicately, statistically from complex data sets, it’s important to ask if this is a real feature of the data, with real implications for how black hole binary systems are assembled, or if it’s a result of our models or statistical analyses.

a diagram illustrating positive and negative effective spin

Top: The black hole spins are aligned with the system’s orbital angular momentum (positive effective spin). Bottom: The black hole spins are misaligned with the system’s orbital angular momentum (negative effective spin). [Kerry Hensley]

Statistical Investigation

Christian Adamcewicz (Monash University and OzGrav) and collaborators approached this question by applying a new statistical treatment to detections of black hole mergers. This new treatment features a new model for effective spin and allows for a subpopulation of black hole binaries with zero effective spin, which hasn’t yet been ruled out and might have an impact that hasn’t been accounted for.

The team applied their population model to the third catalog of gravitational wave signals from the LIGO and Virgo detectors and used Bayesian statistical methods to extract the properties of the overarching population of black holes. They found that the previously reported anticorrelation between effective spin and mass ratio is likely real, ruling out the possibility of there being no correlation at 99.7% probability.

More Work, a Paradox, and Astrophysical Possibilities

Adamcewicz and collaborators acknowledge that this work doesn’t provide a final verdict on this question (as they put it, “a modeler’s job is never done”), and that other statistical effects need to be rooted out. One lingering possibility is that this result is due to the amalgamation paradox, which arises when trends present in different factors disappear or flip when the factors are considered together.

If the observed anticorrelation holds up to further statistical scrutiny, a number of astrophysical phenomena could be responsible for this effect. Extensive mass transfer between black hole progenitor stars, stars evolving within a common envelope and accreting matter at a high rate, or even black hole binary systems assembled within the accretion disks of active galactic nuclei should all be investigated with future black hole population models.

Citation

“Evidence for a Correlation Between Binary Black Hole Mass Ratio and Black Hole Spins,” Christian Adamcewicz et al 2023 ApJ 958 13. doi:10.3847/1538-4357/acf763

A photograph of a concrete tube extending to the horizon into low shrubs and swamp.

Joseph Giaime, a Professor of Physics at Louisiana State University, doesn’t sleep as soundly as he used to. The cause is obvious: every so often, his phone will emit a random, loud ping, regardless of the hour and without care for his desire to rest. He could turn these notifications off, but he purposefully chooses not to. In fact, he’s grateful for the recent disturbances: that’s because Giaime isn’t being woken up by a mindless news alert or a sleepless colleague, but by an automated system within the Laser Interferometer Gravitational-Wave Observatory (LIGO) informing him that it has just spotted a collision at the edge of eternity.

Giaime, like many physicists and astronomers, had to wait many decades to get these notifications, and now savors each one. What was once a just-out-of-reach dream has morphed into a steady stream of discoveries and phone alerts. Every few days LIGO reports that it heard the “sounds” of another merging pair of black holes, or occasionally a set of neutron stars that just smashed each other apart.

To better understand these extreme phenomena and how humanity got so good at detecting them, the AAS Press team led a group of reporters to the LIGO Livingston facility in southern Louisiana after the recent 243rd meeting of the AAS in New Orleans. Here, we provide a summary of that tour and what we learned from the dedicated scientists and support staff that worked for so long to hear the universe’s most distant rumbles.

Ripples in Reality

The “O” in LIGO denotes that the facility is an observatory, but unlike the vast majority of astronomical facilities throughout the world and in space, LIGO does not look for electromagnetic light. Instead, the massive 4-km-long arms that make up the Livingston facility were constructed to detect a different kind of wave: ripples in spacetime itself.

Two white spheres against a black background surrounded. by a purple spiral.

An artist’s rendition of two neutron stars spiraling towards each other and emitting gravitational waves. [ESA, CC BY-SA 3.0 IGO]

These wrinkles were first predicted by the theory of General Relativity nearly 100 years ago, though scientists were not audacious enough to go looking for them until the middle of the 20th century. The mathematics suggest that they are emitted when two massive objects collide and that they should travel at the speed of light. They also suggest that they should be extremely tiny, and therefore difficult to detect. There is an extreme contrast between the violence of the collisions and the strength of the signal detected here on Earth: after two objects both more massive than the Sun slam into each other at relativistic speeds, scientists can measure the lengths of their laser beams changing by only a fraction of a proton’s width.

