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30 Doradus star-forming region

From its name alone, HD 222925 is no different from the 350,000 other stars in the Henry Draper Catalogue. What sets it apart is how closely we’ve studied it: we’ve measured the abundances of more elements in HD 222925 than any other star except for the Sun. This makes it an excellent target for studies of how stars inherit chemical elements from their birth environments.

Making Heavy Metals

Periodic table of elements showing the likely origin of each element

Periodic table of elements showing the likely origin of each element in our solar system. Click to enlarge. [Wikipedia user Cmglee; CC BY-SA 3.0]

The 65 elements identified in HD 222925’s spectrum include 42 elements heavier than iron. These heavy elements form when atoms capture neutrons, a process that can be either fast (multiple neutrons fusing onto an atom at once, before it can split apart into more stable atoms) or slow (single neutrons captured at a time). The rapid capture or r-process happens in extremely hot and dense environments like those created during supernovae, mergers of two neutron stars, or when a neutron star is ripped apart by a black hole.

Since stars form out of the gas left behind by previous generations of stars, the relative amounts of r-process elements in a star’s spectrum can tell us about the events that preceded it. In comparing HD 222925’s chemical abundance pattern to the Sun’s, researchers found that while the stars have similar heavy-metal abundance patterns overall, their patterns differ among the lighter r-process elements.

plot comparing the chemical abundance pattern of HD 222925 to that of the Sun

Comparison of HD 222925’s abundance pattern (black squares) to the Sun’s abundances due to slow neutron capture and rapid neutron capture, denoted s and r, respectively. Click to enlarge. [Holmbeck et al. 2023]

Going for the Gold (and Platinum and Iridium and…)

Researchers often model the Sun’s abundance pattern to test their understanding of how events like neutron star mergers create r-process elements. However, HD 222925 is older than the Sun and has a smaller amount of metals — elements heavier than helium — than the Sun does. This could mean that HD 222925’s chemical makeup reflects the r-process elements created in a single cataclysmic event, like two neutron stars colliding, rather than containing material enriched by multiple events. This would make HD 222925 a better template for the chemical abundance pattern created by a single source of r-process elements.

To address this possibility, Erika Holmbeck (Observatories of the Carnegie Institution for Science) and collaborators used computational models to determine the likeliest source — or sources — for the pattern of chemical elements seen in HD 222925.

More Adjustments Necessary

Holmbeck‘s team found that it was challenging to reproduce some of the star’s elemental abundances with models that included only one source of r-process elements, but they found two possible ways to bring the model into better agreement with the observations. In the first, HD 222925’s light and heavy r-process elements come from two different sources. In the second, there’s only one source — neutron star mergers — but conditions must be tuned in a very specific way so that the energy released when atoms split apart into lighter elements doesn’t re-heat the surrounding material and prompt further fusion.

While these options could explain HD 222925’s observed chemical abundances, the authors suggest that we might just need to pin down certain physical quantities used by the model more precisely. Regardless of the answer, there’s more work to be done — either laboratory work, improvements to our models, or making more measurements of metal-poor stars to understand if HD 222925’s chemical abundances are representative of metal-poor stars as a whole.

Citation

“HD 222925: A New Opportunity to Explore the Astrophysical and Nuclear Conditions of r-process Sites,” Erika M. Holmbeck et al 2023 ApJ 951 30. doi:10.3847/1538-4357/acccf3

photograph of water ice plumes on Enceladus

Saturn’s moon Enceladus is a promising place to look for life in our solar system. A recent research article introduces the Astrobiology eXploration at Enceladus mission concept, which aims to examine the icy moon’s habitability and geological history.

A Promising Target

visible-light and infrared images of Enceladus

Composite optical and infrared images of Enceladus taken at various angles. The “tiger stripes” from which the plumes originate overlap with a warm region at the moon’s south pole. [NASA/JPL-Caltech/University of Arizona/LPG/CNRS/University of Nantes/Space Science Institute]

When the Cassini spacecraft examined the Saturn system from 2004 to 2017, it made surprising discoveries about Enceladus, a tiny, frosty moon that holds the title of most reflective object in the solar system. Enceladus’s sparkling ice shell is marred by long fissures, from which jets of water ice spray. That in and of itself is already exciting, but Cassini’s instruments picked up on something even more thrilling: the plumes are salty and contain organic molecules. Some of these molecules appear to be chunks of larger, more complex molecules that broke apart as they entered Cassini’s instruments.

Further data suggested that Enceladus’s plumes emanate from a global ocean that sloshes beneath the moon’s icy surface, warmed by tidal stresses from Saturn’s gravitational pull. The combination of water, organic molecules, and heat makes Enceladus an enticing target in the search for life beyond Earth. But the Cassini mission is far in our rear-view mirror — where do we go from here?

Mission to an Icy Moon

Long before spacecraft drop into orbit around planets or pass by moons, long before launch or loading or assembly, teams of scientists craft detailed mission plans that outline what the mission will accomplish and how. Today’s research article introduces one such plan developed during a session of the Jet Propulsion Laboratory’s Planetary Science Summer School, which invites teams to put their heads together to devise missions to explore the solar system. (Astrophysicists and solar physicists have their own versions of this program, too!)

illustrations of Enceladus's plumes and surrounding regions

Illustration of Enceladus’s plume environment and the specific regions targeted by AXE. Click to enlarge. [Adapted from Seaton et al. 2023]

Marshall Seaton (Jet Propulsion Laboratory) and collaborators concocted an Enceladus-exploration mission called Astrobiology eXploration at Enceladus (AXE): a New Frontiers–class mission (i.e., costing about a billion dollars) that would take our understanding of Enceladus to a whole new level.

