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photograph of scientists examining new JWST images

How do galaxies grow? It’s a simple question, but answering it is complicated. A recent publication suggests that JWST observations might upend what we think we know about galaxy growth.

Evolution Seen from Afar

comparison of Hubble and JWST image of the Pillars of Creation

Comparison of Hubble (left) and JWST (right) images of the Pillars of Creation. The different wavelength ranges spanned by Hubble and JWST have the potential to illuminate different aspects of many cosmic settings. [Science: NASA, ESA, CSA, STScI, Hubble Heritage Project (STScI, AURA); Image processing: Joseph DePasquale (STScI), Anton M. Koekemoer (STScI), Alyssa Pagan (STScI)]

From our vantage point in the local universe, it’s hard to tell how the galaxies we see today evolved into their current forms. To understand how galaxies grow, we need high-resolution observations of galaxies billions of light-years away, and a succession of increasingly precise space telescopes have made these measurements possible.

Observations with the Hubble Space Telescope helped establish some fundamental rules of galaxy growth: galaxies were smaller in the past than they are today, galaxies that are more massive are usually larger, and galaxies that are actively forming stars are larger than those that are not. But constraints set by Hubble’s observing wavelength range might mean that these rules are due for a reassessment, and JWST is poised to put them to the test.

JWST Enters the Scene

When Hubble observes galaxies in the early universe, it’s seeing light emitted at optical and ultraviolet wavelengths, while JWST sees light that originated in the near-infrared. This small difference might have a big impact: near-infrared light is a better tracer of stellar mass than optical or ultraviolet light, and it’s less sensitive to spatial changes in the mass-to-light ratio seen in some galaxies. Ultimately, when combined with its exceptional resolution, this means that JWST should provide more reliable measurements of galaxy sizes than other telescopes.

Using data from the JWST Cosmic Evolution Early Release Science (CEERS) program, Katherine Suess (University of California, Santa Cruz, and Stanford University) and collaborators studied galaxies during an era commonly nicknamed cosmic noon, which is marked by an abundance of star formation. The team’s goal was to determine the sizes of galaxies during this epoch at two different wavelengths (1.5 and 4.4 microns; 1 micron = 10-6 meter) that correspond to light emitted in the optical and near-infrared, respectively.

Hubble image of the Extended Groth Strip region of the sky

The galaxies surveyed are situated in the Extended Groth Strip, which was previously observed by Hubble. Click to enlarge. [NASA, ESA, and M. Davis (University of California, Berkeley)]

How Do They Measure Up?

comparison of galaxy sizes in two JWST wavelength bands

Comparison of galaxy sizes in 4.4-micron JWST images to 1.5-micron JWST images. [Suess et al. 2022]

Suess and coauthors selected 1,179 bright galaxies with redshift, z, between 1.0 and 2.5 and used a computer algorithm to measure the sizes of these galaxies in the 1.5- and 4.4-micron JWST images. For the 703 galaxies successfully fit by this method, there was a definite size difference between the two wavelengths: the galaxies were, on average, 9% smaller in the 4.4-micron images than in the 1.5-micron images. This means that galaxies are more compact than rest-frame optical observations (e.g., Hubble observations) would suggest. Intriguingly, the difference in size between the two wavelengths appears to be a function of galaxy mass and color — the lightest, bluest galaxies surveyed scarcely show a size change, while those that are redder and more massive show a 30% size decrement at the longer wavelength.

This seemingly straightforward finding might play a role in rewriting the rules of galaxy evolution. For instance, the mass-dependent size decrease between the 1.5- and 4.4-micron images might mean that massive galaxies aren’t actually much larger than their lighter counterparts! While the authors stress that there’s more analysis to be done, it’s clear that JWST observations will have an outsize impact on our understanding of galaxy growth.

Citation

“Rest-frame Near-infrared Sizes of Galaxies at Cosmic Noon: Objects in JWST’s Mirror Are Smaller than They Appeared,” Katherine A. Suess et al 2022 ApJL 937 L33. doi:10.3847/2041-8213/ac8e06

Illustration of a neutron star emitting a jet

When a massive star explodes as a supernova, its core collapses into a city-sized sphere of neutrons called a neutron star. These extraordinarily dense stars — just one teaspoon of a neutron star would weigh billions of tons in Earth’s gravity — exhibit some of the most intriguing behavior in the universe: rapid rotation, beams of radio emission, and extremely strong magnetic fields. Today, we’ll introduce four recent research articles that explore different aspects of these stars.

