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X-ray image of Kepler's Supernova remnant

Observations of Type Ia supernovae underpin our measurements of distances to other galaxies and even our understanding of the expansion of our universe. There’s just one problem: researchers still don’t know exactly how Type Ia supernovae happen. New research explores what a Type Ia supernova in an unusual environment could tell us about the source of these explosions.

Studying Supernovae

Illustration of a white dwarf accreting gas from a red giant companion star

Accretion from a companion star onto a white dwarf is one way to trigger a supernova. [Still image from an animation by NASA’s Goddard Space Flight Center Conceptual Image Lab]

Type Ia supernovae happen in binary systems containing at least one white dwarf: the crystallized core of a low- to intermediate-mass star. In theory, Type Ia supernovae happen when a white dwarf grows larger than 1.4 solar masses and explodes.

But research increasingly points to more than one way for white dwarfs to reach this limiting mass — two white dwarfs in a binary system could collide, or a white dwarf could bulk up by accreting matter from a stellar companion. The exploding white dwarf may not even need to reach 1.4 solar masses: if a thin shell of helium siphoned from a companion star ignites on the surface of a white dwarf, it could trigger a second, catastrophic detonation in the white dwarf’s core, regardless of the white dwarf’s mass. Pinpointing these different scenarios is critical to interpreting Type Ia supernova observations.

How can we examine the progenitor of a Type Ia supernova explosion? Researchers can’t just sift through archival observations after a Type Ia supernova to see what caused it, since white dwarfs in other galaxies are too faint to see — and we haven’t spotted a Type Ia supernova in our own galaxy since 1604. We need a different approach to uncover the origins of these explosions.

Cluster Candidate

In a recent article, Joel Bregman (University of Michigan) and collaborators proposed a path forward: find a Type Ia supernova in a globular cluster. Globular clusters are roughly spherical collections of tens of thousands to millions of stars that orbit within a galaxy’s halo. Because all stars in a globular cluster have roughly the same age and composition, the age and composition of the stars involved in the explosion could also be known — even if we can’t observe the stars directly.

Hubble Space Telescope image of supernova SN 2019ein

The location of the supernova SN 2019ein relative to the globular cluster candidate. Click to enlarge. [Bregman et al. 2024]

Bregman’s team searched for records of a Type Ia supernova occurring within about 100 million light-years in a galaxy with globular clusters. The team pinpointed one promising candidate: SN 2019ein, which was discovered in May 2019 on the outskirts of the elliptical galaxy NGC 5353. The supernova popped up far from the center of this galaxy but close to a faint object that may be a globular cluster.

Detonation Scenarios

Hubble observations show that the supernova happened about 192 light-years from the globular cluster candidate. Statistical tests suggested that the supernova is likely associated with the cluster — the authors put the odds of a chance alignment at 3% — allowing researchers to use the age and composition of the cluster to constrain the properties of the supernova progenitor system.

There’s another interesting wrinkle that could further constrain the identity of the progenitor: given the large separation between the supernova and the candidate cluster, the star going supernova may not have been bound to the cluster when the explosion happened. In this scenario, dynamical interactions between binary systems could have kicked the supernova progenitor system out of the cluster, which is more likely if both stars in the binary were white dwarfs. Then, after careening through space for 3–10 million years, 1) the white dwarfs collided or 2) one white dwarf stole a thin layer of helium from the other, leading to a two-stage detonation on its surface and in its core.

The proposed scenario is rare but not impossible, but there’s not yet enough data to draw firm conclusions. This study highlights the challenge of studying the origins of Type Ia supernovae and shows what can be gained by seeking supernovae in unusual places.

Citation

“A Type Ia Supernova near a Globular Cluster in the Early-Type Galaxy NGC 5353,” Joel N. Bregman et al 2024 ApJL 968 L6. doi:10.3847/2041-8213/ad498f

artist's impression of the first stars in the universe going supernova

The galaxy RX J2129–z8He II is remarkable for its high redshift, sloped spectrum, and strong emission lines. New research suggests that the elusive first stars could be responsible for the galaxy’s properties.

The Hunt for the First Stars

Long ago, the universe was substantially less picturesque than it is today; neutral hydrogen gas absorbed all visible light and rendered the universe opaque. When the first stars glimmered into existence and assembled into the first galaxies, they began to chip away at the opaque universe, carving out increasingly large bubbles of transparent gas.

The nature of the first stars, also called Population III stars, is of great interest to astronomers, but tracking them down is easier said than done. While galaxies composed only of the first generation of stars may be out of reach, shielded from view by clouds of neutral gas, galaxies in which Population III stars mingle with later stellar generations might be easier to spot.

Thirteen Billion Years Ago in a Galaxy Far, Far Away

JWST image of the galaxy RX J2129–z8He II

JWST image of the galaxy RX J2129–z8He II, which is gravitationally lensed by the foreground galaxy cluster RX J2129. [Wang et al. 2024]

Recently, a research team led by Xin Wang (University of Chinese Academy of Sciences) may have identified such a galaxy. Using images from JWST, Wang and collaborators picked out the galaxy RX J2129–z8He II, which even in the eyes of the world’s most powerful infrared telescope is a barely discernible red smudge. JWST spectra of this galaxy place it at a redshift of z = 8.1623, just 613 million years after the Big Bang. At this time, stars and galaxies were still busily ionizing the universe, transforming it from opaque to transparent.

