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two images of Dracula's Chivito

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: Dracula’s Chivito: Discovery of a Large Edge-On Protoplanetary Disk with Pan-STARRS
Authors: Ciprian T. Berghea et al.
First Author’s Institution: US Naval Observatory
Status: Accepted to ApJL

Where Planets Are Born

Studying protoplanetary disks helps us understand how planets, including those in our solar system, are born. These disks are vast and flared structures, consisting of dust and gas orbiting a young star. Protoplanetary disks contain the remnants of the stellar birth process, in which a collapsing molecular cloud gives rise to a central star surrounded by a swirling disk of material. Protoplanetary disks are vital to observe as they are the birth sites of planets. The tiny dust particles come together, sticking to each other and forming larger bodies. This process, influenced by gravity, gas, and radiation, leads to the birth of planets in developing planetary systems.

Meet the Vampire Sub

This research article features a large edge-on protoplanetary disk that was stumbled upon when going through images from the Pan-STARRS research project as a part of  a study of active galactic nucleus candidates. This disk is one of the largest known disks in the sky and is oriented edge on, completely obscuring its central star. Associated with a source of infrared light in the same region of the sky, IRAS 23077+6707, the disk spans approximately 11″ in apparent size, with a very faint structure in the disk’s northern part extending out to about 17″. It is possibly the largest protoplanetary disk (by angular extent) discovered to date. The structure of this disk is reminiscent of the popular Gomez’s Hamburger, which is not associated with any star-forming region, just like the subject of this article. The similarity to a sandwich, along with the fang-like structures in the northern part of the disk as seen in the images in Figure 1, earned the IRAS 23077+6707 protoplanetary disk the name “Dracula’s Chivito” (chivito is a type of sandwich and the national dish of Uruguay, where one of the co-authors is from).

Left: Pan-STARRS image of Dracule’s Chivito with a bright blue-purple light in the center, as a structure resembling a sandwich. It has faint filaments extending out like “fangs”. Right: A model of the disk on the left, which looks smoother and the fangs are shown to be a part of larger disk envelope, which is a hazy blue light emanating from the disk in the center which has a pink hue

Figure 1: Pan-STARRS image of Dracula’s Chivito (left) and the associated radiative transfer model (right). In the northern part of the disk, very faint filaments are seen extending 17″ out from the edges of the disk. These filaments have been nicknamed “fangs.” These are interpreted as the edges of the disk envelope, as reproduced in the model image. These fangs are likely obscured by a cloud in the southern part of the disk. [Adapted from Berghea et al. 2024]

Decoding DraChi

Analysis: Images of Dracula’s Chivito (henceforth referred to as DraChi) were obtained in the grizy filters of Pan-STARRS (Figure 1). These images and data from the Galaxy Evolution Explorer (GALEX) (ultraviolet), the Two Micron All-Sky Survey (2MASS) (infrared), the Infrared Astronomical Satellite (IRAS) (infrared), and AKARI (infrared) were used for photometric analysis, i.e., flux or brightness measurements. These data were used to construct the spectral energy distribution of the disk (Figure 2), together with a radiative transfer model generated using the code HOCHUNK3D. Radiative transfer models help us understand the disk geometry and how light is scattered by the dust grains in the disk. This light, which we see along our line of sight, is plotted as a spectral energy distribution. Spectral energy distributions give information about how much energy an object gives off at different wavelengths.

A plot of the spectral energy distribution of the disk as obtained from brightness measurements from different instruments and compared to the spectrum from the model. The different colored dots representing data are mostly fit well by the model except in near and mid infrared wavelengths (from about 10 to 50 microns)

Figure 2: The spectral energy distribution of the disk, using photometric data from the image, the model, and other sources as mentioned in the article. Brightness is plotted as a function of wavelength. The data (colored dots) match the model (the solid red line shows the model without extinction, and the dashed red line shows the model with extinction) in most places except in near- and mid-infrared wavelengths, which could be due to discrepancies between instruments or possible variability in the disk’s luminosity. [Berghea et al. 2024]

Distance: It is hard to estimate DraChi’s distance because it is not associated with a known star-forming region. Accurate distances to local molecular clouds are vital to locating protoplanetary disks and comprehending planet formation processes. Therefore, using the Gaia DR3 data for nearby stars, the extinction of the disk was estimated to find the distance to the nearest interstellar clouds, and DraChi is hence estimated to be 978 light-years away.

Basic Properties: The spectral energy distribution suggests that the host star of the disk is a pre-main-sequence star of type A with a temperature of about 6500–8500K. The images of the disk and the resultant radiative transfer model constrain the disk inclination to be between 80° and 84°. The scale height of the disk is about 25–50 au at a radius of 500 au (astronomical units), where 1 au is the distance from Earth to the Sun. Using the distance and the angular extent of the disk in the sky, the disk’s radius is estimated to be 1,650 au. The radiative transfer model, based on the scattering of the light, quantifies the mass of the disk to be about 0.2 times that of the Sun.

The Fangs: The authors noticed two “fang-like” features in the northern part of the disk, and these features were also reproduced in the model of the disk. The “fangs” closely resemble the “edge” of the shadow created by the disk in the bright surrounding envelope. They could be filaments due to a possible outflow from the central part of the disk, which is characteristic of a young disk at the end of the Class I phase (~0.5 million years old). It is possible that the fangs are present in the south, but this region is likely obscured in the images from Pan-STARRS and could be perhaps seen in infrared (longer-wavelength) imaging.

Is DraChi the Only One?

The short answer is no. DraChi is certainly different from most other protoplanetary disks, given its size and large distance from any known star-forming regions. But the existence of Gomez’s Hamburger proves there are more such disks awaiting discovery. A disk as young as DraChi is vital to understanding planet formation in its earlier stages, and its large size makes for interesting future observations using more sensitive instruments.

Original astrobite edited by Kylee Carden and Jessie Thwaites.

About the author, Maria Vincent:

Maria is a PhD candidate in astronomy at the Institute for Astronomy, University of Hawai’i at Manoa. Her research focuses on adaptive optics and high-contrast imaging science and instrumentation with ground-based telescopes. Driven by a fascination with planet formation and the intricate processes shaping our solar system, she uses the Subaru Coronagraphic Extreme Adaptive Optics suite to observe and study morphological features of protoplanetary disks in near-infrared wavelengths, aiming to understand disk structure and processes governing planet formation. On the instrumentation side, she is working on designing and constructing an optical testbed to test and characterize a new deformable mirror as part of the upcoming High-order Advanced Keck Adaptive Optics upgrade. Outside of work, she enjoys blogging, mystery, historical and science fiction literature and cinemedia, photography, hiking, and travel.

Illustration of a binary system undergoing mass transfer

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: Expansion of Accreting Main-Sequence Stars During Rapid Mass Transfer
Authors: Mike Y. M. Lau et al.
First Author’s Institution: Heidelberg Institute for Theoretical Studies
Status: Accepted to ApJL

In our universe, most stars live in binaries. These stars live together, orbiting around each other, evolving and influencing each other’s lives. For many stars, this can be a tumultuous relationship with periods of mass transfer, altering the stars’ lives from the typical single-star evolution from a main-sequence star to a red giant to a stellar remnant. Mass transfer allows stars to grow and shrink in mass through their evolution, changing their identities and speeding up or slowing down their evolution. Mass transfer can also cause stars to go supernova.

diagram of the stages of Roche lobe filling

Figure 1: An Illustration of Roche lobe filling of binary stars in three different scenarios, A, B, and C. The left column shows the gravitational potential wells for each star, while the right column outlines the stars and their Roche lobes (the teardrop shapes around them). In scenario A, the matter of both stars is within each potential well and the stars are detached. In scenario B, the left star has expanded and filled its Roche lobe, causing matter to accrete onto the right star. In scenario C, both stars have filled their Roche lobes and orbit in a contact binary. [Storm Colloms, adapted from Pringle and Wade, Interacting Binary Stars (1985, Cambridge University Press)]

Mass transfer occurs when one of the stars in a binary system expands in radius during its evolution. If this expansion pushes the outside layers of the star beyond the gravitational influence of the star, the outer layers will be pulled from the expanded donor star onto the other star in the binary. The region of gravitational influence of each star is called its Roche lobe, illustrated in Figure 1.

