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overlapping rings of panels surround 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: Evolutionary and Observational Consequences of Dyson Sphere Feedback
Authors: Macy Huston and Jason Wright
First Author’s Institution: The Pennsylvania State University
Status: Published in ApJ

Are we truly alone? Or rather, where is everybody? After all, there are many billions of stars and exoplanets in the Milky Way, and our galaxy has had billions of years to evolve. Although the search for extraterrestrial life has so far yielded nothing, we may still be able to detect signs of past or present intelligent life within our galaxy by detecting artificial megastructures.

Technosignatures

In order for a civilisation to make the technological leap to conquer the stars, it must be able to acquire and harness sufficient energy. This is the basis of the Kardeshev scale, which ranks technological advancement based on energy utilisation. One way to harness a massive amount of energy is to partially or even fully surround a star with solar panels or solar collectors. This type of megastructure is known as a Dyson sphere, named after physicist Freeman Dyson who proposed it as a thought experiment, arguing that such megastructures were a logical next step to meet the energy needs of a space-faring civilisation. Dyson postulated that it may be possible to detect the presence of megastructures by looking for changes in a star’s electromagnetic spectrum. A change in a measurable property, like a star’s spectrum, due to the presence of some artificial structure is referred to as a technosignature. So, what is the technosignature of a Dyson sphere?

So far, studies have mostly focused on changes to the spectrum of the star — particularly in the infrared — and/or light dimming as the megastructure rotates around the star. However, there is a more subtle effect at play too: the radiative feedback from the Dyson sphere itself. Today’s article examines this feedback and the subsequent effects on the star’s evolution, and whether such effects constitute a detectable technosignature.

Feedback

The goal of a Dyson sphere is to collect as much energy as possible from a star, but it is likely that some of this energy will be reflected back onto the star. Additionally, the material of the Dyson sphere will ultimately heat up and emit thermal radiation. As such, Dyson proposed conducting observations in the infrared to try to detect this heat signature. We can gain further insight into this signature by understanding how the star behaves when it is subjected to Dyson sphere feedback. The authors of today’s article use the open-source Modules for Experiments in Stellar Astrophysics (MESA) tool to simulate the evolution of a star subject to external irradiation — in this case, the radiative feedback from the surrounding Dyson sphere. The authors simulate several stars with different masses and with different degrees of feedback, modelled as the fraction of the star’s luminosity that is reflected back.

As seen in Figure 1, feedback from the Dyson sphere generally results in a decrease in nuclear luminosity (the rate of energy production due to nuclear fusion) and an increase in radius. In other words, the star cools and expands. The effect is considerably more pronounced for the 0.4-solar-mass star as it is primarily convective (energy transfer is dominated by convection), whereas the 1-solar-mass star is mostly radiative with only a small outer convection layer. The drop in luminosity also means that the star lasts longer on the main sequence before exhausting its supply of hydrogen.

radius and nuclear luminosity as a function of the age of the star

Figure 1: The nuclear luminosity (left panels) and stellar radius (right panels) as a function of the age of the star for a 0.4 solar mass star (top row) and 1 solar mass star (bottom row). Different coloured lines correspond to different levels of Dyson sphere feedback, with the blue line corresponding to the star before the construction of the megastructure. [Adapted from Huston & Wright 2022]

Dyson Sphere Program

The feedback from Dyson spheres is thus able to change the properties of the host star, with more substantial changes occurring in low-mass stars. How does this translate into observations? To find out, the authors use the AGENT formalism, which characterises a Dyson sphere with five parameters. These include the power of the intercepted starlight, denoted α, and the characteristic temperature of the waste heat, T. The authors consider two types of Dyson spheres: hot Dyson spheres, which are coloured black and absorb all starlight, and cold, mirrored Dyson spheres, which reflect all starlight without heating up.

In Figure 2, we see mock colour–magnitude diagrams for two instruments, ESA’s Gaia spacecraft and NASA’s WISE spacecraft. GBP, GRP and G-W4 refer to the blue and red Gaia filters and the infrared WISE filter, respectively. At high feedback levels, the temperature of the sphere is close to that of the star, so it appears bluer. At low feedback levels, the Dyson sphere is cooler and contributes to the dimming and reddening of the star. For cold, mirrored spheres, the reflected light makes the star appear bluer, but its overall luminosity is unchanged.

magnitude versus color

Figure 2: Colour–magnitude diagrams for potential Dyson spheres systems around a 1 solar mass star for Gaia observations (left panel) and WISE observations (right panel). Coloured lines denote different fractions of feedback, with the black lines denoting Dyson spheres at 0.1 au and 1 au. [Adapted from Huston & Wright 2022]

To Change a Star

Low-mass stars are significantly affected by Dyson sphere feedback; only 1.3% feedback is required to alter the nuclear luminosity of a 0.4-solar-mass star by 1%, while 45% feedback is required to produce the same change in a solar-mass star. In general, the Dyson spheres themselves must also have extremely high temperatures in order to generate sufficient feedback. Figure 3 shows that even low-mass stars would require temperatures well in excess of 1000K to result in even a slight change of nuclear luminosity. There is no significant change in nuclear luminosity for Dyson spheres with temperatures in the hundreds of kelvin.

fraction of captured starlight versus the effective temperature of the sphere

Figure 3: The fraction of captured starlight versus the effective temperature (in Kelvin, K) of the Dyson sphere. Solid lines correspond to the values required for a 1% change in nuclear luminosity, and dotted lines correspond to a 1% change in the effective temperature of the star. [Huston & Wright 2022]

Astroengineering

This study has demonstrated that in extreme cases, the feedback from Dyson spheres can directly influence a star’s evolution: it cools, reddens, expands, and its lifetime on the main sequence is extended. The authors suggest that advanced civilisations could therefore use Dyson spheres as part of stellar engineering projects to extend a star’s life or siphon material (star lifting). The search for technosignatures has ramped up in recent years thanks to improvements in instrumentation. Modern instruments are sensitive enough to measure a star’s light dimming, most notably in Boyajian’s star, where one explanation proposed for its unusual light fluctuations is a transiting megastructure. Today’s article shows that Dyson spheres can result in measurable changes to stellar properties. Megastructures have long been confined to science fiction, imagination, and certain video games. However, if there are indeed Dyson spheres out there waiting to be found, we could soon be in a position to find them.

Original astrobite edited by Luna Zagorac.

About the author, Mitchell Cavanagh:

Mitchell is a PhD student in astrophysics at the University of Western Australia. His research is focused on the applications of machine learning to the study of galaxy formation and evolution. Outside of research, he is an avid bookworm and enjoys gaming, languages, and code jams.

Simulated image of a black hole warping light from background stars

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

Title: The Active Fraction of Massive Black Holes in Dwarf Galaxies
Authors: Fabio Pacucci, Mar Mezcua, and John A. Regan
First Author’s Institution: Center for Astrophysics | Harvard & Smithsonian
Status: Published in ApJ

Dwarf galaxies, with masses less than 10 billion times the mass of the Sun (M☉), host massive black holes. Unlike the central black holes in more massive galaxies, those in dwarf galaxies may not have quite become supermassive. This presents an opportunity to study analogs of supermassive black hole “seeds,” giving us a glimpse at the growth of supermassive black holes — a process that is not quite understood yet.

Active galactic nuclei (AGN) get their name from the active accretion of the supermassive black holes that power them, but not all supermassive black holes are “active.” Some, like the one in our own galaxy, are inactive. What causes AGN to shut off or turn on is still a mystery, but there’s no denying their activity makes them easier to observe. In dwarf galaxies, this is especially useful since their central black holes are usually smaller and therefore harder to detect, so many of the observations of massive black holes in dwarf galaxies have been of low-mass AGN. This means that current observations aiming to obtain the occupation fraction — the fraction of dwarf galaxies that have massive black holes, active or inactive — could actually just be measuring the fraction of dwarfs with active massive black holes.