RX J2129–z8He II’s spectrum is tilted sharply toward short wavelengths, more so than any other known galaxy beyond a redshift of z = 7, and it’s marked by several prominent emission lines, including one from singly ionized helium atoms. These factors suggest that the galaxy contains a powerful source of ultraviolet radiation. In the local universe, emission from singly ionized helium is somewhat rare, arising from massive stars that have lost their atmospheres, binary systems containing a star and either a black hole or a neutron star, and galaxies with accreting supermassive black holes. None of these sources are likely to be the cause of the galaxy’s helium emission line. Instead, Wang’s team posits, massive Population III stars could be the source of the ionizing ultraviolet photons.

plot of the spectrum of the galaxy RX J2129–z8He II

JWST spectrum of RX J2129–z8He II, with an inset image showing the He II line. The flux increases toward shorter wavelengths before being attenuated by the intergalactic medium (IGM). Click to enlarge. [Wang et al. 2024]

Are Population III Stars Responsible?

To test this theory, Wang’s team used photoionization models to simulate the properties of a galaxy containing Population III stars as well as stars from later generations. They found that a collection of Population III stars with a total mass of 780,000 solar masses could reproduce the observed emission lines in the galaxy’s spectrum. Previous modeling suggests that this quantity of Population III stars is reasonable for a galaxy at that time period.

As for how a galaxy could host Population III stars alongside their stellar descendants, Wang’s team suggests that new Population III stars could form belatedly in pockets of pristine gas that weren’t swept up into stars in the first round of star formation, or in gas that spilled into the galaxy from the surrounding circumgalactic medium.

This study marks a promising advance in the search for the first generation of stars. In addition to identifying RX J2129–z8He II as a possible home for Population III stars, Wang’s team gained a better sense for the spectral signatures of these stars, which can be used to identify other galaxies that may host them.

Citation

“A Strong He II λ1640 Emitter with an Extremely Blue UV Spectral Slope at z = 8.16: Presence of Population III Stars?” Xin Wang et al 2024 ApJL 967 L42. doi:10.3847/2041-8213/ad4ced

spiral galaxy NGC 6744 as seen by Euclid

Small galaxies in the early universe might have had centers dominated by dark matter. New research dives into cosmological simulations to explore what these galaxies would look like in the universe today and what this might mean for the Milky Way’s dark matter past.

Dark Matter History of Small Galaxies

Recently, a research team led by Anna de Graaff (Max Planck Institute for Astronomy) used JWST to examine six small, low-mass galaxies in the early universe. They discovered something surprising: the centers of these galaxies appeared to be dominated by dark matter, a hypothetical form of matter that neither emits nor absorbs light, among other strange qualities. Though dark matter is thought to make up roughly 85% of the matter in our universe, research shows that the cores of massive galaxies in the universe today contain mostly normal matter.

To check whether the intriguing new JWST observations match our theoretical understanding of the universe’s evolution, and to explore how these dark matter–dominated galaxies might evolve to the present day, de Graaff and collaborators turned to TNG50: a massive cosmological simulation that tracks the universe from its infancy to today.

Enter the Simulation

plot comparing the fraction of normal matter in the cores of galaxies in the TNG50 simulation and the galaxies observed with JWST

Comparison of the fraction of normal matter (fbaryon) in the cores of galaxies in the TNG50 simulation (small circles) to the galaxies observed with JWST (large hexagons). Click to enlarge. [de Graaff et al. 2024]

Using TNG50, the team selected 381 simulated galaxies with redshifts and stellar masses similar to the real galaxies in the JWST sample. They found that the lower-mass galaxies in the simulated sample — those with stellar masses less than about 300 million solar masses — had centers dominated by dark matter, containing just 10–30% normal matter by mass. The higher a galaxy’s stellar mass, the higher the fraction of normal matter in the galaxy’s center.

This result agrees well with what de Graaff’s team observed in real galaxies with JWST, though the team pointed out that it’s possible that the galaxies they studied with JWST only appear to have dark matter–dominated cores. For example, if the gas in these galaxies’ centers is disrupted by outflows from newly forming stars, it could skew the estimate of the amount of dark matter present. Because the simulated galaxies do have dark matter–dominated cores, this finding suggests that this may be a real phase in galaxy evolution.

plot showing how the dark matter fraction evolves with redshift for simulated and observed galaxies

Redshift evolution of the core dark matter fraction. The colored line and shaded areas show the results from the TNG50 simulation, while the colored symbols show the results from observational studies. Click to enlarge. [de Graaff et al. 2024]

Following Evolving Galaxies

What would these early dark matter–dominated galaxies look like later in the universe’s history? Going forward in time in the simulation, the galaxies merged and grew, dwindling from 381 galaxies to just 213 due to mergers. At a redshift of z = 3 — during a period called cosmic noon, when star formation was at its peak — the low-mass dark matter–dominated galaxies had grown to contain 10 billion solar masses of stars, similar in size to other galaxies spotted in that time period.

By the present day, these same galaxies had packed on additional stellar mass, reaching about 30 billion solar masses of stars — roughly equal to the mass of stars in the Milky Way — and their centers no longer contained mostly dark matter. While more work is needed to explore this scenario, this suggests that the Milky Way and galaxies like it in the universe today were once small, low-mass galaxies with dark matter cores.

Citation

“An Early Dark Matter–Dominated Phase in the Assembly History of Milky Way–Mass Galaxies Suggested by the TNG50 Simulation and JWST Observations,” Anna de Graaff et al 2024 ApJL 967 L40. doi:10.3847/2041-8213/ad4c65

A photograph of an asteroid on a black background. Superimposed on the asteroid's surface is a colormap that ranges from red to green, denoting the reflectance.

