Astrobites RSS

Photograph of a meteor streaking across the night sky

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: On the Proposed Interstellar Origin of the USG 20140108 Fireball
Authors: Peter G. Brown and Jiří Borovička
First Author’s Institution: University of Western Ontario
Status: Published in ApJ

Since the unambiguous identification of ʻOumuamua, the first interstellar visitor to our solar system, popular interest in astronomy and astrobiology has exploded. Theories (however unlikely) that ʻOumuamua represented some artifact of an extraterrestrial intelligent civilization not only turned significant public attention to the study of small solar system bodies, but also that of so-called Unidentified Anomalous Phenomena (UAPs). Long speculated to be sightings of alien spacecraft, the study of UAPs is now being tackled by several groups, ranging from the Pentagon, to NASA, to private institutions. Perhaps the best-advertised of these is the Galileo Project, headed by Dr. Avi Loeb, lately Chair of Harvard’s Department of Astronomy, originator of the ʻOumuamua derelict spaceship theory, and second author of an article claiming the archival detection of an interstellar meteor (USG 20140108) from US government (USG) monitoring data.

ʻOumuamua’s Predecessor?

Most meteors we see come from orbits bound to the solar system. However fast these bolides (fiery meteors) appear and disappear in their starry streaks, they typically don’t fly fast enough to indicate unbound, extrasolar origins. USG 20140108 was originally detected by classified government sensors and released as part of the Center for Near-Earth Object Studies (CNEOS) Fireball Catalog, and its interstellar nature was proposed by Siraj and Loeb (2022) based on the government-measured velocity, about 45 km/s. The CNEOS data don’t come with uncertainties, so Siraj and Loeb rely on a Department of Defense statement that “the velocity estimate reported to NASA is sufficiently accurate to indicate an interstellar trajectory.” Armed with this letter, Siraj and Loeb confidently claimed the object to be interstellar in origin, and Loeb’s Galileo Project chartered an expedition to attempt to recover fragments of the bolide from the bottom of the ocean.

Independent Estimations

Rather than try to hunt down the original, classified source of the 2014 event, the authors of today’s article try to estimate the uncertainty on the bolide’s measured speed from other, independently observed fireballs. Aside from the inflammatory claims regarding its origin, USG 20140108 does stand out in the CNEOS catalog, with the second highest speed and sixth lowest altitude of peak brightness out of nearly a thousand recorded events. Independent estimates should help inform the precision of the CNEOS data and determine whether a proposed interstellar origin is likely.

To do this, the authors examined 17 other fireball events, all present in the CNEOS catalog but also independently observed by non-classified space- and ground-based cameras (a full list of these events and their references is present in Table 3 of the article). By comparing these independent events to the CNEOS data, the authors identified two critical parameters — speed and radiant direction (from which the meteor came) — that may be particularly unreliable in the CNEOS data. The 17 events span a decade in time, 20 km/s in speed, and were roughly evenly distributed across the sky.

As we see in Figure 1, as the bolide’s USG-reported speed increases, so does the difference between the USG-reported speed and independently measured speed. No clear speed-dependent trend is apparent in the radiant data, but the USG-reported and independently measured radiants can differ widely. Following these data, the authors predict that for the fireball’s measured speed of 45 km/s, there should be a measurement uncertainty of 10–15 km/s. They also note that given the range of radiant discrepancies, there may be up to a 30-degree error in the apparent radiant.

plots comparing speed and radiant reported by the USG and the difference between those quantities and the ground-based quantities.

Figure 1: Speed (left) and radiant (right) differences between USG and independent sensing of 17 bolides plotted against USG-reported speed. [Brown and Borovička 2023]

Interstellar or Intrastellar?

So, armed with their new uncertainties, the authors compare USG 20140108 to other bolides in the catalog on supposedly unbound orbits. As seen in Figure 2, assuming the other CNEOS measurements have uncertainties comparable to the trends identified in the article, all the unbound bolides have significant chances of actually being on bound orbits. USG 20140108 is again the most extreme of these, but given the velocity and radiant errors estimated in the article, the authors conclude that the simplest answer is that USG 20140108 originated within our own solar system.

Radiant versus velocity plot

Figure 2: Radiant elongation versus velocity plot showing putatively unbound bolides in the CNEOS catalog. USG 20140108 is shown as the red cross. Unbound orbits are found to the right of the line and bound orbits are to the left of the line. [Brown and Borovička 2023]

Hard Rock or Heavy Metal?

While the reliability of the government measurements is the main focus of the article, the authors also used their analysis to reexamine the meteorite’s composition. Siraj and Loeb claimed that in order to produce the observed flashes and flares, the meteor must have had a metallic composition of anomalous strength, possibly indicating extrasolar material. If, in fact, the meteor’s speed was much slower (as the new wider uncertainties would allow for), then a stony meteorite is capable of producing the observed light curve.

Without invoking exotic materials or alien destinations, it appears USG 20140108 may just be yet another simple shooting star, burning brightly yet briefly. Articles like this exist in a long tradition of academic discourse, but few other topics draw as much public and private interest (both scientific and sensational). While we’ll almost certainly never know the bolide’s true provenance, the recently reported recovery of supposed fragments from the meteor (possibly in contravention of international treaties) means this result will definitely be critical to interpreting any new claims about its nature.

Original astrobite edited by Evan Lewis.

About the author, Yoni Brande:

I’m a fourth-year PhD candidate at the University of Kansas, working on exoplanet discovery and characterization. I primarily work with TESS transit data and Hubble Space Telescope exoplanet transmission spectroscopy data, and I’m also interested in enabling more collaborative science with open source astronomical software tools. When I’m not doing research or writing Astrobites, I can be found in a sci-fi streaming binge, running, lifting, cooking, or on Twitter @YoniAstro.

Globular cluster Liller 1

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Demographics of Hierarchical Black Hole Mergers in Dense Star Clusters
Authors: Giacomo Fragione and Frederic A. Rasio
First Author’s Institution: Northwestern University
Status: Published in ApJ

Observations of gravitational waves from binary black hole mergers made by LIGO, Virgo, and KAGRA have revolutionized our understanding of the demographics of compact objects like neutron stars and black holes. At the range detectable by these instruments, for example, they have shed light on the distribution of black hole masses, which gives us a glimpse into the late-stage evolution of massive stars. In particular, stellar evolution models predict a dearth of black hole remnants with masses between fifty and a few hundred times the mass of our Sun, because of the so-called pair-instability process. However, LIGO–Virgo–KAGRA observations have indicated the existence of black hole binaries where one member of the binary is in this pair-instability range, meaning that the simple stellar evolution picture doesn’t fully explain the observed population!

Binary black holes are believed to form by two main channels: in isolation, from a binary star system, and dynamically, as depicted in Figure 1. In the former (left), the binary system evolves through a common-envelope phase, which, if the conditions are right, will result in a binary black hole system. In the dynamical pathway (right), interactions between stars in the dense centers of clusters are frequent and can lead to the formation of binary black hole systems. Subsequent interactions can tighten the orbit, leading to mergers.

Cartoon depiction of the two primary channels for binary black hole formation

Figure 1: Cartoon depiction of the two primary channels for binary black hole formation (left and right columns), with time increasing from top to bottom in each column. At left, a binary star system enters a common-envelope phase that shrinks the orbit of the binary. If this results in ejection of the envelope, the system will eventually form a binary black hole system that will ultimately merge. At right, a binary system of a black hole and a star will become a system of binary black holes through a three-body interaction in a dense star cluster. Later interactions will shrink the orbit until the two black holes merge. [Mapelli 2020; CC BY 4.0]

Focusing on the latter channel, today’s authors explore how such a mechanism could produce intermediate-mass black holes (those with masses between 100 and 1,000 solar masses), specifically through hierarchical black hole mergers. Hierarchical black hole mergers are those in which one (or both) of the component black holes is a so-called “later-generation” black hole: the remnant of a previous binary black hole merger in the center of a dense star cluster. (A first-generation black hole is one produced at the end of a star’s life, a second-generation one is formed from two first-generation black holes, and so on.) These sorts of repeated mergers of stellar-mass black holes would yield black holes in the intermediate-mass black hole range, providing a natural answer to the provenance of these black holes. Today’s authors use a new modeling framework to predict the properties of star clusters that can produce a detectable distribution of hierarchical black hole mergers.

