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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

Four images of black holes in a line

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 Image of the M87 Black Hole Reconstructed with PRIMO

Published April 2023

Main takeaway:

three black hole images side by side

Comparison of a black hole image released by the Event Horizon Telescope collaboration (left) with the results from PRIMO (middle and right). [Medeiros et al. 2023]

A team led by Lia Medeiros (Institute for Advanced Study and University of Arizona) reanalyzed observations of the supermassive black hole at the center of the galaxy Messier 87. Using a new algorithm, the team created a much sharper image of the black hole, opening the door for more precise determination of its properties.

Why it’s interesting:

In April 2019, the Event Horizon Telescope collaboration released the first images of a black hole, constructed from data taken at observatories across the planet. The images allowed researchers to study the properties of the black hole and test our theories of gravity. While these first black hole images were a huge triumph, the work didn’t stop there: researchers continued to develop new algorithms to be applied to the complex interferometric data from the Event Horizon Telescope. And while initial analyses of the Event Horizon Telescope data were careful to be model agnostic — that is, not assuming anything about the black hole’s shape or properties — the excellent agreement between the images and our theory-based expectations allowed researchers to adjust their analysis methods to produce more precise results.

What’s special about PRIMO:

The Event Horizon Telescope consists of telescopes across multiple continents, but even with these powerful observatories working in concert, the data set is still considered “sparse,” interferometrically speaking. This presents a challenge for modelers that PRIMO, which refers to principal-component interferometric modeling, meets by filling in the gaps with simulations. PRIMO is trained on a database of more than 30,000 simulated images of accreting supermassive black holes, and this specialized training set enables the creation of images with resolution up to the nominal resolution of the Event Horizon Telescope.

Citation

Lia Medeiros et al 2023 ApJL 947 L7. doi:10.3847/2041-8213/acc32d

A photograph of a bunny suit-clad engineer standing besides 6 large hexagonal mirrors, one of which is coated gold.

As we approach the second anniversary of JWST’s triumphant liftoff and migration into deep space, astronomers are starting to get a handle on how to approach the data it beams back. The unprecedented precision and stability of its images and spectra necessitated the invention of new analysis tools and motivated reexaminations of old ones. Recently, a team of astronomers made significant progress in this effort: using methods usually reserved for large ground-based telescopes, they successfully detected carbon monoxide in an exoplanet’s atmosphere.

One Positive, One Inconclusive

About one year ago, a large collaboration of astronomers published the first analyses of several observations taken with JWST’s Near Infrared Spectrograph of a planet named WASP-39 b. This world, a scorchingly hot Jupiter-sized planet that lives just 11 stellar radii from its host star, was an early target for exoplanet astronomers looking to characterize atmospheres subjected to extreme conditions. One of these observations revealed carbon monoxide (CO) floating in the atmosphere. Another, which focused on a smaller wavelength range, did not see the same statistically significant signal.

This was a puzzling mismatch, and an unfortunate one: the abundance of CO is a key ingredient in models that predict how far away a planet was born from its star, and without a detection in both wavelength ranges, any attempt to constrain that distance would be forced to accept large uncertainties. Thus motivated to check the data again, a team led by Emma Esparza-Borges (Institute of Astrophysics of the Canary Islands) decided to take a new angle of attack to extract this faint, but suspected, signal.

Cross-Correlation

The team’s key insight was that they could pretend that JWST was actually a high-resolution ground-based spectrograph. Historically, the techniques used to analyze an exoplanet’s atmosphere depended on the facility at which the data were taken. Ground-based spectrographs can have much higher resolutions than their space-based counterparts, meaning they can more cleanly resolve narrow spectral features. To determine which molecules are present using a ground-based instrument, astronomers usually construct high-resolution “templates” of different species, then compare these templates to the actual data to see if, and where, they line up the best. This technique is known as cross-correlation, and it has been a workhorse technique for extracting atmosphere compositions for many years.

Six wavelength vs flux plots at various resolutions, all of which describe the absorption of CO and its isotopologues.

Steps detailing the construction of CO templates that the team then compared to the data. Click to enlarge. [Esparza-Borges et al. 2023]

However, at lower resolutions, such a technique is overkill and unlikely to succeed. If the instrument isn’t extremely stable, small jitters will overwhelm the signal and a template likely won’t line up well.