The Double Asteroid Redirection Test (DART) mission marked humanity’s first attempt to alter the course of an asteroid, a technique that could one day avert a catastrophic impact and save countless lives. But to fully understand the outcome of the experiment and to determine the impact of the human-caused impact, astronomers need a detailed characterization of their target. A recent study aims to bridge observations taken by DART moments before its demise and those taken from back on Earth in order to measure surface properties that neither dataset could constrain alone.

Intentional Collision and In-Situ Measurements

In September 2022, NASA’s DART spacecraft approached a near-Earth asteroid named Didymos and its moon, Dimorphos. With cameras trained on Dimorphos specifically, DART took pictures with ever-increasing resolution as the distance to the moon’s surface steadily shrank. Unfortunately for both DART and Dimorphos, though, this was not a flyby mission. Instead of changing course, DART plowed straight into the moon with enough momentum to dig a fresh crater and to change the satellite’s orbital period by more than 30 minutes.

A greyscale photograph of a rocky surface.

The surface of Dimorphos, as seen by the DART spacecraft 2 seconds before impact. [NASA/Johns Hopkins APL]

The main purpose of this intentional collision was to test if it’d be possible to redirect an asteroid’s path through a brute-force impact. This is a potentially humanity-saving skill, should astronomers ever discover a large asteroid destined to collide with Earth absent a dramatic intervention, and scientists involved with the mission want to be confident that everything they learn from this single collision is applicable to any future high-stakes redirections. To more confidently extrapolate the results of this test to larger asteroids with structures and compositions that might differ from Dimophos’s, astronomers need to measure every material property of this one asteroid that they can.

Some of these properties are best measured from up close to the asteroid, and the images beamed back by DART moments before its abrupt end contain much useful information. Others, though, such as how the surface scatters light, are better constrained by observing the asteroid from many different angles, which one can do by taking observations from Earth over many months. By combining these different types of measurements, a fuller picture emerges than could be built by either dataset alone.

Adding in Ground-Based Observations

A scatter plot of phase angle vs absolute brightness showing a tight linear relationship.

The solar phase curve of Dimorphos as measured from Table Mountain Observatory. [Buratti et al. 2024]

This combination is what Bonnie Buratti, Jet Propulsion Laboratory, and collaborators accomplished in a recent study. The researchers took observations from Table Mountain Observatory of the Didymos/Dimorphos system over about five months in order to build a solar phase curve, or how the brightness of the two asteroids changes with respect to our viewing angle. This allowed them to build a particle phase function, or a model of how the surface scatters light. Then, by poring over and reprocessing the DART images, Buratti and colleagues were able to quantify the surface roughness of the asteroid, a property that would have been impossible to disentangle from the phase function without the extra information gathered from the ground.

This synthesis demonstrates that although the missions we send up to gather exquisite in-situ data, sometimes important, fundamental properties require patient observations from back home as well. In 2026, the European Space Agency’s Hera spacecraft will arrive at Didymos to survey the damage, and analysis of its data will rely heavily on context learned through studies like these in the intervening years.

Citation

“Pre-impact Albedo Map and Photometric Properties of the (65803) Didymos Asteroid Binary System from DART and Ground-Based Data,” B. J. Buratti et al 2024 Planet. Sci. J. 5 83. doi:10.3847/PSJ/ad2b60

Illustration of a temperate exoplanet

Choosing a favorite planet has never been more challenging: researchers have discovered more than 5,600 exoplanets that range from super-heated super-puffs skimming the surfaces of their host stars to Mercurian worlds of pure iron. As telescopes dedicate even more time to the search for distant worlds, the exoplanets discovered are getting smaller, cooler, and more like Earth. The allure of temperate, Earth-like exoplanets is undeniable, but so are the roadblocks to finding and characterizing these planets. In today’s Monthly Roundup, we’ll give an overview of four recent studies of temperate exoplanets, ranging from ways to detect technological civilizations on these worlds to the importance of fine-tuning model inputs.

Temperate Earth-Sized Planet Discovered

TESS light curves for transits of Gliese 12 b

TESS light curves for transits of Gliese 12 b. [Adapted from Kuzuhara et al. 2024]

The first of today’s articles examines a potentially Earth-like planet orbiting an M dwarf — the smallest, coolest, and most common type of star in our galaxy. The star in question, Gliese 12, is about a quarter of the mass of the Sun, has a temperature around 3000K, and sits just 40 light-years away. In 2023, the Transiting Exoplanet Survey Satellite (TESS) spotted a planet passing in front of the star but couldn’t pin down its orbital period. Now, a team led by Masayuki Kuzuhara (National Institute of Natural Sciences, Japan) and Akihiko Fukui (University of Tokyo and Institute of Astrophysics of the Canary Islands) has analyzed follow-up observations to characterize this nearby world.

The team found that Gliese 12 b swings around its host star every 12.76 days and is just a hair smaller than Earth. Its mass might be far greater, though, with a tentative upper limit of 3.9 Earth masses. The large mass range means the planet could be extraordinarily dense — nearly as dense as pure iron — or it could be rocky with a lightweight hydrogen atmosphere. The planet’s equilibrium temperature is a relatively cool 315K (107℉/42℃), but if this planet is blanketed by a thick, Venus-like atmosphere, its true surface temperature could be far hotter.

