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Main-sequence stars with brighter than expected mid-infrared emission can signal the presence of a debris disk, rubble from planetary collisions, or even a theorized sign of a technologically advanced civilization. New research demonstrates a data-driven method to identify mid-infrared excesses in main-sequence stars.

An Excess of Emission

Fomalhaut debris disk

This image combines observations from the Hubble Space Telescope and the Atacama Large Millimeter/submillimeter Array to show the dusty debris disk surrounding the star Fomalhaut. [ALMA (ESO/NAOJ/NRAO). Visible light image: the NASA/ESA Hubble Space Telescope A. Fujii/Digitized Sky Survey 2. Acknowledgment: Davide De Martin (ESA/Hubble); CC BY 4.0]

Young stars swaddled in gas and dust are known to shine extra brightly in the mid-infrared, but as stars age, this mid-infrared exuberance is expected to fade. When it doesn’t, that signals something interesting. Extreme debris disks resulting from collisions between planets or planetesimals provide one explanation for excess infrared light from mature stars; as rubble and dust billow from the collision, the dust captures and reprocesses the star’s light, re-emitting it in the mid-infrared and causing a potentially detectable excess. Only a handful of extreme debris disk candidates have been identified.

Mid-infrared excesses are hypothesized to signal something even wilder: the presence of a Dyson sphere — a hypothetical artificial structure created by an advanced civilization to harness the power of their home star. Similar to dust and rubble, the components of a Dyson sphere would collect starlight and re-emit it at infrared wavelengths, potentially producing a mid-infrared bump.

Regardless of the cause, excess mid-infrared emission from mature Sun-like stars is something to investigate. But how do we find stars with this feature?

Taking Cues from Data

Gabriella Contardo (International School for Advanced Studies, Italy) and David Hogg (New York University; Flatiron Institute; Max Planck Institute for Astronomy) began their search for mid-infrared excesses with an expansive set of observations from the Gaia spacecraft, the Two Micron All Sky Survey, and the Wide-field Infrared Survey Explorer (WISE). After trimming these data sets down to include only main-sequence Sun-like stars, and to exclude objects that might be contaminated by close neighbors or are too dusty, they reduced the number of stars in their sample from 18,751,187 to 4,898,812.

plot showing the difference between observed and predicted magnitudes in two infrared bands

Difference between observed and predicted magnitude in the WISE W1 and W2 bands (3.4 and 4.6 microns, respectively). Click to enlarge. [Adapted from Contardo & Hogg 2024]

To identify mid-infrared excesses in this sample, the team needed an estimate of what the mid-infrared fluxes of these stars should be. Rather than using models, which can be computationally intensive and require making assumptions about the objects, Contardo and Hogg let the data lead the way.

Their data-driven method involves splitting the five million stars into eight sub-samples, each of which is used to train a separate random forest algorithm. Each algorithm “learns” what the mid-infrared emission “should” be from the stars in its sample, then predicts the mid-infrared emission of the stars in the other seven sub-samples. When a star’s actual mid-infrared emission is brighter than predicted, it gets flagged.

To Be Continued

locations of the 53 stars in the final sample

Locations of the 53 stars in the final mid-infrared excess sample. Click to enlarge. [Contardo & Hogg 2024]

This analysis yielded a preliminary sample of 127 objects with mid-infrared excess. Ultimately, after applying additional cuts to remove crowded objects, duplicate sources, and other complications, Contardo and Hogg landed on a sample of 53 objects with interesting infrared behavior. These objects’ mid-infrared emission ranged from 0.5% to 10% higher than expected, spanning the values predicted for extreme debris disks and rubble left over from planetary collisions. In fact, one of the 53 objects has already been highlighted by previous work as an extreme debris disk candidate.

What happens now? To identify the stars that are the most promising hosts of extreme debris disks, Contardo and Hogg listed ways to pin down the ages of the stars in their sample, which may rule out stars whose mid-infrared excess is due to their youth. They also proposed to compare the mid-infrared behavior of their stellar sample to Dyson sphere models, exploring whether the observed stellar behavior matches the predictions for these hypothetical structures.

Citation

“A Data-Driven Search for Mid-infrared Excesses Among Five Million Main-Sequence FGK Stars,” Gabriella Contardo and David W. Hogg 2024 AJ 168 157. doi:10.3847/1538-3881/ad6b90

Artist's rendition of white dwarf merger

As the Gaia spacecraft has mapped more and more of the Milky Way, astronomers have uncovered some of the fastest-moving stars in the galaxy. Can simulations link these stars to the elusive origins of Type Ia supernovae? 

Type Ia Supernova Origins

Occurring in binary star systems with at least one white dwarf, Type Ia supernovae are key cosmological distance indicators and have allowed astronomers to study the expansion of the universe. Despite their importance, the details behind these explosions and the characteristics of their progenitor systems remain unclear. 

One proposed mechanism to launch a Type Ia supernova is the double-detonation scenario, in which the white dwarf accretes helium from a helium-rich donor star. Forming a thin shell around the carbon-oxygen core, the siphoned helium eventually detonates, sending shock waves through the core, causing it to also detonate. In the wake of the powerful explosion, the donor star launches across the Milky Way, forever changed. 

Recent Gaia discoveries of a runaway helium-burning star and hypervelocity stars — stars that zoom through the galaxy much faster than the general stellar population — suggest that the double-detonation scenario may be responsible for a number of Type Ia supernovae. Can double-detonation simulations predict the observed properties of these fast-moving stars, further uncovering Type Ia supernova origins?   