The authors of today’s paper developed a theoretical model to predict the fraction of dwarf galaxies that contain active massive black holes (103 M☉ < M < 107 M☉), based on galaxy properties and observational constraints. In the model, the fraction of active black holes depends on the number density of the gas in the galaxy and the available angular momentum at its center. However, these two parameters are not necessarily available from observations, so the authors use proxies instead. The stellar mass (the mass of the galaxy that comes from stars) is used as a proxy for the number density of the gas. Instead of angular momentum, the authors use rotational support, a quantity determined from the rotational velocity and velocity dispersion of gas and stars. In the analysis, a black hole is considered active if it meets the criteria for efficient accretion.

Constraining a Physical Model

Fraction of dwarf galaxies with a massive black hole detected in the X-ray versus the stellar mass of the galaxy

Figure 1: The y-axis shows the fraction of dwarf galaxies that have a massive black hole detected in the X-ray. The x-axis shows the mass of stars in the dwarf galaxy. The points with error bars show data from observations in previous works. The blue line shows the expected X-ray detected fraction as a function of stellar mass based on the model developed in this paper. [Pacucci, Mezcua & Regan 2021]

Accreting black holes emit radiation across the electromagnetic spectrum, so they have been detected in dwarf galaxies at many different wavelengths. Some of the most extensive studies have been conducted at X-ray wavelengths, encompassing both wide surveys (which cover lots of targets) and deep observations (which see faint objects). These data provide valuable constraints on the authors’ theoretical model. The authors use the X-ray observations to constrain the emission from star formation and whether the AGN are Compton-thick, or surrounded by enough hydrogen gas to absorb most of the X-rays emitted. These factors affect the detectability of the AGN in observations by outshining or obscuring (respectively) the AGN X-ray emission. By adjusting the model parameters, the authors match their calculated fraction of active massive black holes with detectable X-ray emission to what we actually observe. Their results are shown in Figure 1.

After calibrating their model to match observations in the X-ray, the authors then remove the detectability constraints to calculate the fraction of all active massive black holes, not just those that are detectable in X-rays. The results are shown in Figure 2, along with results from simulations and semi-analytical models. The model presented in today’s paper predicts the active fraction of black holes in dwarf galaxies, since that is more likely what is being measured by current observations, rather than the total occupation fraction computed by previous simulations.

Fraction of dwarf galaxies with a massive black hole vs stellar mass of the galaxy.

Figure 2: Fraction of dwarf galaxies with a massive black hole vs. stellar mass of the galaxy. The blue lines show the results of this paper, the active fraction of massive black holes as a function of mass. The solid and dotted blue lines differ in metallicity of the galaxy. The rest of the curves and colored regions show the occupation fraction of massive black holes in dwarf galaxies from observations in other work. [Pacucci, Mezcua & Regan 2021]

The range of active fractions estimated with this model is 5–22%, which is lower than the occupation fractions from other work. The authors note that this is expected, based on the definitions of the two fractions. Assuming not all massive black holes are active, the active fraction should be lower than the overall fraction of massive black holes found in dwarf galaxies.

The authors explore the effects of the galaxy’s metallicity on the predicted active fraction. Metallicity refers to the types of elements that are present in the gas: if you have mostly hydrogen and only small amounts of elements more massive than helium — what astronomers call metals — you have low metallicity. Lower-metallicity gas leads to more active star formation, which produces a lot of X-rays that can wash out the X-ray emission from the active massive black hole. Accounting for the effects of low metallicity, the calculated active fraction can be higher for this model (blue dotted line in Figure 2) — up to 30% for the most massive dwarf galaxies.

Dwarf galaxies allow us to study possible seeds of supermassive black hole growth, which is necessary to understand black hole growth and galaxy evolution. One way to check if our models of black hole growth are correct is to see if the predicted fraction of active massive black holes in galaxies matches observations. The authors of today’s paper developed a model that predicts the fraction of massive black holes in dwarf galaxies that are active, based on galaxy properties, and compare it to observations. Models like this could be useful to compare with results from upcoming observatories like JWST and Athena, among others, which could actually observe supermassive black hole seeds in the early universe.

Original astrobite edited by Jamie Sullivan.

About the author, Gloria Fonseca Alvarez:

I’m a fourth-year graduate student at the University of Connecticut. My research focuses on the inner environments of supermassive black holes. I am currently working on measuring black hole spin from the spectral energy distributions of quasars in the Sloan Digital Sky Survey. As a Nicaraguan astronomer, I am also involved in efforts to increase the participation of Central American students in astronomy research.

Four bright circles appear at varying distances from a central 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: The First Dynamical Mass Measurement in the HR 8799 System
Authors: G. Mirek Brandt et al.
First Author’s Institution: University of California, Santa Barbara
Status: Published in ApJL

Astronomers have caught the first directly imaged exoplanet gravitationally tugging at its host star — thereby revealing its own weight.

Did you know that we can now take pictures of planets orbiting another star hundreds of light-years away? The HR 8799 system is famous for hosting one of the first directly imaged exoplanets. The four imaged planets are giants with masses greater than 5 times the mass of Jupiter (MJup) and wide orbits. They are named planet e, d, c, and b in order of proximity to the star. In the 13 years since their first detection back in 2008 by the Keck and Gemini telescopes in Hawaii, astronomers have tracked the planets’ orbital motions around HR 8799 with great precision (Figure 1).

This has enabled the authors of today’s article to calculate the mass of an exoplanet in this system using simple dynamics such as Newton’s and Kepler’s laws of motion. In particular, they concentrate on planet e, the one closest to HR 8799.

Four points of light slowly orbit a central point of light at varying distances

Figure 1: The HR 8799 planetary system. Blocking out the light from the central star allows us to observe and track the motions of four of its exoplanets. The given scale of 20 au, a little more than the orbit of the first planet (planet e), is equal to the distance between the Sun and Uranus. [Jason Wang (Northwestern)/William Thompson (UVic)/Christian Marois (NRC Herzberg)/Quinn Konopacky (UCSD); CC BY 4.0]

Precision Astrometry as a Weighing Scale

The measurement of planet e’s mass rides on the fact that the motion of the four planets produces minute tugs on host star HR 8799, thereby changing the star’s position in the sky ever so slightly. The process of making extremely precise measurements of a star’s sky coordinates is known as astrometry, a technique that has become especially prominent in recent years with new sky surveys like Gaia complementing their predecessor Hipparcos. The researchers used data from Gaia’s latest data release (DR3) to track the acceleration of HR 8799 caused by the net gravitational forces of all four planets.

Knowing the masses of all four major planets accurately makes it possible to predict how much they can accelerate HR 8799. In turn, precise observations of the star’s acceleration enabled the researchers to reverse this calculation and infer the exoplanet masses.

Probability curves for the likely mass of HR 8799 e for various ratios of its mass to the masses of the other planets in the system

Figure 2: The inferred probability distribution of the mass of planet e, based on varying the assumed mass ratios of other planets with respect to it (top: d, middle: c, bottom: b). The top panel shows that planet e’s mass estimation is most sensitive to the second-closest planet, planet d, being heavier (yellow curve) or lighter (teal curve). It does not depend much on the relative weights of the farther two planets. [Brandt et al. 2021]

How Heavy Is Planet e?

As the closest planet to HR 8799, planet e plays the principal role in shaking up its host star. However, the contributions from the other planets in the system are needed to determine how prominent planet e’s tugs are, and in turn how heavy the planet may be. Without knowledge of any of the planets’ masses, it is necessary to make assumptions about how much they weigh relative to each other. Figure 2 shows the authors’ best estimates for planet e’s mass, shown as probability distributions assuming different ratios of its mass to the masses of the other planets in the system.