Demographics of Cluster Hosts

The primary challenge associated with this channel of intermediate-mass black hole formation is the “recoil kick” that results from asymmetry in the merger process. In some cases, the velocity with which the resulting black hole forms can exceed the escape velocity of the star cluster in which it formed, leading to ejection (and thus limiting the possibility of a subsequent merger). Using the modeling framework developed in a previous work, today’s authors are able to predict the properties of clusters that will be able to retain a binary black hole remnant after the merger, as displayed in Figure 2. From these panels it is clear that hierarchical black holes are likely only produced and retained in the most massive and densest clusters, as these will be the ones with the deepest potential wells and highest rates of interaction.

probability that a merger remnant will survive in the cluster as a function of cluster mass and density

Figure 2: The probability that a merger remnant will survive in the cluster (color scale) as a function of cluster mass and density, going from the merger of two first-generation black holes (left), to the merger of a first- and second-generation black hole (middle), and the merger of two second-generation black holes (right). The points correspond to observations of different types of star clusters from the literature. Notice how the remnant of two first-generation black holes merging is relatively easy to retain, but it becomes progressively more difficult as we introduce later generations. [Adapted from Fragione and Rasio 2023]

In these massive and dense clusters, repeated black hole mergers can eventually result in the formation of a single massive black hole >1,000 times the mass of our Sun that dominates the interactions and binary merger process. It turns out that these massive black holes generally grow as a result of mergers with first-generation black holes, as mergers of two hierarchically produced black holes (i.e., two second- or third-generation black holes) tend to impart a strong kick on the remnant, driving it to escape from the cluster. Therefore, robustly modeling the effect of kicks is crucial to understanding the rates of intermediate-mass black hole formation by this hierarchical merger process.

Merger Rates and Assorted Sundries

With their framework in hand and the properties of the cluster hosts understood, today’s authors then average over the distribution of star clusters of different masses as a function of time to predict merger rates of various generations of hierarchical black holes. In Figure 3, the authors demonstrate that massive clusters (with masses up to 107 solar masses) are necessary to produce later generations of black hole mergers. That is, they find that the rates of hierarchical black hole mergers fall as the maximum cluster mass is lowered from 10 million to 1 million solar masses, as the lower-mass clusters are less likely to hold onto the resulting black holes. In kind, the merger rates for component black holes later than third-generation disappear.

Plots of merger rates as a function of cosmological redshift

Figure 3: The merger rates as a function of cosmological redshift (i.e., over time in the universe’s history) for combinations of merging black holes of different generations (e.g., 1G refers to a first-generation black hole). The left panel is the prediction including star clusters with masses up to 10 million solar masses, while the right panel only includes clusters up to 1 million solar masses. There are no mergers beyond 3G in the right panel, demonstrating that only the most massive clusters can host late-generation black-hole mergers. [Adapted from Fragione and Rasio 2023]

Relatedly, they compare the distribution of merging black hole masses detected with the LIGO–Virgo–KAGRA observatories to those predicted by their model. From such analysis, they demonstrate that within their framework, several of the observed events can only be produced by accounting for hierarchical black hole mergers, with a few appearing to come from mergers of second- and third-generation black holes!

These results, while preliminary, can be extended (by incorporating other metrics, such as the black hole spins) to statistically measure the likelihood of individual gravitational wave events being associated with hierarchical mergers. However, they represent an exciting step towards understanding where these intermediate-mass black holes come from and provide a compelling, natural explanation for how a large population of massive black holes can form and continue to evolve over cosmic time!

Original astrobite edited by Mark Popinchalk.

About the author, Sahil Hegde:

I am an astrophysics PhD student at UCLA working on using semi-analytic models to study the formation of the first stars and galaxies in the universe. I completed my undergraduate at Columbia University, and am originally from the San Francisco Bay Area. Outside of astronomy you’ll find me playing tennis, surfing (read: wiping out), and playing board games/TTRPGs!

three views of Jupiter's moon Io

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Io’s Optical Aurorae in Jupiter’s Shadow
Authors: Carl Schmidt et al.
First Author’s Institution: Boston University
Status: Published in PSJ

Galileo image of Jupiter's moon Io

Figure 1: Jupiter’s innermost Galilean moon, Io. Its yellowish color mainly comes from sulfur on the surface. [NASA/JPL/University of Arizona]

Jupiter’s innermost Galilean moon, Io, has a bunch of really weird properties, in addition to looking like a long-expired pizza (see Fig. 1). Io is being pushed and pulled from all sides, not only by Jupiter itself, but also by the other Galilean moons, Europa, Ganymede, and Callisto. This gives Io some breathtaking tidal effects that make our ocean’s tides look like a few ripples in a pond. These extreme tidal effects cause Io to be very volcanically active, more so than any other object in the solar system. Here on Earth, volcanoes are more or less predictably distributed at the edges of tectonic plates, but they appear randomly over Io’s surface, with no clear patterns. You would think all this volcanic activity would heat up Io’s surface a bit, but no: Io has an average surface temperature of around –130°C. So if you don’t get cooked by randomly erupting volcanoes, the cold won’t be comfortable either. Also, you can’t breathe there, which might be another challenge. Therefore it is best to observe the semi-hellscape surface of Io from afar. There is certainly always enough to see.

Iororae?

In addition to volcanoes, Io has aurorae (not iororae, sadly, missed opportunity there), but they are different from Earth’s. Io’s version is not directly caused by the Sun, but rather by Jupiter’s enormous magnetic field, which is about 20,000 times stronger than Earth’s. Earth’s atmosphere is also very different from Io’s (luckily). The many volcanoes on Io spew out large amounts of sulfur dioxide (SO2), which makes up a large part of the moon’s atmosphere. This molecule gets shredded by a multitude of processes, releasing electrons and ions all around, causing aurorae to appear and helping to form Io’s plasma ring around Jupiter.

If we want to know more about this plasma ring and Jupiter’s enormous magnetic field, we need to figure out how the SO2 in Io’s atmosphere behaves and interacts with other molecules and atoms. In daylight, the Sun’s radiation splits up (dissociates) most of the SO2, but what happens to SO2 during night time? There’s no way we can see Io’s night side without sending very expensive spacecraft up there. However, as it so happens, Io passes through Jupiter’s shadow approximately every 42 hours. During the shadow passage, the surface cools down a lot, and the Sun’s radiation will no longer break down molecules. This closely resembles night time on Io, allowing us to see how the rest of the atmosphere reacts at night, if at all.

Turns out, Io has impressive aurorae during the Jupiter eclipse. Now, to see these aurorae we mostly need to be able to observe ultraviolet light, which we can only see with space telescopes. Also, ultraviolet radiation doesn’t tell us everything, and there could be more to see with optical light as well. To view Io’s aurorae in optical light is challenging, though: we can’t directly see Io in Jupiter’s shadow, which makes it hard for telescopes to track the moon’s position. The authors of today’s article managed to see Io’s aurorae in optical light using ground-based telescopes. The authors waited for Io to pass into Jupiter’s shadow as only then are the optical aurorae visible (similar to here on Earth, Io’s aurorae aren’t very visible during the day).

