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A graphic of a spacecraft with a large dish antenna flying past Jupiter. The Milky Way and numerous stars are in the background.

Using a network of faraway telescopes in the outskirts of the solar system, astronomers could measure the distance to much farther away galaxies with exquisite precision. A recent study describes how this tactic works and explores what else we could learn with such a bold experiment.

Very, Very Long Baselines

Distance is notoriously a tricky quantity to measure in astrophysical contexts, and astronomers have struggled to size up the universe since Hubble first drew his famous diagram. While they have certainly made progress over the last century, it’s natural to wonder if modern technology could enable an entirely new, more precise way to measure the gaps between galaxies.

A plot showing a source of radio emission surrounded by concentric circles, each meant to represent a wavefront. Below the source are 3 detectors in a horizontal line. The same wavefront is touching, ahead of, and behind the three detectors respectively, illustrating how it strikes them at different times.

A sketch of three detectors and a fast radio burst source. Since the wavefront is slightly curved, the same emission will strike each detector at different times. Using measurements of those differences, astronomers can back out the distance to the source. [Boone and McQuinn 2023]

This thinking led Kyle Boone and Matthew McQuinn (University of Washington) to propose a bold new experiment. Their idea, described in a recent publication in the Astrophysical Journal Letters, is to scatter a fleet of radio telescopes throughout the solar system and instruct them to all observe the same flashing, repeating fast radio burst at the same time. Since each flash is emitted equally in all directions at the same time, the wavefront will be slightly curved when it arrives and will strike each satellite at a very slightly different time. Add up these nanosecond delays between each, and with some geometry you can back out the distance to the source.

Such a mission would require solving numerous intense, but feasibly surmountable, engineering challenges. Chief among these, astronomers would have to know the distances between the telescopes to within just a few centimeters, a demanding requirement considering the millions of miles separating them and the many subtle forces that affect their motion. Also, each satellite would need to nurture an ultra-precise atomic clock in the face of the unforgiving vacuum of space. But, should engineers resolve these hindrances, a constellation of four or more telescopes drifting in the outer solar system could pin down the distance to each observed flash to within 1% uncertainty.

Spanning Distances and Disciplines

A line plot of fractional uncertainty vs. distance. Five curves are shown for five different spacecraft baselines, ranging from 12.5 AU to 200 AU. Generally, the larger the baseline, the smaller the fractional uncertainty, and the closer the source, the smaller the fractional uncertainty.

The uncertainty in a measurement of the distance to a source as a function of the true distance to the source for a number of different satellite configurations. Each color represents a different possible baseline separation, and the thickness of each region marks how the uncertainty changes if the resolution of their separation varies between 0.5 and 2 cm. Note that for a source closer than 100 megaparsecs (approximately 300 million light-years), a 25 AU baseline could measure its distance to better than 1%. [Boone and McQuinn 2023]

This experiment was conceived explicitly with precision cosmology in mind, and as Boone and McQuinn show, would be demonstrably revolutionary in that field. However, should astronomers be audacious enough to build a solar system–sized hammer, there are more than a few outstanding nails the same hardware could bludgeon. Take dark matter, for example: several models suggest that invisible clumps of the stuff should occasionally fly through the solar system at high speed. This experiment would necessarily be sensitive enough to notice the slight gravitational tug of such an encounter, meaning even a non-detection of occasional jostles could help constrain our theories of dark matter’s form. Similarly, the much debated “Planet 9” would be unable to evade such an exquisitely sensitive instrument: over time, even from hundreds of AU away, any large planets lurking in the outer solar system would eventually nudge these radio telescopes out of place.

While this study may never grow into more than a thought experiment, such an exercise is constructive nonetheless and gives the astronomical community a chance to reflect on its current capabilities and muse about its future. That said, a more hopeful interpretation is to take this as a starting point for a grand, exacting, colossal mission that could one day uncover secrets of the universe, and our own backyard, all at once.

Citation

“Solar System-scale Interferometry on Fast Radio Bursts Could Measure Cosmic Distances with Subpercent Precision,” Kyle Boone and Matthew McQuinn 2023 ApJL 947 L23. doi:10.3847/2041-8213/acc947

JWST image of the cartwheel galaxy

JWST has given us a new look at galaxies as they were in the first few billion years of the universe. Among the newly discovered galaxies is a population of flat, red, extended disks that may have been entirely missed by previous surveys.