Bursting, Cooling, and Bursting Again

simulated light curves showing the results of different simulations

Simulated light curves during an X-ray burst, showing the effects of incorporating different physics. A model without neutrino cooling (labeled “No DU” in reference to the neutrino cooling pathway called direct Urca), peaks at a lower luminosity than models incorporating neutrino cooling. [Adapted from Dohi et al. 2022]

Sometimes, neutron stars reveal themselves by interacting with other stars. When a neutron star gathers gas from a stellar companion, the gas can ignite on the star’s scorching surface, resulting in a sudden burst of X-rays. After this sudden influx of heat, how does the neutron star cool, and how is the cooling reflected in the star’s light curve? While this may seem like a simple question, the answer hinges on our understanding of the conditions within the neutron star’s interior as well as the characteristics of the gas being accreted.

In a recent publication, a team led by Akira Dohi (土肥明; Kyushu University, Japan) explored the issue of neutron star cooling with general relativistic stellar evolution models. Specifically, the team investigated the effects of cooling by emitting neutrinos — chargeless, nearly massless particles that scarcely interact with matter — which is expected to speed up the cooling rate. The authors found that neutrino cooling increases the time between outbursts but makes them brighter at their peak, though additional physics to be included in future modeling might suppress this effect.

demonstration of subpulse drifting in simulated pulses

Simulated pulses showing a change in the phase of the pulse due to the shifting motion of the sparks. [Adapted from Basu et al. 2022]

Simulating Pulsar Sparks

Rahul Basu (University of Zielona Góra, Poland) and collaborators reported on simulations of conditions very close to the surface of a neutron star that emits beams of radio emission. Neutron stars that emit beamed radio waves are called pulsars for the way the beams sweep across our field of view, generating what we see as pulses of emission. Near a pulsar’s surface, extremely high temperatures and strong magnetic and electric fields combine forces to summon a sea of charged particles that are then accelerated to relativistic speeds.

Basu and collaborators focused on a phenomenon called sparking, in which charged particles jump the gap between the pulsar’s surface at its poles and its plasma-rich magnetosphere. The team’s modeling demonstrated that a pulsar’s poles are tightly filled with constant sparks, and the arrangement of these sparks slowly shifts over time. By modeling the emission associated with the simulated sparks, the team showed that the shifting motion of the sparks appears to be responsible for the observed periodic variations in the phases and amplitudes of some pulsars’ pulses.

Pulsars Probing Gravitational Waves

example of a pulsar radio pulse

Example of a pulse observed with the Giant Metrewave Radio Telescope. [Adapted from Sharma et al. 2022]

By studying large groups of pulsars, astronomers hope to learn about something seemingly unrelated: gravitational waves. Pulsars provide a method to detect gravitational waves by way of these stars’ impeccable timekeeping abilities — because a pulsar’s radio beat is so reliable, the slight distortion of space caused by a passing gravitational wave should impact the arrival times of a pulsar’s pulses.

However, there’s a complication to this technique: spatial and temporal changes in the interstellar medium plasma can also affect when a pulsar’s radio pulses arrive at Earth. In order to compensate for the effect of the interstellar medium, we need to be able to make precise observations of pulsars across a range of radio frequencies. In a recent research article, Shyam Sharma (Tata Institute of Fundamental Research, India) and collaborators tested a pulsar-timing measurement technique using the Giant Metrewave Radio Telescope, which is highly sensitive to low-frequency radio waves. Sharma and coauthors showed that observing using a wide frequency band yields results comparable to typical narrowband observations, indicating that this technique could be used to disentangle the effects of the interstellar medium and more accurately time the pulses of arrays of pulsars, opening a new window onto gravitational waves.

simulated magnetar temperature maps

Temperature maps of the top of a magnetar’s crust (top) and the magnetar’s surface (bottom) after a hotspot is injected. [De Grandis et al. 2022]

Magnetic Outbursts

As if neutron stars could get any wilder: some neutron stars, dubbed magnetars, have extremely strong magnetic fields and exhibit frequent X-ray flares. While the cause of these X-ray outbursts is still unknown, some researchers have suggested that they arise from a sudden upwelling of magnetic energy beneath the magnetar’s crust, creating a hot spot that cools gradually over days or months.

To understand how the injection of heat into a magnetar’s crust might create the spectral features seen during X-ray outbursts, Davide De Grandis (University of Padova, Italy) and coauthors employed a three-dimensional magnetothermal model of hotspot formation and cooling. This model allowed the team to study the effects of asymmetrical hot spots under a magnetar’s crust for the first time. The team was able to confirm that these hot spots can be responsible for outbursts, though we’ll have to wait for future research to fully explore the evolution of the spectral features generated during these events.

Citation

“Impacts of the Direct Urca and Superfluidity inside a Neutron Star on Type I X-Ray Bursts and X-Ray Superbursts,” A. Dohi et al 2022 ApJ 937 124. doi:10.3847/1538-4357/ac8dfe

“Two-dimensional Configuration and Temporal Evolution of Spark Discharges in Pulsars,” Rahul Basu et al 2022 ApJ 936 35. doi:10.3847/1538-4357/ac8479

“Wide-band Timing of GMRT-discovered Millisecond Pulsars,” Shyam S. Sharma et al 2022 ApJ 936 86. doi:10.3847/1538-4357/ac86d8

“Three-dimensional Magnetothermal Simulations of Magnetar Outbursts,” Davide De Grandis et al 2022 ApJ 936 99. doi:10.3847/1538-4357/ac8797

A image of a globular cluster taken by the Hubble Space Telescope.