Spotting Io’s Aurorae 101

As you might imagine, seeing Io’s aurorae is even harder than seeing Earth’s. Most of the time when Io is eclipsed by Jupiter, there’s this giant planet in the way blocking our view. But luckily, this is not always the case. When the positions of the Sun, Earth, and Jupiter form a right triangle (fancy astronomer slang: Earth and Jupiter being at quadrature), we can still “see” Io once it enters Jupiter’s shadow (more fancy astronomer slang: at ingress) or just before exit (egress). This is shown in Fig. 2, where a distinction is made between the full shadow (umbra) and the partial shadow (penumbra).

schematics of Io's location relative to Jupiter during different parts of the observing process

Figure 2: Panel A shows where in Io’s orbit around Jupiter you would see panels C and D, as seen from Earth. Panel B shows Io’s orbit from above the ecliptic plane. Panels C and D show the observations of Io when it’s in partial and in full shadow, respectively (the yellow dot is where the instrument thinks Io is). [Schmidt et al. 2023]

All right, we have Io in Jupiter’s shadow. Now we still need to spot some aurorae, and again, Jupiter doesn’t go easy on us; sunlight scattering off Jupiter’s atmosphere further spoils the observation. Luckily, we know quite well what Jupiter looks like, so we can isolate Io’s aurorae, going so far as to even single out some very interesting spectral lines of oxygen and sodium. The intensity of this light shows how much oxygen and sodium are in Io’s atmosphere. Fig. 3 shows the oxygen, sodium, and SO2 trends during three different eclipses.

Plot of disk-averaged brightness as a function of time after ingress

Figure 2: Light intensity of oxygen (red) and sodium (orange) on three different observation nights (circles, diamonds, squares), along with SO2 and SO intensities on one night. [Schmidt et al. 2023]

It seems oxygen doesn’t really care much about the eclipse at all, doing different things on different dates. How come? The authors claim that the amount of visible emission from oxygen depends on the position of Io in the ring of plasma around Jupiter rather than on whether sunlight reaches Io or not.

On the other hand, sodium and SO2 do seem to align, both declining once Io passes into Jupiter’s shadow. To understand why, it’s important to know that the atomic sodium comes mostly from molecules on Io, namely sodium chloride (aka salt). Once the eclipse starts, SO2 gas in the atmosphere freezes out as SO2 ice and subsequently falls down on Io’s surface almost like snow (no snow sculptures here, though; this stuff is very bad for your health). This snow buries the sodium chloride on Io, preventing sodium from showing up in the spectral lines — hence the decline in sodium brightness.

Despite all the difficulties with spotting them, we can nevertheless pull a lot of information from Io’s optical aurorae. Today’s authors demonstrated this by shedding light on the otherwise unobservable night side on Io. The fact that the authors managed to see Io’s aurorae in the optical and from the ground also opens up the possibility to study Io more frequently, which will undoubtedly lead to more interesting results in the future. Io never fails to surprise!

Original astrobite edited by Konstantin Gerbig.

About the author, Roel Lefever:

Roel is a second-year PhD student at Heidelberg University, studying astrophysics. He works on massive stars and simulates their atmospheres/outflows. In his spare time, he likes to hike/bike in nature, swimming, video games, to play/listen to music, and to read (currently The Wheel of Time, but any fantasy/sci-fi really).

multi-wavelength image of a galaxy known as a Lyman-alpha blob

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: UV and Lyα Halos of Lyα Emitters across Environments at z = 2.84
Authors: Satoshi Kikuta et al.
First Author’s Institution: National Astronomical Observatory of Japan and University of Tsukuba
Status: Published in ApJ

Astrophysics is an observational science. At its most basic level, the science we do requires light from the object we’re studying to reach our telescopes. If the light is produced only at a specific wavelength, as is the case for a spectral line, that’s even better, because it can be used to probe the properties of the object we’re trying to study in detail.

A Useful Escapee

Hydrogen, which is the most common atom in galaxies (and in the universe as a whole, by a lot), produces several spectral lines. The one we’re looking at today is produced by an electron transitioning from its first excited state down to its lowest-energy state. This is known as the hydrogen Lyman-alpha (Lyα) transition, and it produces light in the ultraviolet range at a wavelength of 121.6 nanometers in its rest frame. If the transition happens far enough away, however, the expansion of the universe redshifts the light to longer wavelengths in the visible range. Because there’s so much hydrogen in galaxies, this spectral line is very bright, and because many modern-day telescopes are designed to capture visible light, this line is very easy to see.

Let Me OUTTTTT

Based on all this, Lyα should be the perfect type of light to use when studying galaxies! There’s one issue, however — that light still has to reach the telescope. This is harder than it sounds, for the same reason that so much Lyα emission is produced by galaxies in the first place. Hydrogen is very, very common in galaxies, and hydrogen atoms can suck the Lyα light back up (absorb it) just as easily as they emit it in the first place. Because hydrogen is so common, Lyα light has to rattle around for a long time, being absorbed and re-emitted, before it can finally escape the galaxy in which it was produced. This means that by the time it reaches our telescopes, Lyα light isn’t a great tracer of the regions where it was emitted, but it is a very good tracer of the size and shape of the hydrogen gas across the entire galaxy.

Some Very Efficient Observations

In today’s article, the authors investigate a resolved image of the Lyα emission from a still-forming cluster of galaxies at redshift z = 2.84 (light that was emitted 11.4 billion years ago). Because so many galaxies are clumped together in this galaxy cluster, the 3,490 Lyman-alpha-emitting galaxies in the image could all be observed in just 8.5 hours with the Hyper-Supreme Cam on the Subaru telescope. Observations used the g-band optical filter (which shows redshifted ultraviolet emission) and a narrowband filter intended to pick up emission exclusively from the Lyα spectral line. All of the Lyman-alpha-emitting galaxies detected in the image are shown in Figure 1, colored by their equivalent width, which is a measure of the strength of the emission. The dense region at the center of the image contains an extremely bright quasi-stellar object (QSO; a galaxy with a black hole in its center sucking up matter so fast that the matter becomes brighter than the galaxy itself), which is how the cluster was discovered in the first place.

plot of sky positions of galaxies surveyed for this research article

Figure 1: The positions on the sky of the 3,490 Lyman-alpha-emitting galaxies being studied in today’s article. The points are colored by the strength of the Lyα emission from each galaxy, and their size corresponds to the strength of each galaxy’s ultraviolet emission. The gray contours show the regions where the galaxies are most densely grouped together. [Kikuta et al. 2023]

Stacking Up the Evidence

The authors of this article wanted to investigate how various galaxy properties change the way Lyα photons escape from a galaxy. To do this, they split the full sample of Lyman-alpha-emitting galaxies into subsamples based on other galaxy properties, such as the luminosity of the Lyα and ultraviolet emission, the equivalent width of the Lyα line, or the distance between the galaxy and the central QSO. They then stacked together the images of all the galaxies in each subsample to produce a single average image with a much better signal-to-noise ratio for each subsample. From these averaged images, shown in Figure 2, the authors can calculate the amount of Lyα emission as a function of distance from the center (a radial profile, like the ones shown in Figure 3) to calculate the average size and shape of the Lyα emission from the included galaxies. They can thus deduce how Lyα escape is affected by the galaxy properties that change between subsamples.

Stacked Lyman-alpha images of galaxies in the authors' sample

Figure 2: Stacked images of the Lyα emission from the subsamples of Lyman-alpha-emitting galaxies defined by the authors to explore the effects of various galaxy properties on Lyα emission. From top to bottom, the subsamples are split based on: the magnitude of the ultraviolet emission, the Lyα luminosity, the Lyα equivalent width, and the distance from the central QSO. Contours are provided to show the Lyα shape more clearly. [Adapted from Kikuta et al. 2023]

Factors Affecting Escape

The authors found that Lyα escape matched their expectations well in general: galaxies with more ultraviolet or Lyα luminosity, or with lower equivalent widths (all indicators that a galaxy is making new stars quickly), had much more extended Lyα emission than their lower-luminosity counterparts. This makes sense — bigger galaxies form more stars, so the ones forming stars the fastest should have the most extended hydrogen gas, and thus the most extended Lyα emission.