Dusty Galaxies in the Distant Universe

To understand how today’s galaxies came to be as they are, we need to study galaxies in the distant past. Among the galaxies we know to have existed at redshift (z) greater than 2 — up to when the universe was a little more than 3 billion years old — are massive, dusty galaxies forming stars at a furious rate.

comparison of galaxies seen by the Hubble Space Telescope to those seen by JWST

JWST has the potential to reveal galaxies that were invisible to Hubble, like the red galaxy shown here. Click to enlarge. [Adapted from Nelson et al. 2023]

To study the structure and evolution of these galaxies, we need a telescope that can resolve fine details and is sensitive to dust-reddened photons. The Hubble Space Telescope has the resolving power but doesn’t span the necessary wavelength range. The Spitzer Space Telescope could see the sought-after infrared wavelengths but lacked the ability to pick out the fine details. JWST marries these two requirements, opening a window onto the “Hubble-dark” universe of dusty galaxies.

Three-color infrared images of the 12 elongated galaxies in the sample

Three-color infrared images of the 12 elongated galaxies in the sample. [Nelson et al. 2023]

JWST Spies Hidden Galaxies

In a recently published article, a team led by Erica Nelson (University of Colorado) has reported on their analysis of JWST observations from the Cosmic Evolution Early Release Science (CEERS) Survey. This survey was conducted at infrared wavelengths of a few microns (1 micron = 10-6 meter). The same field of view surveyed by CEERS was also visited by the Hubble Space Telescope during the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS), though Hubble viewed the area at shorter wavelengths.

Nelson and collaborators noticed that the new JWST images contained galaxies that were absent in the Hubble images of the same region. By selecting for galaxies with certain color characteristics, the team picked out 26 galaxies that were bright in the JWST images but missing in the Hubble images. Among these newfound galaxies are a dozen that are remarkably extended rather than compact — a potentially unexplored population of galaxies present 1–3 billion years after the Big Bang.

Red Through and Through

plot comparing the colors of the newly detected galaxies to other galaxies at similar redshift

Comparison of the colors of the newly discovered ultra-red flattened objects (UFOs; red circles) and other galaxies at a similar redshift (black points). Redder galaxies are found at higher F227W-F444W values. [Nelson et al. 2023]

These 12 galaxies are surprising in two key ways. First, these galaxies are not just red — they are also red from their cores to their outskirts. Typically, we think of disk galaxies as having reddish cores that host older stars and bluish spiral arms that are home to young, hot stars. These newly discovered galaxies are red through and through, which Nelson and collaborators interpret to mean they are incredibly dusty.

Second, these galaxies are remarkably flat, and they appear to be seen nearly edge on. Nelson and collaborators suggest that the color criteria they used to select the galaxies might be biased toward edge-on, dusty galaxies, which would appear redder than their face-on counterparts. The team notes that galaxies in this size and mass range would be expected to evolve into the massive galaxies we see today, but their lack of a distinct central bulge of stars is surprising. Luckily, spectroscopy, radio-wavelength observations, and simulations all have the potential to improve our understanding of these curious galaxies.

Citation

“JWST Reveals a Population of Ultrared, Flattened Galaxies at 2 ≲ z ≲ 6 Previously Missed by HST,” Erica J. Nelson et al 2023 ApJL 948 L18. doi:10.3847/2041-8213/acc1e1

Artist's impression of a blazar

Astronomers have spotted a distant blazar behaving in a new way. Understanding its behaviour could lead to a deeper knowledge of these immensely powerful objects.

The Quintessential Blazar

BL Lacertae is the active centre of a distant galaxy almost one billion light-years from Earth. Despite this large distance, it can match the brightness of objects within our own solar system such as Pluto. When it was first discovered in 1929, BL Lacertae was thought to be a star in our own Milky Way. As such, it is an example of a quasi-stellar object — a quasar.

plot of BL Lacertae's magnitude over time

An example of BL Lacertae’s optical variability over less than one day in 2020. [Adapted from Kalita et al. 2023]

BL Lacertae was also the first known blazar — a quasar that varies its brightness. In fact, the BL from its name was combined with quasar to coin the term blazar in the first place.