The Road to Detection

Today’s reality where LIGO regularly hears these signals (one was detected just three days before our visit) is very unlike the previous several decades of quiet. LIGO’s origins lie in a proposal to the National Science Foundation (NSF) submitted in the late 1980s. The team broke ground at the two identical facilities in Washington state and Louisiana in 1994 and 1995, respectively, before many of the graduate students currently working on the data analysis were born. Then, from the early 2000s through 2010, both facilities “listened” to the universe, but heard nothing.

A photograph of 6 television monitors, each displaying a graph.

Monitors in the LIGO control room to track the real-time noise contributions of various sources, including “human activity,” “ocean waves,” and “earthquakes.” [Ben Cassese]

The detectors were simply not sensitive enough to pick out a gravitational wave from background sources of noise that could hide a signal, including vibrations in the ground caused by ocean waves rocking against the continent, occasional nearby logging operations, or moderately-sized earthquakes happening anywhere in the world (the recent large and deadly earthquake in Japan temporary halted all observations all the way in Louisiana).

Instead of backing away from their commitment, already one of the largest projects in their history, the NSF doubled down in 2010 and approved another tranche of funding to upgrade the facilities. LIGO went dark for five years as the collaboration overhauled numerous components, implementing both lessons learned during previous observations and new technologies that had been invented in the past decade. In 2015, they were ready to begin testing their new setup, dubbed “Advanced LIGO.”

Immediate and Continuing Payoff

In September, the collaboration was in the middle of an “engineering run,” meaning the detectors were fully operational but the team hadn’t yet transitioned into science operations. As has been well documented elsewhere, the impatient universe did not bother with their commissioning plans, and almost immediately the team recorded the distinctive and amazingly clear “chirp” of two merging black holes. “This [signal]was so big that we didn’t even need theorists” says Giaime, and immediately it became clear that the long wait for a gravitational wave signal was over.

Five printed photographs of technicians in bunny suits interacting with intricate components.

Photographs hung on the walls of the LIGO Livingston site depicting scenes from upgrades and maintenance over the years. Click to enlarge. [Ben Cassese]

Since that first Nobel-prize-winning detection, LIGO has undergone several minor upgrades, including a prolonged shutdown during the COVID-19 pandemic. These have steadily increased its sensitivity, which in turn has increased its rate of discoveries. After years of non-detections and over $1 billion of NSF funding, it stands as one of the US government’s greatest achievements in science funding and has opened a new window through which we can observe the cosmos.

Other facilities have already joined the search for gravitational waves, and have now detected them at other frequencies. The NANOGrav collaboration and their discovery of a gravitational wave background, for example, was the focus of the kickoff plenary session at the 243rd meeting of the AAS. Other proposed projects, such as LISA, should continue exploring yet more frequencies in the coming years. Each of these missions will stand on the shoulders of scientists working near the Louisiana swamps, and the LIGO Livingston facility will rightly be remembered as a historical site in science.

photo of the supernova remnant 30 Doradus B with a pulsar

A pilot survey using the world’s largest radio dish has led to the discovery of four pulsars, two of which are ultra-precise millisecond pulsars. This survey highlights the wealth of pulsars that await discovery at intermediate galactic latitudes.

Small Stars with a Big Impact

artist's impression of a pair of pulsars

An artist’s impression of a pair of pulsars. [Michael Kramer (Jodrell Bank Observatory, University of Manchester)]

When massive stars explode as supernovae, they can leave behind their extremely dense, collapsed cores in the form of neutron stars. Neutron stars spin rapidly and have strong magnetic fields, leading many of them to produce beams of radio emission along their poles. When these beams sweep across our field of view, we see brief, regular pulses of emission and call the objects pulsars.

Several thousand pulsars have been discovered in our galaxy, but there’s a need to find even more: pulsars provide a path to studying stellar evolution, the interiors of neutron stars, and even gravitational waves. Millisecond pulsars — those with the shortest rotation periods, around 10 milliseconds or less — are especially precious, as their pulses are exceptionally regular. By monitoring the arrival times of the pulses from many millisecond pulsars at once, researchers have found evidence for the gravitational wave background, which is thought to be the combined signals of millions of distant supermassive black hole binaries.