Trends in the subsamples split up by the galaxy’s environment, however, weren’t quite so well behaved. The size of the Lyα emission didn’t change much as the galaxies got more distant from the central QSO, but the shape (the spatial distribution) of the emission definitely did. Galaxies far from the QSO had a lot of Lyα escape in the center of the galaxy, but the amount dropped off very fast towards the outskirts (the blue and green lines in Figure 3). Galaxies close to the QSO, however, didn’t have as steep a drop-off (the red line in Figure 3).

Plot of radio profiles of Lyman-alpha emission for the galaxy subsamples

Figure 3: The average radial profiles (amount of emission as a function of distance from the center) of the different subsamples, split based on distance from the QSO. Note that, in the circled region, there’s more emission from the close-in galaxies (the red line) than the distant ones (the blue and green lines). [Adapted from Kikuta et al. 2023]

The authors believe that these galaxies have the same amount of hydrogen gas surrounding them as the ones farther away from the QSO, but that gas has been excited by the energetic photons emitted by the QSO. This would change how the hydrogen atoms absorb, and let the Lyα escape more easily even far away from the center of the galaxy. This trend is subtle, and the authors do note that it could be due to an observational bias where galaxies moving quickly in relation to the QSO are missed by the narrowband filter they use.

What’s Next?

High-redshift protoclusters like the one studied here are very important to understanding galaxy evolution, because they will evolve into galaxy clusters like the one we live in now. They are very difficult to study effectively, however, because they’re so far away. It’s therefore critical to develop techniques like the one being used by the authors here, that study many galaxies at once using a minimum amount of telescope time. In the future, the authors plan to do similar analyses on different clusters using different telescopes (such as JWST) to really nail down the factors affecting Lyα escape.

Original astrobite edited by Karthik Yadavalli.

About the author, Delaney Dunne:

I’m a PhD student at Caltech, where I study how galaxies form and evolve by mapping their molecular gas! I do this using COMAP, a radio-frequency Line Intensity Mapping experiment based in California’s Owens Valley.

artist's impression of an exoplanet

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: There’s More to Life than O2: Simulating the Detectability of a Range of Molecules for Ground-based, High-resolution Spectroscopy of Transiting Terrestrial Exoplanets
Authors: Miles H. Currie, Victoria S. Meadows, and Kaitlin C. Rasmussen
First Author’s Institution: University of Washington
Status: Published in PSJ

Searching for Life Elsewhere

The need for characterizing planetary atmospheres grows with the increasing number of discovered exoplanets. In the quest to find life on other planets, astronomers turn to one of the main techniques we can use to study the atmosphere of an exoplanet: transmission spectroscopy. This technique can be used if an exoplanet transits in front of its host star, meaning it passes in front of the star and blocks part of the light that we receive from the star. If the transiting exoplanet has an atmosphere, astronomers can use this technique to try to infer the composition of the atmosphere. This technique is promising in the search for life because we can use it to search for biosignatures, which are molecules that could help us determine whether life is present on these exoplanets!

On Earth, oxygen is present in our atmosphere due to photosynthesis, which occurs in plants, algae, and some bacteria. This process takes in carbon dioxide and produces oxygen and energy stored as a sugar. We, as human beings, breathe oxygen to live. Therefore, in our knowledge of what life is on Earth, we look for oxygen on other planets as a sign of life. However, today’s authors argue that there’s more to life than oxygen and explore what other molecules could help indicate whether life could be present on exoplanets. They also explore which of those molecules can be detected by extremely large ground-based telescopes including the European Southern Observatory’s Extremely Large Telescope, the Thirty Meter Telescope, and the Giant Magellan Telescope.

A Novel Detectability Pipeline

The authors of today’s research article aim to estimate how detectable different molecules can be in the atmospheres of terrestrial exoplanet atmospheres by using a technique called cross-correlation analysis (see Figure 1). This technique works by comparing the simulated transmission spectrum put in at the beginning of the pipeline to a model template of the molecular absorption bands from other studies. To assess how they compare to each other, the authors calculate a correlation coefficient. A higher value of the correlation coefficient means a better match between the two spectra. For this article, the authors chose seven gases — oxygen, methane, carbon dioxide, water vapor, ozone, carbon monoxide, and ethane — to be studied in four main types of atmospheres. The four types of atmospheres they focused on were 1) pre-industrial Earth, 2) Archean Earth, 3) an atmosphere with an accumulation of oxygen due to the breakdown of carbon dioxide (also known as photolysis) with no lightning, and 4) an oxygen-containing atmosphere with lightning occurring. Lastly, the authors chose to study the seven selected molecules in these four different types of atmospheres around five different M-dwarf stars at two different distances from the observer. This was all done through a new detectability pipeline made by the authors and can be seen in Figure 1.

flowchart of the new pipeline

Figure 1: This figure illustrates the new pipeline set forth by the authors for studying the detectability of different molecules. The red rounded boxes show the beginning and end points of the pipeline, the green boxes show the different models or software that they used, the white parallelograms show the input or output data, and the white rhombuses represent the parameters used for this study. [Currie et al. 2023]

What Can We See?

By using this new pipeline, the authors were able to determine which of the seven molecules is more likely to be detected with the extremely large ground-based telescopes. The best two potential biosignatures that could be discovered on terrestrial exoplanets are carbon dioxide and methane followed by oxygen, water vapor, and carbon monoxide. If carbon dioxide is detected with methane in the atmosphere of an exoplanet, this constitutes a “disequilibrium biosignature pair” that could be driven by life and only requires between 5 and 20 transits to be detected for their most optimistic scenario of Earth-like planets at two different distances from observer (see Figure 2). Similarly, if oxygen and methane are detected together in an exoplanet’s atmosphere, that is also a disequilibrium biosignature pair that could be driven by life. Both of these pairs could easily be acquired in as few as 39 transits for their “most optimistic scenario of an Earth-like planet transiting a star 5 parsecs (16 light-years) away.” The other two molecules, ozone and ethane, are not accessible with ground-based transmission spectroscopy.

plot of the number of transits required to get a three-sigma detection of certain molecules under certain conditions

Figure 2: This figure shows the total number of observed transits required for a three-sigma detection of the five molecules detectable with extremely large telescopes: oxygen, methane, carbon dioxide, carbon monoxide, and water vapor. All four types of atmospheres are depicted in this figure orbiting a variety of different host stars at a distance of 12 parsecs (39 light-years) from the observer. If there is no marker it means that the atmosphere doesn’t have that molecule or it requires more than 300 transits to be detected. [Currie et al. 2023]

This study is an exciting step in our understanding and search for life on terrestrial exoplanets using transmission spectroscopy through extremely large ground-based telescopes. This article demonstrates the need for these ground-based observatories and highlights how they can complement our current observations with space telescopes such as JWST. By using and studying these observations together, we are one step closer to being able to characterize these exoplanets and potentially find life.

Original astrobite edited by Pranav Satheesh.

About the author, Junellie Gonzalez Quiles:

Junellie Gonzalez Quiles is a PhD Student and NSF Graduate Research Fellow in the Department of Earth and Planetary Sciences at Johns Hopkins University. Her current research focuses on modeling geochemical cycles and outgassing on exoplanets to help us understand the evolution of the atmospheric composition and its effect on planetary climate. She is deeply passionate about outreach, science communication, and diversity, equity, and inclusion (DEI) initiatives. Outside of work, she loves to knit, embroider, and do other arts and crafts. She also plays the trombone and enjoys practicing yoga.