There are light curves, then there are light curves. Recently, astronomers untangled a particularly complex signal and revealed its surprisingly elegant cause: not one, two, or even three, but four stars locked into a never-ending dance.

A Mystery Light Curve

Over the past 20 years, astronomers have piled up quite the hoard of stellar light curves. Most of these are predictable, fairly simple time series: sure, the brightness of a given star might oscillate pseudo-regularly around its average as star spots come and go or the star shrinks and swells, but by and large, most curves don’t reveal anything surprising. A handful of exciting curves shelter the telltale signature of a transiting planet. Another handful hosts the unfortunately similar signature of eclipsing binary stars. Almost all of them can be explained by fairly simple models, just one or two stars and their planets going about their usual lives.

A three panel plot, each of which show brightness over time. In the top panel, the brightness changes dramatically and seemingly randomly, but in the lower two it oscillates regularly with a small amplitude

Measurements of TIC 114936199’s brightness over time, as observed in three different TESS sectors. Note the changes to the y-axis scale after the first sector. [Powell et al. 2022]

However, a few light curves among the hundreds of thousands recorded so far are bafflingly weird. Take the measurements of the source named TIC 114936199, which the Transiting Exoplanet Survey Satellite (TESS) watched in three disconnected chunks of about 30 days.

The second and third of these chunks look like a standard eclipsing binary. But that first stare… What could cause such deep, non-repeating dips?

To find out, a team led by Brian Powell (NASA Goddard Space Flight Center) started looking into arrangements of more and more eclipsing stars to explain how nature could create such strange dips in the first sector but not the others. A few clues led them to consider assemblages of four stars, which had been spotted by TESS before. But actually describing the size, location, and velocities of those four stars proved quite the challenge.

Fitting Challenges

A top-down schematic of the star system. Stars are represented as dots, and their motion is denoted by arrows. Stars Aa and Ab circle each other, star B circles the A pair, and star C circles the inner three.

A cartoon illustration of TIC 114936199’s components: four stars all bound together. The deep eclipses were the result of stars Aa and Ab passing in front of star C at the same time. [Powell et al. 2022]

 

Wandering the landscape of such a broad parameter space, Powell and collaborators found that standard Monte Carlo fitting routines got lost in the hills of local minima and could not find a reasonable solution. The team first increased their computational firepower by switching to a NASA supercomputer, then they deployed other algorithms such as Particle Swarm Optimization and Differential Evolution. In this exploration phase, the team churned through millions of possible combinations over many hundreds of thousands of computing hours.

All of this effort got them within the ballpark of a reasonable solution, and when they sensed they were close, the team again unleashed their fitting algorithm. This time, it made a beeline for their final solution, a configuration of four stars circling three different centers.

Successful Solution

A plot showing brightness over time of the first TESS sector. The data is shown as blue dots, and the final model is shown as a red line. The model faithfully follows each of the many dips in the data.

A zoom-in of data from the first TESS sector (blue) compared with the final model (red line). The residuals are shown below the time series. [Powell et al. 2022]

The model precisely predicted every one of the many dips in the intricate pattern in the first TESS sector, and it successfully explained why the pattern did not repeat: the innermost stars, Aa and Ab, eclipse each other every 3 days, but for 12 days in that first sector, they both also occasionally eclipsed star C as it drifted through the background. That particular arrangement should happen again in 2025, but after that we’ll have to wait much longer for the next lineup in 2071. While this isn’t the first quadruple star system observed by TESS, it is the first in this 2+1+1 configuration. Hopefully, astronomers will be able to observe the next series of complex eclipses in three years’ time — if not, they’ll have to wait a half century before enjoying such a dramatic show again.

Citation

“TIC 114936199: A Quadruple Star System with a 12 Day Outer-orbit Eclipse,” Brian P. Powell et al 2022 ApJ. 938 133 doi:10.3847/1538-4357/ac8934

Artist's impression of a young planetary system

A recent study suggests that protoplanetary disks may tend to linger longer than we thought, meaning that planets likely have at least 5 million years to form before their building materials vanish.

Disk Dispersal Deadlines

illustration of a protoplanetary disk being evaporated by a nearby massive star

One way that protoplanetary disks are dispersed is by radiation and winds from massive stars, as shown in this illustration. [NASA/JPL-Caltech]

Planets arise from gaseous disks called protoplanetary disks. While the details of planet formation are hidden from view within these dusty disks, the big picture is clear: the timeline for planet formation is set by the lifetime of the disk — once the disk disperses, planet formation must come to a halt. Determining how long planets have to form should be a simple task, then: researchers can measure the ages of star clusters and determine whether the stars in those clusters have disks, thus establishing a cutoff point at which disks typically disperse.