Supernova Ejecta Effects

Simulation snapshots of helium white dwarf donor model.

Simulation snapshots showing fraction of donor material (left) and total density (right) for a helium white dwarf donor model. The bottom panel shows that, though much of the donor’s material has expanded, a large fraction is still bound to the donor star as indicated by the gray lines in the left panel. The impacts of shock waves can be seen as concentric shells in the density distribution on the right. Click to enlarge. [Wong et al. 2024]

As a helium-rich donor star is bombarded with material and energy from its exploding white dwarf companion, interactions with the supernova ejecta can leave lasting impacts on the donor star’s trajectory through the galaxy as well as the star’s properties and evolution. Motivated by this interaction and the Gaia observations of hypervelocity stars, Tin Long Sunny Wong (University of California, Santa Barbara) and collaborators performed hydrodynamical simulations that track, with novel clarity, the lasting imprints supernova ejecta leave on their companions. 

The authors’ analysis shows that as the supernova ejecta crashes into the donor star, some of the donor star’s material is swept up and pulled in the direction of the supernova’s propagation. The supernova shock wave passes through the donor star, both compressing and pushing the star away from the explosion center. As the shock front moves on, the donor star attempts to return to equilibrium, contracting and expanding, sending smaller shock waves into its surroundings. 

For each progenitor stellar type simulated, the authors find that the donor stars become puffed up with lower densities and larger radii. The donors also lose some of their original mass but acquire a small portion of supernova ejecta material — consistent with the observed metal-polluted atmospheres and larger radii of hypervelocity stars. 

HR diagram showing evolutionary tracks of simulated donor stars

Postexplosion evolution for each simulated donor star type (labeled in figure legend) in luminosity-temperature space (Hertzsprung-Russell diagram). Four observed stars of interest are plotted, showing intriguing agreement between the well-studied hypervelocity star D6-2 and the expected evolution for a helium white dwarf donor companion. Click to enlarge. [Wong et al. 2024]

Postexplosion Evolution

Particularly important to the identification of donor stars in the field is how these stars evolve over longer timescales and how we may observe them today. The authors performed further simulations to track the temperature and luminosity changes for each simulated donor star from ~10 years to 100 million years after the supernova event. Intriguingly, some of the observed hypervelocity stars seem to fall near the predicted evolutionary tracks, suggesting that these stars could have been ejected by Type Ia supernovae.

This study provides important evidence for the possible double-detonation scenario of Type Ia supernovae. As simulations continue to improve, the ability to identify the progenitor systems of these energetic events becomes more promising. 

Citation

“Shocking and Mass Loss of Compact Donor Stars in Type Ia Supernovae,” Tin Long Sunny Wong et al 2024 ApJ 973 65. doi:10.3847/1538-4357/ad6a11

brown dwarf Gliese 229 B as seen by the Hubble Space Telescope

Astronomers recently discovered a companion to Gliese 229 B, the first confidently identified brown dwarf. This discovery resolves the conflict between Gliese 229 B’s observed mass and the predictions of evolutionary models, potentially illuminating the nature of other poorly understood brown dwarf systems as well.

First in Its Class

illustration of a brown dwarf

An illustration of a brown dwarf. Brown dwarfs aren’t actually brown, likely spanning a range of colors from reddish-orange to nearly black. [NASA/JPL-Caltech]

In 1995, Gliese 229 B became the first object to be unambiguously classified as a brown dwarf: an object that bridges the gap between planets and stars. At roughly 13–80 times the mass of Jupiter, brown dwarfs aren’t massive enough to sustain fusion of hydrogen in their cores, as stars do, but they are able to burn a heavier form of hydrogen called deuterium, setting them apart from planets. (The most massive brown dwarfs can burn lithium as well.) After exhausting their supply of deuterium, brown dwarfs steadily cool, sliding down the spectral-type ladder. The youngest and most massive brown dwarfs occupy late M spectral types, while older or less massive brown dwarfs are classified as L, T, or Y dwarfs.

While improved telescopes have advanced our understanding of brown dwarfs, there’s still much we don’t know about these objects, and attempts to study and classify brown dwarfs have been confounded by their complex properties. This is the case for the first confirmed T-class brown dwarf, Gliese 229 B, which recently became the subject of an astronomical mystery.

A Mass Mystery

Soon after Gliese 229 B was discovered, researchers used substellar evolution models to interpret the object’s spectrum and luminosity and estimate its mass at 30–50 Jupiter masses. More than two decades later, refined observations of the brown dwarf’s orbit around its red dwarf host star allowed researchers to calculate its mass dynamically. The newly calculated mass — 71 Jupiter masses — was troubling. According to models of how substellar objects cool as they age, it simply wasn’t possible for a 71-Jupiter-mass object of Gliese 229 B’s age to have cooled to its present temperature.

plot of count density versus radial velocity

The large relative radial velocity between Gliese 229 A and 229 B and the large difference in Gliese 229 B’s radial velocity between the two time periods provides firm evidence for the existence of an unseen companion. [Whitebook et al. 2024]

This conflict between dynamical mass measurements and evolutionary model predictions led researchers to suspect that Gliese 229 B is actually a binary system — a brown dwarf harboring an unseen companion. In March and November of 2022, Samuel Whitebook (University of California, Santa Barbara; California Institute of Technology) and coauthors turned one of the giant telescopes of Keck Observatory toward the Gliese 229 system, using the sensitive High Resolution Echelle Spectrometer to search for evidence of a companion tugging on Gliese 229 B. The team found a clear difference in Gliese 229 B’s radial velocity compared to expectations for an orderly orbit around its host star. Its radial velocity changed by 11σ between the observations, completely ruling out the possibility that Gliese 229 B is a single object.

