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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.

Hubble image of the star at the center of the Bubble Nebula

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: Constraints on Magnetic Braking from the G8 Dwarf Stars 61 UMa and τ Cet
Authors: Travis S. Metcalfe et al.
First Author’s Institution:  White Dwarf Research Corporation
Status: Published in ApJL

How to Brake a Star

You might be familiar with the classic example of conservation of angular momentum where an ice skater is spinning on a (frictionless) piece of ice with their arms tucked in. When they extend their arms, they increase their moment of inertia. Conservation of angular momentum then requires that the rotation rate of the skater decrease — the skater “spins down.”

However, conservation of angular momentum is only valid if the system (the spinning ice skater) is isolated. Things are a little different if, for instance, the skater is holding weights as they spin. When they extend their arms, they again increase their moment of inertia and slow their rotation rate. Then, the skater can drop the weights. The instant that the skater drops the weights, the system is no longer isolated, and angular momentum is no longer conserved. The reduction in the moment of inertia caused by dropping the weights also reduces the angular momentum.

This same principle is at work in stars. Instead of a figure skater holding weights, there is a stellar magnetic field holding plasma. Winds from the star can push the plasma farther and farther out, causing the star to spin down like a skater with extended arms. Eventually, the plasma is pushed so far away that the magnetic field isn’t strong enough to contain it anymore, and the plasma is lost along with some of the star’s angular momentum. This is called magnetic braking.

Because magnetic braking gradually removes the star’s angular momentum and slows down the star’s rotation, a star’s rotation rate can be used to estimate its age. This principle drives a field of study called gyrochronology. More specifically, stars’ ages are characterized by their Rossby number: the ratio between the star’s rotation period and its convective overturn timescale (the time it takes for a bubble of plasma to move through the convective zone). As a star ages and its rotation rate decreases, its Rossby number increases. Eventually, a star’s rotation slows down so much that the critical Rossby number is reached. At this point, the star experiences weakened magnetic braking and the star spins down at a slower rate than it had previously experienced.

This Work

The authors of this research article investigate the transition to weakened magnetic braking for stars cooler than the Sun — the first time this has been done. This is an important distinction; cooler stars have deeper convective zones and thus longer overturn timescales. This means their Rossby number is smaller than it is for hotter stars with the same rotation period. It also means that at the critical Rossby number, their rotation period will be longer than hotter stars’ periods.

For this work, the authors look at two G8 dwarf stars — these are stars a few hundred degrees cooler than the Sun. In order to investigate the transition to weakened magnetic braking, they chose two stars of very different ages: the younger of the two stars is named 61 UMa and is about 1 billion years old. The older star is named tau Ceti and is ~9 billion years old, which is about twice the age of the Sun! To figure out the effects of magnetic braking on these stars, the authors consider two major parameters besides age: the stellar magnetic field shape and strength and the mass-loss rate of the wind. Stronger magnetic fields can hold plasma out to greater distances and hence provide a larger torque than weak fields do. Additionally, the higher the mass-loss rate is, the faster the angular momentum is lost.

For 61 UMa, the authors determine the magnetic field properties from previously collected Zeeman–Doppler imaging data and calculate the mass-loss rate from the X-ray luminosity. For tau Ceti, they collected data with the PEPSI instrument on the Large Binocular Telescope to estimate the star’s magnetic field and determined its mass-loss rate from previously collected Lyman alpha measurements. With all of this information, they can estimate the torques from the fields and winds that are braking the stars.

The results of their calculations show that 61 UMa experiences a torque about 300 times stronger than tau Ceti’s torque. This is consistent with the idea that older stars, especially those with Rossby numbers above the critical Rossby number, are much less efficient at braking than younger stars that haven’t yet reached the critical Rossby number. The torque they calculated is plotted against the Rossby number in Figure 1, along with hotter stars that had been previously studied. Besides a trend for stars with higher Rossby numbers to have weaker torques, this figure shows that this work extended the stellar sample to include both smaller and larger Rossby numbers and torques than had previously been investigated in this context.