Quasars and blazars are so bright because they are powered by a supermassive black hole that holds sway over the centre of the galaxy. It’s devouring, ripping, and swirling material around. Its brightness varies depending upon what it is snacking on. Starting in August 2020, and lasting for over a year, BL Lacertae was particularly bright in wavelengths stretching from the optical to very high-energy gamma rays.

plot of BL Lacertae's color

Illustration of how BL Lacertae’s color changed over time. The color bar indicates the number of days since the modified Julian date 2459122. [Adapted from Kalita et al. 2023]

Old Blazar, New Tricks

A team led by Nibedita Kalita (Shanghai Astronomical Observatory and Polar Research Institute of China) used the 1.26-metre telescope at the Xinglong Observatory to observe BL Lacertae between October and November 2020. The team found that even within this short period it got up to 30% brighter. Sometimes these brightness variations unfolded in less than an hour.

This short variation time implies that the source is fairly compact. The team estimated the source area to measure about 100 times the radius of the black hole. As well as intra-night variability, they also saw a longer variation pattern that lasted approximately 11 days. They also noticed colour changes in the blazar’s spectrum as it brightened that had never been seen in other blazars before.

composite optical and radio image of the galaxy Hercules A and its jets

Magnetic fields can help shape and power narrow jets, like those of the radio galaxy Hercules A shown here. [NASA, ESA, S. Baum and C. O’Dea (RIT), R. Perley and W. Cotton (NRAO/AUI/NSF), and the Hubble Heritage Team (STScI/AURA)]

Reasons for Change

Kalita’s team suspects that this longer-term variability is due to changes in the jet of material associated with the black hole. Strong magnetic fields corral material into a narrow column that if pointing at Earth can be particularly bright. It can quickly get brighter if particles within it are accelerated to relativistic speeds by some sudden change. This could be shock waves within the material or perhaps a process called magnetic reconnection. Also responsible for flares on the Sun, reconnection happens when two oppositely orientated magnetic fields are forced together. They snap into a new configuration releasing a lot of stored energy.

Citation

“Optical Flux and Spectral Variability of BL Lacertae During Its Historical High Outburst in 2020,” Nibedita Kalita et al 2023 ApJ 943 135. doi:10.3847/1538-4357/aca801

WISE image of the Circinus Molecular Cloud Complex

The radioactive atoms found in meteorites tell a subtle and complicated story about the Sun’s birth. In a recent article, researchers translated this story to discover that our solar system could have formed in a dense molecular cloud buffeted by a supernova.

Meteorites as Messengers from the Early Solar System

illustration of the hub–filament model

In the hub–filament model of star formation, low-mass stars form in dense molecular cloud filaments, while high-mass stars form where filaments meet. [Adapted from Arzoumanian et al. 2023]

Meteorites — remnants of primordial solar system rubble that fall to Earth — contain information about the early days of the solar system, including when it formed and what elements were present at that time. Among the information contained in a meteorite comes in the form of radioactive atoms, as well as the stable atoms they decay into. These atoms are produced by massive stars and spread throughout the galaxy by winds and supernova explosions. As a result, the abundance of radioactive atoms in meteorites can tell us something about the environment in which the Sun was born, and models of solar system formation must be able to explain the abundances of these elements.

In a recent research article, Doris Arzoumanian (National Astronomical Observatory of Japan) and collaborators discussed the radioactive content of meteorites in the context of what’s known as the hub–filament model. In this model, Sun-like stars form along narrow, dense clouds of molecular hydrogen gas known as filaments, while massive stars form where two or more filaments meet — at a hub. Arzoumanian and coauthors suggested that the hub–filament model might provide a natural way to explain the amounts of radioactive species found in meteorites.

diagram of high-mass star evolution providing feedback to nearby stellar systems

Nearby high-mass stars of spectral type O and B suffuse their surroundings with high-energy photons, heating the gas. A dense filament might shield a young star from this radiation. [Adapted from Arzoumanian et al. 2023]

Forming on a Filament

Radioactive atoms can be incorporated into a budding solar system in a few ways. They can be present in the cloud material from which the planetary system forms, placed there by previous generations of massive stars; they can be produced when atoms within the cloud are bombarded by high-energy charged particles from outside the galaxy; or they can be injected by a nearby massive star through winds or a supernova explosion. Based on the particular blend of radioactive atoms found in meteorites, Arzoumanian’s team favors the last explanation, though that opens a new question: how did our nascent solar system survive a supernova next door?

This is where the hub–filament model shows its usefulness. While a planetary system in the early stages of formation should be disrupted by a supernova, a dense filament could shield the young Sun and its planet-forming material. Not only does the filament protect the planetary system, it may also provide a natural way to funnel radioactive-species-rich material to the system via accretion streamers, which have been observed in young star systems.