A Small FAST Survey

Where and how do we find pulsars? The word pulsar is short for pulsating radio source, and most pulsars are identified in surveys by their characteristic pulses of radio emission. Like most stars, pulsars are concentrated in the thin disk of our galaxy, but interstellar clouds of gas and dust in this region can scatter pulsar signals. Searching the area just above the galactic plane makes for easier pulsar discovery, and current evidence suggests that millisecond pulsars may be more common in these higher-latitude regions.

Plots of the four newly discovered pulsars' average pulse profiles

Pulse profiles of the four newly discovered pulsars. Click to enlarge. [Zhi et al. 2024]

Using the Five-hundred Aperture Spherical Telescope (FAST) — the world’s largest radio dish — Qijun Zhi (Guizhou Normal University) and collaborators searched for pulsars in a small area of the sky about 5 degrees above the galactic midplane. The survey discovered four new pulsars and recovered all seven of the known pulsars in the search area. Of the four newly discovered pulsars, two are of the coveted millisecond variety, with rotation periods of 3.9 and 4.6 milliseconds. One of these two millisecond pulsars especially warrants further study, since it is bright enough to possibly be included in pulsar timing arrays in the future.

More Pulsars to Come

Illustration of how galactic latitude is measured

Illustration of how galactic latitude is measured. [AAS Nova/Kerry Hensley]

The pilot survey described in this study complements the efforts of other pulsar surveys. FAST is currently at work on the Commensal Radio Astronomy FAST Survey and the Galactic Plane Pulsar Survey, both of which aim to find pulsars at galactic latitudes below 10 degrees. These surveys have led to the discovery of roughly 800 pulsars so far, about 200 of which are millisecond pulsars.

Zhi and collaborators expect that many more pulsars await discovery at intermediate galactic latitudes, 5 to 15 degrees above the midplane of the Milky Way. Considering the success of their limited pilot study, the team expects that roughly 900 millisecond pulsars could be found in that region.

Citation

“Discovery of Four Pulsars in a Pilot Survey at Intermediate Galactic Latitudes with FAST,” Q. J. Zhi et al 2024 ApJ 960 79. doi:10.3847/1538-4357/ad0eca

infrared image of stars near the center of the Milky Way

This was a year of superlatives in astronomy — researchers studied the brightest known gamma-ray burst, modeled the strongest material in the universe, and announced the finding of the first compelling evidence for the gravitational wave background. As 2023 draws to a close, let’s take a look back at some of the amazing science we covered on AAS Nova this year. Here are the top 10 most-read posts of 2023:

10. Focusing on the Brightest Gamma-ray Burst of All Time

An X-ray image of GRB 221009A's emission scattering off of dust

An X-ray image of GRB 221009A’s emission scattering off of dust. [Adapted from Williams et al. 2023]

The gamma-ray burst GRB 221009A exploded onto the scene in October 2022 and earned the moniker the BOAT — Brightest Of All Time. A Focus Issue of the Astrophysical Journal Letters highlighted the multifaceted, multinational, and multi-wavelength efforts to study this burst, including the hunt for an accompanying supernova, the search for neutrinos produced during the burst, and an assessment of whether GRB 221009A truly deserves its nickname.

Artist's impression of a pulsar

Artist’s impression of a neutron star, the core of a massive star that has exploded as a supernova. [ESO/L. Calçada; CC BY 4.0]

9. How to Model the Strongest Material in the Universe

The crystalline crust of a neutron star is the strongest material in the universe, and its extreme strength and density pose a challenge for modelers. Using a smoothed-particle hydrodynamics code tuned to include material strength, Irina Sagert and collaborators modeled waves in a neutron star’s crust. These waves might explain some properties of neutron star X-ray flares, and they may have an impact on the gravitational waves produced as neutron stars spiral toward a collision.

8. A New Way to Constrain Dark Energy

Andromeda Galaxy in ultraviolet

This ultraviolet mosaic of our galactic neighbor, the Andromeda Galaxy, is constructed from observations by NASA’s Swift Observatory. [NASA/Swift/Stefan Immler (GSFC) and Erin Grand (UMCP)]

Dark energy is thought to be responsible for the accelerating expansion of the universe. David Benisty, Anne-Christine Davis, and Wyn Evans measured the influence of dark energy in an entirely new way by modeling the orbits of the Milky Way and our neighboring galaxy, Andromeda. This method takes into account the outward pressure that dark energy exerts on the two galaxies as they slowly orbit one another. While the constraint placed by this method isn’t yet particularly stringent, the result agrees with measurements made on much larger scales, and upcoming data should allow the team to refine their results.