From Idea to Implementation

AXE has some big goals, like figuring out if there’s life in Enceladus’s subsurface ocean. For a mission proposal to be successful, it has to outline exactly how that mission will achieve its goals. This means laying out the physical quantities the instruments must measure, and how precisely the measurements must be made. It even has to wrestle with possible complications, like instrument degradation or failure.

plots showing how biotic and abiotic processes have different molecular abundance distributions

Demonstration of how the abundance of molecules differs between biotic processes (i.e., life) and abiotic processes. Click to enlarge. [Seaton et al. 2023]

As an example, in order for AXE to achieve its objective of determining whether the organic molecules sprayed out in Enceladus’s plumes are the result of life in the oceans, it must carry a mass spectrometer capable of detecting chemical compounds with masses between 2 and 600 atomic mass units down to a concentration of just ten parts per billion. Despite these exacting requirements, Seaton and collaborators have determined that AXE should be able to achieve its lofty science goals using just 30 flybys worth of data. Choosing multiple flybys rather than an orbiter or lander (what’s known as “mission architecture”) simplifies certain aspects of the mission.

As you can imagine, there’s far more involved in crafting a mission plan than can be included in this short summary — be sure to check out the full article linked below to learn more!

Citation

“Astrobiology eXploration at Enceladus (AXE): A New Frontiers Mission Concept Study,” K. Marshall Seaton et al 2023 Planet. Sci. J. 4 116. doi:10.3847/PSJ/acd119

illustration of a supernova and a companion star

The jury’s still out on whether stripped-envelope supernovae arise from single stars or binary pairs. New research explores how to find the surviving binary companion (should it exist) of a recent supernova, offering a potential way to reach a verdict.

A Supernova Subclass

An illustration of the binary origin for stripped-envelope supernovae

An illustration of the binary origin for a stripped-envelope supernova. In the first panel, two main-sequence stars inhabit a binary system. In the second panel, one of the stars swells to become a red supergiant and loses its atmosphere to its companion. Finally, the evolved star goes supernova. [Adapted from NASA, ESA, Leah Hustak (STScI)]

When a massive star expires as a supernova, the light from the explosion can tell us about the star’s life, death, and the environment that surrounds it. Typical supernova light curves show prominent emission lines from hydrogen and helium, but some supernovae lack these characteristic lines.

Researchers believe that supernovae lacking hydrogen or helium lines result from stars that lost their outer envelope of gas before exploding. These events, dubbed stripped-envelope supernovae, might arise from stars that blew away their own atmospheres through ferocious stellar winds, or they might come from stars that have had their atmospheres stolen by a binary companion. Surprisingly, simulations show that a binary companion to a stripped-envelope supernova should actually be able to withstand the explosion, and new research explores whether we might be able to track down these survivor stars.

Imagining an Explosion’s Impact

A team led by Hsin-Pei Chen (陳昕霈) from National Tsing Hua University in Taiwan explored the possibility of tracking down the binary companion of a stripped-envelope supernova that was detected in 2020. Because of the low amount of mass ejected during the explosion, simulations suggest that the supernova, denoted SN 2020oi, likely happened in a binary system.

snapshots of simulated gas density

Gas density at four points in time during a simulation of supernova ejecta impacting a 5.5-solar-mass star. Click to enlarge. [Chen et al. 2023]

To understand what would have happened to SN 2020oi’s binary companion, Chen and collaborators simulated the impact of an exploding 10-solar-mass star on the evolution of a lower-mass stellar companion. The team ran several simulations with companion stars of 3.0, 5.5, and 8.0 solar masses, and they also varied the separation between the exploding star and its companion. After modeling the impact, the team switched to a second simulation to understand how the companion star’s physical properties and observational characteristics might change.

Ready, Set, Observe!

Plot of the luminosity, size, and temperature of the companion star as a function of time after the supernova

Luminosity, size, and temperature of the companion star as a function of time. The gray shaded area indicates the three-day period during which the supernova heats the star. [Chen et al. 2023]

These simulations captured the interaction between the companion star and the cloud of ejecta created during the explosion. When the supernova crashes into the companion star, it heats up the star, peels away a tiny bit of its mass, and kicks it away from the explosion. Over the course of a few years, the energy injected by the supernova makes the star larger and more luminous.

These changes might be observable by JWST and the Hubble Space Telescope, but only under certain circumstances; Chen and teammates estimate that if SN 2020oi’s binary companion has a mass between 3 and 8 solar masses, and if the two stars were initially separated by no more than five times the radius of the star that went supernova, we can hope to identify the companion star by 2030.

It’s always exciting when a prediction can be tested in years rather than gigayears. With luck and a little telescope time, we might track down SN 2020oi’s stellar companion, giving us new insight into the causes of stripped-envelope supernovae.

Citation

“Exploring the Observability of Surviving Companions of Stripped-envelope Supernovae: A Case Study of Type Ic SN 2020oi,” Hsin-Pei Chen et al 2023 ApJ 949 121. doi:10.3847/1538-4357/acc9af

Illustration of a star being torn apart by a black hole

Not everything astronomers observe has firmly supported explanations. Recently, however, advanced simulations have supported the hypothesis that certain flashes are the sign of a white dwarf in trouble.