Single No More

likelihood distribution of orbital period and mass of the companion object Gliese 229 Bb

Likelihood distribution of the orbital period and mass for the companion object. [Whitebook et al. 2024]

What do these observations tell us about the newfound companion? While it’s not possible to fully pin down the properties of the companion object from current observations, Whitebook’s team estimated the companion’s mass to be somewhere between 15 and 35 Jupiter masses with an orbital period between a few days and 60 days. Future observations will refine the companion’s orbit and provide an accurate estimate of the masses of the two components.

In addition to solving the mystery of Gliese 229 B, this discovery may help to explain other seemingly over-massive T dwarfs orbiting main-sequence stars, several of which have been discovered in the past decade. If future work reveals that these too-massive T dwarfs are actually pairs of brown dwarfs, that may suggest that T dwarfs orbiting main-sequence stars are more likely to host companions than T dwarfs in the field, which are usually solo.

Citation

“Discovery of the Binarity of Gliese 229B, and Constraints on the System’s Properties,” Samuel Whitebook et al 2024 ApJL 974 L30. doi:10.3847/2041-8213/ad7714

V838 Monocerotis Hubble image

Stars orbiting one another on extremely wide orbits seem destined to remain apart forever — but new research shows that the subtle influence of the Milky Way’s gravitational pull can bring them crashing together.

Loosely Connected Stars

Many stars in our galaxy travel through space with a binary partner, but some binary pairs are closer than others. Surveys suggest that as many as 12% of nearby Sun-like stars are members of wide binary systems with separations of 100 to 100,000 au or more. Unlike close stellar binaries that might influence each other’s evolution through accretion, stellar winds, and radiation, wide binaries are expected to evolve as if their stellar partner didn’t exist.

Though unaffected by their partners, stars in loosely connected binary systems can be swayed by the subtle influence of the Milky Way’s background gravitational pull, or they can be knocked off course by passing stars. How do these factors affect the evolution of wide binary systems?

Unexpectedly Close

A team led by Jakob Stegmann (Max Planck Institute for Astrophysics) used relativistic N-body simulations to study how the background gravitational pull of the Milky Way and nudges from passing stars affect the orbits and evolution of stars in wide binary systems.

Plot showing outcomes for wide binaries with different separations and distances from the galactic center

Typical outcomes for wide binaries as a function of initial binary separation and distance from the galactic center. Click to enlarge. [Stegmann et al. 2024]

The team simulated 100,000 binary systems with varying initial separations and allowed them to evolve for as long as 14 billion years. The outcome depended on the initial separation of the binary. Binaries orbiting within 100 au of one another weren’t much affected by the galactic tide or by stars whizzing by. Wide binaries with separations of 100,000 au (about 1.6 light-years) or greater were swept up in the galactic tide and torn apart. In between these two extremes, though, the binaries had the potential to end up closer than before.

Swept Up in the Galactic Tide

Many of the binaries underwent wild eccentricity oscillations that brought their orbits from nearly circular to narrow and elongated. This means that initially widely separated stars can undergo close encounters, mass-transfer events, and even mergers.

plot showing the evolution of a widely spaced binary system's separation over time

An example of the evolution of a binary’s separation. This particular binary collided after 2.3 billion years, producing gravitational waves. Click to enlarge. [Adapted from Stegmann et al. 2024]

The precise outcome depended on the masses of the stars involved. High-mass stars evolved into neutron stars or black holes long before the slow eccentricity evolution brought them close, leading to compact-object mergers. The authors estimated that 0.4–5% of observed binary black hole gravitational-wave events could be due to mergers brought about this way. Slightly less massive stars evolved into white dwarfs before merging, generating Type Ia supernovae, though this process likely accounts for just 0.01–0.1% of observed Type Ia supernovae. Low-mass stars interacted while they were still on the main sequence, leading to divergent outcomes: they either tidally interacted and were drawn into close, stable orbits, or they collided, producing luminous red novae.

There are more intricacies to the model results than can be covered in this brief highlight, so be sure to check out the full research article for all the details!

Citation

“Close Encounters of Wide Binaries Induced by the Galactic Tide: Implications for Stellar Mergers and Gravitational-Wave Sources,” Jakob Stegmann et al 2024 ApJL 972 L19. doi:10.3847/2041-8213/ad70bb

Globular cluster 47 Tucanae

Massive star clusters have moved about the Milky Way for billions of years, and a recent study finds that more complicated dynamics must be considered when using these clusters to investigate the evolutionary history of the galaxy.

Globular Cluster and Galaxy Evolution

Born from giant molecular gas clouds, globular clusters are large, dense collections of tens of thousands to millions of stars. These systems are very stable and long-lived, making them some of the oldest residents of the Milky Way and important relics of our galaxy’s ancient structure that has long since evolved. 