Illustration of TRAPPIST-1 and its seven rocky planets

Illustration of TRAPPIST-1 and its seven rocky planets. [NASA/JPL-Caltech/R. Hurt (IPAC)]

7. Monthly Roundup: TRAPPIST-1 Through the Eyes of JWST

In the first entry in the new Monthly Roundup series, we examined five research articles that tackled recent JWST observations of the TRAPPIST-1 system. The M-dwarf star TRAPPIST-1 became a household name when seven Earth-size planets were discovered in orbit around it. This large number of Earth-like, potentially habitable planets — as many as four of the seven may lie in the star’s habitable zone — makes TRAPPIST-1 a tempting target for atmospheric characterization, which is a challenge even for JWST’s gigantic mirror and sensitive instruments. The articles describe studies of JWST spectra of the innermost two planets, modeling of the possible atmospheres of the outer planets, and an exploration into whether we’d be able to detect life on these planets at all, if it exists.

6. First Look at Extragalactic Cepheid Variable Stars with JWST

Cepheid variable stars provide a powerful way to measure the distances to other galaxies. These stars vary in brightness in a predictable way, and how quickly their brightness varies is linked to their intrinsic luminosity. This method has been used to measure the rate of expansion of our universe, but the result disagrees with the value measured through other means, raising a long-standing problem dubbed the “Hubble tension.” Researchers plan to re-observe the Cepheid variable stars previously observed with the Hubble Space Telescope, in the hopes that JWST’s superior infrared capabilities can ease this tension — an effort that will take years. Wenlong Yuan and collaborators got a sneak preview of JWST’s abilities when the telescope observed a galaxy containing multiple Cepheids.

JWST image of the galaxy cluster SMACS

JWST image of the galaxy cluster SMACS J0723.3-7327. [NASA, ESA, CSA, STScI]

5. Update on JWST Observations of Galaxy Cluster SMACS 0723

The first JWST image seen by the public was of the spectacular galaxy cluster SMACS J0723.3–7327 (SMACS 0723). In the months following, astronomers studied the image from every possible angle, examining the structure of the cluster itself as well as the faint, distant galaxies whose light was curved into our line of sight by the cluster. This article introduced five research articles that advanced our understanding of the SMACS 0723 field, including weighing the galaxy cluster, investigating individual star clusters billions of light-years away, and measuring the chemical makeup of distant galaxies.

Wolf–Rayet star

The dusty ejecta of Wolf–Rayet stars makes for fantastic images. These stars will eventually explode as supernovae. [NASA, ESA, CSA, STScI, Webb ERO Production Team]

4. A Cosmic Dust Factory Ramps Up Production

Have you ever wondered where all the dust in the universe comes from? Megan Peatt’s team investigated the dust production in a rare binary system composed of a Wolf–Rayet star — a massive star that has lost its entire hydrogen envelope, leaving behind a scorching hot core wreathed in shells of gas — and an O-type star rotating so fast that it’s losing its grip on its atmosphere. Each time these stars get close, the collision of the Wolf–Rayet star’s fierce stellar winds with the O star’s decretion disk produces dust. The stars are scheduled for their next close approach in 2024, and astronomers have seized the opportunity to collect infrared measurements of the pair that reveal the amount and type of dust being created as production ramps up.

graphic showing the locations of stars in the pulsar timing array

Locations of pulsars (blue stars) in the NANOGrav pulsar timing array relative to the location of the Sun (yellow star). Some pulsar locations are approximate. Click to enlarge. [NANOGrav]

3. First Compelling Evidence for the Gravitational Wave Background

Using a closely monitored collection of rapidly spinning stellar remnants called pulsars, multiple international collaborations including the North American Nanohertz Observatory for Gravitational Waves, or NANOGrav, announced their discovery of compelling evidence for the long-sought-after gravitational wave background. This background signal, which is too low in frequency to be detected by gravitational wave observatories on Earth, is thought to be the rumblings of supermassive black hole binaries. The exact source of the gravitational wave background remains undecided, though, and more observations and modeling are needed to rule out the possibility that this background signal is the result of new physics.