This discovery is significant because most of the exoplanets known to orbit M-dwarf stars are fried by high-energy radiation, making them inhospitable to life. Unlike many M dwarfs, Gliese 12 appears to be inactive and unlikely to unleash planet-searing flares. Thus, Gliese 12 b joins the short list of nearby temperate exoplanets whose atmospheres could be scrutinized with JWST, and future observations should reveal more about this world.

Inputs Under Investigation

Let’s say you’ve discovered a temperate exoplanet and collected a spectrum of its atmosphere. What happens next? Researchers rely on complex models to interpret their observations of planetary atmospheres. Models of exoplanet atmospheres take in myriad inputs, and tweaking any one of these inputs can have significant impact on the output, as recent research led by Wynter Broussard (University of California, Riverside) shows.

Broussard’s team tested the impact of varying the water photolysis cross section on models of temperate planets without oxygen and without life. Photolysis is the splitting apart of a molecule by a photon; the photolysis cross section describes how readily a molecule is broken apart by a photon of a given wavelength. When a photon cleaves apart a water molecule, the interaction leaves behind a hydrogen atom and a radical, OH. OH is highly reactive and can destroy trace atmospheric gases. Because of this, the photolysis cross section of water molecules has a potentially large impact on the predicted composition of a planet’s atmosphere.

modeled exoplanet spectra

Modeled spectra of an exoplanet orbiting a Sun-like star with the new water cross section (purple) and the old water cross section (coral). Click to enlarge. [Broussard et al. 2024]

Broussard and collaborators showed that this one input can have a huge impact on the model output. The team used three versions of the water photolysis cross section that were obtained through laboratory work, including a new version that extends the cross section to longer wavelengths. For temperate, rocky exoplanets around Sun-like stars, the choice of cross section changed the amount of gases like methane and carbon monoxide by 1–3 orders of magnitude, which visibly affected these planets’ spectra. This work shows how even a small change to a single model input can have a large impact, and Broussard’s team emphasizes how important molecular photolysis cross sections are to interpreting exoplanet spectra.

Signals from Sulfur on Water-World Exoplanets

Our ability to interpret atmospheric spectra is critical: it might be our best hope of detecting life from light-years away. In September 2023, the temperate sub-Neptune planet K2-18b made a splash when researchers announced that it’s likely an ocean-covered world whose atmosphere contains methane, carbon dioxide, and maybe, tentatively dimethyl sulfide, which on Earth is only produced by life. Subsequent research suggested K2-18b fits the model of a lifeless, gas-rich planet equally well, highlighting the difficulty of this type of research.

Now, a team led by Shang-Min Tsai (University of California, Riverside) has examined the chemistry and detectability of sulfur-containing gases — like dimethyl sulfide — that are produced by life. Tsai and collaborators simulated the atmospheres of sub-Neptune-sized water-world exoplanets to understand how sulfur compounds might circulate within a planet’s atmosphere and be detectable through transmission spectroscopy.

modeled transmission spectra of K2-18b

Modeled transmission spectra of K2-18b containing the same flux of sulfur produced by life as Earth (blue), 20 times the biogenic sulfur of Earth (orange), and 20 times the biogenic sulfur of Earth but with the signal from dimethyl sulfide removed (green). The purple line shows the spectrum of a planet with no biogenic sulfur. Click to enlarge. [Tsai et al. 2024]

On Earth, sulfur-containing molecules produced by life are quickly split apart by photolysis or through reactions with OH radicals, making it difficult for these types of gases to build up to detectable levels. Tsai’s team found that in order for a sulfur-containing molecule like dimethyl sulfide to be detectable in an exoplanet’s atmosphere, 20 times more of the compound must be generated than is produced on Earth. The best chance for detecting dimethyl sulfide is in the mid-infrared, around 9–13 microns (1 micron = 10-6 meter). At 3.4 microns, where researchers picked out the tentative signal from dimethyl sulfide in K2-18b’s spectrum, signals from methane complicate the identification of dimethyl sulfide. With more JWST observations of K2-18b planned, we’ll certainly hear more about this planet and its intriguing atmosphere!

From Biosignatures to Technosignatures

The final article of this Roundup examines a way to find intelligent life. Searches for intelligent life beyond Earth focus on technosignatures, or signs of technology, like radio signals, enormous structures created to capture starlight, or atmospheric gases from industrial processes.

Edward Schwieterman (University of California, Riverside, and Blue Marble Space Institute of Science) and collaborators considered the possibility of detecting artificial greenhouse gases injected into a planet’s atmosphere to alter its climate, such as to stave off an oncoming ice age or to make a chilly neighboring world more hospitable. The gases studied in this work — hexafluoroethane (C2F6), octafluoropropane (C3F8), sulfur hexafluoride (SF6), nitrogen trifluoride (NF3), and carbon tetrafluoride (CF4) — are unlikely to be produced by non-technologically advanced life and may linger in planetary atmospheres for millennia, increasing the odds that we could detect these gases.

transmission and emission spectra of technosignature gases

Illustration of modeled transmission (top) and emission (bottom) spectra from a hypothetical Earth-like exoplanet whose atmosphere contains a combination of three technosignature gases: C3F8, C2F6, and SF6. Click to enlarge. [Schwieterman et al. 2024]

Schwieterman’s team modeled the spectra of Earth-like exoplanets with 1, 10, and 100 parts per million of one of the five technosignature gases added. They found that JWST’s Mid-Infrared Instrument could tease out the spectral signals from these gases in as little as six planetary transits (in the case of 100 parts per million of C2F6), though lower concentrations of certain gases could require more than 100 transits. A combination of C2F6, C3F8, and SF6 would be more readily detectable, needing just 5–25 transits, depending on the amount present. A future mid-infrared imaging mission could also detect these gases, Schwieterman’s team found. Ultimately, with the right tools, technosignature gases may be easier to detect than biosignature gases — and in many cases, would require no more telescope time than standard atmospheric characterization.