A Supernova Solution

Illustration of the effects of a massive evolved star being near a molecular cloud filament

Massive stars add short-lived radionuclides (SLRs) like aluminum-26 to their surroundings. [Adapted from Arzoumanian et al. 2023]

Arzoumanian and collaborators delved deeper into the model by estimating the abundance of aluminum-26 (a radioactive form of aluminum) initially present in the molecular cloud where the Sun formed. They then added an extra dose of aluminum from either a supernova explosion or a nearby massive star with powerful winds. This material diffuses into the cloud and snakes onto the young star via accretion streamers.

These calculations show that a 25-solar-mass star exploding as a supernova or an even more massive (40–60 solar masses) star shaking off material through stellar winds could provide the abundance of aluminum and its daughter elements that we find in meteorites. Future simulations should explore these scenarios in more detail.

Citation

“Insights on the Sun Birth Environment in the Context of Star Cluster Formation in Hub–Filament Systems,” Doris Arzoumanian et al 2023 ApJL 947 L29. doi:10.3847/2041-8213/acc849

An image of the night sky featuring a wispy gas cloud and many stars. Tiny and at the center, the supernova remnant is flanked by a pair of rings of debris.

These days, astronomers find so many possible supernovae each night with automated photometric surveys that it’s impossible to follow up on all of them. Recently, however, a new article takes the first steps toward using unrelated spectroscopic surveys to fill in the gaps when luck allows.

Industrial Flash Detection

When a distant, massive star explodes as a supernova, the only sign of the monstrous violence seen from Earth is a tiny, modest flash in the night sky. Consequently, observations of these brief and easy-to-miss eruptions used to be pretty rare: astronomers would have to patiently and manually check the same patch of sky over and over again, hoping that in one of their images they’d see a bright speck of light that wasn’t there before.

But no more. With the advent of large telescopes, advanced imagers, and sophisticated software, this tedious process has been supplanted by a much more efficient workflow. These days, large surveys such as the Zwicky Transient Facility (ZTF) image huge swaths of the sky every night and automate the flash-detection process. While astronomers previously treasured each “transient” as a unique discovery, observers tapped into the data stream of these programs have the luxury to examine any number of the million or so transients detected each night. 

And yet, even as these surveys churn out transients on an industrial scale, astronomers usually want to know more about each one than the fact of their existence. Historically, they’ve gone about this by recording not just images, but also spectra of each object. Unfortunately, although spectroscopic surveys have also grown immensely more efficient, they have not kept pace with their photometric counterparts, meaning that most transients discovered by ZTF will never see their spectra documented.

When Telescopes Align

9 small images of a patch of sky, each with a bright transient in the center circled in red. Several appear next to small galaxies, and several appear next to a large region of masked-out pixels.

Cutout images of the nine transients which were “active” according to ZTF when HETDEX happened to observe them. [Vinkó et al. 2023]

As a team led by József Vinkó (University of Texas at Austin) demonstrated in a recent article, however, sometimes we get lucky. Vinkó and collaborators looked at data collected through the Hobby-Eberly Telescope Dark Energy eXperiment (HETDEX) survey, which from 2018 to 2022 was minding its own business taking spectra of high-redshift galaxies in one corner of the sky while ZTF frantically and repeatedly snapped away at the whole northern hemisphere. By comparing ZTF’s alerts with logs of where HETDEX was pointing each night, the team found that 538 transients went off in the exact same area the unrelated project was already observing. Even more fortuitously, nine of these transients were still glowing as HETDEX took its unrelated measurements.

A multi-panel plot depicting wavelength on each x axis and flux on each y. The data is shown in black, and the best fitting model, which tracks the data closely, is shown in red.

The HETDEX spectrum of ZTF20aatpoos compared to the best-fitting template of a supernova spectrum. [Vinkó et al. 2023]

Out of all of these overlapping events, Vinkó and collaborators successfully identified two supernovae and managed to classify hundreds of others as either fussy active galactic nuclei or other known astronomical objects using the HETDEX spectra. While there was nothing particularly remarkable about the supernovae themselves, the circumstances of its classification are much more exciting: this discovery marks the first serendipitous classification of a transient event by HETDEX and a step forward into an era of automated transient follow-up. As more industrial-style surveys come on line in the coming decade, we can hopefully look forward to more of these lucky alignments in the near future.

