AAS 244: Day 3

Editor’s Note: This week we’re at the 244th AAS meeting in Madison, WI, and online. Along with a team of authors from Astrobites, we will be writing updates on selected events at the meeting and posting each day. Follow along here or at astrobites.com for daily summaries, or follow @astrobites.bsky.social on Bluesky for more coverage. The usual posting schedule for AAS Nova will resume on June 17th.

Table of Contents:

Helen B. Warner Prize Lecture: The Lives and Deaths of Star Clusters, and the Black Holes They Make Along the Way, Carl Rodriguez (University of North Carolina at Chapel Hill) (by Catherine Slaughter)

The Wednesday morning plenary lecture was given by this year’s Warner Prize recipient, Carl Rodriguez (UNC Chapel Hill). Rodriguez is a theoretical and computational astrophysicist, whose recent research focuses on the gravitational waves that could be produced by black holes in globular clusters. He opens the lecture emphasizing how our population of observed black holes has changed dramatically since 2015, when LIGO first came online. While gravitational waves have been used to observe a huge number of new black holes, Rodriguez highlighted a smaller set has been identified using radial velocity and (using Gaia) astrometry.

Rodriguez highlighted that newly identified populations often include black holes in the known upper mass gap; there are few black holes found with masses ranging from 65 to 135 times the mass of the Sun. The mass gap is a predicted result of late-stage stellar evolution, when stellar remnants of these masses are torn apart in highly energetic pulsational pair instability supernovae. We simply don’t expect to see isolated black holes of this size. Additionally, the observed astrometric binaries have masses and separations that are largely inconsistent with the expected parameters of common-envelope evolution of isolated binary stars. Rodriguez explained, however, that both of these oddities may be explained by the unique conditions inside globular clusters.

Rodriguez goes on to describe how black holes move throughout a globular cluster. Generally, dynamical friction causes very massive, dense objects to preferentially move toward the center of the cluster over relatively long timescales (~100 million years). Once they’re in generally the same space, black hole binaries can form through three-body system mechanics. In classical Newtonian mechanics, it is not possible for two unbound point particles to enter a bound orbit via standard gravitational interactions — there is simply too much energy in the system. It is possible, however, with the introduction of a third “interloper” star, which experiences a recoil effect, carrying the extra energy away with it. This phenomenon can explain the seemingly strange orbital parameters of the astrometric binaries. The remaining binary is either kicked from the cluster, merges while in the cluster, or gets disrupted by the cluster.

Of course, this all relies on a strong understanding of how globular clusters form in the first place. In order to better understand this, Rodriguez and his research group make use of their Great Balls of FIRE simulations. This project focuses on “daisy-chaining” three types of models. First, a high-resolution cosmological simulation shows how individual molecular clouds form in galaxies. Then, a cloud-collapse simulation takes in that output and models the creation of stars from the material. Finally, an N-body simulation shows how those stars interact and dynamically evolve over time. Their final result is a simulated galaxy, with mass similar to the Milky Way, containing a similar number of globular clusters to the Milky Way. These clusters are shown to have a similar age-alignment relationship to those in the Milky Way, and — most notably — a distribution of black hole masses that includes the upper mass gap. The question then becomes how the simulated mass-gap black holes formed. Rodriguez speaks primarily about two possible mechanisms: black hole mergers and progenitor star mergers.

An image of one of Rodriguez’s slides, outlining the structure of the Great Balls of FIRE simulation. [Slide by Carl Rodriguez]

Analysis of the simulation found that a given black hole is expected to experience, in the most extreme case, two or three mergers. This is because rapidly spinning black holes release gravitational waves in a preferred direction when they merge, causing the merger remnant to be “kicked” in the opposite direction. The more mergers a black hole experiences, the higher its spin, and the harder it gets kicked, eventually getting kicked from the cluster altogether. Indeed, they find only three of these most extreme hierarchical mergers manage to survive in the cluster. It is also possible that stars themselves merge over time, creating very massive objects that could, in turn, form significantly more massive stellar black holes. Using the simulation, Rodriguez and his research group find that because these clusters form hierarchically, they at some point have density profiles that are flat and relatively unstable, leading to runaway merger events.

An image of one of Rodriguez’s slides, showing the output of the Great Balls of FIRE simulation. [Slide by Carl Rodriguez]

Ultimately, Rodriguez explains, new populations of black holes look unlike how we expect because they were likely formed under fundamentally different conditions than we expect. His simulations show that black hole binaries are ten times more efficiently formed in cluster environments, compared to isolated environments. Importantly, recent observations of binary black holes in the upper mass gap can be well explained by the conditions in globular clusters.