Intermediate Mass, Extreme Danger

Intermediate mass black holes, though several thousand times smaller than their supermassive cousins, share many of the same egotistical personality traits. The more famous gargantuans tend to make themselves the center of attention by living in the middle of large galaxies and surrounding themselves with a dense core of stellar sycophants. Intermediate mass black holes similarly enjoy the spotlight, but on a smaller scale: they inhabit the centers of dwarf galaxies, or even smaller stellar clusters, but also surround themselves with many tightly-packed stars.

As a result of this dense environment, every now and then a star will get gravitationally bumped by its neighbors onto a trajectory that will carry it too close to the central black hole. Once within a certain distance, the star is doomed: as punishment for crossing an unseen barrier, the black hole will stretch the star into a long string of gas, which it will then consume. An even grislier fate awaits hardy white dwarf stars bumped onto very special trajectories that only graze this minimum distance. These stars will continue to circle the black hole on elongated, eccentric orbits, but each time they reach their closest distance, their outermost material will be peeled off and stripped away. Instead of destroying them quickly, the black hole will extend their suffering, slowly consuming them layer by layer, all the while burping out X-rays with each snack.

A Simulated Feast

A 2D heatmap displaying gas density. Stripped gas traces out a figure shaped like the numeral "6", while the dense, still bound gas concentrates at a point along the path.

A snapshot of a hydrodynamical simulation. The white dwarf core is shown in the inset; the long, spiraled streamer of gas represents material that has already been tidally stripped. [Chen et al. 2023]

That’s the story, anyway. Although astronomers have guessed that some strange X-ray flashes and quasi-repeating flares are the signs of the drawn-out ends to white dwarfs, they’ve never been sure since the process has mostly been studied only with analytic approximations. To more confidently attribute these strange observations to the slow deaths of white dwarfs near intermediate mass black holes, a team led by Jin-Hong Chen (Sun Yat-sen University) completed detailed hydrodynamical simulations that more accurately mimic the gruesome process.

A log-log plot of mass loss rate vs. time. The the line appears linear for nearly 1 year, following a t proportional to 5/2 slope, but the diverges towards infinity at the time when the star is destroyed.

The rate at which a white dwarf loses mass to the black hole. Over time, tidal stripping becomes more and more effective, until a certain point at which the white dwarf cannot maintain its structural integrity and is completely disrupted. [Chen et al. 2023]

The team found that yes, if intermediate mass black holes really were feasting on unsuspecting white dwarfs, they would periodically emit bright bursts of X-rays that we could detect with specialized space-based telescopes. Equally exciting, the team also found that if the dance of death were close enough to Earth (within about 100 million light-years, “nearby” by cosmic standards), next-generation gravitational wave detectors could also likely record the inspiral.

Though the instruments needed to record such a signal are still several years away, these accurate simulations of white dwarf tidal stripping will help future astronomers make sense of the strange, somewhat frightening processes that make things flash in the night.

Citation

“Tidal Stripping of a White Dwarf by an Intermediate-mass Black Hole,” Jin-Hong Chen et al 2023 ApJ 947 32. doi:10.3847/1538-4357/acbfb6

the Sun in extreme-ultraviolet and X-ray light

From spacecraft that dive into the Sun’s atmosphere to insights from complex models, we’re learning more about the Sun than ever before. Today’s highlight introduces five recent research articles that tackle hot topics in solar physics.

Tracking Down Dark Regions in the Corona

examples of darkened regions of the corona

An example of two darkened regions (circled) of the corona seen by Parker Solar Probe. The vertical dashed line indicates the moment when the spacecraft passed closest to the Sun, at a distance of just 13.29 solar radii (5.7 million miles or 9.2 million kilometers). Click to enlarge. [Adapted from Stenborg et al. 2023]

The first two articles touch on phenomena seen by the Parker Solar Probe, a NASA spacecraft launched in 2018 that aims to study the Sun’s hot and tenuous upper atmosphere, or corona, up close. In the first article, Guillermo Stenborg (Johns Hopkins University) and coauthors examined white-light images of the solar corona taken by Parker Solar Probe. These images show light that has been scattered off of electrons and dust within the corona. Many of the images show areas where the corona is very faint in white light, which could indicate a depletion of electrons, dust, or both.

To investigate further, Stenborg and coauthors compared the locations of the darkened regions to the locations of coronal holes — places in the corona where the solar magnetic field extends out into the solar system rather than looping back to the solar surface, allowing particles to stream out from the Sun. The team found that many of the darkened regions can be explained as the spacecraft’s line of sight passing through the “zone of influence” of a coronal hole. Other darkened regions had a different cause: as coronal mass ejections exit the corona, they leave a plasma-depleted region in their wake. The largest coronal mass ejection witnessed by the spacecraft excavated both plasma and dust.

an animation of the Parker Solar Probe crossing a solar switchback

An animation showing the Parker Solar Probe traversing a solar switchback. [NASA’s Goddard Space Flight Center/Conceptual Image Lab/Adriana Manrique Gutierrez]

Seeking the Source of Switchbacks

When the Parker Solar Probe made its first journeys toward the Sun, it discovered rapid back-and-forth changes in the direction of the solar wind magnetic field that researchers dubbed switchbacks. There are many theories as to the origins of solar switchbacks, including magnetic field lines rearranging into a new configuration (i.e., magnetic reconnection), turbulence in the solar wind, or a combination of both.

