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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

 

coronal loops over a sunspot group

What happens just as a solar flare begins to produce X-rays? New research leverages data from an Earth-orbiting X-ray telescope to examine the early stages of solar flares.

image of the sun's surface

The Solar Dynamics Observatory spacecraft captured this image of a large solar flare in 2014. [SDO/NASA]

A Common Occurrence

With the help of Sun-gazing observatories in space and on Earth, researchers have witnessed thousands of solar flares. Solar flares are powered by the explosive rearrangement of magnetic fields, accelerating charged particles to breakneck speeds and generating a brilliant flash of X-rays.

While the overall picture of flare production is clear, the details are still under scrutiny. To understand how solar flares happen, researchers must study each stage of the process closely. Today’s article examines the onset phase, which happens just before a flare ramps up to maximum brightness.

plot of counts per second during different phases of a solar flare

The counts per second (CPS) measured by DAXSS during several phases of a flare. [Adapted from Telikicherla et al. 2024]

A Three-Phase Process

Solar flares tend to happen in three stages:

  1. The precursor phase, in which pent-up magnetic energy is released and the first signs of the flare appear in the form of non-thermal particle motions
  2. The impulsive phase, in which charged particles get accelerated to extremely high energies, triggering the release of radio waves, hard (high-energy) X-rays, and gamma rays
  3. Finally, the decay phase, in which soft (low-energy) X-rays gradually brighten and then fade

Anant Telikicherla (Laboratory of Atmospheric and Space Physics, University of Colorado Boulder) and coauthors focused on the onset phase, which refers to the moments just before the impulsive phase, when the first soft X-rays are emitted but before the hard X-ray emission begins. Using data from the Dual-zone X-ray Solar Spectrometer (DAXSS), the team analyzed the soft X-ray spectra of six solar flares. The sampled flares took place in 2022 and had a range of characteristics, from average-intensity C-class flares to more powerful M-class flares. Most of the flares in the sample were eruptive, meaning they were accompanied by explosions of plasma into space, while one was confined, lacking a plasma explosion.

Flare and Back Again

The observations revealed that during the onset phase, when the first soft X-rays are produced, the solar plasma is already extremely hot, in the range of 10–15 million kelvin. The temperature then dips before rising again during the impulsive phase. In this way, the onset phase parallels the rising and falling temperatures of the main flare phase, leading Telikicherla’s team to suspect that the onset phase acts as “preconditioning” for the main event.

image of two coronal loops during the onset phase

Example of two coronal loops (indicated with red arrows) forming during the onset phase. [Adapted from Telikicherla et al. 2024]

The authors also examined extreme-ultraviolet images of the flares to understand the underlying motions of the plasma. They saw two general types of behavior, which they call one-loop and two-loop onset flares. For a one-loop onset flare, a single arc of coronal plasma brightens during the onset phase. This coronal loop then interacts with a second loop that brightens during the impulsive phase. During a two-loop onset flare, two coronal loops brighten, merging into a single loop during the impulsive phase.

Remarkably, the observed behavior appears to mimic the evolution of the main phase of a solar flare, suggesting that the onset phase offers a preview of what’s to come. Future work analyzing hard X-ray emission will explore the connection further, helping to understand whether the characteristics of the onset phase can be used to predict the properties of the main flare stage.

Citation

“Investigating the Soft X-Ray Spectra of Solar Flare Onsets,” Anant Telikicherla et al 2024 ApJ 966 198. doi:10.3847/1538-4357/ad37f6

A computer rendering of a spacecraft with a large radio dish against a black background studded with stars.

A little over nine years ago, a plucky robot with a huge radio dish for a head and a heart made of radioactive plutonium completed one of the most daring space missions conceived of to date. After hibernating through nearly a decade of interplanetary travel, the probe, earnestly dubbed New Horizons by its dedicated creators, woke up as it neared the dwarf planet Pluto. By design it was then beelining towards its destinations at over 50,000 miles per hour, far too fast to slow down using its limited remaining propellant. And so, it didn’t. In barely a blink of an eye, New Horizons snapped as many pictures as it could, furiously recorded data from its other instruments, and served as humanity’s first ambassador to the king of the Kuiper Belt before continuing zipping onward into our outer solar system.