You can read Astrobites’s interview with Carl Rodriguez here.

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Press Conference: More Stars and Distant Worlds (by Ben Cassese)

Wednesday morning, Madison, Wisconsin. As some clouds gathered over Lake Monona just outside the convention center, a comparatively friendly crowd of attendees of the 244th meeting of the AAS assembled to hear the final research-focused press briefing of the conference. Titled “More Stars and Distant Worlds,” this session had presentations split between individual interesting objects and looks towards future observations.

First up was Mary Barsony, who along with collaborators discovered an unusual and striking pair of young stars using the combined powers of ALMA and JWST. The researchers came upon this rare find after noticing something strange in archival images of a triple star system. Although two of the three members appeared about two million years old, measurements indicated that the third was somehow much younger. A closer look at this outlier revealed that what had previously appeared to be one strange star was actually a tightly bound pair, both of which are spewing out long jets of material. Intrigued, they observed the system again, this time using ALMA. This second look revealed nearly edge-on, skirt-like disks of gas around each star. This unique arrangement is an interesting laboratory to test theories of stellar formation and outflows, and a powerful demonstration of the benefits of combining multiple cutting-edge datasets. [Press release]

Next was Breanna Binder, California State Polytechnic University, Pomona, who jumped beyond the cutting-edge facilities of today and instead spoke about the work required to build the next generation of Great Observatories. The Habitable Worlds Observatory (HWO) currently exists only on paper, but by the 2040s, it should hopefully be a real, gigantic space telescope hard at work discovering and characterizing Earth-like planets. Before engineers and technicians can begin bolting together the observatory, however, astronomers need to figure out exactly where they want to aim the thing and what performance specs they need to achieve their ambitious goals. As part of that effort, Binder and collaborators went through a list of about 200 promising target stars to check which of these were amenable to hosting a habitable planet. They found that about a third of them had previously been observed by X-ray telescopes, and in many cases, these archival observations revealed a disappointing but important reality. Although many of their potential target stars appeared docile in the visible wavelengths, a good portion of them are furiously emitting X-rays that are strong enough to (likely) doom any chance of life emerging on the surfaces of their planets. Her presentation emphasized that while the road to HWO will be long, by the time the telescope reaches the launchpad, it’ll be powered on its journey to space both by not only its rocket but also by decades of planning. [Press release]

Next was Roman Gerasimov, University of Notre Dame, who quickly informed the audience that he is “Paid to figure out where the periodic table came from.” He goes about this substantial task by trying to find ancient brown dwarfs and measure their compositions. Brown dwarfs are too small to sustain nuclear fusion in their cores, which means that after being born bright and hot, they fade and cool over time. If one can find a collection of brown dwarfs that all should be around the same age, say a collection that lives in the same globular cluster, then measure their temperatures, the scientist could use them as “galactic chronometers” to estimate the age of the system. That’s exactly what Gerasimov and collaborators did: using images from JWST, they identified three ancient brown dwarfs in a globular cluster named NGC 6397. These puny, sub-Jupiter-sized worlds are likely among the oldest objects in our galaxy, and also likely are just the first of hundreds of similar discoveries. Gerasimov estimates that just one more set of JWST observations taken about three years from now would reveal hundreds more, since the longer baseline would allow the team to distinguish between members of the cluster and faint objects that happen to lie in front of or behind it.

Finally was Juliette Becker, University of Wisconsin-Madison, who described recent modeling of the “perilous” journey a planet with an ocean must embark on as its star grows old. This journey has elements of a good action movie: in order to reach a happy ending where the planet retains its ocean and travels on a stable orbit around its aging white dwarf, it must survive two distinct challenges. First, as its host expands from a typical main-sequence star into a red giant, the planet must avoid getting engulfed by the suddenly enormous central body (for those curious, when our own Sun becomes a red giant, Mercury and Venus will almost certainly fall prey to the ballooning star, though the jury is still out on whether Earth will similarly suffer). Should it successfully avoid getting eaten, then patiently wait out the star’s inevitable shrinkage from a red giant into a puny white dwarf, the planet will now find itself far too far away and cold for anything liquid to remain on its surface. In order to save its ocean, the planet must be kicked inwards towards the white dwarf by some other object in the system, then survive the resulting high-eccentricity migration and tidal dissipation required to land on a close-in, stable, warm orbit.