To delve into the origins of switchbacks, Pankaj Kumar (American University and NASA Goddard Space Flight Center) and collaborators investigated a closely associated phenomenon called microstreams. Microstreams are changes in the speed and direction of the solar wind that last about 10 hours and seem to be linked to switchbacks. Using data from the Parker Solar Probe as it surfed the solar wind, and combining it with photographs taken from afar by the Solar Dynamics Observatory, Kumar’s team found that microstreams in the solar wind and jets produced deeper in the solar atmosphere vary over time in the same way. The jets are also associated with close-set opposing magnetic field lines and bursts of protons and ions — clear signs that magnetic reconnection is underway. This suggests that magnetic reconnection creates the jets, and the jets in turn create switchbacks and microstreams.

Improving Forecasts with Machine Learning

Correctly forecasting the arrival times of coronal mass ejections — immense explosions of tangled solar plasma and magnetic fields — at Earth is important for testing our understanding of how these eruptions travel through space as well as for developing an early warning system. Even as simulations of coronal mass ejections unfurling across space have grown more complex, however, the typical error in the arrival times they predict has remained the same, around 12 hours.

Example of a modeled coronal mass ejection

Example of a modeled coronal mass ejection before it erupts into interplanetary space. [Singh et al. 2023]

A team led by Talwinder Singh (The University of Alabama in Huntsville) developed a new method to predict when a coronal mass ejection will arrive at Earth. Singh and collaborators used observations to shape their modeling of coronal mass ejections when they are poised to erupt, as well as their modeling of the background solar wind. This new model cut the arrival-time error by a third, which the authors attributed to their model’s ability to capture realistic interactions between the coronal mass ejection and the solar wind. Using machine-learning methods improved the model further, yielding a typical arrival-time error as low as 4 hours.

Solving the Cool Chromosphere Problem

Samuel Evans (Boston University) and collaborators focused on the chromosphere, the region between the sunspot-dotted solar surface and the tenuous, superheated corona. The chromosphere represents a unique challenge for modelers because one of our best tools — fluid dynamics models — can’t quite capture what’s going on there; the corona, where fluid dynamics models excel, is so rarefied that different components of the plasma can be treated individually, as they rarely interact. The chromosphere, however, is dense enough that the interactions between different components of the plasma can no longer be ignored. So far, fluid dynamics models have struggled to reproduce the temperature in the solar chromosphere — likely somewhere in the 3000–4000K range, based on observations — predicting a relatively chilly 2000K.

Simulation results showing where the instability grows and what the temperature of the chromosphere is

Top: Areas where the instability is at work. Bottom: Simulated chromospheric temperature. The instability grows fastest where the chromosphere is coolest. Click to enlarge. [Evans et al. 2023]

Evans and collaborators suggested that the mismatch between theory and observations is due to a missing heating process in the chromosphere. Specifically, the team proposed that a plasma instability is at work, creating fleeting meter-sized waves too small to be captured by existing fluid models — but perhaps impactful enough to warm our chromospheric models to the right temperature. The team used simulations to understand how this instability would affect chromospheric plasma composed of multiple interacting components. The instability appears to ramp up fastest in regions where the chromospheric temperature is low, heating those regions and potentially solving the cool chromosphere problem.

Gaining X-ray Insights into Charged-Particle Acceleration

Solar flares are likely the most well-known solar phenomenon, but there’s still plenty we don’t know about them. We know that solar flares accelerate charged particles to high velocities by releasing pent-up magnetic energy, but the details of this process are fuzzy.

Cartoon illustrating the location of the particle acceleration region

Cartoon demonstrating the location of the particle acceleration region and the source locations for X-ray emission and hard X-ray (HXR) emission. Click to enlarge. [Adapted from Stores et al. 2023]

Morgan Stores (Northumbria University) and collaborators explored this issue by modeling the acceleration of charged particles as they encounter a region of turbulent plasma in the Sun’s outer atmosphere. The team’s goal was to determine how factors like the size of the turbulent region, the distribution of the turbulent plasma, and the timescale of particle acceleration affect observable properties like X-ray brightness. Based on the results of this modeling, Stores and coauthors found that images and X-ray spectra of the Sun can be analyzed together to determine not just where electrons are being accelerated, but also when and how quickly. Next, the team plans to use their simulations to analyze solar flare observations made by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and Solar Orbiter.

Citation

“Investigating Coronal Holes and CMEs as Sources of Brightness Depletion Detected in PSP/WISPR Images,” Guillermo Stenborg et al 2023 ApJ 949 61. doi:10.3847/1538-4357/acd2cf

“New Evidence on the Origin of Solar Wind Microstreams/Switchbacks,” Pankaj Kumar et al 2023 ApJL 951 L15. doi:10.3847/2041-8213/acd54e

“Improving the Arrival Time Estimates of Coronal Mass Ejections by Using Magnetohydrodynamic Ensemble Modeling, Heliospheric Imager Data, and Machine Learning,” Talwinder Singh et al 2023 ApJ 948 78. doi:10.3847/1538-4357/acc10a

“Multifluid Simulation of Solar Chromospheric Turbulence and Heating Due to Thermal Farley–Buneman Instability,” Samuel Evans et al 2023 ApJ 949 59. doi:10.3847/1538-4357/acc5e5

“Spectral and Imaging Diagnostics of Spatially-Extended Turbulent Electron Acceleration and Transport in Solar Flares,” Morgan Stores et al 2023 ApJ 946 53. doi:10.3847/1538-4357/acb7dc

JWST image of a field of distant galaxies

JWST observations are poised to help astronomers study the surroundings of quasars — extremely luminous cores of galaxies in the early universe. The first analysis of a new survey of quasars reveals the structure of the cosmic web in the first billion years after the Big Bang.