Artist's impression of a gas giant exoplanet peeking out from behind its parent star

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Flares, Rotation, Activity Cycles and a Magnetic Star-Planet Interaction Hypothesis for the Far Ultraviolet Emission of GJ 436
Authors: R. O. Parke Loyd et al.
First Author’s Institution: Eureka Scientific
Status: Published in AJ

You Go, GJ 436

GJ 436 is an old, fairly typical dwarf star orbited by a short-period Neptune-sized planet, GJ 436b. GJ 436b is a pretty famous planet among those who research gas giants with orbits faster than that of Mercury around the Sun. It has become a high-interest target for JWST with the hopes of characterizing its hot atmosphere. The type, intensity, duration, and frequency of stellar activity all impact the observations and physical evolution of planetary atmospheres. This motivates stellar astronomers, including the authors of today’s article, to understand what stellar activity looks like for stars like GJ 436.

Stellar activity can take many forms, as listed in the title of today’s article: flares, stellar rotation, stellar activity cycles, and magnetic star-planet interactions.

  • Flares are bursts of activity that typically occur on short timescales (minutes to hours).
  • Stellar rotation refers to the changing view of the surface of a star as it spins, usually over the course of days to weeks.
  • A star’s activity cycle is a long-term (several years) shift between low and high amounts of activity.
  • Magnetic star–planet interactions, or SPI, are a proposed type of activity that can occur when a planet orbits close enough to its host star that its magnetic field interacts with the star’s magnetic field.

All forms of activity lead to changes in a star’s spectrum — the amount and energy of light emitted — which can then lead to changes in an orbiting planet’s atmosphere.

Help Me, Hubble Space Telescope, You’re My Only Hope

Today’s authors focus specifically on the far-ultraviolet portion of GJ 436’s spectrum. The far ultraviolet covers wavelengths between 1150 and 1450 Angstroms — highly energetic light that would give you one heck of a sunburn if Earth’s atmosphere did not protect us from it. The far ultraviolet is important because it drives the creation or destruction of atmospheric molecules (i.e., atmospheric photochemistry). This means that stellar and planetary astronomers are interested in how the far ultraviolet varies with stellar activity. To observe this region of the electromagnetic spectrum, one needs the Hubble Space Telescope, as it is currently the only observatory with the ability to observe ultraviolet light at high resolution.

The authors compiled all far-ultraviolet observations of GJ 436 taken with Hubble’s Cosmic Origins Spectrograph. This includes three separate groups of observations that span a total of 5.5 years, which allows the authors to look for stellar activity on short and long timescales. Figure 1 is an average spectrum of GJ 436. The authors added up all the far-ultraviolet light, excluding the gray regions shown in Figure 1, to see how GJ 436’s output at those wavelengths changed over the 5.5 years of observations. The emission line behavior over time was analyzed separately.

Average spectrum of GJ 436

Figure 1: Average spectrum of GJ 436 observed by Hubble with the Cosmic Origins Spectrograph. Gray regions mark the areas of contamination from Earth’s reflected light. Each strong emission line is labeled. [Loyd et al. 2023]

A Flare for the Magnetic

Emission lines like those of silicon and nitrogen are sensitive to flaring behavior, so the authors identified all fourteen flares present in the data by looking at the total light in all of the far-ultraviolet emission lines over time. The authors calculated the durations and energies of each flare, included in Table 1 in today’s article. The flares were then removed from the data so the authors could look for the other forms of variability.

The authors used visible-light observations from Fairborn Observatory’s Automatic Photoelectric Telescope to measure the periods of the star’s rotation and the star’s activity cycle. Using the periods measured from the visible-light observations, the authors fit sine waves to the far-ultraviolet light curves for stellar rotation and activity cycle. They allowed the amplitudes and phases of those sine waves to vary so they could find the best fit. They also included a potential third sine wave with a period equal to the orbital period of the planet to search for any existing magnetic star–planet interactions.

Results and the Bigger Picture

Unfortunately, no magnetic star–planet interactions were directly detected in the far-ultraviolet data presented in this work, though the authors were able to place an upper limit on the planetary magnetic field of 10 Gauss (Earth’s magnetic field strength is around 0.5 Gauss). However, the team did detect a bunch of frequent, low-energy flares. Stars like GJ 436 typically exhibit more energetic flares, so the existence of these lower-energy flares may be a hint of magnetic star–planet interactions! More work on star–planet interaction signatures and disentangling them from plain ol’ stellar activity is needed to tease out the answer.

The variability from all of the forms of activity explored in today’s article were compared to each other, as shown in Figure 2. The stellar activity cycle dominates GJ 436’s far-ultraviolet variability, followed by flares and noise. This means that the largest changes in GJ 436’s far-ultraviolet emission come from the star’s activity cycle.

variability amplitude of different forms of activity in different regions of the far-ultraviolet spectrum

Figure 2: The variability amplitude — or the contribution to the change in the far-ultraviolet emission — for each form of activity investigated by today’s article, separated by each region of the far-ultraviolet spectrum. [Loyd et al. 2023]

GJ 436 is a typical planet-hosting star — meaning that how it behaves is representative of most planet-hosting stars. The results from today’s article show that most older planet-hosting stars have ultraviolet emission that is likely dominated by their activity cycles. This means that exoplanet astronomers can interpret current exoplanet observations knowing the planets likely experienced this history within their stellar environment, contributing to their atmospheric evolution.

Original astrobite edited by Archana Aravindan.

About the author, Keighley Rockcliffe:

Keighley is a PhD candidate at Dartmouth College, which resides on unceded Abenaki land. She studies young exoplanet atmospheres with Dr. Elisabeth Newton. She firmly believes in making science a more inclusive space for all humans, especially those traditionally excluded and oppressed. Keighley loves to meet and support people, so please reach out to chat!

Hubble image of the galaxy cluster Abell 1689 with a map of its dark matter distribution

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: SPT-CL J2215-3537: A Massive Starburst at the Center of the Most Distant Relaxed Galaxy Cluster
Authors: Michael S. Calzadilla et al.
First Author’s Institution: Massachusetts Institute of Technology
Status: Published in ApJ

The largest gravitationally bound structures in the universe are galaxy clusters — hundreds to thousands of individual galaxies bound together by gravity and surrounded by dark matter and gas. Lurking at the center of these galaxy clusters are the brightest galaxies in the universe: the aptly named brightest cluster galaxies (BCGs).

Because BCGs are right at the center of their galaxy cluster’s gravitational field, the clusters themselves act like a well, funneling new material onto the BCGs. This means that BCGs grow very large, and the evolution of the BCG is intimately linked to the evolution of the full galaxy cluster. Traditionally, it has been thought that the clusters feed the BCG with other galaxies full of pre-made stars (this mechanism is referred to as, no joke, galactic cannibalism — here’s a video on the topic). However, many BCGs have been observed to actively grow new stars. In these cases, it seems like the cluster is feeding free-floating gas known as the intracluster medium (ICM) to the BCG. This ICM contains the ingredients for star formation (mostly hydrogen gas), and the BCG can process these ingredients into new stars itself.

Going with the Flow

In order for this second type of BCG growth to be happening, the galaxy cluster needs to obey some fairly specific criteria. There needs to be one specific BCG in the cluster, and it needs to be sitting at the center of the cluster’s gravitational field, in order to direct the ICM onto the BCG. The cluster itself also needs to be large enough to have a lot of concentrated ICM, and old enough that a lot of the initial heat (kinetic energy) in the ICM has had time to escape (if not, it will be moving around too fast to get caught by the BCG’s gravity). These types of clusters are known as “cool-core” clusters, and the flow of ICM onto the BCG is known as a “cooling flow.” All of these things typically happen naturally in clusters, but they all require time, so they’re far more common in much older clusters at times much closer to the present day. These clusters are called “relaxed.” That’s what makes today’s article so exciting — the authors of this article have found a relaxed cluster funneling material onto its BCG at redshift 1.16 (only about 5.3 billion years after the Big Bang). This is the earliest example of such a cluster found to date — so this cluster must have relaxed faster than previously thought possible.