A plot of the wind braking torque versus the Rossby number for several stars

Figure 1: Rossby number (Ro) vs the torque caused by stars’ stellar winds. Blue triangles are for stars hotter than the Sun, yellow circles are for stars around the same temperature as the Sun, and the red squares are for the two stars studied in the paper that are cooler than the Sun. The black dashed line is the empirically derived critical Rossby number. [Metcalfe et al. 2023]

Aside from the magnetic field and mass-loss rate parameters, other stellar parameters like rotation period and stellar size are also used to determine torque. The authors were able to investigate the contributions of the various evolutionary model parameters by varying them, one at a time. What this work confirmed is that, by far, the evolutionary change in mass-loss rate and magnetic-field properties dominate the effects of braking. Changing other parameters like the evolution of the rotation period, the stellar mass, and stellar radius contribute about 2–10 times less to the decrease in torque over time.

This research serves as an important expansion to our understanding of stellar spin evolution as a function of spectral type. This is crucial for understanding both stellar histories and futures and provides important insight to the environment of young and old stars alike. It also represents the beginning of work dedicated to understanding these cooler stars and solar analogs; the authors have plans to collect spectropolarimetric data to map magnetic fields on cooler K-dwarf stars so that they can extend their analysis to a broader sample. Although these stars are slowing down, the authors are moving full speed ahead!

Original astrobite edited by Sarah Bodansky.

About the author, Ivey Davis:

I’m a third-year astrophysics grad student working on the radio and optical instrumentation and science for studying magnetic activity on stars. When I’m not crying over RFI, I’m usually baking, knitting, harassing my cat, or playing the banjo!

ultraviolet image of the Andromeda Galaxy

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 Gamma-Ray Emission of the Andromeda Galaxy M31
Authors: Yi Xing et al.
First Author’s Institution: Shanghai Astronomical Observatory, Chinese Academy of Sciences
Status: Published in ApJL

Gamma rays are the highest-energy photons in our universe. Naturally, they come from some of the most extreme environments in the universe, such as pulsars, active galactic nuclei, supernovae, and potentially even dark matter. Though many gamma-ray sources have been detected both in the Milky Way and extragalactically, the nature of gamma-ray emission from our closest neighbouring galaxy, Andromeda (or Messier 31), remains somewhat of a mystery.

The Fermi Large Area Telescope (Fermi-LAT) is an instrument on the Fermi Gamma-ray Space Telescope that has been surveying the sky for high-energy gamma rays since 2008, with ample data taken on Andromeda throughout its flight. Many groups have analyzed these data, with more data giving more insight into what’s making these gamma rays.

To Extend or Not to Extend?

Up until today’s article, it looked like gamma rays from Andromeda were coming from a blob-like shape (i.e., extended emission) surrounding the centre of the galaxy (similar to Figure 1, left). This was particularly exciting, since extended structure in gamma-ray emission often suggests either a distribution of cosmic rays or the presence of a massive dark matter halo.

significance map of the Andromeda Galaxy in two gamma-ray energy ranges

Figure 1: Significance maps of Andromeda at energies from 0.1 to 500 gigaelectronvolts (left) and 2 to 500 gigaelectronvolts (right). The region of optical emission is represented by the white contour. The colorbar corresponds to test statistic, which is similar to significance. A test statistic of 25 corresponds to a detection. Green markers correspond to nearby sources found in the SIMBAD database. The left figure shows a hint of additional structure in the southeast region of Andromeda, but both point sources emerge out of the seemingly extended region only with the lowest energies cut out. [Xing et al. 2023]

Cosmic rays are charged particles that travel at relativistic speeds through the universe but get easily diverted by magnetic fields, making it very difficult to trace their origin from Earth. Luckily, since there are processes that produce gamma rays from charged particles (hadronic processes), identifying regions of extended gamma rays can trace regions where populations of cosmic rays are interacting with their environments. On the other hand, clumps of massive dark matter located in the centre of Andromeda could decay or annihilate, producing gamma rays in the process.