As common as mass transfer is in the lives of stars, there are a lot of unknowns about mass transfer processes. Astronomers make simulations of stars transferring mass to try to understand these unknowns better. Simulating a fully accurate model of these processes requires many computational resources to model the hydrodynamic processes in all dimensions, and so models always make some simplifying assumptions.

One big simplifying assumption is how much of the donor star’s mass can be packed onto the accreting star. Previous models assumed that the accreting star does not change with time when accepting mass from the donor star. However, today’s authors are actually able to investigate how the properties of the accreting star change through the mass-transfer period using a stellar evolution code called Modules for Experiments in Stellar Astrophysics (MESA).

The authors show that how the accretor’s mass and radius respond to mass transfer depends on two timescales: 1) the thermal timescale of the accretor — this is the timescale on which the accretor can radiate away all of its thermal energy, and 2) the timescale of the accretion — this is the rate of mass transfer from the donor to the accretor. The thermal timescale of the accreting star is generally much longer than the mass-transfer timescale, meaning that during mass transfer the accreting star is fed mass much more quickly than it can “swallow” it, or have it thermally cool and settle onto the star. However, the accreting star can store this accreted matter in an envelope around it, stuffing it in its cheeks much like a hamster, as long as this does not spill over the edge of the Roche lobe — hence why the authors coin the name “hamstars” for these stars. This “cheek stuffing” makes the accreting star grow in radius and cool much like a typical main-sequence star becoming a red giant, as shown in Figure 2. As the star accretes more and more and its luminosity increases, its thermal timescale decreases, and it can radiate away the energy of accreting mass and “swallow” the matter faster. The star then shrinks from its expanded radius, having grown several times in mass.

HR diagrams showing the evolution of the accreting star during mass transfe

Figure 2: Hertzsprung–Russell diagrams showing the evolution of the accreting star during mass transfer. The three panels show the temperature versus luminosity for accretors of different initial masses, and each track in the panels is a different rate of mass transfer in the simulation. Most of the stars cool at some point in evolution, expanding in radius, before the accreted matter can cool onto the star and it shrinks and heats again, resuming its usual evolution having grown several times in mass. [Lau et al. 2024]

How much the radius expands during mass transfer changes the fraction of mass lost by the donor that is added to the accretor. This is called the accretion efficiency, and this quantity is often assumed to be an arbitrary constant. With the authors’ prescription for the change in radius of the accretor, they describe a new way of calculating the accretion efficiency that considers the accretor’s expansion. This depends on the initial mass of the accreting star, and it’s found to be higher than typical assumptions for low-mass (less than 15 solar masses) accreting stars, and lower for high-mass (more than 20 solar masses) stars.

The accretion efficiency determines how much of the mass accreted actually makes it onto the accreting star, versus how much might be lost during mass transfer. While low-mass stars can pack a lot of accreting matter in their hamster cheeks, high-mass stars have a lower limit for how much matter they can chew. These higher-mass accreting stars make up observed populations of X-ray binaries and the progenitors of pairs of black holes and/or neutron stars. Compared to current models, the authors’ prescription for accretion efficiency is in better agreement with the observed population of X-ray binaries and predicts fewer merging black hole binaries that could be observed with gravitational waves, something we could investigate further with more observations.

While today’s article considers the accretor’s response to mass transfer (pinning down the similarities between stars and rodents), it does not account for the accreting star’s spin or mass loss. If a star is spinning fast, it could limit how much the accreting star can expand. Some mass can also be swept away by stellar winds, meaning that not all of the mass lost by one star is accreted onto the other. Future work is needed to incorporate these additional factors before we can look up into the cosmos for a great hamstar hunt!

Original astrobite edited by Lucie Rowland.

About the author, Storm Colloms:

Storm is a postgraduate researcher at the University of Glasgow, Scotland. They work on understanding populations of binary black holes and neutron stars from the gravitational wave signals emitted when they merge, and what that tells us about the lives and deaths of massive stars. Outwith astrophysics they spend their time taking digital and film photos, and making fun doodles of their research.

Illustration of a Neptune-like exoplanet

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

Title: Scaling K2. VII. Evidence for a High Occurrence Rate of Hot Sub-Neptunes at Intermediate Ages
Authors: Jessie L. Christiansen et al.
First Author’s Institution: NASA Exoplanet Science Institute, California Institute of Technology
Status: Published in AJ

Exoplanets come in all shapes and sizes. Some are large and gaseous, like Jupiter or Neptune, while others are small and rocky, like Earth. Curiously, there seems to be a lack of a certain size of planet — those that have radii 1.5–2 times Earth’s radius  — and no one is sure why. Since we can detect exoplanets with sizes above and below this radius range, this dearth of planets is not likely due to a detection bias, but rather some physical mechanism at play. Astronomers believe that planets that form in this intermediate size range have a hard time holding onto their atmospheres and shrink due to some mass-loss mechanism. The authors of today’s research article investigate which mechanism is the most likely culprit for this observed phenomenon.

Mind the (Radius) Gap

Since the first detection of an exoplanet in 1992, the number of confirmed exoplanets has been increasing astronomically. Now astronomers have detected enough planets (5,612 and counting) to try to understand the underlying distributions between different populations of planets. When plotting the radius distribution of planets detected by the Kepler space telescope, Fulton et al. 2017 found that the radius distribution of small planets is bimodal — that is, there are two distinct peaks corresponding to two different planet populations, known as super-Earths and sub-Neptunes, and few planets in between them (see Figure 1). Super-Earths are rocky planets with sizes around 1.5 times that of Earth, while sub-Neptunes are gaseous planets, similar in composition to Neptune, but with radii between 2 and 3 times the radius of Earth. The scarcity of planets between these two populations, between 1.5 and 2 times the size of Earth, is often referred to as the “radius gap” or “radius valley,” where the occurrence rate of known exoplanets is much lower than expected.

plot of planet occurrence per star versus radius for planets with periods less than 100 days

Figure 1: The histogram of planet radii for a sample of Kepler planets with orbital periods shorter than 100 days. The super-Earth and sub-Neptune regimes are highlighted in light red and light cyan, respectively. The “radius gap” corresponds to the decreased occurrence rate between 1.5 and 2.0 Earth radii. [Adapted from Fulton et al. 2017]

The cause of this radius valley is still an unsolved mystery. Many astronomers believe that planets that form in this valley aren’t massive enough to hold on to their gaseous outer layers, causing them to shrink over time until they are left with only their rocky cores. The two leading theories for what causes this atmospheric loss are photoevaporation and core-powered mass loss. In the photoevaporation scenario, planets of this size have their gaseous atmospheres almost entirely stripped away by the high-energy radiation from their host stars. This process is predicted to occur in the first 100 million years following the planet’s formation.

On the other hand, the core-powered mass-loss scenario predicts that the energy generated from the planet’s own core as it cools propagates to its outer layers and blows away its atmosphere if the planet is not initially massive enough to hold onto it. This process is much slower, taking a whopping 0.5–2 billion years to complete. Both theories have a lot of compelling evidence, so which is more likely? In order to address this question, the authors of today’s article turned to NASA’s K2 mission.

