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spiral galaxy with winds emanating from the center in a broad cone

From the boundary of our solar system to the surroundings of exploding stars, astrophysical shocks can accelerate particles to relativistic speeds and produce high-energy radiation. Now, astronomers may have detected a new source of gamma rays from the shock that forms where ultrafast outflows and interstellar gas meet.

Outer-Space Outflows

Centaurus A

This composite image reveals Centaurus A, a galaxy with an active nucleus spewing fast-moving jets into its surroundings. Active galactic nuclei like this one produce extremely high-energy photons. [ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray); CC BY 4.0]

When supermassive black holes accrete material, they can generate powerful outflows in the form of narrow relativistic jets or wide-angle winds called ultrafast outflows (UFOs). UFOs can reach 30% of the speed of light and disrupt the outer reaches of a galaxy, sending gas spraying into intergalactic space and influencing the galaxy’s long-term evolution.

If UFOs billowing out into interstellar space create shocks where particles can be accelerated, they should generate gamma-ray luminosities of 1033 Joules per second, but so far astronomers have only detected them at X-ray wavelengths. The vast distances between galaxies are working against us: from our earthly vantage point, a collecting area of roughly 8,400 square meters would capture just one photon with an energy greater than a gigaelectronvolt per hour from a typical UFO.

stacked profile for the test sample

Test-statistic (TS) values derived for the stacked Fermi LAT observations. Large TS values indicate that the observations are likely, given the model. [Fermi LAT Collaboration 2021]

Stacking Gamma-Ray Snapshots

To detect these elusive UFOs at gamma-ray wavelengths, an international group of scientists used data from the Fermi Large Area Telescope (LAT), an instrument on the Fermi Gamma Ray Space Telescope spacecraft. A single UFO is about 2.5 times fainter than the dimmest source detected by Fermi LAT, but the team hoped to draw out a signal by combining observations of multiple galaxies for which UFOs have been detected at X-ray wavelengths.

The team approached this problem by modeling the combined gamma-ray spectrum. For each set of model parameters, they calculated a test statistic — a quantity that describes how likely it is to have obtained the data if the emission from the source follows the model they tested. The larger the test statistic, the more likely it is that the signal is well-described by the model. The combined observations yielded a maximum test statistic of 30.1 — equivalent to a 5.1-sigma detection.

Double Checking

gamma-ray map of the Milky Way plane with a large bubble above and below the plane

Fermi observations revealed the presence of two bubbles extending 25,000 light-years above and below the plane of the Milky Way. [NASA/DOE/Fermi LAT/D. Finkbeiner et al.]

Before declaring success, the authors also analyzed a control sample to make sure that the emission they attributed to UFOs didn’t come from a different source. The team stacked observations of 20 galaxies with distances and X-ray luminosities similar to the UFO sample — but without previously detected black-hole winds — and searched for a signal within this stack. This analysis returned a maximum test statistic of 1.1, confirming that the signal is due to outflows.

The detection of gamma-ray emission from UFOs supports the theory that these massive outflows produce shocks that are sites of particle acceleration. Although today’s article explored distant galaxies, UFOs might have had an impact closer to home as well; the Fermi LAT team suggests that past outflows from the Milky Way’s supermassive black hole may have created the enormous bubbles extending above and below our galaxy seen previously by Fermi and the eROSITA instrument.

Citation

“Gamma Rays from Fast Black-Hole Winds,” M. Ajello et al 2021 ApJ 921 144. doi:10.3847/1538-4357/ac1bb2

the hazy crescent of Titan

Astronomers have detected dozens of molecules in the hazy atmosphere of Saturn’s moon Titan, but they’re still figuring out how these molecules formed. What can radio observations tell us about where Titan’s molecules came from?

A Trio of Molecular Forms

five spheres representing different atoms squished together in a line

A model of a cyanoacetylene molecule. [Ben Mills and Wikipedia user Jynto]

Today’s article focuses on a little molecule with a big name: cyanoacetylene. These molecules consist of three carbon atoms sandwiched between a hydrogen atom and a nitrogen atom, all arrayed in a line. Cyanoacetylene is chemically interesting because any one of its three carbon atoms can be replaced with carbon-13 — a stable isotope that has an extra neutron.