Where are the Gamma Rays Coming From?

A plot of observed and modeled spectral energy distributions

Figure 2: A spectral energy distribution showing flux (quantity of gamma rays received) plotted against energy of Andromeda’s centre (black) and southeast (red) emission regions, along with the Milky Way’s galactic centre (blue). It is apparent that both sources are not only similar in brightness but are also producing significantly more gamma rays than our galactic centre. Click to enlarge. [Xing et al. 2023]

A reanalysis of 14 years of Fermi-LAT data by the authors reveals that the emission of gamma rays isn’t extended after all. In fact, it seems that it’s constrained to two point sources: one located right at the centre of the galaxy and another ~20,000 light-years to the southeast (see Figure 1). This only became apparent when the authors cut out the lowest-energy gamma rays, which still make the data appear more or less extended when they’re included. Even more curiously, the authors found that both of these regions are significantly brighter than expected when compared to the gamma-ray emission of our own galactic centre (see Figure 2).

This new picture of Andromeda’s gamma rays changes a lot about our understanding of the galaxy. It’s no longer likely that Andromeda’s central gamma-ray hotspot is coming from a dark matter halo or cosmic ray distribution, so the authors looked to the Milky Way’s galactic centre to figure out what sorts of objects could be responsible for the gamma rays. One of the leading theories for our own galactic centre gamma rays is a population of old, unresolved objects, such as millisecond pulsars. However, in the case of Andromeda, at least 15,000 millisecond pulsars would be needed to account for the especially bright gamma-ray emission. While it’s still uncertain whether or not the centre of Andromeda can host this huge number of pulsars, we’ve only detected around 200 in the Milky Way’s centre, so this explanation seems unlikely.

The authors also investigate the southeast source that appeared in their new analysis. Since galaxies are pretty far apart from one another, the chance of finding two or more galaxies by coincidence in a circle drawn around both the central and southeast sources is only ~0.4%. This means that the emission is most likely coming from within Andromeda. As seen in Figure 2, the off-centre source is almost exactly the same brightness as Andromeda’s centre source (which is peculiar in its own right!), leading to the same problem of identifying sources capable of emitting such bright emission. After looking through X-ray and optical surveys, the authors determined that there weren’t any good counterparts for this region in other wavelengths either. Even considering the low probability of this being an extragalactic source behind Andromeda, there aren’t any known counterparts in the region of the sky where this hotspot is located.

The results are certainly unexpected and open up a whole new can of worms when it comes to figuring out the origin of the gamma rays in our neighbouring galaxy. Even though there are still a lot of unknowns, future observations and analyses of these newly constrained regions will help us understand how bright gamma rays are produced near the centres of galaxies and may even help us better understand our own galactic centre.

Original astrobite edited by Ivey Davis and Katya Gozman.

About the author, Samantha Wong:

I’m a graduate student at McGill University, where I study high-energy astrophysics. This includes studying all sorts of extreme environments in the universe like active galactic nuclei, pulsars, and supernova remnants with the VERITAS gamma-ray telescope.

An image of Saturn with a white circle to show the planet's oblateness

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 Effects of Planetary Oblateness on Exoplanet Studies
Authors: David Berardo and Julien de Wit
First Author’s Institution: Massachusetts Institute of Technology
Status: Published in ApJ

The world is a complicated place, and all scientists are just storytellers trying their best to explain it with more and more accurate fables. One of their most commonly accepted tropes in the canon of physics is assuming that anything round-ish, even a cow, is a perfect sphere. Astronomers implicitly do this all the time when it comes to exoplanets: we assume that all of them, big and small, hot and cold, are immaculate orbs.