Luckily, JWST is extremely stable: even after staring at a target for many hours, it is unlikely to have drifted by more than a fraction of a pixel, and the temperature is unlikely to have changed by more than a fraction of a fraction of a degree. Realizing this, Esparza-Borges and colleagues decided to take cross-correlation for a spin on JWST data for the first time.

Alarm Triggered

A corner plot of well-constrained, mostly Gaussian looking posterior distributions.

A selection of the final fits to the data. [Adapted from Esparza-Borges et al. 2023]

When the dust settled and their computers stopped churning, the team was left with a 6.6–7.5-sigma detection of CO in WASP-39 b’s atmosphere. This not only resolved a year-old mystery, it also validated a new path for future analysis. The next time an astronomer puzzles over a JWST spectrum, perhaps they’ll reach for the cross-correlation approach.

Citation

“Detection of Carbon Monoxide in the Atmosphere of WASP-39b Applying Standard Cross-correlation Techniques to JWST NIRSpec G395H Data,” Emma Esparza-Borges et al 2023 ApJL 955 L19. doi:10.3847/2041-8213/acf27b

image of the Sun with a 3D effect

Editor’s Note: The Monthly Roundup is a new series that takes a broad view of one astronomical topic each month, exploring it through the lens of 3-5 recent research articles. The first post in this series, published in November, presented JWST observations of the TRAPPIST-1 system.

As we collect more and more high-resolution data of our home star, we find new phenomena that require explanation. This month, we’ll take a look at four research articles that examine different aspects of the Sun, from fine structures to strange flares.

Throwing Turbulence into the Mix

Solar flares are eruptions of high-energy radiation, occurring as frequently as several times a day during the most active part of the solar cycle. Flares are thought to be powered by magnetic reconnection, in which the local solar magnetic field springs into a new configuration and releases magnetic energy. While this model has been hugely successful in explaining various features of solar flares, increasingly precise data have enabled researchers to examine flares more closely, and adjustments to this model may be required to explain the variety of solar flare characteristics across large and small spatial scales.

comparisons of simulated output and real data

Comparison of synthesized X-ray Telescope (XRT) images (a) and observations (b and c). Click to enlarge. [Wang et al. 2023]

Yulei Wang (Nanjing University) and collaborators examined magnetic reconnection in three dimensions using magnetohydrodynamics simulations with high spatial resolution. The team explored turbulent reconnection, in which random, chaotic motions of the solar plasma spur constant reconnection. The simulations showed that turbulent reconnection arises naturally in a solar flare’s current sheet — the surface that separates regions of oppositely directed magnetic field — and generates dynamic structures like the vortices and fingers seen in recent flare observations. What’s more, Wang’s teams model produced small-scale structures not yet seen in observations, though the team expects that even higher-resolution observations by facilities like the Daniel K. Inouye Solar Telescope (DKIST) or the Multi-slit Solar Explorer (MUSE) will reveal them.

extreme-ultraviolet image of the Sun

This 19.3-nanometer image, taken by the Solar Dynamics Observatory on December 11, 2023, shows at the center of the Sun’s disk the remnant of a large coronal hole. [NASA/SDO and the AIA, EVE, and HMI science teams]

Structure Within a Coronal Hole

R. J. Morton and R. Cunningham, both of Northumbria University, examined the small-scale structures seen in observations of a coronal hole by the Solar Dynamics Observatory spacecraft. Coronal holes are most apparent when viewing the Sun at extreme-ultraviolet and X-ray wavelengths, which showcase the Sun’s extremely hot and rarefied upper atmosphere, or corona.

Coronal holes aren’t really holes, but instead areas where the Sun’s magnetic field lines extend into space rather than looping back to the surface. The outstretched magnetic field lines carry plasma out into the solar system, leaving a depleted region of less dense, cooler plasma behind. The cooler plasma emits less radiation and appears as a dark “hole” in the Sun’s bright atmosphere.

image of plumes from a polar coronal hole

Example of plumes associated with a polar coronal hole. [Adapted from Morton and Cunningham 2023]

The plasma within coronal holes forms column-shaped plumes, within which rapidly varying small-scale structures called plumelets have been found. These structures are thought to be connected to jets that form during magnetic reconnection. To learn more about these structures, Morton and Cunningham analyzed images of a coronal hole from 2011. They found that detailed structures are widespread in plumes and the region between plumes, and the structures are faint, contributing just 1% of the total radiation from that region. The structures are tiny, spanning just a few arcseconds, and their size pushes the resolution limit of current observations. Future observations, such as those made by the Parker Solar Probe as it swings close to the Sun, may be able to reveal structures on even smaller scales, illuminating the connection between plume structure and magnetic reconnection.