The authors determine the resulting mass of planet e to be 9.6 times that of Jupiter, within an error bound of 2 MJup. They can say with a 95% certainty that its mass is below 13 MJup, above which it would have enough pressure at its core to start fusing deuterium and trigger nuclear fusion to become a (proto)star itself!

Implications for the Stellar System

In spite of being the planet closest to HR 8799, planet e is still three times farther away than Jupiter is from the Sun. Are there any smaller planets in the HR 8799 system, closer to the star than planet e but small enough to escape detection via direct imaging?

A shaded area indicates the mass and semi-major axis combinations that are disallowed

Figure 3: Can a hypothetical planet with a certain mass (y-axis) exist within the 16 au distance (x-axis) between HR 8799 and the orbit of planet e? The purple region represents heavier masses that are excluded since their effect is not seen in observations. [Brandt et al. 2021]

A hypothetical interior planet can additionally perturb the astrometric motion of HR 8799. But with the lack of such observed perturbations, the authors discard the presence of any planet weighing more than 6 MJup within 8 au (a little less than the distance between the Sun and Saturn), and any additional planets weighing greater than 7 MJup between a distance of 8 au and planet e’s orbit (Figure 3).

This study is a marker for how far we have come in studying exoplanets. It is a fine example of applying simple dynamics to study the gravitational dance and motions of an exoplanetary system — techniques that, until recent times, could only be applied to study our own solar system.

Original astrobite edited by Olivia Cooper.

About the author, Sumeet Kulkarni:

I’m a third-year PhD candidate at the University of Mississippi. My research revolves around various aspects of gravitational-wave astrophysics as well as noise characterization of the LIGO detectors. It involves a lot of coding, and I like to keep tapping my fingers on a keyboard even in my spare time, creating tunes instead of bugs. I run a science cafe featuring monthly public talks for the local community here in Oxford, MS, and I also love writing popular science articles. My other interests include reading, cooking, cats, and coffee.

A beautiful barred spiral stretches out from a round, bright yellow glowing core. The stars shift from yellow to white to bluish, and dark filaments are laced with pink active star-forming regions.

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: Extragalactic Magnetism with SOFIA (Legacy Program) – II: A Magnetically Driven Flow in the Starburst Ring of NGC 1097
Authors: Enrique Lopez-Rodriguez et al.
First Author’s Institution: Kavli Institute for Particle Astrophysics & Cosmology, Stanford University
Status: Published in ApJ

Galaxies throughout the cosmos display a delightful diversity of morphologies, from elegant and complex spirals to bizarre irregular galaxies. More and more, it seems that the structure and evolution of galaxies are strongly influenced by magnetic fields. The invisible magnetic fields that permeate interstellar space have long been suspected to play a critical role in the formation of stars, buoying clouds against gravitational collapse, steering gas flows throughout galaxies, as well as feeding supermassive black holes in galactic centers. Despite their importance, these magnetic fields are notoriously difficult to measure and map. State-of-the-art instruments and techniques have opened a window into a golden age of magnetic field measurements from local molecular clouds within our Milky Way to the distant maelstroms of gas swirling around galactic nuclei.

How Do We Measure Magnetic Fields?

These magnetic fields cannot be detected directly, so we have to rely on measuring their effects on gas and dust within the environments we aim to study. One way to reveal the magnetic field in the interstellar medium is dust polarimetry. Individual grains of dust amidst the gas in molecular clouds tend to orient themselves relative to the magnetic field that is present, and these organized dust grains emit light with a certain polarization. One can measure the polarization of that light and then infer the orientation of the glowing dust grains to map out the magnetic field lines influencing the dust (Figure 1). This process requires sensitive measurements, but it’s possible using instruments like those aboard SOFIA (the Stratospheric Observatory For Infrared Astronomy).

Cartoon showing a gas cloud with a vertically oriented magnetic field. Elongated dust grains aligned parallel to the magnetic field (i.e., their long dimension is parallel to the magnetic field lines) are labeled “unlikely dust grain orientation.” Elongated dust grains aligned perpendicular to the magnetic field are labeled “likely dust grain orientation.” A sine curve representing the thermal emission from the dust grain emerges from the cloud, traveling toward a telescope.

Figure 1: A cartoon schematic demonstrating how a magnetic field in a molecular cloud influences the likely orientation of dust grains, which emit light with a polarization indicating their orientation. Measurements of this polarization can then be used to infer the original magnetic field. [H Perry Hatchfield]

The Magnetic Heart of NGC 1097

Today’s paper explores the magnetic-field structure of the center of NGC 1097, a barred spiral galaxy with an active galactic nucleus and a brilliant starburst ring fed by a pair of linear gas structures called dust lanes that are aligned with the bar. The ring of dense gas orbiting about 3,000 light-years from the galaxy’s nucleus is forming stars at more than twice the rate of the entire Milky Way! In addition to its spectacular spiral arms, prominent dust lanes, and glowing core, NGC 1097 is also known to host one of the strongest interstellar magnetic fields in a nuclear starburst ring. This galaxy provides an excellent testbed for studying the interactions of star formation, galactic-scale flows and structures, and powerful magnetic fields.

The authors use a combination of far-infrared (89-μm) polarimetry from SOFIA’s High-resolution Airborne Wideband Camera Plus (HAWC+) instrument and radio (3.5-cm and 6-cm) polarimetry from the Very Large Array (VLA) to trace the orientation of dust grains shifted by the magnetic field. They also explore the velocity structure of the gas using molecular line emission from carbon monoxide. By understanding both the gas motions and the magnetic field properties, they can see how the gas flows and magnetic structure of the galaxy might be related. The magnetic field traced by the far-infrared emission appears to have a different structure from the field revealed by the radio observations; the 89-μm dust emission seems to indicate a compressed field, while the radio observations clearly suggest a spiral structure to the field (Figure 2).

A central blob of carbon monoxide emission surrounded by a fragmented ring with a fainter spiral arm trailing off to either side. Yellow and red polarization vectors are scattered across the center of the image, as well as red and yellow contours.

Figure 2: The magnetic field structure traced by far-infrared observations from SOFIA’s HAWC+ instrument (in yellow) and 3.5-cm radio observations from the VLA (in red). In each case, the lines show the orientation of the magnetic field, and the contours show the intensity of the polarized light. The background color-scale image is the integrated carbon monoxide emission (J=2–1 transition) from ALMA. [Lopez-Rodriguez et al. 2021]

The authors interpret this apparent difference in magnetic-field morphology as the two observations tracing separate modes of the magnetic field associated with different phases of the gas: the far-infrared polarization reveals the magnetic field compressed by a shock wave crashing through the dense gas in the starburst ring, while the radio polarization shows the magnetic field being twisted in a spiral by shearing motions in the more diffuse gas. This suggests that the gas motions, whether diffuse or dense, may be guided by the strong magnetic fields. Unraveling the intricate dance of magnetic fields, kinematics, and gravity is no easy task, but multi-wavelength polarization studies like this provide an exciting window into the diversity of the magnetic structures of galaxies — and when it comes to magnetic fields, it seems like there is always more than meets the eye.

Original astrobite edited by Sasha Warren.

About the author, H Perry Hatchfield:

I’m a PhD candidate in Physics at the University of Connecticut, where I study star formation and gas structure in the Milky Way’s galactic center. I do this using radio observations of molecular clouds as well as hydrodynamic simulations, and I’m all about trying to find ways to compare these two exciting means of exploring the universe.

Where Does a Dragonfly Land?