The Picture(s) of Relaxation

This cluster, known as SPT-CL J2215-3537, or SPT2215, was originally found using the Sunyaev–Zeldovich effect in a South Pole Telescope survey. Optical and ultraviolet imaging (Figure 1) from the Hubble Space Telescope and the Magellan Telescopes were used to find the galaxies associated with the cluster, and optical spectroscopy from Magellan was used to make sure that the galaxies were all associated with the cluster in all three dimensions. This optical spectroscopy also measured the distance to the cluster, using redshifting of spectral lines, and therefore confirmed that we’re observing this cluster earlier in the universe’s history than any other cluster of its kind. A faint radio-wavelength source (probably an active galactic nucleus) was also found to be associated with the cluster using the Australian Square Kilometre Array Pathfinder (ASKAP) (Figure 1).

Example observations of the galaxy cluster

Figure 1: Some of the wide variety of observations required to study this galaxy cluster. Clockwise from the top left, they are: the ASKAP radio observations showing the active galactic nucleus, the Chandra X-ray observations showing the ICM, a Hubble composite image showing the cluster, and zoom-ins on the BCG in optical and ultraviolet wavelengths (respectively), also from Hubble. [Calzadilla et al. 2023]

A plot of radius versus the Boltzmann constant k times temperature T

Figure 2: The temperature profile of the ICM of the galaxy cluster, measured from the X-ray observations shown in Figure 1. The temperature is shown in energy units, because in this case it’s essentially a measure of the kinetic energy of the gas. The grey line shows the actual data points, and the green region is a fit to a known model of the temperature profile in cool-core clusters. [Adapted from Calzadilla et al. 2023]

A Cool Customer

The ICM is very diffuse, and it isn’t typically visible in optical or ultraviolet measurements. In order to measure this cluster’s ICM properties, the authors had to take observations using the Chandra X-ray Observatory. From this, they noticed that the ICM is distributed extremely regularly in the cluster, and that the ICM’s luminosity peaks very strongly in the center. As mentioned above, both of these characteristics are good indicators that the cluster is relaxed. The authors also measured the spectrum of the X-rays in order to determine the temperature of the ICM. By measuring different X-ray spectra at different distances from the center of the cluster, the authors developed a temperature profile (Figure 2). This showed that the ICM in the middle of the cluster in particular had a very low temperature, making it a cool-core cluster. Filaments of gas are also visible surrounding the BCG in the ultraviolet imaging from Hubble, suggesting that ICM is indeed falling onto the BCG.

Relaxed, but Working Hard

Finally, the authors measured the spectral energy distribution of the BCG itself (Figure 3). This is a technique where the amount of light emitted from a galaxy is measured at as many different wavelengths as possible, and then the luminosity at these different wavelengths is compared. Different components of a galaxy (such as new stars, old stars, or gas) emit light at different wavelengths, so scientists can estimate how fast a galaxy forms stars by fitting measurements to models of these different components. In this case, the authors used the Hubble and Magellan measurements mentioned above (at optical and ultraviolet wavelengths), additional near-infrared Magellan measurements, and far-infrared (very long-wavelength) Spitzer Space Telescope observations to construct their spectral energy distribution. From the spectral energy distribution, they determined that the BCG in this cluster was forming 320 solar masses of new stars every year (about 300 times the Milky Way’s rate)!

spectral energy distribution for the brightest cluster galaxy

Figure 3: The spectral energy distribution of the BCG inside SPT2215. The blue points show the observed values for this galaxy, and the red points show the model that was fit to these values. Using this technique, the authors determined that the BCG is forming stars at a much higher rate than expected. [Calzadilla et al. 2023]

All of this evidence seems to point to a BCG forming stars out of fuel from the cluster itself. If this is the case, this will be the earliest ever example of such a cluster, and it has some pretty exciting implications. The authors suggest that clusters relaxing this quickly may have a totally separate mechanism for BCG formation, independent from the cannibalism-driven growth we expect. It also implies that active galactic nuclei (such as the one seen in the ASKAP imaging of this cluster in Figure 1) could start powering on earlier than expected, feeding energy back into the BCG and the cluster and disrupting star formation. The authors are working with more X-ray observations to characterize the physics of this cluster more precisely, and hopefully figure out some of the specifics of these implications.

Original astrobite edited by William Lamb.

About the author, Delaney Dunne:

I’m a PhD student at Caltech, where I study how galaxies form and evolve by mapping their molecular gas! I do this using COMAP, a radio-frequency Line Intensity Mapping experiment based in California’s Owens Valley.

artist's impression of a magnetar outburst

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Neutron Star Phase Transition as the Origin for the Fast Radio Bursts and Soft Gamma-ray Repeaters of SGR J1935+2154
Authors: Jun-Yi Shen et al.
First Author’s Institution: Huazhong University of Science and Technology
Status: Published in ApJ

Astronomers are infamously bad at naming things. So when we observed unexplained quick radio pulses in 2007, we imaginatively dubbed them fast radio bursts (FRBs). However, a name does not a physical theory make: the exact cause of these FRBs is still unknown. FRBs are bonkers energetic, putting out as much energy in about a millisecond as the Sun does in three days. The huge energy involved likely implies that FRBs originate from the most extreme objects or events in our universe, such as neutron stars, black holes, and supernovae. Also, some FRBs repeat their bursts, but these repetitions are not evenly spaced in time. This rules out cataclysmic one-time events such as mergers or supernovae, while their uneven spacing is confusing as orbital events have a regular period.

artist's impression of a magnetar

Figure 1: An artist’s impression of a magnetar, a neutron star with an extremely strong magnetic field and possible source of fast radio bursts. [ESO/L. Calçada; CC BY 4.0]

A breakthrough came when an FRB was observed from SGR 1935+2154, a highly magnetic neutron star called a magnetar (Figure 1). While this discovery strongly suggests that at least some FRBs come from magnetars, we are still unclear exactly how magnetars generate these pulses. The authors of today’s article have an interesting hypothetical explanation that will take us deep into the cores of neutron stars: could phase transitions from neutrons to quark matter be causing these mysterious events?

Neutron Star Anatomy Crash Course

To understand the cores of neutron stars we first have to understand quarks. You may have heard of quarks — protons and neutrons are each made of three quarks — and we call any particle made up of quarks a hadron. In fact, we have never observed quarks outside a hadron due to a phenomenon called quark confinement. (A proof of quark confinement is one of the millennium prizes in mathematics, so if you think you have it, you should collect your one million dollars!) However, in extreme enough environments, hadrons “melt” into a quark–gluon plasma (QGP), affectionately referred to as quark–gluon soup. Delicious! In the plasma, quarks are freed from their enforced trios and can travel unconfined. As you might have guessed given the topic of this bite, it is hypothesized that the extreme density at the core of neutron stars may be enough to create QGP. However, at the surface of the neutron star, called the crust, the energy densities are too low for this plasma to exist, and all of the quarks are locked up in neutrons. Hence, there must be some transition point between the hadronic phase (the neutrons) and the quark phase (QGP) within the neutron star.