Quasars in the Spotlight

illustration of a quasar

An artist’s illustration of a young galaxy hosting a quasar: an extremely luminous galactic nucleus powered by a supermassive black hole. [S. Munro; CC BY 4.0]

In the first few billion years of our universe, black holes at the centers of galaxies gained an incredible amount of mass. Their growth is apparent from the existence of quasars: blazingly bright centers of young galaxies powered by the rapid accretion of gas onto supermassive black holes.

Exactly how these supermassive black holes grew so quickly, bulking up to a billion solar masses in less than a billion years, is still up for debate. Researchers predict that in order for these black holes to reach their immense masses in such a short time, they must be located in areas densely populated with galaxies, where they’re constantly fed streams of cold gas.

Won’t You Be My Neighbor?

However, only some quasars in the early universe have been found to live in crowded regions. This might mean that the intense radiation from a quasar prevents galaxies from forming around it, or it might mean that the surrounding galaxies are simply very dusty, making them difficult to detect without a powerful infrared telescope.

JWST image of the quasar field

An infrared JWST image of the field containing J0305–3150. The colored circles indicate the 41 newly identified reionization-era galaxies. Red galaxies are at approximately the same redshift as the quasar, while orange and magenta indicate galaxies at redshifts of z = 5.4 and 6.2, respectively. All other galaxies in the sample are in green. Click to enlarge. [Wang et al. 2023]

Now, researchers are using JWST to carry out A SPectroscopic survey of biased halos In the Reionization Era (ASPIRE), a program that aims to understand whether quasars truly reside in crowded neighborhoods. The ASPIRE program will survey the surroundings of 25 quasars during the epoch of reionization — the period when the first stars illuminated the universe — searching for concentrations of galaxies known as galaxy overdensities. In today’s article,

Feige Wang (University of Arizona) and collaborators introduced the first results of this survey, focusing on a quasar called J0305–3150.

Using JWST’s Near Infrared Camera (NIRCam), Wang and collaborators discovered 41 epoch of reionization galaxies in the field of view containing J0305–3150. And while previous observations found no sign of a galaxy overdensity, the new observations revealed three galaxy overdensities! One of the galaxy clumps is located at the same redshift as J0305–3150, suggesting that this quasar resides in a crowded neighborhood.

Signs of the Cosmic Web

a three-dimensional map of the galaxies located near the quasar

A three-dimensional map of the galaxies located near J0305–3150. Galaxies identified via carbon emission lines are shown as orange circles while those identified via their oxygen emission lines are indicated with red circles. Click to enlarge. [Adapted from Wang et al. 2023]

The galaxies in J0305–3150’s neighborhood aren’t distributed randomly. Instead, they’re lined up in a narrow column. Wang and collaborators suggest that this arrangement is evidence for the formation of the filamentary structure of the cosmic web — the strands along which galaxies form, separated by vast bubbles called cosmic voids.

The filament found in this work happens to be about the size of protoclusters seen in cosmological simulations. After comparing their results against the predictions from simulations, Wang and collaborators suggest that J0305–3150 and the galaxies that surround it could someday collapse into a massive galaxy cluster like those present in the universe today. This study gives us a first look into what the ASPIRE team will discover — stay tuned for observations of 24 more quasars!

Citation

“A SPectroscopic Survey of Biased Halos in the Reionization Era (ASPIRE): JWST Reveals a Filamentary Structure around a z = 6.61 Quasar,” Feige Wang et al 2023 ApJL 951 L4. doi:10.3847/2041-8213/accd6f

illustration of a magnetar

New research proposes a way for fast radio bursts to escape the confines of a magnetized star and jet out into space — by getting help from theoretical particles called axions.

How to Make Cosmic Fireworks

example pulse and spectrum of a fast radio burst

An example band-averaged pulse (top) and spectrum (bottom) of a fast radio burst, FRB 170107. [Bannister et al. 2017]

Fast radio bursts — intense flashes of radio waves lasting a fraction of a second — were discovered by chance in 2007 and have puzzled astronomers ever since. Now, nearly two decades on from the discovery of these cosmic fireworks, we have plenty of theories for what causes them. Many theories have homed in on magnetars, the highly magnetized, extremely dense remnants of exploded massive stars.

Magnetar-based fast radio burst theories come in two main flavors: those in which the burst is generated close to the magnetar’s surface, and those in which the burst forms in the tangled stream of plasma that constantly flows out from the magnetar. The latter crop of models easily explains how fast radio bursts jet out into space, but they’re stymied by the bursts’ rapid variability. The former group of models naturally produces highly variable bursts, but they can’t yet explain how a burst escapes the magnetar’s surroundings, where there is a sea of plasma that scatters the burst and saps its energy.

There and Back Again

In a recent research article, Anirudh Prabhu (Princeton University) proposed a way for fast radio bursts to break free with a little help from theoretical particles called axions. Axions are thought to be extremely light, and their existence might solve a lingering problem in particle physics related to the properties of neutrons. These particles have also been proposed to be an important component of cold dark matter. (Prabhu emphasized that while axions happen to be a dark matter candidate, the validity of the fast radio burst model doesn’t hinge on axions actually being dark matter.)