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: Selection and Characteristics of the Dragonfly Landing Site near Selk Crater, Titan
Authors: Ralph D. Lorenz et al.
First Author’s Institution: Johns Hopkins University Applied Physics Laboratory
Status: Published in PSJ

An Aptly Named Mission

In 2005, the Huygens probe descended onto Saturn’s moon Titan and gave us our first ever view from a surface in the outer solar system (see Figure 1). Even though it was only designed to last a few hours after landing, this landmark achievement returned valuable datasets explaining various properties of Titan’s atmosphere and surface. This time, NASA is sending a mission to Titan with much more power and instrumentation — and better yet, it can fly!

Dragonfly is a lander, but it also leverages Titan’s relatively low gravity and high atmospheric pressure to fly like a drone, scouting new targets and exploring a wide variety of destinations over the course of the mission. This unique ability will allow us to study diverse landscapes over an area possibly covering hundreds of miles, investigate pathways for prebiotic chemistry, analyze habitability (e.g., climate, geological processes), and search for biosignatures.

In order to propose this mission to NASA’s highly competitive New Frontiers Program, scientists and engineers performed early studies of where Dragonfly could land and what location would best suit its science goals. Today’s article explains the basics of the selected landing site and how the team arrived at their chosen location.

Left image: Illustration of the roughly flying-saucer-shaped Huygens probe descending onto Titan’s surface. First, its parachute deploys as its heat shield glows from the friction of entering the atmosphere. Then, it descends more gently to the surface as the parachute slows its descent. Finally, it rests on the surface. Right image: The Dragonfly spacecraft goes through a similar series of maneuvers to land in a flat area between dunes. After landing, its propellors begin to spin, lifting it off the surface.

Figure 1: Artists’ depictions of the Huygens probe (left) compared to Dragonfly (right) arriving on Titan. [Left: ESA/C. Carreau; right: NASA]

Descending onto Titan

Like any lander mission to a planetary surface, safety is the first consideration for narrowing down landing sites. In the mid-2030s — the arrival time planned for the mission — Titan’s northern hemisphere will be pointed away from Earth and the Sun. Since Dragonfly relies on a direct line of sight for communication, and NASA requires active tracking during the spacecraft’s entry, descent, and landing phase, this mission therefore can’t be sent to the intriguing methane seas at Titan’s north pole. Instead, dune fields near the equator make for yet another enigmatic target on Titan, where the composition of the organic sand and its origin are not yet well understood.

To avoid the need for additional propellant, the spacecraft will sneak up behind Titan (rather than hitting head-on) and use an aeroshell to slow down in Titan’s thick atmosphere. Assuming that Dragonfly enters at the same 65-degree angle as the Huygens probe, these constraints further narrow down potential landing sites on Titan to a donut-shaped “viable target region” (see Figure 2). From this region, the team was soon drawn to a large impact crater that may provide a variety of interesting terrains for study.

A grayscale projection of Titan's surface. The equatorial region is spanned by several large dark patches. Several lakes and seas, known as maria, dot the north pole. Yellow annotations indicate the reasons for considering or not considering various locations for a landing site. Lack of daylight excludes most areas with longitudes between -120 and 60. The steepness or shallowness of the spacecraft's entry further restricts options to a donut-shaped region centered on 110 degrees longitude and 10 degrees latitude.

Figure 2: A map of Titan from the Cassini spacecraft’s Imaging Science Subsystem with annotations concerning various considerations for landing. Dragonfly’s proposed landing site, near Selk Crater, is at 3.7°N, 161.8°E. [Lorenz et al. 2021]

Preparing for Landing

The chosen location on Titan, Selk Crater, makes for an interesting study region, in part because of the fascinating chemical reactions possible there. Though Titan is extremely cold and icy, it is wrapped in a haze of organic molecules (long chains of carbon, hydrogen, and nitrogen) that have likely reacted to form the basic building blocks of life. Following the extreme heat of an impact, ice melted into liquid water would have created a fascinating organic soup that may have even been briefly habitable before freezing back into place. Besides the astrobiological potential, the crater and nearby dunes have obvious geological interest. For example, ongoing erosion by winds and methane rains has likely degraded the crater rim.

Despite the sometimes sparse data available for Titan, Selk Crater also has the benefit of having relatively good coverage for mission planning, thanks to several flybys during the Cassini mission. Visible and near-infrared images indicate the chemical composition of the surface based on the brightness of reflections at different wavelengths, but often the highest-resolution data for studying morphology comes from radar images (see Figure 3). Some limited topographic data further suggest that the region has relatively minor slopes, rather than steep mountains that could be hazardous to the mission.

To study the region around Selk Crater, the team divided the area into a grid with quarter-degree (~10 km or ~6 mi wide) cells. Making a map for each of the three main Cassini datasets (visible light, infrared, and radar), each cell is given a classification, indicating, for example, whether that cell contains dunes, less-certain dunes, ice-rich impact ejecta, or insufficient data, among other classes similar to those used in previous maps. These grid cells can then be used to define mission plans, including a specific landing location. Based on simulations of the atmosphere and its winds, the team defined a 149 by 72 km (93 by 45 mi) landing ellipse south of the crater, within which the probe has a 99% chance of landing.

Three images of Selk Crater. Left: A black and white image showing the bright crater rim and ejecta blanket extending down and to the right against the dark background. Center: A representative-color image of the same region in slightly greater detail. Right: A zoomed-in image of the same region with the landing ellipse marked below the crater.

Figure 3: Selk Crater seen in various datasets. Left and center images with black borders are from the near-visible-wavelength ISS (Imaging Science Subsystem, left) and infrared-wavelength VIMS (Visual and Infrared Mapping Spectrometer, center). Zoomed in somewhat closer, the radar mosaic (right) also shows the location of two topographic profiles (XY and JK) and the landing ellipse in white, with colors representing the microwave emissivity. [Adapted from Lorenz et al. 2021]

Setting Your Expectations

In addition to the available data, scientists can also make inferences about the surface conditions by drawing analogies to similar geologic features on Earth (see Figure 4). The “interdune” area between dunes is expected to be flat and safe, so Dragonfly won’t tip over! Since Titan’s dunes are expected to have the same morphology as dunes on Earth, there should be wide sections of each dune that are shallow in slope so that Dragonfly can safely take samples of the sand. Ultimately, if things go well among the dunes, the craft will progressively work its way north into the ejecta blanket and Selk’s interior. Terrestrial analogs for Selk similarly give researchers confidence that the interior of the crater will be relatively flat and hospitable to Dragonfly.

With several years remaining before launch, further changes to the plan might occur, but the team expects that any remaining changes to the landing ellipse will be relatively minor. Ongoing studies will dig deeper into the Cassini–Huygens data in preparation for this ambitious mission. Although Dragonfly’s launch was delayed to 2027 (with arrival at Titan in 2034), it looks like a visit to Selk Crater will certainly be worth the wait!

Top left: Two long, straight, radar-bright dunes extend from the upper left to the lower right of the image. Bottom left: A sinuous dune with flat sand to either side. Right: A flat area with winding streams lies in front of a crater rim.

Figure 4: Terrestrial analogs for the landing site on Titan. Linear dunes in Egypt seen from above in radar (top left) and near the ground (bottom left) are likely very similar to those on Titan. The Haughton impact structure in northern Canada (right) is likely similar to Selk, with a flat area inside the crater rim. [Adapted from Lorenz et al. 2021]

Original astrobite edited by Haley Wahl.

