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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; CC BY 3.0]

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

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

X-ray image of supernova remnant Cassiopeia A

Astronomers can get a better view of more distant supernova explosions by searching for high-energy neutrinos, according to a study conducted by researchers at Uppsala University in Sweden.

Detecting Ghostly Particles from Collapsing Stars

A photograph of the IceCube Neutrino Observatory

The IceCube Laboratory at the Amundsen-Scott South Pole Station in Antarctica. [Felipe Pedreros, IceCube/NSF]

A core-collapse supernova represents the cataclysmic end of a massive star’s life. As the star collapses, its iron core disintegrates, producing vast numbers of sub-atomic particles called neutrinos that stream outwards in unimaginable quantities — somewhere around 10 billion trillion trillion trillion trillion of them. Their outwards pressure blasts the star apart and detonates the supernova.

There are various neutrino detectors scattered across the planet, including IceCube in Antarctica. It can detect the neutrinos produced by core-collapse supernovae, which typically have energies in the mega-electronvolt range. However, the way its detectors work means that supernova neutrinos from farther away than the Magellanic Clouds — the largest satellite galaxies of the Milky Way — are too faint for it to see.

Circumstellar Collisions and Choked Jets

Nora Valtonen-Mattila and Erin O’Sullivan (both Uppsala University) think that there may be higher-energy neutrinos on offer, which would extend that range to more than a hundred million light-years. That would include supernovae in many other galaxies and local galaxy clusters.

These neutrinos — in the giga- to tera-electronvolt range — aren’t produced by the supernova itself, though. Instead Valtonen-Mattila and O’Sullivan consider two alternative production mechanisms.

Before stars go supernova they tend to eject vast amounts of material in fierce stellar winds. When the supernova detonates, the debris from the explosion hits this circumstellar material. Protons collide like they do in particle accelerators here on Earth and should produce high-energy neutrinos in the process. However, these neutrinos have yet to be observed.

A similar thing could be achieved through a jet of material that becomes trapped behind the star’s outer shell. This is called a choked jet. Ordinary particles may be trapped, but neutrinos are famously ghost-like and can stream through dense material with ease. In fact, a light-year of lead would only have fifty-fifty chance of stopping a neutrino.

Plot showing the predicted number of detectable neutrinos as a function of supernova distance

The predicted number of observable neutrinos in the northern sky based on the choked-jet model (top) for two detection methods. The bottom panel shows the cumulative number of supernovae observed by the Zwicky Transient Facility each year. [Adapted from Valtonen-Mattila and O’Sullivan 2023]

Expanding Our Horizons

It’s these high-energy neutrinos from choked jets that offer the greatest possible extension to IceCube’s range, pushing it out to 277 million light-years in the northern sky and 65 million light-years in the southern sky. However, choked jets are rare, with just 1–4% of core-collapse supernovae thought to have one.

These are important considerations, particularly in light of the ongoing upgrade to IceCube-Gen2, which should be completed by 2033. Perhaps then we’ll have a clearer picture of core-collapse supernovae and the neutrinos they produce.

Citation

“Prospects for Extending the Core-collapse Supernova Detection Horizon Using High-Energy Neutrinos,” Nora Valtonen-Mattila and Erin O’Sullivan 2023 ApJ 945 98. doi:10.3847/1538-4357/acb33f

a photograph of a supernova on the outskirts of a dusty galaxy

JWST observations of abundant galaxies in the early universe have the potential to upend our theories of when the first galaxies formed. New research suggests that some of these candidate galaxies are actually individual supernovae, slightly easing — but not eliminating — the tension between theory and reality.

Eyeing Early Galaxies

JWST image of the galaxy cluster SMACS

JWST image of the galaxy cluster SMACS J0723.3-7327. [NASA, ESA, CSA, STScI]

Understanding how and when galaxies formed in the early days of the universe is a central goal of astronomy, and the exceptional observations from JWST should help us advance toward that goal. As early data from JWST rolled in, researchers compiled lists of objects that appeared to be distant galaxies. As the lists grew, though, our theories of how quickly galaxies assembled after the Big Bang began to bend under the strain — JWST found more (and more massive) galaxies in the early universe than we expected.