example of the red wings

Example of the red wings on CII emission lines. The region of interest is marked by the horizontal dashed line. [Adapted from Xu et al. 2023]

Red Wings of a Solar Flare

In March 2022, researchers witnessed a solar flare with some unusual properties. Like many solar flares, this event exhibited two flare ribbons — bright, linear features that appear in the chromosphere, the region between the Sun’s surface and its corona — but it also had an unexpected compact bright region near the ribbons. A team led by Yan Xu (New Jersey Institute of Technology) reported on their investigation of this feature, analyzing images and spectra from several spacecraft. Spectra of the compact emission region show that its emission lines are not symmetrical, but rather have a “wing” that extends in the red (longer-wavelength) direction.

Modeling of these lopsided spectral lines suggested that they were produced by gas plunging downward at 160 kilometers per second — far faster than expected for a typical solar flare. But what could cause the plasma to race downward? Xu’s team suggests that this is an extreme example of a chromospheric condensation, in which plasma in the chromosphere flows downward, propelled by energy output higher in the atmosphere. Though velocities as high as those seen here aren’t unheard of, their rarity and the fact that models of chromospheric condensations struggle to produce such high speeds pose a challenge for future works.

And Now, for Our Space Weather Forecast…

Jihyeon Son (Kyung Hee University) and collaborators applied machine-learning techniques to one of the most pressing problems in solar physics: how to forecast space weather. Space weather refers to the ever-changing plasma environment in our solar system, from the gentle solar wind to powerful coronal mass ejections. By using models to forecast the plasma environment near Earth, researchers explore how conditions near the Sun correlate with conditions near Earth, test their theories of how plasma and magnetic fields behave, and, ultimately, provide a way to predict when damaging space weather will reach us.

Son’s team used a deep-learning model to predict the speed of the solar wind at Earth from extreme-ultraviolet images of the Sun and past measurements of the solar wind speed. Deep learning is a subset of machine learning that uses computing techniques that imitate the way the human brain learns. The team’s model used two neural networks: one to address the extreme-ultraviolet image inputs, and one to handle the past values of the solar wind speed. The two networks are then linked to give a prediction of the solar wind speed at Earth for the next three days.

comparison of model output to another prediction and to data

Predictions of the model used in this work (red line) compared to the output from the model used by the Space Weather Prediction Center (green line) and the actual solar wind speed (gray line). Click to enlarge. [Son et al. 2023]

The team found that the model predicts the solar wind speed well, especially during solar minimum when complicating events like coronal mass ejections are uncommon. Not only is the model able to predict the speed overall, it can also forecast the times at which the speed will abruptly change. The authors suggest that their model can be used to forecast the solar wind speed nearly in real time, and future work will address the relationship between solar wind speed and coronal mass ejections and explore other avenues to improve the model.

Citation

“Three-Dimensional Turbulent Reconnection Within the Solar Flare Current Sheet,” Yulei Wang et al 2023 ApJL 954 L36. doi:10.3847/2041-8213/acf19d

“The Fine-Scale Structure of Polar Coronal Holes,” R. J. Morton and R. Cunningham 2023 ApJ 954 90. doi:10.3847/1538-4357/acea7c

“Extreme Red-Wing Enhancements of UV Lines During the 2022 March 30 X1.3 Solar Flare,” Yan Xu et al 2023 ApJ 958 67. doi:10.3847/1538-4357/acf8c6

“Three-Day Forecasting of Solar Wind Speed Using SDO/AIA Extreme-Ultraviolet Images by a Deep-Learning Model,” Jihyeon Son et al 2023 ApJS 267 45. doi:10.3847/1538-4365/ace59a

X-ray and optical image of light echoes surrounding Circinus X-1

New high-cadence observations of Circinus X-1, a binary system containing an extremely young neutron star, have led researchers to propose a unified model for the system’s complicated X-ray emission.

A Young Neutron Star

composite X-ray, optical, and radio image of Circinus X-1

A composite X-ray, optical, and radio image of Circinus X-1 that shows the surrounding supernova remnant. [X-ray: NASA/CXC/Univ. of Wisconsin-Madison/S.Heinz et al; Optical: DSS; Radio: CSIRO/ATNF/ATCA]

Researchers discovered Circinus X-1, an X-ray source in the southern constellation Circinus, during the flight of a sounding rocket in 1969. With the help of 50 more years of X-ray observations, researchers have found that Circinus X-1 is a binary system containing an extremely young neutron star — likely just 4,600 years old, judging by the surrounding supernova remnant created by the explosion that birthed the neutron star.