Phase Transitions

A phase transition is when the state of matter changes, such as liquid water freezing or boiling away. The authors predict that a phase transition in the magnetar could cause FRBs. To show this, they need a neutron star’s equation of state, which describes how its pressure is related to its density. Finding the equation of state of a neutron star is an area of active research, so the authors use a combination of many articles working on the problem to give expressions for the equation of state of the outer hadronic crust and the gooey quark soup center.

neutron star equation of state

Figure 2: The model of neutron star equation of state used by the authors. H, M, and Q stand for the hadronic, mixed, and quark regimes, respectively. The red line represents the metastable hadronic state. When the neutron star undergoes the sudden phase transition at a pressure around p, the matter jumps from the red line to the normal mixed regime line, causing the star to shrink and theoretically generating an FRB. [Shen et al. 2023]

What happens where these two regions meet? Previous models assumed that these two regimes met and didn’t interact much, but the authors argue that the two phases meet and mix slightly. The authors use thermodynamic arguments to show that droplets of the quark soup could appear in the hadronic matter. This mixing allows matter to stay in the hadronic phase deeper into the star than expected, i.e., beyond the point where it would normally become QGP: this phenomenon of the hadronic matter trespassing into what should be the QGP regime is called a metastable state. Metastable is a fancy way of saying kinda-stable. It only takes a little bit of a bump for a metastable state to collapse to the more stable of the two phases it borders, and it does so quite rapidly. You may have seen videos of people squeezing water bottles that then freeze very quickly — this is an example of water coming out of a metastable state.

The authors hypothesize that some of the hadronic matter will start in the metastable state, but as the neutron star’s spin slows down over the course of its life, the pressure will increase in the core and cause the hadronic matter to fully convert to quark matter (see Figure 2). This change to the denser phase would cause the star to suddenly shrink in size, and the lost gravitational potential energy would be released as an FRB. Could this be enough energy to power the FRBs we observe?

Modeling the Collapse

To find out, the authors solve the Tolman–Oppenheimer–Volkoff equation — yes, that Oppenheimer with the new movie coming out, though unfortunately trailers imply it’s not about his contributions to astrophysics — which describes a rotating sphere of material in gravitational equilibrium in general relativity, an excellent model of a neutron star. The authors use the equation to calculate the energy released by the neutron star as it shrinks under the phase shift for a variety of possible equations of state and models of the mixing region. They find that for an appropriate choice of the equation of state, the energy released by the phase transition would be enough to power SGR 1935+2154, and the energy release would occur roughly often enough to explain observations of repeated bursts! The authors also note that the star would only shrink by about a micrometer, and yet the system is so extremely dense that this is enough to release the energy required.

In addition, this rapid shift in radius would cause a starquake called a glitch, where the magnetar suddenly increases its spin thanks to conservation of angular momentum. Their model predicts a glitch size that is comparable to glitches observed in SGR 1935+2154. In addition, neutron stars have been observed to undergo seemingly random decreases in their spin, called anti-glitches. Since the exact timing of the transition in this model is dependent on the spin of the neutron star, this could explain the variability in the period of FRBs.

While these results are exciting, there are many hurdles to overcome. For instance, the authors note that while the energy scales match, they do not yet have a proposed mechanism for how this energy actually becomes radiation in the form of a radio pulse, while other models do. Nevertheless, the authors highlight that future gravitational wave observations of an FRB would be an excellent way of testing their model, as their use of a general relativistic model of an FRB would allow them to directly predict the type of gravitational wave we see. Thus, while the mystery may be unresolved, the hunt to understand FRBs will push us to understand the most extreme environments in our universe and unite disparate areas of physics and astrophysics.

Original astrobite edited by Pranav Satheesh.

About the author, Cole Meldorf:

I am a Master’s student at the University of Cambridge, currently studying a bit of both observational and theoretical cosmology, particularly in the avenue of using machine learning methods and cosmological data to constrain cosmological parameters. I also do some research with the Dark Energy Survey on galaxy evolution and supernova cosmology. When I’m not dying under the crushing weight of finals and PhD applications, I play the violin, do a little theater, and like to cook!

Simulation of heat transport in a hot Jupiter's atmosphere

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: The Effect of Interior Heat Flux on the Atmospheric Circulation of Hot and Ultra-hot Jupiters
Authors: Thaddeus D. Komacek et al.
First Author’s Institution: University of Maryland
Status: Published in ApJL

One of the defining, and most puzzling, features of hot Jupiter exoplanets is that they often have much larger radii than expected. These giants are thought to be created by strong stellar flux from their host stars heating the deep interiors of the planets and inflating them. There’s evidence of this theory in observations, too, with hot Jupiters with the largest radii often being highly irradiated by their host stars. Because the most inflated exoplanets also have puffy atmospheres, they typically make great targets for characterisation since larger atmospheres produce bigger signals. Therefore, understanding the impacts of hot interiors on the circulation patterns and structure of an atmosphere could be an important step to figuring out exactly what makes hot Jupiters tick.

Fire Up the Models!

To study the impacts of internal heat on exoplanet atmospheres, the authors produce two varieties of general circulation models (GCMs). The first, a “fixed flux” model, uses an interior temperature comparable to those typically used in previous studies. The second, a “hot interior” model, better matches the expected deep temperatures from evolutionary models of hot Jupiters given the strong heating they receive from their host stars. For each version of the GCM, various simulations are produced of exoplanets at different orbital radii and surface gravities, with the atmosphere in each scenario allowed to equilibrate for the equivalent of 3,500 Earth days. In total, the various setups resulted in a grid of 28 GCM simulations.

Figure 1 shows a comparison between the final atmosphere resulting from the fixed flux and the hot interior GCMs for the hottest exoplanet in the model grid. Here, the difference between each GCM is shown for various pressure depths within the atmosphere in the right-hand column, with the highest pressures deeper in the atmosphere. These results show that a hot interior leads to differences in both the wind speed and the temperature, with changes in temperature of up to hundreds of Kelvin. These changes in atmospheric dynamics are seen at all depths in the atmosphere, but the changes are not necessarily consistent throughout the atmosphere. At pressure depths of 1 millibar (those probed by the transmission spectroscopy technique often used to study exoplanet atmospheres), the temperature differences are very localised, with the largest differences occurring in chevron-shaped features. The changes in wind speed also impact the region studied in transmission spectroscopy. The differences in wind speeds at the limb of the atmosphere (the region studied in transmission) at these pressures are comparable to the typical uncertainties being achieved in ground-based high-resolution observations.

Figure 1: Maps of the hottest exoplanet in the model grid in the final 500 Earth days of the GCM simulation. The gradient colouring highlights the local temperature across the latitudes and longitudes of the atmosphere, while the arrows illustrate the circulation patterns in the atmosphere. Each row shows the temperature map at a different pressure depth within the atmosphere, with the deep interior at the top of the plot and the upper atmosphere at the bottom. On the left, the GCM results for the fixed flux version are shown, while the GCM results for the hot interior version are shown in the centre. On the right, the difference between the two setups at each pressure depth is shown. [Komacek et al. 2022]

What Does This Mean for Other Exoplanets?

Expanding their studies across the whole model grid, the authors find that similar patterns in the atmospheric dynamics are seen for all the orbital radii and surface gravities considered. There are, however, some differences between the impacts at low and high gravities. The hot-interior GCM leads to differences in atmospheric temperature of up to 10% compared to the fixed flux GCM for the lowest gravity case, while the high-gravity case sometimes leads to temperature differences of more than 20%.

With all the potential changes seen when considering a hot interior, particularly with differences occurring in the region probed by transmission spectroscopy, might the current standard “fixed flux” models make it harder to interpret these and similar observations? By observing exoplanets throughout their entire orbit in a phase curve, JWST is expected to constrain the pressure–temperature profiles of hot Jupiter atmospheres to tens of Kelvin. Given that the hot-interior GCM results differed in places by up to hundreds of Kelvin, it does indeed seem possible that such assumptions could be problematic.

Original astrobite edited by Katy Proctor.

About the author, Lili Alderson:

Lili Alderson is a PhD student at the University of Bristol studying exoplanet atmospheres with space-based telescopes. She spent her undergrad at the University of Southampton with a year in research at the Center for Astrophysics | Harvard-Smithsonian. When not thinking about exoplanets, Lili enjoys ballet, film and baking.

image of HD 163296's disk

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: A Gap-Sharing Planet Pair Shaping the Crescent in HD 163296: a Disk Sculpted by a Resonant Chain
Authors: Juan Garrido-Deutelmoser et al.
First Author’s Institution:  Pontifical Catholic University of Chile and the Millennium Nucleus for Planet Formation
Status: Published in ApJL

An extra protoplanet might be lurking in the dust around a nearby star.