Citation

“Gliese 12 b: A Temperate Earth-Sized Planet at 12 pc Ideal for Atmospheric Transmission Spectroscopy,” Masayuki Kuzuhara et al 2024 ApJL 967 L21. doi:10.3847/2041-8213/ad3642

“The Impact of Extended H2O Cross Sections on Temperate Anoxic Planet Atmospheres: Implications for Spectral Characterization of Habitable Worlds,” Wynter Broussard et al 2024 ApJ 967 114. doi:10.3847/1538-4357/ad3a65

“Biogenic Sulfur Gases as Biosignatures on Temperate Sub-Neptune Waterworlds,” Shang-Min Tsai et al 2024 ApJL 966 L24. doi:10.3847/2041-8213/ad3801

“Artificial Greenhouse Gases as Exoplanet Technosignatures,” Edward W. Schwieterman et al 2024 ApJ 969 20. doi:10.3847/1538-4357/ad4ce8

Deneb and the North America Nebula and IC 5070

Deneb, the brightest star in the constellation Cygnus and the 19th brightest star in the night sky, has surprised astronomers: it’s not just a variable star — its polarization changes, too!

Full of Surprises

constellation of Cygnus

Location of Deneb, in the constellation Cygnus. Click to enlarge. [International Astronomical Union/Sky & Telescope, CC BY 4.0]

It’s not every day that a discovery involves a recognizable star rather than one with an inscrutable name. Deneb is a blue supergiant star with a mass roughly 20 times that of the Sun. Its brightness varies by about 0.08 magnitude in an irregular way, making it the prototype for the Alpha Cygni class of variable stars. The irregular brightness changes are thought to come from the combination of many different pulsation modes acting simultaneously, including non-radial pulsations in which some parts of the star expand while others contract.

Recently, researchers started a new survey of bright stars in the northern celestial hemisphere to learn more about these stars’ polarization. Polarization refers to the orientation of light waves as they travel through space. If the light waves are oriented in a particular direction, the light is polarized. If instead the waves are oriented randomly, the light is unpolarized. A star’s polarization can reveal critical information about its winds, pulsations, and surroundings.

plots of brightness and polarization of Deneb

Deneb’s normalized brightness (top panel), degree of polarization (middle panel), and angle of polarization (bottom panel). Click to enlarge. [Adapted from Cotton et al. 2024]

Polarization Patrol

Daniel Cotton (Monterey Institute for Research in Astronomy and Western Sydney University) and collaborators presented more than a year of polarization data for Deneb, revealing for the first time that the star’s polarization varies. Deneb’s degree of polarization — how much of its starlight is oriented in an orderly way rather than every which way — is considerable, about 3,000–5,000 parts per million. The variability is large as well, bouncing around by hundreds of parts per million.

Deneb is a prominent star in the northern celestial hemisphere: how did this variability go unnoticed for so long? Like its brightness changes, Deneb’s polarization changes are irregular, changing significantly but unpredictably over the course of several weeks. Because few measurements of the star’s polarization were made until 2022, these changes escaped notice until now.

Reasons for Change

Cotton’s team identified two possible causes for the polarization changes: variable winds and non-radial pulsations. Other supergiant stars are known to have clumpy winds that can scatter and polarize starlight in a way that varies over time. This is likely the most straightforward explanation for Deneb’s polarization variations. In this case, the star’s polarization variations and its brightness variations would not be strongly correlated.

Another possibility is that Deneb’s non-radial pulsations could distort the surface of the star, causing polarization changes that are correlated with the star’s brightness and radial velocity changes. After examining the available data, Cotton’s team concluded that winds are the most likely cause of Deneb’s variable polarization, but non-radial pulsations might play a small role as well.

This marks just the beginning of our understanding of Deneb’s polarization properties — future photometric, polarimetric, and spectral observations of the star are planned, helping to illuminate the star’s behavior further.

Citation

“Deneb Is a Large-Amplitude Polarimetric Variable,” Daniel V. Cotton et al 2024 ApJL 967 L43. doi:10.3847/2041-8213/ad4b0f

star cluster IC 348 as seen by JWST

Last year, a researcher claimed the first detection of an amino acid, tryptophan, in an interstellar cloud. New research that leverages laboratory experiments and data from JWST refutes this finding, suggesting that the hunt for the first amino acid in the interstellar medium isn’t over yet.

A Hunt for Amino Acids

Researchers have long sought evidence for amino acids, the building blocks of all life on Earth, in the dusty clouds of the interstellar medium. Several amino acids have turned up in meteorites and the gaseous envelopes of protostars, but claimed detections of these critical molecules in cold, star-forming clouds have been refuted. Now, another potential detection awaits confirmation.

diagram of a tryptophan molecule

The structure of a tryptophan molecule. [Dhariwal et al. 2024]

The amino acid in question is tryptophan, which is perhaps best known for being unfairly blamed for post-Thanksgiving sleepiness due to its presence in turkey meat. Research published in 2023 found evidence for tryptophan in the gas suffusing the star cluster IC 348, based on a comparison of an infrared spectrum from the Spitzer Space Telescope to a laboratory spectrum of the molecule. How does this finding hold up to further tests?

comparison of results from new and previous laboratory spectra of tryptophan

Comparison of the Spitzer spectrum used in the claimed detection of tryptophan (orange) to the new laboratory spectrum (blue) and the previously used laboratory spectrum (red). The panels at the bottom show individual spectral features. Click to enlarge. [Dhariwal et al. 2024]

A Three-Pronged Investigation

Aditya Dhariwal (University of British Columbia) and collaborators used several methods to evaluate the claimed detection, beginning with a laboratory spectrum of the molecule. It’s no easy task to simulate the conditions of the interstellar medium in a lab: the density of interstellar gas can be far lower than the best lab vacuum, and temperatures in cold clouds hover just above absolute zero. This makes it challenging to generate spectra that match what would be seen from molecules in interstellar space.