In reality, however, this technique has produced a wide range of estimates for the lifetimes of protoplanetary disks, and thus widely varying constraints on how long planets have to form — and the shortest estimates, in the 1.0–3.5 million year range, set a tight deadline for models of planet formation to meet.

Plot of disk fraction as a function of cluster age and distance

Fraction of stars with disks as a function of cluster age and distance. More distant clusters tend to have smaller disk fractions. [Adapted from Pfalzner et al. 2022]

Young, Massive, and Misleading?

In a recent publication, a team led by Susanne Pfalzner (Jülich Supercomputing Center and Max Planck Institute for Radio Astronomy, Germany) suggested that careful application of existing techniques can provide a little more wiggle room for modelers, lengthening the typical lifetime of a protoplanetary disk. Researchers often study disks around stars in clusters, since it’s more straightforward to determine their ages than stars outside of clusters. However, it’s easier to identify young, compact clusters than it is to find old, dispersed clusters, especially at large distances from Earth. Since bright, massive stars are easier to detect at large distances, studies biased toward younger clusters are also biased toward more massive stars — which are known to have shorter-lived disks.

As a demonstration of this effect, Pfalzner and coauthors examined how the results of previous studies varied with the properties of the clusters in each study’s sample. They found that samples containing mostly distant (>650 light-years away), young clusters resulted in short estimates for disk lifetimes, while samples containing nearby, old clusters were linked to long disk lifetimes.

Modelers Everywhere Breathe a Sigh of Relief

Plot showing the effect of stellar mass and initial disk fraction on the median disk lifetime in star clusters

The effect of stellar mass and initial disk fraction (IDF) on the median disk lifetime in star clusters. [Adapted from Pfalzner et al. 2022]

To counteract this issue, the team constructed a new sample that is evenly balanced between young and old clusters that are located within 650 light-years of Earth. Analysis of this sample suggested a median disk lifetime of 6.5 million years, with a substantial fraction of disks enduring for 10–20 million years — meaning that in many star systems, planets have far longer to form than expected.

While this result provides much-needed leeway for our models of planet formation, there are still plenty of open questions to explore. For example, it’s important to pin down the fraction of stars that are born with disks; assuming that all stars are initially shrouded in disks implies typical disk lifetimes in the 5–6 million year range, while allowing for a small fraction of stars to be born diskless would allow planets 8–10 million years to form around low-mass stars and 4–5 million years to form around high-mass stars. Regardless of the exact timeframe, understanding how high-mass stars form planets under stricter timescales than low-mass stars will remain a challenging question to answer.

Citation

“Most Planets Might Have More than 5 Myr of Time to Form,” Susanne Pfalzner et al 2022 ApJL 939 L10. doi:10.3847/2041-8213/ac9839

multiwavelength image of the galactic center region

The massive young stars at the center of the Milky Way have long puzzled astronomers. Did these stars form in a supermassive black hole’s backyard, or did they travel to the galactic center after their birth? In a recent publication, researchers proposed an entirely new scenario, in which the demise of one star prompts the formation of many more.

Born in a Black Hole’s Shadow

diagrams indicating the locations of stars within the disks

Locations of stars belonging to the putative clockwise and counter-clockwise disks at the center of the Milky Way. Distances are given in arcseconds. Click to enlarge. [Paumard et al. 2006]

At the center of our galaxy, massive young stars trace tight orbits around a supermassive black hole. When researchers charted these stars’ paths, a curious pattern emerged: several dozen stars were arrayed in one or more narrow disks that are off kilter from the plane of the galaxy.

The presence of young stars in the hostile inner regions of our galaxy is mysterious enough, and this arrangement is even more perplexing. It’s not yet clear how stars might form so close to a black hole — the tidal forces should prevent new stars from coalescing — and the disk-like arrangement shouldn’t arise naturally if the stars migrated from elsewhere in the galaxy. How did these stars come to be where they are?

diagram of the authors' jet–cocoon system

Diagram of the jet–cocoon configuration. Because the pressure within the cocoon exceeds the pressure of the surrounding gas clouds, the cocoon sweeps outward and compresses the clouds nearby. [Perna & Evgeni 2022]

From Disruption to Disk Formation

Rosalba Perna (Stony Brook University and Flatiron Institute) and Evgeni Grishin (Monash University and Australian Research Council Centre of Excellence for Gravitational Wave Discovery) proposed that when a star is tidally disrupted by a supermassive black hole, it creates the conditions for new stars to form.

Here’s how it works: in rare cases, as a star is being pulled apart by a black hole, it shoots out a jet of material. A cocoon of gas enclosing this jet expands perpendicular to the jet, compressing the surrounding gas and providing enough pressure for gas clumps to overcome the black hole’s tidal pull and form new stars.