About the author, Anthony Maue:

Anthony is a PhD candidate at Northern Arizona University in Flagstaff studying planetary geology. In particular, his research focuses on Titan’s fluvial processes through analyses of Cassini radar data, laboratory experiments, and terrestrial field analog studies. He is also one half of the planetary science podcast Out of the Silent Planet. Back on Earth, Anthony enjoys skiing, cycling, running, music, and film.

illustration of a brown dwarf

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: Ross 19B: An Extremely Cold Companion Discovered via the Backyard Worlds: Planet 9 Citizen Science Project
Authors: Adam C. Schneider et al.
First Author’s Institution: United States Naval Observatory, Flagstaff Station
Status: Published in ApJ

Typically thought of as stars that didn’t quite make it, brown dwarfs are a class of object without enough mass to undergo hydrogen fusion like other main-sequence stars, but heavy enough to fuse deuterium, differentiating them from their gas giant planet cousins. With faint spectra and complex evolutionary tracks, uncovering the properties of a brown dwarf can be much more challenging than your typical star or planet, particularly if the brown dwarf does not have a companion to help determine its age and mass.

However, ever the classifiers, astronomers often separate brown dwarfs into one of three spectral types — L, T, and Y — based on their temperature. The coldest and newest category of brown dwarfs, the Y dwarfs, have so far proven especially hard to study. Out of the only 25 known Y dwarfs, only one, WD0806-661B, has a stellar-mass companion. Since the presence of companions enables the use of techniques like radial velocities to refine system parameters, this means that the ranks of the coldest brown dwarfs are severely lacking in well-constrained masses and ages.

Enter Stage Left: CWISE J021903.84+352112.2

Meet Ross 19B, the newest discovery of the Backyard Worlds: Planet 9 citizen science project. As its “B” designation suggests, Ross 19B is a companion to the nearby M-type star Ross 19A, and it was first identified as a candidate brown dwarf (and labelled CWISE J021903.84+352112.2) by citizen scientists Samuel Goodman, Léopold Gramaize, Austin Rothermich, and Hunter Brooks. By searching through sets of images from NASA’s Wide-field Infrared Survey Explorer (WISE) mission, the citizen scientists were able to identify that Ross 19A and the candidate object had similar proper motions — the way in which nearby objects appear to move across the sky with respect to more distant stars, which remain comparatively constant — as seen in Figure 1. This kind of co-movement suggests that Ross 19A and the candidate are likely physically associated, and they could be binary companions to each other.

Two negative-color images from the WISE mission arrayed vertically, showing black stars and other objects against a tan background. The top image is labeled 2010 and the bottom image is labeled 2019. Each image has two pairs of blue and white circles, showing the positions of Ross 19A and Ross 19B, as well as a square with a zoomed in image of Ross 19B.

Figure 1: Images from the WISE mission demonstrating the similar proper motion of the M-type star Ross 19A and the candidate brown dwarf, taken in 2010 (top panel) and 2019 (bottom panel). Ross 19A (labelled as Ross 19) is seen in the top right-hand corner of each panel, while the candidate object that would become Ross 19B (labelled as CWISE J021903.84+352112.2) is in the bottom left-hand corner. To highlight the proper motion of each object, the white circles in both panels indicate their positions in 2010, while the blue circles do the same for 2019. In both panels, inserts show a zoom-in of the location of Ross 19B, seen as a small brown dot. [Schneider et al. 2021]

To confirm that the candidate object was indeed a companion to Ross 19A and not aligned with it by chance, the authors estimated the probability that the two objects are co-moving using the CoMover code. CoMover makes use of the BANYAN Σ software’s ability to determine how well the position, proper motion, and parallax of an object match with the galactic coordinates and velocities of nearby groups such as stellar associations. With the new CoMover tool, the authors find a 100% probability that Ross 19A and the candidate are physically associated, and the team officially designates the new brown dwarf as Ross 19B.

What Makes Ross 19B So Special?

With the available photometry and the known distance to the Ross 19 system, the spectral type of Ross 19B can be found by calculating its J-band magnitude and converting this to a spectral classification. Ross 19B was found to have a J-band magnitude of ~19.6, corresponding to the boundary between T and Y dwarfs, and a temperature of around 500K. As seen in Figure 2, this makes Ross 19B the coldest known brown dwarf companion within 20 parsecs of the Sun, with the exception of the only known Y-dwarf companion, WD0806-661B.

Plot showing the J-band magnitudes and spectral types of brown dwarfs within 20 parsecs of the Sun. The x-axis (spectral type) ranges from L0 to just beyond Y0. The y-axis (J-band magnitude) ranges from 24 to 10. The placement of the gray rectangle indicating the J-band magnitude of Ross 19B, places it at a spectral type around Y0.

Figure 2: Plot showing the J-band magnitudes and spectral types of brown dwarfs within 20 parsecs of the Sun, with cooler objects towards the right and brighter objects towards the top. Brown dwarfs which have companions are highlighted by the blue circles, while brown dwarfs without companions are shown by the black points. The J-band magnitude of Ross 19B and its error bars are shown by the grey region. With a spectral type near the T/Y boundary, Ross 19B is likely the coldest local brown dwarf companion, with the exception of the Y dwarf WD 0806-661B. [Adapted from Schneider et al.]

Using Ross 19B’s companion to their advantage, the authors are also able to make estimates about the physical parameters of the system. Ross 19A is a low-metallicity star with no signs of activity such as stellar flares, indicating that the star is several billion years old. Using this estimate along with the spectral type of Ross 19B, evolutionary models suggest that Ross 19B could have a mass around 15–40 times that of Jupiter, towards the lower end of typical brown dwarf masses. Meanwhile, at over 11,000 au, the separation of Ross 19A and B makes the system one of the widest found to date!

Given all these factors, Ross 19B provides a fascinating new case study, helping us to expand our understanding of brown dwarfs into colder and wider territories.

Original Astrobite edited by Pratik Gandhi.

About the author, Lili Alderson:

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

Image of a background quasar that has been smeared into a ring, with 4 bright dots visible within the ring. A foreground galaxy can be seen in the ring's center.

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: ALMA Observations of the Sub-kpc Structure of the Host Galaxy of a z = 6.5 Lensed Quasar: A Rotationally-Supported Hyper-Starburst System at the Epoch of Reionization
Authors: Minghao Yue et al.
First Author’s Institution: Steward Observatory, University of Arizona
Status: Published in ApJ

The party starts at dawn, and you all are invited! You’ll be surrounded by new, up-and-coming stars and a big fireworks show! There’s even a house of mirrors, where you can see a distorted, magnified version of yourself. Be careful of the action in the very center — it’s bright and shiny, but if you get too close, you could get ejected from the party entirely. Looking back, your view of the party might get a little warped, but it will surely be a bright point. Oh, and it’s hosted by a luminous quasar (z = 6.5) that is powered by a supermassive black hole — you can’t miss it!

Into the Cosmic House of Mirrors

In today’s paper, the authors target a gravitationally lensed quasar and its host galaxy in the early universe, the first of its kind to be observed so far back in spacetime (during cosmic dawn). Gravitational lensing occurs when mass along the observer’s line of sight distorts light before it reaches the observer, turning the universe into a sort of cosmic house of mirrors. During the process, some clump of mass (in this case, another galaxy) in the foreground magnifies and warps the light — a consequence of general relativity — coming from the background object (here, the quasar and its host galaxy). To get a sense of how this works, see Figure 1 below, or check out the iOS app GravLens3!

Schematic diagram illustrating how gravitational lensing works.

Figure 1: Diagram demonstrating gravitational lensing. Light from the more distant quasar is intercepted by a massive foreground galaxy, causing the light to be bent and magnified. The resulting image seen by the telescope shows the foreground galaxy surrounded by multiple images of the lensed quasar. [NASA/ESA/D. Player (STScI)]

This lensing is like having an extra, natural telescope amplifying the signal and providing a higher resolution image. For this dataset, the galaxy’s light is boosted by about 4 times, and stretched out enough to resolve structure on small scales. But the amplification doesn’t come for free — the lensing also manipulates the image from the background source. While the lensed object is actually a single blob, the lensed image we get is stretched out and appears multiple times, depending on how the mass of the foreground lens is distributed and positioned with respect to the background lensed object.