Citation

“Searching for Supernovae in HETDEX Data Release 3,” József Vinkó et al 2023 ApJ 946 3. doi:10.3847/1538-4357/acbfa8

image of Venus featuring swirling clouds

The next decade will bring several new spacecraft missions to our neighboring planet, Venus. In a recent research article, scientists explored how these missions might use radio signals to detect tiny amounts of sulfur-containing molecules, illuminating the dynamics of Venus’s thick atmosphere and potentially helping us track down active volcanoes.

Venus-Bound Spacecraft

diagram showing the geometry of a radio occultation

An (extremely exaggerated!) example of how the spacecraft’s radio signal is affected by its passage through Venus’s atmosphere. The atmosphere weakens the radio signal, and the refraction, or bending, of the signal imposes an apparent frequency shift that is detected by the ground station on Earth. [Kerry Hensley]

In 2021, NASA and the European Space Agency selected a total of three spacecraft missions that will be launched to Venus in the late 2020s and early 2030s. These missions will carry a variety of instruments that will map Venus’s surface with radar, study the content of its clouds, and probe the composition of its surface before beaming the data back to Earth via a radio signal.

Cleverly, scientists realized decades ago that by purposefully transmitting radio signals through a planet’s atmosphere, we can study the density, temperature, and makeup of that atmosphere. This technique, which is called a radio occultation, gives us an additional way to study Venus’s atmosphere that requires no additional instruments.

3D rendering of Maat Mons on Venus

This 3D rendering shows Maat Mons, Venus’s tallest volcano. Archival data from the Magellan mission showed changes to a vent on Maat Mons, indicative of a volcanic eruption in the 1990s. [NASA/JPL-Caltech]

In Search of Sulfur Species

Recently, a collaboration led by Alex Akins (NASA’s Jet Propulsion Laboratory) explored the possibility of using radio occultations to study sulfur-containing compounds in Venus’s atmosphere. Observations by previous missions to Venus have revealed traces of sulfuric acid (H2SO4) and sulfur dioxide (SO2). SO2 is a particularly intriguing target as it’s produced by volcanoes and plays a role in the formation of Venus’s H2SO4 clouds.

Akins’s team expects that future spacecraft may be more sensitive to H2SO4 and SO2 than past missions were. To estimate just how precisely future missions’ radio occultations can measure these species, the team used observations from previous missions to construct models of Venus’s atmosphere. They then simulated how radio signals of two frequencies would be affected by passing through this model atmosphere. Finally, the team developed a new method to extract atmospheric properties from a radio occultation signal, applied this method to their simulated radio occultations, and compared the derived properties to the “real” properties of the model atmosphere.

A Challenging Measurement

Sulfuric acid and sulfur dioxide abundances extracted by the new method

Abundances of H2SO4 and SO2 as well as the density of the H2SO4 clouds retrieved using the authors’ method, compared to the modeled profiles. Click to enlarge. [Adapted from Akins et al. 2023]

Ultimately, Akins and collaborators found that accurately measuring H2SO4 and SO2 in Venus’s atmosphere is challenging, even with the improved capabilities of upcoming missions. Part of the challenge lies in the fact that the method relies on prior knowledge or assumptions about Venus’s atmosphere. If the assumptions used in the analysis are valid, the abundances of several compounds can be determined simultaneously and precisely; if the assumptions are poor, the accuracy suffers. Despite this, the team suggests that it is possible to measure the SO2 abundance with an uncertainty of just 20 parts per million, aiding the study of Venus’s volcanism and atmospheric dynamics.

Luckily, we won’t have to wait too long to see these ideas put into play — NASA’s Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy (VERITAS) mission is scheduled to launch in 2029, and the European Space Agency’s EnVision mission will follow soon after, in 2031.

Citation

“Approaches for Retrieving Sulfur Species Abundances from Dual X–Ka-band Radio Occultations of Venus with EnVision and VERITAS,” Alex B. Akins et al 2023 Planet. Sci. J. 4 71. doi:10.3847/PSJ/accae3

Artist’s impression of star being torn apart by a supermassive black hole.

Astronomers have seen an extraordinarily bright, long-lasting radio flare in the center of a nearby galaxy. After investigating further they found evidence of a relativistic jet most likely caused by the immense gravitational pull of the galaxy’s supermassive black hole tearing a star apart.