Swapping a carbon-12 atom (the most common isotope of carbon) for a more massive carbon-13 atom has a subtle but measurable effect on the molecule’s spectrum. Spectra of isotopologues — molecules containing one or more isotopes — can be used to calculate the abundance of each form of the molecule and understand how that molecule forms. For example, if a molecule containing a carbon-13 atom reacts to create a larger molecule, it passes the atom to the newly formed molecule. The location of the carbon-13 atom within the new molecule gives us a hint as to what smaller molecules it was made from, so measuring which form of cyanoacetylene is most common can tell us how it tends to be made.

spectra of two isotopologues (intensity vs frequency offset)

Spectral signatures of two forms of cyanoacetylene. [Iino et al. 2021]

Molecules Detected in Millimeter Waves

Cyanoacetylene was discovered in Titan’s atmosphere during the Voyager 1 flyby in 1979, but its isotopologues were detected decades later. Luckily, we now have many observations of these molecules since the Atacama Large Millimeter/submillimeter Array (ALMA) frequently uses Titan as a calibration target.

Takahiro Iino (University of Tokyo, Japan) and coauthors used these archival ALMA observations to measure the abundances of three isotopologues of cyanoacetylene. Iino and collaborators sifted through the observations to find those that were suitable for their needs: high-resolution observations in a frequency range that contains spectral lines from multiple isotopologues. By measuring the intensity of each spectral line, the authors were able to determine the abundances of the molecules.

Cyanoacetylene Across the Universe

ALMA data of one large starless core and one smaller, fragmented core

Starless cores, like the two shown here in ALMA observations, are also sites of molecular formation. [Bill Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO)]

The authors found that the three forms of cyanoacetylene are roughly equally abundant, which is strikingly different from what has been observed in some other astrophysical settings. Previous studies of cyanoacetylene molecules around young stars found ratios similar to those seen at Titan, but the molecule’s three isotopologues were found to be out of balance in the cold, starless cores of molecular clouds.

Even studies of cyanoacetylene in similar environments — such as comparisons between two or more molecular clouds — have returned conflicting results, suggesting that cyanoacetylene might form in several different ways. While many of the pathways proposed for cold environments in starless cores involved reactions between neutral molecules, formation in Titan’s atmosphere — relatively warm at 100–200K — might involve molecular ions as well.

Citation

“13C Isotopic Ratios of HC3N on Titan Measured with ALMA,” Takahiro Iino et al 2021 Planet. Sci. J. 2 166. doi:10.3847/PSJ/ac134c

visualization of a black hole ripping apart a neutron star

Heavy metals are thought to form in space in only a few specific ways. Which scenario makes more of these elements: two neutron stars colliding or a neutron star merging with a black hole?

A Universal Supply of Metals

two jets of debris extend out from the site of a collision of two neutron stars. rings of gravitational waves emanate from the collision

This digital illustration depicts the aftermath of a collision between two neutron stars. [NASA’s Goddard Space Flight Center/CI Lab]

Here’s a fun fact for you: nearly all the gold on Earth — in wedding rings, tooth fillings, and Olympic medals — likely formed in either a supernova explosion or a collision between compact objects like neutron stars. Gold is one of many r-process elements, which form when atomic nuclei capture multiple neutrons in rapid succession, bulking up into heavier elements and isotopes. Roughly half of the elements heavier than iron are considered r-process elements, some of which — like gold — form almost exclusively through this process.

The possible origins of r-process elements are limited, since they form in hot, extremely dense environments. Core-collapse supernovae and collapsars are contenders, but lately astronomers have considered collisions between neutron stars or neutron star–black hole pairs as potential sources of heavy elements. But which of these two types of collisions makes the most heavy metals?

neutron star equation of state curves

The authors used multiple different neutron star equations of state in their models. Of the nine models used, wwf2 is the most compact or “stiffest.” These equations of state are based on observations of pulsars. [Chen, Vitale & Foucart 2021]

Computing Collisions

A team led by Hsin-Yu Chen (Massachusetts Institute of Technology) addressed this open question by estimating how much each type of merger contributes to the universal supply of r-process elements. In order to make this estimate, the team drew upon gravitational-wave observations and existing models to calculate how much metal a single merger creates and how frequently these mergers occur.

Chen and coauthors explored six different models with different spin and mass distributions for the black holes and neutron stars, as well as various neutron star equations of state — a description of how loose or compacted the neutron star is — all of which affect how much r-process material forms in the collision. For each set of model parameters, they simulated the collisions of 100,000 neutron star and neutron star–black hole pairs.

An Array of Possibilities

fraction of mass ejected in a collision between two neutron stars compared to all collisions

The neutron star equation of state affects the amount of material ejected during the collision. “Stiffer” equations of state (labeled ap4 and wwf2) result in the most material ejected. [Chen, Vitale & Foucart 2021]

The authors found that the amount of r-process elements contributed by neutron star–black hole mergers ranged from 1% to 77% of the contribution from colliding neutron-star binaries, depending on the model used. However, neutron star–black hole mergers only outperformed neutron star–neutron star collisions in one of the models the authors tested. In this case, the black holes had low masses (2–3 times the mass of the Sun) and rapid spins, a combination that current gravitational-wave observations suggest is rare.