As the neutron star steals material from its binary companion, it results in long-term, orders-of-magnitude changes in the system’s X-ray flux, fleeting bursts, and changes over the course of each orbit. To understand the complex changes that take place in this system over each orbit, a research team led by Mayu Tominaga (Japan Aerospace Exploration Agency and The University of Tokyo) turned to high-cadence X-ray monitoring.

X-ray light curves of Circinus X-1

The new high-cadence NICER observations (top panel) show the same phases as the previous lower-cadence data from the Monitor of All-sky X-ray Image (MAXI; bottom panel). Click to enlarge. [Tominaga et al. 2023]

High-Cadence Monitoring

Using the Neutron Star Interior Composition Explorer (NICER), a sensitive X-ray telescope on the International Space Station, Tominaga’s team observed Circinus X-1 roughly every four hours for an entire orbital period, 16.6 days. The object’s light curve over this time shows three distinct phases: stable, dipping, and flaring.

To link these phases to the properties of the binary system, the team modeled spectra from each phase. At first glance, the three phases have completely different spectra, but the team was able to unite the seemingly disparate spectra under a single model in which an accretion disk, emitting light across the electromagnetic spectrum, is periodically blocked by a cloud of neutral gas. Surrounding this whole system is ionized gas.

A Unified Model

diagram of the proposed unifying model

Diagram of the model proposed by the authors. Here, “diskbb” is the blackbody emission from the accretion disk. Click to enlarge. [Tominaga et al. 2023]

In this model, the stable phase arises when light from the accretion disk isn’t blocked by the neutral clouds, and simply trickles through the surrounding gas that is ionized by X-rays from the blazing surface of the neutron star and the superheated gas of the accretion disk. Sustained dips occur when the neutral cloud blocks some of the emission from the disk. Tominaga’s team suggests that this neutral cloud forms where the stream of gas accreted from the companion star meets the disk. Over an orbital period, this cloud swings in and out of our line of sight. As the neutral cloud moves out of the line of sight, the transmission of light through clumpy material trailing the cloud produces the rapid changes seen during the flaring period.

This picture of how the system’s emission is moderated over the course of its orbit explains nearly all of the NICER spectra of Circinus X-1. Tominaga and collaborators hope that this simple model can be used to explain short- and long-term behaviors of the system in other parts of the electromagnetic spectrum as well.

Citation

“X-ray Spectral Variations of Circinus X-1 Observed with NICER Throughout an Entire Orbital Cycle,” Mayu Tominaga et al 2023 ApJ 958 52. doi:10.3847/1538-4357/ad0034

illustration of black hole sizes

A chance observation during a survey of active galactic nuclei opened a new window onto an ultra-luminous X-ray source that may be an intermediate-mass black hole.

Looking for the Missing Black Hole Pieces

illustration of two black holes that merged to form a larger black hole

The Laser Interferometer Gravitational-Wave Observatory (LIGO) announced in 2020 the detection of a merger that resulted in a 142-solar-mass black hole, the largest merger product detected by LIGO to date. [LIGO/Caltech/MIT/R. Hurt (IPAC)]

Astronomers often search for universal superlatives: the smallest, the largest, the oldest, the brightest. But when it comes to black holes, it’s the search for the objects in the middle of parameter space that’s most compelling. Intermediate-mass black holes, which have masses in the 100–105-solar mass range, have proved more elusive than their stellar-mass or supermassive counterparts; it’s not uncommon to detect gravitational waves from the birth of a black hole in this mass range, but electromagnetic detections are less certain.

Intermediate-mass black holes likely announce themselves the same way that stellar-mass and supermassive black holes do: by shining brightly across the electromagnetic spectrum when they consume material from their surroundings. Bright X-ray emission may have exposed the location of an intermediate-mass black hole in the outskirts of the galaxy NGC 5252. The source, named CXO J133815.6+043255, or CXO J1338+04 for short, is thought to be surrounded by swirling metal-poor gas. This hints that CXO J1338+04 could have been the central black hole of a metal-poor dwarf galaxy that merged with NGC 5252.

position of the intermediate-mass black hole candidate relative to the center of its host galaxy

Radio image showing CXO J1338+04’s location relative to the center of NGC 5252. [Adapted from Smith et al. 2023]

A Long-Wavelength Look at CXO J1338+04

Recently, Krista Lynne Smith (Texas A&M University and Southern Methodist University) and collaborators used the Very Large Array to observe the nucleus of NGC 5252 as part of their study of nearby active galactic nuclei: luminous galactic centers powered by accreting supermassive black holes. By chance, their field of view encompassed CXO J1338+04, which shines brightly at the 22 gigahertz frequency of their observations.