An Unusual Signal

Photograph of an array of radio dishes against a sunset-illuminated sky.

The radio flare VT J0243 was discovered using the Karl G. Jansky Very Large Array, located in Socorro, NM. [NRAO/AUI/NSF]

A team led by Jean Somalwar (California Institute of Technology) made the initial discovery by trawling through two archived radio surveys of the sky conducted decades apart. Between 1993 and 2018, a radio flare designated VT J0243 appeared in the nucleus of a nearby galaxy, growing one hundred times brighter over this time and continuing to brighten even now.

Intrigued, Somalwar and colleagues conducted a series of follow-up observations across multiple parts of the electromagnetic spectrum. Radio observations revealed the presence of a powerful relativistic jet. The spectrum of the jet revealed it to be young. Whatever caused it must have happened fairly recently.

X-ray analysis hinted at the presence of an accretion disk of material in the galactic center. Matter falling from this disk into the black hole could account for the radio flares, and material corralled by the black hole’s magnetic field could launch the jet.

illustration of an active galactic nucleus emitting a jet

Active galactic nuclei can light up the radio sky — but VT J0243 doesn’t quite fit with a typical active galaxy picture. [NASA/JPL-Caltech]

Exploring the Options

The team examined two main possibilities for what happened. The first suggests a long-standing accretion disk that existed long before the flare was fired — in other words, this galaxy is an active galactic nucleus. However, if this picture is correct, the team says it isn’t clear what caused the accretion rate to suddenly spike and generate the flare. The energy distribution across the radio spectrum also differs from most other active galactic nuclei with young jets. If it is an active galactic nucleus, it would be an unusual one.

artist's impression of a tidal disruption event

Artist’s impression of a tidal disruption event — the ripping apart of a star by a black hole. [NASA/JPL-Caltech]

The second option is that a star passed too close to the black hole and was ripped apart in what’s known as a tidal disruption event. However, there was no X-ray flare associated with this event — and astronomers have only seen one jetted tidal disruption event without such a flare before. It would also have taken an unusually long time for the radio emission to start after the tidal disruption event.

For now it remains a mystery, but this discovery is a clarion call for astronomers ahead of an influx of data from the Very Large Array Sky Survey (VLASS). Started in 2017 and due to finish in 2024, this survey is trawling 80% of the sky and is expected to catalog approximately 10 million radio sources. Among them should be many young jets like the one associated with VT J0243. Perhaps then astronomers will better understand the true triggers of such dramatic radio flares.

Citation

“A Candidate Relativistic Tidal Disruption Event at 340 Mpc,” Jean J. Somalwar et al 2023 ApJ 945 142. doi:10.3847/1538-4357/acbafc

Hubble image of a supernova remnant

Researchers searched the remains of an exploded star for signs of its one-time companion. Though they found a suitable candidate, the star is in some ways an unlikely participant in a Type Ia supernova explosion.

A Companion to an Exploding Star

illustrations of the two main Type Ia supernova pathways

Illustrations of the two main Type Ia supernova pathways: the single-degenerate model (top) and the double-degenerate model (bottom). [Both images from NASA’s Goddard Space Flight Center Conceptual Image Lab]

Astronomers use a class of exploding stars called Type Ia supernovae to measure the distances to other galaxies and even determine the universe’s expansion rate. In theory, all Type Ia supernovae are equally luminous, making them useful signposts. However, research increasingly suggests that Type Ia supernovae are not all alike, and their varied formation mechanisms might lead to varied brightnesses as well, which complicates their role as cosmic distance markers.

In general, Type Ia supernovae can result from two main pathways: two white dwarfs colliding (the double-degenerate scenario) or a white dwarf stealing matter from a companion star (the single-degenerate scenario). In order to determine the event that triggered a supernova, astronomers search the rubble of these cataclysmic events for signs of a companion star scurrying from the site of the explosion; neither white dwarf survives the double-degenerate scenario, while in the single-degenerate scenario the companion star might withstand the blast and live on.

And Yet It Lives

In a recent research article, a team led by Pilar Ruiz-Lapuente (Institute of Fundamental Physics, Spanish National Research Council; Institute of Cosmos Sciences of the University of Barcelona) analyzed 3,082 stars in the vicinity of a nearby supernova remnant called G272.2-3.2 to search for signs of a surviving stellar companion. If present, the companion star would 1) have a high velocity, 2) be traceable back to the center of the supernova remnant when the explosion occurred about 7,500 years ago, and 3) potentially be chemically enriched by catching debris from the exploding star.