Astronomers should be able to refine these estimates in the future. Pulsar observations should help us gain a better understanding of the compactness of neutron stars, and as we detect more gravitational waves from collisions between compact objects, we can make better estimates of the frequency of these collisions. Chen and coauthors also point out that our current gravitational-wave observations mainly probe the past 2.5 billion years, but future detectors will allow us to peer back into the distant past to understand how metals formed long ago.

Citation

“The Relative Contribution to Heavy Metals Production from Binary Neutron Star Mergers and Neutron Star–Black Hole Mergers,” Hsin-Yu Chen, Salvatore Vitale, and Francois Foucart 2021 ApJL 920 L3. doi:10.3847/2041-8213/ac26c6

More than 10 radio telescopes point toward the sky in front of the Milky Way

Image reveals a tilted oval structure of an orange disk containing a number of concentric gaps and rings.

This ALMA image of the protoplanetary disk surrounding the star HL Tauri reveals the detailed substructure of the disk, including gaps that may have been cleared by planets. [ALMA (ESO/NAOJ/NRAO); C. Brogan, B. Saxton (NRAO/AUI/NSF); CC BY 4.0]

The Atacama Large Millimeter/submillimeter Array (ALMA) has provided a decade’s worth of detailed views of cold gas and dust throughout the universe. Its images of protoplanetary disks are particularly noteworthy: the rings and gaps present in these dusty nebular cradles are tantalizing reminders that new worlds are forming all around us — and ALMA gives us a front-row seat.

Now, a team of astronomers has carried out a new observing program to understand the physical and chemical conditions in planet-forming disks around young, nearby stars: Molecules with ALMA at Planet-forming Scales (MAPS). The MAPS program consists of high-resolution observations of protoplanetary disks around five stars — IM Lupi, GM Aurigae, AS 209, HD 163296, and MWC 480 — in which planet formation is thought to be happening right now.

This sample of protoplanetary disks and their host stars spans different spectral types and contains a variety of intriguing structures seen in both gas and dust emission. With ALMA, we can see small-scale (7-30 au) details stretching from less than 10 au from the star to the outer reaches of the disk at 1,000 au. In total, the team measured roughly 40 spectral lines arising from the presence of 20 chemical species, giving an unprecedented view of the chemistry of these disks. A new special issue of the Astrophysical Journal Supplement Series presents the first results in a collection of 20 articles.

Some of the key findings described in these articles include:

  • Radial and vertical distribution of line emissions
    six images of a protoplanetary disk in different emission lines. the apparent size and structure of the disk changes between the images

    A small subset of the observations of the disk around HD 163296, showcasing a variety of chemical structures. [Adapted from Öberg et al. 2021]

    The MAPS program revealed that emissions from different chemical species — and even different emission lines of a single chemical species — can arise from a variety of locations within a disk. These chemical structures also vary widely between the five disks in the MAPS sample. Because the MAPS observations probe the vertical direction (above and below the midplane of the disk) as well as the radial direction, the team was able to determine whether the organic molecules detected are present in the midplane of the disk, where planets form.
  • Molecular abundances
    ALMA observations of molecular line emissions allow for calculations of elemental and molecular abundances, which increase our understanding of the gaseous material from which planets form. An especially enticing result of the MAPS program is the discovery that these protoplanetary disks contain reservoirs of nitriles, a class of organic compound that may have played a key role in the development of life on Earth. Nitriles and other small organic molecules were found to be distributed unevenly throughout the disks, suggesting that the location at which a planet forms has profound implications for its chemical makeup and potential for development of life.
  • Disk structure and properties of the gas
    Five representative-color images of disks, each with several rings and gaps labeled with white lines

    MAPS revealed rings and gaps (labeled) in all five disks surveyed. [Adapted from Law et al. 2021a]

    Previous studies with ALMA have revealed the presence of structures like rings, gaps, and spirals in protoplanetary disks. MAPS detected roughly 200 of these features, with some amount of structure visible in every molecule surveyed. The structures visible in the molecular-line emission often coincide with structures seen in the dust continuum emission, but not always, suggesting that a variety of physical processes are responsible for the presence of disk substructure seen in both gas and dust.

The MAPS observations provide among the most detailed views to date of the planet-forming environment around young stars. Further analysis of these observations promises to illuminate the interplay between gas and dust and advance our understanding of how the chemistry of protoplanetary disks influences planet formation.