The pre-main-sequence star HD 163296 plays host to an extensive circumstellar disk, with gas reaching out beyond 500 astronomical units (au). Observations of this disk have revealed it to be quite the playground for the young planets forming within — and these authors claim there are more planets than previously thought.

Image of the disk surrounding HD 163296

Figure 1: The observed structure of the disk around HD 163296, as reported in Isella et al. (2018). The crescent-shaped region is visible at the bottom left of the inner gap, as indicated by the white box and as shown in the zoomed-in image (a). It appears to be a cloud of gas and dust within the gap, distinct from the overdense ring around it. [Adapted from Isella et al. 2018]

HD 163926’s disk has a series of rings — bumps and dips in surface brightness, corresponding to over- and under-densities of material at different distances from the host star (Figure 1). Observations of this sort of ringed structure have become common fare since the advent of high-precision millimeter imaging in the last 15 years. These rings imply the presence of protoplanets or other large substellar bodies, which can clear out gaps through their growth and alter what would otherwise be a smooth(er) profile.

Trying to figure out the exact setup of bodies that gives a disk its observed structure is an interesting problem — and in the case of HD 163296, it’s a problem that has proven a bit tricky.

It only took low-resolution images of this particular disk (like those taken with the Hubble Space Telescope at the turn of the century) for Grady et al. to suggest that a giant planet might be present in the outer reaches of this system. Jumping ahead to 2018, Teague et al. used rotation curves of observed gas to claim that there were likely two planets out there — both roughly as massive as Jupiter, at 87 and 113 au. Just a few months later, high-precision observations by the Atacama Large Millimeter/submillimeter Array (ALMA) revealed that the disk contains not just a series of gaps but also an intriguing substructure within the innermost one.

Today’s article focuses on this innermost gap, which extends from 38 to 62 au, and the crescent-shaped region near the edge of it that appears to have more material than it should (Figure 1).

The standard idea that gaps form along the orbit of a protoplanet doesn’t allow for this sort of uneven, crescent-shaped structure; the protoplanet should clear the material evenly all the way around the orbit, but it seems to have missed a spot. Modeling this gap, and the substructure within, would complete our current understanding of the HD 163296 system.

Luckily, the crescent has a fairly straightforward explanation. To understand it, though, we need to talk about Lagrange points.

diagram of the Lagrange points of a star–planet system

Figure 2: A diagram of the Lagrange points of a star–planet system. Smaller objects, like asteroids and dust, often accumulate at L4 and L5 due to the long-term stability of these points. [Mark Dodici]

If you’ve taken a class on classical mechanics, you might remember that Lagrange points are sort of like gravitational islands. For any pair of massive bodies (say, a star with a protoplanet), there are five points where the gravitational forces of the bodies balance nearly perfectly to keep much-less-massive things at those points, fixed relative to the more massive bodies. Two of these (L4 and L5) are stable against small displacements, meaning smaller things like asteroids and dust often accumulate at or around these two points. L4 lies almost exactly on the orbit of the less-massive body, in front of it by ~60 degrees. L5 trails behind the less-massive body by the same angle (Figure 2).

In the HD 163296 system, this crescent-shaped region with extra dust and gas could perhaps be explained as a build-up of material in the L4 or L5 of some massive protoplanet, which had otherwise cleared out the gap. In 2021, Rodenkirch et al. simulated the interactions between a gap-clearing planet and the dust around it, and they showed that this could work: a Jupiter-mass planet orbiting the star at 48 AU would both open up the observed gap and trap a significant amount of dust at its trailing L5.

And so the system was solved. The crescent-shaped substructure in HD 163296’s disk was the result of a Jupiter-mass planet. The other gaps were caused by two other planets farther out.

And yet, today’s featured article came out just recently. Why?

It turns out there were two problems with the Lagrange point idea. First, the crescent is centered at a radial distance of 55 au, which requires a lot of dynamical hoop-jumping-through to make sense for a Jupiter-mass planet at 48 au. Second, a Jupiter-mass planet would open up a deep gap in the gaseous disk. Less than a year after the first submission of Rodenkirch et al., observations by Zhang et al. of the disk’s carbon monoxide (CO) surface density — a great tracer for the overall gas density throughout a disk — showed that the gas gap between 38 and 62 au is ten times shallower than it would be if it were carved by a Jupiter-mass planet.

Enter Garrido-Deutelmoser et al. Last year, they studied the effects of having two planets in the same gap in a protoplanetary disk. Through hydrodynamical simulations, they showed that if two sub-Jupiter-mass planets are close enough to each other, their gravitational interactions would create relatively stable “vortices” at L4 and L5 of either of the planets. These vortices could maintain over-densities of dust and gas for thousands of orbits — plenty of time for us to have observed one of them.

In today’s featured article, Garrido-Deutelmoser (and a slightly different) et al. applied this concept to HD 163296. They set up simulations of the system mostly matching those of Rodenkirch et al., with the two proposed outer planets and a smooth disk of gas and dust. But in place of one Jupiter-mass planet at 48 au, they implanted two planets with a few times the mass of Neptune in that region. Since these two combine for a much smaller mass than Jupiter, they would create a much shallower gap in the gas density profile — ideally matching that found in Zhang et al.

A plot showing normalized gap depth as a function of semimajor axis

Figure 3: The radial gas density profile of HD 163296. Dark Purple: observed profile from Zhang et al. (2021). Magenta: simulated profile with one planet opening the 38–62 AU gap (the Rodenkirch et al. (2021) model). Orange: simulated profile with two planets opening said gap (this paper). Neither model fits well beyond ~85 AU, but the two-planet model matches the CO gap depth much more closely up to that point. [Garrido-Deutelmoser et al. 2023]

Through trial and error, they found that planets at 46 and 54 au could, in fact, carve out the appropriate density profile for this gap in both dust and gas (Figure 3) over the course of a half-million-year simulation. And in line with expectations from their previous work, material congregated at L5 for the outer super-Neptune (though they note that this ebbed and flowed over time). They do point out that neither their model nor the Rodenkirch et al. captured the density profile accurately beyond ~85 au, which they explain might be an issue with gas dynamics beyond that point. Regardless, their two-planet model for the gap of interest seems to be a winner.

They close the article with a final proposition, suggesting where in their orbits one might find each of the protoplanets, based largely on the fact that they seem to be close to a mean motion resonance chain — that is, they seem to have orbital periods that are roughly integer multiples of each other. Using a relationship for the orbital angles of objects in such a resonance, along with the location of the crescent and observations of kinematic features in the gas, the authors infer the precise locations of each of the protoplanets within the disk (Figure 4).

In the end, this might provide one final check for this finicky system. If the protoplanets are where they say they are, we’re golden. If not, the saga of HD 163296 will go on.

Two images of the surface brightness of a disk

Figure 4: The observed structure of the disk around HD 163296 (left) and the faux-observed structure from this simulation (right). The proposed locations of the four protoplanets are labeled on the left panel. While the simulated disk isn’t a perfect replica, it recreates most of the important details of the interior portions of the observed disk. [Garrido-Deutelmoser et al. 2023]

Original astrobite edited by Lucie Rowland and Zili Shen.

About the author, Mark Dodici:

Mark is a first-year PhD student in astronomy and astrophysics at the University of Toronto. His space-based interests include planetary systems, from their births to their varied deaths, as well as the dynamics of just about anything else. His Earth-based interests include coffee, photography, and a little bit of singing now and again. You can follow him on Twitter @MarkDodici.

1 2 3 4 5 6 … 38