Photoevaporation or Core-Powered Mass Loss?

While NASA’s Kepler space telescope was originally designed to observe just one patch of the sky, its high-precision pointing began to fail in 2013, causing it to stray away from its original field of view. Luckily, it was able to be repurposed into the K2 mission, where it instead began to observe stars along the ecliptic plane. Its new field of view contained the Praesepe and Hyades star clusters, two populations of stars between 600 million and 800 million years old. Because planets are roughly the same age as their host stars, astronomers were able to detect a large number of planets old enough to experience photoevaporation, but not old enough to have undergone core-powered mass loss. The authors looked for sub-Neptune-sized planets in these two clusters (i.e., the planet population just above the radius valley) and predicted that if they observed a high occurrence rate of sub-Neptunes, they can conclude that photoevaporation could not be the leading cause of this mysterious mass loss.

And that is exactly what they observed. As shown in Figure 2, the occurrence rate of sub-Neptunes is 79%–107% for stars in the Praesepe cluster. This occurrence rate is much higher than the occurrence rate of ~20% for sub-Neptunes around field stars (i.e., stars not associated with a cluster), which can be much older. This result is also consistent with a previous result from clusters observed by the Transiting Exoplanet Survey Satellite (TESS), where the authors found a fractional occurrence rate of ~100% for sub-Neptunes that were roughly 50 million years old. If photoevaporation were the driving mechanism of mass loss, the occurrence rate of sub-Neptunes should be closer to that of planets around field stars, since their atmospheres would have been completely stripped by photoevaporation by this age.

occurrence rates of sub-Neptunes over time

Figure 2: The occurrence rates of sub-Neptunes over time, from the young cluster stars detected with TESS (15–450 Myr) from Fernandes et al. 2023, to the intermediate-age (600–800 Myr) Praesepe stars detected with K2 from this paper, to the old field K2 stars (3–9 Gyr). The shaded pink region indicates the timescales where photoevaporation is predicted to occur (<100 Myr) and the orange shaded region shows the timescale where core-powered mass loss is predicted to occur (0.5–2 Gyr). The drop in occurrence rate occurs after the core-powered mass-loss timescale, making it the more favorable mass-loss mechanism. [Christiansen et al. 2023]

So, does that mean core-powered mass loss is the mechanism that is guilty of stealing these planet’s atmospheres and causing this radius valley? Perhaps! But the jury is still out. While this result does leave core-powered mass loss as the more favorable mass-loss mechanism, there may be many more possibilities we haven’t yet considered. The picture is still incomplete, but this research gives us a little more insight into what happens to small planets as they evolve.

Original astrobite edited by Junellie Gonzalez Quiles.

About the author, Tori Bonidie:

I am a 4th-year PhD candidate studying exoplanets at the University of Pittsburgh. Prior to this, I earned my BA in astrophysics at Franklin and Marshall College, where I worked on pulsar detection as a member of NANOGrav. In my free time you can find me cooking, napping with my cat, or reading STEMinist romcoms!

photograph of the ultra-faint dwarf galaxy Leo IV

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: Extended Stellar Populations in Ultra-Faint Dwarf Galaxies
Authors: Elisa A. Tau, A. Katherina Vivas, and Clara E. Martínez-Vázquez
First Author’s Institution: University of La Serena
Status: Published in AJ

The Runts of the Galaxy Litter

The first galaxies formed only a few hundred million years after the Big Bang. Galaxies start out small, then grow by pulling in the surrounding gas and merging with each other. Most galaxies today are about 100,000 light-years across (roughly the size of the Milky Way), but some of the largest galaxies can be millions of light-years across. At the other end of the scale, we have dwarf galaxies, which are closer to 10,000 light-years across. As a result of their small size, dwarf galaxies have few stars and aren’t very luminous. Dwarf galaxies often orbit larger galaxies as satellites, similar to how moons orbit planets and planets orbit stars.

Dozens of dwarf galaxies have been discovered orbiting the Milky Way. You’ve likely already heard of the most famous of these — the Large and Small Magellanic Clouds, which are visible from the Southern Hemisphere. However, most of the dwarf galaxies orbiting the Milky Way are much smaller and dimmer than the Magellanic Clouds, making them difficult to make out with the naked eye. Astronomers often identify dwarf galaxies by finding a collection of stars and gas that moves together with star-mapping surveys like Gaia. Ultra-faint dwarf galaxies are particularly difficult to detect because they have so few stars; where a normal dwarf galaxy may have millions or billions of stars, ultra-faint dwarfs usually only contain thousands or tens of thousands. Ultra-faint dwarf galaxies lack stars because most of the mass in these galaxies consists of dark matter.

A topic of discussion among astronomers studying dwarf galaxies is how large these galaxies can be. The size of a dwarf galaxy can be determined by measuring the distance to the farthest stars that belong to the galaxy. Stars located near the edges of dwarf galaxies would belong to a stellar halo, which contains older stars. Stellar halos around dwarf galaxies are incredibly difficult to detect because they contain so few stars, so it is not clear how often ultra-faint dwarfs host stellar halos or what the halo properties are. One way to trace stars that may belong to stellar halos is with RR Lyrae variable stars. In addition to being older stars, meaning we expect them to be more common in stellar halos, these stars are a type of “standard candle,” meaning they have a known luminosity or energy output. RR Lyrae stars pulsate, and the variation in their luminosities is related to the period of their pulsations, so astronomers can infer an RR Lyrae star’s luminosity by measuring changes in its brightness. Previous studies have attempted to look for stellar halos in ultra-faint dwarfs with RR Lyrae stars, but this method was challenging to carry out before the data and telescopes that became available in the last few years. The authors of today’s article are continuing the search for RR Lyrae stars in the outer reaches of ultra-faint dwarfs to probe the presence of extended stellar populations and stellar halos.

Finding Ultra-Faint Dwarfs

The authors compiled a list of known ultra-faint dwarfs around the Milky Way, many of which were discovered by the Dark Energy Survey (DES). They also compiled a sample of RR Lyrae stars that were detected by a series of stellar surveys, including Gaia, the Zwicky Transient Facility (ZTF) surveys, DES, and the first Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) survey.

But how do astronomers actually determine which stars belong to an ultra-faint dwarf, especially when these galaxies contain so few stars? Astronomers use a couple of quantities to determine this. First, they are looking for a collection of stars that are in the same area of the sky and are all roughly the same distance from Earth. The authors measure the distance to each star by using the star’s luminosity, since it is related to the distance by the equation L = 4πFd2, where L is the luminosity (total power emitted), F is the flux (power per unit area detected at Earth), and d is the distance. Again, astronomers know the luminosities of these stars because the luminosity of an RR Lyrae star is related to how quickly its light output varies. By comparing this known luminosity to how much light they actually detect, astronomers can estimate the distance between us and the RR Lyrae star.

The next main variable the authors use is proper motion, which is a measurement of how quickly an object moves across the sky. A group of stars with similar proper motions are likely moving together, which is necessary for the collection to be called a dwarf galaxy. Astronomers consider the angular distance between each RR Lyrae star and the center of the dwarf galaxy. Then, they use the distances and proper motions they measured to determine if that RR Lyrae star belongs to the dwarf galaxy or the Milky Way.

Lastly, we need to discuss a quantity called the half-light radius, which the authors denote by Rh. The half-light radius is the radius out to which half the light emitted by a galaxy is contained. Since galaxies don’t have clear edges, the half-light radius provides a useful scale for measuring galaxy sizes. Note that the full radius of a galaxy isn’t 2 Rh since galaxies are brighter in the center and dimmer near the edges. The authors are looking for RR Lyrae stars that are several half-light radii from the centers of their ultra-faint dwarfs, which is where they believe the stellar halos would exist.