Within the galaxy, globular clusters tend to fall into two groups — those that formed in the Milky Way (in situ) and those that were accreted during mergers. One clue often used to identify a globular cluster’s origin is its current location in the Milky Way: globular clusters formed in situ are typically found near the galactic center and accreted globular clusters tend to lie more so in the galactic halo. But could the Milky Way’s long-term evolution have also played a role in these globular clusters’ present positions?

Trapped in Resonance

Milky Way Artistic Rendition

An artist’s rendition of the Milky Way showing a strong central bar. Click to enlarge. [NASA/JPL-Caltech]

The presence of a central bar in the Milky Way has strong dynamical implications for objects that orbit it. Over time, as friction slows the rotating bar, objects can be pushed and pulled, altering their orbits and even forcing them to migrate to different locations altogether. Globular clusters that happen to be trapped in resonance with the bar — having orbital speeds that match or are multiples of the rotation speed of the bar — may be susceptible to more dramatic changes over time than those that reside outside of resonances. 

Using Gaia Data Release 3 measurements and dynamical simulations, Adam Dillamore and collaborators (University of Cambridge) explore how the Milky Way’s barred center influences its surroundings. The authors find that the globular clusters most significantly impacted are those caught in resonance. As the central bar slows, these clusters are transported farther away from the galactic center, making a cluster’s current location less indicative of its origins.

Understanding Observations

How do the authors’ findings fit in with previous observations? Of note is Messier 22, a globular cluster with a metallicity spread akin to those anticipated in the Milky Way’s first massive star clusters. After becoming entangled in resonance with the central bar, Messier 22 likely migrated from the very center of the Milky Way outward, further indicating an ancient origin.

Simulation snapshots of the globular cluster 47 Tucanae, where the simulated stars are shown in on-sky coordinates. From left to right, the panels show the impacts of no central bar, a steady central bar, and a slowing central bar on the resulting spatial distribution of stars in the cluster. The presence of a central bar creates a diffuse halo, corresponding to the prior observations of the central region of the cluster marked with the black circles. Click to enlarge. [Dillamore et al 2024]

In addition to exploring the impact of a central bar on globular clusters’ orbits, the authors also investigated how a bar affects globular clusters’ shape and density. Their simulations show that being trapped in resonances with the central bar strips stars and spreads them out into a diffuse halo — a picture that’s consistent with observations of clusters like 47 Tucanae.

The Milky Way does not rest in a steady state, but instead changes over time, impacting the dynamics of its constituents. When it comes to using globular clusters to characterize the galaxy’s past, taking resonances and other perturbations into account is imperative to accurately determine the Milky Way’s evolutionary history. 

Citation

“Trojan Globular Clusters: Radial Migration via Trapping in Bar Resonances,” Adam M. Dillamore et al 2024 ApJL 971 L4. doi:10.3847/2041-8213/ad60c8

A rendering of two dark planets in front of a bright yellow star.

Although some planets calmly follow the lead of their host stars and always stay more or less aligned with the star’s equator, others are more unruly and are found circling their parent in any direction they choose. A recent study adds evidence to the claim that “warm Jupiters”, or massive planets that are a little farther from their host star than their hot-Jupiter cousins, are almost always well-behaved no matter what else is going on around them.

Disorderly and Dramatic

Hot Jupiters, or massive planets that orbit incredibly close to their host stars, have pretty tough lives. Thanks to a ceaseless stellar blowtorch constantly scorching one hemisphere, their atmospheres can be hot enough to melt steel. Even among this tortured group, some have it worse than others. While some stars, like our sun, are relatively cool, others are more massive and therefore hotter. Hot Jupiters around these hot stars must tolerate even more extreme conditions than their (relatively) cooler counterparts.

Perhaps, then, we can forgive this charred group for acting out. In recent years, astronomers have noticed that hot Jupiters around hot stars tend to orbit within planes that are severely misaligned to the one set by their stars’ equators. Some of these planets have been found on nearly polar orbits, meaning they move nearly perpendicularly to the direction of the star’s spin. Their counterparts around cooler stars, however, seem to mostly follow the rules and stay well aligned with their star.

This pattern invites some obvious questions: Why the difference? Is there something about a star’s temperature that would determine the geometry of its planets’ orbits?

Cool and Collected

To answer these and others, a team of astronomers have spent the past few years measuring the alignments of “warm Jupiters”, or planets about as massive as hot Jupiters but slightly farther from their host stars. The latest study in this effort, led by Xian-Yu Wang of Indiana University, brings the total sample of measured warm Jupiters up to 23 planets. None of these worlds are misaligned; in other words, warm Jupiters, even when around hot stars, never set off on their own.

A plot showing stellar temperature on the X-axis and angle of misalignment on the Y-axis. Hot Jupiters are shown on the top half, where we see more misaligned systems around hotter stars. Warm Jupiters, which are all aligned, are shown on the bottom. Click to enlarge. [Wang et al. 2024]

This consistency allows the team to put together a self-consistent story that not only explains their data, but also the outstanding mystery of how hot Jupiters ended up on such inhospitable orbits. In their model, most planets form in quiet disks that are aligned with the star’s spin. In some systems, the planets jostle one another around, sending one or more careening toward the star on wild, eccentric orbits. These unlucky worlds will eventually end up on tight, circular, but tipped-over orbits thanks to tidal interactions and gravitational dynamics: in other words, they’ll become misaligned hot Jupiters. As for the hot Jupiters we see that are aligned, that’s where the stellar temperature comes into play. Cool stars will slowly, over time, wrestle any misaligned planets onto lower-inclination orbits, again thanks to tides, while hot stars are powerless to alter the orbits of their planets.