2. Sandy, Briny Water on Mars Has a Better Chance of Remaining Liquid

image of the "dragon scale" texture on the surface of Mars

The “dragon scale” texture seen in this Mars Reconnaissance Orbiter image of Mars’s surface is the result of water interacting with bedrock, forming clay-containing rock. [NASA/JPL-Caltech/UArizona]

The presence of persistent liquid water on Mars is a matter of intense debate — it even merited its own round-table discussion at this year’s Division for Planetary Sciences meeting. Recently, laboratory experiments led by Andrew Shumway demonstrated that whether or not water is briny (i.e., salty, in the chemical sense) and mixed with Martian surface material, or regolith, plays a huge role in the conditions under which water remains a liquid. Briny water that seeps into regolith can remain a liquid under colder and drier conditions than pure water alone, suggesting that water could be more widespread on Mars than previously thought.

image of the black hole at the center of the Milky Way

The first image of the supermassive black hole at the center of our galaxy, constructed from data from the Event Horizon Telescope. [EHT Collaboration]

1. Black Holes as the Source of Dark Energy

The most-read article on AAS Nova in 2023 linked two hot topics in astronomy: black holes and dark energy. Duncan Farrah and collaborators found that the supermassive black holes at the centers of galaxies across the universe are growing too quickly than could be accounted for by their galaxies’ supplies of gas and dust. Instead, Farrah’s team proposed that the growth of these black holes is linked to the expansion of the universe — a property belonging to a theorized type of black hole filled with vacuum energy. The expansion-linked growth of these black holes produces outward pressure that accelerates the expansion of the universe or, in other words, produces dark energy.

 

Thank you for joining us for another year of great science — we can’t wait to see the discoveries that 2024 will bring!

location of the X7 gas cloud relative to other sources near the center of the Milky Way

Editor’s Note: In these last two weeks of 2023, 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 in January.

The Swansong of the Galactic Center Source X7: An Extreme Example of Tidal Evolution near the Supermassive Black Hole

Published February 2023

Main takeaway:

photographs of X7's thermal dust emission once a year from 2002 to 2021

X7’s thermal dust emission from 2002 to 2021. Click to enlarge. [Ciurlo et al. 2023]

Using two decades of observations, a team led by Anna Ciurlo (University of California, Los Angeles) characterized a galactic center source called X7, a cloud of gas and dust found near the Milky Way’s supermassive black hole. Over time, the cloud has become dramatically elongated, which the authors attribute to tidal forces from the black hole. While the origins and age of the cloud are uncertain, the authors propose that it is the gas ejected in a glancing collision between stars in a binary system.

Why it’s interesting:

The neighborhood surrounding a supermassive black hole is an exciting place: tightly packed stars undergo collisions or near misses, and some even get tidally shredded by the black hole itself. This dynamic environment changes considerably on short timescales, and not just in the astronomical sense — the stars closest to the Milky Way’s central supermassive black hole take scarcely longer than a decade to complete their orbits. Unlike other dusty objects in its neighborhood, like the “G” objects labeled in the header image above, X7 doesn’t appear to be bound to a star and instead looks to be a 50-Earth-mass blob of dusty gas roaming the galactic center.

More details on X7 and its possible age and origin:

The rapid stretching of X7 over the past two decades suggests that the cloud is unlikely to remain intact as it nears the Milky Way’s supermassive black hole. This sets an expiration date for the cloud of 2036, when it will make its closest approach, and it also sets a maximum age for the cloud of around 200 years, given its orbital period. X7 is likely the result of a recent event, then, and the similarity between the orbits of the tip of the X7 gas cloud and a dusty stellar source called G3 raises the intriguing possibility that X7 was ejected when G3 was created in a stellar merger.

Citation

Anna Ciurlo et al 2023 ApJ 944 136. doi:10.3847/1538-4357/acb344

photograph of the solar corona

Editor’s Note: In these last two weeks of 2023, 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 in January.

Coronal Heating as Determined by the Solar Flare Frequency Distribution Obtained by Aggregating Case Studies

Published May 2023

Main takeaway:

With the help of numerous undergraduate physics students, a team led by James Paul Mason (University of Colorado at Boulder and Johns Hopkins University) analyzed an enormous sample of solar flares to determine the flare frequency distribution — the number of flares as a function of flare energy — and assess the importance of nanoflares as a way of heating the Sun’s outer atmosphere. This analysis showed that nanoflares cannot be the only source of heat, and plasma waves must play an important role as well.