Combining their new data with archival radio observations at lower frequencies, Smith’s team measured the slope of CXO J1338+04’s spectral energy distribution and found it to be consistent with expectations for an outflowing jet that could arise from an accreting black hole. The team noted that the observed spectral slope could also match that of a star-forming region, but they find that unlikely, especially since previous studies found active galactic nucleus–like optical emission lines that are hard to explain with star formation.

Further Support

plot of radio to X-ray luminosity ratio

Ratio of radio and X-ray luminosities at two frequencies (red stars) for CXO J1338+04 compared to radio-loud quasars, radio-quiet quasars, and other active galactic nuclei and ultra-luminous X-ray sources. Click to enlarge. [Smith et al. 2023]

Smith’s team investigated the ratio of the target’s radio luminosity to its X-ray luminosity, which can distinguish between different types of objects, such as radio-loud and radio-quiet active galactic nuclei. (As the names suggest, active galactic nuclei are often grouped based on the strength of their radio emission.) CXO J1338+04’s radio-to-X-ray luminosity ratio suggests that it’s a lower-mass version of a radio-loud active galactic nucleus. Radio-loud active galactic nuclei produce radio emission via their powerful outflowing jets, just as is suspected for CXO J1338+04.

Taken together, CXO J1338+04’s spectral slope and radio emission match what’s expected for an accreting intermediate-mass black hole, supporting its inclusion on the small but steadily growing list of promising intermediate-mass black hole candidates.

Citation

“The Nature of the IMBH Candidate CXO J133815.6+043255: High-Frequency Radio Emission,” Krista Lynne Smith et al 2023 ApJ 956 3. doi:10.3847/1538-4357/acf4f8

Illustration of the Milky Way's disk and halo

Galaxies are embedded within halos of dark matter, the invisible matter thought to make up 85% of the mass of our universe. New research investigates how a tilt in a galaxy’s dark matter halo can affect its stellar halo and stellar disk.

Nestled in a Halo of Dark Matter

Illustration of the components of the Milky Way's halo

Illustration of the components of the Milky Way’s halo. [NASA, ESA, and and A. Feild (STScI)]

Spiral galaxies like the Milky Way have a beautifully nested structure. A thin and extended disk of stars is enveloped within a thicker disk of older stars that is then wrapped up within an expansive galactic halo. Underlying all of this structure is the part of the galaxy that we can’t see: the dark matter halo in which our galaxy is embedded.

While the goings-on in our galaxy’s dark matter halo might seem far removed from life in the thin stellar disk, research by Jiwon Jesse Han (Center for Astrophysics | Harvard & Smithsonian) and collaborators has explored the connection between the two. In a recent article, Han’s team examined the alignment between galaxies’ stellar disks and the inner regions of their dark matter halos and probed what it might mean if the two are misaligned.

Tilting Halos with Ancient Mergers

Using the TNG50 cosmological simulation — part of the IllustrisTNG suite of cosmological simulations — Han’s team selected synthetic galaxies that are similar to the Milky Way and our neighboring galaxy, Andromeda. The simulated galaxies have masses similar to the Milky Way’s present-day mass, a disk-like appearance, and don’t have any other massive galaxies within 500 kiloparsecs (1.6 million light-years).

Dark halo tilt angle versus stellar halo tilt angle

Dark halo tilt angle versus stellar halo tilt angle, derived from Milky Way–like galaxies in the TNG50 simulation. The measured Milky Way stellar halo tilt is marked with the pink shaded area. The plot to the right shows the most likely tilt angle for the Milky Way’s dark halo. Click to enlarge. [Han et al. 2023]

For each of these Milky Way siblings, 198 in all, the team measured the angle between the inner dark halo (the part of the halo within 50 kiloparsecs or 160,000 light-years of the stellar disk), the stellar halo, and the stellar disk. Over 6 billion years of simulation time, the tilt angles between the galaxies’ inner dark halos and their stellar disks, and between their stellar halos and their stellar disks, varied. But the inner dark halos and the stellar halos showed similar behavior, tilting and whirling in tandem.