Living on the Edge

The authors identify more than 100 RR Lyrae stars in ultra-faint dwarfs from the surveys mentioned above and find that nearly half of the ultra-faint dwarfs contain at least one RR Lyrae star. Some examples of the RR Lyrae stars and their locations in the galaxies are shown in Figure 1.

Plots showing the positions of RR Lyrae stars in four of the ultra-faint dwarfs used in this study

Figure 1: A sample of ultra-faint dwarf galaxies and their associated RR Lyrae stars. Symbols correspond to the catalog that the star’s position comes from, and the contours represent different radii. Note that some of the stars for the Bootes dwarf galaxies may actually belong to the Sagittarius Stream, which is a part of the Milky Way. [Tau et al. 2024]

These ultra-faint dwarfs all contain at least one RR Lyrae star at a distance of more than 4 Rh, which is what the authors consider to be the boundary between the main body of the galaxy and the stellar halo surrounding it. Since RR Lyrae stars exist here, we suspect other stars do too, indicating that these ultra-faint dwarfs do contain stars that have a large spatial extension from the main body of the dwarf galaxy and may belong to a stellar halo.

A more comprehensive showcase of the RR Lyrae stars is highlighted in Figure 2. While most RR Lyrae stars (and, therefore, most stars) are expected to be within 2 Rh of the center, several have been found well beyond this distance. This again highlights that RR Lyrae stars can be located beyond the main body of the ultra-faint dwarfs in what are likely stellar halos.

Histograms of the distances between RR Lyrae stars and the centers of their host galaxies

Figure 2: Histograms of RR Lyrae stars and their distances from the center of their ultra-faint dwarf (UFD) host galaxy in units of Rh. Blue represents all RR Lyrae stars, and orange represents a cleaned sample that has removed stars from ultra-faint dwarfs for which the authors are concerned about contamination (e.g., stars in the Bootes dwarf galaxies due to contamination from the Sagittarius Stream). The right panel is a zoom-in of the left panel, focusing on distances of up to 15 Rh. [Tau et al. 2024]

The authors note that there are opportunities for further studies in this area. One thing that remains unclear is why only some ultra-faint dwarfs are observed to host RR Lyrae stars, and why some don’t contain RR Lyrae stars at distances that would suggest the existence of a stellar halo. The authors propose that gravitational interactions with the Milky Way may tidally disrupt the dwarfs, changing their shape and potentially leading to extended stellar populations. This means that dwarfs that have close encounters with the Milky Way may be more likely to host a stellar halo. In addition to further observational surveys, cosmological simulations of dwarf galaxy formation may help shed light on how and why extended stellar populations develop.

Original astrobite edited by Alexandra Masegian.

About the author, Brandon Pries:

I am a graduate student in physics at Georgia Institute of Technology (Georgia Tech). I do research in computational astrophysics with John Wise, using machine learning to study the formation and evolution of supermassive black holes in the early universe. I’ve also done extensive research with the IceCube Collaboration as an undergraduate at Michigan State University, studying applications of neural networks to event reconstructions and searching for signals of neutrinos from dark matter annihilation.

Illustration of a supermassive black hole snacking on a 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: Optical Appearance of Eccentric Tidal Disruption Events
Authors: Fangyi (Fitz) Hu et al.
First Author’s Institution: Monash University
Status: Published in ApJL

While the typical image of a supermassive black hole swallowing up everything in its path may be a trope of science fiction, there is some truth to the violent nature of these enigmatic cosmic objects. Namely, the ultra-strong gravitational fields surrounding supermassive black holes have been observed to shred stars, tearing them apart and producing bright and dramatic transient phenomena known as tidal disruption events, or TDEs. In the process, much of the stellar material becomes unbound and is flung away from the system, with the remainder surrounding the black hole in an accretion disk, the source of the emitted radiation.

As per usual, though, nothing in astrophysics is ever straightforward. Over the years, a disparity between what we expect and what we actually see has emerged when it comes to these events that are reminiscent of a Shakespearean tragedy. More specifically, due to the very-high-temperature gases involved, X-rays are expected to dominate the spectrum of most TDEs. However, observers have found that most TDEs are instead observed at lower energies, producing significant but poorly understood optical emission.

Enter today’s article from researchers at Monash University in Australia, who have simulated the tidal disruption of a Sun-like star in the hopes of uncovering the origin of the perplexing optical emission.

The Nitty Gritty of the Simulation

For simulations to be realistic, theorists typically attempt to account for as many physical processes as possible. Thus, the authors of today’s article employed the PHANTOM code, which includes both general relativistic and hydrodynamical effects. The protagonists of this astrophysical tragedy are a solar-mass star and a million-solar-mass, non-spinning black hole with a highly eccentric, or elliptical, orbit.

Cut to several days later, and the simulation has evolved to a point where the star is in tatters and the supermassive black hole is surrounded by a bright and brand-new accretion disk composed of stolen material from the star. By tracking the time evolution of the system, the authors were able to understand how the accretion disk formed, with the hope of unmasking the source of the optical emission.

The Last Dance

Figure 1 shows the evolution of the system’s column density, indicating the location of the stolen stellar material. In the first two panels at 0.55 and 0.91 day, the stellar debris stream is visible. In the third panel, the debris stream collides with itself, leading to the formation of the accretion disk. In the fourth panel at 2.74 days, the accretion disk is clearly seen as a circular structure surrounding the supermassive black hole at the centre.

time evolution of stellar material tidally disrupted by a black hole

Figure 1: The column density of the stellar material surrounding the supermassive black hole at different times. Larger column densities are indicated by more yellow colours, while lower column densities are shown in purple and black. From left to right, the panels show the simulation at 0.55, 0.91, 1.28, and 2.74 days. [Adapted from Hu et al. 2024]

By considering the system from afar, the authors gained a new perspective on TDE accretion disk formation. They found significant quantities of low-density material ejected out to large distances. This material is shown in Figure 2, which gives the column density of the system at 2.74 days as in Figure 1, but on a much larger scale.

density of stellar material surrounding the supermassive black hole

Figure 2: The column density of the stellar material surrounding the supermassive black hole at t = 2.74 days as in Figure 1, but from a zoomed-out perspective (note the larger scale compared to Figure 1). [Adapted from Hu et al. 2024]

This low-density material is not part of the accretion disk but instead flows out asymmetrically from the centre of the system. Previous studies have hypothesised that the source of the optical emission from TDEs might be some form of reprocessing layer that surrounds the accretion disk and converts X-ray photons into lower-energy optical photons. And that appears to be exactly what is seen in the simulation. The surrounding low-density material in Figure 2 acts as the reprocessing layer with which X-ray photons interact, causing them to lose energy and be observed as optical emission.

But how does this match with actual TDE observations? To test this, the authors constructed spectra and light curves from their simulation results. They found that their synthetic spectra were broadly consistent with the genuine TDE spectra, as were their inferred luminosities from their synthetic light curves. However, their calculated blackbody radii and temperatures were systematically higher or lower than the observations, albeit of the correct order of magnitude. Therefore, the authors conclude that while the model is realistic, it does not yet accurately account for certain physical processes such as radiation transport, which can be improved upon in future simulations.

And thus ends the tragic tale of a supermassive black hole and its stellar neighbour that came a little too close. Clearly, surrounding outflows play an important role in this astrophysical tragedy that rivals Hamlet or Macbeth, processing the high-energy X-rays from the accretion disk into the lower-energy optical photons seen by observers. Unfortunately for us, though, we’ll have to wait for future simulations to find out how the story truly ends.

Original astrobite edited by Archana Aravindan.