Hyper-Starbursting and Supersized

Quasars, some of the brightest and most extreme objects in the universe, are found in galaxies during a temporary mega-bright phase (see this astrobite for more about how they grow). During this phase, their central supermassive black holes vigorously funnel in material, building a disk of hot, luminous material around it and ejecting material via energetic jets. While quasars are bright and easy to detect, the galaxies that host and fuel them are often hidden behind the shining quasar in their active centers. However, if we can peer behind the quasar and target the gas and dust in the host galaxy, we can better understand the nature and origin of the galaxy–quasar system.

Today’s authors use millimeter wavelength observations from the Atacama Large Millimeter/submillimeter Array (ALMA) to measure the host galaxy’s dust and [CII] emission. [CII] is a molecular emission line that is commonly used to trace the motions and content of gas, the main fuel source for luminous galaxies. These observations provide clues about the structure, motions, and star formation activity within the host galaxy. After building a model of the lens to disentangle the multiple images into the original single source, the authors were able to confirm their previous lens model based on HST data (see the group’s earlier paper), and model the new ALMA data.

Based on their reconstruction of the background host galaxy (see Figure 2), the light profile looks like your everyday disk galaxy, and the kinematic tracers show a smooth velocity structure. These clues together suggest that the host galaxy is a regular, rotating disk, like a vinyl spinning around on a record player. However, the dynamics are a bit more complicated, as the galaxy doesn’t appear to be symmetric along its axes, nor does it seem to be a thin disk — so, not quite a vinyl on a record player. It could just be clumpy, but more likely it has a thick geometry and is more spheroidal than disky. Higher resolution observations are needed to better understand the complex kinematics going on here, which are a common feature of high redshift quasars.

Two plots describing velocity structure of the quasar's galactic host, first as a Dec vs. RA plot with velocity represented by colors, and then as a velocity vs. position plot.

Figure 2: Left: velocity structure of the host galaxy, which shows a smooth rotation with blue moving towards and red moving away from the observer. The red curves trace the caustic, which describes the gravitational lensing and marks the curve of highest magnification. The black line corresponds to the right panel. Right: breakdown of the position and velocity across the galaxy. This also shows generally smooth rotation, with a minor irregular feature in the yellow box (region corresponding to black dots in the left panel). [Yue et al. 2021]

The kinematics also provide key measurements that can be used to estimate properties of the galaxy’s supermassive black hole. In the local universe, the mass of a central supermassive black hole and the mass of its host galaxy follow a simple positive relation. In this paper, the authors describe their galaxy’s black hole as oversized, as it’s relatively large compared to the host galaxy. This has been found for other similar distant quasars, and suggests a fundamental difference in the coevolution of black holes within quasar systems and their host galaxies at early times.

A Case Study for the Extreme

This system is the farthest lensed quasar in the universe (redshift z = 6.5) discovered so far, among the very first stars and galaxies (to learn about the most distant quasar currently known, which is non-lensed, check out this astrobite.) It’s a pretty intense system, with an extreme star formation rate and efficiency, and an overly massive black hole in the middle — definitely a real rager of a party. Yet these extreme characteristics are not necessarily unique, and they have been observed for the small sample of (non-lensed) quasars at cosmic dawn with ALMA. What makes this galaxy in particular so special is the fact that it’s gravitationally lensed, allowing for observations with much higher detail and signal. Moreover, the authors refer to this quasar as a case study for probing the nature, origin, and structures of quasars at cosmic dawn, and for understanding how supermassive black holes and their host galaxies co-evolve. Looking ahead to future studies, the authors hope to use higher resolution data to measure the influence of the central black hole more directly, confirm the geometry of the galaxy, and more precisely map its velocity field. Back into the cosmic house of mirrors at the party at dawn!

Original astrobite edited by James Negus.

About the author, Olivia Cooper:

I’m a first year grad student at UT Austin studying the obscured early universe, specifically the formation and evolution of dusty star-forming galaxies. I recently graduated from Smith College where I studied astrophysics and climate change communication. Besides doing science with pretty pictures of distant galaxies, I also like driving to the middle of nowhere to take pretty pictures of our own galaxy!

Illustration of a faint, red gas giant planet orbited by a dark, rocky body.

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

Title: On the Detection of Exomoons Transiting Isolated Planetary-Mass Objects
Authors: Mary Anne Limbach et al.
First Author’s Institution: Texas A&M University
Status: Published in ApJ

When faced with the sheer beauty of shimmering stars emblazoned across a night’s sky, it’s hard not to imagine that there are innumerable other worlds out there — entire planets teeming with alien life, or ruins of once-great civilisations. Until recent decades, such imagination was confined to the realm of science fiction, but in 1992 came the first detection of an exoplanet: a planet outside our solar system.

Illustration of a rocky body next to a gas giant planet, with a distant star and another small body in the background.

Artist’s depiction of an Earth-like exomoon orbiting a gas-giant planet. [NASA/JPL-Caltech]

The study of exoplanets provides important insights into the physics that govern the formation and evolution of planetary systems — in addition to raising many philosophical questions — but exoplanets are notoriously difficult to detect. The primary method of detection relies on measuring the dimming of a parent star during a planetary transit. Even more difficult to detect are exomoons: moons orbiting exoplanets. They are almost impossible to image directly due to the glare of the host star and, due to their smaller size, harder to detect via transit photometry. However, exomoons are believed to be at least as common as exoplanets. The authors of today’s paper point to a source where exomoons may be more easily detected: isolated planetary-mass objects (IPMOs), also known as rogue or nomadic planets.

Sunless Skies

Rogue planets are not bound to a planetary system. Instead, they roam free throughout the galaxy, bathed in darkness and lit by no sun. This makes it much easier to detect exomoons that orbit IPMOs compared to those orbiting ordinary exoplanets. The authors estimate that for over half of all known IPMOs, the sensitivity of the upcoming James Webb Space Telescope (JWST) is enough to detect the presence of an exomoon with similar characteristics to Jupiter’s moon Ganymede. Before planning an exomoon hunt, one must first consider observational constraints. One of the most important is the probability of observing a transit in the first place. Simulations have demonstrated that the formation of moons is common — with at least one large moon forming in roughly 80% of planetary systems — and that these moons often resemble the Galilean moons with respect to their sizes and orbital configurations.

Moonlight Sonata

It is therefore sensible to model probabilities based on moons in our own solar system, such as those orbiting the gas giants. The authors first model the transit probabilities for the solar system gas giant moons — that is, they find the probabilities that a moon will cross the face of the gas giant — with respect to some randomly oriented observer located outside the solar system.

Figure 1 shows the probabilities of observing at least one transit in some system of planets, as well as the observed transit depths. In this case, the transit depth refers to the fractional dip in the light curve of a planet as it is transited by its moon. Supposing that we observed 10 planets with moons, the probability of observing a transit of an Io-like moon is about 85%. For a Europa-like moon, the transit probability drops to 70%, while transits of Mercury-like and Jupiter-like moons are much rarer, at 10% and less than 1% probability respectively.

two side by side plots show transit probability vs. number of objects and transit depth vs. orbital period

Figure 1: Left: Probability of observing at least one transit as a function of the number of objects (exoplanets) in the sample. The curves represent different solar system objects. Right: Percentage transit depth as a function of orbital period for various planets and moons in our solar system. [Limbach et al. 2021]

Figure 2 shows the orbital periods and durations of possible transits as functions of orbital radius. In general, the closer the exomoon is to the exoplanet, the more likely it is that we’ll observe an exomoon transit. Based on Figure 2, the authors suggest that most exomoons will have a transit probability of 10 to 20%, and that they will likely be detected within a minimum of 50 hours of observation.

two side by side plots show orbital period and transit duration vs. semimajor axis

Figure 2: Left: Orbital period of an exomoon versus its orbital distance (in both astronomical units and kilometres). The curves denote super-Jupiter mass IPMOs with transit probabilities listed. The Roche limit is the closest the moon can orbit the planet. Right: Similar to the left panel, but with transit duration on the vertical axis. [Limbach et al. 2021]

Roguelike Exomoons

The main advantage of looking for moons around IPMOs is that high-contrast and high-resolution imaging is not needed; there is no interference from a nearby host star to drown out any signal. Instead, current ground telescopes can observe IPMOs with very high precision. Next-generation space-based telescopes, such as the imminent JWST, will further improve this precision by an order of magnitude.