Becker has practical motivations for fleshing out the odds of each step in this string of hypotheticals, since planets around white dwarfs are significantly easier to characterize than planets around more generic solar-type stars. That’s because the primary technique astronomers use to measure the atmospheres of close-in planets, the transit technique, returns stronger signals as the radii of the star and planet become more similar. So, Becker and collaborators wanted to assess whether it would be worth our time to put the time into surveying these stars in the first place. Their conclusion? Although an ocean could technically survive the journey laid out above, “it’s not an easy process,” and it’s unlikely that a large portion of white dwarf planets have oceans.

You can view a video recording of this press conference or take a look at the presenters’ slides.

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Plenary Lecture: When Data is Not Enough: Illustrating Astrophysics for the Public, Robert Hurt (Caltech/IPAC) (by Will Golay)

Robert Hurt is a researcher at IPAC, an institution based at Caltech that supports four central data archives and 12 different NASA missions. Hurt is a self-described “AstroVizicist,” an expert in visualizing astronomical data for researchers and the public. In his talk, he aimed to motivate the importance of artistic visualization and its role in science communication.

Hurt shows one of the first pieces of acrylic artwork he made early on in his life, an image of a galaxy with several key features aiming to communicate how they are structured. However, Hurt points out that his galaxy painting from 1979 was missing a central bulge region. He noted that this was likely a product of the time: the images of galaxies at the time were being collected on photographic film plates, which would highly overexpose the galaxy’s central regions to have enough sensitivity to capture the lower surface brightness components near the edge. Hurt’s message is that scientific illustration is a sort of hypothesis representing our current understanding of the universe.

To determine if a scientific illustration is successful, we can ask if it is accurate, clear, and, most importantly, amazing. These three components — representing the science, communications, and art “Essential AstroViz Tensions” — can shape how we approach creating a new illustration. In any new science result, there are known facts, known falsehoods, and possibilities consistent with the data but not wholly affirmed. As a scientific illustrator, Hurt aims to capture the most important new known facts of the result, but he also mentions that we can’t possibly know everything. Sometimes, an illustrator must fill in the gaps left behind to paint a complete picture. They might even choose to specifically modify and include some “falsehoods,” such as synthetic colors or rescaling an object so it is visible in the image, to clarify the illustration (trading off accuracy for the value of communication).

Hurt then discusses several case studies of his work and how the scientific illustration process works. In one example, he shows an early image of the Milky Way that resulted in a few inaccuracies as new research emerged. So, he aimed to create a new, better illustration of what our own galaxy might look like to an external observer. Illustrating our galaxy in this way can provide insights that are otherwise challenging to understand with our “inside-out” view of the galaxy. This new image was informed by several scientific results, such as gas simulations, spiral arm models, observations along specific sightlines, and even some unpublished results. The new image ended up being so well-constructed that a feature on the far side of the galaxy, the Scutum-Centaurus Arm of the Milky Way, was so accurate that a different researcher ended up confirming some key details and contacted Hurt because he was concerned he had access to his unpublished data! The artist had simply used some basic symmetries.

The updated illustration of the galaxy incorporates updates to the original image informed by accurate science results. This image is our best guess of the Milky Way's structure.

The updated illustration of the galaxy incorporates updates to the original image informed by accurate science results. This image is our best guess of the Milky Way’s structure. [Slide by Robert Hurt]

Hurt concludes by highlighting his non-traditional career path and the importance of fostering non-traditional and traditional academic career skills in our students. He states that we never know where a student will end up and how their skills outside of the classroom may contribute to a new, unique future role in astronomy that we can’t even predict. It is important for mentors to foster both of these skillsets in their students, even though that might mean a student is working on something beyond their research. Hurt’s illustrations are available online and are free to share with anyone.

You can read Astrobites’s interview with Robert Hurt here.

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Press Conference: Citizen Science from the 2023/2024 Solar Eclipses (by Catherine Slaughter)

Wednesday afternoon brought with it a press conference highlighting citizen science from the 2023 (annular) and 2024 (total) eclipses. Speaking at the conference were Amir Caspi (Southwest Research institute), Angela Des Jardins (Montana State University), Dawn Davies (Hill Country Alliance) and Kate Russo (Being in the Shadow).

A slide from Caspi's press release. It shows a composite image of the solar corona taken by his citizen science teams. The colors indicate the angle of polarization. Caspi's zoom image can be seen on the right.