Even in our own solar system, though, we know that this is not strictly true. Consider the hypothetical cosmic prospector: if they wanted to drill a hole to the center of Saturn, they’d better carefully consider their starting location. If they broke “ground” on the equator, they’d have to dig about 10% farther than if they had started from the north pole (setting aside the difficulties of “digging” through a gas giant). This is because Saturn is slightly squashed: its equatorial radius is larger than its polar radius, and it bulges out in the middle due to its rapid rotation.

Even though our investigations of worlds beyond our neighborhood have turned up strange and unexpected systems wholly different from our own, we have no reason to assume that there wouldn’t be similarly squished planets out there. Today’s authors take that tension between our simple models and expectation of non-spherical planets and answer two resulting questions: can we do better and actually measure a planet’s bulge, and, if not, what are the implications of using the wrong model?

Can We Detect It?

diagram demonstrating how the area of a star that is blocked by a transiting planet varies from a spherical planet to an oblate planet

Figure 1: The difference in the area of the star blocked by a spherical planet and an oblate planet. Note that as the oblate planet crosses the stellar limbs, it blocks different amounts of stellar area than a spherical planet would. The bigger this difference is, the easier it would be to detect that a planet is oblate. [Adapted from Berardo and de Wit 2022]

Actually measuring the degree of squishedness, more technically called oblateness, of a planet is tricky work. When we observe the transit of an exoplanet, the only thing we can measure at any moment is the area of the star blocked by the planet, not the shape of the photobomber blocking the light. As Figure 1 shows, the only time an oblate planet gives itself away are the moments it enters and exits our stage, just as it’s crossing the stellar limbs. Only then is the area blocked by the star different than we’d expect for a spherical interloper.

To assign a quantitative measure of this difference, the authors created fake data of oblate and spherical planets passing in front of their stars, zoomed in on just the beginning and end of the transits, then measured the root mean square of the difference between the two types of planets. They found that this value depends strongly on the cadence, or amount of time between measurements, of their fake data. This is illustrated in Figure 2. All told, they estimate that several tens of currently known planets would differ from a spherical model by a few tens of parts per million if they were actually oblate.

Figure 2: The number of known planets (y axis) that would differ from a spherical model by a given amount (x axis) if they all actually had an oblateness of 0.2. Note that shorter-cadence data can reveal more oblate planets than longer-cadence data. This is because the authors focus only on the ingress/egress of the planet, and shorter-cadence data allows for more measurements during that time. [Adapted from Berardo and de Wit 2022]

Unfortunately, most of our transit measurements aren’t that precise, meaning it would be very difficult to measure the oblateness of a single planet using existing data. That said, even though it isn’t easy to measure the oblateness of one planet, Kepler data are good enough to make statements about population-level trends. After fitting some real Kepler data with their oblate model, the authors concluded that most planets aren’t severely oblate, and short-period planets don’t seem to be systematically more oblate than long-period planets.

How Wrong Is Circular?

If we can’t really measure the oblateness of a moderately squished planet, does it even matter?

Unfortunately, the authors show that it might. Much like a slightly square peg can still be jammed through a round hole, we can still fit an oblate light curve with a circular model, but it won’t be a great match. Our estimates for many of the model parameters will be biased — that is, they’ll lean away from their true value in a given direction. For example, in Section 3 of the research article, the authors show that we’re likely to underestimate the true radius of a planet and overestimate its inclination.

This is unfortunate, but it’s important when we consider the downstream effects of how we use these transit-derived parameters to construct other descriptions of a planet. Astronomers have measured the masses and radii of a growing number of planets using the transit technique and by careful monitoring of the host star’s radial velocity, respectively. It is tempting to combine these measurements into an estimate of the density of the planet, but doing so requires assuming the volume of the planet. This requires assuming the planet is spherical, or at least that your measured radius is correct. If either of those are false, your final density will be incorrect. The authors estimate that available data can only rule out oblateness over about 0.25, but if that planet was even slightly rounder than that, we’d get its density wrong by more than 10%.