While the expanding cocoon spurs star formation perpendicular to the jet, the jet itself creates a cone of superheated gas that suppresses star formation along its length. The combination of these factors promotes star formation in a thin disk, and the orientation of the disk is linked to the orbit of the disrupted star. In other words, the team expects that a tidally disrupted star will lead to a disk of stars forming at a random angle with respect to the galactic plane — exactly the arrangement we see at the center of the Milky Way.

More to Learn

artist's impression of a tidal disruption event

An artist’s impression of a tidal disruption event. [ESA/C. Carreau]

The Milky Way’s supermassive black hole likely tidally disrupts a star once every 10,000–100,000 years, with jetted tidal disruption events occurring every 1–10 million years. Why, then, have we only found evidence for two misaligned disks of stars at the galactic center? Because this method tends to form massive stars, the disks should disappear quickly; stars formed in the wake of a tidal disruption event would survive only a few million years.

While this scenario laid out by Perna and Grishin appears to answer many of the questions regarding the stars arrayed in disks near the center of the Milky Way, the authors acknowledged that their hypothesis needs to be tested thoroughly. Hopefully, future numerical simulations will help us close in on the formation mechanism for these galactic center stars!

Citation

“Disks of Stars in the Galactic Center Triggered by Tidal Disruption Events,” Rosalba Perna and Evgeni Grishin 2022 ApJL 939 L17. doi:10.3847/2041-8213/ac99d8

An artist's impression of an X-ray burst

When a neutron star snares material from a stellar companion, we see a flash of X-rays called an X-ray burst. What can an analysis of 51 bursts from a single source tell us about the physics behind these events?

Bursting Binary Systems

photograph of the NICER telescope on the International Space Station

A view of the Neutron star Interior Composition Explorer (NICER), seen at the center of this image, in its berth on the International Space Station. [NASA]

Binary systems containing a neutron star — the extremely dense core of an expired massive star — and a main-sequence, supergiant, or white dwarf star are called X-ray binaries for the short bursts of X-rays they emit. These outbursts are thought to arise when the neutron star accretes gas from its stellar companion, forming an accretion disk from which the neutron star siphons a stream of material that ignites in a brief flash of nuclear fusion. Studying X-ray bursts allows researchers to pin down the properties of neutron stars and understand the physics that governs accreted gas.

One of our best tools for studying these bursts is the Neutron star Interior Composition Explorer (NICER), which has monitored X-rays from its vantage point on the International Space Station since 2017. Among NICER’s many targets is the highly active binary 4U 1636–536, which was discovered just over 50 years ago. Researchers have cataloged hundreds of X-ray bursts from 4U 1636–536, finding that it averages one burst every four hours!

example of an X-ray burst

An example of an X-ray burst from 4U 1636–536 as seen by NICER. [Adapted from Güver et al. 2022]

Accretion Increases and Disk Reflections

In a recent publication, a team led by Tolga Güver (Istanbul University) searched for evidence of additional X-ray bursts from 4U 1636–536 during a monitoring campaign with NICER. Güver and collaborators identified 51 X-ray bursts during 138 observations and collected spectra for 40 of them, allowing the team to characterize 4U 1636–536’s bursting behavior and understand how X-ray bursts affect their surroundings.

Güver and collaborators found that all of the bursts for which they acquired spectra had an excess of soft (i.e., low-energy) X-ray emission. Modeling of this spectral feature indicated that it likely arises from either an increase in the rate at which matter is accreted onto the neutron star or from the burst scattering off the disk and/or being absorbed and re-emitted at a different wavelength, a process referred to as reflection. However, many of the bursts were fit well by models of both scenarios, and the authors pointed out that both processes likely occur simultaneously.

Further X-ray Investigations

To learn even more about 4U 1636–536’s frequent outbursts, Güver and collaborators analyzed data from India’s multi-wavelength space telescope AstroSat and the Nuclear Spectroscopic Telescope Array (NuSTAR). Using NuSTAR data, the team searched for evidence of Compton cooling, in which high-energy photons lose some of their energy through collisions with nearby electrons. The team discovered decreases in the hard (i.e., high-energy) X-ray emission shortly after the onset of several bursts, but the low count rate prevented a firm detection.

chi-squared distributions for the two models tested on the NICER data

Comparison of reduced χ2 values for best fits to the NICER spectra using the disk reflection model (blue) and the increased accretion model (red). [Güver et al. 2022]

The authors also used observations of several bursts made by AstroSat and NuSTAR to probe the causes of the excess soft X-ray emission further. Similar to their investigations of the NICER spectra, the team found that they could fit the spectra with either a disk reflection model or an increased accretion model — but simultaneously modeling both of these effects will require brighter X-ray bursts or a larger telescope.

Citation

“Burst–Disk Interaction in 4U 1636–536 as Observed by NICER,” Tolga Güver et al 2022 ApJ 935 154. doi:10.3847/1538-4357/ac8106

Three side by side images with the same galaxy in the center. In the center panel, a supernova is visible to the right of the galaxy.