Now, researchers are taking a closer look at these purported “high-redshift” galaxies, testing whether some of them are actually less-distant galaxies reddened with dust or even single objects in our own galaxy, like brown dwarfs. In a recent research article, a collaboration led by Haojing Yan (University of Missouri) has proposed a new type of object that might be contaminating our high-redshift galaxy samples: extragalactic supernovae.

Examples of four point-like galaxy candidates as seen in six JWST filters

Examples of four point-like galaxy candidates as seen in six JWST filters. The wavelength range of the observations increases from left to right. Click to enlarge. [Adapted from Yan et al. 2023]

Galactic or Stellar?

Yan and collaborators hunted for high-redshift galaxies in the first JWST image ever released, an expansive scene showing the galaxy cluster SMACS J0723.3-7327 (SMACS J0723 for short). Yan’s team collected a sample of nearly 90 candidate high-redshift galaxies with potential redshifts ranging from 12.7 to 24.7, which corresponds to roughly 340 to 130 million years after the Big Bang. Upon inspection, 10 of these sources appeared point-like, more like stars than galaxies.

The team suggested that these point-like “galaxies” might instead be extragalactic supernovae. To test this theory, they attempted to fit the photometric JWST observations with models of Type Ia (thermonuclear runaway) and Type II (core-collapse) supernovae.

Still in Disagreement

Two plots showing model fits to JWST data of a high-redshift galaxy candidate

An example of the model fitting. This source, F200DB-086, is well fit by both a Type Ia supernova (top) and a Type IIP supernova (bottom) model. Click to enlarge. [Adapted from Yan et al. 2023]

Most of the sources were well fit by either or both supernova models, with potential redshifts between 1 and 15. These results mean that the point sources could be supernovae, but it doesn’t mean that they are. Yan and coauthors noted that none of the sources are associated with a background galaxy, though that wouldn’t be unexpected if the supernovae occurred in faint dwarf galaxies. And while predictions based on star-formation rates suggest that only five such supernovae would be expected in the SMACS J0723 field, the observed number fits within the large uncertainties.

Ultimately, as is often the case, more data are needed to inspect these mysterious sources further. The team suggested that revisiting the SMACS J0723 field should clarify the situation, confirming that the objects are supernovae by watching them fade over time. And as for our understanding of galaxy growth in the early universe, the solution is still unclear — removing a handful of supernova sources still leaves more galaxies than our theories can handle.

Citation

“Pointlike Sources among z > 11 Galaxy Candidates: Contaminants due to Supernovae at High Redshifts?,” Haojing Yan et al 2023 ApJL 947 L1. doi:10.3847/2041-8213/acc93f

An image taken from orbit around the moon of Shackleton crater. The image is taken from an oblique angle such that only the very edge of the crater is visible; the surrounding landscape and crater interior are in shadow and too dark to resolve.

Three and a half billion years ago, a mile-wide rock haphazardly slammed into the south pole of the Moon. About three years from today, a human-made ship carrying the Artemis III crew will land much more gingerly in the same spot and survey the damage. To make the most of their limited time there, scientists are mapping out where they should go and what they might see.

Back to the Moon

While most exploration within astronomy centers on observing places beyond the physical reaches of humanity, there’s an undeniable romanticism behind our vision to visit our nearest neighbors in person. In the last 50 years, though, we’ve been collectively reclusive; although our robotic emissaries have been busy, no human has left the confines of Earth’s gravitational dominance since before the first personal computers hit shelves.

That’s likely about to change soon, however. While the political winds that govern such endeavors could shift, the previous two presidential administrations have stood by the Artemis program, which aims to send astronauts around the Moon in 2023, and to land on the lunar south pole in late 2025 the earliest. 