The north pole of Pluto, as seen by New Horizons during its 2015 flyby. [NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute]

And then it was over. Though the New Horizons probe was traversing a region never before visited by another mission, there was no planned second destination in mind. Pluto had been the goal, and after the successful flyby, the mission could quite fairly have been declared a triumph and the team could have let New Horizons rest again.

But the probe still had some fuel in its tanks — in a very literal sense. Though New Horizons was traveling on a safe path that wouldn’t take it near any known objects, its controllers could potentially use the onboard thrusters to steer the probe closer to another Kuiper Belt object for a first-of-its-kind investigation. The only flaw in this plan was that when New Horizons left Earth in 2006, astronomers didn’t know of any objects near enough to Pluto to be reachable once the primary flyby was complete. The very first Kuiper Belt objects (besides Pluto) had only been discovered a few years earlier in 1992, and although scientists were beginning to suspect that millions of small planetesimals could inhabit the backwoods of our solar system beyond our planets, they hadn’t found many of them yet.

Unable to let the opportunity to study one of these likely ancient, preserved relics from the formation of our solar system slip away, an international team of astronomers took it upon themselves to search for a second target for New Horizons. Incredibly, the bulk of this work took place while the spacecraft was actively flying towards its eventual destination. The New Horizons team had thrown their hats over the wall, and with limited time before their robot drifted into interstellar space or succumbed to any one of the large number of hazards of the void, they needed to discover a new object quickly.

To spoil the ending: this industrious team was ultimately successful. They discovered a strange, 32-km-long, snowman-shaped object called Arrokoth, and New Horizons performed its second flawless Kuiper Belt flyby on New Year’s Day, 2019. However, though this end result is well known, the effort that went into this search was previously less well documented. This has been recently remedied with a narrative-like article published in The Planetary Science Journal, which details the decade of frantic, build-the-plane-while-flying-it type of work that led to Arrokoth’s discovery, along with the creation of an entire set of tools and best practices that guide many of today’s solar-system mapping efforts.

Early Days

The astronomers’ work began in 2004, in the heady early days of Kuiper Belt research when all of the models were constantly being revised as new discoveries poured in. How many Kuiper Belt objects were out there? How big were they? What orbits did they travel on? These were all uncertain foundational questions, but armed with the Subaru Telescope on Mauna Kea and optimism for quick discoveries, the team set out to answer them.

A photograph of a large telescope with an open rectangular covering.

The Subaru Telescope, a major component of the search efforts. [NAOJ]

From 2004 to 2005, the researchers took deep images of wide swaths of the sky as they searched for just the right object along New Horizons’ eventual path. With every new telescope pointing, they imaged stars that had yet to be recorded in any catalog and likely had never been observed by any human before them. While thrilling, this bounty of side-effect discoveries actually complicated their search. There were so many stars in their images, it was exceedingly difficult to spot the faint, moving Kuiper Belt objects that darted among them.

Painstaking manual inspection of all of the images did eventually yield 24 new objects, but none were close enough to New Horizons’ path to be feasible destinations. After dabbling with some citizen science efforts and a handful of new software routines to automate a portion of their workflow, the team regrouped and devised a new strategy for their search.

Heating Up

Between 2011 and 2015, the astronomers began a more intensive observing campaign. Several factors had emerged in the intervening years that tipped the odds of a suitable detection in their favor. For one, new instruments had been designed and installed on several large telescopes, which accelerated the search. For another, the area of the sky they had to sweep was shrinking thanks to the shifting arrangement of the Earth, Pluto, and New Horizons.

Adding to their advantages, this time around the researchers incorporated complex software tools into their analysis from the very start. The enormous quantity of data anticipated from this new search would place immense pressures on the old method of carefully comparing two images by eye to look for the moving objects; it was now time for computers to take the lead.

These new instruments and analysis techniques, along with yet more manual vetting, produced more than 50 additional discoveries. Again though, further analysis on all of these objects confirmed that they were just out of New Horizons’ reach. The probe could wave as it sailed by, but it wouldn’t get close enough to take any useful images of the surfaces. By now, as the mission team approached the actual Pluto flyby and an impending deadline for any final course corrections, time was running short.