About the author, Sonja Panjkov:

I’m a second-year PhD student at the University of Melbourne. My research focuses on the high-energy emission from the supernova remnants in the Magellanic Clouds. In my spare time, I enjoy hanging out with my cats and going to see live music.

illustration of a jet from an exploding 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: Search for 10–1,000 GeV Neutrinos from Gamma-ray Bursts with IceCube
Authors: IceCube Collaboration
Status: Published in ApJ

Particle Accelerators, but in Space!

Gamma-ray bursts are some of the most powerful explosions in the universe, releasing a “fireball” of particle-filled plasma in a powerful jet that accelerates these particles through the universe. Gamma-ray bursts are understood to originate either from compact object (neutron star or stellar-mass black hole) mergers (these are called short gamma-ray bursts and last less than two seconds!) or from core-collapse supernovae (these are called long gamma-ray bursts but are just defined as anything longer than two seconds). Gamma-ray bursts are the most powerful particle accelerators in the universe and are really useful for looking for new particles and new particle interactions!

Today’s authors look at gamma-ray bursts as a possible source of ghost particles, i.e., neutrinos. Neutrinos rarely interact with other matter, which makes them really hard to detect — like a ghost! The IceCube Neutrino Observatory sees neutrinos all over the sky but can’t pinpoint where they’re coming from. Since gamma-ray bursts could produce neutrinos in their outbursts, the authors search through all of IceCube’s data to see if there are any bursts of high-energy neutrinos that came in at the same time as a gamma-ray bursts.

How Many Gamma-ray Bursts Does It Take to Find a Neutrino?

Today’s authors search for coincident neutrinos in the time periods surrounding the 2,298 bursts that happened during the lifetime of IceCube-DeepCore (IceCube’s highest-energy neutrino detector). They do this by looking at each time window individually and by combining many time windows to add faint signals that might not be seen in individual windows, but together might show an association between neutrinos and gamma-ray bursts.

histogram showing the duration of prompt gamma-ray burst emission

Figure 1: A histogram of the initial gamma-ray burst emission (called prompt emission) duration for all 2,268 gamma-ray bursts used in this study. The time windows investigated in this article are shown as red arrows. [Adapted from IceCube Collaboration et al. 2024]

In the first search, the authors define search windows before and after each burst to look for neutrinos (see Figure 1). Since neutrinos don’t interact with matter very often, they can easily stream out of dusty environments from which photons struggle to escape, meaning that the neutrinos could actually be expected to arrive at Earth before gamma-ray (and other photon) emission. The authors search the entire sky for neutrinos in these windows and assess the probability that there is an excess of neutrinos coming from the source location compared to the neutrino background that we see all over the sky.

The second search looks at groups of gamma-ray bursts that are associated in location and time with neutrino events. The authors look at the combined probability of burst/neutrino association of all the events in this group. This makes it possible to correlate gamma-ray bursts with neutrinos even if the events don’t individually stand out. Using this method, the authors didn’t find any groups of neutrinos that are any more statistically significant than individual neutrinos that fall within gamma-ray burst time windows.

Trials Factors and Tribulations

The winning burst of the first search (i.e., the most significant neutrino–gamma-ray burst correlation) is GRB bn 140807500. (Since there are a lot of gamma-ray bursts recorded by burst-hunting instruments like the Fermi Gamma-ray Burst Monitor (Fermi-GBM) and the Swift Burst Alert Telescope (Swift-BAT), it’s too much of a hassle to give the bursts individual names. Instead, the bursts get “telephone numbers” corresponding to the date they were detected.) The corresponding neutrino falls within 100 seconds of GRB bn 140807500 and has a p-value of 4.6 x 10-5, which is the probability that the correlation between the burst and the neutrino is just a lucky coincidence and not from actual correlation (i.e., small p-values mean a more likely detection of neutrinos from gamma-ray bursts!).

This probability seems really small, and at first glance it seems like the neutrino and the gamma-ray burst are most likely connected here! Unfortunately, this doesn’t take into account trials factors (also called the look-elsewhere effect), which quantify the statistical statement that if you look at enough gamma-ray bursts and neutrinos, there will be some events that line up with each other in space and time, just by chance. To account for this, the authors must correct for 2,268 trials, one for each burst. After correcting for trials, this leaves us with a much larger p-value of 0.097, meaning there’s a one in ten chance that the gamma-ray burst and the neutrino aren’t really connected. Generally particle (and astroparticle) physicists require a p-value of 3×10-7 (about 1 in 3.5 million, corresponding to the [in]famous 5-sigma threshold!) to feel confident in saying that these events are actually correlated.

Don’t Forget About the BOAT

plot of neutrino flux for more than 2,000 gamma-ray bursts compared to the flux from just the single brightest burst

Figure 2: Estimated neutrino flux (number of neutrinos detected at a given energy per area) for 2,264 gamma-ray bursts combined (blue) compared with the BOAT gamma-ray burst (orange). (Four of the bursts used in this study were excluded from this analysis.) [IceCube Collaboration et al. 2024]

At the same time as this article was being prepared, the brightest of all time (BOAT) gamma-ray burst was detected. The authors didn’t include the BOAT directly in their dataset, but they made some predictions as to how it would measure up to the other gamma-ray bursts that were considered. The BOAT was so bright that the authors calculated the expected neutrino signal to be 6–8 times the combined expected signal of all 2,264 gamma-ray bursts used in this part of the study (see Figure 2)! This is because the BOAT is so much more energetic in gamma rays than other gamma-ray bursts, implying a large flux of high-energy neutrinos, which are localized to more precise regions in the sky than the lower-energy neutrinos expected to accompany other bursts. This means that we can more confidently associate any observed neutrinos with the location of the burst.

There’s still work to be done to see if there are any neutrino events that seem to come from the BOAT gamma-ray burst, but this leaves the idea open that neutrinos could come from gamma-ray bursts (or at least, that we can more confidently say that they don’t)! The universe sometimes throws surprises like the BOAT at us, allowing astronomers to study the high-energy universe a lot more easily. Luckily, we’ve entered into the era of time-domain astronomy where instruments like Fermi-GBM and Swift-BAT allow us to catch more bursts and explosions than ever before, giving us an increasingly large sample of gamma-ray bursts to study!

Original astrobite edited by Cole Meldorf.

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.

image showing the change in brightness of a star that is microlensed by a black hole

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: Disentangling the Black Hole Mass Spectrum with Photometric Microlensing Surveys
Authors: Scott Ellis Perkins et al.
First Author’s Institution: Lawrence Livermore National Laboratory
Status: Published in ApJ

Lonesome Black Holes

Seeing the darkness is something that is a great mystery in modern physics, from dark matter to black holes. How do you see something that doesn’t emit light?

Current methods of detecting black holes include observing their accretion from surrounding matter, whether that’s supermassive black holes in the centres of galaxies or smaller black holes that are stealing matter from a stellar companion. Black holes that don’t have any glowing surrounding matter may have a black hole or neutron star partner to spiral into, releasing gravitational waves that we detect with the LIGO, Virgo, and KAGRA detectors. But lonely black holes sitting in the darkness are significantly harder to uncover. In the Milky Way, it’s expected that there are 100 million isolated and binary black holes that are born from dead massive stars. Of those, only ~50 have been detected.

Twinkling Lights

Fortunately, there’s a detection method that can fill this gap: microlensing. If an isolated black hole is located in front of a star or galaxy much farther away, that background object’s brightness will vary with time as its light is lensed by the black hole moving in front of it (see Figure 1). This is similar to a magnifying glass passing in front of the star’s tiny pinprick of light, but instead the lens of the glass is the curvature of spacetime due to the mass of the black hole. Because the lens magnifies the background object, it will appear to change in brightness, twinkling like fairy lights.