Why it’s interesting:

The outer layer of the Sun’s atmosphere, called the corona, is extremely sparse and extremely hot. How exactly the solar corona reaches its million-degree temperature is a matter of great interest, as the corona is the site of significant solar activity and the source of the solar wind. The investigation has closed in on two main mechanisms that convert magnetic energy to heat: tiny solar flares called nanoflares and plasma waves called Alfvén waves. Nanoflares are fleeting bursts of high-energy electromagnetic radiation, and Alfvén waves are oscillations in which energy is transferred back and forth between ions and magnetic fields. Neither of these critical mechanisms can be observed directly.

How the flare frequency distribution helps settle the score:

plot of the flare frequency distribution

The resulting flare frequency distribution and measured slope. [Mason et al. 2023]

The flare frequency distribution is a way of tallying the number of solar flares that occur at a given energy. Previous studies have shown that more powerful solar flares are less common than less powerful solar flares. Even though we can’t see nanoflares, we can guess how common they are — and therefore how much energy they can donate to the solar corona — by measuring the slope of the flare frequency distribution. Mason’s team found that nanoflares aren’t frequent enough to heat the corona to millions of degrees, so Alfvén waves must make up the difference.

Citation

James Paul Mason et al 2023 ApJ 948 71. doi:10.3847/1538-4357/accc89

illustration of a brown dwarf and its magnetic field

Editor’s Note: In these last two weeks of 2023, 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 in January.

Periodic Radio Emission from the T8 Dwarf WISE J062309.94–045624.6

Published July 2023

Main takeaway:

observations of the radio emission from the source

Observations of WISE J062309.94–045624.6 from the MeerKAT array. Click to enlarge. [Rose et al. 2023]

A research team led by Kovi Rose (The University of Sydney) detected circularly polarized radio emission from a brown dwarf named WISE J062309.94–045624.6. The radio emission rose and fell with the object’s 1.9-hour rotation period, suggesting that the way the emission is produced is similar to the mechanism that produces aurorae on gas giant planets. WISE J062309.94–045624.6 is the coolest sub-stellar object from which radio emission has been detected.

Why it’s interesting:

One of the many questions about brown dwarfs, which sit in the gap between the smallest stars and the largest planets, is the nature of their magnetic fields. The magnetic fields of stars and sub-stellar objects can be probed through their radio emission. Magnetic fields threaded through extremely hot coronal gas help to power radio emission from stars, but brown dwarfs are too cool to produce radio emission this way. Instead, their rapid rotation helps to generate currents that in turn create aurora-like radio emission.

Why more brown dwarf radio emissions are likely to be detected:

Brown dwarfs are faint at radio wavelengths, but their radio emission is expected to be strongly circularly polarized, making them stand out in surveys of circularly polarized radiation. WISE J062309.94–045624.6 was discovered through a search for strongly circularly polarized radio waves, and future searches with the exquisitely sensitive Square Kilometre Array (currently being constructed) should uncover even more — and even cooler — sources.

Citation

Kovi Rose et al 2023 ApJL 951 L43. doi:10.3847/2041-8213/ace188

Illustration of Earth-like planets transiting an M-dwarf star

Editor’s Note: In these last two weeks of 2023, 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 in January.

Terminator Habitability: The Case for Limited Water Availability on M-dwarf Planets

Published March 2023

Main takeaway:

Ana Lobo (University of California, Irvine) and collaborators use global climate models to show that the abundance of water on a planet orbiting an M-dwarf star can determine the fraction of the planet’s surface that maintains habitable temperatures. The study focuses on whether temperatures along the terminator, the line that separates day and night, can remain habitable when daytime temperatures on a tidally locked planet become too hot. Perhaps counterintuitively, planets with more water can have less habitable area in this zone.