Big Guns, Big Future

The first images of Arrokoth, taken with the Hubble Space Telescope. Click to enlarge. [Buie et al. 2024]

Since “…we absolutely had to find an object in 2014,” (Buie et al. 2024), it was time to leave the ground-based telescopes behind and bring in a closer: the most famous telescope in (and off of) the world, the Hubble Space Telescope. The team was ultimately awarded over 200 orbits’ worth of observing time in 2014, and finally, they were rewarded for their efforts with a handful of viable potential targets. One of these would eventually come to be known as Arrokoth, and the rest is history.

This decade of surveying, searching, waiting, and re-scheming ultimately left the planetary science community with another flyby and 80 new Kuiper Belt Objects, yes, but also with techniques less tangible than these discoveries. As astronomers push ever deeper into the night sky and search more and more crowded regions of stars, they often use the observing plans, algorithms, and sometimes the actual code developed as a part of this search. It was a tremendous effort that paid off handsomely, and as we near the 10-year anniversary of our robotic arrival at the Kuiper Belt, a reflection and celebration of how we have learned to study this strange place is in order.

Citation

“The New Horizons Extended Mission Target: Arrokoth Search and Discovery,” Marc W. Buie et al 2024 Planet. Sci. J. 5 196. doi:10.3847/PSJ/ad676d

JWST image of Wolf–Rayet star WR 140

Instead of fading smoothly, some supernova light curves take a bumpy road from brilliance to obscurity. Can unusual binary systems containing a rapidly spinning, wind-emitting magnetar and a stellar companion explain these light curves?

Light Curve Wiggles

illustration of a magnetar

Artist’s impression of a highly magnetized stellar remnant called a magnetar. [ESO/L.Calçada; CC BY 4.0]

When a star explodes, researchers record the light curve from its final moments and attempt to understand its life and death. Certain supernovae show bumps and wiggles in their light curves, the cause of which is not yet agreed upon. Researchers suspect that some of these light-curve bumps crop up when the expanding shock wave of the supernova slams into gas and dust surrounding the star. Other brightness increases might occur when the explosion leaves behind a magnetar — an extremely dense, city-size stellar remnant that spins rapidly and has a strong magnetic field — that injects energy into its surroundings.

In a recent research article, Jin-Ping Zhu (Monash University) and collaborators expanded on the latter possibility, pairing a powerful magnetar with an unlucky companion star to explain bumpy features in the light curves of certain supernovae.

diagram illustrating the stages of the magnetar–star binary engine model

A diagram illustrating the stages of the magnetar–star binary engine model. Click to enlarge. [Zhu et al. 2024]

What Goes Bump in a Supernova Light Curve

The proposed theory starts with an ordinary star and a massive star in a close binary system. As the massive star evolves, it sheds its outer layers through rapid rotation and fierce winds, exposing its super-hot core and becoming a rare Wolf–Rayet star. As the Wolf–Rayet star continues to evolve, tidal interactions between the stars in the binary system spin the Wolf–Rayet star up to high speeds. It eventually explodes in a core-collapse supernova, leaving behind a rapidly spinning magnetar.

Other models have invoked magnetars to explain bumpy supernova light curves, but this theory goes a step further, giving the companion an important role to play. As the newborn magnetar and the companion star swing around each other on their tight orbits, the magnetar’s powerful particle wind collides with the other star, evaporating some of the unlucky companion. The evaporated stellar material is then heated and accelerated by the magnetar wind, producing a bump in the light curve.

A Fitting Theory

plot of supernova light curves and model results

Example of multi-band light curves for a supernova that is well fit by the authors’ model. [Adapted from Zhu et al. 2024]

That’s the theory — how does it compare to observations? Zhu and collaborators applied their magnetar–star binary engine model to the light curves of supernovae with a single bump after maximum brightness. They found that the model generally fits the observations well, with the best-fitting results implying that a significant chunk — about 25–60% — of the companion star gets evaporated.

Zhu and collaborators suspect that their model may apply to light curves with multiple bumps, as well. If the companion star remains bound to the magnetar after the supernova explosion but is kicked into a new, highly eccentric orbit, a bump could be created each time the stars draw close to one another on their orbits.

The team notes that there isn’t yet firm observational or theoretical evidence that rapidly rotating massive stars leave behind magnetars, and it’s not clear whether a magnetar embedded within a supernova remnant can sustain a magnetar wind, as is required here. Future work may shore up the needed evidence, and in the meantime, this work provides a new way to interpret bumpy light curves.