Figure 1: An animation of a black hole passing in front of a background star. Because the black hole bends spacetime along the path from the star to the observer, two images of the star are created. The lensing also changes the apparent brightness of the star, which is still detectable photometrically even when the different images are too close to be separately resolved by the observer. [NASA’s Goddard Space Flight Center Conceptual Image Lab]

However, there are a few different kinds of objects that can lens background stars. Black holes, white dwarfs, neutron stars, and free-floating planets have all been found through microlensing. If we can know the mass of the lens, then we can characterise a lens as a stellar-mass black hole (usually 5–100 times the mass of the Sun). However, an individual microlensing light curve doesn’t carry any useful information about the mass of the lens without also knowing the astrometric shift in the lensed object — something we don’t have for many microlensing observations. However, studying a larger group of microlensing events can break the degeneracies present in individual light curves, helping researchers classify lensing events and search for the hidden black holes in the dark. Today’s article defines a framework for determining the class of individual detections and groups of microlensing events using probabilistic models and Bayesian statistics.

Looking at Populations of Black Hole Lenses

Different types of lenses will be distributed differently in the effects they create in the light curves that we detect. For example, there will be more black holes over a certain range of masses and more white dwarfs over a different range of masses. This means that the distributions of some of the measurements of the lensing profile, such as the angle between the image of the source and the actual source location, will be different because of correlations between these parameters and the lens mass.

This article looks at three kinds of lenses: free-floating planets, stellar-mass black holes, and primordial black holes. Accounting for primordial black holes in the model separately from stellar-mass black holes allows this method to demonstrate a measure of the abundance of primordial black holes, which could provide an explanation for dark matter or evidence for the first black hole seeds in the early universe. The authors construct five simulated universes with the same number of mock microlensing events for free-floating planets and stellar-mass black holes, but each with a different number of primordial black holes. For each of these universes, the distribution of the distances and the kinematics of the events are the same, but there is a different distribution of lens masses caused by the varying fraction of primordial black holes.

Using these mock events, the authors construct a probability distribution for events belonging to a certain class for each universe. If you assume a fixed group of certain types of lenses (meaning a fixed mock universe and lens mass distribution), and you know the profiles of the microlensing light curves that are likely to be produced by each of those different lens types, you can say how probable it is that a microlensing event is caused by a specific type of object. Their method reliably classifies most of the true black hole events from each population’s mock events, as shown in Figure 2. While their method has fewer correct black hole candidates, or “true positives,” than previous methods, their identified population of black holes has many fewer false black hole identifications and therefore a much higher purity of detection.

plot comparing the results of this study with the results from previous studies

Figure 2: The number of mock microlensing events correctly categorised as black holes (top panel), incorrectly categorised as black holes (e.g., these were simulated free-floating planets but appeared to be black holes in their framework; middle panel), and the fraction of correctly identified black holes divided by the total number of black hole identifications (bottom panel), versus the fraction of primordial black holes in each population. This method is shown in the green band, while previous methods are shown in orange, purple and pink. The bottom edge of each band represents the number for which the classification probability is above a 50% confidence, while the upper edge is 90% confidence. While this method does not reach the true number of correctly identified black hole events (black crosses), it does have the highest purity of all methods. [Perkins et al. 2024]

Alternatively, if you have a group of microlensing events and a model for the universe, you can say how probable it is that you are observing events in that universe. When drawing a group of events from their five mock universes, the authors characterise how well they can recover the fraction of primordial black holes from these groups of events, as seen in Figure 3. The probability of the fraction of primordial black holes follows the true value, meaning if there really were lots of primordial black holes in the Milky Way that match these models well, we could say that primordial black holes exist with high confidence. If the fraction of primordial black holes is smaller, then we would be able to place upper limits on what we can detect.

plot showing the predicted values for the fraction of primordial black holes with the authors’ framework for five simulated universes

Figure 3: The predicted values for the fraction of primordial black holes (fPBH) with the authors’ framework (green), for five simulated universes (Λ 0-4). The true value is marked with the black vertical line for each universe. The predicted fraction follows the true fraction well, and the width of this distribution represents the uncertainty determined by the framework. [Adapted from Perkins et al. 2024]

The Future of Black Hole Microlensing Surveys

The next step for the authors’ statistical framework is to test it with real photometric measurements of microlensing from the Optical Gravitational Lensing Experiment (OGLE), a survey of more than a billion stars for time-domain astrophysics. By combining this method and real observations with more realistic model universes and more sophisticated simulations of primordial black holes, stellar-mass black holes, and other types of lenses, this could tell us more about the population of stellar-mass black holes alone in the wild. Pairing these results up with astrometric measurements that will come from the Nancy Grace Roman Space Telescope in the next few years will provide even more knowledge about the kinds of black holes making stars (or fairy lights?!) twinkle in the night sky.

Original astrobite edited by Samantha Wong.

About the author, Storm Colloms:

Storm is a postgraduate researcher at the University of Glasgow, Scotland. They work on understanding populations of binary black holes and neutron stars from the gravitational wave signals emitted when they merge, and what that tells us about the lives and deaths of massive stars. Outwith astrophysics they spend their time taking digital and film photos and making fun doodles of their research.

illustration of a neutron 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: Long-Period Radio Pulsars: Population Study in the Neutron Star and White Dwarf Rotating Dipole Scenarios
Authors: Nanda Rea et al.
First Author’s Institution: Institute of Space Sciences (ICE-CSIC), Barcelona, Spain
Status: Published in ApJ

Earth is constantly bombarded by radio waves from all across the Milky Way. The shorter time-scale signals, known as “radio transients,” can sometimes be periodic. These periodic signals are often attributed to rotating neutron stars, which act as cosmic lighthouses, sending out radio waves that appear to us as “pulses” of emission. Normally, the slowest of these sources have pulses separated by milliseconds or seconds. However, the recent detection of two signals with astounding periods of 18 and 21 minutes has completely surpassed even the slowest signals previously detected. What could be causing these ultra-long-period radio signals, and what can they tell us about the populations of extreme compact objects they come from? The authors of today’s article investigate these very questions.

Time for a Mystery

GLEAM-X J1627–52 and GPM J1839–10 (astronomers love brevity) are the names given to these two ultra-long-period radio transients. Amazingly, we have archival data dating back to 1988 showing that GPM J1839–10 has been active for more than 30 years! It is still a great mystery what kind of object could produce such a long-period signal, but two kinds of stars might be the culprits.

White dwarfs, Earth-sized stars that form toward the end of the lifecycle of intermediate-mass stars like the Sun, are the first potential source of these signals. White dwarfs often have slow enough rotation periods to account for the ultra-long-period signals, but there is no known mechanism by which lone white dwarfs could produce bright enough radio signals. It is thought that a companion star could possibly enhance the white dwarf’s radio emission via the companion’s stellar wind. As the white dwarf’s emission beams cross through this stellar wind, the emission accelerates particles within the stellar wind, releasing radio waves as accelerated electrons interact with the white dwarf’s magnetic field.

Only two radio-emitting white dwarfs have ever been observed, and both were in binary systems. Based on the lack of an optical or infrared component, researchers thing that GLEAM-X J1627−52 is likely not in a main-sequence binary system similar to these radio-emitting white dwarfs. GLEAM-X J1627−52 could still have a low-mass companion, similar to the AR Scorpii system, which is a white dwarf pulsar binary containing a low-mass, red dwarf companion star (learn more about AR Scorpii in this Astrobite). GPM J1839–10 has not yet been constrained to be in a binary using optical and infrared observations.