Why did a supernova observed back in 2021 initially emit so much ultraviolet light, then abruptly shift color? A recent study suggests a change in ejecta velocity, a cosmic speed bump, may be the culprit.

Brief Flashes

If you want to study the moments after a white dwarf explodes, speed is key. Each view of these violent ends, dubbed Type Ia supernovae, is temporary: we see a point of light grow brighter, then fainter over the course of months, and many of the interesting science questions can only be answered by catching the earliest moments after the eruption. This is an extremely difficult task considering that no one knows where a Type Ia supernova might go off, and consequently astronomers must scan the entire sky looking for tiny pinpricks of light that weren’t there before.

Although the adoption of automated telescopes in recent years has made this continuous needle-in-a-haystack search easier, it is still rare for astronomers to collect thorough measurements of a supernova many days before it reaches peak brightness. Recently, however, a team led by Chris Ashall (University of Hawaiʻi at Manoa) did just that, and what they found was somewhat surprising.

Too Blue

A two panel plot, both showing time on the x. The bottom panel y axis is B-V color, and the top panel y axis is normalized flux.

Top: the u-band photometry of SN 2021aefx measured in days until peak brightness, along with several possible fits. Bottom: the color evolution of SN 2021aefx, which reverses direction about 16 days before maximum brightness. Another supernova that followed a similarly non-monotonic path, SN 2018aoz, is included for comparison. [Adapted from Ashall et al. 2022]

The team’s target, named SN 2021aefx, initially emitted lots of ultraviolet light (u-band light, in observational astronomy parlance). However, in the first days after the explosion as the remnants grew brighter across all wavelengths, the growth of the u-band light didn’t follow the expected t2 power law. Instead, after its strong start, the ultraviolet emission stumbled: it still grew brighter, but followed a gentler two-component model with two different power indices. While the u-band emission faltered, all other wavelengths grew as expected, and this relative difference in growth rates changed color of the supernova soon after initial detection.

To figure out why this offbeat supernova generated so much ultraviolet light before changing its tune, the team collected spectra as well as multiband photometry. These data revealed that the supernova ejecta had a very high initial velocity of about 30,000 km/s 17 days before peak brightness, but that it slowed to 21,000 km/s less than two days later.

Slow It Down

A plot of flux vs. wavelength. The initial spectrum is duplicated many times and blueshifted to different amounts to demonstrate that a velocity change could alter the amount of light in the u band. The bandpasses of the u and B bands are shown behind the spectra.

A spectrum of SN 2021aefx taken 17.3 days before maximum brightness, artificially blueshifted by different amounts. The u and B bandpasses are marked in blue and red, respectively. Note on the inset that blueshifts affect the observed flux within the u band, but only weakly affect the B-band values. [Adapted from Ashall et al. 2022]

That speed change, combined with the difference in spectral shape across the u and B bands, was key to the authors’ explanation of the excess ultraviolet light. By artificially adjusting the velocity of their spectrum, Ashall and collaborators demonstrated that they could blueshift their way to a higher u-band measurement without affecting their B-band values. Putting it all together, the team claimed that the guts of this star hit a speed bump: when the ejecta were bolting out of the gate, more of their emitted light fell into the u band, but after abruptly slowing down, their emission tumbled back into the B band.

This study only focused on observations between 17 and 6 days before peak brightness, though the authors plan to publish fuller light curves they acquired. In the meantime, we’ll have to wonder about any other obstacles these supernova remnants might face on their journey through the universe.

Citation

“A Speed Bump: SN 2021aefx Shows that Doppler Shift Alone Can Explain Early Excess Blue Flux in Some Type Ia Supernovae,” C. Ashall et al 2022 ApJL 932 L2. doi:10.3847/2041-8213/ac738c

white-light coronagraph image of a coronal mass ejection

The Sun launches tangled masses of plasma and magnetic fields in the form of coronal mass ejections. But these explosions don’t occur in a (literal or metaphorical) vacuum — how is the passage of a coronal mass ejection affected by the eruptions that preceded it?

Capturing Coronal Mass Ejections

Illustration of the sun setting off a coronal mass ejection headed to Earth and it hitting Earth's magnetosphere

Illustration of a coronal mass ejection headed toward Earth. [SOHO/LASCO/EIT (ESA & NASA)]

When the Sun’s activity cycle ramps up to its maximum, as it will in 2025, the Sun will unleash two or three coronal mass ejections every day. These explosions blast out into the solar system, speeding toward Earth and the other planets at hundreds of kilometers a second. When a coronal mass ejection collides with Earth’s protective magnetic field, the ensuing magnetic tussle can fling high-energy particles into Earth’s atmosphere, creating the aurora — and potentially damaging spacecraft electronics.