Figure 3 shows the radius of moons orbiting Jupiter-mass exoplanets versus the photometric precision required to accurately detect their transit. Current high-contrast imaging of stellar-bound exoplanets can only detect moons at least as big as Neptune. With IPMOs, ground-based imaging allows for the detection of Earth-sized moons. With the JWST, it will be possible to detect Ganymede-sized moons, and even moons down to the size of Titan or Io! The authors calculate that the JWST will be able to detect Ganymede-sized moons around over half of all known IPMOs.

plot of moon radius vs. photometric precision for a 5 sigma detection

Figure 3: Exomoon radius (in units of Earth radii) versus the photometric precision required for an accurate transit detection (logarithmic scale). Smaller precision values correspond to higher resolutions. The solid and dotted curves denote Jupiter mass and 1.5 x Jupiter mass IMPOs, with the shapes corresponding to the sizes of orbiting moons. Vertical lines denote the sensitivity limits of current/future observations. [Adapted from Limbach et al. 2021]

Sic Mundus Creatus Est

Exomoons are believed to be just as common as exoplanets, with at least one large moon forming around approximately 80% of simulated planets. Rogue exoplanets can be observed with a higher photometric precision since there is no glare/interference from a nearby host star. Moon–planet transits are also relatively common, and more than half of all known IPMOs are bright enough to allow for the detection of a Ganymede-sized moon. However, the authors propose a minimum observing time of 50 hours, which adds up to nearly 17 weeks of combined observing time for the 57 known IPMOs. The authors note that IPMO exomoons are among the most accessible candidates for detecting habitable worlds (in this case, the moon is warmed by tidal heating rather than by a star). Potentially habitable IPMO exomoons would not only lend insight into the processes governing planet formation, but could also offer a window into the conditions of the primordial Earth, thus allowing us to place a timescale on the development of life itself.

Original astrobite edited by Ryan Golant.

About the author, Mitchell Kavanagh:

Mitchell is a PhD student in astrophysics at the University of Western Australia. His research is focused on the applications of machine learning to the study of galaxy formation and evolution. Outside of research, he is an avid bookworm and enjoys gaming, languages and code jams.

pulsar

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

Title: The Location of Young Pulsar PSR J0837−2454: Galactic Halo or Local Supernova Remnant?
Authors: Nihan Pol et al.
First Author’s Institution: West Virginia University; Vanderbilt University
Status: Published in ApJ

In 1967, a graduate student named Jocelyn Bell Burnell discovered the first pulsar, opening a window into a universe teeming with many wild and wonderful varieties of these extreme objects. Pulsars are fast-spinning neutron stars with powerful magnetic fields, emitting beams of radio waves. They give rise to many interesting phenomena: some pulsars cannibalize companion stars, others emit pulses of high-energy gamma rays, and still others exhibit sudden changes in their rotation period.

Despite this diversity, pulsar astronomers have managed to establish some typical properties of pulsars. For example, pulsars — especially very young pulsars — are rarely found far from the plane of the Milky Way. The stars that form them usually live in the galactic disk, and many pulsars simply don’t have enough time to move away. Today’s paper, however, focuses on a pulsar that seems to be much farther from the galactic plane than we should expect. What could be going on?

The Paradoxical Pulsar

This particular pulsar, designated J0837–2454, was discovered in 2011 using the Parkes Radio Telescope. However, it took six more years to produce enough data to adequately constrain its properties. The authors found that the pulsar was young, estimating its age to be a mere 28,600 years, with a reasonable rotation period and magnetic field. What made it stand out like a sore thumb was its dispersion measure, or DM. The dispersion measure describes how many free electrons lie between us and the pulsar, so it acts as a sort of distance proxy: if you know the DM, the location of the pulsar in the sky, and how the galaxy’s electrons are distributed, you can calculate its distance from us.

The astronomers picked two widely used models of the Milky Way’s electron density, derived from, among other things, measurements of dispersion measures and distances to known pulsars. One, the NE2001 model, predicted that J0837–2454 should lie 6.3 kpc (a bit over 20,000 light-years) from Earth, placing it a whopping 1.1 kpc above the galactic plane — much higher than any pulsar of comparable age. The other, the YMW model, predicted that it should lie beyond the galactic disk. Both would be a surprising result, so the authors considered other means of estimating the distance to the pulsar.

plot of vertical distance from the galactic plane vs. planar distance from galactic center for known pulsars

Figure 1: Most pulsars (shown in red) lie close to the galactic plane, and previously known pulsars less than 50,000 years old (in green) tend to be even closer. According to a standard model of the Milky Way’s electron distribution, PSR J0837-2454 (marked with a blue asterisk) appears to lie at least twice as far away as any other known pulsar of comparable age. [Pol et al. 2021]

Getting a Second Opinion — and a Third

Many young pulsars are associated with supernova remnants (SNRs), so it made sense to see if there was one near J0837–2454. The group used the Australia Telescope Compact Array (ATCA) to search for radio emission from a remnant, supplementing their observations with archival images from the radio Galactic and Extra-galactic All-sky MWA Survey (GLEAM) and the Southern Hα Sky Survey (SHASSA).

The ATCA observations were unable to detect any SNR-like emission, but the GLEAM images showed a faint structure near the pulsar, which might represent an SNR. Assuming this is actually an SNR associated with the pulsar, this structure could provide an independent estimate of the distance to J0837-2454: models of SNR evolution could estimate its physical size and compare that to its angular size, thus yielding the distance to the source. Using this method, the authors found a distance of only 0.9 kpc from Earth, placing the structure — and the pulsar, if the structure is, indeed, an associated SNR — at a more reasonable offset from the plane.

image showing the location of the pulsar, remnant, and nearby second pulsar

Figure 2: Stacked images from the Galactic and Extra-galactic All-sky MWA Survey (GLEAM) show an excess of radio emission in a round region overlapping with PSR J0837–2454, marked by a red cross. This excess has been interpreted as a possible supernova remnant associated with the pulsar. A second nearby pulsar, PSR J0838–2621, is marked by a purple cross and is believed to be unassociated with the pulsar and the candidate supernova remnant. [Pol et al. 2021]

The SHASSA data also provided another way out of the quandary. The images showed a diffuse Hα structure near the position of the pulsar, distinct from the radio emission detected by GLEAM. The authors suggested that this emission was unrelated to the pulsar, but instead lay between it and Earth. This could be contributing to the surprisingly high dispersion measure. If this is true, the SHASSA results would place the pulsar as close as 0.2 kpc from us and close to the plane.

Tying Up Loose Ends

There are still several issues to consider. Assuming the distance derived from the NE2001 model is correct, how could the pulsar have traveled so far from the disk? To have traveled 1.1 kpc in 28,600 years would require the pulsar to be moving unreasonably fast. However, this rests on the assumption that the pulsar was born within the disk; it’s possible that it actually formed much closer to its present position. The massive progenitor could have been a runaway star, propelled away by a companion star going supernova. One way of confirming or refuting this scenario would be to measure the velocity of the pulsar out of the plane — not an easy task, but not impossible.