The Milky Way’s stellar halo is currently tilted relative to its stellar disk at an angle of about 20–30 degrees. Given the parallel behavior of dark halos and stellar halos, this implies that the Milky Way’s inner dark halo is tilted relative to its stellar disk at about 20 degrees. What effect does a tilted dark halo have?

Warped Disks and the Milky Way

Plot of dark halo tilt angle and disk warp amplitude as a function of time

Dark halo tilt angle and disk warp amplitude as a function of time for a simulated Milky Way–like galaxy. [Han et al. 2023]

Han and collaborators delved deeper into the simulation, focusing on the behavior of a single Milky Way analog that experienced a merger 7 billion years ago. The merger tilted the galaxy’s dark matter halo by 50 degrees, and the stellar halo followed suit, swayed by the gravitational pull of the dark halo. Over time, and under the influence of friction, torque, and other factors, the tilts of the dark and stellar halos shrank to 20 degrees.

The effects of the tilted dark matter halo were felt in the thin stellar disk, as well: a few billion years post merger, a warp appeared in the previously flat disk. This warp lessened slightly over time but persisted until the present day in the simulation.

Like the simulated galaxy studied here, the Milky Way experienced a merger billions of years ago and today sports a warped disk. Based on the results of the simulation, Han’s team proposed that halo-tilting mergers induce the long-lasting warps in galactic disks seen in more than half of all spiral galaxies today.

Citation

“Tilted Dark Halos Are Common and Long-Lived, and Can Warp Galactic Disks,” Jiwon Jesse Han et al 2023 ApJL 957 L24. doi:10.3847/2041-8213/ad0641

An artist’s rendering of a small satellite in orbit above the earth. A telescope barrel extends from an otherwise uninterrupted rectangular prism.

Astronomers have collectively cataloged more than 5,000 planets beyond our Sun, a feat borne from the immense efforts of thousands of scientists. However, not all of these discoveries required the same amount of perseverance: while some planets neatly fell out of abundant high-quality data, other worlds required intensive analysis and years of additional study to earn their places in our archives. A recently discovered planet named TOI-2010 b falls firmly into the latter of these categories.

TESS and the Struggles of Single-Transit Planets

The majority of exoplanets discovered so far were initially flagged by space-based telescopes looking for the repeating dips in starlight caused by circling, intermittently photobombing planets. One of the most productive of these telescopes is NASA’s Transiting Exoplanet Survey Satellite, or TESS, which stares are one chunk of the sky for about a month before moving on to observe a new patch. This is a productive strategy that has netted thousands of exoplanet candidates across the whole sky, but it also lets some planets fall through the cracks. Astronomers need to observe more than one transit of a planet to nail down its period, and many of the most interesting cold planets take longer than a month to complete a lap around their star. Often, TESS catches only one transit of these worlds before it proceeds to the next area of the sky, leaving astronomers in a maddening predicament: they know an interesting planet is there, but they have no idea when it will transit again.

A number of astronomers have taken up the challenge of chasing down these “single-transit” planets, and recently a team led by Christopher Mann (University of Montréal) achieved a significant victory: the discovery and characterization of a Jupiter-like planet with a 141-day period.

Introducing: TOI-2010 b

Two side-by-side time series of a high-quality planetary transit.

Left: The first recorded transit of TOI-2010 b as seen by TESS. Right: Another TESS transit, caught after NEOSat refined the orbital period and late into the team’s analysis. Click to enlarge. [Mann et al. 2023]

Back in 2019, TESS captured a single, beautifully clear transit of a Jupiter-sized planet around a star named TOI-2010. Although only one transit could not uniquely determine the planet’s period, reconnaissance spectra and high-contrast imaging revealed a path forward: TOI-2010 was a good candidate for radial velocity follow-up. So, the team embarked on a three-year campaign to measure the planet’s period. Unfortunately, the resulting constraints were only strong enough to predict that the next transit could take place anytime within a week-long window. No Earth-bound telescope could monitor the sky uninterrupted for that long.

Enter the Near-Earth Object Surveillance Satellite (NEOSat), a suitcase-sized telescope launched and operated by the Canadian Space Agency and the Department of National Defence/Defence Research and Development Canada. In December of 2021, this satellite aimed its 15-cm telescope at TOI-2010 and didn’t look away for six straight days. It caught another transit of TOI-2010 b right near the middle of the predicted window, and in doing so ended the years of initial characterization. Astronomers finally had all of the parameters they needed to follow up the planet sometime in the future.