Citation

Special ApJS Issue on the MAPS program

“Molecules with ALMA at Planet-forming Scales (MAPS) I: Program Overview and Highlights,” Karin Öberg et al 2021 ApJS 257 1. doi:10.3847/1538-4365/ac1432

Small beads of sunlight during a total solar eclipse

How big is the Sun when seen from Earth? Though it may seem like a simple question, it’s important for predicting when exactly a total solar eclipse will occur. Using data from the 2017 total solar eclipse, a group of amateur astronomers have recalculated a measurement that hasn’t been updated in more than a century.

Timing Totality 

Solar eclipses provide a unique opportunity to study the Sun’s outer layers (which are usually hidden), the structure of the Moon, and even general relativity. Determining the precise moment a solar eclipse will happen is crucial to these scientific endeavors, and the most accurate way to predict when totality will occur is by determining the exact radius of the Sun so we know when it will be fully covered by the Moon.  

Path of the 2017 total solar eclipse

The path of the total solar eclipse just south of Vale, Oregon. Click to enlarge. [Quaglia et al. 2021]

In the late 1800s, a scientist named Auwers published a measurement of the apparent radius of the Sun from Earth’s distance of 1 au. Despite improvements in the accuracy of values needed for the calculation, the value has remained unchanged, and is still used for all published eclipse predictions today. Led by Luca Quaglia, a team of amateur astronomers from Australia, the UK, Greece, and Italy assembled in Oregon during the 2017 solar eclipse and obtained an updated eclipse solar radius by capturing a video of a solar eclipse and analyzing both its spectrum and its light curve.

Elements in the flash spectrum, labeled individually and by which layer they belong to

The spectrum of elements in the different layers of the Sun during the flash spectrum. Panel (a) shows the different elements and (b) shows which layer the elements belong to. Click to enlarge. [Adapted from Quaglia et al. 2021]

Eclipsed Elements 

Though the Sun is mostly made of hydrogen, traces of other elements are present. When the overwhelming glare of the Sun’s disk is blocked out during an eclipse, emission lines from these elements become visible, arising from the Sun’s fainter outer layers. Different layers of the Sun have different abundances of these elements, so a spectrum taken during a total solar eclipse changes as the photosphere, chromosphere, and inner corona come into view.

From their vantage point at the edge of the eclipse’s path, Quaglia and collaborators captured a video of the eclipse and extracted a spectrum from the moments seconds before and after totality when the chromosphere briefly becomes visible. From the video, they detect when the photosphere spectrum appears and disappears and use that to estimate how long the Sun is in the Moon’s shadow, which helps estimate the radius.  

Lunarious Light Curves 

After observing the spectrum, the team employed another method to calculate the eclipse solar radius: using the light curves obtained during the solar eclipse and comparing them to models. Precise eclipse calculations depend not only on the Sun’s radius but also on getting the positions of the Earth, Moon, and center of the Sun using the latest ephemeris and modeling the surface of the Moon correctly. 

Model of the moon's surface (which has a lot of peaks and valleys, and is magnified to show imperfectios) against the data (which is smooth)

The average lunar surface (green) versus the model (black). The discrepancies on the surface are magnified by 100x so the peaks and valleys can be highlighted. [Quaglia et al. 2021]

The Moon isn’t totally smooth, so the sunlight needs to go through the mountains and valleys of the Moon, which are what cause the chromosphere to briefly shine through just before and after totality. To accurately model the surface, they used data from the Lunar Orbiter Laser Altimeter, which measures lunar topography and elevations, and were able to model the exact moment the chromosphere would appear.   

The team was able to estimate the eclipse solar radius value to within a tenth of an arcsecond, showing that it’s slightly larger than the standard solar radius value used for the last 100 years. During the next total solar eclipse in 2023 in Western Australia, the authors hope to record a higher-resolution flash spectrum video of the Sun to better constrain their estimate. 

Bonus 

Check out the video below that shows the momentary “flash spectrum” captured by Quaglia and collaborators when the chromosphere was briefly visible during the August 2017 eclipse. 

Citation

“Estimation of the Eclipse Solar Radius by Flash Spectrum Video Analysis,” Luca Quaglia et al 2021 ApJS 256 2. doi:10.3847/1538-4365/ac1279

A rounded volcanic peak rises in the center of the image. The warmest part of the heat pattern overlay is found near the peak and on one side of the volcano

After decades of studying Venus, many questions remain about our planetary next-door neighbor. One question has particularly intrigued astronomers: which, if any, of Venus’s 1,600 volcanoes are still active?

Two hot springs melt the surrounding snow in Yellowstone National Park

Today’s article focuses on Imdr Regio, which is thought to be analogous to hotspots on Earth, where heat rises from below the surface and can cause volcanism as well as thermal features like the hot springs shown here. [National Parks Service/Damon Joyce]

Ancient History or Today’s News?