This still leaves an obvious problem: if NE2001 distance estimate is correct, why didn’t ATCA detect a supernova remnant? It’s possible that the structure detected by GLEAM is completely unrelated to the pulsar, and that the real SNR is simply too faint. If the circumstellar medium around the pulsar is too diffuse, or the magnetic field is too weak, the remnant would be unable to generate strong enough synchrotron emission at radio frequencies for it to be detectable by ATCA. A combination of the two seems to be a feasible explanation. As next-generation radio telescopes like the Square Kilometre Array come online, maybe we can find the final pieces of this pulsar puzzle.

Author’s note: While Nihan Pol, Sarah Burke-Spolaor and Harsha Blumer are or have been part of my department, I was not involved with this research.

Original astrobite edited by Sabina Sagynbayeva.

About the author, Graham Doskoch:

I’m a first-year graduate student at West Virginia University, pursuing a PhD in radio astronomy. My focus is on neutron stars and pulsar timing, a method of detecting gravitational waves by monitoring arrays of pulsars over the course of many years. I’m an associate member of NANOGrav, and I’m starting to help with their ongoing timing efforts. I love running, hiking, reading, and just enjoying nature.

Aerial image of a large rectangular structure surrounded by a circle in the landscape. Circular tanks are visible to the right of the structure.

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: Discovery of the Ultra-High Energy Gamma-Ray Source LHAASO J2108+5157
Authors: Zhen Cao, F. A. Aharonian, Q. An, et al. (the LHAASO collaboration)
First Author’s Institution: Institute of High Energy Physics, Chinese Academy of Sciences, China
Status: Published in ApJL

The Hunt for PeVatrons

composite image of a complex-structure spherical bubble of emitting gas of different colors

Figure 1: An image of the supernova remnant Cassiopeia A from NASA’s Chandra X-ray Observatory.  [NASA/CXC/SAO]

What are our galaxy’s most energetic and extreme particle accelerators? Could it be supernova remnants, the remains of the violent death of a massive star? Or might it be the nebulae that form around pulsars, rapidly rotating and highly magnetized neutron stars? Perhaps surprisingly, this is still an open question in high-energy astrophysics and the list of possible suspects is much larger than just those mentioned above.

However, we know that these particle accelerators exist. We know this because we are constantly being bombarded by high-energy cosmic rays — protons or heavier atomic nuclei that are traveling at nearly the speed of light. Now, over 100 years since Austrian physicist Victor Hess discovered cosmic rays by taking an electroscope on a hot air balloon flight, we still don’t know what astrophysical objects produce the highest energy cosmic rays. Some of these cosmic rays have an energy over 1 Petaelectronvolt (PeV = 1015 electronvolts), comparable to that of a mosquito’s kinetic energy, but packed into a single proton.

The cosmic objects responsible for producing PeV cosmic rays are so called “PeVatrons,” and pinpointing these sources is no small feat. However, as these cosmic rays are so energetic, they are akin to a bull in a china shop, and they frequently collide with gas or material that surround the PeVatrons. These interactions produce gamma rays that carry about 10% of the energy of the initial cosmic ray. Thus, finding gamma rays with energy of about one tenth of a PeV (100 TeV) is a smoking gun for a PeVatron.

Photo of a series of circular tanks located in the ground on the side of a large rectangular structure.

Figure 2: Another aerial view of the Large High Altitude Air Shower Observatory (LHAASO). The cylindrical tanks help detect muons, which can be used to distinguish gamma rays from unrelated cosmic rays. [Liu Kun/Xinhua/Alamy]

This is exactly what the authors of today’s paper set out to accomplish. The paper discusses using a detector that is still being constructed in Sichuan, China, the Large High Altitude Air Shower Observatory (LHAASO). LHAASO consists of many small detectors spread out over an area greater than a square kilometer. When a high-energy gamma ray from a PeVatron enters the atmosphere, it can collide with a particle in the atmosphere and create a very large shower of subatomic particles. The array of detectors on the ground can detect these particles and reconstruct the shower to infer properties about the initial gamma ray. A picture of part of LHAASO is shown in Figure 2.

Although LHAASO is not even done with construction, its immense size already makes it an extremely efficient gamma-ray detector. To date, LHAASO has detected 12 sources of these ultra-high-energy (UHE) gamma rays, and has even detected the highest energy photon from an astrophysical source yet found. While all of these sources provide a wealth of information on galactic PeVatrons, one of the objects — LHAASO J2108+5157, the focus of today’s paper — is particularly interesting because of its mysterious origins.

Figure 3 shows the detection of this object at energies between 25 TeV and 100 TeV, as well as for higher energies. The source is significantly detected in both of these energy ranges. However, what is strange is that while all of the other 12 sources have been detected by other telescopes in the “very-high-energy” band (around 1 TeV or so, which is high energy but still below the energy detection limit of LHAASO), this is the only object that is significantly detected at higher energies but does not have a counterpart at 1 TeV.

side by side 2D heatmaps showing bright sources in the middle of each

Figure 3: Significance map in the region around the newly detected source, LHAASO J2108+5157. The left shows gamma rays with energies between 25 TeV and 100 TeV and the right shows gamma rays above 100 TeV. The cross denotes the best-fit location of the source and the red circles show an approximate uncertainty on this location. The white circles show the average directional uncertainty for individual gamma-ray events. [Cao et al. 2021]

The authors spend some time in the paper exploring what could be causing this UHE gamma-ray signal by looking for counterparts in other wavelengths, even if there aren’t counterparts detected in the 1 TeV gamma-ray band. While many of the other 12 sources were coincident with supernova remnants (SNRs) or pulsar wind nebulae (PWNe), no objects like this lie anywhere close to LHAASO J2108+5157.

However, the authors note that this object is spatially coincident with a molecular cloud. Distinguishing between the molecular cloud scenario and the SNR or PWN scenario is crucial, as the presence of a molecular cloud could indicate this source is accelerating protons to these high energies. In the case of SNRs or PWNe, it is only believed that these objects might accelerate electrons (not protons) to high energies. The distinction between these two acceleration scenarios has dramatic implications for the spectra of cosmic rays. Figure 4 explores these two options by modeling what proton acceleration might predict versus electron acceleration. Although both models are consistent with the data, the authors believe the presence of a catalogued molecular cloud is promising for the proton scenario. Hopefully, future observations will be able to distinguish between these two cases.

SED over many decades in energy. Data points are consistent with lines that show various model predictions

Figure 4: LHAASO data (red) compared to various models that would predict gamma-ray emission. The solid red line shows a model based on the acceleration of protons (hadronic) and the dashed green is based on photons scattering off of accelerated electrons (leptonic, labeled IC CMB in the plot). Although the data are consistent with both, distinguishing between these models is crucial for finding the sources of the highest energy cosmic rays. [Cao et al. 2021]

With the first dozen detected sources at these ultra-high energies, there is already a vast amount of insight that has been gained on the nature of PeVatrons. However, as LHAASO is still being constructed and will only improve and continue to take more data, the future of ultra-high energy gamma-ray astronomy will undoubtedly be bright.

Original astrobite edited by Katy Proctor.

About the author, Alex Pizzuto:

Alex is a PhD candidate at the Wisconsin IceCube Particle Astrophysics Center at the University of Wisconsin-Madison. His work focuses on developing methods to locate the Universe’s most extreme cosmic accelerators by searching for the neutrinos that come from them. Alex is also passionate about local science outreach events in Madison, and enjoys hiking, cooking, and playing music when he is not debugging his code.

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