Highly magnetized neutron stars known as magnetars are the second potential source. Magnetars are the extremely dense, extremely magnetic leftovers of stellar explosions. How dense and how magnetic, you might ask? Just one tablespoon of magnetar matter weighs as much as Mount Everest, and their magnetic fields, which are a thousand trillion times stronger than Earth’s magnetic field, make them the most magnetic objects known. Magnetars tap directly into their magnetic fields to fuel their powerful beams of radio waves.

Not So Perfect Sources

Despite the amazing properties of both white dwarfs and magnetars, the authors find several issues when trying to explain GLEAM-X J1627–52 and GPM J1839–10 using these types of objects. While observations from back in 2018 show that GLEAM-X J1627−52 does have a brightness and polarization similar to other radio magnetars, X-ray measurements put limits on the source that challenge a magnetar being responsible. The ultra-long period is also not in line with the rest of the neutron star population. On the other hand, white dwarfs are not known to produce the observed bright emission.

Crossing the Line

To figure out whether neutron stars or white dwarfs might be responsible for these long-period signals, the authors use two methods: death-line analyses and population-synthesis simulations.

“Death line” sounds like an ominous term, but it simply defines the threshold at which radio emitters no longer put out bright enough signals for us to detect with our telescopes, meaning they are effectively “dead.” The authors create a range of death lines (shown in Figure 1) based on models ranging from the simplest models available to more extreme models that incorporate complicated physics like twisted magnetic field lines.

plot of surface magnetic field at poles versus spin period

Figure 1: The two death “valleys” of the neutron star and white dwarf populations. An object that falls below the death line no longer emits strongly enough for detection using our current radio telescopes or no longer emits at all, hence why it is “dead.” The neutron star death lines are marked in red and white dwarf death lines are marked in blue. The bounds of the valleys come from the range of different models. For both populations, GPM J1839-10 falls below even the most extreme death line. [Rea et al. 2024]

Based on these death lines, the authors conclude that neutron stars could create the type of signal seen from GLEAM-X J1627−52, but not the signal seen from GPM J1839–10, since it falls below even the most extreme death line. The same is true for magnetized white dwarfs.

The second method is called population synthesis, which uses statistics from the already existing sample of neutron stars and white dwarfs to try and simulate the total population of these stars. The authors vary different parameters like magnetic field strength, birth rate, and the angle between the magnetic field axis and rotational axis to try and figure out if a special population of neutron stars or white dwarfs with these slow periods might exist in the galaxy.

The authors find that a large population of long-period radio emitters cannot be easily explained by neutron stars, even when assuming extremes like no magnetic field decay, stronger magnetic fields, etc. The white dwarf population synthesis shows that magnetized white dwarfs with long periods are more common, but isolated white dwarfs are not expected to be able to emit bright, coherent (constant phase) radio emission, based on the existing population.

This is likely just the first step in uncovering an entire unknown population of ultra-long-period radio transients. Many more sources like the two discussed in today’s article could be out there, but for now the authors recognize that the sample size is small. If either of the two sources, GLEAM-X J1627−52 or GPM J1839–10, is confirmed to be a neutron star or white dwarf in the future, it will tell us a lot about the physics of these extreme objects. It may even require a revision of our understanding of neutron stars or white dwarfs, both in terms of how exactly they emit radio waves, and what their populations look like in the galaxy!

Original astrobite edited by Maryum Sayeed and Annelia Anderson.

About the author, Magnus L’Argent:

Magnus is a first-year Master’s student and Trottier Fellow at McGill University. When not searching for new pulsars, fast radio bursts, and other radio transients, he enjoys going on hikes, reading sci-fi, and watching movies.

active galaxy Hercules A

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: Exploring Changing-Look Active Galactic Nuclei with the Sloan Digital Sky Survey V: First Year Results
Authors: Grisha Zeltyn et al.
First Author’s Institution: Tel Aviv University
Status: Published in ApJ

Challenging the Original Picture

Supermassive black holes are among the most powerful objects in the universe. Those that are accreting gas from their surroundings (called active galactic nuclei, or AGN for short) release an enormous amount of energy back to their surroundings — enough to unbind an entire galaxy! Historically, AGN have been classified by the emission lines in their optical spectra: Type 1 AGN show broad emission lines, especially from the Balmer series of hydrogen, whereas Type 2 AGN do not show broad emission lines (see this Astrobite for details on the different classifications). As early as the late 1980s/early 1990s, it was hypothesized that these two classifications could be unified into a single model, where the difference in AGN optical spectra was related to the viewing angle to the nucleus. In this model, broad emission lines can only be seen if the observer is seeing the AGN nearly face-on, whereas these lines tend not to be seen in edge-on systems where the observer is probably looking through dense clouds of gas and dust called the torus (see this Astrobite for more on the dusty torus).

This view is supported by the existence of Type 2 AGN with broad emission lines in their polarized spectra. These spectra only contain polarized light, which can contain emission from the broad lines being scattered off of the gas and dust. However, recently there have been a subset of AGN that show changes between these different spectral types on timescales of months to decades. This new class is called “changing-look” AGN, and they challenge the idea that the viewing angle alone determines the type of AGN you see. An example of a changing-look AGN (newly found in today’s article) is shown in Figure 1. The black spectrum was taken 20 years after the blue one and clearly shows a newly formed broad Hβ line as well as an increasingly strong Hα line.

Example changing-look AGN spectrum

Figure 1: Example of a changing-look AGN spectrum. The blue spectrum shows the original spectrum from 2002, and the other spectra are from 2021–2022, where there is now a strong, broad Hβ line and a blue continuum. [Adapted from Zeltyn et al. 2024]

What causes these strange changes, you may ask? There are two main ideas behind what causes these changing-look events — changes to the amounts of dust and gas along our view to the black hole, or changes to something intrinsic about the black hole. For example, a change in how fast the black hole gobbles up the surrounding material could be responsible. Researchers have argued that individual events point to either one of these two scenarios, but building up a large sample is crucial to telling which is the dominant cause.

Introducing: Changing-Look AGN in SDSS-V

Detecting changing-look AGN is difficult because the changes occur on relatively long timescales and require multiple optical spectra of the same AGN (which is rather rare to have!). However, no need to fear, today’s article is here! The authors of today’s article present one of the first systematic searches for changing-look AGN using a new survey, the Black Hole Mapper program as part of SDSS-V, the fifth generation of the Sloan Digital Sky Survey. This survey was designed to take numerous optical spectra of hundreds of thousands of AGN. Today’s article focuses on the first year of data, which contains almost 30,000 AGN with multiple optical spectra. From this sample, the authors find 116 changing-look AGN that vary on timescales ranging from 2 months to 19 years!

It’s All About the Rate!

By comparing their new, robust sample of changing-look AGN to a control sample of ordinary AGN, the authors investigated if there were any crucial differences between the two samples that could help illuminate the origin of changing-look AGN. They found that the black hole mass and total energy output were not significantly different between the two samples, but the accretion rate normalized to the black hole mass (also known as the Eddington ratio) was different. This is shown in Figure 2, with the red showing the changing-look AGN and the blue showing the control sample of ordinary AGN. The two different panels show different control samples, but importantly, they both show this same exact trend. This confirms what other studies have suggested — that the most important factor for determining whether an AGN will undergo a changing-look event is its relative accretion rate!

comparison of the Eddington ratio for ordinary AGN and changing-look AGN

Figure 2: Comparison of the Eddington ratio (the accretion rate normalized to the black hole mass) of ordinary AGN (blue) and changing-look AGN (red). The ordinary AGN (control sample) are drawn from a past research article and are matched to other quantities — the bolometric luminosity (left) and black hole mass (right) — of the changing-look AGN to control against. The changing-look AGN show a statistically lower Eddington ratio than both control samples. [Adapted from Zeltyn et al. 2024]

plot showing the change in strength of the Hα and Hβ for active galactic nuclei

Figure 3: Comparing the relative change in the Hα and Hβ lines for AGN where both can be observed. The two are positively correlated, which is expected, but the Hβ lines seem to vary more than the Hα on average. This could indicate that the observed changes are due to some alterations to the gas and dust along our sight line to the supermassive black hole. [Adapted from Zeltyn et al. 2024]

To investigate the dominant cause of these changing-look AGN, the authors used two different measurements. First, they looked at the relationship between flux changes in different spectral lines. For example, in sources where both Hα and Hβ were observed, they found that the two seemed to change in a correlated manner, but the Hβ changes more than Hα on average (Figure 3). They hypothesized that this could be due to variable dust and gas that would affect the bluer Hβ line more. However, the infrared data for most of these objects showed significant changes as well, which is not consistent with this variable dust and gas model. Thus, the authors concluded that the majority of the changing-look AGN in their sample are the result of changes to the accretion rate.