Composite image of the solar corona taken by his citizen science teams. The colors indicate the angle of polarization. [Slide by Amir Caspi, photo by Catherine Slaughter]

First up was Amir Caspi, reporting on the results of the Citizen CATE project. Citizen CATE featured an observing network of community participants and aimed to study polarization in the solar corona. He highlights that total eclipses provide better ground-based coronal observing than a coronagraph does because eclipses also create pseudo-dark-sky conditions. Because the coronal activity the team was searching for lasts longer than the few minutes an eclipse can be observed at a single site, the Citizen CATE team gave identical, specialized telescopes to 35 teams along the eclipse path. He highlights that the communities that participated were all allowed to keep the equipment for future science and outreach purposes. In all, more than 85% of the sites were able to obtain full or partial eclipse data for the project.

Next up to the podium was Angela Des Jardins, speaking about her eclipse ballooning project, which aimed to measure the atmospheric response to the eclipse leading up to and after totality. Stratospheric ballooning is a multi-disciplinary endeavor, and is useful because it creates space-like conditions without the need for a rocket. For the eclipse project, two teams of researchers — an atmospheric science team and an engineering team — spent the 24 hours preceding the eclipse and 6 hours proceeding, launching stratospheric weather balloons with weather sensors, accelerometers, cameras, and other student-designed experiments. Her primary goal was to observe atmospheric gravity waves (which, she highlights, are not the same as gravitational waves). The experiment was successful, with the first confirmed observations of eclipse-driven gravity waves. [Press release]

Then Dawn Davies stepped up to speak about her project, LightSound. LightSound is a solar eclipse sonification device, made for those in the blind and low-vision community to better experience eclipses. Leading up to the 2023 and 2024 eclipses, LightSound received more than 2,400 requests for devices internationally. Over the course of eight workshops, 420+ volunteers of a broad range of ages and previous building experience created more than 900 devices for libraries, museums, community task forces, blind and low vision services, national parks, senior centers, and more. The team also set up a live-streamed webinar in conjunction with the American Council for the Blind, which reached an audience of more than 2,500 people. As of now, the LightSound team is looking ahead to the 2026 and 2027 eclipses, and are hoping to implement a haptic feedback feature to the device. [Press release (PDF)]

Finally, Kate Russo took the stage to describe her project, Being in the Shadow. A clinical psychologist by trade and eclipse chaser by hobby, Russo and her collaborators work to observe the feeling of awe inspired by viewing a solar eclipse through brain wave signatures. She defines awe by two specific experiences. The first is vastness, an understanding of something greater than ourselves, and the second is accommodation, the way we re-align our perspectives accordingly.  In previous phenomenological research, Russo identifies six experiences that people most often report experiencing during a total eclipse: a sense of wrongness, a primal fear, awe, connected insignificance, euphoria, and a desire to repeat (yielding the appropriately themed acronym “SPACED”). In a pilot project conducted during the 2023 annular eclipse, their primary goal was to simply test the efficacy of the brain-wave mapping technology. What Russo found, however, was that even in a non-total eclipse, they were able to observe awe in every single participant’s data. Looking forward, they are hoping to repeat the experience at the 2028 eclipse in Australia (where Russo primarily works), and possibly extend the project to other astronomical events. [Press release (PDF)]

An image of the panel at the eclipse outreach press conference

An image of the panel at the eclipse outreach press conference. Shown (left to right) are Susanna Kohler (at the podium), Angela De Jardins, Dawn Davies, and Kate Russo. Amir Caspi presented from zoom and is not shown. [Photo by Catherine Slaughter]

You can view a video recording of this press conference or take a look at the presenters’ slides.

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Royal Astronomical Society Gold Plenary Lecture: Challenges to the Cosmological Model, John Peacock (University of Edinburgh) (by Nathalie Korhonen Cuestas)

Over the course of 67 years, modern cosmology was born and matured into the current standard model, known as Λ cold dark matter (ΛCDM). In his plenary lecture, Dr. John Peacock laid out the history of cosmology, highlighting the measurements that got us to where we are now, explained some of the possible problems with ΛCDM, and outlined how recent results address these challenges.

There’s a plethora of evidence supporting ΛCDM. From redshift measurements of Type Ia supernovae, to the large-scale structure of the universe, to the anisotropies of the cosmic microwave background (CMB), many signs point towards ΛCDM. However, there are some areas of the model that we don’t fully understand — including but not limited to the nature of dark energy and dark matter — and some areas of the model that appear to be in conflict with observations. Peacock touched on four major sources of tension — large-scale streaming velocities, the CMB dipole, Hubble tension, and lensing of the CMB — and focused primarily on the last two.