The previous study used a lab spectrum of tryptophan in the form of a solid pellet. Dhariwal’s team pointed out that unlike in the interstellar medium, tryptophan in a solid state likely takes the form of a zwitterion: a molecule that has no electric charge overall but is composed of parts that carry electric charge.

The team used a new method to prepare a sample of tryptophan in non-zwitterionic form and compared the collected infrared spectrum to the Spitzer observations. Crucially, the spectral lines seen by Spitzer and assigned to tryptophan have no matches in the new spectrum.

Paths Forward

JWST spectrum of IC 348

Spectrum of IC 348 from JWST (top) and individual spectral lines from that spectrum (bottom). The blue dotted lines mark the locations of the reported tryptophan lines. Click to enlarge. [Dhariwal et al. 2024]

Next, the team analyzed new infrared spectra of IC 348 from JWST. The JWST spectra clearly contained spectral lines from hydrogen and other elements in the cloud, but none of the lines attributed to tryptophan were present.

Finally, Dhariwal’s team revisited the Spitzer observations used in the previous analysis. Their examination of the spectrum led them to suspect that the purported emission lines of tryptophan were instrumental artifacts rather than true spectral lines.

This study demonstrates the fascinating back and forth of science at work. Although the latest investigation finds no compelling evidence for tryptophan in the interstellar medium, the authors identified paths forward. Laboratory spectra of tryptophan gas at radio wavelengths allow for searches of the sky at longer wavelengths, and though challenging to collect, spectra of tryptophan gas at infrared wavelengths could open yet another window. The search goes on!

Citation

“On the Origin of Infrared Bands Attributed to Tryptophan in Spitzer Observations of IC 348,” Aditya Dhariwal et al 2024 ApJL 968 L9. doi:10.3847/2041-8213/ad4d9a

photograph of runaway star Zeta Ophiuchi

A substantial number of O and B stars say goodbye to their birth clusters and spend their lives zooming through space alone. New research uses the most recent Gaia data to investigate the properties of these runaway stars.

Massive Stars Far from Home

The Trapezium cluster

OB stars are typically born in clusters, like the Trapezium Cluster at the heart of the Orion Nebula, shown here. [ESO/IDA/Danish 1.5 m/R.Gendler, J.-E. Ovaldsen, and A. Hornstrup; CC BY 4.0]

The majority of the most massive stars in our galaxy — those with spectral types O and B, also called OB stars — live and die within their natal star clusters, never traveling far from where they were born during their brief, luminous lives. But roughly 20–30% of OB stars lead significantly more adventurous lives. So-called runaway stars careen through space on a solo adventure or with a single companion, traveling light-years from their birthplaces.

Stars can be ejected from their birth clusters by dynamical interactions or supernova explosions. Dynamical ejection happens when stars in a crowded cluster pass too close to one another, slingshotting one or more stars into space. A star that explodes as a supernova can expel its close binary companion from the cluster. In rare cases, the exploding star and its companion can escape the cluster together. It’s not yet known which process creates more runaway stars or how the two populations of runaways differ.

Map of the Small Magellanic Cloud with stellar proper motion vectors for potential runaway stars

Vectors showing the motion across the sky of the stars in the authors’ sample. Eclipsing binaries (EB) and double-lined spectroscopic binaries (SB2) were ejected dynamically. High-mass X-ray binaries (HMXB) have experienced a supernova. Click to enlarge. [Phillips et al. 2024]

A Search for Runaway Stars

Diving into this question, a team led by Grant Phillips (University of Michigan) investigated more than 300 potential runaways in a nearby dwarf galaxy, the Small Magellanic Cloud. The stars in their sample were chosen for being at least 90 light-years from other OB stars (with the exception of stars in binary systems, which may have escaped their home cluster together).

The binary systems in the sample immediately gave away how they were launched into space: binary systems that don’t contain a neutron star or a black hole can’t have experienced a supernova — those binaries must have been kicked out of their clusters through dynamical interactions. Binary systems that do contain a neutron star or a black hole must have experienced a supernova. As for solo runaway stars, the team’s earlier research suggested that typical OB stars tend to be expelled through dynamical interactions, while stars with emission lines in their spectra due to rapid rotation — OBe stars — tend to be ejected by supernovae.

Population Comparison

Mass and velocity distributions for stars ejected dynamically and via supernovae

Mass and velocity distributions for stars ejected dynamically (left) and via supernovae (right). Click to enlarge. [Phillips et al. 2024]

Using the third release of Gaia spacecraft data, Phillips’s team calculated the velocities of the stars in their sample. They found that the velocities of the single OBe stars overlapped well with those of the post-supernova binary systems, although the OBe stars’ velocities extended to higher values. While this could be an effect of small-number statistics, it could also be due to some stars reaching high velocities through a two-step process: first, dynamical ejection as a member of a binary system; then, an extra velocity kick when the binary companion goes supernova.