Venus’s surface is dotted with volcanoes and puddled with lava flows, but it’s challenging to discern which of these features are ancient and which are more recent. Using data from Venus-orbiting spacecraft, we can search for the subtle changes to Venus’s surface and atmosphere that signal the presence of erupting volcanoes.

Aside from the excitement of adding another item to the list of known active volcanoes in the solar system, figuring out which of Venus’s volcanoes are active is important because volcanoes are a potential source of phosphine, a compound that arises from biological processes on Earth and is thought to be an important biosignature on Venus. In order to interpret detections of phosphine, we need to know how many volcanoes are actively producing it.

Two aerial views of Idunn Mons, showing an area roughly 80 km across. The area near the top of the volcano is radar bright, as well as several cracks or stripes arrayed around the volcano.

Two radar views of Idunn Mons from the Magellan spacecraft. [Adapted from D’Incecco et al. 2021]

Evidence from Multiple Avenues

A team led by Piero D’Incecco (D’Annunzio University of Chieti–Pescara, Italy) rounded up spacecraft observations of Venus’s surface and atmosphere and combined them with findings from laboratory studies to build a compelling case for the active volcanism of Idunn Mons, a 2.5-km high, 200-km wide volcano in Imdr Regio. The team bolstered their case with three key pieces of evidence:

  1. Surface observations: The region surrounding Idunn Mons shows signs of overlapping lava flows, the uppermost of which is coincident with a region of unusually high thermal emission, which is thought to indicate a surface that hasn’t yet been corroded by Venus’s caustic atmosphere.
  2. Laboratory work: Recent laboratory studies, which recreate the hot, high-pressure environment of Venus’s surface to understand how it affects different minerals, have shown that chemical weathering — alteration of the surface material through chemical reactions with atmospheric gases — happens more quickly than previously thought. This means we’ve overestimated the ages of the lava flows surrounding Idunn Mons.
  3. Atmospheric observations: Venus’s surface also interacts with the atmosphere on a macroscopic scale; landforms like volcanoes generate standing waves in the atmosphere called gravity waves (not to be confused with gravitational waves!). These waves can cause Venus’s winds to slow down as they travel above a volcano. In the case of Idunn Mons, the winds slow down more than expected given the size of the volcano, which could be due to heat radiating from recent lava flows.
An artist's impression of the EnVision spacecraft in front of an image of the Earth and an image of Venus that is half clouds and half radar data

The European Space Agency’s upcoming EnVision mission, which is planned to launch in 2031, seeks to understand why Earth and Venus are so different. [NASA / JAXA / ISAS / DARTS / Damia Bouic / VR2Planets]

Venusian Explorations Ahead

Combining all the available evidence, D’Incecco and collaborators conclude that Idunn Mons has been active recently, perhaps within our lifetimes — anywhere from 10,000 years ago to just a few years ago. Recently selected spacecraft missions should soon allow us to study Idunn Mons further. In particular, NASA’s Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy (VERITAS) orbiter and the European Space Agency’s EnVision orbiter both plan to map Venus’s surface in extremely high resolution, which is key for detecting surface changes due to volcanic activity.

Citation

“Idunn Mons: Evidence for Ongoing Volcano-Tectonic Activity and Atmospheric Implications,” P. D’Incecco et al 2021 Planet. Sci. J. 2 215. doi:10.3847/PSJ/ac2258

A large planet silhouetted against a small red star and a larger yellower star with starspots

Astronomers and science-fiction fans alike were delighted to discover that planets can exist in stable orbits around binary stars. Today’s article looks at circumbinary planets from a new angle — a 90-degree angle, to be exact.

Rings of emission that are misaligned with the binary orbit (left) and aligned with the binary orbit (right)

Most circumbinary disks (orange) are aligned with the orbital plane of the binary (e.g., AK Sco, right), but we’ve also found disks oriented perpendicular to the orbital plane (e.g., HD 98800 B, left). Here orbits of the binary system are drawn in white for clarity. Click to enlarge. [ALMA (ESO/NAOJ/NRAO), I. Czekala and G. Kennedy; NRAO/AUI/NSF, S. Dagnello; CC BY 4.0]

Planets at an Extreme Angle

The vast majority of disks observed around binary stars are aligned with the orbital plane of the binary, since angular momentum tends to be conserved as gas clouds collapse to form star systems and their surrounding protoplanetary disks. However, recent observations have found that binary star systems with elongated, eccentric orbits can maintain a protoplanetary disk perpendicular to the orbital plane of the binary system.

These observations suggest that perpendicular circumbinary planets exist, but astronomers have yet to detect any. What can simulations tell us about the likelihood of perpendicular planets forming, especially rocky, Earth-like planets close to their parent stars?