It’s nice to pretend that all cows are spheres. But we learn a lot, and can be more honest about the accuracy of our measurements, when we consider that some planets might be more complicated than we’d wish.

Original astrobite edited by Lili Alderson.

About the author, Ben Cassese:

I am a second-year Astronomy PhD student at Columbia University working on simulated observations of exomoons. Prior to joining the Cool Worlds Lab I studied Planetary Science and History at Caltech, and before that I grew up in Rhode Island. In my free time I enjoy backpacking, spending too much effort on making coffee, and daydreaming about adopting a dog in my NYC apartment.

a near-infrared view of hundreds of galaxies

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: CEERS Epoch 1 NIRCam Imaging: Reduction Methods and Simulations Enabling Early JWST Science Results
Authors: Micaela B. Bagley et al.
First Author’s Institution: The University of Texas at Austin
Status: Published in ApJL

Astronomers around the world breathed a sigh of relief when the JWST, a telescope more than two decades in the making, successfully launched and aligned its mirrors last year. However, there are still several challenges and kinks to work out in the imaging processing. In particular, there are many intermediate steps involved in turning JWST’s raw data into the beautiful final images released to the public. This article, part of the JWST Directors Discretionary Early Release Science (DD-ERS) public program, is one of the first to try to calibrate and measure some of these issues using real data taken in space.

The 13 DD-ERS programs are the first to receive science data from JWST, and in turn, they promise to publicly share all their data products, tools, software, simulations, and documentation to the community to help astronomers learn how to utilize and analyze JWST data. This article describes in detail one of these programs: the Cosmic Evolution Early Release Science Survey (CEERS), which is obtaining imaging and spectroscopy of the Extended Groth Strip Hubble Space Telescope legacy field. The Extended Groth Strip is an image of a small region between the Ursa Major and Boötes constellations, covering about 100 square arcminutes. The CEERS team’s main science goal was to test efficient observation strategies for extragalactic surveys with JWST.

This article presents the analysis of imaging from JWST’s Near Infrared Camera (NIRCam). This is one the first official public dataset releases from any DD-ERS programs. NIRCam has two channels that observe in parallel: one short-wavelength and one long-wavelength channel. Each channel has two modules: A and B, each having a field of view of 2.2 arcmin × 2.2 arcmin. Each module has four short-wavelength detectors, A1-A4 or B1-B4, tuned for observations in the range 0.6–2.3 microns (1 micron = 10-6 meter), and one long-wavelength detector, ALONG or BLONG, tuned for observations in the range 2.4–5 microns. The long-wavelength channel has a resolution of 0.06 arcseconds per pixel and the short-wavelength channel has a resolution of 0.03 arcseconds per pixel. The A and B modules have unique fields of view that lie adjacent to one another. As shown in Figure 1, the same field is simultaneously observed with A1-A4 and B1-B4 (short-wavelength) and ALONG and BLONG (long-wavelength), respectively.

Demonstration of fields of view and wavelength ranges for various JWST and Hubble filters

Figure 1: Top left: field of view of the eight short-wavelength detectors A1-A4 and B1-B4. Bottom left: field of view of the two long-wavelength detectors ALONG and BLONG. The transmission throughput curves (which show which wavelengths of light are filtered through a given filter) are shown in the middle-left panel, where F115W, F150W, and F200W are short-wavelength filters and F277W, F356W, F410M, and F444W are the long-wavelength filters. Example images from the F200W and F277W filters are shown on the bottom right. An image from the Hubble filter F160W is displayed on the top right for reference. The improvement in signal-to-noise and sensitivity from Hubble to JWST is striking. [Bagley et al. 2023]

This article describes the team’s challenges and solutions during the JWST image reduction process. Image reduction in astronomy is processing and converting the raw photons that a detector receives into a flux value from astronomical objects. However, sometimes after processing the images, non-astronomical artifacts can appear in the images. Below is a summary of the three main issues found in the CEERS team imaging:

  • Snowballs are large cosmic ray events that can affect hundreds of pixels and appear as a circular feature on the NIRCam detectors (see Figure 2 for an example of snowball removal). The team saw an average of 25–30 snowballs per detector in their images. To remove the snowballs, the CEERS team identified large contiguous pixels that had jumps in flux and then masked them out of the final image.
example of snowball correction

Figure 2: Example of snowball correction. The left image shows a count-rate map, where snowballs — large cosmic ray events — are present. There is a particularly large snowball in the lower left corner of the image. The middle image shows the identified snowballs. The right image is the result after subtracting the snowballs from the image. [Adapted from Bagley et al. 2023]

  • Wisps are created from stray light reflected off the secondary mirror supports. The strength of the wisp features depends on the source of the reflected light. They are visible on detectors A3, A4, B3, and B4 (see Figure 3 for an example). The CEERS team used wisp templates provided by JWST to fit for wisps in their images and then masked out the wisp features. The wisp templates will continue to improve as more programs obtain NIRCam imaging to characterize the wisps.
before and after images showing the effect of wisp removal

Figure 3: Top panel: an example of a wisp in NIRCam imaging in the black box. Wisps are visible in detectors A3, A4, B3, and B4 (inner four panels). The images are fit with wisp templates and then the wisps are masked out from the final images. The bottom panel shows a cleaned version of the image where the wisp feature has been removed.[Adapted from Bagley et al. 2023]

  • 1/f noise, also called pink noise, is correlated noise introduced in the images when the detectors are read out. In many modern detectors, incoming photons create photoelectrons that are trapped in local potential wells in a given pixel. After a specific time, the photoelectrons are counted, or read out, and the pixels are emptied and reset. In contrast to white noise, which has equal intensity per frequency, pink noise dominates at lower frequencies. In the NIRCam images, this noise presents as a horizontal and vertical striping pattern, as shown in Figure 4. To remove the noise, the CEERS team corrected pixel values more than two standard deviations away from the median of a given row or column.
illustration of removing 1/f noise

Figure 4: Illustration of removing 1/f noise, which presents as horizontal and vertical striping patterns in the NIRCam images. [Adapted from Bagley et al. 2023]

In conclusion, the CEERS team was able to robustly characterize and tackle many of the image-processing problems present in the NIRCam detectors. With more data and time, errors due to the reduction procedures will decrease. The CEERS team put in diligent work to better understand NIRCam, to the benefit of all astronomers using JWST!

Original astrobite edited by Katya Gozman.

About the author, Abby Lee:

I am a graduate student at UChicago, where I study cosmic distance scales and the Hubble tension. Outside of astronomy, I like to play soccer, run, and learn about fashion design!

a foreground star shining in front of the galaxy NGC 7250

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: Uranium Abundances and Ages of R-process Enhanced Stars with Novel U II Lines
Authors: Shivani P. Shah et al.
First Author’s Institution: University of Florida
Status: Published in ApJ

As someone who has recently left their 20’s, I think a lot about how age shows up in my body. I can look up my birth date on the calendar, even count all the minutes of my existence, but I don’t need to go through all that work. Something inside of me just feels… older. While the self realization of the unyielding passage of time on my mortal form may be daunting, I find solace in the fact that I’m no different than the stars — they also carry around their own clocks. Today’s article is about an interesting technique to determine how old a star is by looking at how much uranium is “ticking” in its atmosphere.

The Smallest Hand of the Clock

The technique is based on radioactive dating, a tool used in a variety of ways but most famously in carbon dating. Living things on Earth have carbon atoms in their bodies, some of which are carbon-14 (C14). C14 is a radioactive isotope of carbon, meaning it has a slightly different mass than other carbon atoms, and it decays over time. Even though it decays, C14 is regularly replaced while an organism is alive, and so the ratio between regular carbon and C14 within living things stays mostly steady. Once the flow of C14 stops (aka something dies), the ratio between carbon and C14 changes as the latter decays. By measuring the remaining ratio and comparing it to the normal ratio, we can calculate how much time has passed for the correct amount to decay.