You may know that the universe is expanding at an ever increasing rate, and the Hubble constant describes the speed of this expansion. There are a number of different ways you can estimate the Hubble constant, but they generally fall into two categories. One method involves measuring the distance to and velocity of distant galaxies. These two quantities are linearly related and the slope of the line can give you the Hubble constant. Alternatively, you can calculate the Hubble constant from the temperature of the CMB, which is very precisely measured. But there’s a problem: these two methods yield different values for the Hubble constant. Typically, Type Ia supernovae or Cepheid variable stars are used to calculate the distance measurements, but, Peacock said, these methods might be introducing unknown systematic uncertainties. He also showed that when a different standard candle (J-region asymptotic giant branch stars in this case), you can get a value for the Hubble constant that more closely agrees with the CMB value.

The second source of tension Peacock discussed was the results from weak gravitational lensing studies. Mass warps spacetime, and this leads to a lensing effect that produces distorted images. If you know what the undistorted image should look like, you can use the distortions to calculate the mass that must have been along the line of sight and produced the lensing. The technique can be applied to the CMB; by measuring distortions in the CMB, you can back out the amount of mass that must have been along the line of sight. Since the CMB is the edge of the visible universe, the mass along the line of sight probes the distribution of mass throughout the universe, which is fundamentally determined by cosmology.

Peacock showed that we do in fact observe slight distortions to the CMB, and the overall distortion is in good agreement with ΛCDM. However, when distortion was broken down into different redshift intervals, astronomers found something different. When only considering the mass at high redshift, the results were consistent with ΛCDM, but this was not the case at lower redshifts, in the local universe. The result might point towards a potential time evolution in ΛCDM, specifically in the dark energy component of the model (that’s the Λ). However, Peacock was careful to emphasize that these results are still in their early stages, and further study is needed before we make any major adjustments to our cosmological models.

You can read Astrobites’s interview with John Peacock here.

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Solar Physics Division Harvey Prize Plenary Lecture: Dissipation and Excitation: The Role of Kinetic-Scale Waves and Instabilities in the Evolution of the Solar Wind, Kristopher G. Klein (University of Arizona)

The Karen Harvey Prize is awarded by the Solar Physics Division of the AAS in recognition of significant contributions to solar physics early in one’s career. This year, the award went to Kristopher Klein of the University of Arizona. Klein studies the solar wind — the tenuous plasma that constantly streams out from the Sun and fills the solar system. Because the Sun is by far the closest star to Earth, there are types of data that we can collect from the Sun that we simply can’t get for any other star or distant plasma environment. While Klein jokingly laments that many solar physicists interact not with pretty pictures of the Sun but instead with densely packed plots of particles and electric and magnetic fields, studies of the Sun have far-reaching implications.

Klein’s work uses simulations to understand how energy is transferred within the solar wind plasma. Energy can be transmitted by waves, instabilities, and turbulence, which transfers energy from large spatial scales to small spatial scales. Studying these factors can help researchers understand electron and proton heating, which can in turn help to interpret observations of distant plasma environments, such as the pictures of Sagittarius A*, the supermassive black hole at the center of the Milky Way.

In addition to simulations, solar physicists work with an abundance of spacecraft data: there are currently 20 missions (27 spacecraft in total) capable of making in situ measurements of the solar wind, and 14 more missions are currently being formulated or implemented. Using this fleet of spacecraft, researchers can extract the basic properties of the solar wind and derive more complex parameters, like the plasma beta (the ratio of the thermal pressure of a plasma to its magnetic pressure). Because the plasma beta varies greatly with position in the solar system, different locations within the solar wind can be used as analogs for many different plasma environments across the universe.

Klein expanded on the projects that he and his research team have undertaken to track down the ways in which energy flows within the solar wind. While important advances have been made with existing data and simulations, most current spacecraft missions measure the properties of the solar wind at a single point, taking measurements as the solar wind blows across the spacecraft. (A few missions like the Magnetospheric Multiscale mission involve multiple spacecraft working in tandem.) Klein introduced the upcoming HelioSwarm mission, which will be composed of nine spacecraft that explore the solar wind, magnetosheath, and foreshock on a variety of spatial scales to study these regions in an entirely new way. With new views of the solar wind incoming, we can expect our knowledge of the solar wind and other plasma environments across the universe to increase in leaps and bounds!

image summarizing current heliophysics missions

A summary of the current (as of 2022) fleet of heliophysics missions. [NASA’s Goddard Space Flight Center]

You can read Astrobites’s interview with Kristopher Klein here.

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