Simulating Systems in Formation

To explore the planet-forming potential of perpendicular disks relative to other configurations, Anna Childs and Rebecca Martin (University of Nevada, Las Vegas) simulated three scenarios involving circumbinary disks:

  1. Simulated disk material distributions for each of the three scenarios. The eccentric perpendicular case has the most material close to the stars while the eccentric coplanar case has the least.

    Distribution of disk material as a function of distance from the system center of mass for the eccentric perpendicular (EP), circular coplanar (CC), and eccentric coplanar (EC) scenarios. When the disk is oriented perpendicular to the orbital plane of the binary, more material is found closer to the binary. [Childs & Martin 2021]

    In the same plane as a binary pair with a circular orbit (eccentricity e=0)
  2. In the same plane as a binary pair with an eccentric orbit (e=0.8)
  3. Perpendicular to the plane of a binary pair with an eccentric orbit (e=0.8)

Childs and Martin first performed hydrodynamic simulations to determine how the disk material would be distributed in the early stages of planet formation for each of these three cases. The authors then used these results to guide the placement of 26 Mars-sized and 260 Moon-sized objects within the simulated disk — the mass and size distribution thought necessary to form the terrestrial planets in our solar system. In the final N-body simulation, the authors let these planetesimals loose and watched their planet-forming journey.

Simulation of particle orbits. After 10,000 years, there are many small bodies, while after 7 million years there are fewer bodies that are larger.

Simulated particle orbits for the case of a planet-forming disk perpendicular to the orbit of an eccentric binary after 10,000 years (top) and seven million years (bottom). [Adapted from Childs & Martin 2021]

More Likely Than You Might Think

Over the course of seven million simulated years, material in the fledgling planetary systems collided with the host stars, clumped together to form larger bodies, or was ejected from the systems altogether. The result? Disks oriented perpendicular to the orbital plane of an eccentric binary formed more planets than disks in the same plane as the binary orbit.

On average, the perpendicular disks formed 4.8 terrestrial planets while the coplanar disks formed only 3.4. The difference likely arises from the increased torque that the binary applies to the disk in the coplanar case — more torque means more material gets ejected from the disk. Neither scenario formed as many terrestrial planets as the circular coplanar case, but it’s clear that close-in, rocky planets can exist in orbits perpendicular to their host binary system. Even taking into account the complicating effects of general relativity, which cause eccentric binary systems to precess over time, the perpendicular system retained its terrestrial planets.

Childs and Martin note that detecting planets in this configuration with current techniques is difficult, but not impossible — hopefully the discovery is just around the corner!

Citation

“Formation of Polar Terrestrial Circumbinary Planets,” Anna C. Childs and Rebecca G. Martin 2021 ApJL 920 L8. doi:10.3847/2041-8213/ac2957

A globular cluster, densest at the center, with stars of several different colors

Globular clusters are among the oldest structures in the universe, but astronomers are just starting to figure them out. A new study uses precise measurements of chemical elements to explore the formation history of one of the oldest globular clusters in the Milky Way.

New Understanding of Old Objects

Close-up of the stars at the center of a globular cluster

It can be challenging to study the stars in the densely packed centers of globular clusters. [NASA, Hubble Space Telescope, ESA]

Early theories suggested that all the stars in a globular cluster form at the same time from the same material, resulting in a single stellar population that is chemically identical. However, more recent work has found that some stars in globular clusters show signs of chemical evolution, hinting that they formed from gas enriched by previous generations of stars.

In order to learn more, we need to measure the chemical abundances of hundreds of cluster stars. Performing this chemical tagging is challenging — detecting subtle enhancements in elemental abundances requires precise measurements, and the usual methods have serious drawbacks. Spectroscopy is a powerful tool for determining chemical abundances, but spectra of stars so densely packed are often contaminated. Broadband photometry, on the other hand, doesn’t suffer from contamination but isn’t sensitive enough.

Top panel: Transmission curves for three different filters as a function of wavelength. Bottom panel: a stellar spectrum with calcium and CH features. The calcium filter encompasses the calcium features while excluding the CH feature.

The new filters, some of which are shown in the blue and red lines in the top panel, allow for better isolation of important spectral features. [Lee 2017]

Hundreds of Stellar Measurements

Jae-Woo Lee (Sejong University, South Korea) approached this problem by developing a new photometric system to measure chemical abundances in closely spaced stars. Lee’s system of narrow-band filters is designed to capture the absorption bands of the molecules CN, CH, and NH, which are important for determining the abundances of carbon and nitrogen.