X-ray image of the supernova remnant with current and past positions of a star marked on it

An X-ray image of the supernova remnant G272.2-3.2 with the position of the fast-moving star MV-G272 as seen today (red circle) and 8,000 years ago (cyan circle). [Ruiz-Lapuente et al. 2023]

Using data from the Gaia spacecraft, the team identified a single fast-moving star that was likely to have been in the right place at the right time. Curiously, the star is an M dwarf — the coolest, smallest type of star. And though the star didn’t show the expected chemical enrichment, Ruiz-Lapuente’s team notes that M dwarfs, unlike more massive stars, are fully convective, which means that any enriched material that falls onto the star would be mixed throughout it rather than remaining near the surface.

Small Star, Big Questions

M dwarfs don’t typically participate in Type Ia supernovae. Because of their small size, they can only transfer a small amount of mass, meaning that a white dwarf with an M-dwarf companion is more likely to undergo repeated nova outburst than a one-time, cataclysmic supernova. However, researchers suspect that the strong magnetic fields of an M dwarf and a white dwarf might help funnel material between the stars, cranking up the transfer rate and triggering a supernova. If it’s possible for M dwarfs to facilitate Type Ia supernovae, we can expect to find more cases like that of G272.2-3.2, since M dwarfs are the most common stars in the universe.

Citation

“A Possible Surviving Companion of the SN Ia in the Galactic SNR G272.2-3.2,” P. Ruiz-Lapuente et al 2023 ApJ 947 90. doi:10.3847/1538-4357/acad74

A photorealistic illustration of the Milky Way viewed edge on. A large red planet similar to Jupiter sits in front of it in the foreground.

Tough-to-find and mysterious once they’re spotted, brown dwarfs blur the clean distinction between stars and planets. Basic questions, such as how they form, remain unanswered. However, JWST’s suite of near-infrared detectors is perfectly suited to their study, and astronomers are now making the first of what will likely become a new wave of brown dwarf discoveries.

Uncategorized Objects

As much as astronomers (and many of their cousins in other scientific disciplines) love recording data and gathering observations, their true passion is to sort and label their specimens into neat little categories back in the lab. Most of the time, this is fairly straightforward: stars, those massive assemblages of scorchingly hot plasma, go over here, while planets, smaller chunks of cooler material, go over there. The spreadsheets can be sorted, and a sense of order rules.

But what happens when they find objects that fall awkwardly into the space between these seemingly clean camps? Such oddities aren’t hypothetical: in recent decades, astronomers and citizen scientists have discovered numerous “brown dwarfs,” or objects too big and warm to be a planet, but too small and cool to be a star. These curiosities defy neat classification, and how they form remains an open question. Do they collapse out of giant molecular clouds, which would make them the runts of a stellar litter, or do they condense from circumstellar disks, which would imply they’re actually gargantuan planets?

A two-panel image of a small region of the sky imaged in two different filters. The target brown dwarf is labeled in each and appears much brighter in the longer wavelength image

Cutouts of the JWST images of WISE J033605.05–014350.4 shown in different wavelength filters. The temperature of the brown dwarf is low compared to most stars, making it appear brighter in the image taken near a wavelength of 4 microns than the image taken closer to 1 micron. [Calissendorff et al. 2023]

JWST Weighs In

One way to help settle this question is to check if any known brown dwarfs have companions. If it turns out that brown dwarfs sometimes come in pairs, like stars, then both populations likely form in the same way. Motivated by this thought, a team led by Per Calissendorff (University of Michigan) used JWST’s groundbreaking near-infrared camera to survey 20 nearby brown dwarfs for companions that could have been missed by less sensitive instruments.

After null results for 11 of the 20 targets, the team turned their attention to a ~400 K brown dwarf succinctly named WISE J033605.05–014350.4. This time, the team noticed that the image of the object was somewhat lopsided: it could be better described by a pair of sources too close together to fully resolve than by a single object. Using models to reconstruct the images, the team concluded that they were looking at not one, but two brown dwarfs bound together on a ~7 year orbit, with each weighing in at less than 20 times the mass of Jupiter.

A 3x3 grid of 10x10 pixel cutouts from the images of the target brown dwarf. Each pixel is colored by its intensity.