Wondering how these cosmic curiosities fit into the grand picture of AGN? Keep your eyes peeled because this is coming next! The authors stress that continued SDSS-V/Black Hole Mapper observations will lead to larger samples of changing-look AGN and will help us better understand how these changing-look events relate to normal AGN variability.

Original astrobite edited by Janette Suherli.

About the author, Megan Masterson:

I’m a 4th-year PhD student at MIT studying transient accretion events around supermassive black holes, including tidal disruption events and changing-look active galactic nuclei. I primarily use multi-wavelength observations to study from the inner accretion flow to the obscuring material in these transients. In my free time, you’ll find me hiking, reading, and watching women’s soccer.

artist's impression of the view from the surface of Sedna

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: Primordial Orbital Alignment of Sednoids
Authors: Yukun Huang (黄宇坤) and Brett Gladman
First Author’s Institution: University of British Columbia
Status: Published in ApJL

The outermost region of our solar system is home to a small group of distant objects with orbits that deviate significantly from the rest of the objects at similar distances from the Sun: Sedna, 2012 VP113, and Leleākūhonua — collectively known as sednoids — have peculiar orbital characteristics that have intrigued astronomers and led to questions about the earliest conditions of the solar system.

The Remarkable Orbits of Sednoids

plot of the orbits of the three sednoids

Figure 1: Orbital configuration of the three sednoids. (Leleākūhonua is also cataloged as 2015 TG387.) [Wikipedia user Tomruen; CC BY-SA 4.0]

Trans-Neptunian objects, located beyond Neptune’s orbit, are a diverse collection of icy bodies that include dwarf planets like Pluto and make up the Kuiper Belt — a vast region of space starting at Neptune’s orbit extending outward, populated by countless objects often no larger than a few kilometers across. Among these, sednoids are remarkable due to their incredibly high perihelion distances (see Fig. 1), which means they remain very far from the Sun even at their closest approach (Sedna’s perihelion distance is about 80 times Earth’s distance to the Sun). The sednoids also remain far from our solar system’s planets, and this detachment from the gravitational influence of the major planets raises questions about the sednoids’ origins and the forces that shaped their current trajectories.

Today’s authors set out to determine whether the current orbital alignment of the sednoids could be traced back to an event, such as the possible encounter with a rogue planet or a close stellar flyby, that occurred during the planet formation epoch approximately 4.5 billion years ago. This event would have had the power to imprint a lasting apsidal orientation on these distant objects, guiding them into the peculiar orbits we observe today.

The authors used computer simulations to trace the orbits of the three sednoids backward in time, thus effectively simulating the dynamical evolution of Sedna, 2012 VP113, and Leleakuhonua over billions of years. These simulations calculated the gravitational influence of the Sun and the giant planets — Jupiter, Saturn, Uranus, and Neptune — on the sednoids’ trajectories. To enhance the accuracy of their model, the researchers also took into account a range of initial conditions that reflect the current observational uncertainties in the sednoids’ orbits. This approach allowed them to assess how these uncertainties could influence the backward-in-time orbital paths of these distant objects, ensuring a robust analysis of the sednoids’ dynamical history while minimizing the impact of potential observational biases.

The simulations revealed that 4.5 billion years ago, the sednoids’ orbits may have aligned such that the lines connecting their closest approaches to the Sun, known as their apsidal lines, converged at a perihelion longitude of 200° (see Fig. 2). The perihelion longitude is a specific angle measured in the plane of the solar system that pinpoints the direction toward which each object makes its closest approach to the Sun in its elliptical path. The fact that these apsidal lines clustered so closely in the past suggests that the sednoids did not arrive at their current orbits by random processes alone. Instead, it hints at a shared historical influence or event, potentially during the formation of the solar system, that nudged these objects into such precisely aligned paths. This event could be the gravitational disturbance from a passing star, which may have been common in the densely packed environment where the Sun formed.

plot of the simulated perihelion longitude over time

Figure 2: Past evolutions of perihelion longitudes for the three sednoids. The only time that the three apsidal lines converge is about 4.5 billion years ago. [Huang & Gladman 2024]

Alternatively, the authors of the study consider the possibility of a rogue planet — a massive planet that was once part of our solar system but was ejected due to gravitational interactions. This hypothetical planet could have been in a position to significantly influence the orbits of the sednoids, aligning their paths to the observed perihelion longitude of 200° before the planet was itself cast out into the galaxy as seen in Fig. 3. The concept of such a rogue planet adds a fascinating layer to the history of our cosmic neighborhood, suggesting that our solar system’s architecture could have been quite different in the distant past.

plots of longitude of perihelion for simulated trans-Neptunian objects

Figure 3: Simulation demonstrating the influence of a hypothetical rogue planet on the orbits of trans-Neptunian objects by plotting longitude of perihelion versus perihelion distance. The left panel shows the orbital configurations shortly after the rogue planet’s departure 185 million years post-formation, while the right panel depicts the current arrangement. The ejection of the rogue planet leads the initially clustered longitudes of perihelion to homogenize, except for bodies where precession periods are comparable to the age of the solar system. As this is the case for sednoids, this may hint at the presence of an additional planet in the early solar system. [Huang & Gladman 2024]

Bad News for Planet Nine

The discovery of unusual orbital patterns among trans-Neptunian objects led to the Planet Nine hypothesis, which posits a yet-undetected ninth planet far beyond Neptune that influences the orbits of these distant celestial bodies. However, the apparent one-time alignment of the sednoids’ orbits 4.5 billion years ago suggests a singular historical event, not consistent with the ongoing perturbations expected from a persistent ninth planet. Such a planet would actively sculpt the orbits over time, potentially erasing the signatures of past orbital configurations. The lack of current apsidal clustering supports the idea that today’s orbital architecture of sednoids stems from an early solar system incident, rather than the continuous presence of a large, undiscovered planet.

Future Discoveries and Solar System Secrets

Nonetheless, due to the intricate nature of the solar system’s dynamics, a more comprehensive dataset is essential to substantiate these findings. The advent of advanced telescopic technology and extended astronomical surveys holds promise for the detection of additional sednoid-like objects. Such observations would be crucial, as they could either corroborate the theory of a primordial event influencing sednoid orbits or compel us to reconsider our current understanding of the solar system’s evolution.

The study on sednoids offers a glimpse into the conditions of the early solar system. The sednoids’ unique orbital alignment could serve as a record of a major event that occurred shortly after the solar system’s formation. As more data become available, we may be able to piece together a clearer picture of our solar system’s history, from its most chaotic beginnings to its current state.

Original astrobite edited by Mark Dodici.

About the author, Konstantin Gerbig:

I’m a PhD student in astronomy at Yale University. I’m interested in the theory of (exo)planets and protoplanetary disks and do hydro simulations thereof. I also like music, as well as dancing salsa and tango.

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