The velocities of typical OB stars are similar to the velocities of binary systems ejected dynamically, supporting the hypothesis that these stars are also ejected this way. Intriguingly, the velocity distribution for typical OB stars may have two peaks. The authors speculate that the peaks could point to two groups of dynamical interactions: stars ejected by just a single other star versus stars ejected by a binary system.

Underpinning these findings is the rich, precise dataset from Gaia, which allowed the team to identify systematic offsets from their previous work based on earlier data releases from the spacecraft. Hopefully, future data releases can give even more clues to the origins of these runaway stars!

Citation

“Runaway OB Stars in the Small Magellanic Cloud. III. Updated Kinematics and Insights into Dynamical versus Supernova Ejections,” Grant D. Phillips et al 2024 ApJ 966 243. doi:10.3847/1538-4357/ad3909

Illustration of ejecta from the collision of two neutron stars

Roughly thirteen billion years ago, the periodic table was easy to memorize: hydrogen, helium, and lithium were the only chemical constituents of the infant universe. Today, things are a little more complicated. Through billions of years of stellar alchemy, the universe is now awash with an abundance of metals — what astronomers call elements heavier than helium. Some of these metals are forged in the cores of stars, while others require explosive events to form. In today’s post, we’ll take a look at three research articles that examine the creation of heavy elements in exotic environments across the universe.

Making Metals in Supernovae

Core-collapse supernovae are one of the sites of element formation, which is also called nucleosynthesis. Core-collapse supernovae are triggered by the collapse of a massive star as the star exhausts its ability to hold off the inward pressure of gravity with the outward pressure of radiation generated by nuclear reactions in its core. When the star’s core collapses, its outer layers recoil from the condensed core and explode into space.

The hot, dense ejecta of a supernova explosion may be a good place for elements to be created through r-process nucleosynthesis. In this process, multiple free-wheeling neutrons pack onto nearby atoms, creating heavier isotopes and elements fast enough that unstable isotopes don’t have the chance to decay. (The counterpart to the rapid r-process is the slow s-process, in which a trickle of neutrons builds elements and isotopes more gradually. The s-process tends to take place in stellar interiors, without the need for a cataclysmic explosion.) It’s not yet clear how much element creation via the r-process happens in core-collapse supernovae or how this quantity depends on the mass of the star or other factors.

plots of modeled supernova light curves

Modeled supernova light curves showing the impact of increasing the amount of r-process material (Mr) and the degree of mixing (fmix). Click to enlarge. [Patel et al. 2024]

Anirudh Patel (Columbia University) and collaborators used simulations to understand how r-process nucleosynthesis might leave its mark on the light curves of core-collapse supernovae. Patel’s team produced one set of models in which no r-process reactions take place and another in which r-process elements are produced deep within the explosion and then mixed throughout the ejected material. The team varied the amount of r-process material and how thoroughly it’s mixed with other material.

Patel and coauthors found that if the amount of r-process material is small — less than a hundredth of the mass of the Sun — the light curve looks scarcely different than if there is no r-process material at all. For larger amounts of r-process material and greater degrees of mixing, the “plateau” phase of the light curve shortens and dims, and the supernova appears redder than it otherwise would. The team’s research suggests that r-process-enriched supernovae should be distinguishable from regular supernovae but may be confused with certain types of rare supernovae.

The Influence of Magnetic Fields

Many core-collapse supernovae leave behind a neutron star: an extraordinarily dense sphere of neutrons about 10 kilometers in radius and about the mass of the Sun. When two neutron stars collide, the merger creates ideal conditions for element formation through the r-process. In a recent article, Kelsey Lund (North Carolina State University and Los Alamos National Laboratory) and collaborators examined element creation in the case of a neutron star merger than produces a black hole surrounded by a hot, dense accretion disk.

plots of modeled total ejecta mass, lanthanide mass, and actinide mass as a function of magnetic field strength

Total ejecta mass, lanthanide mass, and actinide mass as a function of magnetic field strength (lower β corresponds to a stronger magnetic field) and the angle of the outflow. Note the different y-axis scales in the middle and bottom plots. [Adapted from Lund et al. 2024]

The merger of a binary pair of neutron stars can create a bright electromagnetic signal called a kilonova. The kilonova emission is powered by the radioactive decay of r-process elements that are produced when the stars collide. One of the many uncertainties that surround the creation of a kilonova is the impact of the magnetic field. The magnetic field is thought to control how quickly material flows from the accretion disk onto the black hole as well as how rapidly the material flows out from the disk — two factors that influence the production of r-process material.

Lund’s team used general relativistic magnetohydrodynamics simulations to trace the evolution of the disk that forms after the stars merge. The three simulations capture the nucleosynthesis that occurs within about 127 milliseconds of the collision. When the magnetic field is stronger, more mass is ejected by the merger and larger amounts of elements in the lanthanide and actinide groups — the two rows of heavy elements separated from the rest of the periodic table — are produced. The amounts of lanthanide-group and actinide-group elements both increase with increasing magnetic field strength, but the increase is larger for the actinide-group elements.

This last finding could explain a curious feature of some old stars: while many old stars contain about the same amount of lanthanide-group elements, there is a broad range of actinide-group element abundances. This may reflect the different magnetic field conditions in the neutron star mergers where the elements formed, long before the stars themselves were born.