Through this combination of new observations and intensive modeling of planetary dynamics, astronomers continue to build a fuller picture of planetary formation, and ultimately, how any planets got to where they are today.

Citation

“Single-star Warm-Jupiter Systems Tend to Be Aligned, Even around Hot Stellar Hosts: No Teff–λ Dependency,” Xian-Yu Wang et al 2024 ApJL 973 L21. doi:10.3847/2041-8213/ad7469

Artist's impression of a pulsar

The realm of high-energy astrophysics is populated with extreme objects like accreting supermassive black holes and exploding stars. Among the most intriguing objects in the high-energy category are pulsars and magnetars, both of which fall under the umbrella of neutron stars: extremely dense, city-sized remnants of collapsed massive stars.

schematic describing the characteristics of pulsars and magnetars

Infographic describing some of the characteristics of pulsars and magnetars. Click to enlarge. [NASA/JPL-Caltech]

Pulsars, short for pulsating radio sources, get their name from their characteristic repeating pulses that are caused by powerful beams of radio emission sweeping across our field of view as the pulsar spins. The rare few neutron stars whose magnetic fields grow to a trillion times the strength of Earth’s magnetic fields are called magnetars. (In rare cases, a neutron star can be both a pulsar and a magnetar!) Today, we’re examining four research articles that tackle various aspects of pulsar and magnetar science, from pinning down pulse periods to probing energized nebulae.

Crab Nebula pulsar wind nebula

A multiwavelength image of the center of the Crab Nebula supernova remnant, showing the distinct jet and torus of the pulsar wind nebula. [X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA-JPL-Caltech]

Tracing a Pulsar Wind Nebula

More than a decade ago, X-ray observations revealed the presence of an energetic pulsar, PSR J1849-0001, and its surrounding pulsar wind nebula: a glowing cloud energized by the high-energy charged particles shed by a young pulsar as it spins down. Pulsar wind nebulae are sometimes found at the centers of supernova remnants, as is the case for the famous Crab Nebula.

Seth Gagnon (George Washington University) and coauthors used the Chandra X-ray Observatory to study PSR J1849-0001 and its surrounding pulsar wind nebula. The observations showed that the structure of the nebula isn’t well defined, lacking the distinct jet and torus regions of other pulsar wind nebulae. PSR J1849-0001’s nebula does exhibit a faint jet, making it similar in appearance to the pulsar wind nebula of another young pulsar, J1811-1925. The team concluded that the angle between the pulsar’s spin and magnetic axes is likely small, potentially placing it in the class of “MeV pulsars” that emit pulses of high-energy X-rays.

X-ray images of two pulsar wind nebulae

Comparison of the pulsar wind nebulae of J1849-0001 and J1811-1925. [Gagnon et al. 2024]

In addition to illuminating the amorphous nature of the nebula, the team’s Chandra observations also picked up bright X-ray emission from a pair of stars that orbit each other widely. Given the stars’ wide separations, it’s not clear what kind of interaction could be powering the observed bright and variable X-ray emission — leaving a mystery to be solved by future observations!

Tracking a Magnetar

Roughly 20% of known and candidate magnetars have been observed at radio wavelengths, emitting pulses of radio emission that are distinct from the signals from pulsars. Hao Ding (National Astronomical Observatory of Japan) and collaborators used the Very Long Baseline Array to monitor the radio emissions of Swift J1818.0−1607, the fastest-spinning and youngest magnetar known.

Why monitor a magnetar? The formation mechanism for these extreme objects isn’t yet known, though several of them are thought to be associated with supernova remnants, making core-collapse supernovae their likely origin. One way to test different formation theories is by measuring how quickly magnetars move through space, as different origin stories impart “kicks” of different strengths on a newborn magnetar. Ding’s team used their three-year observing campaign to determine J1818.0−1607’s parallax and proper motion, which together yielded the magnetar’s distance and velocity perpendicular to our line of sight.

plot of transverse space velocities of magnetars and pulsars

Top: Transverse space velocities of eight magnetars. Bottom: Cumulative distribution of transverse velocities of magnetars and young pulsars. Click to enlarge. [Ding et al. 2024]

They found the object to be about 31,000 light-years away, on the opposite side of the galactic center, and moving at 48 km/s across our line of sight. Combining this measurement with data from other magnetars, Ding and collaborators found that on average, magnetars move more slowly than young pulsars, hinting at different formation mechanisms — though the small sample size limits the conclusions that can be drawn.

As for the origins of J1818.0−1607, the team found that a radio-emitting shell of gas that was previously discovered nearby is located at roughly the same distance as the magnetar, strengthening the possibility that this shell is a supernova remnant.

Pulsars: Not Just Radio Sources

plot showing NICER, NuSTAR, Swift, IXPE, and MAXI observations of J0243.6+6124

The timeline of the IXPE, NICER, and NuSTAR observations along with observations from the Swift Observatory Burst Alert Telescope (BAT) and the Monitor of All-sky X-ray Image (MAXI). NuSTAR and NICER observations are marked by the vertical dashed lines. The IXPE measurements are marked with blue circles. Click to enlarge. [Majumder et al. 2024]

Ultraluminous X-ray pulsars are a rare subclass of pulsars; only nine of these extreme objects are known. The X-ray emission is generated when the pulsar accretes material. The accreted material is funneled by powerful magnetic fields to the pulsar’s surface, where it creates hot spots that blaze with X-rays. Seshadri Majumder (Indian Institute of Technology Guwahati) and collaborators studied Swift J0243.6+6124, an ultraluminous X-ray pulsar in our galaxy, using the Imaging X-ray Polarimetry Explorer (IXPE), the Nuclear Spectroscopic Telescope Array (NuSTAR), and the Neutron star Interior Composition Explorer (NICER).