Lee applied this photometric system to 842 red giant stars in Messier 5, a globular cluster in the Milky Way that is estimated to be more than 12 billion years old. Previous studies of this cluster suggested that it is home to two populations of stars, one of which is enhanced in nitrogen.

Application of the new photometric system revealed that Messier 5 is composed of not two but three stellar populations. The more precise photometry revealed that the nitrogen-enhanced population is really two separate populations that differ in both their level of carbon depletion as well as their distribution within the cluster. What does the presence of multiple stellar populations mean for the formation of Messier 5?

Three Stages of Formation

Plot of the cumulative radial distribution of the different populations. One population is more centrally concentrated than the other populations, which have indistinguishable distributions.

One of the nitrogen-enhanced populations (CN-sE) has a significantly different distribution within the cluster than the other nitrogen-enhanced population (CN-sI) and the nitrogen-normal population (CN-w). [Lee 2021]

This result suggests that the stars in Messier 5 formed in several stages rather than all at once. The author suggests that the nitrogen-normal population is likely made up of first-generation globular cluster stars, while the nitrogen-enhanced populations formed later.

Specifically, Lee argues that the centrally concentrated population likely formed in the dense inner regions of the cluster after the original highest-mass stars exploded as supernovae, while the more evenly distributed population formed somewhat later out of gas containing smaller amounts of recycled supernova material. This hypothesis explains the distributions of the two populations as well as their chemical abundances.

Hopefully, this method can be used to search for multiple stellar populations in many other globular clusters — clearly, these objects aren’t as simple as they seem!

Citation

“Formation of Multiple Populations of M5 (NGC 5904),” Jae-Woo Lee 2021 ApJL 918 L24. doi:10.3847/2041-8213/ac1ffe

False-color image of the Sun

A sunquake might sound like something that would be pitted against a tsunami in a B-level disaster movie, but these ripples on the Sun’s surface aren’t likely to affect us here on Earth. Instead, astronomers study sunquakes to understand their connection to another important solar phenomenon: solar flares.

Disturbances on the Solar Surface

Four panels showing circular ripples expanding on the Sun's surface

Sunquakes appear as expanding circular wavefronts in Doppler images. The event shown here was the first flare-generated sunquake ever detected. [ESA/NASA SOHO]

When the Sun unleashes a huge burst of electromagnetic radiation in the form of a solar flare, the flare sometimes generates a flurry of seismic waves in the Sun’s interior and on its surface, in the photosphere, called a sunquake. Though sunquakes are often described as “waves” or “ripples,” these terms belie the full force of these events: the first sunquake discovered to be connected to a solar flare was estimated to be equivalent to an 11.3-magnitude earthquake — roughly 50 times stronger than the strongest earthquake ever recorded.

Somehow, solar flares power these massive seismic events, but the exact mechanism isn’t known. So far, the majority of sunquakes have been observed in the photosphere, but recent observations suggest they also occur in the chromosphere, the region above the photosphere where the temperature begins to rise. One key to understanding these events is to pin down where they occur, leading Sean Quinn (Queen’s University Belfast, Ireland) and collaborators to seek them out.

Snapshots of the Sun

Quinn and coauthors began their search with a set of 62 sunquakes that had previously been discovered in pictures of the solar photosphere. Their goal was to find evidence of these same sunquakes in 160- and 170-nm ultraviolet images from the Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly, which probe higher temperatures — and therefore higher altitudes — than the photospheric images.

A grayscale image of the Sun with a small, curved wavefront

An example of a sunquake wavefront (indicated by the red arrow) in a 170-nm image from SDO. [Adapted from Quinn et al. 2021]

Using the locations of the 62 photospheric sunquakes to guide their search, Quinn and collaborators first examined the SDO ultraviolet images by eye. The team found that 25 of the photospheric sunquakes also produced some kind of motion in the ultraviolet images, and nine of these events were confirmed based on a secondary analysis of the motion of the sunquake wavefronts.

Because the SDO ultraviolet images capture photons from a range of altitudes, it’s challenging to discern the exact location of these sunquakes. However, previous analysis of SDO images found that flare-associated 160- and 170-nm emission arises from the chromosphere. This suggests that the nine sunquakes studied in this article also occurred in the chromosphere and can be used as a starting point for future studies.

The Investigation Continues

Solar flare at three wavelengths

These Solar Dynamics Observatory images of the Sun show a solar flare in three extreme ultraviolet wavelengths. From left to right: 17.1, 30.4, and 13.1 nanometers. [NASA/GSFC/SDO]

There’s still much we don’t know about sunquakes. Why are only some solar flares accompanied by sunquakes while others are seismically quiet? Similarly, why do some sunquakes only ripple through the photosphere, while others extend up into the chromosphere? The most widely accepted model of sunquake production suggests that nearly all solar flares should generate sunquakes, so alternative models may be necessary. With a little luck — and a lot of image analysis — these questions and more may be answered by further investigations of data from Solar Dynamics Observatory and other spacecraft.