A geologic map of the area surrounding the likely Artemis III landing site. Individual boulders, rocky craters, and rock exposures mapped in this study are included as dots. Click for high-resolution version. [Adapted from Boazman et al. 2023 and Bernhardt et al. 2022]

Remote Cartography

These future moonwalkers will need a careful plan to maximize the scientific return of their precious few days on the surface. Recently, a team led by Sarah Boazman (European Space Research and Technology Centre) used data from the Lunar Reconnaissance Orbiter to get started on this necessary preparation. Published in the Planetary Science Journal, Boazman and collaborators created a new map of potential geologic sampling sites near the likely Artemis III landing site. Their survey specifically flags isolated boulders, rocky craters, and rock exposures that could be reached by astronauts on foot, adding context to previous geologic maps of the polar regions.

After a thorough analysis (they team flagged over 86,000 individual boulders alone), Boazman and collaborators conclude that many of the boulders near the probable Artemis III landing site are associated with the impact that formed Shackleton crater, the 21-kilometer depression that hosts several permanently shadowed regions packed with ices.

Map of boulders in the region studied

Isolated boulders flagged in the study, overlaid on a lower-resolution image of the same region. [Adapted from Boazman et al. 2023]

They also find several positions within the landing zone that would make ideal touchdown spots: flat, safe ground that would put the astronauts within walking distance of the maximum number of geologic targets. Although they were limited by the resolution of the spacecraft’s camera and the awkward viewing geometries, the team points out that “all errors in our mapping can be reduced with ground-truthing efforts by future surface missions.”

Although the Artemis III crew will be the only humans physically on the Moon once they land, they’ll carry with them the curiosity of thousands of planetary scientists back here on Earth. It is a thrilling time when astronomers can make maps of other worlds in preparation for a journey there, and hopefully the wait won’t be long for an astronaut to size up one of the targets on this map in person.

Citation

“The Distribution and Accessibility of Geologic Targets near the Lunar South Pole and Candidate Artemis Landing Sites,” Sarah. J. Boazman et al 2023 Planet Sci. J. 3 275. doi:10.3847/PSJ/aca590

artist's impression of a white dwarf accreting material from a companion star

Extremely compact stellar remnants made of a mixture of normal matter and dark matter could explain a variety of puzzling observations, but it’s not clear exactly how these objects might form. Now, researchers have modeled a potential formation pathway and proposed a way to track them down.

Where Dark Matter and Normal Matter Meet

infographic describing the gravitational wave event GW190814

An infographic describing the gravitational wave event GW190814, which contained a 2.50–2.67-solar-mass object of unknown type. Click to enlarge. [LIGO Scientific Collaboration]

In some parts of the universe, where dark matter is especially dense, normal matter and dark matter might intermingle and swirl together to form stars. If these dark-matter-containing stars evolve into neutron stars — extremely dense stellar remnants about the size of a city — such an object might explain the too-heavy neutron star thought to have participated in the gravitational wave event GW190814, among other curious observations.

A team led by Ho-Sang Chan (The Chinese University of Hong Kong) has proposed that neutron stars containing a small amount of dark matter might form through a circuitous route. First, a low- to intermediate-mass star composed of normal and dark matter evolves to become a white dwarf: an Earth-sized sphere containing roughly the mass of the Sun. If this white dwarf has a giant stellar companion, it can steal some of the companion’s gas and become so massive that it collapses under its own gravity. Usually, this would lead to a supernova explosion, but under certain conditions, the white dwarf might shrink down to become a neutron star instead.

Chan and collaborators suggest that observing these events, called accretion-induced collapse, might yield a way to study the properties of these unusual neutron stars and of dark matter itself.

Conceptualizing Collapse

While the visible-light signature of a white dwarf collapsing to form a neutron star would be faint, Chan and collaborators have suggested that we might be able to track them down via their gravitational wave emission. The team used two-dimensional fluid dynamics simulations to study how a dark-matter-containing white dwarf would collapse into a neutron star and estimated the gravitational waves that would be emitted in the collapse.