The IXPE observations provided the first measurement of this pulsar’s X-ray polarization, or the orientation of its light waves as they travel through space. Like other X-ray pulsars monitored with IXPE, J0243.6+6124’s X-ray emission is weakly polarized (polarization fraction of ~2–3%) compared to what models predict. Based on the current IXPE, NuSTAR, and NICER data, Majumder’s team suggests that the unexpectedly low polarization fraction of J0243.6+6124 is due to vacuum resonance at the interface between the pulsar’s crust and interior, though other mechanisms are possible.

Millisecond Monitoring

The final study of this Monthly Roundup concerns millisecond pulsars. All pulsars spin incredibly fast, but millisecond pulsars are the fastest of the fast, with spin periods less than about 10 milliseconds. Although pulsars tend to spin more slowly as they age, researchers believe that millisecond pulsars buck this trend. Instead, millisecond pulsars are likely older pulsars that have been “spun up” to high speeds by accreting matter from a close binary companion. While many millisecond pulsars have been found to have binary companions, some of them fly solo, and the formation mechanism of these single millisecond pulsars isn’t yet clear. Regardless of whether they are singletons or in pairs, more observations are needed to characterize these extreme objects.

globular cluster Messier 3

Hubble Space Telescope image of the globular cluster Messier 3. Messier 3 is home to 500,000 stars and six known millisecond pulsars. It’s also the first Messier object to have been discovered by Charles Messier himself. [ESA/Hubble & NASA, G. Piotto et al.; CC BY 4.0]

Baoda Li (Guizhou University; Yunnan Key Laboratory) and coauthors investigated the properties of the six known millisecond pulsars in the globular cluster Messier 3, which is about 34,000 light-years away. Using the Five-hundred-meter Aperture Spherical Telescope (FAST) — the largest filled-aperture radio telescope in the world — the team detected signals from five of the six pulsars in the cluster. One pulsar named M3C failed to appear in 41 observations, likely due to scintillation, or the scattering of the pulsar’s radio signal by the interstellar medium.

For the remaining five pulsars, Li’s team obtained updated or new timing solutions: models of a pulsar’s pulses that can be used to predict future pulse arrival times. Additionally, they showed for the first time that pulsars M3E and M3F have low-mass companion objects and travel on circular orbits of 7.1 and 3.0 days, respectively.

In future work, Li’s team plans to focus on scintillation; while scintillation can be an annoyance to researchers when it disperses pulsar signals, it also provides a way to study the intervening interstellar material.

Citation

“Chandra X-Ray Observations of PSR J1849-0001, Its Pulsar Wind Nebula, and the TeV Source HESS J1849-000,” Seth Gagnon et al 2024 ApJ 968 67. doi:10.3847/1538-4357/ad3e6d

“VLBA Astrometry of the Fastest-Spinning Magnetar Swift J1818.0−1607: A Large Trigonometric Distance and a Small Transverse Velocity,” Hao Ding et al 2024 ApJL 971 L13. doi:10.3847/2041-8213/ad5550

“First Detection of X-Ray Polarization in Galactic Ultraluminous X-Ray Pulsar Swift J0243.6+6124 with IXPE,” Seshadri Majumder et al 2024 ApJL 971 L21. doi:10.3847/2041-8213/ad67e5

“Timing and Scintillation Studies of Pulsars in Globular Cluster M3 (NGC 5272) with FAST,” Baoda Li et al 2024 ApJ 972 43. doi:10.3847/1538-4357/ad5a82

JWST photograph of a white dwarf

What will happen to Earth and the other planets in our solar system when the Sun dies? Finding exoplanets around white dwarfs can help answer this pressing question, and researchers have discovered a possible giant exoplanet around a nearby white dwarf.

At the End of the Solar System

Hubble Space Telescope images of stars and a white dwarf

A white dwarf is faintly visible at the center of this Hubble Space Telescope image, tucked between the two brightest stars. [NASA, ESA and H. Richer (University of British Columbia); CC BY 4.0]

When the Sun exhausts its supply of core hydrogen, its interior will undergo a series of transitions that transform our home star into a cool and puffy red giant. Eventually, the Sun will shed its outer layers and leave behind a scorching, crystallized, Earth-sized remnant called a white dwarf. How the planets in our solar system will weather these changes is an open question (though experts are in agreement that Mercury and Venus will be engulfed by the expanding star, sadly).

One way to probe the answer to this question is to study planetary systems orbiting white dwarfs. This may reveal the distances at which planets find safe harbor from the red giant, as well as whether any planets change or undergo migration as the result of their host star’s evolution. Historically, detecting planets around white dwarfs has been challenging — but luckily, JWST makes this task much easier.