Citation

“Flare-induced Sunquake Signatures in the Ultraviolet as Observed by the Atmospheric Imaging Assembly,” Sean Quinn et al 2021 ApJ 920 25. doi:10.3847/1538-4357/ac0139

The Sculptor Dwarf Galaxy, a small galaxy that looks like a clump of stars.

Editor’s note: Haley Wahl is a fifth-year graduate student at West Virginia University, and she was recently selected as the 2021–2022 AAS Media Fellow. We’re excited to welcome Haley to the team and look forward to featuring her writing on AAS Nova regularly!

The first stars to light up the cosmos have so far eluded astronomers. However, new research may now have uncovered these obscure objects in the dusty recesses of a dwarf galaxy 300,000 light-years away.

An Elusive Population

The birthplaces of stars are clouds of gas and dust hundreds of times bigger than our solar system. These nebulae can be formed in a variety of ways and are often the leftover material from a previous star that ended its life in a violent supernova explosion. If this is the case, the nebula can contain elements manufactured in the star that exploded, such as carbon and magnesium, as well as heavier elements forged in the supernova itself, such as silver. Stars that were formed in the aftermath of a supernova contain traces of all these elements.

pop iii stars

Artist’s impression of the first stars in the universe. [NASA/WMAP Science Team]

Based on their composition, stars can be categorized into three populations: Population I stars, which are young metal-rich stars (“metals” here are elements heavier than helium); Population II stars, which are metal-poor stars that started forming early on in the universe when it was only a few billion years old; and Population III stars, which are extremely bright, hot stars that contain almost no metals, only hydrogen, and formed in the earliest stages of the universe. Population III stars represent some of the first stars formed after the Big Bang, and they could provide new insights into what our universe was like when it was only ~800 million years old.

Population III stars have never been observed, only hypothesized, but a team of astronomers led by Ása Skúladóttir (University of Florence, Italy) may be closer than ever to observing these elusive astronomical wonders.

Searching Spectra

The star AS0039 was discovered in the dwarf galaxy Sculptor as part of a large survey done by the European Southern Observatory and observed more in-depth via spectroscopy with the Very Large Telescope/X-Shooter instrument, which reveals the composition of the star. Using the effective temperature of the star obtained from Gaia data, the gravity obtained through photometry, the microturbulent velocity measured through empirical calibration, and a stellar atmospheric model, the team performed spectral analyses on the star to determine its chemical composition.

A plot showing Fe/H vs. Mg/Ti and Fe/H vs. Mg/Ti. Both lines have a slope of around 0.

The ratio of abundances of different metals in stars in AS0039’s host galaxy (blue), dwarf spheroidal galaxies (dSph; green), ultra-faint dwarf galaxies (UDF; yellow), and the Milky Way (gray). AS0039’s metal abundances, shown in red, are unlike the stars in any of these galaxies. [Skúladóttir et al. 2021]

From this analysis, Skúladóttir and collaborators determined that AS0039 has very few metals, and it is different from other metal-poor stars because it is especially lacking in carbon and magnesium, which are formed by nuclear fusion in massive stars. This has intriguing implications for the origin of the nebula from which AS0039 formed.

These abundances of AS0039 are characteristic of elements formed in a hypernova, a giant explosion bigger than a supernova that occurred at the beginning of the universe when stars were more massive. By comparing AS0039’s abundances to those from simulations of Population III supernovae over a wide range of progenitor masses and explosive energies, the authors determined that AS0039 was likely born from the material left over after a star 21 times the mass of the Sun exploded as a hypernova. This discovery provides a link between the current generation of stars and their elusive Population III ancestors.

Future Work

Before this star was spotted, there was only weak evidence of Population III stars from the imprints of low-energy supernovae seen in carbon-enhanced metal-poor stars. The massive star that enriched the material from which AS0039 formed is unlike any star seen before, but this single discovery allows us to probe only a tiny subset of all the possible properties of Population III stars. The discovery of AS0039 will allow astronomers to study these stars in a different light, by probing them through the remnants of the massive explosions of the first stars. Future developments in theoretical simulations and larger spectroscopic surveys will bring astronomers one step closer to solving the mystery of what the first stars were like.

Citation

“Zero-metallicity Hypernova Uncovered by an Ultra-metal-poor Star in the Sculptor Dwarf Spheroidal Galaxy,” Ása Skúladóttir et al 2021 ApJL 915 L30. doi:10.3847/2041-8213/ac0dc2

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