Collapsar Jets and Nucleosynthesis

The final article of today’s post examines a variety of element-making methods in the aftermath of a collapsar: a rapidly spinning, massive star that collapses into a black hole, slinging jets of material into space in the process. Zhenyu He (Beihang University) and collaborators used an extensive network of nuclear reactions to model the fusion, fission, and decay taking place in the collapsar’s jets.

He’s team found that in the immediate aftermath of a collapsar, the r-process is in full swing, packing neutrons onto outflowing atoms and forming elements like gold and platinum. After about five seconds, the outflow has cooled enough that the r-process stalls. This doesn’t mean that nucleosynthesis stops, though: the team found evidence that the slower s-process and the intermediate-speed i-process take over and continue to churn out heavier atomic species for several hours. The later, slower nucleosynthesis is mainly powered by neutrons from the fission of fermium and rutherfordium.

plot of i-process fraction as a function of atomic number

Percentage of element yield (A = atomic mass in atomic mass units) from the i-process. Many elements’ yields are greatly enhanced by the i-process. The two sets of symbols represent results from simulations with different expansion rates. Click to enlarge. [He et al. 2024]

Previous modeling of other r-process sites like supernovae has not revealed this later stage of nucleosynthesis, but this work suggests that the s-process and i-process are important for shaping the chemical makeup of collapsar outflows. In particular, the pattern of elements with even numbers of protons being more abundant than elements with odd numbers of protons might be enhanced by these processes. To learn more, He’s team proposed, researchers will need to study old, metal-poor stars in the distant halo of the Milky Way, which may have formed from gas enriched by collapsars rather than neutron star mergers.

Citation

“The Effects of r-Process Enrichment in Hydrogen-Rich Supernovae,” Anirudh Patel et al 2024 ApJ 966 212. doi:10.3847/1538-4357/ad37fe

“Magnetic Field Strength Effects on Nucleosynthesis from Neutron Star Merger Outflows,” Kelsey A. Lund et al 2024 ApJ 964 111. doi:10.3847/1538-4357/ad25ef

“Possibility of Secondary i– and s-Processes Following r-Process in the Collapsar Jet,” Zhenyu He et al 2024 ApJL 966 L37. doi:10.3847/2041-8213/ad444c

debris disk surrounding Fomalhaut C

JWST has achieved another first: the telescope has spotted light scattered by dust grains in the debris disk surrounding the star Fomalhaut C for the first time. These observations add to our knowledge of the planet formation process around the smallest and most common stars in our galaxy.

A Fleeting Phase

As stars and planets form out of collapsed clouds of gas and dust, nascent planetary systems pass through a short and poorly understood phase. Lasting just 10 million years, the fleeting debris disk stage is marked by collisions between protoplanets that create disks or rings of dust and rubble.

Studying debris disks is key to understanding how planetary systems form. Very few debris disks have been seen around the smallest, coolest, and most common type of star in the Milky Way — M dwarfs —and detailed observations of the few known M-dwarf debris disks have been scarce. Studies are split on whether low-mass stars are less likely to host debris disks or if these disks are simply more challenging to detect. Luckily, JWST is capable of spotting these elusive disks, and recent observations have given researchers a new perspective on a disk surrounding an M dwarf just 25 light-years away.

Disk Detected

photograph of the debris disk surrounding Fomalhaut A

ALMA’s view of the debris disk around Fomalhaut A. [ALMA (ESO/NAOJ/NRAO); M. MacGregor; CC BY 3.0]

Fomalhaut is a triple-star system most famous for the debris disk and highly debated planet candidate around the largest and brightest star in the system, Fomalhaut A. Astronomers previously spotted thermal emission from a debris disk surrounding an M dwarf in the system, Fomalhaut C, in observations by the Atacama Large Millimeter/submillimeter Array (ALMA), but follow-up observations failed to spot the disk in scattered light. Scattered-light observations give researchers valuable information about the size and makeup of dust grains in the disk.

Recently, a team led by Kellen Lawson (NASA’s Goddard Space Flight Center) turned JWST toward Fomalhaut C and eight other nearby M dwarfs to search for planets around these stars. In JWST’s 3.56 and 4.44 micron (1 micron = 10-6 meter) filters, the authors detected a faint disk extending beyond the star. In the shorter-wavelength filter, the outline of the disk matched the location of the debris disk seen previously by ALMA. In the longer-wavelength filter, the disk extended slightly beyond the emission seen by ALMA.

Challenging Observations

Fomalhaut C is the smallest and coolest star for which a disk has been detected in scattered light. The new observations highlight JWST’s ability to track down debris disks around small, cool stars, as well as the inherent difficulty of observing these structures: even under JWST’s watchful gaze, the star’s debris disk is seen only faintly, with background objects and noise interfering with the telescope’s view.

several views of Fomalhaut C's debris disk

Fomalhaut C’s disk as seen in two different filters and with several different methods of removing diffracted starlight. Click to enlarge. [Lawson et al. 2024]

The difficult observing conditions complicated analysis of the disk’s properties — a challenging background-light subtraction might be responsible for the unaccountably red color of the disk — but the team was still able to search for planets. Lawson’s team ruled out the presence of a planet with a mass higher than Saturn’s mass orbiting within a distance of 10 au or a planet with a mass higher than Jupiter’s orbiting the star within 5 au.

Follow-up observations of Fomalhaut C may refine our understanding of its debris disk, and future observations with JWST are sure to add to the small but critical sample of debris disks surrounding the smallest stars.

Citation

“JWST/NIRCam Detection of the Fomalhaut C Debris Disk in Scattered Light,” Kellen Lawson et al 2024 ApJL 967 L8. doi:10.3847/2041-8213/ad4496

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