Discovering Exoplanets with MEOW

The MIRI Exoplanets Orbiting White dwarfs (MEOW) survey is one of several surveys using JWST’s sensitive infrared instruments to search for planets around white dwarfs. In a recently published article, a team led by Mary Anne Limbach (University of Michigan) reported early results from the MEOW survey, focusing on a candidate planet around the nearby white dwarf WD 0310–688.

plot of infrared excess as a function of wavelength

The measured infrared excess of WD 0310–688 (yellow line) compared to other white dwarfs in the MEOW sample (green and blue lines). [Limbach et al. 2024]

The survey is designed to uncover white-dwarf exoplanets through either direct imaging or detection of infrared excess: an unexpectedly large flux at infrared wavelengths that indicates the presence of an object cooler than the white dwarf, like a planet. Limbach and coauthors detect an infrared excess around WD 0310–688 that is most easily explained by a cold (248K) planet companion with a mass of about 3 Jupiter masses. Curiously, the observations place this potential planet between 0.1 and 2 au from the white dwarf — even though planets interior to 2 au are thought to be destroyed when their stars balloon into red giants. This may suggest that the planet migrated to its current location after its host star’s red giant phase.

Candidate Considerations

Researchers have discovered a handful of exoplanets around white dwarfs already — what makes this discovery special? WD 0310–688 is only 34 light-years away, making it the closest white dwarf with a planet candidate, and no planet has ever been discovered 0.1–2 au from a white dwarf. Additionally, this marks the first planet around any type of star to be discovered with the infrared-excess method.

schematic of the potential disk around the white dwarf

Schematic of the best-fitting disk from the team’s modeling. The disk must be very cold, highly inclined, and quite narrow to match the data. Coincidentally, the disk’s emitting area must be roughly the area of a giant planet. [Limbach et al. 2024]

However, the authors cautioned that a planet isn’t the only possibility for the observed infrared excess; a small, cold disk of debris could also be responsible. If the object is a disk, it would be one of the coldest disks ever found around a white dwarf, making this possibility intriguing in its own right.

Follow-up spectroscopy is needed to discern between the giant planet and cold disk hypotheses. Spectral features commonly found in the atmospheres of exoplanets would support the giant planet hypothesis, while a silicate feature would point toward the debris disk. Future work should illuminate the nature of this planet candidate as well as bring us new results from the MEOW survey!

Citation

“The MIRI Exoplanets Orbiting White dwarfs (MEOW) Survey: Mid-infrared Excess Reveals a Giant Planet Candidate around a Nearby White Dwarf,” Mary Anne Limbach et al 2024 ApJL 973 L11. doi:10.3847/2041-8213/ad74ed

Enhanced Hubble image of Comet ISON

Are the icy objects that inhabit the outer solar system truly pristine remnants of the early solar system, or have they been altered by the chaotic process of planet formation? Recent research that compares the chemistry of solar system comets and planet-forming systems may provide an answer to this key question.

Time Capsules from the Early Solar System(?)

Diagram showing the locations of the Kuiper Belt and Oort Cloud

Diagram showing the locations of the Kuiper Belt and Oort Cloud. Click to enlarge. [Adapted from ESA (Acknowledgement: work performed by ATG under contract to ESA); CC BY-SA IGO 3.0]

The Kuiper Belt and Oort Cloud contain unknown numbers of icy planetesimals left over from the formation of the solar system. One of the most pressing questions in planetary science today is whether these objects represent unaltered material from the dusty nebula from which the Sun and the planets formed, or if the process of planet formation and the intervening billions of years have changed their properties. The answer has profound implications for understanding the formation of our own solar system and planetary systems in general.

There’s a good reason that this question poses such a challenge: these far-flung objects are difficult or impossible to observe directly. Luckily, they make occasional journeys into the inner solar system, transformed by warm sunlight into comets sporting twin tails of dust and ions. Now, a team led by Manuela Lippi (National Institute for Astrophysics, Italy) has amassed a sample of these icy interlopers to assess their chemistry and understand whether they’ve changed since their formation.

calculated chemical abundances for the comets in the sample

Abundances of methanol (CH3OH), formaldehyde (H2CO), and ammonia (NH3), expressed as mixing ratios with respect to water. From left to right, the sections separated by dotted lines show the overall abundances; the difference between comets inside and outside 1 au; differences between dynamically new (DN), long-period (LP), and Jupiter-family (JF) comets; pre- and post-perihelion (perihelion is the closest distance to the Sun reached on a comet’s orbit); and pre- and post-perihelion for comet 67P. Click to enlarge. [Lippi et al. 2024]

Comet Calculations

Using infrared and sub-millimeter observations, Lippi and collaborators studied the chemical compositions of 35 solar system comets. They found that the abundances of methanol, formaldehyde, and ammonia didn’t depend on the comet’s dynamical family. (A dynamical family is a group of comets with similar orbital characteristics.) For example, dynamically new comets — those making their first trek into the inner solar system — had similar chemistry to Jupiter-family comets whose orbits remain within the orbit of Jupiter and have visited the inner solar system many times.

The similarity in chemical composition between comets in different dynamical families suggests that comets retain their chemical makeup after they form. If this is the case, comets in the solar system today should be chemically similar to the material surrounding young, planet-forming Sun-like stars.

Throwing Disks into the Mix

As a test of this theory, Lippi’s team compared the typical chemistry of solar system comets to that of 11 planet-forming systems. The systems in the comparison sample ranged in age from 10,000-year-old hot molecular cloud cores to million-year-old protoplanetary disks. Overall, the abundance ratios of solar system comets were similar to those of planet-forming systems. This is the first statistical evidence that planet-forming systems of all ages are chemically similar and that comets are indeed unaltered remnants of the early solar system.