Reconstructions of the data using different models. The top row shows a single-source model and its residuals, while the middle rows show the authors’ binary model and its cleaner residuals. The last row shows the primary from the binary model subtracted from the data to emphasize the companion. [Calissendorff et al. 2023]

Both brown dwarfs fall into the “Y” spectral type, which makes this the first-ever discovered Y+Y binary. While the statistics are still dealing with small numbers and other complications remain, the discovery portends a coming revolution in our understanding of brown dwarf formation. With more JWST observations of brown dwarfs still to come, both to look for companions and to better characterize their atmospheres, astronomers will soon be able to answer some of the fundamental questions about these classification-resistant enigmas.

Citation

“JWST/NIRCam Discovery of the First Y+Y Brown Dwarf Binary: WISE J033605.05–014350.4,” Per Calissendorff et al 2023 ApJL 947 L30. doi:10.3847/2041-8213/acc86d

an infrared image of the Pleiades star cluster

NASA’s newest planet-hunting telescope, the Transiting Exoplanet Survey Satellite (TESS), has been tracking down exoplanets since 2018. Its high-cadence observations make it adept at untangling the workings of bright, nearby stars, and a recent research article has used TESS data to study stars in one of the most familiar clusters: the Pleiades.

The Persistent Mystery of Pulsating Stars

diagram showing the location of stars on the instability strip with respect to other stars

Approximate temperatures and luminosities of stars on the instability strip compared to main-sequence and other categories of stars. [Adapted from ESO; CC BY 4.0]

Astronomers have known for centuries that many stars in the night sky vary in brightness. Over time, we’ve refined our understanding of variable stars, and researchers now recognize that most variable stars lie within a small range of temperatures and luminosities referred to as the instability strip. Many — but not all — of the stars on the instability strip pulsate, their luminosities changing periodically as their radii grow and shrink over time.

A persistent question in the study of variable stars is why some stars on the instability strip are variable and others are not — how is it possible for two stars with nearly identical temperatures and luminosities to behave so differently? As a step toward answering this question, a collaboration led by Timothy Bedding (University of Sydney) used precise spacecraft data to search for pulsating stars in the nearby Pleiades cluster.

plot of the pulsation spectra of 35 stars

Pulsation spectra of the 35 pulsating stars observed by TESS. The stars are ordered according to their color as measured by the Gaia spacecraft, with bluer stars near the top and redder stars near the bottom. Click to enlarge. [Bedding et al. 2023]

Gaia and TESS

First, the team used precise stellar positions and distances cataloged by the Gaia spacecraft to determine which stars belonged to the Pleiades cluster, which is about 450 light-years away. Combining these likely cluster members with a handful of nearby stars that appear to have recently escaped the cluster, Bedding and collaborators collected a sample of 89 stars. Ten stars in this sample are already known to vary in brightness.

TESS observed most of these stars over a period of several weeks in 2021. Using the finely time-resolved TESS data (and some Kepler data for a star in the sample that moved off the edge of the TESS detector), the team detected a particular kind of behavior called δ Scuti pulsations in 36 stars, 30 of which were not previously known to vary. The pulsation spectra of these stars demonstrate that stars with similar ages, temperatures, and chemical compositions can pulsate in dramatically different ways.

Questions Continue

Two plots show the colors and magnitudes of the pulsating stars relative to the non-pulsating stars.

Top: Colors and magnitudes of 89 Pleiades stars with spectral classes A and F. The filled purple circles indicate pulsating stars. Bottom: A histogram showing the colors of pulsating (blue) and not pulsating (orange) stars in the sample. Click to enlarge. [Bedding et al. 2023]

By plotting the observed brightness of these stars against their colors, Bedding and coauthors determined where the stars fell with respect to the instability strip. The pulsating stars spanned a band 0.45 magnitude wide, and 72% of Pleiades stars falling within this band showed pulsations. For stars located in the center of the instability strip, 84% are pulsators, which the team notes is an unusually high percentage.

Given the remarkably high percentage of pulsators in the cluster, Bedding and collaborators noted that the question isn’t so much why some stars pulsate and others do not, but rather why similar stars have such different pulsations. Though the question remains open, one thing is clear: Gaia and TESS data are powerful tools for tracking down variable stars.

Citation

“TESS Observations of the Pleiades Cluster: A Nursery for δ Scuti Stars,” Timothy R. Bedding et al 2023 ApJL 946 L10. doi:10.3847/2041-8213/acc17a

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