Axions are thought to couple to photons, and under the right conditions, they can convert into photons and vice versa. Because of this property, Prabhu suggested that fast radio burst photons can sneak through the plasma surrounding the magnetar disguised as axions. When the coast is clear, so to speak, the axions convert back to photons and the burst continues on its journey into space.

diagram showing the axion-mediated fast radio burst model

A schematic of the proposed process. Axions (black dashed line) are produced close to the magnetar’s surface, at the distance indicated by the yellow circle. They travel outward through the magnetar wind (purple) and reconvert into radio photons (blue wavy line). [Prabhu 2023]

An Axion Answer

Here’s how that might work, physically. After a radio burst is generated near the magnetar’s surface, strong electric fields pointing parallel to the background magnetic field transform photons into axions, creating an axion burst. The axion burst travels outward, streaming along with the outflowing plasma wind as the magnetar’s magnetic field weakens. At a certain point, the axion burst begins to resonate with the surrounding plasma, regenerating a radio burst with properties similar to the original radio burst.

Prabhu notes that this theory works for any process that generates a strong parallel electric field close to the magnetar’s surface, such as a magnetar flare. The clincher for the axion theory would be the detection of axions associated with a fast radio burst, the feasibility of which Prabhu plans to explore in upcoming work.

Citation

“Axion-Mediated Transport of Fast Radio Bursts Originating in Inner Magnetospheres of Magnetars,” Anirudh Prabhu 2023 ApJL 946 L52. doi:10.3847/2041-8213/acc7a7

artist's impression of the gravitational wave background from a supermassive black hole binary sweeping across an array of pulsars

For the first time, researchers using pulsar timing arrays have found evidence for the long-sought-after gravitational wave background. Though the exact source of this low-frequency gravitational wave hum is not yet known, further observations may reveal it to be from pairs of supermassive black holes orbiting one another or from entirely new physics at work in our universe.

A New Window onto Gravitational Waves

In 2016, researchers reported the first detection of gravitational waves by the Laser Interferometer Gravitational-Wave Observatory (LIGO), opening a new window onto a universe’s worth of collisions between extreme objects like black holes and neutron stars. Though this discovery marked the beginning of a new observational era, many sources of gravitational waves remained beyond the reach of our current detectors on Earth.

simulation of a supermassive black hole binary system

Visualization of a simulation of a supermassive black hole binary system. [NASA’s Goddard Space Flight Center/Scott Noble; simulation data, d’Ascoli et al. 2018]

In particular, while existing gravitational wave observatories can spot the collision of two stellar-mass black holes, they can’t access the low-frequency gravitational waves expected to come from massive objects like pairs of supermassive black holes slowly spiraling toward each other at the centers of merging galaxies. The overlapping low-frequency rumblings of supermassive black hole binaries across the universe are theorized to create a constant low-amplitude hum known as the gravitational wave background.

Decades ago, researchers proposed that large numbers of stellar remnants called pulsars could provide a way to detect the low-frequency signal of the gravitational wave background. Today, several international collaborations announced their discovery of significant evidence for the gravitational wave background as seen by pulsar timing arrays.

Keeping Time with the Best Clocks in the Universe

Let’s break down exactly what that means, starting with the precise timing of pulsars. Pulsars are extremely dense, rapidly spinning remnants of massive stars that exploded as supernovae. Just one teaspoon of pulsar matter, brought to Earth, would weigh billions of tons. As pulsars spin, they emit narrow beams of radio waves along their poles. When these beams sweep past Earth, we detect the pulses of radio emission from which pulsars get their name. Typical pulsars spin once every few seconds to milliseconds, and they do so with incredible regularity, varying by only one part in 10 quadrillion from pulse to pulse.

composite image of the Crab Nebula

A composite X-ray, optical, and infrared image of the Crab Nebula, which is energized by the pulsar at its center. [X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA-JPL-Caltech]

Even among pulsars, the most precise timepieces in the universe, there are superlatives — millisecond pulsars that spin hundreds of times each second keep the best time of all. Surprisingly, though pulsars tend to slow down over time, millisecond pulsars are among the oldest of their kind; these pulsars have spun up to their incredible speeds by accreting gas from a stellar companion. Because millisecond pulsars are so regular, it’s theoretically possible to identify a passing gravitational wave by keeping tabs on their pulses; a minuscule compression in spacetime between us and a pulsar will hasten the approach of a pulse, while a tiny expansion in spacetime will delay it.

An Array of Spinning Stars

In reality, there are several reasons why an individual pulse’s arrival time at Earth might vary. The many light-years between us and these radio beacons is filled with ever-shifting clouds of gas and dust, which scatter and delay radio signals. Many millisecond pulsars are constantly swinging in a gravitational dance with their stellar companions. Some pulsars also undergo sudden increases in their spin rate, called glitches, due to starquakes or changes in their interiors.

This is where keeping tabs on many pulsars at once comes in handy. If you survey a collection of pulsars scattered across our galaxy, intervening dust or glitches will affect some pulsars and not others. Gravitational waves from distant sources, on the other hand, send ripples through spacetime that will affect the pulse arrival times of widely separated pulsars in a coordinated way. By searching for correlations among the arrival times of pulses from dozens of pulsars at once — i.e., using a pulsar timing array — researchers hope to uncover the signature of gravitational waves.

Bringing the Background to the Foreground

graphic showing the locations of stars in the pulsar timing array

Locations of pulsars (blue stars) in the NANOGrav pulsar timing array relative to the location of the Sun (yellow star). Some pulsar locations are approximate. Click to enlarge. [Ross Jennings / NANOGrav; CC BY 4.0]

International collaborations located in North America, Europe, India, Australia, and China have surveyed multiple pulsar timing arrays for this purpose, and today several collaborations have announced their discovery of significant, compelling evidence for the gravitational wave background. In a new Focus Issue published in the Astrophysical Journal Letters, a pulsar timing array monitored by radio telescopes across North America — the North American Nanohertz Observatory for Gravitational Waves, or NANOGrav — has laid out their new findings and the implications of their work.