Given the risk of damaging effects on Earth-orbiting spacecraft, researchers have developed models to predict the path a coronal mass ejection will take after detaching from the Sun. These models estimate how a loop of magnetized plasma twists, expands, and is deflected as it travels through the tenuous swirls of the solar wind. However, many such models trade accuracy for speed in order to quickly assess the danger to Earth, failing to capture the variable and turbulent nature of the space between Sun and Earth, which may affect how coronal mass ejections travel through that space.

A Series of Solar Events

A team led by Chin-Chun Wu (Naval Research Laboratory) used magnetohydrodynamics simulations to explore how a coronal mass ejection moves through the wake left by previous solar eruptions. Wu and collaborators opted to model a series of five coronal mass ejections that occurred in two and a half weeks in July 2012, for which we have extensive data.

Modeled radial solar wind speed for four time periods

Modeled radial solar wind speed for four time periods. The red areas indicate coronal mass ejections. Click to enlarge. [Adapted from Wu et al. 2022; full time series available here]

The team’s model uses observations of these events to determine each coronal mass ejection’s initial speed, trajectory, and the time it departs the Sun. By solving fluid dynamics equations to understand how each event evolves over time, the model outputs key parameters like the plasma density and temperature, the speed of the background solar wind, and the magnetic field strength, all of which can be compared against measurements made by satellites.

Creating a Path to Follow

comparison of observed and modeled plasma parameters

Observed (black points) and modeled (red and purple lines) plasma parameters. The model results for a series of three coronal mass ejections are shown as red dotted lines, and the model results for a single coronal mass ejection are shown in purple lines. From top to bottom, the parameters shown are the radial velocity (vr), the plasma density (np), the plasma temperature (Tp), and the magnetic field strength (B). Click to enlarge. [Wu et al. 2022]

Previous work has suggested that when one coronal mass ejection closely follows another, the second event moves faster than it would otherwise. To test this theory, Wu and coauthors compared a model of a series of coronal mass ejections to another of just the final event in the series. These simulations showed that a coronal mass ejection following in the wake of other explosions travels faster than one forging ahead solo — the passage of a previous shock wave reduces the density and increases the speed of the solar wind, allowing the final coronal mass ejection to surf its way to Earth’s orbit 30 minutes faster.

Ultimately, the authors concluded that their model was able to match the observed parameters of the five coronal mass ejections fairly well. Their simulations allowed them to show that coronal mass ejections are affected by those that came before, suggesting that multiple events should be accounted for in modeling these eruptions.

Citation

“Magnetohydrodynamic Simulation of Multiple Coronal Mass Ejections: An Effect of Pre-events,'” Chin-Chun Wu et al 2022 ApJ 935 67. doi:10.3847/1538-4357/ac7f2a

An image of the TW Hydrae disk, zoomed out far enough that the entire disk is visible and a set of concentric dark rings neatly surround the central star.

The gorgeous, nearby, face-on TW Hydrae protoplanetary disk has proved challenging to quantify: just how much gas is sitting between those rings? A new study presents a possible answer and develops a new technique for addressing the question along the way.

Covert Disks

Much like babies cling to their blankets, young stars wrap themselves in disks of gas and dust when they’re very young. Although this state of infancy lasts only a short few million years, this temporary cosmic comforter is a profoundly important artifact of a star’s birth, at least from our earthly perspective. That’s because all planets, asteroids, and humans formed from this material: we are the worn remains of a structure long ago dispersed, clumps of matter that couldn’t get blown away in our Sun’s angsty teenage years when it scattered the rest of its protoplanetary disk.

Basic measurements such as how gas is distributed around young stars are therefore vital for our models of planet formation. Unfortunately, this measurement is notoriously tricky to make since the most dominant component of the disk, hydrogen gas, stubbornly refuses to emit light while at the temperatures in this environment. Most of astronomy relies on catching photons flung our way across the void, but if the hydrogen refuses to play catch, we can’t see it.

An image showing emission from the small region of sky surrounding TW Hydrae. Equal brightness contours appear as concentric rings centered on inner region of the disk.

Emission from dust in the TW Hydrae protoplanetary disk, colored by brightness temperature of the dust. Axes are in arcseconds. [Tsukagoshi et al. 2019, reproduced in Yoshida et al. 2022]

Luckily, although we cannot detect most of the disk, other components besides hydrogen are more amenable to electromagnetic communication. Dust, for example, glows even while embedded in the hydrogen, so we can measure its distribution and use it as a proxy for the gas distribution. However, this method relies on an assumed gas-to-dust ratio and profile, which isn’t always certain. Alternatively, various forms of CO also shine through the gas, but the ratio of CO to hydrogen is likely unique to each disk. As a result of these uncertainties, previous estimates of the total mass of the nearby TW Hydrae disk have spanned more than an order of magnitude.

 

Pressure Wings

A plot of flux vs. velocity shift. The data is a broad downward-opening V, while the comparison Gaussian profile is a much narrower V. The model closely follows the data.