Radioactive dating isn’t just for living things. C14 decays at such a rate that you can use it to measure the ages of objects up to around 50,000 years old, but the same technique works for any element with a radioactive isotope. Our best estimate for the age of our planet and solar system comes from radioactive dating using different elements that have radioactive isotopes that decay at slower rates.

Uranium is an element that is perfect for this, as its radioactive isotope takes billions of years to decay. Today’s article is all about trying a new way to measure the abundance of uranium in stars to find an age estimate for stars with radioactive dating. It’s known as nucleocosmochronometry (a word that spans one and a half Scrabble boards).

Tiny Atoms in Massive Stars

plot of an example uranium feature in the spectrum of a star

Figure 1: An example uranium feature in a star’s spectrum. The black points are the data. The red line is the best model for the spectrum that includes uranium. The authors also fit a model that doesn’t include uranium, which is the blue dashed line. By measuring the difference in the two models, the authors estimated an abundance of uranium responsible for absorbing the missing light. [Adapted from Shah et al. 2023]

It might seem incredibly difficult to detect atoms in a massive star that is light-years away, but it’s actually one of astronomy’s oldest tricks (astrobites has a whole guide about it!). Each element and molecule has its own characteristic fingerprint — its spectral signature — in the form of the specific wavelengths of light that it absorbs (called spectral lines). These can be measured in a lab, and then by looking at the light coming from a star and seeing which wavelengths are being absorbed, we can tell what’s in the star’s atmosphere.

This works great until elements and molecules have lines very close to one another, which is a major challenge with uranium. As it turns out, the typical line used to measure uranium abundance is blended with both an iron line and a cyanide feature (Figure 1). It’s still possible to get a measurement, but today’s authors wanted to use two new uranium lines to measure abundances and see how well they agreed with the single-line method. Even though these new lines are also blended, three measurements allow the authors to do a better job of describing the certainty of the measurement by using statistics to compare the abundances measured between the three lines.

plot of age estimates for four stars

Figure 2: The age estimate of the four stars (names on the x axis) from this work (colored squares) compared to other work (white squares). The accepted age of the universe is included as a black dashed line. The age estimate is not a ratio of uranium to a uranium isotope since only total uranium was measured, but uranium against europium, which does not have any radioactive isotopes so should be a constant and appropriate comparison. [Adapted from Shah et al. 2023]

Do You Have the Time?

The authors measured the abundances of uranium for four stars and compared the results from a fit using just a single-line measurement in each of the stars to one using all of the uranium lines. They found that the abundances from both methods were within a reasonable range of one another.

When the authors used the abundances to measure an age (Figure 2), they found ages that were similar to those calculated with just the single measurement. You might notice that the age estimates have big error bars — some stretch to an age older than the universe! There are clearly still some challenges with the method in general, in part because it’s hard to know how much uranium was in the star to begin with. The authors chose these four stars for the study because they are examples of stars that should have had more uranium. Regardless, the production rates of uranium remain a big question mark.

The Clocks Keep Spinning

It’s worth mentioning that the uranium that makes these stellar clocks tick formed in merging neutron stars, the dramatic burst of atomic creation when two “dead” stars collide. A star’s clock, and even its very existence, is due in part to the stars that came before them. Makes me think about how even if my body feels old, my time being alive has been traced through thousands of lifetimes similar to my own. 30(+) be damned, I’m going for a walk.

Original astrobite edited by Jessie Thwaites.

About the author, Mark Popinchalk:

I’m a PhD Candidate at CUNY/Hunter College based at the American Museum of Natural History. I study the age of stars by measuring how quickly they rotate. I enjoy ultimate frisbee, baking bread, and all kinds of games. My favorite color is sky-blue-pink.

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