Why it’s interesting:

M dwarfs are the smallest, coolest, and most common type of star in our universe. Their cool temperatures result in close-in habitable zones, making for easy detection and characterization; rather than waiting for a once-annual transit of an Earth-like planet around a Sun-like star, habitable-zone M-dwarf planets zip around in days or weeks. However, many if not all of these planets are tidally locked, orbiting with one side permanently facing the host star and one side in permanent shadow. While both the day and night sides of these planets are unlikely to have habitable temperatures, the terminator could be livable.

simulation results showing the surface temperature of each simulated planet

Surface temperatures for aquaplanets (simulation names beginning with “Aq”) and land planets (names starting with “L”). The sub-stellar point is at the center of each image. Click to enlarge. [Lobo et al. 2023]

Why water plays an important role:

On a water-poor tidally locked planet, daytime temperatures can be extremely hot, and nighttime temperatures can be below freezing, leaving just the terminator at a comfortable temperature. Even when water-poor planets are close enough to their host stars that daytime temperatures are too hot to sustain life, there isn’t enough transport of the blazing daytime heat around the planet to render the terminator inhospitable. When more water is splashed in, resulting in an “aquaplanet,” the energy transport increases, extending the uninhabitable region to the terminator.

Citation

Ana H. Lobo et al 2023 ApJ 945 161. doi:10.3847/1538-4357/aca970

Editor’s Note: In these last two weeks of 2023, 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 in January.

The Dark Energy Camera Plane Survey 2 (DECaPS2): More Sky, Less Bias, and Better Uncertainties

Published January 2023

Main takeaway:

maps of source density

Map of the source density from DECaPS2. Click to enlarge. [Saydjari et al. 2023]

A team led by Andrew Saydjari (Harvard University; Center for Astrophysics | Harvard & Smithsonian) reports on the second data release of the Dark Energy Camera (DECam) Plane Survey, or DECaPS2. Phases one and two of this survey have covered the entirety of the plane of the Milky Way, and the DECaPS2 catalog contains 3.32 billion sources extracted from 260 hours of observations.

Why it’s interesting:

Situated as we are within the plane of the Milky Way, it’s difficult to study objects within the disk of our galaxy — the galactic plane is crowded with stars and clouded with dust and gas. However, most of the objects in our galaxy lie within this plane, so it’s critical to design surveys to investigate this rich environment. In addition to cataloging billions of objects, the team behind DECaPS developed techniques to identify the faint nebulosity that suffuses the galactic disk and complicates our characterization of objects within it.

What’s next for nebulosity:

Knowing the distribution of faint filamentary gas clouds in our galaxy is important for any study of galactic plane objects, since dust makes objects appear redder than they are, and, if unaccounted for, can skew our understanding of these objects’ properties. Looking ahead, the DECaPS team hopes to use their knowledge of nebulosity to infer the three-dimensional structure of gas and dust in the Milky Way.

Citation

Andrew K. Saydjari et al 2023 ApJS 264 28. doi:10.3847/1538-4365/aca594

image of the newly discovered nebula and the Andromeda Galaxy

Editor’s Note: In these last two weeks of 2023, 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 in January.

Discovery of Extensive [O III] Emission Near M31

Published January 2023

Main takeaway:

image of the newly discovered nebular feature next to Andromeda

The discovery image showing the nebulous feature to the left of Andromeda. [Adapted from Drechsler et al. 2023]

Amateur astronomers Marcel Drechsler and Xavier Strottner discovered a glowing arc of gas near the Andromeda Galaxy in images taken by Yann Sainty with a 500.7-nanometer filter, which highlights emission from doubly ionized oxygen atoms. The nature of this filamentary feature is unknown.

Why it’s interesting:

The Andromeda Galaxy is one of the nearest galaxies to the Milky Way, and its proximity and gorgeous spiral structure make it a common target for astrophotographers. Despite the attention paid to this galaxy, the new feature has escaped our attention until now. While the wispy nebula is visible at 500.7 nanometers, it doesn’t show up when an H-alpha filter is used. The object’s faint emission, present in only a narrow wavelength range, likely explains why it’s escaped notice until now.

On its nebulous nature:

The nature of this feature, as well as its origins and whether it’s associated with the Milky Way or Andromeda, remains unknown. Possibilities within the Milky Way include a supernova remnant or a planetary nebula — the ghostly, glowing atmosphere shed by a low- to intermediate-mass star that has ceased to fuse hydrogen in its core — but both of these options lack supporting data, such as the presence of radio or ultraviolet emission from a supernova remnant. Another possibility is that the feature formed through the gravitational interaction between Andromeda and the Milky Way or another galaxy that has been torn apart.

Citation

Marcel Drechsler et al 2023 Res. Notes AAS 7 1. doi:10.3847/2515-5172/acaf7e

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