The team set the mass of their dark matter particles to a little more than a tenth of the mass of a proton, and they considered white dwarfs containing 1–20% dark matter by mass. Additionally, they considered different rotation profiles for the stars: rigid rotation (like a spinning top) and Keplerian rotation (like planets in the solar system, the velocity is highest near the center and lowest farther out).

Observational Prospects

Gravitational wave strain for different percentages of dark matter

Normalized gravitational wave strain (related to the wave amplitude) from the collapse of Kepler-rotating white dwarfs containing, from top to bottom, 0%, 1%, 5%, 10%, and 20% dark matter by mass. [Chan et al. 2023]

Chan and collaborators found that the rotation of the star is key to determining the shape of the gravitational wave profile. For rigid rotators, there was essentially no difference between the gravitational waves emitted by stars containing dark matter and those made of solely normal matter. For Keplerian rotators, though, the presence of dark matter softens some peaks in the gravitational wave signal, and these differences are likely detectable with current gravitational wave facilities.

Hopefully, future gravitational wave observations will yield new information about these theorized neutron stars, potentially illuminating the nature of dark matter.

Citation

“Accretion-induced Collapse of Dark Matter-admixed Rotating White Dwarfs: Dynamics and Gravitational-wave Signals,” Ho-Sang Chan et al 2023 ApJ 945 133. doi:10.3847/1538-4357/acbc1d

Researchers have detected hot molecular cloud cores in the Small Magellanic Cloud for the first time. This discovery enhances our understanding of star formation in the nearby universe and will guide future explorations of extragalactic star-forming regions.

From Cold Cloud to Hot Core

an infrared image of the Small Magellanic Cloud

A view of the Small Magellanic Cloud from the European Southern Observatory’s Visible and Infrared Survey Telescope for Astronomy. [ESO/VISTA VMC; CC BY 4.0]

New stars form in massive clouds of molecular hydrogen gas. As the cloud swirls, gas collects in cold, dense clumps, creating the conditions for star formation. When a massive star begins to form in one of these clumps, the gas heats up, creating a hot molecular cloud core. Researchers have previously detected hot molecular cloud cores in the Milky Way and in several nearby galaxies, but they have remained elusive in one of our nearest neighbors: the Small Magellanic Cloud.

The Small Magellanic Cloud is an interesting place to search for hot cores because this small, irregularly shaped galaxy is poor in metals — elements heavier than helium — compared to galaxies like the Milky Way. If we find hot cores in such a metal-poor galaxy, we can study how the formation of massive stars varies between metal-rich and metal-poor galaxies in the universe today. This can also help us understand star formation billions of years ago, when the universe was substantially less metal rich than it is now.

locations of the core candidates within the Small Magellanic Cloud

Locations of the two protostars/hot core candidates, S07 and S09, within the Small Magellanic Cloud. The observations were made at infrared wavelengths. [Shimonishi et al. 2023]

Core Candidates

Takashi Shimonishi (Niigata University) and collaborators began their search for hot cores with two sources in the Small Magellanic Cloud that had been flagged as high-mass protostars. Previous observations found that these two soon-to-be stars are swathed in clouds containing dust and ice, which suggests that the protostars might be embedded within dense gas.

The team combined new and archival data from the Atacama Large Millimeter/submillimeter Array (ALMA) to determine the properties of the gas surrounding the two sources. They detected spectral lines from numerous molecules and molecular ions, including carbon monoxide, methanol, and sulfur dioxide. The data suggested that the gas surrounding the protostars is dense, hot (here, “hot” means warmer than 100K), and concentrated in a small region around each protostar — exactly the characteristics of a hot core!