Over the past 15 years, the NANOGrav collaboration has collected radio data from 68 pulsars in the Milky Way to search for gravitational waves. This long baseline of observations is necessary to capture the years- or decades-long undulation of gravitational waves from circling supermassive black holes.

Not only does the search for the gravitational wave background require years of observations, it also needs the utmost precision: the signal caused by passing gravitational waves is incredibly small, causing spacetime to stretch or compress by just one part in one quadrillion. This is far smaller than the normal fluctuations in an individual pulsar’s timing due to intervening gas and dust.

Plot of correlation strength as a function of the angle between pulsars in the array

Observed correlation between pulse arrival time discrepancies (blue points) and the correlation predicted by general relativity from a background due to supermassive black hole binaries (black dashed line). [NANOGrav]

To extract the gravitational wave signal from the array, researchers with the NANOGrav collaboration modeled the expected arrival time for the pulses from each pulsar in the array, then searched for correlations among the deviations in the pulse arrival times. The correlations that emerged from this analysis appear to match the predictions of Einstein’s general theory of relativity for the impact of gravitational waves. By running statistical tests on their data, the team concluded that the odds of the correlations they see being random are less than 1 in 1,000 — in other words, it’s highly likely that they’ve detected the anticipated low-frequency gravitational wave background and the rumblings of supermassive black hole binaries at last.

Are supermassive black hole binaries the only possible source of the gravitational wave background? No, though they’re still the leading candidate; the NANOGrav team reports that their observations are consistent with black holes that sink toward each other purely due to emitting gravitational waves as well as those that are interacting with their environment, engulfing and flinging nearby gas as the black holes swing around each other. However, exotic new physics, such as certain cosmic inflation scenarios or the existence of ultra-light dark matter, are a potential source as well. Some of these new physics models fit the data better than models of supermassive black hole binaries do, but the team notes that the models rely on major assumptions that are not yet proven valid.

From Evidence to Detection

The NANOGrav collaboration has been careful to refer to their findings as evidence for the gravitational wave background rather than a detection of it. What’s needed to push this discovery into coveted 5-sigma territory? Luckily, the answer is probably just that we need more time — time for the NANOGrav team to combine their observations with those of other pulsar timing array collaborations, or simply time to let the already impressive 15-year baseline of the project increase. In addition to declaring a firm detection of the gravitational wave background, the NANOGrav team expects that future observations should allow us to disentangle the source of the background hum and even pinpoint individual supermassive black hole binaries for further study. It’ll be fascinating to watch the frontier of this new field advance in leaps and bounds!

For more information, check out the Focus Issue in the Astrophysical Journal Letters here: Focus on NANOGrav’s 15 yr Data Set and the Gravitational Wave Background

Citation

“The NANOGrav 15-year Data Set: Evidence for a Gravitational-Wave Background,” Sarah Vigeland et al 2023 ApJL 951 L8. doi:10.3847/2041-8213/acdac6

“The NANOGrav 15-year Data Set: Observations and Timing of 68 Millisecond Pulsars,” Joseph Swiggum et al 2023 ApJL 951 L9. doi:10.3847/2041-8213/acda9a

“The NANOGrav 15-Year Data Set: Detector Characterization and Noise Budget,” Jeffrey Hazboun et al 2023 ApJL 951 L10. doi:10.3847/2041-8213/acda88

“The NANOGrav 15-year Data Set: Search for Signals from New Physics,” Andrea Mitridate et al 2023 ApJL 951 L11. doi:10.3847/2041-8213/acdc91

An image of a large, bright white star, center, and a much fainter, smaller, red star at lower left.

Having caught the gigantic, golden eye of JWST several times now, brown dwarfs are having a moment. A new article in The Astrophysical Journal Letters describes the latest planetary-mass companion to take the spotlight, and how our understanding of these mysterious objects is rapidly evolving.

A Popular Runt

Of all the known tiny, barely glowing “failed stars” known to astronomers as brown dwarfs, TWA 27B stands out as a particularly rich target for a number of reasons. As a member of the TW Hydrae association, the youngest group of stars within the nearest few hundred light-years of the Sun, TWA 27B is both exceptionally young and helpfully close by. Also, as a product of the earliest successful campaign to directly image small companions of larger objects, it has the longest track record of observations that can compared with new measurements. Acting on these temptations, JWST took a look at TWA 27B and its slightly larger partner, TWA 27A, in February of this year.

A six-panel arrangement of images, each of which shows close-up images of what looks like two stars. In the bottom row, the brighter star is partially subtracted away, emphasizing the dimmer one.

Images of the TWA 27 system created using three different grisms aboard the NIRSpec instrument. The top row shows both objects, while the bottom shows an enhanced view of TWA 27B created by subtracting out its brighter companion. [Luhman et al. 2023]

Methane-Free Zone

For about half an hour, JWST aimed at the TWA 27 system and dutifully collected photons with its onboard Near-Infrared Spectrograph (NIRSpec) instrument across three different wavelength ranges. Piecing the data together back on the ground, a team led by Kevin Luhman, Pennsylvania State University, revealed the atmosphere of a 10 million years young, 5–6-Jupiter-mass object, interestingly devoid of any methane and with only a small whiff of carbon monoxide. Both of these chemical species are common in older brown dwarfs but seem to vanish among younger ones due to non-equilibrium chemistry — a recently noted trend that these observations bolster.