A two-panel plot of radial velocity amplitudes. The top plot shows a time series, while the lower plot shows the phase-folded measurements. The best-fitting model is plotted alongside the data in both plots.

The radial velocity measurements of TOI-2010, which reveal the tell-tale pattern of a circling planet. Click to enlarge. [Mann et al. 2023]

That information will likely be put to good use soon. As a relatively cold (and therefore rarely found) planet that circles a bright (and therefore easy to follow-up) star, it’s amenable to several different measurements including Doppler spectroscopy and infrared spectroscopy to target the planet’s emission. All of these exciting possibilities are only possible thanks to the determination and patience of this collaboration who made sure that this planet didn’t slip away.

Citation

“Giant Outer Transiting Exoplanet Mass (GOT ‘EM) Survey. III. Recovery and Confirmation of a Temperate, Mildly Eccentric, Single-transit Jupiter Orbiting TOI-2010,” Christopher R. Mann et al 2023 AJ 166 239. doi:10.3847/1538-3881/ad00bc

illustration of a brown dwarf

Somewhere in between stars, which sustain hydrogen fusion in their cores, and giant gaseous planets, which do not, lies an intermediate class of cool, cloudy objects called brown dwarfs. A new look at a nearby brown dwarf shows how challenging it can be to classify these objects.

Neither Star nor Planet

photograph of a brown dwarf among a field of stars

The green object at the center of this image is the first ultra-cool brown dwarf discovered by NASA’s Wide-field Infrared Survey Explorer. [NASA/JPL-Caltech/UCLA]

Brown dwarfs are assigned spectral types M, L, T, and Y, ranging from objects that just barely miss the mass cutoff to be able to fuse hydrogen into helium to those that overlap in mass with the largest planets. The coolest brown dwarfs, those in class Y, might have cloudy atmospheres similar to those of giant planets but easier to study; tiny, faint, and cool, planets are hard to pick out against the bright light of their host stars.

Finding and classifying Y-type brown dwarfs has its own challenges, though. Part of the challenge comes from the fact that even objects assigned the same spectral type show significant differences. To understand whether these variations are due to metallicity, formation mechanism, or other factors, researchers must amass a sample of these hard-to-find objects.

Under Spectral Scrutiny

Enter CWISE J105512.11+544328.3, or W1055+5443 for short. Researchers initially assigned this object a spectral class of T8 — slightly more massive and warmer than a Y-type brown dwarf — but an updated distance measurement placed it closer to Earth, implying a lower luminosity and nudging its spectral type down to Y0. To confirm this spectral-type assignment and learn more about this nearby brown dwarf, Grady Robbins (University of Florida) and collaborators analyzed archival photometry from the Spitzer Space Telescope and collected new spectra using the Keck II telescope.

color and spectral type of brown dwarfs

W1055+5443 (blue star) compared to other brown dwarfs for which Spitzer data exist. [Robbins et al. 2023]

Oddities were immediately apparent, in both its photometry and its spectra: W1055+5443 appears unusually blue in Spitzer’s filters compared to other Y0 brown dwarfs, and its spectral features were difficult to fit with a single template. Previous research has shown that when brown dwarfs are difficult to classify from their spectra, it can mean that what seems to be a single brown dwarf is actually two — but modeling shows that W1055+5443 is most likely a single Y0 brown dwarf, though a peculiar one.

Unusual Properties

As brown dwarfs age, they cool and contract. This means that brown dwarfs that are young tend to have earlier spectral types (M and L) and lower surface gravity, and brown dwarfs that are old tend to have later spectral types (T and Y) and higher surface gravity. Based on its spectral type, W1055+5443 should be an old brown dwarf with high surface gravity, but modeling suggests that its surface gravity is low — is this object unexpectedly young?

near-infrared spectrum

Near-infrared spectrum of W1055+5443 from the Keck II telescope. [Robbins et al. 2023]

Robbins and collaborators found that W1055+5443 is likely a member of the Crius 197 moving group, a collection of ten stars moving together through space. Many of the stars in this group have ages around 180 million years, implying that W1055+5443 is similarly youthful, though some group members are much older. If the 180-million-year estimate proves correct, this suggests that W1055+5443 is a young brown dwarf with a mass just 4–6 times that of Jupiter, placing it well within the realm of planet masses!