Testing Molecular Tracers

Finding hot cores in the metal-poor Small Magellanic Cloud suggests that hot core formation is an expected part of massive star formation for galaxies with a wide range of metal abundances. Specifically, Shimonishi and collaborators have shown that hot cores can form in galaxies in which metals are 80% less abundant relative to hydrogen than they are in the gas from which the Sun formed.

comparison of sulfur dioxide and methanol emission for one of the hot cores

Comparison of the sulfur dioxide (SO2) and methanol (CH3OH) emission for the hot core S07. The source region of the sulfur dioxide emission is more compact and warmer. Click to enlarge. [Adapted from Shimonishi et al. 2023]

Interestingly, the team found key differences between the Small Magellanic Cloud hot cores and those in other galaxies. Generally, researchers use methanol emission to find hot cores, but the methanol emission from the newly found cores was extended and cool — not what we’d expect for a hot core. Instead, it was sulfur dioxide emission that traced the cores effectively. Why might methanol be a poor core tracer in the Small Magellanic Cloud when it’s so effective in other environments? This might point to differences in how methanol and sulfur dioxide form in metal-poor hot cores, making sulfur dioxide a better indicator of hot cores in these regions.

Citation

“The Detection of Hot Molecular Cores in the Small Magellanic Cloud,” Takashi Shimonishi et al 2023 ApJL 946 L41. doi:10.3847/2041-8213/acc031

a photograph of a total solar eclipse with the solar corona showing

A recent research article describes a new way to measure the magnetic field of the Sun’s tenuous upper atmosphere, or corona, from images taken during total solar eclipses.

Illuminating the Solar Corona

An illustration of the regions of the Sun's atmosphere and interior.

An illustration of the regions of the Sun’s atmosphere and interior. Click to enlarge. [NASA/Goddard]

The outermost layer of the Sun’s atmosphere is normally hidden from view, but a total solar eclipse reveals ghostly tendrils flowing out from the Sun: the corona. The solar corona is exceptionally hot, sparse, and dynamic, and it’s the source of a variety of exciting space weather phenomena. It’s this last point that makes understanding the properties of the corona important, especially the strength and direction of its magnetic field.

Because direct measurements of the coronal magnetic field are hard to come by — the only spacecraft yet designed to travel into the harsh coronal environment, the Parker Solar Probe, has barely dipped a toe into the region — researchers must use complex models to reproduce measurements made from a distance to understand this important region of the Sun.

comparison of output from the two methods

A projection of the magnetic field strength derived using the total solar eclipse (TSE) method and the magnetohydrodynamic (MHD) modeling method. Click to enlarge. [Bemporad 2023]

A Simpler Solution?

Alessandro Bemporad (National Institute for Astrophysics, Italy; Purple Mountain Observatory) explored a simpler way. Bemporad’s analysis method starts with polarized-light images of the corona from the 21 August 2017 total solar eclipse. The author first used an established technique to calibrate the images and convert the brightness of each pixel to an important physical quantity: how densely packed or rarefied the plasma is within the corona.

To convert from plasma density to magnetic field strength, Bemporad made a bit of a leap. In many physical systems, energy is distributed fairly equally among different types — like thermal, kinetic, magnetic, or potential energy. For a system like the solar corona, the dominant forms of energy are magnetic and gravitational potential. By making the assumption that these forms of energy are balanced, Bemporad estimated the magnetic field strength without needing complex modeling.

Looking Forward, Looking Back

comparison of output from the new method and the more complex fluid dynamics modeling

Left: Relative difference between the magnetic field strength derived using the total solar eclipse method and the fluid modeling. Right: Cumulative distribution of relative differences between pixel values. Click to enlarge. [Bemporad 2023]

To test the validity of the new method, Bemporad compared his results against those from fluid dynamics models. In general, the output from the two methods differed by less than 50%. Given that the results from the more complicated fluid models aren’t ground truth, Bemporad considers these discrepancies small.

Bemporad notes that the new method can be applied to any white-light images of the solar corona, not just those taken during total solar eclipses. This means that any ground- or space-based images made using a coronagraph (an instrument that blocks the light from the Sun’s bright disk so that the fainter corona can be photographed) are ripe for further exploration, including images taken decades ago — opening a new window into the magnetic conditions of past solar cycles.

Citation

“Coronal Magnetic Fields Derived with Images Acquired during the 2017 August 21 Total Solar Eclipse,” A. Bemporad 2023 ApJ 946 14. doi:10.3847/1538-4357/acb8b8

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