An illustration of the pressure-broadened wings. The dashed line marks a typically used Gaussian shape, while the green line shows the best-fitting Voigt profile, which captures broadening. The data are shown in black, and the 3-sigma significance level is denoted by the dotted line. [Yoshida et al. 2022]

A new study led by Tomohiro C. Yoshida (National Astronomical Observatory of Japan; The Graduate University for Advanced Studies) develops a workaround to these challenges: rather than relying on a tracer, the team instead looked at the shape of CO lines at the inner region of the disk.

Typically, astronomers assume these lines follow a roughly Gaussian shape. However, Yoshida and collaborators found that in the inner regions of the disk, the “wings” of the lines were wider than expected. After considering various explanations for what might cause this broadening, they concluded that the wings are the result of higher pressures along the midplane of the disk.

This new method should be applicable to other disks moving forward, so astronomers can hope for similarly refined measurements in the near future.

Citation

“Discovery of Line Pressure Broadening and Direct Constraint on Gas Surface Density in a Protoplanetary Disk,” Tomohiro C. Yoshida et al 2022 ApJL 937 L14. doi:10.3847/2041-8213/ac903a

Cassini spacecraft image of the surface of Saturn's moon Enceladus

The icy moons of the outer solar system are a promising place to look for life beyond Earth, and a new research article shows that we already have the tools to start our search.

Biosignature Possibilities

Cassini image of plumes on Enceladus

An image of Enceladus’s water plumes taken by Cassini. The small moon has more than a hundred such geysers. [NASA/JPL/Space Science Institute]

While the search for life beyond Earth often focuses on our neighboring planet Mars or the promise of distant, Earth-like exoplanets, icy moons in our solar system have recently come to the forefront of this discussion. Researchers suspect that many of the moons in the outer solar system have oceans hidden beneath their icy or rocky crusts — and the plumes of water ice observed shooting from fissures in the surface of Saturn’s moon Enceladus may provide an excellent way to study one of these oceans directly.

If life is present in the oceans of Enceladus or other icy moons in the solar system, it’s possible that plumes could deposit life-signaling molecules — biosignatures — on the surfaces of these worlds. A recent publication tests our ability to detect a certain class of biosignature compounds: lipids.

chemical structure of cholecalciferol

The chemical structure of a lipid called cholecalciferol. Cholecalciferol is better known as vitamin D. [Wikipedia user Calvero; Public Domain]

Measuring Samples with Mass Spectrometry

Though the term lipids might not be familiar, the molecules themselves likely are: lipids include fats, waxes, and certain vitamins. Lipids are also important components of cell membranes, making them essential to all life on Earth. Their size and complexity mean they’re unlikely to form through simple chemistry, making them potential biosignatures. But how would we detect these molecules on another world?

Rather than attempt to detect these molecules from afar, we’ll likely need to sample them directly by sending an instrument down to the surface of the planet or moon we want to study. In a recent publication, a team led by Nikita Boeren (University of Bern, Switzerland) tested the abilities of a candidate lipid-detecting spectrometer called the ORganics Information Gathering INstrument (ORIGIN).

ORIGIN is a laser desorption ion mass spectrometer, a type of instrument that uses a pulse of laser light to remove (desorb) molecules from a surface and ionize them. These electrically charged molecules are then channeled toward a detector by electric and magnetic fields, and the time that each molecule takes to reach the detector is related to the ratio of the molecule’s mass to its electrical charge. Using the mass-to-charge ratio of the molecule and any fragments it might have split into, researchers can determine which molecules were present in the sample.

Looking for Lipids

Boeren and collaborators tested ORIGIN’s ability to detect pure samples of six varieties of lipids, as well as combinations of those same lipids with other organic molecules: amino acids (compounds often referred to as “the building blocks of life”) and polycyclic aromatic hydrocarbons (molecules containing rings of carbon atoms).

mass spectra for mixtures of different substances

Mass spectra for a mixture of six lipids (top), four lipids mixed with two amino acids (middle), and four lipids mixed with two polycyclic aromatic hydrocarbons (bottom). Click to enlarge. [Boeren et al. 2022]

The team’s results show that the instrument is capable of detecting and differentiating between several different lipid molecules, even when those substances were mixed with other compounds. While more work remains to be done, including testing the procedure on other lipids and exploring the effects of using different types of materials to hold the sample, ORIGIN already shows great promise as a way to detect biosignature compounds. In fact, the detection limits determined in this study meet the requirements of the Enceladus Orbilander, a high-priority mission proposed in the 2023–2032 Planetary Science Decadal Survey.

Citation

“Detecting Lipids on Planetary Surfaces with Laser Desorption Ionization Mass Spectrometry,” Nikita J. Boeren et al 2022 Planet. Sci. J. 3 241. doi:10.3847/PSJ/ac94bf

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