The authors note that these estimates are preliminary and await further investigation. Spectroscopy from JWST would allow researchers to pin down whether the object belongs to the Crius 197 moving group, determine its metallicity, and investigate its unusual spectral features, and longer-wavelength photometry would provide an accurate estimate of its temperature.

Citation

“CWISE J105512.11+544328.3: A Nearby Y Dwarf Spectroscopically Confirmed with Keck/NIRES,” Grady Robbins et al 2023 ApJ 958 94. doi:10.3847/1538-4357/ad0043

neutron stars approaching a merger

Gravitational waves from colliding neutron stars have improved our understanding of the interiors of these fantastically compressed objects and helped us measure their radii. How much more precisely will we be able to measure neutron stars with future gravitational wave observatories?

Measuring a Neutron Star

Hubble Space Telescope image of a neutron star

This image from the Hubble Space Telescope shows a neutron star. It was estimated to be no more than 28 kilometers (16.8 miles) across and have a temperature of 1,200,000℉ (670,000℃). [Fred Walter (State University of New York at Stony Brook) and NASA/ESA; CC BY 4.0]

When stars more massive than about eight times the mass of the Sun explode as supernovae, they often leave behind a neutron star: the rapidly spinning, magnetized remnant of the star’s core. Neutron stars are immensely dense and strong, packing more than the mass of the Sun into a sphere the size of a city. Counterintuitively, the more massive the neutron star, the smaller it is. Exactly how a neutron star’s size varies with its mass is described by its equation of state: the relationship between mass, radius, and density.

Already, observations of gravitational waves from colliding neutron stars have helped us hone our estimates of the neutron star equation of state. A 1.4-solar-mass neutron star — around the lower limit of a neutron star’s mass — will have a radius between 10.5 and 13 kilometers. Researchers suspect that future gravitational wave observations will narrow this range further, and new work explores how precisely we’ll be able to measure neutron stars in the future.

Computing Collisions

To probe this question, Daniel Finstad (University of Washington; University of California, Berkeley; Lawrence Berkeley National Laboratory) and collaborators Laurel White and Duncan Brown (both Syracuse University) simulated the gravitational waves produced by many pairs of colliding neutron stars.

plot of neutron star mass versus radius

Soft, medium, and stiff equations of state (blue, orange, and green lines, respectively), as well as the full set of equations of state used in the analysis. [Finstad et al. 2023]

The team modeled three populations of colliding neutron stars with different equations of state, labeled “soft,” “medium,” and “stiff.” These equations of state cover the range of neutron star interiors currently allowed by observations. The stiffness of the equation of state affects both the neutron stars’ sizes and how much they’re deformed by tidal forces as they approach a collision. These changes leave an imprint on the gravitational waves produced in a collision, allowing us to extract the equation of state from gravitational wave observations. Modeling a range of equations of state is important because our ability to measure a neutron star’s equation of state depends on the equation of state itself; “soft” interiors produce signals that are fainter than “stiff” interiors do.

Upcoming Observations

With a suite of simulations in hand, Finstad’s team modeled what future gravitational wave observatories would detect if faced with these synthetic signals. They considered future upgrades to the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo detectors that would bring them up to their maximum sensitivity as well as the proposed Cosmic Explorer, which would have arms 10 times as long as LIGO’s and therefore be more sensitive.

plot showing how long it would take LIGO–Virgo to measure the neutron star equation of state to within 2%.

Number of years needed for LIGO–Virgo to observe enough mergers to measure the neutron star equation of state to a precision of 2%. Results are shown for stiff, medium, and soft equations of state (green, orange, and blue, respectively), as well as for different values for the neutron star merger rate, shown with the timescales at the top. [Finstad et al. 2023]

Finstad and collaborators found that an upgraded LIGO-Virgo would be able to measure the neutron star equation of state to within a precision of 1.9–0.7%, depending on the stiffness — but it would take 10, 20, or 57 years to observe enough mergers of stiff, medium, or soft neutron stars (respectively) to reach that precision. Cosmic Explorer, on the other hand, would require only a year to amass a similarly large collection of observations, measuring the equation of state to within a precision of 0.56% or better.

Citation

“Prospects for a Precise Equation of State Measurement from Advanced LIGO and Cosmic Explorer,” Daniel Finstad et al 2023 ApJ 955 45. doi:10.3847/1538-4357/acf12f

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