Comet 67P/Churyumov–Gerasimenko

Comet 67P/Churyumov–Gerasimenko as seen by the Rosetta spacecraft. [ESA/Rosetta/NAVCAM, CC BY-SA IGO 3.0]

However, a note of caution: when Lippi and coauthors compared their solar system comet sample against measurements of the well-studied comet 67P/Churyumov–Gerasimenko, which hosted the Rosetta spacecraft in 2014–2016, they found substantial differences. If comet 67P stands apart from the average solar system comet, its use as a benchmark could bias studies of planet-forming systems.

Citation

“The Ice Chemistry in Comets and Planet-Forming Disks: Statistical Comparison of CH3OH, H2CO, and NH3 Abundance Ratios,” Manuela Lippi et al 2024 ApJL 970 L5. doi:10.3847/2041-8213/ad5a6d

Image of JWST's First Deep Field

The powerful space telescope JWST is peering far into space, allowing astronomers to observe distant galaxies whose light can shine upon the evolutionary history of the universe. A recent study uses JWST spectroscopy to characterize some of the first galaxies and their role in heating up the universe. 

Reionization and Leaky Galaxies

A few million years after the Big Bang, the atoms of the infant universe cooled enough for stars to flicker on and galaxies to coalesce. Many of these early galaxies housed enormous young stars and massive black holes that produced such powerful radiation as to completely ionize the interstellar medium of their hosts, leaving the rest of the high-energy ultraviolet photons nowhere to go but out. Ionizing radiation leaked from these galaxies into the intergalactic medium, stripping the electrons from the previously neutral hydrogen atoms filling up space and setting alight the epoch of reionization.

While we know that massive stars and accreting black holes pump the most energetic photons into the universe, prior to JWST, observations have not been able to narrow down which of these sources primarily fueled cosmic reionization. Each ionizing source impacts the evolution of its host galaxy and the early universe in its own way, so a better understanding of the conditions in reionization-era galaxies is imperative to answer important cosmological questions. Now with JWST’s ability to observe with higher signal-to-noise and spectral resolution than ever before, the characteristics of these galaxies are well on their way to being uncovered.

Clues in Emission-Line Ratios

While reionization-era galaxies are unfortunately much too distant to directly observe their ionizing sources, we can characterize the conditions of their interstellar medium based on the relative strengths of the emission lines observed in their spectra. Especially of interest are the emission-line ratios associated with ultraviolet photons, as those photons have enough energy to ionize the universe. With this in mind, Weida Hu (Texas A&M University) and collaborators leveraged publicly available JWST data from the Cosmic Evolution Early Release Science Survey (CEERS) and the JWST Deep Extragalactic Survey (JADES) to explore the interstellar medium conditions in galaxies at redshifts between 5.6 and 9. This redshift range corresponds to roughly 500 million to 900 million years after the Big Bang — about the time in which the reionization of the universe occurred. 

1D and 2D Composite Spectrum

2D and 1D composite spectrum obtained from stacking 63 high-redshift galaxies. Emission lines of interest are labeled in blue. The lower flux UV lines are zoomed in to show their peaks. Click to enlarge. [Hu et al. 2024]

Combining Near-Infrared Spectrograph (NIRSpec) data for 63 galaxies, the authors built a composite spectrum covering both ultraviolet and optical emission lines from which they measured multiple critical line ratios. Of particular importance was determining the carbon-to-oxygen (C/O) ratio of the composite spectrum because C/O probes stellar wind and outflow activity as well as star formation history, which are critical elements of galaxy evolution. The authors measure a C/O ratio that is smaller than those found in lower-redshift galaxies, indicating the reionization-era galaxies likely experienced rapid star formation, building up their stellar masses rather quickly.    

Ionization Origination

What do the measured emission-line ratios tell us about the ionizing source in reionization-era galaxies? Through comparing the ratios with those of other galaxies across a range of redshifts, Hu’s team finds that, on average, the galaxies in this study are ionized by highly energetic sources — mostly star formation with possible weak black hole activity — but the exact ionization driver is still unclear. However, based on ultraviolet diagnostics, the composite spectrum exhibits similar line ratios to local extreme dwarf galaxies, suggesting that observations of the ionization sources of nearby galaxies may provide insightful information about the conditions of galaxies from the distant past. 

Plot of emission-line ratios.

Plot of emission-line ratios. The composite spectrum’s position is marked with the large red circle, and it falls near local extreme star forming galaxies, green peas and blueberries. Other data points are plotted representing other galaxies across a range of redshifts. The black dashed line shows the separation between star forming galaxies (below the line) and active central black holes (above the line). Click to enlarge. [Hu et al. 2024]

Though the ionization source remains ambiguous, the authors estimate that ~25% of reionization-era galaxies in their sample are leaking high-energy photons into the intergalactic medium, heating up the universe and contributing to cosmic reionization. Determining the exact fraction of galaxies that leak photons and the amount of photons being leaked requires continued high-resolution spectroscopy on individual high-redshift galaxies. With future surveys and targeted observations with JWST, more light from the distant past will illuminate the pressing questions surrounding the epoch of reionization.     

Citation

“Characterizing the Average Interstellar Medium Conditions of Galaxies at z~5.6–9 with Ultraviolet and Optical Nebular Lines,” Weida Hu et al. 2024 ApJ 971 21. doi:10.3847/1538-4357/ad5015

 

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