Citation

“Bumpy Superluminous Supernovae Powered by a Magnetar–Star Binary Engine,” Jin-Ping Zhu et al 2024 ApJL 970 L42. doi:10.3847/2041-8213/ad63a8

photograph of spiral and elliptical galaxies

plot of a signal from a nearby fast radio burst

Example of a signal from a nearby fast radio burst. The top panel shows the overall intensity over time, while the bottom panel shows the frequency of the burst over time. [Adapted from Bhardwaj et al. 2024]

Where do fast radio bursts come from? New research shows that the nearby host galaxies of these fleeting flashes have something in common, which may help researchers understand the origins of fast radio bursts.

Mysterious Bursts

Fast radio bursts are powerful, milliseconds-long flashes of radio waves of unknown origin. Since the first fast radio burst was discovered in 2007, astronomers have detected roughly a thousand of these mysterious signals. The source of these bursts is still up for debate, with supernovae, magnetars, colliding objects, and other energetic phenomena tapped as candidates.

To understand the origin of these bursts, it helps to know what kind of galaxies they happen in. If fast radio bursts emerge from spiral galaxies with active star formation, it could mean that the bursts are linked to “prompt” formation channels such as the deaths of short-lived massive stars. If instead bursts come from elliptical galaxies with little or no star formation, that could imply that bursts come from “delayed” channels like the slowly progressing mergers of stellar remnants. So, what kind of galaxies do fast radio bursts tend to come from?

In the Neighborhood

Mohit Bhardwaj (Carnegie Mellon University and McGill University) and collaborators turned to data from the Canadian Hydrogen Intensity Mapping Experiment (CHIME) to answer this question. Their aim was to study fast radio bursts in the local universe since a sample of nearby bursts is less likely to be affected by observational biases. To find nearby bursts, Bhardwaj’s team searched the first CHIME fast radio burst catalog for signals with low dispersion measure. On average, the farther the source, the larger the dispersion measure — essentially, because there’s more stuff in between the source and Earth to disperse the radio signal.

visible-light images of candidate host galaxies

Visible-light images of the localization regions of the four fast radio bursts in this study. The red boxes indicate the most likely host galaxy for each burst. Note that FRB 20181223C has four potential host galaxies (red and cyan boxes in the upper-left image), but only one that satisfied the source’s maximum redshift limit. Click to enlarge. [Bhardwaj et al. 2024]

The team found four cataloged bursts with dispersion measure excess (the amount left over after the contribution from the Milky Way is subtracted off) less than 100 parsecs per cubic centimeter, which corresponds to a distance of about 1.3 billion light-years. After searching the localization regions of these bursts in deep optical images from the Panoramic Survey Telescope and Rapid Response System survey, the authors found only one plausible host galaxy for each burst.

A Spiral Sample

Including the four bursts with newly identified host galaxies from this work, researchers have now localized the positions of 18 nearby fast radio bursts. What ties these burst-hosting galaxies together, if anything? As it turns out, they’re all spiral galaxies. That’s an intriguing result, but does it necessarily mean that fast radio bursts are more likely to come from spiral galaxies, or is it just easier to detect fast radio bursts from spiral galaxies?

images of the 18 fast radio burst host galaxies

The host galaxies of all 18 local universe fast radio bursts used in this study. Click to enlarge. [Bhardwaj et al. 2024]

Bhardwaj and collaborators explained that their sample selection is actually biased against bursts from spiral galaxies because these galaxies tend to have more signal-dispersing material than elliptical galaxies do, making spiral galaxies more likely to be eliminated by the dispersion measure cutoff. Given the typical ages of stars in spiral galaxies, the team suggests that the dominant formation pathway for fast radio bursts in the local universe is through core-collapse supernovae, which mark the explosive end of stars more than about eight times the mass of the Sun.

Bhardwaj’s team noted that this doesn’t mean all fast radio bursts must come from supernovae; because a small number of known bursts have arisen in unusual locations like globular clusters and non-star-forming spiral galaxies, delayed pathways like mergers of stellar remnants may be responsible for certain bursts.

Citation

“Host Galaxies for Four Nearby CHIME/FRB Sources and the Local Universe FRB Host Galaxy Population,” Mohit Bhardwaj et al 2024 ApJL 971 L51. doi:10.3847/2041-8213/ad64d1

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