 

A flux vs. wavelength plot, spanning 1 to 5 microns on the X axis and 0-1 on the Y axis, in units of (10^(-17) times erg per cm squared per second per angstrom). The data are shown in black, while the best fitting model is overplotted in red. The model traces the largest features of the data quite well.

The full 1–5-micron spectrum obtained of TWA 27B, compared to the best-fitting model spectrum derived from a simulation of a cloudless atmosphere. Although the model qualitatively follows the spectrum well, it overpredicts both the strength of the methane absorption and the object’s temperature. The discrepancies might be corrected with a more complex model that includes clouds. [Luhman et al. 2023]

Hungry Brown Dwarf?

Tantalizingly, the team also noticed that while their spectra did not indicate the smoking-gun signatures of a large circumstellar disk (an apparent “excess” of infrared emission), they did reveal that TWA 27B was emitting at specific wavelengths usually associated with accretion. This raises the possibility that the object is still growing slowly, and that it’s ringed by a tiny disk never before inferred over the long and distinguished trail of previous studies.

Thankfully, this cliffhanger provides a possible resolution: if such a disk exists, it will be obvious in observations taken at longer wavelengths. JWST, once again demonstrating its abilities to see what has never before been seen, has already taken these measurements. Data collected with its Mid-Infrared Instrument (MIRI) are being processed now, meaning the flood of unprecedented observations and accompanying discoveries about brown dwarfs is unlikely to stop soon.

Citation

“JWST/NIRSpec Observations of the Planetary Mass Companion TWA 27B,” K. L. Luhman et al 2023 ApJL 949 L36. doi:10.3847/2041-8213/acd635

Illustration of a gaseous exoplanet orbiting a Sun-like star

Researchers have measured the alignment between a sub-Saturn-mass exoplanet and its massive host star for the first time. This measurement, which shows that the planet is misaligned relative to its host star’s spin, prompted a new theory for how misalignment arises in planetary systems.

Stellar Spin vs. Planetary Orbit

illustration of spin–orbit alignment and misalignment

Star–planet systems demonstrating spin–orbit alignment (top) and spin–orbit misalignment (bottom). [AAS Nova/Kerry Hensley]

Observations of exoplanetary systems have revealed a wide variety of orbital geometries, including planets that orbit in a completely different direction than their host stars are spinning. Researchers have measured the spin–orbit misalignment for dozens of hot Jupiter exoplanets and unearthed a trend: hot Jupiters orbiting small, cool stars are aligned with their stars’ spins, while hot Jupiters orbiting larger, hotter stars can be either aligned or misaligned.

This trend in spin–orbit misalignment hints at how these planetary systems formed and evolved. Previously, researchers suggested that hot Jupiters around small, cool stars tend to be aligned because tidal forces acting on the star’s atmosphere cause it to reorient its spin to align with the orbit of the planet. Larger, hotter stars are less susceptible to this effect, making misaligned hot Jupiters in these systems more common. This seems to tidily explain what’s going on for massive planets, but what about smaller planets?

An animation of the Rossiter–McLaughlin effect

An animation of the Rossiter–McLaughlin effect. [Wikipedia user Amitchell125; CC BY-SA 4.0]

Into a New Domain

In a recent research article, a team led by Kyle Hixenbaugh (Indiana University) measured the spin–orbit misalignment between TOI-1842, a 1.45-solar-mass star, and its planet that is slightly less massive than Saturn. This is the first time this measurement has been made for a sub-Saturn-mass planet around a high-mass star (here, high mass means more than 1.2 solar masses).

To determine TOI-1842’s spin, the team observed the slight change in the star’s apparent radial velocity as the planet passed in front of the oncoming and receding sides of the star, known as the Rossiter–McLaughlin effect. Using models to simulate the star’s light curves and radial velocity measurements, the team found that the planet is considerably misaligned from its star.

Making Sense of Misalignment

Based on this measurement, Hixenbaugh and collaborators proposed a new framework to explain the observed trends in spin–orbit misalignment. Instead of tidal dissipation being the driving force, the team proposed that the number of planets in the system plays a defining role. Because the mass of a protoplanetary disk increases with the mass of the star, low-mass stars are likely only able to form a single Jupiter-mass planet, while higher-mass stars have enough material to create multiple Jupiter-mass planets. A single planet around a low-mass star thus forms in a calm environment and is likely to remain aligned to its star, while a planet with multiple Jupiter-mass siblings is knocked off kilter by gravitational interactions.

plot showing alignment versus misalignment for several exoplanetary system

Planetary mass versus stellar mass for systems for which the spin–orbit angle has been measured. Aligned systems are plotted as open circles and misaligned systems are plotted as filled triangles. Click to enlarge. [Hixenbaugh et al. 2023]

While low-mass stars can likely only form a single Jupiter-mass planet, they have enough material to form multiple Saturn-mass or sub-Saturn-mass planets. This means that a sub-Saturn-mass planet around a small, cool star could experience gravitational kicks and nudges from its siblings, knocking it out of alignment, while a Jupiter-mass planet could not. A sub-Saturn-mass planet around a large, hot star is also likely to have multiple planetary siblings and be misaligned. This framework is consistent with observations, though it doesn’t exclude the possibility that tidal forces play a role as well. Hixenbaugh’s team notes that more observations are needed to test the new hypothesis.

Citation

“The Spin–Orbit Misalignment of TOI-1842b: The First Measurement of the Rossiter–McLaughlin Effect for a Warm Sub-Saturn around a Massive Star,” Kyle Hixenbaugh et al 2023 ApJL 949 L35. doi:10.3847/2041-8213/acd6f5

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